United States
Environmental Protection
Agency
Office of Solid Waste and
Emergency Response
(5102G)
EPA-542-R-97-004
March 1997
vvEPA Recent Developments for
In Situ Treatment of Metal
Contaminated Soils
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DISCLAIMER
The information in this document has been funded wholly or in part by the United
States Environmental Protection Agency under contract number 68-W5-0055 to
PRC Environmental Management, Inc. It has been subjected to the Agency's peer
and administrative review and has been approved for publication as an EPA
document. Mention of trade names or commercial products does not constitute
endorsement or recommendation for use.
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TABLE OF CONTENTS
Abstract 1
Executive Summary 3
Introduction 7
1.0 Purpose 8
2.0 Report Organization 9
Overview of In Situ Technologies for Remediation of Soils
Contaminated with Metals 11
Status of Electrokinetic Remediation Technology 13
1.0 Description 13
2.0 Overview of Status 16
2.1 Electrokinetics, Inc 16
2.2 Geokinetics International, Inc 17
2.3 Isotron Corporation 18
2.4 Battelle Memorial Institute 18
2.5 Consortium Process 19
3.0 Performance and Cost Summary 22
3.1 Louisiana State University - Electrokinetics, Inc 22
3.2 Geokinetics International, Inc 25
3.3 Battelle Memorial Institute 25
3.4 Consortium Process 25
4.0 Analysis and Applications 26
5.0 References 27
Status of In Situ Phytoremediation Technology 31
1.0 Description 32
1.1 Phytoextraction 32
1.2 Phytostabilization 33
1.3 Rhizofiltration 35
1.4 Future Development 35
2.0 Overview of Status 35
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TABLE OF CONTENTS (CONTINUED)
3.0 Performance and Cost Summary 36
3.1 Results of Testing 36
3.2 Cost 37
3.3 Future directions 37
4.0 Analysis of Application 39
4.1 Site Conditions 39
4.2 Waste Characteristics 39
5.0 References 40
Status of Soil Flushing Technology 43
1.0 Description 43
2.0 Overview of Status 45
2.1 Cation Displacement 46
2.2 Lead Removal 46
2.3 Chrome Flushing 46
2.4 Twin Cities Army Ammunition Plant 47
3.0 Performance and Cost Summary 48
4.0 Analysis of Applications 50
5.0 References „ 51
Status of In Situ Solidification/Stabilization Technology 53
1.0 Description „ 54
1.1 Reagent-based S/S Processes 54
1.2 Thermal-based S/S Processes 55
2.0 Overview of Status 55
3.0 Performance and Cost Summary 57
3.1 Reagent-based S/S Processes 57
3.2 Thermal-based Processes 58
4.0 Analysis of Applications 58
5.0 References 59
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TABLE OF CONTENTS (CONTINUED)
3
4
5
6
7
8
9
Figures
Frequency of the Most Common Contaminants in All Matrices at NPL Sites
withRODs
A Schematic Diagram of One Electrode Configuration and Geometry Used
in Field Implementation of Electrokinetic Remediation
Schematic View of Contaminated Plume Stopped by an Electrokinetic Fence.
Setup of Electroheating with Vapor/Water Extraction: Results in
Unsaturated Zone and in Saturated Zone
Electroacoustical Soil Decontamination Process
Schematic Diagram of the Lasagna™ Process
Integrated In-Situ Remediation: Consortium
Typical Soil Flushing System (Surface Sprinklers)
Geosafe In Situ Vitrification Process
Page
.15
.18
.19
.20
.21
.22
.45
.56
Tables
1 Overview of In Situ Technologies for Remediation of
Soils Contaminated with Metals
2 Overview of Electrokinetic Remediation Technology
3 Performance Summary of Electrochemical Soil Remediation Technology
Applied at Five Field Sites in Europe (1987 - 1994)
4 Overview of Phytoremediation Technology
5 Types of Phytoremediation Technology: Advantages and Disadvantages..
6 Examples of Metal Hyperaccumulators
7 Overview of Soil Flushing Technology
8 United Chrome Products Superfund Site Extraction and
Treatment System Summary
9 Overview of Solidification/Stabilization Technology
Page
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.14
.24
.31
.34
.36
.44
.49
.53
Appendix
A Methodology
B Engineering Bulletin: Technology Alternatives for the Remediation of Soils
Contaminated with Arsenic, Cadium, Chromium, Mercury, and Lead
in
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Abstract
Metals contamination is a common problem at hazardous waste sites. This report assists
the remedy selection process by providing information on four in situ technologies for
treating soil contaminated with metals. The four approaches are electrokinetic
remediation, phytoremediation, soil flushing, and solidification/stabilization. The
report discusses different techniques currently hi practice or under development;
identifies vendors and summarizes performance data; and discusses technology
attributes that should be considered during early screening of potential remedies.
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Executive Summary
Metals are prevalent at most Superfund
sites. At sites with signed Records of
Decision (ROD), metals are the sole
contaminants (approximately 16 percent)
or are found in combination with other
contaminants such as volatile or semi-
volatile organic compounds (approxi-
mately 49 percent). In general, in situ
remedies are more cost efficient when
compared with traditional treatment
methods, but relatively few alternatives
exist for the hi situ treatment of metals.
This report presents an overview of four
of the most promising technologies for in
situ soil treatment:
i Electrokinetics
ii. Phytoremediation
Hi. Soil Flushing
iv. Solidification/stabilization
The report is intended to assist in screen-
ing these technologies early in the rem-
edy evaluation and selection process.
Electrokinetics
Electrokinetic remediation relies on the
application of low intensity direct current
between electrodes placed in the soil.
Contaminants are mobilized in the form
of charged species, particles, or ions.
Several organizations are developing
technologies for the enhanced removal of
metals by transporting contaminants to
the electrodes where they are removed
and subsequently treated above ground.
A variation of the technique involves
treatment without removal by transport-
ing contaminants through specially
designed treatment zones that are created
between electrodes. This process is
undergoing early field testing and is
initially being targeted to treat chlori-
nated volatile compounds in low-perme-
ability clay. Electrokinetics also can be
used to slow or prevent migration of
contaminants by configuring cathodes
and anodes in a manner that causes
contaminants to flow toward the center of
a contaminated area of soil. The practice
has been named "electrokinetic fencing."
Experience with electrokinetics is limited
to bench and pilot scales, with the no-
table exception of a metals removal
process for copper, lead, zinc, arsenic,
cadmium, chromium, and nickel that has
been commercially operated by a single
vendor in Europe and recently licensed in
the United States. Limited performance
data from this vendor illustrate the
potential for achieving removals greater
than 90 percent for some contaminants.
A broad range of metals can be treated.
There is also potential application for
radionuclides and some types of organic
compounds. The electrode spacing and
duration of remediation is site-specific.
The process requires adequate soil
moisture in the vadose zone, and the
addition of a conducting pore fluid may
be required due to a tendency for soil
drying near the anode. Specially de-
signed pore fluids can be added at the
anode or cathode to enhance the migra-
tion of target contaminants.
Phytoremediation
This technology is in the early stage of
commercialization for treating soils
contaminated with metals, and may prove
to be a low cost option under specific
circumstances. At the current stage of
development, this process is best suited
for sites with widely dispersed contami-
nation at low concentrations where only
treatment of soils at the surface (in other
words, within depth of the root zone) is
required.
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Two basic approaches for metals reme-
diation include phytoextraction and
phytostabilization. Phytoextraction relies
on the uptake of contaminants from the
soil and their translocation into above-
ground plant tissue, which is harvested
and treated. Although hyper-accumulat-
ing trees, shrubs, herbs, grasses, and
crops have potential, crops seem to be
most promising because of their greater
biomass production. Nickel and zinc
appear to be the most easily absorbed,
although preliminary tests with copper
and cadmium are encouraging. Signifi-
cant uptake of lead, a commonly occur-
ring contaminant, has not been demon-
strated in any of the plants tested thus far.
However, one researcher is experiment-
ing with soil amendments that would
facilitate uptake of lead by the plants.
Phytostabilization achieves risk reduction
by stabilizing contaminants located near
the surface. This result is achieved by
the secretion of compounds by plants to
affect soil pH and to form metal com-
plexes with reduced solubility. In addi-
tion, the plants help control surface
erosion and reduce leaching through
increased evapotranspiration. Laboratory
studies indicate the potential effective-
ness of this approach for lead.
Soil Flushing
This technology involves extraction of
contaminants from soil using water or
other suitable aqueous solutions. Al-
though additives such as acids and
chelating agents have had some commer-
cial use for full-scale ex situ soil washing
projects, they have not been demon-
strated as feasible for in situ applications.
Soil flushing has been selected at seven
Superfund sites with metals present;
however, at six of those sites, organic
contaminants are the primary targets. For
metals, soil flushing would be most
effective in removing water-soluble
species, such as hexavalent chrome. Two
soil flushing remedies are currently
ongoing at Superfund sites, with some
preliminary data available from a
hexavalent chrome application.
Leached contaminants are typically
recovered from the underlying ground
water by pump-and-treat methods. Site-
specific conditions must be carefully
considered to address the possible spread
of contamination.
Solidification/Stabilization
This process (also referred to as immobi-
lization) changes the physical and chemi-
cal characteristics of the waste in order to
immobilize contaminants. Metals are
commonly remediated by ex situ solidifi-
cation with pozzolans and sometimes
other additives. This technology has
been adapted to in situ applications
through the use of various proprietary
augers which provide reagent delivery
and mixing. In situ treatment will likely
have a cost advantage over ex situ appli-
cations for larger volumes and for depths
greater than 10 feet. However, this
technology has been only occasionally
selected for Superfund use, largely
because of concerns with long-term
reliability.
A second solidification technique in-
volves vitrification where an electrical
current is passed between electrodes to
melt soil and incorporate metals into a
vitrified product. This technology is
commercially available and has been
successfully used at two Superfund sites,
one of which was contaminated with
metals.
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Both processes are broadly applicable to
a range of metals. Vitrification uses a
hood to capture mercury and other
volatile metals, such as lead and arsenic,
which may be partially vaporized during
operations. Vitrification is best suited for
wastes that are difficult to treat, such as
mixtures of organics and metals.
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Introduction
Metals account for much of the contami-
nation found at hazardous waste sites.
They are present in the soil and ground
water at approximately 65 percent of the
Superfund sites for which the U.S.
Environmental Protection Agency (EPA)
has signed records of decisions (ROD).
The metals most frequently identified are
lead, arsenic, chromium, cadmium,
nickel, and zinc. Other metals often
identified as contaminants include copper
and mercury. Figure 1 shows the most
common contaminants in all matrices at
Superfund sites. In addition to the
Superfund program, metals make up a
significant portion of the contamination
requiring remediation under the Resource
Conservation and Recovery Act (RCRA)
and contamination present at federal
facilities, notably those that are the
responsibility of the Department of
Defense (DoD) and the Department of
Energy (DOE).
Since the reauthorization of Superfund in
1986, there has been a significant in-
crease in the treatment of soil at
Superfund sites. In the early days of the
program, EPA selected conventional
technologies (for example, incineration,
solidification and stabilization, and
groundwater pump-and-treat systems).
Subsequently, new and improved pro-
cesses were developed, especially for
soils, that are capable of providing more
cost-effective cleanups. In fiscal year
1993, EPA for the first time selected
innovative technologies as remedies
more frequently than conventional
processes. The innovative technologies
most often selected are in situ soil vapor
extraction, various bioremediation pro-
cesses, and thermal desorption for soils
and in situ air sparging and bioremedia-
tion for ground water. All of these
technologies target the treatment of
organic compounds.
Experience under the Superfund program
clearly demonstrates the successful
development of new technologies to treat
organic compounds. In addition, statis-
tics show that more than half of the new
technologies selected for soil treatment
are in situ processes. In situ techniques
have the potential to provide significant
cost savings and are generally considered
to represent a promising direction for the
development of new technologies.
Few commercial alternatives exist,
however, to treat metals in soil, espe-
cially in situ. The most frequently
selected treatment process in the
Superfund program is solidification/
stabilization, which was selected 203
times through fiscal year 1994. This
accounts for nearly 30 percent of all soil
treatment technologies. By contrast,
other technologies available to address
metals in soil were selected only 18
times. No treatment technologies have
been selected for sites with low-level
radioactive metals, where excavation and
either on-site or off-site disposal are
typically chosen.
The difference between the availability of
new technologies for the treatment of
metals versus new technologies for the
treatment of organic compounds is
illustrated by data from EPA's Vendor
Information System for Innovative
Treatment Technologies (VISITT). The
system, which is distributed on request to
more than 12,000 users, contains infor-
mation submitted by vendors of new
technologies about the capabilities of
their processes. EPA recently released
the fifth version of the database, which
contains information on 346 innovative
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technologies offered by more than 210
vendors. Information provided by ven-
dors indicates that 226 technologies treat
volatile organics, 208 technologies treat
semi-volatile organics, and 66 technolo-
gies treat metals (some technologies can
treat several waste groups). While a
substantial portion (about 40 percent) of
the organic treatment technologies are in
situ processes, only 9 of the 66 technolo-
gies that treat metals are designed to treat
soil or groundwater in situ.
1.0 Purpose
This document surveys treatment tech-
nologies with the potential for providing
in situ treatment of soil contaminated
with metals. The report updates project
managers and cleanup professionals
about the status of four technologies
which are currently available or under
active development. The information
should be useful in screening technolo-
gies early in the remedy evaluation and
selection process.
This document is not meant to provide a
rigorous scientific examination. This
document focuses only on contamination
in soils; EPA recently published a series
of booklets summarizing bench- and
field-scale efforts for in situ treatment of
organics and metals in groundwater. [In
Situ Remediation Technology Status
Reports. EPA542-K-94-003/005/006/
007/009. April 1995]
500--
Contaminants
Source: U.S. EPA, Office of Solid Waste and Emergency Resonse, ROD Information Directory, 1995.
Figure 1. Frequency of the Most Common Contaminants in All Matrices at NPL SHes with RODs
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IZAUOIY
2.0 Report Organization
This document focuses on the treatment
of metals, such as cadmium, copper,
chromium, lead, mercury, arsenic, nickel,
and zinc. The four in situ technologies
presented are electrokinetic remediation,
phytoremediation, soil flushing, and
solidification/stabilization (S/S) tech-
niques. The second chapter of this
document presents a brief summary of
the attributes of these technologies.
Electrokinetic remediation, discussed in
the third chapter separates contaminants
from soil through selective migration
upon application of an electric current.
Phytoremediation, discussed in the
fourth chapter is an emerging technology
that uses plants to isolate or stabilize
contaminants. Soil flushing techniques,
described in the fifth chapter promote
mobility and migration of metals by
solubilizing contaminants so that they
can be recovered. The sixth chapter
describes two types of S/S techniques,
one based on addition of reagents and the
other based on the use of energy.
The four chapters that address specific in
situ technologies are organized in four
sections. The first table of each technol-
ogy chapter presents an overview of the
technology. The general characteristics
of the technology are summarized in the
table, and are discussed in greater detail
in Section 4 of the chapter, Analysis of
Applications. Section 1, Description,
provides a detailed description of the
principle of the technology. The ap-
proaches described in the summary table
are discussed further in Section 2, Over-
view of Status. The available perfor-
mance data for each of the technologies
are provided in Section 3, Performance
and Cost Summary.
Appendix A contains a description of the
methodology followed in the preparation
of this report and includes a list of techni-
cal experts that were contacted. It also
contains treatment options not discussed
here, such as the use of treatment trains.
Appendix B contains a
copy of an engineering bulletin titled
Technology Alternatives for the
Remediation of Soils Contaminated with
Arsenic, Cadmium, Chromium, Mercury,
and Lead. This bulletin provides a
background description of physical
properties of metals and discussions of
S/S, soil washing, and soil flushing.
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Overview of In Situ Technologies
for Remediation of Soils Contami-
nated with Metals
This chapter presents an overview compari-
son of the four in situ technologies. The
key factors that were considered in this
analysis are: status, range of metals treated,
major limiting factor, and site-specific
considerations. Status refers to the stage of
development of the technology. Range of
metals treated specifies whether the tech-
nology can address a broad range of metals
or focuses on a Limited range of metals.
Major limiting factor refers to process
considerations which may limit broad use
of the technology. Site-specific consider-
ations refers to those site characteristics that
can influence the effectiveness of the
technology. Table 1 provides an overview
of the key factors for each of the four
technologies.
As Table 1 indicates, electrokinetics, soil
flushing, and solidification/stabilization are
in more advanced stages of development
than phytoremediation. Soil flushing
currently is applicable to a limited range of
metals. Soil flushing requires consideration
of the potential risk of aquifer contamina-
tion by residual flushing solution at the site.
The permeability of the soil and the charac-
teristics of the groundwater flow are the
main site-specific considerations affecting
the applicability of soil flushing. Electroki-
netics is most applicable to sites at which
the soil is homogeneous and the moisture
level is relatively high. Phytoremediation
requires longer treatment times than other
treatment technologies and may potentially
be applied at sites at which the contamina-
tion is shallow and the concentration of the
contaminants relatively low. Solidification/
stabilization is limited by the lack of data
concerning the long-term integrity of the
treated material. The technology is most
effective at sites at which little or no debris
is present.
flETAlS
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P " ' V -H • *i |
",' I * «
ITT'
TABLE 1
OVERVIEW OF IN SITU TECHNOLOGIES FOR REMEDIATION OF SOILS
CONTAMINATED WITH METALS
EVALUATION ELECTROKINETICS
FACTOR
•••••BI^^^H
Status
Range of
Metals Treated
Major Limiting
Factor(s)
Site-Specific
Considerations
^^^^^r^m^^fm^mmmm^mm
Full-scale applications
in Europe
Recently licensed in
U.S.
Broad
State-of-the-art
Homogeneity of soil
Moisture level in soil
Pilot-scale
Currently being field-
tested in Trenton, NJ;
Butte,MT;INELat
Femald, OH; and
Chernobyl, Ukraine
Broad
State-of-the-art
Longer time required
for treatment
Crop yields and growth
patterns
Depth of contamination
-! *
Concentration of
contamination ', * '
Commercial
Selected at 7
Superfund sites
Limited
"^ .-, '^5
Potential contami-
nation of the aquifer
from residual flushing
solution
Permeability otsoil *
-' '*<..
Groundwaterflov?"
anddeptrT v -
Commercial
Broad „. J" ,
t y , •'
Concern with long-
term integrity
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OVERVIEW OF IH-SITU ti
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Status of Electrokinetic
Remediation Technology
Electrokinetic remediation involves the
application of low density direct current
between electrodes placed in the soil to
mobilize contaminants in the form of
charged species.
Attempts to leach metals from soils by
electro-osmosis date back to the 1930s.
In the past, research focused on removing
unwanted salts from agricultural soils.
Electrokinetics has been used for dewa-
tering of soils and sludges since the first
recorded use in the field in 1939 [1],
Electrokinetic extraction has been used in
the former Soviet Union since the early
1970s to concentrate metals and to ex-
plore for minerals in deep soils. By
1979, research had shown that the con-
tent of soluble ions increased substan-
tially in electro-osmotic consolidation of
polluted dredgings, while metals were
not found in the effluent [2]. By the mid-
1980s, numerous researchers had realized
independently that electrokinetic separa-
tion of metals from soils was a potential
solution to contamination [3].
Table 2 presents an overview of two
variations of electrokinetic remediation
technology. Geokinetics International,
Inc.; Battelle Memorial Institute; Electro-
kinetics, Inc.; and Isotron Corporation all
are developing variations of technologies
categorized under Approach #1, En-
hanced Removal. The consortium of
Monsanto, E.I. du Pont de Nemours and
Company, General Electric, DOE, and
the EPA Office of Research and Devel-
opment is developing the Lasagna Pro-
cess, which is categorized under Ap-
proach #2, Treatment Without Removal.
1.0 Description
Electrokinetic remediation, also referred
to as electrokinetic soil processing,
electromigration, electrochemical decon-
tamination, or electroreclamation, can be
used to extract radionuclides, metals, and
some types of organic wastes from
saturated or unsaturated soils, slurries,
and sediments [4]. This in situ soil
processing technology is primarily a
separation and removal technique for
extracting contaminants from soils. An
in situ bioremediation technology by
electrokinetic injection is under develop-
ment, with support from EPA and DOE
[16].
The principle of electrokinetic remedia-
tion relies upon application of a low-
intensity direct current through the soil
between two or more electrodes. Most
soils contain water in the pores between
the soil particles and have an inherent
electrical conductivity that results from
salts present in the soil [5]. The current
mobilizes charged species, particles, and
ions in the soil by the following pro-
cesses [6]:
• Electromigration (transport of
charged chemical species under
an electric gradient)
• Electro-osmosis (transport of pore
fluid under an electric gradient)
• Electrophoresis (movement of
charged particles under an electric
gradient)
• Electrolysis (chemical reactions
associated with the electric field)
Figure 2 presents a schematic diagram of
a typical conceptual electrokinetic reme-
diation application.
REM
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TABLE 2
OVERVIEW OF ELECTROKINETIC REMEDIATION TECHNOLOGY
General Characteristics
• Depth of soil that is amenable to treatment depends on electrode placement.
• Best used in homogeneous soils with high moisture content and high permeability.
APPROACH #1 - ENHANCED REMOVAL
Description: Electrokinetic transport of contami-
nants toward the polarized electrodes to
concentrate the contaminants for subsequent
removal and ex-situ treatment.
Status: Demonstration projects using full-scale
equipment are reported in Europe. Bench- and
pilot-scale laboratory studies are reported in the
U.S. and at least two full-scale field studies are
ongoing in the U.S. Recently, full-scale field
studies also have been initiated in the U.S.
Applicability:
Pilot scale: lead, arsenic, nickel, mercury, copper,
zinc.
Lab scale: lead, cadmium, chromium, mercury,
zinc, iron, magnesium, uranium, thorium, radium.
No performance data available for completed full-
scale applications.
Comments: The efficiency and cost-effectiveness
of the technique have not been fully evaluated at
full scale in the U.S. by any federal agency.
Field studies are under evaluation or recently
have been initiated by EPA, DOE, DoD, and
Electric Power Research Institute (EPRI). The
technique primarily would require addition of
water to maintain the electric current and
facilitate migration; however, there is ongoing
work in application of the technology in partially
saturated soils.
APPROACH #2 - TREATMENT
WITHOUT REMOVAL
Description: Electro-osmotic transport of
contaminants through treatment zones placed
between the electrodes. The polarity of the
electrodes is reversed periodically, which
reverses the direction of travel of the contami-
nants back and forth through treatment zones.
The frequency with which electrode polarity is
reversed is determined by the rate of transport
of contaminants through the soil.
Status: Demonstrations are o'ngoing.
Applicability: Technology developed for organic
species. Research underway for metals.
Comments:' Time required for treatment is - I
relatively independent of the-concentration of'
the contamination. This technology is being.,
developed'for deep day formations." ^
EIECTBOKINETJC REMEDIATON^
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-DESCRIPTION
Figure 2. Schematic Diagram of One Electrode Configuration and Geometry Used in Field Implementation
of Electrokinetic Remediation [13,23]
Electrokinetics can be efficient in extract-
ing contaminants from fine-grained,
high-permeability soils. A number of
factors determine the direction and extent
of the migration of the contaminant.
Such factors include the type and concen-
tration of the contaminant, the type and
structure of the soil, and the interfacial
chemistry of the system [7]. Water or
some other suitable salt solution may be
added to the system to enhance the
mobility of the contaminant and increase
the effectiveness of the technology. (For
example, buffer solutions may change or
stabilize pore fluid pH). Contaminants
arriving at the electrodes may be re-
moved by any of several methods, in-
cluding electroplating at the electrode,
precipitation or coprecipitation at the
electrode, pumping of water near the
electrode, or complexing with ion ex-
change resins [7].
Electrochemistry associated with this
process involves an acid front that is
generated at the anode if water is the
primary pore fluid present.
The variation of pH at the electrodes
results from the electrolysis of the water.
The solution becomes acidic at the anode
because hydrogen ions are produced and
oxygen gas is released, and the solution
becomes basic at the cathode, where
hydroxyl ions are generated and hydro-
gen gas is released [8]. At the anode, the
pH could drop to below 2, and it could
increase at the cathode to above 12,
depending on the total current applied.
The acid front eventually migrates from
the anode to the cathode. Movement of
the acid front by migration and advection
results in the desorption of contaminants
from the soil [4]. The process leads to
temporary acidification of the treated
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OF STATUS
-
soil, and there are no established proce-
dures for determining the length of time
needed to reestablish equilibrium. Stud-
ies have indicated that metallic electrodes
may dissolve as a result of electrolysis
and introduce corrosion products into the
soil mass. However, if inert electrodes,
such as carbon, graphite, or platinum, are
used, no residue will be introduced in the
treated soil mass as a result of the pro-
cess. The electrodes can be placed
horizontally or vertically, depending on
the location and shape of the plume of
contamination.
Before electrokinetic remediation is
undertaken at a site, a number of different
field and laboratory screening tests must
be conducted to determine whether the
particular site is amenable to the treat-
ment technique.
• Field conductivity surveys: The
natural geologic spatial variability
should be delineated because
buried metallic or insulating
material can induce variability in
the electrical conductivity of the
soil and, therefore, the voltage
gradient. In addition, it is impor-
tant to assess whether there are
deposits that exhibit very high
electrical conductivity, at which
the technique may be inefficient.
• Chemical analysis of water: The
pore water should be analyzed for
dissolved major anions and
cations, as well as for the pre-
dicted concentration of the
contarninant(s). In addition,
electrical conductivity and pH of
the pore water should be mea-
sured.
• Chemical analysis of soil: The
buffering capacity and geochem-
istry of the soil should be deter-
mined at each site.
i pH effects: The pH values of the
pore water and the soil should be
determined because they have a
great effect on the valence,
solubility, and sorption of con-
taminant ions.
i Bench-scale test: The dominant
mechanism of transport, removal
rates, and amounts of contamina-
tion left behind can be examined
for different removal scenarios by
conducting bench-scale tests.
Because many of these physical
and chemical reactions are inter-
related, it may be necessary to
conduct bench-scale tests to
predict the performance of elec-
trokinetics remediation at the
field scale [3,4].
