EP A/600/A-94/078
Chapter 15
Contaminant Leaching from Solidified-Stabilized
Wastes
Overview
Paul L Bishop
Department of Civil nnd Environmental Engineering, University
of Cincinnnti, Cincinnati, Oil 45221
The current state-of-the-art of solidification/stabilization (S/S)
technologies is reviewed. This includes the legal impetus and basis
for use of solidification/stabilization for hazardous wastes or
contaminated soils, the principles and chemistry of contaminant
immobilization within the waste form matrix, leaching mechanisms,
and environmental factors affecting leachability. It is shown that S/S
processes can be very effective at immobilizing certain waste
materials, but other wastes may not be amenable to these processes.
Stabilization/solidification (S/S) processes have been developed to
concurrently eliminate land disposal of liquid wastes and minimize leaching
of the resultant solid waste after disposal. These processes are also being
used to remediate existing hazardous waste sites by markedly reducing the
rate of leaching of pollutants from contaminated soils and debris.
Land disposal of wastes should not be the primary means of waste
Jisposal if other alternatives are available. Waste reduction, recycle and reuse
ire much superior alternatives. Where this is not possible, destruction or
lctoxification options should be considered. There will always be some
vastcs, though, where these options are not viable. Most reuse or destruction
>perations will result in some residue which cannot be further reduced and
vhich must be disposed of on land. This includes flyash and bottom ash from
ncineration processes, mixed metal sludges, foundry sands, heavy metal
rontanunated soils, etc. Direct land disposal of these wastes could lead to
>otcntially serious consequences, however, if contaminants in the waste leach
mo ground or surface waters. Many of these wastes can be effectively treated
>y stabilization/solidification processes so as to minimize leaching to
nvironmcntally acceptable levels.
5. 1IISIIOI' I^eachinf'from Solidified—Stabilized Wastes
303
Much of the impetus for S/S of hazardous wastes has been provided by
the Resource Conservation and Recovery Act (RCRA) of 1976, including the
1984 amendments, and the Comprehensive Environmental Response, Liability
and Recovery Act (CERCLA) of 1980, later reauthorized in 1986 as the
Supcrfund Amendments and Reauthorization Act (SARA). RCRA deals
primarily with the generation, handling, treatment and disposal of hazardous
wastes, while CERCLA and SARA established a massive remedial program
for the cleanup of existing sites that threaten the environment.
In 1985, under RCRA authority, the U.S. EPA banned the disposal of
bulk hazardous liquids into landfills, necessitating solidification of the waste.
Stabilization/solidification technologies have been specified by EPA as "best
demonstrated available technologies" for a number of waste streams, and
some can be used as a basis for "delisting" a waste as hazardous under RCRA.
Under SARA provisions, permanent treatment of contaminated soil and
debris is being emphasized rather than the use of nontreatment containment
systems such as covers, grout walls and similar methods. A large number of
Superfund sites are now using S/S treatment processes for soil treatment.
Stabilization/solidification technology refers to treatment processes that
are designed to (1) improve the handling and physical characteristics of the
waste (2) decrease the surface area of the waste mass across which transfer
or loss of contaminants can occur, and/or (3) limit the solubility of any
hazardous constituents of the waste such as by pH adjustment or sorption
phenomena.
Stabilization processes attempt to reduce the solubility or chemical
reactivity of a waste by changing its chemical state or by physical entrapment.
The hazard potential of the waste is reduced by converting the contaminants
to their least soluble, mobile or toxic form. Solidification refers to techniques
that encapsulate the waste in a monolithic solid of high structural integrity.
Solidification does not necessarily involve a chemical interaction between the
wastes and the solidifying reagents, but may mechanically bind the waste into
the monolith. Contaminant migration is restricted by vastly decreasing the
surface area exposed to leaching and/or by isolating the wastes within a
relatively impervious capsule (/].
The most important factor in determining whether a particular
stabilization/solidification process is effective in treating a given waste is the
reduction in the short- and long-term leachability of the waste [2], Leaching
can be defined as the process by which a component of waste is removed
mechanically or chemically into solution from the solidified matrix by the
passage of a solvent such as water. Resistance to leaching will depend on
both the characteristics of the solidified/stabilized waste and on those of the
leaching medium it will come into contact with.

