A LITERATURE REVIEW SUMMARY OF
METALS EXTRACTION PROCESSES
USED TO REMOVE LEAD FROM SOILS
by
IT Corporation
Cincinnati, Ohio 45246
Contract No. 68-C9-0036
Work Assignment- No. 3-87
Project Officer: Ronald J. Turner
Water and Hazardous Waste Treatment Research
RISK REDUCTION ENGINEERING LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
CINCINNATI, OHIO 45268
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DISCLAIMER
monta.^m^ A be*1 Junded wholly or in part by the United States Environ-
mental Protection Agency under Contract Number 68-C9-0036 to IT Corporation It
has been subject to the Agency's review and K has been approved tof*3£En as
an EPA document. Ment.on of trade names or commercial products does not
constitute endorsement or recommendation for use.
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FOREWORD
Today's rapidly developing and changing technologies and industrial products
and practices frequently carry with them the increased generation of materials that, if
Improperly dealt with, can threaten both public health and the environment. The EPA
is charged by Congress with .protecting the Nation's land, air, and water resources.
Under a mandate of national environmental laws, the Agency strives to formulate and
implement actions leading to a compatible balance between human activities and the
ability of natural systems to support and nurture life. These laws direct the EPA to
perform research to define our environmental problems, measure the impacts, and
search for solutions.
The Risk Reduction Engineering Laboratory is responsible for planning,
implementing, and managing research, development, and demonstration programs to
provide an authoritative, defensible engineering basis in support of the policies,
programs, and regulations of the EPA with respect to drinking water, wastewater,
pesticides, toxic substances, solid and hazardous wastes, and Superfund-related
activities. This publication is one of the products of that research and provides a vital
communication link between the researcher and the user community.
This report presents the major findings of a literature review to investigate
metals extraction processes for recovering lead from soils. Key issues such as soil
characterization and use of chelating agents are addressed, and descriptions of
various metals extraction technologies are discussed in this report. The information
provided in this report will assist in identifying lead extraction process options for
treating lead-contaminated soils.
E. Timothy Oppelt, Director
Risk Reduction Engineering Laboratory
HI
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ABSTRACT
This report was submitted in fulfillment of Contract No
iv
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CONTENTS
Page
Disclaimer • jj
Foreword yj
Abstract jv
Rgures vi
Tables Vjj
Acknowledgement jx
Executive Summary x
1. Introduction -j
2. Metals Extraction Technologies and Soil Characterization 3
2.1 Metals extraction technologies 3
2.2 Soil characterization • 25
3. Chelating Agents 30
3.1 Physical properties 30
3.2 Treatability tests 40
4. Membranes 45
4.1 Properties, of membranes. 46
4.2 Operating parameters 47
5. Summary 52
5.1 Metals extraction technologies and soil characterization 52
5.2 Chelating agents 54
5.3 Membranes 55
References 56
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6 Reaction Time for a 1:1 Molar Chelant Wash Using EDTA
FIGURES
Mimber
1 Bureau of Mines Acid Leaching Process
4
2 Bureau of Mines Fluosilicic Acid Leaching/Electrowinnina
Process for Treatment of Lead-Contaminated So™ 9
5
3 TerraMet™ Lead Removal Process
6
4 Surfactant Liquid Membrane System
8
5 Electromembrane Reactor for PEI Lead Recovery Study
12
25
7 Structure of Metal-EDTA Chelate
31
8 Stability Constants for EDTA, DTPA, and NTA
9 Stability Constants for Metal-EDTA Complexes as a Function of pH 35
10 Stability Constants-tor Metal-NTA Complexes as a Function of pH 36
11 Lead-EDTA Distribution as a Function of pH
] 38
12 Lead-NTA Distribution as a Function of pH |
39
13 Effect of Electrolytes on pH-Dependent Recovery of Lead 42
43
15 Concentration Gradients in the Electromembrane Reactor ] 4Q
VI
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TABLES
Number Page
1 Lead Removal Efficiency from Contaminated Soil 10
2 Lead Removal Efficiency for a Three-Step Extraction Process 11
3 Summary of Lead Recoveries for EMR Experiments 13
4 Results of Soil Washing on Battery Breaker Wastes 14
5 Selectivity of Various Sodium EDTA Forms in Leaching
Various Lead Species 15
6 Results of Bench-Scale Soil Washing of Various Lead-
Contaminated Soils 16
7 Summary of Removal Efficiencies Obtained For Bench- and
Pilot-Scale Evaluations of Soil Washing 17
8 Effect of Reaction Time on Treatment of Contaminated Soil 18
9 Analytical Results for TCLP Lead in Soil Samples 19
10 Methods of Lead Extraction 21
11 Recovery of Added Lead in Various Extracts 22
12 Soil Washing Results: SARM III (High Organics, Low Metals) 23
13 Total Lead Content of Treated Soil Recovered on Number
10 and Number 60 Screens 27
14 Maximum Metal Removals From Sludge Solids for an Acid
Extraction Study 28
15 Stability Constants of Metal Chelates 32
VII
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TABLES (continued)
Number
16 Solubility of EDTA and Its Sodium Salts in Water at Different
Temperatures
; d7
17 Single-Agent Shaker Table Extraction Efficiencies
45
18 Three-Agent Sequential Extraction Efficiencies- Soil
Column Tests >,c
45
viii
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ACKNOWLEDGEMENT
This report was prepared for the U.S. Environmental Protection Agency bv IT
Corporation Cincinnati, Ohio, under EPA Contract No. 68-C9-0036, Work Assignment
3-87. The EPA Work Assignment Manager was Mr. Ronald J. Turner. The IT Project
Manager was Mr. E. Radha Krishnan, P.E., and the IT Work Assignment Manager was
Ms. Mary Beth Foerst. The principal authors were Ms. Mary Beth Foerst Ms Diane
Roush, and Mr. Maqsud Rahman, Ph.D. : '
ix
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EXECUTIVE SUMMARY
The objective of this report is to review and evaluate literature relating to metals
extraction technologies, soil characterization, chelating agents and membranes
anfal^H6 llteraJUre !ssessment- P°tentia' operating problems could be identified
and avoided regarding the extraction of lead from soils.
The information presented in the following report is derived from journal articles
books, research documents, and phone conversations with experts in the fieTd of lead
recovery, chelating agents, and membranes. !
The findings from a treatability study performed by IT for the National Science
Foundation are also presented. The remainder of the report discusses melalse^rac-
Sor^?168',8011 c*aractrization> Delating agents, and membra^TS5*Se
5255 Si? £ fhk8y r° 6, 'n the SUCC6SS Of lead extraction from soils and the
of lead through the use of various metals recovery processes.
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SECTION 1
INTRODUCTION
Numerous Superfund sites throughout the United States are contaminated with
toxic metals. Battery reclamation, lead smelting, and lead-based paint manufacturing
are examples of processes that can result in lead-contaminated soils. Metals, unlike
many hazardous organic constituents, cannot be degraded or readily detoxified. Toxic
metals represent a long-term threat in the soil environment. The cleanup of metal-
contaminated sites has traditionally involved excavation of the wastes and contami-
nated soils with subsequent disposal at an off-site, RCRA-approved landfill, in accor-
dance with hazardous waste regulations. This process is expensive because of the
special precautions (e.g., double liners) required to prevent leaching of toxic metals
from the landfills. In addition to increasing costs and dangers to public safety from
large-scale transportation of wastes, long-term environmental liability is also a concern
associated with the landfilling approach. Furthermore, periodic monitoring may be
necessary. Thus, there is great incentive for the development of alternative methods
for cleanup of contaminated sites.
Chemical or physical fixation of the contaminated soils, which immobilizes the
lead (or other heavy rnetals), is required before landfilling. The drawbacks of this
approach include 1) the need for future monitoring of heavy metals on site, 2) ques-
tionable longevity of fixation chemicals, 3) unknown biosystem (plant/animal uptake)
effects, and 4) the potential need of a soil cap to prevent wind erosion problems.1
The on-site treatment of wastes and contaminated soils to recover heavy metais
(e.g., lead) is a more attractive alternative. The goal of the on-site approach is to treat
the contaminated soil to an acceptable level, protecting groundwater and surface
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water resources, without physicallyjempving or jsplating the contaminated soil from
the contiguous environment.
A literature review was conducted in order to provide the latest available
information on lead removal/recovery processes including use of chelating agents
and/or membranes and to define important parameters for treatability testing. The
literature search was conducted through a review of the DIALOG database. Relevant
journal articles, conference papers, research reports, and books that describe soil
washing, chelating agents, and electromembrane and metals recovery processes were
reviewed.
The report is divided into sections that describe each major parameter to be
studied. Section 2 presents a summary of the literature containing information on
metals extraction technologies and soil characteristics for use in soil washing studies.
