Environmental Protection Technology Series
MOVEMENT OF SELECTED METALS,
ASBESTOS, AND CYANIDE IN SOIL:
APPLICATIONS TO WASTE DISPOSAL PROBLEMS
Municipal Environmental Research Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Cincinnati, Ohio 45268
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RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development, U.S. Environmental
Protection Agency, have been grouped into nine series. These nine broad cate-
gories were established to facilitate further development and application of en-
vironmental technology. Elimination of traditional grouping was consciously
planned to foster technology transfer and a maximum interface in related fields.
The nine series are:
1. Environmental Health Effects Research
2. Environmental Protection Technology
3. Ecological Research
4. Environmental Monitoring
5. Socioeconomic Environmental Studies
6. Scientific and Technical Assessment Reports (STAR)
7. Interagency Energy-Environment Research and Development
8. "Special" Reports
9. Miscellaneous Reports
This report has been assigned to the ENVIRONMENTAL PROTECTION TECH-
NOLOGY series. This series describes research performed to develop and dem-
onstrate instrumentation, equipment, and methodology to repair or prevent en-
vironmental degradation from point and non-point sources of pollution. This work
provides the new or improved technology required for the control and treatment
of pollution sources to meet environmental quality standards.
This document is available to the public through the National Technical Informa-
tion Service. Springfield. Virginia 22161.
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EPA-600/2-77-020
April 1977
MOVEMENT OF SELECTED METALS, ASBESTOS, AND CYANIDE IN SOIL;
APPLICATIONS TO WASTE DISPOSAL PROBLEMS
by
Wallace H. Fuller
Department of Soils, Water and Engineering
The University of Arizona
Tucson, Arizona 85721
Contract No. 68-03-0208
Project Officer
Mike H. Roulier
Solid and Hazardous Waste Research Division
Municipal Environmental Research Laboratory
Cincinnati, Ohio 45268
MUNICIPAL ENVIRONMENTAL RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
CINCINNATI, OHIO 45268
CftEfl -
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DISCLAIMER
This report has been reviewed by the Municipal Environmental Research
Laboratory, U.S. Environmental Protection Agency, and approved for publica-
tion. Approval does not signify that the contents necessarily reflect the
views and policies of the U.S. Environmental Protection Agency, nor does
mention of trade names or commercial products constitute endorsement or
recommendation for use.
ii
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FOREWORD
The Environmental Protection Agency was created because of Increasing
public and government concern about the dangers of pollution to the health
and welfare of the American people. Noxious air, foul water, and spoiled
land are tragic testimony to the deterioration of our natural environment.
The complexity of that environment and the Interplay between its components
require a concentrated and Integrated attack on the problem.
Research and development 1s that necessary first step in problem solu-
tion and 1t involves defining the problem, measuring its impact, and search-
ing for solutions. The Municipal Environmental Research Laboratory develops
new and Improved technology and systems for prevention, treatment, and man-
agement of wastewater and solid and hazardous waste pollutant discharges from
municipal and community sources, for the preservation and treatment of public
drinking water supplies, and to minimize the adverse economic, social,
health, and aesthetic effects of pollution. This publication 1s one of the
products of that research; a most vital communications link between the re-
searcher and the user community.
This report presents a discussion of the physical, biological and
chemical reactions that affect the movement of certain hazardous substances
1n soils. The information is applicable to the problem of safely disposing
of wastes on land.
Francis T. Mayo, Director
Municipal Environmental Research
Laboratory
111
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PREFACE
Much solid waste, whether transported by air, water, or man's
activities, reaches the soil and Interacts 1n a way characteristic of the
particular environment 1n which 1t deposits. With such an explosive
increase in quantity and variety of wastes paralleling the bulge 1n human
population, the potential pollution hazard to the quality of surface and
underground water as well as to the food-producing soil Itself has reached
serious proportions.
The soil 1s a dynamic system where conditions are constantly changing
and where numerous reactions are taking place at any one time. The great
number of attenuation mechanisms which are operative when trace contami-
nants from wastes are put on or Into the soil can be qualitatively Identi-
fied with reasonable assurance, but quantitative data relating to specific
mechanisms are not available. Such a lack, however, does not make 1t Impos-
sible to develop a workable program for controlling potentially hazardous
pollutants. The soil can be treated as a muscle where work 1s performed
which can be measured with some degree of precision, even though not all the
muscle enzymes and mechanisms of Input are Identified and quantltlzed.
There 1s no doubt that additional research will refine and even alter
some of the concepts put forth in this report. However, this Information
1s presented as the best available 1n November 1974 for guiding management
of solid and hazardous waste disposal on land and research on the problems
associated with such disposal.
N1c Korte, Elvla Nlebla. Bruno Alesii, Colleen McCarthy, and Joe Skopp
have been responsible for the laboratory progress of the Arizona research
program. I am Indebted to them, particularly, and to others attached to
1v
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the project: Frank Wlersma, Giles Marlon, Gordon Dutt, and Ted McCreary.
The Invaluable help so generously provided by M1ke Rouller 1n supplying key
reference material and manuscript review 1s greatly appreciated. I wish to
thank Emery Lazar of the U.S. EPA Office of Solid Waste Management Programs
and Robert H. Dowdy, U.S. Department of Agriculture, Agricultural Research
Service, for their helpful suggestions 1n the manuscript development.
Supplements to this publication will be Issued as additional Information
on the subject becomes available. Critical comments, suggestions for Im-
provements 1n format and presentation, and citations of pertinent literature
that was omitted or that has recently been published would be appreciated.
Address such comments to:
M1ke Rouller
EPA Municipal Environmental Research Laboratory
26 West St. Clalr Street
Cincinnati, Ohio 45268
Certain research reported herein and this review were supported, 1n
part, by U.S. EPA Contract (No. 68-03-0208) funds from the Solid and Hazard-
ous Waste Research Division, Municipal Environmental Research Laboratory,
Cincinnati, Ohio 45268, 1n cooperation with the Arizona Agricultural Experi-
ment Station, Journal No. 202, The University of Arizona, Tucson, 85721.
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ABSTRACT
This report presents Information on movement of selected hazardous sub-
stances 1n soil which can be applied to problems of selecting and operating
land disposal sites for wastes containing arsenic, asbestos, beryllium, cad-
mium, chromium, copper, cyahldei Iron, lead, mercury, selenium, and zinc.
The information 1s based on a literature review, laboratory studies of move-
ment of hazardous substances through Soil in municipal landfill leachate, and
the author's experience 1n soil science and waste disposal.
In addition to a discussion of the soil and waste-related factors to be
considered 1n selecting and managing disposal sites for minimum migration
hazard, the report also presents general Information on soils and geological
materials and specific Information on the chemistry of the selected hazardous
substances which 1s relevant to an understanding of their migration in soil.
Critical Information gaps are Identified, particularly as regards the chemis-
try and soil adsorption behavior of mixtures of several hazardous substances
in the presence of high concentrations of other organic and Inorganic solutes,
a situation commonly encountered 1n leachates from municipal and hazardous
solid wastes. In spite of these Information gaps, it Is concluded that waste
disposal practice can be Improved by application of present Information. The
report contains 250 references and a bibliography of 81 related citations.
The report was submitted 1n partial fulfillment of Contract No.
68-03-0208 by the University of Arizona under the sponsorship of the U.S.
Environmental Protection Agency. The work was completed November 30, 1974.
v1
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CONTENTS
Foreword 111
Preface 1v
Abstract v1
List of Tables v111
List of Figures x1
Sections
I. Summary 1
II. Introduction 2
III. Soil and Waste Related Factors to be Considered in
Selecting and Managing Disposal Sites for Minimum
Migration Hazard 13
IV. Soil and Geological Materials 41
V. Mobility of Selected Constituents 1n Soil and Specific
Mechanisms Involved 80
VI. Specific Reactions of Selected Hazardous Constituents . . . .119
VII. References 182
VIII. Bibliography 209
Appendices
A. Pollutant Attenuation Research at the University
of Arizona 219
B. Supplementary Soil Classification Information 234
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LIST OF TABLES
Number Page
3.1 Characteristics of the Soils Used in Arizona Soil Column
Research 28
3.2 The Cation Exchange Capacity, Exchangeable Cations, Surface
Area, Free Iron Oxides, and Total Mn of Soils Used in the
Column Research 29
3.3 The Exchange Acidity, Exchange Capacity and Certain Exchange-
able Elements Found in the Neutral to Acid Soils Used in the
Soil-Column Research 30
3.4 Concentration Ranges of Some Common Cations and Anions of the
Soil-Solution Displacements from Columns Leached with Deion-
ized Water 31
3.5 Article-Size Distribution of the Soil Samples Used in the Column
Research 32
3.6 Total Analysis of Soils for Trace Metals and Free Iron Oxides. . . 33
3.7 Textural Classes and Clay Mineral Composition of the Clay
(< 2y) Separate of Soils Used in the Pollution Attenuation
Research 34
3.8 The Soil Orders Represented in the Arizona Study 35
3.9 Relationship between Certain Measurable Parameters of 11 Repre-
sentative Soils and Attenuation of Selected Trace Contaminants
of Landfill Leachates 37
3.10 The Pore Volumes in which the Element First Appeared in the
Soil-Column Effluent 39
3.11 The pH Value, Clay and Surface Area of Soils used 1n the Column
Research 39
4.1 Comparisons in Number of Particles and Surface Area per Unit
Weight for Different Soil Separates 46
4.2 Dominant Clay Minerals found in Different Soil Orders of the
Comprehensive Classification System 47
viii
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Number Page
4.3 Comparative Properties of Three Major Types of Colloids 48
4.4 Present Soil Orders and Approximate Equivalents in Revised
Classification 53
4.5 Mean Contents and Ranges of Arsenic in Well Drained New
Brunswick Podzal Profiles 60
4.6 Ranges of Total Fe, As, Cu, Mn, Pb, Sb, Sn and Zn Found in
Podzol and Podzol-like Soil Profiles of Humid Temperate
Climate 61
4.7 Some Trace Elements Found in Rock Samples 62
4.8 Concentrations of Trace Metals in Igneous and Sedimentary Rocks,
Seawater, and Soils 63
4.9 Abundances of Elements in Typical Humid Temperate Region
Mineral Soils 66
4.10 Data from Analyses of 68 Soil Samples Representing 30 Soil
Series in California 68
4.11 Total Copper, Lead, and Zinc in Some Humid Temperate Region
Soil Profiles 69
4.12 Abundances of Micronutrient Elements 76
4.13 The Range of Micronutrient Content Commonly Found in Soils and
a Suggested Analysis of a Representative Surface Soil 78
4.14 Relative Mobilities of the Elements in the Supergene
Environment 79
5.1 Some Microbial Transformations of Inorganic Substances 90
6.1 -The Formation Constants and Solubility Products used in
Calculations 125
6.2 Effect of Cation and Anion Hydrolysis on the Conditional Solu-
bility and Solubility Product at Three pH Values 126
6.3 Solubilities and Intrinsic Solubilities of Zn, Cd, Hg and Pb
Hydroxides 131
6.4 Effect of Chloride Concentration on the Conditional Solubility
and Solubility Product 132
ix
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Page
Adsorption of Cadmium by Selected Minerals 134
Heavy Metal Content 1n Soils and Plants 154
Selected Soil Properties and Results of Soil and Plant Lead
Analyses 155
6.8 Mercury Content of Soils and Glacial Materials 159
6.9 Measured Values of Eh, pH and total ^Se, Fe Concentrations of
a 1:10 soH-0.01 M Ca(N03)2 Extract for Estimating pH + pH
Se03 and pH - l/3~Fe . 7 7 173
7C
6.10 Percent Se Extracted from 0.8 g Subsamples of Woodburn s.l.
by 8 ml of 0.1 N Anlon Solutions In 1 M KN03 during 24 Hours
at 25C ..'.." 7 . 173
A-l Some Characteristics of the Soils Used 1n Research at the
University of Arizona 221
A-2 Partitioning of Materials 1n the Municipal Waste-Type Landfill
Used to Generate Leachate 223
A-3 Ranges of Constituents Detected 1n the Natural Leachate
Generated from Municipal Solid Waste and Used 1n the Soil
Column Research 224
B-l Major Divisions, Soil Type Symbols, and Type Descriptions for
the Unified Soil Classification System (USCS) 236
B-2 U.S. Department of Agriculture (USDA) and Unified Soil
Classification System (USCS) Particle Sizes 237
B-3 Corresponding USCS and USDA Soil Classifications 238
B-4 Corresponding USDA and USCS Soil Classifications 239
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LIST OF FIGURES
Number Page
4.1 Textural classification of soil based on the percentage of
different sizes of particles it contains 43
4.2 General relationship between particle size and kinds of minerals
present. Secondary silicates (Fe and Al) dominate the fine
colloidal clay. The hydrous oxides of Fe, Al, and Mn are prom-
inent in the fine silt and clay fractions 44
4.3 General conditions for the formation of the various silicate
clays and oxides of iron and aluminum. Clay genesis is accom-
panied by the removal of solubilized elements such as K, Na,
Ca, and Mg 45
4.4 Classification of soil structure 49
4.5 Classification of soil pH ranges 51
4.6 Geographic distribution of low-, variable-, and adequate-Se areas
in the USA 72
4.7 A typical soil profile 74
5.1 General nitrogen cycle illustrating nitrogen transformations in
the soil 93
5.2 The nitrogen cycle in nature 94
5.3 The carbon cycle and organic matter cycle showing decay release
of mineral nutrients and humus formation 95
5.4 The phosphorus cycle in nature showing its universal distribution 96
5.5 The sulfur cycle in nature showing oxidation of sulfur to sul-
furic acid and sulfate formation and reduction to sulfides . . 97
6.1 Distribution of molecular and ionic species of divalent Cd at
different pH values 127
6.2 Distribution of molecular and ionic species of divalent Hg at
different pH values 128
xi
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Number
Page
6.3 Distribution of molecular and ionic species of divalent Pb at
different pH values 129
6.4 Distribution of molecular and ionic species of divalent Zn at
different pH values 130
6.5 Cadmium electrode titration curves for soil humic acid, organo-
clay, and illite. 50 mg samples in 100 ml of 0.1 M Ca(N03)2. 133
6.6 Relative adsorption capacities of clay minerals for low levels
of cadmium in calcium solutions at 26°C 135
6.7 The solubility of various Cu minerals compared to that of soil
Cu 140
6.8 Soluble Cu species in solution equilibrium with soil Cu . . . . 141
6.9 Eh (pE) vs. pH stability field diagram from iron (aFe = 10 M;
PCD = 0;Ptot = ] atm> T" = 25°C> 149
£ M
6.10 Eh (pE) vs. pH stability field diagram for iron (aFe = 10 M.;
PCQ = 10~°'81, 10"2'7, and 10~3t5atm; Pt t = 1 atm; T. = 25°C) 150
6.11 The Fe (III) species in solution in equilibrium with hydrous
ferric oxides 151
6.12 The solubility of various Fe(II) minerals compared to that of
soil Fe 152
6.13 The cycle of mercury interconversions in nature 165
6.14 An anaerobic biological synthesis mechanism for methyl mercury. 166
6.15 Generalized geochemistry cycle in mercury 167
6.16 Stable fields of selenium 169
6.17 Solubility diagram for the ferric selenites, and the solubility
data obtained from 1:10 soil - 0.01 M Ca(N03)2 extracts of
seven soils shown in Table 171
6.18 The oxidation-reduction potentials of selenium couples and some
redox couples that could affect oxidation state of selenium
in soils as a function of pH 174
6.19 The solubility of various Zn minerals compared to the solubil-
ity of Zn in soils 179
xii
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Number Page
6.20 Soluble Zn species 1n solution 1n equilibrium with soil Zn. . 180
6.21 The solubility of Zru(POA)« compared to that of other Zn
minerals and soil Znf . 7 7 181
A-l Diagram of the municipal solid waste leachate generator . . . 222
A-2 System used at the University of Arizona for circulating and
applying leachate anaeroblcally to soil columns 227
A-3 Relative mobility of cations 1n soils used 1n the University
of Arizona study 231
A-4 Relative mobility of anlons used 1n the University of
Arizona study 232
B-l Geographic distribution of Great Soil Groups 1n the 1938
U.S. Department of Agriculture Soil Classification System . . 240
B-2(A) Geographic distribution of Orders and Suborders 1n the 1960
U.S. Department of Agriculture Soil Classification System . . 241
B-2(B) Legend for F1g. B-2(A) "Geographic distribution of Orders and
Suborders" 242
xiii
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SECTION I
SUMMARY
A critical review of the literature pertinent to biological, chemical,
and physical reactions and mechanisms of attenuation (decrease in the maxi-
mum concentration for some fixed time or distance traveled) of selected
elements — As, Be, Cd, Cr, Cu, Fe, Hg, Pb, Se, and Zn — asbestos and
cyanide in soil systems is presented.
A "state of the art" on migration rate through soil of potentially
hazardous pollutants contained in leachates generated from solid waste and
wastestream disposal was developed. This presentation originated from (a)
a critical review of the literature, (b) ongoing research at The University
of Arizona, and (c) the author's best knowledge and expertise in the area of
soil and water sciences.
Only a limited amount of information concerning the attenuation in
soils of the 10 selected elements, cyanide, and asbestos originating from
solid wastes or any other sources is available. From the standpoint of
practical application, the literature is still contradictory, fragmentary,
and confusing.
Despite the limited usable data in the literature and only partially
completed research program at The University of Arizona, certain guidelines
have been developed which are useful for initiating a pollution control
program for wastes disposed-of on land.
Throughout the report, needs for supporting research data are
suggested.
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Solutions (leachates) containing hazardous constituents must migrate
through soil and geologic material to reach the capillary fringes of under-
ground water reservoirs. Because the soil is a far more reactive and
dynamic system than geologic material, the soil's capacity to immobilize
and retain (attenuate) pollutants is far greater than that of geologic
materials.
Numerous factors have been suggested as influencing mobility of ele-
ments in soils. Among those most frequently mentioned are: Physical —
texture (particle size distribution), structure (pore size distribution),
bulk density, temperature, moisture regime, aeration; Biological — aerobic
and anaerobic microbial transformations of inorganic and organic substances,
addition of certain microbial products to the system, removal of inorganic
ions from solution by, and transitory immobilization in, microbial tissues;
changing of the quantity of an element in the soil by such mechanisms as
volatilization; Chemical — pH or hydrogen ion activity, oxidizing/reducing
conditions, lime, organic matter, concentration of ions or salt, and certain
hydrous oxides.
Under appropriate conditions any of these factors could become domi-
nant and exert the controlling influence on mobility. However, in general,
the following will be the most significant:
1. Soil texture or particle size distribution (sand,.silt, and clay).
2. Pore space distribution (soil structure, bulk density, compaction,
etc.) is closely related to Number 1, above.
3. Content and distribution of Fe, Al, and Mn hydroxy oxides and
oxides in soil and coating particles.
4. pH (pH of soil and leachates and buffering capacity).
5. Reduction/oxidation potential in the soil in micro- as well as
macro-pores (anaerobic and aerobic, oxic and anoxic conditions).
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6. Soil organic matter and amount and concentration of organic
constituents in the wastes which may become an energy source for
microorganisms.
7. Concentration of hazardous ions.
These are not listed in order of importance because the intensity with
which a factor is expressed will determine its influence, relative to other
factors, on mobility. Overlappings and interactions among physical, biologi-
cal, and chemical factors are to be expected as a rule, not as an exception.
One of the most critical factors in attenuation of the selected haz-
ardous constituents in soils is pore size distribution. Since this will
vary significantly even at short distances within a single soil type (or
series), a valid laboratory evaluation of the suitability of a disposal
site can be made only on disturbed soil and geologic material where minimum
heterogeneity is established. Disturbing the soil is to be viewed as a very
favorable and even an essential practice in land disposal of wastes because
such disturbance will increase attenuation by increasing the number of smal-
ler pores, reduce channeling, and improve contact conditions with soil
particles.
Among the many soil parameters studied at The University of Arizona
with selected soils representing seven major soil orders in the U.S.,
attenuation and mobility were significantly correlated with particle size
distribution (texture) and the amount of extractable iron, probably domina-
ted by hydrous oxides of Fe. Because texture varied so widely among the
soils, an evaluation of soil characteristics other than Fe content may not
have been made satisfactorily.
Based on information in the literature, the 12 selected constituents
may be grouped as follows with respect to mobility in soil under aerobic
conditions:
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Mobility Class Element
Comments
I. Relatively mobile
Cyanide - CN"
Selenium - HSeO^ & SeOl
II.
Moderately mobile
Iron
Zinc
Lead
Copper
Beryllium - Be
III. Slowly mobile
Arsenic - h
c.
Cadmium - Cd
Chromium - Cr
(or Cr"°)
LJL
Mercury - Hg
Asbestos - < 2y
IV. Immobile
Asbestos - > 2y
Not strongly retained by
the soi1.
Not strongly retained by
the soil, at normal pH
levels.
Absorbed more strongly by
the soil in the order of
Cu"H">Pb"H">Zn'H' Fe"1"1". Stabil-
ity for complexes of any given
type should be increasing
in the order of Fe>Zn>Pb>Cu.
b
(Chemistry in soils probably
similar to aluminum.)
Mobility similar to phospho-
rus.
Forms insoluble precipitates
in oxidizing conditions.
Forms insoluble precipitates
in oxidizing conditions.
Retained in the surface layer
of most aerated soils.
Particles less than 2y, are
retained in surface layer
of soils like clay.
Particles >2y, or greater
than clay size are retained
on the surface of soils.
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Based on the results to date (1974) at The University of Arizona using
municipal landfill leachate as the transporting medium the relative
mobility of eight selected constituents under anaerobic conditions is
as follows:
Soil Series
Element in Leachate
Acid Soils
Ava
Kalkaska
Wagram
Davidson
Molokai
Chalmers
Zn, Cd, As, Cr, Se, Cu
Be Pb
In order of decreasing mobility
Neutral to Alkaline Soils
Anthony
Fanno
Mohave (limy)
Mohave
Nicholson
Cr, As, Se, Cd, Be, Cu
Zn Pb
In order of decreasing mobility
An expected grouping of the soils for immobilization of the selected
elements in municipal landfill leachates under anaerobic conditions is
as follows:
Mobility Class
I. Strongly immobilizing
II. Usually strongly
immobilizing
III. Moderately immobilizing
IV. Weakly immobilizing
Texture
clay
silty clay
silty clay
loam
clay
clay
sandy clay
loam
silty clay
loam
sandy loam
sandy loam
sand
loamy sand
Soil Series
Molokai
Nicholson
Chalmers
Davidson
Fanno
Mohave
(limy)
Ava
Mohave (lime
free)
Anthony
Kalkaska
Wagram
Order
Oxisol
Alfisol
Molisol
Ultisol
Alfisol
Aridisol
Alfisol
Aridisol
Entisol
Spodosol
Ultisol
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SECTION II
INTRODUCTION
There is no doubt that almost all waste materials, sooner or later,
reach the soil and interact with soil constituents in a way characteristic
of the particular habitat in which they deposit. Since air, rivers, and
streams are only vehicles for waste transport, and sensitivity over what
enters the ocean is keen, the soil continues to be the primary means of mass
disposal. The soil is an experienced "old-timer" at effective digestion and
disposal of plant and animal wastes. Recently, however, because of the
quantity and variety of wastes, and variations in methods of disposal, the
leachates from land disposal sites are presenting a potential threat to the
quality of surface and underground water as well as to the soil itself. In
view of this threat, individuals from industry and local, state, and fed-
eral government are asking for answers to questions about the best methods
of disposing of wastes on land.
To speculate in the absence of at least some available data for back-
ground carries a certain tag of professional hazard. Yet, not to speculate
at all carries an even worse penalty of stagnation in professional progress.
This report on the potential for migration of twelve hazardous substances in
soil takes the middle road between complete speculation and the conservative,
traditional scientific approach. Having enjoyed many years of university
and federal research and college teaching, I do not welcome having to take
this middle position. On the other hand, when confronted with the growing
demand by industry and society for some guideline ... any guideline ...,
planning for the safe disposal of wastes must be initiated; my conscience
dictates a program of action. This action is in the form of a report which
presents an interpretation of the literature along with our best knowledge
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of the major biological, chemical and physical reactions in soil of some
potentially hazardous pollutants. Since this information relates to pollut-
ants which are already in solution, this report applies both to liquid wastes
and the leachates from solid wastes disposed of on land. The constituents to
be reviewed are: arsenic, asbestos, beryllium, cadmium, chromium, copper,
cyanide, iron, lead, mercury, selenium, and zinc.
The many threads of the "state of the art" of migration and attenuation
of these constituents must be pulled together as a point of departure for
future research planning as well as for initiating a functional land pollu-
tion control program for protecting the quality of our environment and
insuring the future survival of man. To meet these objectives most effec-
tively, research information available in the literature as of 1974 and that
currently developed at The University of Arizona have been combined in this
report. The result is a hybrid of a review and a research report.
The specific objectives of this work were to (a) provide a critical
review of the existing literature concerned with biological, chemical and
physical reactions of certain hazardous materials in soil systems, with
emphasis on the migration potential of these materials through soils; and to
(b) map out areas for future research aimed at providing a base for develop-
ment of pollution-control procedures and guidelines regarding land disposal
of the designated hazardous materials.
This review-report is presented in eight sections. Section I provides
a summary for transmitting the gist of the report for those who do not have
a need to delve deeply into the subject matter and the multisciences bearing
on molecular and ion migration through the soil.
Section II is the Introduction.
In Section III, some recommendations for disposal of wastes containing
the selected hazardous constituents which may be solubilized and appear in
aqueous leachates are discussed. The impact of the ongoing research at The
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University of Arizona is incorporated most distinctly in this part.
In Section IV, background information on the general characteristics
of soils and geologic material as may influence the migration of hazardous
substances in soils is given. It is prepared principally for the uniniti-
ated in soil science and geoscience. The natural content of the 12 hazard-
ous constituents in soil is also given in this section.
In Section V, the potential for migration of the hazardous constitu-
ents through soil is discussed in broad terms and an estimation of the rela-
tive rates of mobility through soils of these constituents is presented.
The attenuation classification of soils and migration potential classes of
hazardous materials are based largely on work with metals In the matrix of
municipal landfill leachate. Results may differ if the metals are carried
in a different matrix or were present in higher concentrations, as, for
example, leachates from some industrial waste streams. A discussion of
attenuation mechanisms is included in this section.
Section VI discusses some of the chemistry of the 12 hazardous constit-
uents. This information is presented not as having direct application to
disposal problems but as forming a basis for understanding the attenuation
mechanisms discussed in Section V.
The literature on trace and heavy elements as it may relate to any
aspect of the soil is dominated by the following fields of interest to the
almost complete exclusion of other fields.
1. Concentration evaluation of trace elements necessary as nutrients
for plant growth in soils.
2. Uptake of trace nutrient elements from soil by plants.
3. Chance uptake of a few heavy elements from soil by plants, as
related to growth inhibition and toxicity.
8
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4. Chance uptake by plants of trace and heavy elements contained in
sewage effluents, sludges, and municipal composts applied to soils.
5. Concentration evaluation of trace and heavy elements in bottom
mucks and muds of lakes, streams, and seas where waste disposal has
occurred over a period of time.
6. Surface soil contamination from industrial stack gases and particu-
lates, automobile emission, industrial substances, etc.
The geochemist has focused attention, almost wholly, in areas of ele-
ment behavior not closely related to soil or soil problems. Accumulation
mechanisms of indefinite forms of elements often occurring in soils have
been avoided as a field of study.
Soil scientists also have failed to consider trace and heavy elements in
the broader aspects of soil interaction except as essential plant nutrients
or, to a limited extent, as toxic factors in normal plant growth. The agri-
cultural approach has dominated to the complete exclusion of all other
approaches, particularly pollution control.
Certain terms used in this report have such a wide variety of meanings
that they require defining. Some of the most conspicuous ones are:
Absorption - is confined to biological uptake of elements and constituents.
Adsorption - relates to a physico-chemical process of holding or the immobi-
lization mechanisms against extraction by salt solution.
Adsorption complex - The group of substances in soil capable o,f adsorbing
other materials. Organic and inorganic colloidal substances form the
greater part of the adsorption complex; the noncolloidal materials,
such as silt and sand, exhibit adsorption but to a much lesser extent
than the colloidal materials.
-------
Attenuation - Attenuation is defined by looking at the movement of a pulse
of a solute through a soil. As the pulse migrates, the maximum con-
centration decreases. Attenuation can then be defined as the decrease
of the maximum concentration for some fixed time or distance traveled.
Fixation Fixation is defined for use in this report as that portion of
trace contaminant retained against salt extraction. (Some investi-
gators, particularly those in the plant sciences, use another criter-
ion, i.e., that portion of constituent retained against acid extrac-
tion.) The Soil Science Society of America (1964) definition Is:
"The process or processes in a soil by which certain chemical elements
essential for plant growth are converted from a soluble form to a much
less soluble or to a non-exchangeable form; for example, P "fixation."
Contrast with N "fixation."
Hazardous Substance - A substance which may cause or contribute to an in-
crease in mortality or serious illness on account of its toxicity,
persistence, mobility, or potential for accumulation or concentration
in tissue. Such substances may be harmful even in small quantities.
See, for example, the allowable concentrations of metals in the
Interim Primary Drinking Water Standards (Federal Register, March 14,
1975, 40(51):11994). The hazardous substances considered in this
report were selected from the pollutants covered by the Clean Air Act
and the Federal Water Pollution Control Act, reasoning that tighten-
ing restrictions on discharge of these into air and water must inevi-
tably lead to their increased disposal on land, the only remaining
sink for wastes. Iron, though not strictly hazardous, was included
because it is known to be one of the major pollutants in leachates
from municipal landfills.
Heavy Metals - In the chemical literature the term "heavy metals" generally
refers to those metals which have denisities greater than 5.0-
Landfill Leachate - originates primarily from natural rainfall indigenous to
the climate in which the landfill is located- It accumulates as rain
10
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wets the landfill, infiltrates the solid waste pore spaces, and
finally occupies the lower levels of the landfill varying in depth
according to the balance between infiltration rate of the surrounding
soil and/or geologic material and intensity of the rainfall. Biologi-
cal transformations within the leachate are dominated by anaerobic
processes. Landfill solid waste may be municipal (resident wastes),
industrial (primarily from chemical, mining, agricultural, food-
processing, or forestry processes). The landfill leachate used in
The University of Arizona research program originated from typical
municipal solid waste.
Sorption - relates to loosely as well as strongly fixed ions or molecules by
the soil constituents. A host of mechanisms is involved.
Trace Elements - These elements, especially metals, are used by organisms in
minute quantities but nevertheless are believed to be essential to
their physiology. (In a looser sense, the term has been used to desig-
nate elements with no known physiological function.) More recently it
has been used also to refer to elements which, though present in only
minute quantities, are toxic to living systems.
Trace Contaminants - Trace contaminant in this text refers to the ten ele-
ments listed (As, Be, Cd, Cr, Cu, Fe, Pb, Hg, Se, Zn), asbestos, and
cyanide. They may or may not be essential for plant growth; but, if
present in the soil solution in great enough concentration, are
absorbed by, and toxic to, plants.
In this report "trace elements" will refer to As, Be, Cd, Cr, Cu, Fe,
Hg, Pb, Se, and Zn. "Heavy metals," "heavy elements," "transition elements,"
in this report, will refer to As, Cd, Cr, Pb, Se, and Hg even though As and
Se are not transition series. Asbestos and cyanide will be considered poten-
tially hazardous pollutants along with the elements just listed but will be
referred to as substances, materials, or constituents, as well as by their
given names.
11
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The soil is a dynamic system where conditions are constantly changing
and where numerous reactions are taking place at any one time. The great
number of attenuation mechanisms which are operative when trace contami-
nants are put on or into the soil can be qualitatively identified with
reasonable assurance, but quantitative data relating to specific mechanisms
are not available. The lack of such data, however, does not make it impossi-
ble to develop a workable program for controlling the spread of potentially
hazardous pollutants from land disposal sites. The soil can be treated as
a muscle where work is performed which can be measured with some degree of
precision, even though not all the muscle enzymes and mechanisms of input
are identified and quantitized. This type of research program, currently
underway at The University of Arizona, Tucson, is described in the appendix.
Attenuation predictions are best made on a relative basis, e.g., by compar-
ing one element with another and by comparing various soils. Predictions
derived from such laboratory data provide maximum attenuation information
for any given element. Attenuation in undisturbed field soil may be less
because of differences in pore size distribution, a more "open" soil struc-
tural condition, and other possible soil physical variations.
There is no doubt that additional research and experimentation will
refine and even alter some of the concepts put forth in this report. How-
ever, guidelines for safe disposal of wastes on land are needed now and this
report presents the best information available as a basis for their
formulation.
12
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SECTION III
SOIL AND WASTE RELATED FACTORS TO BE CONSIDERED IN SELECTING AND MANAGING
DISPOSAL SITES FOR MINIMUM MIGRATION HAZARD
The information presented here has its origin in a synthesis of data
obtained from the literature review, the ongoing University of Arizona
research program, and from the author's experience. This is a preliminary
attempt to provide information useful in the selection of land disposal sites
and in the management of existing disposal sites so as to minimize the hazard
of migration through soil. This part contains two broad subsections. The
first is a discussion of some factors affecting migration with emphasis on
how the waste or soil at a particular site can be managed to minimize migra-
tion. The second contains a detailed listing and discussion of the chemical
and physical properties and attenuation ratings for soils used in a labora-
tory study of pollutant migration in municipal landfill leachate; it is
intended to aid in selecting the best of several soils for attenuation of
pollutants. The information from both subsections is repeated and explained
in greater detail in other parts of this report.
FACTORS AFFECTING MIGRATION RATE
The many factors affecting migration rate have been sorted out to iden-
tify those most often dominant in movement of the 12 selected potentially
hazardous pollutants (trace contaminants) through soil. Before discussing
specific factors, two important general conditions should be mentioned.
The first is that the soils themselves contribute to the sum of the
materials, including the 10 elements listed as trace contaminants, that move
through the soils, finally entering underground water or lodging in the soil
as insoluble or slowly soluble compounds. Elements in these compounds may
13
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be re-released by man's activities, specifically, by the disposal of acidic
or acid-forming wastes on the soil. In some instances, particularly newly
developed agricultural lands in arid regions which have not been previously
leached, the natural migration in itself may be sufficient to pollute the
groundwater beyond human use. In other instances (depending on the compo-
sition of the overburden of soil and geologic debris) the quantity and quali-
ty of such migration is of little or no significance to water quality. Cli-
mate, particularly rainfall, is a highly significant factor in natural soil
pollution since it may provide the vehicle for movement or may have previ-
ously leached most of the readily soluble materials out of the soil.
The second important condition to recognize is that the soil-contami-
nant relationship is dynamic and time must be a consideration in any assess-
ment. For example, ions are not permanently attached to cation exchange
positions on the clay minerals. As the concentration of the ion in solution
decreases or the concentration of a competing ion increases, some or all of
the ions on exchange positions will be exchanged and free to migrate. Thus,
as the waste or the soil environment changes, the attenuation becomes time
dependent and the soil must be viewed not as a capacitor which only stores
contaminants but as a parallel-connected capacitor and resistor which "leaks"
at some rate which changes with time and with the forces applied to it. In
this same context, while only very small concentrations of contaminants,
even near the limit of detectability, may be in a mobile status, accumula-
tion in soil and later release in significant quantities may take place over
an extended period of time. Thus, very small concentrations may have to be
considered significant because of the time factor.
Specific Factors Influencing Migration
There is no definite division between those factors which decelerate
or accelerate migration of the trace contaminants through soils. The influ-
ence depends on the intensity with which all factors are expressed in the
soil. A factor which may decelerate or attenuate a constituent in one soil
may have either no effect or an opposite effect in another soil. Moreover,
14
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a factor which may inhibit movement of one element may have no influence on
the movement of another element.
The following listings, however, are based on what may be considered
as general or broad trends suggested in the literature. The factors are
discussed not necessarily in the order of importance because relative im-
portance depends on the other factors present in the soil.
1. Hydrogen ion activity (pH)
a. Soils - Except for selenium, chromium, and some valency states of
arsenic, attenuation (decelerated migration) may be expected to be
greater in soils of neutral to alkaline pH values.
b. Aqueous wastes - An aqueous solution containing some level of one or
more of the trace contaminants originating from solid or liquid
wastes deposited on land. The soil is very well buffered but pro-
longed discharge of highly acidic (pH below 2.0) or highly alkaline
solutions (above 8.5) alter the soil pH and cause the release of
natural soil constituents (among which will be some of the trace con-
taminants) for migration. This effect is most pronounced in sandy
soils with no free lime and least pronounced in fine-textured cal-
careous soils. Highly alkaline aqueous wastestreams keep many of
the contaminants immobilized until the pH of the soil rises above
8.5. Except for selenium, chromium, and possibly some valency states
of arsenic the more acidic the soil-waste medium, the greater is the
solubility of the constituents both in the soil and in the liquids
associated with the waste. Consequently, as the acidity increases
from neutral (pH value decreases) the attenuation tends to decrease,
i.e., migration accelerates.
c. Selenium, arsenic, and chromium - The literature is quite clear in
that selenium is more available to plants when the soil is alkaline,
Brown and Carter (1969). It may be inferred that it will also be
more mobile under these conditions. Research at The University of
Arizona has indicated, in general, that Mohave soil which is high
in native lime retards migration of the trace elements studied. How-
ever, Se, As, and Cr have been unexpectedly mobile through this soil.
15
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With Se, this is probably due to the high pH stabilizing soluble
selenates in the soil solution. In the case of As and Cr, no such
corroboration has been found in the literature. However, on a
quantitative basis both are much more mobile in alkaline soil than
was expected. In a separate experiment, Cr was leached through a
layer of lime and there was no observable difference between the
presence or absence of the lime layer over a sandy soil. In contrast,
the difference between lime and no lime was very great for Ni and Cd.
d. Management - To minimize migration, the soil, or the liquids associ-
ated with the waste, or both can be managed to maintain neutral to
slightly alkaline conditions by adding agricultural limestone or
other liming material and by excluding or pretreating acidic wastes.
In agriculture, remedying plant diseases due to an excess of soluble
trace metals is done by raising soil pH to decrease the solubility/
availability of the trace metal. Likewise, in the mining industry,
pH control is a well established practice for treatment of trace
metal-bearing waste waters, Pearson, et al. (1975).
2. Oxidation/reduction (redox)
A reaction in which an electron transfer takes place is called an oxida-
tion-reduction process. An element or substance giving off electrons is
,111 4"i*
being reduced. For example, Fe + e_ * Fe or Ox + ne_ * Red. The
intensity of the oxidizing or reducing action of a system is determined
by a standard electrode placed in the solution system. The potential
difference between the electrode and solution is called redox potential.
Under reducing conditions, i.e., usually the absence of free molecular
oxygen, chemical or biological oxygen-demanding reactions are consuming
oxygen at a greater rate than it is being transported into the system.
Microorganisms find a substitute for Og in metabolic processes. These
substitutes may contain combined oxygen as nitrates (NOl) or sulfates
o •*
(SOj: ) or may involve electron transfer without the involvement of oxygen
as with Ferric (Fe ) or manganic (Mn +) substances.
Oxidizing conditions favor attenuation as opposed to reducing conditions.
The transfer of electrons need not include oxygen. Cyanide is an
16
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exception. When it is oxidized to NO^, it is highly mobile. The
reduced form, NhL is less mobile. Precipitates develop in anoxic land-
fill leachates upon exposure to air (oxic conditions). Many of the
trace contaminants become a part of this insoluble residue, Korte et al.
(1976).
Reducing (anoxic) conditions favor accelerated migration of heavy metals
as compared with oxidative (oxic) conditions. For example, trace con-
taminants As, Be, Cr, Cu, Fe, Ni, Se, V, Zn, are much more mobile under
anaerobic than aerobic soil conditions, all other factors the same.
Water logging such as exemplified by the soil below the leachate level
of sanitary landfills favors accelerated mobility of most trace constitu-
ents. The effect of aerobic (oxidizing) and anaerobic (reducing) condi-
tions on mobility is discussed in greater detail later in this section.