2.0 Overview of Status
Various methods, developed by combin-
ing electrokinetics with other techniques,
are being applied for remediation. This
section describes different types of
electrokinetic remediation methods
currently under development for use at
contaminated sites. The methods dis-
cussed were developed by Electrokinet-
ics, Inc.; Geokinetics International, Inc.;
Isotron Corporation; Battelle Memorial
Institute; a consortium effort; and P&P
Geotechnik GmbH.
2.1 Electrokinetics, Inc.
Electrokinetics, Inc. operates under a
licensing agreement with Louisiana State
University. The technology is patented
by and assigned to Louisiana State
University [17] and a complementing
process patent is assigned to Electroki-
netics, Inc. [18]. As depicted in Figure 2,
groundwater and/or a processing fluid
(supplied externally through the
-------
boreholes that contain the electrodes)
serves as the conductive medium. The
additives in the processing fluid, the
products of electrolysis reactions at the
electrodes, and the dissolved chemical
entities in. the contaminated soil are
transported across the contaminated soil
by conduction under electric fields. This
transport, when coupled with sorption,
precipitation/dissolution, and volatiliza-
tion/complexation, provides the funda-
mental mechanism that can affect the
electrokinetic remediation process.
Electrokinetics, Inc. accomplishes extrac-
tion and removal by electrodeposition,
evaporation/condensation, precipitation,
or ion exchange, either at the electrodes
or in a treatment unit that is built into the
system that pumps the processing fluid to
and from the contaminated soil [20].
Pilot-scale testing was carried out with
support from the EPA under the
Superfund Innovative Technology Evalu-
ation (SITE) program, and a design and
analysis package for the process was
developed with the support of the Gulf
Coast Hazardous Substance Research
Center of the EPA Office of Research
and Development [19].
2.2 Geokinetics International, Inc.
On July 18, 1995, Geokinetics Interna-
tional, Inc. (Gil) was awarded a patent
for an electroreclamation process. The
key claims in the patent are the use of
electrode wells for both anodes and
cathodes and the management of the pH
and electrolyte levels in the electrolyte
streams of the anode and the cathode.
The patent also includes claims for the
use of additives to dissolve different
types of contaminants [9]. Fluor Daniel
is licensed to operate Gil's metal removal
process in the United States.
Gil has developed an alternative that
combines containment, remediation, and
prevention in electrokinetic fencing.
Laboratory experiments have demon-
strated that, with an electrokinetic fence,
it is possible to:
• Capture electrically charged
(polar) contaminants while treated
water passes through the fence
• Influence the pH and redox
potential of the groundwater
• Introduce microorganisms and
nutrients through the electrode
system or injection well
• Increase soil temperatures in the
area inside the fence to accelerate
biodegradation processes
• Retard and prevent migration
Electrokinetic fences can be installed
both horizontally and vertically and at
any depth [10], as Figure 3 shows.
Another alternative developed by Gil,
electroheating, uses heat generated by
electrokinetics in combination with
extraction methods to remove volatile
and semivolatile compounds. Figure 4
presents a schematic and performance
data for this electroheating process.
Electroheating and extraction can be used
to remove dense non-aqueous phase
liquids (DNAPL), such as chlorinated
solvents, that have sunk deep into the
aquifer. Field trials by Gil using electri-
cal current have shown that soil and
groundwater between the electrodes are
heated uniformly. In combination with
vacuum or groundwater extraction, the
vendor claims the removal of the con-
taminants identified above can be accom-
plished very effectively [10].
ierww*?-
-------
"T '
OVERVIEW OF STATUS,
current lines
Figure 3. Schematic View of Contaminated Plume Stopped by an Electrokinetic Fence [10]
GEE has developed and patented electri-
cally conductive ceramic material
(EBONEXR) that has an extremely high
resistance to corrosion. It has a lifetime
in soil of at least 45 years and is self-
cleaning. OH also has developed a batch
electrokinetic remediation (BEKR)
process. The process which incorporates
electrokinetic technology, normally
requires 24 to 48 hours for complete
remediation of the substrate. BEKR is a
mobile unit that remediates ex situ soils
on site. Gil also has developed a solution
treatment technology (EIXR) that allows
removal of contamination from the anode
and the cathode solutions up to a thou-
sand times faster than can be achieved
through conventional means [14].
2.3 isotron Corporation
Isotron Corporation is participating in a.
pilot-scale demonstration of electrokinetic
extraction supported by DOE's Office of
Technology Development. The demonstra-
tion is taking place at the Oak Ridge K-25
facility in Tennessee. Laboratory tests
completed in 1994 showed that the Isotron
process could effect the movement and
capture of uranium present in soil from the
Oak Ridge site [12].
Isotron Corporation also is involved with
Westinghouse Savannah River Company
in an ongoing demonstration of electroki-
netic remediation. The demonstration,
supported by DOE's Office of Technol-
ogy Development, is taking place at the
old TNX basin at the Savannah River site
in South Carolina. Isotron is using the
ElectrosorbR process with a patented
cylinder to control buffering conditions
in situ. An ion exchange polymer
matrix called IsolockR is being used to
trap metal ions. The process is being
tested for the removal of lead and
chromium, although the low concentra-
tions of mercury (5 milligrams per
kilogram [mg/kg]) at the site have not
been reduced appreciably [12].
2.4 Battelle Memorial institute
Another method that uses electrokinetic
technology is electroacoustical soil
decontamination. This technology
ELECTROKINETIC REMEDIATION •
-------
$T*iy=r~"
generator
pump stripper carbon filter
Set-up Electro-heating with vapor/water extraction
Depth Concentration Concentration Temperature
at beginning at end
(m) (mg/kg) (mg/kg) (°C)
9,000
9,000
9,000
220
9
18
40
55
70
Results of Electro-heating in combination with soil vapor
extraction in the unsatorated zone (diesel)
Compound
Concentration
at beginning
(mg/L)
Concentration
at end
(mg/L)
Benzene
Toluene
Ethylbenzene
Xylenes
610
1,900
2,400
8,500
<0.20
<0.20
<0.20
<0.20
Total aromatics 13,410
Naphthalene 310 <0.20
Mineral oil 7,300 <50
Results of Electro-heating with low yield groundwater extraction
in the saturated zone at: 80-90 "C
Figure 4. Setup of Electroheating with Vapor/Water Extraction: Results in Unsaturated Zone and in Saturated Zone [10]
combines electrokinetics with sonic
vibration. Through the use of sonic or
ultrasonic vibration, the properties of a
liquid contaminant in soil can be altered
in a way that increases the level of
removal of the contaminant. Battelle
Memorial Institute of Columbus, Ohio
developed the in situ treatment process
that uses both electrical and acoustical
forces to remove floating contaminants,
and possibly metals, from subsurface
zones of contamination. The process was
selected for EPA's SITE program; the
technology demonstration was completed
in May 1989 [13]. Figure 5 illustrates the
process.
2.5 Consortium Process
Monsanto Company has coined the name
Lasagna™ to identify its products and
-------
FLUSHING (OPTIONAL)
INJECTION WELL
(ANODE)
GROUND SURFACE
EXTRACTION WELL
(CATHODE)
UNSATURATEDZONE
SATURATED ZONE
FLOATING '
CONTAMINANT
L. J
Source: Adapted from H.S. Mutahdhara et al., Battelle Memorial Institute.
Figure 5. Electroacousitical Soil Decontamination Process [13]
services that are based on the integrated
in-situ remediation process developed by
a consortium. The proposed technology
combines electro-osmosis with treatment
zones that are installed directly in the
contaminated soils to form an integrated
in-situ remedial process, as Figure 6
shows. The consortium consists of
Monsanto, E.I. du Pont de Nemours and
Company (DuPont), and General Electric
(GE), with participation by the EPA
Office of Research and Development and
DOE, as Figure 7 shows.
The consortium's activities are being
facilitated by Clean Sites, Inc., under a
cooperative agreement with EPA's
Technology Innovation Office (TIO)
[12].
The in-situ decontamination process
occurs as follows:
• Creates highly permeable zones
in close proximity sectioned
through the contaminated soil
region and turns them into sorp-
tion-degradation zones by intro-
ducing appropriate materials
(sorbents, catalytic agents, mi-
crobes, oxidants, buffers, and
others).
• Uses electro-osmosis as a liquid
pump to flush contaminants from
the soil into the treatment zones
of degradation.
• Reverses liquid flow, if desired,
by switching the electrical polar-
ity, a mode that increases the
efficiency with which contami-
nants are removed from the soil;
allows repeated passes through
the treatment zones for complete
sorption or destruction.
Initial field tests of the consortium
process were conducted at DOE's gas-
eous diffusion plant in Paducah, Ken-
tucky. The experiment tested the combi-
nation of electro-osmosis and in situ
sorption in treatment zones. In Novem-
ber 1994, CDM Federal Programs Corpo-
fiarmowenc Rmwmtai,
-------
A. Horizontal Configuration
Borehole
Ground Surface
t
APPLIED ELECTRICAL
POTENTIAL
Electro-osmotic^" i
Liquid Flow
Contaminated
Soil
Degradation Zone
Granular Electrode
B. Vertical Configuration
Ground Surface
Degradation
Zone
Contaminated Degradation
Soil Zone
Note: electro-osmotic flow is reversed upon switching electrical polarity.
Figure 6. Schematic Diagram of the Lasagna™ Process [11]
-------
IP i i liiil'iii br^Mg^.^^^roBlii|Mi^OST SUMMAR¥_
DuPont (Anaerobic Biodegradation/
Vertical Zone Installation)
DOE (Site Selection and
Field Support)
General Electric
(EKand Physicochemical
Treatment)
EPA (Hydrofracture/
Biodegradation)
Integrated In-situ
Remediation Technology
Monsanto (Lasagnam/Electo-
Osmosis/Biodegradation)
Rgure 7. Integrated In-Srtu Remediation: Consortium [11]
ration installed field demonstration
equipment [12]. Technology develop-
ment for the degradation processes and
their integration into the overall treatment
scheme were carried out in 1994 and
1995 at bench and pilot scales, with field
experiments of the full process planned
for 1996 [11].
3.0 Performance and Cost
Summary
Work sponsored by EPA, the U.S. Army
Waterways Experiment Station (WES),
DOE, the National Science Foundation,
and private industry (for example, Dow
Chemical, Du Pont, Monsanto, and GE),
when coupled with the efforts of re-
searchers from academic and public
institutions (for example, Sandia National
Laboratories, Argonne National Labora-
tory, Louisiana State University, the
Massachusetts Institute of Technology,
Texas A&M University, West Virginia
University, and the University of Massa-
chusetts Lowell [12]), have demonstrated
the feasibility of moving electrokinetics
remediation to pilot-scale testing and
demonstration stages [4].
This section describes testing and cost
summary results reported by Louisiana
State University, Electrokinetics, Inc.,
Gil, Battelle Memorial Institute, and the
consortium.
3.1 Louisiana State University •
Electrokinetics, inc.
The Louisiana State University (LSU) -
Electrokinetics, Inc. Group has conducted
bench-scale testing on radionuclides and
on organic compounds. Test results have
been reported for lead, cadmium, chro-
mium, mercury, zinc, iron, and magne-
sium. Radionuclides tested include
uranium, thorium, and radium. Experi-
mental data on the transport and removal
of such polar organic compounds as
phenol and acetic acid have been re-
ported, and information about transport
of nonpolar organic compounds such as
-------
JC--
J.7..
PERFORMftiyCE A1«D COST SUMMARY
«*»A»"Sai«i™Av!if j^ \ -r ,&, i. w. x j V^PM^,. ? —
benzene, toluene, ethylene, and xylene
(BTEX) below their solubility values also
has been disseminated.
In collaboration with EPA, the LSU-
Electrokinetics, Inc. Group has com-
pleted pilot-scale studies of electrokinetic
soil processing in the laboratory. WES,
in partnership with Electrokinetics, Inc.,
is carrying out a site-specific pilot-scale
study of the Electro-Klean™ electrical
separation process. Pilot field studies
also have been reported in the Nether-
lands on soils contaminated with lead,
arsenic, nickel, mercury, copper and zinc.
A pilot-scale laboratory study investigat-
ing the removal of 2,000 mg/kg of lead
loaded onto kaolinite was completed in
May 1993. Removal efficiencies of 90
to 95 percent were obtained. The
electrodes were placed one inch apart
in a two-ton kaolinite specimen for four
months, at a total energy cost of about
$15 per ton [13].
Currently (in 1996), with the support of
DoD's Small Business Innovative Re-
search Program and in collaboration with
WES, Electrokinetics, Inc. is carrying out
a comprehensive demonstration study of
lead extraction from a creek bed at a U.S.
Army firing range in Louisiana. EPA is
taking part in independent assessments of
the results of that demonstration study
under the SITE program. The soils are
contaminated with levels as high as 4,500
mg/kg of lead; pilot-scale studies have
demonstrated that concentrations of lead
decreased to less than 300 mg/kg in 30
weeks of processing. TheToxicity
Characteristic Leaching Procedure
(TCLP) values dropped from more than
300 milligrams per liter (mg/L) to less
than 40 mg/L within the same period. At
the site of the demonstration study,
Electrokinetics, Inc. is using the
CADEX™ electrode system that pro-
motes transport of species into the cath-
ode compartment, where they are precipi
tated and/or electrodeposited directly.
Electrokinetics, Inc. uses a special elec-
trode material that is cost-effective and
does not corrode. Under the supervision
and support of the Electric Power Re-
search Institute and power companies in
the southern U.S., a treatability and a
pilot-scale field testing study of soils hi
sites contaminated with arsenic has been
initiated, in a collaborative effort between
Southern Company Services Engineers
and Electrokinetics, Inc [20].
With support from a Small Business
Innovative Research (SBIR) Phase I
grant from DOE, Electrokinetics, Inc., in
collaboration with the Argonne National
Laboratory, has initiated a project to
assess the potential for electrokinetic
transport processes to supplement,
enhance, and engineer in situ bioremedia-
tion systems in contaminated soils that
are characterized by numerous zones of
significantly different hydraulic and
electrical conductivities [14]. Pilot-scale
development of the project is underway
at Electrokinetics, Inc., with support from
the EPA's National Risk Management
Research Laboratory in Cincinnati, Ohio,
under the SITE program [20].
The processing cost of a system designed
and installed by Electrokinetics, Inc.
consists of energy cost, conditioning cost,
and fixed costs associated with installa-
tion of the system. Power consumption
is related directly to the conductivity of
the soil across the electrodes. Electrical
conductivity of soils can span orders of
magnitude, from 30 micro reciprocal
ohms per centimeter (umhos/cm) to more
than 3,000 umhos/cm, with higher values
being in saturated, high-plasticity clays.
A mean conductivity value is often
^MEDIATION
-------
TABLE 3
PERFORMANCE SUMMARY OF ELECTROCHEMICAL SOIL REMEDIATION
TECHNOLOGY APPLIED AT FIVE FIELD SITES IN EUROPE (1987-1994)
Sm= DESCRIPTION
VOLUME (FT3) • CONTAMINANT(S)
INITIAL FINAL
CONCENTRATION CONCENTRATION
(MG/KG) J(MG/KG)
Former paint factory
Operational
galvanizing plant
Former timber plant
Temporary landfill
Military air base
8,1 00 peat/clay
soil
1,350 clay soil
6,750 heavy
clay soil
194,400
argillaceous sand
68,000 clay
Cu
Pb
Zn "; ;
As
,Cd ; :
Cd
Cr
Cu
Ni
Pb
Zn
1,220
>3,780
. ;>i.4«> . .
>250
'• v. ' f^ 'V, ,1
•*; >180 •"
660
7,300
770
860
730
2,600
<200
<280
, . -.600
<30
,. -^
47
755
98
80
108
289
approximately 500 |jmhos/cm. The
voltage gradient often is held to approxi-
mately 1 volt per centimeter (V/cm) in an
attempt to prevent adverse effects of
temperature increases and for other
practical reasons [4]. It may be cost-
prohibitive to attempt to remediate high-
plasticity soils that have high electrical
conductivities. However, for most
deposits having conductivities of 500
fimhos/cm, the daily energy consumption
will be approximately 12 kilowatt hours
(kWh)/cubic meter (m3) per day or about
$0.40/m3 per day, (@ $0.03 /kWh) and
$12/m3 per month. The processing time
will depend upon several factors, includ-
ing the spacing of the electrodes, and the
type of conditioning scheme that will be
used. If an electrode spacing of 4 m is
selected, it may be necessary to process
the site over several months.
Ongoing pilot-scale studies using "real-
world" soils indicate that the energy
expenditures in extraction of metals from
soils may be 500 kWh/m3 or more at
electrode spacings of 1.0 m to 1.5 m [19].
The vendor estimates that the direct cost
of about $15/m3 (@ $0.03 /kWh) sug-
gested for this energy expenditure,
together with the cost of enhancement,
could result in direct costs of $50/m3 or
more. If no other efficient in situ tech-
nology is available to remediate fine-
grained and heterogenous subsurface
deposits contaminated with metals, this
technique would remain potentially
competitive.
-------
PERFORMANCE AND COST SUMMARY >
^£ *siK ~£^«&.* a,^,f-.,. ^.^w^mffZ^'^^^A^.^*
3.2 Geokinetics International, Inc.
Gil has successfully demonstrated in situ
electrochemical remediation of metal-
contaminated soils at several sites in
Europe. Geokinetics, a sister company of
Gil, also has been involved in the electro-
kinetics arena in Europe. Table 3 sum-
marizes the physical characteristics of
five of the sites, including the size, the
contaminant(s) present, and the overall
performance of the technology at each
site [22].
Gil estimates its typical costs for 'turn
key' remediation projects are in the range
of $120-$200/cubic yard (yd3) [22].
3.3 Battelle Memorial Institute
The technology demonstration through
the SITE program was completed in May
1989 [13]. The results indicate that the
electroacoustical technology is techni-
cally feasible for the removal of inor-
ganic species from clay soils (and only
marginally effective for hydrocarbon
removal) [24].
3.4 Consortium Process
The Phase I-Vertical field test of the
Lasagna™ process operated for 120 days
and was completed in May 1995. Scale-
up from laboratory units was successfully
achieved with respect to electrical param-
eters and electro-osmotic flow. Soil
samples taken throughout the test site
before and after the test indicate a 98%
removal of trichloroethylene (TCE)
from a tight clay soil (i.e., hydraulic
conductivity less than IxlO'7 cm/sec).
TCE soil levels were reduced from the
100 to 500 mg/kg range to an average
concentration of 1 mg/kg [25]. Various
treatment processes are being investi-
gated in the laboratory to address other
types of contaminants, including heavy
metals [25].
'af~Sfr ~~ jHgff " vp-;~s^- JK
j%ECT^KI|ffiriC PlMEOIWiW ,
-------
4.0 Analysis and Applications
Electrokinetic remediation may be
applied to both saturated and partially
saturated soils. One problem to over-
come when applying electrokinetic
remediation to the vadose zone is the
drying of soil near the anode. When an
electric current is applied to soil, water
will flow by electro-osmosis in the soil
pores, usually toward the cathode. The
movement of the water will deplete soil
moisture adjacent to the anode, and
moisture will collect near the cathode.
However, processing fluids may be
circulated at the electrodes. The fluids
can serve both as a conducting medium
and as a means to extract or exchange the
species and introduce other species.
Another use of processing fluids is to
control, depolarize, or modify either or
both electrode reactions. The advance of
the process fluid (acid or the conditioning
fluid) across the electrodes assists in
desorption of species and dissolution of
carbonates and hydroxides. Electro-
osmotic advection and ionic migration
lead to the transport and subsequent
removal of the contaminants. The con-
taminated fluid is then recovered at the
cathode.
Spacing of the electrode will depend
upon the type and level of contamination
and the selected current voltage regime.
When higher voltage gradients are
generated, the efficiency of the process
might decrease because of increases in
temperature. A spacing that will generate
a potential gradient in the order of one
V/cm is preferred. The spacing of elec-
trodes generally will be as much as three
meters. The duration of the remediation
will be site-specific. The remediation
process should be continued until the
desired removal is achieved. However, it
should be recognized that, in-cases in
which the duration of treatment is re-
duced by increasing the electrical poten-
tial gradient, the efficiency of the process
will decrease.
The advantage of the technology is its
potential for cost-effective use for both in
situ and ex situ applications. The fact
that the technique requires the presence
of a conducting pore fluid in a soil mass
may have site-specific implication. Also,
heterogeneities or anomalies found at
sites, such as submerged foundations,
rubble, large quantities of iron or iron
oxides, large rocks, or gravel; or sub-
merged cover material, such as seashells,
are expected to reduce removal efficien-
cies [4].
-------
5.0 References
The following vendors were contacted during the preparation of this report:
CONTACTS
Name
Dr. Roger Seals
Steve Clark
Robert Clark
Dr. B. Mason Hughes
(Project Manager)
Lasagna™ Process
Dr. Sa Ho
(Contractor
Principal
Investigator)
Lasagna™ Process
Agency/Company
Electrokinetics, Inc.
Louisiana State University
South Stadium Drive
Baton Rouge, Louisiana, 70803-6100
Telephone Number
(504) 388-6503
Geokinetics International, Inc.
Geokinetics International, Inc
Monsanto
Monsanto
Environmental Science Center
St. Louis, Missouri 63167
(510) 254-2335
(510)254-2335
(314) 694-1466
(314) 694-5179
LITERATURE CITED
1. Pamukcu, S. and J.K. Wittle. 1992. "Electrokinetic Removal of Selected Metals
from Soil." Environment Progress. Volume n, Number 3. Pages 241-250.
2. Acar, Y.B. 1992. "Electrokinetic Cleanups." Civil Engineering. October. Pages
58-60.
3. Mattson, E.D. and E.R. Lindgren. 1994. "Electrokinetics: An Innovative Tech-
nology for In Situ Remediation of Metals. " Proceedings, National Groundwater
Association, Outdoor Acnon Conference. Minneapolis, Minnesota. May.
4. Acar, Y.B., R.J., Gale and others. 1995. "Electrokinetic Remediation: Basics and
Technology Status." Reprinted from Journal of Hazardous Materials, Volume 40.
Pages 117-137.
5. Will, Fritz. 1995. "Removing Toxic Substances from the Soil Using Electro-
chemistry." Chemistry and Industry. May 15. Pages 376-379.
T^Sf" ^ija^^e?»!r •^JW^i^D'sr-.irS. ;:*2?^-=xm5i?^;«*=s?>»"^^~''rJ^>~^^ "2^1
IliSilBIiR^^^ i
-------
6. Rodsand, T. and Y.B. Acar. 1995. "Electrokinetic Extraction of Lead From
Spiked Norwegian Marine Clay." Geoenvironment 2000. Volume 2. Pages 1518-
1534.
7. Lindgren, E.R., M.W. Kozak, and E.D. Mattson. 1992. "Electrokinetic Remedia-
tion of Contaminated Soils: An Update." Waste Management 92. Tuscon,
Arizona. Page 1309.
8. Jacobs, R.A., M.Z. Sengun, and others. 1994. "Model of Experiences on Soil
Remediation by Electric Fields." Journal of Environmental Science and Health.
29A (9).
9. U.S. Patent Office. 1995. Patent #5,433,829, "Process for the Electroreclamation
of Soil Material." Wieberen Pool, July 18.
10. Geokinetics International, Inc., (GU). Undated. "Electro-Remediation: A Clean-
Up Technology for The Present and The Future." GIL Orinda, California.
11. U.S. Department of Energy. 1995. "Development of an Integrated In-Situ Reme-
diation Technology." Technology Development Data Sheet. DE-AR21-
94MC31185.
12. EPA Office of Solid Waste and Emergency Response. 1995. "In Situ Remedia-
tion Technology Status Report: Electrokinetics." EPA 542-K-94-007.
13. "Innovative In Situ Cleanup Processes." 1992. The Hazardous Waste Consult-
ant, September/October.
14. Environmental Technology Network. 1995. Volume 39, Number 1.
15. Lockheed Missiles and Space Company, Inc., Research and Development Divi-
sion, Electroremediation Group, LTD. Undated. "Batch Electrokinetic Remedia-
tion of Contaminated Soils, Muds and Sludges." Prepared for the U.S. Environ-
mental Protection Agency (EPA), Office of Research and Development, Risk
Reduction Engineering Laboratory.
16. Acar, Y.B., E. Ozsu, A. Alshwabkeh, and others. 1996. "In situ Bioremediation
by Electrokinetic Injection." Chemtech. April.
17. Acar, Y.B., R. J. Gale. 1992. "Electrochemical Decontamination of Soils and
Slurries," Commissioner of Patents and Trademarks, Washington, D.C., U.S.
Patent No.: 5,137,608. August 15.
18. Marks, R., Y. B. Acar, R. J. Gale. 1995. "In situ Bioelectrokinetic Remediation
of Contaminated Soils Containing Hazardous Mixed Wastes," Commissioner of
Patents and Trademarks, Washington, D.C., U.S. Patent No. 5,458,747. October
17.
-------
*
19. Acar, Y.B. and A. N.Alshawabkeh. 1996. "Electrokinetic Remediation: I. Pilot-
Scale Tests with Lead Spiked Kaolinite, H Theoretical Model, Journal of
Geotechnical Engineering, Volume 122, Number 3. March. Pages 173-196.
20. Acar, Y.B. Letter addressed to Dr. Walter W. Kovalick, Jr. April 6,1996.
21. Denisor, Gennady and others. 1996. "On the Kinetics of Charged Contaminant
Removal from Soils Using Electric Fields." Journal of Colloid and Interface
Science. Volume 178. Pages 309-323.
22. Geokinetics International, Inc. Marketing brochure provided by John B.
Parkinson Jr., SRI International to Dr. Walter Kovalick, Jr. EPA Technology
Innovation Office.
23. EPA Office of Solid Waste and Emergency Response. 1995. "Emerging Tech-
nology Bulletin: Electrokinetic Soil Processing. Electrokinetics, Inc." EPA 540-
F-95-504.
24. EPA Office of Research and Development. 1994. "Superfund Innovative Tech-
nology Evaluation Program Technology Profiles. Seventh Edition." EPA 540-R-
94-526.
25. EPA Office of Research and Development. 1996. "Lasagna™ Public-Private
Partnership." EPA 542-F-96-010A.
OT>Fr"S5M»T't'
-------
-------
Status of In Situ
Phytoremediation Technology
Phytoremediation is the use of plants to
remove, contain, or render harmless
environmental contaminants. This
definition applies to all biological,
chemical, and physical processes that are
influenced by plants and that aid in the
cleanup of contaminated substances [1].
