PB84-140169
Fate of Selected Toxic Compounds Under Controlled Redox Potential
and pp Conditions tn Soil and Sediment-Water Systems
Louisiana State Univ., Baton Rouge
Prepared for
Environmental Research Lab., Athens, GA
Jan 84
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EPA-600/3-84-018
January 1984
FATE OF SELECTED TOXIC COMPOUNDS UNDER CONTROLLED REDOX POTENTIAL
AND pH CONDITIONS IN SOIL AND SEDIMENT-WATER SYSTEMS
by
Robert P. Gambrell
Barbara A. Taylor
K. S. Reddy
W. H. Patrick, Jr.
Laboratory for Wetland Soils and Sediments
Center for Wetland Resources
Louisiana State University
Baton Rouge, Louisiana 70803-7511
Grant No. R-807018
Project Officer
Harvey W. Holm
Environmental Systems Branch
Environmental Research Laboratory
U. S. Environmental Protection Agency
Athens, Georgia 30613
ENVIRONMENTAL RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U. S. ENVIRONMENTAL PROTECTION AGENCY
ATHENS, GEORGIA 30613
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1 REPORT NO
EPA-600/3-G4-Q18
3 RECIPIENT'S ACCESSION NO.
140169
4 Tl
TJ.E AND SUBTITLE
Fate of Selected Toxic Compounds under Controlled
Redox Potential and pH Conditions in Soil and
Sediment-Water Systems
5 REPORT DATE
January 1984
6. PERFORMING ORGANIZATION CODE
7 AU
;O.R(1iambrel 1, B.A. Taylor, K.S. Reddy and
W.H. Patrick, Jr.
8 PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
Laooratory for Wetland Soils and Sediments
Center for Wetland Resources
Louisiana State University
Baton Rouge LA 70803
10 PROGRAM ELEMENT NO
CCBE1A
11 CONTRACT/GRANT NO
R-807018
12 SPONSORING AGENCY NAME AND ADDRESS
U.S. Environmental Protection Agency--Athens GA
Office of Research and Development
Environmental Research Laboratory
Athens GA 30613
13 TYPE OF REPORT AND PERIOD COVERED
Final. 10/79-10/82
14. SPONSORING AGENCY CODE
EPA/600/01
IB. SUPPLEMENTARY NOTES
16. ABSTRACT1
A study was conducted to determine the effects of pH and redox potential con-
ditions on the degradation of selected synthetic organics. Also, the effects of
these physicochemical parameters as well as other physical and chemical properties
of soils and sediment-water systems on the adsorption of selected organics were
measured. Compounds used in degradation studies included methyl parathion, 2,4-
dichlorophenoxyacetic acid (2,4-D), and Aroclor 154 (a polychlorinated biphenyl
formulation). Compounds used in adsorption studies included methyl parathion, 2,4-D
and pentachlorophenol. Soils and sediments used for both the degradation and
adsorption studies were selected to include materials having a wide range of physi-
cal and chemical properties. .
Degradation studies conducted on soil and sediment suspensions amended with
the synthetic organics and then extracted and analyzed by gas chromatography
showed important redox potential effects on the degradation of methyl parathion and
2,4-D, but not Aroclor 1254. Redox potential was shown to have a significant effect
on the adsorption of pentachlorophenol as this compound was partitioned more with
the solid phase under oxidized conditions than reduced conditions.
IT.
KEY WORDS AND DOCUMENT ANALYSIS
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c COSATI I icId/Group
IB DISTRIBUTION STATEMENT
RELEASE TO PUBLIC
19 SECURITY CLASS (I hit Kefi
UNCLASSIFIED
21 NO OF PAGES
112
20 SECURITY
UNCLASSIFIED
22 PRICE
EPA Perm 2220-1 (0-73)
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DISCLAIMER
Although the research described in t,his report has; been funded wholly
or in part by the United States Environmental Protection Agency through
Grant Number R-807013 to Louisiana State University, it has not been subjected
to the Agency's required peer" and policy review and therefore does not
necessarily reflect the views of the Agency and no official endorsement
should be inferred.
ii
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FOREWORD
Environmental protection efforts are increasingly directed toward
preventing adverse health and environmental effects associated v/ith specific
compounds' or natural or human origin. As part of this laboratory's research
on the occurrence, movement, transformation, impact, and control of environ-
mental contaminants, the Environmental Systems Branch studies complexes of
environmental processes that control the transport, transformation, degrada-
tion, fate, and impact of pollutants or other materials in soil and water
and develops models for assessing exposures to chemical contaminants.
Two key processes influencing the fate and potential human health or
environmental effects of pesticides or other synthetic organic chemicals
in soil or sediment-water systems are degradation and transport. In this
report, the effects of pH and redox potential conditions on the degradation
of selected synthetic organics are examined. The degradation and adsorption
studies demonstrated that redox potential has a significant sorption effect
and that physicochemical properties are important in the persistence and
mobility of selected organics in soils and sediment. This information has
potential application in the development of models for predicting chemical
fate and impact in the environment.
William T. Donaldson
Acting Director
Environmental Research Laboratory
Athens, Georgia
111
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ABSTRACT
A study was conducted to determine the effects of pH and redox
potential conditions on the degradation of selected synthetic organics.
Also, the effects of these physicochemical parameters as well as other
physical and chemical properties of soils and sediment-water systems on
the adsorption of selected organics were measured. Compounds used in
degradation studies included methyl parathion, 2,4-D, and Aroclor 1254
(a polychlorinated biphenyl formulation). Compounds used in adsorption
studies included methyl parathion, 2,4-D, and pentachlorophenol. Soil
and sediment materials used for both the degradation and adsorption
studies were selected to include a wide range of physical and chemical
properties.
Degradation studies conducted on amended soil and sediment suspen-
sions extracted and analyzed by gas chromatography showed important
redox potential effects on the degradation of methyl parathion and
2,4-D, but not Aroclor 1254. Methyl parathion was removed much more
rapidly under reducing conditions. Oxidized conditions contributed to
mgre rapid degradation of 2,4-D. Degradation studies conducted with
C-labeled compounds also showed important effects of oxidation-
reduction conditions on mineralization of the synthetic organics.
However, if collection of the labeled carbon as evolved carbon dioxide
is the criteria for measuring degradation, the rate of degradation and
the treatment effects, in the case of methyl parathion, were found to be
very different compared to degradation studies using extraction and gas
chromatography for analysis. Differences due to analytical procedures
used were discussed in terms of whether one was looking for initial
modification of the parent molecule during degradation or mineralization
of the C-containing portion of a labeled compound. The latter would
usually involve a sequence of degradative steps before the label was
converted to carbon dioxide.
Redox potential was shown to have a significant effect on the
adsorption of pentachlorophenol as this compound was partitioned more
with the solid phase under oxidi/.ed conditions than reduced conditions.
Correlation coefficients between methyl parathion arid 2,4-D adsorption
and several soil physical and chemical properties were determined. In
particular, an effort was made to identify important properties contrib-
uting to adsorption in soils and sediments with a low organic matter
content.
These degradation and adsorption studies demonstrated the importance
of physicochemical properties on the persistence and mobility of selected
synthetic organics in soils and sediments. Information of this type
should improve the capability to predict the fate and potential impacts
of synthetic organics in various environmental compartments. However,
additional work should be done to establish the effect of these physico-
chemical conditions on the rate of degradation under natural conditions.
This report was submitted in fuJfillment of Grant No. R-807018 by
Louisiana State University under the sponsorship of the U.S. Environmental
Protection Agency. This report covers a period from October 20, 1979 to
October 19, 1982, and work was completed as of September 30, 1983.
iv
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CONTENTS
Abstract iv
Figures vi
Tables x
Acknowledgment xi
1. Introduction 1
Background 1
Synthetic Organics Selected for Study 2
Literature review 2
2. Materials and Methods 13
Characterization of soil materials 13
Concentration effects of methyl parathion and
2,4-D on the rate of soil reduction 13
Degradation studies 14
Adsorption studies -21
. 3. Results and Discussion 24
Properties of soil materials and sediment
materials studied 24
Concentration effects of methyl parathion and
2,4-D on the rate of soil reduction 24
Degradation studies 38
Adsorption studies 74
4. Summary and Conclusions 86
Pesticide concentration effects on the rate of
soil reduction 86
Degradation studies 86
Adsorption studies 89
5. Recommendations 91
Literature Cited 93
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FIGURES
Number page
1 Methyl (a) and ethyl (b) parathion 3
2 Schematic representation of parathion metabolism via
nitro reduction (R) or hydrolysis (H) (Sethunathan,
1973c 4
3 2,4-dichlorophenoxy acetic acid 6
4 One possible adsorption mechanism for 2,4-D 8
5 2,4'-dichlorobiphenyl, one of more than 200 possible PCB
isomers 10
6 Laboratory microcosm for incubating soil and sediment
suspensions under controlled pH and redox potential
conditions 15
7 Biometer flask used for degradation studies with C-
labeled compounds 20
8 Redox potential levels (Eh) with time in a Crowley soil
material amended with varying concentrations of
2,4-D and subjected to flooding 26
9 Redox potential levels (Eh) with time in a Cecil soil
material amended with varying concentrations of
2,4-D and subjected to flooding 27
10 Redox potential levels (Eh) with time in a Crowley
soil material amended with varying concentrations
of methyl parathion and subjected to flooding 28
11 Redox potential levels (Eh) with time in a Cecil soil
material amended with varying concentrations of
methyl parathion and subjected to flooding 29
12 pH values with time in a Crowley soil material amended
with varying concentrations of 2,4-D and subjected
to flooding 30
13 pH values with timr in a Cecil soil material .intended with
varying concentrations of 2,4-D and subjected to
flooding 31
14 pH values with timr in .1 Crow ley soil material amended
with varying concentrations of methyl parathion and
subjected to Hooding 33
VI
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FIGURES (continued)
Number Page
15 pH values with time in a Cecil soil material amended
with varying concentrations of methyl parathion and
subjected to flooding 34
16 The effect of redox potential on the recovery of methyl
parathion from a Calcasieu River sediment material
incubated at pH 8 39
17 The effect of redox potential on the recovery of methyl
parathion from the aqueous phase of a Calcasieu
River sediment suspension incubated at pH 8 40
18 The effect of redox potential on the recovery of methyl
parathion from a Baratana Bay sediment material
incubated at pH 7 and 8 ppt salinity ' 41
19 The effect of redox potential on the recovery of methyl
parathion from a Barataria Bay sediment material
incubated at pH 5 and 8 ppt salinity 42
20 The effect of redox potential on the recovery of methyl
parathion from a Hartwel1 Koservoir sediment material
incubated at pH 7 43
21 The effect of redox potential on the recovery of methyl
parathion from a Hartwell Reservoir sediment material
incubated at pH 5 44
22 The effect of redox potential on the recovery of methyl
parathion from a Cecil topsoil material incubated at
pH 7 45
23 The effect of redox potential on the recovery of methyl
parathion from a Barataria Bay sediment material
incubated at pH 7 and 25 ppt salinity 46
24 The effect of redox potential on the recovery of methyl
parathion from a Barataria Bay sediment material
incubated at pH 5 and 25 ppt salinity 47
25 The effect of redox potent id 1 on the recovery of 2,4-D
from a Cecil topsoil material incubated at pll 7 50
26 The effect of redox potential on the recovery of Aroclor
1254 from a Hartwell Reservoir sediment material
incubated at pH 6 51
27 The effect of redox potential on the recovery of Aroclor
1254 from a Hartwell Reservoir sediment material
incubated at pH 8 52
vii
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FIGURES (continued)
Number Page
28 Carbon dioxide production as an indication of relative
microbial activity in a methyl parathion-amended
Hartwell Reservoir sediment material incubated at
four redox potential levels at pH 5.0 54
29 Carbon dioxide production as an indication of relative
microbial activity in a methyl pdrathion-amendcd
Cecil topsoil material incubated at tour redox
potential levels at pH 7.0 55
30 Carbon dioxide production as an indication of relative
microbial activity in an Aroclor 1254-amended sediment
material incubated at pH 6 and four redox potential
levels 56
31 Carbon dioxide production as an indication of relative
microbial activity in an Aroclor 1254-amended
sediment material incubated at pH 8 and four redox
potential levels 57
32 Oxidation effects on degradation of labeled methyl
parathion as indicated by recovery of CO. from
moist, unstirred Cecil subsoil material 60
33 Oxidation effects on degradation of labeled methyl
parathion as indicated by'recovery of CO- from
moist, unstirred Cecil topsoil material 61
34 Oxidation effects on degradation of labeled methyl
parathion as indicated by recovery of CO. from
moist, unstirred Crowley soil material 62
35 Oxidation effects on degradation of labeled methyl
parathion as indicated by recovery of CO. from
moist, unstirred Hartwell Reservoir sediment
material 63
36 Oxidation effects on degradation of labeled methyl
parathion as indicated by recovery of CO. from
moist, unstirred Lake Providence material 64
37 Oxidation effects on degradation of l.ihcled methyl
parjthion as indicated hy recovery of CO. from
moist, unstirred Metis topsoil material 65
38 Oxidation effects on degradation,of labeled 2,4-D
as indicated by recovery of CO. from moist,
unstirred Cecil subsoil material 67
viii
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FIGURES (continued)
Number Page
39 .Oxidation effects on degradation,of labeled 2,4-D
as indicated by recovery of C0_ from moist,
unstirred Cecil topsoil material 68
40 Oxidation effects on degradation,of labeled 2,4-D
as indicated by recovery of CCL from moist,
unstirred Crowley topsoil material 69
41 Oxidation effects on degradation,of labeled 2,4-D
as indicated by recovery of CCL from moist,
unstirred Hartwell Reservoir sediment material 70
42 Oxidation effects on degradation,of labeled 2,4-0
as indicated by recovery of CO. from moist,
unstirred Lake Providence sediment material 71
43 Oxidation effects on degradation,of labeled 2,4-D
as indicated by recovery of CCL from moist,
unstirred Betis topsoil material 72
44 Oxidation effects on degradation of labeled 2,4rD
(chain labeled) as indicated by recovery of CO.
from moist, unstirred Cecil topsoil material 73
LX
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TABLES
Page
Gas chromatography parameters for methyl parathion,
2,4-D, and Aroclor 1254 18
2 Selected physicochemical properties of soil and
sediment materials 25
3 Soluble plus exchangeable Fe, Mn, and Zn levels on
selected sampling dates in two soils amended with
varying concentrations of methyl parathion 35
4 Soluble plus exchangeable Fe, Mn, and Zn levels on
selected sampling dates in two soils amended with
varying concentrations of 2,4-D 36
2
5 R values for Fe, Mn, and Zn as influenced by soil pH
and redox potential 37
6 Notes on carbon dioxide collection studies to indicate
relative raicrobial respiration 58
7 Distribution coefficients for methyl parathion
equilibrated with 19 soil and sediment materials .... 75
8 Distribution coefficients for 2,4-D equilibrated with
19 soil and sediment materials 76
9 Percent organic carbon and distribution coefficients on
an organic carbon basis for methyl parathion and
2,4-D in 19 soil and sediment materials 77
10 Means, standard deviation, and coefficient of variation
for K and K values of methyl parathion and 2,4-D
adsorption in 19 soil and sediment materials 78
2
11 R values for KA and KB for 2,4-D and methyl parathion
vs. selected independent variables 80
12 Adsorption coefficients for methyl parathion in soil and
sediment materials incubated under controlled pH and
redox potential conditions 81
13 Adsorption coefficients for 2,4-1) in soil and sediment
m.ileri;] 1 s incubated under controlled pH and redox
potential conditions 83
14 Adsorption-desorption ot PCP from Shell Beach sediment ... 84
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ACKNOWLEDGMENTS
The authors wish to acknowledge Ms. Steffanie Scott for operating
the microcosms and analyzing samples early in the project, Ms. Rita
Strate for typing and editorial assistance, and Ms. Sherri Stewart for
help in processing the data. Also, we are grateful to a number of
student assistants who washed glassware and ground soil and sediment
samples.
XI
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SECTION 1
INTRODUCTION
BACKGROUND
The threat of adverse environmental impacts associated with organic
pesticide residues or industrial waste depends on several factors.
These include: (1) the particular compounds present, (2) the concentra-
tions of the compounds present, (3) the environmental compartments in
which the materials are found and the organisms exposed, (4) the persis-
tence or rate of degradation of the material in various environmental
compartments, and (5) the transport of the material in the environment.
Degradation and transport are two key processes influencing the fate and
potential environmental or human health impacts of synthetic organics in
soil or sediment-water systems.
A substantial amount of research has been conducted on the degradation
and transport of synthetic organics in the last 20 years. However, much
of this work has been limited to insecticides and herbicides in typical
agricultural soil materials. Until recent years, little work had been
done with sediment-water systems or with industrial organics. The
sediment-water systems of streams, rivers, lakes, and coastal waters
have been major recipients of pesticide residues, industrial organic
wastes, and chemicals from transportation accidents (point and nonpoint
sources). The physicochemical properties of sediment-water systems
affecting the degradation and transport of synthetic organics are often
substantially different than the properties of agricultural soils in the
area. In deep or quiescent sections of a sediment-water system, the
sediments will tend to have a greater clay content, greater humic material
content, and strongly anaerobic conditions prevailing in the bulk of the
sediment material. However, the oxidation-reduction conditions of
suspended particulates and the top few millimeters or centimeters of
sediments are aerobic. The microbial and chemical processes occurring
in oxidized sediments that may influence the degradation of synthetic
organics are similar to processes occurring in typical upland soils.
Thus sediment-water systems usually include a wide range of oxidation-
reduction conditions.
It has been reported for more than a decade that the degradation
rate of some synthetic organics is affected by physicochemical conditions
of their environment such as pH, oxidation-reduction intensity, and
salinity. However, relatively little research has been done on this
topic. Some researchers have made the generalization that the degradation
of synthetic organics is faster under anaerobic conditions. The research
conducted by this Laboratory over the last few years indicate the effect
of oxidation-reduction conditions on degradation rates is very compound
specific. Even within the t>amc chemical class (organophosphate insecti-
cides, for example), we hnvr seen one- compound degrade more quickly
under aerobic conditions while the recovery of another will decrease
most rapidly under anaerobic conditions. Because of the recent emphasis
on nodeling the fate of synthetic organics in the environment and the
wide range of physical, chemical, and microbial properties encountered
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in soils and sediment-water systems that may affect the persistence of
synthetic organics, it is important to understand the effects of physico-
chemical conditions on the degradation of pesticide residues and industrial
organic wastes.
It is well established in the literature that soil and sediment
hiunic material is a primary component associated with the adsorption of
synthetic organics. Humic materials are also qualitatively and
quantitatively affected by oxidation-reduction conditions. Recent
literature has clearly demonstrated the partitioning of hydrophobia
organics between bound and dissolved forms is associated.with the amount
of humic materials present. However, the possibility also exists that
oxidation-reduction conditions affect the chemical properties of humic
materials affecting adsorption of synthetic organics. Other soil components
such as hydrous iron oxides that may influence the adsorption of synthetic
organics are also affected by oxidation-reduction conditions. One
objective of this project was to conduct preliminary investigations into
oxidation effects on adsorption.
