United States
Environmental Protection
Agency
Risk Reduction
Engineering Laboratory
Cincinnati, OH 45268
Research and Development
EPA/600/SR-94/006
March 1994
EPA Project Summary
A Literature Review Summary of
Metals Extraction Processes
Used to Remove Lead from Soils
Numerous Superfund sites through-
out the United States are contaminated
with toxic metals. Battery reclamation,
lead smelting, and lead-based paint
manufacturing are examples of pro-
cesses that can result in lead-contami-
nated soils.
The objective of the report summa-
rized here is to review and evaluate
literature relating to metals extraction
technologies, soil characterization,
chelating agents, and membranes. The
literature assessment provides insight
regarding potential operating problems
that can be identified and avoided when
extraction processes are used to re-
cover lead from soils.
This Project Summary was developed
by EPA's Risk Reduction Engineering
Laboratory, Cincinnati, OH, to announce
key findings of the research project
that is fully documented in a separate
report of the same title (see Project
Report ordering information at back).
Introduction
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 contaminated soils with sub-
sequent disposal at an off-site, RCRA-
approved landfill, in accordance with
hazardous waste regulations. This pro-
cess 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.
Thus, there is great incentive for the de-
velopment of alternative methods for
cleanup of contaminated sites.
Chemical or physical fixation of the con-
taminated soils, which immobilizes the lead
(or other heavy metals), is required before
landfilling. The drawbacks of this approach,
however, include (1) the need for future
monitoring of heavy metals on site, (2)
questionable longevity of fixation chemi-
cals, (3) unknown biosystem (plant/animal
uptake) effects, and (4) the potential need
of a soil cap to prevent wind erosion prob-
lems.
The use of extraction processes to re-
cover heavy metals (e.g., lead) from con-
taminated soils is a more attractive
alternative. The goal of this approach is to
treat the contaminated soil to an accept-
able level, protecting groundwaterand sur-
face water resources, and then to return
the "clean" soil to the site from where it
originated.
Metals Extraction Technologies
and Soil Characterization
Several technologies have been or are
currently being developed to remove met-
als from contaminated soils.
The Bureau of Mines (BOM) has devel-
oped a process that employs acid leach-
ing to convert lead sulfate and lead dioxide
to lead carbonate, which is soluble in ni-
tric acid. Lead is recovered by precipita-
tion with sulfuric acid to produce a lead
sulfate product. The BOM has also devel-
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oped a process to convert the lead com-
pounds to lead carbonate with ammonium
carbonate and ammonium bisulfite, fol-
lowed by leaching with fluosilicic acid. Lead
is then recovered using an electrowinning
process.
The TerraMet™* soil remediation sys-
tem developed by COGNIS, Inc., leaches
and recovers lead from contaminated soils,
sludges, or sediments by using a propri-
etary aqueous leachant. Various forms of
lead, including metallic lead, soluble ions,
and insoluble lead oxides and salts, are
amenable for leaching via this process.
A soil recycling process developed by
the Toronto Harbor Commission employs
a treatment train using three technolo-
gies. The first stage involves soil washing
to reduce the volume and concentrate the
contaminants into a fine slurry. The sec-
ond stage employs acidification and se-
lective chelation to dissolve heavy metals.
The third stage involves chemical hydroly-
sis followed by biodegradation to destroy
organic contaminants in the slurry. All met-
als may be recovered in their pure form
by using this process.
The U.S. Environmental Protection
Agency (EPA) has conducted research on
a lead extraction process involving the
following steps: (1) conversion of lead sul-
fate to lead carbonate with ammonium
carbonate, (2) conversion of lead carbon-
ate to lead acetate and oxidation of lead
to lead acetate with acetic acid and oxy-
gen, (3) conversion of lead dioxide into
lead acetate, and (4) conversion of lead
acetate to lead sulfate with sodium sul-
fate. Table 1 presents the results of the
lead extraction process used to treat a
synthetic lead-contaminated soil. The
amount of lead recovered was approxi-
mately 80% for the experiments performed
with 1,000 and 5,000 mg/kg lead-contami-
nated soils.
A lead recovery process developed by
Kaur and Vohra uses a surfactant liquid
membrane to recover lead (II) from waste-
waters. The lead first diffuses through a
stagnant film and reacts with di(2-
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, 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 solu-
' Mention of trade names or commercial products does
not constitute endorsement or recommendation for
use.
tion (Table 2). Bench-scale tests were per-
formed with actual chelate generated with
the use of lead-contaminated soil from a
battery reclamation site.