Plants can be used in site remediation,
both to mineralize and immobilize toxic
organic compounds at the root zone and
to accumulate and concentrate metals and
other inorganic compounds from soil into
aboveground shoots [2]. Although
Phytoremediation is a relatively new
concept in the waste management commu-
nity, techniques, skills, and theories devel-
oped through the application of well-
established agro-economic technologies are
easily transferable. The development of
TABLE 4
OVERVIEW OF PHYTOREMEDIATION TECHNOLOGY
General Characteristics
• Best used at sites with low to moderate disperse metals content and with soil media that will support plant
growth.
• Applications limited to depth of the root zone.
• Longer times required for remediation compared with other technologies.
• Different species have been identified to treat different metals.
'.-'-f i-i-As^;-': '•• >.A~-£'\. :-.-v
tsfeSSfcfi" iMwiBasft^Sfes':*fi ;»> «W ,srf«Ms;^-,*«S:'-v\;:^
Description: Uptake of contaminants from soil into
aboveground plant tissue, which is periodically
harvested and treated.
V* """<*" '
' State Pi
ina for effae$«mess on irfdioao»e
s,-m6tatersoftgo|oi'P the vicinitysfthe damaged -••'
n Cft^mobPJ^heC fteld <
' „_ -CSt
Applicability: Potentially applicable for many
metals. Nickel and zinc appear to be most easily
absorbed. Preliminary results for absorption of
copper and cadmium are encouraging.
proctuce'ci that may require frea
04*' ~ N *~!~_ ^ *" ... . *
f, required,.-
Description: Production of chemical compounds by
the plant to immobilize contaminants at the
interface of roots and soil. Additional stabilization
can occur by raising the pH level in the soil.
Applicability: Potentially applicable for many
metals, especially lead, chromium, and
mercury.
Comments? Long-term maintenance;is required..
" '
-------
plants for restoring sites contaminated with
metals will require the multidisciplinary
research efforts of agronomists, toxicolo-
gists, biochemists, microbiologists, pest
management specialists, engineers, and
other specialists [1,2]. Table 4 presents an
overview of phytoremediation technology.
1.0 Description
Metals considered essential for at least
some forms of life include vanadium (V),
chromium (Cr), manganese (Mn), iron
(Fe), cobalt (Co), nickel (Ni), copper
(Cu), zinc (Zn), and molybdenum (Mo)
[2]. Because many metals are toxic in
concentrations above minute levels, an
organism must regulate the cellular
concentrations of such metals. Conse-
quently, organisms have evolved trans-
port systems to regulate the uptake and
distribution of metals. Plants have
remarkable metabolic and absorption
capabilities, as well as transport systems
that can take up ions selectively from the
soil. Plants have evolved a great diver-
sity of genetic adaptations to handle
potentially toxic levels of metals and
other pollutants that occur in the environ-
ment. In plants, uptake of metals occurs
primarily through the root system, in
which the majority of mechanisms to
prevent metal toxicity are found [4]. The
root system provides an enormous sur-
face area that absorbs and accumulates
the water and nutrients essential for
growth. In many ways, living plants can
be compared to solar-powered pumps
that can extract and concentrate certain
elements from the environment [5].
Plant roots cause changes at the soil-root
interface as they release inorganic and
organic compounds (root exudates) in the
area of the soil immediately surrounding
the roots (the rhizosphere) [6]. Root
exudates affect the number and activity
of microorganisms, the aggregation and
stability of soil particles around the root,
and the availability of elements. Root
exudates can increase (mobilize) or
decrease (immobilize) directly or indi-
rectly the availability of elements in the
rhizosphere. Mobilization and immobili-
zation of elements in the rhizosphere can
be caused by: 1) changes in soil pH; 2)
release of complexing substances, such
as metal-chelating molecules; 3) changes
in oxidation-reduction potential; and 4)
increase in microbial activity [7].
Phytoremediation technologies can be
developed for different applications in
environmental cleanup and are classified
into three types:
• Phytoextraction
• Phytostabilization
• Rhizofiltration
1.1 Phytoextraction
Phytoextraction technologies use
hyperaccumulating plants to transport
metals from the soil and concentrate
them into the roots and aboveground
shoots that can be harvested [1, 2, 6]. A
plant containing more than 0.1 percent of
Ni, Co, Cu, Cr, or one percent of Zn and
Mn in its leaves on a dry weight basis is
called a hyperaccumulator, regardless of
the concentration of metals in the soil [2,
10].
Almost all metal-hyperaccumulating
species known today were discovered on
metal-rich soils, either natural or artifi-
cial, often growing in communities with
metal excluders [2,11]. Actually, almost
all metal-hyperaccumulating plants are
endemic to such soils, suggesting that
-------
hyperaccumulation is an important
ecophysiological adaptation to metal
stress and one of the manifestations of
resistance to metals. The majority of
hyperaccumulating species discovered so
far are restricted to a few specific geo-
graphical locations [2,10]. For example,
Ni hyperaccumulators are found in New
Caledonia, the Philippines, Brazil, and
Cuba. Ni and Zn hyperaccumulators are
found in southern and central Europe and
Asia Minor.
Dried or composted plant residues or
plant ashes that are highly enriched with
metals can be isolated as hazardous waste
or recycled as metal ore. The goal of
phytoextraction is to recycle as "bio-
ores" metals reclaimed from plant ash in
the feed stream of smelting processes.
Even if the plant ashes do not have
enough concentration of metal to be
useful in smelting processes,
phytoextraction remains beneficial
because it reduces by as much as 95
percent the amount of hazardous waste to
be landfilled [14]. Several research
efforts in the use of trees, grasses, and
crop plants are being pursued to develop
phytoremediation as a cleanup technol-
ogy. The following paragraphs briefly
discuss these three phytoextraction
techniques.
The use of trees can result in extraction
of significant amounts of metal because
of their high biomass production. How-
ever, the use of trees in phytoremediation
requires long-term treatment and may
create additional environmental concerns
about falling leaves. When leaves con-
taining metals fall or blow away, recircu-
lation of metals to the contaminated site
and migration off site by wind transport
or through leaching can occur [15].
Some grasses accumulate surprisingly
high levels of metals in their shoots
without exhibiting toxic effects. How-
ever, their low biomass production
results in relatively low yield of metals.
Genetic breeding of hyperaccumulating
plants that produce relatively large
amounts of biomass could make the
extraction process highly effective;
however, such work has not yet begun.
It is known that many crop plants can
accumulate metals in their roots and
aboveground shoots, potentially threaten-
ing the food chain. For example, in May
1980 regulations proposed under RCRA
for hazardous waste (now codified at 40
CFR Part 264) include limits on the
amounts of cadmium and other metals
that can be applied to crops. Recently,
however, the potential use of crop plants
for environmental remediation has been
under investigation. Using crop plants to
extract metals from the soil seems practi-
cal because of their high biomass produc-
tion and relatively fast rate of growth.
Other benefits of using crop plants are
that they are easy to cultivate and they
exhibit genetic stability [14].
1.2 Phytostabilization
Phytostabilization uses plants to limit the
mobility and bioavailability of metals in
soils. Ideally, phytostabilizing plants
should be able to tolerate high levels of
metals and to immobilize them in the soil
by sorption, precipitation, complexation,
or the reduction of metal valences.
Phytostabilizing plants also should
exhibit low levels of accumulation of
metals in shoots to eliminate the possibil-
ity that residues in harvested shoots
might become hazardous wastes [5]. In
addition to stabilizing the metals present
in the soil, phytostabilizing plants also
can stabilize the soil matrix to minimize
erosion and migration of sediment. Dr.
-------
DESCRIPTION
Gary Pierzynski of Kansas State Univer-
sity is studying phytostabilization in
poplar trees, which were selected for the
study because they can be deep-planted
and may be able to form roots below the
zone of maximum contamination.
Since most sites contaminated with
metals lack established vegetation, metal-
tolerant plants are used to revegetate such
sites to prevent erosion and leaching [16].
However, that approach is a containment
rather than a remediation technology.
Some researchers consider
phytostabilization an interim measure to
be applied until phytoextraction becomes
fully developed. However, other re-
searchers are developing
phytostabilization as a standard protocol
of metal remediation technology, espe-
cially for sites at which removal of
metals does not seem to be economically
TABLE 5
TYPES OF PHYTOREMEDIATION TECHNOLOGY:
ADVANTAGES AND DISADVANTAGES
w. ..." "at1? ' . .•
: - —TYPE OF • .
PHYtORERflEIDATION
Phytoextraction by trees
Phytoextraction by grasses
Phytoextraction by crops
Phytostabilization
Rhizofiltration
High biomass production
High accumulation
growth rate
High biomass and
increased growth rate
No disposal of contaminated*
biomass required
Readily absorbs metals
Potential for off-site migration and
leaf transportation of metals to surface
Metals are concentrated in plant
biomass and must be disposed of
eventually.
"Low biomass production""§nd slow;
Metals aje concentrated m pfinf
i and must be dtsposed-of >*,, •-<,
eventually, ** ' :" ^ ->- -"
% ^/^ " * **--'
Potential threat to the food chain
through ingestion by herbivores
Metals are concentrated in plant
biomass and must be disposed of
eventually.
,'Refnaining liability issues, iiicluding'
" rnaintenance forjndef inttl #§fIpd of
time* (contalnmenfta|ier than removal)
Applicable for treatment of water only
Metals are concentrated in plant
biomass and must be disposed of
eventually.
-------
feasible. After field applications con-
ducted by a group in Liverpool, England,
varieties of three grasses were made
commercially available for
phytostabilization [5]:
• Agrostis tenuis, cv Parys for
copper wastes
• Agrosas tenuis, cv Coginan for
acid lead and zinc wastes
• Festuca rubra, cv Merlin for
calcareous lead and zinc wastes
1.3 Rhizofiltration
One type of rhizofiltration uses plant
roots to absorb, concentrate, and precipi-
tate metals from wastewater [5], which
may include leachate from soil.
Rhizofiltration uses terrestrial plants
instead of aquatic plants because the
terrestrial plants develop much longer,
fibrous root systems covered with root
hairs that have extremely large surface
areas. This variation of
phytoremediation uses plants that remove
metals by sorption, which does not
involve biological processes. Use of
plants to translocate metals to the shoots is
a slower process than phytoextraction [16].
Another type of rhizofiltration, which is
more fully developed, involves construc-
tion of wetlands or reed beds for the treat-
ment of contaminated wastewater or
leachate. The technology is cost-effective
for the treatment of large volumes of
wastewater that have low concentrations of
metals [16]. Since rhizofiltration focuses
on treatment of contaminated water, it is
not discussed further in this report.
Table 5 presents the advantages and
disadvantages of each of the types of
phytoremediation currently being re-
searched that are categorized as either
phytoextraction on phytostabilization [5].
1.4 Future Development
Faster uptake of metals and higher yields
of metals in harvested plants may be-
come possible through the application of
genetic engineering and/or selective
breeding techniques. Recent laboratory-
scale testing has revealed that a geneti-
cally altered species of mustard weed can
uptake mercuric ions from the soil and
convert them to metallic mercury, which
is transpired through the leaves [23, 24,
25]. Improvements in phytoremediation
may be attained through research and a
better understanding of the principles
governing the processes by which plants
affect the geochemistry of their soils. In
addition, future testing of plants and
microflora may lead to the identification
of plants that have metal accumulation
qualities that are far superior to those
currently known [17].
2.0 Overview of Status
Plants have been used to treat wastewater
for more than 300 years, and plant-based
remediation methods for slurries of
dredged material and soils contaminated
with metals have been proposed since the
mid-1970s [1, 13]. Reports of successful
remediation of soils contaminated with
metals are rare, but the suggestion of
such application is more than a decade
old, and progress is being made at a
number of pilot test sites [11]. Success-
ful phytoremediation must meet cleanup
standards in order to be approved by
regulatory agencies.
No full-scale applications of
phytoremediation have been reported.
One vendor, Phytotech, Inc., is develop-
-------
SUMMARY
TABLE 6
EXAMPLES OF METAL HYPERACCUMULATORS
ZN
Cu
Ni
Pb
Co
Thlaspi calaminare
Viola species
Aeolanthus biformifolius
Phyllanthus serpentinus
Alyssum bertoloniand 50
other species of alyssum
Seberiia acuminata
Stackhousia tryonii
Brassucajuncea
Haumaniastrum robertii
<3
1
1
3.8
>3
25 (in latex)
4.1
<3.5
1
Germany
Europe
Zaire
New Caledonia
Southern Europe and
Turkey
New Caledonia
Australia
India
Zaire
ing phytostabilization for soil remedia-
tion applications. Phytotech also has
patented strategies for phytoextraction
and is conducting several field tests in
Trenton, New Jersey and in Chernobyl,
Ukraine [14]. Also, as was previously
mentioned, a group in Liverpool, En-
gland has made three grasses commer-
cially available for the stabilization of
lead, copper, and zinc wastes [5].
3.0 Performance and Cost
Summary
Currently, because it has not been used in
any full scale applications, the potential
of phytoremediation for cleanup of
contaminated sites cannot be completely
ascertained. However, a variety of new
research approaches and tools are ex-
panding understanding of the molecular
and cellular processes that can be em-
ployed through phytoremediation [3].
3.1 Results of Testing
Potential for phytoremediation
(phytoextraction) can be assessed by
comparing the concentration of contami-
nants and volume of soil to be treated
with the particular plant's seasonal
productivity of biomass and ability to
accumulate contaminants. Table 6 lists
selected examples of plants identified as
metal hyperaccumulators and their native
countries. [10, 12]. If plants are to be
effective remediation systems, one ton of
plant biomass, costing from several
hundred to a few thousand dollars to
produce, must be able to treat large
-------
volumes of contaminated soil. For
metals that are removed from the soil and
accumulated in aboveground biomass,
the total amount of biomass per hectare
required for soil cleanup is determined by
dividing the total weight of metal per
hectare to be remediated by the accumu-
lation factor, which is the ratio of the
accumulated weight of the metal to the
weight of the biomass containing the
metal. The total biomass per acre then
can be divided by the productivity of the
plant (tons[t]/hectare[ha]/year[yrj) to
determine the number of years required
to achieve cleanup standards—a major
determinant of the overall cost and
feasibility of phytoremediation [3].
As discussed earlier, the amount of
biomass is one of the factors that deter-
mines the practicality of
phytoremediation. Under the best cli-
matic conditions, with irrigation, fertili-
zation, and other factors, total biomass
productivity can approach 100 t/ha/yr.
One unresolved issue is the trade-off
between accumulation of toxic elements
and productivity [20]. In practice, a
maximum harvest biomass yield of
10 to 20 t/ha/yr is likely, particularly for
plants that accumulate metals.
These values for productivity of biomass
and the metal content of the soil would
limit annual capacity for removal of
metals to approximately
10 to 400 kg/ha/yr, depending on the
pollutant, species of plant, climate, and
other factors. For a target soil depth of
30 cm (4,000 t/ha), this capacity amounts
to an annual reduction of 2.5 to 100
mg/kg of soil contaminants. This rate of
removal of contamination often is accept-
able, allowing total remediation of a site
over a period of a few years to several
decades [3].
3.2 Cost
The practical objective of
phytoremediation is to achieve major
reductions in the cost of cleanup of
hazardous sites. Salt and others [5] note
the cost-effectiveness of
phytoremediation with an example:
Using phytoremediation to clean up one
acre of sandy loam soil to a depth of 50
cm typically will cost $60,000 to
$100,000, compared with a cost of at
least $400,000 for excavation and dis-
posal storage without treatment [5]. One
objective of field tests is to use commer-
cially available agricultural equipment
and supplies for phytoremediation to
reduce costs. Therefore, in addition to
their remediation qualities, the agronomic
characteristics of the plants must be
evaluated.
The processing and ultimate disposal of
the biomass generated is likely to be a
major percentage of overall costs, par-
ticularly when highly toxic metals and
radionuclides are present at a site.
Analysis of the costs of phytoremediation
must include the entire cycle of the
process, from the growing and harvesting
of the plants to the final processing and
disposal of the biomass. It is difficult to
predict costs of phytoremediation, com-
pared with overall cleanup costs at a site.
Phytoremediation also may be used as a
follow-up technique after areas having
high concentrations of pollutants have
been mitigated or in conjunction with
other remediation technologies, making
cost analysis more difficult.
3.3 Future directions
Because metal hyperaccumulators gener-
ally produce small quantities of biomass,
they are unsuited agronomically for
phytoremediation. Nevertheless, such
-
*» 37
-------
ft II tf I *
PERFORMANCE AMD COST SUMMARY
™j—°..^;Ji^iaivAth.fr-1 t« **« *,~iw>ia& J ™»™SS4» —fe, 3*
plants are a valuable store of genetic and
physiologic material and data [1]. To
provide effective cleanup of contami-
nated soils, it is essential to find, breed,
or engineer plants that absorb, translo-
cate, and tolerate levels of metals in the
0.1- to 1.0-percent range. It also is
necessary to develop a methodology for
selecting plants that are native to the
area.
Currently, phytoremediation is generally
not commercially available (although
three grasses are commercially available
for the stabilization of lead, copper, and
zinc wastes [5]). Relatively few research
projects and field tests of the technology
have been conducted. An integrated
approach that involves basic and applied
research, along with consideration of
safety, legal, and policy issues, will be
necessary to establish phytoremediation
as a practicable cleanup technology [1].
According to a 1994 DOE report titled
"Summary Report of a Workshop on
Phytoremediation Research Needs," three
broad areas of research and development
can be identified for the in situ treatment
of soil contaminated with metals [3]:
• Mechanisms of uptake, transport,
and accumulation: Research is
needed to develop better under-
standing of the use of physiologi-
cal, biochemical, and genetic
processes in plants. Research on
the uptake and transport mecha-
nisms is providing improved
knowledge about the adaptability
of those systems and how they
might be used in
phytoremediation.
• Genetic evaluation of
hyperaccumulators: Research is
being conducted to collect plants
growing in soils that contain high
levels of metals and screen them
for specific traits useful in
phytoremediation. Plants that
tolerate and colonize environ-
ments polluted with metals are a
valuable resource, both as candi-
dates for use in phytoremediation
and as sources of genes for
classical plant breeding and
molecular genetic engineering.
• Field evaluation and validation:
Research is being conducted to
employ early and frequent field
testing to accelerate implementa-
tion of phytoremediation tech-
nologies and to provide data to
research programs. Standardiza-
tion of field-test protocols and
subsequent application of test
results to real problems also are
needed.
Research in this area is expected to grow
over the next decade as many of the
current engineering technologies for
cleaning surface soil of metals are costly
and physically disruptive.
Phytoremediation, when fully developed,
could result in significant cost savings
and in the restoration of numerous sites
by a relatively noninvasive, solar-driven,
in situ method that, in some forms, can
be aesthetically pleasing [1].
-------
4.0 Analysis of Applications
Phytoremediation is in the early stage of
development and is being field tested at
various sites in the U.S. and overseas for
its effectiveness in capturing or stabiliz-
ing metals, including radioactive wastes.
Limited cost and performance data are
currently available. Phytoremediation has
the potential to develop into a practicable
remediation option at sites at which
contaminants are near the surface, are
relatively nonleachable, and pose little
imminent threat to human health or the
environment [1]. The efficiency of
phytoremediation depends on the charac-
teristics of the soil and the contaminants;
these factors are discussed in the sections
that follow.
4.1 Site Conditions
The effectiveness of phytoremediation
generally is restricted to surface soils
within the rooting zone. The most
important limitation to phytoremediation
is rooting depth, which can be 20, 50, or
even 100 cm, depending on the plant and
soil type. Therefore, one of the favorable
site conditions for phytoremediation is
contamination with metals that is located
at the surface [3].
The type of soil, as well as the rooting
structure of the plant relative to the
location of contaminants can have strong
influence on uptake of any metal sub-
stance by the plant. Amendment of soils
to change soil pH, nutrient compositions,
or microbial activities must be selected hi
treatability studies to govern the effi-
ciency of phytoremediation. Certain
generalizations can be made about such
cases; however, much work is needed in
this area [1]. Since the amount of biom-
ass that can be produced is one of the
limiting factors affecting
phytoremediation, optimal climatic
conditions, with irrigation and fertiliza-
tion of the site, should be considered for
increased productivity of the best plants
for the site [3].
4.2 Waste Characteristics
Sites that have low to moderate contami-
nation with metals might be suitable for
growing hyperaccumulating plants,
although the most heavily contaminated
soils do not allow plant growth without
the addition of soil amendments. Unfor-
tunately, one of the most difficult metal
cations for plants to translocate is lead,
which is present at numerous sites in
need of remediation. Although signifi-
cant uptake of lead has not yet been
demonstrated, one researcher is experi-
menting with soil amendments that make
lead more available for uptake [5].
Capabilities to accumulate lead and other
metals are dependent on the chemistry of
the soil in which the plants are growing.
Most metals, and lead in particular, occur
in numerous forms in the soil, not all of
which are equally available for uptake by
plants [1]. Maximum removal of lead
requires a balance between the nutritional
requirements of plants for biomass
production and the bioavailability of lead
for uptake by plants. Maximizing avail-
ability of lead requires low pH and low
levels of available phosphate and sulfate.
However, limiting the fertility of the soil
in such a manner directly affects the
health and vigor of plants [1].
-------
ten
M-J
REFERENCES
, ~f, ^*Si*,
5.0 References
The following vendors were contacted during the preparation of this report:
CONTACTS
Name
Burt Ensley
Company/Research Center
Phytotech, Inc.
One Deer Park Drive
Suite 1
Monmouth Junction, NJ 08852
Telephone Number
(908) 438-0900
Dr. S. Cunningham Central Research
and Development
DuPont Glasgow, Suite 301
Newark, DE 19714
(302) 451-9940
LITERATURE CITED
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with Green Plants: An Overview." In Vitro Cell. Dev. Biol. Tissue Culture
Association. Volume 29. Pages 207-212.
2. Raskin, I. and others. 1994. "Bioconcentration of Metals by Plants."
Environmental Biotechnology. 5:285-290.
3. U.S. Department of Energy. 1994. Summary Report of a Workshop on
Phytoremediation Research Needs. Santa Rosa, California. July 24-26.
4. Goldsbrough, P. and others. 1995. "Phytochelatins and Metallothioneins:
Complementary Mechanisms for Metal Tolerance?" Abstract Book: Fourteenth
Annual Symposium 1995 in Current Topics in Plant Biochemistry, Physiology and
Molecular Biology. Pages 15-16.
5. Salt, D.E. and others. 1995. "Phytoremediation: A Novel Strategy for the
Removal of Toxic Metals from the Environment Using Plants." Biotechnology.
Volume 13. May. Pages 468-474.
6. Kumar, P.B. A; and others. 1995. "Phytoextraction: The Use of Plants to Re-
move Metals from Soils." Environmental Science & Technology. Volume 29.
Pages 1232-1238.
-------
7. Morel, I.L. 1995. "Root Exudates and Metal Mobilization." Abstract Book:
Fourteenth Annual Symposium 1995 in Current Topics in Plant Biochemistry,
Physiology and Molecular Biology. Pages 31 -32.
8. Farago, M. 1981. "Metal-Tolerant Plants." Chemtech. Pages 684-687.
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Copper...and Zinc." New Scientist. Volume 1603. Pages 44-48.
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Contaminated Soil and Sediments." Prepared for 1994 Publication the Soil Sci-
ence Society of America from Proceedings of a Symposium in Chicago, Illinois.
November 1993.
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Lures Venture Funds." The Bioremediation Report. Volume 4, Number 1.
Washington, D.C. January.
15. PRC Environmental Management, Inc. (PRC). 1995. Interview about
phytoremediation between Dana H. Mun, Chemical Engineer, PRC, and Dr.
A.J.M. Baker, Scientist. June 8.
16. Ensley, B.D. 1995. "Will Plants Have a Role in Bioremediation?" Abstract
Book: Fourteenth Annual Symposium 1995 in Current Topics in Plant Biochem-
istry, Physiology and Molecular Biology. Pages 1-2.
17. PRC. 1995. Record of telephone conversation about phytoremediation between
Dana H. Mun, Chemical Engineer, PRC, and S.D. Cunningham, Scientist, Central
Research and Development, E.I. du Pont de Nemours and Company. June 26.
18. Chaney, R.L. and J.S. Angel. 1994. "Green Remediation: Potential Use of
Hyperaccumulator Plant Species to Phyto-remediate Soils Polluted with Zinc and/
or Cadmium" (The Revival Field Project).
-------
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19. Berti, W.R. and S.D. Cunningham. 1993. "Remediating Soil Pb With Green
Plants." Presented at the International Conference of the Society for Environmen-
tal Geochemistry and Health. New Orleans, Louisiana. July 25-27.
20. Parry, I. 1995. "Plants Absorb Metals." Pollution Engineering. February. Pages
40-41.
21. Cunningham, Scott D. and David W. Ow. 1996. "Promises and Prospects of
Phytoremediation." Plant Physiology. Volume 110. Pages 715-719.
22. Vasudev, Dev and others. 1996. "Removal of Radionuclide Contamination from
Water by Metal Accumulating Terrestrial Plants." Prepared for presentation at the
Spring National Meeting at New Orleans, Louisiana. February 25-29. Unpub-
lished.
23. Rugh, Clayton and others. 1996. "Mecuric Ion Reduction and Resistance in
Transgenic Arabidopsis Thaliana Plants Expressing a Modified Bacterial
MerAGene." Proceedings of the National Academy of Sciences. Volume 93.
April 1996. Pages 3182-3187.
24. "Phytoremediation Gets to the Root of Soil Contamination." 1996. The Hazard-
ous Waste Consultant. May/June.
25. Langreth, Robert. 1996. "Altered Weeds Eat Mercury Particles in Lab Experi-
ments on Toxic Waste." The Wall Street Journal. April 16.
-------
«*„ ,1*
DESCRIPTION
Status of Soil Flushing Technology
One approach to treating contaminated
sites is physical separation and removal
of the contaminants from the soil. Physi-
cal separation can be achieved in situ by
introducing a fluid to the soil that will
flush out the contaminants, leaving the
soil matrix intact. In situ soil flushing is
the extraction of contaminants from the
soil with water or other suitable aqueous
solutions. In situ soil flushing has been
used most often at sites contaminated
with organics. This chapter focuses on
the application of in situ soil flushing to
sites contaminated with metals.