The objectives of this research were to: (1) determine the effects
of pH and oxidation-reduction conditions on the degradation rate of
selected toxic synthetic organic compounds, (2) to examine the effects
of oxidation-reduction conditions and other soil properties on the
adsorption of the selected synthetic organics, and, (3) to demonstrate
an experimental approach for evaluating the effects of physicochemical
conditions of soil and sediment-water systems on the environmental
chemistry of synthetic organics.
SYNTHETIC ORGANICS SELECTED FOR STUDY
The compounds selected for study included methyl parathion (0,
0-Dimethyl-O-p-nitrophenyl phosphorothioate), an organophosphorus
insecticide; 2,4-D (2,4-Dichlorophenoxyacetic acid), a chlorinated
hydrocarbon herbicide; and Aroclor 1254, one formulation of a mixture of
variously chlorinated biphenyl compounds where the chlorine content is
about 54 percent by weight. Methyl parathion and 2,4-D are pesticides
that have been widely used for years and continue to be important chemical
pest control agents. Aroclor 1254 is an industrial organic liquid
widely used for its non-flammability and for its electrical insulating
properties. The PCB's, of which Aroclor 1254 is one formulation, are
noted for the large quantities that have been produced and released in
the environment, their persistence in the environment, .ind the health
risks of these materials when found in drinking w.iter or food supplies.
Another compound, HCP (Pentachlorophenol) was included for adsorption
studies only.
LITERATURE REVIEW
Methyl Parathion
The persistence and accumulation of chlorinated hydrocarbon insecti-
cides such as DDT in the environment has led to the use of less persistent
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insecticides such as the organophosphates. Though more readily degraded
than chlorinated hydrocarbon compounds, this compound is relatively more
resistant to chemical hydrolysis compared to other organophosphates.
There are two kinds of parathion, both of which are used extensively.
Both have a similar structure except for the methyl and ethyl groups.
They are: 1) Ethyl Parathion (0, 0, diethyl-0-p-nitrophenyl phosphorothioate)
and 2) Methyl Parathion (0, 0, dimethyl-0-p-nitrophenyl phosphorothioate).
Methyl parathion has a water solubility of about 60 ppm as compared to
the ethyl parathion solubility of 25 ppm, and a vapor pressure of 37.8
on Hg (Weber, 1972). The structures of ethyl and methyl parathions are
given in Figure 1 below:
S
-* cf»f^
Methyl Parathion Ethyl Parathion
Figure 1. Methyl (a) and ethyl (b) parathion.
Most of the research done on parathion has been with the ethyl
form. Hence the literature reported here is mostly on ethyl parathion.
Though certainly not identical, the environmental chemistry of these two
compounds is somewhat similar. It appears that parathion residues on
the soil surface degrade quickly, however, if the residues penetrate
into deeper soil layers, the residues are more stable. Toxic residues
can be transported to the edible parts of crops. Surface water can also
be contaminated if soil particles containing adsorbed parathion residues
are transported by runoff water (Nicholson et al., 1962). Since the
organophosphates are more water soluble than the chlorinated hydrocarbons,
transport of both soluble and adsorbed forms may contribute to surface
water contamination. Wershaw and Goldberg (1972) reported that processes
affecting the mobilization and immobilization of parathion include: 1)
adsorption through H-bonding, 2) adsorption through van der Waal's
forces, 3) solubilization reactions, 4) chemisorption, 5) ion exchange,
and 6) other chemical reactions. Humic and fulvic acids are the most
active components of soil organic matter influencing the fate of para-
thion. Several functional groups present in humic and fulvic acids
appear to interact with parathion.
There is limited information on the degradation pathway of methyl
parathion, but there are several studies on the ethyl parathion degrada-
tion pathway in flooded and non-flooded soils. Figure 2 shows the
proposed pathway of parathion metabolism in flooded soils. As indicated
in the schematic below, there are two pathways involved in the biodegra-
dation of parathion: 1) nitrogroup reduction of parathion as occurs in
non-flooded and flooded soil systems or in microbial cultures
(Lichtenstein and Schulz, 1964; Zuckerman et al., 1970; Sethunathan and
Yoshida, 1973) and 2) hydrolysis of parathion to p-nitrophenol as an
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C2H3°V'
J >P-0
C2H30
Figure 2.
Para
^""2 "— - /
ihion C2H3°
C2H3°\S '
^>P— OH"*-^
O.O.diethylphosphorothiate
Amii
. — -—
acid
c=TNH2
Aminoparathion
p_-nitrophenol
HO
£-aminophenol
Schematic representation of parathion metabolism via nitro
reduction (R) or hydrolysis (H) (Sethunathan, 1973c).
intermediate degradation product in a flooded soil (Sethunathan and Yoshida,
1972).
Oxidation and hydrolysis are the most important mechanisms of
chemical degradation of parathion. Oxidation of parathion at the P = S
bond has been reported to occur under ultraviolet light and in most
common oxidation systems. But Lichtenstein and Schulz (1964) and Miyamoto
et al. (1966) reported that the chemical oxidation of parathion in soils
and waters is not significant. Ahmed et al. (1958) reported that the
reduction reactions were more important than the oxidation reactions in
metabolizing organophosphate compounds such as parathion.
Sethunathan (1973a) reported that the rate of parathion degradation
in soil depends on soil properties. It appears parathion degrades
faster in soils that have higher organic matter contents. It was reported
to degrade rapidly in acid sulphate soils that had the highest organic
matter content of several soils studied, but slowly in lateritic and
alluvial soils with low organic matter content. Generally organic
materials appear to accelerate the reduction of the nitrogroup in para-
thion. Rajaram and Sethunathan (1975, 1976) reported the influence of
several organic carbon sources on the degradation of parathion in the
following order: glucose > rice straw > algal crust > farmyard manure.
In soils inoculated with a parathion-hydrolyzing enrichment culture,
organic amendments appear to inhibit hydrolysis of parathion - the rate
of hydrolysis with different organic matter amendments following a
reverse order of that of nitrogroup reduction.
Sethunathan (1973) reported that the addition of rice straw as a
source of organic matter inhibited the hydrolysis of parathion to amino-
parathion. He also reported rapid transformation of parathion into
aminoparathion and an unidentified metabolite in the first 3 days. This
was because of a rapid decrease in the redox potential of soil after the
addition of organic matter. Sethunathan and Yoshida (1973) reported
that parathion degrades faster in near neutral pH soils under flooded
conditions than under non-flooded conditions.
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Generally, organophosphates are relatively non-persistent. However,
Graetz et al. (1970) reported the rate of hydrolysis of parathion in
•lightly acid lake sediment as 0.15 to 0.18 percent of added parathion
per day. This indicates that parathion is very resistant to degradation
under acidic conditions (Suffet and Faust, 1972), whereas the hydrolysis
rate in calcareous sediments appear to be more rapid. Stewart et al.
(1971) reported that 0.1 percent of applied parathion remained in soil
even after 16 years because of the association of parathion in lipids of
soil organic matter which apparently offered some protection from bacterial
degradation and hydrolysis. Lichtenstein and Schutz (1964) reported
that the methyl parathion was more readily degradable than ethyl parathion.
Under laboratory conditions, nearly 95 percent of applied methyl parathion
and 30 percent of applied ethyl parathion were lost within 12 days in a
loam soil.
Parathion appears to be more persistent in estuarine water than in
soil. Walker (1976) reported that nearly 94 percent of added parathion
remained in estuarine water even after 25 days of incubation. Salinity
was reported to affect the degradation of organophosphate insecticides
like malathion - the degradation rate increasing with an increase in
salinity. On the other hand, the degradation rate of parathion may be
retarded by increases in salinity. Even after 129 days incubation at a
salinity of 40 parts per thousand and at a pH of 8.1, only 45% of the
added parathion was lost.
Clay minerals and organic matter are the main soil components that
affect the behavior of parathion in the soil environment. The adsorption
of parathion on the clay surface is affected by the saturating cation.
The sorption sequence for parathion is Li > Al > Mg > Na > K > Ca (Yaron,
1978). Adsorption of parathion is also affected by the type of clay,
bydration status of the mineral, and the temperature. Parathion adsorp-
tion on clay minerals in a hydrated form decreases with an increase in
temperature. The effect of temperature is due to an increase in the
solubility of parathion with an increase in temperature, and also due to
the fact that desorption, being an exothermic process, will decrease
with increasing temperature. Yaron (1978) reported that in a partially
hydrated-attapulgite system, parathion molecules cannot replace the
strongly adsorbed water molecule, such that parathion adsorption occurs
on water-free surfaces only. This led to the decreased adsorption of
parathion on attapulgite. The clay-parathion complex is coordinated
through cations, the type of cation determining the structure of the
complex.
Clay surfaces appear to catalyze the degradation of parathion.
Yaron (1978) reported that Kaolinite enhances the degradation of parathion,
which is mainly moisture and cation-dependent. He further reported
relative parathion degradation was 93 percent for Ca-saturated clays and
16 percent for Na and Al-Kaolinites. Whereas for methyl parathion, the
degradation was 64 percent for Ca-saturaled clays, and less than 15
percent for Na and Al-Kaolinites. Finely et al. (1977) studied the
absorption of methyl parathion by clothes worn by workers in the cotton
field sprayed with this insecticide. It appears that all-cotton clothing
absorbs less methyl parathion as compared to cotton-polyester blends.
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Raja ram and Sethunathan (1976) reported that certain toxic products
produced (Patrick, 1971) during the decomposition of organic matter
appear to block hydrolysis of parathion in rice-amended soil under
flooded conditions. These toxic substances produced were not inactivated
by heat treatment of the soil.
Several microorganisms are responsible for the degradation of
parathion in soils and sediments (Sethunathan, 1973b). Lichtenstein and
Schulz (1964) reported that a yeast was capable of degrading parathion.
Miyamoto et al. (1966) studied the major degradalive pathway of methyl
parathion incubated with the bacterium Bacillus subliiis. This resulted
in the reduction of methyl parathion to an amino compound and then
hydrolysis to dimethylphosphorothioic acid. Zuckerman et al. (1970)
reported parathion metabolized into four compounds, the major metabolite
being aminoparathion. Rao and Sethunathan (1974) reported that a fungus
PenciIlium walksmani Zaleski degraded parathion in acid sulphate soil
under flooded conditions. This fungus was reported to be very tolerant
to high concentrations (1000 ppm) of parathion. In flooded soils that
become predominantly anaerobic within a few days after flooding, the
facultative anaerobe Flavobacterium sp. decomposes parathion.
2.4-D
The herbicide 2,4-D (2,4-dichlorophenoxy acetic acid) is one of the
most widely used chemicals for weed control (Figure 3). Extensive
research has been conducted on its fate and behavior in soil-plant
systems (Newman et al., 1952; Ogle and Warren, 1954; Harris and Warren,
1964; Wiese and Davis, 1964; Lavy et al., 1973; CAST, 1975). Its
chemical structure is given below. It has a molecular weight of 222,
and a water solubility of about 650 ppm. This compound and its esters
are also widely used for controlling some species of aquatic weeds in
lakes, ponds, and reservoirs.
-CH2-COOH
Figure 3. 2,4-dichlorophenoxy acetic acid.
Residues of 2,4-D in water can come from several different sources
including: 1) runoff water from agricultural and non-agricultural land,
2) drift from aerial or ground applications, 3) applications to control
floating, submerged and marginal aquatic vegetation, 4) control of
ditchbank vegetation, 5) emptying and washing of herbicide application
equipment, and 6) discharge of industrial wastewater (Frank, 1972).
Residues of 2,4-D may accumulate in soils as spray, either directly to
the soil surface or as the material on aerial parts of plants drip from
-------
the leaves on to the soil surface, or, as a result of the decay of weeds
to which 2,4-D was applied.
Residues of 2,4-D that reach the soil from leaves and from direct
application are tightly bound to several adsorptive soil components and
thus are usually found in the upper part of the soil. The adsorption of
herbicides by soil humus and clay colloids causes some difficulties in
assessing their long-term fate in soils and sediments. The herbicides
adsorbed on soil particles may be transported by soil erosion to lakes
or reservoirs and may persist there due to unfavorable conditions for
microbial degradation. Thus the strong adsorptive properties of soils
and sediments as well as degradation rates under different physicochemical
conditions must be considered to evaluate the impact and fate of herbicide
residues.
It appears that the adsorption of 2,4-D by soil colloids can reduce
its effectiveness and mobility (Crafts, 1960), resulting in the reduction
of leaching of 2,4-D into groundwater. But the 2,4-D sorbed on soil
particles can be carried into surface waters by erosion of contaminated
soil particles. The adsorbed 2,4-D may eventually desorb and contaminate
surface waters.
Leaching and movement of a herbicide like 2,4-D are affected by the
solubility of the herbicide, adsorption of herbicide on the soil, moisture
level of the soil at the time of application, and the amount of evaporation
between rains (Hartley, 1960). The rate of leaching of 2,4-D is also
influenced by soil organic matter, being leached more in soils low in
organic matter than in soils of greater organic matter content (Hernandez
and Warren, 1950). The molecular weight of the herbicide and the type
of salts likely to be found in the soil also influence leaching. Residues
of 2,4-D have been reported to leach readily in a sandy soil and very
slowly in a muck soil.
. The 2,4TD molecule is a weak acid with a dissociation constant, Kd,
of about 10* (Haque and Sexton, 1968). This acidic herbicide can
readily dissociate in solution to form anionic species. In aqueous
solution, both the dissociated and undissociated species exist. The
proportion of each species depends on the pH of soil solution. The
dissociated anions are not readily adsorbed directly by the negatively
charged soil colloids. Weber (1970) reported that these organic anions
may be adsorbed by positively charged oxyhydroxides of iron and aluminum
through anion-exchange reactions. He also reported that the phenoxy
herbicides may precipitate as calcium salts in calcareous soils. These
processes appear to reduce the mobility and effectiveness of 2,4-D.
The 2,4-D molecule can be bonded to several soil colloids through
the aromatic rings and functional groups. The adsorption strength
appears to be weaker for .in acidic herbicide Like 2,4-D than for basic
herbicides such as triazincs (Weber rt al., 1965). Reactions of the
following type may be responsible Tor adsorption of 2,4-D on an anion-
exchange site such as ferric oxyhydroxides (Figure 4).
Miller and Faust (1972) reported that even though clays are negatively
charged, they can indirectly adsorb the negatively charged 2,4-D molecules.
-------
2,4-D
-CH2COOH Dissociation
-CH2COO"
Dissociated 2,4-D
-CH2COO~
Cl
Anion
Exchanger
Z 2,4-D
Cl"
Dissociated 2,4-D Anlon Exchanger
Figure 4. One possible adsorption mechanism for 2,4-0.
Adsorbed 2,4-D on
Anion Exchange site
It appears clays adsorb soiJ organic molecules in a specific orientation
and the resulting soil organic matter coated clays then adsorb 2,4-D.
They also reported that several industrial wastes such as cationic
detergents, disinfectants, cationic dyes, floatation chemicals, and
various anions bond to clays strongly, thereby forming an organic
surface on which organic molecules such as 2,4-D can sorb. Miller and
Faust (1972) reported that the sorption of 2,4-D by Bentone-24 (Wyoming
bentonite coated by sorbed dimethylbenzyl octadecyl ammonium chloride)
increases with an increase in clay:water ratio and a decrease in pH from
7.0 to 3.4. Sorption was also affected by the type of cation present.
Harris and Warren (1964) reported the adsorption of 2,4-D by an
organic muck soil, bentonite, an anion-exchanger and a cation-exchanger.
Their results show that the adsorption depends upon several factors
including the nature of adsorbent, pH, temperature, and the nature of
herbicide. Their results also show that 2,4-D was readily desorbed from
bentonite compared to a muck soil. These findings suggest that 2,4-D is
more strongly adsorbed by organic matter than by clay minerals like
bentonite. Weber (1972) reported that 2,4-D is adsorbed on these compounds
in molecular form under acidic conditions. The adsorption mechanisms
for the anionic herbicides in acidic medium involve hydrogen bonding
with multiple sites being available on the surfaces of both organic
matter and herbicide. Whereas under neutral or alkaline soil conditions,
the anionic herbicides are bound through salt linkage (Stevenson, 1972;
Khan, 1973).
The two major types of adsorbing surfaces available for herbicides
are the clay-humus surface nnd clay surface .ilone. The relative contri-
bution of humus and clay to the ;xlsorpLion of 2,4-D depends upon the
extent of coating of clay minerals by humus. Stevenson (1972) reported
that if the soil contains more than 8 percent organic matter, the organic
surfaces are primarily responsible for adsorption of herbicides. The
clay minerals are mainly responsible tor the adsorption of herbicides in
soils containing less than 8 per cent organic matter. But in soils with
similar organic matter content, the contribution of humus to the adsorption
-------
of herbicides will be more than the clay where Kaolinite is the predominant
clay mineral, and less than the clay where montmorillonite is the predominant
clay mineral. Adsorption capacity for three dominant clay minerals in
soils follows the order: montmorillonite > illite > kaolinite.
Acidic herbicides like 2,4-D and 2,4,5-T may leach readily in
coarse textured soils and in soils low in organic matter (Ogle and
Warren, 1954; Wiese and Davis, 1964; Weber, 1972; White et al., 1976).
Several bonding mechanisms such as H-bonding, ion exchange, van der
Waal's forces and coordination through the attached metal ion (salt
linkage) are responsible for the adsorption of herbicides by soil
colloids. But for anionic herbicides like 2,4-D, H-bonding and salt
linkage are especially important (Stevenson, 1972).
The adsorption of 2,4-D depends upon several factors like temperature,
solubility of the compound, moisture content and other surface properties
of soil colloids. Since the adsorption process is an exothermic reaction,
it decreases with increasing temperature. Temperature also affects the
adsorption indirectly by its effect on the solubility of 2,4-D. The
type of cation associated with 2,4-D affects its solubility and hence
its adsorption. Aly and Faust (1964) reported the solubility of calcium
salt of 2,4-D as 4000 mg/2, and that of magnesium salt of 2,4-D as
11,000 mg/JK.
The degradation of 2,4-D has generally been attributed to microbial
activity. Soil and environmental factors which can affect microbial
activity could also influence 2,4-D degradation indirectly. Persistence
of 2,4-D has been reported to vary from 10 days to 14 weeks. The varia-
tions in persistence are attributed to different soil types, rainfall,
temperature, application rates, formulations, and other factors. Brown
and Michell (1948) reported that a low moisture content of 5 to 10
percent or below, and a low soil temperature of about 50°F prolong the
toxic effects of 2,4-D in soils. It also persists for a long period of
time under field conditions in cold weather. Factors such as soil
organic matter content, soil moisture, and temperature apparently
influence the rate of 2,4-D inactivation indirectly through their effect
on the growth of soil microorganisms. The breakdown of 2,4-D is also
affected by soil type. Ogle and Warren (1954) reported the degradation
of 2,4-D was lowest in sandy soil, intermediate in silt loam, and highest
in muck soil. Bioactivity, persistence, biodegradability and leachability
have been found to be directly related to the humus content of the soil
(Bailey and White, 1964).
