United States
Environmental Protection"
Agency
Office of Emergency and
Remedial Response
Washington, DC 20460
Office of
Research and Development
Cincinnati, OH 45268
Superfund
EPA/540/S-97/500
August 1997
<&EFA Engineering Bulletin
Technology
Alternatives for the
Remediation of Soils
Contaminated with As, Cd, Cr,
Hg, and Pb
Purpose
Section 121(b) of the Comprehensive Environmental Re-
sponse, Compensation, and Liability Act (CERCLA) mandates
the U.S. Environmental Protection Agency (EPA) to select rem-
edies that "utilize permanent solutions and alternative treatment
technologies or resource recovery technologies to the maximum
extent practical" and to prefer remedial actions in which treat-
ment "permanently and significantly reduces the volume, toxic-
ity, or mobility of hazardous substances, pollutants, and contami-
nants 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 re-
lated issues. They provide summaries and references of the lat-
est information to help remedial project managers, on-scene
coordinators, contractors, and other site cleanup managers un-
derstand the type of data and site characteristics needed to evalu-
ate a technology for potential applicability to their hazardous waste
sites. Documents that describe individual site remediation tech-
nologies focus on remedial investigation scoping needs. Addenda
are issued periodically to update the original bulletins.
Introduction
This bulletin provides remedial project managers, on-scene
coordinators, and other state or private remediation managers
and their technical support personnel with information to facili-
tate 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 Reme-
dial Options at Selected Metal-Contaminated Sites [1]".
Common compounds, transport, and fate are discussed for
each of the five elements. A general description of metal-con-
taminated Superfund soils is provided. The technologies cov-
ered are immobilization [containment (caps, vertical barriers, hori-
zontal barriers), solidification/stabilization (cement-based, poly-
mer 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 docu-
ment, but not here, since it had not been demonstrated at full
scale in the U.S. for metals remediation. Also, an update on the
status of in situ electrokinetics for remediation of metal-contami-
nated soil is in progress and should be available in the nearfu-
Contents
Section
Purpose 1
Introduction 1
Overview of As, Cd, Cr, Hg, Pb, and Their Compounds 2
General Description of Superfund Soils
Contaminated with As, Cd, Cr, Hg, and Pb 3
Soil Cleanup Goals for As, Cd, Cr, Hg, and Pb 3
Technologies for Containment and Remediation of
As, Cd, Cr, Hg, and Pb in Soils 4
Specific Remedial Technologies 5
Use of Treatment Trains 15
Cost Ranges of Remedial Technologies 16
Sources of Additional Information 17
Acknowledgements 18
References.... ... 18
-------
ture [2]. Another change from the original technical resource docu-
ment is that physical separation is addressed in the bulletin un-
der soil washing, whereas it was previously covered as a sepa-
rate topic.
It is assumed that users of this bulletin will, as necessary,
familiarize themselves with (1 ) the applicable or relevant and ap-
propriate regulations pertinent to the site of interest; (2) appli-
cable 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 Pb battery (Pb, As), wood preserv-
ing (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 EPASuperfund
documents [3][4][5][6][7][8]. The greatest emphasis is on reme-
diation of inorganic forms of the metals of interest. Organometal-
lic compounds, organic-metal mixtures, and multimetal mixtures
are briefly addressed.
At the time of this printing, treatment standards for Resource
Conservation and Recovery Act (RCRA) wastes that contain met-
als (in 40 CFR 268) and for contaminated media (in 40 CFR
269) are being investigated for potential revisions. These revi-
sions may impact the selection of the technology for remediating
sites containing these metal-bearing wastes.
Overview of As, Cd, Cr, Hg, Pb
Their Compounds
This section provides a brief, qualitative overview of the physi-
cal characteristics and mineral origins of the five metals, and
factors affecting their mobility. More comprehensive and quanti-
tative reviews of the behavior of these five metals in soil can be
found in other readily available EPA Superfund documents
Overview of Physical Characteristics
Mineral Origins
As is a semimetallic element or metalloid that has several
allotropic forms. The most stable allotrope is a silver-gray, brittle,
crystalline solid that tarnishes in air. As compounds, mainly As2O3,
can be recovered as a by-product of processing complex ores
mined mainly for copper, Pb, zinc, gold, and silver. As occurs in a
wide variety of mineral forms, including arsenopyrite (FeAsS4),
which is the main commercial ore of As worldwide.
Cd 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 Pb, zinc, and copper. Cd is recovered as a by-
product when these ores are processed.
Cr is a lustrous, silver-gray metal. It is one of the less com-
mon elements in the earth's crust, and occurs only in compounds.
The chief commercial source of Cr is the mineral chromite
(FeCr2O4). Cr 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 U.S..
Hg 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 cur-
rent demand for Hg is met by secondary production (i.e., recy-
cling and recovery).
Pb is a bluish-white, silvery, or gray metal that is highly lus-
trous 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 ex-
truded. The most important Pb ore is galena (PbS). Recovery of
Pb from the ore typically involves grinding, flotation, roasting,
and smelting. Less common forms of the mineral are cerussite
(PbCO3), anglesite (PbSO4), and crocoite (PbCrO4).
Overview of Behavior of As, Cd, Cr, Pb,
Since metals cannot be destroyed, remediation of metal-
contaminated soil consists primarily of manipulating (i.e., exploit-
ing, 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 con-
centration 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 con-
stituents of the waste matrix degrade, or as pH or redox
conditions change, either through natural processes of
weathering or human manipulation, the potential mobil-
ity 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.
Cd, Cr(lll), and Pb are present in cationic forms under natural
environmental conditions [9]. These 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, carbon-
ates, 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 ex-
periments involving various solids with oxide surfaces (y FeOOH,
cc-SIO2, and y-AI2O3). In several experiments, Cd adsorption was
decreased by the addition of Pb or Cu for all three of these sol-
Technology Alternatives for Remediation of Soils Contaminated with As, Cd, Cr, Hg & Pb
-------
ids. 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].
As, Cr (VI), and Hg behaviors differ considerably from Cd,
Cr (III), and Pb. As and Cr(VI) typically exist in anionic forms
under environmental conditions. Hg, although it is a cationic
metal, has unusual properties (e.g., liquid at room tempera-
ture, easily transforms among several possible valence states).