When wastes are disposed-of in ponds, lagoons, or deep fills, management
of redox status is not always practicable. However, in land spreading
or spray irrigation operations, oxidizing (aerobic) conditions can be
promoted by allowing the soil-waste system to dry between waste appli-
cations. The determination of the tendency for soils towards oxidation
or reduction conditions should play a part in establishing management
practices for waste and wastewater disposal sites.
3. Particle size distribution of soils
Many attenuation mechanisms involve physical and chemical reactions on
surfaces. The greater the surface area available, the greater is the
potential for attenuation by these mechanisms. Because of greater sur-
face area per unit weight, finer soil materials (silts, clays, and col-
loids) have greater attenuating characteristics than the coarser material
(sands and gravels). In general, the finer the soil texture, the less is
the migration of trace constituents. One should keep in mind, however,
that the soil contains a great number of colloidal species both organic
and inorganic which vary several-fold from each other both with respect
to ion exchange capacities and ability to hold the trace contaminants
against removal by solutions of salts, acids, alkalies, etc. The
17
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colloidal hydroxy oxides and oxides of Fe, Mn, and Al react strongly
with most of the trace contaminants and retain them against exchange
much more tenaciously than the clay minerals. Hydroxy oxides (Fe, Al,
and Mn) coat particles such that a small amount can have a profound
influence on attenuation. Whenever possible, disposal sites should be
located on fine-textured soils containing significant amounts of hydroxy
oxides. Alternately, disposal sites can be lined with such soils or with
a clay.
4. Pore size distribution
The pore size distribution is the volume of the various sizes (diameter)
of pores in a soil expressed as a percentage of the bulk volume (soil
particles plus pore space). Fine-textured soils generally have a
greater total volume of pore space than coarse-textured soils but the
pores in fine-textured soils are usually much smaller than the pores
in coarse-textured soils. Some clay soils can Impede water from verti-
cal flow almost completely because of the very small pore spaces while
sandy soils, on the other hand, transmit water rapidly.
Because water in soil pore spaces 1s the vehicle in which soluble con-
stituents (colloids also, but to a much lesser extent) move and because
soil water travels more rapidly through larger than through smaller pore
spaces, the pore size distribution of a soil has a profound influence on
migration of trace contaminants. Soils with small diameter pores will
restrict the migration of trace contaminants by slowing the rate of
movement of water through the soil which, 1n turn, allows more time for
the contaminants to react physically or chemically with the soil parti-
cles. Attenuation reactions proceed at some finite rate and, for the
same amount of solution passed through soil, the greater amount of
attenuation would be associated with the slower flow rate.
Waste disposal sites should be selected and constructed to maximize the
percentage of small pores in the soil. Fine-textured soils are preferred.
Any soil can be improved by disturbing 1t and compacting it as deeply as
practical to minimize the total pore space and maximize the amount of
18
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small pores. Disturbance and compaction are desirable not only because
they will reduce the rate of flow of water through the soil but also
because fresh soil particle surfaces will be exposed which may be more
reactive with contaminants.
5. Lime
Because of the effect on pH and carbonate ion concentration, the presence
of lime in soil, either as a result of natural soil-forming processes or
man's addition retards migration of certain hazardous elements except
for Se, Cr, V, and possibly As, item Ic above. Lime added in sufficient
quantity to raise the pH value of soil also is particularly effective in
attenuation of certain elements as indicated in item Ic. The effect of
the carbonate ion generally is to decelerate migration by combining with
heavy metals to form poorly soluble precipitates (e.g., PbCO~, ZnCO~,
CuC03).
6. Organic matter
Organic matter in soils, as well as in the aqueous leachates and other
wastestreams, has a generally decelerating influence on trace contaminant
mobility. For example, heavy metals and certain hazardous trace elements
are found at enrichment levels in sludges and at low levels in the efflu-
ent solution. In addition to organic matter's high cation exchange
capacity, which holds ions temporarily, it possesses mechanisms which
strongly retain heavy metals. Organic colloids retain Zn and Cd as
strongly as they retain Ca. Copper, Pb, Hg, and Cr are retained more
strongly than Ca. Organic matter has the capacity to form some rather
insoluble precipitates with trace contaminants. Chelation, though, can
undo some of the beneficial immobilization properties of organic colloids
and organic acids by forming soluble compounds with trace contaminants.
The organic link to the trace contaminant (complex, chelate, or precipi-
tate), however, is limited not only by its chemical stability, but by
its susceptibility to microbial attack which can release the element for
chemical reaction with soil constituents and/or further microbial
incorporation.
19
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Because of uncertainty about the rate and extent of specific reactions
between organic compounds and trace contaminants and uncertainty about
the ultimate fate of the compounds formed, no recommendations are offered
about organic matter and selection or management of land disposal sites.
7. Concentration of ions or salts
The wide variety of reactions each trace contaminant might undergo makes
relative concentrations of ions very important with respect to mobility.
Low concentrations of salts favor more complete attenuation by simple
attachment to soil ion adsorption positions. Also, many of the trace
contaminants form very insoluble precipitates at low concentration.
+*^+ XA
Examples are lead, arsenate, and sulfides of Pb and Cd .
Where concentrations of some salts are high, the effect of competing
reactions can be especially important. In a leachate containing some of
the hazardous pollutants in small amounts, as well as a large amount of
++ ^ <
Ca , the Ca might effectively dominate the exchange reaction to the
exclusion of the trace contaminants. On the other hand, if the sulfides
were also present in high concentration, the trace elements would be
immobilized by precipitation.
High concentrations of certain ions can also dramatically reduce solubil-
ities due to the "common ion" effect. A leachate high in sulfate or
chloride would shift the equilibrium of those elements precipitated by
chloride or sulfate far to the right or less soluble state, in what is
known as "salting out."
Concentration of ions or salts may either increase or decrease attenua-
tion depending on (a) the kinds and concentrations of ions present in
the soil solution, (b) the concentration of the trace contaminant in the
soil solution and the leachate from the waste, and (c) hydrogen ion
activity or pH. Each case has to be analyzed separately.
No general statement about ion concentration and attenuation can be made
because the subject is too complex and very little work has been done
with the chemistry of high concentration-multiple ion systems. Whenever
20
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possible, consider mixing wastes to promote formation of insoluble
compounds. Be aware that attenuation observed for a given soil and con-
centration of contaminant in a specific waste may not be a constant;
attenuation is likely to be different if another waste with the same
contaminant concentration but differing composition of other substances
is disposed of on that soil.
8. Certain hydrous oxides
++ +++ "H"
Adsorption reactions with hydrous oxides of Fe , Al and Mn , in
general, are considered by many investigators to furnish a major mecha-
nism for the attenuation of heavy metals in soils. The abundance of Fe,
Al, and Mn in soils and their chemistry which is sensitive to slight
changes in redox make them prime mechanisms for removal of trace contami
nants from circulation. Some of the trace contaminants at first may be
adsorbed on the surface of the hydrous oxide and later buried by the con
tinued formation of hydrous oxides-trace metal combinations. Leeper
(1972), for example, writes an equation such as:
2FeO.OH + Zn2+ -»• ZnFe0 + 2H+
to explain a possible adsorption reaction.
Whenever a choice is possible, soils with a significant content of hydrous
oxides should be selected for disposal sites. It is suggested, as a
result of preliminary data from the University of Arizona research that
soils at disposal sites be treated with iron compounds to form artificial
hydrous oxide coatings on the soil particles. Based on present knowledge
of the role of hydrous oxides in attenuation, this approach seems promis-
ing. However, it has not been examined in detail or field tested.
9. Climate (weathering)
Climate, as expressed in rainfall and temperature, influences attenuation.
Wetting and drying, waterlogging or droughting (aerobic/anaerobic) are
controlled in the soil by climate. Wetting and drying generally decrease
mobility of heavy metals, particularly if they occur during short
21
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intervals of time. Warm and hot temperature conditions, as occur where
lateritic (thermic) soils form (Hawaii, southern Florida), favor the
formation of the hydrous oxides and oxides in soils which are important
in slowing migration of trace contaminants. High rainfall may dilute the
contaminant or promote its migration through soils, depending on the rate
at which the contaminant can be leached from the waste.
Management of wetting-drying or converting back and forth from anaerobic
to aerobic is not practicable except when wastes are disposed of by land
spreading or sprinkler irrigation systems; although it is possible to
control the leaching of contaminants from waste into the soil either by
covering the waste to exclude rainfall or by lining the disposal site
to facilitate collection and treatment of liquids that have passed
through the waste. Different climatic conditions will dictate the site
selection and management system chosen.
10. Aerobic and Anaerobic Conditions
Literature is not particularly helpful because little is known about move-
ment of heavy and trace elements through soils under anaerobic or anoxic
conditions.
Oxygen-stressed soil and geologic material will receive greater research
attention as soil scientists shift effort more from the historical
emphasis on food and fiber production to controlled waste disposal.
Soils and geologic materials surrounding and below sanitary landfills
and ponds and lagoons developed for animal waste disposal are highly
anoxic. Deep-well injections also create anoxic waste-disposal condi-
tions. Excessive sprinkling or irrigation of land surfaces with sewage
effluents and aqueous waste streams may conceivably develop intermittent
oxygen-stressed soil as a result of waterlogging and/or partial soil sat-
uration. Even in unsaturated soil conditions a great demand for oxygen
by biodegradation processes and limited access to air by filled as well
as partially filled pore spaces may create areas of anaerobic microspace
and reducing conditions. Thus, in waste and leachate disposal, oxygen-
stressed soil and geological material are, indeed, a common and expected
22
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situation for which special attention is required.
Mobility of heavy metals and trace elements, in general, will be accel-
erated in oxygen-stressed compared with oxygen-rich soil. Some of the
factors which influence mobility will be briefly reviewed to illustrate
how anaerobic conditions may promote mobility of some contaminants while
not affecting the mobility of others.
Some characteristics of anaerobic systems are:
a. Gas production, CCL, CH., and HpS predominates; other gases,
as \\2 and N«, form in low concentrations and occur only under
special circumstances, or are not significantly involved in
contaminant mobility.
1). Methane (CH*) - Methane is one of the most characteristic
gases produced during microbial decomposition of organic
compounds under anaerobic conditions. The methane bacteria
are most active between pH values of 6.4 to 7.2. Below pH
6 and above pH 8 the growth rate (and gas production) falls
off rapidly. Methane does not measurably affect trace
contaminant mobility.
2). Hydrogen sulfide (H^S) - Under reducing conditions in the
absence of "free" Og, hydrogen sulfide is produced by
reduction to HgS in soils. Unlike CH4, H^S is highly
reactive with certain trace contaminants. The solubility
products of some of the sulfides are:
FeS = 3 x 10"17 - 1 x 10"16; ZnS = 1 x 1(T24;
CdS = 5 x 10"27; PbS - 7 x 10"28;
CuS = 4 x 10"36; Hg2S = 1 x 10"45;
and HgS = 3 x 10"52
in a saturated solution of H^S at [H J = 1.
This gives a relative indication of attenuation
expected by sulfide formation of the
various selected elements in anaerobic
soils where HgS is produced. Exceptions
can occur under special circumstances.
23
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For example, sodium sulfide may make mercuric
sulfide more soluble if present in any
appreciable quantity.
3). Carbon dioxide (C02) - Carbon dioxide is produced in
abundance in all landfills and in landfill leachates as
well as other wastes having a favorable microbiological
habitat and a decomposable carbon source. This gas unites
with water to form carbonic acid. Carbonic acid produc-
tion reduces the pH (often as low as 5.5) and in this way
contributes to accelerated migration. The abundance of
bicarbonate ions (HCO^) aids in keeping certain trace con-
taminants more soluble. Selenium and probably arsenic are
not influenced as much by bicarbonate as are other trace
constituents. Neither asbestos nor cyanide mobility is
affected by bicarbonate or acid development. Maintenance
of high C02 concentrations in the leachate from sanitary
landfill was shown at The University of Arizona to be nec-
essary to keep trace and heavy metals from precipitating
during sampling and experimentation. Carbon dioxide pro-
duction thus tends to enhance mobility, Korte et al. (1976).
4). Other gases - Volatile compounds of Hg and As may be pro-
duced under anaerobic conditions. Amounts of mono- and
methyl-mercury produced in an anaerobic system are depend-
ent on temperature, pH, organic loading, mercury compounds,
and microbial species present (Jensen and Jernelov, 1969).
Trimethylarsine also is a volatile reduction product of
microorganisms. The reaction is as follows:
0 0
As (OH), ^H,C — As — OH ^H,C — As — OH —,
3 3 i O i
OH CH3
RCHq
As(CH3)3< *
Trimethylarsine
24
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5). Hydrogen (H«) - Small amounts of hydrogen gas evolve from
anaerobic leachates. It does not appear to be an important
factor in attenuation of the trace contaminants as such.
b. Reducing conditions (redo* effect) - Reducing conditions in soil
promote mobility of most of the trace contaminants. Cadmium, Pb,
and Hg mobility may be little affected by the lack of oxygen as
compared with As, Be, Cr, Cu, CN, Fe, and Zn, which will migrate
at a greater rate. Cyanide, a readily mobile constituent, will
denitrify and evolve as an N« gas. The mobility (relative to
each other) of the elements, As, Be, Cr, Cu, Fe, Zn, and
asbestos and cyanide, as suggested elsewhere in this paper, for
"usual" aerated soil conditions probably will not change much.
Though the actual mobility of Cd, Pb and Hg will not change,
their position relative to the other elements will be changed.
Cyanide will be at the top and asbestos at the bottom of the list
as originally, their mobility also being little altered by the
presence or absence of 0« in the soil.
c. Organic acid production (pH effect) - Organic acids will be pro-
duced when organic materials decompose in a limited oxygen
environment. This is the case in a municipal sanitary landfill or
in an industrial waste landfill so long as the concentration of
salts or toxic materials does not limit microbiological activity.
Organic acids enhance the rate of mobility of most of the trace
contaminants through the soil. Even slight lowering of the pH
value of the soil solution in the region between slightly alka-
line pH 8 to slightly acid pH 6 will markedly influence attenua-
tion of most heavy elements. Asbestos, As, Se, and Probably CN
mobility should not be appreciably influenced within this range.
Lowering the soil pH to 3 or below (somewhat unlikely in most
soils except sands and gravels), however, will greatly influence
solubility of heavy and most trace elements.
Organic acids, fulvic, humic, uronic and others such as the vola-
tile organics and carboxy (-COOH) and acid hydroxy (-OH-) radicals
on large organic molecules are available to react with heavy and
25
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trace metals. The downward movement of the trace contaminant
metals through soil and geologic material as soluble metal-organ-
ic matter complexes can be of considerable importance. Fraser
(1961), for example, describes Cu metal-organic-complex mobility
as a factor in contaminant accumulation in underground seepage
water. The organic acids produced under anaerobic conditions
form chelates with many heavy and trace metals. These metals are
then protected and available for accelerated movement through
soils. Our knowledge in this area is still fragmentary, leaving
opportunity for quantitative evaluation.
d. Retardation of biodegradation - Anaerobic degradation of organic
matter proceeds more slowly than aerobic degradation. Not only
is it slower but often stops at some intermediate stage of oxi-
dation leaving an accumulation of organic products which are free
to react with the soil and constituents in the waste stream and
soil solution. Slime and gum accumulation, however, tend to be
less severe under strictly anoxic as compared with oxic condi-
tions. Clogging and filling of the soil pore spaces may be
expected to be less severe as compared with aerobic or partly
aerobic conditions. The nature of anaerobic flora is less con-
ducive to clogging of pore spaces than when fungi, algae and
slime bacteria accumulate in the presence of oxygen. All of the
anaerobic conditions tend to permit water to pass through the
soil more rapidly. This effect may be counterbalanced by accum-
ulation of unoxidized sludges, and original organic debris which
would tend to have an opposing influence.
e. Water movement not retarded by slime or gum formation - As
described above, water movement may be expected to be impeded
less under anaerobic than aerobic conditions. If the waste is
strictly inorganic, water movement should not be affected by the
presence or absence of oxygen.
As indicated previously, it is only practicable to manage the
aeration status of the soil-waste environment when the dis-
posal operation is conducted at the soil surface (land spreading
26
-------
or spray irrigation of wastes). However, the effects of
anaerobic conditions should be taken into consideration in any
disposal operation, particularly for wastes with a significant
content of heavy metals or organics. For example, heavy metal-
bearing wastewater treatment sludges formed by alkaline precipi-
tation should not be disposed of in a municipal refuse or other
organic waste environment; the organic acids formed during
anaerobic decomposition would interact with the sludge, rever-
sing the treatment procedure and freeing the heavy metals for
migration.
RELATIVE ATTENUATION OF SELECTED TRACE CONTAMINANTS
A soil class or group represents a segment of a spectrum of soil
properties with sufficient like characteristics as to have been separated
out on the basis of practical agricultural use and genetic characteristics.
In predicting how each trace contaminant may be attenuated relative to
another contaminant in a soil, it must be kept in mind that within a given
soil-group class, properties such as texture, clay mineral, pH value,
structure and organic matter vary from place to place. As an aid in
selecting the best of several soils for attenuation of contaminants, infor-
mation is presented here on the chemical and physical properties and the
attenuation classifications of the soils used in the study at The University
of Arizona. This information can be compared to the properties of the soils
in question in order to rank them according to their attenuation potential.
The ranking of the soils used in The University of Arizona study is based on'
work with municipal landfill leachate; there is indication from the prelimi-
nary results of another EPA study that the ranking is also valid for atten-
uation of trace contaminants in leachates from some industrial wastes.
*
Dugway Proving Ground, "Migration of Hazardous Substances through Soil." Un-
published Progress Report prepared for the Solid and Hazardous Waste
Research Division, EPA Municipal Environmental Research Laboratory,
Cincinnati, Ohio. Interagency Agreement EPA-IAG-D4-0443. January
1975.
27
-------
Table 3.1. CHARACTERISTICS OF THE SOILS USED IN ARIZONA SOIL-COLUMN RESEARCH
ro
Soil
Series
Soil
Order
Soil
Paste
pH
Cation
Exch.
Capac.
meq/lOOg
Wag ram
Ava
Kalkaska
Davidson
Molokai
Chalmers
Nicholson
Fan no
Mohave
Mohaveca
Anthony
Ultisol
Alfisol
Spodosol
Ultisol
Oxisol
Mollisol
Alfisol
Alfisol
Aridisol
Aridi sol
Entisol
4.2
4.5
4.7
6.2
6.2
6.6
6.7
7.0
7.3
7.8
7.8
2
19
10
9
14
26
37
33
10
12
6
Elec.
Cond.of
Extract
uhos/cm
225
157
237
169
1262
288
176
392
615
510
328
Column
Bulk
Density
g/cmZ
1.
1.
1.
1.
1.
1.
1.
1.
1.
1.
2.
89
45
53
89
44
60
53
48
78
54
07
Sand
88
10
91
19
23
7
3
35
52
32
71
Silt
v
8
60
4
20
25
58
47
19
37
28
14
Clay
4
31
5
61
52
35
29
46
11
40
15
Texture
Class
loamy
sand
silty
clay
loam
sand
clay
clay
silty
clay
loam
silty
clay
clay
sandy
loam
clay
sandy
loam
Predominant ^
Clay Minerals
Kaolinite, chlorite
Vermiculite, kaolinite
Chlorite, kaolinite
Kaolinite
Kaolinite, gibbsite
Montmorillonite,
Vermiculite
Vermiculite
Montmorillonite, Mica
Mica, Kaolinite
Mica, Montmorillonite
Montmorillonite, Mica
* Listed in order of importance
-------
Table 3.2. THE CATION EXCHANGE CAPACITY,
FREE IRON OXIDES, AND TOTAL Mn
EXCHANGEABLE CATIONS, SURFACE AREA,
OF SOILS USED IN THE COLUMN RESEARCH
Soil Series
Wagram l.s.
(N. Carolina)
Ava si. c.l.
(Illinois)
Kalkaska s.
(Michigan)
Davidson c.
(N. Carolina)
Jo Molokai c.
(Hawaii)
Chalmers si. c.l.
(Indiana)
Nicholson si.c.
(Kentucky)
F.anno c.
(Arizona)
Mohave s.l.
(Arizona)
MohaveQa c.l.
(Arizona)
Anthony s.l.
(Arizona)
Surface
Area
in*
8.0
61.5
8.9
51.3
67.3
125.3
120.5
122.1
38.3
127.5
19.8
Free
Iron
Oxides
0.6
4.0
1.8
17.0
23.0
3.1
5.6
3.7
1.7
2.5
1.8
Total
Mn
ppm
50
360
80
4100
7400
330
950
280
825
770
275
Cation
Exchange
Capacity
2
19
10
9
14
26
37
33
10
12
6
Exchangeable Cations
Na
0.20
0.43
0.10
0.31
0.20
0.20
0.70
0.35
0.40
0.41
0.15
K
•meq/lOOg
0.40
0.23
0.06
0.16
0.70
0.10
0.19
0.87
0.50
0.88
0.38
Ca
cnil
0.60
1.33
0.80
3.40
6.00
20.60
21.80
17.10
8.90
6.20
3.85
Mg
0.60
2.63
0.10
1.58
4.10
8.60
5.82
7.82
1.80
4.30
1.23
Free lime (Ca^) is present as a natural constituent,
-------
GO
O
Table 3.3. THE EXCHANGE ACIDITY, EXCHANGE CAPACITY AND CERTAIN EXCHANGEABLE ELEMENTS
FOUND IN THE NEUTRAL TO ACID SOILS* USED IN THE SOIL-COLUMN RESEARCH
Soils
Wagram
Ava
Kalkaska
Davidson
Molokai
Chalmers
Nicholson
Exchange
Acidity
meq/100 ml
0.7
9.5
2.7
0
0
0
0
Exchange
Capacity
meq/100 ml
0.7
4.4
1.1
5.1
10.9
28.5
36.5
Exchangeable Element
Ca
100
350
190
690
1400
3800
5800
Mg
20
320
20
195
480
1140
900
N
3
9
12
3
35
3
3
P
2.0
2.0
17.0
4.0
21.0
1.0
26.0
K
-yg/ml soil-
16
50
34
38
214
36
50
Cu
1.0
7.0
1.0
2.5
13.5
3.0
1.5
Fe
44
100
+150
16
14
7
14
Mn
0.5
3
0.5
16
49
3
3
Zn
0.05
2.1
3.2
0.8
7.0
1.0
1.3
* Analysis provided by Hunter, Agricultural Environmental Systems, Inc., Raleigh, N.C.
-------
Table 3.4. CONCENTRATION RANGES OF SOME COMMON CATIONS AND AN IONS OF THE SOIL-SOLUTION
DISPLACEMENTS FROM COLUMNS LEACHED WITH DEIONIZED WATER*
Soil
Series
Wagram l.s.
Ava sl.c.1.
Kalkaska s.
Davidson c.
Molokal c.
Chalmers sl.c.1.-
Nicholson sl.c.
Fanno c.
Mohave s.l.
Mohaveca c.1.
Anthony s.l.
Effluent
pH
5.3-6.2
4.8-5.3
4.5-5.0
6.0-6.8
7.1-7.8
7.0-7.4
6,4-7.1
7.2-7.7
6.7-8.0
7.4-7.9
6.8-7.5
Elect.
Cond.
«itnh/\c //*m
ymnos/ciTi
13-270
24-160
40-200
36-303
120-1500
37-310
30-180
170-370
45-850
85-430
40-330
K
.6-11
1-3.2
.5-8
.5-2.4
7-20
T-l.l
.2-. 6
1.5-5
2-15
1-4
5-14
Na
T-15®
T-8
T-6
T-4.8
2-107
4-12
.4-20
6-21
2-79
3-25
T-10
Ca
1-26
2-5
2-17
3-35
6-105
3-37
2-15
9-39
5-170
15-200
4-53
Mg
ppm-
.1-1.6
.5-7
.3-3
1-14
5-58
1-12
.2-3
9-39
1-22
1-22
1-3
Cl
.1-31
1-22
3-11
1-27
1-320
2-17
1-24
*
1-28
2-20
2-11
NH4
.2-. 5
.06-. 2
.4-2.6
.1-.2
.4-5
.3-. 4
.2-1.6
*
.1-1.6
.2-. 3
.1-.8
P Si
* T-.6
# .6-2.8
I .1-.25
1 .1-.2
* . o— 1.3
# T-8
I 1-2
T T-.7
T-5 1.4-1.6
# .7-. 9
T-3.6 .7-1.3
* Data represent ranges of constituents for * 28 pore-space displacements
I Below detectable limit
-------
Table 3.5. PARTICLE SIZE DISTRIBUTION OF THE SOIL SAMPLES USED IN THE COLUMN RESEARCH
Very
Coarse
Series 2-lmm
Wagram 7.48
Ava 0.53
Kalkaska 0.19
Davidson 0.71
Molokai 1.29
Chalmers 0.74
Nicholson 0.67
Fanno 8.45
Mohave 15.28
Anthony 18.05
Coarse
1-0. 5mm
20.70
0.56
1.79
2.38
2.64
0.62
0.31
4.87
11.30
13.71
SAND
Medium
0.5-0. 25mm
32.06
1.25
47.99
6.52
4.57
1.67
0.44
2.40
12.40
17.68 "
SILT CLAY
Very
Fine Fine
0.25-O.lmm 0.1-0. 05mm 50-20y 20-10w 10-5y 5-2u <2y
21.81 5.84 2.00 1.37 1.59 3.31 3.84
0.82 0.67 12.80 21.69 15.71 9.63 30.63
36.26 5.19 1.34 1.01 1.38 0.18 4.67
6.02 3.39 3.32 4.83 4.08 7.43 61.32
6.64 7.91 5.78 8.30 6.08 4.88 52.00
1.38 2.52 19.42 20.18 11.18 7.44 34.89
0.42 1.18 12.90 13.47 15.27 5.41 49.89
9.96 9.06 5.92 4.27 3.05 5.56 46.46
8.02 5.42 30.36 5.00 1.34 0.43 10.45
12.93 8.92 7.41 2.69 2.20 1.37 15.04
CO
ro
-------
Table 3.6. TOTAL ANALYSIS OF SOILS FOR TRACE METALS
AND FREE IRON OXIDES
Soil
Series
Wag ram 1 . s .
Ava si. c.l.
Kalkaska s.
Davidson c.
Molokai c.
Chalmers si. c.l .
Nicholson si.c.
Fanno c.
Mohave s.l .
MohaveCa c.l.
Anthony s.l.
Mn
50
360
80
4100
7400
330
950
280
825
770
275
Co
*
50
25
120
310
60
50
45
50
50
50
Zn
ygy
40
77
45
110
320
100
130
70
85
120
55
Ni
fn
9
80
110
50
120
600
130
135
100
100
120
80
Cu
62
80
46
160
260
83
65
60
265
200
200
Cr
*
55
15
90
410
68
68
38
18
40
25
Fe
Oxides
Of
0.6
4.0
1.8
17.0
23.0
3.1
5.6
3.7
1.7
2.5
1.8
* Below detectable limit
33
-------
Table 3.7. TEXTURAL CLASS AND CLAY MINERAL COMPOSITION OF THE CLAY (< 2v) SEPARATE OF
SOILS USED IN THE POLLUTION ATTENUATION RESEARCH
Soil
Series
Wagram
Ava
Kalkaska
Davidson
Molokai
Chalmers
Nicholson
Fanno
Mohave
Mohaveca
Anthony
Texture Mont-
Class morillonite
Loamy sand
Silty clay loam
Sand
Clay
Clay
Silty clay loam
Silty clay
Clay
Sandy loam
Clay loam
Sandy loam
0
20
0
0
0
4
0
3
2
3
4
Bei del lite
0
0
0
0
0
0
0
0
0
0
0
Vermiculite Chlorite
0
3
0
0
0
2
5
0
0
0
1
3*
0
3*
1
0
2
0
0
0
0
1
Mica
1
2
1
0
2
0
1
2
4
4
3
Kaolinite Other
4
3
2
5
4
2
1
1
3
2
2
Quartz-3
Quartz-2
Quartz-2
Gibbsite-1
G1bbsite-3
Quartz-2
Quartz-2
Quartz-2
Quartz-2
Quartz-2
Quartz-2
* Chloritic intergrade
@ The amount of each mineral present is represented as: predominant 5, large 4, moderate 3, small 2, trace 1,
and looked-for but not detected 0.
-------
Chemical and physical characteristics of the soils used in the
Arizona study are presented in Tables 3.1-3.7. The soil orders represented
are shown in Table 3.8 below.
TABLE 3.8. THE SOIL ORDERS REPRESENTED IN THE ARIZONA STUDY
Soil
Order
Ultisol
Alfisol
Spodosol
Ultisol
Oxisol
Mollisol
Alfisol
Alfisol
Aridisol
Aridi sol
Entisol
Great Soil Group
description
Red Yellow Lateritic
Gray Brown Podzolic
Podzol
Red Brown Lateritic
Red Lateritic
Prairie
Gray Brown Podzolic
Reddish Brown
Red Desert
Red Desert/ lime
Alluvial
Series
name
Wag ram
Ava
Kalkaska
Davidson
Nolokai
Chalmers
Nicholson
Fanno
Mohave
Mohave (Ca)
Anthony
State
location
N. Carolina
Illinois
Michigan
N. Carolina
Hawaii
Indiana
Kentucky
Arizona
Arizona
Arizona
Arizona
Attenuation Comparisons Between Soils
One of the most critical factors affecting attenuation is particle-
size distribution. For example, Anthony which has the same dominant clay
mineral, montmorillonite, as Fanno.attenuated the trace contaminants As, Be,
Cd, Cr, Cu, Fe, Pb, Hg, Se and Zn to the least extent of all soils (except
Wagram which is even coarser textured), and Fanno, most often, appeared at
the top of the list for attenuation. Anthony is classed as a sandy loam, and
Fanno as a clay. See Tables 3.5 and 3.7 for information on the texture and
35
-------
clay mineralogy of the soils. Wagram, a loamy sand whose clay mineralogy is
dominated by kaolinite, and kalkaska, a sand that contains a small amount of
kaolinite but is dominated moderately by chlorite, attenuate the trace ele-
ments somewhat similarly and to an extent only slightly better than Anthony
and much poorer than Davidson, a clay dominated by kaolinite.
The ion exchange capacity appears to be less well correlated with
trace element retention than other soil parameters as colloid and "free"
iron oxide. Those soils highest in cation exchange capacity (CEC), however,
also were those which contained more clay and retained the most trace contam-
inants. Additional study must be conducted before CEC can be said to
relate directly to attenuation of the selected trace elements.
The nature of the soil colloid fraction (i.e., that measured as < 2y
clay) appears to have a highly significant effect on attenuation. There is
some evidence in our preliminary studies that Molokai, which is high in hyd-
rous oxides and oxides of Fe (the clay is kaolinitic-like) also is unusually
active in attenuation. It contains only slightly more colloid than Fanno and
Nicholson, but less than Davidson.
The only soil that contained appreciable quantities of organic matter
was Kalkaska, a Spodosol. It is so very sandy that leachates contained infil-
trated organic matter and therefore the effect of organic matter could not be
evaluated readily.
Thus, the major properties which appeared to influence attenuation
for the soils studied are: (a) texture, (b) iron oxides, (c) content of the
< 2y colloid, (d) lime, and (e) pH value.
Lime effects are being studied with two soils (one containing natural
lime and one without lime) of the same series, Mohave. Early data lead to
the belief that the effect of natural lime in soil is an important factor in
attenuation since the pore space displacements of leachate from lime Mohave
are freer of the trace metals than those of lime-free Mohave. Furthermore,
research involving the use of a 2-cm layer of ground Kentucky agricultural
36
-------
Table 3.9. RELATIONSHIP BETWEEN CERTAIN MEASUREABLE PARAMETERS OF 11 REPRESENTATIVE
SOILS AND ATTENUATION OF SELECTED TRACE CONTAMINANTS OF LANDFILL LEACHATES
CO
Soil
Series
Davidson
Molokai
Nicholson
Fanno
MohaveCa
Chalmers
Ava
Anthony
Mohave
Kalkaska
Wagram
Soil
pH
6.2
6.2
6.7
7.0
7.8
6.6
4.5
7.8
7.3
4.7
4.2
Colloid
<2M
(clay)
%
61
52
49
46
40
35
31
15
11
5
4
Silt
20
25
47
19
28
52
60
14
37
4
8
Cation
Exchange
Capacity
meq/lOOg
9
14
37
33
12
26
19
6
10
10
2
Mineral
Nature of
Colloid
Dominant
Kaolinite
Kaolinite (Fe203)
Vermiculite
Montmorillonite
Mica
Montmorillonite
Kaolinite
Montmorillonite
Mica
Chlorite
Chlorite
"Free"
Iron Attenuation
Oxide Class*
%
17.0
23.0
5.6
3.7
2.5
3.1
4.0
1.8
1.7
1.8
0.6
2
1
1
2
3
3
3
5
4
5
5
Rough estimate for all trace contaminants (1 is high attenuation, 5 is low).
-------
limestone over a 10-cm soil layer greatly reduced the migration of several
trace elements through the soil columns.
The soils representing seven major soil orders are listed in Table
3.9 in a decreasing order of percentage of soil colloid to allow comparison
on the basis of some of the soil factors affecting attenuation. According
to data listed, the proportion of silt and clay in soils is quite variable.
Ava, for example, assumes sixth position (31%) from the top in colloid, but
first (60%) in silt. Nicholson, which is one of the best attenuator soils,
ranks third in colloid and second in silt. It contains very little sand
(^ 4%). Silt also exerts important adsorption properties and must not be
overlooked as contributing substantially to attenuation of trace metals.
The last column in Table 3.9 is an attempt to broadly evaluate the
soils according to their attenuation characteristics of the trace contami-
nants considered in this manuscript. To further evaluate the soils for
attenuation characteristics of individual trace elements "spiked" into land-
fill leachate, Table 3.10 is presented which gives the pore-volume in which
the element first appeared in the soil-column effluent. The obvious conclu-
sion is that texture is a critical factor in attenuation in soils; perhaps
the most important. Because of its wide variation among soils, even within
the same order, texture always will play a leading position in attenuation
of trace contaminants.
Soil structure may also be important since it, too, like texture,
relates to pore-size distribution and available surfaces for chemical and
physical reactions. Because of structure, soil surface area does not relate
exactly with the clay content, although in a broader comparison as the soil
particles become smaller, the soil surface area increases. Compare the last
two columns in Table 3.11. Soil structure is almost impossible to describe.
Even in soils taken into the lab, after carefully sampling in what often is
termed "undisturbed conditions," the structure is altered as compared with
its natural state in the field.
38
-------
TABLE 3.10. THE PORE VOLUME IN WHICH THE ELEMENT FIRST APPEARED IN THE SOIL-
COLUMN EFFLUENT
Soil
Series pH As Be
Wagram 4.2 1 1
Ava 4.5 5 3
Kalkaska 4.7 1 4
Davidson 6.2 13* 6
Molokai 6.2 13* 12*
Nicholson 6.7 12 5*
Fanno 7.0 1 4*
Mohave 7.3 1 13
Mohave^ 7.8 10* 6*
Anthony 7.8 1 5
Cd Cr
1 1
1 7
2 1
3 17*
30 19*
13 5
14* 1
13 1
8 1
2 1
Cu
20*
10
19*
27*
13*
12*
15*
19*
15*
15
Pb
1
10
17*
26*
22*
16*
16
16*
23*
17
Se
4
21*
10
17*
14*
8*
3*
2
6
2
Zn Hg
1 1
1 1
1 1
5 1
27* 7
7 1
13* 4
17 5
11* 6
2 1
None of the elements appeared in
volumes.
TABLE 3.11. THE pH VALUE, CLAY
the effluent for the listed
AND SURFACE
RESEARCH
AREA OF
number of pore
SOILS USED IN
THE COLUMN
Soil Series
Wagram loamy sand
Ava silty clay loam
Kalkaska sand
Davidson clay
Molokai clay
Chalmers silty clay loam
Nicholson silty clay
Fanno clay
Mohave sandy loam
Mohave- clay loam
Anthony sandy loam
pH
4.2
4.5
4.7
6.2
6.2
6.6
6.7
7.0
7.3 .
7.8
7.8
<
2u Clay
%
4
31
5
61
52
35
49
46
11
40
15
Soil surface
area
cnrVg
8
62
9
51
67
126
121
122
38
128
20
39
-------
Because certain soil orders, such as Spodcsol, are very sandy, they
usually are poor attenuators of trace contaminants compared with other soils.
Oxisols, on the other hand, tend to be good attenuators since they are char-
acteristically fine textured, contain a high amount of hydrous oxides, and
are deep. Entisol (alluvial) is one order that is quite variable in texture.
It represents a dominant position in landfill sites since alluvial soils
occur on low river terraces, river bottoms, coarse-textured alluvial fans,
gravel pits, and often represent the least economic land. Entisols also may
represent highly productive (agriculturally) soils bordering large streams
and rivers and pocketed along small water channels.
In conclusion, it is encouraging that certain readily measurable soil
parameters "stand out" as critical factors affecting attenuation and broad
predictions may be drawn with some confidence at an early date with respect
to soil attenuation of trace contaminants. It is important that the research
proceed further, comparing soils having similar textures (and soil surface
areas) but dissimilar clay minerals, pH, lime, cation exchange capacity,
soluble salt and hydrous oxides in an orderly and variably controlled fashion,
if the most effective disposal control practices are to evolve and we are to
acquire sufficient evidence to establish the best practices for waste dis-
posal on land.
40
-------
SECTION IV
SOILS AND GEOLOGICAL MATERIAL
This discussion on soils and geological materials is presented as
background for understanding the processes which affect the migration rate
of metals in soil. Additionally, it is hoped that the reader will gain some
appreciation of the complexity of the soil environment and of the consequent
difficulty of obtaining simple, general solutions for the problems involved
in land disposal of wastes.
THE SOIL
The soil is a complex, dynamic biological, chemical, and physical
system which transforms all matter, including waste materials. Some compon-
ents of the wastes are permanently incorporated into the system while others
pass through and eventually enter underground water or are expelled into the
air as gases. It is necessary to become acquainted with the major compon-
ents of soils which are involved in the transformations of wastes to under-
stand why soils attenuate. Waste components and a soil's effectiveness in
functioning as an attenuator are related to its physical, chemical, and bio-
logical properties.
Major Components
Soils contain inorganic and organic solids, water, air, and micro-
organisms. Generally, the soil scientist thinks of a good agricultural or
garden soil as being medium textured (silt loam) consisting of about 45 per-
cent mineral matter, 5 percent organic matter, and 50 percent pore space of
which half is filled with water and half with air.
41
-------
Organic Solids--
Organic solids constitute the bulk of organic matter in soils.
Organic matter originates in part from plant constituents resistant to decay,
such as degraded lignin, wa*es, and resins, and in part from living and dead
microorganisms, synthesized microbial gums and slimes, and other microbial
cell debris. Organic matter concentrates almost wholly in the surface layer
of soils. Landfills are characteristically excavated into subsoil material
which is low in organic matter. The discussion in this manuscript, there-
fore, will deemphasize the soil organic fraction. Interactions of the
hazardous constituents with degraded and synthesized organic constituents
contained in leachates from municipal refuse, however, will be included.
The biochemical route that decay takes is about the same, but the rate may
vary from soil to soil. For example, under aerobic conditions paper will
decay primarily into gas and water and leave about the same kind and amount
of residue regardless of the soil in which it is placed.
Inorganic Solids—
The characteristics of inorganic solids in soil vary considerably,
and each soil has a nearly unique set of properties. One of the important
physical properties of soil inorganic solids is the distribution of particle
sizes between sand (2.00-0.02 mm diameter), silt (0.02-0.002 mm), and clay
(0.002 mm). The textural classification of a soil is based on the percent-
age of different sizes of particles it contains (Figure 4.1). See Appendix B
for additional information on particle size and texture in the USDA and
USCS systems. Sand and silt particles are composed mostly of quartz and
primary silicate minerals which are relatively insoluble and complex in
structure and composition. The types and amounts of primary minerals in
soil are governed by the mineral composition of the parent material from
which the soil originated. The clay-sized and colloidal materials are com-
posed mostly of secondary materials (Figure 4.2), which are weathering prod-
ucts of primary minerals. The types of secondary minerals in a soil are
governed not only by the primary minerals but also by the weathering condi-
tions and the rate of removal of the weathering products (Figure 4.3).