Table 7 presents an overview of soil
flushing technology.
1.0 Description
Soil flushing techniques promote mobil-
ity and migration of metals by solubiliz-
ing the contaminants so that they can be
extracted. Soil flushing is an in situ
process that is accomplished by applying
the flushing fluid to the surface of the site
or injecting it into the contaminated zone.
The resulting leachate then typically is
recovered from the underlying groundwa-
ter by pump-and-treat methods. Figure 8
presents schematics of different soil
flushing systems [1,2].
Soil flushing can solubilize contaminants
using either water as the flushing fluid or
chemical additives to enhance the solu-
bility of the contaminant. Water alone
can be used to remove certain water-
soluble contaminants (for example,
hexavalent chromium). The use of soil
flushing chemicals may involve adjusting
the soil pH, chelating metal contami-
nants, or displacing toxic cations with
nontoxic cations. The in situ flushing
process requires that the flushing fluids
be percolated through the soil matrix.
The fluids can be introduced by surface
flooding, surface sprinklers, leach fields,
vertical or horizontal injection wells,
basin infiltration systems, or trench
infiltration systems.
Several chemical and physical phenom-
ena control the mobility of metals in
soils. The finer-sized soil fractions
(clays, silts, iron and manganese oxides,
and organic matter in soil) can bind
metals electrostatically as well as chemi-
cally [3]. Numerous soil factors affect
sorption of metals and their migration in
the subsurface. Such factors include pH,
soil type, cation exchange capacity
(CEC), particle size, permeability, spe-
cific types and concentrations of metals,
and types and concentrations of organic
and inorganic compounds in solutions.
Generally, as the soil pH decreases,
solubility and mobility of cationic metals
increase. In most cases, mobility and
sorption of a metal are likely to be con-
trolled by clay content in the subsoils and
by the organic fraction in topsoils. Clays
can adsorb metals present in the soils. It
has been reported that surface soils high
in organic matter retained significantly
more metal than subsurface soils that
contained less organic matter [4]. Or-
ganic matter in soil is of significant
importance because of its effect on CEC
[5]. CEC, which measures the extent to
which cations in the soil can be ex-
changed, often is used as an indication of
a soil's capacity to immobilize metals [6].
Once the infiltrated or percolated solution
has flushed the contaminants to a certain
location, the contaminated fluids must be
extracted. Extraction techniques include
vacuum extraction methods in the vadose
zone and pump-and-treat systems in the
saturated zone.
-------
TABLE 7
OVERVIEW OF SOIL FLUSHING TECHNOLOGY
General Characteristics
• Best used in soils with high permeability.
• Different delivery systems available to introduce flushing solutions.
• Cost is primarily influenced by potential need for interim containment, the depth of contamination, and the
time required for operation.
• Associated risk of contamination of underlying aquifer with unrecovered flushing solution that contains
solubilized contaminants; best used at sites with aquifers that have low specific yields.
Description: Use of water to solubilize the
contaminants prior to extraction.
Status: Commercial.
Applicability: Chromium (VI); potentially applicable
for other water-soluble metals.
Comments: Applicable only for water-soluble metals;
focus of water flushing often is on organics.
In situ flushing has been selected at 7 Superfund
sites at which soils are contaminated with metals
(most of the sites also are contaminated with
organics).
Description: Use of a chemical reagent to solubilize
the contaminants for extraction.
Status: Umitedjesea.rcri; 4 * "--J_" ^"/'
Applicability: Bench-scale: lead, uranium.
Comments:-. .,- .,%,- -^<-.
•Some^mall-soate testing has been -•« -•/,» *
conducted with 'chetators as thVprlmajy reagent
for removal of metals from soils;-the results" of Hjpae
tests have 'flat le&to further testing on a largetscafe.
»pH adjusters and~chemfca| binders also are being".-
studied for potentiaf applicability to metals. " ;
-Surfactants are primarily targeted for removal of ''~>
Recovered groundwater and flushing
fluids containing the desorbed contami-
nants may require treatment to meet
appropriate discharge standards before
such fluids are recycled or released to
publicly owned wastewater treatment
works or receiving streams. If state
regulations so allow, recovered fluids
should be reused in the flushing process
to reduce disposal costs.
The treatment system will be configured
to remove specific contaminants of
concern. For treatment of inorganics, the
system may include standard precipita-
tion systems, electrochemical exchange,
ion exchange, or ultrafiltration systems.
The contaminants of concern may in-
clude organics and inorganics in the same
waste stream. In posttreatment, once the
recovery system (that is, pump-and-treat
system) has been shut down, it may be
necessary to control infiltration through
the use of caps or covers to prevent
further migration of residual contami-
nants.
-------
* ~f^*f&4* ~ "1?^ * J% —' ^
^^J5JMiEW< OF $1*011
Spray Application
Low Permeability
Zone
Figure 8. Typical Soil Flushing System (Surface Sprinklers)
2.0 Overview of Status
For treatment of metals, soil flushing has
been employed for a limited number of
projects, using the treated effluent from a
pump-and-treat operation for reinjection
and improved mobilization of contami-
nants. The use of chelating additives for
treating metals in soil has not yet been
found to be effective.
Limited information is available on the
use of soil flushing to remediate soils
contaminated with metals. Most infor-
mation is related to treatment of organic
contaminants rather than metals. Soil
flushing has been selected at seven
Superfund sites which contain metals. At
two sites, Lipari Landfill in New Jersey
and the United Chrome Products site in
Oregon, in situ soil flushing is opera-
tional [7]. At one other site, in situ soil
flushing is listed as the technology in
design, and at four other sites, in situ
flushing is listed as the technology in the
predesign stage [7].
One literature reference summarizes a
bench-scale soil flushing technology
called metal extraction that was devel-
oped by Scientific Ecology Group, Inc. of
Pittsburgh, Pennsylvania for removing
heavy and radioactive metals from soil
and groundwater through cation displace-
ment [10]. Another literature reference
describes the bench-scale use of organic
-------
OVERVIEW OF STATUS
and inorganic flushing agents to remove
lead from in situ soils. Solutions of
hydrochloric acid (HC1), ethylenediamine
tetraacetic acid (EDTA), and calcium
chloride (CaCl2) were used as flushing
agents [3]. A third literature reference
describes an ongoing, full-scale in situ
soil flushing technology that uses water
as the flushing agent to treat chromium
[8]. The information in these representa-
tive references is summarized in the
following paragraphs.
2.1 Cation Displacement
The metal extraction method is based on
demonstrated in situ uranium mining
technology. Continuous injection and
recapture of an extraction solution
flushes heavy or radioactive metals from
the subsurface. The metal extraction
process consists of the following steps
[11]:
• Introduction of extraction solu-
tion: The remedial process begins
with the injection of a solution
containing sufficient cation
concentrations to displace the
contaminants from the soil.
• Removal of contaminants: The
solution migrates through the
treatment zone, selectively dis-
placing the target contaminants.
Cations that occur naturally, or
that are present in the extraction
solution, remain in the soil.
• Recovery of solution: The con-
taminated solution is pumped to
surface equipment through a
network of recovery wells. A
subsequent treatment process
precipitates the contaminants.
• Stabilization of residual contami-
nants: If necessary, a stabilizing
solution is injected after soil
flushing has been completed. The
solution reacts with the remaining
contaminants, produces an immo-
bile species, and prevents further
migration of residual metals.
2.2 Lead Removal
Organic and inorganic flushing agents to
remove lead have been tested on a small
scale. In a bench-scale experiment,
contaminated soil columns (coarse, sandy
loam with a favorable hydraulic conduc-
tivity and relatively low organic content)
were flushed separately with solutions of
0.1 moles per liter (M) HC1, 0.01 M
EDTA, and 1.0 M CaCl2. Each soil
column was packed under saturated
conditions by maintaining the water level
above each successive soil layer during
the packing procedures. Significant
amounts of lead were removed from the
soil when HC1 and EDTA were used.
When HC1 and EDTA were used as
flushing solutions, the pH levels of the
effluent appeared to be related directly to
the rate of removal of lead. The mecha-
nisms of lead removal appeared to be
desorption caused by a decrease in pH,
dissolution of Pb(OH)2 or other lead
precipitates, metal chelation, and cation
exchange for HC1, EDTA, and CaCl2,
respectively [3].
This approach is not practical for use in
full scale applications due to the high
costs of reagents.
2.3 Chrome Flushing
A full-scale in situ soil flushing technol-
ogy is being implemented at the United
Chrome Products site, a Superfund site in
Corvallis, Oregon. The site is a former
industrial hard-chrome electroplating
shop. Leaks from plating tanks and the
discharge of rinse water into a disposal
;„>. SollflOSHIpl
-------
SjATOS
pit during the shop's operation from 1956
to 1985 contaminated soil and groundwa-
ter underlying the facility. Contamina-
tion of soil at levels higher than 60,000
mg/kg chromium and contamination of
groundwater at levels exceeding 19,000
mg/L chromium were detected in areas
adjacent to the plating tanks. In 1985,
EPA began remediation activities that
have continued to the present time
(1996). Those activities include con-
struction of two infiltration basins to
flush contaminated soils, a 23-well
groundwater extraction network in low-
permeability soils, and an injection and
groundwater extraction network in a deep
gravel aquifer, as well as on-site treat-
ment of wastewaters containing high
concentrations of chromium [8].
At this site, Cr(III) is found in high
concentrations in the soils of the upper
zone, but, because of its very low solubil-
ity, it is only a minor groundwater con-
taminant. In contrast, Cr(VI), a potential
carcinogen, is found in high concentra-
tions in the upper zone, aquitard soils,
and groundwater, because of its high
solubility in water. EPA has established
a maximum concentration level (MCL)
of 0.05 mg/L (total chromium) as a
drinking-water standard. Thus far,
chromium levels in groundwater have
been reduced from more than 5,000 mg/L
to less than 50 mg/L in areas of high
concentration [8].
This in situ, full-scale cleanup is unique
because: 1) soil flushing has been ap-
plied in low-permeability silt soil, 2) both
the shallow and deep aquifer have been
treated, and 3) flushing of the clay
aquitard has been accomplished indi-
rectly by using the deep aquifer injection
wells in conjunction with the upper zone
extraction wells to create upward vertical
gradients.
Three methods of infiltration have been
employed: infiltration basins, an infiltra-
tion trench, and injection wells. The two
basins are abovegrade structures that
have open bottoms that permit infiltration
of water to the underlying soils. They
were placed at the sites of the highest
observed levels of soil contamination (the
former plating tank and disposal pit
areas) [8]. The basins have been success-
ful in delivering water to the upper zone,
averaging approximately 7,600 gallons
per day in Basin No. 1 and 3,000 gallons
per day in Basin No. 2 during the dry
summer months. During the winter
months, infiltration rates decrease to 50
percent or less of the summer rates [8].
The infiltration trench was constructed
approximately 22 months after the
project began. The trench is positioned
and operated primarily to increase dis-
charge rates of the extraction wells along
the longitudinal axis of the plume during
the dry summer months. The trench is
approximately 100 feet long and 8 feet
deep, and a float valve maintains the
water level at 4 feet below grade. Infil-
tration rates have averaged 2,500 gallons
per day [8].
Another type of groundwater recharge
used at the site is water injection. To
reverse the downward vertical gradient
present between the upper zone and the
deep aquifer, clean water has been in-
jected into the deep aquifer through two
wells [9].
2.4 Twin Cities Army Ammunition Plant
In 1993, the Twin Cities Army Ammuni-
tion Plant (TCAAP) soil remediation
demonstration project for removal and
recovery of metals (lead was the main
contaminant) began in New Brighton,
Minnesota. The TCAAP project used the
IH1HB'
-------
•Z
COGNIS TERRAMETR process and was
the first project in which cleaned soil
from a soil washing process was returned
on-site. Although the COGNIS process
currently is operated as a soil washing
system rather than an in situ soil flushing
technology, research is being considered
to assess the viability of adapting the
COGNIS process for in situ remediation
applications [10]. No process water is
discharged during operation of the
COGNIS process; all leachant is recycled
within the plant. Targets for removal of
lead were not achieved; therefore, the
treatment was only partially successful.
3.0 Performance and Cost
Summary
According to Scientific Ecology Group,
Inc., the metal extraction technology
demonstrates removal efficiencies as
high as 90 percent. Concentrations of
uranium in groundwater of 5 to 20 mg/L
were reduced to 1 to 2 mg/L. Groundwa-
ter contaminated with 250 to 500 mg/L of
ammonium contained only 10 to 50 mg/L
after treatment [11].
In the soil column experiment, initial
concentrations of lead during the bench-
scale study were 500 to 600 mg/kg. Lead
removal efficiencies for HC1, EDTA, and
CaCL, were 96, 93, and 78 percent,
respectively. In the soil used in the
study, background concentrations of lead
were approximately 20 mg/kg. Final
concentrations of lead, after flushing with
the three test solutions, were 23.3 mg/kg
(HC1), 37.8 mg/kg (EDTA), and 135.6
mg/kg (CaCl2) [3]. It should be noted
that, if the soils contain relatively high
levels of calcium, substantial amounts of
the HC1 flushing solution would be
consumed in neutralization reactions.
At the United Chrome Products site, the
use of water as a flushing solution to
remove chromium (VI) from in situ soils
appears to be a successful treatment
option. The full-scale cleanup has
achieved hydraulic containment of the
plume, while extracting significant
amounts of chromium from the subsur-
face. Table 8 presents a summary of
recent available performance data [13].
The performance of the two infiltration
basins constructed at the United. Chrome
Products site has been confirmed by the
increase in pumping rates and concurrent
decreases in concentrations of Cr(VI)
observed in the extraction wells around
the basins. In many of the wells, pump-
ing rates have increased from less than
0.5 gallon per minute (gpm) to 2 or more
gpm [8]. Concentrations of Cr(VI)
decreased from more than 2,000 mg/L to
approximately 18 mg/L [13].
According to the developers of the metal
exchange process, the cost of such a
project is estimated to be approximately
50 percent of that of a typical pump-and-
treat method.
Because in situ soil flushing has had only
limited field application, it is difficult to
obtain comprehensive, detailed estimates
of the cost of this treatment technology.
The factors that most significantly affect
costs are the initial and target concentra-
tions of contaminants, permeability of the
soil, and depth of the aquifer [11].
Capital costs for chemically enhanced
solubilization (CES) are similar to those
for traditional pump-and-treat systems,
except for the initial expense of equip-
ment needed to handle the flushing
solution. Operating costs also are simi-
lar, except for the cost of handling and
replacement of flushing solutions and
-------
additives. Overall, for the life of the
treatment process, CES should be signifi-
cantly less expensive than pump-and-
treat systems because of the much shorter
time frames for treatment and smaller
volumes of water to be extracted and
treated [2].
A hypothetical analysis in a recent
engineering monograph on soil washing
and soil flushing compares cost and time
estimates for CES with those for pump-
and-treat systems. Based on interpreta-
tion of data from a test site, the effective
aqueous solubility of a contaminant was
compared to the amount of flushing
solution needed to solubilize the contami-
nant. The pore volumes required by the
two systems to attain similar levels of
cleanup differed dramatically; the CES
system would require 21 pore volumes
and the pump-and-treat system would
require more than 2,000 pore volumes.
Likewise, the time frames for treatment
using the two systems also differed.
Using the specified injection rates of the
two systems to calculate time required
for treatment, the CES system would
require 4 years and the pump-and-treat
system would require 400 years. [2].
TABLE 8
UNITED CHROME PRODUCTS SUPERFUND SITE
EXTRACTION AND TREATMENT SYSTEM SUMMARY
AUGUST 1988 THROUGH DECEMBER 1995
Groundwater Extracted
influent cr (VI) concentration
Range
Mass of Cr (VI) Removed
Infiltration Recharge
Average Effluent Cr (VI)
Concentration
58,000,000 gal
•1 /1ft mn/l fn •! QOO mn/l
I*K> rng/L TO i ,y^o rny/L
31.200R)
4,700,000 gal
* > * V*^"""^ t A^S?^ *"*" ' ~^ '' * '
' . - ' ^ye^A^^J*- ,/ >-'s^
j?~>s % jiffej ^ >v ', ^ 5>ti>«? '" *"? '""
11, 400 gal
"7 "^ > \T /,„ s-<- " ^ f!" * ^ *
V - % ^> ^ " -^^'<.*^- ^
^^.^•» / , ^,^^, •*$" (' \™~
^?s vs , o^--;s -v « '-*»v» ^ '^! "N ^
41 Ib
8,000 gal
1. 7 mg/L (monthly)
-------
T ANALYSIS
4.0 Analysis of Applications
The performance of an in situ soil flush-
ing system depends largely upon the
amount of contact achieved between the
flushing solution and the contaminants.
The appropriateness of the flushing
solution, the soil adsorption coefficients
of the contaminants, and the permeability
of the soil are also key factors.
Best results will be achieved in highly
permeable soils.
The following types of data are required
to support selection of the flushing
solution and to predict the effectiveness
of soil flushing:
• Soil hydrogeology (physical and
chemical properties of the soil),
subsurface vertical and horizontal
flow and velocity, characteristics
of the aquifer, and vadose zone
saturation
• Areal and vertical concentration
gradients for contaminants.
Effective application of the process
requires a sound understanding of soil
chemistry (the manner in which target
contaminants are bound to soil), relative
permeability, and hydrogeology. In
general, soil flushing is most effective in
homogeneous, permeable soils (sands
and silty sands with permeabilities
greater than IxlO'3 centimeters per
second [cm/sec]). The relationships
among capillary processes, water content,
and hydraulic conductivity must be
understood before any flushing solution
can be used effectively. In addition,
because soil flushing increases the
mobility of contaminants, the hydrology
of the site must be well understood.
-------
REFERENCES,
5.0 References
The following vendors were contacted during the preparation of this report:
CONTACTS
Name
Donald R. Justice
Ken Wyatt
Agency/Company
Horizontal Technologies, Inc.
(Soil Flushing- In Situ)
Surtek, Inc.
(Soil Flushing - In Situ
Surfactant Enhanced Recovery)
Telephone Number
813/995-8777
303/278-0877
LITERATURE CITED
1. DoD Environmental Technology Transfer Committee. 1994. "Remediation
Technologies Screening Matrix and Reference Guide." EPA/542/B-94/013, NTIS
PB95-104782.
2. American Academy of Environmental Engineers. 1993. "Innovative Site Reme-
diation Technology: Soil Washing/Soil Flushing."
3. Moore, Roderic E. and Mark R. Matsumoto. 1993. "Investigation of the Use of
In-Situ Soil Flushing to Remediate a Lead Contaminated Site." Hazardous and
Industrial Waste: Proceedings, Mid-Atlantic Industrial Waste Conference. Tech-
nical Publishing Company, Inc. Lancaster, Pennsylvania.
4. Semu, E., B.R. Singh, and A.R. Selmer-Olsen. 1987. "Adsorption of Mercury
Compounds by Tropical Soils." Water, Air, and Soil Pollution. Volume 32,
Number 1-10.
5. Elliot, H.A., M.R. Liberati, and C.P. Chuang. 1986. "Competitive Adsorption of
Metals by Soils." Journal of Environmental Quality. Volume 15, Number 3.
6. Dragun, J. 1988. "The Soil Chemistry of Hazardous Materials." Presented at a
Meeting of the Hazardous Materials Control Research Institute. Silver Spring,
Maryland.
7. EPA. 1995. "Innovative Treatment Technologies: Annual Status Report."
Seventh Edition. September. EPA-542-R-95-008.
8. McPhillips, Loren C. and others. 1991. "Case History: Effective Groundwater
Remediation at the United Chrome Superfund Site." Prepared for presentation at
§=
. FtUSfipB, „'
BtJ
K. a •*. £
-------
-
the 84th Annual Meeting and Exhibition of the Air and Waste Management
Association. Vancouver, British Columbia. June 16-21.
9. McKinley, W. Scott and others. 1992. "Cleaning up CHROMIUM." Civil
Engineering. Pages 69-71.
10. Fristad, William E. 1995. "Full-scale Soil Washing/Terramet® Soil Leaching."
Presented at Environment Week's Second Annual Soil Remediation Conference:
Metals Innovative Technology Contracting. Washington, D.C. April 24-25.
11. "Innovative Li-Situ Cleanup Processes." 1992. The Hazardous Waste Consult-
ant. September/October.
12. Cline, S.R. 1993. "Soil Washing Fluid Efficiencies for the Treatment of Lead and
Organically Contaminated Soil." To be Published in the Proceedings of the 48th
Annual Purdue University Industrial Waste Conference. West Lafayette, Indiana.
13. EPA. 1996. "Superfund Fact Sheet: United Chrome Products Inc. Corvallis,
Oregon." June 10.
-------
Status of In Situ Solidification/
Stabilization Technology
Solidification treatment processes change
the physical characteristics of the waste
to improve its handling and to reduce the
mobility of the contaminants by creating
a physical barrier to leaching. Solidifica-
tion can be achieved through the use of
conventional pozzolans, such as Portland
cement. Stabilization (or immobiliza-
tion) treatment processes convert con-
taminants to less mobile forms through
chemical or thermal interactions.
(Vitrification of soil is an example of a
solidification/stabilization (S/S) process
that employs thermal energy.) S/S
treatment processes can be performed in
situ or ex situ.
Although many vendors provide S/S
technologies for ex situ applications,
relatively few companies offer in situ S/S
treatment processes. This chapter fo-
cuses on the in situ applications of S/S
remediation techniques.
Table 9 presents an overview of solidifi-
cation/stabilization technology.
TABLE 9
OVERVIEW OF SOLIDIFICATION/STABILIZATION TECHNOLOGY
General Characteristics
• Commercially available
• Cost is affected by the depth of the contamination, the degree of homogeneity of soil, the presence of debris,
and excess moisture.
Description: Addition of pozzolanic reagents with
or without additives to physically and chemically
convert contaminants to less mobile forms.
'Status; Comm§reial. " " **- " „
^i- 'v-^yr ~' ^ ._ ', -^_ _ ^ " _^
Applicability: Broad general applicability to most
metals; applicability to arsenic and mercury
should be tested on a case-by-case basis.
Hexavalent chromium requires additives that
ensure its conversion to the trivalent state during
mixing.
""Comments: Serfortnarice fe Jfighly, dependent on ?
' f \ mixing efficiency;- SolWiavfrfg hig'rr'cfay content
52- iot $$ hlltcant debrip,rnay;'Se difficult to mk< ; -*
' ' " Varl0us-
Applicability; Broad general applicability to most
metals.
Full-scale: arsenic, lead, chromium.
Potential: cadmium, copper, zinc, asbestos,
radioactive metals.
Conwtfeits: ltma|be"necessarystoJiat» "^
^fimdyerorjltef,mercury or^Qther volatjle metals^
' irom ,prosess oft-gases/ ||(gh rnoistiire Content;'
will|ricr|ase,c>9slssCib|tantjally. OeMs't^high^
conceritratteSsOl orgariic.contamiriants niay ll
"' '" > '
%..*
-------
DESCRIPTION
c
1.0 Description
S/S technologies are used to change the
physical characteristics and leaching
potential of waste. The term S/S refers to
treatment processes that utilize treatment
reagents or thermal energy to accomplish
one or more of the following objectives
[1]:
• Reduce the mobility or solubility
of the contaminants to levels
required by regulatory or other
risk-based standards
• Limit the contact between site
fluids (such as groundwater) and
the contaminants by reducing the
permeability of the waste, gener-
ally to less than IxlO'6 cm/sec
• Increase the strength or bearing
capacity of the waste, as indicated
by unconfined compressive
strength (UCS) or measured by
the California bearing ratio
There are two basic types of S/S treat-
ment processes: reagent-based systems
and thermal-based systems. Reagent-
based systems use chemicals to solidify
and stabilize the contaminants in the soil
matrix. Thermal-based systems use heat
to melt the soil to solidify and stabilize
the contaminants after cooling.
1.1 Reagent-based S/S Processes
In situ reagent-based S/S technologies
consist of a reagent formulation and a
delivery system. With the exception of
near-surface applications (that is, to
depths of 15 feet deep), a reagent-based
S/S delivery system usually consists of a
slurry batch plant, delivery hoses, and
one or more augers. Most reagent formu-
lations for in situ S/S applications consist
of ordinary pozzolanic reagents, although
proprietary reagents are often used in
conjunction with or instead of pozzolanic
reagents [7]. Pozzolanic mixtures are
based on siliceous volcanic ashes similar
to substances used to produce hydraulic
cement. Depending on the characteristics
of the waste to be treated and the desired
properties of the treated wastes, additives
such as bentonite or silicates may be
added to the cement and/or fly ash
mixture. For example, addition of bento-
nite increases the ease of pumping of the
wet reagent slurry and decreases the
permeability of the treated waste. Sili-
cates form chemical complexes with
metals, often providing greater insolubil-
ity than do hydroxide, carbonate, or
sulfate precipitates. (Other additives or
proprietary reagents, such as activated
carbon or organophilic clays, can be used
to stabilize semivolatile organic com-
pounds in wastes).
Wastes containing lead can be stabilized
with the addition of trisodium phosphate;
the resulting lead phosphate precipitate is
insoluble in water. Although solidifica-
tion of the waste treated with trisodium
phosphate is not necessary to provide a
barrier to leaching, it may be done for
other purposes such as providing suffi-
cient bearing strength to support a cap.
Additionally, lead phosphate is toxic by
inhalation. Solidification or other means
of encapsulation may be used to prevent
air-borne particulates from escaping the
treated waste. Alternatively, solidifica-
tion may be used to provide a barrier to
acids or alkaline solutions which could
solubilize the lead phosphate.
Each of the vendors contacted has a
patented auger consisting of blades or
paddles studded with injection ports
through which the reagent mixture flows.
Some vendors emphasize the kneading
and shearing action of their augers, while
-------
JD vERviEw OF STATUS
other vendors emphasize grout (reagent
mixture) control and the capability to
deliver two or more mixtures simulta-
neously. The vendors also differ with
respect to the size of injection ports and
their operating pressure.
