United States
Environmental Protection
Agency
Risk Reduction
Engineering Laboratory
Cincinnati, OH 45268
Research and Development
EPA/600/S-92/023 May 1992
^ EPA Project Summary
An Evaluation of Factors
Affecting the Stabilization/
Solidification of Heavy
Metal Sludge
R. Mark Bricka and Larry W. Jones
Solidification/stabilization (SIS) of
hazardous waste involves mixing the
waste with a binder material to enhance
the physical properties of the waste
and to immobilize contaminants that
may be detrimental to the environment.
Many hazardous wastes contain mate-
rials that are known to inhibit the set-
ting and strength development proper-
ties of cement and pozzolan binding
agents commonly used in S/S pro-
cesses.
The study summarizes the results of
an evaluation of the effects of 10 inter-
fering substances (oil, grease, lead,
copper, zinc, sodium hydroxide, sodium
sulfate, phenol, trichloroethylene, and
hexachlorobenzene) on the physical
and contaminant mobility properties of
a solidified/stabilized heavy metal
sludge. Three binder materials (Port-
land cement, CEM; lime/fly ash, LFA;
and cement/fly ash, CFA) were used to
solidify/stabilize a specially prepared
sludge containing substantial concen-
trations of four metals (cadmium, chro-
mium, nickel, and mercury). The ef-
fects of these interfering materials were
evaluated using five physical tests
(unconfined compressive strength,
cone index, bulk density, wet/dry cy-
cling, and permeability). Contaminant
leaching properties were evaluated us-
ing the U.S. Environmental Protection
Agency's (EPA) extraction procedure
(EP) test. Microchemical/ micromorpho
logical analyses were also performed
on the samples.
Test results indicated that copper,
lead, zinc, grease, oil, and phenol have
a significant detrimental effect on the
physical properties of the solidified/sta-
bilized sludge. In contrast, the effects
of hexachlorobenzene, trichloroethyl-
ene, and sodium sulfate on the physi-
cal properties were much less signifi-
cant. The EP leaching test indicated
that contaminant leaching, except for
mercury, was highly dependent on the
extraction solution's final pH. No de-
finitive conclusions could be drawn
from the microchemical/micromorpho-
logical examinations.
This research confirms the need for
waste-binder specific studies before
selecting a chemical S/S process for
the treatment of hazardous waste.
This Project Summary was developed
by EPA's Risk Reduction Engineering
Laboratory, Cincinnati, OH, to announce
key findings of the research project
that is fully documented in a separate
report of the same title (see Project
Report ordering information at back).
Introduction
S/S is a process that involves mixing a
hazardous waste with a binder material to
enhance the physical and chemical prop-
erties of the waste and to chemically bind
any free liquid. Several binder systems
are currently available and widely used for
the S/S of hazardous wastes.
Most common S/S techniques are built
around either Portland cement or some
type of pozzolan as the basic S/S re-
agent. GEM is widely available, relatively
economical, and well known to the gen-
eral public as producing a very durable
product. Pozzolans are siliceous materi-
als that, when added to a source of free
lime, will go through a cementation pro-
UjQ Printed on Recycled Paper
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cess much like GEM but at a much lower
rate. Fly ash and blast-furnace slags are
common pozzolans that are generally con-
sidered as waste materials themselves.
Wastes most amenable to S/S treat-
ment are water-based sludges. Most
wastes are a complex and variable mix-
ture of many precipitated and dissolved
materials, some of which are expected to
Interfere with the S/S process and cause
undesirable consequences. Effects that
might be expected are a breakdown of
the solidified matrix, flash or retarded set,
or spalllng and disintegration. Such reac-
tions could cause contaminant loss to the
environment.
Vary few quantitative data are currently
available concerning the effects of poten-
tial Interfering compounds on particular S/
S processes. This study was undertaken
to help fill this data void. An earlier report
from this study reviewed the literature and
discussed theories and details of the ef-
fects of known Interfering materials with
cement, pozzolanic, and asphaltic S/S sys-
tems and products.1
General Approach to the
Investigation
In Phase I of the study, an initial litera-
ture survey was conducted to identify any
existing data describing the effects on
materials that interfere with cementation
or pozzolanic setting reactions.
