-------
typtoally used to define the distribution of materials
ranging from fine sand to gravel sizes (0.1 mm and
larger). The hydrometer analysis is typically used to
define the distribution of materials ranging from fine
sand and silt to clay (0.1 mm and smaller).
The results of both of these tests are required for a
complete evaluation of the distribution of particle sizes
of soils containing both coarse- and fine-grained frac-
tions. For materials containing less than 20 percent
fines, the hydrometer analysis is often disregarded.
Evaluation of the particle size distribution of a material
by sieve analysis is performed by first washing a dried
and weighed sample through a No. 200 (0.075 mm)
sieve to determine the percentage of fine-grained
particles, and then redrying the sample and passing it
through a series of circular screens with various mesh
sfzes. The sieving is usually done on a Ro-tap shaker,
but sometimes by hand. The machine produces uni-
form circular and tapping motions that cause the par-
ticles to be sorted through the sieves. The apparatus
can use as many as 13 screens at a time for maxi-
mum grain size differentiation. After sieving, the weight
of the material on each screen is measured and is cal-
culated as a percentage of the total. The results are
plotted on a grading curve, as shown in Figure 4-1.
The use of this test is straightforward because the
procedures are mechanical. Errors that do occur
often relate to improper sampling of the material.
These errors can be overcome by proper sampling
techniques, such as quartering, or by using several
samples and comparing their particle size distribu-
tions. Analysis may be difficult or unreliable when soil
grains are contaminated with oils or other organic
materials that have a tendency to agglomerate.
The hydrometer test is used to obtain an estimate of
the distribution of particle sizes from the No. 200 sieve
size (0.075 mm) to around 0.001 mm. This encom-
passes silt and clay-sized particles. According to
Stoke's Law, which considers the terminal velocity of
a single sphere falling in an infinity of liquid, the sizes
calculated represent the diameter of spheres that would
fall at the same rate as the soil particles (Note 14,
ASTM D422-63).
In this test, a sample of approximately 50 grams of air-
dried soil in 125 ml sodium hexametaphosphate
("Calgon") solution (40 g/liter) is dispersed to neutral-
ize soil-particle charges. After it soaks in the dispers-
ing solution, the sample is further dispersed by mixing
it in a mechanical or air-jet mixing apparatus. The dis-
persed mix is then transferred to a sedimentation
cylinder, and distilled water is added to a total volume
of 1000 ml. The mixture is again agitated briefly, and
a hydrometer is placed in, the cylinder to monitor
changes in specific gravity of the solution over time.
The measured values of specific gravity of the solu-
tion and known (or assumed) specific weights of the
solid and fluid phases are used to calculate the grain
sizes still in suspension as larger sizes fall out of sus-
pension. Although the specific weight of water is
typically used for calculations, contaminants desorbed
from the solids may affect this value to some degree.
Figure 4-1. Typical particle-size grading curve.
Boul-
ders
Cob-
bles
Gravel
Coarse | Fine
Sand
Can« I Medium
Fine
Fines
Silt sizes Clay sizes
U.S. Standard Sieves
j 3"2"1" %" Vt" 46 10 20 XO 60 100 200
90
80
70
eo
so
40
30
20
10
0
il I
i
I!
HI
ill
III
Illl
Hi!
HI!
•' I '
*,i!
II
X
^
^"^
\
Gap-graded
I!
II
\
s
N.
\
"N
\
••*•
\
A
1
Uniform 1
s
v
\
X,
<\
\
N
\
\
A
\
\
V
,
"x.
\
X
eathered uniform soil
X
V
•--
rs.
^ •
-•-
Well-graded
J^HUiil
M
0)
>>
JD
c:
a>
o
i_
Q)
CL
1000
100
10 1 0.1
Grain diameter in millimeters
0.01
0.001
Source: Introductory Soil Mechanics and Foundations, by G. B. Sowers and G. F. Sowers. MacMillan Publishing Co., New York. 1970.
4-4
-------
4.1.1.2 fnterpretation and Application of Results 4.1.2 Atterberg Limits (ASTM D4318-84)
The shape of the gradation curve represents the par-
ticle size uniformity of the sample. A steep curve
indicates a soil in which nearly all the grains are the
same size. A flatter curve shows a wider variation in
grain size and thus a well-graded soil. Figure 4-2
gives representations of well-graded (poorly sorted)
and poorly graded (well-sorted) materials.
The absolute size and shape of the waste particles
affect the feasibility of various stabilization/solidifica-
tion processes and the ultimate strength of the stabi-
lized/solidified product (Wiles 1987). Very fine or very
coarse particles can increase the difficulty of stabiliz-
ing/solidifying wastes.- For example, fine-grained
wastes have been shown to produce poor stabilized/
solidified materials because they lack the size needed
to form a stabilized/solidified product with adequate
durability (Cullinane et al. 1986). Vick et al. (1987),
however, have shown that very hydrophobic contami-
nants such as dioxin tend to bind preferentially to
small soil particles. Very large particles may also
preclude the use of certain processing equipment or
require particle size reduction prior to stabilization/'
solidification.
A well-graded soil that does not contain extremely
large or extremely small particles is likely to show
favorable physical Characteristics after stabilization/
solidification. The strong matrix formed by the inter-
locking of diversely sized particles often produces
characteristics such as high strength, low permeabil-
ity, and low teachability.
The Atterberg Limits are a simple and useful series of
tests originally developed for classification and char-
acterization of clays used in ceramics.
Atterberg Limits are the moisture contents that mark a
material's liquid and plastic states. The liquid limit of a
material is the moisture content at which it will flow as
a viscous liquid. The plastic limit is the moisture^
content at the boundary between the plastic and brittle"
states. The plasticity index is the difference between
the liquid limit and plastic limit. Atterberg Limit tests
are applicable to fine-grained materials only, and their
results are useful not only for classification, but also
for correlation with a broad range of engineering prop-
erties and to indicate clay mineralogy.
Atterberg Limits are used to estimate such properties
as compressibility, strength, and swelling characteris-
tics, and to indicate how the material will behave
when stresses are applied (Cullinane 1986). In addi-
tion, the plasticity index can be used to determine the
proper amount of stabilization/solidification agent (e.g.,
lime) to be added to a waste. Atterberg Limit tests are
applicable to any natural or artificial mixture of soil-like
particles.
The tests are performed on both unstabilized/unsol-
idified and freshly stabilized/solidified soil-like wastes.
The resulting data can be particularly useful during
the compacting of friable materials after stabilization/
solidification. To date, these tests have not been
completed extensively on hazardous waste materials;
however, they are expected to be used more fre-
quently in the future.
Figure 4-2. Illustration of material gradation.
MATERIAL GRADATION
Poorly graded
Well-graded
Source: Adapted from Caterpillar Performance Handbook, Caterpillar, Inc., Peoria, IL. October 1987.
4-5
-------
4.1.2.1 Definitions
The liquid limit is the moisture content of a soil at the
arbitrarily defined boundary between the liquid and
plastic states. The liquid limit can be regarded as the
water content at which soil no longer acts as a plastic,
but begins to act as a liquid (i.e., flows).
The plastic limit is the moisture content of a soil at the
boundary between the plastic and brittle states. The
plastic limit is considered the water content at which a
soil no longer acts brittle, but can be molded and
deformed without falling apart.
The plasticity index is the difference between the
liquid limit and the plastic limit. Reduction in the
plasticity index is used to judge the effect of lime
addition to clays.
4.1.2.2 Test Description
Liquid Limit Determination—The material is mixed
with water, placed in a brass cup, and divided in two
with a grooving tool. The liquid limit is the water
content at which the divided sample flows together
after the cup is dropped by an apparatus 25 times
from a height of 10 centimeters. The water content of
the material is determined by the method described in
Subsection 4.1.3.
Plastic Limit Determination—A sample is alternately
pressed and rolled into a 1/8-inch-diameter thread.
The sample slowly dries out until the thread can no
longer be pressed together or rerolled; that water
content is the plastic limit.
Although procedurally defined, the definitions of the
liquid and plastic limits are at least partially subjective.
For good consistency, the same tester should run
duplicate trials of identical samples.
4.1.2.3 Interpretation and Significance of the Test
The test is part of several engineering classification
systems to characterize fine-grained fractions of a
material. These test results characterize material-
handling properties and the variation in the properties
as a function of water content. Liquid limit and plastic
limit are used individually or together with other soil
properties to correlate with other engineering behav-
iors such as compressibility, permeability, compacti-
bility, shrinking/swelling, and shear strength.
Liquid limits for various waste/stabilization/solidifica-
tion agent mixtures typically range between 40 and 55
percent water content. Typical plastic limits for vari-
ous waste/stabilization/solidification agent mixtures are
between 20 and 50 percent water content (Morgan et
al. 1984).
4.1.3 Moisture Content (ASTM D2216-80)
The moisture content test determines the amount of
free water (or fluid) in a given amount of material.
This test is often used to determine if pretreatrnent is
necessary in the design of the stabilization/solidifica-
tion process (Cullinane 1986). An example of waste
pretreatrnent would be sludge drying, dewatering, or
consolidation prior to stabilization/solidification. Mois-
ture in a waste is not always detrimental. For ex-
ample, the presence of water may be needed to pro-
vide a reaction mechanism (e.g., hydration) for cor-
rect stabilization of the waste.
In this test method, the term "water" refers to "free" or
"pore" water, not waters of hydration. Also, water in
discontinuous pores is not measured by this test.
It is also important to note that water is often not the
only liquid-phase constituent in contaminated materi-
als. The fluids may also include a broad range of
liquid wastes present in solution or as nonaqueous
phase liquids. This can have several effects on the
performance and results of moisture content determi-
nations. For example, if VOCs are present, samples
should be aerated to allow volatilization of flammable
VOCs before samples are oven-dried. The type and
level of contamination may also influence the relation-
ship between "free" and adsorbed water.
4.1.3.1 Test Description
A preweighed sample is dried in an oven for 24 hours
at a constant temperature of 110°G or at a tempera-
ture below the dehydration temperature. The sample
is allowed to cool to room temperature in a desiccator.
The difference between the original sample weight
and the weight after drying is used to determine the
moisture content.
During the collection of samples for analysis, the mois-
ture content (free water) must not change during han-
dling. Samples must be placed in appropriate jars
and sealed with plastic, foil, or wax. Maintenance of a
constant temperature in the drying oven is also impor-
tant for achieving accurate results. A 5°C range is
allowable.
Moisture content results for soils high in organic mat-
ter require careful interpretation. Organic material, in-
cluding VOCs, may be driven off during the drying
process. Any weight loss due to the organic matter
will be recorded as water loss.
This method does not give truly representative results
for materials containing significant amounts of hal-
loysite, montmorillonite, and gypsum; highly organic
soils; or materials with pore waters containing dis-
4-6
-------
solved solids. For these soils, vacuum drying at 60°C
may be better. Another source of error in moisture
content determination of stabilized materials is the
accelerated hydration reactions that may occur at the
elevated temperatures used during the test. These
reductions reduce the amount of free water available
for evaporation and cause erroneous weight meas-
urements.*
4.1.3.2 Interpretation and Application of Results
The results of this test are usually expressed as fluid
representing a percentage of total mass (Cullinane
1986). This basic information is required forthe plan-
ning and execution of a stabilization/solidification proj-
ect. Excess moisture may have to be removed by fil-
tration. Low moisture content may indicate the need
to add water to stabilize the waste correctly. For
example, water is sometimes added to incinerator
ash to stabilize/solidify the ash.**
In a study by Stegemann et al. (1988) of 69 wastes,
water content in the untreated wastes ranged from
close to 100 percent for wastewaters to less than 10
percent for soils. After waste stabilization/solidifica-
tion, water content ranged from 64 percent to less
than 1 percent. In this study, the water content was
determined from weighing a ground sample before
and after drying to a constant weight at 60°C.
4.1.4 Suspended Solids (USEPA Method 208C)
"Suspended solids" is a term originally used in the
monitoring of the quality of wastewater (both the influ-
ent and effluent of a wastewater treatment plant) and,
by inference, the performance of the plant. 'Total
solids" include all solids, suspended and dissolved,
present in a sample. Suspended solids are those
removed by filtration. The difference between total
solids and suspended solids is known as the "dis-
solved solids." One must know the suspended solids
content of supernatants in lagoons or other impound-
ments to plan dewatering operations.
4.1.4.1 Test Description
A well-mixed sample is filtered through a preweighed
standard glass-fiber filter (Gelman Type A or equiva-
lent). The filter is then dried to a constant weight at
103° to 105°C, cooled in a desiccator, and reweighed.
.Suspended solids, expressed as a percentage, is the
increased weight of the filter for a known volume of
sample.
4.1.4.2 Interpretation and Applicability
Suspended solids content is an important parameter
for determining the materials handling requirements
for a waste material, i.e., to determine if the waste can
be pumped. Suspended solids content also can be
used to estimate the decrease in volume that can be
achieved by dewatering. Table 4-2 presents consis-
tency categories for various waste types based on
approximate suspended solids content. Although
these categories are approximate, they give an indi-
cation of how a waste can be handled and the opera-
tions that can occur in and on the material.
Table 4-2. Liquid waste consistency classification.
Consistency
category
Liquid waste
Pumpable waste
Flowable waste
Nonflowable waste
Characteristic properties
Less than 1% suspended solids,*
pumpable liquid, generally too dilute for
sludge dewatering operations.
Less than 10% suspended solids,*
pumpable liquid, generally suitable
for sludge dewatering.
Greater than 10% suspended solids,* not
pumpable, will flow or release free liquid,
will not support heavy, equipment, will
undergo extensive primary consolidation.
Solid characteristics, will not flow or
release free liquids, will support heavy
equipment, may be 100% saturated, may
undergo primary and
secondary consolidation.
'Suspended solid ranges are approximate.
Source: Cullinane 1986.
4.1.5 Paint Filter Test (USEPA Method 9095-
SW846)
The Paint Filter Test is used to determine the pres-
ence of "free liquids" in a representative sample of
bulk (or noncontainerized) waste. The test is required
by RCRA's 40 CFR 264.314 and 265.314 and is used
to determine if a material releases free liquids.
The American Nuclear Society has a test similar to
the Paint Filter Test, the Allowable Drainable Liquid
Test (ANS 55.4). The EPA's Office of Solid Waste
and Emergency Response is proposing that the Liq-
uid Release Test be used in conjunction with the Paint
FJIterTest. Inthe December24,1986. Federal Regis-
ter, the EPA proposed the use of the Liquid Release
Test to test for release of liquids from nonbiodegradable
absorbent mixtures when a waste is under compres-
sive forces in a landfill. The proposed Liquid Release
Test calls for the application of 50 psi pressure to the
waste sample to determine if liquids will be released
under compressive forces.
.. Personal communication from Dr. Paul L. Bishop, University of Cincinnati, to M. Arozarena, PEI, September 30,1988.
Personal communication from M. John Cullinane. P.E., U.S. Army Corps of Engineers, Waterways Experimental Station, to T.
Whipple, Earth Technology Corporation.
4-7
-------
4.1.5.1 Test Description
In the Paint Filter Test, the material is placed in a paint
filter, which rests in a funnel attached to a ring stand.
If any portion of the material passes through and
drops from the filter within the 5-minute test period,
the material is deemed to contain free liquids.
4.1.5.2 Interpretation and Application of Results
As of May 8,1985, the placement of bulk (or noncon-
tainerized) liquid hazardous waste or hazardous waste
containing free liquids in any landfill was prohibited
[Land Ban HSWA Section 3004(c)(1)]. Absorbents
(in contrast to stabilization/solidification agents) can-
not be added to liquid wastes to achieve a temporary
liquid-free status. In addition, no credit is given for the
liner or leachate collection system that the landfill
uses.
The Paint Filter Test may be performed after a waste
is stabilized to determine whether the waste may be
disposed of in a RCRA-authorized landfill. If it does
not pass the Paint Filter Test, it must undergo further
treatment before it can be disposed of in a landfill.
(For further information on the Land Ban regulations,
see Section 2.)
4.2 Density Testing
Bulk density is the ratio of the total weight (solids and
water) to the total volume. Bulk density, along with
specific gravity and moisture content measurements,
can be used to calculate a material's porosity. More
commonly, bulk density values are used to convert
weight to volume for materials-handling calculations
and are essential for characterizing the rates at which
a so!) can be excavated. In addition, bulk density data
provide a comparison between stabilized and unsta-
bilized waste. Calculated increases in volume of a
material due to changes in bulk density after excava-
tion may determine if the stabilized materials can be
disposed of on site or must be shipped offsite.
Four methods of bulk density measurement are pre-
sented. The data from each are sufficiently accurate
for calculating densities. Selection of a method is
usually based on ease of use. Laboratory determina-
tion of specific gravity can supplement these mea-
surements.
4.2.1 Bulk Density—Drive-Cylinder Method
(ASTM D2937-83)
The Drive-Cylinder Method is designed to determine
the in-place density and moisture content of soils or
soil-like wastes. Because the test is not appropriate
for nondeformable materials, it is limited to unstabil-
ized, freshly stabilized, or soil-like stabilized waste
that have been compacted by conventional earth-
moving equipment.
A thin-walled cylinder is driven into the soil to obtain a
sample. The weight per unit of volume of the sample
in the cylinder is then determined.
4.2.2 Bulk Density—Sand-Cone Method
(ASTM D1556-82)
For determination of bulk density via the Sand-Cone
Method, a template is placed on a level surface of the
unstabilized/unsolidified waste and the material is
excavated through a hole in the template with small
digging tools. A volume of roughly 1000 to 2500 cm3
is typically excavated through the 165-mm-diameter
hole in the template. An apparatus filled with a sand
of known density is weighed and placed on the tem-
plate above the hole created by the excavation. Sand
is released from the apparatus and allowed to fill the
hole. The apparatus is then reweighed. From the
sand's previously determined density and the weight
of sand remaining in the apparatus, the volume of the
sand needed to fill the hole is calculated. The ratio of
the weight of material removed from the hole to the
volume of sand needed to fill the hole is the material's
bulk density. The moisture content is determined on a
sample of the excavated material to evaluate dry
density.