Bell (1956) reported that photodegradation is not the major route
of dissipation of 2,4-D in soil. Andus (1964) discussed in detail the
behavior of herbicides in soil. Research done in the past indicates
that biodegradation is mainly responsible for its dissipation.
Crafts (1964) reported that metabolism by plants also contributed
to the disappearance of 2,4-D from soil. The bacterium Sporocytophaga
congregata, an obligate aerobic organism has been shown to be effective
in degrading 2,4-D in soil under aerobic conditions (Jensen and Peterson,
1952). Lavy et al. (1973) reported rapid dissipation of 2,4-D under
both aerobic and anaerobic conditions in the soil. This, they attributed
to the presence of facultative microorganisms.
-------
From the literature available, it is apparent that both the adsorp-
tion and degradation of 2,4-D in soils and sediments must be considered
in evaluating its fate and environmental impact.
Polychlorinated Biphenyls (PCBs)
Polychlorinated biphenyls, usually called PCBs, consist of two
benzene rings joined with a C-C bond between one carbon atom of each
ring, and up to 10 chlorine substitutes for hydrogen on the rings (Figure
5). Considering the varying number and positions of possible chlorine
Cl—,
Figure 5. 2,4'-dichlorobiphenyl, one of more than 200 possible PCB
isomers.
substitutions, there are more than 200 possible compounds or isomers.
PCBs were produced as mixtures of various isomers, thus they are found
in the environment as mixtures of isomers. PCBs have been manufactured
since 1929 and it has been estimated that total production has exceeded
800 million pounds before their manufacture ceased in the U. S. in the
late 1970s. Because PCBs are non-flammable fluids (if there are four or
more chlorine substitutions) with excellent dielectric properties, they
have been widely used in the manufacture of countless industrial and
consumer items. Some of the industrial applications include high pressure
and temperature lubricants, hydraulic fluids, plasticizers, heat transfer
and electrical insulation fluids, and others (Griffin et al., 1977).
Although there are now restrictions on the uses of PCBs, its persis-
tence coupled with accidental releases, waste discharges, and improper
disposal of old equipment has resulted in its widespread distribution in
the environment. Transport of PCBs can be from industrial sources and
other contaminated areas by streams ami rivers or by atmospheric movement
(Armstrong and Swackhamer, 1982). The importance of atmospheric transport
is indicated by results of a study suggesting atmospheric transport and
subsequent deposition may be the primary input mechanism for PCB into
Lake Superior (Eisenreich et al., 1979).
Because of their widespread distribution in the environment, their
toxicity to aquatic organisms and man, and their persistence, there is
much interest in the rate of degradation of PCBs in different compartments
10
-------
of the environment as well as in factors affecting degradation. Degrada-
tion of PCBs in soils and sediment-water systems occurs primarily either
by microbial action or photochemical pathways, the photochemical route
being the only significant non-biological degradative pathway. Photo-
chemical degradation may be a factor for a portion of the trace levels
of PCBs dissolved in shallow surface waters, suspended on particulates
in shallow reaches of surface waters, or moving by atmospheric transport
during air transport. However, much of the PCB contamination of the
environment is associated with sediments where photodegradation is not
an effective dissimulative mechanism. In sediments and all but the very
surface layer of soils, microbial degradation will be the primary degra-
dative mechanism.
PCBs are strongly adsorbed to the sediment solid phase, but there
is some low level of PCBs in the aqueous phase, though the concentrations
are very low (Fulk et al., 1975). However, the fact that most of the
PCBs are bound with the sediment phase does not mean that the PCBs no
longer represent an environmental or health threat. When PCB contaminated
sediments have been introduced into an uncontaminated sediment-water
system, high levels of PCBs have accumulated in fish indicating transport
by one or more processes from the sediment phase (Sherwood, 1976).
Pure culture studies have demonstrated the potential for PCB degra-
dation by microorganisms. It has been reported, using pure culture
techniques, that bacteria capable of degrading PCBs were present in all
of numerous water and sediment samples taken from many areas of the east
coast of the United States (Sayler et al., 1978). The more contaminted
areas had greater numbers of PCB degrading bacteria and a greater diver-
sity of organisms capable'of degrading PCBs.
Both the position of chlorine substitution and the number of chlorine
substitutions on a biphenyl ring have been reported to affect microbial
degradation rates. Wong and Kaiser (1974) reported the order of degradation
rates of biphenyl and two mono-chloro isomers as biphenyl > 2-chlorobiphenyl
> 4-chlorobiphenyl.
Shiaris and Sayler (1982) reported a natural, mixed population of
microorganisms in lake water could oxidize 2-chlorobiphenyl, but not
2,4'-dichlorobiphenyl. The number of chlorine substitutions is probably
a more important factor affecting degradation rate. Veight (1970) noted
that Aroclor 1242 levels decreased faster in surface waters than Aroclor
1260 and suggested Aroclor 1260, which has greater chlorine substitution,
was more persistent. In a study with one microbial genus, Gibson et al.
(1973) found that biphenyl and monochlorinated derivations of Aroclor
1221 were degraded more rapidly than the more highly chlorine substituted
isomers. Others have also reported that increasing chlorination levels
result in greater persistence of biphonyl (Ahmed and Focht, 1973; Tucker
et al., 1975).
Most PCB degradation work has been done with pure cultures under
oxidized conditions. The relatively lit.tic work done on natural systems
suggest PCBs may be more resistant under anaerobic conditions. Fries
(1972) reported no apparent degradation of Aroclor 1254 in silage sub-
jected to typical anaerobic fermentation for several months. Labeled
11
-------
tetrachlorobiphenyl has been incubated with reduced marsh soil and sea
water for 45 days with no apparent transformation of the original mole-
cule, though some degradation was measured within 10 days under aerobic
conditions (Carey and Harvey, 1978). Johnson (1973) incubated soil and
cattle manure-enriched soil with selected single PCB isomers under
flooded conditions for one month and reported no degradation was observed.
Griffin et al. (1978) reported aerobic degradation of water soluble
Aroclor 1242 was 92 percent complete within 1 day and that isomers with
less than four chlorines were degraded while those with four or more
chlorines did not degrade to a significant extent. In view of this
apparent rapid degradation rate, two things should be pointed out: 1)
the water soluble Aroclor 1242 may have been predominately the isomers
with the least chlorine substitution that are known to degrade most
quickly, and, 2) the amount of water soluble PCB in a typical contami-
nated sediment-water system represents a very small proportion of the
total. In field plots where PCB-contaminated sewage sludge had been
applied for 7 years, the PCBs recovered were primarily the high
chlorine substituted isomers.
12
-------
SECTION 2
MATERIALS AND METHODS
CHARACTERIZATION OF SOIL MATERIALS
Soil and sediment materials were air-dried and ground to pass
through a 2 mm sieve for characterizing selected physical and chemical
properties. Soil samples were characterized for pH, cation exchange
capacity, organic carbon, particle size distribution, free iron oxides,
and Ca, Al, and Mn extracted with the free iron oxides. Sample pH was
determined on a 1:1 soil-water suspension (Peech, 1965). Cation exchange
capacity was determined by the ammonium acetate method (Chapman, 1965).
Organic carbon was determined by the Walkley and Black method (Jackson,
1967), and organic matter content was calculated by multiplying organic
carbon by a factor 1.724 (Wilson and Staker, 1932). The particle size
distribution was determined by the hydrometer method (Day, 1965), and
free iron oxide was extracted by an ammonium oxalate extraction procedure
(McKeague and Day, 1966). Iron, Al, Ca, and Mn were measured in the
ammonium oxalate extract by an ICP emission spectrometer.
CONCENTRATION EFFECTS OF METHYL PARATHION AND 2,4-D ON THE RATE OF SOIL
REDUCTION
The insecticide methyl parathion (0,0-dimethyl-O-p-nitrophenyl
phosphorothioate) and the herbicide 2,4-D (2,4-dichlorophenoxy acetic
acid) were used in this study. Two surface soil materials were used: a
Crowley silt loam and a Cecil clay loam. Five concentration levels were
studied: 0, 5, 10, 25, and 75 pg/g, oven dry solids basis. To simulate
the flooded soil system in the laboratory, 10 g aliquots of air-dried
and ground soil amended with specified concentrations of the two compounds
were placed in test tubes and 15 ml of deionized water was added to
flood the soil. A fresh sample of bayou water containing a small quantity
of bottom sediment was collected, mixed well, and allowed to settle.
One-half ml of the supernatent was then added to each test tube to
innoculate the soil-water mixtures, then the tubes were shaken and
allowed to settle. Eighty tubes were used for each compound (8 sampling
dates x 2 subsamples x 5 concentration levels). Replicate subsamples
were taken for measurements at 0, 2, 4, 6, 9, 12, 16, and 20 days for
methyl parathion and 0, 6, 12, 18, 24, 30, 40, and 50 days for 2,4-D.
Temperature was maintained at 28°C C±2°. The test tubes were sealed and
were purged with nitrogen once a day.
Redox potential was measured by placing bright platinum electrodes
in the soil material in those tubes removed for sampling. A measurement
was recorded after allowing one hour for equilibration of the electrode
in the soil.
Soil pH was measured using a calibrated, combination pH electrode
that was inserted directly into the soil material.
13
-------
Exchangeable metals were extracted by shaking the soil materials
with 30 ml of a IN ammonium acetate solution for one hour (15 mis of 2N
ammonium acetate were added to the soil-water mixture, as described by~
Howeler and Bouldin, 1971). The pH of the ammonium acetate solution was
adjusted to approximately the pH of the soil-water mixture. After
shaking, the soil suspensions were centrLfuged at 2,000 rpm for 10
minuteu (IEC 11277 rotor) and then filtered. Extractable Fe, Mn, and Zn
were measurer! by an ICP emission spectrometer. This extraction would
include both water soluble and exchangeable metals.
DEGRADATION STUDIES
Controlled pH-Redox Potential Microcosms Using Gas Chromatography for
Analysis
*
Microcosms—
Soil- and sediment-water mixtures amended with synthetic organics
were maintained as suspensions in 2-liter flasks by continuous stirring
using a motor driven magnetic stirrer (Figure 6). To minimize abrasive
wear, flexible polyvinyl chloride (Tygon) tubing was placed over the
2-inch (5.08 cm) teflon coated magnetic stirring bar. The 3-necked
flasks were equipped with two platinum electrodes for monitoring redox
potential, a combination electrode for measuring pH, a thermometer, a
septum through which suspension samples could be obtained and acid or
basic solutions could be added to adjust pH, separate inlet tubes for
air and nitrogen, and an outlet tube that was connected to a water trap
to prevent gaseous oxygen diffusion into the flask.
All suspensions were maintained at 28-30°C. Thin layers of insulating
material were placed between the flask and the stirring motor as necessary
to control heat transfer from the motor.
A saturated calomel reference electrode used with the platinum
electrodes was connected to the suspensions with a saturated potassium
chloride-agar salt bridge. The platinum electrodes were connected to a
millivolt/pH meter (Beckman Zeromatic III or IV models) operated in the
millivolt mode for monitoring and control of redox potential. A meter
relay (General Electric Type 195 or 196) was connected to the recorder
output of the millivolt meter. This meter relay switched on an aquarium
air pump when the redox potential became more reducing than the selected
redox level for each incubation flask. In the absence of oxygen, soil
and sediment-water systems tend to become anaerobic or more reducing due
to the activity of microbial populations. A low flow (1 to 4 ml/minute)
of air during the air-pumping cycle slowly increased redox potential to
the preset level at which time the meter relay would switch off the air
pump. By regulating the addition of air to soil and sediment systems
with appreciable microbial activity, oxidation-reduction levels can be
controlled over a wide range.
Nitrogen gas was continuously purged through the suspension at a
rate of about 5 ml/minute to: 1) flush excess oxygen from the suspen-
sions at the end of an aeration cycle, 2) prevent an accumulation of
14
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C a lom el
hall call
Meier
relay
w\
1J_
"t
,1 , Air
] pump
- N,
1 pH electrode
2. Platinum electrode*
3. Salt bridge
4. Gat outlet
5. Serum cap
6. Air Inlet
7. N2 Inlet
8. Thermometer
9. Stirring bar
Figure 6. Laboratory microcosm for incubating soil and
sediment suspensions under controlled pH and
redox potential conditions.
r>
-------
gaseous decomposition products such as carbon dioxide or hydrogen sulfide
that might affect microbial activity, and 3) remove air contamination
should there be small leaks in the system.
The pH of the suspensions was checked daily and adjusted as necessary
using a syringe to add a dilute solution of either hydrochloric acid or
sodium hydroxide through the septum.
Each batch of soil or sediment material collected was uniformly
mixed before 300 gram aliquots were added to incubation flasks. Fifteen
hundred ml of water were added and stirring was initiated for the preincu-
bation period. A preincubation period (prior to amending the sediment
with a synthetic organic) was necessary to gradually achieve the desired
pH and redox potential levels. Approximately 7 to 12 days were usually
necessary to achieve the selected redox potential levels and this period
was followed by another week at the desired pH and redox potential
levels before the synthetic organics were added.
On sampling dates, a 50-ml glass syringe equipped with a glass
needle was used to withdraw suspension aliquots from the incubation
flasks. The samples were frozen immediately in 4 ounce (112 ml) glass
jars fitted with aluminum foil-lined caps if they were to be stored
before extraction.
Extraction Methods--
Methyl parathion--!. The samples were thawed if stored. 2. About
0.5 g sodium chloride was added to the thawed sediment-water mixture and
then the sample was centrifuged for 15 minutes (IEC //266 or #277 rotor).
3. The supernatant solution was discarded (or poured into a separately
funnel it the solution phase was to be analyzed). '4. One to 2 ml of
toluene was added, with mixing, to the wet sediment to inhibit microbial
degradation during sample drying, then the sample was placed in a forced
draft oven for drying at a temperature of 35-38°C. 5. When dry, the
soil or sediment materials were finely ground and weighed into cellulose
extraction thimbles. 6. The soil or sediment solids were extracted
with 130 ml of acetone on a Soxhlet extraction apparatus for 4 hours.
7. The extracting solution was rinsed into a flask with acetone, then
moisture was removed by filtering the solution through acetone-rinsed
sodium sulfate. 8. The solution was then made to volume in 200 or 250
ml volumetries with acetone. 9. The sample was concentrated if necessary.
2.4-D--1. The sample (1-2 g dried weight equivalent) was thawed,
if stored. 2. Fifteen ml of I percent potassium hydroxide was added to
the sample and the pH checked to confirm a pH of approximately 11 was
achieved. 3. The samples were extracted by shaking with the alkaline
solution for 15 minutes. 4. The alkaline extract was heated in a water
bath at 45°C for 15 minutes, then centrifuged at 5000 rpm (Sorvall GS-3
rotor) for 30 minutes. 6. The supernatant was decanted through glass
wool into a separatory funnel. 7. The potassium hydroxide extraction
was repeated .ind the extracts combined in a separatory funnel. 8. The
oven dry weight of the extracted soil or sediment material was determined.
9. Twenty ml of diethyl ether was added to the alkaline extract, the
16
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separatory funnel was shaken for 1 minute and the phases allowed to
separate. 10. The potassium hydroxide phase (bottom layer) was trans-
ferred to a second separatory funnel and the ether layer discarded. 11.
Steps 9 and 10 were repeated twice more. 12. The potassium hydroxide
extract was transferred to another separatory funnel, and 1 ml (or more
if necessary) of 9N sulfuric acid added to reduce the pH to 3. 13.
Twenty ml of diethyl ether was added, the mixture shaken and the phases
allowed to separate. 14. The ether layer was transferred to a flask.
15. Steps 13 and 14 were repeated and the ether extracts combined. 16.
A pinch of acidified sodium sulfate was added to the combined ether
extract and the ether evaporated to 5-10 ml under a stream of nitrogen.
17. The ether extract was then transferred to a 15 ml centrifuge tube
and evaporated to dryness. 16. One ml of a fresh solution of diazo-
methane was added to each tube, the tube capped tightly, then heated for
15 minutes at 45°C in a water bath. 17. After methylation, one ml of
hexane was added and the volume evaporated to 1 ml. 18. The methylated
sample was then diluted to an appropriate concentration for GC analysis
with hexane that had been dried by filtering through anhydrous sodium
sulfate.
Aroclor 1254--1. The sample was thawed if stored. 2. About 0.5
grams of sodium chloride was added to the sediment-water mixture, then
the sample was centrifuged for about 15 minutes to facilitate separation
of the aqueous phase, and the clear supernatant was discarded. One ml
of toluene was added to the sediment to retard microbial activity, then
the sediment material was dried at 35-38°C in a forced draft oven.
4. The dried solids were finely ground and were transferred to tared
cellulose thimbles and the sample weight determined. 5. The thimbles
were placed in a Soxhlet extraction apparatus and extracted Tor 16 hours
with a 40% hexane/60% acetone mixture. 6. The extract was transferred
to a separatory funnel and the acetone removed by shaking with an addition
of 100 ml of sodium chloride-saturated water. 7. This acetone removal
step was repeated twice more with 50 ml of the sodium chloride saturated
water. The Aroclor 1254 partitioned into the hexane was passed through
anhydrous sodium sulfate to remove moisture and collected in a volumetric
flask. The flasks were brought to volume with hexane.
Gas Chromatography--
For the degradation studies under controlled pH-redox potential
conditions, all samples were analyzed by gas chromalography (Perkin-Elmer
Model 3920B). A summary of the instrumental parameters is given in
Table 1. Sample concentration was determined by comparing sample and
standard peak areas, when an intergrator was used (Parkin-Elmer Model
M-2), or by comparing sample peak height times the instrument attenuation
with the peak height times instrument attenuation values of standards.
For Dost methyl parathion and 2,4-1) samples, the intergrator was used to
record peak areas. For Aroclor 1254 which gives a multipeak chromatogram,
five of the major characteristic peaks were identified, and the sum of
peak height times attenuation for these peaks were compared for sample
and standards to give sample concentration. The concentrations of
samples were kept within the linear response range of the instrument, or
dilutions were made as appropriate.
17
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TABLE 1 . GAS CHROMATOGRAPHY PARAMETERS FOR METHYL PARATHION. 2.4-D. AND AROCLOR 1254
c=
Parameter
Column
Column dimensions
Carrier gas
Carrier gas flow rate
Injector temperature
Column temperature
Interface temperature
Detector
Methyl Parathion
1.5% OV-17, 1.95% QF-1
glass, 6' x V O.D.
(1.8 m x 6.35 mm O.D.)
0-grade N2
70 ml/nun
230°C
200°C
250°C
Nitrogen/Phosphorus specific
(thermoionic)
2,4-D
3% OV-210
glass, 6' x V O.D.
