Technical Report
Advances in Encapsulation
Technologies for the
Management of Mercury-Contaminated
Hazardous Wastes
Contract GS-10F-0275K
Task Order No. 0001
Submitted to
U.S. Environmental Protection Agency
National Risk Management Research Laboratory
26 W. Martin Luther King Drive
Cincinnati, OH 45268
Paul M. Randall
Task Order Project Officer
Prepared by
Sandip Chattopadhyay
Wendy E. Condit
Battelle
505 King Avenue
Columbus, OH 43201-2693
August 30, 2002
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Notice
The U.S. Environmental Protection Agency through
its Office of Research and Development funded and
managed the research described here under GS-10F-
0275K, Task Order No. 001 to Sandip Chattopadhyay
and Wendy E. Condit, Batelle, Environmental
Restoration Department, 505 King Avenue,
Columbus, OH 43201. It has been subjected to the
Agency's peer and administrative review and has been
approved for publication as an EPA document.
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Foreword
The U.S. Environmental Protection Agency (EPA) is charged by Congress with protecting
the Nation's land, air, and water resources. Under a mandate of national environmental laws,
the Agency strives to formulate and implement actions leading to a compatible balance
between human activities and the ability of natural systems to support and nurture life. To
meet this mandate, EPA's research program is providing data and technical support for
solving environmental problems today and building a science knowledge base necessary to
manage our ecological resources wisely, understand how pollutants affect our health, and
prevent or reduce environmental risks in the future.
The National Risk Management Research Laboratory (NRMRL) is the Agency's center for
investigation of technological and management approaches for preventing and reducing risks
from pollution that threaten human health and the environment. The focus of the Laboratory's
research program is on methods and their cost-effectiveness for prevention and control of
pollution to air, land, water, and subsurface resources; protection of water quality in public
water systems; remediation of contaminated sites, sediments and ground water; prevention
and control of indoor air pollution; and restoration of ecosystems. NRMRL collaborates with
both public and private sector partners to foster technologies that reduce the cost of
compliance and to anticipate emerging problems. NRMRL's research provides solutions to
environmental problems by: developing and promoting technologies that protect and improve
the environment; advancing scientific and engineering information to support regulatory and
policy decisions; and providing the technical support and information transfer to ensure
implementation of environmental regulations and strategies at the national, state, and
community levels.
This publication has been produced as part of the Laboratory's strategic long-term research
plan. It is published and made available by EPA's Office of Research and Development to
assist the user community and to link researchers with their clients.
E. Timothy Oppelt, Director
National Risk Management Research Laboratory
111
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Contents
Figures iii
Tables iv
Abbreviations and Acronyms v
Abstract vii
1.0 Introduction 1
2.0 Encapsulation Materials 3
2.1 Sulfur Polymer Cement Encapsulation 4
2.2 Chemically Bonded Phosphate Ceramic Encapsulation 10
2.3 Polyethylene Encapsulation 17
2.4 Other Encapsulation Materials 22
2.4.1 Asphalt 22
2.4.2 Polyester andEpoxy Resins 23
2.4.3 Synthetic Elastomers 23
2.4.4 Polysiloxane 23
2.4.5 Sol-Gels 25
2.4.6 Dolocrete™ 25
2.4.7 Materials Used With Other Metals 25
3.0 Cost and Vendor Information 28
4.0 Future Development and Research Needs 30
5.0 References 32
11
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Figures
Figure 2-1. Sulfur Polymer Cement Encapsulation 5
Figure 2-2. Chemically Bonded Phosphate Ceramic Process 12
Figure 2-3. Polyethylene Macroencapsulation 18
Figure 2-4. Materials and Additives for Solidification/Stabilization of Other Metals 26
in
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Tables
Table 2-1. Key Performance Data for Sulfur Polymer Cement Encapsulation 7
Table 2-2. Key Performance Data for Chemically Bonded Phosphate Ceramic Encapsulation. 13
Table 2-3. Key Performance Data for Polyethylene Encapsulation 19
Table 2-4. Key Performance Data for Various Encapsulation Materials 24
Table 3-1. Summary of Cost and Vendor Information for Encapsulation and Other Treatment
Technologies 29
IV
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Abbreviations and Acronyms
ANL
ANS
ANSI
ARD
ASTM
BDAT
BNL
BP
CBPC
CFR
CSF
DOE
DOT
HOPE
Hg2SO4
HgCb
HgS
INEL
KH2PO4
LDPE
LDR
MA
MgO
Argonne National Laboratory
American Nuclear Society
American National Standard Institute
acid rock drainage
arsenic trioxide
American Society for Testing and Materials
best demonstrated available technology
Brookhaven National Laboratory
bench-scale/pilot-scale
chemically bonded phosphate ceramics
Code of Federal Register
ceramic silicon foam
United States Department of Energy
United States Department of Transportation
full-scale
haematite
hydrogen sulfide
high-density polyethylene
mercurous sulfate
mercuric chloride
mercuric sulfide
Idaho National Engineering Laboratory
monopotassium phosphate
low-density polyethylene
Land Disposal Restrictions
macroencapsulation
magnesium oxide
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MI microencapsulation
NA not applicable
Na2S sodium sulfide
Na2S.9H2O sodium sulfide nonahydrate
Na2SO sodium sulfite
NR not reported
NRC Nuclear Regulatory Commission
ORNL Oak Ridge National Laboratory
ppm parts per million
psi pounds per square inch
RCRA Resource Conservation and Recovery Act
SAIC Science Application International Corporation
SEM scanning-electron microscope
SITE Superfund Innovative Technology Evaluation
SPC sulfur polymer cement
STC Silicate Technology Co.