The section describes the findings from full-scale soil treatment processes and the soil
characteristics that affect lead removal from soil. Section 3 presents a summary of
information on chelating agents and their properties. This section also presents the
physical properties that influence chelation of lead, including formation constants,
solubility, and stability constants. Results of treatability studies using various chelating
agents to remove metals are also provided in this section. Section 4 presents a
summary of the literature on membranes. Physical properties such as permeability,
capacity, and burst strength are discussed. This section also discusses important
operating parameters such as membrane potentials and current density. -Section 5
presents a summary of the key findings of the literature survey. \
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SECTION 2
METALS EXTRACTION TECHNOLOGIES AND SOIL CHARACTERIZATION
At the onset of the literature survey effort, articles on soil characterization and
soil washing of lead-contaminated soils were reviewed to determine the conditions that
are conducive to extraction and recovery of lead from soil. The following soil proper-
ties must be taken into consideration when soil washing is performed: soil pH, clay
content, contamination levels, and moisture content. The following subsections
present a summary of the important considerations regarding the recovery of lead by
various metals extraction processes.
2.1 METALS EXTRACTION TECHNOLOGIES
A paper by William Schmidt (1990) of the Bureau of Mines (BOM) summarizes
the results of EPA's investigation into alternatives for treating contaminated soils found
at many battery reclamation sites. The BOM process employs acid leaching to
convert lead sulfate and lead dioxide to lead carbonate, which is soluble in nitric acid.
Lead is recovered by precipitation with sulfuric acid to produce a lead sulfate product.
Figure 1 presents a flow diagram of the acid leaching process. The BOM has also
developed a process to .convert the lead compounds to lead carbonates with
ammonium carbonate and ammonium bisulfite, followed by leaching with fluosilicic
acid. Rgure 2 shows the process flow diagram of the fluosilicic acid leaching process.
Lead is recovered from the acid solution by electrowinning. Any remaining lead in the
soil would be leached with 0.5 percent nitric acid solution.2
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Feed
Casings
Soil
Makeup
HN03
Figure 1. Bureau of Mines Acid Leaching Process.2
4
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Lead-Contaminated
Water
Ammonium
Hydroxide
Filtrate
Fluosilicic
Acid
HNO
Water
4 inch
+ 1/2 inch
Ammonium Carbonate/
Ammonium Bisulfate
To Wastewater
Treatment
Filtrate
Electrowinning
Lead
To Wastewater
Treatment
Clean Soil
Figure 2. Bureau of Mines Fluosilicic Acid Leaching/Electrowinning
Process for Treatment of Lead-Contaminated Soils.2
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The TerraMet™- soil remediation systemf eyeloped by COGNIS, Inc leaches
and recovers lead from contaminated soil, sludge, or sediment. Lead in the form of
metallic lead, soluble ions, and insoluble lead oxides and salts may be leached with a
proprietary aqueous leachant. The first step of the process involves dry screening of
the soi. to remove any oversized material. In the second step, the lead contaminants
are dissolved and recovered from the aqueous leachate by a metals recovery process
such as ion exchange or reduction. The aqueous leaching solution is recovered and
the lead can be regenerated and recycled. Any residual leachate in the treated soil is
nontoxic and biodegradable. Figure 3 presents a flow diagram of the TerraMet™
metals leaching process. Contaminated soils that may be treated by this process
.nclucle battery recycling sites, scrap yards, and metal plating shops. A bench-scale
test was conducted using 17,000 ppm lead-contaminated soil, which was treated to
less than 300 ppm residual lead. A pilot-scale system is currently being assembled for
additional testing.3
Leachant
Soil-
* ^
Clean Sol! Recovered Metal
Figure 3. TerraMet™ Lead Removal Process3
*>- "°< °ons«ute endorsement
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A soil recycling process developed by the Toronto Harbor Commission employs
three technologies operating in series. The first stage involves soil washing for
reducing the material volume and concentrating the contaminants in a fine slurry. The
second stage employs acidification and selective chelation for dissolution of heavy
metals. All metals may be recovered in their pure .form. The third stage involves
chemical hydrolysis followed by biodegradation to destroy organic contaminants in the
slurry. The soil recycling process was demonstrated at a Toronto Port Industrial
District site used for metals finishing and refinery and petroleum storage.3
A lead recovery process developed by Kaur and Vohra (1992) uses a surfactant
liquid membrane to recover lead (II) from wastewaters. The process consists of four
steps: 1) emulsification of the stripping phase into the carrier containing the mem-
brane, 2) continuous contact of the effluents with the emulsion, 3) separation of the
emulsion, and 4) splitting of the emulsion. The surfactant liquid membrane (SLM)
system consists of a membrane phase [di(2-ethyl hexyi) phosphoric acid (D2EHPA), n-
hexane, and span-80], an aqueous internal phase (sulfuric acid), and an external
phase [Pb(ll) in water]. Figure 4 presents a schematic of the three phases. The SLM
system involves the following steps: 1) Pb(ll) ions in the external phase diffuse
through the aqueous stagnant film and react with the D2EHPA at interface 1, 2) a
Pb(H) complex is formed at interface 1, which diffuses through the membrane phase,
and the lead transports to the organic interface 2, and 3) the lead is then stripped by
the internal phase reagent at interface 2. Bench-scale studies showed that with a 2
percent D2EHPA concentration, approximately 87 percent of the Pb(ll) was extracted.
These tests also determined that up to 100 percent Pb(ll) extraction could be achieved
with a 10 percent D2EPHA concentration. Acidification of the aqueous medium was
observed to substantially decrease the amount of metal ion extracted. The decrease
in metal ion extraction with increased acidification is caused by the increase in
common ion concentration.4
Krishnamurthy (1992) of the U.S. Environmental Protection Agency (EPA) has
conducted research on a lead extraction process involving the conversion of lead
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Pb2*
H*
1
Ext<*ma\ Phase
(Feed Soln.).
Pb2*
H*
2
Membrane
\
Interna
(Strip
H*
Pb2+
•
! t
•««BW>9BXE»BBB9iW*M
Phase
Soln.)
Figure 4. Surfactant Liquid Membrane System.3
8
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sulfate to lead carbonate with ammonium carbonate, lead carbonate to lead acetate
and oxidation of lead to lead acetate with acetic acid and oxygen, conversion of lead
dioxide into lead acetate, and the final conversion of lead acetate to lead sulfate with
sodium sulfate.5
Two separate bench-scale studies were performed to determine the recovery of
lead sulfate. The first set of tests examined each form of lead individually to determine
the reaction completion after the addition of the various reagents. Up to 94 percent of
the lead was solubilized in acetic acid solution. The solubility of lead in oxygenated
acetic acid was determined to range from 72 to 96 percent. The final test of the first
series of experiments showed that approximately 33 to 60 percent of lead dioxide is
converted to lead acetate with manganese acetate.5
The second series of bench-scale tests involved treatment of a synthetic lead-
contaminated soil. A clean soil was spiked with 10,000 mg/kg total lead in the
following percentages: lead sulfate - 60 percent, basic lead carbonate
(PbCO2.Pb(OH)2) - 20 percent, elemental lead -10 percent, and lead dioxide -10
percent. The soil was first treated with ammonium carbonate for 30 minutes and
filtered. Acetic acid and oxygen are then added followed by water washing of the
filtrate. The final step involves the addition of manganese acetate and acetic acid
followed by filtration. Table 1 presents the results of this first treatability study.
Approximately 80 to 89 percent of the lead was recovered in the three-step process.
The experiments were repeated using soil spiked with 1,000 and 5,000 mg/kg total
lead. The amount of lead recovered in these experiments was approximately 80
percent. Table 2 presents the results of all three lead concentration tests.5
In 1986, PEI Associates, in a study for the National Science Foundation, used
an electromembrane reactor (EMR) process to recover lead from an ethylenediamine
tetraacetic acid (EDTA)-lead chelate solution. Figure 5 presents a diagram of the
EMR. The bench-scale test was performed with actual chelate generated by use of
lead-contaminated soil from a battery reclamation site. The study examined the effect
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TABLE 1. LEAD REMOVAL EFFICIENCY FROM CONTAMINATED SOIL
(250 g; Total Pb = l.CUQOJ) mg/kg;v;Sol1/$oln. Ratio 1:8)'
Step
No.
I
II
III
Reagent
Ammonium carbonate
Oxygen + acetic acid
Washing
Manganese acetate
SUBTOTAL
Lead (ppm)
in filtrate
5.401
847.262
207.268
183.215
Filtrate Vol.