EPA Region V and PEI Associates have
developed an on-site soil washing pro-
cess for recovery of lead from contami-
nated soils. Contaminated soil from a
battery reclamation site was washed with
a chelating agent followed by addition of
sodium sulfide to precipitate the chelating
agent from the wash solution. Two chelat-
ing agents were evaluated for the soil
wash: EDTA and NTA (nitrilotriacetic acid).
The ratio of soil to chelating solution de-
pends on how contaminated the soil is.
EDTA was determined to be the more
efficient chelating agent for lead removal.
Soil characterization performed before
soil washing was used to treat the metal-
contaminated soils showed that a majority
of the metals are adsorbed on the fine soil
fraction (less than 250 urn). The predomi-
nant species of lead found at many of the
examined battery breaking and reclama-
tion sites were lead sulfate, lead carbon-
ate, and lead dioxide. Because the
chelating agent may chelate both the metal
and soil particles, techniques to eliminate
the formation of colloids will need to be
employed to effectively separate the che-
late solution and solid fractions. 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 repre-
sent a critical value for lead solubilization.
Chelating Agents
Many metals extraction processes for
soils involve the use of chelating agents,
and selecting appropriate chelating agents
is 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 selective complexation of one metal
in the presence of other metals depends
on there being a large difference between
the stability constants of the two metals.
Carboxylic acids such as EDTA and NTA
are hydrolytically stable at high tempera-
tures and pH levels. EDTA forms stable
1-to-1 complexes with most metals, espe-
cially those of the transition metal group.
Table 3 presents the stability constants
for chelating agents that are commonly
used in soil washing. Enough chelating is
needed to combine with the target metal
ions as well as with any competing metal
ions that could displace the target metal.
Factors affecting the stability of metal che-
late include the size and number of rings,
substituents on the rings, and the nature
of the metal and donor atoms. The pH at
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 concen-
trations of clay, calcite (CaCO3) may be
present in concentrations of up to 30% to
60%. High quantities of calcium carbon-
ate may affect the equilibrium constants
of the metal chelates.
Metals may be removed from the che-
late through acidification or by precipita-
tion with a hydroxide, sulfide, or oxalate.
Where EDTA is used as the chelating
agent, mineral acids may cause dissocia-
tion of the metal-EDTA complex as a re-
sult of the strong competition for the
formation of a protonated EDTA specie
from H+. Precipitation of EDTA can only
occur on acidification if the concentration
of the protonated EDTA species is greater
than its solubility. Under alkaline condi-
tions (pH >9), the complex ion can be
dissociated by precipitation of the metal
as a hydroxide. Based on the stability
constant for EDTA and the solubility of
the hydroxide product, the cation would
be completely dissociated from the metal-
EDTA complex by direct hydroxide pre-
cipitation in only a few cases [e.g., Fe(lll)].
Membranes
Membrane technologies are often used
to recover metals from waste streams.
For the successful separation of cation
and anions, the selection of an appropri-
ate membrane is critical. A membrane
must be durable, able to withstand harsh
chemical and physical treatment, stable at
high temperatures, and possess low elec-
trical resistance. Membranes are manu-
factured from a variety of materials ranging
from polymers to sulfonic acids. Polymeric
membranes are characterized by the fol-
lowing polymer properties: the large, av-
erage size of their macromolecules, their
size distribution, their architecture, the spe-
cific nature of their chemical groups in the
chain, and the aggregate state of the mac-
romolecules. Although the integrity of the
membrane tends to increase with molecu-
lar weight, the higher molecular weight
polymer membranes may decrease the
ion selectivity.
Not only are the properties of a mem-
brane important, but the operating condi-
tions are also important. Process
parameters such as pH and fluid turbu-
lence affect the rate in which the ions are
transported across the membrane. In
electromembrane processes, which utilize
a membrane to separate the cathode and
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Table 1.
Step No.