Choice of auger diameter varies among
the vendors of reagent-based S/S tech-
nologies, but generally depends on depth
of drilling, consistency and hardness of
the soil, and soil porosity. For example,
augers from 4 to 12 feet in diameter
generally can be used to a depth of 40
feet. The diameter selected will depend
on the porosity of the soil. Augers of
larger diameter may be used in sludges
and sands, while silts and clays require
augers of a smaller diameter. One vendor
uses small-diameter augers for all depths
because large-diameter augers affect
mixing efficiencies. Beyond a depth of
35 to 40 feet, a smaller-diameter auger
(from 2.5 to 4 feet in diameter) is sug-
gested. Using a smaller auger permits
treatment to depths as great as 100 feet or
more. Only two of the five vendors
contacted treat soil at depths of more than
40 feet; however, most soil contamina-
tion is encountered at depths of 10 to 20
feet and only rarely deeper than 40 feet
[6] (although treatment to depths of more
than 40 feet is becoming more common).
1.2 Thermal-based S/S Processes
The only thermal-based S/S treatment
process commercially available is in situ
vitrification. In situ vitrification uses
electrical power to heat and melt soils
contaminated with organic, inorganic,
and metal-bearing wastes. The molten
material cools to form a hard, monolithic,
chemically inert, stable product of glass
and crystalline material that incorporates
and immobilizes the inorganic com-
pounds and metals. The resultant vitri-
fied product is a glassy material, with
very low leaching characteristics. Or-
ganic wastes initially are vaporized or
pyrolyzed by the process. Those con-
taminants migrate to the surface, where
they are treated in an off-gas treatment
system [2].
2.0 Overview of Status
The vendors that were identified as
potential providers of in situ S/S pro-
cesses were contacted to determine
whether they have available data that can
be used in the status report for this
technology. Vendors of ex situ stabiliza-
tion equipment also were contacted to
determine whether any has made
progress in developing an in situ version
of the technology.
A single vendor, Geocon, accounts for
most of the in situ applications reported;
however, little data on applications are
available. Each vendor's system is well
established, tracing its roots to estab-
lished construction technologies. (Deep
soil mixing and the installation of cement
footers and grout curtains or slurry walls
are construction techniques that have
been employed for many years.) An
emerging development for American
vendors is the injection of dry reagents
when high levels of moisture in the soil
preclude the use of liquid reagents.
Although this variation of the technology
has been employed in Europe for more
than 20 years, only one U.S. vendor
(Hayward Baker) has used it. Although
conveying dry reagents pneumatically
requires some expertise, both Millgard
and Geocon currently are experimenting
with the technique. It is notable that in
situ application of dry reagents tends to
decrease the effective depth of treatment
for a given auger diameter and soil
STABILIZATION
-------
JIM i it i» in
» °Jl STATUS
porosity. (Conversely, wet slurries help
extend the depth of treatment.)
Only one vendor offers in situ thermal-
based S/S treatment processes. Geosafe
Corporation of Richland, Washington
offers the in situ vitrification (ISV)
technology commercially. Figure 9 is a
schematic of the Geosafe ISV process.
ISV uses electrical current to heat and
vitrify the contaminated material in
place. A pattern of electrically-conduc-
tive graphite containing glass frit is
placed in the soil between the electrodes.
When power is supplied to the electrodes,
the mixture of graphite and glass frit
conducts the current through the soil,
heating the surrounding area and melting
the soil between and directly adjacent to
the electrodes.
Molten soils are electrically conductive
and can continue to transmit the electrical
current, melting soil downward and
outward. The electrodes are lowered
further into the soil as the soil becomes
molten, continuing the melting process to
the desired depth of treatment. One
setting of four electrodes is referred to as
a melt. For the Geosafe system, the
melting process occurs at an average rate
of approximately three to four tons per
hour.
Off-gas hood
ttt
Utility or
dtesel
generated
Scrubber water flow
Off-gas treatment system
To atmosphere
(if necessary)
Figure 9. Geosafe In Situ Vitrification process [3]
-------
3.0 Performance and Cost
Summary
Information on the testing and costs of
reagent-based and thermal-based S/S
processes are discussed separately in the
following subsections.
3.1 Reagent-based S/S Processes
In most cases involving in situ S/S, the
site cleanup manager independently
contracts with a testing laboratory to
develop and optimize a suitable reagent
formulation that will meet the desired
performance objectives for the site of
concern. Vendors then submit bids for
delivering the specified formulation in
situ. Occasionally, the vendor of the in
situ technology will develop the formula-
tion at the bench scale to achieve the
desired immobilization of contaminants
and posttreatment permeability and
unconfined compressive strength. There-
fore, testing at the bench scale consists of
optimizing the reagent formulation.
Testing at the pilot or full scale consists
of quality control of grout and confirma-
tion sampling to determine whether the
treated material is meeting required
performance specifications.
Although published data generally are
limited to those developed in demonstra-
tion projects sponsored by EPA, in situ
S/S is likely to be effective in reducing
leachable concentrations of metals to
within regulatory or risk-based limits.
The goal of vendors (and site managers)
is to meet the performance specifications
at the lowest cost. Failure to meet the
design specifications in the field most
often stems from poor grout control (that
is, inconsistently formulated slurries or
clogged injection ports that cause incom-
plete mixing or a spray pattern that is not
uniform).
Interviews with five vendors indicated
that costs for in situ S/S are likely to be
below ex situ treatment under certain
circumstances. For contaminated depths
of less than eight feet, excavation and ex
situ treatment are likely to be cheaper. In
situ S/S treatment is likely to be cheaper
for larger volumes because of the high
cost of mobilization and demobilization
for in situ S/S technologies (four to five
times that of ex situ technologies.) For
this reason, vendors of in situ S/S tech-
nologies are not likely to use augers or
bid jobs in cases where the depth of
treatment is 10 feet or less. (Geocon, for
example, uses a backhoe-mounted attach-
ment for depths to 10 feet). In addition,
auguring requires a level, stable base. At
sites that are not level, backfill must be
brought in to level the site to support the
auguring equipment. Eventually, the cost
of bringing in backfill can make the cost
of ex situ treatment competitive with that
of in situ S/S.
According to the vendors consulted, the
cost of in situ S/S can range from as low
as $20 to $40 per cubic yard to as much
as $100 to $200 per cubic yard, depend-
ing on the volume to be treated, the
structure of the soil (porosity), the treat-
ment depth, the type of contaminant, and
the post-treatment objectives (leachabil-
ity, permeability, or bearing ratio) de-
sired. The low end of the cost range
would apply to solidifying dredge spoils,
while the high end would apply to treat-
ment of high concentrations of contami-
nants at great depths. For application at a
hazardous waste site consisting of sands
to silts at a depth of 25 feet, $75 to $90
per cubic yard would be typical (20
percent of that figure would be the cost of
reagent).
-------
ANALYSIS OF APPLICATIONS
3.2 Thermal-based Processes
The Geosafe IS V process was demon-
strated under the SITE program at the
Parsons Chemical/ETM Enterprises
Superfund site in Grand Ledge, Michigan
from May 1993 to May 1994. The ISV
system that was used at the Parsons site
included an air emissions control and
treatment system to treat the eight-melt
operation. This project was the first
application of in situ vitrification at a
Superfund site to treat soils and sedi-
ments contaminated with pesticides,
metals, and dioxins.
The Geosafe ISV system used at the
Parsons site included eight melt cells and
an air emissions control system. Because
contamination was shallow, contami-
nated soil was excavated and staged at
the site. The melt cells were installed in
a treatment trench. Eight melts were
completed, ranging in duration from 10
to 20 days. Mercury concentrations in
the treated waste were reduced by more
than 98 percent when compared with
untreated soil. In addition, TCLP con-
centrations of arsenic, chromium, lead,
and mercury in the treated waste were
below regulatory levels of concern.
ISV also subsequently was applied
successfully the Wasatch Chemical
Superfund site, where ISV was used to
treat dioxin, pentachlorophenol, pesti-
cides, and herbicides.
The major factors affecting cost of ISV
are the amount of water present, the
treatment zone, depth, combustible waste
load, scale of operation and price of
electricity. The vendor estimates costs
between $375 and $425 per ton, which
makes this process especially suited for
hard to treat wastes, such as mixtures of
metals and organics.
4.0 Analysis of Applications
The most commonly stabilized metal
contaminants for reagent-based systems
are chromium, arsenic, and lead, fol-
lowed by cadmium, copper, zinc, and
mercury. Site managers may specify that
hexavalent chromium be treated in two
stages (the first to reduce the chromium
and the second to stabilize it); however,
vendors may add reducing agents to their
formulations to treat hexavalent chro-
mium in one stage.
Limited experience with ISV suggests
that it should not be recommended at
sites at which organic content in the
soil exceeds 10 percent by weight. In
addition, it is not recommended at sites
at which metals in the soil exceed 25
percent by weight or where inorganic
contaminants exceed 20 percent of the
soil by volume. The cost of ISV is
influenced principally by the need for
electric ppwer, which increases sub-
stantially with increasing moisture in
the soil [8].
-------
••**
REFERENCES
5.0 References
The following vendors were contacted during the preparation of this report:
CONTACTS
Name
Steve Day
David Coleman
David Yang
George Burke
Neville Kingham
Kent Saugier
James Hansen
Agency/Company
Geocon
Millgard Environmental
S.M.W. Seiko
Hay ward Baker
Kiber Environmental
Services, Inc.
Brown & Root
Geosafe Corporation
Telephone Number
(412) 856-7700
(313) 261-9760
(510)783-4105
(410) 551-1995
(770) 455-3944
(713) 575-4677
(509) 375-0710
LITERATURE REFERENCES
1. EPA. 1994. "Selection of Control Technologies for Remediation of Soil Con-
taminated with Arsenic, Cadmium, Chromium, Lead, or Mercury." Revised Draft
Engineering Bulletin. January 31.
2. EPA. 1994. "In Situ Vitrification Treatment." Engineering Bulletin. October.
3. Geosafe Corporation. 1994. "In Situ Vitrification Technology." SITE Technol-
ogy Capsule. November.
4. Member Agencies of the Federal Remediation Technologies Roundtable. 1995.
"Remediation Case Studies: Thermal Desorption, Soil Washing, and In Situ
Vitrification." March.
5. Yang, David S., Sigeru Takeshima, Thomas A. Delfino, and Michael T. Rafferty.
1995. "Use of Soil Mixing at a Metals Site." Proceedings of Air & Waste Man-
agement Association, 8th Annual Meeting. June.
6. Bates, Edward. U.S. Environmental Protection Agency. 1995. Letter to Carl Ma. May 20.
7. PRC Environmental Management, Inc. 1996. Telephone call with Neville
Kingham. August 8.
8. U.S. Environmental Protection Agency. 1995. Geosafe Corporation In Situ
Vitrification Innovative Technology Evaluation Report. Office of Research and
Development, Washington, DC. EPA/540/R-94/520. March.
-------
-------
Methodology
-------
-------
METHODOLOGY
Technologies discussed in this report
were chosen because they were at or near
a point of being commercially available.
The survey work to prepare this report
consisted of the following activities:
• Literature searches of several on-
line databases, including EPA's
Clean Up Information Bulletin
Board (CLU-IN) and Alternative
Treatment Technology Informa-
tion Center (ATTIC) databases
• Searches of the EPA record of
decision (ROD) database
• Searches of back issues of various
technical journals and shelf
material in EPA's libraries not
available on-line
• Communication with experts at
federal agencies, such as DoD, the
DOE, and the Bureau of Mines,
who are involved in research and
development of environmental
restoration technologies
• Contacts with technology vendors
identified in EPA's Vendor
Information System for Innova-
tive Treatment Technologies
(VISITT)
• Interviews with authors of articles
relevant to each technology
Several technology vendors and authors
identified from the searches were con-
tacted via telephone calls. They were
asked to comment on the status of the
technology, the amount of performance
data available from field applications of
the technology, and cost estimates for
performing remedial actions with the
technology. Vendors were chosen to
contact to provide representative infor-
mation on different technologies. Refer-
ence information on the vendors con-
tacted is included in each technology
chapter. No attempt was made to iden-
tify all vendors and their inclusion or
exclusion is purely coincidental. Re-
searchers and technical experts that were
also contacted are listed on the following
pages.
^APPENDIX A
BE™«&. vSfe— '
-------
I) 1 ('{ '"!•
*i il • ,.i.n .,..av,i
ELECTROKINETICS
Name
Randy Parker
Jack Hubbard
Kelly D. Pearce
Mark Bricka
Eric R. Lindgren
Ronald F. Probstein
Dr. Dennis Kelsh
Agency/Company
U.S. Environmental Protection Agency (EPA)
National Risk Management
Research Laboratory (NRMRL)
Cincinnati, Ohio
EPA NRMRL
Project Manager
(Electrokinetics Technology Site
Demonstrations)
U.S. Department of Energy (DOE)
Project Manager
Gaseous Diffusion Plant
Paducah, KY
U.S. Army Corps of Engineers Waterways
Experiment Station
Vicksburg, MS
Sandia National Laboratories
P.O. Box 5800
Mailstop0719
Albuquerque, NM 87185
Massachusetts Institute
of Technology
Department of Mechanical Engineering
Cambridge, MA 02139
SAIC
Gaithersburg, Maryland
-------
PHYTOREMEDIATIOiU
Name
Dr. Alan Baker
Dr. Gary Pierzynski
Dr. Ilya Raskin
Steve McCutcheon
Steve Rock
Agency/Company
Department of Animal and Plant Sciences
The University of Sheffield
Sheffield S10 2TN, United Kingdom.
E-mail: A.Baker@sheffield.ac.uk
and 100577.1360@compuserve.com
Department of Agronomy
Kansas State University
Manhattan, KS 66506-5501
AgBiotech Center and Department of
Environmental Sciences
Rutgers University
Cook College, P.O. Box 231
New Brunswick, NJ 08903-0231
EPA
EPA - NRMRL
SOIL FLUSHING
Name
John Mathur
Jesse Yow
JeffWalke
Eduardo Gonzales
Alan Goodman
Agency/Company
DOE
Program Manager
Office of Technology Development
MS EM-141
Washington, DC 20585
DOE
DOE
EPA
EPA
~r~1t~ w
-------
EZ
SOIL FLUSHING (CONTINUED)
Name
Neville Kingham
Dr. Brian E. Reed
Dr. M. R. Matsamato
Roderic E. Moore
Lome G. Everett
Agency/Company
Kiber Environmental Services, Inc., Inc.
West Virginia University
University of California at Riverside
West Virginia University
Geraghty & Miller, Inc.
3700 State Street
Suite 350
Santa Barbara, CA 93105
SOLIDIFICATION/STABILIZATION
Name
Ed Bates
Trish Erickson
Bob Thurnau
Mike Royer
JeffMarquesse
Len Zintak
Terri Richardson
Agency/Company
EPA-NRMRL
EPA - NRMRL
EPA - NRMRL
EPA - NRMRL
U.S. Department of Defense
Office of Assistant Deputy
Undersecretary of Defense
(Environmental Technology)
EPA - Region 5
(Parsons project)
EPA - SITE Program
(Parsons project)
ApffNBIX A":
-------
^ -x^^^w^vto-^k^
Engineering Bulletin: Technology Alternatives for the Remediation
of Soils Contaminated with Arsenic, Cadium, Chromium, Mercury, and Lead
-------
-------
Enwpncy
su&pome -
' Washington. D(X20AgG
Bulletin_; •' - '"'• • ••=""..,. .'..
.Technology Alternatives for
' .W ^ .• ^A- ** f ^ff&f '
Remediation of Soils :7
.jw .- "• ^ ~ f ^ f ff - fff f ff
inated with
&EPA Cadmium, Chrqmiijfn;
iMerctiry, and beaqT ;.?
Purpose
Section 121(b) of the Comprehensive Environmental
Response, Compensation, and Liability Act (CERCLA)
mandates the U.S. Environmental Protection Agency (EPA)
to select remedies that "utilize permanent solutions and
alternative treatment technologies or resource recovery
technologies to the maximum extent practical" and to prefer
remedial actions in which treatment "permanently and
significantly reduces the volume, toxicity, or mobility of
hazardous substances, pollutants, and contaminants as a
principal element." The EPA Engineering Bulletins are a
series of documents that summarize the available
information on selected treatment and site remediation
technologies and related issues. They provide summaries
and references of the latest information to help remedial
project managers, on-scene coordinators, contractors, and
other site cleanup managers understand the type of data
and site characteristics needed to evaluate a technology for
potential applicability to their hazardous waste sites.
Documents that describe individual site remediation
technologies focus on remedial investigation scoping needs.
Addenda are issued periodically to update the original
bulletins.
Introduction
This bulletin provides remedial project managers (RPM),
On-Scene Coordinators (OSC), and other state or private
remediation managers and their technical support personnel
with information to facilitate the selection of appropriate
remedial alternatives for soil contaminated with arsenic (As),
cadmium (Cd), chromium (Cr), mercury (Hg), and lead (Pb).
This bulletin primarily condenses information that is included
in a more comprehensive Technical Resource Document
(TRD) entitled "Contaminants and Remedial Options at
Selected Metal-Contaminated Sites [1]".
Common compounds, transport, arid fate are discussed
for each of the five elements. A general description of
metal-contaminated Superfund soils is provided. The
technologies covered are: immobilization [containment
(caps, vertical barriers, horizontal barriers),
solidification/stabilization (cement-based, polymer
microencapsulation), and vitrification]; and separation and
concentration (soil washing, pyrometallurgy, and soil
flushing). Use of treatment trains is also addressed.
Electrokinetics is addressed in the technical resource
document, but not here, since it had not been demonstrated
at full-scale in the United States for metals remediation.
Also, an update on the status of in situ electrokinetics for
remediation of metal-contaminated soil is in progress and
should be available in the near future [21. Another change
from the original technical resource document is that
physical separation is addressed in the bulletin under soil
washing, whereas it was previously covered as a separate
topic.
Contents
Section
Page
Purpose 1
Introduction 1
Overview of As, Cd, Cr, Hg, Pb, and Their Compounds .... 2
General Description of Superfund Soils and Sediments
Contaminated with As, Cd, Cr, Hg, and Pb 4
Soil Cleanup Goals for As, Cd, Cr, Hg, and Pb 5
Technologies for Containment and Remediation of
As, Cd, Cr, Hg, and Pb in Soils 5
Specific Remedial Technologies 6
Use of Treatment Trains 19
Cost Ranges of Remedial Technologies 19
Sources of Additional Information 21
Acknowledgements 21
References 22
-------
It is assumed that users of this bulletin will, as
necessary, familiarize themselves with: (1) the applicable
or relevant and appropriate regulations pertinent to the site
of interest; (2) applicable health and safety regulations and
practices relevant to the metals and compounds discussed;
and (3) relevant sampling, analysis, and data interpretation
methods. The majority of the information on which this
bulletin is based was collected during 1992 to 1994.
Information on lead battery (Pb, As), wood preserving (As,
Cr), pesticide (Pb, As, Hg), and mining sites is limited, as it
was in the original technical resource document. Most of
these site types have been addressed in other EPA
Superfund documents [3][4][5][6][7][8]. The greatest
emphasis is on remediation of inorganic forms of the metals
of interest. Organometallic compounds, organic-metal
mixtures, and multimetal mixtures are briefly addressed.
At the time of this printing, treatment standards for
RCRA wastes that contain metals (in 40 CFR 268) and for
contaminated media (in 40 CFR 269) are being investigated
for potential revisions. These revisions may impact the
selection of the technology for remediating sites containing
these metal-bearing wastes.
Overview of As, Cd, Cr, Hg, and Pb
and Their Compounds
This section provides a brief, qualitative overview of the
physical characteristics and mineral origins of the five
metals, and factors affecting their mobility. More
comprehensive and quantitative reviews of the behavior of
these five metals in soil can be found in other readily
available EPA Superfund documents [1][9][10].
Overview of Physical Characteristics
and Mineral Origins
Arsenic is a semi-metallic element or metalloid that has
several allotropic forms. The most stable allotrope is a
silver-gray, brittle, crystalline solid that tarnishes in air.
Arsenic compounds, mainly As2O3, can be recovered as a
by-product of processing complex ores mined mainly for
copper, lead, zinc, gold, and silver. Arsenic occurs in a
wide variety of mineral forms, including arsenopyrite
(FeAsS^J, which is the main commercial ore of As
worldwide.
Cadmium is a bluish-white, soft, ductile metal. Pure Cd
compounds rarely are found in nature, although occurrences
of greenockite (CdS) and otavite (CdCO3) are known. The
main sources of Cd are sulfide ores of lead, zinc, and
copper. Cd is recovered as a by-product when these ores
are processed.
Chromium is a lustrous, silver-gray metal. It is one of the
less common elements in the earth's crust, and occurs only
in compounds. The chief commercial source of chromium
is the mineral chromite (FeCr204). Chromium is mined as a
primary product and is not recovered as a by-product of any
other mining operation. There are no chromite ore reserves,
nor is there primary production of chromite in the United
States.
Mercury is a silvery, liquid metal. The primary source of
Hg is cinnabar (HgS), a sulfide ore. In a few cases, Hg
occurs as the principal ore product; it is more commonly
obtained as the by-product of processing complex ores that
contain mixed sulfides, oxides, and chloride minerals (these
are usually associated with base and precious metals,
particularly gold). Native or metallic Hg is found in very
small quantities in some ore sites. The current demand for
mercury is met by secondary production (i.e., recycling and
recovery).
Lead is a bluish-white, silvery, or gray metal that is highly
lustrous when freshly cut, but tarnishes when exposed to
air. It is very soft and malleable, has a high density (11.35
g/cm3) and low melting point (327.4°C), and can be cast,
rolled, and extruded. The most important lead ore is galena
(PbS). Recovery of lead from the ore typically involves
grinding, flotation, roasting, and smelting. Less common
forms of the mineral are cerussite (PbCO3), anglesite
(PbS04), and crocoite (PbCrO4).
Overview of Behavior of Arsenic, Cadmium,
Chromium, Lead, and Mercury
Since metals cannot be destroyed, remediation of metal-
contaminated soil consists primarily of manipulating (i.e.,
exploiting, increasing, decreasing, or maintaining) the
mobility of metal contaminant(s) to produce a treated soil
that has an acceptable total or leachable metal content.
Metal mobility depends upon numerous factors. As noted
in reference [9]:
"Metal mobility in soil-waste systems is determined by
the type and quantity of soil surfaces present, the
concentration of metal of interest, the concentration and
type of competing ions and complexing ligands, both
organic and inorganic, pH, and redox status.
Generalization can only serve as rough guides of the
expected behavior of metals in such systems. Use of
literature or laboratory data that do not mimic the
specific site soil and waste system will not be adequate
to describe or predict the behavior of the metal. Data
must be site specific. Long term effects must also be
considered. As organic constituents of the waste matrix
degrade, or as pH or redox conditions change, either
through natural processes of weathering or human
manipulation, the potential mobility of the metal will
change as soil conditions change."
Based on the above description of the number and type of
factors affecting metal mobility, it is clear that a
comprehensive and quantitative description of mobility of
the five metals under all conditions is well beyond the scope
of this bulletin. Thus, the behavior of the five metals are
described below, but for a limited number of conditions.
Cadmium, chromium (III), and lead are present in cationic
forms under natural environmental conditions [9]. These
Technology Alternatives for Remediation of Soils Contaminated with As, Cd, Cr, Hg, & Pb
-------
cationic metals generally are not mobile in the environment
and tend to remain relatively close to the point of initial
deposition. The capacity of soil to adsorb cationic metals
increases with increasing pH, cation exchange capacity, and
organic carbon content. Under the neutral to basic
conditions typical of most soils, cationic metals are strongly
adsorbed on the clay fraction of soils and can be adsorbed
by hydrous oxides of iron, aluminum, or manganese present
in soil minerals. Cationic metals will precipitate as
hydroxides, carbonates, or phosphates. In acidic, sandy
soils, the cationic metals are more mobile. Under conditions
that are atypical of natural soils (e.g., pH <5 or >9;
elevated concentrations of oxidizers or reducers; high
concentrations of soluble organic or inorganic complexing or
colloidal substances), but may be encountered as a result of
waste disposal or remedial processes, the mobility of these
metals may be substantially increased. Also, competitive
adsorption between various metals has been observed in
experiments involving various solids with oxide surfaces (y-
FeOOH, ff-SiO2, and y-M 2O3). In several experiments, Cd
adsorption was decreased by the addition of Pb or Cu for all
three of these solids. The addition of zinc resulted in the
greatest decrease of Cd adsorption. Competition for surface
sites occurred when only a few percent of all surface sites
were occupied [11].
Arsenic, chromium (VI), and mercury behaviors differ
considerably from cadmium, chromium (III), and lead.
Arsenic and Cr(VI) typically exist in anionic forms under
environmental conditions Mercury, although it is a cationic
metal, has unusual properties (e.g., liquid at room
temperature, easily transforms among several possible
valence states).
In most arsenic-contaminated sites, arsenic appears as
As2O3 or as anionic arsenic species leached from As203,
oxidized to As (V), and then sorbed onto iron-bearing
minerals in the soil. Arsenic may be present also in
organometallic forms, such as methylarsenic acid
(H2As03CH3) and dimethylarsinic acid ((CH3)2AsO2H), which
are active ingredients in many pesticides, as well as the
volatile compounds arsine (AsH3) and its methyl derivatives
[i.e., dimethylarsine (HAs(CH3)2) and trimethylarsine
{As(CH3)3)]. These arsenic forms illustrate the various
oxidation states that arsenic commonly exhibits {-III, 0,lll,
and V) and the resulting complexity of its chemistry in the
environment.
As (V) is less mobile (and less toxic) than As (III). As (V)
exhibits anionic behavior in the presence of water, and
hence its aqueous solubility increases with increasing pH,
and it does not complex or precipitate with other anions.