(ml)
1860
1866
440
1880
: % Pb
removed
\ 0.375
65.07
3.65
13.78
! 82.88
a Reference 5.
of system variables such as electrode potential, current density, pH, current efficiency,
and chelate concentration. The EDTA chelate solution was adjusted to pH levels of 4,
8, and 11 with sulfuric acid or sodium hydroxide. The amount of lead in solution was
varied from 0.2, 1, and 3 percent by weight to determine the effect of lead con-
centration on plating efficiency. From the PEI study, the following conclusions were
made: 1) the higher lead concentrations (1, 2, and 3 percent by weight) yielded nearly
90 percent lead recovery, whereas the 0.2 percent concentration recovered 40 percent
of the lead during the same experimental time period; 2) the optimum EDTA/Pb molar
ratio for the chelation reaction is 1.5 to 2, and the reaction is essentially complete
within one hour; 3) a pH of 12 effectively prevented the chelation of other metals,
particularly iron; 4) the initial pH of the chelate solution had no apparent effect on lead
removal; 5) the current density of the electromembrane reactor showed a slight
inverse relationship with respect to the current efficiency; and 6) higher current
densities resulted in faster plating rates, but yielded a spongy lead deposit on the
cathode. Table 3 presents lead recoveries for the electromembrane bench-scale
experiments.
The economics of the electromembrane process is driven by the recovery of
lead metal and regeneration of the chelating agent, which makes the process cost
competitive with other treatment/disposal methods (e.g., landfilling). Comparative
10
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TABLE 2. LEAD REMOVAL EFFICIENCY FOR THREE-STEP EXTRACTION
PROCESS
Total Pb -
• - •
Step No.
-"
I
II
III
Total Pb -
I
II
III
Total Pb =
I
II
III
a Reference
. i 3 •"* • •
10,000 rag/kg;
Reagent
Ammonium carbonate
Oxygen + acetic acid
Washing
Manganese acetate
Washing
TOTAL
5,000 mg/kg
Ammonium carbonate
Oxygen + acetic acid
Washing
Manganese acetate
TOTAL
1,000 mq/kg
Ammonium carbonate
Oxygen + acetic acid
Washing
Manganese acetate
TOTAL
5.
/^vtuuiun r a L. IU
Lead (ppm)
in filtrate
0.776
62.598
32,774
12.225
5.202
5.545
428.55
94.373
96.623
2.583
75.347
24.116
21.316
i ;o i
Filtrate
Vol. (mL)
1980
1820
545
1910
495
1850
1860
465
1840
————————
1800
1910
450
?<565
% Pb
removed
0.0165
4.5557
0.714
0.934
0.103
6-»7n
. J/U
~""~— ^"— — ™^—
0.6883
63.77
3.51
14.21
82 18
1.860
57.56
4.36
15.90
79.68
11
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ANODE (+)
Na2CO3 SOLUTION
-CO
2-
GASKET
\
LEAD ANODE
COVER
PLATE
MAGNETIC STIRRER
c
CATHODE (-)
Pb-EDTA SOLUTION
CATION
•TRANSFER
MEMBRANE
OH*
LEAD CATHODE
Pb'
MAGNETIC STIRRER
C~^"
1/4
2 In.
In. j**j**j 1/4 In.
2ln.
12 In.
Figure 5. Electromembrane reactor for PEI lead recovery study:1
12
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TABLE 3. SUMMARY OF LEAD RECOVERIES FOR EMR EXPERIMENTS3
Experiment
number
1
2
3
4
5
6
7
8
9
Starting
lead, %
1.0
3.0
•0.2
0.2
0.2
0.2
0.2
1.0
3.0
PH
11
4
8
4
11
11
11
11
11
Current
density,
ma/cm
15
15
15
15
5
15
25
25
25
Time, hr
2.0
6.0
0.5
0.5
1.25
0.5
0.3
1.25
3
% Lead
removed
88
93
25
31
42
30
26
88
87
a Reference 1.
t-
p
and nature of the material. The higher the lead content, the higher is the potential
credit for lead recovery. Although the processing time and the amount of raw
materials will be proportionally higher than for disposal methods, the lead credit makes
the electromembrane process cost competitive with land disposal methods.1
Soil washing is an established technology for removing metals from con-
taminated soils. Table 4 presents the soil washing results for several battery breaking
wastes. The following extraction agents were investigated in the soil washing
experiments: water, a surfactant, and tetra-sodium EDTA. Soil washing did not
remove significant amounts of lead in any of the soil particle size categories. EDTA
was found to be effective in leaching lead from the soil; lead removal, however, was
shown to be highly dependent on the form of lead present. Different forms of EDTA
appear to be effective for various lead species. While testing the process, the Bureau
of Mines determined that the predominant species found at lead-contaminated battery
13
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breaking sites were lead metal, lead sulfate, lead carbonate, and lead dioxide.2 Table
5 presents the results of the laboratory-scale tests that were performed by the Bureau
of Mines (BOM) in order to select the use of disodium or tetrasodium EDTA.2
TABLE 5. SELECTIVITY OF VARIOUS SODIUM EDTA FORMS IN LEACHING
VARIOUS LEAD SPECIES"
EDTA form
(5% solution)
pH
Percent removed
PbO2b
Di-sodium 5 100
Tetra-sodium 10 0
PbS04b
0
100
Pb metal0
2.7
23.6
a
Reference 2.
D After 3 hours at 20 °C.
c After 2 hours at 60 °C and 72 hours at 20 °C.
Another drawback encountered in the soil washing studies involved the extreme
difficulties in filtering or otherwise effecting a solid-liquid separation. Because most of
the lead is found on the greater than 200-mm-size particles, the EDTA tends to chelate
with both the lead and the soil particle creating a colloid that is difficult to separate.2
Soil washing experiments were performed with tap water, an anionic surfactant
(0.5 percent), and tetrasodium EDTA (3-to-1 molar ratio) for 30-minute contact times at
a 10-to-1 solution-to-soil ratio. The experimental results indicated that total lead
content was not appreciably reduced by the soil washing process. Table 6 presents
the results of the soil washing study. The results of the tests with synthetic soils
determined that soil washing efficiency depends upon the length of time a metal is in
contact with soil particles.6
A paper by EPA discusses the techniques of soil washing using EDTA at two
pH levels. Table 7 presents the results of the soil washing study. The results show
that 1) contaminant removal capabilities are highly dependent on particle size (most
contamination was found on the fines), 2) fine particles have a high adsorption
15
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TABLE 7. SUMMARY OF REMOVAL EFFICIENCIES OBTAINED FOR
BENCH- AND PILOT-SCALE EVALUATIONS OF SOIL WASHING
Study
PEI°'b
Leeds, Alabama0
Contaminant
Arsenic
Cadmi urn
Chromium
Copper
Lead
Nickel
Zinc
Lead
Wash solutions Removal efficiencies
tested obtained
Tap water > 2 mm soil
< 2 mm soil
5.4% EDTA > 2 mm soil
< 2 mm soil
13% EDTA > 2 mm soil
< 2 mm soil
- 96%
- 44%
- 97%
- 77%
- 94%
- 97%
a Reference 7.
Removal efficiencies presented for this study are an average of organic and
metals contaminant reduction efficiencies obtained under the washing
conditions specified.
c Reference 8.
capacity for contaminants and can be difficult to separate from washing fluid,
3) materials handling is a critical issue for effective treatment, and 4) the wash solution
must be tailored to the site. The predominant lead species found in the six soils
studied were lead carbonates, lead sulfates, land lead oxides.9
An article by U.S. EPA Region V and PEI Associates discusses the results of an
on-site soil washing process for recovery of lead from contaminated soils. The soil was
washed with a chelating agent followed by use of sodium sulfide to precipitate the
chelating agent from the wash solution. Two chelating agents were evaluated for the
soil wash- EDTA and NTA (Nitrilotriacetic acid). EDTA was mixed with water to form a
10 percent by weight solution. The contaminated soil was treated by a 30-to-70 soil-
to-solution ratio. The ratio of soil to chelating solution depends on how contaminated
the soil is. EDTA was determined to be the more efficient chelating agent for lead
removal. In order to determine the optimum reaction time, samples were removed at
10, 20, 30, and 60 minutes and every hour thereafter for up to a 4-hour reaction time.
Table 8 presents the rate of lead removal. Approximately 95 percent removal was
achieved within 2 hours.10
17
-------
TABLE 8. EFFECT OF REACTION TIME ON TREATMENT
CONTAMINATED SOIL"
Sample treatment time (min.)
10
20
30
60
120
180
240
Lead concentration
3290
4330
4120
3720
4710
5120
4820
(mg/L)
;
i
a Reference 10.