42
-------
100
CO
90
siltloemi^ coarse x
sand
coarse loamy *
•&•?>%
percent sand
Figure 4.1.
Textural classification (USDA) of soil based on the percentage
of different sizes of particles it contains.
-------
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PRIMARY
SILICATE
MINERALS
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SAND
SILT
CLAY
Figure 4.2. General relationship between particle size and kinds of minerals present.
Secondary silicates (Fe and Al) dominate the fine colloidal clay. The
Hydrous oxides of Fe, Al, and Mn are prominent in the fine silt and cl*"
fractions. (From Brady, 1974)
-------
CO
UJ
J-
o
CO
1
o
?
2E
J3
z
X
CO
x
—
i£
(0
o
o"
o
d>
z
X
o
X
MICROCLINE
ORTHOCLASE
a OTHERS
MUSCOVITE
MICAS
BIOTITE
SODA -LIME
FELDSPARS
AUGITE
HORNBLENDE
a OTHERS
HOT WET CLIMATES (-Si)
!
-K
/^ RAPID REMOVAL
^s^ MUCH Mg IN WE,
Mvnpni i<%
-K
OF BASES
*%,
VH Qj» MICAS
-K
VERMICULITE
S^
MONTMORILLONITE
KAOLINITE
OXIDES OF
Fe a Al
-Mq
SLOW REMOVAL
RAPID REMOVAL OF BASES
HOT WET CLIMATES (-Si)
DEGREE OF WEATHERING INCREASES
Figure 4.3. General conditions for the formation of the various silicate clays and oxides of
iron and aluminum. Clay genesis is accompanied by the removal of solubilized
elements such as K, Na, Ca, and Mg. (From Bradv, 1974.)
-------
The colloidal fraction of the soil controls the majority of
biological, chemical, and physical reactions. In subsoils colloidal frac-
tion is composed primarily of clay although the very reactive oxides and
hydrous-oxides of iron, manganese, and aluminum, as well as some organic
matter, may be present in significant amounts. Not only is the surface area
(Table 4.1) of colloidal particles vastly greater than that of sand and silt,
but the surface carries a negative charge which attracts cations (metallic
elements).
TABLE 4.1. COMPARISONS IN NUMBER OF PARTICLES AND SURFACE AREA PER UNIT
WEIGHT FOR DIFFERENT SOIL SEPARATES
Texture *
(Particle
size)
Fine gravel
Coarse sand
Medium sand
Fine sand
Very fine sand
Silt
Clay
Diameter
mm
2.00-1.00
1 . 00-0. 50
0.50-0.25
0.25-0.10
0.10-0.05
0. 05-0. 002
below 0.002
Number
g
90
722
5,777
46,213
722,074
5,776,674
90,260,853,860
Surface
area
g/cmz
11.3
22.7
45.4
90.7
226.9
453.7
11,342.5
*
U.S Department of Agriculture classification
The soil clays are secondary minerals formed by weathering of geo-
logical material. They basically are composed of aluminum silicates. Table
4.2 compares the relative amounts of silicate clays and the hydrous oxides
in soils as related to climate and geographic locations in the USA. Because
clays possess a negative charge they hold cations subject to later exchange
with other cations in solution. In this way, elements become delayed in
their downward migration in the soil solution. The charge on the surface of
some clay-size particles changes, depending on pH of the soil as a whole.
46
-------
TABLE 4.2. DOMINANT CLAY MINERALS FOUND IN DIFFERENT SOIL ORDERS OF THE COMPREHENSIVE
CLASSIFICATION SYSTEM*
Soil
order
Entlsol
Inceptisol
Aridisols
Vert i sols
Mollisols
Alfisols
Spodosols
Ultisols
Ox iso Is
Genera]
Weathering
Intensity
Low
•
High
Typical
1 ocati on
in USA
Variable
Variable
Desert
Ala, Tex
Central
USA
Ohio, Pa,
N.Y.
New
England
SE USA
Tropical
zones
Hydrous Montmor-
oxides Kaolinite illonite
X
X
X
X X
X
X X
X X
IlTite
X
X
X
X
X
X
Inter-
Vermiculite Chlorite grades
X
X XX
X X
X X
X
X X
X
Adapted from Jackson (3).
-------
Table 4.3 shows some of tlie striking differences in the chemical and
physical properties of the three major clay minerals.
TABLE 4.3. COMPARATIVE PROPERTIES OF THREE MAJOR TYPES OF COLLOIDS^
Property
Montmorillonite
Type of Colloid
Illite
Kaolinite
Size (microns) 0.01 - 1.0
Shape irregular flakes
External surface high
Internal surface very high
Cohesion, plasti- high
0.1 - 2.0
irregular flakes
medium
medium
medium
0.1 - 5.0
hexagonal crystals
low
none
low
city
Swelling capacity
Cation exchange
capacity (*me/
100 gm)
high
80-100
medium
15-40
low
3-15
From Buckman and Brady (1960)
Aluminum and iron hydroxide gels, oxides, and mixed hydroxide/oxide
compounds act strongly to absorb, combine with, and precipitate hazardous
substances and elements. The hydrous manganese oxides also should be
included in this group. This is a group of soil chemicals which coat as
well as form particles and are dominant in numerous attenuation mechanisms.
Soil Water—
The vehicle in which most constituents are transported through soils
is water. The movement of water is a continual, naturally occurring process
governed by potential gradients originating from water addition, gravity,
capillary action, evaporation, temperature differences, variable salt con-
centration, matrix deformation, and plant extraction. The mechanism oper-
ating during the redistribution of water and solutes in soils and reliable
methods for assessing hydraulic properties of field soils must be defined
before transfer processes can be accurately predicted.
48
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The movement of water in soil takes place through the pore spaces. The
rate of flow 1s controlled by the size and distribution of the pore spaces
and the total pore volume. Such gross physical characteristics as texture
(particle size distribution), structure (arrangement of the particles into
crumb-like, blocky, etc., units; see Figure 4.4 below), bulk density (compaction),
swelling and shrinking, wettability, and pore clogging, affect water movement.
Structure can have a pronounced effect on water flow patterns and migration
of pollutants. The spaces between structural units can provide pathways for
liquid movement so that solutions passing through the soil may come into
contact with only a small proportion of the soil material, giving pollutants
in solution less opportunity to be attenuated.
Prismatic
Columnar
Angular
blocky
Subangular
blocky
. .. .
w * \ i * *• •
,'.v .'••-•.*,.!
•'-*.*• • ••*»« v
Platy
Granular
Single grain
Figure 4.4. Classification of soil structures (From Fuller, 1974).
49
-------
The water flow or flux through a soil is an important factor in
attenuation and movement of potentially hazardous pollutants. If water does
not flow, migration of pollutants virtually stops. Attenuation reactions
proceed at some finite rate and for the same amount of solution passed
through soil, the greater amount of attenuation would be associated with the
slower flow rate because sufficient time would be available for attenuation
mechanisms to be fully effective.
Soil Air--
Composition of the soil air differs appreciably from that above the
soil surface because of microbial and chemical activity taking place within
the soil. Carbon dioxide and nitrogen are higher, and under reducing condi-
tions (as would be expected below disposal sites), gases such as hydrogen
sulfide, methane, and some of the oxides of nitrogen may be present in sig-
nificant concentrations.
Generation of any gas within the soil inhibits the flow of water
through the soil by blocking the soil pore spaces.
Evolution of carbon dioxide is particularly significant because of the
role it plays in pH (Figure 4.5). As the carbon dioxide dissolves in water
it forms carbonic acid and buffers the soil solution at a pH below 6.0. A
lowered pH increases the solubility of pollutants from the solid phase of a
waste and increases the mobility of hazardous materials once they are in
solution.
Presumably, if the hazardous materials are present in solution in
high enough concentration, they can sterilize the soil at least in the near
vicinity of the disposal site, greatly reducing the production of gases.
Farther away from the site, because of dilution or other physical and chemi-
cal attenuation processes, concentrations are lowered enough to permit bio-
logical activity and production of gases. Even if microbial activity is
inhibited because of toxic materials in solution, carbon dioxide can still
be formed because of the reaction between leachates from an inherently
acidic waste and carbonates (free lime) present in the soil.
50
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NEUTRALITY
Very
strong
• ACIDITY
ALKALINITY-
Strong
Extreme
pHfor
acid peat
soils
Mod-
erate
Slight Slight
Mod-
erate
T I 'T
8
Range in pH Range in pH
common for common for
humid region arid region
mineral soils mineral soils
— Extreme range in pH for —
most mineral soils
'Attained
only by
alkali
mineral
1 soils
Figure 4.5. Classification of soil pH range.
Soil Microorganisms—
Soils are abundantly supplied with microorganisms capable of
decomposing all organic substances. Although such microbially resistant
materials as rubber, plastics, and waxes decay very slowly, they do decay.
Even metals and metallic compounds are transformed (oxidized or reduced) by
microorganisms. The majority of organic materials (paper, grass clippings,
food scraps, etc.) decay relatively rapidly. If this were not so, the
earth's surface would be literally a huge garbage pile, and life would long
since have been smothered in its own debris.
Microbial degradation of waste and debris is a continual process.
The transformation of wastes takes place most rapidly when they are mixed
with the soil and more slowly when they lie on top of it or layered in it.
Under conditions of plentiful oxygen, the mineral constituents are oxidized
and most become relatively immobile, although those that form soluble organ-
ic complexes or chelates are protected, at least for awhile, from further
oxidation and are thus free to migrate. When microbial processes consume
oxygen faster than it can diffuse into the soil, anaerobic conditions
develop followed by reducing conditions as substances othe r than oxygen are
51
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used as electron acceptors and are reduced. In a reduced state, most metals
are more mobile. Although, oxidation-reduction reactions represent the most
significant involvement of microbes in the mobility of metals, other involve-
ments such as production of organic compounds and degradation of soluble
metal-organic complexes are important.
The microbial flora (bacteria, fungi, actinomycetes, amoeba, protozoa,
algae, and others) capable of carrying out these transformations (i.e., oxi-
dation and reduction reactions) are indigenous to the soil and present in
great abundance. Seeding or inoculation is not needed to change either the
rate or route of biochemical change.
SOIL CLASSIFICATION AND ATTENUATION
This section gives a brief description of the U.S. Department of
»
Agriculture (USDA) system of classifying soils and of some of the soil char-
acteristics used in classification. By knowing certain characteristics of
soils, general predictions can be made concerning trace and heavy metal
reactions. These predictions are assumed to be valid wherever these soils
occur, i.e., are identified and mapped in the United States. Data on attenu-
ation properties of certain classifications of soils are presented in Section
V.
It is unrealistic to expect any element to react or move the same in
a soil developed in Michigan as one developed in Florida. Soils are a
product of both their environment and their parent material and since both
differ from one location to another, soil characteristics also differ, par-
ticularly in their attenuation properties. To elaborate, soils developed
under the same high rainfall conditions in Michigan or Florida, and from the
same parent rock may be quite different. In Michigan, a Spodosol (Podzol)
may form; in Florida, an Oxisol (Laterite) may develop. The Spodosol is
found in temperate to cold climates, the oxisol in subtropical to tropical
climates. The two soils do not attenuate trace and heavy metals in a simi-
lar way. Laboratory data from the study at The University of Arizona
52
-------
indicate that the spodic soil allows far greater migration to take place
than does the oxic soil.
The USDA tentatively recognizes the 10 orders listed in Table 4.4.
TABLE 4.4. PRESENT SOIL ORDERS AND APPROXIMATE EQUIVALENTS IN REVISED
CLASSIFICATION
Present Order*
Approximate equivalents?
1. Entisols
2. Vertisols
3. Inceptisols
4. Aridisols
5. Mollisols
6. Spodosol
7. Alfisols
8. Ultisols
9. Oxisols
10. Histosols
Azonal soils, and some Low Minnie Gley soils.
Grumusols.
Ando, Sol Brun Acide, some Brown Forest, Low-Humic
Gley, and Humic Gley soils.
Desert, Reddish Desert, Sierozem, Solonchak, some
Brown and Reddish Brown soils, and associated
Solonetz.
Chestnut, Chernozem, Brunizem (Prairie), Rendzinas,
some Brown, Brown Forest, and associated Solonetz
and Humic Gley soils.
Podzols, Brown Podzolic soils, and Ground-water
Podzols.
Gray-Brown Podzolic, Gray Wooded soils, Noncalcic
Brown soils, Degraded Chernozem, and associated
Planosols and some Half-bog soils.
Red-yellow Podzolic soils, Reddish-brown Lateritic
soils of the U.S., and associated Planosols and
Half-bog soils.
Late rite soils, Latosols.
Bog soils.
Present (1960, 1968) USDA comprehensive soil classification
#01d (1938) USDA soil classification
In the studies at The University of Arizona, the following orders are repre-
sented, Entisol, Aridisol, Mollisol, Spodosol, Alfisols, Ultisol, and Oxisol.
Vertisol and Histosol were not included because they represent only limited
acreages of suitable potential site locations as compared to the other soils.
Because of the widespread distribution of Alfisols in the United States and
53
-------
certain differences 1n soil characteristics which are expected to critically
Influence attenuation, three Alflsols of quite different climatic location
and chemical and physical parameters were selected for research.
In the present USDA comprehensive soil classification there are 10
orders, the highest category 1n the classification. These are further sub-
divided Into 43 suborders and approximately 200 Great Groups. Successively
lower and more numerous categories of the classification are Subgroups,
Families, and Series. Although the Orders contain soils with similar prop-
erties, 1n some the chemical and physical properties affecting attenuation
may vary as much for soils within the same Order as for soils 1n different
orders. Nevertheless, the groupings 1n the classification are of use 1n pre-
dicting 1on mobility because many of the classification criteria are soil
properties which affect attenuation.
The USDA Soil Conservation Service (1968) points out that the lowest
categories 1n the system, the family and series, serve purposes that are
largely pragmatic: the series names are abstract and the technical family
names are descriptive. The factors used to distinguish families of mineral
soils within a subgroup are listed to Illustrate how family differences can
relate to attenuation characteristics. The justification for their presen-
tation becomes evident after reviewing them; they could significantly In-
fluence movement of trace and heavy metals.
Particle-size classes Soil depth classes
Mineralogy classes Soil slope classes
Calcareous and reaction (pH) classes Soil consistency classes
Soil temperature classes Classes of coatings
Classes of cracks (on sands)
Even when attenuation properties correlate well with one of the
groupings 1n the classification system, man may so alter the soil environment
54
-------
by his activities that the soil classification, 1n the volume of soil
affected, 1s no longer useful as a guide for selecting a waste-disposal site.
For example, the accumulation of leachate 1n soil below a municipal landfill
may change the environment from aerobic to anaerobic and strongly reducing,
thereby promoting the solubility and movement of metals. If the depth of
soil thus affected Includes a water-bearing formation, the disposal site can
contaminate potable water supplies. On the other hand, 1f there 1s a suffi-
cient depth of soil beneath the site, a partially aerobic, oxidizing zone may
remain above the water-bearing formation and the soil classification can have
significance as regards attenuation. This sort of possibility must be con-
sidered when using soil classification as a factor 1n site selection.
The following are descriptions of characteristics of soils 1n the 10
orders, the highest and most general category 1n the classification. These
descriptions are adapted from Information provided by the USDA Soil Conser-
vation Service. See Figure B-2(A) and B-2(B) In Appendix B for a map of the
geographic distribution of Orders and Suborders 1n the United States.
Alfisols
Alflsols have a clayey subsoil horizon and moderate to high base
(cation) saturation. Water 1s held above the wilting point during at least
three of the warm months of the year. Alflsols are higher 1n hydroxy-oxldes
(sesquloxldes) than most soils, as the name Implies, and therefore may have
a fraglpan, durlpan, sodium horizon, petrocaldc (Hme), and pUnthlte (Iron
oxides or sesquloxldes) or other similar features which separate them from
other soils. Where the temperature 1s moderate to cool, alfisols form a belt
between the Mo111sols of the grasslands, and Spodosols and Inceptisols of
very humid climates. In warmer climates, Alflsols form a belt between
AHdlsols of arid regions and Inceptisols, Ultlsols, and Oxisols of warmer
climates. Leaching of bases from the soil may occur almost every year or
may be Infrequent.
55
-------
Aridlsols
Arid!sols occur in arid climates. They do not have water available
to mesophytic plants for long periods as do the Alfisols. Water is held at
less than 15 bars or it is salty. A few Aridisols occur in semiarid climates
because they take up water slowly and most of the rainfall runs off.
Aridisols have one or more pedogenic horizons that may have formed
in the present dry environment or that may be relics of a former pluvial
period. The pedogenic horizon may be the result of translocation and accu-
mulation of salts, lime, or silicate clays or of cementation by carbonates
or silica. The pH usually is neutral to alkaline, sometimes highly alkaline.
Entisols
Entisols do not have horizon or profile development, or at least no
evidence of such. In many of the soils time has been too short for dis-
tinct horizons to differentiate. Other Entisols are on soil slopes too
steep for water to penetrate well or where erosion rate exceeds develop-
ment rate, still others are on flood or glacial outwash plains which con-
tinously accumulate new alluvium. Some are windmoving sand. Not all
Entisols are young. Some are actually very old, consisting mostly of quartz
sand which weathers very slowly. Such materials as organic matter, lime,
gypsum, iron oxides, and clays do not accumulate or at least only to a very
small extent.
Histosols
Most Histosols are saturated or nearly saturated with water most of
the year. They are high in organic matter and represent what is often
described as mucks, peat bogs, high moors, or raised peats. For obvious
reasons these soils are not included with those we are testing. Few dis-
posal sites are located in such unstable -swamps.
56
-------
Inceptisols
By USDA definition (1968), in part at least, "Inceptisols are soils
of humid regions that have altered horizons which have lost bases or iron
and aluminum but retain some weatherable minerals. They do not have an il-
luvial horizon enriched either with silicate clay that contains aluminum
or with an amorphous mixture of aluminum and organic matter." This is a
difficult soil order to visualize from the description. It represents more
what other soils are not. than what they are. Inceptisols develop mainly in
the more clayey parent materials, in contrast to Spodosols, which develop in
materials which have little clay. No Inceptisols are included in the soils
being studied at Arizona.
Mollisols
Very dark colored soils, rich in bases and naturally covered by grass
(steppe land) are called Mollisols. Many soils of this order accumulate
lime and/or sodium, and clay. Mollisols occupy extensive subhumid to semi-
arid areas of the grass plains in the USA. They are located generally
between the Aridisols of arid climates and the Spodosols or Alfisols of
humid climates. These soils are highly productive, constituting rich crop
land in the breadbasket areas of central and west central U.S. Areas of
Mollisols appear in nearly every state. Luxuriant, perennial grass seems
to be essential to their formation. Where waste waters and leachates are
spread on the surface, the high soil organic matter plays an important
part in mobility of trace and heavy elements. Where leachates and aqueous
wastes pass through subsoils only, lime and bases influence mobility in
addition to that of clay minerals.
Oxisols
Those soils which once were called red and yellow laterites are now
named Oxisols. Reddish, yellowish, or grayish soils of tropical and sub-
tropical climates that form on mostly gentle slopes on surfaces of great
age are Oxisols. They are mixtures of quartz, kaolin, free oxides and
57
-------
organic matter. The boundaries of horizons blend into each other so
gradually they are generally arbitrary. Weathering has proceeded to great
depths. Water moves through these soils rapidly. Because of the high
oxide (primarily iron) content that coats particles and forms granular
particulates, these soils attenuate the trace and heavy metals very well.
Oxisols occur in Hawaii.
Spodosols
Two well defined and obvious-to-the-eye horizons distinguish Spodo-
sols. Just below the surface layer of forest litter and partly decomposed
dark organic matter is a bleached layer of uncoated quartz sand. The
second layer, just below, usually is coffee color. Organic matter and iron
complexes accumulate to give the dark brown color. However, this spodic
is one in which amorphous mixtures of organic matter and aluminum may also
occur with or without iron.
Spodosols generally are coarse textured, containly only small quanti-
ties of clay, if any, and usually permit rapid water movement. These soils
occur under high rainfall conditions and coniferous forest, though sometimes
hardwoods are present. Attenuation is poor despite the spodic horizon
where aluminum, and often iron, complexes with organic matter. Usually the
iron and aluminum content is low, though, even when a fragipan (soft when
wet, brittle when dry) is present. The textures are mostly sandy, sandy-
skeletal, coarse loam, loamy-skeletal, and coarse-silty. New England, New
York, Northern lake states, and Alaska are most noted for spodic (Podzolic)
soils.
Ultisols
The concept of Ultisols is that of soils of mid-to-low latitudes
that have a horizon that contains an appreciable amount of translocated
silicate clay but few bases. Highly humid conditions sometime during the
year causes water to move through them. Ultisols are most commonly found
in warm-humid climates that have a seasonal deficit of rain and on older
58
-------
surfaces. They develop on a wide variety of parent materials. Kaolin,
gibbsite, and aluminum-interlaid clays are common in the soil clay fraction.
They usually form under coniferous and hardwood-forest vegetation in the
United States.
Vertisols
Vertisols are clayey soils which crack severely when dry and have
high bulk densities between the cracks. The clay minerals are dominated by
montmorillonite. Most are found under warm climatic conditions, i.e., ther-
mic or warmer. In arid regions they form in closed depressions or playas.
Vertisols often are referred to as churning soils because during swelling,
pressure is exerted, causing them to heave and recycle the soil. Verti-
sols are not included in the Arizona study. They generally do not represent
suitable sites for disposals and are seldom used for this purpose.
NATURAL CONTENT OF ELEMENTS IN SOIL
The natural content of certain elements in soil is of importance
because this is the "background" against which the degree of soil contamina-
tion by waste disposal must be judged. Additionally, under certain condi-
tions, many of the naturally occurring trace and heavy elements in soils may
be solubilized and contribute significantly to pollution of surface and
groundwaters. Hence, a summary of some information on this subject is pre-
sented here.
Arsenic
An average content of arsenic in soils is estimated by Cavender (1964)
and Vinogradov (1959) as being about 5 ppm. Soils contain between 1 and 10
ppm arsenic unless contaminated, according to Warren et al. (1964). Ash of
plant material is much higher in arsenic than soils. For example,
Goldschmidt (1964) reports up to 320 ppm As in the ash from the uppermost
humus layer of a beech forest. There appears to be an arsenic-sulfide rela-
tionship in lower soil horizons above sulfide-rich parent materials.
59
-------
According to Presant and Tupper 0966), who studied 21 Spodic (Podzol) soil
profiles, the mean values of total arsenic ranged from 2 ppm in the surface
AQ horizons above nonraineralized bedrock to 1,100 ppm in the C horizon of
profiles over sulfide deposits (Table 4.5). See Tables 4.6 and 4.7 for As
contents of a large number of soil and rock samples, and Table 4.8 for a
comparison of the concentration of trace metals in igneous and sedimentary
rocks, seawater, and soils.
TABLE 4.5. MEAN CONTENTS AND RANGES OF ARSENIC IN WELL DRAINED NEW BRUNSWICK
PODZOL PROFILES*
Above "normal"
No
A
0
A2
Bl
B2
C
. of Profiles sampled
horizon
horizon
horizon
horizon
horizon
— -»• 13
Mean As
ppm
2
5
22
14
11
bedrock
Range
ppm
0-10
0-40
0-70
0-40
0-30
Above sulfide deposits
— -»• 7
Mean As
ppm
530
620
927
1101
1270
Range
ppm
70-1100
0-1900
10-2900
15-3500
15-3400
From Presant and Tupper (1966).
Asbestos
Asbestos is a fibrous mineral occurring as a serpentine (or chryso-
lite) form. It is a calcium-magnesium silicate used extensively in the pro-
duction of roofing material, water pipes, insulation, fireproof materials,
and clothes. It is not found abundantly in the soil. If it were, it still
would not offer a hazard to man, except when it becomes highly subdivided
as a windborne polluting dust. The dust from processing and mining of ser-
pentine provides a serious respiratory hazard. Soils filter-out asbestos
readily on or near the surface. During soil formation, the rate of
60
-------
TABLE 4.6. RANGES OF TOTAL Fe, As, Cu, Mn, Pb, Sb, Sn and Zn FOUND IN VARIOUS PODZOL, AND PODZOL-LIKE
SOIL PROFILES OF HUMID TEMPERATE CLIMATE
Horizon
A
00
A
0
A2
Bi
B2
B3
C
Cl
C2
C9l
C92
No. of
samples
2
20
19
19
18
1
13
4
4
2
1
PH
3.0-4.5
2.4-5.8
3.2-4.8
3.4-5.4
3.8-6.2
4.0
4.4-6.6
4.6-6.6
4.1-6.9
3.8-6.9
3.2
Org nic
carbon
-------
TABLE 4.7. SOME TRACE ELEMENTS FOUND IN ROCK SAMPLES'
l\3
Samp] e
No.
7e
15e
19e
21d
30f
38e
40h
44h
52d
53e
55e
62g
64f
72d
73g
78f
80e
Pb
50
400
560
40
75
15
5
<10%
15
240
30
25
15
5
< 5
< 5
1.10%
Cu
10
20
45
35
45
20
40
150
5
75
15
TO
< 5
5
< 5
30
2,100
Zn
440
570
170
120
190
30
100
220
30
50
60
30
20
60
30
110
220
As
85
70
85
5
80
<5
15
1,500
0
105
25
20
10
35
20
@
1%
Sb
4
35
3
1
3
<1
1
280
1
6
2
1
<1
<1
<1
2
90
Mn
6,120
1,240
620
465
1,320
< 80
80
390
390
2,320
775
< 80
700
775
540
1.20%
230
Ag
0.9
1.0
13.0
0.2
1.1
0.2
0.8
67.0
0.1
4.6
0.3
0.2
0.1
0.2
0.2
0.2
31.0
Sn
9.0
<0.5
62.0
<0.5
<0.5
0.7
2.8
14.0
11.0
4.4
15.0
4.3
19.0
9.1
0.7
2.6
47.0
Cd
3.9
0
24.0
0
0
0
0
0
0
0
@
@
@
0
0
0
0
S
<200
<200
<200
1,400
1,500
2,100
<200
< 3%
200
600
2,900
< 200
< 200
< 200
600
< 200
Fe CO
V
4.41
4.62
1.12
4.97 <
3.64 <
1.40
3.71
32.13
4.34
3.50
3.99
1.75
0.63
2.87
6.44
10.22
40.15
2
0.49
1.54
3.72
.01
.01
6.00
0.10
0.05
<.01
0.02
0.06
<.01
<.01
<.01
0.01
0.24
From Presant (1971)
'Not determined.
-------
TABLE 4.8. CONCENTRATIONS OF TRACE METALS IN IGNEOUS AND SEDIMENTARY ROCKS,
SEAWATER, AND SOILS
Trace
element
As
B
Cd
Cr
Co
Cu
Pb
Ni
Se
Zn
Concentration in
igneous rocks,
sedimentary rocks,
seawater
PPJT)
2
1-3 ,
3 x 10"J
10
20-1 00
4.6
0.2
0.03-3 -
1.1 x 10"H
100
10-100 ,
5 x 10"D
25
1-20 A
2.7 x 10"H
55
5-45 ,
3 x 10"J
12.5
5-20 ,-
3 x 10"D
75
2-70 ,
5.4 x 10"J
0.05
0.1-1. 0K
9 x 10"°
70
10-100
0.01
Concentration
in soils
ppm
0.1-40
2-100
0.01-7
5-3000
1-40
2-100
2-200
10-1000
0.1-2.0
10-300
Annual
uptake by
grasses
and crops
Kg
107
108
107
107
106
108
107
107
107
109
From All away (1968)
63
-------
movement of clay-sized (< 2y diameter) particles is about 1-10 cm/3,000-
40,000 yr, Berkland (1974), depending on the texture. Asbestos particles of
this size would not be expected to migrate more rapidly, and particles of a
larger size would move more slowly, if at all. Asbestos weathers slowly.
The meager weathered products offer no more hazard to underground waters
than natural silicates in soils or clay minerals.
Beryllium
Beryl is the principal Be-bearing primary mineral, although phena-
cite, chrysoberyl, and bertrandite are more or less widespread. The beryl-
lium content in beryl is about 5%. Most sedimentary rocks have small
amounts of beryllium. According to Beus (1956), Kuroda (1956), and
Sidgwick (1950), the Be content of the earth's lithosphere varies from
0.006 to 0.002%. Soils, where there is no beryllium in the parent material
(and these are most common), contain no beryllium in concentrations detect-
able by the usual atomic absorption spectrophotometer methods, Romney and
Childress (1965). Beryllium appears to be dispersed or dissipated by the
process of weathering.
Cadmium
Cadmium is a normal constituent of soils in trace or trace-trace
amounts. It is present in insoluble form as an impurity in various pri-
mary and secondary minerals. The spodic soils of the New Brunswick area in
Canada contain < 3 ppm, except where the soils overlay a sulfide vein;
Presant (1971), for example, found more than 3 ppm in the Bg and C horizons
of two soils.
Cadmium is a normal constituent of marine deposits and is a notor-
ious contaminant in phosphate rock. It carries through the refinement of
phosphorus and appears as a superphosphate fertilizer, Lagerwerff (1972).
Soil contamination takes place from automobile traffic (tires and oil),
factories and mines handling zinc, use of fungicides, and certain phosphate
64
-------
fertilizers. Cadmium becomes more concentrated than zinc in the air-soil-
plant cycle.
Cadmium, like zinc, unites readily with sulfur, Fairbridge (1972).
Also like zinc, sulfides and sphalerites are the principal carriers of
cadmium. Geochemically, though, it is the nature of Cd to be widespread as
isomorphic impurities in various other minerals, usually as sulfides. Cad-
mium sulfide weathers more slowly than zinc sulfide. Oxidation of the sul-
fide to sulfate causes cadmium to leach out of sulfide deposits since the
sulfate is relatively soluble in acid waters. In fact, according to Hawkes
and Webb (1962), Cd in streams has been a pathfinder in the geochemical
search for zinc. In weathering, Cd follows Zn, except in cases of zinc
sulfide. Secondary CdS remains after zinc has been leached. Fairbridge
(1972) suggests that the average concentration of Cd in soils is ^ 0.5 ppm.
Chromium
Chromium in the earth's crust is derived from deeper levels in the
mantle as a constituent of basaltic magmas, Smith (1972). Chromium precipi-
tates from the magmas at an early stage as chromium spinel or in silicate
minerals. Chromite resists chemical weathering. When it is released, it
concentrates mechanically in laterites or heavy mineral placers. Because
chromium goes into solution sparingly, it appears only in very low concentra-
tions in precipitates and evaporates, Smith (1972). Although it appears to
be fairly abundant in the earth's crust, chromium seems to occur naturally
in soils, like cadmium, only in trace amounts or not at all (last 2 columns,
Table 4.9). For example, chromium was positively identified in only 8 of
the 68 soil samples representing 30 soil series in California (Table 4.10).
The California locations may or may not be representative of general
chromium distribution patterns in most soils. Unfortunately, such data are
limited.
65
-------
TABLE 4.9.
ABUNDANCES OF aEMENTS LN TYPICAL HUMID TEMPERATE REGION
MINERAL SOILS*
(The values shown are estimates which vary widely between soils, but ranges
are thought to be representative for well aerated soils of pH 6-7. Extract-
able indicates adsorbed or exchangeable forms. Soluble represents concen-
tration in soil solution.)
Approximate concentration in ppm (mg/1000 g)or %
Element
Group one
Lithium
Sodium
Potassium
Nitrogen
Rubidium
Cesium
Magnesium
Calcium
Strontium
Barium
Group two
Chlorine
Sulfur
Selenium*
Group three
Fluorine
Phosphorus
Molybdenum
Silicon
Arsenic*
Boron
Group four
Aluminum
Beryllium*
Vanadium
Group five
Iron*
Manganese
Chemical
notation
Li
Na
K
(Nht)
Rb
Cs
Mg
Ca
Sr
Ba
Cl
S
Se
F
P
Mo
Si
As
B
Al
Be
V
Fe
Mn
Earth s
crust
(total )
ppm or %
65
2.8%
2.6%
280
3.2
2.1%
3.6%
150
430
480
520
0.09
800
1200
2.3
27.7%
5
10
8.1%
6
150
5%
1000
Mineral
soil
(total )
ppm or %
5-200
0.15-1.5%
0.75-2.4%
20-800
0.2-0.6%
0.3-1.5%
50-1000
100-5000
250-1000
0.1-2.0
300
100-1000
0.2-5
29-38%
1-50
2-100
3.2-6.9%
0.2-10
20-500
1.4-4.2%
200-5000
Mineral Mineral
soil soil
(extractable) (soluble)
ppm
25-120
20-200
10-100
120-600
750-3700
0.2-10
10-40
0.2-20
0.02-0.2
0.1-5
< 25
0. 05-5
< 5
5-100
mg/1
0.5-5
1-10
5-50
30-300
60-600
50-500
0.001-0.
0.1-0.5
0.002-0.
0.001-0.
10-50
< 0.01
01
03
01
very low
0.1-10
66
-------
Table 4.9 Ccontlnued).
Element
Group six
Copper*
Cobalt
Zinc*
Nickel
Cadmium*
Lead*
Mercury
Chromi urn*
Silver
Group seven
Total organic
matter
Organic-N
Organic-P
Organic-S
Chemical
notation
Cu
Co
Zn
Ni
Cd
Pb
Hg
Cr
Ag
Earth.1 s
crust
20
40
80
100
0.2
16
0.5
200
0.02
Mineral
soil
2-100
1-50
10-300
5-500
2-200
0.02-0.2
5-1000
0.1-1
0.4-10%
0.02-0.5%
50-500
125-500
Mineral Mineral
soil soil
0.05-5 0.03-0.3
0.5-4
2-30 < 0.005
0.1-5
0.05-10
0.01-4
From Murrmamand Koutz (1972)
Values not given when a reasonable estimate could not be formulated.
Copper
The abundant stable minerals of copper in soils are sulfides. The
occurrence of copper as a native element is not infrequent in terrestrial
i i
environments. It is largely held in the Cu adsorbed state. Though copper
appears in the soil solution, its abundance is dependent upon whether (a)
the amount of Cu exceeds the adsorption capacity of the soil, and/or (b)
conditions are acidic and oxidizing. Krauskopf (1972) suggests that copper
remains fairly close to its source (parent material in soils. Yet concen-
trations in soils do not appear to correlate with the lesser concentrations
of Cu in ordinary rocks.
Copper also is abundantly present in the organic horizon of soils and
is probably associated with organic matter. Total copper concentrations in
67
-------
TABLE 4.10.
DATA FROM ANALYSES OF 68 SOIL SAMPLES REPRESENTING 30 SOIL
SERIES IN CALIFORNIA*
Elements
Fe
Mo
Mn
Cu
Zn
Ni
Co
Cr
Hg
Pb
V
B
Sr
Ba
Si
Ti
Al
Mg
Ca
Na
K
Li
SO^sulfur
Cl
Ca
Mg
Na
K
Positive
Occurrences Concentration of saturation extracts
in samples Units Range
65
32
26
67
68
30
2
8
47
19
39
16
51
42
68
0
3
68
68
68
68
27
68
68
68
68
68
68
ppm < 0.01-0.8
< 0.01-22.0
< 0.01-0.95
< 0.01-0.20
0.01-0.40
< 0.01-0.09
< 0.01-0.14
< 0.01-0.017
0.0002-0.0109
< 0.01-0.30
< 0.01-1.20
< 0.1-26.0
< 0.1-10.4
< 0.05-1.20
0.20-24.0
< 0.1-
< 0.1-0.60
0.4-400.0
1.0-930.0
0.9-19,200
0.7-128.
< 0.03-1.08
meq./l 0.025-145.4
0.01-580.0
meq./IOOg.soil 2.8-42.0
(exchangeable+soluble)
0.19-18.0
0.02-40.0
0.12T3.2
Mean
0.05
0.73
0.17
0.04
0.07
0.02
0.06
0.01
0.0024
0.05
0.07
3.06
0.93
0.26
5.0
< 0.1
0.4
38.0
128.0
524.
20.
0.11
8.4
12.6
13.3
4.4
1.6
0.8
Median
0.03
< 0.01
< 0.01
0.03
0.04
< 0.01
< 0.01
< 0.01
0.001
< 0.01
0.01
< 0.1
0.18
0.10
3.50
< 0.1
< 0.1
12.4
60.0
45.
10.
0.05
1.23
0.7
11.0
2.9
0.25
0.6
From Bradford et al. (1971)
68
-------
humid spodic or spodic-like soils range from a few ppm to over 360 ppm
(Table 4.6) and up to 200 ppm in soil studied by Allaway (1968, Table 4.8).
The mean for 84 samples as related to the A , A2, B,, B2, and C horizons
were 29, 19, 24, 61 and 92 ppm, respectively (Presant, 1971). Water-
soluble Cu usually was 0.01 ppm or less, for most of the soils (Table 4.9).
According to the Allaway (1968) review, soils contain between 2 and 100 ppm
Cu (Table 4.8). Podzolic soils of eastern Canada contain 1.06-19.03 and
0.09-0.46 ppm exchangeable and water-soluble Cu, respectively. Small bag
samples of Canadian humid soils varied in median values of total Cu from 8
to 20 ppm, Presant (1971), Table 4.11. Bradford et al. (1971) report 0.01-
0.20 ppm Cu in 67 California samples (Table 4.10).
TABLE 4.11. TOTAL COPPER LEAD, AND ZINC IN SOME HUMID TEMPERATE REGION SOIL
PROFILES
(Median values only)
Horizon
Ao
A & A1
A2
B
Bl
B2
C
Number of
soil samples
analyzed
48
9
48
7
48
48
49
pH and Total Element Concentration
pH
4.2
4.7
4.1
4.8
4.6
4.7
4.9
Cu
10
10
8
20
8
20
25
Pb
25
25
15
260
20
20
25
Zn
70
110
25
140
80
120
100
Summarized from data of Presant (1971).
69
-------
Cyanide
Cyanide is not a natural constituent of soil. Plants can manufacture
quite large amounts of cyanide in their tissues under certain climatic con-
ditions. Incorporation of cyanide-containing plant materials into soils usu-
ally transforms CN into harmless nitrogen gas N^t or into nitrate by microb-
ial oxidation.
Lead
All soils contain some lead. Total lead content of soils is far
greater than water-soluble lead or lead in the soil solution (Table 4.10 and
4.11), and the mean value for the earth's crust is reported to be 16 ppm
(Table 4.9). Native soil lead may be substantially higher, 40-70 ppm,
Davies (1968). Surface values of lead increased substantially during recent
years as a result of human activity. Lead enters the soil from aerial
deposits originating from automobile exhaust, fertilizers, and pesticides.
Lead-bearing soil minerals contribute substantially to lead in some
soils as plants recycle and concentrate lead in the organic matter. Swaine
and Mitchell (1960) report that a Scottish soil contained 20-30 ppm Pb in
lower horizons as compared with 550 ppm in the surface organic layer.
Environmental conditions largely control the behavior of lead in
soils during weathering. Soils, therefore, have highly variable lead con-
tents (2-200 ppm), Wampler (1972). Waterlogged soils and those under
reducing conditions contain lead in the highly insoluble PbS form. In oxi-
dized conditions Pb appears primarily as a sulfate, which is only slowly
soluble. Acidic waters carry small amounts of Pb in solution, probably as
sulfate. In neutral and slightly alkaline media, Pb hydrolizes and co-
precipitates with hydroxides. Clay minerals also absorb Pb. Lead, there-
fore, is one of the less soluble elements in the soils.
70
-------
Estimates of Hg concentrations in soils range from 10 to 500 ppb in
Sweden (Andersson, 1967). Pierce, Botbol, and Learned (1970) consider 100
ppb to be a good average figure in the USA, and Andersson (1967) suggests
60 ppb as an average in Sweden. Shacklette, Boerngren, and Turner (1971)
in a study of 912 soil samples found averages of 96 ppb in eastern US, 55
ppb in western US, and 71 ppb nationwide. Local areas are much higher in
Hg. In belts along major faults, natural (geological) Hg in the soil solu-
tion and water-soluble Hg are much lower than total Hg. In contrast to
Sweden's 60 average total Hg, drainage water from 36 cultivated soil sam-
ples contained concentrations ranging from 0.02 to 0.07 ppb (Wiklander,
1969).
