United States
Environmental Protection
Agency
Risk Reduction
Engineering Laboratory
Cincinnati, OH 45268
Research and Development
EPA/600/S2-89/034 Jan. 1990
4>EPA Project Summary
Cleaning Excavated Soil Using
Extraction Agents:
A State-of-the-Art Review
R. Raghavan, E. Coles, D. Dietz
In response to the RCRA Hazardous
and Solid Waste Amendments of 1984
prohibiting the continued land
disposal of untreated hazardous
wastes, the U.S. Environmental
Protection Agency (EPA) has
instituted a research and develop-
ment program for new technologies
to treat RCRA and Superfund wastes.
As part of this research program,
technologies applicable to cleaning
excavated soils were reviewed.
This report reviews the state-of-the-
art of soil cleaning technologies and
their applicability to Superfund sites
in the United States. The review
includes Superfund site soil and
contamination characteristics; as
well as soil washing technologies,
their principles of operation, and
process parameters. The technical
feasibility of using soil washing
technologies at Superfund sites in
the United States is assessed.
Contaminants are classified as vol-
atile, hydrophilic, or hydrophobic
organics; PCBs; heavy metals; or
radioactive material. Soils are classi-
fied as either sand, silt, clay, or waste
fill.
Three generic types of extractive
treatments are identified for cleaning
excavated soils: water washing aug-
mented with a basic or surfactant
agent to remove organics, and water
washing with an acidic or chelating
agent to remove organics and heavy
metals; organics-solvent washing to
remove hydrophobic organics and
PCBs; and air or steam stripping to
remove volatile organics.
Although extraction of organics
and toxic metal contaminants from
excavated sandy/silty soil that is low
in clay and humus content has been
successfully demonstrated at several
pilot-plant test facilities, extraction
from clay and humus soil fractions is
more complicated and requires
additional pilot-scale testing before
application at Superfund sites.
This Project Summary was devel-
oped by EPA's Risk Reduction
Engineering Laboratory, Cincinnati,
OH, to announce key findings of the
research project that is fully
documented in a separate report of
the same title (see Project Report
ordering information at back).
Introduction
Under the Comprehensive Environ-
mental Response, Compensation, and
Liability Act of 1980 (CERCLA) as
amended by the Superfund Amendments
and Reauthorization Act of 1986 (SARA),
cleanup activities at hazardous waste
sites must reduce the toxicity, mobility,
and volume of hazardous substances.
The 1984 Hazardous and Solid Wastes
Amendment (HSWA) to the Resource
Conservation and Recovery Act (RCRA)
was created in large part in response to
citizen concerns that existing methods of
hazardous waste disposal, particularly
land disposal, were not safe.
The land ban provisions of the 1984
RCRA amendments have given con-
siderable impetus to developing more
economical and effective means of
treating hazardous waste. EPA is now
sponsoring research on new treatment
technologies to destroy, detoxify, or
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incinerate hazardous waste; on ways to
recover and reuse hazardous waste; and
on methods to decrease the volume of
hazardous waste requiring treatment or
disposal. On-site treatment technologies
that remove contaminants or decrease
contaminant levels may achieve better
hazard control than containment
techniques. In addition, as landfill
disposal becomes more expensive and
as hazardous waste transportation is
more stringently regulated, on-site waste
treatment technologies will become more
desirableif they are technically
demonstrated, environmentally safe, and
economical. One of the research areas
initiated by the EPA is use of extraction
agents for washing excavated
contaminated soil. Washing excavated
soil holds promise for being applicable to
all contaminants.
Soil Washing for Safe On-site
Redeposit
Soil washing employing extraction
agents consists of soil excavation, above-
ground treatment, isolation and removal
or destruction of the contaminant, and
redeposit of the cleaned soil. Each of the
above-ground treatment techniques for
separating the contaminant from the soil
uses an extraction agent~a liquid, gas,
chemical additive, or combination of
agents-that mobilizes the contaminant,
which is chemically or physically
attached to the soil particles.