TCE trichloroethylene
TCLP Toxicity Characteristic Leaching Procedure
U.S. EPA United States Environmental Protection Agency
voc
vol%
wt%
WPI
volatile organic compound
volume percent
weight percent
Waste Policy Institute
VI
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Abstract
Although industrial and commercial uses of mercury have been curtailed in recent times, there is
a demonstrated need for the development of reliable hazardous waste management techniques
because of ongoing hazardous waste generation and historic operations that have led to
significant contamination. The focus of this article is on the current state of encapsulation
technologies and materials being used to immobilize elemental mercury, mercury-containing
debris, and other mercury-contaminated wastes, soils, or sludges. The range of encapsulation
materials used in bench-scale, pilot-scale, and full-scale applications for mercury-containing
wastes are summarized in this report. Several studies have been completed regarding the
application of sulfur polymer stabilization/solidification, chemically bonded phosphate ceramic
encapsulation, and polyethylene encapsulation. Other technologies or materials reported in the
literature or under development for encapsulation include asphalt, polyester resins, synthetic
elastomers, polysiloxane, sol-gels (e.g., polycerams), and Dolocrete™. The objective of these
encapsulation methods is primarily to physically immobilize hazardous wastes to prevent contact
with leaching agents such as water. These methods may also include a stabilization step to
chemically fix mercury into a highly insoluble form. Economic information relating to the use
of these materials is provided, along with available vendor information. Future technology
development and research needs are also discussed.
vn
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1.0 Introduction
The development of effective treatment options for mercury-contaminated wastes is a significant
technical and practical challenge due to several factors, including the limited economic benefit
derived from mercury recovery/recycling, the high toxicity, volatility, and environmental
mobility of mercury, and the varied nature and composition of industrial waste products.
Principal sources of mercury-contaminated industrial wastes include chloralkali manufacturing,
weapons production, copper and zinc smelting, gold mining, paint applications, and other
processes (United States Environmental Protection Agency [U.S. EPA], 1997). Although
industrial and commercial uses of mercury have been curtailed in recent times, there is a
demonstrated need for the development of reliable hazardous waste management techniques
because of ongoing hazardous waste generation and historic operations that have led to
significant contamination. This document focuses on the current state of encapsulation
technologies and materials being used to immobilize elemental mercury, mercury-containing
debris, and mercury-contaminated wastes, soils, and sludges.
As an inorganic element, mercury cannot be destroyed, but it can be converted into less soluble
or teachable forms to inhibit migration into the environment after disposal. The management
and ultimate disposal of mercury-contaminated hazardous waste is controlled by U.S. EPA
regulations known as the Land Disposal Restrictions (LDRs) (40 Code of Federal Register
[CFR] Part 268). Under the current LDR program, the U.S. EPA has established thermal
recovery (e.g., roasting/retorting) as the best demonstrated available technology (BDAT) for
treatment of wastes containing greater than 260 mg/kg of mercury. For treatment of wastes with
less than 260 mg/kg of mercury, other extraction technologies (e.g., acid leaching) or
immobilization technologies (e.g., stabilization/ solidification) may be considered (U.S. EPA,
1999). Because mercury contained in radioactive or mixed waste is not suitable for thermal
recovery and recycling, the U.S. EPA also recognizes that stabilization/solidification may be an
appropriate treatment option for heavily contaminated mercury mixed wastes or debris (Waste
Policy Institute [WPI], 1999).
Stabilization/solidification relies upon mobility reduction resulting from a combination of
chemical reaction (e.g., precipitation) and physical entrapment (e.g., porosity reduction).
Encapsulation technologies are based primarily on solidification, and act to prevent hazardous
waste from coming into contact with potential leaching agents, such as water. Hazardous waste
materials can be encapsulated in two ways: microencapsulation or macroencapsulation.
Microencapsulation involves mixing the waste together with the encasing material before
solidification occurs. Macroencapsulation involves pouring the encasing material over and
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around a larger mass of waste, thereby enclosing it in a solidified block. Sometimes these
processes are combined.
The U.S. EPA is considering changes to the LDR program to require a macroencapsulation step
prior to the land disposal of stabilized mercury wastes. Mercury wastes may be stabilized using
sulfide or other chemical fixation processes, but the stabilization process is pH dependent and
may not permanently immobilize mercury for disposal. The optimal pH range is 4 to 8 for
chemical fixation of mercury compounds to the highly insoluble solid form, mercuric sulfide
(HgS). At high pH, the more soluble solids mercurous sulfate (Hg2SO4), mercuric sulfate
(HgSC>4), and mercury sulfide hydrogen sulfide complex (HgS[H2S]2) are formed depending on
oxidizing or reducing conditions; while at low pH, hydrogen sulfide gas escapes from the waste
(Wagh et al., 2000; Clever et al., 1985). Combining stabilization with macroencapsulation to
prevent pH-related degradation of the treated waste may improve its long-term stability and
therefore minimize any potential threats to human health and the environment.
The range of encapsulation materials used in bench-scale, pilot-scale, and full-scale applications
are summarized in the following sections. Economic information for several different
encapsulation materials is provided, along with available vendor information. Future technology
development and research needs are also discussed.
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2.0 Encapsulation Materials
Materials used for encapsulation of mercury must be both chemically compatible with the
hazardous waste and inert to common environmental conditions that may be encountered in a
disposal facility, such as rain infiltration, groundwater flow, and freeze/thaw cycles. Sulfur
polymer stabilization/solidification (SPSS), chemically bonded phosphate ceramic (CBPC)
encapsulation, and polyethylene encapsulation are just three of the techniques that are currently
being tested and used to improve the long-term stability of hazardous wastes. Studies that focus
on the management of elemental mercury, mercury-contaminated debris, and other mercury-
contaminated wastes will be discussed. Each encapsulation material will be reviewed in terms of
the key features of the encapsulation process, current applications and technology status, and
available performance data. The advantages and disadvantages associated with each material
will also be discussed.
Performance data for encapsulated wastes typically include both physical data (e.g., strength,
density, and permeability) and/or chemical data (e.g., teachability). For macroencapsulated
waste, the most important evaluation criteria are the compressive strength, the waste form
density, the presence of void spaces, and the barrier thickness. The primary concern during
macroencapsulation is that an inert surface coating or jacket is created which substantially
reduces the potential for exposure of the waste to leaching media (Mattus, 1998). For
microencapsulated waste, the toxicity characteristic leaching procedure (TCLP) plays an
important role in determining whether or not the material can be accepted by a landfill.