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304
EMERGING TECHNOLOGIES IN HAZARDOUS WASTE MANAGEMENT II
This paper discusses the principles of contaminant immobilization in
solidified/stabilized wastes, leaching mechanisms from these materials, factors
which affect leaching, and models which can be used to predict short- and
long-term leaching rates.
Principles or Immobilization
Stabilization/solidification processes employ systems which both solidify
the waste mass and eliminate free liquids, and stabilize the contaminant in
their least soluble form. The overall objective is to minimize the rate of
leaching of pollutants from the resulting waste form. These processes typically
involve the addition of binders and other chemical reagents to the
contaminated soil or sludge to physically solidify the waste and chemically
bind the contaminants into the monolith.
Binder systems can be placed into two broad categories, inorganic or
organic. Most inorganic binder systems in use include varying combinations
of hydraulic cements, lime, flyash, pozzolans, gypsum and silicates. Organic
binders used or experimented with include epoxy, polyesters, asphalt/bitumen,
polyolefins (primarily polyethylene and polybutadienc) and urea
formaldehyde. Combinations of inorganic and organic binder systems have
also been used. These include diatomaceous earth with cement and
polystyrene, polyurethane and cement, polymer gels with silicates, and lime
cement with organic modified clays [2],
Most immobilization processes currently in use involve hydraulic cements,
such as Portland cement, cement kiln dust, flyash, or other pozzolanic
materials. Consequently, the focus of this review will be on cement-based
processes.
The main components of cement are lime and silicates. Cementation of
the mixture begins when water is added, either directly or as part of the waste
being immobilized. First, a calcium-silicate-hydrate gel forms, followed by
hardening of the material as thin, densely-packed, silicate fibrils grow and
interlace. The hydration reactions form a variety of compounds as the cement
paste sets, including calcium hydroxides and calcium silicate hydrates. The
latter provides the cement's structural stability, while the former supplies large
amounts of entrapped alkaline material.
The water-to-cement ratio (W/C) is very important to the properties of
the final product. The volume of the cement approximately doubles upon
hydration, creating a network of very small gel pores. The volume originally
occupied by the added water forms a system of much larger capillary pores.
As the water-cement ratio increases, the percentage of larger pores increases,
substantially increasing the permeability of the waste form and increasing the
potential for contaminant leaching. A W/C of 0.48 by weight will fully
IS. lUSIIor Leaching from Solidified—Stabilized Wastes
305
hydrate the cement, leaving some free pore water, gel water and air voids.
Above this W/C, permeability increases rapidly, which could lead to increased
leaching rates. Because of economic reasons, though, these low water-binder
ratios are usually not feasible for waste immobilization. Very low
permeability is sacrificed for a decrease in the amount of binder required.
A number of factors affect the degree of immobilization, or fixation, of
constituents in the waste. Major factors include solubility minimization
through pH or redox potential control; chemical reaction to form carbonate,
sulfide or silicate precipitates; adsorption, chemisorption; diadochy
(substitution in the calcite crystal lattice); and encapsulation.
Not all wastes can be effectively treated by solidification/stabilization
technologies. The major category of wastes for which immobilization is
applicable are those which are essentially all inorganic. Cement and
pozzolan-based waste forms rely heavily on pH control for pollutant
containment. Cement-based waste forms typically have a pore water pH of
10-12 because of the excess lime present in the pores. These high pH values
are usually desirable for heavy metal immobilization because most metal
hydroxides have minimum solubility in the range of 7.5-11. Some metals,
though, arc amphoteric and have higher solubility at both low and high pH.
These metals may be soluble at the high pH of the pore water (see Figure 1).
Other contaminants, such as anions (arsenate, selenite, etc.), may be more
soluble at high pH than low. Metals may also precipitate as carbonates,
silicates or metal sulfides [3,4\.
Metal immobilization is primarily dependent on the extent of
solubilization of precipitated metals. This is governed by the solubility
product, K,0. Therefore, the solid metal concentration in the waste does not
affect the concentration in the pore water which can leach out; only the pore
water (leachant) composition will govern the amount of metal which will
leach. It is only the solubilized fraction which can diffuse out of the waste
form into the surrounding environment.