This report reviews the technologies
that may be applicable for cleaning
excavated soil. Physical separation and
extraction technologies are examined and
evaluated for their applicability to soil
washing.
Specifically, this report:
1. surveys the contaminants (by type
and concentration) and soil (by type and
quantity) at the various National Priority
List (NPL) sites to define the most
frequently occurring problems at these
sites,
2. reviews the extractive treatment
technologies that have potential for
cleaning the contaminants from soils, and
3. recommends areas for future
research.
Patterns of Contamination at
NPL Sites
The choice of soil washing method will
depend on the type of contaminant and
type of soil at the site. Therefore, NPL
site information files were surveyed to
determine the contaminants and soil
types prevalent at these sites.
To determine! the patterns of contam-
ination at NPL I sites, contaminants are
categorized into| major groups from a soil
washing perspective, based on the
following soil washing parameters:
« water solubjlity
vapor pressure
octanol/water partition coefficient
density
These parameters are used to create
contaminant categories:
hydrophilic organic compounds
(volatile and nonvolatile)
hydrophobic organic compounds
volatile organic compounds
heavy metals
PCBs i
« radioactive hnaterial
other organ(cs
Soil is classified according to its major
particle size fraction as sand, silt, or clay.
Since the soil and contaminants together
determine the effectiveness of a
particular soil! washing method, the
contaminant arjd soil types are cate-
gorized under one of 32 soil-contaminant
type pairs. Derived from soil and
contaminant data at 82 NPL sites in
USEPA's Region II (consisting of New
York, New Jersjey, Puerto Rico, and the
Virgin Islands); these soil-contaminant
type pairs are listed together with their
frequency of occurrence. Three pairs
occur at significantly greater frequency
than do the remaining 29 pairs. These
are hydrophobic volatile compounds,
hydrophobic nonvolatile compounds, and
heavy metals-all of them in sites with
sandy soil.
Procedure
Three major bxtraction techniques are
used to clean [soil: water washing with
extractants, solvent extraction, and air
stripping. >
Water washing with extractive agents is
applicable for cleaning nonvolatile
hydrophilic and hydrophobic organics
and heavy metals from soil. The solvent
extraction processes show potential for
cleaning nonvplatile hydrophilic and
hydrophobic organics from soil. Air strip-
ping processes are limited to cleaning
soil of volatile organics.
Most of the
involve mixing
soil cleaning processes
the extractant with soil,
followed by solid/liquid separation where
the cleaned so'il is separated from the
extractant fluid.} The extractant is then
cleaned of the contaminant and recycled
as required. j
Water Washing
In water washing with extractive agents,
the washing solutions can be basic
aqueous solutions (caustic, lime, slaked
lime, or industrial alkali-based washing
compounds); acidic aqueous solutions
(sulfuric, hydrochloric, nitric, phosphoric,
or carbonic acids); or solutions with
surfactant or chelating agents. Use of
hydrogen peroxide, sodium hypochlorite,
and other oxidizing agents, which
chemically change the contaminants,
often facilitates the washing process. A
strong (highly ionized) basic or surfactant
solution can be used for some organics
extraction, and strong (highly ionized)
acidic or chelating agent solutions can be
used for metals extraction.
In cleaning soil by aqueous extraction,
large objects -are removed by screening
and then cleaned separately. The soil is
then mixed thoroughly with water and
extraction agents to remove the
contaminants from the soil. This is
followed by solid/liquid separation where
the coarse fraction of the soil is
separated. The extraction agent with
contaminant and smaller soil particles
(clay and fine silt) undergoes further
solid/liquid separation where fine soil
fractions are separated as much as
possible. The extraction agent is cleaned
and recycled. The separated soil fraction
undergoes post-treatment where it is
cleaned of any residual extraction fluid.
Solvent Extraction
Solvent extraction using organic
solvents may be used to clean soil
contaminated with high concentrations of
nonvolatile hydrophobic organics.