Macroencapsulated materials are typically not tested with the TCLP. According to the Resource
Conservation and Recovery Act (RCRA) LDR rules, mercury hazardous waste is defined as any
waste that has a TCLP value greater than 0.2 mg/L. Mercury-contaminated wastes that exceed
this value must be treated to meet the Universal Treatment Standard (UTS) of 0.025 mg/L or less
prior to disposal in a landfill. The TCLP test methodology, Method 1311, is described in detail
in the U. S. EPA guidance document SW-846 Test Methods for Evaluating Solid Waste,
Physical/Chemical Methods. TCLP tests on microencapsulated material may require size
reduction to meet the particle size specifications in Method 1311. Instead of crushing, cutting, or
grinding the microencapsulated material, the particle size requirements are typically met through
subsampling the waste and binder formulation and casting small pellets in the appropriate size.
This methodology helps to meet TCLP test requirements, while maintaining the barrier surface
and integrity of the waste form. The U.S. Nuclear Regulatory Commission (NRC) has also
developed its own waste form testing protocols for mixed wastes. In general, NRC waste form
testing examines the influence of environmental factors on the final waste form stability
including the effect of thermal cycling and immersion on compressive strength and the impact of
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biodegradation and irradiation. However, a detailed discussion of these waste evaluation criteria
are beyond the scope of this report.
2.1 Sulfur Polymer Stabilization/Solidification
Conventional stabilization/solidification methods typically include the fixation of metals using
Portland cement and fly ash, which produces an impermeable, solid waste form and creates a
high pH environment that limits the solubility and teachability of most metals. However, it is
very difficult to stabilize mercury with cement-based processes because it does not form a low-
solubility hydroxide solid (U.S. EPA, 1999). For this reason, a significant amount of research
has gone into the development of alternative binding materials for the stabilization/solidification
of mercury-contaminated wastes. As discussed below, the SPSS process can be used to both
convert mercury compounds into the highly insoluble HgS form and to simultaneously
encapsulate the waste.
The SPSS process relies upon the use of a thermoplastic material which contains 95 wt%
elemental sulfur and 5 wt% of the organic modifiers, dicyclopentadiene and oligomers of
cyclopentadiene. This material is referred to in the literature as sulfur polymer cement (SPC),
although it is not a cementitious material. SPC melts at approximately 115°C (235°F) and sets
rapidly upon cooling. It is relatively impermeable to water compared to conventional concrete
and has a high mechanical strength at approximately double that of conventional concrete. SPC
is also well suited to harsh environments with high levels of mineral acids, corrosive electrolytes,
or salt solutions, according to research completed by van Dal en and Rijpkema (1989), McBee
and Donahue (1985) and others as quoted in Darnell (1996).
Figure 2-1 provides a simplified block-diagram for the SPSS encapsulation process (United
States Department of Energy [DOE], 1994). For macroencapsulation, molten SPC is poured
over and around large debris such as metal scrap and is then allowed to set into a monolithic
waste form. The recommended mixing temperature for SPC is between 127-138 °C (260-280
°F). Operating in this range will minimize gaseous emissions and provide optimum viscosity
(Darnell, 1996).
For microencapsulation of liquid, elemental mercury, a two-stage process referred to as SPSS
has been patented by Kalb et al. of Brookhaven National Laboratory (BNL) under U.S. Patent
No. 6,399,849. First, the elemental mercury is mixed in a heated reaction vessel at 40 °C with
powdered SPC. Other chemical stabilization agents such as sodium sulfide and triisobutyl
phosphine sulfide can also be added during this initial step. The heated reaction vessel helps to
accelerate the reaction between mercury, SPC, and the additives to form HgS and an inert gas
atmosphere helps to prevent the formation of mercuric oxide. Next, additional SPC is added and
the mixture is heated to 130 °C (266 °F) to form a homogenous molten liquid, which is then
poured into a mold and allowed to set into a monolithic waste form. This two-step process
minimizes both the oxidation of mercury to mercuric oxide and the amount of unreacted
mercury. In addition, the researchers have confirmed the formation of two forms of mercuric
sulfide as a result of the treatment process. Both meta-cinnabar and cinnabar phases were
identified using x-ray powder diffraction scans (Fuhrmann et al., 2002). BNL has two patents
related to sulfur polymer encapsulation (U.S. Patents No. 6,399,849 and 5,678,234). BNL
recently has licensed the SPSS technology to Newmont Mining Corporation for the
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encapsulation of liquid elemental mercury generated as a byproduct of gold mining operations.
Newmont and BNL are currently working on scaling-up the technology for industrial use (BNL,
2002).
Several studies have been completed regarding the use of SPC for metal-contaminated wastes
including Fuhrmann et al. (2002), Mattus (1998), and Darnell (1996). The results and
observations from these studies are discussed below, along with a summary of the advantages
and limitations associated with the SPC encapsulation method. Table 2-1 summarizes key
performance data from these studies.
Resize'Shred
Sludge/
Other
Dryer
Pretreatment
Dual
Planetary
Orbital
Mixer
cr-p
Off-Gas
Treatment
(Bag House)
Supplemental
Heating Tapes
Waste Form
Container
Figure 2-1. Sulfur Polymer Stabilization/Solidification
Note: Figure modified from DOE (1994). Additives can be used to decrease the leachability of mercury.
Additives reported in the literature include sodium sulfide and tri-isobutyl phosphine sulfide (Fuhrmann et
al., 2002).