4.2.3 Bulk Density—Nuclear Method (ASTM D2922-
81)
The density of soil and soil aggregate can also be
measured in place by nuclear methods. The total or
wet density of the material is determined by placing a
gamma source and gamrna detector either on, into, or
adjacent to the material to be measured. Nuclear
densiometry works on the principle of Compton scat-
tering. The amplitude of back-scattered gamma ra-
diation depends on the electron concentration in the
material being measured, and this is, in turn, roughly
proportional to the density of the material.
4.2.4 Bulk Density—Stabilized Waste
For determination of the bulk density of a monolithic
stabilized waste, a sufficiently cured cube or cylinder
of the solid is weighed and measured. The bulk
density is then calculated by dividing the volume into
the mass.
Bulk density measurements are used for materials-
handling calculations such as volume removed, vol-
ume stabilized, and volume returned to the site. Bulk
density measurements of stabilized materials are used
to determine the amount of material that will need to
be shipped offsite or to determine the degree of mound-
ing that will occur as a result of stabilization/solidifica-
tion. These applications are similar to cut-and-f ill cal-
culations for construction or road building.
4-8
-------
For 69 stabilized/solidified products tested by Stege-
mann et al. (1988), bulk densities ranged from lighter
than water (0.7 g/cm3 or 0.6 ton/yd3) to very dense
(2.2 g/cm3 or 1.8 ton/yd3).
4.3 Compaction Testing—Moisture/
Density of Soil-Cement Mixtures
This test (ASTM D558-82) determines the relation-
ship between the moisture content and density of soil-
like materials. The test is normally completed on the
stabilized waste before stabilization/solidification has
occurred. The test determines the moisture content
that allows maximum compaction to occur so as to
achieve maximum density.
4.3.1 Test Description
The optimum moisture content is obtained by creating
a series of samples for the particular waste-stabiliza-
tion/solidification agent mix to be tested. Varying
amounts of water are added to these samples, which
are then placed in a standard mold with a volume of
1/30 ft3 and compacted in three equal lifts by use of a
standard 5.5-lb rammer dropping from a height of
12 inches. After compaction, the samples are oven-
dried for 12 hours, and their densities are then meas-
ured. The baseline moisture content (before the addi-
tion of water) is also noted.
Dry densities of the stabilized samples are plotted as
a function of moisture content. This plot produces a
curve as shown in Figure 4-3. Note: The moisture
content of samples may reflect moisture added for
testing purposes.
The moisture content corresponding to the peak 'of
the curve is called the "optimum moisture content."
The density corresponding to the peak of the curve is
called the "maximum density."
4.3.2 Interpretation and Application of Results
The moisture content of a stabilized waste is appli-
cable for soil-like stabilized wastes that must be re-
compacted in place after they are stabilized. Experi-
ence in the construction fields shows that it is difficult
to achieve proper compaction if materials are too wet
or too dry. Water lubricates soil particles, which helps
them slide into a denser position. The stabilized
waste has an optimum moisture content at which a
maximum density can be achieved upon compaction.
It should be noted, however, that the optimum water
content for compaction properties may not be the
optimum water content for hydration reactions.*
Personal communication from Or. Paul L. Bishop, University of Cincinnati, on September 30,1988.
Figure 4-3. Typical soil-cement moisture/density relationship.
120
116
112
108
104
100
10 12 14
Moisture content, %
16
4-9
-------
4.4 Permeability (Hydraulic Conductiv-
ity) Testing
Hydraulic conductivity, often referred to as permeabil-
ity, is a measure of the resistance of a material to the
passage of water. Permeability tests are performed
to estimate the quantity and flow rates of water through
a material under saturated conditions. Laboratory
permeability testing consists of applying a hydraulic
head of water to one end of a specimen and measur-
ing the flow through the specimen (Carter 1983).
There are two basic types of permeability tests: con-
stant-head and falling-head. The constant head test
allows relatively large quantities of water to flow through
the sample and be measured. This test is suitable for
materials with a permeability greater than 10-8 cm/s.
The falling-head test, which allows for more accurate
measurement of small quantities of water, is more
suitable for materials with a permeability of less than
10-* cm/s (Carter 1983). The following are general
descriptions of the testing techniques.
Falling-Head Permeability (EPA Method 9100-
SW8461 (Figure 4-41
In this test, a specimen is sealed in an imper-
meable membrane and placed in a fluid-filled
chamber, within which hydrostatic pressures
can be applied to the specimen (a triaxial
compression chamber with back pressure to
ensure complete saturation is commonly used).
Platens on the top and bottom of the specimen
are connected through tubing to cylinders filled
with air-free water. The water in the chambers
is differentially pressurized to create a head
difference across the specimen and to force
flow through the specimen. The pressure dif-
ferential must be specified. If the differential is
very high, problems can result from expansion
of internal pores and creation of a more con-
tinuous flow network, which results in errone-
ously high permeability readings.* The elapsed
time for the water level to fall between cali-
brated marks on the glass cylinder is recorded.
The test is usually repeated three or four times.
The falling-head permeameter is usually
equipped with a series of glass tubes of vari-
ous diameters to allow a reasonable time inter-
val to be used for the test. Selection of the
correct size tube is based on judgment and
accomplished by trial and error. Figure 4-4 is
a schematic diagram of the apparatus required
for this test.
Constant-Head Permeability (EPA Method
9100-SW846) (Figure 4-5)
In this test, the sample is placed in a water
bath and is connected to a column of air-free
water in a glass tube, with the head of the
water maintained at a constant value by a
* Prootul commonlcttton from Dr. Paul L. Bishop, University of Clndnnall. to M. Arozarana, PEL September 30.1988.
Figure 4-4. Basle layout of apparatus for falling-head parmeablllty test
"• OffFSMNTUU. PRESSURE GAG6 (OPTIONAL)
DtALMOCATOR
+ BACK PRESSURE OAOe
VACUUM UNE (OPTIONAL) FOR
ELECTRICAL PRESSURE TRANSDUCER OR
ACCEPTABLE NO-FLOW PORE PRESSURE
MEASURING SYSTEM
4-10
-------
header tank and a balance tank. Water flows
through the sample by gravity and is collected
in a measuring cylinder. The time taken to
collect a given quantity of water is recorded.
The cylinder containing the soil sample usually
has nipples at a number of points along the
sides and is connected via rubber tubing to
glass manometer tubes so that the piezomet-
ric head can be measured at various points
throughout the specimen (Carter 1983). Fig-
ure 4-5 is a schematic diagram of the appara-
tus required for this test.
Flgura 4-5. Baste layout of apparatus for constant-head
TO REGULATED PRESSURE SOURCE AND
PRESSURE GAGE OR MANOMETER USED TO
MEASURE H .
|-*_ PRESSURE RELEASE VALVE
TOP PLATE
RUBBER "0" RING SEALS
OUTFLOW TO VOLUMETRIC MEASURING DEVICE.
PRESSURE SHOULD BE ATMOSPHERIC OR ZERO
GAGE PRESSURE
Both of these permeability tests are laboratory meth-
ods and are only considered accurate to within one
order of magnitude. If a soil-like waste is stabilized
and sampled in the field for testing in the laboratory, it
is necessary to recompact the sample in accordance
with USAGE methods (USAGE 1980) to obtain a rep-
resentative sample.
The permeability of a stabilized waste is an important
factor, as it indicates the ability of a material to permit
the passage of water and to limit the loss of contami-
nation from the stabilized waste to the environment.
Permeability is examined in conjunction with leach
test results to evaluate the potential of the stabilized
waste to release contaminants into the environment.
The relevance of permeability measurements can be
understood by comparing them with natural materials.
Sand, a highly permeable material, has a hydraulic
conductivity on the order of 10-2cm/s. Clay, a mate-
rial that is used to line lagoons and surface impound-
ments, can have hydraulic conductivity on the order of
10-* cm/s or less and is considered relatively imper-
meable. Thus a stabilized waste with a permeability
similar to clay is desirable because it will not permit
the free passage of water though the stabilized waste.
By slowing the contact of water with the waste, it
reduces the possible transport of contaminants out of
the waste. Typical hydraulic conductivities for stabi-
lized wastes range from 10-" to 10'8 cm/s. Hydraulic
conductivities of less than 1Q-5 cm/s (upflow triaxial
procedure) are recommended for stabilized waste
destined for land burial (USEPA 1986e).
Highly permeable materials are not necessarily unde-
sirable. If the stabilized waste does not readily leach
contaminants into the water, a high permeability is
not as important. High permeability values can also
be addressed through engineering solutions such as
construction of impermeable liners and covers. Fi-
nally, relative permeability can be just as important as
actual permeability. The permeability of stabilized/
solidified materials should be two orders of magnitude
below that of the surrounding materials.
Permeability tests may also be completed on stabi-
lized samples that have undergone durability testing.
The durability tests mimic natural stresses that are
applied to the stabilized samples (e.g., freeze/thaw
weathering) (see Section 4.6). Permeability can po-
tentially increase over time as a result of natural
stresses.
4-11
-------
4.5 Strength Testing
Strength-test values indicate how well a material will
hold up under mechanical stresses created by over-
burden and earth-moving equipment. Strength-test
data also are often used to provide a baseline com-
parison between unstabilized and stabilized wastes.
Unstabilized waste materials generally do not exhibit
good shear strength; however, if the waste is sta-
bilized into a cement-like form, the strength character-
istics can be expected to increase significantly. Cor-
relation between strength and contaminant leachabil-
tty has not been widely demonstrated.
Several other strength tests may be performed in
addition to or in place of the tests described in the
following subsections, depending on the intended use
of the data. These include ASTM D1883-87, Califor-
nia Bearing Ratio of Laboratory-Compacted Soils, and
ASTM C109-86, Compressive Strength of Hydraulic
Cement Mortars.
4.5.1 Unconfined Compressive Strength of
Cohesive Soils (ASTM D2166-85)
The Unconfined Compressive Strength measures the
shear strength of a cohesive, soil-like material in un-
saturated undrained conditions without lateral con-
finement on the sample. The test method provides an
approximate value of the strength of cohesive soils
(wastes) in terms of total stress. The soil (waste) may
be undisturbed, remolded, or recompacted. The test
is applicable to cohesive materials that do not expel
water during the loading portion of the test. (Water is
expelled from soil as a result of deformation or corn-
Figure 4-6. Stress-strain curve.
paction.) Dry, cmmbly, or fissured materials cannot
be tested with this method.
4.5.1.1 Test Description
The test is completed by centering a specimen (pre-
pared in accordance with ASTM D1632-87) on an
apparatus containing upper and lower plates. The
specimen is not supported laterally. The test is usually
performed as a strain-controlled test, in which the
specimen is subjected to a vertical strain rate of 0.5 to
2 percent per minute by applying axial load until the
specimen fails. The test continues until the load
values decrease with increasing strain, or until 15
percent strain is reached. The peak stress (at failure)
is defined as the Unconfined Compressive Strength
of the sample. For the assumptions made for this test
(zero internal friction and zero minor principal stress),
the shear strength equals the Unconfined Compres-
sive Strength divided by two. The strain rate used for
the test should be selected to achieve failure in less
than 15 minutes. Load and deformation are recorded
at sufficient time intervals to be able to plot a stress-
strain curve (example in Figure 4-6). The moisture
content of the sample is also determined (Subsection
4.1.3) because different moisture contents will change
the shape of the curve.
4.5.1.2 Interpretation and Application of Results
Unconfined Compressive Strength results for unsta-
bilized waste have limited application. They do, how-
ever, serve as a baseline for comparison with the
Unconfined Compressive Strength results for stabi-
« 4O.OOO
™ 3O.OOO
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20.0OO
55 10,000
I 1 I
Proporlionol elastic limit
Ultimate strength
Rupture
Strain-shortening in percent
Source: Structural Geology, third edition, by Marland P. Billings. Prentice-Hall, Inc., Englewood Cliffs, NJ.
4-12
-------
lized waste. If the unstabilized product is soil-like and
the stabilized product is cement-like, there should be
a marked increase in the Unconfined Compressive
Strength (10 to 20 psi).
4.5.2 Unconfined Compressive Strength of
Cylindrical Cement Specimens (ASTM D1633-84)
For stabilized cement-like wastes, the Unconfined
Compressive Strength test can provide several pieces
of useful information, including the following:
• The ability of the stabilized waste to with-
stand overburden loads.
The optimum,water/additive ratios and
curing times for cement setting reactions.
• The improvement in strength characteristics
from the unstabilized to the stabilized waste.
This test also is often conducted on samples sub-
jected to durability tests (Subsection 4.6).
4.5.2.1 Test Description
The test is completed on a cylindrical sample of the
materials (ASTM D1632-87). It can be completed
with two different cylinder height-to-diameter ratios:
1.15 and 2.0 (Methods A and B). The cylindrical test
specimen must be cured for a specified time in a room
with 100 percent humidity. Typical curing times for
cement are 1, 7, 14, and 28 days, The age of the
tested sample should be noted.
The two height-to-diameter ratios cause several dif-
ferences in testing and in results. Method A uses
equipment more readily available in soil-testing labo-
ratories; however, this test method may lead to more
complex stress conditions during crushing. There-
fore, Method A gives a relative measure of strength
rather than a rigorous determination as found in Method
B. Method A normally yields a higher Compressive
strength than an identical sample tested by Method B.
Although no consistent preferences in test methods
are noted in the literature, comparisons in Unconfined
Compressive Strength should only be completed for
samples tested by the same method.
The testing apparatus is commonly called a "Com-
pression Testing Machine." The machine may take
on several forms, but most important, it must be able
to control the rate at which load (stress) is applied.
The machine has an upper and lower plate, and the
sample is placed upright (long axis vertically) on the
lower plate.
The upper plate is lowered and brought into contact
with the sample. Load is added continuously without
shock by a screw. A sample fails when it loses its
physical integrity by falling apart. The total load at
failure of the test specimen is recorded to the nearest
10 pounds of force. The Unconfined Compressive
Strength is the ratio of force applied at failure to the
original cross-sectional area of the cylinder, usually
expressed in pounds per square inch (psi). Uncon-
fined Compressive Strength is often expressed in
other units. Table 4-3 presents conversion factors for
the more common units.
In its methods description, ASTM reports that the
average strength difference of two duplicate samples
is 8.1 percent. The samples should be tested by the
same person to give the maximum precision when
interpreting the failure of the sample.
4.5.2.2 Interpretation and Application of Results
The EPA considers a stabilized/solidified material with
a strength of 50 psi to have a satisfactory Unconfined
Compressive Strength (USEPA OWSER Directive,
No. 9437.00-2A). This minimum guideline of 50 psi
has been suggested to provide a stable foundation for
materials placed upon it, including construction equip-
ment and impermeable caps and cover material. A
study by Stegemann et al. (1988) reported Uncon-
fined Compressive Strength values for 69 stabilized/
solidified wastes ranging from 10 to 2900 psi.
The minimum required Unconfined Compressive
Strength for a stabilized/ solidified material should be
evaluated on the basis of the design loads to which
the material will be subjected. The anticipated over-
burden pressure and other loads, along with appropri-
ate safety factors, can be used to calculate this.
Typical construction and compaction equipment can
generate very high contact pressures of 1000 psi or
more (e.g., sheepsfoot rollers), but surface contact
pressures on the order of 50 to 100 psi are more
common. This surface load is attenuated with depth
so that bearing pressures are reduced to values on
the order of 10 to 20 psi at a depth of 2 feet and 3 to 7
psi at a depth of 5 feet below grade. Overburden
pressures will usually be on the order of 0.75 to 1.0
psi per foot of depth. If guidelines such as these are
used, the stresses to which the stabilized/solidified
waste will be subjected can be predicted, and design
criteria can be selected accordingly.
Martin et al. (1987) suspect that one-dimensional
compressibility may be a more useful indicator of
mechanical stabilization/solidification in some situ-
ations than is Unconfined Compressive Strength. The
one-dimensional compressibility test (e.g., ASTM
D2435-80) allows a prediction of fluid expulsion dur-
ing consolidation and evaluation of cover support.
For stabilized/solidified waste forms that are relatively
soft or ductile, this test can provide useful perform-
ance information; however, for stiff, cement-like, sta-
bilized/solidified waste, the Unconfined Compressive
Strength with a rational design criterion is probably
4-1-3
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adequate for most situations.
Compressive Strength of Hydraulic Cement Mortars
(ASTM C109-86) is often used in place of ASTM
D1633-84. It is similar to ASTM D1633-84; however,
cubic specimens, which require less material to form,
are used instead of cylindrical specimens.
The shape of the stress-strain curve is indicative of
the relative stiffness of the stabilized/solidified mate-
rial. Thus, the shape of the curves can provide some
insight as to whether problems with creep or consoli-
dation of ductile behavior or cracking due to brittle or
rigid behavior are possible.
4.5.3 Flexural Strength (ASTM D1635-87)
Flexural strength is a measurement of a material's
ability to withstand loads applied in tension. During
this testing, the loads are considered for the short axis
of the sample. In contrast, the loads are applied to the
long axis of the sample when the unconfined strength
is measured.
4.5.3.1 Test Procedures
This test is completed on apparatus such as that illus-
trated in Figure 4-7. The specimen should be kept
moist after curing and be tested as soon as possible
after removal from the moist environment. The stabi-
lized/ solidified specimen (ASTM D1632-87) is placed
on the apparatus, which supports the specimen at
both ends. The head of the testing apparatus consists
of a mass supported by two half-rods made of steel.
The head is placed at the center of the specimen.
The two end supports and the head produce four
points of contact on the specimen. Load is applied to
the head of the apparatus, which produces a stress at
the center of the specimen. The force is applied per-
pendicularly to the long axis of the specimen to test
the flexibility of the specimen. Load is added slowly to
the specimen until failure. The load necessary to
produce failure of the specimen is recorded to the
nearest 10 pounds. If the machine uses a hydraulic
press for loading stress, the rate of loading should not
be greater than 100 psi per minute. After the speci-
men fails, the average width and depth of the speci-
men at the point of failure are measured to the near-
est 0.01 inch.
4.5.3.2 Interpretation and Application of Results
The results are a measurement of the strength of a
stabilized/solidified waste in terms of cracking in flex-
ure. The results indicate resistance of the stabilized/
solidified waste to cracking due to settlement of the
underlying fill or due to surface loads. Flexural strength
can be viewed as a performance criterion that is
dependent on the disposal conditions. It is most
appropriate when a material is subject to surface
loads after placement or when differential settlement
of subgrade materials is possible.