In Phase II, a synthetic sludge contain-
ing four heavy metal contaminants was
prepared with the use of lime precipita-
tion. The lime sludge was then dewa-
tered to produce the sludge used in this
study.
In Phase III, after the metal sludge was
produced, binding agents were added to
solidify/stabilize the waste and immobilize
the contaminants. Before setting, the
sludge/binder mixture was divided into four
portions. Selected interfering chemicals
were mixed with three of the sludge/binder
portions at three concentration levels.
Molded specimens were prepared for the
three interference/sludge/binder mixtures
and for the sludge/binder mixture (con-
trol).
In Phase IV, an accepted testing re-
gime was employed to evaluate the physi-
cal and contaminant release properties of
the solidified/stabilized specimens.
Sludge Preparation
1 Jones, LW. 1589 Inlortoronco Mechanisms In Waste
SoSdSieallorVSabHIzalion Processes. EPA/600/2-89/
087 (NTIS No. P890-1S6 209/AS). USEPA, Risk
Reduction Engineering Laboraloiy.
Laboratory jar tests were performed on
the synthetic metal waste solution to de-
termine the settling properties of the sludge
and the calcium hydroxide dosage for
maximum sludge formation. The synthetic
metal solution was prepared by dissolving
23.1 g/L of Cr(NO,)«9H,O, 14.9 g/L of
N,(NO,) -8HaO, 1.6g/L of Cd(NO3)2-4H O,
and 0.02g/L of Hg(NO3)2«H2O in tap water.
A lime dosage of 20 g/L produced a
sludge with optimal settling characteristics
and a supernatant with the lowest turbid-
ity. This lime dosage was twice the calcu-
lated stoichipmetric amount required for
metal precipitation. The sludge produced
using this lime dosage contained approxi-
mately 9% solids and had a density of
1.08 g/mL. The supernatant had a final
pH of 11.5.
The maximum solid contents of the fil-
ter cake that could be achieved using
vacuum filtration was 30% to 35%. Based
on these preliminary test results and the
fact that filtrate could be used to dilute the
sludge, a 25% solid sludge was selected
for use in this study. By using a constant
sludge solids content, the water-to-binder
ratio for the solidified/stabilized product
could be tightly controlled. Analysis of
this sludge indicated that it contained 18.1-
mg/g of Cd, 81.3 mg/g of Cr, 1.39 mg/g
Hg, and 81.1 mg/g Ni on a dry weight
basis.
Sludge Treatment and
Interference Addition
Binder-to-sludge ratios were formulated
to produce a solidified/stabilized product
that had a 28-day unconfined compres-
sive strength (DCS) of at least 100 psi
(690 kPa). For the three binder systems
(Portland type I cement [GEM]; Portland
type I cement and Class F fly ash [CFA];
and lime and Class C fly ash [LFA]), vari-
ous sludge-to-binder ratios were prepared.
Based on these results, the binder ratios
shown in Table 1 were selected.
Table 1. Binder-to-Sludge Ratios Selected for
Evaluation
Dilution
Binder Binder/sludge ratio factor1
CEM 0.3:1 Cement:sludge 5.2
LFA 0.5:1 Fly ash:sludge 7.2
CFA 0.2:1 Cementsludge 6.8
0.5:1 fly ash:sludge
* Weight of final mixture/weight of dry sludge.
To limit variability, one interference
chemical was evaluated for each batch of
dewatered sludge prepared. Thus, for
each batch of sludge, specimens were
prepared for one binding agent in combi-
nation with one chemical at ratios (weight/
weight) of 0, 0.02, 0.05, and 0.08. These
specimens were subjected to physical and
chemical evaluations after curing.
Chemical reagents were used to intro-
duce the desired interference into the
sludge/binder system. The desired inter-
ference, the interference reagent, and the
weight correction factors are listed in Table
2. All specimens were cured in the molds
at 23°C and 98% relative humidity until
needed for testing.