In most As-contaminated sites, As appears as As2O3 or as
anionic As species leached from As2O3, oxidized to As (V), and
then sorbed onto iron-bearing minerals in the soil. As may be
present also in organometallic forms, such as methylarsenic
acid (H2AsO3CH3) and dimethylarsenic 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 As forms illustrate the various oxidation states that As
commonly exhibits (-III, O.lll, and V) and the resulting complex-
ity 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
CO2 from the air will result information of CaCO3and 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 al-
kaline environments, while iron arsenate is stable under acidic
soil conditions. In aerobic environments, H3AsO4 predominates
at pH <2 and is replaced by H2AsO4-, HAsO42" and AsO43' as pH
increases to about 2, 7, and 11.5, respectively. Under mildly
reducing conditions, H3AsO3 is a predominant species at low
pH, but is replaced by H2AsO3", HAsO32~, and AsO33" as pH in-
creases. Under still more reducing conditions and in the pres-
ence of sulfide, As2S3 can form. As2S3 is a low-solubility, stable
solid. AsS2 and AsS2- are thermodynamically unstable with re-
spect to As2S3 [12]. Under extreme reducing conditions, elemen-
tal As and volatile arsine (AsH3) can occur. Just as competition
between cationic metals affects mobility in soil, competition
between anionic species (chromate, arsenate, phosphate, sul-
fate, etc.) affects anionic fixation processes and may increase
mobility.
The most common valence states of Cr in the earth's sur-
face and near-surface environment are +3 (trivalent or Cr(lll))
and +6 (hexavalentorCr(VI)). The trivalent Cr (discussed above)
is the most thermodynamically stable form under common en-
vironmental conditions. Except in leathertanning, industrial ap-
plications of Cr 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 at-
tributed to the presence of iron oxyhydroxide. The quantity of
Cr(VI) adsorbed onto the iron solids increases with decreasing
pH.
At metal-contaminated sites, Hg can be present in mercuric
form (Hg2*) mercurous form (Hg22*), elemental form (Hg°), or alky-
lated form (e.g., methyl and ethyl Hg). Hg22+ and Hg2+ are more
stable under oxidizing conditions. Under mildly reducing condi-
tions, both organically bound Hg and inorganic Hg compounds
can convert to elemental Hg, which then can be readily con-
verted to methyl or ethyl Hg by biotic and abiotic processes. Methyl
and ethyl Hg are mobile and toxic forms.
Hg 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 Hg
increases with increasing pH. Mercurous and mercuric Hg also
are immobilized by forming various precipitates. Mercurous Hg
precipitates with chloride, phosphate, carbonate, and hydroxide.
At concentrations of Hg commonly found in soil, only the phos-
phate precipitate is stable. In alkaline soils, mercuric Hg precipi-
tates with carbonate and hydroxide to form a stable (but not ex-
ceptionally insoluble) solid phase. At lower pH and high chloride
concentration, soluble HgCI2 is formed. Mercuric Hg also forms
complexes with soluble organic matter, chlorides, and hydrox-
ides that may contribute to its mobility [9]. In strong reducing
conditions, HgS, a very low solubility compound is formed.
Of
with As, Cd, Cr, Hg, Pb
Soils can become contaminated with metals from direct con-
tact with industrial plant waste discharges; fugitive emissions; or
leachate from waste piles, landfills, or sludge deposits. The spe-
cific type of metal contaminant expected at a particular Super-
fund site would obviously be directly related to the type of opera-
tion that had occurred there. Table 1 lists the types of operations
that are directly associated with each of the five metal contami-
nants.
Wastes at CERCLA 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 trans-
port mechanisms. Similar radical variations frequently occur on
the particle-size scale as well. The waste often consists of a physi-
cal 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 con-
trolled reuse projects. Waste piles can be exposed to natural
disasters or accidents causing further dispersion.
Cleanup for As, Cd, Cr, Hg,
Pb
Table 2 provides an overview of cleanup goals (actual and
potential) for both total and leachable metals. Based on inspec-
Technology Alternatives for Remediation of Soils Contaminated with As, Cd, Cr, Hg & Pb
-------
Table 1.
Principal Sources of As, Cd, Cr, Hg, and Pb
Contaminated Soils
Contaminant
Principal Sources
As
Cd
Cr
Hg
Pb
Wood preserving
As-waste disposal
Pesticide production and applica-
tion
Mining
Plating
Ni-Cd battery manufacturing
Cd-waste disposal
Plating
Textile manufacturing
Leather tanning
Pigment manufacturing
Wood preserving
Cr-waste disposal
Chloralkali manufacturing
Weapons production
Copper and zinc smelting
Gas line manometer spills
Paint application
Hg-waste disposal
Ferrous/nonferrous smelting
Pb-acid battery breaking
Ammunition production
Leaded paint waste
Pb-waste disposal
Secondary metals production
Waste oil recycling
Firing ranges
Ink manufacturing
Mining
Pb-acid battery manufacturing
Leaded glass production
Tetraethyl Pb production
Chemical manufacturing
tion of the total metals cleanup 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 sev-
eral 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 technol-
ogy for meeting total or leachable treatment goals should be
viewed with some caution.
Technologies for Containment
of As, Cd, Cr, Hg, 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 avail-
able from a substantial number of vendors. Bracketed technolo-
gies 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 de-
posit surfaces. The remaining technology, electrokinetics, has
been implemented at full-scale in Europe and not in the U.S. but
is undergoing a Superfund Innovative Technology Evaluation
(SITE) demonstration. As noted above, electrokinetics is not ad-
dressed in the bulletin. Othertechnologies (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.
Technology Class
Containment
Solidification/
Stabilization
Separation/
Concentration
Specific Technology
Caps, Vertical Barriers.
Horizontal Barriers
Cement-Based
[Polymer MicroencapsulationJ
[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 Demon-
strated Available Technology (BOAT) status
• Technologies in the SITE Demonstration Program
• EPA contact for the technology
The BOAT status of the technology (see fourth bullet above)
refers to the determination underthe RCRAofthe BDATforvari-
ous industry-generated hazardous wastes that contain the met-
als 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 pre-
clude the use of Othertechnologies, and (3) a technology identi-
fied as BOAT may not necessarily be the current technology of
choice in the RCRA hazardous waste treatment industry.