Fuhrmann et al. (2002) presents the results from a bench-scale study for the treatment of
radioactive elemental mercury with the patented SPSS process described above. Elemental
mercury and radioactive elemental mercury were obtained from waste stocks at Brookhaven
National Laboratory. The study explored three issues including the leachability of the treated
waste, the formation of mercuric sulfide, and mercury vaporization during processing. Several
treatability tests were conducted on the mercury wastes including microencapsulation with SPC
alone, with 3 wt% triisobutyl phosphine sulfide, 3 wt% sodium sulfide nonahydrate, and a 1.5
wt% combination of these two additives. Microencapsulation of the elemental mercury with
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SPC alone resulted in TCLPs ranging from 20 ug/L to > 400 ug/L. The final formulation that
was chosen was the 3 wt% sodium sulfide treatment which resulted in an average leachate
concentration for mercury of 25.8 ug/L and a range from 1.3 to 50 ug/L. Long-term leaching
studies were also conducted according to the American Society for Testing and Materials
(ASTM) Method C-1308. This test demonstrated a very low release rate with a diffusion
coefficient for mercury in the final waste form on the order of 10"17 cm2/s. The authors also
explored the formation of mercuric sulfide through x-ray diffraction studies and determined that
elemental mercury and SPC reacted to form primarily meta-cinnabar. However, elemental
mercury and sodium sulfide nonahydrate formed primarily cinnabar, which explains the
improved leaching behavior in those tests with 3 wt% sodium sulfide as an additive. The results
of mercury volatilization studies also demonstrated that mercury volatilization was reduced
through the treatment with sodium sulfide. Headspace measurements for elemental mercury
ranged from 9.2 to 12.7 mg/m3 in vapor, ranged from 0.41 to 4.5 mg/m3 with just SPC, and 0.20
to 1.3 mg/m3 with the addition of sodium sulfide. These results suggest that, for adequate
retention of the mercury during processing, the use of additives such as sodium sulfide may be
necessary (Fuhrmann et al., 2002).
Oak Ridge National Laboratory (ORNL) completed a treatability test to scale-up the SPC
process for the macroencapsulation of mixed waste debris, contaminated with mercury and other
metals (Mattus, 1998 and ORNL, 1997). The ORNL treatability study objectives included
scaled-up equipment selection, determination of the size and shape of the final waste form, and
process parameter monitoring and optimization. The treatability study was performed using two
mixed waste streams generated at ORNL:
a 208 kg (457 Ib) of cadmium sheets (Resources Conservation and Recovery Act [RCRA]
waste code D006), and
a 204 kg (448 Ib) of lead pipes contaminated with mercury (RCRA waste codes D008 and
D009).
The cadmium sheets were classified as debris under the alternative debris standards found in 40
CFR 268.45. Macroencapsulation is an option for treatment of this waste code under the
alternative debris treatment standards. Macroencapsulation with SPC also satisfied the LDR
treatment standards for radioactive lead solids (D008) and for mercury (D009) through
amalgamation, as sulfur is one of the elements that is able to form a non-liquid, semisolid when
combined with mercury.
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Table 2-1. Key Performance Data for Sulfur Polymer Stabilization/Solidification
Author/
Vendor
Mattus
(1998)
Mattus
(1998)
Fuhrmann et
al. (2002)
Fuhrmann et
al. (2002)
Fuhrmann et
al. (2002)
Darnell
(1996)(b)
Kalbetal.,
(1996)
Type
MA
MA
Ml
Ml
Ml
Ml
Ml
Scale
BP
BP
BP
BP
BP
BP/F
BP
Waste Type
Mixed waste cadmium
sheets
Mixed waste lead
pipes/gloves contaminated
with Hg
Radioactive Hg°
Radioactive Hg° with 3
wt% triisobutyl phosphine
sulfide additive to SPC
Radioactive Hg° with 3
wt% Na2S.9H2O additive to
SPC
Metal oxides including Hg,
Pb, Ag, As, Ba, and Cr at 5
wt% each
Mixed waste
off-gas scrub solution
Waste
Form
Size
5 gal
5 gal
5 gal
5 gal
5 gal
NR
NR
Waste
Loading
(wt%)
15.8 to
28.6
31. 3 tO
38.8
33.3
33.3
33.3
40
25 to 45
Compressive
Strength
(psi)
NR
NR
NR
NR
NR
4,000
3,850
to
8,160
Density
(g/cm3)
NR
NR
NR
NR
NR
NR
1.86
to
1.94
Before
Hg
TCLP
(mg/L)
NA
NA
2.64
2.64
2.64
250(a)
0.14
After
Hg
TCLP
(mg/L)
NA
NA
0.020
to >0.40
>0.40
0.0013 to
0.050
<0.2
<0.009
BP=Bench-Scale/Pilot-Scale.
F= Full-Scale.
MA= macroencapsulation
Ml= microencapsulation,
NA= not applicable.
NR= not reported.
TCLP = Toxicity Characteristic Leaching Procedure.
(a) Untreated waste TCLP not reported, so estimated based on total Hg level in waste divided by 20.
(b) Sodium sulfide nonahydrate was added to reduce metal leachability.
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The equipment used in the study included primarily a 5-gallon steel container, a rigid wire
basket, a handle spacer to hold the debris in place, a vibrating table, external heater tapes, and a
melting pot and pour pipe for the molten SPC. The size and shape of the final waste form was
developed based upon criteria from the United States Department of Transportation (DOT)
hazardous material shipping regulations (49 CFR 173.12), the hazardous waste disposal facility
(Envirocare), and general safety and handling considerations. The DOT requirements included
the use of an approved container with a total waste form weight limit of 205 kg. The Envirocare
facility specified that the following process requirements would have to be met for waste
acceptance:
a The barrier had to be in intimate contact with the waste,
a The barrier should be at least 2 inches thick around the waste material, and
a The waste had to be encapsulated using a continuous pour.
Because of the small size of the waste form mold, some preparation and size reduction of the
cadmium sheet debris and lead pipe wastes was required. In a radiological fume hood, sheet
metal scissors were used to reduce the size of the cadmium sheets, which ranged in length from 4
inches to more than 40 inches. For the lead pipe wastes, pieces of debris were bent or cut to the
target size (e.g., < 4 inches).
The SPC process was first tested with non-contaminated materials, so the waste form could be
cut transversely and observed to optimize process parameters. Following two practice trials, the
radioactive mixed waste streams were encapsulated in a series of 20 batches. The major steps
involved in the SPC macroencapsulation process included the following:
1) The debris was placed into a secured wire basket, which was centered in the drum to
maintain a 2-inch surrounding layer of SPC,
2) Molten SPC was poured into the drum to provide both the outer layer of SPC and to fill
the voids between the debris,
3) The pour was continued until SPC reached 2 inches from the top of the drum,
4) Once the bottom portion of the waste form had hardened, the spacer holding the wire
basket was removed and a cap of molten SPC was added to fill the drum, and
5) Once the cap layer was set, the drum was sent for land disposal.