(1.8 m x 6.35 mm O.D.)
0-grade NZ
70 ml/min
230°C
170°C
250°C
ECD
(275°C)
Aroclor 1254
3% OV-1
glass, 61 x k" O.D.
(1.8 m x 6.35 mm O.D.)
0-grade N2
70 ml/min
230°C
190°C
250°C
ECD
(275°C)
-------
Research and standard material for methyl parathion and 2,4-D were
obtained from Chem Service, Inc. (West Chester, PA). The Aroclor 1254
was obtained from Applied Science (State College, PA). Identification
and purity of the compounds from commercial sources were confirmed by
comparisons of chromatograms with Analytical Reference Standards from
the EPA Health Effects Research Laboratory.
Carbon Dioxide Analyses--
In selected degradation studies using the controlled pH-redox
potential microcosms, carbon dioxide evolution from the microcosms was
measured as an indication of microbial activity. The microcosms were
operated in the usual manner, except that all air or nitrogen going into
the suspensions passed through two sodium hydroxide traps connected in
series to remove incoming carbon dioxide, and the exit gas stream was
passed through another two alkaline traps (either 0.1 N or 0.2 N NaOH)
to remove that carbon dioxide that was generated in the suspensions.
Standardized acid and base solutions were used and the quantify of
carbon dioxide trapped was measured by titrating the alkaline solution
to determine the amount of base neutralized and then calculating the
carbon dioxide equivalent.
Biometer Flasks Using Mineralization Hate of C-LabeLed Compounds for
Analysis
Biometer flasks.^Figure 7) were used for degradation studies in
which collection of C-labeled carbon dioxide was used to measure
degradation of the compounds studied (Bartha and Pramer, 1965). Ten
grams of soil or sediment material that had been air-dried and ground
were weighed into flasks. To insure an active microbial population, all
of the soil and sediment materials were inoculated after being weighed
into flasks. The inoculate was prepared by obtaining a fresh sample of
bayou water containing a small quantity of bottom sediment, mixing this
well, and allowing the solids to settle. Then 0.5 ml of the supernatent
was mixed with 100 ml of deionized water. Approximately 2 ml of this
water (depending on the amount required to achieve field moisture capacity
of each soil material) was added to the 10 grams of soil. A humidified
nitrogen stream was continuously purged through those flasks designated
for the reduced treatment, and similarly, air was used for those flasks
designated for the oxidized treatment. Prior to adding the labeled
compounds, the flasks were purged with air or nitrogen for 5 days to
obtain reduced or oxidized conditions in the soil materials. Each
experimental combination was replicated three times.
As the labeled compounds were mineralized to labeled carbon dioxide
and purged from the biometer flasks, the labeled gas was collected in
two sodium hydroxide traps connected in series to the outlet gas line
from each flasks. One ml of the alkali trapping solution was transferred
to 15 ml of a commercial liquid scintillation counting reagent in vials
and the activity was measured on a Beckman liquid scintillation counter
(Model LS100C). All compounds (methyl parathion, 2,4-D, and Aroclor
1254) were uniformly labeled in the aromatic ring.
19
-------
humidified air (OX
o r
nitrogen (RE)
^ NaOH solution to
collect 14CO2
-------
ADSORPTION STUDIES
Adsorption of Methyl Parathion and 2,4-D by 19 Soil and Sediment Materials
Soils—
To study the factors affecting pesticide adsorption, 19 soil and
sediment materials were selected to include a broad range of physical
and chemical properties. Samples were air-dried and ground to pass
through a 2 mm sieve for characterization and adsorption studies.
Soil samples were characterized as previously described for pH,
cation exchange capacity, organic carbon, particle size distribution,
free iron oxides, and Ca, Al, and Mn extracted with the free iron oxides.
Pesticides--
The pesticides selected for this study were the insecticide methyl
parathion (0-0-dimethyl-O-p-nitrophenyl phosphorothioate) and the herbicide,
2,4-D (2,4-dichlorophenoxyacetic acid). It would be expected that the
partitioning of these pesticides between dissolved and adsorbed forms
would be affected differently by changes in soil properties. The solubility
of methyl parathion in water at 25°C is 50 ppm (Wauchope, 1978), and the
solubility of 2,4-D in water is approximately 0.062% (Loos, 1969).
Pesticide standard compounds from Chem-Service, Inc. were amended
with C-labeled materials obtained from Pathfinder Laboratories, Inc.
for this study. Stock solutions containing labeled methyl parathion and
2,4-D (specific activity 1 uCi/ml) were prepared in acetone and stored
in the refrigerator. The stock solutions were diluted with water such
that on a soil solids basis, the amended levels of compound and label
were 1 pg compound/g and 0.1 pCi specific activity per g respectively.
A batch equilibrium technique described below was used for this study
(Leenher and Ahlriches, 1971).
Twenty milliliters of a given pesticide solution were added to 2 g
of air-dried soil in a 50 ml stainless steel centrifuge tube, capped
with teflon-lined lids and equilibrated for 2 hrs on a mechanical box
shaker. Each soil treatment combination was replicated three times.
Following equilibration, the tubes were centrifuged for 15 minutes at
12,000 rpm in a DuPont Sorvall SA-600 rotor. AliquoLs of the clear
supernatant solution (1 ml) were transferred to counting vials and
C-labeled material was measured on a Beckman Model LS100C liquid
scintillation counter. To prepare a sample for counting, 1 ml of an
aqueous sample was mixed with 15 ml of a commercial liquid scintillation
counting reagent. Decreases in pesticide solution concentration were
attributed to adsorption by the soil. After the 1 ml sample aliquot was
removed at 2 hours, the caps wore rrpl.ict.Ml on the centrifuge tubes and
the equilibration continued for 22 more hours when another 1 ml aliquot
was sampled. Thus data wen> obtained for 2-hour and 24-hour equilibra-
tions. Each sample was counted for 10 minutes. Standards and blanks
were also counted and included in calculations to get adsorption coeffi-
cients.
21
-------
The amount of pesticides adsorbed on soils has been expressed as
the soil-water distribution coefficient (K) calculated by the formula
given below (Luchini et al., 1981; Wahid and Sethunathan, 1978).
if - Mg adsorbed/g soil
pg dissolved/g solution
When the sorption of compounds is expressed as a function of the
organic carbon content of a soil or sediment, a coefficient (K ) is
generated that is dependent upon the organic component (Briggs°c1973;
Hamaker and Thompson, 1972; Karickhoff et al., 1979; Khan et al., 1979),
K - K
be % OC (decimal equivalent)
K is equal to the distribution coefficient (K) divided by percent
organic carbon in the respective soil or sediment. The K value is a
measure of the partitioning of the compound between an aqueous solution
and a stationary organic phase (humus).
The Effect of pH and Redox Potential on the Adsorption of Methyl Parathion
and 2,4-D
At the conclusion of selected degradation studies, an aliquot of
the soil or sediment suspensions was removed, air-dried, and ground for
adsorption studies on materials that had been incubated under controlled
pH and redox potential conditions. The methods used were essentially
the same as described in the preceding section.
The Effect of Redox Potential and Sample Processing Methods on the
Adsorption of Pentachlorophenol
Collection of Research Material--
On October 9, 1980, sediment and water were collected from the area
surrounding a 22 July 1980 ship collision on the Mississippi River Gulf
Outlet near Shell Beach, Louisiana, where 25,000 pounds of granular PCP
(pentachlorophenol) was spilled into the channel. Sediment samples were
collected using a Peterson dredge. Water sampJes were collected by
immersing 18.9 liter plastic carboys below the water surface and filling.
Adsorption-Desorption Study--
An adsorption-desorption study was conducted with PCP using Shell
Beach sediment material preinrubalcd as suspensions undrr reduced and
oxidizing conditions by continuous purging with nitrogen or air, respec-
tively. The pH of the reduced and oxidized suspensions was maintained
22
-------
at 6.8, the pH of the sediment as collected. A batch equilibrium adsorp-
tion technique was used (Leenher and Ahlriches, 1971) on 2 g-aliquots,
oven-dry solids basis, of the wet sediment material. Based on the
predetermined waterisolids ratio, the solution volume of the sediment
material was made up to a total of 20 mis with a C-labelled PCP solution
having a final activity of 3.5 x 10 pCi/ml in 50 ml stainless steel
centrifuge tubes. Other sample aliquots were dried and ground after the
incubation under oxidized and reduced conditions to determine the effect
of this sample processing step on adsorption and desorption of PCP..
Concentrations of unlabelled PCP in the added solution was 1 |Jg ml" .
The tubes were sealed with teflon-lined stainless steel caps and the
sediment-solution mixture equilibrated for 2 hrs on a reciprocating box
shaker. Following this equilibration, the tubes were centrifuged for 15
minutes at 12,000 rpm (Sowall SA-600 rotor) and a 1 mi-aliquot of the
clear supernatent solution transferred to an LS vial for C assay by
liquid scintillation counting. The C-labelled sediment aliquots were
then resealed, the equilibration continued,by shaking for an additional
22 hours, and a 1 ml aliquot removed for C counting as described
previously. For the desorption phase, the supernatant after the 24-hr
equilibration was discarded and the sediment material was dried at low
temperature, then finely ground within the tube. Twenty ml of deionized
water was added, the suspension equilibrated for 2 hrs by shaking, and a
solution aliquot taken for C counting as done for the adsorption
phase. The adsorption-desorption studies were done using triplicate
subsamples.
The amount of PCP adsorbed or desorbed was expressed as the solid
phase:aqueous phase distribution coefficient (K) calculated by the
formula (Luchini et al., 1981):
,. _ pg adsorbed/g soil
l\ ™
ug dissolved/g solution
23
-------
SECTION 3
RESULTS AND DISCUSSION
PROPERTIES OF SOIL AND SEDIMENT MATERIALS STUDIED
Selected physical and chemical properties of the soil and sediment
materials studied in this project are given in Table 2. Materials
number 1, 2, 3, 4, 10, 12, 13, 17, and 20 are sediments. The remainder
are identified by a soil series name and are topsoils (A or Ap horizons)
except for number 5 which is a subsoil or B horizon material.
CONCENTRATION EFFECTS OF METHYL PARATHION AND 2,4-D ON THE RATE OF SOIL
REDUCTION
When an oxidized soil is flooded such that oxygen transport is
greatly restricted, soils with appreciable microbial activity become
more reducing as indicated by a decrease in redox potential.
In the Crowley soil material amended with varying levels of 2,4-D,
redox potential for all treatments decreased from about 450 to 500 rav
(indicating moderately oxidizing conditions) to 0 to 50 rav in six days
with no clear indication that 2,4-D levels affected reduction (Figure
8). Except for approximately 18 days being required to reach minimum Eh
levels in the Cecil soil material, the results were similar with this
soil in that levels of 2,4-D did not appear to affect the rate of reduction
(Figure 9).
Methyl parathion amendments did apparently affect the rate of
decrease in redox potential in both soils (Figures 10 and 11). Redox
potential levels in control soil materials were generally much lower
than in the amended materials during the phase of the incubations when
the soils were gradually becoming more reduced. At the concentrations
included in these particular soil materials, the data suggest methyl
parathion may stress microbial populations more than 2,4-D even at
levels below 100 ppm.
Although a change in redox potential is certainly a primary indication
of microbial activity, other important changes also occur as a soil
becomes more reduced. The pH of and soils tends to increase toward
neutrality when subjected to flooding that results in development of
anaerobiosis (Patrick and Mikkelsen, 1971). An increase in pH with time
and decreasing redox potential was observed for all combinations of
soils, compounds, and concentrations. Tn the Crowley soil amended with
2,4-D, pH increased more rapidly in the control than the amended soil
material and remained higher (especially when compared to the 75 ppm
treatment) during the first 25 days, but there was considerable switching
of relative pH levels of the various treatments over the next 25 days
indicating considerable experimental variability within the narrow pH
range of the samples (Figure 12). There was also no indication of 2,4-D
concentration effects in the Cecil soil (Figure 13). In contrast,
methyl parathion did appear to retard the rate of increase in pH as both
24
-------
TABLE 2. SELECTED PHYSICOCHEMICAL PROPERTIES OF SOIL AND SEDIMENT MATERIALS
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
Soil material
Airplane Lake ,
Atchafalaya River
Bayou Chevreuil.
Calcasieu River
Cecil Subsoil
Cecil Top Soil
Chastain
Crowley
Gallion ,
Hartwell Lake
Lafitte Muck ^
Lake Pontchartrain
Lake Providence
Leeville
Loring
Mhoon .
Mississippi River
Norwood
Shubuta .
Barataria Bay
pH
6.8
7.6
5.4
7.0
5.6
5.1
3.4
6.4
6.8
5.3
4.0
3.4
7.4
7.1
4.3
7.0
6.8
7.8
3.9
7.3
CEC
meq/100
g soil
23.50
1.00
41.00
20.50
9.00
5.50
39.00
8.88
4.00
13.50
57.00
2.00
21.00
11.50
32.00
6.00
16.00
4.50
1.50
19.20
Organic
matter
%__
6.53
0.42
5.92
3.95
0.74
2.88
5.14
3.71
0.87
3.29
26.58
0.94
2.63
3.47
9.92
0.93
1.99
0.97
0.75
5.17
Particle size distribution
Sand
36.2
91.8
21.3
27.1
21.3
40.0
33.8
33.2
73.7
17.5
56.5
94.2
53.1
65.7
46.9
74.7
27.8
76.2
87.0
39.0
Silt
%____.
52.6
1.7
36.6
45.0
6.8
30.0
23.2
55.6
19.0
39.0
15.3
2.1
45.0
13.8
36.. 2
16.0
59.7
17.1
4.4
33.0
Clay
11.2
6.5
42.1
27.9
71.9
30.0
43.0
11.2
7.3
43.5
28.2
3.7
1.9
20.5
16.9
9.3
12.5
6.7
8.6
28.0
Ammonium oxalate extractable
Fe
4090
2565
8190
4720
8165
1980
14100
9100
1155
37750
10100
699
16550
2940
23800
1758
9140
355
298
=4000
Al
Vgi
2110
422
2032
1553
2560
1755
3380
2220
512
4965
3120
238
1725
1387
2800
589
2030
273
354
N.A/
Ca
la _...-
'g
36
19
42
30
92
2
45
18
29
122
43
63
88
23
65
29
180
26
40
N.A/
Mn
16
126
253
200
36
126
61
499
91
786
43
40
565
90
502
144
559
76
110
N.A.
1
Sediment
Not available
-------
EH
500-L
400-
300-
200-
100-
0-
-100-
2.4-D CROWLEY
&
*
CJ
«
•f
8
g
e
A
"H "-T-
036
12
10 ?l 2« 27 30 33 36 39 42 45 48
FLOODING TIME. DATS
LKGEND: AMT_ADn
* * * 0
« « a 5
a a a 10
-i < + 75
Figure 8. Redox pocenti.il levels (Eh) with time in a Crowley soil material
amended with varying concentrations of 2,4-D and subjected to
flooding.
26
-------
EH
500H
400-
300-
200-
100-
0-
-100-
9
a
2.4-D CECIL
A
*
«
D
6 9 12 15 18 21 24 27 30 33 36 39 «2 45 18
FLOODING TIME. DAYS
LEGEND: AMT_ADD
* * * 0
a a a 5
D D D 10
4 + + 75
a A 25
Figure 9. Rcdox potential levels (Kh) with time in a Cecil soil material
amended with varying concentrations of 2,4-0 and subjected to
flooding.
27
-------
METHYL PARATHION CROWLEY
EH
500-
400-
300-
•
•
200-
100-
0-
-100-
+
D
1 9
. s ; •
D fl
"
* «
0
+
»
•
0 V '« 6 (t \(
fLCKHHNG '
LEOEN11: AMT_ADD * * * 0
a a it 5
*
a
a a a 10
+ * + 75
18 20
a A 25
Figure 10. Redox potential levels (Eh) with time in a Crowley soil material
amended with varying concentrations of methyl parathion and
subjected to flooding.
28
-------
METHYL PARATHION CECIL
EH
50CH
400-
*
A
300-
200-
100-
0-
-100-
a
a
8
a
8
+
A
e
6 6 10 12 14
FLOODING TIME. DAYS
16
LEGEND: AMT_ADD
* * * 0
a a « 5
a a o 10
+ » 4 75
18
a & 25
20
Figure 11. Redox potential levels (Eh) with time in a Cecil soil material
amended with varying concentrations of methyl parathion and
subjected to floodinR.
29
-------
2,4-D CROWLEY
PH
7.5H
7.0-
6.5-
D
6.0-
5.5-
5.0-
4.5-
*
«
D
»
D
tt
D
A
O
A
0 :< 6 v» 1? IV. in /I 7« ?7 ;HO 33 36 39 42 45 48
I-U10IJ1NC. TIME. DAYS
LECKND: AMT_ADU *»*0 DDDlO
att»5 +4+75
25
Figure 12. pH values with Lime in a Crnwlcy soil material amended with
varying concentrations of 2,4-D and subjected to flooding.
30
-------
2,40 CECIL
PH
7.5-
7.0-
6.5-
6.0-
5.5-
5.0-
B
8
* *
a
s
t
8
a
A
«
a
0 3 b 9 I? ir> 10 21 ?J\ 2> 'JQ 33 36 39 42 «5 18
FLOODING TIME. DAYS
LKC.hNIl: AMI'. ADI1
» • * 0
a a a b
n u a 10
* * + 75
Figure 13. pH values with time in a Cecil soil material amended with
varying concentrations of 2,4-D and subjected to flooding.
31
-------
soils became more reducing (Figures 14 and 15). In the Crowley soil,
the pH at 0 ppm was consistently above all other treatment levels and
about 0.5 pH units above the 75 ppm treatment. This was also true of
the changes in pH in the methyl parathion treated Cecil soil, although in
both soils, there was considerable variability in relative pH levels at
the intermediate concentrations.
The levels of soluble and exchangeable Fe and Mn are substantially
affected by changes in pH and redox potential in many soils and sediments
(Gotoh and Patrick, 1972; Gotoh and Patrick, 1974; Gambrell et al.,
1977). Both tend to be more mobile or available with decreasing redox
potential and pH. Flooding the Crowley and Cecil soils resulted in a
reduction in redox potential and an increase in pH. Thus the separate
influence of pH and redox potential changes in these two soils would
tend to have opposite effects on mobilization of Fe and Mn such that
considerable variability in Fe and Mn levels might be expected. All
treatment combinations gave less than 3 ppm of soluble plus exchangeable
Fe at time zero, but after that, there was substantial variability in Fe
levels, both with time and concentration (Tables 3 and 4). However, the
methyl parathion controls (0 ppm) consistently had higher levels of
mobile Fe than materials that were amended with methyl parathion during
the early part of the experiment before substantial degradation may have
occurred. An examination of these Eh, pH, and Fe levels with time
suggest that the changes in redox potential in these soils may have
influenced Fe mobility more than the changes in pH that occurred.