Hydrophilic organics can be removed by
solvent extraction but are most effectively
removed by water washing, as discussed
previously. The choice of a suitable
solvent depends primarily on chemical
structure of the contaminant, solvent
extractive capacity, soil type, and
equilibrium characteristics. In addition to
these, the solvent should be stable and
must have favorable density, viscosity,
and interfacial tension properties. There
should be a sufficient difference between
the boiling points of the contaminated
solute and the solvent to facilitate post-
treatment separation.
Leaching and immersion extraction are
the two general extraction techniques. In
its most typical form, leaching is a batch
extraction operation in which the
screened soil is deposited in a screened-
bottom tank inside retaining walls, and
solvent is sprayed over it. The solvent
leaches the contaminant from the soil.
For low-solubility contaminants, fine
soils like clay and silt or soils with a very
low residual contaminant content, the
leaching process is unacceptable be-
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cause of slow mass transfer rates. For
these cases, the solid is dispersed into
the liquid in an immersion extraction. In
its simplest form, an immersion extractor
is an agitated tank filled with the solvent,
in which the soil is suspended and
thoroughly mixed. When the extraction
equilibrium has been reached, the
agitation is stopped and the solids
allowed to settle.
The most easily treated soil is a coarse
sand that retains, after free gravity
drainage, approximately 2 to 3 wt%
solvent. For finer-grained soils, centrifu-
gation or thermal desorption may be
necessary to obtain low solvent residuals.
Soil/solvent separation must be
effected to recycle solvent. For coarse
easy draining soil, solvent is separated
by gravity drain. For hard-to-settle soil
the operation requires centrifugation or
filtration. Residual solvent is normally
removed from separated soils by either
solvent displacement or gas, steam, or
vapor stripping.
Contaminants are generally removed
from the solvent by distillation, assuming
a difference in boiling point for the sol-
vent and contaminated material; other-
wise an extractive technique may first be
needed. Small amounts of contaminant
may be recycled with the solvent and
may be present in a subsequent soil
extraction.
Air Stripping
Air stripping is normally used to
remove volatile organic compounds
(VOCs) from soil. To strip VOCs from
soil, the VOCs must be vaporized. The
stripping may be done at ambient
temperatures, or heat may be used to
increase the rate of vaporization. Air and
steam are the most commonly used
stripping gases. Adsorption or com-
bustion removes VOCs from a circulating
air stream. When steam is used as the
stripping medium, the steam can be
removed by condensation, and a
relatively concentrated vapor of VOC
remains for disposal.
In general, any system that is
employed to dry solids can also strip
VOCs from soil. These systems consist
of: a gas/solids stripping device; a
stripping gas circulating device; and a
means to remove, recover, or destroy the
VOCs in the stripping gas.
Results and Discussion
Water Washing
To date, several aqueous extraction
systems for cleaning excavated contam-
inated soil have been demonstrated on a
pilot scale; some of these soil pretreat-
ment/extraction methods are listed in
Table 1.
Solvent Extraction
Large quantities of solids (ores, sugar
beets, etc.) have been extracted using
continuous countercurrent extractors
such as Dravo's Rotocel (rotary-type)
Endless-Belt Extractor,* Lurgi's Frame
Belt Extractor, the DeSmet Beit Extractor,
and the BMA Diffusion Tower. Some of
these solvent extraction processes used
for treating soil are listed in Table 2.
Air Stripping
When treating soils that adhere and
form large particles (i.e., are fine-grained
and tend to agglomerate), a Holo-flite
screw, rotary kiln/dryer, or Hereschoff
furnace may be used for stripping.
When processing granular free-flowing
sandy soils, which disperse easily, fluid
bed combustors of the circulating or
bubbling types are applicable. Table 3
describes this equipment and states
process operating conditions.
