United States
Environmental Protection
Agency
Risk Reduction
Engineering Laboratory
Cincinnati OH 45268
Research and Development
EPA/600/S2-89/013 Feb. 1990
&EPA Project Summary
Evaluation of Solidification/
Stabilization as a Best
Demonstrated Available
Technology for Contaminated
Soils
Leo Weitzman, Lawrence E. Hamel, and Edwin F. Barth
This project evaluated the per-
formance of solidification/ stabiliza-
tion as a means of treating soil from
"Superfund" sites. Tests were
conducted on four different types of
artificially contaminated soil that are
representative of the types of
contaminated soils found at Super-
fund sites. For purposes of this
study, the term solidification infers
the conversion of a non-solid to a
solid while stabilization infers reduc-
tion in contaminant leachate. Many
times the terms are used inter-
changeable since both goals are met
The contaminated soils used for this
study were synthetically prepared
and termed Standard Analytical
Reference Matrix (SARM). The soils
were solidified/stabilized using the
following three commonly used
solidification/stabilization agents or
binders: (1) portland cement, (2) lime
kiln dust, and (3) a mixture of lime
and flyash.
At 7, 14, 21, and 28 days after soil
and binders were mixed, samples of
the solidified material were subjected
to Unconfined Compressive Strength
(UCS) testing. Samples of those
mixes that had a UCS minimally
greater than 50 psi (pounds per
square inch), or which showed the
highest UCS below 50 psi, after 14
and 28 days were subjected to
chemical testing such as the Toxicity
Characteristic Leaching Procedure
(TCLP) and Total Waste Analysis
(TWA) to determine if stabilization
occurred. The results follow.
The water-to-total-solids ratio ap-
pears to be a better measure of the
amount of water needed to solidify/
stabilize a given mix than the water-
to-binder ratio that is commonly
used. This was clearly the case for
the SARM's with these binders. This
needs to be confirmed on other
systems.
Solidification/stabilization resulted
in significant reductions in the
amount of metal salt contaminants
released, as measured by the TCLP.
Because of the large losses of
organics during the mixing process,
the effect of solidification/stabili-
zation on the organic leachate via the
TCLP could not be quantitatively
determined. The volatile and semi-
volatile organic contaminants did
appear to decrease because of the
solidification/stabilization process;
however, this decrease can be
attributed to their release to the air
during processing and curing. No
correlation between UCS and the
results of the leaching tests was
observed.
This Project Summary was devel-
oped by EPA's Risk Reduction Engi-
neering Laboratory in Cincinnati, OH,
to announce key findings of (he re-
search project that is fully docu-
mented in a separate report of the
-------
same title (see Project Report
ordering information at back).
Introduction
The Hazardous and Solid Waste
Amendment Act (HSWA) of 1984
requires the U.S. Environmental Pro-
tection Agency (EPA) to develop
treatment standards or treatment
methods (called "Best Demonstrated
Available Technology" or "BOAT") for
listed hazardous waste before it is land
disposed. Treatment methods were to be
evaluated that reduce the toxicity or the
likelihood of the migration of the
hazardous constituents in the waste. The
Superfund Amendment and Reauthori-
zation Act (SARA) requires that remedial
actions meet all applicable, relevant, and
appropriate public health and environ-
mental standards. In order to be
consistent in these requirements, it may
be necessary to establish the level of
performance that the best available
technology can achieve in the treatment
of wastes from remedial actions. This
project evaluated the performance of
solidification/stabilization as a "BOAT" for
treating soil from "Superfund" sites.
Four different types of artificially
contaminated soil, which are represen-
tative of the types of contaminated soils
found at Superfund sites, were
solidified/stabilized using three common-
ly used solidification/stabilization agents
or binders. The products were subjected
to DCS tests, and each blend of soil and
binder that had a UCS minimally greater
than 50 psi, or which showed the highest
UCS below 50 psi after curing, were
subjected to chemical testing such as the
TCLP and TWA to determine stabilization
effectiveness. The 50 psi criterion is
consistent with the Resource Con-
servation and Recovery Act (RCRA)
guidance.
The binders evaluated were commonly
used generic agents that are readily
available. Other binders, both proprietary
and generic, are available and could
possibly enhance the solidification/
stabilization process. There is, at present,
no set protocol for evaluating the efficacy
of solidification/stabilization technologies.