Use of this measurement would be appropriate if a
waste were stabilized/solidified and compacted in a
disposal ceil in a series of lifts or layers. A problem
could arise if the first lift were a stiff layer compacted
over a soft bedding. In such a case, the compacted
layer must have sufficient flexural strength to support
the vehicle operations during the installation of the
upper lifts. The compacted layer also must have
enough strength to support the upper lifts without
flexure. In either case, if the lift of compacted.material
does not have sufficient strength to support surface
loads, the compacted material will crack and zones of
high permeability and increased potential of leaching
will be formed.
Figure 4-7. Schematic of apparatus for flexure test of soil-cement by third-point loading method.
1
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Source: The American Society for Testing and Materials, 1916 Race St., Philadelphia, PA 19103.
4-15
-------
4.5.4 Cone Index (ASTM D3441-79)
A material's cone index is indicative of its stability and
bearing capacity. The cone index test involves forc-
ing a standard cone-shaped device into the stabilized/
solidified waste being tested and measuring the pene-
tration resistance offered by the material (Cullinane
1986).
Cone penetration tests are suited for testing stabi-
lized/solidified sludges or other wastes for landfilling.
The test helps to determine the types of vehicles
needed to move and place material and the curing
time required before other construction equipment
can move over it [time before it can be used as
subgrade (Tittlebaum and Seals 1985)].
4.5.4.1 Test Description
Various types of cone penetration tests have been
used. One commonly used test involves placing a
sample of soil-like stabilized/solidified waste in a cup
at the bottom of a standard testing apparatus and
scraping it level with the top of the cup with a palette
knife. A standard metal cone on a shaft is held in
place by a clamp. At the beginning of the test the tip of
the cone is just touching the surface of the sample.
When the clamp is released, the cone penetrates the
sample. After 5 seconds, the clamp is closed and the
penetration is halted. The distance the cone has
penetrated the sample is measured to the nearest
0.1 mm with a dial gauge. The test is repeated to
check precision.
4.5.4.2 Interpretation and Application of Results
The cone index was originally developed to determine
the "trafficabilily" of compacted materials, which means
it shows the ability of a compacted material to support
construction equipment.* Test results are used to
determine when a material is able to support the load
of specific construction equipment. It is an important
parameter in terms of logistical and cost considera-
tions at a site. It is often imperative for equipment to
be able to move over the stabilized/solidified waste as
quickly as possible to continue work on other sections
of the disposal area.
The cone penetration test is practical and inexpen-
sive. Proponents of this test claim that its results can
be used in lieu of the Unconfined Compressive
Strength test in certain instances (e.g., when a rapid
turnaround is required).* Exclusive use of the cone
index test, however, would require laboratory correla-
tion between it and the Unconfined Compressive
Strength test. The test is also easy to complete in the
field by use of a Pocket Penetrometer (Figure 4-8),
which works on the same principle as the cone index
apparatus just described.
4.6 Durability Testing
Durability testing evaluates the resistance of a stabi-
lized/solidified waste mixture to degradation due to
external environmental stresses. The tests are de-
signed to mimic natural conditions by stressing the
sample through 1) freezing and thawing, and 2) wet-
ting and drying. The stabilized/solidified specimens
undergo repeated cycling during the testing. Uncon-
fined Compressive Strength, flexural strength, per-
meability, or other performance-based tests may be
conducted on the stabilized/solidified samples after
each cycle to determine how the physical properties
of the stabilized/ solidified waste change as a result of
simulated climatic stresses. The number of cycles a
material can withstand without failing can be used to
judge the mechanical integrity of the material.
Figure 4-8. Photograph of a pocket penetrometer.
Photograph courtesy Gilson Company, Inc., Worthington, Ohio.
Personal communication from M. John Cullinane, U.S. Army Corps of Engineers, Waterways Experimental Station, Augusta, 1988.
4-16
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4.6.1 Freezing and Thawing Test of Solid Waste
(ASTMD4842)
Seven molded samples (44 mm in diameter x 74 mm
in length) are cured in moist containers for 28 days.
One sample is selected for moisture content determi-
nation by drying to constant weight in accordance with
ASTM D2216-80, revised to use a temperature of
60°C ± 3°. Three samples are subjected to testing
and three are control samples. Each of the test
specimens is weighed and subjected to 24 hours of
freezing at -20°C ± 3°. The controls are kept in the
moist containers for 24 hours.
After 24 hours, the samples are covered with distilled
water and allowed to sit for another 23 hours. The
samples are then removed from their beakers with
tongs, and loosely attached particulates are removed
by spraying distilled water from a wash bottle onto the
surface of the specimen. The samples are then ob-
served for physical deterioration and their weight loss
is measured (solids content in beakers by evaporat-
ing water at 60°C ± 3° in drying oven). The freeze-
thaw cycle is repeated for a total of 12 cycles or until
the weight loss (corrected against the weight loss of
control samples) of any of the specimens exceeds 30
percent.
4.6.2 Wetting and Drying Test of Solid Wastes
(ASTMD4843)
Seven molded samples (44 mm in diameter x 74 mm
in length) are cured in moist containers for 28 days.
One sample is selected for moisture content determi-
nation by drying to constant weight in accordance with
ASTM D2216-80, revised to use a temperature of
60°C ± 3°. Three samples are subjected to testing
and three are control samples. Each of the test
specimens is weighed and subjected to 24 hours of
drying at 60°C ± 3°.
After 24 hours, the samples are allowed to sit for 1
hour and then covered with distilled water for another
23 hours. The samples are removed from their beak-
ers with tongs, and loosely attached particulates are
removed by spraying distilled water from a wash bottle
onto the surface of the specimen. The samples are
then observed for physical deterioration and their
weight loss is measured (solids content in beakers by
evaporating water at 60°C ± 3° in drying oven). The
wetting and drying cycle is repeated for a total of 13
cycles or until the weight loss (corrected against the
weight loss of control samples) of any of the speci-
mens exceeds 30 percent.
4.6.3 Interpretation and Application of Results of
Durability Tests
These tests relate to the long-term stability of the
sample. If the results show low loss of materials and
retention of physical integrity after testing, the stabili-
zation/solidification process and agent-to-mix ratio are
adequate. If the test results show a large loss of
material and loss of physical integrity, a different waste-
to-agent ratio or different stabilization/solidification
agent should be used to provide the long-term stabil-
ity needed.
No standards are currently established for determin-
ing whether stabilized material has passed durability
testing; however, Vick et al. (1987) suggest that 15
percent weight loss is an acceptable amount. Be-
cause very few materials can withstand the full 12
cycles, the only true measure is a comparison of the
results with another stabilization/solidification test (i.e.,
how many cycles can one mixture withstand versus a
different mixture). Obviously, if a cement-like stabi-
lized/solidified waste requires good strength charac-
teristics for proper disposal and it loses its physical
integrity during testing, one can conclude that a differ-
ent stabilization/solidification agent or waste to agent
ratio needs to be used.
Poor durability results often can be addressed by a
change in design and should not be used as auto-
matic grounds for exclusion. For example, materials
that fail freeze-thaw durability testing can be placed
below the frost line to mitigate their poor durability
property.
4-17
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-------
Section 5
Chemical Testing Procedures
This section is devoted to a discussion of leaching
tests, as these are the tests most often used to evalu-
ate the performance of stabilization/solidification as a
treatment process for hazardous waste. Emphasis is
on the appropriate selection of leaching tests and the
interpretation of laboratory results. Also highlighted
are the experimental conditions affecting the repro-
ducibility of the laboratory data and the limitations in
extrapolating test results to the field.
Chemical testing procedures applicable to untreated
hazardous wastes may also, have application to stabi-
lized/solidified wastes. Table 5-1 lists general para-
meters, test methods, specific applications to untreated
and stabilized/solidified wastes, and references.
5.1 Overview of Leaching Mechanisms
and Leach Tests
In the field, leaching of hazardous constituents from
stabilized/solidified wastes (waste forms) is a function
of both the intrinsic properties of the waste form and
the hydrologic and geochemical properties of the site.
Although laboratory physical and chemical tests can
be used to define the waste form's intrinsic properties,
the controlled conditions of the laboratory environ-
ment are usually not equivalent to changing field con-
ditions. At best, laboratory leaching data can simulate
the behavior of waste forms under "ideal," static (con-
ditions at one point in time), or "worst-case" field con-
ditions. Presently, leach tests can be used to compare
the effectiveness of various stabilization/solidification
processes, but they have not been verified for deter-
mining the long-term teachability of the waste.
5.1.1 Leaching Mechanisms
Leaching of a porous medium in the field is generally
modeled by solute transport equations that incorpo-
rate the following factors:
Chemical composition of waste and leaching
medium
Physical and engineering properties (e.g., par-
ticle size, porosity, hydraulic conductivity) of
the waste and surrounding materials
Hydraulic gradient across the waste
The first factor includes the chemical reactions and
kinetics between the leaching fluid and the waste that
transform contaminants from an immobile form to a
mobile form. The last two factors are used to define
the transport of fluids and mobilized contaminants
through the waste material.
The physical and engineering properties of the waste
material and the hydraulic gradient determine how a
leaching solution contacts the waste material. The
hydraulic gradient, together with effective porosities
and permeabilities, governs the velocity and quantity
of the leaching solution migrating through the waste
form. For example, if the waste is relatively imper-
meable (i.e., has low hydraulic conductivity) com-
pared with the surrounding material, the leaching
solution will tend to flow around the waste form. This
can occur when an intact stabilized/solidified waste
form is disposed of in a medium with a hydraulic
conductivity 100 times greater (i.e., 10"6 to 10'4cm/s).
In such cases, most of the contact between the leach-
ing solution and the waste form occurs at the geo-
metrical surface of the waste form. The permeability
of stabilized/solidified wastes can increase over time
through physical and chemical weathering processes,
however, and increase the amount of liquid flow
through the waste. Therefore, in the long run, con-
tact between the leaching solution and the waste will
occur at the particle surface within the waste form.
The chemistry of the waste and the leaching solution
defines the types and kinetics of the chemical reac-
tions that mobilize or demobilize contaminants in the
stabilized/solidified waste. Reactions that can mobi-
lize contaminants adsorbed or precipitated within the
waste form include dissolution and desorption. Un-
der nonequilibrium conditions, these reactions com-
pete with demobilizing reactions such as precipitation
and adsorption. Nonequilibrium conditions generally
develop when a stabilized/solidified waste is con-
tacted by a leaching solution and can result in a net
transfer, or leaching, of contaminants into the leach-
ing solution.
The following chemical kinetic factors affect molecu-
lar diffusion of pollutants within the waste form:
5-1
-------
Table 5-1. General chemical teat methods that may also have application to •tabllized/i oldifled waste*.
Parameter
FH
Oxidation/reduction
potential (EH)
Major oxidos
Total organic carbon
(TOO)
Oil and grease
Elemental analysis
Volatile organic
compounds (VOCs)
Base, neutral, and
acid compounds
(BNA)
Polychlorinated
bipheny'ls
(PCBs)
Ion measurements
Hoat of hydratlon
Alkalinity
Test
method
EPA Method SW-9045
ASTM D1498-76
ASTMC114
Combustion Method
EPA Method 41 3.2
EPA Method SW-846
EPA Method SW-846
(Methods 5030 and
8240)
EPA Method SW-846
(Methods 3540,
3520 and 8270)
EPA Method SW-846
(Methods 3540, 3520,
680, and 8080)
Std. Method No., 429
ASTM 0186-86
Titrometry
Applicability to untreated and stabilized/solidified wastes
Leachability of hazardous constituents (e.g., metals) may
be governed by the pH of the solid
Changes in EH after treatment can change the teachability
of many elements
Mineralogy of the stabilized/solidified v/aste may aid in
interpretation of leach test results
Used to approximate the nonpurgeable organic carbon
in wastes and treated solids
May be used to compare the teachable oil and grease from
the treated and untreated wastes
Used to determine the fraction of metals leached to the total
metals content of the untreated and stabilized/solidified
wastes
Used to compare VOC concentrations in stabilized/solidified
wastes and untreated wastes with the VOC concentrations
in TCLP extracts to determine relative teachability of the
treated and untreated wastes
Used to compare BNA concentrations of leachates with
respective concentrations in treated and untreated wastes
to determine relative teachability of the treated and untreated
wastes
Same as for VOC with respect to PCB teachability from treated
and untreated wastes
Used to determine leachate ionic species concentrations
Measurement of temperature changes during current mixing
will allow prediction of VOC emissions in the field
Alkalinity changes in leachates may be used to determine
changes in stabilized/solidified waste form
Reference3
1
2
3
4
5
1
1
1
S.
6
7
8
9,10
Reference:
1 U.S. Environmental Protection Agency. 1986. Test Methods for Evaluating Solid Waste. Volumes 1A-1C: Laboratory Manual,
Physical/Chemical Methods; and Volume II: Field Manuals, Physical/Chemical Methods, SwW846, Third Edition, Office of Solid
Waste. Document Control No. 955-001-00000-1.
2 ASTM D1498-76, Standard Practice for Oxidation-Reduction Potential of Water.
3 American Society for Testing and Materials. 1981. Standard Methods for Chemical Analysis of Hydraulic Cement, ASTM
Committee C-1 on Cement, Philadelphia, Pennsylvania. July 1981.
4 Perkin Elmer 240C Users Manual,
5 U.S. Environmental Protection Agency. 1979. Methods for the Chemical Analysis of Water and Wastes. Office of Research
and Development. EPA-600 4-79-020, March 1979.
6 Albord-Stevens, Bellar, Erchelberger and Bubble, Method 680, November 1985, Determination of Pesticides, PCBs in Water
and Sediment by GC-MS, Office of Research and Development, U.S. Environmental Protection Agency.
7 Standard Methods for the Examination of Water and Wastewater, 16th Edition, APHA, AWWA, WPCF, 1985. Washington, D.C.
8 ASTMC186-86. Standard Test Method for Heat of Hydration of Hydraulic Cement.
9 Bishop, P. L. 1986. Prediction of Heavy Metal Leaching Rates from Stabilized/Solidified Hazardous Wastes. In Toxic and
Hazardous Wastes Proceedings of the 18th Mid-Atlantic Industrial Waste Conference.
10 APHA-AWWA-WPCF. 1975. Standard Methods for the Examination of Water and Wastewater, 14th Edition. Method 403. pp.
278-281.
5-2
-------
Accumulation of waste species in the pore
solution at the particle surface.
Concentration of reactive species (e.g., H+,
complexing agent) in the pore solution at the
particle surface.
Bulk chemical diffusion of the waste or reac-
tive species within the leachate pore solution
or waste form.
Polarity of the leaching solution and waste
species.
Oxidation/reduction conditions and competing
reaction kinetics.
Because laboratory leaching tests usually involve stan-
dardized aqueous solutions (neutral, buffered, or di-
lute acidic solutions) rather than site-specific solu-
tions, the results of the laboratory tests may not di-
rectly duplicate leaching in the field. As previously
mentioned, laboratory leaching tests run with stan-
dard solutions can be used to compare the relative
teachability of waste constituents under similar test
conditions and with similar leaching solutions.
Depending on the physical and chemical properties of
the waste and the leaching solution, the kinetics of
contaminant transport (or leaching) in a porous me-
dium are controlled by advective or dispersive/diffu-
sive mechanisms. Advection refers to the hydraulic
flow and subsequent solute transport of highly soluble
contaminants in response to a hydraulic gradient.
Dispersion refers to the transport of contaminants via
mechanical mixing in the pore solution and molecular
diffusion (the transfer of mass between adjacent lay-
ers of fluid in laminar flow). Because of the low per-
meabilities of most stabilized/solidified wastes, the
rate of contaminant transport for adsorbed or chemi-
cally bonded constituents is generally considered to
be controlled by molecular diffusion at the particular
surfaces within the waste form, rather than advection
or dispersion.
The buildup of chemical potential at the interface
between particles and pore solution is the driving
force for the diffusion-controlled transport of waste
constituents within the aqueous solution and the waste
form (Cote et al. 1987). This nonequilibrium condition
is controlled primarily by the chemistry and velocity of
the leaching solution.
Figure 5-1 shows the effect of the velocity of the
leaching solution on the leaching rate of contaminants
that are leached at the particle surface. Leaching
solution velocity (v) is defined as the volume of leach-
ing solution (V) contacted with waste per unit of sur-
face area (SA) per unit of time (T):
v = V/(SA x T)
Leaching rate (L) is the mass of the waste species (M)
leached per unit of surface area per unit of time:
L = M/(SA x T)
The slope of the leaching curve in Figure 5-1 is the
leachate concentration of the waste species in the
leaching solution, or M/V. As shown in Figure 5-1, at
high leaching solution velocities (rapid flow through
the waste form), the leaching rate approaches the
maximum, Lr. In addition, under rapid leaching solu-
tion velocities, leachate concentrations are very low
(approaching zero) if leaching of the waste species is
diffusion-controlled. High leaching rates and low
leachate concentrations occur at the particle surface
under rapid leaching velocities because nonequili-
brium conditions at the particle surface are main-
tained. Rapid leaching rates occur in laboratory stud-
ies when the leaching solution is constantly being
replenished with fresh solutions.
At low leaching solution velocities (i.e., static hydrau-
lic conditions), the amount of a species leached ap-
proaches the saturation limit, SI, or the maximum
leachate concentration. Low leaching solution veloci-
ties and maximum leachate concentrations occur when
the leaching solution is not replenished, and the same
leaching solution is allowed to equilibrate with the
waste.
These relationships between leachate concentrations
and leaching solution velocities are important to the
understanding and interpretation of leaching test re-
sults because the tests vary dramatically with leach-
ing solution velocities, contact surface areas, leach-
ing solution volumes, and time of leaching. The chem-
istry of leaching solutions also varies widely from neu-
tral solutions to very acidic or strong chelating solu-
tions.
5.1.2 Leaching/Extraction Tests
Numerous leaching tests have been developed to test
solid wastes, including those developed specifically
for stabilized/solidified nuclear and hazardous wastes.