Mercury in soils under anaerobic conditions, such as expected
beneath leachate of landfills, aqueous lagoons, trenches, and ponds, is
converted, in part, into soluble monomethyl or gaseous dimethyl Hg com-
plexes. Under the usual aerobic soil conditions, mercury exists in any
form.
Selenium
Selenium in soils has been studied so extensively, a map showing
geographic concentrations has been produced (Figure 4.6). Selenium notor-
iously is known as a natural soil contaminant. According to data from
Murrmann and Koutz (1972), total Se concentrations in typical humid temper-
ate region mineral soils range from 0.1 to 2.0 ppm (Table 4.9). Concen-
tration in the soil solution (soluble Se) ranges from 0.001 to 0.01 ppm.
Native levels of "available" Se in soils are sufficiently high to accumu-
late in plants at levels that are toxic to animals. On the other hand, Se
is a required animal nutrient and some soils do not contain sufficient
quantities to produce plants to satisfy animal requirements. Excesses of
soil Se occur primarily in soils of neutral to alkaline pH levels and
where lime (CaCO-) and gypsum (CaSO-^HgO) appear in the soil profile. In
highly localized areas of the Great Plains and Rocky Mountain states,
71
-------
concentrations exceed 50 ppra (figure. 4.6).
•| Low - approximately 80% of all foraga and grain contain <0.03 ppm
^^ of tolonium.
I 1 Variabla -approximately 50% contain § >0.l ppm.
Adaquote - 80% of all forogat and grain contain > O.I ppm of talanium.
Local trtot whara *alanium accumulator plantt contain >SO ppm.
Figure 4.6. Geographic distribution of low-, variable-, and adequate-Se
areas in the USA. (From Kubata and Allaway, 1972.)
Good reviews on selenium have been produced by Kubota and Allaway
(1972); Rosenfeld and Beath (1964); Anderson, et al. (1961); and Moxon, 01 sen,
and Searight (1939).
Zinc
All soils are considered to contain Zn naturally. The Zn concentra-
tion in most soils ranges between 10 and 300 ppm. Maximum concentrations
have been reported at levels between 4,000 and 6,000 ppm (Table 4.6). The
availability of Zn as a required trace element for plant growth, however,
often is low in some soils, and deficiency symptoms appear. Addition of
72
-------
soluble Zn in soils varies widely. Where sphalerite and sulfides occur as
parent material for soils, Zn is abundant. In most profiles Zn shows a
rather uniform distribution throughout. Zinc differs from the other heavy
metal cations in being one of the least readily absorbed.
Zinc concentrations in some soils are reported in Tables 4.6 and
4.8-4.11.
Research on the oxidative weathering of sulfide ore bodies is the
primary source of knowledge concerning the process of weathering and sedi-
mentation. Zinc sulfate is relatively soluble and moves rapidly in aqueous
transport. This is evidenced by the fact that sea water can contain as
much as 0.01 ppm and Zn deposits in sea and lake sediments have been found
at levels well over twice those of the estimated average crustal abundance
or mean shale content. The great affinity of Zn for the sulfide phase,
however, keeps the mobility somewhat in check (C.D. Curtis, 1972).
THE GEOLOGIC MATERIAL
Soil Formation
Soils develop slowly from geologic material, either consolidated or
unconsolidated. The soil is a natural body synthesized in profile form from
a variable mixture of broken, weathered minerals and decaying organic
matter, which covers the earth in a thin layer. See Figure 4.7 for a rep-
resentative soil profile. The most prominent factors which function in
this transformation of rock into soil are (a) climate, (b) organisms (includ-
ing all life—animal, vegetation, and man), (c) topography, (d) parent
material, and (e) time. When rain is limited, these factors act slowly.
Chemical dissolution of geologic minerals and movement of substances in and
out of the soil profile, or parts of the profile, are almost at a standstill
under arid conditions. Figure 4.4 shows this relationship with geologic
materials. Under humid conditions such processes are accelerated.
73
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slp^i^S-gtp
iS-Gp -<•>? o''-<7 <=Bi. &D-c53?rt
ps§&2ss^>*s?*S
-
A HORIZON
SURFACE SOIL
\ ZONE OF MAXIMUM ORGANIC
ACCUMULATION. COINCIDES
MORE OR LESS CLOSELY WITH
FURROW SLICE
B HORIZON
SUBSOIL
\ CHARACTER IS DETERMINED
r TO A CONSIDERABLE DEGREE
BY NATURE OF SOIL
FORMING FORCES
C HORIZON
SUBSTRATUM
USUALLY MORE OR
LESS WEATHERED
Figure 4.7. A typical soil profile.
74
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The selected potentially.hazardous elements (As, Be, Cd, Cr, Cu, Fe,
Pb, Hg, Se, Zn), cyanide, and asbestos are governed in their distribution
and behavior toward other materials making up the earth., including soil and
vegetation by the laws of geochemistry. Unfortunately, the knowledge of
geochemistry as it relates to the migration of elements through geologic
material is incomplete. The small amount of information available in the
literature may oe of some assistance, however, in understanding in a specu-
lative way the migration of certain elements and substances.
Distribution of Elements
The distribution of Fe, Cu, and Zn and other potentially less hazar-
dous pollutants in some common geologic materials, compared with soils (ave-
rage figure), is given in Table 4.12. Because of the variability of all
materials appearing in the table, the average concentration is probably
reliable only within a factor of 2. Among igneous rocks these metals are
more abundant in basaltic lavas than in granites. The average concentration
in the earth's crust is roughly the mean of that in granite and basalt. The
two kinds of igneous rock are the principal components of the earth's crust.
According to Krauskopf (1972), "Soils, like shale, should have on the average
a composition close to that of the earth's crust as a whole," and the numbers
in Table 4.12 show general agreement with this expectation. But the raw
material from which soils are derived is so variable in composition, and
soil-forming processes are so different from one climatic regime to another,
that an average soil composition can have little significance. The wide
ranges suggested in the table include the majority of soils but certainly not
the extremes. The chief importance of such ranges is simply to identify
soils having concentrations outside these limits as probably anomalous either
in their parent materials or in the geologic processes by which they were
formed.
The contribution that geochemistry now can make to knowledge about
migration of the designated hazardous pollutants through soils is very
limited. The concept just put forth (Krauskopf, 1972) does serve to point
75
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TABLE 4.12. ABUNDANCES OF MICRONUTRIENT ELEMENTS*
a
Igneous rocks
Element
Fe
Mn
Cu
Zn
Ho
B
Crust
56,000
950
55
70
1.5
10
Grani te
27,000
400
10
40
2
15
Basalt
86,000
1,500
100
100
1
5
Sedimentary rocks
Limestone
ppm
3,800
1,100
4
20
0.4
20
Sandstone
9,800
10-100
30
16
0.2
35
Shale
47,000
850
45
95
2.6
100
**
Soils
10,000-100,000
20-3,000
10-80
10-300
0.2-10
7-80
From Krauskopf (1972).
*From Taylor (1964).
'From Turekian and Wedepohl (1961).
**
Compilation from many sources.
-------
out the complexity of the subject and the serious need for development of
considerably more information in both soil science and geochemistry before
attenuation and migration of heavy and trace elements in soils and geologic
material can be predicted with accuracy. The predictions we now make, how-
ever, can serve as a departure for further investigations.
Information on the attenuation of potentially hazardous pollutants
through geologic materials assumes Importance because (a) many landfills and
waste disposal excavations are located below the soil body and in unconsoli-
dated material and (b) leachates which originate in soil sites often must
pass through unconso11 dated geologic material before reaching the capillary
fringes of groundwater. Unfortunately, the geologic material located below
disposal sites often 1s composed of sand, gravel and/or alluvial stone and
debris with little or no attenuating capacity. Site locations in the
stony alluvium of river bottoms can be expected to sieve-out only the larger
partlculates. Soluble constituents and colloidal material in this debris
will pass directly into the underground water virtually unchanged.
Abundance of Elements
Because the data in Table 4.12 overemphasize mean similarities
between soils and geologic material, 1t 1s Important to point out that under-
lying rock has been claimed to have little Influence on the composition of
the more mature soils; I.e., soils having almost identical profile character-
istics may have been derived from different parent geologic material. Soil
scientists generally conclude from studies of soil-forming processes that
trace nutrient elements 1n soils are not closely related to their concentra-
tion In parent rock, except In places where weathering of the rock is slight
(Table 4.9). Suggested ranges of Iron, copper, and zinc in representative
surface soil are compared in Table 4.13.
Mobility of Elements
Thus, elements within the soil profile are not necessarily (In fact,
rarely) found In the same proportion as they appear In the geologic materials
77
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TABLE 4.13. THE RANGE IN MLCRONUTRIENT CONTENT COMMONLY FOUND IN SOILS AND
A SUGGESTED ANALYSIS OF A REPRESENTATIVE SURFACE SOIL.*
Nutrient
Iron
Manganese
Zinc
Boron
Copper
Normal
%
.500-5.000
. 020-1 . 000
.001-. 025
.0005-. 01 5
.0005-. 01 5
Suggested Analysis
of a representative
Soil Range surface soil
ppm
5000-50,000
200-10,000
10-250
5-150
5-150
25,000
2,500
100
50
50
*Estimates based on
L. Mitchell, "Trace
published data
Elements," in
from a number of sources
F.E. Bear, Chemistry of
, especially R.
the Soil (1964K
from which they are derived. They either concentrate or disperse, depending
on the intensity of the factors controlling mobilization within the soil body.
Writers in the field of soil science have attempted to define these factors.
Brooks (1972), on the other hand, gives us the view of the geochemist. He
states that mobilization and distribution of the elements within the soil
profile are controlled by four main factors:
1. Mobilization due to breakdown of soil minerals by various
weathering agents and leaching through the profile.
2. Adsorption of ions onto clay minerals and humus.
3. Surface enrichment of elements by plant materials (the bio-
geochemical cycle).
4. Mobilization or fixation by soil microorganisms.
It is good to note that Brooks' statement of factors believed to
influence mobility of elements in soils, though very broad, is similar to
that suggested by soil scientists. His discussion comparing the importance
of the individual factors also relates well to the concept put forth by soil
scientists. Rules for the biogenic enrichment of elements in the soil pro-
file as suggested by Irving and Williams (1953) are worthy of describing
78
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here, even if only briefly. The stability of complexes between metals and
organic matter is (.a) largely independent of the ligand, (b) for divalent
cations follows the order: Pt > Pd > Hg > IKL > Be > Cu > Ni > Co > Pb >
Zn > Cd > Fe > Mn > Ca > Sr > Ba, (c) for monovalent cations: Ag > Ti > Na
> K > Pb > Cs, and (d) for trivalent cations: Fe > Ga > Al > Sc > In > Y >
Pr > Ce > La.
The relative mobilities of elements in the supergene environment are
given in Table 4.14. Supergene refers to mineral formation by descending
waters, usually taking place below the surface. Some of the conditions in
the supergene environment (e.g., oxidizing conditions in the absence of free
oxygen) are unlike those encountered in the soil environment and some of the
factors in attenuation of metals in the soil environment (e.g., cation
exchange and biological transformations) are absent in the supergene environ-
ment. Consequently, relative mobilities of metals in the two environments
will differ except when solubility is the dominating factor.
TABLE 4.14. RELATIVE MOBILITIES OF CERTAIN ELEMENTS IN THE SUPERGENE
ENVIRONMENT*
Environmental condition
Element
As
Be
Cd
Cr
Cu
Fe
Hg
Pb
Se
Zn
Oxidizing
3
4
3
5
3
5
3
4
2
2
Acidic
3
4
3
5
2
4
2
4
2
2
Neutral -
alkaline
3
4
3
5
5
4
5
4
1
5
Reducing
5
5
5
5
5
4
5
5
5
5
*Adapted from Andrews-Jones (1968), as presented by Brooks (1972).
Note: Values represent descending mobility; 1 - very high; 2 - high; 3
medium; 4 - low; 5 - very low to immobile.
79
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SECTION V
MOBILITY OF SECECTED CONSTITUENTS IN SOIL AND SPECIFIC MECHANISMS
INVOLVED
This section presents a rating of the mobility 1n soil of selected con-
stituents and a discussion of the specific mechanisms which control mobility.
The mobility ratings are based on information from the University of Arizona
study and from the literature; the discussion of specific mechanisms 1s based
on the author's experience and on the literature reviewed In this report. The
information in this section is presented as background for the recommendations
in Section III to indicate the rationale and to aid users 1n applications to
situations not exactly covered.
MOBILITY OF CONSTITUENTS IN SOIL
While the following definitions of attenuation and mobility are not as
precise as desired, they do outline the concepts. A first basis for defining
attenuation is the observed decrease in the concentration of a pulse or front
of solute as it moves through a soil. In this case, attenuation Is quantified
by specifying the time or distance required for the maximum concentration of
one or more of the solutes in the pulse to decrease to some fraction of the
original concentration. The second basis for defining attenuation 1s the soil's
capacity for retaining a solute and the rate at which the solute 1s Immobilized
by the soil. In this case, attenuation Ms quantified by calculating a mass
balance for solutes in the system after attenuation has occurred or by con-
tinuous measurements of solute input and output for the system.
Although the concept of attenuation is useful in visualizing the behavior
of a system, the concept of migration or mobility is often more useful in
management because it can be inferred from the commonly made measurements of
the distance traveled by a given concentration of solute in a fixed time or of
the time required for solute to appear at.a fixed distance from a solute source.
80
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If the system 1s sufficiently well behaved (I.e. linear) data used to esti-
mate mobility can also be used to estimate attenuation. A working definition
of mobility 1s the ease with which a specific element becomes distributed
vertically 1n soil and/or weathered geologic materials as a function of the
rate and direction of movement of soil solution or leachates from wastes de-
posited 1n or on the soil.
Mobility of a constituent 1n a particular soil 1s controlled by the
net effect of all the soil characteristics or factors which Influence mobil-
ity. While one factor, such as a high content of hydrous oxides of 1ron»
aluminum, or manganese, may be very favorable for attenuation (decreased mo-
bility), other factors may be so Intensely unfavorable (e.g., partial pres-
sure of Q£ or low Og content, coarse texture, low pH) as to dominate the re-
actions and allow rapid migration. Thus the mobility of a constituent In a
particular soil should not be estimated by considering only one factor.
Absolute mobility, measured under laboratory conditions cannot be
extrapolated, at this time, to field conditions. Hence the mobility of a
constituent 1n soil 1s defined relative to other constituents. This rating
of relative mobilities 1s generalized, and In a specific situation the order
of constituents may change because the mobility of one or more of them 1s
particularly sensitive to soil factors, such as pH or aeration status. Two
examples of this may be seen In the mobility listings presented later 1n this
section. Anaerobic soil conditions will not only alter the mobility ranking
predicted for aerobic conditions but the absolute mobilities will be greater
under anaerobic conditions and more nearly similar for most constituents. In
the ranking presented for anaerobic conditions, a change 1n soil pH changes
the relative positions of chromium, zinc, and selenium while affecting the
ranking of the other constituents only slightly.
The terms used to describe mobility are qualitative and are not
associated with actual rates of migration. For purposes of reference, how-
ever, the terms for the extremes of migration behavior may be defined as
follows:
81
-------
Relatively .mobile - siroilar to the almost uninhibited movement
of chloride ion, moving at the same rate as soil solution.
Immobile - similar to the rate of movement of clay-sized
(< 2y diameter) particles in soils, 1 to 10 cm per 3,000
to 40,000 years, depending on the texture of the soil.
The two intermediate terms, moderately mobile and slowly mobile,
are empirical groupings of substances which exhibit similar behavior and
whose absolute rates of mobility lie between the two extremes described
above.
Based primarily on information in the literature, the 12 selected
constituents may be grouped as follows with respect to mobility under
aerobic (oxidizing) conditions:
II
III
Element
Relatively mobile
Cyanide - CN"
Selenium - HSeO
Moderately mobile
Iron, zinc, lead
copper
Beryllium - Be
Slowly mobile
Arsenic - h
c.
Cadmium - Cd"1"1"
Chromium - Cr
Mercury - Hg
Comment
Not strongly retained by the soil.
Not strongly retained by the soil, at
normal pH levels.
Absorbed more strongly by the soil in
the order of Cu+* > Pb++ > Zn^
< Fe"1"1". Stability for complexes of
any given type should be increasing
in the order of Fe Zn Pb Cu
Chemistry in soils probably similar to
aluminum.
Mobility similar to phosphorus.
Forms insoluble precipitates in oxidiz-
ing conditions.
Forms insoluble precipitates in oxidiz-
ing conditions.
Retained in the surface layer of most
aerated soils.
82
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Element
Immobile
Asbestos
Comment
Insoluble mineral. Particles will move
at about the same rate as clays and will
be retained at or near the soil surface.
Information in the literature on mobility under anaerobic (reducing)
conditions is scanty. Consequently, the following estimates of mobility
groupings are based primarily on results to date (1975) at the University of
Arizona. Municipal landfill leachate, spiked singly with trace elements,
was passed through columns of disturbed soils maintained in anaerobic
conditions. The soils are described in Section III and the design of the
project is described in Appendix A. The mobility of selected elements in
acid soils and in neutral to alkaline soils grouped was as follows:
Soil type
PH
Elements in the leachate
Wag ram l.s.
Ava si. c.l.
Kalkaska s.
Davidson c.
Molokai c.
Chalmers si. c.l.
Nicholson si.c.
Fanno c.
Mohave s.l.
Mohavec c.l.
Anthony s.l.
J
4.2
4.5
4.7
6.2
6.2
6.6
Neutral
6.7
7.0
7.3
7.8
7.8
Relatively
mobile
Acidic soils
Cadmi urn
Nickel
Mercury
Zinc
to alkaline soils
Arsenic*
Chromium*
Moderately
mobile
Arsenic*
Beryl 1 i urn
Chromi urn
Beryllium
Cadmium
Mercury
Selenium*
Zinc
Slowly
mobile
Copper
Lead
Selenium*
Copper
Lead
Nickel
Arsenic, selenium, and to a certain extent chromium conform to the above
mobility pattern least well. For example, Se was found to move slowly in
Fanno c. and relatively fast through. Wagram l.s. Arsenic moved slowly
through the neutral soils, Nicholson s.c. and Fanno c., and relatively fast
through Davidson and Molokai clays. Chromium moved slowly through
83
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Davidson c., Nicholson si.c. and Molokai. Thus with, those elements which
form anions, the effect of clay content in soils is relatively dominant,
particularly where the pH ranges near neutral conditions (i.e., mildly
acidic or alkaline).
Specific Mechanisms
When wastes are deposited on land, a number of physical, chemical,
and biological reactions take place or are initiated which affect the rate
of movement of contaminants from the waste through the surrounding soil. In
this section, some of the most significant of these reactions are described
to lay a framework for a better understanding of the complex migration and
attenuation phenomena and reaction products of the individual trace contam-
inants, described in other parts of this report.
Although the mechanisms have been grouped (physical, biological, and
chemical) it should be understood that sharp distinctions cannot always be
made between them. For example, biological processes of organic-matter
degradation initiate many chemical reactions. Oxygen consumption and deple-
tion in the substrate for carbon oxidation is one example of such specific
interaction. A multitude of biological transformations of inorganic con-
stituents also occurs in soils. Similarly, poor physical condition of the
soil (heavy clays, poor structure, and compaction) dramatically influences
soil aeration and related redox potential, which in turn influence biologi-
cal and chemical processes.
Biological mechanisms—
Although soil microorganisms exert the primary (most significant)
effect on attenuation of trace contaminants, their influence may not be
immediately apparent. However, consider the following processes which soil
microorganisms either perform entirely or mediate:
1. Degradation of carbonaceous wastes;
2. Transformation of cyanide to mineral nitrogen compounds and
denitrification to inert N2 gas;
84
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3. Initiation of metal ion oxidation-reduction;
4. Production of C02 with subsequent fonnation of weakly ionized
carbonic acid;
5. Production of simple organic acids;
6. Production of large and small molecular species upon which trace
contaminants may be adsorbed;
7. Production of complex organic compounds which may react with
trace contaminants;
8. Production of small-sized organic debris which can infiltrate
small pore spaces and move downward in the soil profile.
Although it may not be possible to fully control the activities of
the soil microbes, their intimate involvement in many attenuation processes
is good reason to be aware of them and their role in the movement of trace
contaminants.
The following excerpt from a review by Patrick Hunt (1972)is pre-
sented as an overview of microbial numbers and activities in the soil:
"The primary microorganisms of the soil can be classified as
bacteria, actinomycetes, fungi, algae and soil animals. These
organisms are the ecological units that may likely have the
largest effect on land-disposed wastewater. The reason they are
so important is that they can transform the wastewater components
extensively from gases, liquids or solids; and the transformations
may be beneficial or detrimental to good wastewater management.
These transformations are accomplished by processes such as oxida-
tion, reduction, mineralization, immobilization, precipitation
and concentration.
"The bacteria are the most numerous and biochemically active
group, especially when oxygen levels are low. They are also
the smallest on a per-cell basis, commonly 1 to 10 microns in
length or diameter .... Bacteria are primarily responsible
for such important processes as nitrification, denitrification,
nitrogen fixation and sulfur transformations.
"The fungi occupy more volume in the soil than any other group,
and they are referred to as the most massive group. They have
extensive mycelial branching and are good competitors for simple
carbohydrates and bteproducts of the less efficient bacteria.
The fungi are involved in humus formation, aggregate stabilization,
85
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and certain mineral transformations. In addition, many of the
plant diseases related to jnolst conditions are caused by fungi.
"The actinomycetes are morphologically intermediate to the
bacteria and fungi. They all belong to the taxonomic order
Actinomvcetales. along with some of the true bacteria, and they
are unicellular. However, they have mycelia that resemble those
of fungi. The actinomycetes produce numerous antibiotics and are
quite important to the pharmaceutical field.. Some of the thermophilic
types are prevalent in rotting organic material such as animal
manure, hay and compost.
"The algae are normally present in fewer numbers than the other
microorganisms. However, in certain conditions, such as flooded
soils, their nitrogen fixing ability is very important ecologically
and agriculturally. Algae are also a prominent part of the desert
ecology. They require the least number of growth factors, and the
blue-green algae even fix both atmospheric nitrogen and carbon
dioxide and carry on photosynthesis. Consequently they are often
the first organisms to appear in a sterile soil.
"The soil animals comprise a very large group of organisms.
Some of the more prominent are the protozoa, earthworms and nema-
todes. The protozoa are generally classified according to their
mode of mobility, and are thought to be most important in their
consumption of bacteria. The earthworms are quite important in
maintaining the soil structure and aeration of certain soils. The
nematodes have been studied mostly because of their plant parasitic
role, but many nematodes are predacious or saprophagous.
"All of these microbial groups are commonly found in soils, but
their numbers vary considerably and are governed by environmental
conditions. Some of the more important environment parameters are
temperature, moisture content, oxygen status, nutrient and energy
sources, and pH.
"Soil temperatures between 25 and 35°C are best for most soil
microbes. However, some organisms (psychrophiles) grow best
between 0 and 20°C, while others (thermophiles) grow best at
45 to 65°C. Low temperatures tend to lengthen the generation
time and slow metabolism, and high temperatures tend to speed
metabolism until temperature becomes an inhibiting factor.
"A certain amount of moisture is required for biological action,
but once soil moisture has increased to the point that it impedes
the diffusion of oxygen in the soil it greatly reduces aerobic
metabolism. When the oxygen supply of soil is low, a considerable
number of microsites are anaerobic, and the populations of
anaerobic microbes increase significantly.
"In order for a microbial population to expand it must have
an energy source-, an electron acceptor, and the essential nutrients
86
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for Us protoplasm. The autotrophs use, oxidation of inorganic
compounds to obtain their energy, wh.il e the heterotrophs
oxidize organic compounds for energy, and the photosynthetic
organisms utilize light energy. Under aerobic conditions
oxygen is the electron acceptor, but under anaerobic conditions,
nitrate, sulfate, or organic compounds are the acceptors. Nitro-
gen and phosphorus are normally the nutrients that limit microbial
growth in soils, but in particular locations other nutrients
can be limiting.
"The physiology of many microorganisms is quite sensitive
to pH, nitrifying bacteria and certain plant pathogenic actino-
mycetes being the most notable. However, many microorganisms
are not greatly affected by a one-unit shift to either side of
their optimum pH. In soils the largest effect of pH is often
the modified availability of nutrients."
The three most important biological influences on migration and
attenuation of trace contaminants (oxidation-reduction, mineralization-
immobilization, and production of organic constituents) are discussed below.
This grouping of mechanisms-effects, while not complete, covers most of the
significant microbial involvement.
Oxidation-Reduction — One of the most critical mechanisms of trace
and heavy metal attenuation is associated with redox effect. Microorganisms
in soil initiate acute reducing conditions where aeration (oxygen movement
into the waste/soil) is poor and an abundant supply of "available" oxidiz-
ing substances, such as garbage, paper, or industrial organic compounds,
is present.
During initial water-saturation of soils, acids and carboxyl radi-
cals accumulate, provided that organic matter is present. The pH values
generally decrease as the hydrogen activity (acids) builds up. The associ-
ated evolution of C0« from degradation of carbonaceous materials also con-
tributes to hydrogen ion activity and/or lowering of pH values. Lowering
soil solution pH values increases the opportunity for trace contaminants to
move through soils. Oxidative processes usually have the opposite effect.
Oxidizing conditions are usually associated with higher pH values than
reducing conditions. Thus trace elements are expected to be less subject to
87
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movement. However there are situations in which, the two are not related.
In some instances h.ighly anaerobic and reducing conditions may be associated
with the latter stages of decay or decomposition when very stable organic
complexes form, and alkali earths and other basic ions accumulate.
The reduction processes in leachates, aqueous waste streams, and
soils are energy-requiring systems. Decomposable cdrbon compounds for this
necessary energy may be either organic or inorganic, but organic matter
must be present for the most active reduction. Orgariic products resulting
from the decomposition of plant and animal material are much more effective
in making heavy metals more mobile under anoxic than oxic conditions. The
influence of reduction on the mobility of trace and heavy metals is well
documented by experimental evidence. For example, the change of Mn , Fe ,
2+ 2+ 2+ +
and Cu to Mn , Fe , and Cu takes place when decomposable organic mater-
ials are added to soils as follows:
Mn02 + 4H+ + 2e ->• Mn2+ + 2H20
Fe2 + 6H* + 2e + 2Fe2+ + 3H20
CuO + 2H = e -" Cu+ + HO
Where drainage is poor in soils associated with disposal lagoons,
ponds, lakes, landfills, waste water spreadings, and industrial aqueous
wastestream discharges redox potentials decrease and trace and heavy metals
become more soluble and mobile (Rao, 1956; Ghanem et al., 1971; Olomu et al.,
1973; Hemstock and Low, 1953; Ponnamperuna, 1955, 1967, 1969; Cheng, 1973).
The reduction of iron in soils is a biologically dominated process.
The reaction is most prominent at low pH and anoxic conditions when readily
available organic matter is present in abundance. Iron also can be used as
an energy source by iron-oxidizing bacteria. Organic matter is not needed
for oxidation to proceed, although some microorganisms use both.
The presence of sul fate-reducing bacteria and sulfate also favors
reduction reaction of iron. The sul fide produced reduces ferric iron to
-------
ferrous sulflde. Sulfides, can cause problems in .management of aqueous
waste disposal because they clog soil pores. If the increased availability
of iron should encounter an aerobic zone, hydroxy iron oxides precipitate.
The suggested use of anaerobic soil zones in a profile for control
of nitrates through the denitrification mechanisms can create such an alter-
nate anaerobic/aerobic layering pattern. The ubiquitous presence in soil of
manganese can influence iron mobility, and like iron, clog pores and prevent
the downward flow of water, Jenne (1968). Iron and manganese precipitate in
a mixed system more than they do singly in pure systems, Collins and Buol
(1970a,b).
Oxidation-reduction reactions of certain inorganic substances are
presented in Table 5.1. Most trace contaminants can be oxidized or reduced
by at least one type of microorganisms, depending on the availability or
lack of oxygen and other substances in the habitat.
Mineralization and Immobilization — When the elements or organic
matter, microbial tissues, and organic complexes are converted into the
inorganic state, the process is called mineralization. Biological immobi-
lization is considered to be the reverse of mineralization. The incorpora-
tion of trace and heavy metals into microbial tissue results in their being
"fixed" with mobility controlled by cell or cell tissues. On the other
hand, for the elements that are relatively immobile in soils as inorganic
complexes, incorporation into cell materials may be thought of as a mechan-
ism for them to migrate as minute particles and cell materials when the
tissues die and decay. Phosphorus movement in organic form is an example,
Hannapel et al. (1964), and probably also arsenic by analogy. See Table
5.1 for some examples of mineralization and immobilization of inorganics.
The mechanisms of immobilization through microbial incorporation
are important in the nitrogen cycle. Indeed, if this were not so, the
dependent animal kingdom would long since have starved. Cyanide, for
example, is oxidized in the soil into ammonium and finally into nitrate.
During the transformation processes, the nitrogen is immobilized, thus
89
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TABLE 5.1. SOME MICROBIAL TRANSFORMATIONS OF INORGANIC SUBSTANCES
(Modified from Microbial Formation and Degradation of Minerals, by Melvin P.
Silverman and Henry L. Ehrlich. Advances in Applied Microbiology, Vol. 6,
p. 153-206, 1964. Copyright 1964 by Academic Press. Specific references
omitted.)
Element Microorganism
Physiological activity9
As
Cd
Cu
Fe
Ni
F. ferrooxidans
Heterotrophic
bacteria
Achromobacter
PseudomonaT
Xanthomonas
M. lactilyticus
Desulfovibrio
T. ferrooxidans
F. ferrooxidans
Desulfovibrlo.
C. nlgrlflcans
M. lactilyticus
T. ferrooxidans
Ferrobadllus spp.
GalHonella
Leptothrlx ochraceal
SpnaerotHus
Protozoa, algae
M. lactilytlcus
B. clrculans
B. polymyxa '
Desulfovibrio.
C. n1grif1cans
T. ferrooxidans
Desulfovibrlo
As2S3 oxidized to AsO^"; AsO]j"; SOj (?)'
oxidized to
" reduced to
CdC03 + SOJj" + 8H+ + 8e~ = CdS + 4H20 + CO2"
Cu2S + 4H20 = 2Cu
CuS ^ 4H20 = C
2"*"
lOe
6H"
8e
2+ 2-
Cu and SOI reduced to CuS; Cu1QSg;
Cu(OH)2 + H e" » CuOH + H20
Fe
2-t-
Adsorption, precipitation
r - ^ - c
Fe •»• e • Fe
2+
Fe3* + SO2"
N1S + 4H20 •
N1CO- + SO?'
<3 H
N1(OH)2 •»• S(
8H
9e" = FeS + 4H20
2"
10H
SO
8e" - N1S
' + 8e"
8e"
NiS + 6H20
,2-
90
-------
Table 5.1 (continued)
Element Microorganism
Physiological activity"
Thiobacteriaceae
Thiorhodaceae
Chlorobacterlaceae
Begglatoaceae
S. natans
Achromatlum
Leucothrlxc
Bacteria,
actlnomycetes,
fungi
All microorganisms
H2S
= S° + 2H + 2e
+ H20 • S0*~ +
8e"
Polysulfldes reduced to thlosulfate and
sulflde
S° + 2e" + 2H+ = H.S
Se
V
Zn
M. selenlcus
M. lactllytlcus
C. pasteurlanum ^
D. desulfuricans
Neurospora
C. albicans
Baker's yeast j
M. lactllytlcus
D. desulfuricans
C. pasteurlanum
T. ferrooxidans
Desulfov1br1o
H2Se + 4H20 = SeoJ" + 10H* + 8e"
Se° + 2e" + H* = SeH"
, HSeO! + 4e" + 5H* = Se° * 3H90
«3 b
1 H2Vo4 + 2e" + 2H* = VO(OH) + HgO
ZnS ^ 4H20 - Zn2"1" + SO^" + B\f + Be'
4|2ZnC03-3Zn(OH)2| + SO^" + 9ft H* + 8e"
» 5^ H,0 + 1 C0?"+ ZnS
52 8 3
Metal HeterotropMc
chelate microorganisms
Oxidation of chelatlng agent with precipi
tation of metal moiety
30x1dat1ve or reductive half-reactions listed most nearly describe the
particular mlcroblal activity cited.
br
Proof of sulfate production lacking.
cBut see Harold and Stanler (1955).
91
-------
preventing i.t from being lost to plants by leaching. The carbon is oxidized
to harmless CC^. As it again becomes .mineralized slowly, plants have an
opportunity to take up the nitrogen and combine it into their protein pool.
Upon death of the cell tissues which contain protein (animal, plant, or
microorganism), the N returns to the soil-organic matter pool where it
slowly mineralizes into a form usable by plants again, is lost to subsurface
depths by leaching, or is denitrified to N2* gas and escapes harmlessly into
the atmosphere.(Figures 5.1 and 5.2).
The examples of other element cycles which illustrate biological
influence on mobility of elements are carbon (Figure 5.3), phosphorus (Figure
5.4),and sulfur (Figure 5.5). Such cycles could be drawn for the 12 poten-
tially hazardous pollutants with existing data and a little educated specu-
lation.
Since those trace and heavy metals which concern us most enter in
organic combination as living cells and tissues of microorganisms of soil,
plants, and animals, the biological mechanisms of mineralization and immobi-
lization act significantly in attenuation of the potentially hazardous
pollutants.
Reactions with Organic Constituents — Organic complexes in soils
which accumulate as a result of microbial synthesis as well as degradation
of organic residues have a relatively high capacity to combine strongly with
trace and heavy elements. Hodgson (1963), for example, delineates three
organic fractions which can be identified in relation to immobility of metals:
1. Lignin-like compounds of high molecular weight and similar
organics, mostly cyclic in nature.
2. Organic acids and bases, short chained in nature (short-
chained humic acids included).
3. Soluble constituents which become insoluble when combining
with heavy metals
92
-------
POOL OF NITROGEN (
I GASSES IN ATMOSPHERE
GASEOUS LOSS OF
NITROGEN FROM SOIL
PRECIPITATION OR
FERTILIZATION
oo
PLANT UPTAKE
CROP
RESIDUES
ANIMAL
MANURE
NITRATJ
DENITRIFICATION I NITRIFYING BACTERIA
NITRITE
I
NITRIFYING BACTERIA
I
'AMMONIUM
HELD BY SOIL PARTICLES
LEACHING LOSS OF NITRATE (N03)
NITROGEN TO GROUND WATER OR
NEARBY LAKES AND STREAMS
ORGANIC MATTER
(CONTAINING NITROGEN)
MICROBIAL DECOMPOSITION
ORGANIC NITROGEN
MICROBIAL DECOMPOSITION
Figure 5.1. General nitrogen cycle illustrating nitrogen transformations in the soil
(From Maine Guidelines)
-------
NITROGEN CYCLE IN NATURE
PLANT PROTEIN
10
Fiqure 5.2 The nitrogen cycle in nature. (From K.U. Thiamann, 1963.
The Life of Bacteria, Ed. 2. Macmillan, New York.)
-------
PLANT
SIMPLE
PLANT
CONSTITUENTS
REUSE
RESISTANT
PLANT
COMPONENTS
(LIGNIN)
SLOW
DECAY
MICROBIAL
TISSUE
vo
WATER
CARBON DIOXIDE
CHLOR1DE
RAPID
DECAY
AMMONIUM
^/NITRITE
PHOSPHATE
POTASSIUM
NITRATE
MANGANESE
STORAGE
^ SOIL
IMMOBILIZATION
SODIUM
CALCIUM
MAGNESIUM
COPPER
ZINC
IRON
MOLYBDENUM
AND
OTHER
ORGANIC
MATTER
(PREHUMUS)
MICROBIAL
TISSUE
THE ORGANIC MATTER CYCLE IN SOIL
Figure 5.3.
The carbon cycle and organic matter cycle showing decay release of
mineral nutrients and humus formation. (From W.H. Purler. Soils
of The Desert Southwest, Univ. of Ariz. Press, Tucson, AZ. 1975.)
-------
PLANTS AND
MICROORGANISMS
(Foca, Fiber, Sro'.-er)
IGNEOUS ROCK
ANIMALS
AND MAN
FERTILIZERS
SOIL
PHOSPHORUS
FISH AND •<
-PLANKTON^?
OCEANS
AND
SEAS
SEWAGE
AND
REFUSE
Figure 5.4. The phosphorus cycle in nature showing its universal distribution.
(From W.H. Fuller, 1972. Phosphorus: Element and Geochemistry.
lr± The Encyclopedia of Geochemistry and Environmental Sciences.
Van Norstrand Reinhold Co. New York.
-------
SULFUR CONTAINING
ROCK
PLANTS AND
MICROORGANISMS
MANUFACTURING
AND FERTILIZERS
ANIMALS
ANDAMAN
AIR
PLANKTON^ _ ^^ REFUSE>
-SEWAGE
AND
IRRIGATION
-WATERS-
Figure 5.5. The sulfur cycle in nature showing oxidation of sulfur to sulfuric acid
and sulfate formation and reduction to sulfides. (From W.H. Fuller,
Soils of the Desert Southwest. Univ. Ariz. Press, Tucson, 1975.)
-------
Organic substances, thus, are an Important factor in attenuation and
immobilization of potentially hazardous jDetal pollutants. Also see Fuller
and L'Annunziata (1968), L'Annunziata and Fuller (1968), and Hannapel et al.
(1964).
The formation of complexes between soil organic matter and metals
can take place through several mechanisms, according to Mortensen (1963).
These are: ion-exchange, surface adsorption, chelation, and complex coagu-
lation and peptization reactions. He suggests that the chelation ligands
(organic reactive groups or radicals which "tie up" or bind elements chemi-
cally in the polymeric components of soil organic matter include carboxyl,
hydroxyl, and ami no groups to list a few. Specific ligands which may be
present not only in soil organic matter but leachates from landfills and
waste from other municipal and certain industrial operations (canning, wood
product manufacturing, tanning, slaughtering) include microbial slimes, gums,
cell debris and humus; polymers of lignin, polysaccharides, tannins, poly-
phenols, proteins, quinones; and low molecular weight substances, aliphatic
acids, ami no acids, organic phosphates, phenolics and volatile acid complexes.
"Wright and Schnitzer (1963) suggest that in podzolic soils fulvic
acid is an important ligand which brings about the translocation or movement
of Fe and Al. The aromatic "nucleus" of fulvic acid (with its attached
hydroxyl, carboxyl, and carbonyl groups) makes it a likely candidate for
critically influencing the mobility and attenuation of metals (Schnitzer,
1969). There is reason to believe that fulvic acid forms through microbial
synthesis during the degradation process in sanitary landfills as well as in
soils since fulvic acid forms readily from plant residues and landfill
materials generally are dominated by substances of plant origin (Schnitzer,
1969). Fulvic acid then may easily find its way into leachates from these
systems. There is little doubt that chelation reactions with the organic
matter of disposal systems exert significant influence on the movement and
attenuation of metals. Microbiological activities which are responsible for
the production of the organic chelating substances are thus an important
factor in attenuation of the hazardous trace contaminants in soils and
geologic materials.
98
-------
Organic complexes In municipal landfill leachates probably Immobilize
many of the trace metal contaminants by organic precipitation In the presence
of air (oxygen), some of the organic precipitates are Irreversible unless
the pH drops to 3.0 or below. The organic constituents in wastestreams and
leachates of low pH values (3 or below) may be expected to immobilize trace
contaminants poorly. The hydrogen ion concentration tends to keep heavy
metals and trace elements in solution. A host of unidentified and identifi-
able biological products present both in soils and aqueous organic "waste-
streams" is capable of immobilizing metals. Flaig (1966) and Allison (1973)
discuss some of these compounds in their review concerning the retention of
metals by organic constituents in soil. Suggested chemical reactions
between some of the better known organic complexes and the selected poten-
tially hazardous waste constituents will be discussed further in the section
on chemical reactions.