The TCLP was used for evaluating the
level of stabilization achieved in this
program. It is one of several leaching
procedures commonly used at present.
Experimental Procedure and
Results
The SARM's were prepared for EPA
under a separate program. They are
identified by the amount of organic and
metals contamination added to the soil
follows:
SARM I low metals, high orgar
concentration
SARM II low metals, low orgar
concentration
SARM III high metals, low orgai
concentration
SARM IV high metals, high orgai
concentration
Table 1 presents the raw waste a
ysis for the SARM's received for
program. Table 2 presents the m
species utilized for the contamination.
As the first step in the program,
apparent water content of the SAP
was determined by drying therr
constant weight and attributing the we
loss to water removed by evapora
although organic material loss not
occurred in addition. The results \
SARM I - 31.4%, SARM II - 8
SARM III - 19.3%, and SARM IV
22.1%. Then the amount of water reqi
to form a satisfactory solidified pro*
defined to be that mix which gave
product most resistant to penetration
U.S. Army Corps of Engineers, <
Table 1. Results of TWA for SARM Samples Received for this Program
Metals Concentration (mg/kg)
Analyte
SARMI
High Organic,
Low Metal
SARM II
Low Organic,
Low Metal
SARM III
Low Organic,
High Metal
SARM l\
High Orga,
High Met
Volatiles
Acetone 3,150
Chlorobenzene 330
1,2-Dichloroethane 380
Ethylbenzene 3,350
Styrene 1,000
Tetrachloroethylene 710
Xylene 4,150
Semivolat/les
Anthracene 940
Bis(2-ethylhexyl)
phthalate 600
Pentachlorophenol 135
Inorganics
Arsenic 18
Cadmium 17
Chromium 27
Copper 193
Lead 190
Nickel 27
Zinc 392
230
9.2
3.9
74
26
16
210
275
34
62
18
23
37
260
240
32
544
220
8.9
3.1
100
24
13
150
265
140
15
904
1,280
1,190
9,650
15,200
1,140
53,400
13,000
27C
83C
2.50C
541
54i
3,70(
77'.
50
7
81
1,43
1,65
13,3C
16,9C
1,31
29,91
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Table 2. Chemical Identification and
Solubility of SARM Metal
Contaminants
Chemical Type
Solubility in
Water
Lead sulfate (PhSOJ
Lead oxide (PbO)
Zinc oxide (ZnO)
Cadmium sulfate
(3CdS04-BH20)
Arsenic trioxide (As20j)
Copper sulfate
(CuSO4-5H20)
Chromium nitrate
Nickel nitrate
Slightly soluble
Insoluble
Insoluble
Soluble
Slightly soluble
Soluble
Soluble
Soluble to very
soluble
Penetrometer test after 24 hr, was
determined.
The testing showed that water-to-
binder ratios were not good indicators of
the amount of water that should be used
to form the SARM's into a monolithic
solid suitable for hardness testing. The
ability of the product to set up could be
correlated reasonably well to the water-
to-total-solids ratio (W/TS) of the mix.
This ratio is simply the mass of water
used versus the sum of the solid
component of the SARM and of the
solidifying agent. In virtually all cases
tested, a W/TS ratio of 0.4 to 0.5 resulted
in an acceptable product, regardless of
the soil or the binder used.
The next phase of the program was
intended to determine the minimum
binder-to-soil (B/S) ratio that would result
in a sample of solidified soil with an
unconfined compressibility greater than
50 psi. Actually, with some binders this
DCS level could not be achieved with any
ratio tested within the 28-day curing time
set under this program. In that case, the
sample that achieved the highest UCS
level was used for subsequent testing.
The B/S ratio tests were performed by
mixing each soil (4 soils) with each
binder (3 binders) at three B/S ratios.
Each mixture was split into a number of
samples. On each of the 7th, 14th, 21st
and 28th days after mixing, the samples
were subjected to UCS testing. On the
14th and 28th days, the samples that had
either minimally satisfied the 50-psi UCS
requirement or, if none had achieved 50
psi, the one that had the highest UCS
reading, were also subjected to TWA and
TCLP analysis. The program resulted in a
total of 648 samples.