It was found that heating the debris to 140 to 150 °C (284 to 302 °F) for six hours prior to the
pour helped to ensure that fast cooling of the SPC did not occur at the waste-binder interface and
helped to reduce the formation of air pockets. Vibrating the container throughout the pouring
sequence and for up to five minutes after the pour also improved setting of the waste form.
Heating tapes were used to maintain a target temperature of 190 °C (374 °F) at the top portion of
the container. This allowed air bubbles from the setting SPC to escape. The optimal additional
heating time was determined to be 10 hours after the pour had ended. During the surrogate waste
test, examination of the waste form cross section revealed good contact between the debris
pieces and SPC and no identifiable interface between the two pour layers (i.e., the top portion of
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the drum and the cap). No H2S or 862 off-gasses were detected during the tests. For
macroencapsulation of the mixed wastes, waste loadings for the cadmium sheets ranged from
15.8 to 28.6 wt% and for the lead pipes ranged from 31.3 to 38.8 wt%. Key performance data
from this study is summarized in Table 2-1.
Darnell (1996) demonstrated the use of SPC for the microencapsulation of up to 5 wt% of metal
oxides including mercury, lead, silver, arsenic, barium, and chromium. Darnell
microencapsulated a variety of metal-contaminated wastes including dehydrated boric acid salts,
incinerator hearth ash, mixed waste fly ash, and dehydrated sodium sulfate salts. These treated
wastes were then subjected to the U.S. EPA TCLP, the Nuclear Regulatory Commission (NRC)
waste form qualification testing, and the American Nuclear Society (ANS 16.1) leaching index
test. Darnell also found that an additional chemical stabilization step was needed to treat
mercury to meet TCLP limits. A 7 wt% sodium sulfide nonahydrate (Na2S.9H2O) was added to
the SPC mixture to convert metal oxides to more leach-resistant metal sulfides. The U.S. EPA
TCLP limits were achieved for all metals. Key performance data from this study is summarized
in Table 2-1.
Darnell (1996) also discussed the issues considered during scale-up of the SPC encapsulation
process for mixed waste incinerator fly ash. The full-scale system proposed consisted of a large
disposal box (1 m on each side) that would be stacked in an above-grade, earth-mounded,
concrete disposal vault. The box would be surrounded by a heated mold-form to prevent
swelling due to the approximately 3,000 Ib of waste and SPC to be placed inside. Both the box
and the waste would be preheated to the melt temperature to prevent the SPC from freezing upon
contact. Automated steam or oil-heated mixers were planned to provide temperature control and
to allow the mixer to be shut down or restarted during a pour. Temperature controls for mixing
and cooling would be computer controlled with the appropriate alarms for safety (e.g., gas
detection alarms).
The following is a list of advantages and limitations associated with the use of SPC for the
encapsulation of hazardous wastes:
Advantages
a SPSS results in the formation of a very insoluble sulfide compound with mercury (HgS).
a SPSS is well-suited to the treatment of elemental Hg.
a No chemical reaction is required for SPC to set and cure; therefore greater waste-to-
binder ratios are allowed than with Portland cement.
a Relatively low temperature process (127-138 °C or 260-280 °F).
a Superior water tightness (e.g., low permeability and porosity) compared to Portland
cement.
a High resistance to corrosive environments (e.g., acids and salts).
a SPC has a high mechanical strength.
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a SPC is resistant to freeze-thaw cycling and has coefficients of expansion compatible with
other construction materials.
a Simple to implement because mixing and pouring equipment is readily available.
a SPC is easier to use than other thermoplastics, like polyethylene, because of its low
viscosity and low-melt temperature.
a It is possible to remelt and reformulate SPC.
Limitations
a Although SPC encapsulation occurs at relatively low temperatures, volatile losses of
mercury may occur and engineering controls are needed. BNL's patented SPSS process
was designed to minimize Hg volatilization and 99.7% of the Hg treated remains in the
waste form, while only 0.3% of the Hg is volatilized and captured in an off-gas system.
a Aqueous wastes must be dewatered prior to processing.
a If cooled too quickly, SPC will develop an excess of voids or air pockets, which could
allow water or gas to penetrate the waste form.
a Metal debris or pieces with large thermal mass may require debris preheating above the
SPC melting point to prevent the formation of air pockets around the debris-binder
interface.
a Not compatible with strong alkaline solutions (>10%), strong oxidizing agents, or
aromatic or chlorinated solvents.
a Expanding clays cannot be used in SPC.
a SPC handling requires the use of engineering controls to mitigate possible ignition and
explosion hazards.
a If excessive temperatures are created, SPC will emit hydrogen sulfide gas and sulfur
vapor.
2.2 Chemically Bonded Phosphate Ceramic Encapsulation
Chemically bonded phosphate ceramics (CBPCs) are well suited for encapsulation because the
solidification of this material occurs at low temperatures and within a wide pH range. The DOE
Argonne National Laboratory (ANL) has six patents covering the use of this material for the
encapsulation of hazardous wastes. The technology has been licensed to Wangtec, Inc., for the
treatment of incinerator ashes from power plants in Taiwan (DOE, 1999a). Similar to the SPC
technology, successful treatment with CBPC is due to both chemical stabilization and physical
encapsulation. Although mercury will form low solubility phosphate solids, stabilization with a
small amount of sodium sulfide (Na2S) or potassium sulfide (K2S) to form HgS greatly improves
the performance of the final CBPC waste form. Hg3(PO4)2has a solubility product of 7.9 x 10"46'
compared to HgS with a solubility product of 2.0 x 10"49 (Wagh et al., 2000).