Because these tests have been developed by several
different groups and specialists, the terminology ap-
plied to leaching tests has not been well established.
Therefore, an attempt is made to clarify the terms
used in this handbook with regard to leaching tests.
Extraction (or batch extraction) tests refer to a leach-
ing test that generally involves agitation of ground or
pulverized waste forms in a leaching solution. The
leaching solution may be acidic or neutral. Also, it
may vary throughout the extraction test. Extraction
tests may involve one-time or multiple extractions. In
either case, leaching is assumed to reach equilibrium
by the end of one extraction period; therefore, extrac-
tion tests are generally used to determine the maxi-
mum, or saturated, leachate concentrations under a
given set of test conditions.
5-3
-------
Figure 5-1. Relationship between velocity of leaching solution and the leaching rate.
L'
SI i Maximum Leochate Concentration
—1-
I
I
-Maximum Leaching Rate
//
/
M (Concentration)
v~
k.
/_! v
Leaching Solution velocity (SA • TJ
Sourca: Cotoetal. 1937.
"Leach test," anothertype of leaching test, involves no
agitation. The leaching of monolithic (instead of
crushed) waste forms is evaluated in these tests.
Leaching may occur under static or dynamic condi-
tions, depending on the frequency of the leaching
solution renewal. In static leach tests, the leaching
solution is not replaced by a fresh solution; therefore,
leaching takes place under static hydraulic conditions
(tow leaching velocities and maximum leachate con-
centrations for monolithic waste forms). In dynamic
leach tests, the leaching solution is periodically re-
placed with new solution; therefore, this test simu-
lates the leaching of a monolithic waste form under
nonequilibrium conditions in which maximum, satura-:
tton limits are not obtained and leaching rates are
high. "Static" and "dynamic," therefore, refer to the
velocity, not the chemistry of the leaching solution.
Results of dynamic leach tests are generally expressed
in terms of a flux or mass transfer parameter (i.e.,
leaching rate), whereas data from extraction tests are
expressed in terms of leachate concentration or cu-
mulative fraction of total mass leached. Another key
difference between these two leaching tests is that
extraction tests are short-term tests lasting from hours
to days, whereas leach tests generally take from weeks
to years. Because of the crushed nature of the waste
and the larger amount of surface area available for
leaching, extraction tests (although short-term) are
used to simulate "worst-case" leaching conditions.
Leach tests on monolithic wastes (although longer in
duration) are often used to simulate leaching under
"well-managed," short-term scenarios in which the
waste form is intact.
The column leach test is another type of laboratory
leaching test. This test involves placing pulverized
waste in a column, where it continuously contacts with
a leaching solution at a specified rate. The leaching
solution is generally pumped through the waste in an
upf low column setup. Column tests are considered to
be more representative of field leaching conditions
than batch extraction tests because of the continuous
flux of the leaching solution through the waste. This
test is not often used, however, because of problems
with the reproducibility of test results. These prob-
lems include channeling effects, nonuniform packing
of the wastes, biological growth, and clogging of the
column (Cote and Constable 1982). Nevertheless,
column tests have been used along with extraction
tests to study the effects of particle size on the leach-
ing of heavy metals (Bishop 1986).
5.2 Leach Test Methods and
Applications
This subsection presents a detailed review of the
methods and general uses of several of the more
common extraction and leaching tests. The following
extraction tests are discussed:
• Toxicity Characteristic Leaching Procedure
(TCLP)
• Extraction Procedure Toxicity Test (EP Tox)
• California Waste Extraction Test (Cal WET)
5-4
-------
Multiple Extraction Procedure (MEP)
Monofilled Waste Extraction Procedure
(MWEP)
Equilibrium Leach Test (ELT)
• Acid Neutralization Capacity (ANC)
Sequential Extraction Test (SET)
Sequential Chemical Extraction (SCE)
The TCLP is used by EPA as the basis for the promul-
gation of best demonstrated available technologies
(BOAT) treatment standards under the land disposal
restrictions program. The EP Tox and Cal WET pro-
cedures are used by EPA and the State of California,
respectively, for characterizing hazardous wastes. The
remaining six tests provide useful information about
maximum leachate concentrations under various con-
ditions and the chemistry and waste constituents of
the waste form.
In addition to the extraction tests, the following leach
tests are also discussed:
Materials Characterization Center Static Leach
Test (MCC-1P)
American Nuclear Society Leach Test
(ANS-16.1)
Dynamic Leach Test (DLT).
The MCC-1 P and ANS-16.1 tests were developed for
stabilized high- and low-radioactive wastes. The
Dynamic Leach Test represents a modification of the
ANS-16.1 leach test, which was developed for stabi-
lized/solidified hazardous waste. These tests provide
data for evaluating leaching rates (DLT and ANS-
16.1) and maximum leachate concentrations (MCC-
1P Static Leach Test) from intact waste forms leached
with water.
Table 5-2. Extraction tests.
Table 5-2 summarizes the main differences in test
conditions among the nine extraction tests listed. The
main experimental test variables in extraction tests
are the leaching medium, the liquid-to-solid ratio, the
particle sizes of the crushed waste sample, and the
number and duration of the extractions. As shown in
Table 5-2, leaching solutions in extraction tests vary
from acids of different strengths and concentrations to
distilled/deionized water. Liquid-to-solid ratios vary
from as low as 3:1 (less acid added per gram of
waste) to as high as 50:1 (more acid added per gram
waste). Particle sizes range from less than 9.5 mm
(larger particles and smaller contact surface area) to
less than 0.15 mm (smaller particles and larger con-
tact surface area). Extraction periods range from 2
hours to 48 hours, and the number of extractions
range from 1 to 15. It is obvious that results from
leach tests must be evaluated with an understanding
of the differences in the experimental conditions.
5.2.7 Toxiclty Characteristic Leaching Procedure
(TCLP) (Federal Register 1986)
Waste samples are prepared by crushing the wastes
to pass through a 9.5-mm screen, and liquids are
separated from the solid phase by filtration through a
0.6- to 0.8-u.m borosilicate glass-fiber filter under 50
psi pressure. Two choices of buffered acidic leaching
solutions are offered under TCLP, depending on the
alkalinity and the buffering capacity of the wastes.
Both are acetate buffer solutions. Solution No. 1 has
a pH of about 5; Solution No. 2 has a pH of about 3.
The leaching solution is added to a Zero Headspace
Extractor (ZHE) at a liquid:solid ratio of 20:1, and the
sample is agitated with a National Bureau of Stan-
Test method
TCLP
EPTox
Cal. WET
MEP
MWEP
Equilibrium
leach test
Add
neutralization
capacity
Sequential
extraction tests
Sequential
chemical
extraction
Leaching medium
Acetic acid*
0.04 M acetic add (pH - 5.0)
0.2 M sodium citrate (pH . 5.0)
Same as EP Tox, then with
synthetic acid rain (sulfuric
acid: nitric add hi 60:40 wt.%
mixture)
Dlstllled/delonized water or
other for specific sit
Distilled water
HNO, solutions of Increasing
strength
0.04 M acetic add
Five leaching solutions
increasing In acidity
Uquldsolid ratio
20:1
16:1
10:1
20:1
10:1 per extraction
4:1
3:1
50:1
Varies from 16:1 to 40:1
Maximum partlde size
9.5mm
9.5mm
2.0mm
9.5mm
9.5 mm or monolith
150 urn
150 urn
9.5mm
150 |tm
Number of extractions
1
1
" 1
9 (or more)
4
1
1
15
5
Time of extraction
18 hours
24 hours
48 hours
24 hours per extraction
18 hours per extraction
7 days
48 hours per extraction
24 hours per extraction
Varies from 2 to 24 hours
Two acetate buffered solutions: 1) pH - 5.2) pH - 3.0
5-5
-------
dards (NBS) rotary tumbler at 30 rpm for 18 hours.
The leaching solution is filtered and combined with
the separated liquid waste fraction for analysis.
The EPA proposed this test 1) to replace EP Tox as
the criterion for defining hazardous and nonhazard-
ous wastes, and 2) to be used for some listed wastes
as the standard criterion for hazardous waste treat-
ment. With the ZHE apparatus, TCLP can be used to
evaluate the leaching of volatile and semivolatile or-
ganic compounds. It has also been used to evaluate
maximum "worst-case" leachate concentrations
achievable in the field; however, several studies
(Bishop 1986, Barich et al. 1987, U.S. EPA 1988c)
show that TCLP leaching of cement-based waste forms
may not necessarily yield maximum concentrations.
Multiple extraction tests, such as MWEP or MEP,
may be needed to assess maximum leachate con-
centrations under different pH conditions.
5.2.2 Extraction Procedure Toxlcity Test (EP Tox)
(U.S. EPA 1986)
Similar to TCLP in experimental design, EP Tox gen-
erally yields comparable results (Newcomer et al.
1986). Considerably different results may be ob-
served if the more acidic TCLP extraction fluid No. 2
(pH « 2.88 ±0.05) is used. Table 5-3 summarizes the
differences between the two test methods. The pri-
mary difference between EP Tox and TCLP is that the
EP Tox leaching solution (only one solution of acetic
acid at a pH of 5) is added periodically (up to a
specified maximum acid addition) as needed to adjust
the pH of the leaching solution during the course of
the extraction. The TCLP solutions are buffered so
that the leaching solution is added only once at the
start of the extraction.
Table 5-3. Summary of procedural differences between EP
TOX and TCLP.
Experimental
parameter
Filter size, urn
Filter pressure, psi
Leaching solution
Period of extraction, h
LlquW:solid ratio
TCLP
0.6-0.8
50
Acetate buffered
solution (pH «3 or 5)
18
20:1
EPTox
0.45
75
Acetic acid
(pH-5)
24
16:1
The EP Tox test also has been used to classify
wastes as hazardous or nonhazardous. This test,
however, is designed to determine semivolatile or-
ganic and heavy metal leachate concentrations; it
does not include analysis of volatile organic com-
pounds. Generally, EP Tox and TCLP yield similar
leachate concentrations of metals. Studies by New-
comer, Blackburn, and Kimmell (1986) and Shively
and Crawford (1987), however, indicate that TCLP
extracts greater metal concentrations. The statistical
mean TCLP leachate concentrations ranged from 1.0
to 3.0 times greater than those for EP Tox (New-
comer, Blackburn, and Kimmell 1986). Although EP
Tox may also be used to evaluate maximum leachate
concentrations, like TCLP, it should be used along
with other extraction tests.
5.2.3 California Waste Extraction Test (Cal
WET)*
As shown in Table 5-2, Cal WET differs from TCLP
and EP Tox in the following parameters:
• Different leaching solution (sodium citrate
buffered solution at pH of 5; or, for hexav-
alent chromium, distilled water)
Smaller liquidisolici ratio (10:1)
• Smaller particle size (less than 2.0 mm)
Longer extraction period (48 hours)
The State of California uses Cal WET to classify
hazardous waste. Because of the different metal
chelating ability of sodium citrate solution, Cal WET is
a more stringent leach test than TCLP for some met-
als.
5.2.4 Multiple Extraction Procedure (MEP) (U.S.
EPA 1986g)
Although MEP is not a regulatory leaching test, it has
been used in some instances for delisting wastes.
This test involves multiple (sequential) extractions of
the crushed sample with a synthetic acid rain solu-
tion. The first extraction is performed with the acetic
acid solution in accordance with EP Tox methods.
The subsequent extractions are performed with the
synthetic acid solution (concentrated sulfuric acid/ni-
tric acid, 60/40 wt.%, diluted to a pH of 3). A total of
nine extractions are usually performed; however, more
extractions are possible if the last three extractions do
not decrease the leachate concentrations.
Results obtained by MEP can be used to determine
maximum leachate concentrations occurring under
acidic conditions. This test can be used with EP Tox
or with MWEP (multiple extraction test with water) to
compare teachability of hazardous constituents under
mild and acidic conditions.
C«HonilaCod«,Tlti*22, Anlda 11. pp. 1800.76-1800.82.
5-6
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5.2.5 Monofill Waste Extraction Procedure (MWEP)
(U.S. EPA 1986Q
Formerly called the Solid Waste Leach Test (SWLT),
MWEP involves multiple extractions of a monolith or
crushed waste with distilled/deionized water. The
sample is crushed to less than 9.5 mm, or it can be left
intact if it passes the Structural Integrity Test (SW-
846). Ultimately, however, monolithic samples of sta-
bilized/solidified wastes can be crushed during the
extraction test as a result of agitation by the rotary
tumbler (Barich et al. 1987). The liquidrsolid ratio is
10:1, and the sample is extracted with water four
times (18 hours per extraction).
This test can be used to derive reasonable leachate
compositions in monofilled disposal facilities, and this
information can be used to assess waste-liner com-
patibility under mild leaching conditions. It also can
be used with TCLP to determine delays in the release
of hazardous constituents. In addition, results from
MWEP can be compared with those from ELT to
assess the maximum leachate concentrations
achieved under mild leaching conditions.
5.2.6 Equilibrium Leach Test (ELT) (Environment
Canada and Alberta Environmental Center 1986)
This leach test involves static leaching of hazardous
constituents in distilled water. The particle size of the
crushed sample (150 urn) is much smaller than that
for TCLP and EP Tox to allow greater contact surface
area and to reduce the time needed to achieve equi-
librium conditions. Water is added once at a liquid:solid
ratio of 4:1, and the sample is agitated for 7 days.
Like MWEP, ELT can be used to determine maximum
leachate concentrations under mild leaching condi-
tions. Although particle size and liquid:solid ratio are
smaller for ELT, leachate concentrations from the two
tests should be comparable if equilibrium conditions
are achieved under both (Cote, Constable, and Mor-
eira 1987). Sample heterogeneity and analytical lim-
itations may cause differences.
5.2.7 Add Neutralization Capacity (ANC) (Envi-
ronment Canada and Alberta Environmental Cen-
ter 1986)
Acid Neutralization Capacity (ANC) involves separate
extractions of several predried, crushed, waste
samples with leaching solutions of varying levels of
acidity. The amount of sample is much smaller, the
extraction is performed in test tubes and a rotary
tumbler, and liquid-solid separation is accomplished
by centrifuging instead of filtering. Particle size is less
than 150 urn (-100 mesh), and the liquid-to-solid ratio
of the extraction is 3:1. Ten samples (labeled 1
through 10) are extracted for 24 hours in one of 10
nitric acid solutions that increase incrementally in the
number of equivalents of acid added per gram of dried
solid.
The ANC test is used to determine the buffering ca-
pacity of the stabilized/solidified waste form. Figure
5-2 illustrates the change in pH as increasing amounts
of acid are added for cement-based waste forms.
Stegmann, Cote, and Hannak (1988) conclude that
for a wide range of metal- and organic-bearing wastes
stabilized with proprietary agents, the amount of acid
required to bring the pH down to 9, (where many
metals are soluble) varied between 2 and 10 mil-
liequivalents per gram (meq/g) of waste. For cement-
-based wastes, the ANC is generally about 15 meq/g
(Cote and Bridle 1987).
Figure 5-2. Acid neutralization capacity for several stabi-
lized/solidified synthetic sludge samples.
X
Q.
Curves represent a variety of
wastes and their varying butfe
capacities
Amount of nitric acid added,
meq/g of dry waste
The higher the buffering capacity of the waste, the
greater the possibility of maintaining alkaline condi-
tions and minimizing the amount of metals leached.
The buffering capacity of the waste form, therefore, is
very important in evaluating the amount and rate of
metals leached from stabilized wastes in the field.
5-7
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5.2.8 Sequential Extraction Test (SET) (Bishop
1986)
The Sequential Extraction Test is also designed to
evaluate the buffering capacity of the waste form.
Unlike ANC, SET involves 15 sequential extractions
of one sample of crushed waste with particle sizes
between 2.0 and 9.5 mm. Each extraction is per-
formed on a shaker table for 24 hours with the same
extraction solution (0.04 M acetic acid solution) and a
!iquid:soHd ratio of 50:1. With each extraction, 2 meq/
g of acid is added to the ground waste. The pH is
measured and the leaching solution is filtered. At the
end of the fifteenth extraction, the remaining solids
are digested with three more extractions in which
more concentrated acid solutions are used. These
last three extractions are combined for analysis.
Bishop (1986) and Shively et al. (1986) used SET to
evaluate the waste buffering capacity and alkalinity of
cement-based waste forms. They also correlated
analyses of pH, alkalinity, dissolved solids, and met-
als to determine metal speciation within the waste
form. Shively et al. (1986) and Bishop (1988) ob-
served that the pH of the leaching solution remained
above 10 during the first three extractions, and only
anionfc arsenic (As) appeared in the leachate. The
leachate pH rapidly dropped to 6 during the next three
extractions and then leveled out at 4 during the last
nine extractions (Figure 5-3a).
The buffering species is primarily calcium hydroxide,
based on gran analysis (Bishop 1986). Also, analy-
ses of the extracts from the last three digestions
indicate that from 75 to 85 percent of chromium, lead,
and silicon dioxide were not leached during the 15
extractions, whereas only 8 percent calcium remained
after 15 extractions (Bishop 1986). Based on similar
release curves, chromium and lead are believed to be
bound to the silicate matrix (Figure 5-3b). These
studies indicate that the buffering capacity for ce-
ment-based waste forms is about 18 meq/g.
5.2.9 Sequential Chemical Extraction (SCE) (En-
vironmental Canada and Alberta Environmental
Center 1986)
This test was developed specifically to evaluate the
species of organic and inorganic waste constituents
In a stabilized matrix. Like SET, the test involves
sequential extraction of a sample. Unlike SET, how-
ever, the leaching solution increases in acidity from
neutral to very acidic with each sequential extraction.
The particle size of the sample is also very small (less
than 45 urn), and the sample is agitated on a Burrell
wrist-arm shaker.
Results from the SCE test are used to interpret the
bonding nature of metals and organics in the stabi-
lized waste matrix. Contaminants leached in Frac-
tions A, B, and C are interpreted to be available for
leaching, whereas those that are leached with con-
centrated acids (D and E Fractions) are considered to
be unavailable for leaching. Studies by Bridle et al.