In the Crowley and Cecil soil materials amended with 2,4-D, soil
reduction did substantially increase soluble plus exchangeable Mn levels,
though no 2,4-D concentration effects were apparent (Tables 3 and 4).
In the methyl parathion-amended soil materials, the soluble plus exchange-
able Mn levels were consistently greater in the controls, probably
reflecting the more rapid rate of reduction in soils receiving no methyl
parathion.
Zinc of course is an essential plant nutrient that is not subject
to valence charge changes as a consequence of changing soil pH or redox
potential conditions. However, the readily available concentrations of
zinc are affected by both pH and redox potential conditions. In flooded
soils and sediments, it is known that increasing pH and decreasing redox
potential tend to decrease levels of soluble and exchangeable zinc
(Gambrell et al., 1977; Jugsujinda, 1975).
Zinc levels tended to decrease slightly with time for all combinations
of soil materials and compounds, but there were no apparent pesticide
concentration effects over the narrow r.ingr of change in zinc concentrations
observed (Tables 3 and 4). A decrease in readily available soil zinc
levels would be expected both from the increase in soil pH and the
decrease in redox potential observed with time when these soil materials
were flooded.
2
Table 5 indicates the association (R values) between water soluble
plus exchangeable levels of Fe, Mn, and Zn with pH and redox potential
changes that occurred upon flooding the Crowley and Cecil soil materials.
32
-------
METHYL PARATHION CROWLEY
PH
7.5-
7.0-
6.5-
D
D
6.0-U
5.5-
5.0-
4.5-
o
A
a
A
D
a
A
•f
B
a
f-. n 10 12 l«
FLOODING TIME. PAYS
LECINH: AMT.ADD
• * » 0
a a « Ji
n LI a 10
+ * * 7b
A
•f
16
18 20
Figure 14. pH values with time in a Crowley soil material amended with
varying concentrations of methyl parathion and subjected to
flooding.
33
-------
METHYL PARATHION CECIL
PH
7.0-
6.5-
6.0-
5.5-
5.0-B
4.5-
*
a
a
A
A
a
2 « 6 6 10 12 14 16 18 20
FLOODING TIME. DAYS
LEGEND: AMT_ADP »*»0 oaalO' aaa25
» * » 0
« « a b
a a a 10
+ t + 75
Figure 15. pH values with time in a Cecil soil material amended with
varying concentrations of methyl parathion and subjected
to flooding.
-------
TABLE 3. SOLUBLE PLUS EXCHANGEABLE FE, UN, AND ZN LEVELS ON SELECTED
SAMPLING DATES IN TWO SOILS AMENDED WITH VARYING CONCENTRATIONS
OF METHYL PARATHION
Soil material
Crow ley
Cecil
Incubation
time, days
2
2
2
9
9
9
16
16
16
2
2
2
9
9
9
16
16
16
Concentrat ion
added
0
10
75
0
10
75
0
10
75
0
10
75
0
10
75
0
10
75
Soluble
Fe
ppm
5.5
.3
.3
129
21
1.1
99
36
2.1
2.9
2.0
1.7
3.2
0.7
2.2
12.0
5.0
1.5
+ exchangeable
Mn
161
33
27
204
153
107
144
126
102
3.6
2.6
3.6
31.5
25.8
29.8
45.2
38.7
38.6
Zn
2.8
2.5
2.3
2.4
1.1
2.4
1.4
1.8
0.7
2.3
2.2
2.3
2.2
1.7
0.8
2.0
0.7
1.3
Mean value of two replicates.
35
-------
TABLE 4. SOLUBLE PLUS EXCHANGEABLE FE, MN, AND ZN LEVELS ON SELECTED
SAMPLING DATES IN TWO SOILS AMENDED WITH VARYING CONCENTRATIONS
OF 2.4-D
Soil material
Crowley
Cecil
Incubation
time, days
6
6
6
24
24
24
40
40
40
6
6
6
24
24
24
40
40
40
Concentration
added
0
10
75
0
10
75
0
10
75
0
10
75
0
10
75
0
10
75
Soluble
Fe
ppm --
1.8
1.9
1.4
20.7
18.1
18.0
6.3
5.2
19.7
3.2
2.4
0.5
2.2
3.4
2.6
13.7
14.7
5.6
+ exchangeable
Mn
330
322
325
340
372
364
144
292
275
44
47
42
50
50
50
54
50
51
Zn
1.1
2.3
1.4
1.4
1.1
1.3
0.8
0.6
1.0
2.6
2.7
2.9
2.6
3.0
3.1
1.6
1.4
1.6
Mean value of two replicates.
36
-------
TABLE 5. R VALUES FOR FE, MN, AND ZN AS INFLUENCED BY SOIL PH AND
REDOX POTENTIAL
Dependent
variable
Fe
Mn
Zn
Independent
variable
redox potential
PH
redox potential
PH
redox potential
PH
Crowley,
2,4-D
irk
•23**
.27
**
•80**
.44
•04**
.10
80 obs1
Cecil,
2,4-D
*
•°8**
.10
J— »„
•"«
.84
*
•09*
.09
78 obs
Crowley,
methyl
parathion
R2
**
•32**
.30
**
•57«
.55
.02
.02
79 obs
Cecil,
methyl
parathion
**
•36**
.37
**
•81**
.66
*
•06**
.13
80 obs
R values followed by a single asterisk are significant at the 5%
level.
** 2
R values followed by a pair of asterisks are significant at the 1%
level.
Number of observations.
37
-------
There was not a good relationship between Fe or Zn levels and the physico-
chemical parameters of pH and redox potential (though many of the Fe
associations were statistically significant). All of the Mn levels were
significantly associated with pH and redox potential conditions and most
of the R values were high.
DEGRADATION STUDIES
Controlled pH-Redox Potential Microcosms
Degradation as Indicated by a Decrease in Recovery of Parent Compound
vs. Time —
The laboratory microcosms using stirred suspensions offer the
advantage of precise control of selected soil and sediment physicochemical
conditions as well as the maintenance of uniform conditions throughout
the material being studied. As discussed elsewhere, the relative treatment
effects on degradation observed with these systems provide useful infonna-
tion that cannot be obtained by other experimental techniques, but the
rate of degradation observed may not be applicable to in situ degradation
rates.
For the controlled pH/redox potential experiments, the data were
plotted as the log of concentration, in Mg/g, vs. time in days. Log
plots of concentration (or percent of compound remaining) vs. time are
frequently used to illustrate degradation rates in the literature (Paris
et al., 1982; Liu et al., 1981; Wolfe et al., 1977; Perdue and Wolfe,
1982). First order degradation kinetics are indicated if these plots
result in straight lines. Correlation coefficients are often given to
indicate how well the data points fit a straight line. In the controlled
pH/redox potential studies, we have presented the data as the log of
concentration vs. time and have also included correlation coefficients.
Methyl parathion--The degradation of methyl parathion under controlled
pH and redox potential conditions has been studied in five soil or
sediment materials (Figures 16-24). Figures 16 and 17 indicate the
decrease in recovery of methyl parathion in sediment and water phases
respectively of a Calcasieu River sediment material incubated at pH 8.0.
It is readily apparent that redox potential substantially affects the
rate of degradation of methyl parathion in this experiment as recovery
had decreased to essentially zero after four days at -150 mv. However,
it is also apparent that under the imposed experimental conditions,
methyl parathion rapidly degrades under oxidizing conditions as well.
While levels decreased to about 1 percent of spiking levels in 2 days at
-150 mv (strongly reducing conditions), 6 days were required to get down
to about 10% of spiking levels at 450 mv (oxidized conditions). Changes
in levels of methyl parathion in the aqueous phase generally correspond
to changes in the sediment phase.
Figures 18 and 19 give the change in recovery of methyl parathion
with time in Barataria Bay sediment-water suspensions incubated at pH 7
and 5 respectively. At pH 7, the rate of degradation as indicated by
compound recovery was much the same as found for the Calcasieu River
38
-------
CALCASIEU RIVER (SEDIMENT)
METHYL TARATHION. PH t?
1.00-
-1.50-
f. 9 10 'I
LEuEND: fcH
T1ML. DAV.
t» «?-€» bl)
Figure 16. The effect of rcdox potential on Che recovery of methyl
parathion from a Calcasieu River sediment material Incubated
at pH 8.
39
Reproduced
beil available
-------
CALCASIEU RIVER (WATER PHASE)
METHYL PARATHION. PH 8
-1.0-
-3.0
LEGEND: EH
Figure 17. The effect of redox potential on the recovery of methyl
parathion from the aqueous phase of a Calcasieu River
sediment suspension incubated at pH 8.
Reproduced ttom
best available copy.
40
-------
BARATARIA BAY (SEDIMENT)
METHYL PARATHION. PH 7. 3 PPT SALINITY
I.OO-d
0.75-
0.1 2J456769101I
Figure 18. The effect of redox potential on the recovery of methyl
parathion from a Barataria Bay sediment material incubated
at pH 7 and 8 ppt salinity. ±.
Reproduced from
best available copy.
41
-------
BARATARIA BAY (SEDIMENT)
METHYL PARATHION. PH 5. 8 PPT SALINITY
l.O-
0.8--
0.6-
L
O
C 02-
C
0
N
0 0-j
U
C
/ -0.2-
C
-0.4-
-0.6-
-0.8-
-l.O
LEGEND: EH
-150
450
Figure 19. The effect of redox potential on the recovery of methyl
parathion from a Barataria Bay sediment material incubated
at pH 5 and 8 ppt salinity.
I Reproduced from
best available copy.
A2
-------
HARTWELL RESERVOIR (SEDIMENT)
METHYL PARATHION. PH 7
0.8-J
l.K'.t-NP: Oi
TIME. I'AY'"-,
50 e-0-0 2r.-,0
5DC
Figure 20. The effect of redox potential on the recovery of methyl
parathion from a Hartwell Reservoir sediment material
incubated at pH 7.
43
-------
HARTWELL RESERVOIR (SEDIMENT)
METHYL PARATHION. FH =
L
0
C
c
0
N
C
c
c
-1.50
TIKE. PAlrS
Mi.'N!)- FH * - » r.o e--u-0 ,1C.^I
50 ^
Figure 21. The effect of rcdox potential on the recovery of methyl
parathion from a Hartwell Reservoir sediment material
incubated at pH 5.
-------
CECIL TOP SOIL
METHYL PARATHION. PH 7
I.OOH
ft / H 'j Hi 11 12
-1.75-1
-2.00-
0
LEOEND: EH
T1MI . [IAYS
50
» — >- 250
500
Figure 22. The effect of redox potential on the recovery of methyl
parathion from a Cecil topsoil material incubated at pH 7.
-------
BARATARIA
METHYL PARATHION
BAY (SEDIMENT)
. PH 7. 25 PPT SALINITY
0.6-1
-1.4-
LEGEND. EH
-IbO
TIME. DAYS
BOO =•()
—*- 250
9 10
450
Figure 23. The effect of redox potential on the recovery of methyl
parathion from a Baratarla Bay sediment material incubated
at pH 7 and 25 ppt salinity.
-------
BARATARIA BAY (SEDIMENT)
METHYL PARATHION. PH 5. 25 PPT SALINITr
1 .2-
1.0-
0.9-
0.6-
0.7-
0.6-j
L '•
0 0.5-
C 0.4-1
o :
N
C 0.3-'
U 0..-J
G
G 0.1-
0.0-
-0. l-j
-0.2-
-0.4-
-0.r^-
LE.-'.ENP: IH
n
'* f\
I'.n
-^Vo-as
\
(. 7 f< fi 1C) II 12 \'i \U
TIME. riA'i'-.
450
Figure 24. The effect of redox potential on the recovery of nethyl
parathion from a Barataria Bay sediment material incubated
at pH 5 and 25 ppt salinity.
A7
-------
sedijaent material, and again, redox potential effects are apparent. At
pH 5.0, the same response to redox potential is observed, but the compound
is considerably more persistent as measurable levels are still present
at 14 days under strongly reducing conditions and approximately 50
percent of spiking levels remain under the most oxidizing conditions.
Figures 20 and 21 give the recovery of methyl parathion from the
Hartwell Reservoir sediment material incubated at pH 7 and 5 respec-
tively. The same large response to pH and redox potential observed in
the previous materials was found with the Hartwell Reservoir sediment
material. However, recovery did decrease more rapidly in this material
than in the others studied.
Figure 22 shows the recovery of methyl parathion from a Cecil soil
incubated at pH 7.0. As observed in the other materials studied, methyl
parathion levels decreased most rapidly under strongly reducing conditions.
After two weeks, recovery was zero or nearly so at all redox potential
levels and this was also generally the case for the pH 7 or 8 treatments
in the other soil materials.
Salinity effects on methyl parathion degradation—Figures 23 and 24
show the decrease in recovery of methyl parathion from Barataria Bay
sediment suspensions incubated at pH 7 and pH 5, but with a salinity
level of 25 ppt instead of 8 ppt as was the case for Figures 18 and 19.
At pH 7, the decrease was more rapid initially at the higher salinity
level, but the salinity effects were less with time and at 10 to 11 days
there was essentially no difference. At pH 5.0, the higher salinity
treatment tended to result in a slightly slower reduction in methyl
parathion recovery with time, but again, after 10 to 14 days, any differ-
ences in levels, if real, were small.
These results indicate that salinity levels may have less effect on
the degradation of methyl parathion than pH and redox potential.
The very rapid decrease in recovery of methyl parathion under
reduced conditions should be addressed. At -150 mv in the Calcasieu
River and Barataria Bay sediments materials, much more than half of the
methyl parathion added was not recovered after 0.2 days into the incuba-
tion. In the Hartwell Reservoir material, measurable levels were not
recovered in sample aliquots taken within minutes after spiking the
stirred suspensions. This results in the plots of log concentration vs.
time appearing to intercept the Y-axis at significantly different points.
However, this was not the case as all suspensions were spiked at the
same level. These loss rates seem too fast for microbial degradation.
Since the initiation of this project, Whalid et al. (1980) have reported
the instantaneous degradation of parathion in anaerobic soils. They
reported equilibration of parathion with one prereduced soil resulted in
the concentration of the parent molecule decreasing to 44 and 11.6
percent of the nriginal level after 5 second- and 30 minute-equilibrations,
respectively. Soil enzymes and/or other heat-labile materials produced
under anaerobic, but not aerobic, conditions were implicated. Parathion
and methyl paralhion are similar enough such that this process reported
for parathion may be a contributing factor in the very rapid disappearance
of methyl parathion in the anaerobic soil and sediment materials in this
study.
48
-------
If the data points of log concentration vs. time are linear, first
order degradation kinetics are suggested. In a few cases, the correlation
coefficients are low indicating poor linearity (in particular, the
Barataria Bay experiment at pH 7, 25 ppt salinity). Many of the remaining
correlation coefficients were high suggesting a reasonably good fit to a
straight line (21 out of the remaining 29 regression lines had an r
value of -.88 or better). However, most of the methyl parathion plots
curved upward toward the end of the incubations indicating processes are
involved giving some deviation from simple first order kinetics. With
the exception of the incubation represented by Figure 23 in which there
appeared to be excessive experimental variability, the lowest measurable
concentration value was consistently above the regression line (24 of 27
regression lines consisting of 3 or more points). The reasons for this
deviation from linearity and first order degradation kinetics were not
pursued further at this time.
2,4-D--Figure 25 shows the recovery of 2,4-D from a Cecil A horizon
(topsoi.1) material incubated at pH 7 and four redox potential levels.
As observed for methyl parathion, redox potential affects the loss of
2,4-D. However, unlike methyl parathion, this compound is removed more
rapidly from oxidized soil materials than reduced soil. From an initial
spiking level of 12 to 15 ppm, 0.3 to 0 ppm were recovered after two
weeks at the two highest oxidation levels, but about half of the material
(greater than 6 ppm) remained in the soil at the two most reducing redox
potential levels.
This agrees with the work of Liu et al. (1981) who, working with
laboratory cyclone fennentors, reported the half-life of 2,4-D was near
40 times greater under anaerobic as compared to aerobic conditions.
A plot of log concentration vs. time was near linear for the two
most oxidizing treatments (r = -0.88 and -0.96 for 500 and 250 rav
respectively), while the remaining two redox potential treatments gave
low correlation coefficients, apparently due to experimental variability.
Aroclor 1254--Aroclor 1254, one PCB formulation, was incubated with
Hartwell Reservoir sediment material at 2 pH levels (Figures 26 and 27).
These incubations were carried out for just over six weeks. The levels
of amended Aroclor 1254 generally decreased to just under half of the
spiking levels after six weeks. There appeared to be no strong pH or
redox potential effects on the recovery of this PCB formulation.
At pH 6, and for three of the four treatments plotted at pH 7, the
slopes of the regression lines were very similar. Only the 250 mv
regression line may have had a significantly different slope, and this
was mostly due to apparent experimental variability in one or two data
points. All but the pH 7, 250 mv treatment gave a good fit to a straight
line as indicated by regression coefficients of -0.96 or better.
CO. Evolution as an Indication of Microbial Respiration--
Hicrobial degradation is believed to be the primary process by
which many synthetic organics are broken down and removed from the
49
-------
CECIL (SOIL)
2.4-D. PH7
1.2-
l.O-
J' '-*0 33 36 ?9
-0.3-
LEC! ND: EH
Figure 25. The effect of rcdox potential on the recovery of 2,4-D
frcm a Cecil topsoil material incubated at pH 7.
50
Reproduced Irom
besr available copy.
-------
HARTWELL RESERVOIR (SEDIMENT)
AKOCHI.OR 125«. PH 0
1.02-
0.99-
0.63-
I " I "
0 3 6 9 12 15 18 21 24 27 30 33 36 39 • «2 45
TIME. DAYS
LEGEND: EH * * * 50 e-e-o 250 •*—i—•- 500
Figure 26. The effect of redox potential on the recovery of Aroclor 1254
from a Hartwell Reservoir sediment material incubated at pH 6.
51
Reproduced from
beil available copy.
-------
HARTWELL RESERVOIR (SEDIMENT)
AROTHLOr 125'). PH /
I.OOH
0.95-
a X\ a
i. X
0.85-
\\
L o ec-
'*,'
£
c o •--
0
N
? 0 '.
1 1
>J
/ 0.65-
C
O.cC-
0.5C-
0.5C--
0.45-
O.'iO
\: \\^V
\ \ *- ^,
v x * \ "" ••
x \« x^* ' ^ » * " " •-- *
^$fc s» ?6
s>» ^f X^
XX '\
\ X
> «
\
0 J *i 0 I .' 1 ' . I >i / 1 //i / / 'JO 33 36 39 4<: uc.
LECCND: E!l
H H ci c>0
•"»— 500
Figure 27. The effect of redox potential on the recovery of Aroclor 1254
from a Hartwell Reservoir sediment material incubated at pH 8.
' Reproduced from
besi available eoov.