10
-------
CBPCs are fabricated through an acid-base reaction between calcined magnesium oxide (MgO)
and monopotassium phosphate (KH2PO4) in solution to from a hard, dense ceramic of
magnesium potassium phosphate hydrate as shown in the reaction below:
MgO + KH2PO4 + 5 H2O -> MgKPO4.6 H2O (MKP)
Iron oxide phosphates can also be used to form a low-temperature ceramic, but research into the
use of this material is limited (Seidel et al., 1998). CBPC waste forms typically have a density of
1.8 g/cm3 and high compressive strengths (>2,000 psi). They also have an open porosity that is
up to 50% less than conventional fabricated cement. Waste loadings up to 78% have been
demonstrated with this technology. Figure 2-2 provides a simplified block-diagram for the
CBPC encapsulation process. First, enough water is added to the waste in the disposal drum to
reach a target or stoichiometric water content. (One advantage of the CBPC process is that it can
be carried out on dry solids, wet sludges, or liquid wastes.) Next, calcined magnesium oxide and
monopotassium phosphate binders are ground to a powder and blended in a one-to-one ratio.
Additional ingredients (e.g., fly ash or K2S for mercury fixation) also are added to the binders.
The water, binders, additional ingredients, and waste are mixed for about 30 minutes. Under
most conditions, heat from the reaction causes the waste matrix to reach a maximum temperature
of approximately 80 °C (176 °F). After mixing is stopped, the waste form typically sets in about
2 hours and cures in about two weeks. Mixing can be completed in a 55-gallon disposal drum
with a planetary type mixer. The waste, water, binder, and additives can be charged to the drum
using hoppers, feeding chutes, and piping as needed (DOE, 1999a).
Several detailed studies have been completed to demonstrate the use of CBPCs for both
macroencapsulation and microencapsulation of hazardous wastes including Singh et al. (1998),
DOE (1999a), Wagh et al. (2000), and Wagh and Jeong (2001). These papers describe the steps
involved in fabricating the CBPC waste forms and also discuss the results of various
performance tests including compressive strength measurements, U.S. EPA TCLP tests, and
leaching index tests. Visual observations of the structural integrity of the waste forms were also
made. The CBPC encapsulation process has been tested on a wide variety of hazardous wastes
including low-level, mixed waste ash, transuranics, fission products, radon-emanating wastes,
salt solutions, and heterogeneous mercury-containing debris (Wagh and Jeong, 2001). Table 2-2
summarizes key performance data from these studies.
11
-------
Solids
V__x
Resize/Shred
Dual
Planetary
Orbital
Mixer
Waste Form
Container
MgO
KH2PO4
Additives
(Fly Ash/
K2S)
Figure 2-2. Chemically Bonded Phosphate Ceramic Process
12
-------
Table 2-2. Key Performance Data for Chemically Bonded Phosphate Ceramic Encapsulation
Author/
Vendor
Singh et al.
(1998)(b)
Singh et al.
(1998)(b)
Sing et al.
(1998)(b)
Singh et al.
(1998)(b)
DOE
(1999a)
Wagh et al.
(2000)(b)
Wagh etal.
(2000)(b)
Wagh etal.
(2000)(b)
Wagh and
Jeong
(2001 )(c)
Wagh and
Jeong
(2001 )(c)
Type
MA
MA
MA
Ml
Ml
Ml
Ml
Ml
Ml
Ml
Scale
BP
BP
BP
BP
BP
BP
BP
BP
BP
BP
Waste Type
Cryofractured debris
Lead bricks
Lead-lined gloves
Hg-contaminated
crushed light bulbs
DOE Surrogate Wastes of
nitrate salts and
off-gas scrub solution
DOE Ash
(HgCI2 at 0.5 wt%)
Delphi DETOX
(with 0.5 wt% each HgCI2,
Ce203, Pb(N03)2)
Soil (HgCI2 at 0.5 wt%)
Detox Wastestream
(HgCI2at0.5wt%)
Detox Wastestream
(Hg at 0.5 wt%)
Waste
Form
Size
1 .2 to 3
gal
NR
5 gal
5 gal
NR
(100 g)
(100g)
(100 g)
(162 to
500 g)
(162 to
500 g)
Waste
Loading
(wt%)
35
NR
NR
40
58 to 70
NR
NR
NR
60 to 78
60 to 78
Compressive
Strength
(psi)
5,000 to
7,000
5,000 to
7,000
5,000 to
7,000
5,000 to
7,000
1,400
to
1,900
NR
NR
NR
NR
NR
Density
(g/cm3)
1.81
1.8
1.8
1.8
1.7
to
2.0
NR
NR
NR
NR
NR
Before
Hg
TCLP,
(mg/L)
NA
NA
NA
0.200
to
0.202
540
to
650
40
138 to
189
2.27
250(a)
250(a)
After
Hg
TCLP
(mg/L)
NA
NA
NA
<0.00004
to
0.00005
<0.00004
to
<0.00005
<0.00085
<0.00002
to
0.01
<0.00015
4.7 to
15.1
7.19 to
7.64
(a) Untreated waste TCLP not reported, so estimated based on total Hg level in waste divided by 20.
(b) Potassium sulfide was added to reduce metal leachability.
(c) Sodium sulfide nonahydrate was added to reduce metal leachability.
-------
Singh et al. (1998) demonstrated the macroencapsulation of four waste streams with CBPC
including cyrofractured debris, lead bricks, lead-lined plastic gloves, and mercury-contaminated
crushed light bulbs. The cyrofractured debris consisted of metals, wood, bricks, rocks, and
plastics. Some material handling and size reduction (e.g., shredding) was needed to fit the
wastes into the waste disposal drum. The study was a bench-scale project with waste form sizes
ranging from 1.2 to 5 gallons. In general, debris fragments were sized to be less than one third
the diameter of the drum. The CBPC fabrication process was approximately the same for each
waste with the exception of minor formula changes in the wt% of water, ash, or binders and the
addition of K2S in the mixture for the mercury-contaminated crushed light bulbs.
Cryofractured Debris
For the cryofractured debris, the phosphate ceramic slurry was created with a premixed powder
of calcined magnesium oxide and fly ash added to an acid phosphate solution in a Hobart mixer.