Physical Testing
Five physical tests were performed on
all solidified/stabilized samples: unconfined
compressive strength (UCS), cone index
(Cl), bulk density (BD), weight/dry cycling
durability (W/D), and permeability (PERM).
Unconfined Compressive
Strength
The UCS test was used to determine
the strength development characteristics
of the various solidified/stabilized wastes.
The UCS of the S/S wastes was deter-
mined using ASTM method C 109-86,
Compressive Strength of Hydraulic Ce-
ment Mortars (using 2-in. or 50-mm cube
specimens). The only deviation from this
method was specimen vibration.
Cone Index
The Cl test was used to evaluate the
strength development characteristics of the
solidified/stabilized materials during their
initial strength development period. The
Cl measures the resistance of a material
to the penetration of a 30-degree right
circular cone. The method is specified in
U.S. Army's "Materials Testing" Technical
Manual 5-530. The Cl is measured using
a penetrometer.
Bulk Density
BD measurements were performed to
evaluate density changes resulting from
the added interfering chemicals. Although
a standard method was not followed, the
method used in this study was tailored
after ASTM method D-558-82.
Wet/Dry Testing
Eleven cycles of W/D testing were per-
formed to measure durability to cyclic
weathering conditions and the effect of
the interference chemicals on sample du-
rability. A modified ASTM method 599-82
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Table 2. Interference Reagents Utilized
Desired
Interference
Oil
Grease
Phenol
Sulfate
Strong base
Degreaser
Pesticide
Lead
Copper
Zinc
Interference
Reagent
30 weight motor oil
Axle grease
Phenol
Sodium sulfate [Na2SOJ
Sodium hydroxide [NaOH]
Trichloroethylene [TCE]
Hexachlorobenzene [HCB]
Lead nitrate [PbfNO^J
Copper nitrate [Cu(NO.)f3Hp]
Tine nitrate [Zn(NOJ2-HsO]
Weight Correction
Factor
1.00
1.00
1.00
1.48
2.35
1.00
1.00
1.60
3.80
4.55
was conducted on specimens after they
had cured for 28 days.
Permeability
The 2.8-in. specimens and a triaxia! cyl-
inder were used in all permeability mea-
surements. Specimens were saturated
using a back-pressure saturation technique
following the method outlined in the U.S.
Army's Corp of Engineers Manual 1110-2-
1906. Triplicate permeability determina-
tions were done on a single individual
specimen that had cured a minimum of 28
days.
Chemical Testing
Extraction Procedure (EP)
Toxicity Test
EPA method 1310 was followed, with
the exception that all materials leached
were ground to pass a 9.5-mm sieve.
Each waste/binder/interference mixture
at each interference concentration was
extracted in duplicate after the solidified/
stabilized materials had cured for 28 days.
The EP extracts were analyzed for Cad-
mium, Chromium, Mercury, Nickel, and
the interference contaminant of interest.
Microchemical/
Micromorphological
Characterization
The Louisiana State University (LSU)
Hazardous Waste Research Center inves-
tigated the microchemical/micromorpho-
logical characteristics of the solidified/sta-
bilized materials to characterize the ef-
fects of the interference chemicals on mi-
croscopic properties of the samples. LSU
employed three analytical techniques:
scanning electron microscopy (SEM), SEM
in conjunction with energy dispersive X-
ray analysis (EDXRA), and X-ray diffrac-
tion (XRD).
Results
The levels of sludge metals contained
in the raw waste were compared with EP
results of the raw waste and specimens
treated by each of the three binder sys-
tems without interferant addition in Table-
3. The levels of metals found in the EP
leachates were excellent, considering the
high concentrations of metals contained
in the sludge. All binders reduced the
metal concentrations in the EP leachates
by 3 to 5 orders of magnitude even though
all the S/S products were ground before
the EP. All three binder systems exhib-
ited good containment characteristics for
the metals in this sludge. Mercury levels
in the EP leachates from the S/S prod-
ucts, however, are about the same as
those from the raw sludge. All three bind-
ers were essentially ineffective at increas-
ing the containment of the Hg in the
sludge.