Cement and pozzolan-based systems rely heavily on hydroxide formation
for metal containment, but other factors can come into play. Shively et al. [5]
demonstrated that even after the alkali was leached from cement-based waste
forms, lead and chromium leaching was much lower than would be expected
from metal hydroxide solubilities. In this case, the metals were probably
bound into the silica matrix itself. Cote [3] also found differences between
calculated hydroxide solubility-pH curves and those determined empirically.
A number of agents which may be present in the waste may interfere
with the binding systems and lead to decreased immobilization of hazardous
constituents. Some of these are inorganic (certain metals, sulfates, etc.), while
others are organic (oil, grease, HCB, TCE, phenol, etc.) [6,7], For example,

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306	EMERGING TECHNOLOGIES IN HAZARDOUS WASTE MANAGEMENT II
Pb
/in
LCd
c
E
w
Cu
0.001
0.0001
6
7
8
9
10
11
12
pH
Figure 1. Solubilities of metal hydroxides as a function of pH.
15. lilSIIOl' Leaching from Solidified—Stabilized Wastes
307
some metals may temporarily inhibit setting of cement-based processes;
chlorides may decrease durability; oils, greases and other nonpolar organics
may inhibit setting and decrease long-term durability. Sulfates are a
particular problem because they cause the expansive compound etringite to
form.
Long-term immobilization of a contaminant incorporated into a cement
matrix depends primarily on the ability of the matrix to maintain its integrity.
Durability refers to the resistance of the matrix to chemical and physical
interactions in the environment. All compounds of cement hydration are
relatively insoluble in neutral water with the exception of Ca(OH)2. Lime will
leach easily, leaving a much more porous structure. Acids can dissolve the
matrix of hydrated cement, releasing many of the bound metals. Freeze-thaw
and wet-dry cycles can cause fracturing of the matrix, resulting in increased
liquid-solid interfaces where leaching can occur. little research has been
conducted on the influence of these factors on long-term containment of
heavy metals.
Very little research has been reported on immobilization of organics in
waste forms, and what has been reported is often contradictory. Several
researchers have reported chemical reactions between organic waste and
binder, resulting in immobilization, but most researchers report that these
positive results may actually be due to sorption effects, volatilization of
organics or dilution by reagent chemicals. Much more research is needed on
the stabilization/solidification of organics, because most principally inorganic
wastes which are suited for stabilization/solidification also contain appreciable
quantities of organics which may leach.
Organic constituents tend to retard cementitious reactions, inhibiting the
formation of a solid monolithic mass. Also, the organic components may be
easily leached from the waste form. Recently, research has been conducted
into the use of organically modified clays in order to overcome these
difficulties [£]. When these clays are mixed with cement-based stabilization
agents, they reportedly adsorb and retain organic pollutants while solidifying
organic wastes into a stable mass with low leaching potential.
The modified organophilic clays are made by mixing quaternary
ammonium ions with the clay. The [R4N]+ ions substitute for metal ions
present between the layers of alumina and silica in the clay minerals. This
yields clays that have both organic and inorganic properties. Introduction of
these ions increases the interplanar distance between clay plates allowing
organics to penetrate, and makes the polarity of the stationary phase more
compatible to that of the organic waste to be stabilized [9,10], Preliminary
studies indicate that chemical bonding between the organophilic clay binder
and certain organic wastes may occur, but it is too early to tell the long-term
fate of these complexes. It is possible that a unique binding mixture may be
required for each organic compound encountered.

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308
EMERGING TKCIINOLOGIES IN HAZARDOUS WASTE MANAGEMENT II
Leaching Mechanisms
A solidified waste is a porous solid at least partially saturated with water.
The pore water in the solid is in chemical equilibrium with the solid phase.
When the solid is exposed to leaching conditions, equilibrium is disturbed.
The resulting difference in chemical potential between the solid and the
leaching solution causes a mass flux between the solid surface and the
leachant. This in turn causes concentration gradients that result in bulk
diffusion through the solid [11,12], Figure 2 depicts the leaching mechanisms
in effect. Transport can either be by diffusion of metal ions from the solid
matrix surface into the bulk aqueous phase, or by dissolution into the water
in matrix pores and microfractures and then diffusion out. Consequently, the
porosity and integrity of the waste form is of major importance.