At 14 and 28 days after mixing, the
organic volatile and semivolatile emis-
sions from the solidified samples were
qualitatively measured to track the loss of
organic components from the samples
into the surrounding air by withdrawing a
sample of the air from the polyethylene
bag in which the samples were allowed to
cure and injecting it into a gas chromato-
graph. Because no gas flux measure-
ments through the plastic bags were
made, these concentrations cannot be
used to calculate the emission rate of the
organics and should be construed as
qualitative in nature.
TCLP leachate analyses were per-
formed for both organics and metals;
however, because of the significant
losses of the organic constituents during
mixing and handling, the results of the
TCLP analyses for organics proved
inconclusive. The results of the TCLP
analyses for metals are presented in
Table 3. It lists the SARM type (I through
IV) and the sample number that was
tested in the first column. The second
column identifies the type of binder used.
"RAW" is the contaminated SARM with-
out solidification/stabilization and PC, KD,
and LF are the three binders. The
numbers in parenthesis identify the day
the analyses were performed 14 or 28
days after mixing. The final columns
present the TCLP results for each metal:
(a) giving the parts per million (ppm) of
that metal found in the extract, and (b)
giving the percent decrease that this
represents over the raw SARM. The
values in the (b) column correct for the
decrease in the concentration of that
metal that is due to dilution by the
binders.
Discussion of Results and
Conclusions
The results indicated that the portland
cement formed a much stronger matrix
than the other two binders. Typically, the
Portland cement resulted in a UCS
exceeding 1,000 psi (the upper limit of
measurement with the available equip-
ment) for three out of the four SARM's.
Further, it achieved these levels at far
lower B/S ratios than the other two
binders, possibly resulting in a smaller
volume of waste requiring ultimate
disposal. The strength of the product
solidified/stabilized with portland cement
was significantly lower for SARM IV than
for the other three SARM's. The SARM
IV had been contaminated with very high
levels of both organic compounds and
metal salts and it appears that this
combination resulted in a large amount of
interference to the solidification/
stabilization process.
The lime kiln dust and the lime/flyash
mixtures used for these tests did not
result in values of the UCS as high as
those observed with portland cement.
The strength (UCS) values were initially
low, however, the values continued to
increase during the course of these tests.
The trend in the data was very clear and
confirmed the general impression that
lime-based binders will continue to
harden over time.
The SARM samples stabilized with
lime and lime kiln dust/flyash continued
to cure over time. The UCS values for
these samples started very low but as
time progressed, they increased. The test
suggests that the curing time for these
binders should have been extended to
determine their ultimate strength. The
trend in the data suggested that these
samples would continue to show
increases in their UCS beyond the 28-
day period.
An observation made during the initial
screening tests of this program appears
to be useful for further work. These tests
showed that a water/solids ratio of
approximately 0.4 would result in a
solidified/stabilized product regardless of
the binder used within the overall
context of the experiment. This observa-
tion, if confirmed with other systems,
may result in a significant reduction in
number of experiments required to test a
given water/binder ratio.
The results of the TCLP for the metals
on the treated SARM's were very
encouraging. In general, the data show
that the metals leaching from the SARM's
are reduced significantly by the
solidification/stabilization process. In fact,
the reduction approached 100% for many
of the compounds.
The TCLP results on the raw
(untreated) contaminated SARM's were
lower than the expected values for almost
all of the metals. This made the data
difficult to interpret as many of the
analyses were being made at or near the
detection limits. Nevertheless, the results
clearly indicate a significant reduction in
the TCLP of the metals in almost all
cases. The TCLP results of the raw
SARM's showed that the matrix itself
prevented a large portion of the metals
from being released to the TCLP. Many
analyses of the raw, contaminated mate-
rial approached the minimum level of
sensitivity of the analyses. Many of the
metal salts appeared to be attenuated by
the SARM itself, possibly because of the
clay portion of the soils.
All of the binders reduced the teach-
ability of the cadmium, copper, nickel,
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nd zinc. In all of these cases, the
olidified/stabilized SARM's resulted in
nly trace amounts or less of these metal
alts in the leachate. The TCLP results
)r lead was less consistent. All of the
amples that were solidified/stabilized
'ith portland cement showed very large
jductions in the teachability of this ion.
he kiln dust caused some reduction in
le leachability of the lead, although the
3duction was not as great as for the
ortland cement.