(1987) and Stegmann, Cote, and Hannak (1988) indi-
cate that metals are leached primarily in Fractions B
and C, and thus are available for leaching in natural
systems. They also conclude that this test cannot be
used to determine speciation and bonding mecha-
nisms for arsenic and mercury. Because of the lack of
sensitivity indicated by the results from Fractions A
through C, results from the SCE test should be used
only to determine whether contaminants are poten-
tially available or unavailable for leaching under the
mild leaching conditions that may be encountered in
the field (personal communication with P. L. Cote on
July 14,1988).
5.2.10 Materials Characterization Center Static
Leach Test (MCC-1P) (MCC1984)
This static leaching test was developed for high-level
radioactive waste. It involves leaching of a monolithic
waste form with water (ASTM Type I or II) at a ratio of
volume of leaching solution to surface area solids (V/
S) of between 10 and 200 cm. The period and tem-
perature of extraction vary, depending on the sched-
ule selected.
For organic and polymer stabilization/solidification
processes, teachability should be evaluated by using
MCC-1P or ANS-16.1, as these waste forms cannot
be ground. In general, however, MCC-1 P test results
can be combined with those from extraction tests
(e.g., TCLP, MWEP) to determine a range of leachate
concentrations in the short term (well-managed site
with waste form intact) and the long run (waste matrix
has been subjected to many years of environmental
stress and is fractured).
5.2.11 American Nuclear Society Leach Test (ANS-
16.1,1986) (ANS 1986)
A "quasi-dynamic" leach test, ANS-16.1, is applied to
stabilized/solidified low-level and hazardous wastes.
A monolithic cylinder (length-.diameter of 0.2 to 5.0) is
leached with demineralized water applied at a V/S
ratio of 10 cm under ambient temperatures. At the
start of the experiment, the sample is rinsed to obtain
zero contaminant concentration at the surface of the
sample. Afterwards, the sample is immersed in wa-
ter, which is replaced after 2 hours, 7 hours, 24
hours, 48 hours, 72 hours, 4 days, 5 days, 14 days,
28 days, 43 days, and 90 days.
The results of the leaching test are recorded in terms
of cumulative fraction leached over the total mass in
the waste form, F. Calculations are then used to
derive an effective diffusion coefficient, De (cmVs),
and a teachability index (LX - -log De). The LX values
range from 5 (De = 5-10, rapid diffusion) to 15 (De =
10-15, very slow diffusion).
5-8
-------
Figure 5-3. Sequential extraction test results: (a) Leachate pH and cumulative alkalinity leached from solidified/stabilized
subjected to multiple-batch extraction procedure, (b) Cumulative heavy metal, silicone, and alkalinity leached from solidified/
stabilized samples subjected to multiple-batch extraction procedure.
Cadmium
Chromium
Lead
Silicon
Alkalinity
31
Q.
12 15
Extraction
5-9
-------
Interpretation of ANS-16.1 results assumes that leach-
ing is diffusion-controlled and that nonequilibrium
conditions are maintained during each leaching pe-
riod. Because leaching rates for hazardous constitu-
ents vary widely and may not be diffusion-controlled,
the leaching rates under an ANS-16.1 renewal sched-
ule may actually be slower as leaching conditions
become static (equilibrium conditions). Hence, the
LX values obtained from ANS-16.1 results may be
greater than those under the modified ANS-16.1 (Dy-
namic Leach) test, discussed in the next subsection.
5.2.12 Dynamic Leach Test (DLT) (Environmental
Canada and Alberta Environmental Center 1986)
The Dynamic Leach Test is a modified version of the
ANS-16.1. The only modification to the leach test is
the renewal frequency of the leaching solution and the
V/S ratio, which is based on known diffusion coeffi-
cients and results from batch extraction tests (e.g.,
ELT). The selected V/S ratio must ensure that the
contaminant can be detected, and the renewal fre-
quency for the leaching solution should ensure that
nonequilibrium leaching conditions prevail. Other-
wise, the extraction procedure, apparatus, and the
subsequent calculations for deriving LX values are
the same.
The leaching data must be checked for the following
criteria:
Cumulative fraction leached, F, increases lin-
early with T1/2, where T is time.
• Concentrations leached during one leaching
period are less than the maximum, or satu-
rated, concentrations.
• F Is less than 0.2.
The linear relationship between F and the square root
of time verifies that leaching rates are diffusion-
controlled. If leaching is occurring through dissolution
or convective mechanisms, the relationship would not
be linear. If leachate concentrations are less than the
saturated concentrations, leaching is occurring under
nonequilibrium conditions and approaching maximum
leaching rates. The final criteria, if met, allow leaching
rates to be calculated by equations that are based on
leaching from a semi-infinite medium.
Figure 5-4 illustrates potential relationships between
cumulative fraction leached, F, and time of leaching
V T. Curve A shows a nonlinear relationship between
F and V T for a very soluble constituent. Curve B is a
straight line with a Y-intercept (F > 0 at T = 0), which
indicates that the contaminant was leached in the
initial rinse (surface wash-off). Curve C is a straight
Figure 5-4. Schematic plot of cumulative fraction leached
vs. the square root of time.
LEGEND
A — Very soluble constituents
B — Constituents In rlnsate
C — Delayed leaching or early
contaminant volatilization
Time, T1/J (sec'/:)
line with an X-intercept (F = 0 at T > 0), which indi-
cates either delayed leaching or early volatilization of
the contaminant from the surface of the waste form
(Cote and Hamilton 1984).
Results from dynamic leach tests conducted on ce-
ment containing heavy metals with different initial
concentrations and under different renewal schedules
show that calculated De's varied within one order of
magnitude (i.e;, LX values varied within 1 unit) (Cote
and Hamilton 1986). Studies on various waste types
and various leaching solution stabilizers show that or-
ganic compounds tend to have LX values between 5
and 10 (higher leach rates), and metals tend to have
higher LX values (slower leach rates) (Cote and
Hamilton 1986; Stegmann, Cote, and Hannak 1988).
Variability in test results for metals generally are at-
tributable to analytical sensitivity, especially for rela-
tively insoluble metals.
5.3 Experimental Factors Affecting
Results and Interpretation
Results of both leaching and extraction tests vary,
primarily because of sample nonhomogeneity. Cur-
ing time may also affect leach test results. The Corps
of Engineers cures waste for 28 days, as opposed to
the minimum curing time of 42 days recommended by
5-10
-------
Bishop (1988). The relative standard deviations
(%RSD) reported for TCLP, EP Tox and MCC-1P
range from 10 to 100 percent (ANS1986; Newcomer,
Blackburn, and Kimmell 1986; Stegmann, Cote, and
Hannak 1988). Numerous studies comparing the test
results obtained by TCLP and EP Tox for their relative
precision (summarized in Newcomer, Blackburn, and
Kimmell 1986) show that, although neither is a pre-
cise test, TCLP is as precise or slightly more precise
than EP Tox in inter- and intralaboratory tests with
metal wastes. Because of the variability in leaching
test results, more than one sample of stabilized/solidi-
fied hazardous waste should be leach-tested.
Other experimental parameters may also affect re-
sults of batch extraction tests (Cote and Constable,
1982; Newcomer, Blackburn, and Kimmell 1986). For
TCLP, varying the acidity of the No. 2 leaching solu-
tion from 190 to 210 meq greatly influences test re-
sults for metals; other variables, such as liquid-to-
solid ratio (19:1 and 21 ;1), extraction time (16 vs. 18
hours), and filter type, have relatively little impact on
test results (Newcomer, Blackburn, and Kimmell 1986).
Cote and Constable (1982) conclude that rotary-tum-
bler agitators used in extraction tests yield better pre-
cision than shaker tables or wrist-arm agitators. In
comparing results of several different extraction tests,
Cote and Constable (1982) concluded that, of five test
variables (leaching medium, liquid :solid ratio, number
of extractions, particle size, and period of Figure 5-4
extraction), the first three have the greatest impact on
extraction test results for metals. In general, cumula-
tive mass fractions of metals leached are higher with
more acidic leaching solutions, with larger liquid:solid
ratios (with neutral solutions), or with multiple extrac-
tions (Cote and Constable 1982; Cote, Constable,
and Moreira 1987). In theory, particle size and period
of extraction have no effect on leachate concentra-
tions if the extraction tests reach equilibrium condi-
tions. Studies by Bishop (1986), however, show that
although the opposite is observed initially, with mul-
tiple extractions, smaller particle sizes leach slightly
more metals than do larger particles.
The effects of the liquid:solid ratio vary with the type of
leaching solution and the solubility of the contaminant
(Cote, Constable, and Moreira 1987). With neutral or
slightly acidic leaching solutions and soluble waste
species, an increase in the liquid:solid ratio increases
the cumulative mass fraction leached, but the leachate
concentrations remains constant (i.e., the ratio of mass
leached to leaching solution volume remains the
same). Leachate concentrations of insoluble species
in the same medium decreased with increasing
liquid:solid ratio. With more acidic leaching solutions,
the leachate concentrations are controlled by the pH
and chemistry of the leaching solution; therefore,
leachate concentrations are not so easily predicted.
In general, liquid:solid ratios must be large enough for
analysis and small enough to allow detection of ana-
lytes above detection limits.
The impacts of these experimental parameters must
be considered in an evaluation of the results of sev-
eral leaching tests on a stabilized/solidified waste.
Also, the pH and chemistry of the leaching solution
must be monitored before and after the leaching test.
For example, leaching tests on stabilized/ solidified
metal wastes should include at least measuring the
pH and alkalinity as well as total dissolved solids or
conductivity of the leaching solution. Leaching tests
conducted by Bishop (1988) include analysis for cal-
cium, iron, aluminum, and other cement elements.
These analyses aid in the interpretation of the leach-
ing test data.
Other analytical techniques used to determine the
speciation of waste constituents, particularly organic
compounds, in the stabilized matrix are polarized light
microscopy, energy-dispersive X-ray diffraction (EDX),
scanning electron microscope (SEM), and electron
probe microanalysis (EPMA) (Tittlebaum et al. 1987;
Campbell et al. 1987). In addition, protocols for evalu-
ating bonding between organic compounds and stabi-
lizer in a stabilized/solidified waste have been devel-
oped by Dr. J. Soundararajan (PRC 1988). These
protocols include four technologies: 1) Fourier trans-
form infrared spectroscopy (FTIR); 2) thermal gravi-
metric analysis (TGA); 3) differential scanning colori-
metry (DSC), and 4) DSC coupled with gas chroma-
tography/mass spectroscopy (GC/MS).
5.4 Leaching Test Selection and
Interpretation
As mentioned in the preceding discussions, leaching
tests produce results that are not directly applicable to
leaching behavior in the field. Nevertheless, the re-
sults of several leaching tests or of leaching tests
combined with physical tests or microscopic tech-
niques can be used as indicators of field performance
and environmental impact.
When used for comparative purposes, results from
several leaching tests can help to identify field condi-
tions that result in higher concentrations of waste con-
stituents. Therefore, these data may be used to site
or design waste facilities that will minimize the leach-
ing of hazardous constituents from the wastes. The
data also may be used to predict the leaching of
stabilized/solidified wastes at different stages in time.
For example, leaching conditions of a well-managed
operational facility where the monolithic stabilized/
solidified waste receives maximum precipitation infil-
tration may be simulated by the use of the DLT, as
this test involves constant renewal of the leaching
5-11
-------
solution and a monolithic waste form. For a closed
facility that has a cover which is maintained (i.e., a30-
year post-closure period) and minimizes precipitation
infiltration, leaching conditions may be similar to those
of the MCC-1P test (i.e., static hydraulic conditions).
In the long run, the liners, cover, and waste form will
degrade, and leaching conditions may be similar to
those found in multiple extraction tests (such as MEP
and MWEP) or equilibrium tests (such as ELT).
In the few cases where the actual field leaching solu-
tion is well known, use of this solution in the laboratory
tests may yield more representative results. When
the site leaching solution is used, however, the results
may only be relevant to field leaching conditions in the
short term because the site hydrogeochemistry will
change over the long run.
5-12
-------
Section 6
Technology Screening
Technology screening is a tool used to organize rele-
vant information for each hazardous waste site for the
purpose of evaluating the applicability of stabilization/
solidification. Such screening is required because of
the imprecise nature of stabilization/solidification and
because most wastes are far from homogeneous. This
section outlines a series of steps for assembling infor-
mation about stabilization/solidification. An understand-
ing of the site and its waste, bench-test evaluations of
the stabilized/solidified waste form through the tests
described in Sections 4 and 5, and an understanding
of the implementation and costs of stabilization/solidifi-
cation will aid in making an informed judgment about
product suitability. The choice of a final waste product
will inevitably reflect a compromise based on chemi-
cal, physical, and cost considerations. This section
does not discuss design considerations in the screen-
ing of stabilization/solidification technologies; rather, it
discusses the feasibility of the technologies.
Selection of a stabilization/solidification treatment tech-
nology for hazardous waste site remediation depends
on the nature of the waste, the physical containment
properties required for the stabilized/solidified mate-
rial, the applicability of the technology to the waste
site, and cost (Wiles, undated). This technology may
become increasingly sophisticated as a result of chang-
ing regulations, which require a controlled product
with specific leachate characteristics (Federal Register
1988). Durability tests such as those described in
Section 4 are also an important aspect of evaluating
the stabilized/solidified product (Jones 1986).
Waste/binder incompatibility also presents a compli-
cation in the application of stabilization/solidification
technology to hazardous wastes. Experience in the
cement, asphalt, and nuclear waste industries demon-
strates that small amounts of some compounds can
significantly reduce the physical strength and contain-
ment characteristics of the waste/binder product (Jones
1986). Some of these interactions have been docu-
mented in stabilization/solidification case studies (Jones
1986; Tittlebaum and Seals 1985; Kyies, Malinowski,
and Stanczyk 1987; JACA 1985; U.S. EPA 1988d).
As of this writing, however, few data quantifying the ef-
fects of interfering compounds on a particular stabili-
zation/solidification process are available (Jones 1986;
Tittlebaum and Seals 1985). Protocols for processing
hazardous waste materials by use of stabilization/
solidification technologies have not been standard-
ized.
6.1 Application of Technology Screening
Technology screening is a process in which both the
limitations and applicability of a technology are sys-
tematically reviewed. With regard to stabilization/
solidification, the technology screening process in-
volves the following:
Review of existing chemical and physical inter-
ferences between the waste and binder.
Identification of previous studies of similar
wastes.
Identification of potential pretreatment options
for the waste or the site that would extend the
application of stabilization/solidification or im-
prove the containment properties of the prod-
uct.
Assessment of site conditions that could af-
fect the stabilization/ solidification of the waste
or its ultimate disposal.
• Review of health and safety and process re-
quirements.
Deciding on the selection of a technology
based on waste disposal requirements and
overall costs.
Available technical information on interferences and
applications of stabilization/solidification technologies
to hazardous waste materials is often incomplete and
sometimes conflicting. Table 6-1 presents a listing of
interferences and applications of stabilization/solidifi-
cation technologies for common hazardous waste
materials. The information in this table should be
viewed as a starting point, as stabilization/solidifica-
tion has been applied to many wastes with varying
contaminants and concentrations. Because of pos-
sible exceptions to the information presented, each
waste should be evaluated individually.
Selection of a technology is often complicated by
gaps in documented waste/binder interferences. Se-
lection can be further complicated by site-specific
conditions and disposal requirements. The technol-
ogy screening methodology uses empirical data and
6-1
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experience when such are available and addresses
site- and waste-specific concerns as they apply to a
certain stabilization/solidification technology. Thus,
limitations and applications of a technology are pre-
viewed before large amounts of resources are com-
mitted to a site. The cost of hazardous waste treat-
ment and disposal depends on several binder-,
waste-.and site-specific conditions.
Site and waste modifications that aid or enhance
stabilization/solidification methods should be consid-
ered. Specifically, binder additives can be used to
reduce the negative effects of organic waste constitu-
ents (especially oils, greases, and solvents) on the
stability of stabilized/solidified materials. Many of
these additives are proprietary or experimental, and
little direct information is usually available. In some
cases, the wastes should be pretreated to increase
their acceptability for stabilization/solidification. Ex-
amples of pretreatment technologies that have been
used at hazardous waste sites and might be appli-
cable to stabilization/solidification are neutralization,
precipitation, adsorption, and chemical oxidation (JACA
1985; USEPA1986; U.S. EPA 1988d). Site modifica-
tions (e.g., water table diversion or depression) may
be able to be practiced at some stabilization/solidifi-
cation sites. An important modification, which relates
less to chemical properties than to physical proper-
ties, Is mixing. Simply mixing a stratified lagoon, for
example, can make processing more effective be-
cause it generates a uniform or constant material.
This reduces the chance of errors related to process
change. How frequently these site modification tech-
niques are used is difficult to determine, but success
stories have been reported (U.S. EPA 1988d).
As a result of site-specific conditions, different types
of hazardous wastes, and increasing land disposal re-
strictions, a complete site characterization is needed
for selection of an appropriate treatment regime for a
particular site. In some cases, specific waste/binder
incompatibilities will rule out a particular stabilization/
solidification technology. Comparisons of various sta-
bilization/solidification technologies with competing
remediation measures such as soil washing are not
straightforward. Selection of the proper remediation
measure often is a compromise based on both engi-
neering and scientific characteristics.
Figure 6-1 presents a technology screening flow chart
that can be used to develop a treatment plan for a
given waste site. As shown, this screening approach
consists of five distinct phases, which may overlap or
run concurrently:
1) Waste Screening. Obvious characteristics,
such as the presence of large quantities of
water or regulated materials, require the waste
be pretreated to make it acceptable for stabili-
zation/solidification.
2) Waste Characterization. The waste is ana-
lyzed for constituent compounds and the pres-
ence of inhibitors is established.
3) Bench-Scale Testing. Trial stabilized/solidi-
fied waste forms are created and tested by
chemical and physical means to determine the
proper agent type and quantity.
4) Site Evaluation. The applicability of the proc-
ess to the particular site is reviewed. For
example, the presence of a near-surface water
table may require the ground water to be low-
ered for stabilization/solidification to proceed.
5) Cost Evaluation. A determination must be
made as to whether stabilization/solidification
can be implemented economically.