Chemical Mechanisms—
This section reports some of the most obvious chemical mechanisms
which relate to attenuation of the trace contaminants:
1. Complexing with organic substances (cation exchange, organic
anion fixation, nonpolar organic reactions, and nonionic polar
reactions).
2. Adsorption by mineral species (not ion exchange).
3. Chemical oxidation-reduction effects.
4. Precipitation reactions and pH effects.
5. Ion exchange reactions.
Complexing with Organic Substances— Organic-metal complexes can be
thought of as a reservoir of potentially hazardous pollutants which mineral-
ize at various rates, depending on the susceptibility of the organic mole-
cules to biological attack, and release, the elements for migration if the
environment is favorable. Since the soil, effluent, sludge, wastestream,
or leachate contains a wide variety of organic compounds and complexes, the
net attenuation or retention effect of this organic fraction is difficult
to evaluate.
99
-------
As previously discussed, aqueous wastestrearas and solid waste
landfill leachates contain a wide variety of organic substances capable of
chemically immobilizing trace contaminants. These organic complexes also
may be further degraded biologically and release contaminants (mineraliza-
tion) at varying rates, depending on the biodegradability of the material.
Immobilization and mobilization, thus, take place at the same time. Gener-
ally, immobilization equals or exceeds mobilization for heavy metals. Some
types of the organic compounds, produced or altered by biological action,
may immobilize trace contaminants by direct chemical bond complexing pro-
cesses in the soil. Organic cations may attach themselves to cation
exchange positions of soil clays as well as to organic matter exchange
sites. Soil organic constituents, for example, possess a high cation
exchange capacity, even much greater than that of montmorillonite. The
large cations attach themselves to the exchange positions with varying
degrees of tenacity (Stevenson and Ardakani, 1972). Some contaminants
become a fixed part of the exchange material. In this manner they may reduce
the total exchange capacity of the soil.
2+ 2+ 3+
Organic anions react with such elements as Ca , Fe , Al , etc.,
and form slowly soluble to nearly insoluble compounds. At low pH values
basic bonds unite with H on soil-particle surfaces.
Another interesting phenomenon or mechanism of attenuation of non-
polar organic compounds (such as oils, grease, and hydrocarbons of low
molecular weight which are insoluble in water) is film formation on the
surface of soil particles. The author's personal experiences with fuel oil
spills have been very convincing as to the great tenacity with which soils
absorb and hold certain nonpolar hydrocarbons against migration through the
soil. Fine-textured soils, particularly clays, because of their relatively
vast surface area, have great capacity to remove and attenuate certain
types of organic molecules. The effectiveness probably increases with the
size and charge of the molecule.
Nonionic polar compounds in aqueous wastestreams and leachates can
be retained to some degree depending on (a) their susceptibility to microbial
100
-------
degradation and (b.) their specific molecular characteristics.
The detailed interest in attenuation by chemical reaction with
organic substances results from the knowledge that (a) wastestreams often
contain substantial levels of organic molecules, (b) the soil at the dis-
posal site often contains organic matter, and (c) organic compounds complex
readily with trace and heavy metals.
Adsorption by Mineral Surfaces—The adsorption of trace and heavy
metals in soils leading to immobilization may be of various types. Indeed,
specific adsorption forces may often dominate the behavior of heavy metals
when they make contact with soil (Tiffin, Lagerwerff, and Taylor, 1973).
Adsorption may be variously defined but, in general, it is the adhesion, in
an extremely thin layer of gas molecules, dissolved substances or liquids, to
the surface of solids with which they are in contact, Ellis and Knezek (1972):
Ion exchange is not considered here in adsorption reactions, except when the
soil has a capacity to retain dissolved chemicals so tightly that they can be
removed from the solid fraction only with difficulty (Murrmann and Koutz,
1972). There is some evidence (De Mumbrum and Jackson, 1956; Wilkinson et al.
1968; Hodgson, 1960) that a certain fraction of the exchange capacity is
specific for certain ions (Cu, Zn, and Co). The fraction that becomes immob-
ile is held by variable bonding strengths and a slow rate of reaction approach-
ing first-order kinetics.
Adsorption, as discussed here, differs from precipitation, which
forms well defined solid phases. The adsorption mechanisms are poorly defined
and often the term is used loosely with no real meaning when no reasonable
explanation can be made for interactions of chemicals with soils. Adsorption
has been used at times to include the soil cation-exchange-capacity of both
loosely and firmly held ions. In this report, adsorption will be considered
as the adhesion of the elements in an aqueous medium, to the surface of solid
materials, including layered silicates, hydroxy oxides, organic matter, and
lime. Adsorption probably is the most important process influencing attenua-
tion of the trace and heavy metals in soils. Evidence for this comes from a
number of sources. The high anion retention by soils cannot be explained by
101
-------
an ion exchange or by precipitation, Jenne 0968). Phosphate is an example.
Phosphorus is thought to react with, surface iron and aluminum exposed by
their layer silicates and with, iron and aluminum hydroxide and hydrous
oxides coating soil particles.
Another possibility is the incorporation of certain elements as
impurities in solid phase accumulations and crystal structures which resist
resolution. Salt formation of lime and gypsum, iron and aluminum hydroxy
oxides, and other precipitates may well assume some solid phase formations
that occlude trace and heavy metals as impurities.
The possibility of forming slowly soluble impurities with organic
constituents should not be overlooked, although this probably should be con-
sidered as a distant "cousin" to the mechanism of adsorption.
The most interesting adsorption mechanisms in soils are those associ-
ated with iron and aluminum hydroxides and hydroxy oxides. Manganese proba-
bly should be included along with Fe and Al but there is much less evidence
for such Mn reactions than for Fe and Al. A very small amount of hydrous
oxides can create a great dimension of reactive surface area. Hydroxy oxides
form into particles of fine (colloidal), coarse, and/or porous aggregates
and increase reactive iron and aluminum adsorption surfaces, (Jenne, 1968;
Jackson, 1963; and Marion et al., 1976).
Another argument favoring the dominance of adsorption reactions over
the conventional ion exchange reactions is that the concentration of heavy
metal cations is so low, compared with that of other ions (H , Ca , Na , K ),
that mass action prevents cation exchange being the most effective means of
removal of trace and heavy metals from aqueous waste solutions.
Adsorption processes take place rapidly. Rather than stopping at
this initial reaction phase, though, the equilibrium continues to shift
slowly toward the more insoluble forms. According to Murrmann and Koutz
(1972), a general equation may be written as:
102
-------
Soluble - -x * adsorbed - x ->• Insoluble - jc
where x represents the component adsorbed from the solution.
Chemical Oxidation-Reduction Effects—Chemical and biological oxida-
tion-reduction reactions, as they relate to retention of the selected hazard-
ous elements, are not often possible to separate in soils. Most oxidation-
reduction reactions in soils are initiated by biological activity. The inor-
ganic ions which are released, however, may be free to take part in a multi-
tude of strictly chemical reactions.
Oxidation/reduction reactions would be important in a management
program only if an anoxic system, such as municipal landfill leachate, could
be oxidized (e.g., chemical treatment on the surface, land spreading, spray
irrigation) to reduce the solubility of metal complexes and compounds.
Reduced forms generally are more soluble than oxidized forms of heavy metals.
A logical research "follow-up" is to try various types of field aeration
systems even to placing perforated plastic tubing below fills through which
air can be pumped into leachate.
Precipitation Reactions and pH Effects—The precipitation reactions
of trace and heavy metals in soils relate so closely to pH levels that dis-
cussion of the two cannot be separated. It is difficult to distinguish
between precipitation and adsorption reactions in soils. (Precipitation here
is used in the strict chemical meaning.) Numerous literature references can
be cited to the effect that trace and heavy metals, in general, form insolu-
ble or very slowly soluble precipitates at neutral or higher pH values.
Since good literature reviews already exist for this type of attenuation
mechanism, Jenne (1968), Hahne and Kroontje (1973), Lagerwerff (1972), Kee
and Bloomfield (1962), Lindsay L972a) and Krauskopf (1972), none will be
given here. Liming results in the formation of insoluble heavy metal com-
pounds. More specifically, Jenne (1968-) states that, "it seems apparent
that the effect ascribed to sorption of zinc and copper by calcium carbonate
is primarily a pH effect and, secondly, an effect of carbonate-bicarbonate
ions on heavy metal solubility."
103
-------
Jenne (1968) proposes that the hydrous oxides of Mn and Fe, furnish
the principal control for the attenuation of Co, Ni, Cu, and Zn and, in gen-
eral, other heavy metals in soil and fresh.water sediments, and are, in his
words, "a sink" for heavy metals. His contention is supported partly by the
ubiquitous occurrence of the hydrous oxides in clays, soils, and sediments
both as coatings on mineral particles and as discrete oxide particles and
specific examples of hydrous oxide reactions. He suggests that sorption
and desorption of heavy metals take place in response to: "(a) solution
concentration of the metal in question; (b) concentration of competing metals;
(c) hydrogen ion concentration; and (d) formation and destruction of organic
chelates and inorganic complexes." He claims only a very small amount of
hydrous oxides of Mn and/or Fe may be sufficient to control the equilibrium
between the heavy metals in solid and aqueous phases. Prediction of this
equilibrium is complicated by the dissolution-precipitation of Mn and Fe
oxides in relation to changes in pH-Eh and to leaching of Fe and Mn which
may be occurring at the same time as adsorption reactions. The results fur-
nished by Tiller, Hodgson, and Peech (1963) provide additional evidence that
fixation of heavy metals is not exclusively related to cation exchange
capacity. They found that by plotting cobalt sorption (from 1.1 x 10~ M_
CoCl2) vs. pH, 15 of the 16 soils are nearly identical in shape to those of
manganese sorption by 6 - MnO« of Morgan and Stumm (1964). Jenne (1968)
believes there is sufficient evidence in the literature indicating that metal
ion-clay mineral surface complex formation is not important, at least for Cu
and Zn.
Another important factor in precipitation of trace and heavy metals
which often is overlooked is concentration. Adsorption reactions seem to dom-
inate with micro-concentrations but precipitation frequently occurs with macro-
concentrations. Bingham, Page and Sims' research (1964) indicates that the
dominant reason for retention of copper cUoride and acetate by montmorillonite
at pH values above 5.0 and 6.5, respectively, is precipitation at Cu concen-
trations twice the cation exchange capacity. The same Zn salts precipitated
at one pH unit higher than the Cu salts.
Eh refers to redox potential as an intensity factor.
104
-------
St1.ll another interesting phenomenon of the hydrous oxide surfaces
is that of catalyzing oxidation reactions Cand thereby precipitation).
Oxides of iron appear to catalyze the oxidation of Fe"1"1" to Fe"H"*', Gasser and
Bloomfield (1955). The same seems to be true for Mn"1"1" to Mn+"HH", Zapffe
(1931).
The pH effect on heavy metal fixation and mobility, according to
Jenne (1968) can be "interpreted in terms of competitive exchange of hydrogen
with heavy metals occluded by the hydrous oxides as well as dissolution-
precipitation and oxidation of the hydrous manganese and iron oxides."
Some of the many investigations concerning pH effect are:
1. Soil samples sorbed less metal in more acid systems (Banerjee,
Bray, and Melsted, 1953; Barrels and Christ, 1965; Gayer and
Woontmer, 1956).
2. Some of the sorbed nonexchangeable heavy metal is acid extract-
able (Banerjee, Bray and Melsted, 1953; Crooke, 1956; Hodgson,
1963; Hodgson and Tiller, 1962; Nelson and Melsted, 1955; and
Tiller and Hodgson, 1962).
3. Lowering the pH of the extractant removes more of the sorbed
metal (Banerjee et al., 1953; Brown, 1950; Dion, Mann, and
Heintze, 1947; Hibbard, 1940; Jones, Gall, and Barnette, 1936;
Nelson and Melsted, 1955; Staker and Cummings, 1941).
4. More manganese is extracted from acidic soil (Walker and Barber,
1960; Williams and Moore, 1952).
Conversely, liming to raise the pH, decreases the solubility of many
heavy metals and their availability to plants. Some references often cited
are: Askew and Dixon (1937); Askew and Maunsell (1937); Camp and Reuther
(1937); Carroll (1958); Christensen andToth, and Bear (1950); Ekman,
Karlsson, Svanberg (1952); Fox and Plucknett (1954); Gall and Barnette (1940);
Hill, Toth and Bear (1953); Jones, Gall and Barnette (1936); Lott (1938)
Mann (1930); Mitchell (1951); Page (1964); Painter, Toth, and Bear (1953);
Percival, Josselyn, and Beeson (1955); Reith and Mitchell (1964); Prince and
105
-------
Toth. 0938); Rogers and ULI (19481; Seatz 0960); Waltz, Toth, and Bear 0953);
Bear (1956).
In summary, the pH effect on the trace and heavy metals must result
from the combined effect of the activity of hydrogen on:
1. The direct precipitation of the metal as the oxide or hydroxide,
or carbonate (PbC03), for example.
2. The concentration of carbonate, phosphate and possibly silicate
ions in the aqueous phases of the system.
3. The precipitation-dissolution of certain oxides, notably Fe and
Mn.
4. The rate of sorption and desorption of heavy metals by the
hydrous oxides.
Items 1,3, and 4 may be very difficult to distinguish experimentally.
The pH range for maximum solubility of trace and heavy metals varies
with the origin of the soil. The optimum for availability of trace metals
to plant's is given as 5.5-5.8 for wood-sedge soils and 5.0 for sphagnum peats
(Lucas and Davis, 1961). This is 1.0 to 1.5 pH units lower than for most
mineral soils. Interactions of heavy and trace metals with phosphorus in
various forms (silicates and carbonates), in addition to hydrous oxides of
Fe and Mn, complicate the straightforward effects of pH on their precipita-
tion and mobility in soils. Also, not to be overlooked is Jackson's (1963)
2+
study, "Aluminum bonding: A unifying principle in soil science." The Al
ion bonds through oxygen to form a number of functional groups which influ-
ence trace contaminant reactions in soils. The layer silicate clays, for
example, provide cation exchange sites. At pH values below about 5.5, Al
is solubilized in soil solution (Lindsay et al., 1959). A number of pH-
controllc^ aluminum species appear in soils which are highly reactive
(Marion et al., 1976).
Ion Exchange Reactions—Ion exchange reactions occupy the bulk of
attention in the soil chemistry literature. The soil chemist and plant
106
-------
nutritionist "team up" to explain plant uptake., absorption, and nutrient
aval 1 ab.i 1 i ty through-the common phenomenon of i.on exchange. Laboratories
throughout the world routinely evaluate soils on the basis of CEC, which is
related primarily to the clay and organic matter of soils. CEC values,
though empirical and originating from a multitude of soil properties (some
yet to be evaluated), provide crop production people as well as plant nutri-
tionists with valuable information. The effect of ion exchange reactions on
trace and heavy metal mobility in soils is another matter. Ion exchange
appears to function, in only a small way, as a temporary or transitory mech-
anism of attenuation. Ion exchange is a phenomenon relating to positively
charged ions (cations) much more than to negatively charged ions (anions).
Soil clay minerals, the primary seat of cation exchange, have small capacity
for anion exchange.
The relationship of ion exchange reactions and retention of elements
in soil is similar to that of a football player who receives the kickoff. As
the ball carrier proceeds up the field, blockers and tacklers slow his prog-
ress but many never stop him completely. If the carrier is stopped, it is
temporary. He has a number of additional chances to proceed to the goal.
Ultimately the goal is reached, the tackier and blocker only relate to time
delay, just as an ion will be delayed on its progress through soil by tempo-
rary contact with the exchange positions. Some ions move more rapidly than
others. They are the better ball carriers. They are the more loosely held
metal ions. The ion exchange complex plays an important part in time delay
but usually does not inhibit the ion from arriving at some ultimate goal of
depth in the soil.
Ion exchange in the mineral fraction of soil is believed to origi-
nate primarily from the exchange sites of layered silicate clays. Substitu-
tion of divalent for trivalent and trivalent for quadravalent cations within
the crystal structure of the mineral can take place to a limited extent. Most
layered aluminum silicate minerals (secondary) in soils possess a permanent
negative charge within the crystal lattice structure. This internal negative
charge presumably is balanced on the surface by positively charged, exchange-
able cations.
107
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There are four primary factors affecting the Ion exchange capacity
of soils: (a) kind of clay mineral present, (b) the quantity of clay min-
eral, (c) the amount of organic matter present, (d) the pH of the soil.
There is a great amount of information in the literature on the CEC
of different clay minerals. The montmorillonite expanding lattice type has
CEC values ranging from 60 to 120 meq/100 g of the exchanger phase, while
the kaolinite nonexpanding lattice type has between 5-15 meq/100 g. The CEC
of illite type clays is between those for montmorillonite and kaolinite.
The second factor, quantity, is reflected by soil texture. CEC
usually increases with an increase in the clay content of soil. Silt also
has a CEC, but the values are lower than those for clay. Sands exhibit
almost no exchange capacity in relative comparison. Therefore, in general,
soils have greater CEC values the finer the textural class. For example,
CEC values of sandy soils may range from 2 to 6 meq/100 g; sandy loams 4 to
15; silt loams 10 to 25, and clay and clay loams 20 to 60.
The third factor, organic matter, is very prominent in enhancing the
total CEC of a soil. Landfills, however, are located below the soil organic
layer, though the leachate from the fill itself contains organic constitu-
ents which exhibit cation exchange capacity.
The fourth factor, pH, was discussed earlier but not in connection
with CEC. The CEC, in general, increases with the soil pH. Over wide
ranges, pH changes the CEC greatly. Over the range of 5 to 7, however, the
increase probably does not exceed 30% of the original value, Murrmann and
Koutz (1972).
In defense of the position that ion exchange in the soil is an impor-
tant mechanism for the attenuation of trace and heavy metals, it is only
fair to reproduce Murrmann and Koutz1 (1972) discussion:
"The type of exchange capacity described above (.i.e., clay
minerals having layer aluminum silicate) is more or less permanent.
108
-------
However, the ... material fraction of soils possesses another
element of exchange capacity which, depends upon the acidity or
pH of the soil. This type of Ion exchange is reflected in the
retention of both cations and anions. The nature of pH-dependent
exchange capacity of the mineral phase is not well understood
but is thought to result primarily as a consequence of the dissoci-
ation or association of hydrogen ions (H+) with structural ele-
ments at the crystal edges of aluminum silicate minerals, with
the more amorphous aluminum silicate phases, and to some degree
with iron and aluminum hydroxide materials. At high pH values of
8 or more, the contribution to the total cation exchange capacity
can be considerable. At pH values below 4 or 5 the retention
of anions such as sulfates (S0^~) can be significant. However,
in the pH range from 5 to 7 characteristic of many soils the reten-
tion of anions by this mechanism is small to insignificant. The
retention of cations would normally not account for more than 10
to 20% of the total cation exchange capacity attributed to the
mineral phase. However, this type of exchange capacity does
account for the tendency of soils to retain anions with decreas-
ing soil pH and, in part, for the increase in cation exchange
capacity with increasing pH."
They go on to say, "Soils with a high cation exchange capacity gen-
erally also have the ability to remove heavy metal cations from solution
due to adsorption by the soil inorganic and organic components. However,
exchange capacity should not be equated with the capacity of a soil to
remove chemicals from wastewater. Since the exchange capacity of a soil is
already saturated with common cations such as Ca and Na , retention of
wastewater chemicals will be accompanied by the release of these cations
into solution. Wastewater also normally contains large amounts of Na and
±t
Ca relative to the amounts of heavy metal-cations. Thus, the net effect
will be some readjustment in the composition of the exchanger and solution
phases but the total soluble salt concentration in the wastewater will
remain fairly constant. Removal of heavy metal cations by cation exchange
will be small due to the competing effects of the common cations present at
109
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much higher levels."
From a practical standpoint, cation exchange does not effectively
lower the total salt concentration in aqueous waste solutions; heavy metal
cations are not significantly retained by cation exchange under soil con-
ditions where concentrations of soluble salts in solution are much higher
than concentrations of the heavy metals.
For practical purposes, the fact that fine-textured soils, in gen-
eral, immobilize trace and heavy metals to a greater extent than coarse-
textured soils may provide useful guidelines for disposal of trace and heavy
metals, even if our knowledge of the exact proportioning of the mechanisms
involved is incomplete.
Physical Mechanisms—
It is not always possible to make a clear-cut separation between phy*
sical and chemical mechanisms of element retention in soil. A few purely
physical factors have roles which can be separated and discussed.
This section lists some of the physical mechanisms/ influences on
trace contaminant movement in soils and discusses their interaction with
biological and chemical mechanisms. The topics covered are:
1. Particle size distribution
2. Pore size distribution
3. Moisture relationships
4. Adsorption
5. Temperature
Particle Size Distribution—Soil particle-size distribution is often
referred to in terms of texture classes. In general, the finer the particles
(texture), the less trace contaminants can move through soils. Suspended
solids in aqueous waste streams and solid waste leachates remain in the soil
near the first point of contact as a result of filter-prig action. The dis-
solved solids move wttfi. the waste and percolate through the soil.
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The constituents which are not attenuated or iranobilized may finally reach
the capillary fringes of underground water tables. Soil texture exerts a
profound effect on this process, since it is an indicator of such parameters
as pore (space) size and distribution, particle size, exposed surface area,
clay content, organic matter, and moisture conditions.
Soil solutions (and leachates, such as those from solid waste) flow
more slowly through finer soils. The intensity of attenuation, though, is
not dependent only on flow rate. Two soils of nearly identical texture
may not have the same attenuation characteristics since attenuation also is
influenced by factors other than flux and particle size and distribution.
The kind of clay mineral, presence of lime, organic matter, hydrous oxides,
and other factors influence mobility and can cause differences as much as
1- or 2-fold between like-textured soils. Proportionally wider differences
occur more commonly between like-textured coarse soils.
Whereas texture is thought of as being unique in any given soil, the
arrangement of soil particles (structure) varies considerably. This mani-
fests itself in the total pore space and the pore size distribution. (For
a general discussion see Baver et al., 1972, and Hillel, 1971). While the
measurement of total pore space is relatively straightforward, that of pore
size usualy involves and idealization as equivalent capillary radii, Baver
et al. (1972), Vomocil (1965), Klock et al. (1969). However, if we assign
a single number to total pore space, a distribution is not so simply charac-
terized (Gary and Hayden, 1973; Sridharan et al., 1971). This is especially
important since pores of different sizes contribute unequally to the flow of
soil solution and the reaction of solute. Frequently it has been useful to
consider the particle size as a measure of pore size distribution
(Scheidegger, I960, who also mentions the relation to surface area). This
approach has been used not only to study Indirectly the pore size distribu-
tion (Graton and Fraser, 1935; Bodraan and Constantin, 1965), but also the
movement of solute (Underbill, 19731.
Ill
-------
The net movement of s.olute is the result of interaction with pores
of all sizes. While we cannot simply add up these effects, it 1s useful to
look at the movement of solute in pores having a uniform radius. The movement
of unreacted solute occurs by convection (bulk transfer fn solution) or
diffusion. The movement of liquids through capillaries can be described by
the analysis of Hagen and Poiseuille (see Bird et al., 1960), from which we
conclude that the larger pores are primarily responsible for the movement of
solution. We should also note that since the ratio of surface area to volume
is smallest for the largest pores, the potential for attenuation is least
and for migration greatest in the larger pores.
The diffusion of solute 1n pores is complicated by the fact that
this is usually superimposed on the convective transfer. Taylor (1953) has
presented an anlysis which implies that the diffusion coefficient is larger
in the larger pores. Following a different line of reasoning Beck and Schultz
(1970) and Saxena et a1.(1974), demonstrated that a reduction of diffusion
coefficients occurs with a decrease in pore size. Thus the effect of dif-
fusion is to accentuate that of convective transfer.
When we start to talk about a more realistic model for soils, we
find that the capillary model is inadequate. In general, we find that the
migration of nonreactive solutes is greater than we would otherwise expect.
This phenomenon is labeled 'dispersion1 (Greenkorn and Kessler, 1970;
Nunge and Gill, 1970; Ogata, 1970; Fried and Combarnous, 1971). While
dispersion includes the effects of diffusion, we emphasize the effects
due to the structure of the soil. Its effects can be categorized roughly
as (a) those due to a random matrix of soil particles and hence, soil pore
spaces, (b) those effects due to aggregates and other large structural
units, and (c) those resulting from the instability of a displacing front.
The presence of a random structure in soils results in the presence
of 'dead-end' pores (Turner, 1958) and tortuous flow paths (Jong, 1958;
Bruch and Street, 1967; and Ogata, 1970). This may be advantageous 1f it
enlarges the volume of soil exposed to the solute, or deleterious if it
112
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results in premature transfer to an aquifer or stream. The work of
Harleman et al. (1963) indicates that, in general, the dispersivity will
be greater for coarser-textured soils. However, this may not significantly
affect attenuation by such soils.
The presence of aggregates or extensive cracks and ped formation1
may result in the primary movement of soil solution between these struc-
tures rather than through them. In general, the result is a decrease in
the efficiency of mixing and an enhancement of migration, Passioura (1971),
Passioura and Rose (1971). Finally, dispersion may be caused by the insta-
bility of a displacing front. This may occur if significant viscosity or
density differences occur between the displaced and displacing solutions,
Nunge and Gill (1970). An analysis of its occurrence is given by Heller
(1966). It results in the movement nonuniformly through the soil of the
invading solution.
If we look at a real soil with a continuum of pore sizes, we might
conclude that a soil of uniform small pores could optimize the attenua-
tion of trace contaminants: uniform, to provide efficient mixing; and
small, to maximize the surface area with which the solution comes in con-
tact. However, in asking for small pores we minimize the rate a solution
can be passed through the soil. In addition, the presence of unsaturated
conditions may offset the effect of large pores.
The soil scientist has long recognized the great variability of
pore size distribution in natural soils. This justifies the recommenda-
tion for mechanical treatment of the soil in preparing the land for dis-
posal of solid waste leachates and liquid wastestreams. This requirement
is not unusual nor impractical since soil manipulation is already a prac-
tice recognized in lagoon, artificial lake, and the more sophisticated
sanitary-landfill operation. Use of the 'cell' system to surround
(enshroud) solid waste on a daily basis is a required practice in well
managed disposal sites. The soil encasing the waste thus is disturbed and
Ped~A unit of soil structure such as an aggregate, crumb, prism, block,
or granule, formed by natural processes.
113
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ends up 1n a more homogeneous mixture than the natural soil with respect
to texture, structure, and pore size distribution. The "soil column"
studies in the laboratory thus more closely resemble what should be expec-
ted in "prepared" disposal sites than that of the "natural"soil. Moreover,
in the disposal of aqueous effluents from municipal sewage plants onto
agricultural land, golf courses, etc., a certain layering or depth of dis-
turbed soil will be encountered which is more homogeneous in physical char-
acteristics than the original soil as a result of tilling, moving, spread-
Ing and other physical management.
Moisture Relationships—It has indirectly been implied from previous
sections that water plays an important role in the attenuation and migra-
tion of trace contaminants. Here, the influence of the water status of the
soil beneath a disposal site is emphasized. Unfortunately, in general, the
soil moisture regime of any geographical area is not well defined. Smith
(1973) has outlined the classification scheme used by the Soil Curvey Staff.
One alternative to this approach is to use climatic data as described by
Scrivner et al. (1973) to estimate the moisture distribution. More relia-
ble, however, would be the actual monitoring of the site. Without some
estimate of how much, when, and where water moves through the soil, it will
be difficult, 1f not impossible, to determine how solutes will migrate or
attenuate.
The primary importance of moisture movement in the soil is simply as
a medium of convective transfer of the trace contaminant. The net movement
of solute depends on the relative magnitudes of the moisture flux and the
rate of reaction. Only limited attempts have been made to model such sys-
tems for time-dependent and unsaturated use (Bresler and Hanks, 1969;
WarHck et al., 1970; and Bresler, 1973-1974).
The secondary effects which unsaturated moisture conditions may have
are varied. Biggar and Nielsen (1960) discuss the effect of diffusion and
dispersion. They find that these effects are magnified as the column
becomes unsaturated. Other effects can be discerned on the precipitation
and dissolution of salts (Garrels and Christ, 1965), redox potentials (Bonn,
114
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1971), biological reactions (McLaren and Skujlns, 1968), and entrapped gases
(Orlob and Radhakr1shna,l958). These effects may be Involved and interrela-
ted; they are not well understood, particularly In their net effect on the
attenuation and migration of solutes.
Initial attempts at modeling this complex system will necessarily be
crude. Several groups of researchers are looking at complex systems of
reacting chemicals, but most assume instantaneous reaction as a basis for
their analysis (Morel and Morgan, 1972; Dutt, 1962).
One point that has not been examined 1s the Influence of solution flux
on the attenuation or migration of trace contaminants. Fluctuating flow
rates and dlspersivlty have been modeled by Warrick et al. (1972). Also from
the work of Skopp and Warrick (1974) we would expect maximum attenuation at
the lowest fluxes; however, the change 1n attenuation with flux 1s most
dramatic at high fluxes. The Influence of pore-size distribution and flux
1s presently being studied using a similar approach as given in Skopp and
Warrick (1974).
Adsorption (Physico-chemical)--Chetnical adsorption received earlier
attention under chemical mechanisms but the more physico-chemical oriented
mechanisms were not emphasized. Discussions of adsorption appearing in the
literature often do not Include a clear delineation between chemical and
physical adsorption processes. Perhaps it 1s not too much to expect, because
the mechanisms of adsorption are not well established. To illustrate,
Murrmann and Koutz (1972) state that "Adsorption 1s the most Important pro-
cess by which chemicals are removed from wastewater applied to soil ...
they are not well understood." The adsorption forces, which often dominate
behavior of heavy metals 1n soils, particularly when the metals are present
in the soil solution in small amounts, may be of various types. Only a small
proportion of the reactions of heavy metals usually quantitized as adsorp-
tlve can be accounted for by chemical bonding. Solid state diffusion of
specific Ions (Cu and Zn) Into octahedral positions 1n layer silicates may
be possible 1f open crystal structure prevails, Banerjee et al. (1953), but
Jenne (1968) believes this to be a minor mechanism of Immobilization for
115
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for most heavy metals.
Heavy metal adsorption most often has been related to sorption by
the layer silicates which include (a) surface complex ion formation, (b)
surface reactions, not ion exchange, (c) lattice penetration or inclusion,
and (d) ion exchange. Other hypotheses concerning heavy metal adsorption
include reactions with organic matter, lime, and hydrous oxides. There is
enough information available now to broadly classify adsorption into four
main groups:
1. Layer silicates
2. Hydrous oxides
3. Organic matter
4. Lime (carbonates)
Layer silicates—Solid-state diffusion known as "lattice penetration"
is not a likely mechanism of consequence for heavy metals. Heavy metals
appear to diffuse to a distance of a few atomic planes only at the crystal
edges and along fractures. This is more of a fixation mechanism (i.e.,
retention against removal by salt solutions) than a sorption mechanism.
Layer silicates may bind soluble chemical species through two princi-
pal mechanisms, (a) ion exchange as the result of electrostatic attractions
and (b) ion adsorption through covalent bonding. Ion exchange is relatively
nonspecific and readily reversible, while ion adsorption may be very spe-
cific and at times irreversible. Ellis and Knezek (1972) have discussed
mechanisms of Zn and Cu reactions with clay surfaces. Involved in these
r ^
-------
of reversibility of heavy metal sorption by the hydrous oxides, as well as
the exact nature of the 'pH effect1 on the reversibility is not entirely
clear." The available data indicate that heavy metal adsorption by the
hydrous oxides is at least partially competitive with and reversible to
exchange with other heavy metals. Jenne (1968) further proposes that Fe and
Mn supply the principal matrix into which less abundant heavy metals are
adsorbed, coprecipitated, or occluded. In addition to the Fe and Mn hydrous
oxides, Al hydroxides may play a role at times. Gibbsite (A1(OH)3) is a com-
mon mineral of our more heavily weathered soils and would undoubtedly play
an important role in the mobility of some of the chemical species considered
in this work (e.g., As forms very insoluble precipitates with Al). Also Al
hydroxide polymers present on clay surfaces (of general occurrence in soil
systems according to Jackson,1963) may also play a role in immobilizing
chemical elements in soil systems.
Organic matter—Reactions between heavy metals and organic matter are
thought to involve (a) the formation of complexes, and (b) chelates. These
are chemical reactions. Physico-chemical adsorption is often alluded to in
the literature but not defined or quantitized. The extent to which organic
matter plays a part in physico-chemical adsorption of the trace contaminants
is not known.
Li me--Retention of trace and heavy metals by lime is more likely to be
surface adsorption or surface precipitation by the carbonate ion. Jurinak
and Bauer (1956) believe that zinc, for example, is adsorbed on the crystal
surface of dolomite and magnesite at lattice sites that normally are occu-
pied by magnesium. Later, Brown and Jurinak (1964) found that additions
of CaCO. (lime) to Yolo fine sandy loam, beyond 5% (w/w), did not influence
either Cu or Zn uptake by plants. The carbonate interaction was considered
to be primarily a pH effect and secondly, an effect of carbonate-bicarbonate
ion on heavy metal solubility.
Temperature Effect—The physical effects of temperature changes are
minimal beyond 3 feet below the soil surface. On or near the surface,
117
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freezing and thawing can directly cause insoluble or slowly soluble
precipitates to form in the aqueous solutions. Trace and heavy metals will
be carried out of solution with the organic matter and/or precipitate as
salts of various crystalline and amorphous structures. Freezing is a con-
centrating and dehydrating mechanism. When freezing concentrates the
solutes sufficiently to exceed solubility constants between constituents
in the wastestreams and leachates, precipitation can occur. Freezing con-
ceivably could be a very useful mechanism for natural control of trace and
heavy metal in cold climates during the winter months. Re-solution of such
precipitates upon thawing usually is slow, if at all. The process of freeze-
drying or "salting-out" undesirable salts is an accepted process for puri-
fication of saline waters in western states. For further physical and
chemical behavior of cationic concentration by freezing, see Malo and
Baker (1973).
Temperature can also influence flow rates of water. There appears to
be little data in the literature on the effect of temperature on flow rates
involving wastestreams and leachates containing trace and heavy metals.
118
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SECTION VI
SPECIFIC REACTIONS OF SELECTED HAZARDOUS CONSTITUENTS
The migration of selected elements, asbestos, and cyanide in soils is
not known precisely and, because of the heterogeneity of field soils, it
cannot be predicted from laboratory work with homogenized and repacked soils.
Except for special cases where migration behavior is known because a specific
waste has been applied to a particular soil, mobility in soil can usually
only be inferred from a knowledge of the soil chemistry of the constituent
in question and from a knowledge of the composition of the waste or leachate
containing the constituent. As an aid in making predictions about migra-
tion and as background for other parts of the report, this section presents
a selection of relevant information on specific chemical reactions of 10
elements, asbestos, and cyanide in pure systems and in soils. No attempt
has been made to quantitize the reacting and resultant chemical species
under specific conditions. Even if such information were available or
could be constructed from a knowledge of the chemistry of the constituents,
the information could not be applied practically because of the difficulty
in measuring reaction conditions in the field. Some of the entries in this
section are much larger than others because of the disproportionate amount
of research attention that has been devoted to some constituents. It is
hoped that the deficiencies of information will be remedied as more atten-
tion is focused on the chemistry and fate of wastes in soils.
ARSENIC
Unlike most of the potentially hazardous elements listed, arsenic has
essentially no aqueous cationic chemistry and arsenate acts as a pH-
dependent weak acid. Only trace amounts of arsenic appear naturally in
soils. Arsenic has entered the soil in the past primarily through the use
119
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of arsenical insect sprays and, to a limited extent, through disposal of
some industrial wastes.
The most prominent mechanism of attenuation for arsenic applied to
soils is adsorption by soil colloids (Murrmann and Koutz, 1972). Presant
and Tupper (1966) studied the arsenic contents of six acid soil (Spodic)
profiles by leaching them with dilute acid (0.1N HC1). They observed that
under acidic soil conditions arsenic, on oxidation of the organic matter,
migrates to the B and C horizons where it is probably associated with free
iron and present in an adsorbed form. Jacobs, Syers, and Keeney (1970)
showed that retention of As against extraction with NHL Ac and Bray P-l
reagents from Wisconsin soils equilibrated with As (0, 80, 320 yg As/g
added as NaHAsO.) increased as the sesquioxide content increased. Also, as
the free Fe^O, content of the soils increased, the amount of As adsorbed
from the solution increased. Amorphous Fe and Al compounds had a similar
effect on As.
According to Tiffin, Lagerwerff, and Taylor (1973) As is known to
associate with sesquioxides in a manner similar to iodine. A large fraction
of As in soils is not extractable by citric acid. Although extractable by
acids, the (Ca(As).)2 form is another rather immobile compound which fre-
quently occurs in soil. Arsenate and arsenite usually appear in equilib-
rium (Misra and Tiwaii, 1963). However, if poor drainage or waterlogging
occurs, As appears in the reduced form which is more soluble and mobile than
the oxidized form (Swaine and Mitchell, 1960). Upon the return of water-
logged soils to oxic conditions, arsenite becomes oxidized to arsenate
through biological oxidation (Questel and Scholefield, 1953). These authors
further state that the adsorption of arsenite at pH 7 is directly propor-
tional to the amount applied and that small amounts of arsenite are irre-
versibly bound to soil. At the low concentrations usually found in waste
waters, landfill leachates, and other aqueous wastestreams, As probably will
not precipitate in soils except possibly as an impurity in phosphorus com-
pounds formed over long periods of time.
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ASBESTOS
Finely divided particles of asbestos are an air pollutant and a
health hazard and cause lung cancer and a variety of other respiratory
problems in human beings. Asbestos is a general term applied to certain
minerals that form soft, flexible fibers in metamorphic rocks. The most
common asbestos is chrysotile. a variety of the mineral serpentine, a mag-
nesium silicate. Because of its fibrous, insoluble nature it has been used
extensively in water filtration processes for clarification and purifica-
tion. Because it is fireproof and flexible and has a fibrous nature, it is
used extensively in fireproof clothing, blankets, heating coil insulation,
and insulation placed around furnaces and heaters. It is also blown into
attics and walls for insulation.
Asbestos is readily retained by the soil and deposits almost wholly
on top of the soil regardless of the vehicle of transport. Colloidal
particles < 2y may penetrate the soil pores but not for any significant
depth. Upon drying, asbestos again may be free for "pick up" by air turbu-
lences unless incorporated into the soil.
Although there are no data on mobility of asbestos in soil, predic-
tions about its behavior can be made with reasonable confidence. Since the
weathering products of asbestos are the common nonhazardous salts of Ca, Mg,
and Si, physical transport is the only mode of movement in soil which is of
significance. The extensive data on movement of clay-sized (< 2y diameter)
particles by strictly physical processes provide a convenient yardstick for
gaging the probable behavior of asbestos in soil. Clay particles 0.1 to
2.0 y in diameter are estimated to move at a rate of 1 to 10 cm per 3,000 to
40,000 years, depending on the soil texture (Berk!and, 1974). There is no
reason to expect that asbestos particles of similar sizes would move differ-
ently from this. Consequently, asbestos migration through soil will not be
a problem of any significance.
Microorganisms and nonbiological geologic weathering will separate
asbestos into its constituent, harmless parts; namely, calcium, magnesium,
and silicate compounds, to join similar naturally occurring soil constituents.
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Asbestos degradation, however, proceeds in soils at a very slow rate.
Except as a possible dust hazard, asbestos does not offer a serious contam-
ination prospect to the soil or underground water supplies and cannot be
classed as a soil pollutant. Plowing or tilling the asbestos into the soil
where it can be mixed in great dilution can control its chances of getting
into the air as dust again. Surface waters can be polluted by asbestos
which is washed into them. Consequently, precautions should be taken to
prevent erosion from land disposal areas receiving asbestos.
Control of soil and water pollution from asbestos is straightforward
and can be carried out with the technical knowledge which is now available.