As(V) can form low solubility metal arsenates. Calcium
arsenate (Ca3(AsO4)2) is the most stable metal arsenate in
well-oxidized and alkaline environments, but it is unstable in
acidic environments. Even under initially oxidizing and
alkaline conditions, absorption of C02from the air will result
in formation of CaC03 and release of arsenate. In sodic
soils, sufficient sodium is available, such that the mobile
compound Na3AsO4 can form. The slightly less stable
manganese arsenate (Mn2(AsO4)2) forms in both acidic and
alkaline environments, while iron arsenate is stable under
acidic soil conditions. In aerobic environments, H3As04
predominates at pH <2 and is replaced by H2AsO4-, HAs042'
and AsO^ as pH increases to about 2, 7, and 11.5, re-
spectively. Under mildly reducing conditions, H3AsO3 is a
predominant species at low pH, but is replaced by H2As03-,
HAsO32-, and AsOs3* as pH increases. Under still more
reducing conditions and in the presence of sulfide, As2S3
can form. As2S3 is a low-solubility, stable solid. AsS2 and
AsS2- are thermodynamically unstable with respect to As2S3
[12]. Under extreme reducing conditions, elemental arsenic
and volatile arsine (AsH3) can occur. Just as competition
between cationic metals affects mobility in soil, competition
between anionic species (chromate, arsenate, phosphate,
sulfate, etc.) affects anionic fixation processes and may
increase mobility.
The most common valence states of chromium in the
earth's surface and near-surface environment are +3
(trivalent or Cr(lll)) and +6{hexavalent or Cr(VI)). The
trivalent chromium (discussed above) is the most
thermodynamically stable form under common
environmental conditions. Except in leather tanning,
industrial applications of chromium generally use the Cr(VI)
form. Due to kinetic limitations, Cr (VI) does not always
readily reduce to Cr (III) and can remain present over an
extended period of time.
Cr (VI) is present as the chromate (CrO42-) or dichromate
(Cr 2O 72-) anion, depending on pH and concentration. Cr
(VI) anions are less likely to be adsorbed to solid surfaces
than Cr (III). Most solids in soils carry negative charges that
inhibit Cr (VI) adsorption. Although clays have high
capacity to adsorb cationic metals, they interact little with
Cr (VI) because of the similar charges carried by the anion
and clay in the common pH range of soil and groundwater.
The only common soil solid that adsorbs Cr(VI) is iron
oxyhydroxide. Generally, a major portion of Cr(VI) and
other anions adsorbed in soils can be attributed to the
presence of iron oxyhydroxide. The quantity of Cr(VI)
adsorbed onto the iron solids increases with decreasing pH.
At metal-contaminated sites, mercury can be present in
mercuric form (Hg2+) mercurous form (Hg22+), elemental
form (Hg°), or alkylated form (e.g., methyl and ethyl
mercury). Hg22+ and Hg2+ are more stable under oxidizing
conditions. Under mildly reducing conditions, both
organically bound mercury and inorganic mercury
compounds can convert to elemental mercury, which then
can be readily converted to methyl or ethyl mercury by
biotic and abiotic processes. Methyl and ethyl mercury are
mobile and toxic forms.
Mercury is moderately mobile, regardless of the soil.
Both the mercurous and mercuric cations are adsorbed by
clay minerals, oxides, and organic matter. Adsorption of
cationic forms of mercury increases with increasing pH.
Mercurous and mercuric mercury also are immobilized by
forming various precipitates. Mercurous mercury
Technology Alternatives for Remediation of Soil Contaminated with As, Cd, Cr, Hg, & Pb
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precipitates with chloride, phosphate, carbonate, and
hydroxide. At concentrations of Hg commonly found in soil,
only the phosphate precipitate is stable. In alkaline soils,
mercuric mercury precipitates with carbonate and hydroxide
to form a stable (but not exceptionally insoluble) solid
phase. At lower pH and high chloride concentration, soluble
HgCI2 is formed. Mercuric mercury also forms complexes
with soluble organic matter, chlorides, and hydroxides that
may contribute to its mobility [9]. In strong reducing
conditions, HgS, a very low solubility compound is formed.
General Description of Superfund Soils
Contaminated with As, Cd, Cr, Hg, and Pb
Soils can become contaminated with metals from direct
contact with industrial plant waste discharges; fugitive
emissions; or leachate from waste piles, landfills, or sludge
deposits. The specific type of metal contaminant expected
at a particular Superfund site would obviously be directly
related to the type of operation that had occurred there.
Table 1 lists the types of operations that are directly
associated with each of the five metal contaminants.
Wastes atCERCLA sites are frequently heterogeneous on
a macro and micro scale. The contaminant concentration
and the physical and chemical forms of the contaminant and
matrix usually are complex and variable. Of these, waste
disposal sites collect the widest variety of waste types;
therefore concentration profiles vary by orders of magnitude
through a pit or pile. Limited volumes of high-concentration
"hot spots" may develop due to variations in the historical
waste disposal patterns or local transport mechanisms.
Similar radical variations frequently occur on the particle-
size scale as well. The waste often consists of a physical
mixture of very different solids, for example, paint chips in
spent abrasive.
Industrial processes may result in a variety of solid metal-
bearing waste materials, including slags, fumes, mold sand,
fly ash, abrasive wastes, spent catalysts, spent activated
carbon, and refractory bricks [13]. These process solids
may be found above ground as waste piles or below ground
in landfills. Solid-phase wastes can be dispersed by well-
intended but poorly controlled reuse projects. Waste piles
can be exposed to natural disasters or accidents causing
further dispersion.
Table 1. Principal Sources off As, Cd, Cr, Hg, and Pb Contaminated Soils
Contaminant
Arsenic
Cadmium
Chromium
Mercury
Lead
Wood preserving
As-waste disposal
Plating
Plating
Textile manufacturing
Chloralkali manufacturing
Weapons production
Ferrous/nonferrous smelting
Lead-acid battery breaking
Ammunition production
Leaded paint waste
Pb-waste disposal
Principal Sources
Pesticide production and application
Ni-Gd battery manufacturing
Leather tanning
Wood preserving
Copper and zinc smelting
Gas line manometer spills
Secondary metals production
Waste oil recycling
Firing ranges
Ink manufacturing
Mining
Mining
Cd-waste disposal
Pigment manufacturing
Cr-waste disposal
Paint application
Hg-waste disposal
Lead-acid battery manufacturing
Leaded glass production
Tetraethyl lead production
Chemical manufacturing
Technology Alternatives for Remediation of Soils Contaminated with As, Cd, Cr, Hg, & Pb
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Soil Cleanup Goals for As, Cd,
Cr, Hg, and Pb
Table 2 provides an overview of cleanup goals (actual
and potential) for both total and leachable metals. Based
on inspection of the total metals clsanup goals, one can
see that they vary considerably both within the same
metal and between metals. Similar variation is observed in
the actual or potential leachate goals. The observed
variation in cleanup goals has at least two implications with
regard to technology alternative evaluation and selection.
First, the importance of identifying the target metal(s),
contaminant state (leachable vs. total metal), the specific
type of test and conditions, and the numerical cleanup goals
early in the remedy evaluation process is made apparent.
Depending on which cleanup goal is selected, the required
removal or leachate reduction efficiency of the overall
remediation can vary by several orders of magnitude.
Second, the degree of variation in goals both within and
between the metals, plus the many factors that affect
mobility of the metals (discussed earlier in the bulletin),
suggest that generalizations about effectiveness of a
technology for meeting total or leachable treatment goals
should be viewed with some caution.
Technologies for Containment and
Treatment of As, Cd, Cr, Hg,
and Pb in Soils
Technologies potentially applicable to the remediation of
soils contaminated with the five metals or their inorganic
compounds are listed below. Underlined technologies have
been implemented (not necessarily in all applicable modes—
ex situ, in situ, off-site, and onsite) on numerous metal-
contaminated soils and are available from a substantial
number of vendors. Bracketed technologies have been
operated or demonstrated on metal-contaminated soil with
some success at full scale on one to approximately five
soils, and some cost and performance data are available. In
situ horizontal barriers are difficult to implement, but are
included to address in situ containment options for all
contaminated soil deposit surfaces. The remaining
technology, electrokinetics, has been implemented at full-
scale in Europe and not in the United States, but is
undergoing a Superfund Innovative Technology Evaluation
(SITE) demonstration. As noted above, electrokinetics is
not addressed in the bulletin. Other technologies (e.g.
phytoremediation and bacterial remediation) are being
evaluated and may provide low-cost remediation for low
concentration, large volume wastes, but these technologies
are not addressed here due to their early stage of
development and application to metal-contaminated soils.
Table 2. Cleanup Goals (Actual and Potential) for Total and teachable Metals
DESCRIPTION
As
Cd
CrfTotal)
Hg
Pb
Total Metals Goals (mg/Kg)
Background (Mean) [1]
Background (Range) [1]
Superfund Site Goals from TRD [1]
Theoretical Minimum Total Metals to Ensure TCLP
Leachate < Threshold (i.e., TCLP x 20)
California Total Threshold Limit Concentration [1]
5
1 to 50
5 to 65
100
500
0.06
.01 to .70
3 to 20
20
100
100
1 to 1000
6.7 to 375
100
500
0.03
.01 to .30
1 to 21
4
20
Leachable Metals Oig/L)
TCLP Threshold for RCRA Waste (SW 846, Method 131 1)
Extraction Procedure Toxicity Test (EP Tox) (Method 1310)
Synthetic Precipitate Leachate Procedure (Method 1312)
Multiple Extraction Procedure (Method 1320)
California Soluble Threshold Leachate Concentration
Maximum Contaminant Level8
Superfund Site Goals from TRD [1]
5000
5000
—
__
5000
50
50
1000
1000
__
—
1000
5
_.
5000
5000
—
—
5000
100
50
200
200
—
—
200
2
.05 to 2
10
2 to 200
200 to 500
100
1000
5000
5000
„
—
5000
15
50
Maximum Contaminant Level = The maximum permissible level of contaminant in water delivered to any user of a public system.
No specified level and no example cases identified.
Technology Alternatives for Remediation of Soil Contaminated with As, Cd, Cr, Hg, & Pb
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Technology Class
Containment
Solidification/
Stabilization
Separation/
Concentration
Specific Technology
Caps, Vertical Barriers,
Horizontal Barriers
Cement-Based
[Polymer Microencapsulation]
[Vitrification]
[Soil Washing]
[Soil Flushing (In Situ Only)]
[Pyrometallurgy]
Electrokinetics (Addressed
in TRD only)
For each technology listed above, the following topics
are discussed:
• Process description
• Site requirements for technology implementation
• Applicability
• Performance in treating metals in soil and Best
Demonstrated Available Technology (BOAT) status
• Technologies in the Superfund Innovative
Technology Evaluation (SITE) Demonstration
Program
• EPA contact for the technology
The BOAT status of the technology (see fourth bullet
above) refers to the determination under the Resource
Conservation and Recovery Act (RCRA) of the BOAT for
various industry-generated hazardous wastes that contain
the metals of interest. Whether the characteristics of a
Superfund metal-contaminated soil (or fractions derived
from it) are similar enough to the RCRA waste to justify
serious evaluation of the BOAT for a specific Superfund soil
must be made on a site specific basis. Other limitations
relevant to BDATs include: (1) the regulatory basis for BOAT
standards focus BDATs on proven, commercially available
technologies at the time of the BOAT determination, (2) a
BOAT may be identified, but that does not necessarily
preclude the use of other technologies, and (3) a technology
identified as BOAT may not necessarily be the current
technology of choice in the RCRA hazardous waste
treatment industry.
The EPA's Superfund Innovative Technology Evaluation
(SITE) program (referred to in the fifth bullet above)
evaluates many emerging and demonstrated technologies in
order to promote the development and use of innovative
technologies to clean up Superfund sites across the country.
The major focus of SITE is the Demonstration Program,
which is designed to provide engineering and cost data for
selected technologies.
Cost is not discussed in each technology narrative,
however, a summary table is provided at the end of the
technology discussion section that illustrates technology
cost ranges, and treatment train options.
Specific Remedial Technologies
Containment
Containment technologies for application at Superfund
sites include landfill covers (caps), vertical barriers, and
horizontal barriers [1]. For metal remediation, containment
is considered an established technology except for in situ
installation of horizontal barriers. This bulletin does not
address construction of onsite landfill liners for placement
of excavated material from the site.
Process Description — Containment ranges from a
surface cap that limits infiltration of uncontaminated surface
water to subsurface vertical or horizontal barriers that
restrict lateral or vertical migration of contaminated ground-
water. In addition to the containment documents refer-
enced in this section, six other EPA containment documents
are noted in the final section of this engineering bulletin on
covers, liners, QA/QC for geomernbrane seams, and QA/QC
for containment construction. Containment is covered
primarily by reference in the original technical resource
document. The text provided here is primarily from refer-
ence [5] on remediation of wood preserving sites.
Caps — Capping systems reduce surface water infiltra-
tion; control gas and odor emissions; improve aesthetics;
and provide a stable surface over the waste. Caps can
range from a simple native soil cover to a full RCRA Subtitle
C, composite cover.
Cap construction costs depend on the number of
components in the final cap system (i.e., costs increase
with the addition of barrier and drainage components).
Additionally, cost escalates as a function of topographic
relief. Side slopes steeper than 3 horizontal to 1 vertical
can cause stability and equipment problems that
dramatically increase the unit cost.
Vertical Barriers — Vertical barriers minimize the
movement of contaminated groundwater off-site or limit the
flow of uncontaminated groundwater onsite. Common
vertical barriers include slurry walls in excavated trenches;
grout curtains formed by injecting grout into soil borings;
vertically-injected, cement-bentonite grout-filled borings or
holes formed by withdrawing beams driven into the ground;
and sheet-pile walls formed of driven steel.
Certain compounds can affect cement-bentonite barriers.
The impermeability of bentonite may significantly decrease
when it is exposed to high concentrations of creosote,
water-soluble salts (copper, chromium, arsenic), or fire
retardant salts (borates, phosphates, and ammonia).
Specific gravity of salt solutions must be greater than 1.2 to
impact bentonite [14][15]. In general, soil-bentonite blends
resist chemical attack best if they contain only 1 percent
bentonite and 30 to 40 percent natural soil fines.
Treatability tests should evaluate the chemical stability of
the barrier if adverse conditions are suspected.
Technology Alternatives for Remediation of Soils Contaminated with As, Cd, Cr, Hg, & Pb
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Carbon steel used in pile walls quickly corrodes in dilute
acids, slowly corrodes in brines or salt water, and remains
mostly unaffected by organic chemicals or water. Salts and
fire retardants can reduce the service life of a steel sheet
pile; corrosion-resistant coatings can extend their
anticipated life. Major steel suppliers will provide site-
specific recommendations for cathodic protection of piling.
Construction costs for vertical barriers are influenced by
the soil profile of the barrier material used and by the
method of placing it. The most economical shallow vertical
barriers are soil-bentonite trenches excavated with
conventional backhoes; the most economical deep vertical
barriers consist of a cement-bentonite wall placed by a
vibrating beam.
Horizontal Barriers — In situ horizontal barriers can
underlie a sector of contaminated materials onsite without
removing the hazardous waste or soil. Established
technologies use grouting techniques to reduce the
permeability of underlying soil layers. Studies performed by
the U.S. Army Corps of Engineers (COE) [16] indicate that
conventional grout technology cannot produce an
impermeable horizontal barrier because it cannot ensure
uniform lateral growth of the grout. These same studies
found greater success with jet grouting techniques in soils
that contain fines sufficient to prevent collapse of the wash
hole and that present no large stones or boulders that could
deflect the cutting jet.
Since few in situ horizontal barriers have been
constructed, accurate costs have not been established.
Work performed by COE for EPA has shown that it is very
difficult to form effective horizontal barriers. The most
efficient barrier installation used a jet wash to create a
cavity in sandy soils into which cement-bentonite grouting
was injected. The costs relate to the number of borings
required. Each boring takes at least one day to drill.
Site Requirements — In general, the site must be suitable
for a variety of heavy construction equipment including
bulldozers, graders, backhoes, multi-shaft drill rigs, various
rollers, vibratory compactors, forklifts, and seaming devices
[18]. When capping systems are being utilized, onsite
storage areas are necessary for the materials to be used in
the cover. If site soils are adequate for use in the cover, a
borrow area needs to be identified and the soil tested and
characterized. If site soils are not suitable, it may be
necessary to truck in other low-permeability soils [18]. In
addition, an adequate supply of water may also be needed
in order to achieve the optimum soil density.
The construction of vertical containment barriers, such as
slurry walls, requires knowledge of the site, the local soil
and hydrogeologic conditions, and the presence of
underground utilities [17]. Preparation of the slurry requires
batch mixers, hydration ponds, pumps, hoses, and an
adequate supply of water. Therefore, onsite water storage
tanks and electricity are necessary. In addition, areas
adjacent to the trench need to be available for the storage
of trench spoils (which could potentially^ contaminated)
and the mixing of backfill. If excavated soils are not
acceptable for use as backfill, suitable backfill must be
trucked onto the site [17].
Applicability — Containment is most likely to be
applicable to: (1) wastes that are low-hazard (e.g., low
toxicity or low concentration) or immobile; (2) wastes that
have been treated to produce low hazard or low mobility
wastes for onsite disposal; or (3) wastes whose mobility
must be reduced as a temporary measure to mitigate risk
until a permanent remedy can be tested and implemented.
Situations where containment would not be applicable
include: (1) wastes for which there is a more permanent and
protective remedy that is cost-effective, (2) where effective
placement of horizontal barriers below existing
contamination is difficult; and (3) where drinking water
sources will be adversely affected if containment fails, and
if there is inadequate confidence in the ability to predict,
detect, or control harmful releases due to containment
failure.
Important advantages of containment are: (1) surface
caps and vertical barriers are relatively simple and rapid to
implement at low cost—can be more economical than
excavation and removal of waste; (2) caps and vertical
barriers can be applied to large areas or volumes of waste;
(3) engineering control (containment) is achieved, and may
be a final action if metals are well immobilized and potential
receptors are distant; (4) a variety of barrier materials are
available commercially; and (5) in some cases it may be
possible to create a land surface that can support
vegetation and/or be applicable for other purposes.
Disadvantages of containment include: (1) design life is
uncertain; (2) contamination remains onsite, available to
migrate should containment fail; (3) long-term inspection,
maintenance and monitoring is required; (4) site must be
amenable to effective monitoring; and (5) placement of
horizontal barriers below existing waste is difficult to
implement successfully.
Performance and BOAT Status — Containment is widely
accepted as a means of controlling the spread of
contamination and preventing the future migration of waste
constituents. Table 3 shows a list of selected sites where
containment has been selected for remediating metal-
contaminated solids.
The performance of capping systems, once installed, may
be difficult to evaluate [18]. Monitoring well systems or
infiltration monitoring systems can provide some
information, but it is often not possible to determine
whether the water or leachate originated as surface water
or groundwater.
With regard to slurry walls and other vertical containment
barriers, performance may be affected by a number of
variables including geographic region, topography, and
material availability. A thorough characterization of the site
and a compatibility study are highly recommended [17].
Technology Alternatives for Remediation of Soil Contaminated with As, Cd, Cr, Hg, & Pb
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Table 3. Containment Selections/Applications at Selected Superfund
Sites With Metal Contamination
Site Name/State
Ninth Avenue Dump, IN
Industrial Waste Control, AK
E.H. Shilling Landfill, OH
Chemtronlc, NC
Ordnance Works Disposal, WV
Industriplex, MA
Specific Technology
Containment-Slurry Wall
Containment-Slurry Wall
Containment-Slurry Wall
Capping
Capping
Capping
Key Metal
Contaminants
Pb
As, Cd, Cr, Pb
As
Cr, Pb
As, Pb
As, Pb, Cr
Associated Technology
Slurry Wall/Capping
Capping/French Drain
Capping/Clay Berm
Capping
Capping
Capping
Status8
S
1
S
S
S
1
• Status codes as of February 1996: S = selected in ROD; I = in operation.
Containment technologies are not considered "treatment
technologies" and hence no BDATs involving containment
have been established.
SITE Program Demonstration Projects — Ongoing SITE
demonstrations applicable to soils contaminated with the
metals of interest include:
• Morrison Knudsen Corporation (High clay grouting
technology)
• RKK, Ltd. (Frozen soil barriers)
Contact — Technology-specific questions regarding
containment may be directed to Mr. David Carson (NRMRL)
at {513) 569-7527.
Solidification/Stabilization Technologies
Solidification/stabilization (SIS), as used in this
engineering bulletin, refers to treatment processes that mix
or inject treatment agents into the contaminated material to
accomplish one or more of the following objectives:
• Improve the physical characteristics of the waste
by producing a solid from liquid or semi-liquid
wastes.
• Reduce the contaminant solubility by formation
of sorbed species or insoluble precipitates (e.g.
hydroxides, carbonates, silicates, phosphates,
sulfates, or sulfides).
• Decrease the exposed surface area across
which mass transfer loss of contaminants may
occur by formation of a crystalline, glassy, or
polymeric framework which surrounds the
waste particles.
• Limit the contact between transport fluids and
contaminants by reducing the material's
permeability [1].
S/S technology usually is applied by mixing contaminated
soils or treatment residuals with a physical binding agent to
form a crystalline, glassy, or polymeric framework
surrounding the waste particles. In addition to the
microencapsulation, some chemical fixation mechanisms
may improve the waste's leach resistance. Other forms of
S/S treatment rely on macroencapsulation, where the waste
is unaltered but macroscopic particles are encased in a
relatively impermeable coating [19], or on specific chemical
fixation, where the contaminant is converted to a solid
compound resistant to leaching. S/S treatment can be
accomplished primarily through the use of either inorganic
binders (e.g., cement, fly ash, and/or blast furnace slag) or
by organic binders such as bitumen [1]. Additives may be
used, for example, to convert the metal to a less mobile
form or to counteract adverse effects of the contaminated
soil on the S/S mixture (e.g. accelerated or retarded setting
times, and low physical strength). The form of the final
product from S/S treatment can range from a crumbly, soil-
like mixture to a monolithic block. S/S is more commonly
done as an ex situ process, but the in situ option is
available. The full range of inorganic binders, organic
binders, and additives is too broad to be covered here. The
emphasis in this bulletin is on ex situ, cement-based S/S,
which is widely used; in situ, cement-based S/S, which has
been applied to metals at full-scale; and polymer
microencapsulation, which appears applicable to certain
wastes that are difficult to treat via cement-based S/S.
Additional information and references on solidification/
stabilization of metals can be found in the source technical
resource document for this bulletin [1] and Engineering
Bulletin: Solidification and Stabilization of Organics and
Inorganics, EPA/540/S-92/015 [20]. Also, Chemical
Fixation and Solidification of Hazardous Wastes [21] is
probably the most comprehensive reference on S/S of
metals (692 pages total, 113 pages specifically on fixation
of metals). It is available in several EPA libraries.
Innovative S/S technologies (e.g., sorption and surfactant
processes, b'ituminization, emulsified asphalt, modified
sulfur cement, polyethylene extrusion, soluble silicate, slag,
8
Technology Alternatives for Remediation of Soils Contaminated with As, Cd, Cr, Hg, & Pb
-------
lime, and soluble phosphates) are addressed in Innovative
Site Remediation Technology - Solidification/Stabilization,
Volume 4 [22].
Process Description — Ex situ, cement-based S/S is
performed on contaminated soil that has been excavated
and classified to reject oversize. Cement-based S/S
involves mixing contaminated materials with an appropriate
ratio of cement or similar binder/stabilizer, and possibly
water and other additives. A system is also necessary for
delivering the treated wastes to molds, surface trenches, or
subsurface injection. Off-gas treatment (if volatiles or dust
are present) may be necessary. The fundamental materials
used to perform this technology are Portland-type cements
and pozzolanic materials. Portland cements are typically
composed of calcium silicates, aluminates, aluminoferrites,
and sulfates. Pozzolans are very small spheroidal particles
that are formed in combustion of coal (fly ash) and in lime
and cement kilns, for example. Pozzolans of high silica
content are found to have cement-like properties when
mixed with water. Cement-based S/S treatment may
involve using only Portland cement, only pozzolanic
materials, or blends of both. The composition of the
cement and pozzolan, together with the amount of water,
aggregate, and other additives, determines the set time,
cure time, pour characteristics, and material properties (e.g.,
pore size, compressive strength) of the resulting treated
waste. The composition of cements and pozzolans,
including those commonly used in S/S applications, are
classified according to American Society for Testing and
Materials (ASTM) standards. S/S treatment usually results
in an increase (>50% in some cases) in the treated waste
volume. Ex situ treatment provides high throughput (100 to
200 yd3/day/mixer).
Cement-based S/S reduces the mobility of inorganic
compounds by formation of insoluble hydroxides,
carbonates, or silicates; substitution of the metal into a
mineral structure; sorption; physical encapsulation; and
perhaps other mechanisms. Cement-based S/S involves a
complex series of reactions, and there are many potential
interferences (e.g., coating of particles by organics,
excessive acceleration or retardation of set times by various
soluble metal and inorganic compounds; excessive heat of
hydration; pH conditions that solubilize anionic species of
metal compounds, etc.) that can prevent attainment of S/S
treatment objectives for physical strength and leachability.
While there are many potential interferences, Portland
cement is widely used and studied, and a knowledgeable
vendor may be able to identify, and confirm via treatability
studies, approaches to counteract adverse effects by use of
appropriate additives or other changes in formulation.
In situ, cement-based solidification/stabilization has only
two steps: (1) mixing and (2) off-gas treatment. The
processing rate for in situ S/S is typically considerably lower
than for ex situ processing. In situ S/S is demonstrated to
depths of 30 feet and may be able to extend to 150 feet.
The most significant challenge in applying S/S in situ for
contaminated soils is achieving complete and uniform mixing
of the binder with the contaminated matrix [23]. Three
basic approaches are used for in situ mixing of the binder
with the matrix: (1) vertical auger mixing; (2) in-place mixing
of binder reagents with waste by conventional earthmoving
equipment, such as draglines, backhoes, or clamshell
buckets; and (3) injection grouting, which involves forcing
a binder containing dissolved or suspended treatment agents
into the subsurface, allowing it to permeate the soil. Grout
injection can be applied to contaminated formations lying
well below the ground surface. The injected grout cures in
place to produce an in situ treated mass.
S/S by polymer microencapsulation can include
application of thermoplastic or thermosetting resins.
Thermoplastic materials are the most commonly used
organic-based S/S treatment materials. Potential candidate
resins for thermoplastic encapsulation include bitumen,
polyethylene and other polyolefins, paraffins, waxes, and
sulfur cement. Of these candidate thermoplastic resins,
bitumen (asphalt) is the least expensive and by far the most
commonly used [24]. The process of thermoplastic
encapsulation involves heating and mixing the waste
material and the resin at elevated temperature, typically
130°C to 230°C in an extrusion machine. Any water or
volatile organics in the waste boil off during extrusion and
are collected for treatment or disposal. Because the final
product is a stiff, yet plastic resin, -the treated material
typically is discharged from the extruder into a drum or
other container.