The EPA's SITE program (referred to in the fifth bullet above)
evaluates many emerging and demonstrated technologies in or-
der to promote the development and use of innovative technolo-
gies 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; how-
ever, a summary table is provided at the end of the technology
Technology Alternatives for Remediation of Soils Contaminated with As, Cd, Cr, Hg & Pb
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Table 2. Cleanup Goals (Actual and Potential) for Total and teachable Metals
Description As Cd
Cr (Total)
Hg
Pb
Total Metals Goals (mg/Kg)
Background (Mean) [1] 5
Background (Range) [1] 1 to 50
Superfund Site Goals from TRD [1 ] 5 to 65
Theoretical Minimum Total Metals to 100
Ensure TCLP
Leachate < Threshold (i.e., TCLP X 20)
California Total Threshold 500
Limit Concentration [1]
teachable Metals (u.g/L)
TCLP Threshold for RCRA Waste 5000
(SW846, Method 1311)
Extraction Procedure Toxicity Test 5000
(EP Tox) (Method 1310)
Synthetic Precipitate Leachate —
Procedure (Method 1312)
Multiple Extraction Procedure —
(Method 1320)
California Soluble Threshold 5000
Leachate Concentration
0.06
0.01 to 0.70
3 to 20
20
100
1000
1000
100
1 to 1000
6.7 to 375
100
500
5000
5000
0.03
0.01 to 0.30
1 to 21
4
20
200
200
10
2 to 200
200 to 500
100
1000
5000
5000
1000
5000
200
5000
Maximum Contaminant Level3
Superfund Site Goals from TRD [1]
50
50
5
100
50
2
0.05 to 2
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.
discussion section that illustrates technology cost ranges and
treatment train options.
Technologies
Containment
Containment technologies for application at Superfund sites
include landfill covers (caps), vertical barriers, and horizontal bar-
riers [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 waterto sub-
surface vertical or horizontal barriers that restrict lateral or verti-
cal migration of contaminated groundwater. In addition to the con-
tainment documents referenced in this section, six other EPA
containment documents are noted in the final section of this en-
gineering bulletin on covers, liners, Quality Assurance/Quality
Control (QA/QC) for geomembrane seams, and QA/QC for con-
tainment construction. Containment is covered primarily by ref-
erence in the original technical resource document. The text pro-
vided here is primarily from reference [5] on remediation of wood
preserving sites.
Caps—Capping systems reduce surface water infiltration;
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 compo-
nents in the final cap system (i.e., costs increase with the addi-
tion of barrier and drainage components). Additionally, cost es-
calates 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 uncon-
taminated groundwater onsite. Common vertical barriers include
slurry walls in excavated trenches; grout curtains formed by in-
jecting grout into soil borings; vertically-injected, cement-bento-
nite 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, Cr, As), 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.
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 retar-
dants 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.
Technology Alternatives for Remediation of Soils Contaminated with As, Cd, Cr, Hg & Pb
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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 aresoil-
bentonite trenches excavated with conventional backhoes; the
most economical deep vertical barriers consist of a cement-ben-
tonite wall placed by a vibrating beam.
Horizontal Barriers—In situ horizontal barriers can under-
lie a sector of contaminated materials onsite without removing
the hazardous waste or soil. Established technologies use grout-
ing techniques to reduce the permeability of underlying soil lay-
ers. 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 bulldoz-
ers, graders, backhoes, multi-shaft drill rigs, various rollers, vi-
bratory compactors, forklifts, and seaming devices [18]. When
capping systems are being utilized, onsite storage areas are nec-
essary 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 identi-
fied 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 utili-
ties [17]. Preparation of the slurry requires batch mixers, hydra-
tion ponds, pumps, hoses, and an adequate supply of water.
Therefore, onsite water storage tanks and electricity are neces-
sary. In addition, areas adjacent to the trench need to be avail-
able for the storage of trench spoils (which could potentially be
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 con-
centration) 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 mea-
sure to mitigate risk until a permanent remedy can be tested and
implemented. Situations where containment would not be appli-
cable 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 and 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 (con-
tainment) 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 sup-
port vegetation and/or be applicable for other purposes. Disad-
vantages of containment include (1) design life is uncertain; (2)
contamination remains onsite, available to migrate should con-
tainment fail; (3) long-term inspection, maintenance and moni-
toring is required; (4) site must be amenable to effective monitor-
ing; and (5) placement of horizontal barriers below existing waste
is difficult to implement successfully.
Performance and BOAT Status—Containment is widely ac-
cepted 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 origi-
nated 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 availabil-
ity. A thorough characterization of the site and a compatibility
study are highly recommended [17].
Containment technologies are not considered "treatment
technologies" and hence no BDATs involving containment have
been established.
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
Chemtronic, 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
Status3
S
I
S
S
S
I
1 Status codes as of February 1996: S = selected in ROD; I = in operation.
Technology Alternatives for Remediation of Soils Contaminated with As, Cd, Cr, Hg & Pb
-------
SITE Program Demonstration Projects—Ongoing SITE
demonstrations applicable to soils contaminated with the metals
of interest include
« Morrison Knudsen Corporation (High clay grouting tech-
nology)
• RKK, Ltd. (Frozen soil barriers)
Contact—Technology-specific questions regarding contain-
ment may be directed to Mr. David Carson (NRMRL) at (513)
569-7527.
Technologies
Solidification/stabilization (SIS), as used in this engineering
bulletin, refers to treatment processes that mix or inject treat-
ment agents into the contaminated material to accomplish one
or more of the following objectives:
• Improve the physical characteristics of the waste by pro-
ducing a solid from liquid or semiliquid wastes.
• Reduce the contaminant solubility by formation of sorbed
species or insoluble precipitates (e.g., hydroxides, car-
bonates, silicates, phosphates, sulfates, orsulfides).
• Decrease the exposed surface area across which mass
transfer loss of contaminants may occur by formation of a
crystalline, glassy, or polymeric framework which sur-
rounds the waste particles.
• Limit the contact between transport fluids and contami-
nants by reducing the material's permeability [1].