For any constituent to leach, it must first dissolve in the pore water of the
solid matrix. The amount of dissolution which occurs is dependent on the
solubility of the constituent and the chemical makeup of the pore water,
particularly its pH. Under neutral pH leaching conditions, the leaching rate
is controlled by molecular diffusion of the solubilized species. Under acidic
conditions, however, the rate will also be governed by the rate of penetration
of hydrogen ions into the solid matrix, since this establishes the speciation and
solubility of the contaminants present. Acid attacks pozzolanic-based paste
through permeation of pore structure and dissolution of ions that must diffuse
back through a chemically altered layer to enter solution. Acid consumes
most of the calcium hydroxide in the leached layer and leaves a highly porous
structure. Diffusion across this layer can be considered as a steady-state
process since the leached layer provides little resistance to diffusion. At the
leaching front, diffusion of hydrogen ions proceeds as if the unleached
medium is infinite and dissolution reactions occur simultaneously in the pores.
Proton transfer reactions arc usually very fast with half-lives less than milli-
seconds. Hence, the dissolution reactions can be treated as diffusion-
controlled fast reactions. The whole process then can be described as steady-
state diffusion across the leached layer and unsteady-state diffusion controlled
fast reactions in the porous leaching front [13).
Figure 3 depicts theoretical concentration gradients produced in the
waste form during leaching [72]. The II* ions in the penetrating leachant
react with metal hydroxides in the waste form, solubilizing the metal and
reducing the H* concentration. The result is a leached layer where II* is
essentially totally consumed. The soluble metal concentration in the pore
water peaks at the leaching front. There is a gradient for the metal to diffuse
out to the surface, but it can also move further back into the matrix where it
reacts with the excess alkalinity and reprecipitates. Thus a narrow zone
behind the leaching front is denser than the bulk solid.
IS. BISHOP Leaching from Solidified—Stabilized Wastes
309
WASTE MATRIX
INTERFACE
SURFACE
DISSOLUTION

DIFFUSION
MATRIX DIFFUSION
SURFACE
PHENOMENA
UPTAKE
TORTUOSITY
LEACH
RATE
f LEACHANT
I RENEWAL
Figure 2. Schematic showing leaching mechanisms from a waste product.
(Reprinted with permission from ref. 12. Copyright 1989 American Society
for Testing and Materials.)
AQUEOUS
SOLUTION
METAL
POROUS MATRIX
INSOLUBLE
METAL
PROFILE
INTERFACE
REGION OF
RE-PRECIPITATION
REGION UNAFFECTED
BY LEACHING
SOLUBLE METAL
PROFILE
H PROFILE
LEACHED
~WE
Figure 3. Concentration gradients during leaching. (Reprinted with
permission from ref. 12. Copyright 1989 American Society for Testing and
Materials.)

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310
KMKKGING TKCIINOl.OC.lKS IN HAZARDOUS WASTK MANAOKMICNT II
Recent research in our laboratories confirms these findings [14],
Measurements of pH and heavy metals across the profile at various stages of
leaching demonstrate the leached layer, leaching zone and reprecipitation
zone. It has been shown that the leaching zone is very narrow, usually less
than 10/im deep. There is little or no change in pH from that of the leachant
all the way through the leached zone to the leaching front, indicating that all
available alkalinity is leached from the leaching zone as the leaching front
progresses.
Once solubilized, the constituent is transported from the solid matrix
through the leached zone to the leaching solution by molecular diffusion. The
flux of the constituent within the solid can be described by Fick's first law:
J=-D—	(1)
dz
where:
C = concentration of the constituent
D = diffusion coefficient
J = flux
z = distance
A semi-infinite medium diffusion model with uniform initial
concentration and zero surface concentration can be used to interpret the
kinetic data generated from serial batch leaching tests [75]. The equation
takes the form
Tam v
—1 - = 2
A. S
D N0 5
t
oj	(2)
where	an	= contaminant loss during leaching period n (mg)
A„	= initial amount of contaminant present in the
specimen (mg)
V	= volume of specimen (cm3)
S	= surface area of specimen (cm2)
tB	= time to end of leaching period n (sec)
De	= effective diffusion coefficient (cm2/sec)
The American Nuclear Society recommends use of a series of seven
batch leaching tests in order to determine the effective diffusion coefficient
in the Godbee and Joy model [16]. They suggest that the results be presented
15. IIISIIOI' Leaching from Solidified—Stabilized Wastes
311
1 - - 1 1 -	O)
as a "Icachability index", LX, equal to the average negative logarithm of Dc
" >4(5;
This index can be used to compare the relative mobility of different
contaminants on a uniform scale that varies from about 5(DC = 10"5 cm2/s,
very mobile) to 15 (De = 1015 cm2/s, immobile) \17\.