6.1.1 Waste Screening
The initial screening step entails identification of waste
characteristics that may preclude the use of a technol-
ogy. It may show that the complex hazardous waste
contains a regulated constituent that cannot be
landfilled (e.g., dioxin) or that the organic levels in the
waste are in excess of vendor or literature speci-
fications for successful stabilization/solidification. In
some cases, engineering solutions may be available
to overcome these initial problems.
The presence of oil or grease in the waste material
slows (or sometimes stops) the setting and curing
reactions of cernentitious and pozzolanic stabilization/
solidification. Vendors and empirical studies show
conflicting results as to the amount of oil and grease
that can be isolated within the solid inorganic product
matrix without creating short- or long-term leachate or
matrix problems. In low concentrations, oil and grease
entrapment appears to take place and the matrix does
not seem to be affected (Tittlebaum and Seals 1985;
Rowe and API 1987). Some vendors indicate up to
25 percent oil and grease, by weight, can be accom-
modated by cement and silicate mixtures or pozzo-
lanic mixtures without inhibiting the stabilization/so-
lidification reactions (JACA 1985). Other studies
(Kyles, Malinowski, and Stanczyk 1987) and vendors
indicate success with lower or higher concentrations
of oil and grease with their treatments. Depending on
the oil and grease content of the material and its
physical state, phase separators (oil/water separa-
tors, centrifuges, dissolved air flotation) or in situ bi-
odegradation may be appropriate for removing ex-
cess amounts of oily material that are not easily treated
by these inorganic stabilization/solidification technolo-
gies. The characteristic limitations and costs of each
pretreatment technology will need to be identified be-
fore further consideration can be given to implemen-
tation. Bench-scale study will normally be required to
confirm the response of the waste to pretreatment
6-4
-------
Figure 6-1. Technology screening flowchart for stabilization/solidification.
MAJOR WASTE CHARACTERISTICS:
Significant amounts of oil/grease
Presence of wastes prohibited from
landfllllng
Waste Is not readily mlxabte
(gummy/Viscous)
Significant amounts of highly volatile
organic materials
Presence of cerdan types of debris
High water content En waste
AVAILABLE ENGINEERING SOLUTIONS?
Oil/water separation
Filtering/screening debris
Chemical/physical pretreatment
Dewaterlng the waste
REJECT
STABILIZATION/
SOLIDIFICATION
VES
DETAILED WASTE
C HARACTE RIZATION:
Physical
Chemical
Review of Known Inhibitors/users for
specific waste
See Matrices/Literature
Select Stabilization binder and
additives
Eperlence
Llteratu re
Vendor Information
AVAILABLE ENGINEERING SOLUTIONS?
REJECT
STABILIZATION/
SOLIDIFICATION
NO
BENCH SCALE TESTING
Define waste/binder ratios
Define mixing requirements
Identify curing requirements
Document volume
Increase/decrease of product
Perform leach tests on product
(see Section S)
Determine Integrity of the end
product (see section 4)
AVAILABLE ENGINEERING SOLUTIONS?
Secondary containment (liners, etc)
Burlar below frontline
Remove to secure landfill
Create secure landfill on site
REJECT
STABILIZATION/
SOLIOIF 1C AT ION
YES
SITE EVALUATION: is THE TECHNOLOGY
AND PRODUCT APPLICABLE TO THE
WASTE SITE/DISPOSAL SITE?
Environmental concerns: (see section 6)
water table/flood plain restrictions
soil permeability
dust control
Equipment/process requirements:
equipment access to site
equipment movement on site
area available for stockpiling
reagents/wastes
Health and Safety Considerations:
for worker protection
for adjacent neighbors
for application of equipment
for materials handling
Community Relations
AVAILABLE ENGINEERING SOLUTIONS?
Water table depression / diversion
Use of liners
Water spray, etc., for dust control
YES
COST EFFECTIVE TECHNOLOGY?
Material costs and avallabllty
Labor costs'
Cost for removal of product to landfill
or
Cost of lining, replacing and covering
product on site
REJECT
STABILIZATION/
SOLIDIFICATION
REJECT
STAB II. IZATtON/
SOLIDIFICATION
IMPLEMENT
STAB ILIZATIO N/
SOLIDIFICATION
6-5
-------
and to stabilization/solidification. Although interfer-
ence from oil and grease is less of a problem when
organic binders are used, the stabilization/solidification
process will nevertheless vary with the type of organic
binder and the level of oil and grease in the waste.
The capability of inorganic and organic binders to
treat organic materials (e.g., pesticides, PCBs, and
solvents) is a somewhat controversial issue in the
hazardous waste literature. Organic chemicals can
act as solvents for some organic binders (Tittlebaum
and Seals 1985), and phenol or organic solvents can
inhibit the setting and curing reactions of inorganic
binders (JACA1985). Different types of organics will
affect the final product in various ways (Tittlebaum
and Seals 1985; Kolvites and Bishop 1987), and the
effect of the interfering material is usually dispropor-
tionate to the amount present in the waste (Jones
1986). Therefore, waste assessment must include
individual chemical constituents and classes of or-
ganics rather than rely on total organic content of the
waste material. Current literature (U.S. EPA 1986h)
and vendor nformation often summarizes total or-
ganic concentrations, which are reported as being
treatable with inorganic binders. One common range
reported is 20 to 40 percent (U.S. EPA 1986h). This
information cannot be directly applied to all classes of
organic chemicals, however. It may be possible and
cost-effective to treat or remove the interfering or-
ganic material or to modify the binder/additive mixture
to achieve successful stabilization/solidification (Tjt-
tiebaum and Seals 1985; U.S. EPA 1986h). Depend-
ing on the type of organics present, several options
are available for removal of a large fraction of the
organic material or for controlling the effects of or-
ganic materials on the inorganic stabilization/solidifi-
cation. These include:
Soil washing
Thermal removal
Chemical oxidation
Extraction removal
Biodegradation
Addition of adsorbent prior to mixing
(limestone, diatomaceous clays, activated
carbon, or fly ash)
For cement-based and pozzolanic processing, small
concentrations of volatile organic carbons may be
trapped within the inorganic matrix (Tittlebaum and
Seals 1985); however, volatilization may occur during
excavation, mixing, or curing because of the agitation
and heat of hydration (Weitzmari, Hamel, and Barth
1988). Air quality standards or health and safety
precautions may require the control of volatile organ-
ics during processing. In addition, many organic binder
technologies require preheating of the binder and
waste as part of processing. For waste constituents
that decompose or volatilize at the temperatures re-
quired fororganfo stabilization/solidification processes,
the use of these processes may be precluded. Pro-
cess modifications (e.g., batch mixing with volatile off-
gas scrubbing) may help to resolve this problem.
When scrubbing is impractical, pretreatment to re-
move volatile organic carbon may be warranted.
Occasionally, the consistency of the waste may pre-
vent mixing. For example, creosote wastes may be
too viscous for easy excavation and mixing. Heating
the material to ("educe the viscosity and to improve
mixing potential can result in noxious organic vapors.
Excavation during the winter freeze is possible, but
the wastes must be heated before they are mixed with
the binder. Many organic binders are somewhat diffi-
cult to mix because of their viscous nature. Mixing
problems can arise when materials having signifi-
cantly different viscosities are combined.
When the wastes are very dilute, dewatering tech-
nologies may prove useful as a pretreatment method.
Depending on the characteristics of the waste mate-
rial, filtration (vacuum, belt filter press, chamber pres-
sure filtration) or membrane separation may make
stabilization/solidification treatment more feasible.
Water may be required for the hydration of both ce-
ment- and pozzolan-based stabilization/solidification,
however. Excess water is a liability for both organic
and thermal binder technologies because it increases
the cost of processing. In both cases, the water must
be vaporized before or during processing, which re-
quires more energy and thus greater cost. If a dewa-
tering technology is used to remove excess water, the
supernatant must be assessed for possible contami-
nation and treatment prior to its disposal.
Removal of debris can be important for equipment
protection and for process quality control. Docu-
mented cases of mixer or conveyor belt damage and
problems with binder encapsulation of large articles of
debris point up the need for this aspect of waste
characterization (U.S. EPA 1988d). Debris can in-
clude common items such as tires and tree stumps
that are buried with other hazardous waste. The
physical separation and washing of these materials
may be both feasible and economical. When debris is
included with the waste material, processing difficulties
may occur in the mixing and conveyer equipment.
Debris can also interfere with quality control of the
final product in all of the stabilization/solidification
technologies.
6.1.2 Waste Characterization
Hazardous wastes are characterized through analy-
sis of major physical and chemical properties. At this
level of scrutiny, possible inhibitors are reviewed and
binders, additives, and appropriate pretreatment op-
tions are chosen. Stabilization/solidification experi-
ence, vendor information, and literature reviews can
serve as bases for the selection of the proper sta-
bilization/solidification system, plausible waste/binder
6-6
-------
ratios, and other options. In some cases, waste pre-
treatment may be required to overcome specific waste
problems.
Interferences from waste constituents can affect the
physical properties of inorganic and organic binder/
waste products. With regard to the inorganic binders,
these constituents may interfere with the setting reac-
tions, the water-to-binder ratio, the porosity, or the
final strength of the product (Jones 1986). Interfering
constituents may soften organic binders or require
larger amounts of binder to overcome their negative
effects, in addition, inhibitors exhibit various time-
related effects. With regard to organic binders, some
materials can slowly deteriorate the organic binder/
waste product or prevent stabilization/solidification from
occurring. Surface-active molecules work on the ce-
ment (or pozzolan) and water immediately. Set-con-
trolling materials ionize and affect the chemical reac-
tion between the cement (or pozzolan) and water after
minutes or hours. Finely ground insoluble minerals
affect the rheological behavior of fresh cement, but
the chemical effects take several days or months to
become manifested (Jones 1986). The following are
examples of the physical manifestations that can oc-
cur when interfering constituents are present in the
hazardous waste and stabilization/solidification is at-
tempted with inorganic binders:
Spelling and cracking
Set retardation; hardening and waste contain-
ment are impeded.
Flash set; mixing of binder and waste is incom-
plete as a result of a very quick set; equipment
can be fouled.
Chelated/complexed toxic constituents may
accelerate leaching, even if the waste is suc-
cessfully stabilized/solidified.
Some waste constituents can react and cause
swelling and disintegration of the stabilized/
solidified mass long after setting reactions are
complete (Jones 1986).
Oxidizing salts can cause slow deterioration of the
organic binder matrix (Tittlebaum and Seals 1985).
Large quantities of sulfur, calcium chloride, sodium
arsenate, sodium borate, sodium phosphate, sodium
iodate, sodium sulfide, as well as the soluble salts of
magnesium, tin, zinc, copper, and lead all adversely
affect inorganic binders (JACA1985; U.S. EPA 1986h).
Salts can often cause swelling and cracking within the
inorganic matrix, which exposes more surface area to
leaching. These effects can occur in the short term or
the long term.
In addition to the specific chemical interferences listed
in Table 6-1, other chemical factors have an impact
on waste/binder compatibility. An alkaline pH is re-
quired for most inorganic binders to set and cure
properly. In the high-pH environment generated with
inorganic binders, amines or aminated compounds
evolve as ammonia. In high concentrations, gaseous
ammonia can cause immediate danger to life and
health. Any environmental or waste condition that
lowers the pH of the inorganic (e.g., lime, cement)
crystalline matrix will eventually cause matrix deterio-
ration. Flocculants such as ferric chloride can inter-
fere with the setting of cements and pozzolans (JACA
1985). Phenols and nitrates cannot be immobilized
with lime/fly ash, cement, and soluble silicates; fly ash
and cement; or bentonite and cement (Stegmann,
Cote, and Hannak 1988).
On hazardous waste sites, concentrations of the waste
components usually vary with location and depth.
Solid waste dumps and lagoons are often stratified or
pocketed with varying waste materials. Records of
the historic deposition of these wastes can reveal the
types of materials that may be encountered at differ-
ent levels or in different cells within a site.
Physical properties of the waste, such as volume,
solids content, particle size distribution, water con-
tent, debris content, viscosity, and pH have an impact
on waste processing. The volume, solids content,
and particle size analysis aid in determining the waste-
to-binder ratio. Moisture content is also important in
determining whether water should be added or re-
moved for processing.
6.1.3 Feasibility Testing
No set protocol has been established for determining
the feasibility of stabilization/solidification at the bench-
scale level. Testing addresses the leaching potential
of the stabilized/solidified waste as well as its durabil-
ity,
During bench-scale testing, different waste-to-binder
ratios are used to stabilize/solidify waste samples.
Selection of the optimum waste-to-binder ratio is usu-
ally based on leach-test and durability-test results.
The desired properties of the stabilized/solidified prod-
uct may vary with the geographic region and type of
final disposal site. For example, EP Toxicity, uncon-
f ined compressive strength, wet-dry, and freeze-thaw
tests may be among those required of a stabilized/
solidified product at a given site. Incompatibilities in
product quality may be resolved on a site-specific
basis. For example, if the optimum EP Toxicity re-
sults are obtained with a stabilized/solidified waste
that erodes readily during freeze-thaw testing, the
solution may be to bury the waste below the frostline.
Bench-scale tests can define process control require-
ments, including mixing requirements, curing time,
and quality control parameters, which can then be
further defined during pilot-scale testing. Defining
these parameters is important and could save time
and money during field operations. The processing
specification is often recognized to be as important as
the product specification.
6-7
-------
Determining whether the volume of the stabilized/
solidified product will increase or decrease is an im-
portant parameter, both for onsite and offsite dis-
posal. At some remediation sites, the excavated
original raw waste must be replaced with stabilized/
solidified waste material. If the treatment results in a
large increase in the volume of the stabilized/solidi-
fied product, some of it may have to be removed from
the site or the waste volume must be reduced prior to
stabilization/solidification treatment. When the waste
product is transported to an approved landfill, an in-
crease in its volume adds to the overall cost of reme-
diation.
Whenever possible, waste materials from the actual
waste site should be used for bench-scale testing.
Differences in the physical and chemical characteris-
tics of the actual waste (such as particle size distribu-
tion and complex chemical constituents) can affect
waste/binder relationships and final product charac-
teristics.
Bench-scale studies offer a significant advantage in
the refinement of the overall system design, including
the establishment of a quality control (QC) program.
Bench-scale test mixtures can be used to verify or
supplement the waste/additive design and provide
both visual and numeric observations for establishing
QC criteria.
During testing, the following information is important
for an evaluation of inorganic and organic binder tech-
nologies:
Leaching tests appropriate forthe waste site.
• Durability tests as appropriate for the final dis-
position of the product (e.g., freeze-thaw test-
ing has little relevance for a material that will
be buried beneath the freeze line or that is in a
part of the country where it does not freeze).
Permeability tests of the final product, which
should be two orders of magnitude below that
of the surrounding materials.
• The waste-to-binder ratio needed to achieve
leaching and durability characteristics appro-
priate forthe waste site.
• Increase or decrease in the volume of the final
product.
Evolution of gases during processing, curing,
or preprocess drying.
The need for dewatering or adding water to the
inorganic waste/binder mixture.
Ability to mix the waste material and binder.
6.1.4 Site Evaluation
The fourth evaluation step involves a review of rele-
vant environmental conditions that may affect waste
containment on the site, during both excavation* and
processing. Environmental parameters that should
be considered include the depth to the water table; lo-
cation of any sensitive environmental conditions within
the study site; access to the site for equipment and
material; processing space requirements; health and
safety issues; and planning for potential community
relations. Standard construction practices can be
evaluated for such site modifications as water table
depression/diversion or access road construction.
Final disposal options are reviewed and confirmed
during this phase. These options may include the
construction of a liner on the site or offsite transporta-
tion and disposal. The availability of adequate soil for
capping and lining the waste site also can be deter-
mined during this phase. Where applicable, the site
evaluation should also include information on the place-
ment and construction of monitoring wells.
Overall site-specific concerns with regard to a reme-
dial action project are geared toward evaluating waste
containment potential. Containment depends in part
on the types of wastes, their characteristics, and their
physical state. The proximity of a waste to the water
table, onsite drainage tiles, or receiving streams offer
significant migration potential before, during, and af-
ter onsite disposal of the waste material. Important
site parameters in this regard include:
Area of the site
• Permeability of the area soils, both for a review
of leaching capabilities and also for possible
liner/cap material
Existing ground-water contamination
• Velocity and direction of both ground water
and ambient air
Site drainage
• Proximity to populated areas
Access routes to the site
• Available work area/stockpiling area on the
site
• Final disposal options and their site-specific
implications
• Postrernediation use of the site
Sensitive environmental areas within the work
site, such as flood plains or marshes.
This discussion applies primarily to processes in which materials are excavated, processed on site, and returned to the excavated
area. In situ processes do not require excavation and replacement of stabilized/solidified materials. This advantage could lead to the
selection of an in situ process. Offsetting this advantage may be limitations due to poor mixing or depth. Section 7.1.8 describes these
In situ processes. Other factors, e.g., the presence of a near-surface water table, relata to all stabilization/solidification processes.
6-8
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Waste product volume increase and its impli-
cations for the capacity of the site to contain
final product if onsite disposal is required.
• Dusting and material stockpiling
Ability to mix the materials adequately on the
site
Availability of the binder/additives in the
amounts required for the entire site.
Access to the waste site for appropriate equipment
for excavating and processing the waste materials
should be evaluated on a site-by-site basis. Numer-
ous standard construction practices provide solutions
to the problem of difficult access. Sometimes, special
roads must be built or existing roads modified to ac-
commodate the needs of the site. Sometimes, con-
tractors have modified their equipment to gain ac-
cess. Such modifications add to the overall cost of
the project, and customized equipment may be diffi-
cult to repair because of the unavailability of standard
replacement parts.
In some cases, the waste site cannot provide suffi-
cient area for the expected processing, binder stock-
piling, and temporary or final waste disposal. Some
kinds of processing require stockpiling of untreated
excavated wastes, the processed wastes, and the
binder, and these materials may have to be covered
to reduce exposure to the elements. Binders increase
the volume of the waste product, and this added
volume, could present difficulties where the waste/
binder product is buried in the original waste site
excavation. Solutions to problems posed by limited
area must be developed on a site-specific basis. De-
livery of preweighed amounts of the binder directly to
the process site is one possible solution. The binder
can then be added directly to the in-place mixing area
rather than being stockpiled in bulk containers.