The CBPC formula consisted of a ratio of 40 wt% ash, 40 wt% binder (MgO and KH2PO4
powders mixed in 1:1 molar ratio) and 20 wt% water. The slurry was mixed at low speed until it
reached the desired consistency. The slurry then was poured into the drum containing the waste
and was stirred continuously to assure homogeneity of the mixture. The temperature was
monitored and peaked at approximately 72 °C (162 °F) and the CBPC set at around 55 °C (131
°F). The final waste forms had a waste loading of 35 wt% and a density of 1.81 g/cm3.
Lead Brick and Lead-Lined Gloves
The low-level radioactive lead brick and lead-lined plastic gloves were encapsulated in CBPC
formulated from 60 wt% ash, 25 wt% binder, and 15 wt% water. Macroencapsulation of the lead
brick involved pouring a 2-inch lower base and allowing it to set for one hour until it could bear
the weight of the lead brick. The macroencapsulation of the glove wastes was accomplished
with the use of a plastic cage suspended in a 5-gallon pail.
Mercury Contaminated Light Bulbs
The mercury-contaminated crushed light bulbs were pre-treated by mixing with a potassium
sulfide solution for approximately 1 hour. The glass was then set in CBPC with a similar
formulation to the cryofractured debris. Mercury levels in the glass waste were around 200 parts
per million (ppm). The crushed glass ranged in size from 2 to 3 cm long by 1 to 2 cm wide.
During the mixing of the waste with the binder, the glass was crushed down to sizes less than 60
mm and a waste loading of approximately 40 wt% was achieved.
Each waste form was allowed to cure for about two weeks prior to performance testing. The
density of the final waste forms was approximately 1.8 g/cm3, the open porosity less than 4%,
and the compression strengths between 5,000 and 7,000 psi. The cross sections of the final
waste forms were observed to be very homogenous, dense, and free of air pockets. A complete,
intact coating with continuous adhesion was observed around the lead brick and other wastes and
no gaps were present at waste-binder interfaces. TCLP tests on the mercury-contaminated
wastes showed 200 to 202 |J,g/L in the untreated wastes compared to <0.04 to 0.05 ng/L for the
treated wastes. Key performance data from this study is summarized in Table 2-2.
14
-------
A DOE study (1999a) was completed to test the effectiveness of CBPCs in the treatment of salt-
containing, mercury-contaminated mixed wastes. A significant proportion of DOE mixed wastes
contain greater than 15 wt% salts and these wastes are very difficult to treat with conventional
methods. Salts adversely impact conventional cement matrices by causing a decrease in
compressive strength and an increase in metal teachability. There is a demonstrated need to find
encapsulation materials that can allow higher waste loadings to be achieved compared to
conventional cement stabilization/solidification. The waste streams used in this study included
saturated salt solutions (NaNO3 and NaCl), activated carbon, ion exchange resins, spent
incinerator off-gas scrub solution, and Na2CO3. These surrogate wastes were spiked with
hazardous constituents including lead, chromium, mercury, cadmium, nickel, and
trichloroethylene (TCE) at levels up to 1,000 ppm.
Waste loadings in CBPC of up to 70 wt% (40 wt% salt) were achieved during the study. Several
performance tests were completed on the CBPC-encapsulated wastes, including compressive
strength, U.S. EPA TCLP tests, and salt anion leaching tests per American National Standards
Institute (ANSI) Method 16.1. The final CBPC waste forms fabricated with the saturated salt
solutions had densities ranging from 1.72 to 1.8 g/cm3 and compressive strengths ranging from
1,800 to 3,500 psi. The binder was amended with K2S, which successfully stabilized mercury to
meet the TCLP limit in these wastes. Anion leaching indexes of 6.9 and 6.7 were measured for
chloride and nitrate, respectively, which barely passed the demonstration's criteria level (6.0).
These results indicate that salt leaching may deteriorate the waste over time and that an
additional binder or coating technique for the surface may be needed. Subsequently, some
CBPC waste forms were coated in a commercial polymer to plug the surface pores and the
combined leaching index of NO3 and Cl was changed to 12.6, which indicated a reduction in
leaching behavior. Key performance data from this study is summarized in Table 2-2.
Wagh et al. (2000) discusses the results of bench-scale studies for the encapsulation of mercury-
contaminated surrogate wastes including DOE ash waste, secondary waste streams from the
DETOXSM wet oxidation process, and contaminated topsoil. The surrogate waste streams were
dosed with mercuric chloride (HgQ2) at 0.1 wt% to 0.5 wt% and also with other metals
including lead and cesium. Initial tests showed that encapsulation with CBPC alone caused
mercury leaching to decrease by a factor of three to five times. However, for adequate mercury
stabilization, Wagh et al. determined that a small amount of Na2S or K2S should be used in the
binder. For use with CBPC, the K2S formulation was initially deemed to be the most appropriate
because the CBPC binder is a potassium-based material. Other potential additives for mercury
stabilization referenced by the author include H2S or NaHS. In this study, K2S was mixed
directly with MgO and KH2PO4 powders to form one binder powder. The optimal range of K2S
in the binder powder was found to be 0.5 wt% and it was also established that levels significantly
above this dose resulted in the formation of Hg2SO/t, which has a much higher solubility than
HgS (Hg2SO4 has a solubility product of 7.99 x 10"7 versus HgS with a solubility product of 2.0
x 10"49). All of the surrogate wastes were successfully treated to levels below the U.S. EPA
TCLP criteria for mercury from initial, untreated TCLP levels ranging from 2.27 mg/L in the soil
to 189 mg/L for the iron phosphate wastes. Long-term (90-day) leaching tests were also
performed on the waste forms. It was determined that the diffusion of mercury through the
CBPC matrix is 10 orders of magnitude lower than in cement systems. Key performance data
from this study is summarized in Table 2-2.