Leachatc generation is an extremely complex process. The free alkalinity
present in the pozzolanic-based paste maintains a high pH environment and
limits the metal Icachability of fixed wastes. Calcium hydroxide, which is
produced by the hydration reactions of the binder, provides most of the
buffering capacity. The leaching model shown above, however, does not
include the factor of acid strength of the leachant and cannot describe the
rate of movement of leaching front into the waste solid.
Cheng and Bishop [/J] have shown that the rate of advance of the
leaching front can be expressed as steady-state diffusion across the leached
layer. Figure 4 depicts the cumulative amount of calcium leached and Figure
5 shows the penetration distance into the waste matrix versus the square root
of time for two leachant acetic acid strengths (5 and 15 meq/g solids). The
plots are linear, as would be expected for diffusion-based processes. The
leachant strengths differ by a factor of 3.0, but the ratios of the amount of
leaching and the penetration distances differ by factors of 1.65 and 1.95,
respectively. This can possibly be explained by comparing the free hydrogen
ions arising from dissociation of the diffusing acetic acid. A 15 meq/g acetic
acid leachant contains approximately 1.73 times more free hydrogen ions than
the 5 meq/g leachant. This is a very close agreement, considering the
complex, heterogeneous nature of the waste matrix. Thus, leachant acid
strength may be very important in determining the rate and extent of leaching
of solidified/stabilized wastes. This concept is currently not included in any
leaching models, however.
Factors Affecting Leachability
There are a number of factors which can affect the leachability of a
particular solidified/stabilized waste form. Table 1 presents a summary of
some of the more important ones.

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312
KMKKC.1NG TECHNOLOGIES IN HAZARDOUS WASTE MANAGEMENT II
60000 -|
rn
o
o
' 4 0000
x
u
<1
5
Q 20000
J
<
o
Z>
O
0 0
I [) meg/g ' 'Ac
b meq/y HAc
~i—i—I—r "t"
10.0
I—1—1—I—I-
70.0
-i—r—i—|
30.0
SQUARE K001 OF TIME, (hr- !/?)
Figure 4. Cumulative calcium hardness leached versus square root of
time during leaching of portland cement solidified/stabilized wastes using
two strengths of acetic acid leachant.
GO
'i 5 0
0	<1 0
-X
1	-
l/l
g 'jo
O
r
o: 2 0
ijj
/
ui
a
1 0 .
o.o
0 0
i/g ,|At
—I—t—»—i—i—r~
. 10 0	20 0
SOUARI K00I or IIMI- (l>r 1/7)
30.0
Figure 5. Acid penetration distance versus square root of time during
leaching of portland cement solidified/stabilized wastes using two
strengths of acetic acid leachant.
15. IUS1IOI' Leaching from Solidified—Stabilized Wastes
313
Tabic 1. Factors That AHect Leaching from Solidified/Stabilized Wastes
Waste Form Factors
Contaminant binding mechanisms
Alkalinity
Surface-to-volumc ratio
Porosity and pore tortuosity
Durability
Leachant Factors
Composition (pH, acidity, Eh, chelating potential, etc.)
Leachant volume to waste form surface area ratio
Flow rate
Temperature
Obviously, the composition of the waste form determines the
physicochemical properties and the leaching mechanisms. Every effort should
be made to minimize the potential for leaching by improving the quality of
the waste form. Alkalinity is needed in the final product to maintain metals
in their most insoluble form and to buffer against acid dissolution. One of the
principle factors governing diffusion of soluble metals from the waste form is
the solid surface-to-volume ratio (see equation 3). The larger the monolith,
the smaller the surface-to-volume ratio and the smaller the potential for
leaching. The role of internal fractures in determination of the applicable
surface-to-volume ratio to use has not as yet been determined. Porosity and
tortuosity, which is a measure of the path length for a diffusing substance to
reach the surface of the waste product through winding and convoluted pores,
governs to a large extent the rate of diffusion to be expected. A highly porous
matrix will have a higher effective diffusion coefficient for a particular
contaminant than a less porous one, while wastes with a large tortuosity factor
will have reduced De. The "effective" diffusion coefficient in equation 3
modifies the true diffusion coefficient for the contaminant to account for these
variations in porosity and tortuosity. Waste durability is very important
because if an initially intact and acceptable monolith weathers poorly over
time, the porosity and surface-to-volume ratios will increase markedly,
resulting in increased leachability.