Total Pb
1
II
III
Total Pb
1
II
III
Total Pb
1
II
III
Lead Removal Efficiency for Three-Step Extraction Process*
Reagent
= 10,000 mg/kg:
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 mg/kg
Ammonium carbonate
Oxygen + acetic acid
Washing
Manganese acetate
TOTAL
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
Filtrate Vol. (mL)
1980
1820
545
1910
495
1850
1860
465
1840
1800
1910
450
1865
% Pb Removed
0.0165
4.5557
0.714
0.934
0.103
6.370
0.6883
63.77
3.51
14.21
82.18
1.860
57.56
4.36
15.90
79.68
"250-g soil; ratio of soil to solution, 1:8.
anode chambers in an electrolytic cell, the
speed and direction of the ionic flow de-
pend on the current potential and density
as wsii as ins resistance OT uoin ins an-
ode and cathode chamber solution char-
acteristics. The solute transport rate of a
liquid adjacent to the membrane can be
controlled by diffusion through the mem-
brane. Film diffusion tends to occur when
membrane 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. The energy re-
quirement for ionic transport in the
electromembrane is a function of the elec-
trical resistance of the solutions and mem-
brane and the back electromotive forces
caused by concentration gradients. The
resistance of the membrane depends on
the transport processes occurring around
the membrane; the electrical resistance of
a solution depends on the solute concen-
tration and solution conductivity. Exami-
nation of the literature showed that there
are currently no commercial or full-scale
membrane technologies for the recovery
of lead from soils.
Table 2. Summary of Lead Recoveries for Bench-Scale Experiments Done by Using an Electromembrane Reactor
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/crrf
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
Table 3. Stability Constants of Metal Chelates
Metal
V(lll)
Fe(lll)
In(lll)
Th(IV)
Hg(ll)
Cu(ll)
VO(II)
Ni(ll)
Y(lll)
Pb(ll)
Zn(ll)
Cd(ll)
Co(ll)
Fe(ll)
Mn(ll)
V(ll)
Ca(ll)
Mg(ll)
Sr(ll)
Ba(ll)
Rare earths
STPP*
8.7
6.7
7.6
6.9
2.5
7.2
5.2
5.7
4.4
3.0
Citric Acid
10.9
6.1
4.8
5.7
4.5
4.2
4.4
3.2
3.4
3.5
2.8
LogK
NTAf
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
EDTAf
25.9
25.1
25.0
23.2
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
" STPP = sodium tripolyphosphate.
t NTA = nitrilotriacetic acid.
f EDTA = ethylenediaminetetraacetic acid.
Summary
Several lead recovery methods have
been or are currently being developed to
extract lead from contaminated soils. Many
of these technologies involve washing the
soil with a reagent to initially extract the
lead into solution, followed by a lead re-
covery step that may include precipitation
of lead as lead sulfate or recovery of lead
by electrowinning. Although most of these
processes are in the developmental stage,
they have potential to progress into pilot-
or full-scale applications.
Soil characterizations performed on
metal-contaminated soils show that a ma-
jority of the metals are adsorbed on the
fine soil fraction (less than 250 |im). Soil
washing with EDTA produces colloids con-
sisting of fine soil particles, which create
difficulties in solid-liquid separation. Add-
ing a filter aid before filtration appears to
allow better separation of the fine par-
ticles from the liquid fraction.
The predominant species of lead found
at many battery breaking and reclamation
sites are lead sulfate, lead carbonate, and
lead dioxide. Because of the slow dissolu-
tion 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
seems to represent a critical value for
lead solubilization.
The tendency for a metal to chelate
with a chelating agent is determined by
the stability constant, which is highly de-
-------
pendent on pH of the solution. In several
studies, EDTA and NTA have been used
to recover metals from waste streams.
Both chelating agents are relatively stable
at high temperatures and pH levels.
Many metal recovery technologies em-
ploy membranes for metals separation.
Important characteristics that need to be
considered in the selection of a mem-
brane include low electrical resistance; high
permselectivity (exclusion of anions); and
durability to withstand high temperatures,
low and high pH solutions, and chemical
and physical treatment for removal of de-
posits. The amount of current, concentra-
tion of ions in the anode chamber, and
stirring rate of solutions in both the anode
and cathode chambers must be controlled
to maintain steady-state conditions in the
boundary layers of the membrane.
The full report was submitted in fulfill-
ment of Contract No. 68-C9-0036 by IT
Corporation under the sponsorship of the
U.S. Environmental Protection Agency.
The report bibliography is available from
the EPA Project Officer.
The Project Summary was prepared by the staff of IT Corporation, Cincinnati,
OH, 45246.
Ronald J. Turner is the EPA Project Officer (see below).
The complete report, entitled "A Literature Review Summary of Metals
Extraction Processes Used to Remove Lead from Soils," (Order No. PB94-
140613; Cost: $19.50, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Risk Reduction Engineering Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
United States
Environmental Protection Agency
Center for Environmental Research Information
Cincinnati, OH 45268
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