S/S process quality control requires information on the
range of contaminant concentrations; potential interferences
in waste batches awaiting treatment; and treated product
properties such as compressive strength, permeability,
leachability, and in some instances, toxicity [20].
Site Requirements — The site must be prepared for the
construction, operation, maintenance, decontamination, and
decommissioning of the equipment. The size of the area
required for the process equipment depends on several
factors, including the type of S/S process involved, the
required treatment capacity of the system, and site
characteristics, especially soil topography and load-bearing
capacity. A small mobile ex situ unit occupies space for
two, standard flatbed trailers. An in situ system requires a
larger area to accommodate a drilling rig as well as a larger
area for auger decontamination.
Applicability — This section addresses expected
applicability based on the chemistry of the metal and the
S/S binders. The soil-contaminant-binder equilibrium and
kinetics are complicated, and many factors influence metal
mobility, so there may be exceptions to the generalizations
presented below.
For cement-based S/S, if a single metal is the
predominant contaminant in soil, then cadmium and lead are
the most amenable to cement-based S/S. The predominant
mechanism for immobilization of metals in Portland and
similar cements is precipitation of hydroxides, carbonates,
and silicates. Both lead and cadmium tend to form insoluble
precipitates in the pH ranges found in cured cement. They
Technology Alternatives for Remediation of Soil Contaminated with As, Cd, Cr, Hg, & Pb
-------
may resolubilize, however, if the pH is not carefully
controlled. For example, lead in aqueous solutions tends to
resolubilize as Pb(OH)3- around pH 10 and above. Mercury,
while it is a cationic metal like lead and cadmium, does not
form low solubility precipitates in cement, so it is difficult to
stabilize reliably by cement-based processes, and this
difficulty would be expected to be greater with increasing
mercury concentration and with organomercury compounds.
Arsenic, due to its formation of anionic species, also does
not form insoluble precipitates in the high pH cement
environment, and cement-based solidification is generally
not expected to be successful. Cr(VI) is difficult to stabilize
in cement due to formation of anions that are soluble at high
pH. However, Cr (VI) can be reduced to Cr (III), which does
form insoluble hydroxides. Although Hg and As (III and V)
are particularly difficult candidates for cement-based S/S,
this should not necessarily eliminate S/S (even cement-
based) from consideration since: (1) as with Cr (VI) it may
be possible to devise a multi-step process that will produce
an acceptable product for cement-based S/S; (2) a non-
cement based S/S process (e.g., lime and sulfide for Hg;
oxidation to As (V) and coprecipitation with iron) may be
applicable; or (3) the leachable concentration of the
contaminant may be sufficiently low that a highly efficient
S/S process may not be required to meet treatment goals.
The discussion of applicability above also applies to in
situ, cement-based S/S. If in situ treatment introduces
chemical agents into the ground, this chemical addition may
cause a pollution problem in itself, and may be subject to
additional requirements under the Land Disposal
Restrictions.
Polymer microencapsulation has been mainly used to
treat low-level radioactive wastes. However, organic
binders have been tested or applied to wastes containing
chemical contaminants such as arsenic, metals, inorganic
salts, PCBs, and dioxins [24]. Polymer micro-encapsulation
is particularly well suited to treating water-soluble salts such
as chlorides or sulfates that generally are difficult to
immobilize in a cement-based system [25]. Characteristics
of the organic binder and extrusion system impose
compatibility requirements on the waste material. The
elevated operating temperatures place a limit on the
quantity of water and VOCs in the waste feed. Low
volatility organics will be retained in the bitumen, but may
act as solvents causing the treated product to be too fluid.
The bitumen is a potential fuel source so the waste should
not contain oxidizers such as nitrates, chlorates, or
perchlorates. Oxidants present the potential for rapid
oxidation, causing immediate safety concerns, as well as
slow oxidation that results in waste form degradation.
Wastes containing more than one metal are not addressed
here, other than to say that cement-based solidification/
stabilization of multiple metal wastes will be particularly
difficult if a set of treatment and disposal conditions cannot
be found that simultaneously produces low mobility species
for all the metals of concern. For example, the relatively
high pH conditions that favor lead immobilization would tend
to increase the mobility of arsenic. On the other hand, the
various metal species in a multiple metal waste may interact
(e.g., formation of low solubility compounds by combination
of lead and arsenate) to produce a low mobility compound.
Organic contaminants are often present with inorganic
contaminants at metal-contaminated sites. S/S treatment of
organic-contaminated waste with cement-based binders is
more complex than treatment of inorganics alone. This is
particularly true with VOCs where the mixing process and
heat generated by cement hydration reactions can increase
vapor losses [26][27][28][29]. However, S/S can be applied
to wastes that contain lower levels of organics, particularly
when inorganics are present and/or the organics are
semivolatile or nonvolatile. Also, recent studies indicate the
addition of silicates or modified clays to the binder system
may improve S/S performance with organics [19].
Performance and BOAT status — S/S with cement-based
and pozzolan binders is a commercially available,
established technology. Table 4 shows a selected list of
sites where S/S has been selected for remediating metal-
contaminated solids. At 12 of the 19 sites, S/S has been
either completely or partially implemented. Note that S/S
has been used to treat all five metals (Cr, Pb, As, Hg, and
Cd). Although it would not generally be expected (for the
reasons noted in the previous section) that cement-based
S/S would be applied to As and Hg contaminated soils, it
was beyond the scope of this project to examine in detail
the characterization data, S/S formulations, and
performance data upon which the selections were based, so
the selection/implementation data are presented without
further comment.
Applications of polymer microencapsulation have been
limited to special cases where the specific performance
features are required for the waste matrix, and
contaminants allow reuse of the treated waste as a
construction material [30].
S/S is a BOAT for the following waste types:
• Cadmium nonwastewaters (other than cadmium-
containing batteries)
• Chromium nonwastewaters such as D007 and
U032 [following reduction to Cr (III)]
• Lead nonwastewaters such as D008, P110, U144,
U145,and U146
• Wastes containing low concentrations (< 260
mg/Kg) of elemental mercury -sulfide precipitation
• Plating wastes and steel-making wastes.
Although vitrification, not S/S, was selected as BOAT for
RCRA arsenic-containing nonwastewaters, EPA does not
preclude the use of S/S for treatment of As (particularly
inorganic As) wastes, but recommends that its use be
determined on a case-by-case basis. A variety of
stabilization techniques including cement, silicate, pozzolan,
and ferric coprecipitation were evaluated as candidate
BDATs for As. Due to concerns about long-term stability
and the waste volume increase, particularly with ferric
coprecipitation, stabilization was not accepted as BOAT.
10
Technology Alternatives for Remediation of Soils Contaminated with As, Cd, Cr, Hg, & Pb
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Table 4. Solidification/Stabilization Selections/Applications at Selected Superfund
Sites With Metal Contamination
Site Name/State
DeRewal Chemical, NJ
Marathon Battery Co., NY
Nascolite, Millville, NJ
Roebling Steel, NJ
Waldick Aerospace, NJ
Aladdin Plating, PA
Palmerton Zinc, PA
Tonolli Corp., PA
Whitmoyer Laboratories, PA
Bypass 601, NC
Flowood, MS
Independent Nail, SC
Pepper's Steel and Alloys, FL
Gurley Pit, AR
Pesses Chemical, TX
E.I. Dupont de Nemours, IA
Shaw Avenue Dump, IA
Frontier Hard Chrome, WA
Gould Site, OR
Specific Technology
Solidification
Chemical fixation
Stabilization of wetland soils
Solidification/stabilization
(34-acre slag area)
S/S, 4,000 cy
Stabilization, 12,000 cy
Stabilization, fly ash, lime, potash
S/S
Oxidation/fixation
S/S
S/S, 6,000 cy
S/S
S/S
In situ S/S
Stabilization
S/S
S/S
Stabilization
S/S
Key Metal
Contaminants
Cr, Cd, Pb
Cd, Ni
Pb
As, Cr, Pb
Cd, Cr
Cr
Cd, Pb
As, Pb
As
Cr, Pb
Pb
Cd, Cr
As, Pb
Pb
Cd
Cd, Cr, Pb
As, Cd
Cr
Pb
Associated Technology
GW pump and treatment
Dredging, off-site disposal
On-site disposal of
stabilized soils; excavatioin
and off-site disposal of
wetland soils
Capping
LTTD, off-site disposal
Off-site disposal
-
In situ chemical limestone
barrier
GW pump and treatment,
capping, grading, and
revegetation
Capping, regrading,
revegetation, GW pump
and treatment
Capping
Capping
On-site disposal
Concrete capping
Capping, regrading, and
revegetation
Capping, groundwater
monitoring
Capping, regrading, and
revegetation
Status8
S
I
S
S
C
C
1
S
S
S
C
C
C
C
C
C
C
S
1
a Status codes as of February 1996: S = selected in ROD; I = in operation, not complete; C = completed.
SITE Program Demonstration Projects — Completed or
ongoing SITE demonstrations applicable to soils
contaminated with the metals of interest include:
Completed
• Advanced Remediation Mixing, Inc. {Ex situ S/S)
• Funderburk & Associates (Ex situ S/S)
• Geo-Con, Inc. (In situ S/S)
• Soliditech, Inc. (Ex situ S/S)
STC Omega, Inc. ( Ex situ S/S)
WASTECH Inc. (Ex situ S/S)
Ongoing
Separation and Recovery Systems, Inc.
(Ex situ S/S)
Wheelabrator Technologies Inc. (Ex situ S/S)
Technology Alternatives for Remediation of So/7 Contaminated with As, Cd, Cr, Hg, & Pb
11
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Contact — Technology-specific questions regarding S/S
may be directed to Mr. Ed Barth (NRMRL) at (513) 569-
7669.
Vitrification
Vitrification applies high temperature treatment aimed
primarily at reducing the mobility of metals by incorporation
into a chemically durable, leach resistant, vitreous mass.
Vitrification can be carried out on excavated soils as well as
in situ.
Process Description — During the vitrification process,
organic wastes are pyrolyzed (in situ) or oxidized (ex situ)
by the melt front, whereas inorganics, including metals, are
incorporated into the vitreous mass. Off-gases released
during the melting process, containing volatile components
and products of combustion and pyrolysis, must be
collected and treated [1H31]. Vitrification converts con-
taminated soils to a stable glass and crystalline monolith
[32]. With the addition of low-cost materials such as sand,
clay, and/or native soil, the process can be adjusted to
produce products with specific characteristics, such as
chemical durability. Waste vitrification may be able to
transform the waste into useful, recyclable products such as
clean fill, aggregate, or higher valued materials such as
erosion-control blocks, paving blocks, and road dividers.
Ex situ vitrification technologies apply heat to a rnelter
through a variety of sources such as combustion of fossil
fuels (coal, natural gas, and oil) or input of electric energy
by direct joule heat, arcs, plasma torches, and microwaves.
Combustion or oxidation of the organic portion of the waste
can contribute significant energy to the melting process,
thus reducing energy costs. The particle size of the waste
may need to be controlled for some of the melting
technologies. For wastes containing refractory compounds
that melt above the unit's nominal processing temperature,
such as quartz or alumina, size reduction may be required to
achieve acceptable throughputs and a homogeneous melt.
For high-temperature processes using arcing or plasma
technologies, size reduction is not a major factor. For the
intense melters using concurrent gas-phase melting or
mechanical agitation, size reduction is needed for feeding
the system and for achieving a homogeneous melt.
In situ vitrification (ISV) technology is based on electric
melter technology, and the principle of operation is joule
heating, which occurs when an electrical current is passed
through a region that behaves as a resistive heating
element. Electrical current is passed through the soil by
means of an array of electrodes inserted vertically into the
surface of the contaminated soil zone. Because dry soil is
not conductive, a starter path of flaked graphite and glass
frit is placed in a small trench between the electrodes to act
as the initial flow path for electricity. Resistance heating in
the starter path transfers heat to the soil, which then begins
to melt. Once molten, the soil becomes conductive. The
melt grows outward and downward as power is gradually
increased to the full constant operating power level. A
single melt can treat a region of up to 1000 tons. The
maximum treatment depth has been demonstrated to be
about 20 feet. Large contaminated areas are treated in
multiple settings that fuse the blocks together to form one
large monolith [1]. Further information on in situ vitrification
can be found in the Engineering Bulletin: In Situ Vitrification
Treatment, EPA/540/S-94/504 [33].
Site Requirements — The site must be prepared for the
mobilization, operation, maintenance, and demobilization of
the equipment. Site activities such as clearing vegetation,
removing overburden, and acquiring backfill material are
often necessary for ex situ as well as in situ vitrification. Ex
situ processes will require areas for storage of excavated,
treated, and possibly pretreated materials. The components
of one ISV system are contained in three transportable
trailers: an off-gas and process control trailer, a support
trailer, and an electrical trailer. The trailers are mounted on
wheels sufficient for transportation to and over a compacted
ground surface [34].
The field-scale ISV system evaluated in the SITE program
required three-phase electrical power at either 12,500 or
13,800 volts, which is usually taken from a utility
distribution system [35]. Alternatively, the power may be
generated onsite by means of a diesel generator. Typical
applications require 800 kilowatt hours/ton (kWh/ton) to
1,OOOkWh/ton [33].
Applicability — Setting cost and implementability aside,
vitrification should be most applicable where nonvolatile
metal contaminants have glass solubilities exceeding the
level of contamination in the soil. Chromium-contaminated
soil should pose the least difficulties for vitrification, since
it has low volatility, and a glass solubility between 1 % and
3%. Vitrification may or may not be applicable for lead,
arsenic, and cadmium depending on the level of difficulty
encountered in retaining the metals in the melt, and
controlling and treating any volatile emissions that may
occur. Mercury clearly poses problems for vitrification due
to high volatility and low glass solubility (<0.1 %), but may
be allowable at very low concentrations (see Performance
and BOAT section that follows).
Chlorides present in the waste in excess of about 0.5
weight percent typically will not be incorporated into and
discharged with the glass, but will fume off and enter the
off-gas treatment system. If chlorides are excessively
concentrated, salts of alkali, alkaline earths, and heavy
metals will accumulate in solid residues collected by off-gas
treatment. Separation of the chloride salts from the other
residuals may be required before or during return of
residuals to the melter. When excess chlorides are present,
there is also a possibility that dioxins and furans may form
and enter the off-gas treatment system.
Waste matrix composition affects the durability of the
treated waste. Sufficient glass-forming materials (Si02)
(>30 wt %) and combined alkali (Na + K) (> 1.4 wt %) are
required for vitrification of wastes. If these conditions are
not met, frit and/or flux additives typically are needed.
Vitrification is also potentially applicable to soils
12
Technology Alternatives for Remediation of Soils Contaminated with As, Cd, Cr, Hg, & Pb
-------
contaminated with mixed metals and metal-organic wastes.
Specific situations where ex situ vitrification would not
be applicable or would face additional implementation
problems include: (1) wastes containing > 25% moisture
content cause excessive fuel consumption; (2) wastes
where size reduction and classification are difficult or
expensive; (3) volatile metals, particularly cadmium and
mercury, will vaporize and must be captured and treated
separately; (4) arsenic-containing wastes may require
pretreatment to produce less volatile forms; (5) metal
concentrations in soil that exceed their solubility in glass;
and (6) sites where commercial capacity is not adequate or
transportation cost to a fixed facility is unacceptable.
Specific situations, in addition to those cited above,
where in situ vitrification would not be applicable or would
face additional implementation problems include: (1) metal-
contaminated soil where a less costly and adequately
protective remedy exists; (2) projects that cannot be
undertaken because of limited commercial availability; (3)
contaminated soil < 6 feet and > 20 feet below the ground
surface; (4) presence of an aquifer with high hydraulic
conductivity (e.g., soil permeability >1 X 10"s cm/sec)
limits economic feasibility due to excessive energy required;
(5) contaminated soil mixed with buried metal that can
result in a conductive path causing short circuiting of
electrodes; (6) contaminated soil mixed with loosely packed
rubbish or buried coal can start underground fires and
overwhelm off-gas collection and treatment system; (7)
volatile heavy metals near the surface can be entrained in
combustion product gases and not retained in melt; (8) sites
where surface slope >5% may cause melt to flow; (9) in
situ voids > 150 m3 interrupt conduction and heat transfer;
and (10) underground structures and utilities < 20 ft from
the melt zone must be protected from heat or avoided.
Where it can be successfully applied, advantages of
vitrification include: (1) vitrified product is an inert,
impermeable solid that should reduce leaching for long
periods of time; (2) volume of vitrified product will typically
be smaller than initial waste volume; (3) vitrified product
may be usable; (4) a wide range of inorganic and organic
wastes can be treated; and (5) there is both an ex situ and
an in situ option available. A particular advantage of ex situ
treatment is better control of processing parameters. Also,
fuel costs may be reduced for ex situ vitrification by the use
of combustible waste materials. This fuel cost-saving
option is not directly applicable for in situ vitrification, since
combustibles would increase the design and operating
requirements for gas capture and treatment.
Performance and BOAT Status — In situ vitrification has
been implemented to date at one metal-contaminated
Superfund site (Parsons/ETM, Grand Ledge, Ml) and was
evaluated under the SITE Program [36]. The demonstration
was completed in April 1994. About 3,000 cubic yards of
soil were remediated. Some improvements are needed with
melt containment and air emission control systems. The
Innovative Technology Evaluation Report is now available
from EPA [37]. ISV has been operated at a large scale ten
times, including two demonstrations on radioactively
contaminated sites at the DOE's Hanford Nuclear
Reservation [31][38]. Pilot-scale tests have been conducted
at Oak Ridge National Laboratory, Idaho National
Engineering Laboratory, and Arnold Engineering
Development Center. More than 150 tests and
demonstrations at various scales have been performed on a
broad range of waste types in soils and sludges. The
technology has been selected as a preferred remedy at 10
private, Superfund, and DOD sites [39]. Table 5 provides
a summary of ISV technology selection/application at metal-
contaminated Superfund sites. A number of ex situ
vitrification systems are under development. The technical
resource document identified one full-scale ex situ melter
that was reported to be operating on RCRA organics and
inorganics.
Table 5. In Situ Vitrification Selections/
Applications at Selected Superfund
Sites With Metal Contamination
Site Name/State
Parsons Chemical, Ml
Rocky Mountain
Arsenal, CO
Key Metal
Contaminants
Hg (low)
As,Hg
Status8
C
S/D
a Status codes as of February 1996: C = completed;
S/D = selected, but subsequently de-selected.
Vitrification is a BOAT for the following waste types:
arsenic-containing wastes including K031, K084, K101,
K102, D004, and arsenic-containing P and U wastes.
SITE Program Demonstration Projects — Completed or
ongoing SITE demonstrations applicable to soils contamin-
ated with the metals of interest include:
Completed
• Babcock & Wilcox Co. (Cyclone furnace - ex situ
vitrification)
• Retech, Inc. (Plasma arc - ex situ vitrification)
• Geosafe Corporation (In situ vitrification)
Ongoing
• Vortec Corporation (Ex situ oxidation and
vitrification process)
Three additional projects were completed in the SITE
Emerging Technology program.
Contact — Technology-specific questions regarding
vitrification may be directed to Ms. Teri Richardson (NRMRL)
at (513) 569-7949.
Technology Alternatives for Remediation of Soil Contaminated with As, Cd, Cr, Hg, & Pb
13
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So/7 Washing
Soil washing is an ex situ remediation technology that
uses a combination of physical separation and
aqueous-based separation unit operations to reduce
contaminant concentrations to site-specific remedial goals
[40]. Although soil washing is sometimes used as a stand-
alone treatment technology, more often it is combined with
other technologies to complete site remediation. Soil
washing technologies have successfully remediated sites
contaminated with organic, inorganic, and radioactive
contaminants [40]. The technology does not detoxify or
significantly alter the contaminant, but transfers the
contaminant from the soil into the washing fluid or
mechanically concentrates the contaminants into a much
smaller soil mass for subsequent treatment.
Further information on soil washing can be found in
Innovative Site Remediation Technology - Soil Washing/Soil
Flushing, Vol. 3, EPA 542-B-93-012[41]. Revised versions
of an EPA Engineering Bulletin and a soil washing treatability
study guide are currently in preparation.
Process Description — Soil washing systems are quite
flexible in terms of the number, type, and order of
processes involved. Soil washing is performed on
excavated soil and may involve some or all of the following,
depending on the contaminant-soil matrix characteristics,
cleanup goals, and specific process employed: (1)
mechanical screening to remove various oversize materials,
(2) crushing to reduce applicable oversize to suitable
dimensions for treatment; (3) physical processes (e.g.
soaking, spraying, tumbling, and attrition scrubbing) to
liberate weakly bound agglomerates (e.g. silts and clays
bound to sand and gravel) followed by size classification to
generate coarse-grained and fine-grained soil fraction(s) for
further treatment; (4) treatment of the coarse-grained soil
fraction(s); (5) treatment of the fine-grained fraction(s); and
(6) management of the generated residuals.
Step 4 above (i.e., treatment of the coarse-grained soil
fraction) typically involves additional application of physical
separation techniques and possibly aqueous-based leaching
techniques. Physical separation techniques (e.g., sorting,
screening, elutriation, hydrocyclones, spiral concentrators,
flotation) exploit physical differences (e.g., size, density,
shape, color, wetability) between contaminated particles
and soil particles in order to produce a clean (or nearly
clean) coarse fraction and one or more metal-concentrated
streams. Many of the physical separation processes listed
above involve the use of water as a transport medium, and
if the metal contaminant has significant water solubility,
then some of the coarse-grained soil cleaning will occur as
a result of transfer to the aqueous phase. If the
combination of physical separation and unaided transfer to
the aqueous phase cannot produce the desired reduction in
the soil's metal content, which is frequently the case for
metal contaminants, then solubility enhancement is an
option for meeting cleanup goals for the coarse fraction.
Solubility enhancement can be accomplished in several
ways: (1) converting the contaminant into a more soluble
form (e.g., oxidation/reduction, conversion to soluble metal
salts); (2) using an aqueous-based leaching solution (e.g.,
acidic, alkaline, oxidizing, reducing) in which the
contaminant has enhanced solubility; (3) incorporating a
specific leaching process into the system to promote
increased solubilization via increased mixing, elevated
temperatures, higher solution/soil ratios, efficient
solution/soil separation, multiple stage treatment, etc.; or (4)
a combination of the above. After the leaching process is
completed on the coarse-grained fraction, it will be
necessary to separate the leaching solution and the coarse-
grained fraction by settling. A soil rinsing step may be
necessary to reduce the residual leachate in the soil to an
acceptable level. It may also be necessary to re-adjust soil
parameters such as pH or redox potential before
replacement of the soil on the site. The metal-bearing
leaching agent must also be treated further to remove the
metal contaminant and permit reuse in the process or
discharge, and this topic is discussed below under
management of residuals.
Treatment of fine-grained soils (Step 5 above) is similar
in concept to the treatment of the coarse-grained soils, but
the production rate would be expected to be lower and
hence more costly than for the coarse-grained soil fraction.
The reduced production rate arises from factors including:
(1) the tendency of clays to agglomerate, thus requiring
time, energy, and high water/clay ratios to produce a
leachable slurry; and (2) slow settling velocities that require
additional time and/or capital equipment to produce
acceptable soil/water separation for multi-batch or
countercurrent treatment, or at the end of treatment. A
site-specific determination needs to be made whether the
fines should be treated to produce clean fines or whether
they should be handled as a residual waste stream.
Management of generated residuals (Step 6 above) is an
important aspect of soil washing. The effectiveness,
implementability, and cost of treating each residual stream
is important to the overall success of soil washing for the
site. Perhaps the most important of the residual streams is
the metal-loaded leachant that is generated, particularly if
the leaching process recycles the leaching solution. Further-
more, it is often critical to the economic feasibility of the
project that the leaching solution be recycled. For these
closed or semi-closed loop leaching processes, successful
treatment of the metal-loaded leachant is imperative to the
successful cleaning of the soil. The leachant must: (1) have
adequate solubility for the metal so that the metal reduction
goals can be met without using excessive volumes of
leaching solution; and (2) be readily, economically, and
repeatedly adjustable (e.g., pH adjustment) to a form in
which the metal contaminant has very low solubility so that
the recycled aqueous phase retains a favorable
concentration gradient compared to the contaminated soil.
Also, efficient soil-water separation is important prior to
recovering metal from the metal-loaded leachant in order to
minimize contamination of the metal concentrate'. Recycling
the leachant reduces logistical requirements and costs
associated with make-up water, storage, permitting,
compliance analyses, and leaching agents. It also reduces
Technology Alternatives for Remediation of Soils Contaminated with As, Cd, Cr, Hg, & Pb
-------
external coordination requirements and eliminates the
dependence of the remediation on the ability to meet POTW
discharge requirements.
Other residual streams that may be generated and require
proper handling include: (1) untreatable, uncrushable
oversize; (2) recyclable metal-bearing particulates,
concentrates, or sludges from physical separation or
leachate treatment; (3) non-recyclable metal-bearing
particulates, concentrates, soils, sludges, or organic debris
that fail TCLP thresholds for RCRA hazardous waste; (4)
soils or sludges that are not RCRA hazardous wastes, but
are also not sufficiently clean to permit return to the site;
(5) metal-loaded leachant from systems where leachant is
not recycled; and (6) rinsate from treated soil. Options for
residuals treatment are listed in Table 8 at the end of the
technology section.
Site Requirements — The area required for a unit at a site
will depend on the vendor system selected, the amount of
soil storage space, and/or the number of tanks or ponds
needed for washwater preparation and wastewater storage
and treatment. Typical utilities required are water,
electricity, steam, and compressed air; the quantity of each
is vendor- and site-specific. It may be desirable to control
the moisture content of the contaminated soil for consistent
handling and treatment by covering the excavation, storage,
and treatment areas. Climatic conditions such as annual or
seasonal precipitation cause surface runoff and water
infiltration; therefore, runoff control measures may be
required. Since soil washing is an aqueous based process,
cold weather impacts include freezing as well as potential
effects on leaching rates.
Applicability — Soil washing is potentially applicable to
soils contaminated with all five metals of interest.