The presence of a water table in the waste area
creates three problems: 1) a water table poses the
possibility of existing ground-water contamination; 2)
excess water (especially flowing water) can cause
excavation difficulties; and 3) a water table creates
the potential need for dewatering a saturated waste
material prior to its processing. All three of these
problems have significant cost implications and must
be resolved before the final technology selection is
made.
Some waste materials are difficult to excavate and
handle. Waste pretreatment can consist of adding
sorbents or inorganic binder to dewater the waste and
to increase its handling ability before it is processed
with different inorganic or organic binders. This adds
both a cost factor and a volume factor to the overall
operation. For selected wastes, excavation in the
winter or summer will alter the consistency of the
material sufficiently to allow its excavation.
Site environmental effects also can be critical. Sea-
sonal timing to remove wastes while they have the
most suitable viscosity level was discussed in Section
6.1.1. Freezing retards the overall reaction rate and
may cause long-term damage of the matrix. Summer
temperatures in excess of 100°F accelerate the reac-
tion rate and may ruin the final product.
6.1.5 Cost Evaluation
In the fifth phase, an assessment is made of the costs
involved with the entire technology as it will be applied
to the waste and final disposal site. This includes the
cost of binders, additives, pretreatment, excavation
and moving equipment, labor, and final disposal of the
waste. Disposal costs may include preparation for
onsite burial (liners, caps, and possibly monitoring
wells) or transportation and dumping fees for offsite
disposal.
6.2 Summary
Choosing a waste disposal method is a complex deci-
sion. The costs, capabilities, and'limitations of differ-
ent technologies and their application to both the
waste site and waste material must be weighed. Tech-
nology screening provides a systematic methodology
for reviewing the most important elements of the waste
material, the site, and the technology.
Few adequately documented studies exist that report
the long-term performance of stabilized/solidified haz-
ardous wastes. Performance in this context is deter-
mined by the physical and chemical stability of the
waste/binder product and related leaching character-
istics. Also, almost no published information is avail-
able on the nature, strength, and permanence of the
bonds formed in the stabilization/solidification pro-
cess. Some data have been compiled from the expe-
rience of the construction and nuclear waste treat-
ment industries. These admixtures are managed in
the construction trade by means of strict limitations on
the amounts of additives allowed for a particular appli-
cation. In addition, batch testing of a product is an
integral part of the total construction project. During
the screening of stabilization/solidification technolo-
gies for their adequacy for isolating toxic constituents
and improving the handling characteristics of hazard-
ous waste materials, a review of known inhibitors is
recommended. The list of inhibitors is incomplete,
however, and even the effects of a single component
additive are still poorly understood. Interfering reac-
tions with mixed or complex wastes cannot be pre-
dicted (Jones 1986; JACA1985).
6-9
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As long as this information is lacking, the practice of
designing stabilization/solidification schemes for haz-
ardous wastes will continue to be primarily empirical
in nature. Such design practices are also likely to be
based on the short-term performance of the stabi-
lized/solidified product (Jones 1986).
6-10
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Section 7
Field Activities
This section covers the application of the stabilization/
solidification process at a site. It includes a discussion
of stabilization/solidification functions and processes,
lime stabilization techniques, and site conditions that
can affect the stabilization/solidification process.
Many of the tests described in Sections 4 and 5 often
relate to stabilized/solidified forms developed in the
laboratory. Whereas these stabilized/solidified test
forms may have excellent waste characteristics, dupli-
cating these forms under field conditions can be diffi-
cult because of the equipment used for field stabiliza-
tion/solidification and the conditions present at the
site. Also, the volume of the materials to be stabilized/
solidified at a site often reaches thousands of cubic
yards. Equipment is usually geared to the characteris-
tics of the site and its materials. For example, at many
sites the material to be stabilized/solidified is in aban-
doned impoundments, and its removal may require
large-scale earth-moving equipment such as tracked
backhoes or draglines. At other sites, the soil itself
needs to be stabilized/solidified, and equipment such
as loaders and bulldozers is used to move this soil.
Regardless of the process chosen, adequate quality
assurance/quality control (QA/QC) procedures should
be implemented to be certain the material produced is
near the desired composition. This seemingly simple
task is difficult to accomplish and requires answers to
questions such as the following: Is the material to be
judged as it is produced? How much time is required
to test the quality of the material? For large-scale op-
erations in which the material could conceivably be re-
processed, the turnaround time for the QA/QC proce-
dures must be quick so as to minimize the amount of
material that must be recovered and to avoid the
possibility of burying stabilized/solidified material that
cannot be easily retrieved and reprocessed.
The operations described in this section frequently
use earth-moving equipment. The manufacturers' lit-
erature contains descriptions of this equipment, such
as the reach of backhoes, the capacity of loaders, and
the associated cycle times of the equipment. The Cat-
erpillar Performance Handbooks (Caterpillar 1987a,
1987b) are excellent sources of information for these
operations. These easily understood handbooks pro-
vide information needed for estimating the costs of the
basic operations required at a site.
With the exception of a few specialized processes,
most hazardous waste stabilization/solidification op-
erations fall into two categories: 1) lime/silicate-based
stabilization/solidification, which results in a product
similar to mortar; and 2) stabilization/solidification of
a soil-like material. These two processes are quite
different. One produces a material that hardens like
mortar, and the process and its QA/QC procedures
are comparable to those used in mortar production.
The waste forms, however, differ greatly from mortar
in physical characteristics and they are usually far
less durable. The other process produces a material
that must be described in terms of soil mechanics
and soil handling.
7.1 Stabilization/Solidification Functions
and Processes
Regardless of the stabilization/solidification process
chosen, typically seven functions must be satisfied
for its successful implementation;
1) Waste removal
2) Untreated waste transportation r
3) Untreated waste storage
4) Chemical reagent storage
5) Waste/reagent mixing
6) Stabilized/solidified waste transportation
7) Stabilized/solidified waste replacement
In addition to the preceding, the waste may have to
be stockpiled, moved, or further processed in pre-
treatment operations (e.g., dewatering or neutral-
ization). Some processes may not require all of the
steps listed. An example would be in situ stabiliza-
tion/solidification, in which the reagent is brought to
the waste and it is then stabilized/solidified in place.
7.1.1 Waste Removal
Waste removal generally involves the use of tradi-
tional earth-moving equipment such as tracked back-
hoes, draglines, bulldozers, and front-end loaders.
Because this equipment has been in use for many
years in the construction industry, its application to
hazardous waste handling is merely one of adapta-
tion. Nevertheless, the characteristics of the waste
and the site must be considered.
7-1
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Waste characteristics of importance are its corrosiv-
ity, Us physical state (liquid or solid), and its potential
hazard. Corrosivity is important with respect to the
materials of construction for the removal equipment.
Most construction equipment is fabricated of steel
and is subject to acid attack. This problem can be
eliminated by use of a neutralization pretreatment
step. Neutralization should be performed carefully.
The neutralization of acidic liquids generates hydro-
gen, an explosive gas. Slowing down the neutralizing
process can control this generation. Also, neutraliza-
tion should not take place in confined spaces. A final
caution—chemicals should be added sparingly to avoid
the Incurrence of excess costs.
The physical state of the waste also may influence the
sequencing of operations necessary for the site. For
example, any liquid present above the waste (super-
natant) can be managed by removing it and treating it
as a separate waste.
The hazard posed by the waste is of concern to the
equipment operators and other workers. Complete
enclosure of the operator space in construction equip-
ment and the provision of breathing air may be essen-
tial at sites where dangerous materials such as hydro-
gen sulflde or cyanide may be present.
Tracked backhoes and draglines are used for the
remote removal of materials. Figure 7-1 shows tracked
backhoes and draglines in operation at a hazardous
waste site. Typically, a backhoe is used in areas
where front-end loaders and bulldozers cannot be
used because of the soil's instability. Backhoe opera-
tions are possible in hazardous waste sludges that
ordinarily could not support equipment. During its
operation, the tracked backhoe rests on mats con-
structed of wooden timbers. As the operation pro-
gresses, additional mats are used or the old mats are
moved. The backhoe is limited by the distance and
depth ft can reach and by the cycle time of the operat-
ing sequence.* Distance and depth are available
from the equipment manufacturer's manuals (Cater-
pillar 1987a, 1987b). Because these vary by machine
size, the size of the site and the required reach of the
backhoe determine the piece of equipment to be used.
The cycle time of the operating sequence depends on
the operator, the material to be removed, the removal
equipment, and the transportation equipment (e.g.,
dump trucks). An efficiently operated site considers
all of these characteristics and their interrelation. For
example, the rate at which waste is removed needs to
be assessed so the proper number of dump trucks
can be used at a site. The rate of material removal
and transportation should be as evenly matched as
possible (Caterpillar 1987a).
Draglines are applicable where reaches longer than
40 feet are necessary. They are delivered to a site in
pieces, usually by truck, and assembled on site. This
delivery and assembly must be considered as it may
require several days. The assembly should take place
before the arrival of workers whose activities are not
related to its use. As shown in Figure 7-1, a dragline
is a very large piece of equipment. Its size permits the
removal of materials at a distance, but at the price of
imprecision. Even a good dragline operator cannot
assure removals more accurate than plus or minus a
couple of feet in depth. Thus, excess material is
usually removed during dragline operations. Although
a dragline is often used to remove materials entrained
with liquids, such removal is not efficient because the
liquids escape from the bucket. Other considerations
required in connection with the use of a dragline are
similar to those for a tracked backhoe.
In contrast to backhoe operation, bulldozers and front-
end loaders maneuver in and on the material that is
being removed. The bulldozer pushes the material by
scraping. In a stabilization/solidification operation,
this movement of material may be to an area where
stabilization/solidification takes place or to an area
where it is loaded. Front-end loaders are capable of
both removal and loading of solid material. This
capability is advantageous when the material must be
placed in a dump truck or a hopper for transportation
or processing. Front-end loaders can also be oper-
ated in sludge material. The use of low-pressure tires
(LPT) allows the equipment to be operated in soft (but
not liquid) materials.
7.1.2 Untreated Waste Transportation
Depending on the nature of the waste and the site,
waste can be transported by dump truck, pump and
hose, or a fixed system such as a conveyor belt or
screw auger system. Dump trucks are commonly
used to transport solids, particularly when the liquid
content is low and travel distance is more than one-
fourth mile. As with the use of a backhoe, the carrying
capacity of the trucks at a site must be matched with
the capacity of the removal equipment. Spillage par-
ticularly must be considered because the waste is
hazardous. Truck beds should be lined with plastic to
prevent escape of waste when the waste contains
liquid or when the trucks will travel offsite. The trucks
can also be covered with tarpaulins; this is essential
when the material consists of small particles that are
subject to wind dispersion. As an alternative, the
material can be sprinkled with water to reduce dust-
ing. Finally, trucks are a commonly accepted means
of conveying waste material when it must be taken out
of a controlled area. When this occurs, the trucks
Pftunc* d ovtrhMd utltlMgrMtly rMtrid* ttw UM o( this equlpmant
7-2
-------
Figure 7-1. Photograph of tracked backhoes and draglines in operation at a hazardous waste site.
Source: Geo-Con Inc., Pittsburgh, PA.
must be thoroughly decontaminated upon exiting the
site, and their beds must be decontaminated upon
project completion. Decontamination usually produces
large quantities of water, which in turn must be treated
before its disposal.
Conveyor transportation is used at a site where large
amounts of waste must be moved over a fixed dis-
tance for long periods of time. Their setup is compli-
cated and costly, but the complexity and expense are
offset by their ease of use once they are set up.
Conveyors cannot move liquids, and spillage can be a
problem if the material to be stabilized/solidified is not
dewatered or its water decanted. Because a con-
veyor is a piece of moving equipment, time must be
allotted for maintenance and repair. Also, the use of
sidewalls may be necessary to prevent unwanted
dispersal of hazardous materials.
Pumping of hazardous waste solids may be feasible if
they are sludge-like in nature. In this instance the
transportation function is also the removal function.
The advantage of pumping is that the waste can be
sent directly to the processing equipment.
Thixotropic sludges, which liquify upon movement,
illustrate the need for thorough preparation (as dis-
cussed in Section 6, Technology Screening). These
sludges can be transported without spillage by the
use of gondola conveyors or protective displacement
pumps. A special example of a pump, a sulfur pump,
is equipped with a set of steam coils that melts the
solid material on which it rests and enables the liquid
to be pumped.
7.1.3 Untreated Waste Storage
Occasionally, untreated hazardous waste must be
stored prior to stabilization/solidification. This storage
should be evaluated for each individual site, but the
following factors should be accounted for in any case:
• A 2 to 3 percent sloping of the storage area to
provide for collection of liquids.
Lining of the storage area with a high-density,
polyethylene, flexible, membrane liner and sand
to prevent contact between liquids and the soil
and to allow for easy drainage of the liquids.
Preparation of the underlying soils to remove
rocks, cobbles, and vegetative matter that could
puncture the liner.
Installation of drainage pipes and a sump for
final collection of liquids.
• Operation of a water-treatment system to re-
move hazardous constituents (from the col-
lected liquids) and to reduce the volumes of
liquids that must be stored.
Covering of the wastes to prevent their disper-
sion.
Berming of the sides of the area to resist slump-
ing of the waste.
Installation of a cement loading ramp for the
unloading of wastes from trucks.
7-3
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• Provision of a rubber-tired front-end loader to
move wastes within the storage area.
Providing a temporary storage area is obviously a
complex task. Inasmuch as the construction of this
area can be both expensive and time-consuming,
planning this aspect of the work is just as worthwhile
as for other engineering projects. Untreated waste
storage requirements can be minimized by matching
excavation capacity with treatment capacity.
7.1.4 Chemical Reagent Storage
The satisfactory operation of a stabilization/solidifica-
tion process requires some material stockpiling. Pri-
mary requirements are 1) sufficient storage of agent
to prevent delays in operation; 2) ability to keep an
agent dry, as virtually all stabilization/solidification
agents are dry; and 3) ease of unloading upon deliv-
ery from the supplier and to the process. Bins and
hoppers are the primary storage vessels for solid
agents. The hoppers are sometimes transportable.
On occasion, solid materials are delivered to the site
in bags, which are palletted, or in bulk for storage in
piles. Liquid storage is occasionally required, and
chemical storage tanks are routinely used for this
function.
7.1.5 Waste/Reagent Mixing
The heart of the stabilization/solidification process is
the mixing of the hazardous waste and its stabiliza-
tion/solidification agent. Unfortunately, this is also the
area in which the process fails to meet expectations.
Therefore, most QA/QC procedures are directed to-
ward this operation.
The objective is to achieve mixing in a practical man-
ner. Although ideally, all hazardous waste should be
mixed and reacted with the stabilization/solidification
agent, Tittlebaum et al. (1986) have shown that, even
under ideal laboratory conditions, complete mixing is
not achieved. Thus, a question of degree of mixing
becomes a consideration before one piece of equip-
ment is sent into the field.
Many of the stabilization/solidification activities that
have occurred in the United States have used what is
known as "area mixing." In area mixing, the stabiliza-
tion/solidification agent is delivered to the area to be
stabilized/solidified and mixed with the waste directly.
Often, tracked backhoes, whose removal capabilities
were described in Section 7.1.1, are used for the mix-
ing. Figure 7-2 shows backhoes being used to mix
sludge and lime. Typically, enough water is present in
the waste to promote reaction with the stabilization/
solidification agent and the waste. The waste is mixed
until the operator judges the mixing to be complete.
The benefits to this type of mixing are 1) the waste
usually need not be removed from the site, 2) the
operation requires only traditional earth-moving equip-
ment, and 3) it is inexpensive. Stabilization/solidifica-
tion accomplished by backhoe mixing at Vickery, Ohio,
has been well documented (Curry 1986).
Offsetting these benefits is the probability of incom-
plete and inadequate mixing of the waste. Virtually no
QA/QC program has been enforced at many stabiliza-
tion/solidification sites, and successful stabilization/
solidification cannot be assured unless it can be dem-
onstrated that proper mixing has occurred. In opera-
tions where area mixing has been used, the degree of
mixing has been left to the operator's judgment.
Backhoe mixing is unlikely to result in complete mix-
ing that assures that the waste is phyically or chemi-
cally trapped. Processing equipment, such as pug
mills, ribbon blenders, etc., provides more efficient
mixing.
A wide range of mixing equipment is available for use
in industries that process solids. This equipment is
described by Culliriane, Jones, and Malone (1986) in
their handbook on stabilization/solidification of haz-
ardous wastes. Typical mixing equipment suitable for
use in stabilization/solidification operations includes
pug mills, ribbon blenders, Mueller mixers, extruders,
and screw conveyors (Green 1984). The waste and
reagent can be metered into this equipment during
mixing for quality control purposes.
Some waste stabilization/solidification contractors re-
port the use of pug mills for mixing operations. Pug
mills are manufactured by several companies and are
occasionally modified by contractors to suit their spe-
cial needs. Pug mills consist of double screws, which
allow one material to be mixed with another on a flow-
through basis. The materials are generally partially
mixed before they are introduced into the pug mill.
Figure 7-3 is a photograph of a pug mill operation. As
shown in the photo, one solid is added at the top by a
screw conveyor, mixed with a second solid, and then
fed to the pug mill.
Many small generators drum inorganic wastes that
are, not suitable for incineration. These drummed
liquid wastes can be stabilized/solidified by methods
discussed by Cullinane, Jones, and Malone (1986).
Although drum mixing has limited application, it may
provide better mixing than other methods used to add
stabilization/solidification reagents to hazardous waste.
7.1.6 Stabilized/Solidified Waste Transportation
Stabilized/solidified waste is transported by the same
means (dump truck, conveyor) as unstabilized/unsol-
idified waste. Transporting stabilized/solidified waste
is usually easier than transporting untreated waste
because it should be water-free and less subject to
wind dispersal. One disadvantage associated with
transporting stabilized/solidified waste is that it may
set up (as cement does) if held too long. This can be
7-4
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Figure 7-2. Photographs of backhoe mixing of sludge and lime.
Photographs courtesy National Lime Association
7-5
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Figure 7-3. Photograph of • wll-bentonlte mixing plant using a pug mill.