The leachant composition determines the reactions which will occur
within the waste form. Acid dissolution, oxidation-reduction reactions and
metal complexation can all occur, depending on the chemical composition of
the leachant. Increased leachant volume to surface area ratios and flow rates
will increase leaching because diffusing substances will be removed from the
monolith surface more rapidly and concentration gradients in the solid will be
greater. Temperature is not normally a factor for buried waste forms once
the exothermic heat of reaction of the cement has dissipated. Increases in
temperature result in increases in all reaction rates, including those involved
in leaching, which can be described by the Arrhenius equation.

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314
EMERGING TECHNOLOGIES IN HAZARDOUS WASTE MANAGEMENT II
Summary
Stabilization/solidification processes are being used to minimize the
potential for groundwater pollution from land disposal of hazardous wastes.
Many variations are used, but most rely on pozzolanic reactions to chemically
stabilize and physically solidify the waste. Portland cement alone or in
combination with fly ash, cement kiln dust, lime or other ingredients is the
principal solidifying agent used.
' f f	... ...
Stabilization/solidification processes are very effective at immobilizing
most heavy metals present in sludges, contaminated soils and other wastes.
They are not as effective at immobilizing toxic organic materials. Organically
modified clays are now being evaluated as an additive to S/S processes in
order to adsorb and retain these organic pollutants in the solidified waste
form.
The environmental acceptability of stabilization/solidification processes
will depend on the long-term ability of the waste form to retain contaminants.
i This will be governed by the chemical binding mechanisms involved and by
the durability of the waste form. Many S/S processes have been developed
which can pass regulatory leaching tests, but these tests do not indicate the
potential for leaching after long-term environmental exposure. Wide spread
acceptance of stabilization/solidification processes will be hampered until the
long-term durability of the waste form can be demonstrated.
Literature Cited
, 1. Cullinane, M., Jones, L. and Malorie, P., Handbook for Stabilization
' i < Solidification of Hazardous Wastes. U.S. EPA, EPA/540/2-86/001,1986.
2.	Poon, C., "A Critical Review of Evaluation Procedures for
Stabilization/Solidification Processes," Environmental	Aspects—o£
Stabilization and Solidification of Hazardous and Radioactive Wastes.
ASTM STP 1033, American Society for Testing and Materials,
Philadelphia, 1989, pp. 114-124.
3.	Cote, P., Contaminant Leaching from Cement-Based Waste Forms Under
Acidic. Conditions. Ph.D. Dissertation, McMaster University, Hamilton,
Ontario, 1986.
4.	Bishop, P., "Leaching of inorganic hazardous constituents from
stabilized/solidified hazardous wastes," Hazardous Wastes and Hazardous
Materials. 1988. voL 5. pp. 129..
5.	Shively, W., Bishop, P., Brown, T. and Gress, D„ "Leaching Tests of
Heavy Metals Solidified and Stabilized with Portland Cement," JmniaL
Water Pollution Control Federation. 1986, vol. 58, pp. 234-241..
IS. IJlSllor Leaching from Solidified—Stabilized Wastes
315
6.	Wiles, C., "A review of solidification/stabilization technology," Journal
of Hazardous Materials. 1987, vol 14, pp. 210.
7.	Jones, L., Interference Mechanisms in Waste Solidification/ Stabilization
. Processes. Final report for U.S. EPA, IAG No. SW-219306080-01-0,1988.
8.	Alther, G., Evans, J. and Pancoski, S., "No Feet of Clay," Civil
Engineering. 1990, vol. 60, pp. 60-61.
9.	Soundararajan, R., Barth, E. and Gibbons, J., "Using an Organophilic
Clay to Chemically Stabilize Waste Containing Organic Compounds,"
Hazardous Materials Control. 1990, vol. 3, pp. 42-45.
10.	P1ZI Associates, Inc., "Use of Organophilic Clays for Organic
Stabilization," unpublished report to U.S. EPA, Cincinnati, OH, 1990.