SIS 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 macro-
scopic 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 treat-
ment 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 physi-
cal 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 em-
phasis 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/sta-
bilization of metals can be found in the source technical resource
document for this bulletin [1] and Engineering Bulletin: Solidifi-
cation and Stabilization ofOrganics and Inorganics, EPA/540/S-
92/015 [20], Also, Chemical Fixation and Solidification of Haz-
ardous Wastes [21] is probably the most comprehensive refer-
ence on S/S of metals (692 pages total, 113 pages specifically
on fixation of metals). It is available in several EPA libraries. In-
novative S/S technologies (e.g., sorption and surfactant pro-
cesses, bituminization, emulsified asphalt, modified sulfur ce-
ment, polyethylene extrusion, soluble silicate, slag, lime, and
soluble phosphates) are addressed in Innovative Site Remedia-
tion Technology—Solidification/Stabilization, Volume 4 [22].
Process Description—Ex situ, cement-based S/S is per-
formed on contaminated soil that has been excavated and clas-
sified to reject oversize. Cement-based S/S involves mixing con-
taminated materials with an appropriate ratio of cement or simi-
lar 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 treat-
ment (if volatiles or dust are present) may be necessary. The
fundamental materials used to perform this technology are Port-
land-type cements and pozzolanic materials. Portland cements
are typically composed of calcium silicates, aluminates,
aluminoferrites, and sulfates. Pozzolans are very small spheroi-
dal 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 accord-
ing to American Society for Testing and Materials (ASTM) stan-
dards. 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 com-
pounds by formation of insoluble hydroxides, carbonates, or sili-
cates; 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 organ-
ics, excessive acceleration or retardation of set times by various
soluble metal and inorganic compounds; excessive heat of hy-
dration; pH conditions that solubilize anionic species of metal
compounds, etc.) that can prevent attainment of S/S treatment
objectives for physical strength and teachability. 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 chal-
lenge 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 forc-
ing a binder containing dissolved or suspended treatment agents
into the subsurface, allowing it to permeate the soil. Grout injec-
tion 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.
Technology Alternatives for Remediation of Soils Contaminated with As, Cd, Cr, Hg & Pb
-------
S/S by polymer microencapsulation can include applica-
tion of thermoplastic orthermosetting resins. Thermoplastic ma-
terials are the most commonly used organic-based S/S treat-
ment materials. Potential candidate resins for thermoplastic en-
capsulation include bitumen, polyethylene and other polyolefins,
paraffins, waxes, and sulfur cement. Of these candidate thermo-
plastic resins, bitumen (asphalt) is the least expensive and by far
the most commonly used [24]. The process of thermoplastic en-
capsulation 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 dis-
posal. 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, teachability, and in some
instances, toxicity [20].
Site Requirements—The site must be prepared forthe con-
struction, operation, maintenance, decontamination, and decom-
missioning 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 ex-
ceptions to the generalizations presented below.
For cement-based S/S, if a single metal is the predominant
contaminant in soil, then Cd and Pb are the most amenable to
cement-based S/S. The predominant mechanism for immobili-
zation of metals in Portland and similar cements is precipitation
of hydroxides, carbonates, and silicates. Both Pb and Cd tend to
form insoluble precipitates in the pH ranges found in cured ce-
ment. They may resolubilize, however, if the pH is not carefully
controlled. For example, Pb in aqueous solutions tends to
resolubilize as Pb(OH)3" around pH 10 and above. Hg, while it is
a cationic metal like Pb and cadmium, does not form low solubil-
ity 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 Hg concentration and with
organomercury compounds. As, due to its formation of anionic
species, also does not form insoluble precipitates in the high pH
cement environment, and cement-based solidification is gener-
ally 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 particu-
larly difficult candidates for cement-based S/S, this should not
necessarily eliminate S/S (even cement-based) from consider-
ation since (1) as with Cr (VI) it may be possible to devise a
multistep 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 pol-
lution problem in itself, and may be subject to additional require-
ments 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 As, metals, inorganic salts, polychlorinated biphenyls
(PCBs), and dioxins [24]. Polymer microencapsulation is particu-
larly 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 volatile organic chemicals (VOCs) in
the waste feed. Low volatility organics will be retained in the bitu-
men 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 per-
chlorates. Oxidants present the potential for rapid oxidation, caus-
ing 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/stabili-
zation of multiple metal wastes will be particularly difficult if a set
of treatment and disposal conditions cannot be found that simul-
taneously produces low mobility species for all the metals of con-
cern. For example, the relatively high pH conditions that favor Pb
immobilization would tend to increase the mobility of As. On the
other hand, the various metal species in a multiple metal waste
may interact (e.g., formation of low solubility compounds by com-
bination of Pband arsenate)to produce a low mobility compound.
Organic contaminants are often present with inorganic con-
taminants at metal-contaminated sites. S/S treatment of organic-
contaminated waste with cement-based binders is more com-
plex 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 con-
tain lower levels of organics, particularly when inorganics are
present and/orthe 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 organ-
ics [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 imple-
mented. 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
(forthe 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 charac-
terization data, S/S formulations, and performance data upon
which the selections were based, so the selection/implementa-
tion data are presented without further comment.
Applications of polymer microencapsulation have been lim-
ited to special cases where the specific performance features
are required forthe waste matrix, and contaminants allow reuse
of the treated waste as a construction material [30].
S/S is a BOAT forthe following waste types:
Technology Alternatives for Remediation of Soils Contaminated with As, Cd, Cr, Hg & Pb
-------
Table 4. Solidification/Stabilization Selections/Applications at Selected Superfund Sites With Metal Contamination
Site Name/State Specific Technology Key Metal Contaminants Associated Technology Status3
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
Solidification
Chemical fixation
Stabilization of wetland soils
Solidification/stabilization
(34-acre slag area)