In a study performed by PEI Associates, Inc., and Bruck, Hartman, & Espossito,
Inc., four different concentrations of EDTA solutions were used to wastisoil samples
from the Pesses Chemical site in Fort Worth, Texas. The test solutions consisted of
water, 0.01 M EDTA solution (pH 7 to 8), 0.015 M EDTA solution (pH 7 to 8), 0.02 M
EDTA solution (pH 7 to 8), and 0.02 M EDTA solution (pH 11 to 12). The soil was
contaminated mainly with cadmium, copper, lead, and nickel. Results of the soil
washing tests indicated that none of the chelate wash solutions substantially reduced
the total metals in any of the soil fractions examined. Table 9 presents the results of
Toxicity Characteristics Leaching Procedure (TCLP) tests for lead. The soil fractions
were divided into greater than 2 mm, 0.25 mm to 2 mm, and less than 0.25 mm. The
highest concentrations of metals were associated with the 0.25- to 2-mrh particle size
fraction. In the study, the leachable lead content of the coarse fraction greater than
2 mm) was reduced to below the detection limit (0.062 mg/L).11 ;
The Bureau of Mines contaminated soil treatment method investigation deter-
mined that the soil fines form colloidal suspensions when mixed with EDTA solutions,
which blind the filters. It was recommended that a filter aid be added to: the
soil/solution mixture in order to improve its dewatering characteristics.11
18
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A treatability study conducted by EPA and PEI Associates (1988) examined the
recovery of lead-contaminated soils from the Lee's &L site in Woodvillev Wisconsin
Information on the physical difficulties of soil washing with EDTA was provided Lead
concentrations in this soil ranged from 1.5 to 6 percent. Excessive material handling
problems were encountered when EDTA was used to treat the soil. According to field
personnel, the agitation combined with EDTA solution caused excessive soil particle
breakdown. These ultra-fine particles clogged the filter and were unsellable in the
process configuration. This condition also resulted in fines carryover into the
EDTA/lead recovery system, which eventually failed because of excessive suspended
solids loading.12 '•
A study performed at the University of Saskatchewan (1976) used 11 different
types of extraction solutions to remove lead from four different soil samples. The
methods of lead extraction are presented in Table 10. The soils were treated with 10
ppm PbCI2 labelled with lead-210 and incubated for 7 weeks. The five extractants that
provided the best lead removal from the soil were 6N HNO3, 3N HNO3, 1N HNO3, 0 5N
EDTA, and diethylenetriamine pentaacetic acid (DTPA). Table 11 presents the results
of the lead extraction study. Approximately 93 to 98 percent of the lead was recov-
ered by use of 6N HNO3, and approximately 80 percent of the lead could be recov-
ered by DTPA and EDTA.13
In 1987, PEI Associates conducted a bench-scale soil washing study using
Standard Analytical Reference Matrix (SARM) prepared soils. The experiments were
conducted with two wash solutions: Dow Chemical Versene 100 (containing EDTA)
and Institutional Formula Tide (produced by Procter and Gamble). Temperature
variations from 78- to 120 -F had little effect on contaminant reduction efficiencies
The variation of pH from 12 to 8 produced no additional metal removal. A 15-minute
reaction time was selected for the chelant washes based on the results presented in
Figure 6. Two EDTA molar ratios were used to wash the higher metal-contaminated
SARM soil: a Lto-1 and 3-to-1 ratio of moles of EDTA to total moles of metals
present. The 3-to-1 EDTA molar ratio solution removed a higher amount of metals in
the middle soil fraction (250 Mm to 2 mm). Table 12 presents the results of the soil
20
-------
TABLE 10. METHODS OF LEAD EXTRACTION
Total Pb
1 N NH.OAC (pH 7)
0.05 H CaCl,
0.05 H EOTA -
1 H HH.tCO,),
OPTA - TEA
0.5 NaHCO, (pH 8.5)
2.5X (v/v) AcOH
0.5 H Bad,
6 H HNO,
3 N HNO,
1 N HNO, .
0.1 N HNO,
Weight of Air-Dry
Soil Used (g)
2
10
10
10
10
5
5
5
5
5
5
5
Soil Extract
-
1:5
1:5
1:2
1:2
;1:20
1:20
1:10
1:3
1:3
1:3
1:3
Outline of Procedure
Digestion with hot concentrated HNO,
with lead taken up in KC1
24-h equilibration and filtration
24-h equilibration and filtration
Shaking for 2 h and filtration
Shaking for 1/2 h and filtration
Shaking for 45 min. and filtration
Shaking for 2 h and filtration
Overnight equilibration and filtration
Reference 13.
21
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en
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-Q- Arsenic a
-®- Cadmium
-»- Chromium
-o- Copper
•*- Lead
•o- Nickel
•*• Zinc
Time, mln.
Arsenic and nickel overlap in this figure.
Figure 6. Reaction time for a 1:1 molar chelant wash using EDTA.14
washing study. The reduction of metal contaminants appears to be affected more by
the use of the various wash solutions in the fine soil fraction (less than 250 Mm) than in
the other soil fractions. It was determined that a significant fraction of the con-
taminants are attached to the fines (silt, humus, and clay). The coarse material may
be cleaned by simply separating out the fines. The data indicate that water alone can
efficiently remove a significant portion of the contamination from the >2-mm soil
particle fraction.14
2.2 SOIL CHARACTERIZATION
Soil characterization is necessary to determine optimum conditions for the
metals extraction'processes. Soil properties that influence the process employed for
lead recovery include soil particle size, clay content, moisture content, quantity and
species of metals, soil pH, and total water-soluble solids. These characteristics will
govern both the physical and chemical operations of the process employed to recover
25
-------
metals from contaminated soils. Thie fallowing subsection describes soil conditions
commonly found at lead-contaminated soil sites. i
As described in Subsection 2.1, a significant amount of metals is adsorbed onto
small particle-size fractions (less than 250 ^m) than is adsorbed onto larger soil
particle fractions (250 /im to 2 mm, and greater than 2 mm). Because the chelating
agent may chelate both the metal and soil particle, techniques to eliminate this
phenomenon will need to be employed in order to effectively separate the chelate
solution and solid fractions. One method suggested called for the addition of a filter-
aid prior to soil dewatering.
Barth and Traver (1989) report on experiments performed by EPA to evaluate a
treatment process consisting of soil washing followed by solidification/stabilization.
Soils from battery cracking sites and synthetic soils were characterized for the
following physical and chemical properties: 1) grain size distribution, 2) predominant
clay species, 3) moisture content, 4) pH, 5) cation exchange capacity, 6)ihumic acid
content, 7) total organic carbon, 8) total lead, 9) leachable lead, and 10) predominant
lead species. Table 13 presents the results of the waste characterization;6
Wozniak and Huang (1982) discuss the results of a process developed to
remove cadmium, copper, lead, nickel, and zinc from sludges. The process consists
of solubilization of heavy metals by acidification followed by the separation of solids
from the liquid fraction. The amount of metal removed from the sludge was found to
be dependent upon pH, solids concentrations, specific types of metals, and length of
acidification times. Removal of lead at pH 3 was much lower than at pH 1.5 and pH 2.
A pH of 2 seemed to represent a critical value for lead solubilization similar to cad-
mium and copper. Table 14 presents the results of the acidification study. Maximum
lead removal rates were 100, 81, and 30 percent at a 0.5 percent solids concentration
for pH 1.5, 2, and 3, respectively.15 ;
A document by PEI Associates, Inc., discusses the feasibility of using soil
washing and stabilization/solidification to reduce lead contamination at several metals
26
-------
TABLE 13. TOTAL LEAD CONTENT OF TREATED SOIL RECOVERED ON
NUMBER 10 AND NUMBER 60 SCREENS*
Site
A
B
C
D
E
F
SSMb
A
B
C
D
E
F
SSHb
Total
Whole soil
untreated
57,150
75,850
3,230
27,150
5,194
210
14,318
Total
lead recovered on No. 10 screen
(>2 mm, mg/kg)
Water wash
153,100
50,150
893
31,350
6,487
42
122
lead recovered
(0.25 - 2 mm
22,300
52,200
.2,160 :
12,800 '
2,020
312
401
Surfactant wash
98,100
66,500
1,580
25,610
Not analyzed
Not analyzed
Not analyzed
on No. 60 screen
, mg/kg)
32,000
46,670
1,755
10,960
Not analyzed
Not analyzed
Not analyzed
Chelate
wash
119,050
164,200
886
8,965
1,081
30
98
24,550
57,350
2,340
8,670
1,516
408
171
Total lead content of treated soil passing through
both No. 10 and No. 60 screens
(<0.25 mm, mg/kg)
A
B
C
D
E
F
SSMb
14,650
49,500
2,949
41,400
13,698
111
30,600
36,450
52,100
2,645
42,700
Not analyzed
Not analyzed
Not analyzed
41,250
24,170
3,995
15,250
4,693
73
1,470
a Reference 15.
b Synthetic Soil Matrix.