BERYLLIUM
Commercial use of beryllium began about 1921. In the early 'forties,
its use increased greatly to keep pace with an expanding air transport
industry and in the late 'forties as part of the atomic energy development.
Today, beryllium is used in many different manufactured products. It endows
other metals with great strength, hardness, and rt •'stance to corrosion. It
is not"capable of magnetization and is nonsparking. . j precious gems,
emerald and aquamarine, are varieties of beryl, BeJMpESigO-jgJ, which con-
tain impurities (Beus, 1956). Chromium impurity yields emeralds, and FeO
impart the blue to aquamarine.
Beryllium in certain chemical combinations presents a serious health
hazard to those industrial workers susceptible to Be effects. It is par-
ticularly dangerous as an air pollutant.
In aqueous solution, only Be valence state occurs. Beryllium has
the smallest radius (r = 0.3A) of the metal cations and a hydration number
of 4. Hydroxides and fluorides complex Be in aqueous solutions and may be
considered as the most important inorganic ligand in such solutions
(Bondietti et al., 1973). Hydrolysis becomes detectable at about pH 3 in
q
concentrations above 10 M. Precipitation of BefOHk takes place with
increasing pH when about one OH~ ion on the average becomes bound per Be
122
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ion. Hydrolysis products are both monomeric and polymeric in solution.
Baes and Mesmer (1973) emphasize the hydrolytic reactions of Be, in their
review of trace contaminants, with these words:
"By far the most important hydrolysis product at beryllium
concentrations about 10~3M is the trimer Be3(OH)3+. Significant
amounts of a dimer Be2OH + are formed in the initial hydrolysis
of more concentrated solutions. At least one other polymeric species,
probably Be6(OH)g , is formed in minor amounts in extensively hydro-
lyzed acidic solutions prior to the precipitation of the hydroxide.
"Because the polynuclear hydrolysis products of Be dominate
at the usual metal concentrations studied (> 10~1), the mononuclear
hydrolysis products BeOH , BeCPHjg. and Be(OH)I can be examined by
the usual equilibrium methods only at very low metal concentrations.
The stability of BeOH has been estimated by a kinetic method at
ordinary concentrations while the stabilities of Be(OH)Z have been
estimated at very low beryllium concentrations from liquid-liquid
extraction measurements and beryllium hydroxide solubility measure-
ments, respectively. Estimates of the stability of these three
monomeric species could be considerably improved by additional,
more accurate measurements of the solubility of beryllium hydroxide.
In strongly basic solutions (> 0.1 M) beryllium hydroxide dissolves
to form the fully hydroxylated species Be(OH)^."
Using Be, Romney and Chi 1 dress (1965) found that the isotope was
strongly adsorbed by soils containing montmorillonite and illite-like clay
minerals (Hanford and Vina soil) but not kaolinite. Furthermore Mg, Ba or
Ca did not effectively replace Be adsorbed to the two soils or bentonite
even at high concentrations under "batch-equilibrium" conditions. These di
valent cations, though, effectively competed with Be for sorption sites in
soil but not in bentonite suspensions. Magnesium, Ba, and Ca readily
replace Be from kaolinite.
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Romney and Chi 1 dress (1965) have this to report about their leaching
column research:
"Additional tests on 300-g lots of Be-treated soil in leaching
columns showed that virtually no Be was extracted from Hanford or
Vina soil by 600 ml of distilled water and 0.1 M BaCU.CaCK, or MgCU
Five successive Teachings with 300 ml increments of 0.1 r[ HC1 were
required to extract significant amounts of Be from the treated soil.
Equilibration-extraction tests showed that Be effectively displaced
Ca, Mg, and Sr from sorption sites; Fe and P adsorbed by Hanford
and Vina soil, however, were not influenced by Be treatments."
Beryllium reacts similarly to Al. It undergoes isomorphic substitu-
tion in the expanding lattice-type clay minerals as well as cation-exchange
reactions. In summary, beryllium is strongly immobilized in soils, partic-
ularly those containing montmorillonite- and illite-like clays. It may dis-
place divalent cations already on common adsorption sites in the exchange
complex.
Evidence for Be organic attenuation or formation of soluble organic
complexes in soil solution was not found in the literature at the time of
this review.
CADMIUM
Adsorption on colloidal surfaces due to coulomb-type forces (Lagerweiff
and Bower, 1972), is claimed to be primarily responsible for the immobility
of cadmium in soils. Cadmium, like Zn, Hg, and Pb, undergoes hydrolysis at
pH values encountered in soil environments, Hahne and Kroontje (1973).
Cadmium belongs to the oxyphilic and sulfophilic group of elements. How-
ever, these elements (Zn, Hg, Pb, and Cd) must be treated differently since
similarities in behavior do not extend to all reactions in soils. For
example, Cd, Hg, Pb, and Zn demonstrate different hydrolysis species (Tables
6.1, 6.2 and 6.3). Compare the distribution of molecular and ionic species
of divalent Cd, Hg, Pb, and Zn at different pH values in Figures 6.1, 6.2,
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TABLE 6.1. THE FORMATION CONSTANTS AND SOLUBILITY PRODUCTS USED IN
CALCULATIONS*
Formation Constants
System
Zn-Cl
Cd-Cl
Hg-Cl
Pb-Cl
Zn-OH
Cd-OH
Hg-OH
Pb-OH
P04-H
S-H
Log 0i
0.43
1.32
6.74
0.88
4.40
4.30
11.86
7.82
Log 61
12.325
14.920
Log 02
0.61
2.22
13.22
1.49
12.89
7.70
22.13
10.88
Log 0i2
19.532
21.960
Log 03
0.53
2.31
14.07
1.09
15.86
10.30
13.94
Log 13
21.693
Log 04
0.20
1.86
15.07
0.94
15.95
12.00
16.30
Solubility Products
Precipitate
Zn(P04)2
ZnS
Zn(OH)2
Cd3(po4)2
CdS
Cd(OH)2
Rb3(po4)2
PbS
Pb(OH)2
HgS
Hg(OH)2
Log Kap
-32.04
-25.15
-15.50
-32.60
-27.92
-13.55
-42.10
-28.15
-19.52
-53.50
-25.40
Adapted from Si 11 en and Martell (1971).
6.3, and 6.4. The very common and mobile chloride ion is a persistent com-
plexing agent with Cd, Hg, Pb, and Zn (Table 6.4). The Cl" concentration
also differentiates these heavy metals markedly, depending on their affinity
for such complex formations according to Hahne and Kroontje (1973).
Preliminary studies by Bondietti et al. (1973) indicate that the
109
silt fraction of a Midwest soil, Dodge silt loam, immobilized IU3Cd as much
as, if not more than, the sand and clay fraction. The greatest concentra-
tion of 109Cd, though, was associated with the organic matter rather than
inorganic mineral fraction of the soil. These same investigators state that
"in soils and sediments the clay minerals, sesquioxides and the humic acids
are the major components involved in adsorptive reactions." Carbonates are
implicated in immobilization of Cd also but the mechanism is not clear.
125
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TABLE 6.2. EFFECT OF CATION AND ANION HYDROLYSIS ON THE CONDITIONS SOLUBILITY AND SOLUBILITY
PRODUCT AT THREE pH VALUES*
ro
01
Precipitate
4.0
Log
S'/S (K'sp/Ksp)
S'/S
pH
6.0
8.0
Log
(K'sp/Ksp)
S1
/S
Log
(K'sp/Ksp)
Cation and Anion Hydrolysis
ZnS
PbS
Cds
HgS
Zn3(P04)2
Pb3(P04)2
Cd3(po4)2
9.554 x 106 13.96
9.586 x 106 13.96
9.554 x 106 13.96
1.378 x 103 16.28
4.125 x 104 23.08
4.141 x 104 13.09
4.125 x 104 23.08
9.981
1.286
9.977
1.162
1.056
1.430
1.055
x
X
X
X
X
X
X
Anion Hydrolysi
MS*
M3(P04)2
9.554 x 104 13.96
4.125 x 104 23.08
9.976
1.055
X
X
104
105
104
103
103
103
103
S
104
103
10.
10.
10.
16.
15.
15.
15.
10.
15.
00
22
00
13
12
78
12
00
12
9.
2.
3.
3.
2.
7.
5.
3.
5.
010
489
068
529
101
115
769
038
701
x
X
X
X
X
X
X
X
X
103
104
103
103
102
102
101
103
10
7.90
8.79
6.97
17.10
.11.61
14.26
8.81
6.97
8.78
*
From Hahne and Kroontje (1973.
a
S' = conditional solubility derived from
derived
M stands for
from solubility product Ksp.
Zn2*, Cd2*, Hg2*, and Pb2*
conditional
solubi
lity product
K'sp;
S =
solubil
ity
-------
CdOH* Cd(OH)§
Cd(OH)2
0.0 -
-10
-8
•6 -4
log [OH]
-2
8
10
PH
12
14
Figure 6.1. Distribution of molecular and 1on1c species of divalent
Cd at different pH values (From Hahne and Kroontje,
1973).
127
-------
-10
log [OH]
-6
I
4
pH
8
Figure 6.2. Distribution of molecular and 1on1c species of divalent
Hg at different pH values. (From Hahne and Kroontje,
1973)
128
-------
0.0 -
-12
I
8
PH
10
12
14
Figure 6.3. Distribution of molecular and ionic species of divalent Pb at
different pH values. (From Hahne and Kroontje, 1973)
129
-------
-8
-6 -4
log [OH]
-2
8
10
pH
12
14
Figure 6.4. Distribution of molecular and ionic species of divalent Zn at
different pH values (From Hahne and Kroontje, 1973).
130
-------
TABLE 6.3. SOLUBILITIES AND INTRINSIC SOLUBILITIES OF Zn, Cd, Hg and Pb
HYDROXIDES a
Component
Solubility
moles/ liters
Zn
Cd
Hg
Pb
(OH)2
(OH)2
(OH)2
(OH)2
4.
1.
1.
2.
292
917
961
151
x 10"6
x 10"5
x 10"7
x 10"9
(s)
ppm
861
384
393
431
Intrinsic
solubility
moles/1 i
x 10"3
x 10"3
x 10"4
x 10"6
2.
1.
5.
2.
454 x
412 x
370 x
291 x
iter
io-3
io-6
ID'4
ID'9
S°
ppm
160
150 x
107
474 x
io-3
io-6
From Hahne and Kroontje (1973).
aThe intrinsic solubility of the metal-ion hydroxides is defined as S° =
[M(OH)2] solution. Also, the metal-ion hydroxide concentration in solu-
tion can be expressed E(OH)2] solution = B2Ksp = S°.
The relative adsorption of Cd by selected minerals appears in Table 6.5 for
comparative purposes. Literature on the hydrolytic behavior of Hg, Zn, Cd,
Pb, Cu, and Be has been reviewed by Bondietti et al. (1973) in a report,
Ecology and Analysis of Trace Contaminants, supported by the NSF-ORNL-EATC
(June 1972-January 1973). Primary references for literature may be found
compiled up to 1968 by Sillen and Martell (1971) under the title, "Stability
Constants of Metal-Ion Complexes."
As contaminants from air, accumulation of Cd and Hg in soil is quite
different. Cadmium adsorbs more to understory plants and litter than Hg
(Baylock et al., 1973). Radiotracer techniques confirmed the expectation
that Hg is more mobile in the Tennessee soils studied than Cd. Zinc, on the
other hand, forms slightly more stable complexes with humic acids than Cd
(Bondietti et al., 1973). These investigators compared Cd adsorption by
humic acid and clays. The materials, represented humic acid extracted from
a local Captiva soil (Tennessee), a clay-organic fraction from Dodge silt
loam, and a reference 11 lite clay. The results of the Cd electrode titra-
tion curves for these materials using 50-mg samples in 100 ml of 0.1 N
Ca(N03)2 appear in Figure 6.5. Both the humic acid and clay-organo fraction
131
-------
TABLE 6.4. EFFECT OF CHLORIDE CONCENTRATION ON THE CONDITIONAL SOLUBILITY AND SOLUBILITY PRODUCT
ro
Chloride concentration
Precipitate
Zn3(P04)2
ZnS
Zn(OH)2
Cd3(po4)2
CdS
Cd(OH)2
Rb3(po4)2
PbS
Pb(OH)2
HgS
Hg(OH)2
0.
S'/S@
1.000
1.000
1.000
1.001
1.001
1.001
1.000
1.000
1.000
4. 082x1 02
5.503x10
0001 M
Log
(K'sp/Ksp)
0.00
0.00
0.00
0.01
0.00
0.00
0.00
0.00
0.00
5.22
5.22
0.
S'/S
1.016
1.014
1.009
1.130
1.017
1.070
1.047
1.039
1.026
4. 423x1 O4
1.21 4x1 O3
01M
Log
(K'sp/Ksp)
0.004
0.01
0.01
0.27
• 0.09
0.09
0.10
0.03
0.03
9.25
9.25
1.
S'/S
4.604
3.569
2.336
39.83
21.55
7.745
11.72
7.77
3.926
3.619xl07
1.094xl05
OOM
Log
(K'sp/Ksp)
3.32
1.11
1.11
8.00
2.67
2.67
5.35
1.78
1.78
15.12
15.12
From Hahne and Kroontje (1973).
"S1 = solubility derived from conditional solubility product K'sp; S = solubility derived from
solubility product Ksp.
-------
Q.
OL
T3
O
O
0.01
0.001
280 270 260 250 240 230 220 210
POTENTIAL (mV)
Figure 6.5. Cadmium electrode titratlon curves for soil humlc acid, organo-
clay, and ilHte. 50 mg samples 1n 100 ml of 0.1 M Ca(N03)2.
(From Bond1ett1 et al., 1973)
133
-------
TABLE 6.5. ADSORPTION OF CADMIUM BY SELECTED MINERALS*
(Solution was 0.025 M calcium acetate.)
Material
Activated alumina powder
Limonite (activated)
Conasauga shale
Fithian illite (< 2y)
Muscovite (20-2y)
Biotite (20-2y)
pH
7.71
7.43
7.59
7.60
7.96
7.90
K@
Kd
5642
9578
2624
1038
363
220
From Bondietti et al. (1973).
Fraction of cadmium adsorbed per gram divided by the fraction in solution
(ml).
display high selectivity as well as high capacity for Cd adsorption. Illite
is the poorest adsorber, though illite also demonstrates selective adsorp-
tion for Cd compared with such elements as Ca. From the data presented in
Figure-6.6, Bondietti et al. (1973) draw this conclusion.. ."it should not
be surprising that the mobility of cadmium in soils is very limited."
Before concluding the discussion on Cd, however, it seems desirable
to return to Table 6.5. The distribution coefficient (Kd) shows that alum-
ina and iron oxide have a higher adsorptive capacity than illite and far
greater capacity than muscovite and biotite. These and other data (Figure
6.6) presented by Bondietti et al. (1973) make it quite evident that Cd has
a selective adsorption affinity for oxides and/or hydroxyoxides of Fe and
Al and strongly suggest that these constituents (often lumped under the
heading of sesquioxides) are responsible for adsorbing most of the Cd
applied to soils in aqueous wastestreams.
CHROMIUM
Chromium has two important oxidation states, trivalent and hexavalent.
The latter is used widely in industries such as metal plating.
134
-------
CO
en
OKLAHOMA
ILLITE
MONTMORILLONITE
8
.Figure 6.6. Relative adsorption capacities of clay minerals for low levels of cadmium in calcium
solutions at 26°C. (From Bondietti et al., 1973)
-------
Very little has been published about the chemistry of Cr in soils.
The dominant species in soils is Chromium (III). Its concentration in the
soil solution is limited by (a) adsorption on organic matter, clay minerals,
Basu and Mukherjee (1964), and hydrous oxides of Fe, Mn and Al , and (b)
precipitation as an oxide, and/or a mixture of the two.
Chromium (III) forms precipitates readily with hydroxides, carbonates,
and sulfides, Murrmann and Koutz (1972). Although precipitation is proba-
bly not a major mechanism for Cr attenuation in soils at low concentration
except at high pH values, it may well be an important immobilization reac-
tion if aqueous wastestreams high in Cr enter the soil. In dilute solution
the main mechanism is retention by adsorption at the surfaces of aluminum
silicate minerals and iron oxides or hydrous oxides, Murrmann and Koutz
(1972). The extent and rate of adsorption reactions probably will be deter-
mined largely by the type of adsorber present, the concentration of other
ions in solution which would compete with chromium for exchange sites, and
the pH. Reduction of solution concentration would be favored in soils with
high contents of clay minerals and hydrous oxides of iron, aluminum, and
manganese, and by a high pH (> 6.0). Precipitation reactions will likewise
be favored by a higher pH.
Laboratory data published by Wentink and Edzel (1972) demonstrated
that three different soils were capable of completely removing trivalent
chromium from dilute solutions, and this suggests that percolation through
soil may be an effective method of treating dilute chromium-bearing solu-
tions. On the other hand, Deutsch (1969) reports instances of groundwater
• q ^.g
contamination by chromium (Cr or Cr ) wherein the soil was not an effec-
tive treatment system, probably due to coarse-textured soil and high con-
i O
centrations of chromium (both Cr
centrations of other soluble salts.
i O .
centrations of chromium (both Cr and Cr ) in the presence of high con-
The role of soil organic matter in controlling chromium concentration
in solution is not well documented. Meta-ls in general may be immobilized
by adsorption as cations on the surface of lignin-like compounds, may be
solubilized by complexing with organic acids and bases of low molecular
136
-------
weight, and may form insoluble metal-organic precipitates. The extent to
which these mechanisms apply to chromium chemistry in soils is not known
although immobilization of Cr by exchange groups of organic matter may be
expected to be fairly important. All of the heavy metals form soluble organ-
ic chelates with organic complexes in soil solution. Just how this balances
out with adsorption is difficult to assess without further study.
At acidic pH values the hexavalent form of chromium is dichromate
(Cr207~), a powerful oxidizing agent (Tiffin et al., 1963). It would be
expected, then, that hexavalent chromium applied to the soil would rapidly
oxidize organic matter, leaving Cr (III). The conversion from hexavalent to
trivalent will proceed more rapidly under acidic conditions. This may be a
factor in the relation between soil pH and plant toxicity. In soils amended
with hexavalent chromium, Patterson (1966) observed increasing toxicity with
increases in soil pH.
The presence of Cr in deep groundwaters of the desert Southwest,
which frequently exceeds the U.S. Public Health acceptable drinking water
quality limits, Dutt and McCreary (1970), might indicate a certain mobility
of Cr not experienced by many of the other heavy metals. The relevance of
this to attenuation of Cr by soils from "wastestreams" is not known.
The only important physical transport mechanisms in soils are downward
movement in solution and erosion of solid material from the soil surface.
Very little quantitative data on movement down through different types of
soil (except as noted earlier) and on the extent of erosion transport are
available, but some qualitative estimates can be made.
Other things being equal, downward transport will be more rapid in
coarse-textured soils than in fine-textured soils because of the larger
pores and therefore faster movement'of the soil water, and smaller amounts
of clay minerals. Similarly, transport through the soil will be faster in
higher rainfall areas because of the potential for more water entering the
soil (Tiffin et al., 1963). Transport in solution will be affected not only
137
-------
by the amount and rate of flow of soil water but also by the previously
discussed chemical mechanisms which control the concentration of chromium
in the soil solution.
For soils where added chromium remains near the surface because chemi-
cal mechanisms limit the concentration in soil solution, there is the possi-
bility that soil particles with adsorbed chromium or precipitates of chromium
will be eroded from the surface by irrigation and/or rainfall and carried
into surface waters (Tiffin et «!.-, 1963). This would be particularly sig-
nificant and should be considered when designing disposal sites for sewage
or industrial sludges where the soil has been selected or treated to maxi-
mize heavy metal retention in the surface.
COPPER
The important oxidation state of Cu in soils is +2. In the presence
of organic acids, organic-complex - COOH groups, -OH groups, etc., in land-
fill leachates, Cu may be present. Cupric ion hydrolyzes to a slight
extent prior to precipitation (Baes and Mesmer, 1973). Cu2(OH)2 is the
only polymeric species whose identity is well established. Investigators
do not agree on the stability of Cu(OH) , though there is evidence support-
j. 4»X
ing interpretation in terms of Cu(OH) and Cu2(OH)2 in acidic solutions.
Schindler et al. (1965), who studied the stable phase of CuO at 25°C
relative to surface area on the solubility of CuO, found that the only basic
species appears to be Cu(OH)T. The mononuclear species, Cu(OH)2(aq) and
Cu(OH)~ are not well established so the minimum solubility of the oxide is
still virtually unknown.
The carbonate complexes of Cu have very high stability constants
(Schindler et al., 1968). Carbonates (CuC03(aq) and Cu(C03)2, occur readily.
Schindler (1968), who further defined the Cu^-CO^HgO system, shows the
solid state to be Cu2(OH)2(C03) malachite and Cu3(OH)2C03)2 azurite.
138
-------
Lindsay (1972a) has prepared a diagram of the solubilities of various
Cu minerals (Figure 6.7) showing the Cu-soil curve which is represented by
the reaction
Cu++ + soil 2 Cu-soil + 2H+
The log K value as developed in five soils by Norvell and Lindsay
(1969) was found to be - 3.2 expressed ty the the equation:
Cu"1"1" = 103>2(H+)2.
This equation gives the approximate concentration of Cu maintained
in soils. According to Figure 6.7 the Cu-soil level falls considerably
below that of the individual Cu compounds. Lindsay (1972a) also prepared
another diagram, Figure 6.8, to show the relationship among soluble Cu
species expected in soil solution in equilibrium with the Cu-soil complex
defined in the above equations. The diagram shows that below pH 7.3, Cu
predominates while above this, CuOH+ is most prevalent. Cu2(OH2)++ and
CufOHjg are less abundant and Cu(OH)7 falls outside the diagram.
Again, the movement of cuprous or reduced forms of Cu through soil,
as may be expected below landfills producing leachates is not documented in
the literature. The present soils information is limited to the oxidized
form. The research of Cu attenuation by soil organic matter, for example,
is fairly extensive. The general consensus is that Cu is strongly com-
plexed to organic matter, Hodgson, Lindsay, and Trierweiler (1966).
Jenne (1968) proposed that the hydrous oxides of Mn and Fe provide the
main control on the immobilization of Cu as well as Co, Ni, and Zn in soils
and fresh water sediments. The hydrous oxide furnishes the principal
matrix in soils into which Cu and Zn (heavy metals) may be adsorbed, copre-
cipitated, and occluded. All of these reactions contribute to attenuation
of Cu.
Gilbert (1952) and Lucas (1948) report that leaching of copper in
agricultural soils is negligible.
139
-------
0
SOLUBILITY OF Cu MINERALS
o 6
E
OJ
<£ 8
o>
o
I
10
12
14
= 0.0003
= O.OI
Cu3(OH)2(C03)2
(AZURITE)
CuO
(TENORITE)
Cu2(OH)2(C03)
(MALACHITE)
Cu-SOIL
I
6 7
pH
8
Figure 6.7. The solubility of various Cu minerals compared to that of soil Cu.
(From Lindsay, 1972)
140
-------
Cu SOLUBILITY IN SOILS
o 8
> 10
en
o 12
14
16
Cu2(OH)|*
6 7
pH
8
Figure 6.8. Soluble Cu species 'in solution equilibrium with soil Cu.
(From Lindsay, 1972)
141
-------
CYANIDE
Cyanides are found in effluents from coal-burning furnaces, gas works,
and coke ovens, from the scrubbing gases of steel plants, from metal clean-
ing and electroplating processes, and certain chemical industries.
Cyanide is an anion (CN~) and, as such, is not strongly retained in
soils. Except for AgCN, cations form at least slightly soluble salts with
CN".
Cyanide appears in nature and industry in so many chemical and biologi-
cal combinations and forms, it requires immediate strict definition in
order that this discussion may move forward in as clear a manner as possible.
In industrial wastes, "cyanide" refers to all CN groups in the cyanide com-
pounds present that can be determined as the cyanide ion, CN", by the
methods used, Taras (1971). The cyanides are further conveniently classi-
fied into (a) simple and (b) complex groups.
The simple forms occur as
A(CN)x (1)
where A = an alkali (Na, K, NHL) or a metal
x = the valence of A and the number of CN groups
and CN = is present as CN"
The complex forms are many and varied but the alkali-metallic cya-
nides have the formula:
AyM(CN) (2)
J\
where A = the alkali present y times
M = a heavy metal (Fe , Fe , Au, Cd, Cu, Ni, Ag, Zn and others)
x = the number of CN" groups and equal to the valence of A taken
y times plus that of the heavy metal.
The anion radicals in the complex cyanides appear as M(CN)x-
142
-------
When the simple cyanides come in contact with acids, HCN forms. The
metal cyanides vary widely in their decomposition to HCN in acid. As a
matter of convenience they may be grouped further, on a basis of rate of
decomposition, as readily decomposable (metallic forms of Ag, Au, Cd, Cu, Ni,
Pt, and Zn) and slowly decomposable (Fe and Co).
The stability of alkali-metallie cyanides also vary in aqueous solu-
tion alone. Many remain rather stable in water. Because of the toxicity
of CN", the formation of the more stable cyanides has been a significant
factor in the activity of biological systems.
Cyanide in Aqueous Wastestreams
Studies involving cyanides and aqueous wastestreams, although only
indirectly related to cyanide attenuation in soils, provide some clues to
chemical and biological reactions which may be expected to occur in soils.
Reactions in most wastewaters (landfill leachates, sewage waters, polluted
streams, and wastewaters from a wide variety of industrial sources) are
more readily definable in aqueous than soil media. Moreover, toxic limits
of CN to biological systems appear to be more readily identified in the
absence of the great sorption and buffering variables of soil materials.
The identification and evaluation of CN" in sewage, leachate, waste, and
polluted waters appears, from our studies at Arizona, to be much more
quantitative than for soils. A number of procedures have been reviewed by
Taras (1971), certain of which are offered as quantitative with the excep-
tion of cobalticyanide. Sulfides, heavy-metal ions, fatty acids, oxidiz-
ing agents, and other interfering substances which often respond to removal
by distillation, however, can seriously influence the qualitative evalua-
tion of CN. These may be expected to be some of the same substances which
influence the migration rate of CN in soils.
Virtually no organic compound is left immune to degradation by the
highly versatile microbial population. Cyanide is no exception despite the
fact that it is highly toxic to biological systems as CN" (Taras, 1971;
Ludzack et al., 1951; and Dodge and Reams, 1949). Simple alkali cyanides
143
-------
and many alkali -metal lie cyanides, which form CN~ in aqueous solution, may
decompose slowly to form varying degrees of toxicity. The level of toxi-
city of the more stable cyanides depends on the metal present and the pro-
portion of CN groups converted into simpler alkali cyanides.
The threshold limit of CN toxicity on biological activity of aqueous
systems also varies widely with such environmental factors as water quality,
temperature, type, and size of the organism. Thus, definite effects cannot
be established except in terms of the nature of the effects. For example,
Lockett and Griffith (1947) report that 5.0 ppm of CN in sewage treated by
the activated process had a marked depressive effect on the purification
process, whereas Ludzack et al . (1951) found inhibitory effects as low as
0.3 ppm under certain other conditions. In concentrations of 6% CN, all
waters studied were purified up to 50% or more of the control within 10 days
of incubation.
Of particular interest because of its toxicity to the cytochrome sys-
tem is the utilization of cyanide by specific microorganisms. Ware and
Painter (1955) isolated an aerobic autotrophic actinomycete contained in
sewage which is capable of growing on silica gel containing only KCN as a
source of carbon and nitrogen. This organism can utilize concentrations
of CN up to 15 mgm/100 ml but grows more favorably at 4 mgm/100 ml concen-
trations. The rate of utilization in colony culture approached a maximum
of 0.5 mgm CN/day. Presumably the general reaction proceeds as follows:
2 KCN + 4H20 + 02 + 2KOH + 2NH3 + 2C02 (3)
Other examples of specific microbial assimilation and transformation
of CN in synthetic media (not soil) are those of Reynolds (1924), and
Strobel (1964). All the active organisms are fungi. Howe and Howe (1966)
patented a process for biological degradation of CN using the biological
masses of the activated sludge system. They claim to have successfully
degraded or detoxified more than 570,000 Ib of CN" in a year. The system
does not require any specific organism and may be written as:
144
-------
Microbial masses + CN~ Enzyme .... . _ ......
(Co PO NH }> Vl':amin B-|2 in biomass and
4* 3 degradation of CN. (4)
Cyanide in Soils
Cyanide finds its way into soils primarily through the activity of
man, although it is actually produced by some fungi (Bach, 1956), at least
one bacterium (Michaels and Corpe, 1965) and many higher members of the
plant kingdom (Robinson, 1962). Cyanide also is utilized as an energy
V
source and/or source of nitrogen by plants and microorganisms (Goldschmidt
and Butler, 1963; Allen and Strobel, 1966; and Ware and Painter, 1955).
In fact, cyanide and such related compounds as cyanamide, dicyanodiamide, and
guanidine nitrate have long been regarded by agriculturalists as potential
nitrogen fertilizers. As early as 1918 Cowie (1919) of the Rothamsted
Experiment Station, Harpenden, England, recognized that cyanamide can serve
as a valuable fertilizer because it forms ammonia readily in soils. Nitrate-
nitrogen then accumulates through the usual microbial-ammonia-oxidation
channel (Cowie, 1919; McCool, 1945; and Fuller et al., 1950a, 1950b).
Fuller et al. (1950), using a calcareous soil and Volk (1950), using
an acid soil, found that cyanamidewas inhibitory to ammonia nitrification
in soil at high concentrations. In the calcareous soil cyanamidewas readily
converted to nitrate when applied at a rate of 100 ppm N. At a rate of
200 ppm N only about half of the nitrogen was converted to ammonium-nitrogen
during the year, and only small amounts of nitrate-nitrogen were detected.
The pH at the high rate was 8.0 or above. At pH values below 7 the reaction
probably is:
4CaCN2 + 9H20 -»• 2Ca(OH)2 + (CaOH)2 CN + 3CO(NH2)2 (5)
See Fuller et al. (1950). The urea may then hydrolyze to yield NH3 + C02-
At pH values between 7 and 8 both reactions may be expected to occur
with the formation of urea exceeding that of dicyanodiamide.
145
-------
Anaerobic soil conditions presumably cause cyanamide to decompose,
yielding nitrogen gas.
Dicyanodiamide applied to a calcareous soil was found by Fuller et al.
(1950a) to yield only small amounts of ammonium-nitrogen over a year's time.
Nitrates did not form in appreciable quantities and were depressed even
below that of untreated soil. Cowie (1919) also claimed dicyanodiamide gave
no evidence of nitrification in soil over periods of several months. Dicyan-
odi amide inhibits oxidation of ammonia, although it may not be toxic to
organisms other than the nitrifiers.
Cyanide (CN~) added to soil in modest amounts (up to 200 ppm NaCN)
appears to be readily transformed and/or degraded depending on the oxida-
tion/reduction conditions. In fact, McCool (1945) suggests it is only
slightly less effective as an N-fertilizer for tobacco, corn and mustard
than sodium nitrate when applied to nitrogen-deficient, acidic soils.
Cyanide as KCN15 was shown by Strobel (1967) to yield C02 and NH3 in non-
sterile soils. He further suggests that cyanide is fixed by various soil
organisms in several ways, all of which give rise to some organic nitrile.
The nitriles yield ammonia plus the corresponding organic acid as a result
of nitrilase activity. Many microorganisms of the soil can utilize ammonia
and "fix" the N in the form of living cells. Strobel's (1967) experiments
with doubly lab labeled CN (C14N15) showed that the N of the cyanide was
retained more firmly than the C. Mobility of CN" through soil according to
McCool also appeared to be slower than that of nitrate from sodium nitrate
sources.
Despite the fair amount of information on cyanide reactions in natural
and waste systems, a number of critical gaps exist which need filling before
predictions concerning the fate of CN" under such a wide variety of habitats
as the soil can be made with confidence. Some of the most obvious deficien-
cies in information are tn the area of anaerobic reactions. Since many of
the CN wastestreams, waste ponds, and leachates end in an anoxic anaerobic
habitat, a research program to obtain data in this area was initiated.
146
-------
Since data concerning the movement of cyanide in different soils are
very limited and fragmentary, further study is needed. In particular, more
information on rates of cyanide to harmless N« gas under anaerobic condi-
tions is needed, since conditions are expected to be predominantly anaerobic
at the leachate-soil junction under a landfill. Analytical procedures for
-CN in a mixed matrix such as soil are unusually difficult and time consum-
ing to carry out.
IRON
Iron is universal. Soils contain large amounts of iron as compared to
other trace and heavy metals. The clay minerals (aluminum silicates), sul-
fides, sulfates, oxides, and hydroxides all react to form compounds or com-
3+
pi exes with iron. Under oxic conditions iron is present in solution as Fe .
Iron first forms the hydroxide and then proceeds to the insoluble Fe203
oxide forms on aging. Iron solubility and therefore mobility in soil is
dependent both on pH and redox potential.
According to Jenne (1968):
"When [(Fe3+)(OH")3Jsoln is greater than the solubility product,
iron precipitates, presumably as Fe(OH3(a), as a coating on clay
surfaces. Discrete iron hydroxide particles appear to form only
after a certain quantity of iron has been precipitated as a coating
on the clay surface, Berg (1960) and Fripiat and Gastuche (1952).
Attempts have been made to relate this phenomenon to the cation
exchange capacity of the clay. For example, Fripiat and Gastuche
(1952) found that the amount of iron oxide occurring as coatings
on kaolim'te was ten times the cation exchange capacity. However,
Berg (1960) demonstrated that the amount of iron precipitated as
coatings, before discrete oxide particles appeared, was about one
weight % and eight weight % iron for the 2 to 0.2 u kaolinite size
fractions, respectively; hence, the amount precipitated as coatings
was related to the surface area of the clay rather than to cation
exchange capacity. Fripiat and Gastuche (1952) observed that hydro-
gen (aluminum?) saturation of the kaolinite led to a definitely
147
-------
amorphous iron oxide precipitate such that iron oxide continued to
precipitate on the surface, whereas alkali or alkaline earth cation
saturation of the clay led to a compact, nonporous coating which
resulted in the latter portions of the iron precipitate forming dis-
crete particles. In addition to iron precipitation on clay surfaces,
colloidal iron hydroxide is readily sorbed onto clay-mineral surfaces,
Follett (1965) and Fripiat and Gastuche (1952)."
Jenne also presents two diagrams of Eh and pH effects on the occur-
rence of iron minerals under equilibrium conditions (Figures 6.9 and 6.10).
These lines are constructed by deriving equations, using Eh and pH, and
relating adjacent phases. The Eh-pH values at which the two phases are in
equilibrium are represented by the lines between the various phases. The
dotted area in Figures 6.9 and 6.10 represents the range of conditions to
which soils are exposed. The anoxic conditions most likely to occur in
soils under landfills would fall on the dotted area at pH 0 to 4 or Eh 0 to
-250 (mv). Eh values of the soil columns receiving leachate under anoxic
or partially anoxic conditions in the University of Arizona study reach
levels as low as -0 mv.
3+ 2+
Because of the possible shift between Fe and Fe in aqueous wastes
and in landfill leachates, the two equilibrium diagrams of Lindsay (1972a)
are reproduced as Figures 6.11 and 6.12. Figure 6.11 illustrates the
o i o •
relationship between Fe and Fe in equilibrium with hydrous Fe (III)
oxides and given 02 partial pressure using the equation:
Log K o
Fe(0h)3(s) Fe3+ + 30H - 39.40
Fe3+ + e- Fe""" 13.02
3H+ + 30H" 3H20 42.00
*5H0 e" + H+ + %0(g) - 20:77
Fe(OH)2(s) + 2H Fe"1"1" + %02(g) + 2^0 - 5.15
In Figure 6.12 the instability of some common Fe (II) minerals in
relation to some hydrous FE (III) oxides in soils is illustrated. Also
148
-------
+ 20-
+ 16 -
+ 12 -
UJ
Q.
- +1200
- +900
- +600
- +300
J
j=
LJ
300
- -600
-12 -
-16 -
-20 -
-24
--900
- -1200
12 14
2468
PH
Figure 6.9. Eh (pE) vs. pH stability field diagram from iron (ap
« •» ^ A. T OPO/»\ '"• • ' •**"
PCO =0i Ptot = 1 atm;T = 25C).
= 10'4M;
Jenne»
149
-------
+20 -
-+I200
LJ
Q.
'''** ^•!*t"!»!•!•!•?*!• !?""!•!•!•
-20 -
-24
- -1200
14
Figure 6.10. Eh (pE) vs. pH stability field diagram for iron (ape = 10" M;
Prn = 10"°'81, 10~2'7, and 10"3'5 atm; Ptnt = 1 atm; T = 25°C).
wU/i cuu
(From Jenne, 1968)
150
-------
Or-
o 8
E
£ 12
o
o»
o
16
20
24
Fe (ED) IN SOIL SOLUTION
Fe (OH)3 COLLOIDAL
—, j
PH
8
Fe(OH)3oq.
Figure 6.11. The Fe(III) species 1n solution In equilibrium with hydrous
ferric oxides. (From Lindsay, 1972)
151
-------
SOLUBILITY OF Fe MINERALS
Or-
8
o
E
ro
-------
according to Lindsay (1972a), "The Fe (II) silicates, carbonates, and
hydroxides are too soluble to persist in soil, [i.e., normal aerobic soils]
... They readily dissolve in soils and release Fe"1"1" which is oxidized and
precipitates as hydrous Fe (III) oxides. [The soil line in Figure 6.12
indicates its solubility.] Small changes in 02 and C02 partial pressure
cause only slight shifts in solubility of the Fe (II) compounds compared to
the very low solubility of hydrous Fe(HI) oxides."
In summary, it should be emphasized that iron is found in great abun-
dance in nature. It appears in most aqueous wastestreams, landfill leachates,
and solid wastes in recognizable amounts. In fact, it is a wonder that geo-
logic and recharge subsurface waters are so free of Fe.
The intense reactivity of Fe keeps it remarkably immobile in soils as
well as waters. The solubility of Fe in soil depends on the solubility of
J.A
the hydrous Fe(III) oxides. Under anoxic conditions the solubility of Fe
salts and complexes varies with the partial pressures of C02 and 02 in aque-
ous solutions. Slight variations can readily cause Fe to precipitate. Fur-
ther, it should be kept in mind the mobility of Fe is strongly pH-dependent
at any Eh level, as oxidation proceeds less slowly with increasing acidity.
Iron as hydrous oxides and oxides attracts a host of elements and is involved
in attenuation mechanisms for most trace and heavy metals. There appears to
be sufficient evidence to support the belief that Fe, Mn, and Al, as hydrous
oxides and oxides may be primarily responsible for attenuation or immobili-
zation of most of the potential hazardous-pollutants here considered.
LEAD
Lead as a "contaminant" of the human environment and as a potentially
hazardous pollutant has long been recognized. Only token abatement measures
have been initated to improve environmental contamination and these only
relate to the most obvious sources of pollution. Sources of Pb in the air,
waters, and soils have been reviewed by Lagerwerff (1972) as they relate to
biological systems and are therefore not discussed here. Suffice it to say
153
-------
That Pb appears in all areas of our environment (soils, waters, and air) as
a "contaminant"; lead in the environment increases in direct proportion to
increases in population. The obvious most serious pollution originates from
energy consumption, primarily fossil fuel utilization. Lead concentration
in soils is given in Table 6.6 as varying from 2 to 200 ppm (Allaway, 1968).
TABLE 6.6. HEAVY METAL CONTENT IN SOILS AND PLANTS*
Plants
Metal
Cadmi urn
Cobalt
Copper
Lead .
Manganese
Nickel
Zinc
Soils
Typical
ppm
0.06
8
20
10
850
40
50
Range
ppm
0.01-7
1-40
2-100
2-200
100-4000
10-1000
10-300
Common
Range
ppm
0.2-0.8
0.05-0.5
4-15
0.1-10
15-100
1
8-15
Toxic limits
(Using
recent work)
ppm
-
-
30
-
-
25
500
From Allaway (1968).