The results of this study conclusively
show that common components of haz-
ardous wastes interfere with the contain-
ment and strength characteristics of so-
lidified/stabilized wastes. All measured
parameters were affected by one or more
of the interferant materials. Table 4 sum-
marizes all the measured effects of the
interferants on the binder systems used in
the study. Each interferant was rated as
to whether it produced a strong, moder-
ate, or slight positive or negative effect on
each of the test results on each binder
system. PERM was not included in the
summary table because of the high vari-
ability in the test results. BD was also
omitted from the table because only slight
differences were observed for this param-
eter, and these were not correlated with
specimen physical or contaminant con-
tainment properties.
UCS, Cl, and W/D
The metals, grease and oil, and phe-
nol were generally deleterious to all
binder systems. The effects of these
interferants generally increased with
increasing concentration.
Sulfate, hexachlorobenzene, and the
chloroethylene had little measurable
effect at any interference concentra-
tion.
Sodium hydroxide had a mixed effect
as it tended to increase early strength
(Cl) at the 2% and 5% interference
addition, but decreased the early
Table 3. Comparison of the Solidified/Stabilized Extracts with Raw Waste Extracts
Parameter
(units)
Cd
Cr
Ni
Sludge concentrations 18,100 81,300
(mg/kg dry wt)
EP of raw sludgs 57.9 242
(mg/l leachate)
Median EP of CEM S/S 0.0021 0.010
controls (mg/l)
Median EP of CFA S/S 0.028 0.078
controls (mg/l)
Median EP of LFA S/S 0.0009 0.007
controls (mg/l)
1,390
81,100
0.84
0.95
0.29
0.69
149
0.083
0.068
0.009
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Table 4. Summary of the Effects of Interferants on the Solidified/Stabilized Samples
Binder
CEM
CFA
LFA
CEM
CFA
LFA
CEM
CFA
LFA
CEM
CFA
LFA
CEM
CFA
LFA
CEM
CFA
LFA
CEM .
CFA
LFA
CEM
CFA
LFA
CEM
CFA
LFA
CEM
CFA
LFA
* Symbol
+++
++
+
0
-
'
Interferant UCS
Copper +++*
Copper
Copper
Lead 0
Lead
Lead
Zinc
Zinc ~
Zinc
Sulfate
Sulfate 0
Sulfate +
NaOH +
NaOH
NaOH 0
Grease
Grease
Grease
Oil
Oil
OH
HCB 0
HCB 0
HCB 0
TCE
TCE
TCE +
Phenol
Phenol --
Phenol
UCS and Cl
(number times
the control)
<2
1.5 to 2.0
12 to 1.5
0.8 to 12
0.8 to 0.66
0.66 to 0.5
<0.5
Wet/dry
Cl cycles
ND
-
0
..
...
...
... ...
...
0
0 0
++ +
.. ...
++ 0
+++
... ...
0
.
.. ...
.
.
0
0 0
+ 0
0
0
0 0
«... _
0
Wet/Dry
(cycles intact
vs. control)
+5
+2 to 5
+1to2
-1 to +1
1 to -2
-2 to -5
-5
Final
EPpH Cd
..
0
..
BDL
,
...
BDL
0
... '
BDL
+ o
0
0 BDL
+ ++
0
0
0
0 0
0 ++
0 +
0
0 0
0
0
.
* +
0 +++
Final EPpH
>+2
+1to2
+0.3 to 1
-0.3 to 0.3
0.3 to -1
-1 to -2
>-2
EP Leachate
Concentrations
Cr
».
_
0
-
...
f
~
...
..
»-
...
...
, -
0
+
0
0
.
-
f-
+++
-
o ;
.
0
0
++
0
EP Leachate Cone.
(number times the
control level)
<0.1
0.25 to 0.1
0.5 to 0.25
2 to 0.5
2 to 4
4 to 10
>10
Hg
+
0
0
0
+
+
.