52
-------
environment. This project and similar work has demonstrated that oxidation-
reduction conditions do affect the degradation rate of many synthetic
organics. Oxidation-reduction conditions also affect the types of
microorganisms that will be active and certain metabolic processes such
as what oxidants are used for terminal electron acceptors in the case of
facultative anaerobic organisms. Thus it would be of interest to know
the relative microbial respiration rates in the laboratory microcosms
used to study degradation of synthetic organics under controlled pH and
redox potential conditions. This was accomplished by scrubbing CO. from
inlet streams of the nitrogen purge gas .ind the air additions used to
control oxidation levels in selected experiments, then measuring the
total carbon dioxide evolved. The primary source of this carbon dioxide
would be soil or sediment humic material being mineralized by microbial
respiration. In some cases, microcosms for all redox potential levels
were amended with ground rice straw in attempts to achieve the low end
of the desired redox potential range/ At certain of the sampling intervals,
there were some problems with leaks, apparent carbonate precipitation,
or other unexplained anomalies. Nevertheless, the data generally indicate
greater microbial activity with increasing redox potential. The results
are given in Figures 28 through 31. A few problems were encountered in
attempting to continuously collect all of the C0_ produced by each flask
over extended periods of time. Table 6 gives some information on the
problems encountered and some additional notes on this study. Though
some problems were evident, it is apparent from examining Figures 28
through 31 that microbial activity as measured by CO- production was
greatest under well oxidized conditions. The total CO. evolved at -150
and 50 tnv ranged from 11 to 30 percent of that evolved at 500 mv in the
four experiments. This is in close agreement with the results,of DeLaune
et al. (1981) who measured pH and redox potential effects on CO.
produced in soil suspensions amended with C-labeled plant material.
Considering these results and the redox effects on degradation rates, it
is clear that one cannot make the blanket statement totdl microbial
respiration accounts for the redox potential effects on the degradation
of synthetic organics in general and methyl parathion in particular in
this study. While this could be true for a few specific organics, this
research shows the degradation of two of the three compounds studied to
behave independently of total microbial respiration. The most rapid
loss in recovery of methyl parathion occurred where the respiration rate
is lowest (reducing conditions) and the degradation of Aroclor 1254 was
not appreciably affected by either redox potential or respiration rates.
The degradation rate of 2,4-D corresponded in a general way with increasing
respiration rate and redox potent id 1, but it is not possible from these
data to say definitely what the influence of respiration rate was on
2,4-D degradation, or even to stale thai (.here is a cause and effect
relationship between respi i;it ton ,nul degrml.it ton rate.
14
Biometcr Flasks Using Miner.i I izat ion Rate of C-Labeled Compounds for
Analysis
The primary advantages of the biometer flask approach for degrada-
tion studies are their simplicity and low cost. Compared to the controlled
pH-redox potential microcosms, the biometcr flasks, using collection of
C0_ as an indication of degradation, permit the study of a larger
53
-------
CARbON DIOXIDE PRODUCTION
METHV I PARATHJON IN HAJRTWELL SEDIMENT, PH 5
5000--J
«500-
4000-
c
0
2 3000
M
C
/ 2500^
H
R
/
F
L 200CH
A
s
K
1500-
1000-
500-
0-
10
*
n
• r-
10
rNll. RLUDX
0 '.> 10 15
[)Ar Kl I M I VI1 1i> CllMIilUNI. AUDITION
* * » IM) ,1 LI c_i SO « » « 250
20
+ + 500
Figure 28. Carbon dioxide production ns .-in indication of relative nicrobial
activity in a methyl pnratliion-amendud Hartwell Reservoir sediment
material incubated at Four rcdox potential levels at pH 5.0.
54
-------
CARBON DIOXIDE PRODUCTION
METHYL PARATHION IN CECIL SOIL. PH 7
3900-
3600-
3300-
3000-
2700-
c
0 2400
2
M 2100-
C
H
R 1800
F
L
A 1500
S
K
1200-
900-
600-
300-
0-
-17.5
LEGEND: REDOX
*
a
«
C
-7.5 7.5 12.5 22.5
DAYS Khl.ATlVL TO COMPOUND ADDITION
* * • - 150 o D n 50 « « a 250 * + + SCO
Figure 29. Carbon dioxide production .is .in indication of relative microhial
activity in a mo thy I p. irnt.li ion-amended Cecil topsoil material
incubated at four redox potential levels at pi I 7.0.
55
-------
CARBON DIOXIDE PRODUCTION
AROCHLOR IN HART WELL SEDIMENT, PH 6
3250-
3000-
2750-
2500-
2250-
C
0
2 2000
M
C 1750-
H
R
/ 1500-
F
L
A
S 1250-
K
1000-
750-
500-
250-
. t i i i i ' I'""""1 ' • "' '
ID '.> M '> U) I1. /() ->'•> '0 y.> *0 45 50 55
|JAY^ Rn.ATIVF- rn COMPOUND ADDITION
LECENU. Kl i;t)X ** two reps, 50 D ° ° 25° " * " 50°
Figure 30. Carbon dioxide production as an indication of relative nicrobial
activity in an Aroclor 1254-amc.nded sediment material incubated
at pH 6 and four redox potential levels.
56
-------
4500-
4000-
3500-
3000-
C
0
2
2500-
M
C
H
R
/ 2000
F
L
A
S
K
1500
1000-
500
CARBON DIOXIDE PRODUCTION
AROCHLOR IN HART WELL SEDIMENT. PU 8
a
*
.,.
T
••-I™
:to
LEGEND: REDOX
'.. 10 1li A} ;."> 1)0 35 40
DAYS Rfcl.ATIVE TO COMPOUND ADDITION
• * * - ll>0 EDO 50 « » « 250
45 50 55
+ + 500
Figure 31. Carbon dioxide production as an indication of relative microbial
activity in an Aroclor 1254-nmondcd sediment material incubated
at pH 8 and four rcdox potential levels.
57
-------
TABLE 6. NOTES ON CARBON DIOXIDE COLLECTION STUDIES TO INDICATE RELATIVE
MICROSIAL RESPIRATION
Corresponding Interval,
Figure days
Redox potential, mv
-150
50
250
500
28(Hartwell,
pH 5 methyl
parathion)
ii
it
29 (Cecil, pH
7 methyl
parathion)
H
it
ii
ii
ii
30(Hartwell,
pH 6, Aroclor
1254)
31(Hartwell,
pH 8, Aroclor
1254)
-5 to
0 to
3 to
8 to
14 to
-17 to
-12 to
-5 to
0 to
7 to
11 to
14 to
18 to
-5 to
0 to
7 to
13 to
21 to
28 to
36 to
-5 to
0 to
7 to
13 to
21 to
28 to
36 to
0 «- precipitate
3 formed in
8 «- leaks trap
14 developed «- traps were
21 I saturated
,n -> j J-. -«
-5
0 < changed from 0.1 to 0.2N NaOH in traps >
-i^ A A ' •- A it r* A **
11
14 «- leak
18 developed
21 4.
0
7
13
21 «- precipitate
28 formed in
36 trap
54
0
7
13
21
28
36
54 <- leaks
developed
Relative to time samples wrrr amended with .-i synLhrtic- organic.
Redox potential was 50 mv for Figure 30 (Hartwell, pH 6.0, Aroclor 1254).
58
-------
number of experimental parameters with adequate replication. Since a
primary objective of this research was to examine redox potential effects
on degradation of synthetic organics in several soil and sediment materials,
we made extensive use of this experimental approach. It should be
noted, however, that this experimental method measures mineralization of
the labeled portion of the molecule rather than simple modification of
the parent compound. This difference will be discussed more thoroughly
elsewhere in this report.
Methyl Parathion--
14
Figures 32 through 37 indicate the recovery of C-labeled carbon
dioxide from the degradation of methyl parathion in six soil and sediment
materials. There was a very large and statistically significant oxidation
treatment effect in all six materials. In every case after the two to
three month incubations, 10 to 20 times or more of the labeled carbon
was evolved as labeled CO. under oxidized conditions compared to reducing
conditions. Also, soil properties greatly affected recovery of methyl
parathion as collected labeled carbon ranged from about 0.7 to 3.2 and
13 to 87 percent of that added for the reduced and oxidized treatments
respectively. In the oxidized materials, there appears to be some
association between mineralization of the methyl parathion and pH. In
all but the Hartwell Reservoir material, the evolution of labeled carbon
dioxide had leveled off or decreased to a slow rate at the end of the
incubation compared to earlier in the incubation.
Although much smaller accumulations of labeled CO were observed
from the reduced treatment when the studies were terminated, all but the
Lake Providence material was still producing labeled c.irbon dioxide at
the highest rate observed when the experiments were terminated (a leveling
off of labeled carbon dioxide production had not occurred in the reduced
treatments).
Upon examination and comparison of the controlled pH-redox potential
microcosm results and the biometer flask results, two major differences
are readily apparent: (1) the apparent degradation rate of methyl
parathion is much slower in the unstirred soil suspensions, and (2) the
redox potential treatment effects are opposite using the two techniques.
These observations must be addressed.
Regarding the rate ol degradation, the biometer flasks were maintained
at about 23°C whereas the stirred suspensions were kept at about 28°C.
This temperature difference would he expected to contribute to some
increase in degradation tate in the stirred suspensions. Also, Parr and
Smith (1974) noted .1 similar stirring effect in laboratory studies of
toxaphene degradation under both aerobic and anaerobic conditions. It
was suggested that stirring some soils provides additional available
carbon to microor gun isms to susldin r.ipid degrddat ion of toxaphene.
Measurements of total c.irhou dioxide evolution showed there was a higher
level of respiratory activity in a stirred anaerobic suspension compared
to a moist, unstirred, anaerobic suspension.
59
-------
METHYL PARATHION, CECIL SUBSOIL
OXIDATION EFFECTS ON MINERALIZATION
OF LABELED COMPOUND
100-1
90-
R
E
C
0
V
E
R
Y
0
F
L
A
B
E
L
P
E
R
C
E
N
T
80-
70-
50-
30-
20-
10-
0-
fl u
a
n
D
r- •[ -
60
1C) /() «) tO L>0 60 70 80
INfUHMION T1MI . DA^S
LKC-LNP: TKKM * » * OX ID u a a REDD
90
Figure 32. Oxidation effects on
indicated by recover-/ of
material .
ton of labeled methyl parathion as
., from moist, unstirred Cecil subsoil
60
-------
100-
90-
00-
R
E
C 70
O
V
E
R
Y 60'
0
F
L 50-
A
B
E
L
P
E
R
C 30
E
N
T
20-
10-
METHYL PARATHION, CECIL TOPSOIL
OXIDATION EFFECTS ON MINERALIZATION
OF LABELED COMPOUND
0
ii li
.......
10
II II
•»•• I ••••
n n
'10
70
hi) 60
INCUBATION TIMC. DAYS
LEGEND: TREAT * » * OXID "DDD PEDU
80
90
Figure 33. Oxidation effects on degradation of labeled methyl parathion as
indicated by recovery of
material.
from moist, unstirred Cecil topsoil
61
-------
METHYL PARATHION, CROWLEY
OXIDATION EFFECTS ON MINERALIZATION
OF LABELED COMPOUND
R
E
C
0
V
E
R
Y
0
F
L
A
B
E
L
t
P
E
R
C
C
N
T
100-
90-
80-
70-
60-
50-
40-
30-
20-
10-
0-
*
*
*
*
*
*
»
»
O II O II 11 LI D I'
i " " ' i i i i
o 10 ?o :KI 'io '
n u
60 70 80
1NCUUATION 11 Ml . DAf?,
: "U'1 Al » t * (i/. Ill n D D REDD
90
Figure 34. Oxidation offocts on do>;r.id.iL Ion of Libeled metliyl parathion as
Indicated Iw recovery of CO from moist, unstirred Crowley
soil material.
62
-------
METHYL PARATHION, HARTWELL RESERVOIR
IOOH
90-
R
E
C
0
V
E
R
Y
0
F
L
A
R
i-:
i.
80-
70
60-
50 i
'HI
R
C 30-]
E
N
T
20-
10-
0-
OXIDAT10N EFFECTS ON MINERALIZATION
OF LABELED COMPOUND
rfflfi n
... .,..
in
f I IJ LI 11
I
II
111
11
,,.,.
'ID
60
70
80
90
I.H.I Ml)
1NO.I|«AI IPN 1 I Ml . HAYS
IKIAT * • * OX Mi u n a REDU
Figure 35. Oxidation efforts on dc>;rnd;iLion of labeled methyl parathion as
from moist, unstirred Hartwell
indicated by recovery of
Reservoir sediment material.
63
-------
METHYL PARATHION, LAKE PROVIDENCE
OXIDATION EFFECTS ON MINERALIZATION
OF LABELED COMPOUND
100-1
80-
R
E
C 701
0
V
E
R
Y 6
O
F
L 50-
A
B
E
L
. 40-
P
E
R
C 30-]
E
N
T
20-
10-
0-
D a i
10
n n
a o
70
70 no 10 50 60
INCUBATION TIME. DAYS
LEGEND- TREAT * * * OX ID a a O REDU
80
90
Figure 36. Oxidation effects on degradation of labeled methyl parathion as
indicated by recovery of ^ CO from moist, unstirred Lake
Providence material.
-------
METHYL PARATHION, BETIS SOIL
OXIDATION EFFECTS ON MINERALIZATION
OP LABELED COMPOUND
IOOH
90-
80-
R
E
C 70
0
V
E
R
Y 60
0
F
L 50
A
B
E
L
. 40
P
E
R
C 30
E
N
T
20-
10-
0-
a LI ci n n [i n u
~* * '• -"i * •••••"•»• »•*• i •••- •-•• • • i»• •»•' *•* • j'
in
(I U
20
70 80
:«) /in so f>n
INCUBATION TlMl . DAYS
J.I(,INH: I'KI AT * • • OXin u a a REPU
90
Figure 37. Oxidation offsets on degradation of labeled methyl parachion as
indicated by rerovcry ol"
topsoil material.
from moist, unstirred Bctis
65
-------
In the biometer flask degradation studies, the labeled carbon was
located in the aromatic ring of the methyl parathion molecule. Literature
discussed elsewhere in this report suggest that several other modifications
to the molecule will occur before degradation of the aromatic ring
(i.e., hydrolysis of the ester group and reduction of the nitro group on
the benzene ring). This recovery of the label as carbon dioxide represents
very substantial modification (degradation) of the parent compound. The
rate of recovery of the label as CO does not say anything about the
rate of initial or partial degradation of the parent compound as several
intermediate degradation products are probably involved before mineraliza-
tion of the aromatic ring to carbon dioxide occurs. Thus it is highly
probable that modification of the parent molecule occurred at a greater
rate than indicated by the rate of recovery of the label. Extraction of
the soil or sediment material and analysis by gas chromatography more
accurately indicates the rate of loss of the parent compound, but,
unless intermediate degradation products are identified and quantified,
does not reveal much about the rate of total mineralization of the
parent molecule. Both C labeled and extraction/specific compound
quantification methods are used extensively in degradation studies of
synthetic organics. The comparison of results by the two methods in
this study clearly indicate a researcher should carefully consider the
compatibility of a particular experimental approach with the research
objectives.
2,4-D--
Figures 38 through 44 indicate the recovery of labeled carbon as
C02 from the same six soil materials used for the biometer flask studies
of methyl parathion. As for methyl parathion, it is clearly apparent
that both oxidation-reduction conditions and soil/sediment properties
affect the mineralization rate of ring-labeled 2,4-D. Recovery of the
labeled material as CO. ranged from about 0.5 to 35 percent and 10 to 85
percent for the reduced and oxidized treatments, respectively. Surpris-
ingly, the results from the reservoir and lake sediments were essentially
on opposite ends of the recoveries obtained for the six soil and sediment
materials. Both are fine textured sediments with moderate amounts of
organic matter. No simple relationship is apparent between the percentage
recovery of the label and soil properties measured. There is likely a
complex interaction between soil properties contributing to the observed
rates of mineralization that might require a larger number of soil and
sediment materials to identify.
The controlled pU-redox potential microcosm studies also indicated
the parent compound was degraded mure quickly under oxidizing conditions.
As expected, the initial modification ot the parent material occurred
more quickly than mineralization of Lhr aromatic ring. For example, in
the two oxidized treatments of the Ceril topsoi1 extracted and analyzed
by gas chromatography, nono of the parent molecule was recovered after
four weeks while 90 percent of the l.ibH had not been recovered from the
oxidized biometer flask alter about 11 weeks. Again, at least two
processes are probably involved with those differences: 1) initial and
subsequent molecular modifications through intermediates may be faster
under stirred rather th.m unstirred conditions, and, 2) recovery of
-------
100-j
90-
R
E
C
0
V
E
R
Y
0
F
L
A
B
L
I.
P
E
R
C
E
N
T
80-
70-
60-
50-
30-
20-
10-
0-
2,4-D, CECIL SUBSOIL
OXIDATION EFFECTS ON MINERALIZATION
OF LABELED COMPOUND
HI n
»
ii
»
ii
M
D
n
id
"—7—
70
n
• • i i I'-
ll) 'Hi '..(i 6P
INI HUM ION i INI . DAY::
I.K-I Nil "I KIM * * * OX I Ii n a a RfcDU
80
90
Figure 38. Oxidation effects on degradation of labeled 2,4-D as indicated
by recovery of ^^C09 from moist, unstirred Cecil subsoil material.
67
-------
2,4-D, CECIL TOPSOIL
OXIDATION EFFECTS ON MINERALIZATION
OF LABELED COMPOUND
R
E
C
0
V
E
R
Y
0
F
L
A
B
E
L
P
E
R
C
E
N
T
100-
90-
80-
70-
60-
50-
40-
30-
20-
10-
0-
*
*
4
»
1
1
1
1
I
*
*
+
»
9
a ft a an LI a
I1* * ' " * " 1 I I
0 10 /O 3D 'I
a a
a
70
50 60
INCUBATION TIMr . DAI'S
: TRCA1 * * * OX ID ODD REDD
80
9C
Figure 39. Oxidntion cffoi-ts on di'pr.icl.ition of inhclccl 2,4-D as indicated by
recovery of ^V.O,, from moist, unstirred Cecil topsoil material.
68
-------
100-1
90-
80-
R
E
C 70
0
V
E
R
Y 60-]
0
F
L
A
B
E
L
P
E
R
C
E
N
T
50-
40-
30-
20-
0-- W
0
2,4-D, CROWLEY
OXIDATION EFFECTS ON MINERALIZATION
OF LABELED COMPOUND
i •
l(i
tj n n n u ii 1.1 u
!•••• I | | | ••• --I
.'() KI /in '.(i f-itj 7(j
INi IIKAI ION riMI . I'M'.
I.I (-1 NIL IKI M * . » O-'lli nun
80
90
Figure 40. Oxid.-ition ol fui-ts on di');r.idaL ion of labeled 2,4-1) as indicated
as indie.ilcd l>y rei-nvi-ry of (!()„ from moist, unstirred Crowley
topsoll ni.iLcri.il.