Lead, like many heavy metals, is present in soils as a cation which
precipitates easily as sulfide, hydroxide, and carbonate. Lead also is sub-
ject to surface adsorption, particularly on aluminum silicates. Comparing
Pb with some other metal cations, sorption occurs more strongly in the gen-
L I It 1 • ±1 XA
eral order of Cu > Pb > Ni > Co > Zn . There is a great affinity
between Pb and organic matter for retention or immobilization (attenuation)
against migration.
In an extensive study of soils and their properties in relation to Pb
availability to plants, John (1972) concluded that Pb availability is
related to soil pH, extractable Al, and total Ni (Table 6.7). He found no
observable relationship with soil organic matter. Plant availability
154
-------
•TABLE 6.7.
SELECTED SOIL PROPERTIES AND RESULTS OF SOIL AND PLANT
LEAD ANALYSES*
Location pH in
and soil water
order0 Soil type (PHW)
F
F
F
F
F
F
F
F
F
F
F
F
F
F
F
F
F
F
F
F
F
F
F
F
F
0
P
C
N
VII
VII
VII
VIII
VII
VIII
VII
VI
VI
VI
VII
V
VI
VII
V
V
VII
VII
V
V
V
VII
VI
VII
V
II
III
III
V
Benson si.l.
Delta si. c.l.
Ladner si. c.l.
Lulu muck
Vinod muck
Lumbum muck
Ann is muck
Fairfield si. c.l.
Grevell s.
Monroe si.l.
Buckerfield si.l.
Marble Hill s.l.
Sumas s.
Vedder si.l.
Abbotsford s.l.
Whatoom si.l.
Cloverdale si.l.
Lang ley 1.
Milner 1.
Nicholson 1.
Sunshine 1 .s
Hjorth 1.
Monroe si.l*
Page si.l.
Ryder v.f.s.l.
Glenmore si.c.
Alcan si.l.
Vanderhoof si. c.l.
Selwyn s.l.
Mean
Standard deviation
5.
7.
4.
5.
5.
4.
4.
5.
7.
5.
5.
5.
5.
6.
5.
5.
5.
5.
5.
5.
5.
5.
5.
5.
6.
7.
6.
6.
5.
5.
0.
3
2
9
1
0
4
5
4
2
7
7
7
6
2
2
7
6
5
7
7
5
0
7
8
5
3
1
5
0
'/
7
Organic Clay
matter fractic
(OMAT) (CLAY)
6
5
12
35
36
61
36
5
1
3
7
6
1
4
6
7
10
16
8
5
10
17
3
4
8
4
8
1
2
11
14
9
17
11
19
20
20
21
35
1
13
32
35
3
23
6
25
21
20
15
15
6
38
30
36
8
66
35
45
4
20
15
Lead
from
1
.*
O r—
O
21
102
1
13
13
13
10
10
6
7
10
8
4
10
8
36
6
2
10
8
8
9
8
7
7
14
7
7
6
13
18
extracted
soil with
N. HN03
•o
0
E
C -I- Q.
•-• "£. Q.
493
493
507
429
482
446
550
457
518
486
536
443
500
500
432
529
482
479
464
442
421
507
482
482
493
489
550
482
471
484
34
c
O-
C +J O
0 c E •»->
°*§>o
-O Q. 0)
to o ~o
-------
information is mentioned as a possibility for providing clues to Pb
mobility and attenuation in soils.
Because many wastes are applied to the soil under conditions conducive
to waterlogging and the creation of anoxic conditions, the studies by Kee
and Bloomfield (1962) should be pointed out. They found that naturally
occurring Fe, Co, Ni, Zn, Pb, V, and Mo in soils became more mobile when
soils were flooded and incubated anaerobically with organic residues (plant
debris). Chromium but not chromite was also mobilized.
Both hydroxy and chloride complexes appear to influence the mobility
of Hg (II), Cd (II), and Pb (II) in soils, according to Hahne and Kroontje
(1973). Hydrolysis of Pb (II) becomes important at pH values above 5.
Lead is in the form of hydroxy complexes at a pH value of 8.5 and Cl con-
centration of 350-60,000 ppm in contrast to Hg (II) and Ce (II) which are
complexed by Cl.
In aqueous wastes containing organic matter, organo-metal-complexes
may form. Certainly they form in soils containing organic matter and may
exist as soluble chelates or colloids; lead is no exception, Brown (1969),
Lindsay and Norvell (1969), Schnitzer (1969), Wallace (1963), and Stevenson
and Ardakani (1972). Some evidence is presented indicating that soil organic
matter chelates of Pb are of low solubility.
Soluble Pb, reaching the soil from aqueous wastestreams, is expected
to form compounds of low solubility readily with SOT, COZ, or P0*~ anions.
Lagerwerff (1972) suggests the following as means of Pb abatement:
"... liming reduces the uptake by plants from soil. Thiis may be due to
several reasons. The ensuing increase in soil pH may result in precipita-
tion of Pb as the hydroxide, carbonate, and phosphate. The Ca ions will
compete with the trace amounts of Pb present for exchange sites on the soil
XA
and root surfaces. The increased pH and-Ca activity may diminish the
physiological capability of plant roots to absorb Pb. Finally, liming may
promote the capacity of organic matter to form complexes with Pb."
156
-------
MERCURY
One of the most complete reviews on mercury in the environment is
that of D'ltri (1972). Because this review was supported by the Michigan
House of Representatives Great Lakes Contamination (Mercury) Committee,
the Data and examples provided in the manuscript are highly localized for
the Great Lakes area. This does not imply, however, that the basic princi-
ples of mercury behavior in the environment, as provided in this document,
cannot contribute to understanding other environments. Indeed, some of
the data may have world-wide application. Information from this book, which
is well documented and international in scope, is not repeated here. This
book is an excellent supplement to this manuscript for those who wish a
more thorough insight into the subject of mercury.
In addition to D'ltri's (1972) review, Jenne (1970) and the manuscript
of Jonasson and Boyle (1971) are suggested for reading.
The chemical behavior of Hg in the environment also has been reviewed
by Bondietti et al. (1973). Some of their comments are:
''The oxidation states (sic) of Hg, 0, +1, and +2, occur in
nature depending on the oxidizing conditions of the local environ-
ment. In aerated water and in nearly neutral solution HgO is
stable and the soluble inorganic mercury is in the +2 oxidation
state. Elemental mercury forms only when the activity of oxygen
nr\
is less than 10 atm (or reducing conditions). The solubility of
Hg° was recently estimated at 2.8 x 10"7M (^ 50 ppb), Spencer and
Voigt (1968).
"Mercuric ion, Hg2"1", hydrolyzes to give the species: Hg(OH ,
Hg(OH)2, Hg2(OH)3+ and Hg3(OH)^ and probably HgfOH)^ in very basic
solutions. ... Our reexamination of the data of Ahlberg (1962) in
relatively concentrated mercury solutions has led us to propose the
trimeric species to replace the two species Hg^OH)2"1" used by Ahlberg
(1962). Also Johansson's (1971) recent X-ray diffraction data on
157
-------
solutions indicate that the polymeric species contains two Hg
neighbors as would be expected for the trimer.
"The neutral dissolved species, Hg(OH)2, has exceptional sta-
bility and exists at pH values above 4 when other complexing
reactions do not compete. The most stable oxide phase at 25°C
is orthorhombic HgO. The red and yellow oxides are but differ-
ent particle sizes of this form and the minimum solubility of
the red HgO at 25° is 2.4 x 10 M. Obvi.ously, natural waters will
not be saturated with respect to this form if, indeed, they are
saturated with respect to any solid phase. The sulfide is very
insoluble and forms when appreciable levels of S~ are present. As
with other trace metals the interactions with the biota and the
exchange with sediments 'or dispersed minerals or clays are not
well understood.
"The methyl mercury ion, CHgHgCtUO) , hydrolyzes to form
CHgHgOH and the species (CH3Hg)2OH , at concentrations in excess of
10~ M (200 ppm as Hg). Above pH 6 methyl mercury exists as the
natural species, CHoHgOH, in the absence of other complexing agents,
+
•Schwarzenbac and Schellenberg (1965). The complexes of CH-Hg with
_ O
S~ and S-containing ligands are uniquely stable. Other alkyl
mercury ions have complexing properties similar to the methyl
derivative for OH~, Br~, and Cl~. The thermodynamic stability of
the CHoHg ion toward decomposition to methane is not known, but
it is presumed to be unstable. The kinetics of this decomposition
has not been studied, although CH3Hg appears to be stable for long
periods at ordinary temperatures."
Mercury is present in the outer earth's crust, in most common ores,
and gangue materials ranging from about 0.01 to several hundred ppm. Mer-
cury also is present in soils at lesser levels, probably between 20-150 ppb
with a mean value of about 70 ppb (Table 6.8). Airborne Hg deposits itself
in the surface of soils along with the natural sources present. The soil
organic matter attenuates Hg to the extent that Hg concentrates in or on
soil surfaces. In natural water sources, free of Cl, Hg° predominates as
158
-------
*
TABLE 6.8. MERCURY CONTENT OF SOILS AND GLACIAL MATERIALS
Description Range Mean
"~ ppbppb"
Normal soils 20 - 150 70
Normal tills, glacial clay, sand, etc. 20-100 50
Soils, tills, etc., near mercury deposits,
sulfide deposits, etc. up to 250 ppm
Soil horizons (normal) : A (humic) 60 - 200 161
B 30 - 140 89
C 25-150 96
Soil horizons (near mercury deposits) :
A (humic)
B
C
200 - 1 ,860
140 - 605
150 - 554
480
475
263
From Jonasson and Boyle (1971).
Examples from Clyde Forks area, Ontario.
a species. Concentration values of Hg in these waters, with only a few
exceptions, fall below 5.0 ppb, the threshold value for acceptability as
drinking water, Lagerwerff (1972).
Mechanisms of attenuation (immobility) in soils include (a) valence-
type ionic adsorption by organic and inorganic materials (b) formation of
covalent bonds with organic compounds, (c) formation of low solubility Hg
salts of phosphate, carbonate, and sulfide.
Transformations in Soils
A fairly large volume of literature is available specifically con-
cerned with mercury in soils. There is also a great deal more information
that may be related to soils, but, unfortunately, there is no agreement on
the fate and movement of mercury in the soil environment. There is no doubt
159
-------
that inorganic mercury in sediments, lake bottoms, etc., is methylated to
mono- or di-methyl mercury, Wood et al. (1968), Jenson and Jernelov (1969).
Wood et al. (1968) have shown that Methanobacterium omelianskii, which forms
an integral part of a sludge ecosystem, methylates mercury through one of
two processes:
(1) Non-enzymatic
CH.
Methylcobaltamine Co + Hg
»-Aquacobaltamine Co + QU— HgX
methyl
mercury
(2) Enzymatic
CH
^CH3Hg"1" + CH4
Ho
Hg
metnane
enzyme
(CH3)2Hg + 2CoH
In the non-enzymatic process, the methyl group is transferred as an an ion
from Co to Hg . Methylcobaltamine is a common co-enzyme in both anaer-
obic and aerobic bacteria. Thus, significant quantities of methyl-B^
derivatives are present in sludge promoting this transfer.
For the enzymatic process, there is first the chemical reduction of
Hg to Hg°. Then, depending on the concentration of Hg° and pH, dimethyl
and/or monomethyl mercury are formed as products.
This allows loss of Hg from the systems in several ways. As will be
seen, all of the products (including Hg°) are volatile, water-soluble,
or both.
Landner (1971) has presented evidence that Neurospora crassa can
methylate mercury aerobically without methylcobaltamine. Furthermore,
Komura et al. (1970) state that types of coli bacteria can reduce Hg to
160
-------
the volatile metal. Escherichia coli is an example of a bacterium derived
from fecal sources and apt to be found in pollutant systems.
Hem (1970) presents an Eh-pH diagram demonstrating that Hg° is slightly
soluble. This characteristic is used in the flameless atomic absorption
technique for the analysis of mercury. The technique depends upon the high
vapor pressure of metallic mercury. Thus, if Hg° appears in a solution
flushed with a gas, the volatile metal is removed. This occurred when land-
fill leachate in the Arizona lab was flushed continuously with C02. Thus,
any organic transformation or chemical reduction of Hg+* inorganic ions can
lead to a Hg loss from the leachate, Niebla et al. (1976, in press).
Lagerwerff (1972) states that the ionic monomethyl compound is solu-
ble in H20 and complexes with anions. The molecular dimethyl compound is
relatively insoluble in HgO and escapes as a vapor. The formation of mono-
methyl mercury apparently requires acidic conditions and high concentra-
tions of Hg . /mis^ftidicates Hg loss due to volatilization of Hg° or
dimethyl mercury. It has also been reported that Pseudomonas vaporizes Hg
and removes it from wastes, Suzuki et al. (1969). Kimura and Miller (1964)
state that monomethyl mercury compounds are by themselves volatile, pre-
senting another mechanism of mercury loss. This is in contrast to what
Lagerwerff reported. It seems likely that monomethyl compounds are stable
in water because of the high concentrations of methyl mercury found in
fish taken from water polluted with mercury.
Some very interesting results have been reported by Frear and Dills
(1967) regarding the use of inorganic mercury salts as insecticides. They
discovered that mercury vapor was the lethal agent in the insecticide.
They mixed mercuric salts with soils to determine the effects of organic
matter, temperature, soil pH, and moisture content. If a soil was ignited
to remove organic matter, no mercury vapor was produced -- no insects were
killed. They also found that low moisture content inhibits reduction.
Moisture content of 17% and greater gave rapid reduction. Mercuric
reduction also increased with increasing soil pH. Soil pH of 5.25 pro-
vided little or no reduction. Furthermore, the maximum rate of
161
-------
transformation was attained at 25°C with reduction being limited at
temperatures less than 10°C, Insect eggs were killed when suspended over
.* i-- _X
v^" a pool of mercury vapor, indicating that elemental mercury was coming off
fcks the soil. Although the experiments were not analytical in nature, they
serve to point out that mercury escapes very easily from soils.
Despite the fact that most landfills are located in inorganic subsoils
and inorganic geologic materials, organic matter still is a key factor in
Hg mobility through soils, Niebla et al. (1976, in press). Landfill leach-
ates are abundantly supplied with organic constituents, acids, and hetero-
geneous organic debris at various stages of degradation and subparticle
division. These may exist as soluble or colloidal forms and therefore differ
in rates of migration, Hahne and Kroontje (1973). Though Hg in aerobic
(oxic) soils may persist in any form, under anaerobic, anoxic, waterlogged
and water-saturated conditions, at least some Hg will be converted by anaer-
obic bacteria into soluble monomethyl or gaseous dimethyl Hg complexes. It
should be pointed out that in the case of Hg it is well known that alkyl
mercury compounds such as methyl mercury are far more toxic than ordinary
Hg salts. On the other hand, if mercury complexes with larger functional
groups, such as aryl or alkoxyalkyl compounds, toxicity is less (Lagerwerff
1972).
2+ 2+
Reactions such as : Hg2 = Hg + Hg° occur readily in soils.
Soil-mercury reactions, as pointed out by the voluminous literature
on the subject, seem to be dominated by mercury-organic interactions, Baes
and Mesmer (1973).
Free Hg or Hg° produced under anaerobic conditions becomes available,
however, for conversion to HgS. Under certain alkaline conditions, HgS may
be converted into Hg2S, which is soluble.
Anionic HgCU and HgCl?" and molecular Hg2Cl2 and HgCl2 may adsorb to
particle coatings of hydrous oxides of Fe and Mn, Jenne (1968).
162
-------
Movement and Retention in Soils
Much work on mercury has been undertaken by Anderson et al. (1961).
They stated that under acidic aerobic conditions with sufficient chloride
present, the predominant Hg species in soil solution is HgCl|. While this
species is only slightly adsorbed, elemental mercury is strongly adsorbed
by Fe-oxides and also scavenged by organic matter. The latter process
occurs most readily'at low pH. As the pH increases Hg is more readily ad-
sorbed on soil minerals, Anderson (1967).
Aomine et al. (1967) report that Hg is adsorbed most on allophanic
soils. Allophane is an amorphous mineral which forms under conditions of
medium to high hydromium ion concentrations in highly moist systems by rapid
weathering of non-crystalline volcanic ash or rapid weathering of feldspars.
Allophane can be described as an amorphous alumino-silicate with which abun-
dant organic matter is complexed in the upper layers, Buol, Hole, and
McCracken (1973). Possibly adsorption of Hg by allophane is a further mani-
festation of the role of organic matter.
Strohal and Makhonia (1971) found that competing cations had no effect
on the release of organically complexed mercury. They extracted soil with
H20 and with H«0 containing varying amounts of leaf extracts. As the amount
of organic matter increased, the amount of mercury released from the soil
increased. This is direct evidence that mercury is more mobile in soil if
it is organically complexed. "Z
Several studies have been made of the fungicide, phenylmercuric acetate
(PMA). Kimura and Miller (1964) state that PMA degraded to Hg° in the soil
and was lost as Hg vapor. They also reported that the tendency of Hg to
escape increased at low soil moisture. This contradicts Frear and Dills'
(1967) finding that inorganic mercury was reduced most easily at high soil
moisture. Another anomalous finding is that air drying soil samples results
in no Hg loss, Hamm and Stewart (1973). It should be noted that this experi-
ment dealt with "naturally abundant" mercury. Aomine et al. (1967) and
163
-------
Inoue et a1.(1969) have worked extensively with PMA. They found that it
did not migrate down into the soil upon which it was applied. In each case
(except with a sandy, very porous soil), no Hg or PMA was found in sub-
surface horizons. There was no way to tell from the work how much mercury
was lost by volatilization.
It is rather difficult to summarize findings such as these. The only
safe assumption is that mercury applied to the environment will not remain
in the same state as applied.
There is ample evidence that mercury applied directly to soils is
volatilized, probably as Hg°. Mercury found in a strictly reducing environ-
ment, such as in a sludge or in a landfill, is alkylated. There is appar-
ently no available data on the amount of methyl mercury in soils. It
appears that from the landfill standpoint, this is the most important
species. Any Hg° or dimethyl mercury formed will be expelled as a gas.
Inorganic mercury can be precipitated as a sulfide and probably does not
migrate greatly. However, in any system containing ample organic matter,
the mercury can be complexed and transformed in a variety of ways. Of the
metals being considered in the landfill project, mercury is clearly the
most complicated.
For the most part, Hg attenuation or movement in soils must relate to
organic carbon or sulfur chemistry either separately or together as humic
substances containing S. Some biologically oriented cycles have been pre-
pared by Jonasson and Boyle (1971) and are reproduced here as Figures 6.13-
6.15.
SELENIUM
A number of good references exist on the subject of selenium mineral-
ogy, and one of particular interest is by N.O. Sindeeva (1964).
Selenium is widely distributed over the earth's surface crust as
selenites and selenates of sodium and calcium. Some Se also occurs as
164
-------
cn
MERCURIC ION,
CHELATED CATIONS 8 AN IONS
SIMPLE COMPLEXES,
OXIDES SULPHIDES
REACTIONS
ELEMENTAL MERCURY
AS VAPOUR LIQUID Hg (0)
OR DISSOLUTE-
BACTERIAL OXIDATION
PLANKTON
PLANTS
INORGANIC
Hg(ll)
BACTERIAL REDUCTION
FUNGI
PLANTS
INORGANIC REACTIONS
SUNLIGHT
BACTERIAL SYNTHESIS
CHELATION
BACTERIA_
SUNLIGHT
BACTERIAL OXIDATION
PLANTS
INORGANIC REACTIONS
DISPROPORTIONATE AND
ELECTRON EXCHANGE
.BACTERIAL REDUCTION
FUNGI
PLANTS
INORGANIC
REACTIONS
t
BACTERIA,
CONVERSION BY
ORGANIC OXIDANTS
ORGANO-MERCURY
COMPOUNDS
R,R' = ALKYL,ARYL,
MERCAPTO,
PROTEIN, etc.
X. - MONOVALENT ANION
EG. HAL IDE, ACETATE,
etc.
Hg(l)
MERCUROUS ION,
CHELATED CATIONS ANIONS,
SIMPLE COMPLEXES
BACTERIAL SYNTHESIS
CHELATION
ORGANIC OXIDANTS
2Hg'l = Hg(0)
Figure 6.13. The cycle of mercury interconversions in nature. (From Jonasson and Boyle, 1971)
-------
H
CHEMICAL
TRANSFER
CYCLE
THF
ENZYMATIC
REGENERATION
CYCLE
CH3Hg'
CH3COOH
NADPH
N3CH3THF B
NADP
Figure 6.14. An anaerobic biological synthesis mechanism for methyl mercury. (From Wood et al., 1968)
-------
DEGRADATION
ATMOSPHERE
BIOSPHERE
PLANTS = ANIMALS
T
DEGRADATION
DEGRADATION
AND
SOLUTION
ABSORPTION
AND
ADSORPTION
EXHALATION
PRECIPITATION
HYDROSPHERE
WATER =T SEDIMENTS
T
PRECIPITATION
SOLUTION
PEDOSPHERE
SOILS
GLACIAL MATERIALS
CHEMICAL PRECIPITATION
AND SEDIMENTATION
OF SOLIDS
EXHALATION
CHEMICAL PRECIPITATION
1
SOLUTION AND
MECHANICAL
WEATHERING
LITHOSPHERE
ROCKS
MERCURY DEPOSITS
VOLCANIC PHENOMENA
SOLUTION AND
MECHANICAL
WEATHERING
PRECIPITATION AND
CONSOLIDATION OF SOLIDS
Figure 6.15. Generalized geochemistry cycle in mercury. (From Jonasson and Boyle,
1971.)
-------
sparsely soluble basic salts of Fe. The presence of organic Se-containing
compounds has been established in soils. According to Sindeeva (1964) the
process of oxidation and reduction may be repeated several times under the
action of soil microorganisms and atmospheric agents.
Selenium is used in the glass industry, electrical engineering, tele-
vision, steel, rubber, photography and numerous other branches of industry.
Selenium is closely related to sulfur both chemically and biologically.
The latter because it can substitute for S in biological assimilation and
in organic molecule substitution. Also like sulfur, it has six electrons
in the outer shell, making it metalloid. Upon the addition of two more
electrons, selenium is transformed into negative bivalent ions, in which
form it combines with metals and forms selenides. In combination with Opt
the valencies of 4 and 6 are expressed. Like sulfur, selenium forms di-
oxides and trioxides and the corresponding acids — selenious and selenic.
The salts of these acids form similarly to sulfurous and sulfuric acid.
Metals of Cu, Pb, Ni, and others in soils" form from the action of selenious
acids. These are stable in soil. Salts of selenic acid rarely appear in
nature. If they form, they readily reduce to selenites and/or to free selen-
ium, Sherilla and Isard (1928).
The equilibrium states of Se are diagrammed by Delahay, Pourbiax, and
Van Kyselberghe (1952) and are reproduced as Figure 6.16. The diagram
shows pH values 0 to 14 plotted in relation, to Eh[(V) - 0.55 to + 1.25]
at + 25°, 1 atm. pressure, and concentration of Se at 10~ moles. The two
lines having dashes are included to point out assumed boundaries of normal
earth surface conditions. Selenium is highly pH sensitive. Lakin (1961)
and Lakin and Trites (1958), who have studied Se in soils, conclude that the
ferric hydroxide in acidic or neutral soils provides an important mechanism
of precipitation which not only attenuates Se but substantially reduces up-
take by plants. In some soils, reduction of Se to elemental Se also pro-
vides an effective mechanism of attenuation. Selenate is the available
form of Se in alkaline soils, Brown and Carter (1969). Less selenium is
available to plants at lower pH, indicating decreased mobility. Preliminary
168
-------
LU
+ 1.2 -
+ 1.0 -
+0.8 -
1-0.6
+ 0.4 -*
+0.2
ASSUMED BOUNDARY
OF NORMAL
SURFACE CONDITIONS -I
-0.2 -
-0.4 -
-0.6 -
PH
Figure 6.16. Stable fields of selenium. (Data from Delahaye, Pourbaix,
Van Russelberghe, diagrammatically arranged by Coleman.)
169
-------
data from the University of Arizona study support the finding that, other
factors being equal, selenium is less mobile in acidic than in neutral to
alkaline soils.
Geering et al. (1968) furnish evidence that Se concentration in the
solution of seven soils is governed primarily by ferric oxide-selenite-
adsorption complex (Se oxidation state +4). They show Se can also exist in
the oxidation states of +6, 0, and -2. They approached the problem of
identifying the selenites that may exist in a soil by obtaining values for
the activities of selenite, ferric and hydrogen ions in soil solution
extracts and referring these values to a solubility diagram of the ferric
selenites (Figure 6.17). The selenious acid potential (pH + pH Se03) is
plotted against the ferric hydroxide potential (pH - 1/3 pFe). They have
this to say about the data plotted in Figure 6.17:
"Had the points fallen along one of the ferric selenite lines,
this would indicate that a crystalline ferric selenite could exist
in the soil. Had the points fallen in the region below the ferric
selenite lines and to the left of the ferric hydroxide lines, this
would indicate that a crystalline ferric selenite could exist in
the soil. Had the points fallen in the region below the ferric
selenite lines and to the left of the ferric selenite lines, this
would indicate that the selenite Se in these soils may be present
in less soluble forms than the ferric selenites considered here.
Less soluble forms of selenite could arise through the formation of
(i) less soluble crystalline ferric selenites, and (ii) anomalous
solid solutions of the ferric selenites in ferric hydroxide.
"For the most part, the experimental points fell along the
ferric hydroxide line and below the ferric selenite lines. This
indicates that crystalline ferric selenites are probably not govern-
ing the observed Se solubility in these soils. If part of the
selenium measured in the soil solution extracts were in some form
other than selenite, the experimental points [...] would again be
located along the ferric hydroxide line but fall still further below
170
-------
ro
O
a*
CO
X
Q.
I
O.
8
10
12
14
16
18
O MADRAS SANDY LOAM, ph 6.38
© CECIL SILT LOAM, ph 6.52
® DALTON SILT LOAM, ph 7.08
A LAKELAND SAND, ph 4.82
> CARRINGTON SILT LOAM, ph 6.72
* WOODBURN SILT LOAM, ph 6.65
O MARDIN SILT LOAM , ph 5.90
O MARDIN SILT LOAM, ph 7.02
Fe2(OH)4Se03
Fe
A
(OH)3
-------
the ferric selenite lines. Thus, the evidence would be even stronger
that the solubility of Se in these soils is not controlled by the
crystalline ferric selenites indicated [...].
"The points that fell to the right of the ferric hydroxide line
3+
were due to the fact that the Fe ion activity values estimated in
the soil extracts were higher than those predicted by the activity
product of ferric hydroxide. This may have resulted from the com-
3+
plexing of Fe ions by soluble ligands in the soil, or simply from
the possibility that the soils extracts were supersaturated with
respect to ferric iron. In such instances, the method used to cal-
3+
culate the Fe activity from total iron measured in solution was not
appropriate.
"Additional evidence [in Table 6.10] appears to support the con-
tention that selenite may form adsorption complexes with ferric
oxides in soils rather than a crystalline ferric selenite. Appar-
ently, anions which tend to dissolve ferric precipitates through
complexing of ferric ion, such as EDTA and citrate, are less effec-
tive in solubilizing radioactive selenite in soils than are anions
such as 'cold selenite', arsenate, arsenite and phosphate, which
tend to form adsorption complexes and/or precipitates with ferric
ion. The mechanism by which these anions are effective may involve
selenite replacement, which would be greater if selenite ions were
surface-adsorbed than if they were in crystal lattice of a pure com-
pound." (See Tables 6.9 and 6.10).
2_
For reactions of Se in soils at other valencies (i.e., SeO, , Se° or
2—
Se ) than 4+, Geering et al. (1968) compared Eh as a function of pH for
some redox couples that may influence Se oxidative states fend mobility) in
soils. They plotted the Eh dividing line between oxidized and reduced
soils, according to Pearsall (1938), as a dashed line. Their comments
regarding Figure 6.18 are as follows:
"Above this [dashed] line are drawn the redox couples for
» Mn02/Mn2+, and Seo£~/Se042~. Intersecting this line are
172
-------
75
TABLE 6.9. MEASURED VALUES OF Eh, pH and TOTAL Xt)Se, Fe CONCENTRATIONS
OF A 1:10 SOIL-0.01 M Ca(N03)2 EXTRACT FOR ESTIMATING pH+
pHSe03 and pH-1/3 pFe*
Soil
Madras sandy loam
Cecil sandy clay
Dalton silt loam
Lakeland sand
Carrington silt loam
Woodburn silt loam
Mardin silt loam
Mardin silt loam
DH
6.38
6.52
7.08
4.82
6.72
6.65
5.90
7.02
Eh
mv
600
590
575
665
570
590
605
565
(75Se) total
M x 108
9.8
12.4
3.2
5.7
8.9
8.6
2.1
4.2
(Fe) total
M x 108
9.8
1.0
15.0
15.0
7.6
79.0
8.5
106
From Geering et al. (1968).
TABLE 6.10. PERCENT 75Se EXTRACTED FROM 0.8-g SUBSAMPLES OF WOODBURN si BY
8 ml OF 0.1 N.ANION SOLUTIONS IN 1 M KN03 DURING 24 HOURS
AT 25C*
Anion
Selenite
Fluoride
Arsenate
Arsenite
Phosphate
Citrate
Molybdate
EDTA
Sulfate
Nitrate
Chloride
Source
H2Se03
NH4HF2
As2°3
As2°3
KH2P04
H3C6H5°7'H2°
Mo03
EDTA
K2S04
KN03
KC1
% 75Se Extracted
33.1
26.6
20.0
19.2
18.2
13.5
8.1
7.3
6.3
6.3
5.5
From Geering et al. (1968).
173
-------
CO
+—
"o
Figure 6.18,
1.20
1.00 -
0.80 -
0.60
0.40
0.20 -.
iij 0.00 -
-0.20 -
-0.40 -
'Cu2Se(Cu*=IOHO) and
CuSe (Cu**= IO'9)
-0.60
The oxidation-reduction potentials of selenium couples and
some redox couples that could affect oxidation state of selen-
ium in soils as a function of pH. (Dashed line is Pearsall's
(1938) dividing line between oxidized and reduced soils.) (From
Geering et al., 1968.)
174
-------
drawn the Se°/Cu2Se and CuSe, H+/ H2, and the Se°/H2Se couples.
The Eh values of the respective couples were calculated from the
standard oxidation potentials given by Latimer [(1952)] and the
dissociation constants [Sillen and Martell (1964)] of H2Se04> H£Se03
and H2SeQaq and the solubility products of Cu2Se and CuSe [Buketov
et al. (1964)]
"The activities of the various ion species in solution were
selected from values reported in the literature. The values for
2+ 2+
Mn and Fe are maximum values reported by Leeper, while the
activity for Se ions is an average, selected by Lakin (1961). The
Cu activity value is based on soil solution data reported by
Hodgson and Geering, and the Cu activity is a calculated value to
allow the Se°/Cu2Se couple to be represented by the same line as the
Se°/CuSe couple.
"[...] one can see the regions of Eh and pH where one might
expect to find one or another valence state of Se existing in the
soil solution. In the region between the lines drawn for the 0«/HoO
2-2-
couple and the SeO^ /Se03 couple, one might expect essentially all
of the Se in solution to be in the selenate form. Between the
2- 2- 2- 2-
SeOj /Se03 and SeO^ /Se° lines, SeOg would be expected. Between
the SeOgSe0 and the H /H2 lines, only elemental Se and possibly some
heavy metal selenides would be expected. No hLSe by itself would
be expected to exist in soils, except either in a metastable state
or in concentrations very much less than 10" M, inasmuch as this
redox couple falls even below the H /H2 couple.
"While the redox potential of a soil may indicate what to expect
regarding the valence state of Se in a soil, it yields little
information about the kinetics or rates of conversion from one
oxidation state to another. Experiments and observations with pure
systems in the laboratory indicate that the rates of transformation
of SeO?" to SeO?" and vice versa are relatively sluggish [Rosenfeld
o 4 2-
and Beath (1964)], whereas the rate of transformation of Se03 to
Se° proceeds very readily. On the other hand, the oxidation of Se°
to SeO*" is very difficult to effect, unless Se° is finely dispersed
175
-------
[Schulek and Koros (I960)]."
The paper by Geering et al. (1968) provides an excellent review and
background of the solubility and redox criteria for the possible forms of
selenium in soils and possible, mechanisms of attenuation of Se as defined
in the manuscript. Another paper concerning Se in soils is that of Brown
and Carter (1969), who studied the leaching of added Se from alkaline soils
as influenced by sulfate. Their leaching solutions for soil columns were
water and gypsum solution.
ZINC
Zinc occurs in abundance in the earth's crust and exists only in the
+2 oxidation state. Unlike cadmium, mercury, and lead, it is essential to
most biological systems, including those of human beings. Also, unlike Cd,
Hg, and Pb, it is not toxic in trace amounts. A few organisms, however,
find it toxic at low levels. Since there is little tendency for Zn polymer
formation, its hydrolysis chemistry is relatively simple. Such species as
+ 3+ 2—
Zn(OH) , Zn(OH) are common. In very basic solutions zincates, Zn(OH) ~),
appear, Baes and Mesmer (1973).
Zinc oxide, ZnO, Zn(OH)9(aq), and Zn(OH)Z stabilities are not precisely
L. O
established. ZnO, though is the stable phase of zinc. Davies and Staveley
(1972) and Schindler et al. (1964) have stablished the solubility of the
most stable of the zinc hydroxides E-Zn(OH)p.
2+
Zinc (Zn ) forms slowly soluble precipitates with carbonate, sulfide,
silicate and phosphate ions. Hem (1970) and Stumm and Morgan (1970) have
stressed the immobilization of the Zn silicates and carbonates in waters
and soils. In waters which are carbonated (C02) Schindler et al. (1964)
have reported the occurrence of ZnOHCO. and
Benson (1966), who studied the mobility of Zn in eight Washington
soils, found that Zn is retained well in the upper part of the 2 cm x 75 cm
soil columns when leached with the equivalent of 2 feet of water after
176
-------
applying Zn at a rate of 0.2 g Zn/cm2. Six of the 8 soils were fine sandy
loams; one was a fine sand, and one was a loam. Clay content was low.
Applications of Zn salts to the soils resulted in a high Zn concentration in
the upper few centimeters (5-10 cm). Zinc and K were shown to be competi-
tive for exchange positions. Mobility of Zn in these soils may be expected
because of their coarse sandy texture, generally low organic matter, and
low clay content. Zinc was precipitated at or near the surface in the soil
containing "free" calcium carbonate or lime. The same was true for the soil
highest in organic matter.
Zinc may be retained in soils in excess to the exchange capacity.
Bingham et al. (1964) explain the immobilization mechanism in terms of pre-
cipitation of Zn(OH)? rather than by assuming the formation and adsorption
^ i
of complex ions Zn(OH) . On the other hand, Tiller et al. (1962) suggest
that the linear relationship they found in a great number of soil-water
systems between the quality/intensity ratio of Zn and the soil pH could be
accounted for only on a basis of the predominance of Zn(OH) .
Only a part of the Zn adsorbed to soils can be removed with neutral
salt. Jones et al. (1936) and Hibbard (1940), studying Zn adsorption in
Florida and California soils, respectively, found that only acid extrac-
tion can remove this tightly bound Zn in soils. Perhaps there is a highly
specific adsorption or complexing mechanism in soils for Zn. In a review
of heavy metals and radio-nuclides in soils, Tiffin et al. (1973) makes the
following statement concerning Zn retention:
"... a significant portion of Zn on montmorillonite, kaolinite and
four soils was not readily extractable against N_NH^-acetate, but
it could be removed with acid. The smaller the amount of Zn that
was added to the soil or clay, the larger was the fraction that
was only exchangeable by acid, Nelson and Melsted (1955). The same
was found for Cu, Peach (194). In both cases, the nonremovable
fraction varied in size with pR and increased in size with time,
Brown (1950). Clark and Graham (1968) observed a strongly increasing
177
-------
diffusivity with increasing Zn concentration in sand, loams and
clays at pH 5.0. They ascribed this partly to the greater fraction
of Zn that is specifically adsorbed at the lower concentrations.
From the relationship between diffusivity and distribution coef-
ficient they concluded that some of the adsorbed Zn diffused at a
significant rate by surface migration, Clark and Graham (1968).
"The rate of immobilization of Zn because of adsorption by
montmorillonite followed first-order kinetics, Nelson and Melsted
(1955). Zinc was found to be more strongly bound by montmorillon-
ite than by attapulgite, and more Zn was available to plants from
attapulgite containing high-pH rendzina soils than from soils con-
taining montmorillonite, Navrot and Gal (1971). In comparison of
calcite and magnesite in soil, Zn showed greater affinity for mag-
nesite, in agreement with calculated ion geometry, Jurinak and Bauer
(1956). Immobilization of Co and Zn by clay mineral adsorption
followed a Freundlich type isotherm, with reactivity decreasing in
the order hectorite > vermiculite > montmorillonite > holloysite >
kaolinite for underground material, and muscovite talc > biotite =
vermiculite > pyrophyllite for ground material, Tiller and Hodgson
(1962). The particle size distribution of primary and secondary
clay minerals have important effects on the mobility of trace
metals in soil."
Zinc carbonate (ZnC03) and Zn(OH)2 are reported by Norvell and Lindsay
(1969) to be ^ 10 times more soluble than the Zn-soil complex responsible
for fixing Zn in five soils. Furthermore, the solubility of Zn species in
2+
equilibrium with soil Zn (Figures 6.19 and 6.20) is seen to be Zn below
pH 7.7, and above this pH the neutral species Zn(OH)2(aq) predominates.
The solubility of Zn3(PO,)2 is compared to ZnC03, Zn(OH)2, and soil Zn in
Figure 6.21. This seems to indicate phosphate may not be the element that
is particularly significant in inducing Zn deficiency in soils as has often
been suggested.
178
-------
SOLUBILITY OF Zn MINERALS
N
£ 8
10
12
0.0003 atm C02
0.01 atm COg
8
4567
. pH
Figure 6.19. The solubility of various Zn minerals compared to the solubil
ity of Zn in soils. (From Lindsay, 1972.)
179
-------
o
E
I-
o
<
o
I
8
10
12
14
Zn SOLUBILITY IN SOILS
a =6.5ppm
b = 0.007 ppb
Zn (OH)"1"
Zn(OH)2(aq)
pH
8
Figure 6.20. Soluble Zn species in solution 1n equilibrium with soil Zn.
(From Lindsay, 1972.)
180
-------
SOLUBILITY OF Zn COMPOUNDS
o 4
E
CJ
c.
ti 6
en
o
8
10
ZnC03
Zn(OH)2
Zn
C02 =0.0003 otm
P = IO'4 M
8
pH
Figure 6.21. The solubility of Zn3(P04)2 compared to that of other Zn
minerals and soil Zn. (From Lindsay, 1972.)
181
-------
SECTION VII
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218
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APPENDIX A
POLLUTANT ATTENUATION RESEARCH AT THE UNIVERSITY OF ARIZONA1
In a research project, directed by W.H. Fuller at The University of
Arizona, the factors which attenuate contaminants (limit contaminant trans-
port) in leachate from municipal solid waste landfills were examined.
Although the work was strongly oriented toward problems with disposal of
strictly municipal wastes, the impact of co-disposal of municipal and haz-
ardous wastes was also considered. This project strongly emphasized the
influence of soil and contaminant properties on migration.
The project was concerned with contaminants normally present in
leachates from municipal solid waste landfills and with contaminants that
are introduced or increased in concentration by co-disposal of hazardous
wastes. These contaminants are: arsenic, beryllium, cadmium, chromium,
copper, cyanide, iron, mercury, lead, nickel, selenium, vanadium, and zinc.
The objectives of the project were to identify:
1. Soil factors that exert the greatest influence on contaminant
migration.
2. Intrinsic mobility of contaminants and their properties which
influence migration (e.g., oxidation state, compounds).
Eleven soils representative of 7 major orders were collected through-
out the U.S. at depths to avoid organic matter in surface layers which would
not be typical of soils below fills. They range in pH from 4.2 for the
Ultisol,Wagram loamy sand, to pH 7.8 for the alkaline Aridisols, Anthony
"Investigation of Leachate Pollutant Attenuation in Soils," Contract 68-03-
0208, Solid and Hazardous Waste Research Division, U.S. EPA Municipal
Environmental Research Laboratory, Cincinnati, Ohio 45268.