Conclusions and
Recommendations
The following conclusions have
emerged from this literature review of
theoretical, bench-scale, and pilot-scale
investigations of state-of-the-art tech-
nologies for the extraction of
contaminants from soil.
Pilot-scale tests conducted by TNO,
Heijmans, HWZ Bodemsanering,
BSN, and Ecotechniek show that
sand or silt can be washed.
Above-ground extraction of organics
and heavy metals from sandy soil
containing very low levels of clay is
feasible.
Above-ground extraction of organics
and heavy metals from clay soil
fractions has not been demonstrated
on a pilot scale.
Separation of the extractant from the
soil and regeneration of the ex-
tractant have not been successfully
demonstrated for clay soils.
Contaminant extraction experience
does provide enough information to
support a decision on the technical
feasibility of applying soil washing at
NPL sites.
More applied pilot-scale testing must
be conducted to support any
statement on the environmental and
economic practicability of extraction
technologies.
Experience with contaminant removal
via water washing at the bench, pilot,
and prototype scales supports
application of the technology for
cleaning sandy and silty soils.
Economic competitiveness of soil
washing compared to other remedial
technologies such as incineration or
fixation is indicated. Further study is
needed to establish fixed and
operating costs for aqueous
extraction of soil contaminants.
A program is needed that would
include the following components:
Characterization of soil at NPL sites
from a soil washing perspective. This
would include particle size dis-
tribution, mineralogical observations,
physical and chemical analyses, etc.
Bench-scale testing to establish the
required processing configurations
and operating conditions for the
various wastewater treatment and
regeneration subsystem options.
Preliminary process design, sizing,
and costing of a modular trans-
portable pilot-plant system to
determine process economics for
comparison with incineration and
other remedial technologies.
Design, construction, and operation
of a modular transportable pilot-scale
unit to demonstrate its applicability at
selected NPL sites.
Research and development efforts
toward broadening the application to
washing of high-clay soils, if eco-
nomically justified.
"Mention of trade names or commercial products
does not constitute endorsement or recom-
mendation for use.
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Table 1.
Aqueous Phase Extraction Processes
Aqueous Extraction Process
Capacity
\tonslhr
Year
Operation
Commenced
Comments
Netherland's bromide removal from sand Pilot scale
(Netherlands Organization for Applied \
Research) '
7982 Organic bromide compounds
removed from sandy soil
containing less than 10% clay
and humus. Extract/on agent
was caustic solution (pH > 11).
Extractant-to-soil ratio: 2:1.
Heijmans Milieutechniek extractive
cleaning of heavy metals and cyanide
from soils
10-15 1985 Process has potential for
I cleaning soil contaminated with
I cyanides, heavy metals, and
water-immiscible and low-
i density hydrocarbons.
HWZ Bodemsanering extractive cleaning
of cyanide-contaminated sandy soils
20 1984 The extracting agent used is a
detergent.
Ecotechniek thermal washing of sandy soil , 20
contiminated with crude oil '
7982 Sands containing 200,000 ppm
of oil were cleaned to approx.
20,000 ppm.
Bodemsanering Netherlands (BSN) high- \ 20
pressure washing of sandy soil \
contaminated with oil i
1983 This plant is transportable.
Klockner Umwelttechnik high-pressure j 75-40
water jet for cleaning contaminated sandy j
so;7s :
7987 This process is a modified
version of the BSN process
and is effective for cleaning
soils with fines (<63 im) not
exceeding 20%. Water
pressure 5,075 psi.
Harbauer soil cleaning system
EWH-Alsen-Breitenburg cleaning of sandy Pilot scale-
soil contaminated with oil
I 40 1987 This wet extraction process
I uses hydraulically produced
: oscillation/vibration to achieve
I initial separation of soil
j particles and contaminants.
I So;7 recovery is approximately
! 95% of input volume.
Not Custom reagents added to
8 to 10 Available water. Water-to-soil ratio is 1:1;
m3lhr cleaning efficiency is 95%.