Photograph courtesy National Lime Association
avoided by the proper addition of stabilization/solidifi-
cation agent or by timely transportation to the stabi-
lized/solidified waste disposal area. Figure 7-4 shows
the loading of lime stabilized/solidified sludge into an
unlined dump truck.
7.1.7 Stabilized/Solidified Waste Replacement
Replacement of the stabilized/solidified waste depends
upon its form. For waste stabilized/solidified in situ,
replacement is of no consequence. Cement-like ma-
terials should immediately be placed in their disposal
area because they set up and harden.
The placement of soil-like materials should be per-
formed in accordance with procedures used in the
road-building industry, where the stabilized/ solidified
waste is placed in lifts of 8 to 10 inches with earth-
moving equipment such as graders or bulldozers.
These procedures are described in detail in Section
7.2. Reportedly, the U.S. Army Corps of Engineers
uses "roller-compacted concrete" in dam construc-
tion. This technique may also be applicable to haz-
ardous waste stabilization/solidification.
After it is placed, the waste is compacted. Determina-
tion of the optimum moisture content of the material
(see Subsection 4.1.3) is required to assure proper
compaction. The optimum moisture content is that
which gives the greatest density to a given material.
Moisture must be added because too little moisture
does not provide the lubrication required for the soil
7-6
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Figure 7-4. Photograph of unllned dump truck being loaded with lime stabilized/solidified sludge.
Photograph courtesy National Lime Association.
particles to slide past one another during compaction.
On the other hand, too much moisture causes the
material to approach the density of water, which is
presumably lower than that of the waste. Thus, for
soil-like, stabilized/solidified, hazardous waste, the
amount of water in the waste must be controlled as it
is compacted.
A second factor that affects the compaction of the
waste is the particle size distribution of the material.
Poorly graded wastes that are predominantly of one
particle size do not compact well. Well^graded mate-
rials compact better because the void volume of the
larger particles is occupied by smaller particles, which
gives a better fit. This factor is usually one that cannot
be controlled unless the particle size distribution of
the stabilization/solidification agent can be regulated.
Figure 7-5 shows the equipment that should be used
for compaction of various-sized materials (Caterpillar
1987a). Use of this information requires the determi-
nation of particle size distribution of the stabilized/so-
lidified material in the laboratory. This, plus an as-
sessment of whether the material has the characteris-
tics of clay, silt, or sand, enables one to make a
proper choice of compacting equipment.
7.1.8 In Situ Processes
Waste stabilization/solidification can also take place
at the site itself. These in situ processes, which have
been in use since 1980, were described by Cullinane
et al. (1986). One major application for them has
been in old lagoons where waste has been present for
decades. Not all are designed for lagoon work, how-
ever, and their specialized application can be both
cost-effective and technically sound. In all of these
processes, the entire range of functions described in
Section 7.1 are combined into one operation.
7.1.8.1 In situ Mixing: Geo-Con System
The Geo-Con Company of Pittsburgh, Pennsylvania,
has developed a system of stabilizing/solidifying waste
materials in situ. This process has been used in
EPA's Superfund Innovative Technology Evaluation
(SITE) demonstration project in Hialeah, Florida. The
equipment also has other geotechnical applications,
such as the stabilization/solidification of dam founda-
tions and in the construction of slurry walls.
The process is based on a combination of an auger
and caisson, which operates in the waste. The stabili-
zation/solidification agent is fed into the auger and
7-7
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Figure 7-5. Chart used to select equipment for compaction of various-sized media.
COMPACTOR ZONES OF APPLICATION
COMPACTIVE METHOD
100%
CI.AY
SILT
SHEEPSFOOT
100%
SAND
ROCK
GRID
VIBRATORY .
SMOOTH STEEL DRUMS
MULTI-TIRED PNEUMATIC
" I
HEAVY PNEUMATIC
VIBRATORY
TAMPING FOOT
TOWED TAMPING FOOT
r
HIGH SPEED TAMPING FOOT
CATERPILLAR
TAMPING FOOT
Static Weight, Kneading
Static Weight, Kneading
Static Weight, Vibration
Static Weight
.__ Static Weight, Kneading
Static Weight, Kneading
Static Weight, Kneading
Static Weight, Kneading, Impact, Vibration
CATERPILLAR
TAMPING FOOT
. Static Weight, Kneading, Impact, Vibration
Source: Caterpillar, Inc. Caterpillar Performance Handbook, October 1987.
then into the waste through a hollow stem. Inside the
caisson, the auger mixes the agent with the waste by
a lifting and turning action. Geo-Con reports that it is
possible to stabilize/solidify wastes to a depth of 100
feet. An important feature of the operation of this
process is the overlapping bore patterns, which, if
properly used, allow for complete coverage of the
waste area.
The Geo-Con process is illustrated in Figure 7-6. The
agent is fed from bulk storage tanks, usually in dry
form. To date, Geo-Con has not specified the agents
for use, but a variety of agents are available for this
purpose. Prior to the mixing step, a liquid can be used
to supply the water necessary for reaction between
the stabilization/solidification agent and the waste and
to improve mixing characteristics. A tracked backhoe
can be used to carry the hydraulic drive that turns the
auger. An exhaust fan draws air from the caisson
through a dust collector and activated carbon for the
removal of particulates and organics, respectively.
Typically, a week to 10 days is needed to mobilize this
equipment. The process requires a crew of about
eight people, and it is capable of processing 600 to
900 cubic yards of waste each day, depending on the
nature of the waste and the stabilization/solidification
agent used.
The Geo-Con process has several advantages. First,
the process is inherently less expensive than one that
uses earth-moving equipment because the waste does
not have to be excavated. Second, the water table
does not have to be lowered. Third, fugitive emis-
sions are controlled by the dust collector and the
activated carbon tanks. Fourth, the quality control as-
pects of this process are good because the rate of
addition of the stabilization/solidification agent can be
regulated and the materials are well mixed. The one
reported disadvantage of the process is its inability to
penetrate masses containing boulders or other de-
bris.
7.1.8.2 In Situ Mixing: ENRECO System
The ENRECO Company of Amarillo, Texas, uses an
injection system for the stabilization/solidification of
waste lagoons. The stabilization/solidification agent
is fed to a lagoon through an injector on a backhoe.
ENREGO manufactures its own equipment, and sev-
eral variations of the basic design are available. Fig-
ure 7-7 is a photograph of ENRECO's equipment.
7-8
-------
Figure 7-6. Crane-mounted mixing system advancing through unstabllized/unsolidifled sludge layer.
BULK
STORAGE
TANKS
TREATMENT
TRANSFER
TANK
ACTIVATED
CARSON
DUST TREATMENT EXHAUST
COLLECTOR TANKS FAN
Source: Geo-Con Inc., Pittsburgh, PA.
Figure 7-7. Photograph of ENRECO backhoe equipped with stabilization/solidifying agent injector.
Photograph courtesy ENRECO Company.
7-9
-------
The stabilizing/solidifying agent is delivered to the
sites by truck, and it is often fed directly from a truck
by a pneumatic system. Mixing is accomplished by
the back and forth action of the injector and by the
force of the pneumatic delivery.
The ENRECO system can stabilize/solidify to depths
of 18 feet, although 10 feet is more common. Even
greater depths have been reached by stabilizing/so-
lidifying in 10-foot lifts, followed by removal. After the
stabilization/solidification of a lift, the lagoons are solid
enough to support the tracked backhoes that perform
the removal. Typical processing rates are from 500 to
1500 cubic yards per day. A crew of three operators
is required for each injection system. The stabilized/
solidified material is usually left on site, but occasion-
ally it is removed.
Typically, ENRECO uses cement, lime, or kiln dust as
the reagent. These materials have been assembled
from sources across the United States, so ENRECO
is able to tailor the agent based on its source. Pro-
prietary agents are also used, although infrequently.
Volume is typically increased by 10 percent after sta-
bilization/solidification. Reportedly, ENRECO has sta-
bilized/solidified a variety of oil- and metal-bearing
lagoons.
Quality control is maintained by sampling for uncon-
f ined compressive strength. A cone penetrometer or
shear vane is used for the measurements. The EPA
paint filter test is used to determine the presence of
free liquids.
7.1.8.3 In Situ Mixing: Envlrlte System
Envirite of Plymouth Meeting, Pennsylvania, formerly
American Resources Corporation, has 3 years' expe-
rience with in situ stabilization/solidification process-
Ing. Envirite uses two types of processing equipment,
based on the depth of the waste. The first, called the
HSS (High Solids Stabilization)System, consists of a
rotary auger placed on the front end of a bulldozer.
This system mixes the soil and stabilization/solidifica-
tion agent in 8- to 10-inch lifts. The mixed waste is
either compacted or transported elsewhere for dis-
posal.
The second system, the PF-5 injector, is placed at the
end of a tracked backhoe. This system can reach into
a lagoon for remote access to the waste materials.
The PF-5 unit uses five injection tubes, each with a
separately controlled feed system, a dust control sys-
tem, and a sludge/additive dispersion mixer. Impel-
lers and augers at the outlet of the PF-5 have been
designed to mix the waste and stabilization/solidifica-
tion agent. The stabilization/solidification agent is
blown into the PF-5, air-separated, transferred by
screw conveyor, and mixed by the auger system. The
PF-5 injector can stabilize/solidify material to a depth
of approximately 10 feet; greater depths can be
achieved with custom units.
Envirite uses cement, lime, or kiln-dust as a stabiliza-
tion/solidification agent. These materials are custom-
tested for each waste in Envirite's laboratory. Pro-
prietary agents are not generally used by Envirite;
however, the equipment is capable of accepting them.
Equipment mobilization at a site requires several days.
A crew of six is required to operate the equipment.
Quality control is performed by sampling each 250
cubic yards of waste, which is measured for EP Toxic-
ity (or TCLP) and unconfined compressive strength.
7.2 Stabilization/Solidification of Materi-
als by Lime Stabilization Techniques
Lime stabilization is used to improve the characteris-
tics of an area such as an airport runway or a highway
roadbed (National Lime Association 1987; National
Research Council 1987). Typically, lime and water
are added to produce a material with increased
strength and decreased plasticity index. Lime is gen-
erally added to clay materials, but other soils are also
appropriate candidates. With respect to hazardous
waste stabilization/solidification, the use of this tech-
nology is attractive for several reasons:
1) Lime is a base for many stabilization/solidifica-
tion processes, either as a raw material itself
or as a constituent of cement.
2) The equipment already exists for the process,
and it can easily be adapted to hazardous
waste use.
3) Much information has already been collected
about the ancillary aspects of the process. For
example, lime dusting and worker safety is-
sues were addressed by the lime industry many
years ago, and these issues can be easily
implemented in the hazardous waste industry.
These stabilization/solidification techniques could be
applied in several ways. First, and most closely re-
lated to the techniques of highway roadbed stabiliza-
tion, would be the application of the stabilization/solidi-
fication agent to the uppermost layer of a soil, where it
would be worked into the soil after being wetted. A
second application of these techniques would be to
place lifts of the waste and stabilization/solidification
agent in a landfill. The stabilization/solidification agent
would be reacted with the waste, possibly through the
addition of water, and the mixture would be com-
pacted so that additional lifts could be placed.
Another application of these techniques would be to
use them with liquid or semiliquid wastes to which no
water addition was required. Typical soil stabilization/
solidification procedures include soil preparation, lime
spreading, soil/lime mixing, compaction to the maxi-
mum practical density, and curing.
7-10
-------
Soil preparation consists of those activities necessary
to prepare the ground for acceptance of the lime
stabilizer. Ground preparation would be required when
a shallow layer of soil needed to be stabilized. In this
instance, a grader-scarifier or disc harrow would be
used for initial scarification. Soil pulverization also
could be necessary. This is accomplished through
the use of a disc harrow or rotary speed mixer.
Lime spreading requires the use of a variety of equip-
ment. When shipment is by truck, self-unloading bulk
tanker trucks are the most efficient for transporting
and spreading the lime because no rehandling is in-
volved. Unloading is effected pneumatically or by one
or more integral screw conveyors. The spreading is
accomplished with a mechanical spreader attached to
the rear of the truck, or through metal downspouts or
flexible rubber boots extending from each conveyor or
airline.
Mixing is required to distribute the lime uniformly
throughout the soil to the proper depth and width and
to pulverize the soil to minus 2 inches. During this
step, water may be added to raise the moisture con-
tent of the soil-lime mixture to the optimum. A rotary
mixer is the best equipment for this purpose because
of its uniform mixing, fine pulverization, and speed of
operation. Blade mixing with a road grader is also
satisfactory for some applications. Proper mixing is
required for complete stabilization of all of the soil.
The sharp contrast between the white lime and all
soils permits visual determination of the uniformity of
the lime and soil mixture.
The lime-soil mixture should be compacted to at least
95 percent of the maximum density. Various rollers
and layer thicknesses have been used for compaction
in lime stabilization. The most common practice is to
compact in one lift by use of a sheepsfoot roller (Fig-
ure 7-8), followed by a multiple-wheel pneumatic roller
(generally a 10-ton roller). A flat-wheel roller is then
used for finishing. Single-lift compaction can also be
accomplished with vibrating impact rollers or heavy
pneumatic rollers; light pneumatic or steel rollers would
then be used for finishing. Compaction should take
place as soon as possible after mixing, but a delay of
several days is not detrimental. During compaction,
light sprinkling may be required to keep the soil at its
optimum moisture content.
After it has been compacted, the soil-lime mixture
must cure. Temperature, moisture, and time are re-
quired for adequate curing. The temperature should
be greater than 50°F, and the moisture should be
close to optimum.
7.3 Site Conditions
Certain site-specific conditions can determine if a sta-
bilization/solidification project is feasible. These in-
clude waste type, land use, safety, and geographic
considerations.
Figure 7-8. Photograph of a sheepsfoot roller being used for compaction.
Photograph courtesy National Lime Association
7-11
-------
7.3.1 Waste Type
The types of wastes most likely to be stabilized/solidi-
fied are liquids, sludges, and solids. Liquids and
sludges are found primarily In lagoons. Solids also
may be present In these lagoons, or they may occur
as soils that have been contaminated by liquids.
Pretreatment of lagoon materials may be necessary
for stabilization/solidification operations. Typical pre-
treatment operations include neutralization and water
removal. Water removal may be a costly and exten-
sive operation, especially if the waste is present be-
neath the local water table. In this case, dewatering
may occur throughout the entire project. The water
produced by dewatering must be treated. Several
companies offer water treatment operations for re-
mote site operation. These operations can remove
solids, dissolved organics, and biological matter.
Access to lagoon materials can be difficult. The dis-
tance to the waste will determine whether a backhoe
or a dragline is used for removal. For operations that
provide for remote removal of the waste, whether the
dikes will support the weight of the equipment must be
considered. Proper width of the roads/dikes also
must be ascertained. Some equipment (e.g., front-
end loaders and bulldozers) may actually operate in a
lagoon. The feasibility of operating this equipment in
the waste must be considered. One consideration
would be whether the waste can support the vehicle.
If not, the use of wide tracks or low-pressure tires
might make this operation feasible. Another consid-
eration is the steepness of the dikes or walls surround-
ing a lagoon. At the one site, front-end loaders could
not be used to carry waste over a dike and into dump
trucks for this reason. Instead, an extra operation
was necessary, which involved the use of backhoes
to transfer the waste from the lagoon bottom to its
side.
Soil treatment operations can disturb ongoing opera-
tions, and stabilization/solidification operations should
be planned to keep this disturbance to a minimum.
Dusting of the soil may be a problem, especially if the
contaminant is present in high concentrations or if a
large receptor population Is nearby. Both dust sup-
pression and the stabilization/solidification operation
require water. Planners should determine if a source
of water is present at the site if it is free of additional
contaminants. If the soil contains concrete or large
rocks, jackhammers will be needed for their removal.
In addition, some soils at a site may not be appropri-
ate candidates for stabilization/solidification even
though they are contaminated. For example, an aqui-
tard at the Frontier Hard Chrome Site was contami-
nated with hexavalent chromium (Dames & Moore
1987). Removal of this aquitard for stabilization/ solidi-
fication could have inadvertently introduced contami-
nation into the aquifer.
7.3.2 Land Use and Safety
Stabilization/solidification applications must include an
appraisal of future site use. If the waste is very toxic,
certain site uses (e.g., residential use) would be pre-
cluded after treatment. If the waste will not support
structures, this option greatly limits the use of the site.
One of the disadvantages of stabilization/solidification
is that it increases the waste volume. A significant
increase in an already large volume of waste could
result in an unexpectedly large waste pile, in which
case offsite disposal of a portion of the waste may be
necessary.
Site operations require complete health and safety
procedures for hazardous waste work. Workers in
machinery may require supplied breathing air; others
may require air-purifying respirators. Provision also
must be made for adequate protective clothing (Level
A, B, C, or D). Decontamination areas are essential
for hazardous waste stabilization/solidification opera-
tions. The proximity of a site to nearby populations
must be determined before the stabilization/solidifica-
tion process is begun. Monitoring for dusts and or-
ganics may be necessary, and the presence of these
materials in the atmosphere could prevent operations
during unfavorable meteorological conditions.
7.3.3 Geography
This topic includes consideration of geology, surface
waters, and meteorology. The principal geologic con-
sideration is that relating to site aquifers. Waste that
extends to aquifers may have to be dewatered as de-
scribed earlier. Weather can have an effect on opera-
tions or the placement of the waste. For example,
stabilization/solidification operations may not be fea-
sible during freezing weather if the stabilized/solidi-
fied waste cannot cure. If the waste is subject to
failure upon repeated freeze/thaw or wet/dry testing,
an engineered solution can be required-yi.e., simply
placing the waste below the frost line. Finally, waste
should never be placed in a f loodplain.
7.3.4 General
Other considerations relate to the peculiarities of a
site. For example, an area may be needed for stock-
piling operations, and the absence of such an area
could preclude stabilization/solidification as a reme-
dial action. Power and utility sources for treatment
operations are also important. If not available locally,
electricity can be generated on site and water can be
imported. Utilities can have an impact on field opera-
tions in another way as well; the presence of over-
head power lines should be determined for safe op-
erations at a site. Finally, any debris that is present in
the waste may have to be removed. It is not unusual
to encounter trees, large rocks, appliances, and other
materials within a waste.
7-12
-------
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