27
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recycling Superfund sites. Soil characteristics were studied to determine their relation-
ship to metals removal. The presence of calcite signifies the presence of a car-
bonatedform of lead such as Pb(COJ2(OH)2 or Pb4SO4(CO3)2(OH)2. Lead in the soil
was distributed among the soil fractions rather than being concentrated in the smaller
particles. The use of agitation during soil washing and the use of filter aids are
recommended to improve dewatering characteristics and filtering rate.16
29
-------
SECTION 3
CHELATING AGENTS
Selecting the appropriate chelating agents is also important when extracting
lead from contaminated soils. The quantity and type of chelating agent used, pH, and
contact time are all important factors because they influence both the process
economics and lead extraction efficiencies. The following subsections describe the
physical properties and results of previous studies in which various chelating agents
were used in metals extraction. ;
3.1 PHYSICAL PROPERTIES
Kirk-Othmer's Encyclopedia of Chemical Technology describes what a chelating
agent is and how it forms bonds with metals. The molecular structure of metal-EDTA
chelate is presented in Figure 7, For the selective complexation of one metal in the
presence of other metals, there must be a large difference between the stability
constants of the two metals. Table 15 presents the stability constants for a number of
chelating agents and metals. Enough chelating agent is needed to combine with the
target metal ion as well as any competing metal ions that could displace the target
metal. Some factors affecting the stability of metal chelate include the size and
number of rings, substituents on the rings, and the nature of the metal and donor
atoms. Five- and six-membered rings are more stable than other ring formations
because the coordination angles prohibit the formation of three-rnembered rings, and
closure is unlikely for more than seyen-membered rings because of competing
reactions to form linear chains. Substituents on the ring may produce steric hindrance
effects or alter the availability of donor atom electrons for coordination.17
30
-------
co — CHZ CH/XV
o
N /
2 +
M
of V
\ l\
CO—CH. CH/XV
(U>
Figure 7. Structure of Metal-EDTA Chelate.17
Extracted from: Encyclopedia of Chemical Toxicology, Third Edition, Volume 5 Grav-
son, M., EcKroth, D., Mark, H. F., Othmer, D. F., Overberger C G
and Seaborg, G. T., Editors. Copyright®, 1979. John Wiley & Sons
Inc. Reprinted by permission of John Wiley & Sons, Inc
31
-------
TABLE 15.
Metal
V(III)
Fe(III)
In(III)
Th(IV)
Hg(II)
Cu(II)
VO(II)
Ni(II)
Y(III)
Pb(II)
Zn(II)
Cd(II)
Co(II)
Fe(II)
Mn(II)
V(H)
Ca(II)
Mg(II)
Sr(II)
Ba(II)
Rare earths
STPPb
8.7
6.7
7.6
6.9
2.5 |
7.2
5.2
5.7
4.4
3.0
• — •• • >*wii^ir^i'
Citric
acid
10.9
6.1
4.8
5.7
4.5
4.2
4.4
3.2
3.4
3.5
2.8
• i •* «i riLinu untu
Log K
NTAC
15.9
15.0
12.4
12.7
12.7
11.3
11.4
11.8
10.5
10.1
10.6
8.8
7.4
6.4
5.4
5.0
4.8
10.4-12.5
nico
EDTAd
25.9
25.1
25.0
23.2
i
21.8
18.8
18.8
18.6
18.1
18.0
16.5
16.5
16.3
14.3
14.0
12.7
10.7
8.7
8.6
7.8
15.1-20.0
Reference 17.
STPP * sodium trlpolyphosphate;!
' NTA >= nitrilotriacetic acid;
EDTA * ethylenediaminetetraaceti'c add.
32
-------
A book entitled "Chemistry of the Metal Chelate Compounds" provides stability
constants for several metal chelates. An increase in oxidation potential is a possible
indication of a metal complex formation. The coordination with a donor group
increases the oxidation potential and therefore increases the relative stability of the
higher valence state.18
A representative of Hampshire Chemical was contacted regarding the chelating
agents the company manufactures: EDTA, DTPA, and NTA. For EDTA solutions at a
pH level above 12, insoluble metal salts are formed, and at a pH above 7, the chela-
tion of iron decreases. For DTPA, the chelation of lead decreases at approximately
pH 9. Figure 8 presents metal chelating results at various pH levels for the three
chelating agents. The pH level in which the soil is washed with a chelating agent is
important if one or several metals are to be selectively chelated. In many soils,
particularly those with high concentrations of clay, calcite (CaCOg) may be present in
concentrations of up to 30 to 60 percent. Soils typically have a pH range of 5 to 8.
High quantities of calcium carbonate may affect the equilibrium constants of the metal
chelates.19'20 Chelation value is defined as milligrams of calcite per gram of chelating
agent used. The manufacturer recommended that 1.4 parts EDTA per part lead be
used, and 1.9 parts DTPA per part lead be used for the chelation study. NTA requires
approximately 4.2 parts NTA per part lead because it forms weaker bonds and is a
less-effective chelating agent than DTPA and EDTA for chelating lead. The cost per
pound of chelating agents was quoted at approximately $1.20/lb for EDTA acid,
$1.40/lb for DTPA acid, and $0.45/lb for NTA.20
Martell and Calvin (1952) suggested that water-soluble chelating agents be
used for regeneration so that the metal chelate may be rapidly formed and dis-
sociated.18 Carboxylic acids such as EDTA and NTA are hydrolytically stable at high
temperatures and pH levels. The solubility levels of the sodium salts of EDTA are
presented in Table 16. The trisodium salt of NTA has a solubility of 50 grams per 100
ml of water at 25 °C. Figures 9 and 10 show how EDTA and NTA chelating abilities
are affected at various pH levels.21'25
33
-------
7 8
PH
9 10 11 12 13 14
Fe (ill)
EDTA
Pb
EDTA
Ca
EDTA
Fe (III)
DTPA
OTPA
DTPA
Fe(lll)
NTA
Pb
NTA
NTA
Figure 8. Stability constants for EDTA, DTPA, and NTA.20
34
-------
Figure 9. Stability constants for metal-EDTA complexes as a function of pH.26
Extracted from: Complexation in Analytical Chemistry: "A Guide for the Critical
Selection of Analytical Methods Based on Complexation Reactions."
Ringbom, A. Copyright ©, 1963. Interscience Publishers. Reprinted
by permission of John Wiley & Sons, Inc.
35
-------
00
o
pH
Figure 10. Stability constants for metal-NTA complexes as a function of pH.
27
36
-------
TABLE 16. SOLUBILITY OF EOTA AND ITS SODIUM SALTS
IN WATER AT DIFFERENT TEMPERATURES
Substance
H4L
NaHjL
Na2H2L
Na3HL
Na,L
PH
2.2
3.5
4.7
8.4
10.6
22*C
0.2
1.4
10.8
46.5
60.0
40'C
0.2
1.4
13.7
46.5
59.0
80*C
0.5
2.1
23.6
46.5
61.0
References 23 and 24.
Ku and Yen (1991) discuss their experimental results involving the precipitation
of metals from wastewater in the presence of chelating agents.22 Precipitation of
copper and lead by hydroxide precipitation was greatly inhibited by the presence of
NTA at pH levels of less than 9 because of the formation of metal-NTA complexes. In
the presence of strong chelating agents such as EDTA and NTA, the formation of lead
chelates is more favorable than the formation of lead sulfide. Decreasing the pH
intensifies this effect because the formation of hydrogen sulfide competes with the
formation of metal suifide compounds. Chelating agents such as citrate and tartrate
form weaker bonds with metals than do EDTA and NTA and show little influence on
residual lead concentration. At low pH levels, hydrogen suifide tends to form and thus
decreases the level of lead sulfide. In the neutral pH ranges, lead forms stable
chelates with EDTA and NTA. For pH levels above 10, the formation of lead hydroxide
complexes becomes a competing reaction for some weak chelating agents and
increases the soluble lead concentration. For strong chelating agents, however, no
significant decrease In chelating ability occurs above pH 10. The predominant specie
between lead and EDTA above pH level 4 is PbEDTA-2, whereas for pH levels of less
than 3, the predominant form is PbEDTA-1. For NTA, a similar phenomenon occurs:
PbNTA-1 is the predominant lead chelate form for pH levels between 3 and 11.
Figures 11 and 12 present the lead distribution of lead chelates at various pH levels for
EDTA and NTA, respectively.22
37
-------
12
Figure 11. Lead-EDTA distribution as a function of pH.22
38
-------
10'
Pb, = 100 mg/l
Si = 16mg/t
NTAr 25 mg/l
Figure 12. Lead-NTA distribution as a function of pH.