Conditions that particularly favor soil washing include: (1)
a single principal contaminant metal that occurs in dense,
insoluble particles that report to a specific, small mass
fraction(s) of the soil; (2) a single contaminant metal and
species that is very water or aqueous leachant soluble and
has a low soil/water partition coefficient; (3) soil containing
a high proportion (e.g., >80%) of soil particles >2 mm are
desirable for efficient contaminant-soil and soil-water
separation.
Conditions that clearly do not favor soil washing include:
(1) soils with a high (i.e., >40%) silt and clay fraction; (2)
soils that vary widely and frequently in significant
characteristics such as soil type, contaminant type and
concentration, and where blending for homogeneity is not
feasible; (3) complex mixtures (e.g. multi-component, solid
mixtures where access of leaching solutions to contaminant
is restricted; mixed anionic and cationic metals where pH of
solubility maximums are not close); (4) high clay content,
cation exchange capacity, or humic acid content, which
would tend to interfere with contaminant desorption; (5)
presence of substances that interfere with the leaching
solution (e.g., carbonaceous soils would neutralize
extracting acids; similarly, high humic acid content will
interfere with an alkaline extraction); and (6) metal
contaminants in a very low solubility, stable form (e.g., PbS)
may require long contact times and excessive amounts of
reagent to solubilize.
Performance and BOAT Status — Soil washing has been
used at waste sites in Europe, especially in Germany, the
Netherlands, and Belgium [42]. Table 6 lists selected
Superfund sites where soil washing has been selected
and/or implemented.
Acid leaching, which is a form of soil washing, is the
BOAT for Hg (D009, K071, P065, P092, and U151).
SITE Demonstrations and Emerging Technologies
Program Projects — Completed SITE demonstrations
applicable to soils contaminated with the metals of interest
include:
• Bergmann USA (Physical separation/leaching)
• BioGenesisSM (Physical separation/leaching)
• Biotrol, Inc (Physical separation)
• Brice Environmental Services Corp.
(Physical separation)
• COGNIS, Inc. (Leaching)
• Toronto Harbour Commission
(Physical separation/leaching)
Four SITE Emerging Technologies Program projects have
been completed that are applicable to soils contaminated
with the metals of interest.
Contact — Technology-specific questions regarding soil
washing may be directed to Mr. Richard Griffiths at (513)
569-7832 or Mr. Michael Borst (NRMRL) at (908) 321-
6631.
So/7 Flushing
Soil flushing is the in situ extraction of contaminants
from the soil via an appropriate washing solution. Water or
an aqueous solution is injected into or sprayed onto the area
of contamination, and the contaminated elutriate is collected
and pumped to the surface for removal, recirculation, or
onsite treatment and reinjection. The technology is
applicable to both organic and inorganic contaminants, and
metals in particular [1]. For the purpose of metals
remediation, soil flushing has been operated at full-scale,
but for a small number of sites.
Process Description — Soil flushing uses water, a
solution of chemicals in water, or an organic extractant to
recover contaminants from the in situ material. The
contaminants are mobilized by solubilization, formation of
emulsions, or a chemical reaction with the flushing
solutions. After passing through the contamination zone,
the contaminant-bearing fluid is collected by strategically
placed wells or trenches and brought to the surface for
disposal, recirculation, or onsite treatment and reinjection.
During elutriation, the flushing solution mobilizes the sorbed
contaminants by dissolution or emulsification.
Technology Alternatives for Remediation of Soil Contaminated with As, Cd, Cr, Hg, & Pb
15
-------
Table 6. Soil Washing Selections/Applications at Selected Superfund
Sites With Metal Contamination
Site Name/State
Ewan Property, NJ
GE Wiring Devices, PR
King of Prussia, NJ
Zanesvilla Well Reid, OH
Twin Cities Army
Ammunition Plant, MN
Sacramento Army Depot
Sacramento, CA
Specific Technology
Water washing
Water with Kl solution additive
Water with washing agent
additives
Soil washing
Soil washing
Soil washing
Key Metal
Contaminants
As, Cr, Cu, Pb
Hg
Ag, Cr, Cu
Hg, Pb
Cd, Cr, Cu, Hg, Pb
Cr.Pb
Associated Technology
Pretreatrnenf: by solvent
extraction to remove
organics
Treated residues
disposed onsite and
covered with clean soil
Sludges to be land
disposed
SVE to remove organics
Soil leaching
Offsite disposal of wash
liquid
Status3
S
S
C
S
C
S/D
Status codes as of February 1996: S = selected in ROD; C = completed; S/D = selected, but subsequently de-selected.
One key to efficient operation of a soil flushing system is
the ability to reuse the flushing solution, which is recovered
along with groundwater. Various water treatment
techniques can be applied to remove the recovered metals
and render the extraction fluid suitable for reuse. Recovered
flushing fluids may need treatment to meet appropriate
discharge standards prior to release to a POTW or receiving
waters. The separation of surfactants from recovered
flushing fluid, for reuse in the process, is a major factor in
the cost of soil flushing. Treatment of the flushing fluid
results in process sludges and residual solids, such as spent
carbon and spent ion exchange resin, which must be
appropriately treated before disposal. Air emissions of
volatile contaminants from recovered flushing fluids should
be collected and treated, as appropriate, to meet applicable
regulatory standards. Residual flushing additives in the soil
may be a concern and should be evaluated on a site-specific
basis [43]. Subsurface containment barriers can be used in
conjunction with soil flushing technology to help control the
flow of flushing fluids. Further information on soil flushing
can be found in the Engineering Bulletin: In Situ Soil
Flushing [43] or Innovative Site Remediation Technology -
Soil Washing/Soil Flushing, Volume 3, EPA 542-B-93-012
[41].
Site Requirements — Stationary or mobile soil-flushing
systems are located onsite. The exact area required will
depend on the vendor system selected and the number of
tanks or ponds needed for washwater preparation and
wastewater treatment. Certain permits may be required for
operation, depending on the system being utilized. Slurry
walls or other containment structures may be needed along
with hydraulic controls to ensure capture of contaminants
and flushing additives. Impermeable membranes may be
necessary to limit infiltration of precipitation, which could
cause dilution of the flushing solution and loss of hydraulic
control. Cold weather freezing must also be considered for
shallow infiltration galleries and above-ground sprayers [44].
Applicability — Soil flushing may be easy or difficult to
apply, depending on the ability to wet the soil with the
flushing solution and to install collection wells or subsurface
drains to recover all the applied liquids. The achievable
level of treatment varies and depends on the contact of the
flushing solution with the contaminants and the
appropriateness of the solution for contaminants, and the
hydraulic conductivity of the soil. Soil flushing is most
applicable to contaminants that are relatively soluble in the
extracting fluid, and that will not tend to sorb onto soil as
the metal-laden flushing fluid proceeds through the soil to
the extraction point. Based on the earlier discussion of
metal behavior, some potentially promising scenarios for
soil flushing would include: Cr(VI), As (III or V) in
permeable soil with low iron oxide, low clay, and high pH;
Cd in permeable soil with low clay, low CEC, and
moderately acidic pH; and, Pb in acid sands. A single target
metal would be preferable to multiple metals, due to the
added complexity of selecting a flushing fluid that would be
reasonably efficient for all contaminants. Also, the flushing
fluid must be compatible with not only the contaminant, but
also the soil. Soils that counteract the acidity or alkalinity
of the flushing solution will decrease its effectiveness. If
precipitants occur due to interaction between the soil and
the flushing fluid, then this could obstruct the soil pore
structure and inhibit flow to and through sectors of the
contaminated soil. It may take iong periods of time for soil
flushing to achieve cleanup standards.
A key advantage of soil flushing is that the contaminant
is removed from the soil. Recovery and reuse of the metal
16
Technology Alternatives for Remediation of Soils Contaminated with As, Cd, Cr, Hg, & Pb
-------
from the extraction fluid may be possible in some cases,
although the value of the recovered metal would not be
expected to fully offset the costs of recovery. The
equipment used for the technology is relatively easy to
construct and operate. It does not involve excavation,
treatment, and disposal of the soil, which avoids the
expense and hazards associated with these activities.
Performance and BOAT Status — Table 7 lists the
Superfund sites where soil flushing has been selected and/or
implemented. Soil flushing has a more established history
for removal of organics, but has been used for Cr removal
(e.g., United Chrome Products Superfund Site, near
Corvallis, Oregon). In situ technologies, such as soil
flushing, are not considered RCRA BOAT for any of the five
metals.
Soil flushing techniques for mobilizing contaminants can
be classified as conventional and unconventional.
Conventional applications employ water only as the flushing
solution. Unconventional applications that are currently
being researched include the enhancement of the flushing
water with additives, such as acids, bases, and chelating
agents to aid in the desorption/dissolution of the target
contaminants from the soil matrix to which they are bound.
Researchers are also investigating the effects of
numerous soil factors on heavy metal sorption and migration
in the subsurface. Such factors include pH, soil type, soil
horizon, CEC, particle size, permeability, specific metal type
and concentration, and type and concentrations of organic
and inorganic compounds in solutions. Generally, as the soil
pH decreases, cationic metal solubility and mobility
increase. In most cases, metal mobility and sorption are
likely to be controlled by the organic fraction in topsoils, and
clay content in the subsoils.
SITE Demonstration and Emerging Technologies Program
Projects — There are no in situ soil flushing projects
reported to be completed or ongoing either as SITE
demonstration or Emerging Technologies Program Projects
[44].
Contact — Technology specific questions regarding soil
flushing may be directed to Mr. Jerry N. Jones (NRMRL) at
(405) 436-8593.
Pyrometallurgical Technologies
Pyrometallurgy is used here as a broad term
encompassing elevated temperature techniques for
extraction and processing of metals for use or disposal.
High-temperature processing increases the rate of reaction
and often makes the reaction equilibrium more favorable,
lowering the required reactor volume per unit output [1].
Some processes that clearly involve both metal extraction
and recovery include roasting, retorting, or smelting. While
these processes typically produce a metal-bearing waste
slag, metal is also recovered for reuse. A second class of
Pyrometallurgical technologies included here is a
combination of high temperature extraction and
immobilization. These processes use thermal means to
cause volatile metals to separate from the soil and report to
the fly ash, but the metal in the fly ash is then immobilized,
instead of recovered, and there is no metal recovered for
reuse. A third class of technologies are those that are
primarily incinerators for mixed organic-inorganic wastes,
but which have the capability of processing wastes
containing the metals of interest by either capturing volatile
metals in the exhaust gases or immobilizing the nonvolatile
metals in the bottom ash or slag. As noted in the
introduction, mixed organic-metal waste is beyond the
scope of this bulletin. However, since some of these
systems may have applicability to some cases where metals
contamination is the primary concern, a few technologies of
this type are noted that are in the SITE program.
Vitrification is addressed in a previous section. It is not
considered Pyrometallurgical treatment since there is
typically neither a metal extraction nor a metal recovery
component in the process.
Process Description — Pyrometallurgical processing
usually is preceded by physical treatment to produce a
uniform feed material and upgrade the metal content.
Table 7. Soil Flushing Selections/Applications at Selected SuperfundSites With Metal Contamination
Site Name/State
Lipari Landfill, NJ
United Chrome Products,
OR
Specific Technology
Soil flushing of soil and wastes
contained by slurry wall and cap;
excavation from impacted watlands
Soil flushing with water
Key Metal
Contaminants
Cr, Hg, Pb
Cr
Associated Technology
Slurry wall and cap
Electrokinetic Pilot test,
Considering in situ
reduction
Status3
I
I
Status codes as of February 1996: I = in operation, not complete.
Technology Alternatives for Remediation of Soil Contaminated with As, Cd, Cr, Hg, & Pb
17
-------
Solids treatment in a high-temperature furnace requires
efficient heat transfer between the gas and solid phases
while minimizing particulate in the off-gas. The particle-size
range that meets these objectives is limited and is specific
to the design of the process. The presence of large clumps
or debris slows heat transfer, so pretreatment to either
remove or pulverize oversize material normally is required.
Fine particles also are undesirable because they become
entrained in the gas flow, increasing the volume of dust to
be removed from the flue gas. The feed material is
sometimes pelletized to give a uniform size. In many cases
a reducing agent and flux may be mixed in prior to
pelletization to ensure good contact between the treatment
agents and the contaminated material and to improve gas
flow in the reactor [1].
Due to its relatively low boiling point (357°C) and ready
conversion at elevated temperature to its metallic form,
mercury is commonly recovered through roasting and
retorting at much lower temperatures than the other metals.
Pyrometallurgical processing to convert compounds of the
other four metals to elemental metal requires a reducing
agent, fluxing agents to facilitate melting and to slag off
impurities, and a heat source. The fluid mass often is called
a melt, but the operating temperature, although quite high,
often is still below the melting points of the refractory
compounds being processed. The fluid forms as a lower-
melting-point material due to the presence of a fluxing agent
such as calcium. Depending on processing temperatures,
volatile metals such as cadmium and lead may fume off and
be recovered from the off-gas as oxides. Nonvolatile
metals, such as chromium or nickel, are tapped from the
furnace as molten metal. Impurities are scavenged by
formation of slag [1]. The effluents and solid products
generated by pyrometallurgical technologies typically include
solid, liquid, and gaseous residuals. Solid products include
debris, oversized rejects, dust, ash, and the treated medium.
Dust collected from particulate control devices may be
combined with the treated medium or, depending on
analyses for carryover contamination, recycled through the
treatment unit.
Site Requirements — Few pyrometallurgical systems are
currently available in mobile or transportable configurations.
Since this is typically an off-site technology, the distance of
the site from the processing facility has an important
influence on transportation costs. Off-site treatment must
comply with EPA's off-site treatment policies and
procedures. The off-site facility's environmental compliance
status must be acceptable, and the waste must be of a type
allowable under their operating permits. In order for
pyrometallurgical processing to be technically feasible, it
must be possible to generate a concentrate from the
contaminated soil that will be acceptable to the processor.
The processing rate of the off-site facility must be adequate
to treat the contaminated material in a reasonable amount
of time. Storage requirements and responsibilities must be
determined. The need for air discharge and other permits
must be determined on a site specific basis.
Applicability — With the possible exception of mercury,
or a highly-contaminated soil, pyrometallurgical processing
where metal recovery is the goal would not be applied
directly to the contaminated soil, but rather to a concentrate
generated via soil washing. Pyrometallurgical processing in
conventional rotary kilns, rotary furnaces, or arc furnaces is
most likely to be applicable to large volumes of material
containing metal concentrations (particularly, lead,
cadmium, or chromium) higher than 5 to 20%. Unless a
very concentrated feed stream can be generated (e.g.,
approximately 60% for lead), there will be a charge, in
addition to transportation, for processing the concentrate.
Lower metal concentrations can be acceptable if the metal
is particularly easy to reduce and vaporize (e.g., mercury) or
is particularly valuable (e.g., gold or platinum). Arsenic is
the weakest candidate for pyrometallurgical recovery, since
there is almost no recycling of arsenic in the U.S. Arsenic
[$250 to $500 per metric ton (mt)] for arsenic trioxide) is
also the least valuable of the metals. The reported price
range [1] for the other metals are: Cd ($5,950/mt); Cr
($7,830/mt); Pb ($700 to $770/mt); and, Hg ($5,295 to
$8,490/mt).
Performance and BOAT Status — The technical resource
document (1) contains a list of approximately 35
facilities/addresses/contacts that may accept concentrates
of the five metals of interest for pyrometallurgical
processing. Sixteen of the 35 facilities are lead recycling
operations, 7 facilities recover mercury, and the remainder
address a range of RCRA wastes that contain the metals of
interest. Due to the large volume of electric arc furnace
(EAF) emission control waste (K061), extensive processing
capability has been developed to recover cadmium, lead,
and zinc from solid waste matrices. Permitting is being
expanded to cover other hazardous waste types. The
currently available process technologies for K061 and
similar materials include:
• Waelz kiln process (Horsehead Resource
Development Company, Inc.)
• Waelz kiln and calcination process (Horsehead
Resource Development Company, Inc.)
• Flame reactor process (Horsehead Resource
Development Company, Inc.)
• Inclined rotary kiln (Zia Technology)
Plasma arc furnaces currently are successfully treating
K061 (EAF waste) at two steel plants. These are site-
dedicated units that do not accept outside material for
processing.
Pyrometallurgical recovery is a BOAT for the following
waste types:
• Cadmium-containing batteries, D006
• Lead nonwastewaters such as K069 in the
noncalcium sulfate subcategory
• Mercury wastes, P065, P092, and D009 (organics)
prior to retorting [45]
18
Technology Alternatives for Remediation of Soils Contaminated with As, Cd, Cr, Hg, & Pb
-------
• Lead acid batteries such as D008
• Zinc nonwastewaters such as K061 in the high
zinc subcategory
• Mercury from wastewater treatment sludge such
as K106 in the high-mercury subcategory
• Mercury such as U151 in the high-mercury
subcategory.
SITE Demonstration and Emerging Technologies Program
Projects — Completed SITE demonstrations applicable to
soils contaminated with the metals of interest include:
• RUST Remedial Services, Inc. (X-Trax Thermal
Desorption)
• Horsehead Resource Development Company, Inc.
(Flame Reactor)
Four SITE Emerging Technology Program projects that are
applicable to the metals of interest have been completed or
are ongoing.
Contact — Technology-specific questions regarding
pyrometallurgical treatment may be directed to Mrs. Marta
K. Richards (NRMRL) at (513) 569-7692.
Use of Treatment Trains
Several of the metal remediation technologies discussed
are often enhanced through the use of treatment trains.
Treatment trains use two or more remedial options applied
sequentially to the contaminated soil and often increase the
effectiveness while decreasing the cost of remediation.
Processes involved in treatment trains include soil
pretreatment, physical separation designed to decrease the
amount of soil requiring treatment, additional treatment of
process residuals or off-gases, and a variety of other
physical and chemical techniques, which can greatly
improve the performance of the remediation technology.
Table 8 provides examples of treatment trains used to
enhance each metal remediation technology that has been
discussed.
Cost Ranges of Remedial Technologies
Estimated cost ranges for the basic operation of the
technology are presented in Figure 1. The information was
compiled from EPA documents, including Engineering
Bulletins, SITE Demonstration Reports, and EPA electronic
databases. The reader is cautioned that the cost estimates
generally do not include pretreatment, site preparation,
regulatory compliance costs, costs for additional treatment
of process residuals (e.g., stabilization of incinerator ash or
disposal of metals concentrated by solvent extraction), or
profit. Since the actual cost of employing a remedial
technology at a specific site may be significantly different
than these estimates, data are best used for order-of-
magnitude cost evaluations.
Containment8
10 to 90
S/S
-> 60 to 290
Vitrification
-> 400 to 870
Soil Washing
-> 60 to 245
Soil Flushing13
-> 60 to 163
Pyrometallurgical
-> 250 to 560
0 TOO 200300 400 500 600 700 800 900 TOOO
COST ($/Ton)
a Includes landfill caps and slurry walls. A slurry wall depth of 20' is assumed.
b Costs reported in $/Yd3 , assumed soil density of 100 Ib/ft3
Figure 1. Estimated Cost Ranges of Metals Remediation Technologies
[Source: VISITT (Ver. 3.0)j various EPA Engineering Bulletins and TRD]
Technology Alternatives for Remediation of Soil Contaminated with As, Cd, Cr, Hg, & Pb
19
-------
Table 8. Typical Treatment Trains
Contain-
ment
S/S"
Vitrification"
Soil
Washing
Pyromet-
allurgical
Soil
Flushing
Pretreatment
Excavation
Debris removal
Oversize reduction
Adjust pH
Reduction (e.g., Cr(v1) to Cr(lll))
Oxidation (e.g., As(lll) to As (V))
Treatment to remove or destroy organics
Physical separation of rich and lean fractions
Dawaterlng and drying for wet sludge
Conversion of metals to less volatile forms
(e.g., ASjOa to Ca3(AsO4)2]
Addition of high temperature reductants
Palletizing
Flushing fluid delivery & extraction system
Containment barriers
•
•
•
•
•
•
E,P
E,P
E,P
I,E,P
I.E
I,E
I,E
I,E,P
P
I.E.P
I,E
E
E
E
E
E
1
•
•
•
•
•
•
•
•
•
•
•
•
•
•
Post-treatment/Reslduals Management
Disposal of treated solid residuals (preferably below
the frost line and above the water table)
Containment barriers
Off-gas treatment
Reuse for onsite paving
Metal recovery from extraction fluid by aqueous
processing (ion exchange, electrowinning, etc.)
Pyrometallurgical recovery of metal from sludge
Processing and reuse of leaching solution
S/S treatment of leached residual
Disposal of solid process residuals (preferably
below the frostline and above the water table)
Disposal of liquid process residuals
S/S treatment of slag or fly ash
Reuse of slag/vitreous product as
construction material
Reuse of metal or metal compound
Further processing of metal or metal compound
Rushing Hquld/groundwater treatment/disposal
I,E,P
I.E.P
I,E,P
P
E
I,E
I,E
E
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
0 Technology has been divided into the following categories: I = In Situ Process; E = Ex Situ Process;
P» Polymer Microencapsulation (Ex Situ)
20
Technology Alternatives for Remediation of Soils Contaminated with As, Cd, Cr, Hg, & Pb
-------
Sources of Additional Information
The following databases, reports, and EPA hotlines offer
additional information on the remediation of metal-
contaminated soil. The reader is also encouraged to review
sources referenced in this paper.
Alternative Treatment Technology Information Center
(ATTIC) database. U.S. EPA Assistance, (908) 321-
6677. ATTIC modem contact, (703) 908-2138 (1200 or
2400 baud), and the modem settings are no parity, 8
data bits, 1 stop bit, and full duplex.
The Clean-Up Information Bulletin Board System (CLU-
IN). System Operator, (301) 589-8368. Online
communication, (301) 589-8366.
EPA Online Library System (OLS). Includes the following
applicable databases: The National Catalog, The
Hazardous Waste Superfund Data Collection, and The
Chemical Collection System. Online communication,
(919) 549-0720. Public Information Center, (202) 260-
2080.
Records of Decision System (RODS) database. RODS
staff and registration, (703) 603-9091. RODS database
searches, (703) 538-7234.
Subsurface Remediation Technology (SRT) Database.
Database information, contact Dr. David S. Burden,
(405) 436-8606.
Cost of Remedial Action (CORA). PC-based database
available on disk.
Hazardous Waste Superfund Data Collection (HWSDC).
PC-based database available on disk. For information,
Felice Sacks, (202) 260-3121.
RiskReduction Engineering Laboratory (RREL) Treatability
Database. Available on disk and through the ATTIC
database. Contact Glenn Shaul, (513) 569-7589.
Vendor Information System for Innovative Treatment
Technologies (VISITT) database. PC-based database
available on disk, (800) 245-4505 or (703) 883-8448.
ReOpt/Remedial Action Assessment System (RAAS)
databases. U.S. Department of Energy. For government
projects only-a contract number must be filed with PNL
for each copy received.
EPA Home Page on World Wide Web (http://www.
epa.gov).
Marks, Peter J., Walter J. Wujcik, and Amy F. Loncar.
Remediation Technologies Screening Matrix and
Reference Guide, Second Edition. U.S. Army
Environmental Center. October 1994.
RCRA/Superfund Assistance Hotline. Washington, D.C.,
(800) 424-9346.
U.S. Environmental Protection Agency. Lining of Waste
Containment and Other Impoundment Facilities,
EPA/600/2-88-052. 1988.
U.S. Environmental Protection Agency. Design,
Construction, and Evaluation of Clay Liners for Waste
Management Facilities, EPA/530/SW-86/007F.
November 1988.
U.S. Environmental Protection Agency. Technical
Guidance Document: Final Covers on Hazardous Waste
Landfills and Surface Impoundments, EPA/530-SW-89-
047. July 1989.
U.S. Environmental Protection Agency. Technical
Guidance Document: Inspection Techniques for the
Fabrication of Geomembrane Field Seams, EPA/530/SW-
91/051. May 1991.
U.S. Environmental Protection Agency. Technical
Guidance Document: Construction Quality Management
for Remedial Action and Remedial Design Waste
Containment Systems, EPA/540/R-92/073. October
1992.
U.S. Environmental Protection Agency. Technical
Guidance Document: Quality Assurance and Quality
Control for Waste Containment Facilities, EPA/600/R-
93/182. 1993.
Acknowledgements
This Engineering Bulletin was prepared by the U.S.
Environmental Protection Agency, Office of Research and
Development (ORD), National Risk Management Research
Laboratory (NRMRL), Edison, New Jersey, with the
assistance of Science Applications International Corporation
(SAIC) under Contract No. 68-C5-0001. Mr. Michael D.
Royer served as the EPA Technical Project Manager. Ms.
Margaret Groeber was the SAIC Work Assignment Manager
and author. The author is especially grateful to George
Wahl, Joe Tillman, and Kristin Meyer of SAIC, who
contributed significantly to the development of this
document.
Technology Alternatives for Remediation of Soil Contaminated with As, Cd, Cr, Hg, & Pb
21
-------
The following EPA personnel
the document:
have contributed their time and comments by participating in peer reviews of sections of
Edwin Earth, NRMRL-Ci
Edward Bates, NRMRL-Ci
Benjamin Blaney, NRMRL-Ci
Michael Borst, NRMRL-Edison
David Burden, NRMRL-Ada
David Carson, NRMRL-Ci
Harry Compton, ERT, OSWER
Patricia Erickson, NRMRL-Ci
Frank J. Freestone, NRMRL-Edison
Richard Griffiths, NRMRL-Ci
Jerry N. Jones, NRMRL-Ada
Richard Koustas, NRMRL-Edison
Norm Kulujian, ORD-Region III
Ann Leitzinger, NRMRL-Ci
Shaun McGarvey, OSWER
Robert Puls, NRMRL-Ada
Marta Richards, NRMRL-Ci
Teri Richardson, NRMRL-Ci
Larry Rosengrant, OWSER
James Ryan, NRMRL-Ci
Robert Stamnes, Region X
Mary Stinson, NRMRL-Edison
Andre Zownir, ERT, OSWER
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Soils Contaminated with Metals - Status Report
(Draft). U.S. Environmental Protection Agency, Office
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Research and Development, 1994.
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Solutions to Waste Management, Calgary, Alberta,
Canada, 1992,533-545.
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Environmental Technology Development Guidelines
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