S/S,4,OOOcy
Stabilization, 1 2,000 cy
Stabilization, fly ash, lime, potash
S/S
Cr, Cd, Pb
Cd, Ni
Pb
As, Cr, Pb
Cd, Cr
Cr
Cd, Pb
As, Pb
GW pump and treatment
Dredging, off-site disposal
On-site disposal of stabilized
soils; excavation and off-site
disposal of wetland soils
Capping
LTTD, off-site disposal
Off-site disposal
—
In situ chemical limestone
S
I
S
S
C
C
I
S
Whitmoyer Laboratories, PA
Bypass 601, NC
Oxidation/fixation
S/S
As
Cr, Pb
barrier
GW pump and treatment,
capping, grading, and
revegetation
Capping, regrading,
revegetation, GWpump
and treatment
Flowood, MS
Independent Nail, SC
Pepper's Steel and Alloys, FL
Gurley Pit, AR
Pesses Chemical, TX
E.I. Dupontde Nemours, IA
Shaw Avenue Dump, IA
Frontier Hard Chrome, WA
Gould Site, OR
S/S, 6,000 cy
S/S
S/S
In situ S/S
Stabilization
S/S
S/S
Stabilization
S/S
Pb
Cd, Cr
As, Pb
Pb
Cd
Cd, Cr, Pb
As, Cd
Cr
Pb
Capping
Capping
On-site disposal
Concrete capping
Capping, regrading, and
revegetation
Capping, groundwater
monitoring
Capping, regrading, and
revegetation
C
C
C
C
C
C
C
S
I
1 Status codes as of February 1996: S = selected in ROD; I = in operation, not complete; C = completed.
• Cd nonwastewaters (other than Cd-containing batteries)
• Cr nonwastewaters such as D007 and U032 [following
reduction to Cr(lll)]
• Pb nonwastewaters such as D008, P110, U144, U145,
and U146
• Wastes containing low concentrations (< 260 mg/Kg) of
elemental Hg—sulfide precipitation
• Plating wastes and steelmaking wastes
Although vitrification, not S/S, was selected as BOAT for
RCRA As-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 ce-
ment, silicate, pozzolan, and ferric coprecipitation were evalu-
ated 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.
SITE Program Demonstration Projects—Completed or on-
going 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)
WheelabratorTechnologies Inc. (Ex situ S/S)
Technology Alternatives for Remediation of Soils Contaminated with As, Cd, Cr, Hg & Pb
-------
Contact—Technology-specific questions regarding S/S may
be directed to Mr. Ed Earth (NRMRL) at (513) 569-7669.
¥itrification
Vitrification applies high temperature treatment aimed pri-
marily 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, or-
ganic wastes are pyrolyzed (in situ) or oxidized (ex situ) by the
melt front, whereas inorganics, including metals, are incorpo-
rated into the vitreous mass. Off-gases released during the melt-
ing process, containing volatile components and products of com-
bustion and pyrolysis, must be collected and treated [1][31]. Vit-
rification converts contaminated soils to a stable glass and crys-
talline 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 melter
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 re-
fractory compounds that melt above the unit's nominal process-
ing temperature, such as quartz or alumina, size reduction may
be required to achieve acceptable throughputs and a homoge-
neous 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 mechani-
cal 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 in-
serted 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 re-
gion 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 vitrifi-
cation can be found in the Engineering Bulletin: In Situ Vitrifica-
tion Treatment, EPA/540/S-94/504 [33].
Site Requirements—The site must be prepared forthe mo-
bilization, 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 pre-
treated materials. The components of one ISV system are con-
tained in three transportable trailers: an off-gas and process con-
trol 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 1,000 kWh/ton [33].
Applicability—Setting cost and implementability aside, vit-
rification should be most applicable where nonvolatile metal con-
taminants have glass solubilities exceeding the level of contami-
nation in the soil. Cr-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 Pb, As, and Cd, depending on the level of difficulty
encountered in retaining the metals in the melt, and controlling
and treating any volatile emissions that may occur. Hg clearly
poses problems for vitrification due to high volatility and low glass
solubility (<0.1%) but may be allowable at very low concentra-
tions (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 resi-
dues 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 (SiO2) (>30 wt %) and
combined alkali (Na + K)(>1.4wt%)are required for vitrification
of wastes. If these conditions are not met, frit and/or flux addi-
tives typically are needed. Vitrification is also potentially appli-
cable to soils contaminated with mixed metals and metal-organic
wastes.
Specific situations where ex situ vitrification would not be
applicable orwould face additional implementation problems in-
clude (1) wastes containing > 25% moisture content cause ex-
cessive fuel consumption; (2) wastes where size reduction and
classification are difficult or expensive; (3) volatile metals, par-
ticularly Cd and Hg, will vaporize and must be captured and
treated separately; (4) As-containing wastes may require pre-
treatment to produce less volatile forms; (5) metal concentra-
tions 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 orwould face addi-
tional implementation problems include (1) metal-contaminated
soil where a less costly and adequately protective remedy ex-
ists; (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"5 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) con-
taminated soil mixed with loosely packed rubbish or buried coal
10
Technology Alternatives for Remediation of Soils Contaminated with As, Cd, Cr, Hg & Pb
-------
can start underground fires and overwhelm off-gas collection and
treatment system; (7) volatile heavy metals nearthe 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 feet from the
melt zone must be protected from heat or avoided.
Where it can be successfully applied, advantages of vitrifi-
cation 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 vol-
ume; (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 ad-
vantage of ex situ treatment is better control of processing pa-
rameters. 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 com-
bustibles 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 Super-
fund site (Parsons/ETM, Grand Ledge, Ml) and was evaluated
underthe 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 Evalua-
tion Report is now available from EPA [37]. ISV has been oper-
ated 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 Labo-
ratory, and Arnold Engineering Development Center. More than
150 tests and demonstrations at various scales have been per-
formed 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 sum-
mary of ISV technology selection/application at metal-contami-
nated Superfund sites. A number of ex situ vitrification systems
are under development. The technical resource document iden-
Table 5. In Situ Vitrification Selections/Applications at Selected Su-
perfund Sites With Metal Contamination
Site Name/State
Key Metal
Contaminants
Status3
Parsons Chemical, Ml
Rocky Mountain Arsenal, CO
Hg (low)
As, Hg
S/D
Status codes as of February 1996: C = completed; S/D = selected,
but subsequently de-selected.
tified one full-scale ex situ melter that was reported to be operat-
ing on RCRAorganics and inorganics.
Vitrification is a BOAT for the following waste types: As-con-
taining wastes including K031, K084, K101, K102, D004, and
As-containing P and U wastes.
SITE Program Demonstration Projects—Completed or on-
going SITE demonstrations applicable to soils contaminated with
the metals of interest include
Completed
• Babcock & Wilcox Co. (Cyclone furnace—ex situ vitrifica-
tion)
• Retech, Inc. (Plasma arc—ex situ vitrification)
• Geosafe Corporation (In situ vitrification)
Ongoing
* Vortec Corporation (Ex situ oxidation and vitrification pro-
cess)
Three additional projects were completed in the SITE Emerging
Technology program.