22
39
-------
3.2 TREATABILiTY TESTS
Bhat and Gokhale (1964)!discuss the results of experiments conducted to
recover EDTA from metal-EDTA complex solutions. EDTA forms stable i-to-1 com-
plexes with most metals, especially those of the transition metal group. Because
EDTA has a low solubility, it may be easily recovered even in dilute solutions. Metal
may be removed from the chelate through acidification or by precipitation with a
hydroxide, sulfide, or oxalate. Mineral acids may cause dissociation of the metal-EDTA
complex (Mr4*"1) as a result of the strong competition for the formation! of a pro-
tonated EDTA specie from H*. The following equilibrium reaction shows this complex
dissociation:
nH+ <™> M*m + YH
n
Precipitation of EDTA can only occur on acidification if the concentration ;of the
protonated EDTA species is greater than its solubility.28
Under alkaline conditions (pH > 9), the complex ion can be dissociated by
precipitation of the metal as a hydroxide. EDTA becomes completely dissociated and
a hydroxy-metal-EDTA complex may be formed. Based on the stability constant for
EDTA and the hydroxide product's solubility, the cation would be completely dis-
sociated from the metal-EDTA complex by direct hydroxide precipitation in only a few
cases such as Fe(lll).28 ; • i •
The sulfide precipitation method was effective in recovering both EDTA and
metal. In all of the experiments, over 98 percent of the metal and 80 percent of the
EDTA were removed. The metal-chelate had to be separated at 90 'C because at
lower temperatures, EDTA precipitated with the metal sulfides as a result of low
solubility. The experiment was performed by first heating sulfuric acid to 90 °C to
acidify the solution at a pH of 2; then hydrogen sulfide was added to the heated
solution. The metals were then precipitated and the hot solution decanted. Copper,
40
-------
bismuth, cadmium, and lead also may be precipitated as sulfides from the EDTA
solutions by this method.28
Although oxalate precipitation effectively recovered the metal, its low solubility
limited its ability to recover EDTA. The oxalate precipitation method cannot be applied
to very dilute (<0.002 M) solutions.28
A patent was developed by Lindstrom and Winget for the recovery of EDTA
from ion exchange effluent solutions containing copper-EDTA complexes. In this
process, rare earth elements are first adsorbed on a cation exchange resin bed. The
bed is physically connected to a second cation exchange bed containing adsorbed
copper. EDTA solution is then passed in sequence through the first bed to adsorb the
rare earth elements, and then through the second bed containing the resin in the
cupric cycle. High-calcium hydrate lime was used to separate the copper from the
complex. Calcium replaces copper and forms an EDTA-calcium complex and Cu(OH)2
precipitate. The filtrate containing the calcium-EDTA complex is recovered by filtration
and is treated with sulfuric acid down to a pH below 3 to precipitate EDTA.29
A study performed by Pennsylvania State University examined the use of
electrolytes as a means to modify EDTA extraction of lead from highly contaminated
soil. In a 5-hour extraction using 0.04 M EDTA (1-to-1 ratio of lead to EDTA) at a pH
range of 5 to 9, approximately 65 percent of the lead was released from the soil.
Electrolytes consisting of Na+, Li+, and MH4ClO4 salts in a 0.5 M solution were used to
increase the lead desorption to approximately 80 percent. This increase in lead
desorption was attributed to the exchange displacement of soil-bound lead and
increased the solubility of lead-containing species at higher ionic strength. Figure 13
shows the relative lead recovery rates for the various electrolytes studied. With the
divalent electrolytes [(Ca(CIO4)2 and Mg(CIO4)2] at a 0.167 M solution, a similar
recovery was observed. At higher pH levels* however, calcium and magnesium
suppressed the lead dissociation because of competing chelation with EDTA. Figure
14 presents the results obtained with divalent electrolytes.30
41
-------
100
90
80
o
UJ
£ 70
60
1 T
,EDTA withNH4CI04(A)
EDTAwifhNoCI04(»)
£DTAwilhLiCI04
EDTA only (o)
50' 1 1 •- i '
2 4 6
8
PH
IO
12 (4
Figure 13. Effect of electrolytes on pH-dependent recovery of lead.30
42
-------
100
90
8O
70
S 6O
I 50
UJ
-------
The U.S. EPA performed a treatability study to evaluate in situ methods for
treating releases of hazardous materials from uncontrolled waste disposal sites. The
study was conducted by using a three-agent sequential extraction technique to
recover lead and other metals from contaminated soil. The soil contained five toxic
heavy metals: cadmium, chromium, copper, lead, and nickel. Preliminary shaker table
studies using EDTA, hydroxylamine hydrochloride, acidic buffer, and DTPA showed
that EDTA was the best extractant for all metals. Table 17 presents the results of the
shaker table study. The three-agent sequential extraction study was conducted with
soil washing solutions of EDTA followed by hydroxylamine hydrochloride and then
citrate buffer. Table 18 presents the results of the three-agent study. Lead appeared
to be easily removed by the EDTA; further removal occurred with citrate. This study
demonstrated that weaker agents do not remove the metals as efficiently as EDTA
alone.31
A study was conducted to investigate how the solubility of lead is affected by
the presence of iron in soil during soil washing with EDTA. The results indicate that an
abundance of iron in the soil does not interfere with the solubility of lead: When iron
solubility was inhibited by increases in pH, the solubility of lead was unaffected. The
kinetically slow dissolution of iron oxides in the soil in the presence of EDTA is thought
to be the limiting factor for the interference by iron in lead displacement.?2
In an experiment conducted by Norvell (1984), five chelating agents [DTPA,
EDTA, ethyleneglycol-bis(2-aminpethylether) tetraacetic acid (EGTA), hydroxyethylene-
diaminetriacetic acid (HEDTA), and NTA] were compared as extractants for aluminum,
iron, manganese, zinc, copper, cadmium, and nickel in 25 soils. The experiments
were conducted with 0.005 M chelating agent, 0.01 M CaCI2, and 0.1 M acetic acid-
ammonium acetate buffer at a solution pH of 5.3 and a 5-to-1 extractant-to-soil ratio.
DTPA,, EDTA, and HEDTA were the most effective agents for extracting all seven of the
metals used in testing. The extracting solutions were formulated to maintain a
relatively constant pH and concentration of Ca+2 in all soils. These conditions were
intended to retard dissolution of CaCO3 in soils and to yield clear filtrates by promoting
effective flocculation.33
f
44
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TABLE 17. SINGLE-AGENT SHAKER TABLE EXTRACTION EFFICIENCIES*
Cd
Soil metals (ppm) 47
EDTA (0.1 H at pH 6), 114
% extracted
Hydroxylamine hydrochloride 86
(0.1 M in acetic acid), %
extracted
Citrate buffer (0.1 M at 77
pH 3), % extracted
Pyrophosphate (0.1 M), 5.4
% extracted
DPTA (0.005 M in 0.1 M 59
Triethanolamine),
% extracted
a Reference 31.
Cr
349
24
32
24
9.6
2
Cu
219
62
43
48
29
48
TABLE 18. THREE-AGENT SEQUENTIAL EXTRACTION
SOIL COLUMN TESTS8
Cd
Soil metals (ppm) 47
Water, 0.2
% extracted by water
EDTA (0.1 H at pH 6), 60.5
% extracted by agent
Hydroxylamine hydrochloride 23.8
(0.1 M in acetic acid)
% extracted by agent
Citrate buffer (0.1 M at 3.6
pH 3), % extracted by agent
Water wash, % extracted by 0.4
water
Total % extracted 88.5
Cr
349
0
12.2
8.9
12.2
1.1
34.4
Cu
219
0
47.1
0.7
0.2
0.1
48.1
Ni Pb
214 2,480
14 106
20 80
14.5 65
2.9 9.7
2 67
EFFICIENCIES:
Ni Pb
214 2,480
0 0.1
6.8 60.1
8.7 2.3
4.8 8.8
0.5 0.5
20.8 71.8
a Reference 31.
45
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SECTION 4
MEMBRANES
In an electromembrane process, the selection of a membrane is critical for the
successful separation of cations and anions. A membrane must be durable, able to
withstand harsh chemical and physical treatment, stable at high temperatures, and
possess low electrical resistance. Several articles were reviewed to provide infor-
mation on the selection of membranes. The following subsections provide an over-
view of the pertinent information contained in these articles.
4.1 PROPERTIES OF MEMBRANES
Membranes are manufactured from a variety of materials ranging from polymers to
sulfonic acids. Polymeric membranes are characterized by the following polymer
properties: large average size of the macromolecules, their size distribution, their
architecture, the specific nature of their chemical groups, the arrangements of these
groups in the chain, and the aggregate state of the macromolecules. The highest
molecular weight (MW) polymer should be evaluated before A particular polymeric
membrane is selected. Although the integrity of the membrane tends to increase with
MW, the higher MW polymer membranes may show a decrease in ion selectivity.
Membranes produced by p dry phase-inversion process generally need a higher MW
polymer than those produced by a wet process. Also, membranes made of polymers
with narrow MW distributions are preferable to those with broad distributions, and
single-viscosity grades are preferable to blends of two viscosity grades.21;
In a journal article by Houng Loh, et al. (1990), two microporous membranes,
polypropylene Celgarde® and Teflon Plastolon®, were selected to evaluate the mor-
phology of polymeric membranes. The membranes were examined for conductivity,
stability, surface morphology (determined by scanning electron microscope), and
46 ;
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tensile strength. This study determined that Teflon membranes exhibit higher conduc-
tivity than polypropylene membranes. The tests also determined that the ionic
permeability of an electromembrane can be regulated with the electrically controllable
oxidation state of the polymer.34
DuPont manufactures a Nafion® perfluorosulfonic acid cation-exchange membrane
that is reinforced with Teflon, is 0.43 mm thick, and has a weight of 6.3 g/dm. The
membrane has excellent stability properties to withstand the most harsh conditions.