The lime/flyash binder did not appear
> reduce the leachability of the lead. In
ict, the results actually showed an
icrease in the leachability after cor-
seting for dilution. The increase is most
w well metals are immobilized for the
\RM's. The UCS of the samples formed
th these binders continued to increase
en though there was little change in the
JLP.
The variability owing to the analytical
method use for metal analysis can be
estimated by examining the difference
between the TWA for the metals for each
mix after 14 and 28 days. For example,
SARM III solidified/stabilized with
Portland cement (PC) after 14 days
showed 528-ppm arsenic. The same
sample at 28 days showed 584 ppm.
Generally, comparison of the metals
analyses for each sample at 14 and 18
days showed a similar consistency. This
type of variability is quite small, indi-
cating that the mixing procedures used in
this program resulted in a homogeneous
product and that the analytical protocol
appeared to give reasonably consistent
results.
The analysis of the volatile and semi-
volatile organic compounds in the head-
space by gas chromatography/flame ion
detector (GC/'FID) seemed to indicate that
the volatile organic emissions occur
mostly during mixing and then continue
at a steady rate after curing in a sample,
dropping as the organic content of the
solidified/stabilized material is reduced.
The solidified/stabilized SARM's gener-
ally showed a lower TCLP value for the
volatile organic contaminants than the
original SARM's. This should not,
however, be attributed to the solidifica-
tion/stabilization process binding the
volatile compounds. Rather, this is most
likely because of a simple release of the
volatile compounds during the mixing
process and during the sample prepa-
ration prior to extraction.
The TWA analyses for the volatile
organics showed the same pattern as the
TCLP. The TWA analyses, however,
showed the results magnified. That is, the
solidified/stabilized SARM's contained on
the order of 80% to 90% less volatile
organics than the original material. This is
consistent with the hypothesis that the
volatile organics were released to the air
rather than trapped in the solid. Had the
volatile organics truly been stabilized,
then the TWA would have shown a
constant value for these materials while
the TCLP would have shown a decrease.
The TCLP for the semivolatile organics,
in general, showed a significant decrease
because of solidification/stabilization. The
results show that the percent decrease in
the TCLP analyses for the semivolatile
organics is greatest for SARM's I and IV
(those contaminated with high levels or
organics) and least for II and III. SARM's
If and III also show a greater variability for
the semivolatile reduction, but this is
most likely because of analytical errors
caused by the low concentration of the
semivolatile compounds.
The TWA results for the semivolatile
organics was unexpected. Solidification/
stabilization appeared to result in an
apparent increase in almost all of the
values. This is most likely an artifact of
the analytical method. The TWA results
appear to have a very wide variation in
them. The reason for this is unclear, but it
may be because of the physical nature of
the semivolatile compounds. They are
heavy solids that go into solution slowly.
As a result, the amount of each con-
stituent in the liquid after the extraction
for analysis may be more of a function of
how much of the material actually
dissolves than of the total amount of that
compound in the waste. Under normal
conditions, this error is not significant;
however, m this case, the TWA values
are corrected for dilution. This results in a
"leveraging" of any error and a much
higher degree of uncertainty for the TWA
results.
In conclusion, it appears that
solidification/stabilization can significantly
reduce the leachability of many metals of
the SARM's matrices. In this specific
case, when no effort was made to match
the solidification/stabilization process to
the contaminant, most of the contami-
nants were effectively immobilized as
determined by the TCLP. It is therefore
likely that with a proper choice of binder,
it may be possible to better stabilize the
inorganic contaminants and, possibly,
even some of the organics.
The full report was submitted in
fulfillment of Work Assignment No. 2-18
under Contract No. 68-03-3241 by
Acurex Corporation under the spon-
sorship of the U.S. Environmental Protec-
tion Agency.
-------
Leo Weitzman and Lawrence E. Hamel are with Acurex Corporation, Research
Triangle Park, NC 27709; and the EPA author, Edwin F. Barth (also the EPA
Technical Project Manager, see below), is with the Risk Reduction Engineering
Laboratory, Cincinnati, OH 45268.
The complete report, entitled "Evaluation of Solidification/Stabilization as a Best
Demonstrated Available Technology for Contaminated Soils," (Order No. PB 89-
169 908/AS; Cost: $15.95, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Technical Project Manager can be contacted at:
Risk Reduction Engineering Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
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