Contact—Technology-specific questions regarding vitrifica-
tion may be directed to Ms. Teri Richardson (NRMRL) at (513)
569-7949.
Soil washing is an ex situ remediation technology that uses
a combination of physical separation and aqueous-based sepa-
ration unit operations to reduce contaminant concentrations to
site-specific remedial goals [40]. Although soil washing is some-
times used as a stand-alone treatment technology, more often it
is combined with other technologies to complete site remedia-
tion. Soil washing technologies have successfully remediated
sites contaminated with organic, inorganic, and radioactive con-
taminants [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 contami-
nants into a much smaller soil mass for subsequent treatment.
Further information on soil washing can be found in Innova-
tive Site Remediation Technology—Soil Washing/Soil Flushing,
Vol. 3, EPA542-B-93-012[41]. Revised versions of an EPAEngi-
neering Bulletin and a soil washing treatability study guide are
currently in preparation.
Process Description—Soil washing systems are quite flex-
ible in terms of the number, type, and order of processes in-
volved. Soil washing is performed on excavated soil and may
involve some or all of the following, depending on the contami-
nant-soil matrix characteristics, cleanup goals, and specific pro-
cess employed: (1) mechanical screening to remove various over-
size materials, (2) crushing to reduce applicable oversize to suit-
able dimensions fortreatment; (3) physical processes (e.g. soak-
ing, 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) treat-
ment 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 frac-
tion) typically involves additional application of physical separa-
tion 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 orderto pro-
duce 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 solu-
Technology Alternatives for Remediation of Soils Contaminated with As, Cd, Cr, Hg & Pb
11
-------
bility, 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 solu-
bility 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, alka-
line, oxidizing, reducing) in which the contaminant has enhanced
solubility; (3) incorporating a specific leaching process into the
system to promote increased solubilization via increased mix-
ing, 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 com-
pleted on the coarse-grained fraction, it will be necessary to sepa-
rate the leaching solution and the coarse-grained fraction by
settling. A soil rinsing step may be necessary to reduce the re-
sidual leachate in the soil to an acceptable level. It may also be
necessary to re-adjust soil parameters such as pH or redox po-
tential before replacement of the soil on the site. The metal-bear-
ing 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 residu-
als.
Treatment of fine-grained soils (Step 5 above) is similar in
concept to the treatment of the coarse-grained soils, but the pro-
duction rate would be expected to be lower and hence more
costly than forthe coarse-grained soil fraction. The reduced pro-
duction rate arises from factors including (1) the tendency of clays
to agglomerate, thus requiring time, energy, and high water/clay
ratios to produce a teachable slurry; and (2) slow settling veloci-
ties that require additional time and/or capital equipment to pro-
duce acceptable soil/water separation for multi-batch or coun-
tercurrent 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 orwhetherthey should be handled
as a residual waste stream.
Management of generated residuals (Step 6 above) is an
important aspect of soil washing. The effectiveness, implement-
ability, and cost of treating each residual stream is important to
the overall success of soil washing forthe 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. Furthermore, it is often critical to the eco-
nomic feasibility of the project that the leaching solution be re-
cycled. Forthese 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 forthe metal so that the metal reduction goals
can be met without using excessive volumes of leaching solu-
tion; 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 re-
tains a favorable concentration gradient compared to the con-
taminated soil. Also, efficient soil-water separation is important
prior to recovering metal from the metal-loaded leachant in or-
der to minimize contamination of the metal concentrate. Recy-
cling the leachant reduces logistical requirements and costs as-
sociated with makeup water, storage, permitting, compliance
analyses, and leaching agents. It also reduces external coordi-
nation requirements and eliminates the dependence of the re-
mediation on the ability to meet Publicly Owned Treatment Works
(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 slud-
ges from physical separation or leachate treatment; (3) non-re-
cyclable metal-bearing particulates, concentrates, soils, sludges,
or organic debris that fail toxicity characteristic leaching proce-
dure (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 com-
pressed air; the quantity of each is vendor- and site-specific. It
may be desirable to control the moisture content of the contami-
nated 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 par-
ticularly favor soil washing include (1) a single principal contami-
nant metal that occurs in dense, insoluble particles that report to
a specific, small mass fraction(s) of the soil; (2) a single contami-
nant metal and species that is very water or aqueous leachant
soluble and has a low soil/water partition coefficient; (3) soil con-
taining 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 blend-
ing for homogeneity is not feasible; (3) complex mixtures (e.g.
multicomponent, solid mixtures where access of leaching solu-
tions to contaminant is restricted; mixed anionic and cationic met-
als 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) pres-
ence 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 Neth-
erlands, 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
forHg(D009, K071, P065, P092, andU151).
SITE Demonstrations and Emerging Technologies Pro-
gram Projects—Completed SITE demonstrations applicable to
soils contaminated with the metals of interest include
• Bergmann USA (Physical separation/leaching)
12
Technology Alternatives for Remediation of Soils Contaminated with As, Cd, Cr, Hg & Pb
-------
Table 6. Soil Washing Selections/Applications at Selected Superfund Sites With Metal Contamination
Key Metal
Site Name/State Specific Technology Contaminants Associated Technology
Status3
Ewan Property, NJ
GE Wiring Devices, PR
King of Prussia, NJ
Zanesville Well Field, OH
Twin Cities Army
Ammunition Plant, MN
Sacramento Army Depot
Sacramento, CA
Water washing
Water with Kl solution additive
Water with washing agent
additives
Soil washing
Soil washing
Soil washing
As, Cr, Cu, Pb
Hg
Ag, Cr, Cu
Hg, Pb
Cr, Pb
Pretreatment by solvent
extraction to remove
organics
Treated residues
disposed onsite and
covered with clean soil
Sludges to be land
disposed
SVE to remove organics
Cd, Cr, Cu, Hg, Pb Soil leaching
Offsite disposal of wash
liquid
S
C
S/D
Status codes as of February 1996: S = selected in ROD; C = completed; S/D = selected, but subsequently de-selected.
• BioGenesisSM (Physical separation/leaching)
• Biotrol, Inc. (Physical separation)
• Brice Environmental Services Corp. (Physical separation)
• COGNIS, Inc. (Leaching)
« Toronto Harbour Commission (Physical separation/leach-
ing)
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.