The Nafion 324 membrane is a reinforced membrane composed of 1500 and 1100
equivalent weight copolymer layers. This membrane is primarily used to produce 10
to 20 percent NaOH and to recover metal ions. The Nafion 417 reinforced membrane
is a 1100 equivalent weight polymer used in the electroplating industry to regenerate
chromic acid solutions and to recover metals.35
The Ionics CR 67 membrane is an acrylic fiber-backed cation-transfer membrane
with a specific weight of 13.7 mg/cm2, a thickness of 0.6 mm, a burst strength of 7
kg/cm2, and a minimum capacity of 2.10 meq/dry gram resin. The membrane is
primarily used in electrodialysis reversal units and was used in the electromembrane
reactor for the 1986 PEI bench-scale study. These membranes must be kept wetted
at all times to prevent the membrane from cracking.1'36'37
4.2 OPERATING PARAMETERS
Both the properties of a membrane and the conditions under which the membrane
is operated are important. Process parameters such as pH and fluid turbulence affect
the rate in which the ions are transported across the membrane. The following articles
were reviewed to determine the operating conditions that should be considered for the
design of an electromembrane reactor.
A book entitled "Synthetic Polymeric Membranes" provides an overview of the
various membranes and the mechanism in which they operate. In electromembrane
processes, the speed and direction of the ionic flow depend on the current potential
and density as well as the resistance of both the anode and cathode chamber solution
characteristics, such as charge classification and valence state. The solute transport
47
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rate of a liquid adjacent to the membrane can be controlled through the' membrane?.
Film diffusion tends to occur when filrm diffusion .coefficients are high, or where the
membrane is very thin, and when there is little difference between the concentration of
the species in solution and in the membrane.21
Tlhe membrane potential is the difference in electric potential between two electro-
lyte solutions separated by a permeable or semipermeable membrane. The mem-
brane potential is dependent upon the properties of the mobile and fixed ions and is
usually independent of membrane thickness or cross-sectional area. The membrane
potential is the sum of the diffusion potential within the membrane, the Donnan (phase
boundary) potential, and the diffusion potential of the films with partial or total film
control. Ideal membrane potentials (Ei) can be calculated by the following equation:21
Ei = 2RJ_ln (a1/a2) ;
F
where |
Ei = membrane potential
R = ideal gas law constant
T = temperature. :
F = electricity, Faradays
a1, a2 .= mean activities of the solutions in the anode and cathode
chambers
I
i
Diffusion of a substance across a concentration gradient is defined by Pick's first
law of diffusion that the flux (J) is proportional to the concentration gradient:21
J = -D ac/3x
where D = diffusion coefficient '
dc/dx = change in concentration
The current density [calculated as milliamps (ma)/cm2] that can be applied
across a membrane is limited by concentration polarization. Figure 15 presents an
48 ;
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o-
ENRICHING
SOLUTION
CATION
EXCHANGE
MEMBRANE
DEPLETING
SOLUTION
BOUNDARY LAYERS
Figure 15. Concentration gradients in the electromembrane reactor.38
Extracted from: "Physiochemical Aspects of Electromembrane Processes," Chapter 2,
in Industrial Processing With Membranes. Davis, T. A. and Brockman,
G. F. Lacey, P.E., and Loeb, S. (editors). Copyright ©, 1972. Wiley-
Interscience. Reprinted by permission of John Wiley & Sons, Inc.
49
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illustration of concentration gradient. The cation exchange membrane in the center is
highly permeable to cations and almost impermeable to anions. Cations are trans-
ferred through the membrane faster than they can arrive from the solution on the right
or leave the solution on the left by electrical transfer. Thus, the concentration at the
left interface increases until concentration gradients are established. At this point,
additional fluxes are needed to maintain steady-state conditions in the boundary
layers.
If the current is raised high enough, the concentration at the right interface will
approach zero. During the operation of the electromembrane reactor, the ion con-
centration in the anode chamber is depleted as ions are transported across the
membrane. If the concentration decreases too much in the anode chamber, the ion
concentration will approach zero; therefore, excessive energy will be required to
transfer any additional ions. In order to avoid the anion depletion, the bulk con-
centration in the anode chamber must be maintained.1 :
The energy requirement for ionic transport in the electromembrane is a function
of the electrical resistance of the solutions and membrane and the back electromotive
forces caused by concentration gradients. The resistance of the membrane depends
on the transport processes taking place around the membrane. The electrical
resistance of a solution depends on the solute concentration and solution conduc-
tivity.1 ;
The velocity in which an ionic species is transferred by an electrochemical
driving force or potential gradient (ui) in the x direction is defined by the following
equation: I
ui = - k dE/dx
where k = the proportionality constant
E = potential
50
-------
The electrochemical potential is the sum of the chemical and electrical potential. The
velocity is proportional to the ionic mobility, mi, expressed in units of cm2/V-sec;
therefore, the velocity may be expressed as:
ui = -mi Zi dE/dx
where Zi = the ionic charge
If the potential is to be expressed in chemical terms, the proportionality factor of
velocity is D cmVsec:39
ui = -D dlnCi/dx (Pick's first law)
where D = diffusion coefficient
Ci = concentration of ionic species
51
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SECTION 5
SUMMARY
A literature review of metals extraction technologies, soil characterization,
chelating agents, and membranes was conducted to define important parameters
regarding lead recovery from soils. The following is a summary of the important
findings that may impact lead recovery.
5.1 METALS EXTRACTION TECHNOLOGIES AND SOIL CHARACTERIZATION
Several technologies have been or are currently being developed for removal of
metals from contaminated soils and wastewaters. The goals of the metal extraction
processes are to recover enough lead from the soil so that the soil will pass the
Toxicity Characteristic Leaching Procedure (TCLP) test for metals, and to recover a
lead product that is marketable. :
The Bureau of Mines (BOM) has developed a process that employs acid
leaching to convert lead sulfate and lead dioxide to lead carbonate, which is soluble in
nitric acid. Lead is recovered by precipitating with sulfuric acid to produce a lead
sulfate product. The BOM has also developed a process to convert the lead com-
i
pounds to lead carbonate with ammonium carbonate and ammonium bisulfite,
followed by leaching with fluosilicic acid. Lead is then recovered using an electro-
winning process. Lead remaining in the soil after leaching will be leached with a 0.5
percent nitric acid solution. ;
The TerraMet™ soil remediation system developed by COGNIS, Inc. leaches
and recovers lead from contaminated soils, sludges, or sediments. Lead jn the form
of metallic lead, soluble ions, and insoluble lead oxides and salts may be leached with
a proprietary aqueous leachant. >
52 !
-------
A soil recycling process developed by the Toronto Harbor Commission employs
three technologies operating in series. The first stage involves soil washing to reduce
the volume and concentrate the contaminants into a fine slurry. The second stage
employs acidification and selective chelation for dissolution of heavy metals. All metals
may be recovered in their pure form using this process. The third stage involves
chemical hydrolysis followed by biodegradation to destroy organic contaminants in the
slurry.
EPA has conducted research on a lead extraction process involving the
conversion of lead sulfate to lead carbonate with ammonium carbonate, lead car-
bonate to lead acetate and oxidation of lead to lead acetate with acetic acid and
oxygen, conversion of lead dioxide into lead acetate, and the final conversion of lead
acetate to lead sulfate with sodium sulfate. ;
A lead recovery process developed by'Kaur and Vohra uses a surfactant liquid
membrane to recover lead (II) from wastewaters. The lead first diffuses through a
stagnant film and reacts with di(ethylhexyl) phosphoric acid to form a lead complex.
The lead complex then diffuses through a membrane and is transported to an organic
interface. Lead is then stripped by an internal phase reagent and recovered.
In 1986, PEI Associates conducted a study for the National Science Foundation
using an electromembrane reactor (EMR) process to recover lead from an ethylene-
diamine tetraacetic acid (EDTA)-lead chelate solution. The bench-scale test was
performed with actual chelate generated using lead-contaminated soil from a battery
reclamation site.
U.S. EPA Region V and PEI Associates have developed an on-site soil washing
process for recovery of lead from contaminated soils. The soil was washed with a
chelating agent followed by use of sodium sulfide to precipitate the chelating agent
from the wash solution. Two chelating agents were evaluated for the soil wash: EDTA
and NTA (Nitrilotriacetic acid). The ratio of soil to chelating solution is highly depen-
dent on the site because of varying levels of contamination. EDTA was determined to
be the more efficient chelating agent for lead removal.
53
-------
The soil characterizations performed prior to the soil washing to treat the metal-
•4*-'* - - 'i~-^k
contaminated soils showed that a majority of the metals are adsorbed on the fine soil
fraction (less than 250 /um). The coarse material (greater than 2 mm) was found to
have fewer contaminants than the other soil fractions, and could be cleaned by
separating the soil fractions and treating the smaller soil fractions. Soil washing with
EDTA formed colloids consisting of fine soil particles that created difficulties in solid-
liquid separation. It was suggested that the addition of a filter aid prior to filtration
would enable better separation of the fine particles from the liquid fraction. EDTA was
used extensively as a soil washing solution at various concentrations and pH levels. In
most cases, EDTA was able to remove approximately 80 percent of the lead from the
metals-contaminated soils.