-------
2,4-D. HARTWELL RESERVOIR
OXIDATION EFFECTS ON MINERALIZATION
OF LABELED COMPOUND
IOCH
90-
80-
70-
60-]
5CH
R
E
C
0
V
E
R
Y
0
F
L
A
B
E
L
P
E
R
C 30-1
E
N
20-J
10-
0-
atffi n
cu i r) ri
a o
10
/o
-jo 'io ?»:••
INf.UHATMN TIML. ll
IklAI * * * 0/11)
70 8C
90
a u u RED'J
Figure 41. Oxidation I'fU'cts on di-^r.idnL ion of lahcLi-cJ 2,4-D as indicated by
rocoverv ol ^CO., from moist, unstirred llnrtwcll Reservoir sediment
rn.iteri.il .
70
-------
100-1
80-
R
E
C 70^]
0
V
E
R
Y 60H
0
F
L
A
B
E
L
P
E
R
C
E
N
T
50-
40-;
30-
20
10
2,4-D, LAKE PROVIDENCE
OXIDATION EFFECTS ON MINERALIZATION
OF LABELED COMPOUND
»
LI
n>
«n '.»()
INCUUM LON TIM1 ,
: TKI Al • • * OX ID
60
70
80
90
ODD REDU
Figure 42. Oxid.itIon i-ffivLs on do'.r.id.iL inn of Labeled 2,4-U as indicated
by recovery of CO from moist, unstirred Lake Providence sediment
material.
71
-------
90-
80-
R
E
C 70
0
V
E
R
Y 60-
0
F
L 50-
A
B
E
L
P
E
R
C
E
N
T
2,4-D, BETIS SOIL
OXIDATION EFFECTS ON MINERALIZATION
OF LABELED COMPOUND
40-
30J
20-
10
()-] HI ft n II II M II
I I I" '
0 10 /() ill
n
LKC-I
(i ii n u
1 * " i « * *-| • • »-r .-ri ( fT , , , ,
au SO 60 70
INfUBATlUM TIM1 . 'AT-.
TKI A: » » • OX!M n n o KEDU
80
90
Figure 43. Oxidation efforts on di-nr.ul.it ion of lahcLod 2,4-U as indicated
by recovery of ^ C0,; fri>ni moist, unstirred Metis topsoil material.
72
-------
90-
80-
R
E
C 70-
0
V
E
R
Y 60-
0
F
L 50-
A
B
E
L
. 40
P
E
R
C 30-
E
N
T
20-5
10-
0-
2,4-D, CECIL CHAIN LABELED
OXIDATION EFFECTS ON MINERALIZATION
OF LABELED COMPOUND
n aci
11 •
10
n n n
i
Cl
Cl
•1 •
•.n
T"
W)
70 80
1NC.UHM KIN I I Ml . MAi'.
I.I i.l Nli- 1KIAI • . * OX in ana REDU
90
Figure 44. Oxid.'itton i-fl'i-ri-s on (.Icc.r.ul.iL ion of inbuJud 2,4-1) (chain labeled)
as indiirnlcd by recovery of CU7 from noist, unstirred Cecil
topsoil mntorl.il .
-------
labeled carbon as carbon dioxide occurs only after substantial modifica-
tion of the molecule, probably involving several intermediates.
All of the biometer flask 2,4-D degradation results presented to
this point have used ring-labeled 2,4-D. Figure 44 gives the recovery
of labeled carbon dioxide from chain labeled 2,4-D incubated in a Cecil
topsoil material. It was observed that the rate of mineralization of
the ring and side chain organic units were essentially the same.
ADSORPTION STUDIES
Adsorption of Methyl Parathion and 2,4-D by 19 Soil and Sediment Materials
The physical and chemical properties of the soil and sediment
materials studied are given in Table 2. Adsorption/desorption coefficients
for methyl parathion equilibrated with the 19 soils and sediments are
given in Table 7. Distribution coefficients were calculated for 2 hrs
and 24 hrs adsorption. Soils 1, 3, 4, 7, 11, 14, and 15 adsorbed methyl
parathion relatively strongly after a 2 hr equilibration and soils 2, 5,
9, 18, and 19 did not strongly adsorb this compound. Those soils that
adsorbed relatively strongly were generally those with the highest
organic matter content and those in the group that weakly adsorbed
tended to have the lowest organic matter contents. The adsorption
coefficients were generally about the same or slightly higher after 24
hrs equilibration for most of the soils tested compared to the 2 hr
equilibration except for the muck which adsorbed substantially more at
24 hrs. Distribution coefficients of 2,4-D are presented in Table 8.
The values in the table showed the same trend from soil to soil as
methyl parathion except that the coefficients were less indicating 2,4-D
is less tightly bound than methyl parathion. The data show that with
the exception of the Shubuta series, adsorption coefficients for 2,4-D
were greater as the pH of the soil decreased.
The relationship between sorption and the soil organic carbon
content can be expressed by dividing the individual distribution coeffi-
cients of the samples by their respective organic i.irbon contents to
produce K values (Hamaker and Thompson, 1972; Krinrkhoff et al.,
1979). Calculation of K _ values Ivis thr effect of putting the sorption
on a uniform organic car Ron h;jsis, .issuming Lh;tt all the organic carbon
is equally effective (F.ambrrt, 1968) in sorbing methyl parathion and
2,4-D. The methyl parntluon K values obtained for 4 to 2831 for 2 hrs
and 24 hrs adsorption respectively with average values of 1374 and 1516
respectively (Table 9).
The K values for 2,4-D were .ilso lower than for methyl parathion
ranging frBm 9 to .HO and 40 to 41S for 2 hrs and 24 hrs adsorption,
respectively with .in average value of 'J3 and 160 respectively. The
substrinlu 1 ly decreased coell i cienl s of v.iriaLion | (.S/x) x 100| of K
values compared to coefficients of v.iri.ition of adsorption coefficients
for both compounds indicates the reduced variability between soils when
adsorption is considered on .in or K.I me carbon basis (Table 10). This
supports what others have reported in the literature.
74
-------
TABLE 7. DISTRIBUTION COEFFICIENTS FOR METHYL PARATHION EQUILIBRATED
WITH 19 SOIL AND SEDIMENT MATERIALS
Soil material
1. Airplane Lake
2. Atchafalaya River
3. Bayou Chevreuil
4. Calcasieu River
5. Cecil Subsoil
6. Cecil Topsoil
7. Chastain
8. Crowley
9. Gallion
10. Hartwell Lake
11. Lafitte Muck .
I
12. Lake Pontchartra in
13. Lake Providence
14. Leeville
15. Loring
16. Mhoon
17. Mississippi River
18. Norwood
19. Shubuta
Mean (x)
Standard deviation(s)
Coefficient of variation (CV, %)
.»-
KA"
70.6310.3011
2.0310.08
60.8510.56
56.3511.40
2.5810.12
16.1710.09
39.5810.98
21.7810.26
3.4610.14
18.1610.10
79.9313.69
14.0810.59
18.7710.93
50.4813.92
162.4513.25
5.8010.23
23.3210.54
2.6910.L2
2.0510.12
34.3
rj«) . 9
1 14
**
KB
64.2riO.5811
2.5210.07
64.1510.40
49.2811.17
4.6210.52
23.3610.65
48.9410.39
19.5310.11
3.8510.13
20.3210.11
251.4118.93
15.5710.91
20.4010.63
40.1712.05
153.8812.59
7.5410.14
24.5410.45
3.1010.09
3.1310.19
43.2
61.8
143
n,
KA: 2-hour adsorption coefficient
v
KB: 24-hour adsorption coefficient
Sediment
Mean of triplicate subsamples and standard deviation
75
-------
TABLE 8. DISTRIBUTION COEFFICIENTS FOR 2,4-D EQUILIBRATED WITH
19 SOIL AND SEDIMENT MATERIALS
Soil material
1. Airplane Lake
2. Atchafalaya River
3. Bayou Chevreuil
4. Calcasieu River
5. Cecil Subsoil
6. Cecil Topsoil
7. Chastain
8. Crowley
9. Gallion .
10. Hartwell Lake
11. Lafitte Muck L
12. Lake Pontchartrain
13. Lake Providence
14. Leeville
15. Loring
16. Mhoon
17. Mississippi River
18. Norwood
19 . Shubuta
Mean (x)
Standard deviation(s)
Coefficient of variation (CV, %)
.».
KA"
0.8510.05^
0. 1610.07
0.6910.07
1.4210.11
0.6910.04
0 K310.04
8.4510.13
0.5010.07
0. 3310.07
3.7510.10
20.1610.24
1.8210.08
0.1910.09
1.2910.02
8.9610.31
0.0510.08
0.5110.03
0.3110.07
0.0910.02
2.7
5.0
185
**
KB
1.5210.06^
0.6710.05
1.3710.01
1.8910.07
1.3110.08
1.6710.12
10.8510.09
0.9310.06
0.8710.08
4.4610.07
39.0910.67
2.2810.07
0.8510.05
1.9310.09
9.9910.11
0.3810.05
1.1710.08
0.7010.04
0.4410.03
4.3
8.9
206
fl,
KA: 2-hour adsorption inefficient
t
KB: 24-hour adsorption roptficient
Sediment
Mean of triplicate subsamples and standard deviation
76
-------
TABLE 9. PERCENT ORGANIC CARBON AND DISTRIBUTION COEFFICIENTS ON AN ORGANIC CARBON BASIS FOR METHYL
PARATHION AND 2.4-D IN 19 SOIL AND SEDIMENT MATERIALS
Methyl parathion
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11 .
12.
n.
14.
15.
16.
17.
18.
19.
Mean
Soil material
Airplane Lake ,
Atchafalaya River
Bayou Chevreuil.
Calcasieu River
Cecil Subsoil
Cecil Topsoil
Chastain
Crowley
Gallion ,
Hartwell Lake
Lafitte Muck
Lake Pontchartrain
Lake Providence
Leeville
Lor ing
Mhoon ,
Mississippi River
Norwood
Shubuta
(x)
% oc
3.80
0.24
3.44
2.30
0.43
1.67
2.99
2.16
0.51
1.91
15.45
0.55
1.53
2.02
5.77
0.54
1.16
0.56
0.44
Standard deviations)
Coefficient of variation
ICV, %)
2 hrs
adsorption
K
oc
1859
846
1769
2450
600
968
1324
1008
678
951
517
2560
1227
2499
2815
1074
2010
480
466
1374
782
57
24 hrs
adsorption
K
oc
1690
1050
1865
2143
1074
1399
1637
904
755
1064
1627
2831
1333
1989
2667
1396
2116
554
711
1516
642
42
2,4-D
2 hrs
adsorption
K
oc
22
67
20
62
160
50
283
23
65
196
130
330
12
64
155
9
44
55
20
93
93
100
24 hrs
adsorption
Koc
40
279
40
82
305
100
363
43
171
234
253
415
56
96
173
70
101
125
100
160
115
72
1
Sediment
-------
TABLE 10. MEANS, STANDARD DEVIATIONS, AND COKKHCIKNT OK VARIATION TOR
K AND K VALUES OF METHYL PARATHION AND 2,4-D ADSORPTION IN 19 SOIL
00 AND SEDIMENT MATERIALS
Compound
Equilibration
time, hr
Parameter
CV
methyl parathion
methyl parathion
methyl parathion
methyl parathion
2,4-D
2,4-D
2,4-1)
2,4-D
2
2
24
24
2
2
24
24
K
K
oc
K
K
oc
K
K
oc
K
K
i\i-
•»ft . 'J
1374
4'J.2
Ir>l6
2.7
93
4. J
IdU
39.9
782
61.8
642
5.0
93
H.'J
llr>
116
57
143
42
185
100
207
72
78
-------
2 Table 11 gives the results of a statistical analysis to determine
R values, the amount of variability in a dependent variable that is
explained by the variation in one or more independent variables. Where
all 19 soil and sediment materials were included, organic matter content
gave the highest R values (except for methyl paralhion with a 2 hr
adsorption period). Soil pH and ration exchange capacity arc also
reasonably well associated with 2,4-D adsorption where all 19 soils were
included, while CEC contributed significantly for methyl parathion. The
soil property most often correlated with 2,4-D adsorption is organic
matter (Bailey and White, 1964). Our data on 19 soil and sediment
materials indicate that low pH also promotes 2,4-D adsorption.
In soil and sediment materials where the organic matter content is
very low, other soil properties may then become substantially more
important in regulating the mobility of synthetic orgariics. One objective
of this work was to examine the importance of properties other than
organic matter content in adsorbing these pesticides in materials with
low organic matter content. In particular, we were interested in examining
the role of reducible iron levels as well as the Al extracted by an
ammonium oxalate extractant. Therefore, R values describing the relation-
ship between adsorption and soil properties were also determined on
those soil and sediment materials with less than 1% organic carbon.
2
In these low organic matter materidls, pH gave low R values between
.31 and .36 for both 2 and 24 hr adsorption coefficients for both compounds
and all of the other properties we originally planned to examine gave
much lower values In particular, oxalate extractable Fe and Al levels
gave especially low R values. In these low organic matter materials,
however, we observed that the relationship between adsorption and oxalate
extractable Mn and Ca increased, especially for 2,4-D. Where the best
three (1st, 2nd, and 3rd highest R values) models with two independent
variables were examined tor both equilibration times and both compounds
in the low organic matter materials, Mn and C.i appeared in 11 of the 12
pairs and the R values were siihsl.ml i.il ly higher tli.ni where the best
single independent variables were considered in these low organic matter
soils.
Thus in soil and sediment materials with low organic matter content,
Ca and Mn or factors associated with weak chelate extractable levels of
these elements may be important for regulating the mobility of these
synthetic organics.
The Effect of pH and Redux PoleiiLi.il on the Adsorption of Methyl Parathion
and 2j4-J)
Table 12 gives ailsorption (ocltu lents lor 2 and 24 hr equilibrations
of methyl p.ir.ithion with five soil .nut sediment materials that had been
incubated under controlled pll and rcdox potential conditions. Although
the adsorption proiedure was applied to materials incuhated at -150 mv,
the data are not Mutinied because the dcgr;idation studies indicated that
most ol tlie ,1 tided methyl paralhion w.is not recovered within hours of
amending the suspensions, thus mm h ol the Olabelc-rl material measured
in the aqueous phase may not have been the parent compound. Whether or
79
-------
TABLE 11. R2 VALUES FOR KA1 AND KB2 FOR 2.4-D AND METHYL PARATHION VS. SELECTED INDEPENDENT VARIABLES
Dependent
R for single independent variables
Organic
Compound parameter Soils matter Clay Sand pH CEC Al Fe Mn Ca
Best pair. 2nd best 3rd best
222
R pair, R pair, R
**
** *
2,4-D K 2 hr. 19(all) .84 .15 .13 .30 .60 .26 .12 .00 .00
**
**
2,4-D K 24 hr. 19(all) .90 .10 .07 .22 .56 .18 .05 .02 .00
pH-OM,
**
.90
PH-OM,
**
.92
iron-OM,
**
.87
OM-sand,
**
.91
Al-OM,
*•*
.85
Ca-OM,
-A-v
.91
2,4-D K 2 hr. 7
(1 ow OM 1
Uow U"J
.03
.01 .01 .31 .00 .00 .00 .62" .32
Mn-sand,
**
.82
Mn-clay,
*•*
.78
Mn-Al ,
*
.73
2,4-D K 24 he. 7
(lowOM)
methyl K 2 hr. 19(all)
parathion
.02
.04 .00 .34 .00 .02 .03 .66" .40 Mn-sand, Mn-clay, Mn-CEC,
-;— v. j. .^f.
.34 .17 .26 .08 .46 .16 .14 .02 .00 iron-CEC, Mn-CEC, CEC-sand,
methyl K 24 hr. 19(all) .93
.
parathion
.14 .15 .17 .70 .18 .07 .00 .00
CEC-OM, OM-clay, iron-OM,
.4_t. .'..*. .*M*.
.93~" .93"" .93""
methyl K 2 hr. 7 .10
parathion (low OM)
.07 .16 .32 .03 .07 .06 .15 .08
Ca-clay,
?4**
Ca-Al, Mn-sand,
methyl K 24 hr. 7 .11
parathion (low OM)
.03 .12 .36 .01 .02 .02 .17 .14
Ca-clay, Ca-Al, iron-Ca,
74Vrtr 66** 50
2-hour adsorption coefficient
2
**
24-hour adsorption coefficent
Significant at 1% level
Significant at 5% level
-------
TABLE 12. ADSORPTION COEFFICIENTS FOR METHYL PARATHION IN SOIL AND SEDIMENT MATERIALS
INCUBATED UNDER CONTROLLED PH AND REDOX POTENTIAL CONDITIONS
oo
Soil Material
Hartwell Reservoir
sediment
Cecil clay loam
Lake Providence
sediment
Crowley silt loam
Airplane Lake
sediment
Adsorption Redox
Coefficient 50
18.49
K ±0.04
22.08
Kfi ±0.42
34.69
K ±0.86
KV 39.52
± 1.61
21.66
K ±0.54
K? 25.84
B ± 0.46
5.06
K ±0.32
1C 6.10
B ± 0.23
74.35
K ±0.15 '
K£ 75.56
± 0.18
pH 7.0
potential
250
19.98
± 0.21
26.24
±0.64
25.90
±0.45
30.38
± 0.26
19.49
±12.64
20. 11
±12.58
4.56
±0.33
5.80
± 0.63
74.33
± 3.07
80.91
± 0.84
, mv
500
18.97
± 0.20
22.87
± 0.34
22.87
±1.36
25.91
± 0.27
26.27
±1.49
28.68
±1.89
6.50
±1.65
7.73
±1.78
66.89
±1.99
72.05
± 5.55
Redox
50
17.07
± 0.43 ±
21.25
±0.27 ±
32.36
± 0.27 ±
33.80
± 0.23 ±
6.45
± 0.16 ±
8.20
±0.20 ±
92.21
±1.29 ±
98.39
±5.29 ±
pH 5.0
potential
250
22.00
0.69
27.87
0.16
33.39
0.28
36.35
0.33
5.86
0.57
6.49
0.32
134.32
2.47
145.64
1.78
, mv
500
21.97
± 0.48
28.12
± 0.67
40.36
± 0.26
42.84
± 0.68
5.34
± 0.49
6.95
± 0.47
118.46
± 10.63
138.33
± 5.25
2-hour adsorption coefficient
>
"24-hour adsorption coefficient
-------
not rapid modification of methyl parathion occurred in strongly reduced
soil materials subject to air drying and grinding prior to equilibration
with the adsorbate is not known, but until this possibility is checked,
the -150 mv data should not be included.
In the four soil materials where adsorption studies were done at
two pH levels (7.0 and 5.0), there tended to be more adsorbed at the
lower pH. Only 4 of 24 measurements at pH 5.0 for which there were
corresponding values at pH 7.0 gave a lower adsorption coefficient at pH
5.0, and all of these four values were only slightly lower. Adsorption
coefficients were lower at +50 mv than higher redox potential values in
three of four materials at pH 5.0, but there were no consistent redox
potential effects apparent over all soil materials at both pH levels.