Lee's Farm lead extraction from soils Pilot scale-
30
1985 Crushed soil (lead contarni-
(for short nated) was washed with a 30%
duration) EDTA aqueous solution using
an inclined-screw washing unit.
Tests were used to specify
equipment that can handle
clays.
USEPA's extraction of spilled hazardous Pilot scale-
matdrials from excavated soil ! 6
7984 Process using EDTA removed
(limited 97% of the lead in soil
operation) containing 47,000 ppm. The
plant is mobile.
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Table 2. Solvent Extraction Processes
Solvent Extraction Process Capacity (scale)
Year Operation
Commenced
Comments
So/Vex
CF Systems Corporation
Cambridge, MA
Pilot Scale
Commercial scale, 1,000
barrels/day
Basic Extraction Sludge
Treatment (BEST)
Prototype commercial scale,
WO tons/day design
1984 A kerosene-water solvent removes PCBs from
soil. The PCB leaching percentage is 84%.
Kerosene is recovered, decontaminated, and
recycled. Kerosene residuals in soil have been
about 25% of the kerosene charged.
1988 Propane at or near its critical point is used to
dissolve organic contaminants present in a
sludge-water slurry. Typically, 99% of the
organics are extracted from the sludge.
Propane is separated from the organics by
flashing, and then is recompressed, cooled,
and recycled to the extractor.
1986 Triethylamine (TEA) extracts oil from oily
sludges. TEA is soluble in water below 65°F,
insoluble above 65 "F. Hazardous oil is
recovered, not destroyed. Operation of this
multi-step process is highly sophisticated.
Table 3.
Volatile Organic Stripping
Equipment Name
Equipment for Air Stripping VOCs from Soil
Equipment Description
Process Operating Conditions
Holo-Flite Screw
Rotary Kiln/Dryer
Hereschoff Furnace
Circulating Bed Combustor
Bubbling Bed Combustor
A jacketed trough houses a double-screw
mechanism. Heat transfer medium enters the
hollow screw shafts and flights (indirect
heating). Air contracts soil directly. Removal
efficiency 99%.
Rotating Drum. VOCs can be evaporated
using direct or indirect heating.
Soil fed to the center of the top tray is moved
by rotating flights to the outer edge, falls to
second tray, moves to center on second tray,
falls to third tray, etc.; gas moves counter-
current to the soil. ID fan required.
Hot gas flows countercurrent to soil and
entrains the soil. Entrained soil is separated
from hot flue gases in a cyclone and
recirculated to the bed. A solids draw-off is
provided.
Gas is blown from a distributor at bottom of
bed. Bed is maintained below fluidization.
Soil discharge temp. = 50° to 150°C. Soil
residence time = 30 to 90 mm. Air inlet temp.
= ambient to 90°C. Circulating oil temp. =
100° to 300°C.
Temperature in the kiln controlled at 100° to
400°C if the character of the soil is to be
maintained (or to avoid fouling the walls)
Temperatures to 500°C are attainable.
Requires free-flowing soil feed.
Residence time controlled by bed height or
soil feedrate.
U. S. GOVERNMENT PRINTING OFFICE: 1990/748-012/07207
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A Raghavan, E. Coles, and D. Dietz are with Foster Wheeler Enviresponse, Inc.,
Livingston, NJ 07039.
Dar/ene Williams is the EPA Project Officer (see below).
The complete report, entitled "Cleaning Excavated Soil \Using Extraction Agents:
A State-of-the-Art Review," (Order No. PS 89-212 757/AS; Cost: $15.95,
subject to change) will be available only from: \
National Technical Information Service '
5285 Port Royal Road
Springfield,VA22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Releases Control Branch
Risk Reduction Engineering Laboratory :
U.S. Environmental Protection Agency
Edison, NJ 08837
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
BULK RATE
POSTAGE & FEES PAID
EPA
PERMIT No. G-35
Official Business
Penalty for Private Use $300
EPA/600/S2-89/034
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