The predominant species ;of lead found at many of the battery breaking and
reclamation sites examined were lead sulfate, lead carbonate, and lead dioxide.
Calcite signifies a presence of carbonated forms of lead. Because of the slow
dissolution of iron oxides in soil, the presence of iron in soil does not appreciably
affect the chelation of lead with EDTA. A pH of 2 appears to represent a critical value
for lead solubilization. !
5.2 CHELATING AGENTS
The tendency for a metal to chelate is determined by the stability constant,
which is highly dependent on the pH of the solution. EDTA and DTPA have approxi-
mately the same lead-chelation abilities and are relatively similar in cost ($1.20/lb and
$1.40/lb, respectively). Approximately three times as much NTA is required to chelate
lead than is required for EDTA and DPTA, but the cost of NTA is substantially lower
($0.45/lb). All three chelating agents are relatively stable at high temperatures and pH
levels. In several studies, EDTA, NTA, and DTPA have been used to recover metals
from waste streams.
54
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5.3 MEMBRANES
Important characteristics that need to be considered in the selection of a mem-
brane include low electrical resistance, high permselectivity (exclusion of anions),
durability to withstand high temperatures, low and high pH solutions, and chemical
and physical treatment for removal of deposits. The amount of current, concentration
of ions in the anode chamber, and stirring rate of solutions in both the anode and
cathode chambers must be controlled in order to maintain steady-state conditions in
the boundary layers of the membrane. Both the Ionics CR 67 and the DuPont Nafion®
membranes are stable at extreme pH levels, exhibit high burst strength, and are
permeable to cations and highly impermeable to anions. Examination of the literature
has shown that there is no commercial or full-scale membrane technologies for the
recovery of lead from soils.
55;
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REFERENCES
1. PEI Associates, Inc. Innovative Electromembrane Process for Recovery of Lead
from Contaminated Soils. Project conducted under a grant from the National
Science Foundation, Grant No. ISI-8560730. July 1986.
2. Schmidt, W. Assessment of Current Treatment Techniques at Superfund Battery
Sites. Bureau of Mines. 1990. ',
3. U.S. Environmental Protection Agency, Office of Research and Development.
The Superfund Innovative Technology Evaluation Program: Technology Profiles
Fifth Edition. EPA/540/R-92/077. November 1992.
4. Kaur, S., and D. K. Vohra. Recovery of Lead (II) From Acidic Solution by Sur-
factant Liquid Membrane. Presented at the Emerging Technologies for Haz-
ardous Waste Management Symposium. September 21-23, 1992. '
5. Krishnamurthy, S. Extraction and Recovery of Lead Species from Soil. Environ-
mental Progress, Volume 11, No. 4. November 1992. pp. 256-260.
; ' f
6. Earth, E. F., and R. P. Traver. Treatment of Lead Battery Contaminated Soil
Utilizing Soil Washing and Solidification/Stabilization Technology. U.S. Environ-
mental Protection Agency. 1989.
7. Fox, R. D., et al. Quick Response Feasibility Testing of Lead Removal From Con-
taminated Fill Materials by extraction with EDTA Using the EERU Mobile Drum
V/asher Limit. IT Corporation, Knoxville, Tennessee. 1984.
8. PEI Associates, Inc. CERCLA BOAT SARM Preparation and Results of Physical
Soils Washing Experiments Prepared for the U.S. Environmental Protection
Agency, Hazardous Waste Engineering Research Laboratory, Release Control
Branch, under Contract No. 68-03-3413. 1988.
9. PEI Associates, Inc. Soil Washing. Presented at the U.S. EPA Region X Engi-
neering Forum on Lead Battery Reclamation Sites. May 1988.
56
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REFERENCES (continued)
10. Castle, C., and J. Bruck, et al. Research and Development of a Soil Washing
System for Use at Superfund Sites. U.S. Environmental Protection Agency,
Research & Development. 1988.
11 PEI Associates, Inc., and Bruck, Hartman, and Esposito, Inc. Results of Bench-
Scale Soil Washing Evaluation of Metals-Contaminated Soils from the Pesses
Chemical Site, Fort Worth, Texas. U.S. Environmental Protection Agency. 1984.
12. PEI Associates, Inc. Lee's Farm, Woodville, Wisconsin. Summary prepared for
U.S. EPA Region X Engineering Forum on Lead Battery Reclamation Sites. May
1988.
13. Karamanos, R. E., et al. Extractability of Added Lead in Soils Using Lead-210.
Canadian Journal of Soil Science, Volume 56. 1976. pp. 37-42.
14. PEI Associates, Inc. CERCLA BOAT Standard Analytical Reference Matrix
(SARM) Preparation and Results of Physical Soils Washing Experiments. U.S.
Environmental Protection Agency. 1987.
15. Wozniak, D. J., and J. Y. Huang. Variables Affecting Metal Removal from Sludge.
Journal WPCF, Volume 54, Number 12. December 1982. pp. 1574-1580.
16. PEI Associates, Inc. Lead Battery Site Treatability Studies. U.S. Environmental
Protection Agency. 1987.
17. Kirk-Othmer's Encyclopedia of Chemical Technology, Volume 5. John Wiley &
Sons. New York, New York. 1981. pp. 339-362.
18. Martell, A. E., and M. Calvin. Chemistry of Metal Chelate Compounds. Prentice-
Hall, Inc. 1952.
19. Vendor literature from Hampshire Chemical Company on chelating agents. 1992.
20. Personal conversation with Mr. Malcolm Forbes of Hampshire Chemical Com-
pany. October 24, 1992.
21. Kesting, R. E. Synthetic Polymeric Membranes: A Structural Perspective,
Second Edition. John Wiley & Sons. New York, New York. 1985.
22. Ku, Y., and M. Yen. The Effect of Chelating Agents on the Removal of Lead
from Wastewaters by Sulfide Precipitation. Journal of the Chin. I. Ch. E., Volume
22, No. 2. 1991. pp. 81-88.
57
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REFERENCES (continued)
23. Smith, R. 1. The Sequestration of Metals. Chapman & Hall, Ltd. 1959.
24. Pribil, R. Analytical Application of EDTA and Related Compounds. [Pergamon
Press. 1972.
25. McCrary, A. L, and W. L Howard. Chelating Agents. In: Encyclopedia of
Chemical Technology, 3rd ed., Vol. 5. John Wiley & Sons. 1979.
26. Ringbom, A. Complexation in Analytical Chemistry: A Guide for the Critical
Selection of Analytical Methods Based on Complexation Reactions., Interscience
Publishers. 1963.
27. Hampshire Chemical Company. Conditional Stability Constants for!HAMPSHIRE
Chelating Agents. Technical Information Sheets. Organic Chemical Division.
1982.
28. Bhat, T. R., and Y. W. Gokhale. Recovery of Metal & EDTA Contents from
Metal-EDTA Solutions. Indian Journal of Chemistry, Volume 2. March 1964. pp.
105-107. I
29. Lindstrom, R. E., and J. O. Winget. Process for Recovering Ethylenediamine-
Tetraacetic Acid (EDTA) from Copper-EDTA-lon Exchange Effluent Solutions.
United States Patent No. 3,138,637. June 23, 1964. ;
\
30. Brown, G. A., and H. A. Elliot. Influence of Electrolytes on EDTA Extraction of
Pb from Polluted Soil. Water, Air, and Soil Pollution, Volume 62. 1992. pp. 157-
165. :
31. Ellis, W. D., et al. Treatment of Soils Contaminated with Heavy Metals. U.S.
Environmental Protection Agency. 1987.
32. Elliot, H. A., et al. Role of Iron in Extractive Decontamination of Lead-Polluted t
Soils. Hazardous Waste & Hazardous Materials, Volume 6, Number 3. 1989.
pp. 223-229.
33. Norvell, W. A. Comparison of Chelating Agents as Extractants for Metals in
Diverse Soil Materials. Soil Science Society American Journal, Volume 48. 1984.
pp. 1285-1292.
34. Loh, H., et al. Electrically Conductive Membranes: Synthesis and Applications.
Journal of Membrane Science, Volume 50. 1990. pp. 31-49.
58
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REFERENCES (continued)
35. Vendor literature from DuPont on Nafion® membranes. 1992.
36. Vendor literature from Ionics on CR 67 membranes. 1992.
37. Personal conversation with Mr. Tom Wey of Ionics Corporation. October 22
1992.
i
38. Tseng, D. Regeneration of Heavy Metal Exhausted Cation Exchange Resin with
a Recoverable Chelating Agent. A Thesis submitted to the Faculty of Purdue
University, Dr. James E. Etzel, School of Civil Engineering, August 1983.
39. Lacey, R. E., and S. Loeb. Industrial Processing with Membranes. Wiley-
Interscience. New York, New York. 1972.
59
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