Soil Flushing
Soil flushing is the in situ extraction of contaminants from
the soil via an appropriate washing solution. Water or an aque-
ous solution is injected into or sprayed onto the area of contami-
nation, 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 in-
organic contaminants, and metals in particular [1], For the pur-
pose 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 con-
taminants from the in situ material. The contaminants are mobi-
lized by solubilization, formation of emulsions, or a chemical re-
action with the flushing solutions. After passing through the con-
tamination 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 con-
taminants by dissolution or emulsification.
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 extrac-
tion fluid suitable for reuse. Recovered flushing fluids may need
treatment to meet appropriate discharge standards prior to re-
lease to a POTW or receiving waters. The separation of surfac-
tants 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 appropri-
ately treated before disposal. Air emissions of volatile contami-
nants 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 con-
tainment barriers can be used in conjunction with soil flushing
technology to help control the flow of flushing fluids. Further in-
formation on soil flushing can be found in the Engineering Bulle-
tin: In Situ Soil Flushing [43] or Innovative Site Remediation Tech-
nology—Soil Washing/Soil Flushing, Volume 3, EPA 542-B-93-
012 [41].
Site Requirements—Stationary or mobile soil-flushing sys-
tems 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 struc-
tures may be needed along with hydraulic controls to ensure
capture of contaminants and flushing additives. Impermeable
membranes may be necessary to limit infiltration of precipita-
tion, which could cause dilution of the flushing solution and loss
of hydraulic control. Cold weather freezing must also be consid-
ered for shallow infiltration galleries and above-ground sprayers
[44],
Applicability—Soil flushing may be easy or difficult to ap-
ply, depending on the ability to wet the soil with the flushing solu-
tion 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 con-
taminants and the appropriateness of the solution for contami-
nants, 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 ex-
traction point. Based on the earlier discussion of metal behavior,
some potentially promising scenarios for soil flushing would in-
Technology Alternatives for Remediation of Soils Contaminated with As, Cd, Cr, Hg & Pb
13
-------
elude 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
cation exchange capacity (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 flush-
ing fluid that would be reasonably efficient for all contaminants.
Also, the flushing fluid must be compatible with not only the con-
taminant, 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 long 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 from
the extraction fluid may be possible in some cases, although the
value of the recovered metal would not be expected to fully off-
set the costs of recovery. The equipment used for the technol-
ogy is relatively easy to construct and operate. It does not in-
volve 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 Super-
fund sites where soil flushing has been selected and/or imple-
mented. 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. Uncon-
ventional applications that are currently being researched in-
clude 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 sub-
surface. Such factors include pH, soil type, soil horizon, CEC,
particle size, permeability, specific metal type and concentra-
tion, and type and concentrations of organic and inorganic com-
pounds in solutions. Generally, as the soil pH decreases, cat-
ionic 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 Pro-
gram Projects—There are no in situ soil flushing projects re-
ported to be completed or ongoing either as SITE demonstra-
tion 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.
Pyrometallurgy is used here as a broad term encompass-
ing elevated temperature techniques for extraction and process-
ing of metals for use or disposal. High-temperature processing
increases the rate of reaction and often makes the reaction equi-
librium 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 pyro-
metallurgical 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 recov-
ered 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 ex-
haust 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 technolo-
gies of this type are noted that are in the SITE program. Vitrifica-
tion is addressed in a previous section. It is not considered pyro-
metallurgical treatment since there is typically neither a metal
extraction nor a metal recovery component in the process.
Process Description—Pyrometallurgical processing usu-
ally is preceded by physical treatment to produce a uniform feed
material and upgrade the metal content.
Solids treatment in a high-temperature furnace requires ef-
ficient heat transfer between the gas and solid phases while mini-
mizing 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 undesir-
able 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 pallati-
zation to ensure good contact between the treatment agents and
the contaminated material and to improve gas flow in the reactor
[1]-
Table 7. Soil Flushing Selections/Applications at Selected Superfund Sites With Metal Contamination
Key Metal
Site Name/State Specific Technology Contaminants Associated Technology
Status3
Lipari Landfill, NJ
United Chrome Products, OR
Soil flushing of soil and wastes Cr, Hg, Pb
contained by slurry wall and
cap; excavation from impacted
wetlands
Soil flushing with water Cr
Slurry wall and cap
Electrokinetic Pilot test,
Considering in situ
reduction
I
' Status codes as of February 1996: I = in operation, not complete.
14
Technology Alternatives for Remediation of Soils Contaminated with As, Cd, Cr, Hg & Pb
-------
Due to its relatively low boiling point (357°C) and ready con-
version at elevated temperature to its metallic form, Hg is com-
monly recovered through roasting and retorting at much lower
temperatures than the other metals. Pyrometallurgical process-
ing to convert compounds of the other four metals to elemental
metal requires a reducing agent, fluxing agents to facilitate melt-
ing 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-melt-
ing-point material due to the presence of a fluxing agent such as
calcium. Depending on processing temperatures, volatile metals
such as Cd and Pb may fume off and be recovered from the off-
gas as oxides. Nonvolatile metals, such as Cr 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 trans-
portation costs. Off-site treatment must comply with EPA's off-
site treatment policies and procedures. The off-site facility's en-
vironmental 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 contami-
nated soil that will be acceptable to the processor. The process-
ing rate of the off-site facility must be adequate to treat the con-
taminated material in a reasonable amount of time. Storage re-
quirements 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 Hg, 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 appli-
cable to large volumes of material containing metal concentra-
tions (particularly, Pb, Cd, or Cr) higher than 5 to 20%. Unless a
very concentrated feed stream can be generated (e.g., approxi-
mately 60% for Pb), there will be a charge, in addition to trans-
portation, for processing the concentrate. Lower metal concen-
trations can be acceptable if the metal is particularly easy to
reduce and vaporize (e.g., Hg) or is particularly valuable (e.g.,
gold or platinum). As is the weakest candidate for pyrometallur-
gical recovery, since there is almost no recycling of As in the
U.S. As [$250 to $500 per metric ton (mt)] for As 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/ad-
dresses/contacts that may accept concentrates of the five met-
als of interest for pyrometallurgical processing. Sixteen of the 35
facilities are Pb recycling operations, 7 facilities recover Hg, 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 ca-
pability has been developed to recover Cd, Pb, and zinc from
solid waste matrices. Permitting is being expanded to cover other
hazardous waste types. The currently available process tech-
nologies 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 Develop-
ment 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 forthe following waste
types:
• Cd-containing batteries, D006
• Pb nonwastewaters such as K069 in the noncalcium sul-
fate subcategory
• Hg wastes, P065, P092, and D009 (organics) prior to re-
torting [45]
• Pb acid batteries such as D008
• Zinc nonwastewaters such as K061 in the high zinc sub-
category
• Hg from wastewater treatment sludge such as K106 in
the high-Hg subcategory
• Hg such as U151 in the high-Hg subcategory.