11.	Conner, J., Chemical Fixation and Solidification of Hazardous Wastes.
New York: Van Nostrand Reinhold, 1990.
12.	Cote, P., Bridle, J. and Bcnedek, A., "An approach for evaluating long-
term leachability from measurement of intrinsic waste properties,"
Hazardous and Industrial Solid Waste Testing and Disposal. ASTM S IT
933, American Society for Testing and Materials, Philadelphia, 1989, pp.
63-78.
13.	Cheng, K. and Bishop, P., "Developing a kinetic leaching- model for
solidified/stabilized hazardous wastes." Journal of Hazardous Materials, in
press.
14.	Cheng, K. and Bishop, P., unpublished data, University of Cincinnati,
Cincinnati, Oil.
15.	Godbee, H. et al., "Application of mass transport theory to the leaching
of radionuclides from solid waste," Nuclear and Chemical Waste .
Management. 1980, vol. 1, pp. 29.
16.	American Nuclear Society, "Measurement of the Leachability of
Solidified Low-Level Radioactive Wastes by a Short-Term Procedure,"
1986.
17.	Cote, P. and Hamilton, D., "Leachability comparison of four hazardous
waste solidification processes," Proceedings of the 38th Annual Purdue
Industrial Waste Conference. 1983, vol. 38, pp. 221.
RlLCr.lvi.lj April 5, 1991

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TECHNICAL REPORT DATA
(Please read Inunctions on the reverse before comple
1. REPORT NO. 2.
F.PA/600/A-94/078
3.
4. TITLE AM O SUBTITLE
Contaminant Leaching from Solidified-Stabilized Wastes
5. REPORT DATE
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
Paul L. Bishop
8. PERFORMING ORGANIZATION REPORT NO.
9 PERFORMING ORGANIZATION NAME AND ADDRESS
Department of Civil and Environmental Engineering
University of Cincinnati
Cincinnati, OH 45221
10. PROGRAM ELEMENT NO.
1 1. CONTRACT/GRANT NO.
12. SPONSORING AGENCY NAME AND ADDRESS
Risk Reduction Engineering Laboratory — Cincinnati, OH
Office of Research and Development
U.S. Environmental Protection Agency
Cincinnati, OH 45268
13.	TYPE OF REPORT AND PERIOD COVERED
Bk Chapter
14.	SPONSORING AGENCY CODE
EPA/600/14
15. SUPPLEMENTARY NOTES
Diana Kirk (513) 569-7674 Emerging Technologies in Hazardous Waste Management II,
American Chemical Society, 1991r Chester 15, d:302-315
16. abstract stabilization/solidification processes are being used to minimize the po-
tential for groundwater pollution from land disposal of hazardous wastes. Many var-
iations are used, but most rely on pozzolanic reactions to chemically stabilize and
physically solidify the waste. Portland cement alone or incombination with fly ash,
cement kiln dust, line or other ingredients is the principal solidifying agent used.
Stabilization/solidification processes are very effective at immobilizing most heavj
metals present in sludges, contaminated soils and other wastes. They are not as effect-
ive at immobilizing toxic organic materials. Organically modified clays are now being
evaluated as an additive to S/S processes in order to adsorb and retain these organic
pollutants in the solidified waste form.
The environmental acceptability of stabilization/solidification processes will de-
pend on the long-term ability of the waste form to retain contaminants. This will be
governed by the chemical binding mechanisms involved and by the durability of the waste
form. Many S/S processes have been developed which can pass regulatory leaching tests,
but these tests do not indicate the potential for leaching after long-term environmen-
tal exposure. Wide spread acceptance of- stabilization/solidification processes will be
hampered until the long-term durability of the waste form can be demonstrated.
17. KEY WORDS AND DOCUMENT ANALYSIS
a. DESCRIPTORS
b.IDENTIFIERS/OPEN ENDED TERMS
c. COSATI Field/Group

contaminant leaching
solidified/stabilized
waste

13. DISTRIBUTION STATEMENT
Release To Public
19. SECURITY CLASS (This Report)
Ilnrl ^ssif i<="i
21 NO. OF PAGES
15
20 SECURITY CLASS (This page)
Unclassified
22. PRICE
EPA Form 2220-1 (9-73)

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