+
0
+
+
0
+
0
0
+
0
+
+++
+++
0
0
-
+
+
0
+
0
0
Ni
BDL
-
0
..
BDL
_
...
BDL
BDL
0
BDL
+
0
+
0
+
0
0
0
0
0
+
-
0
0
, +
0
0
ND " not detected; BDL * below detection limit
strength at the 8% level. Sodium
hydroxide also had less effect on long-
term strength (UCS) and durability.
The strength and durability of S/S
products are strongly related, as might
be expected.
A highly significant correlation was
found between the UCS and C) mea-
surements.
Permeability
Because of their inherent variability, per-
meability measurements did not appear
meaningful for evaluating solidified/stabi-
lized solid samples. This information, how-
ever, is thought to be of basic importance
for evaluating the leaching potential of S/
S waste forms.
Bulk Density
Significant volume changes with increas-
ing interference concentrations were ob-
served for the phenol, sulfate, and copper
interferences. Although these volume
changes were significant, generally these
changes were less than 10%.
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EPA Extraction Procedure
The changes in final EP leachate pH
correlate with the EP leaching losses
of cadmium and chromium, and to
some extent with those of nickel.
These metals were leached from the
solidified/stabilized waste materials
with an indirect relationship to the fi-
nal EP leachate pH.
The final pH values of the EP
leachates were lowered appreciably
by the metal nitrate and sulfate
interferants. These interferants greatly
increased the leaching rates of cad-
mium and chromium, and to a lesser
extent that of nickel.
Addition of sodium hydroxide raised
the final pH as would be expected
and caused an increased loss of chro-
mium, and to some extent cadmium.
The organic interferants had little ef-
fect upon the final EP pH. Generally
the organic interferants did not ap-
preciably affect the leaching of the
metals, although hexachlorobenzene
and phenol additions may have low-
ered the leaching rates of cadmium
and chromium slightly.
The mercury concentrations in the EP
leachates were independent not only
of the added interferants, but also of
S/S treatment itself. Mercury was
leached from the S/S products at
about the same levels as from the
raw sludge. Only the addition of oil
as an interferant appeared to increase
the concentration of mercury in the
EP leachates.
Microchemical/
Micromorphological
Examinations
No definitive conclusion can be drawn
from the results of the microchemical/
micromorphological examinations. More
work needs to be done to perfect these
methods as a diagnostic tool for hazard-
ous waste evaluation.
Recommendations
Since the effect of waste constituents
on the integrity of the final product
cannot be predicted from current
knowledge, tests of strength, durabil-
ity, and teachability should be required
of all S/S-treated wastes before dis-
posal.
Cl measurements are recommended
as a rapid and inexpensive method to
estimate early strength of solidified/
stabilized waste. These measure-
ments correlate well to 28-day
unconfined compressive strengths.
Longer term tests have not been
evaluated.
The variability associated with the per-
meability measurements needs to be
addressed. Alternative measurements
that address attributes such as con-
nected pore volume or gas perme-
ation may be of greater value for
highly impermeable materials.
Additional studies addressing long-
term durability of solidified/stabilized
materials must be conducted. These
studies should include at least a small
portion of the samples evaluated by
this study. This will provide a basis
for correlation of the short-term test-
ing with long-term S/S treatment suc-
cess.
fru.S. GOVERNMENT PRINTING OFFICE: 1993 - 750-071/80025
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R. M. Bricka and LW. Jones are with the U.S. Army Engineers Waterways
Experiment Station, Vicksburg, MS 39180-6199.
Car/ton C. Wiles is the EPA Project Officer (see below).
The complete report, entitled "An Evaluation of Factors Affectinq the Stabiliztion/
Solidification of Heavy Metal Sludge," (Order No. ADA 264-128; Cost: $27.00,
subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Off her can be contacted at:
Risk Reduction Engineering Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
United States
Environmental Protection
Agency
Center for Environmental
Research Information
Cincinnati, OH 45268
BULK RATE
POSTAGE & FEES PAID
EPA
PERMIT NO. G-35
Official Business
Penaity for Private Use $300
EPA/600/S-92/023
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