SITE Demonstration and Emerging Technologies Pro-
gram Projects—Completed SITE demonstrations applicable to
soils contaminated with the metals of interest include
• RUST Remedial Services, Inc. (X-Trax Thermal Desorp-
tion)
• 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 pyromet-
allurgical treatment may be directed to Mrs. Marta K. Richards
(NRMRL) at (513) 569-7692.
of 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 treat-
ment trains include soil pretreatment, physical separation de-
signed to decrease the amount of soil requiring treatment, addi-
tional treatment of process residuals or off-gases, and a variety
of other physical and chemical techniques, which can greatly
Technology Alternatives for Remediation of Soils Contaminated with As, Cd, Cr, Hg & Pb
15
-------
Table 8 Typical Treatment Trains
Contain- Vitrifi- Soil Pyromet- Soil
ment S/Sa cation3 Washing allurgical Flushing
Pretreatment
Excavation • E,P I,E
Debris removal E,P E
Oversize reduction E,P E
Adjust pH • I,E,P
Reduction [e.g., Cr(VI) to Cr(lll)] • I,E
Oxidation [e.g., As(lll) to As (V)] • I,E
Treatment to remove or destroy organics I,E
Physical separation of rich and lean fractions I,E,P E
Dewatering and drying for wet sludge P E
Conversion of metals to less volatile forms
[e.g.,As203toCa3(As04)2] E
Addition of high temperature reductants
Pelletizing
Flushing fluid delivery and extraction system
Containment barriers • I.E.P I
Post-treatment/Residuals Management
Disposal of treated solid residuals (preferably
below the frost line and above the water table) I.E.P E
Containment barriers I,E,P I,E
Off-gas treatment I,E,P I,E
Reuse for onsite paving P
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 E
Reuse of metal or metal compound
Further processing of metal or metal compound
Flushing liquid/groundwater treatment/disposal •
a Technology has been divided into the following categories: I = In Situ Process; E = Ex Situ Process; P= Polymer Microencapsulation (Ex Situ)
improve the performance of the remediation technology. Table 8 onstration Reports, and EPA electronic databases. The reader is
provides examples of treatment trains used to enhance each cautioned that the cost estimates generally do not include pre-
metal remediation technology that has been discussed. treatment, site preparation, regulatory compliance costs, costs
for additional treatment of process residuals (e.g., stabilization
Cost Ranges Of Remedial Technologies of incinerator ash or disposal of metals concentrated by solvent
extraction), or profit. Since the actual cost of employing a reme-
Estimated cost ranges for the basic operation of the tech- dial technology at a specific site may be significantly different
nology are presented in Figure 1. The information was compiled than these estimates, data are best used for order-of-magnitude
from EPAdocuments, including Engineering Bulletins, SITE Dem- cos^ evaluations.
16 Technology Alternatives for Remediation of Soils Contaminated with As, Cd, Cr, Hg & Pb
-------
Containment3
S/S
Vitrification
Soil Washing
Soil Flushing11
Pyrometallurgical
10 to 90
-> 60 to 290
-> 400 to 870
-> 60 to 245
60 to 163
250 to 560
0 100 200 300 400 500 600
Cost ($/ton)
3 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
700
800
900
1000
Figure 1. Estimated Cost Ranges of Metals Remediation Technologies
[Source: VISIT! (Ver. 3.0), various EPA Engineering Bulletins and TRD
of Additional Information
The following databases, reports, and EPA hotlines offer ad-
ditional 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(AT-
TIC) database. U.S. EPAAssistance, (908)321-6677. AT-
TIC modem contact, (703) 908-2138 (1200 or2400 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 communi-
cation, (301)589-8366.
EPA Online Library System (OLS). Includes the following
applicable databases: The National Catalog, The Haz-
ardous Waste Superfund Data Collection, and The Chemi-
cal 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.
Risk Reduction Engineering Laboratory (RREL)Treatabil-
ity Database. Available on disk and through the ATTIC
database. Contact Glenn Shaul, (513) 569-7589.
Vendor Information System for Innovative Treatment Tech-
nologies (VISITT) database. PC-based database avail-
able on disk, (800) 245-4505 or (703) 883-8448.
ReOpt/Remedial Action Assessment System (RAAS) da-
tabases. 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. Re-
mediation 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, Construc-
tion, and Evaluation of Clay Liners for Waste Manage-
ment Facilities, EPA/530/SW-86/007F. November 1988.
U.S. Environmental Protection Agency. Technical Guid-
ance Document: Final Covers on Hazardous Waste Land-
fills and Surface Impoundments, EPA/530-SW-89-047.
July 1989.
U.S. Environmental Protection Agency. Technical Guid-
ance Document: Inspection Techniques for the Fabrica-
Technology Alternatives for Remediation of Soils Contaminated with As, Cd, Cr, Hg & Pb
17
-------
tion of Geomembrane Field Seams, EPA/530/SW-91/051.
May 1991
U.S. Environmental Protection Agency. Technical Guid-
ance Document: Construction Quality Management for
Remedial Action and Remedial Design Waste Contain-
ment Systems, EPA/540/R-92/Q73. October 1992.
U.S. Environmental Protection Agency. Technical Guid-
ance Document: Quality Assurance and Quality Control
for Waste Containment Facilities, EPA/600/R-93/182.
1993.
Acknowledgments
This Engineering Bulletin was prepared by the U.S. Envi-
ronmental Protection Agency, Office of Research and Develop-
ment (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 WorkAs-
signment 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.
The following EPA personnel have contributed their time and comments by participating in peer reviews of sections of the
document:
Edwin Barth, 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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20
Technology Alternatives for Remediation of Soils Contaminated with As, Cd, Cr, Hg & Pb
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