219
-------
sandy loam and MohaveCa clay loam. The clay ranges from 3 to 61% and cation
exchange capacity (CEC) from 2 to 37 meq/lOOg. Two Mohave soils were
included (because the presence of lime varies with depth), providing an
opportunity to compare two genetically similar soils, one with and the other
without lime. Physical and chemical characteristics of the soils are sum-
marized in Table A-l.
The simulated municipal landfill leachate used in the study was gen-
erated in a 3,800 liter septic tank (Figure A-l) filled with representative
municipal refuse (Table A-2). To insure uniformity, a pump was connected to
the outlet to recirculate leachate back through the solid waste fill. Clear
leachate was withdrawn from the bottom of the tank under high C02 pressure
and in the exclusion of air (Korte et al., 1976). The characteristics of
the leachates used in this research are shown in Table A-3.
The leachate was analyzed repeatedly until concentrations of most
constituents reached near steady state. Atomic adsorption spectrometry was
used to analyze for Fe, Mn, Zn, Cu, Ni, Cr, Pb, Co, Cd, Al, and Mg. Na, K,
and Ca were measured by flame emission. Si, Cl, N03, NH^, P04, and COD were
measured by standard colorimetric procedures. The pH was measured in the
field at the tank outlet and electrical conductivity (for dissolved solid
evaluation) was measured in the laboratory. The composition of the solution
is within the range for natural landfill leachates (Garland and Mosher,
1974).
Since no industrial wastes were included in the tank, the content of
trace elements is low relative to that of mixed municipal and industrial
wastes. This made it advantageous to "spike" or add to the leachate the
specific trace element needed for study of migration characteristics.
Where spiked leachates were used, the pH was adjusted with HC1 to
5.0, to facilitate handling and retard precipitation of dissolved ions, then
spiked with the element of interest to a concentration of 70-120 ppm. These
concentrations exceed that which might be expected for strictly municipal
landfill leachates but resemble what might be found in mixed municipal-
220
-------
TABLE A-l. SOME CHARACTERISTICS OF THE SOILS USED IN RESEARCH AT THE UNIVERSITY OF ARIZONA
CO
-^
p^»
(D
Wag ram
Ava
Kalkaska
Davidson
Molokai
Chalmers
Nicholson
Fanno
Mohave
Mohavera
La
Anthony
0
^^_
0)
-5
-t-
Ultisol
Alfisol
Spodosol
Ultisol
Oxisol
Mollisol
Alfisol
Alfisol
Aridisol
Aridisol
Entisol
CO
o
_J
•o
DC "0
Q)
in
rt-
(D
4.2
4.5
4.7
6.2
6.2
6.6
6.7
7.0
7.3
7.8
7.8
o o
0* CD 0>
"O X c+
a» o -••
03-0
-*• CU 3
ft 3
*< tfl*
CD
meq/
IQOg
2
19
10
9
14
26
37
33
10
12
6
r>
o o m
-t> 3 — •
rt (D
n> c o
X O r+
c* rf -J
D> < n
o -*• o>
C^" K™^1 "^
V£
yhos/
cm
225
157
237
169
1262
288
176
392
615
510
328
o
^^J
Q.C
ft) 3
3 3
(/)
-i. cr
^T ^K
V^ ^_J
7^"
g/cma
1.89
1.45
1.53
1.89
1.44
1.60
1.53
1.48
1.78
1.54
2.07
CO
c
^> — fl
^ 0*
(D O
01 (D
mz/g
8.0
61.5
8.9
51.3
67.3
125.6
120.5
122.1
38.3
127.5
19.8
o 7
X fl>
-•• n>
o.
(D I-H
c/> ~^
0
3
%
0.6
4.0
1.8
17.0
23.0
3.1
5.6
3.7
1.7
2.5
1.8
—i
o
o>
^mj
%£
^3
ppm
50
360
80
4100
7400
330
950
280
825
770
275
Texture
CO
o>
3
Q.
%
88
10
91
19
23
7
3
35
52
32
71
co
«j.
«j
{-f>
%
8
60
4
20
25
58
47
19
37
28
14
o
pj
<^
%
4
31
5
61
52
35
49
46
11
40
15
*3P
-••OS
3 — -0)
a> a> <->•
-s «< o
Q| ^
«J
in
*
*
Kaolinite,
Chlorite
Vermiculite,
Kaolinite
Chlorite,
Kaolinite
Kaolinite
Kaolinite,
Gibbsite
Montmorillonite,
Vermiculite
Vermiculite
Montmorillinite,
Mica
Mica, Kaolinite
Mi ca ,
Montmorillonite
Montmorillonite,
Mica
ro
ro
t U.S. Department of Agriculture
* U.S. Department of Agriculture
**The dominant mineral is listed
Comprehensive Soil Classification System.
System: Sand, 2mm-0.05mm; Silt, 0.05mm-0.002mm; Clay, <0.002mm diameter
first
-------
X^L
/ "
A i
r;j
V
EACHATE
ECYCLING
MLET -^
"•it**'
~s
10 '
TO
24"
SOLI
4"
P
NLET FOR^X
OS LOADING A
\k-'-
t
I
6 INLET FOR
LIQUID RECYCLING
v
1
COVERS
LJ
™
U
i
i
i
L
i; ;l II I
0 1 0 ! I
I ENTIRE INSIDE SURFACE !
EPOXY RESIN COATED ',
^J i ° L_ RECYCLING !
! ° r~LEACHATE PIPE V
L.B.j
4" PERFORATED 8 FILTERED
/lEACHATE RECEIVER
.
Ll
's
6 OUTLET FOR
'LIQUID OVERFLOW
RECYCLING
PUMP LOCATION
Is"
-4 REINFORCED CONCRETE
3 CONCRETE BOTTOM
SIDE
LEACHATE GENERATOR
Figure A-l. Diagram of the municipal solid waste leachate generator.
222
-------
TABLE A-2. PARTITIONING OF MATERIALS IN THE MUNICIPAL WASTE-TYPE LANDFILL
USED TO GENERATE LEACHATE
Solid waste material Amount loaded in 1000 gal. generator
Tbs~:I
Paper (mostly newspaper) 1,400 45.7
Food waste 450 14.7
Garden waste 376 12.3
Plastic 34.
Rubber 109
Leather 60
Textiles 109-
Metal (mostly cans) 187 6.1
Glass 177 5.8
Ash #59 ) 127 4 1
Soil #68 }
Calf manure 35 1.1
10.2
industrial waste leachates. Furthermore, these concentrations were neces-
sary to assure migration through the soil columns in a reasonable period of
time. The precipitation of Pb was retarded by further decreasing the pH of
the solution to 3.0. Where possible, the appropriate chloride was used for
spiking. Oxides were used for As, Cr, V, and Se, and a nitrate for Pb. The
+3 +2 +2 +3 +2 +2
oxidation states for the elements were As , Be , Cd , Cr , Cu , Hg ,
Ni+2, Pb+2, Se*4, and V+5.
Other displacing solutions used for evaluating the contribution of
the soil to the pool of soluble potentially hazardous elements were (a)
aqueous H2S04 adjusted to pH 3.0, (b) 0.025 M A1C13 plus 0.025 M FeCl2 with
enough HC1 added to attain a pH of 3.0, and (c) deionized water.
One major problem encountered in this project was to preserve the
leachate in its natural state for extended periods of time. The variation
in analyses of landfill leachates (Brunner and Keller, 1972) reported in the
223
-------
TABLE A-3. RANGES OF CONSTITUENTS DETECTED IN THE NATURAL LEACHATE
GENERATED FROM MUNICIPAL SOLID WASTE AND USED IN THE SOIL COLUMN RESEARCH
Constituent
pH
Al
Ca
Cd
Co
Cr
Cu
Fe
K
Hg
Mn
Ni
NH4-N
P
Pb
Si03-Si
Zn
Cl
COD
TDS
Overall Range
3/1/74 - 7/7/75
6.6-6.8
ppm
bdl*
90-275
bdl
bdl
bdl
bdl
48-120
150-950
bdl
0.6-1.8
bdl
70-190
0.8-7.9
bdl
19-31
0.1-3.4
93-3,900
150-500
2,400-2,800
Stabilized Range
used in
the research
7/1/75 - 7/7/75
6.6-6.8
ppm
bdl
160-225
bdl
bdl
bdl
bdl
60-120
850-950
bdl
0.6-1.8
bdl
125-190
--
bdl
20-25
0.4-0.65
780-**
1 60-200
2,400-2,600
bdl - means below detectable limits; the bdl for atomic adsorption method
used, in yg/fc are: Cd = 0.005, Cr - 0.05, Co = 0.05, Cu = 0.05,
Pb = 0.5, Ni = 0.05, Mn = 0.05, Zn - 0.005, Al = 0.5, and Fe = 0.05.
**
Where leachates were spiked with a trace element to simulate levels
more closely related to that of industrial waste streams, the Cl content
may be as high as 5,000 ppm.
224
-------
literature, leads to speculation that some of this variation is due to
inadequate techniques for sample collection and preservation. Current prac-
tices in acidification of samples is not sufficient to keep all ions in
solution. For example, acid will not prevent ferrous iron from being slowly
oxidized and precipitated (Strumm and Morgan, 1970; and Cotton and Wilkinson,
1966). Simple acidification clearly is not acceptable for long-term research
involvement with natural leachates.
In this project, samples were collected and maintained under CCu by
first purging the collection vessel with the gas and then adding leachate
while continuing to flush with C0«. Once the collection flask was filled,
the C0« hose was withdrawn and the flask capped. For large samples and for
preservation in the laboratory, C02 was constantly and vigorously bubbled
through the leachate both during transport and storage.
Precautions were taken to preserve neutral natural leachates. Even
momentary contact with the air resulted in precipitation and an increase in
pH. When leachate was continually flushed with COp, it could be preserved
indefinitely without precipitation or changes in pH, color, and concentration
of dissolved inorganic ions.
The soils were passed through a 2 mm sieve and then uniformly packed
into 10 cm diameter x 20 cm PVC or 5 cm diameter x 10 cm PVC columns for
irrigation studies. Packing into the columns was undertaken one centimeter
at a time until full using a solid glass rod ^ 1 cm diameter for tamping.
Bulk densities (Table A-l) for silt and clay soils exceeded 1.5 depending on
the texture, and sands exceeded 2.0. These values were greater than those
under natural field conditions and insured against channeling either intern-
ally in the soil body or along the sides of the columns.
The soil columns were saturated while inverted to exclude air and pro-
vide uniform wetting. Carbon dioxide was used to pump the solution to a
constant head device where landfill leachate "spiked" with the potentially
hazardous trace element was being studied. The height of the column was
225
-------
adjusted to maintain as uniform a flux as possible. To retard stagnation in
the manifold the solution could be recirculated back to the main reservoir.
The leachate application and recirculation system is shown in Figure A-2.
The landfill leachate was continuously flushed with gas and the
deionized H20, H2S04, and Al-Fe solutions with N2 gas to keep the 02 content
at a minimum during the infiltration process. In the smaller columns (deion-
ized H20 and landfill leachate) the flow from each column was adjusted so
that approximately one pore volume flowed from each column in 24 hours. The
flow rate for the H^SCh and Al-Fe solutions in the larger columns was main-
tained at ^ one-half pore volume per day. Columns were leached until 15 or
more pore volumes of water, natural leachate, or sulfuric acid solution had
passed through the soil. The Al-Fe solution was allowed to flow until the
effluent concentration of Al and Fe equalled that of the influent. Each sam-
ple was divided after collection so one part could be acidified to preserve
it for trace element analysis and another for routine constituents.
Leaching was continued until one of three conditions were met; first,
breakthrough (effluent cone. influent cone.); second, steady state (un-
changing or very slowly changing effluent concentration at value below that
of influent); third, continued absence of the element after leaching.
At the conclusion of the leaching experiments the columns were seg-
mented into ten sections of 1-cm each. Each segment was oven dried so that
a material balance could be easily calculated and so that no variations in
moisture content would be involved. Preliminary results showed that vari-
ations in extractability with dried versus saturated samples were slight.
Untreated soils were shaken with a solution containing the trace ele-
ment to sorb the maximum amount on the soil. This soil was dried and used
to determine optimum extraction times. Extractions were done with 0.1 N^
HC1 and deionized water in a 1:10 soil solution rate. Samples were then
centrifuged and the concentration of trace element in the extracting solution
was measured by atomic absorption spectrometry. A material was calculated
from the data and the percentage extracted was correlated to soil properties.
226
-------
ro
ro
MANIFOLD
RETURN
TO BULK
RESERVOIR
TO TRAP
RETURN
SOIL
COLUMNS
BULK
RESERVOIR
Figure A-2.
System used at The University of Arizona for circulating and applying leachate
anaerobically to soil columns.
-------
Free iron oxides of the soils were determined by the method of Kilmer
(1960), surface area by the method of Heilman et al. (1965), and manganese by
modified procedure of Bernas (1968). For total analysis a sample size of 0.1
g of finely ground soil, 1 ml of aqua regia and 6 ml of HF were used for
digestion. Boric acid (2.0 g) then was added and the sample diluted to a
final volume of 50 ml.
The trace elements (As, Be, Cd, Cr, Cu, Hg, Ni, Pb, Se, V, and Zn) and
Al, Fe, and Mn of the spiked leachates (influents) and soil column effluents
were measured by atomic absorption spectrophotometry using standard proced-
ures except for As, Se, and Hg. Because of the relatively high concentra-
tions of these elements, the more specialized techniques of analysis, such
as cold vapor mercury analysis (Hatch and Ott, 1968), and hydride generation
of As and Se (Chu et al., 1972), were not required. Since these elements are
subject to severe interference in atomic absorption analysis, chemical inter-
ferences were minimized by matrix adjustment. Lanthanum (1000 ppm) and K
(100 ppm) were added to samples and standards alike to equalize the solution
matrix for all samples. Non-atomic adsorption which was especially notice-
able for Hg was measured with a H-continuum lamp (Varian Techtron), and sub-
tracted from the sample reading.
Common ions (Ca, Mg, K, Na, Cl, NH.-N, P, and Si) were determined by
the standard U.S. EPA recommended methods (1971) and COD by the conventional
Technicon Autoanalyzer method.
The pH values were measured using the glass electrode. Where pH,
common ions, total dissolved solids were evaluated on water-saturated soils
(soil-paste) and its extract, the method as recommended by the USDA (1954)
was used as was the total dissolved solids (TDS) and cation exchange capacity
(CEC).
Standard X-ray (Jackson, 1964) and mechanical analysis procedures
(Day, 1965) were used to identify the < 2y clay minerals and the particle
size distribution of the soils.
228
-------
Two types of variables were considered for the statistical analysis
of the results: first, those representing soil properties -- clay, sand,
percentage of free iron oxides, surface area, total manganese, pH, and elec-
trical conductivity of the saturation extract — and second, those measure-
ments characterizing the migration and/or attenuation of the trace elements;
mass adsorbed per gram of soil per milliliter of added leachate and maximum
concentration. The mass balance for each soil column was calculated from
daily measurements of the effluent and influent.
These data were first used to calculate a matrix of simple correla-
tion coefficients (Korn and Korn, 1968) from which independent variables
(clay, pH, etc.) having statistically significant relationships with the
dependent variable (mass of element adsorbed per gram of soil per milliliter
of added influent) were selected. To quantitatively decompose the effect of
cross-correlation between independent variables (i.e., correlation between
surface area and clay content) a stepwise multiple linear regression analy-
sis (Efroymson, 1960) was performed using only independent variables which
had statistically significant simple correlation coefficients. In this pro-
cedure a predicting equation of the form:
y= bo + b.x. + b2x2 ... + bnxn
where y = dependent variable
x.= independent variables
b.= coefficients
is derived by adding one variable at a time (stepwise) to the predicting
equation, selecting as the next variable to be added, the one which most
reduces the sum of squares deviation between observed and predicted y. The
procedure is therefore a least squares analysis which is capable of "best
fitting" linear combinations of independent variables to a dependent varia-
ble. The result is the evaluation and measurement of overall dependence of
a variable on a set of other variables. This can be very useful for pre-
dictive purposes and also for providing the actual contribution of cross-
correlated dependent variables.
229
-------
The most significant findings were:
1. Soil properties most useful in predicting attenuation (retarda-
tion of migration) of contaminants by soils are:
a. Clay content
b. Content of hydrous oxides, primarily iron oxides
c. pH and content of free lime
d. Surface area per unit weight of soil
2. The mobility of the eleven contaminants studied may be classified
as follows:
a. Most generally mobile - Cr, Hg, Ni
b. Least generally mobile - Pb, Cu
c. Mobility varies with conditions - As, Be, Cd, Se, V, Zn
3. Although the effect of soil solution flux (flow rate) was not
thoroughly evaluated, attenuation appears to increase as flux
decreases.
As a qualitative summary, Figures A-3 and A-4 present a ranking of the
soils used in the project according to their attenuation properties and a
ranking of the contaminants according to their mobility in the various soils.
Separate rankings are given for contaminants present in the leachate as cat-
ions and as anions because of the differences in migration behavior shown in
Tables 2 and 3. Note that every change in the ordering of soils when going
from cations to anions involves a higher ranking for soils having a lower pH
and/or a higher content of free iron oxides.
Based on the data analyses completed to date it is concluded that clay
content, surface area, and content of hydrous oxides and free lime will be
the soil properties most useful in selecting safe disposal sites for munici-
pal and hazardous wastes. Additionally, the data suggests that use of lime
and iron oxides should be examined as practical management tools for mini-
mizing the movement of contaminants from landfills. Although information
230
-------
INCREASING MOBILITY
t
INCREASING
ATTENUATION
CAPACITY.
ro
CO
NICHOLSON si.c
HIGH MOBILITY
Figure A-3. Relative mobility of cations in soils used in the University of Arizona study,
-------
INCREASING MOBILITY
I
INCREASING
ATTENUATION
CAPACITY
ro
co
ro
NICHOLSON si.c
:S MODERATE
£ MOBILITY
Figure A-4. Relative mobility of anions in soils used
in the University of Arizona study.
-------
about the relation between solution flux (flow rate) and attenuation is
limited, it appears that at the low solution fluxes expected in unsaturated
soils contaminant attenuation will be much greater than that observed in the
saturated soils used in this study. This effect could also be used in man-
agement of landfills by lining with low permeability materials to reduce the
rate of flow of leachate into soils below the landfill.
Further information on this project is contained in reports by Fuller
(1975), Fuller and Korte (1976), and Korte et al. (1976).
233
-------
APPENDIX B
SUPPLEMENTARY SOIL CLASSIFICATION INFORMATION
This appendix contains additional details on soil classification that
will be helpful 1n relating this report to other reports on waste disposal,
site selection, and soils research.
There are three systems under which soils are most likely to have been
classified 1n the United States: The Unified Soil Classification System, the
old (1938) U.S. Department of Agriculture System, and the present U.S.
Department of Agriculture System.
The Unified Soil Classification System (USCS) serves engineering uses
of soils and the criteria for soil types 1n the system are based on the
grain (particle) size and response to physical manipulation at various water
contents. Major divisions, soil type symbols, and type descriptions are
shown 1n Table B-l. This 1s an abbreviated description of the system and
does not Include complete Information on the use of the manipulation tests
(liquid limit and plasticity Index) 1n the classification.
The U.S. Department of Agriculture System (USDA) serves agricultural
and other land management uses and the criteria for classification 1n the
system are based on both chemical and physical properties of the soil. The
USDA system 1n general use between 1938 and 1960 was based on soil genesis
- how soils formed or were thought to have formed. The present USDA
comprehensive soil classification system 1s based on quantitatively measur-
able properties of soils as thev exist 1n the field. Although the present
USDA system 1s Incomplete ai.d 1s being continually refined, 1t 1s generally
accepted by U.S. soil scientists and Us nomenclature 1s used 1n most of the
234
-------
current literature. The present (1960) USDA system is described on pages 52
through 59. Table 4.4 on page 53 lists the Orders of the present system along
with their approximate equivalents 1n the 1938 system. The geographic dis-
tribution of the Great Soil Groups of the old system 1s shown 1n Figure B-l;
the geographic distribution of Orders and Suborders 1n the present system 1s
shown 1n Figures B-2(A) and B-2(B). Additionally, Figure B-2(B) lists the
approximate 1938 classification equivalents of the Suborders 1n the present
system.
The part of the USDA classification which may be compared most directly
with the soil types 1n the USCS system Is soil texture (distribution of grain
or particle size) and associated modifiers such as gravelly, mucky, dlatom-
aceous, and micaceous. The size ranges for the USDA and the USCS particle
designations (e.g. sand, gravel) are listed 1n Table B-2. The soil texture
(USDA - sandy loam, silt loam, etc.) or the soil type (USCS - GC clayey
gravel, SC clayey sand etc.) 1s based on the relative amounts of different
sized particles 1n a soil. The USDA system for classifying soil texture 1s
described on page 42 and 1n Figure 4.1 on page 43; an abbreviated description
of the USCS classification 1s listed 1n Table B-l. An unpublished correlation
of the USCS and USDA systems on the basis of texture 1s presented in Tables
B-3 and B-4. It should be emphasized that this correlation 1s not precise
because texture 1s a high level (major) criterion In the USCS while texture
1s a low level (minor) criterion 1n the USDA system. A soil of a given
texture can be classified Into only a limited number of the 15 USCS soil
types while in the USDA system, soils of the same texture may be found in
many of the 10 Orders and 43 Suborders because of differences 1n their
chemical properties or the climatic areas 1n which they are located.
* Personal communication. N. B. Schomaker, U.S. EPA Municipal Environmental
Research Laboratory, Cincinnati, Ohio to W. H. Fuller, University of
Arizona, Tucson, Arizona, Aug. 10, 1976.
235
-------
TABLE B-l.
MAJOR DIVISIONS, SOIL TYPE SYMBOLS, AND TYPE DESCRIPTIONS FOR
THE UNIFIED SOIL CLASSIFICATION SYSTEM (USCS)
o
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-h*< VI
rt-
_j.
I/I 3 <
~j re re
io v> *<
3-01
rt 3 -h
CL _•.
•o vi 3
Oi O
I/I -$ VI
rt 0)
n o.
-^ vi
-!? '
Coarse-grained soils
More than half of material is
larger than No. 200 sieve size
Sands 1
More than half of coarse More t
Gravels
han half of coarse
fraction is smaller than fraction is larger than
No. 4 sieve size. | No. 4 sieve size.
Sands with
Fines
(appreciable
fines)
CO
o
o
•^
Ol
re
i/i
Ol
3
CX
I/I
w
VI
Ql
3
CX
1
O
Ql
%^
3^
^
rt
-1
re
VI
•
co
CO
Mj
r+
•V-C
VI
Ql
3
CX
VI
«
I/I
Ol
3
CX
1
VI
^J«
M^
r*
^
^j*
X
rt-
C
•^
re
VI
Clean Sands bravels with
(little or
no fines)
CO
-o
I/I -O
Ql O
3 O
CX -I
VI — J
- *<
••Jir*>
-J--J
rt Qi
rt CX
— • re
re cx
O VI
-S Q"
3
3 CX
o vi
~"h O
— •• ~J
3
re «o
VI -I
. Ol
^
re
•^
M^
^^
CO
— • s:
^J» ^^
rt — •
rt — •
m^
re vi
o o
-j -i
3 tO
O -J
Ol
—^1 ^
••*• re
3 -J
0> 1
VI
CD
s:
3 C
«j« (p
X — •
rt — •
C
"1 *****
re -j
I/I Ql
- o.
fO
— «cx
— *•
rt- (O
rt -J
— « Oi
re <
re
o — •
-i VI
w
3
O (O
^
-hoi
^"* <
3 re
re ^^
vi i
• i/i
Q>
3
CX
Oi
o
-J
o
<_
VI
o
3
VI
CO
1
O"
0
CJ
CD
(A
o
^
^•n
^J
c+
^J*
ro
co
cr>
-------
TABLE B-2. U.S. DEPARTMENT OF AGRICULTURE (USDA)
AND UNIFIED SOIL CLASSIFICATION SYSTEM (USCS) PARTICLE SIZES
USDA
Particle
Cobbles
Gravel
coarse gravel
fine
SaM
very
gravel
coarse sand
coarse sand
medium sand
fine
very
snt
Clay
sand
fine sand
USCS
Size Range
76.
2.
12.
2.
0.
1.
0.
0.
0.
0.
0.
2 -
0 -
7 -
0 -
05 -
0 -
5 -
25 -
1 -
05 -
002
<0.
(mm)
254.
76.
76.
12.
2.
2.
1.
0.
0.
0.
- 0.
002
Particle
2
2
7
0
0
0
5
25
1
05
Cobbles
Gravel
coarse
gravel
fine gravel
Sand
coarse
medium
sand
sand
fine sand
Fines*
"isllt
& clay)
Size Range
4.
19.
4.
0.
2.
0.
0.
>76.2
76 -
1 -
76 -
074 -
0
42 -
074 -
(mm)
76
76
19
4
4
2
0
.2
.2
.1
.76
.76
.0
.42
<0.074
USCS silt and clay designations are determined by response of the soil to
manipulation at various water contents rather than by measurement of size.
237
-------
TABLE B-3. CORRESPONDING USCS AND USDA SOIL CLASSIFICATIONS
Unified Soil Classification
System (USCS) Soil Types
Corresponding United States Department of
Agriculture (USDA) Soil Textures
1. GW
2. GP
3. GM
4. GC
5. SW
6. SP
7. SM
8. SC
9. ML
10. CL
11. OL
12. MH
13. CH
14. OH
15. PT
Same as GP--gradat1on of gravel sizes not a
criteria.
Gravel, very gravelly* sand less than 5% by
weight silt and clay.
Very gravelly* sandy loam, very gravelly* loamy
sand very gravelly* silt loam, and very grav-
elly* loam#.
Very gravelly clay loam, very gravelly sandy
clay loam, very gravelly sllty clay loam, very
gravelly sllty clay, very gravelly clay#.
Same - gradation of sand size not a criteria.
Coarse to fine sand; gravelly sandA (less than
20% very fine sand).
Loamy sands and sandy loams (with coarse to fine
sand), very fine sand; gravelly loamy sandA and
gravelly sandy loamA.
Sandy clay loams and sandy clays (with coarse, to
fine sands); gravelly sandy clay loams and
gravelly sandy clays^.
S1lt, silt loam, loam very fine sandy loam*.
S1lty clay loam, clay loam, sandy clays with
<50% sand*.
Mucky silt loam, mucky loam, mucky sllty clay
loam, mucky clay loam.
Highly micaceous or diatomaceous silts, silt
loams — highly elastic.
Sllty clay and clay*.
Mucky sllty clay.
Muck and peats.
Also Includes cobbly, channery, and shaly.
^Also Includes all of textures with gravelly modifiers where >l/2 of total
held on No. 200 sieve 1s of gravel size.
AGravelly textures Included 1f less than 1/2 of total held on No. 200 sieve
1s of gravel size.
*Also Includes all of these textures with gravelly modifiers where >l/2 of
the total soil passes the No. 200 sieve.
238
-------
TABLE B-4. CORRESPONDING USDA AND USCS SOIL CLASSIFICATIONS
United States Department of Agriculture
(USDA) Soil Textures
Corresponding Unified Soil Classi-
fication System (USCS) Soil Types
1. Gravel, very gravelly loamy sand
2. Sand, coarse sand, fine sand
3. Loamy gravel, very gravelly sandy
loam, very gravelly loam
4. Loamy sand, gravelly loamy sand,
very fine sand
5. Gravelly loam, gravelly sandy clay
loam
6. Sandy loam, fine sandy loam, loamy
very fine sand, gravelly sandy
loam
7. S1lt loam, very fine sandy clay loam
8. Loam, sandy clay loam
9. S1lty clay loam, clay loam
10. Sandy clay, gravelly clay loam,
gravelly clay
11. Very gravelly clay loam, very
gravelly sandy clay loam, very
gravelly sllty clay loam, very
gravelly sllty clay and clay
12. Sllty clay, clay
13. Muck and peat
GP, GW, GM
SP, SW
GM
SM
GM, GC
SM
ML
ML, SC
CL
SC, GC
GC
CH
PT
239
-------
I'm N fc •—•- ntrtlf* f W •)"» »•
<•< r1* •"•' • "» w '»• •• "" f*™
Fiaure B-l Geographic distribution of Great Soil Groups in the 1938 U.S. Department of Agricultural Soil
Classification System. (Courtesy of Soil Survey Division, U.S. Soil Conservation Service)
-------
Figure B-2{A).
Geographic distribution of Orders and Suborder
Agriculture Comprehensive Sol n- ifir, £ I960 U'S< Apartment of
^
-------
ALFISOL8 . . . SolU with (ray to brown aurfaca horl-
a-ona, awdlua to hl|fc baae (apply. and eubeurface horliana
o» cUy accae»Ullan; uully owUI but My to dry darln(
Al AQUALPS (eeaaanally aauratod with water ) (ently aloplny
IMwnl crop* K drained. peatura ud woodland If end™ toed
(Soaw Low-Hueric OUr "Uo ud PlenoaoU)
A2
ro
El
E2
E3
EJS
E4
BORALFS (cool or cold) (ently alopinc anally woodland.
putun, ud aoaw wall (rain ( Cray Voodad toil* )
A2S BORALFS alaap; anally woodland
A3 UDALFS (temperate or wan, aad aiolal) (anlly or andarotaly
aloplaa; atoally farawd, con, aoybeana, aawll (rain, aad
paetora (Gray-Brown Podiollc eolla)
M USTALFS (warn and intermittently dry lot Inn (pailoda)
(antly M moderately aloplne; ranea, auli (rain, aad lrrl(acad
cropa (Soaw Raddiah Chnlnvt and Rad-Yallow Podiollc aoila)
ASS XERALFS (warn and eonllnuooaly dry la aumawr lor loot
parloda. awlat In winter ) (antly alopln( lo alaap; aioally
raaaa, lull (rain, and Irrifatad cropa (Noacalclc Brow*
lolla)
ARIOI8OL8 . . . SolU with padoeantc horixooa, low la
organic Batter, aad dry man than 6 monlha of the yaar la
01 AROIDS ( with horira of clay accomuUlloo ) (anlly or awdar-
ataly sloping; anally ran(a, aoow Iniaatad cropa (Some)
Daoart, Raddlak Daaart, RaddUh-Brown, aad Brawn aolla
•nd aaaoclatad Solonati aolU)
D1S ARGIOS (aatly alopla( lo alaap
02 ORTHIDS ( without horlion of clay accumulation ) (aotly or mod-
arauly alopUgj anally range aad soaw irrl(atad cropa
(Saw Daaart, Raddlah Daaart. Slaranai, and Brawn aolU,
and aoaw CalclaoU and Solonchak lolU )
D2S ORTHIDS (ently aleplai to ataap
^^H| HISTOSOLS . . . Oraaalc aolla
HI nBRlSTS (fibrava or woody paata. Ur(aly undacoaipoaad)
anally voooad or Idla ( Paata)
H2 SAPRISTS (dacompoaad amcka) track cropa il drained, Idla II
aadrainaO(Hocka)
INCEPTISOLS . . . SolU that ara aaaally awlat, wltk
paooftnlc horUona of allatalloa of parant aularlaU bat
not of accuHulalion
IIS ANDEPTS (with aanrphoaa clay or vltrlc volcanic aah and
puaiica) (aotly alopla( lo aiaap, anally woodland; In
Hawaii anally aa(ar cana, plnaappla, aad ranaa ( Ando
aolU, aoaw Tundra aolU )
12 AQUEPTS (aaaaonally aaluratad with walar) (antly aloplaa; if
dralnad, anally row cropa, con, aoybaana, and cotton; If «n-
drttnad, anally woodland or poatnra (Soaw Low-HnaUc Clay
aolU and Allovlal aolU)
DP AQUEPTS (wUheonllnaooa or aporadic prmalroat) (anlly
«lopU( to ataap; woodland or Idla ( Tandra aolU)
13 OCHREPTS (with thin or U(hl-colorad aarfaca horuona and
Illtla or(anlc auttar) fantly to awdaralaly aloplnc anally
paatora, aaull (rain, and hay (SoU Bniaa Acldaa and aoaw
Alluvial aoila )
US OCHREPTS (antly alopin( to ataap; woodland, paatun, aaiall
(raina
MS UMBREPTS (with thick dark-colorad aurfaca kocUona rich In
orfanlc awllar) awdaralaly alopta( lo ataap; anally wood-
land (Soaw Rafoaola)
MOLLISOLS . . . SolU with oaarly block, oraanlc-rlch
aurfaca horUona and hi(h baaa aapply
::::::::>J ENTISOLS . . . soiia withoot padocanic horuona
AQUENTS (seasonally saturated with water) gently sloping;
aoaw
ORTHENTS ( loaaiy or cUyay laxtwa* ) dawp to hard rock;
(antly to •odarataly aloptag; ranee or IrrifBiad farmui(
(RaapaoU)
ORTHENTS shallow lo hard rack; (antly to awdarataly aloplnfc
•oatly raa(a ( UtkoaoU )
ORTHENTS ahallow lo hard rock; ataap: araaUy raafla
PSAIMENTS ( aaad or loaaqr aand tastun ) (aatly lo andar-
alaly alopia(; anally ranaa in dry cliaataa, woodland or
cropland la haaild cllautaa ( RaapaoU )
•1 AQUOLLS (Maaonally aaturatad with w.tar) (aatly aloplnc
•oatly dralnad and farawd (HuaUc Clay aoiU)
H2 BOROLLS (cool or cold) (aatly or awdarataly aloplnt, aonw
ataap alopaa In Utah; anally aaull (rain in North Cantral
Slalaa, nn(a and woodland U •aatam Stataa (Son
Chamaxaaai)
M3 UDOLLS (laoiparala or warn, and wolal) (antly or andaralary
•loplnc aoatly com, aoybaana, and aaull (ralna (Sooia
Bmalsaau)
•4 USTOLLS ( intamlttantly dry for lon( parioda darin( amnwr )
(tolly to awdaiataly aloplaa; aioally whaot and ran(a in
waalara part, whaal and corn or aorfhuai in aaalarn part,
aoaw Inlaatvd cropa ( Chaataut aolU and aoaia Chamoiaau
and Brown «olU)
H4S USTOLLS awdarataly alopin( lo alaap; aioatly rang, or woodland
US XEROLLS (contlnuonaly dry in auoiawr for lon( parloda, awUt
in winter) (anlly lo awdarataly alopla(. anally wnaat, ranaa.
and Inlaalad cropa (Soaw BranUaoui, Chaalnut, aad Brown
lolU)
HSS XEROLLS ondarataly ilopini to ataap; aioatly ranaa
SPODOSOLS . . . SolU with accuMlationa of aanrphoon
arUU in aabaarfaca horiioni
SI AQUODS (uaionally aaluralad with watar) (antly alooiof,
anally mn(a or woodUnd; where drainad la Florida, cltrua and
apacUl cropa ( Ground-Watar PodioU )
S2 ORTHODS (with aabaarfaca accoaraUlkMa of Iron. alaaUnai. and
orfanlc utter ) (anlly lo nwdarataly aloptaaj woodland,
paature, aaiall (ralna. apacUl cropa (PodioU, Brawn Podiollc
•oiU)
S2S ORTHODS ataap; anally woodUad
ULTISOLS . . . Soils ih«i ere usually molal with horiion
of cUy accumuUnon and a low baae supply
Ul AQUULTS (aaaaonally aaluraled with water) gently sloping,
woodland and paature if undrained, feed and truck cropa if
U2S
UJ
,
drainad (Soaw Low-Huaiic Clay toili
HUMUI.TS (with hi(h or very hi(h or(anic-ulter contant) and-
arataly ilopinf to aleep, woodland and pasture il alaap,
• u|ir cane and pineapple in H>w«n. truck and aead cropa in
Waalam Slatea (Soaw Reddiah-Browa Lelrnlic aolla)
UOULTS (wilh low or(anic- Kaltar content; tenperate or warm,
and noiat ) (antly lo moderately alopln(; woodUnd, paalora.
faad cropa. tobacco, and cotton ( Red-Yellow Podiolic aoiU,
aoaw Reddiah-Brown Latariikc soila)
U3S UOULTS moderately eloping to steep. woodUnd. paalara
U4S XERULTS (wltk low to moderate organic-matter content, con-
tinuously dry (or long periods la aoawwr) rang* end woodUnd
(Some Reddish-Brown Laterillc aoila 1
VERTISOL8 . . . SoiU with high content of awaiting cUya
and wide deep cracka at aoaw season
VI UDERTS (cracka open for only abort parloda, less thin 3 months
in a year) (antly alopUa; cotton, com, paatnre, and aome
rice (Soow GmmaooU )
V2 USTERTS (cracka open end clone twice a yaar and remain open
more than 3 months ); general cropa, range, and aome irrigated
cropa (Soow GramuaoU )
AREAS will) Hill* *oll . . .
XI Salt data
X2 Rockland, ice fialda
Figure B-2(B). Legend for Fig. B-2(A) "Geographic Distribution of Orders and Suborders"
-------
TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1. REPORT NO.
EPA-600/2-77-020
2.
3. RECIPIENT'S ACCESSION-NO.
4. TITLE AND SUBTITLE
Movement of Selected Metals* Asbestos, and Cyanide in
Soil: Applications to Waste Disposal Problems
5. REPORT DATE
April 1977 (Issuing Date)
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
8. PERFORMING ORGANIZATION REPORT NO.
Wallace H. Fuller
9. PERFORMING ORGANIZATION NAME AND ADDRESS
Department of Soils, Water, and Engineering
University of Arizona
Tucson, Arizona 85721
10. PROGRAM ELEMENT NO.
1DC618
11. CONTRACT/GRANT NO.
68-03-0208
12. SPONSORING AGENCY NAME AND ADDRESS
Municipal Environmental Research Laboratory—Cin.,OH
Office of Research and Development
U.S. Environmental Protection Agency
Cincinnati, Ohio 45268
13. TYPE OF REPORT AND PERIOD COVERED
Final
14. SPONSORING AGENCY CODE
EPA/600/14
15. SUPPLEMENTARY NOTES
is. ABSTRACT mis report presents i merman on on movement of selected hazardous sub-
stances 1n soil which can be applied to problems of selecting and operating land dis-
posal sites for wastes containing arsenic, asbestos, beryllium, cadmium, chromium,
copper, cyanide, Iron, lead, mercury, selenium, and zinc. The Information is based on
a literature review, laboratory studies of movement of hazardous substances through
soil 1n municipal landfill leachate, and the author's experience in soil science and
waste disposal.
In addition to a discussion of the soil and waste-related factors to be considered 1n
selecting and managing disposal sites for minimum migration hazard, the report also
presents general Information on soils and geological materials and specific informa-
tion on the chemistry of the selected hazardous substances which is relevant to an un-
derstanding of their migration 1n soil. Critical Information gaps are Identified,
particularly as regards the chemistry and soil adsorption behavior of mixtures of sev-
eral hazardous substances In the presence of high concentrations of other organic and
inorganic solutes, a situation commonly encountered in leachates from municipal and
hazardous solid wastes. In spite of these Information gaps, it 1s concluded that waste
disposal practice can be Improved by application of present Information. The report
contains 250 references and a bibliography of 81 related citations.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS C. COS AT I Field/Group
Hazardous Materials
Transport Properties
*So1l Chemistry
Contaminants
Arsenic
Beryllium
Cadmium
Copper
Chromium
Iron
Lead (metal)
Mercury (metal)
Selenium
Z1nc
Pollutant Migration
Attenuation
Groundwater Pollution
Industrial Waste
Municipal Landfill
Leachate
13B
18. DISTRIBUTION STATEMENT
Release to public
19. SECURITY CLASS (This Report)
Unclassified
21. NO. OF PAGES
257
20. SECURITY CLASS (Thispage)
Unclassified
22. PRICE
EPA Form 2220-1 (9-73)
243
* u i GWONMOIT HMHTMIG omcfc in;- ? ^ ? -
------- |