In the highly organic Airplane Lake sediment at pH 5.0, there did
appear to be an increase in adsorption as redox potential increased
above 50 mv.
Two- and 24-hour adsorption coefficients for 2,4-D under different
redox potential conditions are given in Table 13 for four soil materials
at pH 5.0 and five soil materials at pH 7.0. In three of the four soil
materials for which data are available at both pH levels, adsorption was
greater at pH 5.0 than 7.0. As for methyl parathion, there was no
general redox potential effect apparent over all soils. However, as
noted for methyl parathion, both pH and redox potential may have substan-
tially affected adsorption in the highly organic Airplane Lake sediment
where 2,4-D was much more strongly adsorbed under oxidized conditions at
pH 5.0 than under any other combination of experimentally imposed conditions.
The Effect of Redox Potential and Sample Processing Methods on the
Adsorption of Pentachlorophenol
The results of the adsorption/desorption study using PCP and Shell
Beach sediment material incubated under oxidized and reduced conditions
is given in Table 14. It is apparent that a 2-hour shaking period did
not give adequate time for the adsorption process to equilibrate as the
distribution coefficient (K) was greater after 24 hours than two hours
in every case.
Regardless of the sample processing technique used, the data indicate
PCP was more tightly bound to oxidized sediment solids than to reduced
sediments. Thus there would be some tendency for PCP in a contaminated
sediment-water system to preferentially become associated with the thin
oxidized, surface sediment horizon as well as suspended colloidal particu-
lates, which would also tend to he oxidized. Preferential adsorption to
suspended particulates in predominantly oxidized, shallow coastal water-
bodies could also favor transport I rom cont.imin.iLcd .ireas, but would
also result in enhanced degradation since degradation was shown to be
faster in oxidized Shell Beach sediment-water systems (DeLaune, Gambrell,
and Reddy, 1983).
The relatively soluble nature of PCP is indicated by the fact that
sediment materials in the presence ot uncontdminated water may desorb
H2
-------
TABLE 13. ADSORPTION COEFFICIENTS FOR 2,4-D IN SOIL AND SEDIMENT MATERIALS INCUBATED
UNDER CONTROLLED PH AND REDOX POTENTIAL CONDITIONS
pH 7.0 pH
Soil Material
Hartwell Reservoir
sediment
Cecil clay loam
Lake Providence
sediment
Crowley silt loam
Airplane Lake
sediment
Adsorption Redox potential, mv
Coefficient -150 50 250 500
0
K. ±0
A, 1
T
KB ±0
0
K ±0
K;! 1
B ±0
K.
Kj
B
0
K ±0
KD °
B ±0
0
K ±0
^** i
E\n 1
B ±o
.46
.13
.29
.08
.26
.07
.25
.01
.25
.03
.77
.09
.84
.08
.60
.11
0.52
±0.02
1.48
±0.27
0.20
±0.02
1.25
±0.10
0.65
±0.04
1.20
±0.06
0.45
±0.03
0.92
±0.10
1.09
±0. 11
1.73
±0.09
0.18
±0.14
0.93
±0.05
0.71
±0.06
1.53
±0.04
0.44
±0.04
1.08
±0.04
0.64
±0.36
1.33
±0.25
1.06
±0.09
1.75
±0.11
0
±0
1
±0
0
±0
1
±0
0
±0
1
±0
0
±0
2
±1
1
±0
1
±0
.59
.06
.59
.09
.25
.07
.24
.06
.88
.06
.43
.08
.81
.07
.17
.29
.18
.04
.86
.03
5.0
Redox potential, mv
-150 50 250 500
0
±0
1
±0
1.21 0
±0.12 ±0
1.79 0
±0.06 ±0
1.30 2
±0.03 ±0
1.88 3
±0 . 08 ±0
.83
.05
.60
.06
.22
.03
.83
.09
.59
.13
.21
.04
1
± 0
2
± 0
1
± 0
1
± 0
0
± 0
0
± 0
11
± 0
14
± 0
.76
.05
.67
.02
.01
.14
.79
.05
.11
.01
.69
.02
.94
.68
.93
.59
1.50
±0.04
2.26
±0.24
2.13
±0.02
2.69
±0.05
0.21
±0.04
0.58
±0.06
13.49
± 0.32
16.58
±0.43
2-hour adsorption coefficient
"24-hour adsorption coefficient
-------
TABLE 14. ADSORPT10N-DESORPTION OF PCP FROM SHELL BEACH SEDIMENT
Method of
sediment sample Oxidation
preparation conditions
PH
Distribution coefficients
2 hrs 24 hrs 2 hrs
adsorption adsorption desorption
Not dried1
Not dried
Reduced
Oxidized
6.8
6.8
76.76a3
b
111.08
117. 15a
b
169.80
%___
6.393
b
4.49
Dried
Dried
& ground Reduced 6.8 31. 863
h
& ground Oxidized 6.8 72.57
50.673 14.023
b b
105.81 8.38
Wet method: Sediment materials were not dried prior to amending with
labeled PCP.
Dry method: Sediment materials dried and ground prior to amending
with labeled PCP.
Means within columns for each method of sample preparation not
followed by the same letter are significantly different at the 5%
level using Duncan's Multiple Range procedure.
84
-------
several percent of the initially adsorbed PCP into the aqueous phase
within two hours. This also suggests that PCP may be relatively mobile
in sediment-water systems. The somewhat greater attraction of an oxidized
solid phase (sediment or suspended particulates) for PCP and its rapid
degradation under oxidized conditions are complimentary processes enhancing
the rate of removal from contaminated sediment-water systems.
85
-------
SECTION 4
SUMMARY AND CONCLUSIONS
PESTICIDE CONCENTRATION EFFECTS ON THE KATE OF SOIL REDUCTION
A simple technique was demonstrated for determining the potential
for synthetic organics to stress microhial populations. Oxidized Crowley
and Cecil soil materials were amended with varying concentrations of
2,4-D and methyl pa rath ion, flooded, and then analyzed for changes in
pH, redox potential, and levels of soluble plus exchangeable Fe, Mn, and
Zn, all of which may be directly or indirectly influenced by the activity
of soil microorganisms. At the concentrations tested (up to 75 ppm),
there was little effect of 2,4-D, hut methyl parathion apparently did
affect nucrobial activity contributing to changes in the measured soil
properties upon flooding. This approach-may be a useful technique for
screening various compounds for their potential to stress microbial
activity that, for many researchers, would be easier than direct observa-
tions of microbial parameters such as population numbers and classifica-
tions, or enzyme levels.
DEGRADATION STUDIES
Two types of degradation studies were conducted. In one, soil and
sediment materials were maintained under controlled pH and redox potential
conditions and amended with either methyl parathion, 2,4-D, or Aroclor
1254. Then the loss of these compounds with time was measured by extraction
and gas chromatography analysis as .in indication of degradation. In the
other type of degradation study conducted, methyl parathion and 2,4-D
uniformly labeled with C in the aromatic ring were incubated under air
and oxygen-free nitrogen conditions in severjl soil and sediment materials.
Collection of labeled carbon dioxide w.is used as a measure of degradation.
These are commonly called biometcr flask degradation studies.
Comparing the results of the extraction/gas chromatography studies
and the biometer flask studies revealed major differences in the conclu-
sions one might draw on the effects of oxidation conditions on degradation
rates. One difference is that the observed degradation rate of methyl
parathion is much slower in the unstirred soil and sediment materials
(biometer flasks) compared to I he stirred suspensions (extraction/gas
chromalogr.iphy analysis) Another more sinking difference is that
oxidation-reduction treatment el feels .ire opposite for methyl parathion
using the two degradation study terliin<|iies. These observations must be
addressed.
Regarding the r.ite of ilexr.id.il inn, the biometer flasks were maintained
at rfhout 2J°C wherc.is the stirred suspensions were kept at about 28°C.
The temper.iture difference would he expected to contribute to some
increase in degrail.it ion r.ile in the- stirred suspension, but it is unlikely
temperature differences .ire-minted for mosl of the degradation differences
observed.
-------
The rate of degradation may be enhanced in stirred suspensions
compared to unstirred conditions. Parr and Smith (1974) noted a stirring
effect in laboratory studies of toxaphene degradation under both aerobic
and anaerobic conditions. It was suggested that stirring some soils
provides additional available carbon to microorganisms to sustain rapid
degradation of toxaphene. Measurements ol total carbon dioxide evolution
showed there was a higher level of respiratory activity in a stirred
anaerobic suspension compared to a moist, unstirred, anaerobic suspension.
Next, the apparent opposite treatment effects, depending on the
type of degradation study, must be addressed. In the hiometer flask
studies, the labeled carbon WHS located in the arom.it ir ring of the
methyl parathion molecule. Literature discussed elsewhere in this
report suggest thai several other modiIicutions to the molecule may
occur before degradation of the .ironi.itic ring (i.e., hydrolysis of the
ester group and reduction of the nitro group on the benzene ring). Thus
recovery of the label as carbon dioxide represents very substantial
modification (degradation) of the parent compound. The rate of recovery
of the label as carbon dioxide does not say anything about the rate of
initial or partial degradation of the parent compound as several sequential
intermediate degradation products are probably involved before mineraliza-
tion of the aromatic ring to carbon dioxide occurs. Thus it is highly
probable that modification of the parent molecule occurred at a greater
rate than indicated by the rate of recovery of the label. Extraction of
the soil or sediment material and analysis by gas chromatography more
accurately indicates the rate of loss of the parent compound, but,
unless intermediate degradation products are identified and quantified,
does not reveal much about the rate of total mineralization of the
parent molecule. Both C labeled and extraction/specific compound
quantification methods are used extensively in degradation studies of
synthetic organics. The comparison of results by the two methods in
this study which indicate opposite treatment effects clearly indicate a
researcher should carefully consider Die c<>ni|>;ilil>i 11 ly ol a particular
experimental approach with the research objectives.
Controlled pH-Redox Potential Microcosm Studies
The primary purpose of the stirred suspension studies was to determine
relative effects of redox potential on the degradation of the three
synthetic organics. Continuous stirring insures that uniform conditions
are maintained throughout the media.
Salinity effects on the loss of methyl parathion from an estuarine
sediment were minim.nl in this si inly. Soil
-------
The apparent very rapid degradation of methyl parathion under
strongly reduced conditions is believed to be due to processes other
than direct microbial metabolism as substantial Losses and, in one case,
a complete loss of the compound was observed within hours. Plots of the
log of concentration vs. time were nearly Linear as indicated by high
correlation coefficients in most cases. However, careful examination of
these plots revealed a slight upward curve was present indicating some
deviation from first order degradation kinetics.
One degradation study using stirred suspensions and controlled pH
and redox potential conditions was conducted with 2,4-D. As observed
for methyl parathion, redox potential conditions greatly affected the
loss of 2,4-D, but, unlike methyl parathion, 2,4-D was removed much more
rapidly from oxidized than reduced soil material.
Aroclor 1254 was incubated at two pH and four redox potential
levels for six weeks. Although recovery decreased to about half of
initial levels after six weeks in the stirred suspensions, there was no
apparent effect of pH or redox potential on the recovery of this PCB
formulation.
In selected studies, carbon dioxide production and evolution was
measured from mircocosms incubated under controlled redox potential
conditions. The data indicate greater microhiaJ activity with increasing
redox potential. It was not possible to relate the degradation rate of
any of the compounds studied directly to microbial activity levels.
With methyl parathion in particular, it is obvious factors and processes
other than microbial respiration rates are involved with the rate of
degradation.
Biometer Flask Studies
An advantage of using biometer flasks for degradation studies is
their simplicity. Compared to the controlled pH-redox potential micro-
cosms, the biometer flasks, using collection of C-labeled carbon
dioxide as an indication of degradation, permits the study of a larger
number of experimental parameters with adequate replication. It should
be noted that this experimental method measures mineralization of the
labeled portion of the molecule rather than simple modification of the
parent compound. How this ;i ft rets experimental observations in degradation
studies compared to ex tr,id ion/gas i hi om.itography methods is discussed
elsewhere in this report.
Carbon-14 labeled methyl p.ir.ithion w.is incubated two to three
months in six soil ami sediment materials under air and nitrogen atmos-
pheres. There was a very large and statistically significant oxidation
treatment effect in all six materials. In every rase, 10 to 20 or more
times of the labeled carbon from the aromatic ring was evolved as labeled
carbon dioxide under oxidizing conditions compared to reducing conditions.
The observed oxidation treatment effects were opposite the results
obtained from the stirred siispens inn/ext r.ict ion/ana I ys is by gas chromatog-
raphy studies for methyl p.ir.ilhmn. The redsoris for this are discussed
elsewhere.
88
-------
Recovery of the ring-labeled carbon in 2,4-D as carbon dioxide was
also much greater under oxidized conditions. For the one soil material
studied using both ring- and chain-labeled 2,4-D, no difference was
noted in the rate of mineralization of the aromatic ring or the alkyl
chain portion of the molecule.
ADSORPTION STUDIES
Methyl Parathion and 2,4-D Adsorption Study Using 19 Soi[ aiid__S_r:dijnent
Materials
Adsorption coefficients fur 2,4-1) :ind methyl pa rath ion were determined
in 19 soil and sediment materials selected tu give a wide range in
physical and chemical properties. Organic matter content of the materials
was generally indicated to be the most important property affecting
adsorption. As expected, 2,4-D was less strongly associated with the
sediment phase than methyl parathion, and pH was more closely associated
with the adsorption of 2,4-D than with methyl parathion.
One objective of this study was to examine soil properties that may
be important in immobili/ing synthetic compounds where the soil organic
matter content is Low. Data were presented indicating that in these low
organic matter content soils, the association between adsorption and
oxalate extractable Mn and Ca may be an important relationship. Additional
work should be done in low organic matter soil material to better quantify
the relationship between adsorption and soil gcochemical properties. As
our work has demonstrated, those properties closely associated with the
nobility of some synthetic orxanics in typical surface soils may not be
useful in predicting the immoh 11 i/;ii i on of synllirtit or panics in subsoils.
Understanding the relat lonship between gcoc-hemic.il properties and the
mobility of synthetic organics is especially important in typical,
low-organic matter subsoil materials that have been contaminated with
hazardous organics such as has occurred at many hazardous waste disposal
sites.
pH and Redox Potential Effects on Adsorption
Metljyl parathion tended to adsorb to the soil materials tested more
strongly at pH 5.0 than 7.0. There were no consistent redox potential
effects for methyl parathion ove.r .ill soil materials and pH levels, but
in the highly organic Airplane Lake sediment at pH 5.0, adsorption
appeared to increase .is redox potential increased above 50 mv.
For 2,4-1), adsorpl i on w.is .11 so gieater .it pll 5.0 than 7.0 in most
soil materials and t lie re w.is no gcuri.il redox potenti.il effect over all
soils. However, as noted for methyl p.iratluou, both pll :iud redox potential
may have substantially .1 Heeled adsorption in the highly organic Airplane
Lake sediment where 2,4-0 w.is nun h more strongly adsorbed under oxidized
conditions at pH 5.0.
89
-------
Effects of Oxidation Conditions and Sample Processing Methods on the
Adsorption of Pentachlorophenol (PCP)
The method of processing sediment materials prior to conducting the
adsorption procedures (testing wet sediment materials vs. aLiquots that
had been dried and ground) did make a difierence in the 2- and 24-hour
adsorption coefficients measured. Sample preparation methods did not
affect the relative oxidation treatment effects. PCP was more tightly
bound to oxidized estuarine sediment solids than to reduced sediment
solids.
•JO
-------
SECTION 5
RECOMMENDATIONS
Many pesticide and other environmental scientists now recognize
that oxidation conditions affect the persistence of synthetic organics
in soil and sediment-water systems, though there is relatively little
published information on the role of oxidation-reduction conditions on
the degradation rate of specific synthetic organics. Where information
is available on oxidation-reduction effects on degradation of synthetic
organics, the effect of this parameter is usually great enough such that
soil and sediment oxidation conditions should be considered in evaluating
the persistence and potential impacts of the compounds in the environment.
Where suitable information is available, it should be incorporated into
modeling efforts to predict the fate of synthetic organics in the environ-
ment.
Work done in this laboratory and by a very few other groups has
clearly demonstrated the importance of redox potential conditions in
soil and sediments on the rate of degradation of many compounds as well
as the fact that different compounds respond differently to redox condi-
tions. Unfortunately, the information available to date generally does
not provide the information needed to quantify redox potential effects
on degradation rates under natural conditions. For example, it appears
stirred suspension studies may indicate an artifically high rate of
degradation. On the other hand, most degradation studies using carbon-14
labeled compounds in unstirred soil materials make conclusions on degra-
dation rates based on recovery of the labeled carbon as carbon dioxide.
In most cases, depending on the molecular structure of the compound and
the position of the label, recovery of the label as carbon dioxide
certainly indicates the compound has degraded such that nothing of
environmental consequence remains. However, this approach tends to be
overly cautious as labeled carbon dioxide is collected only after
substantial modification of the parent compound, usually through a
series of many degradation products. For many compounds, modification
of the molecule has proceeded beyond the point where it presents an
environmental threat long before the label is recovered as carbon dioxide.
Also some of the label, transformed to harmless decomposition products,
may be incorporated into the humus fraction of the soiI and retained as
soil organic matter for extended periods of time. Methods used for
degradation studies, particularly those examining pliys icorhemical effects,
have been successful in documenting the importance of redox potential,
but probably unreliable in indicating effects of physicochemical conditions
on the rate of degradation. We have conducted degradation studies using
both methods in this project to demonstrate how the experimental approach
can affect the results and conclusions of degradation studies. Thus one
recommendation coming from this project is that more be done alerting
environmental organic chemists doing degradation research to the importance
of matching experimental techniques to the research objectives such that
erroneous conclusions on treatment effects and degradation rates are
minimized.
«M
-------
The sequence of research findings to date have lead to the conclusion
that physicochemical conditions do substantially affect the degradation
rate of many compounds, but research methods should be devised and
employed to provide information on how physicochemical properties of
soils and sediments affect the degradation rate of compounds under
natural conditions. This information would be most useful to modelers
for predicting the persistence of synthetic organics in various environ-
mental compartments. The technique developed should be relatively
inexpensive and capable of including a wide range of soil and sediment
physical and chemical properties. The experimontaI material for these
degradation studies should be placed in the field where the experimental
units are subjected to natural climate, microbial, and hydrologic condi-
tions. Soil and sediment materials should be characterized for most of
the commonly measured properties, much as was done in the adsorption
study included in this report using 19 soil and sediment materials. One
important difference in the degradation studies would be that the soils
should be characterized for certain parameters in their natural condition,
not after sampling, drying, and grinding. Then degradation rates and
soil properties should be evaluated using appropriate statistical proce-
dures to determine the relationship between degradation rate and the
measured soil properties. If a large number of soils were included,
statistically valid prediction models could be developed for degradation
rates under natural conditions based on soil and sediment properties.
-------
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-------
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