15
-------
Wagh and Jeong (2001) continued work related to the encapsulation of DETOXSM wastes. The
study was concerned with the effect of haematite (Fe2C>3) on the fabrication and setting of the
CBPC waste form. The DETOX™ wastes contained approximately 95 wt% Fe2C>3; which was
found to be highly reactive and caused the CBPC slurry to set too quickly before mercury could
be effectively fixed into HgS. Additional tests were conducted in order to modify the CBPC
fabrication process to account for the reactive nature of these wastes. Two surrogate wastes were
created including a waste stream with 0.5 wt% HgCb and 94.32 wt% Fe2C>3 and a waste stream
with 0.5 wt% HgO and 95 wt% Fe2O3. Two samples of each surrogate waste were pretreated
with sodium sulfide nonahydrate (Na2S.9H2O) for two hours, which allowed sufficient time for
the mercury to form HgS. The binder was then added and the slurry was mixed until it set. The
CBPC samples were cured for three weeks and subjected to the U.S. EPA TCLP test. Final
TCLP results for the treated HgCb waste ranged from 4.7 to 15.1 |j,g/L and the HgO wastes
ranged from 7.19 to 7.64 [ig/L. Waste loadings ranged from 60 to 78 wt%. Setting times were
rapid (10 to 18 minutes) and the authors suggested that it may be possible in large-scale systems
to slow down the reaction by adding boric acid (at <1 wt%). Key performance data from this
study is summarized in Table 2-2.
The following is a list of advantages and limitations associated with the use of CBPC for the
encapsulation of hazardous wastes:
Advantages
a Waste stabilization is due to both chemical stabilization and physical encapsulation.
a Low temperature process (<80 °C or 176 °F).
a CBPC can be used to treat dry solids, sludges, and liquids.
a Unlike SPC, CBPC requires no additional heat input.
a High waste loading (up to 78 wt%) minimizes disposal volumes.
a Superior water tightness and chemical resistance compared to Portland cement.
a Simple to implement since mixing and pouring equipment is readily available.
a Nonflammable and stable and safe with oxidizing salts.
a No secondary wastes are generated.
a The process does not generate potentially hazardous off-gasses.
Limitations
a Pretreatment with K2S or other compounds is needed for chemical stabilization of
mercury; CBPC alone is not enough.
a Excess sulfide will increase the teachability of mercury, so careful processing is needed.
a Some waste constituents (e.g., haematite) may accelerate setting times and decrease
workability of the CBPC slurry.
a Only limited data is available to support the long-term effectives and durability of CBPC
waste forms.
a For high salt wastes, the leaching of salt anions over time could deteriorate the integrity
of the waste. A polymer coating of the waste form may be needed to decrease the
leaching of salt anions.
16
-------
2.3 Polyethylene Encapsulation
Polyethylene is a thermoplastic material or a noncross-linked linear polymer that melts and
liquefies at a specific transition temperature (120 °C or 248 °F). Polyethylene physically
encapsulates the waste and does not interact with or chemically alter the waste materials.
Polyethylene is readily available as a post-consumer recycled material (e.g., low-density
polyethylene [LDPE] and high-density polyethylene [HDPE] used in commercial
packaging/containers). It also has good chemical resistance and is water insoluble. According to
Kalb et al. (1997) the physical properties of LDPE are better suited to encapsulation because
HDPE requires greater temperatures and pressures during processing and mixing with wastes.
LDPE with a high melt index from 50 to 55 g/10 minutes is reported to provide the optimal melt
viscosity for mixing with wastes (Kalb et al. under U.S. Patent No. 5,649,323).
Figure 2-3 provides a simplified block diagram for the polyethylene macroencapsulation process.
The key equipment used in this process typically includes a polymer extruder and feed hoppers.
There are three types of extruder units including a single screw extruder, an intermeshing
counter-rotating twin-screw extruder, and an intermeshing co-rotating twin-screw extruder.
These extruders melt the polyethylene feed through both heat generated by friction from the
rotating screw and supplemental barrel heaters. Single screw extruders are well-suited to both
macroencapsulation and microencapsulation and have been used in the plastics industry for over
50 years (Kalb et al., 1992). Kinetic mixers have also been used for polyethylene encapsulation
(Jackson, 2000). Polyethylene macroencapsulation typically involves the use of a basket placed
inside a drum to allow at least a 1-inch barrier around the waste material. Molten polyethylene is
then poured from an extruder over and around the waste in the drum. The drum can be rotated to
ensure a more uniform distribution of the molten plastic. (An alternative to on-site pouring is the
use of pre-manufactured containers.) Polyethylene microencapsulation typically involves
directly mixing the waste material and polyethylene at an elevated temperature (typically 120 to
150°C or 248 to 302°F) in an extruder. The mixture of waste material and polyethylene is then
poured into a drum and allowed to set. Microencapsulation may require several pretreatment
steps, including drying of wet wastes and physical separation to resize or improve the particle
distribution of the waste (Faucette, 1994). At the Envirocare facility in Utah, the optimal
processing parameters for microencapsulation in a single screw extruder were determined to be a
maximum of 2% moisture content and a 3-mm particle size limit (Jackson, 2000). In addition,
off-gas treatment is needed for any water vapor, volatile organic compounds (VOCs), or volatile
metals (e.g., arsenic and mercury) in the waste (Faucette, 1994). Polyethylene
microencapsulation and macroencapsulation services are commercially available. In 1998, the
Envirocare facility in Utah installed and permitted a single screw extruder system that can
process up to 5 tons of waste per day. The final waste forms are typically set in 30- to 55-gallon
drums and have a minimum exterior surface coating of LDPE of 1 to 2 inches (Jackson, 2000).
Several studies have been carried out using polyethylene for both macroencapsulation and
microencapsulation of hazardous wastes, including Faucette et al. (1994), Burbank and
Weingardt (1996), and Carter et al. (1995). In addition, several commercial vendors (e.g.,
Chemical Waste Management, Boh Environmental, and Ultra-Tech, International) provide
macroencapsulation services with pre-manufactured HDPE containers. Encapsulation with
polyethylene has been demonstrated with numerous waste streams including mixed waste salts,
17
-------
sludges, and ash. Because macroencapsulation is BDAT for radioactive lead solids, several
studies deal with macroencapsulation of lead brick and shielding waste materials including
Faucette et al. (1994) and DOE (1998). One study was found which dealt with the encapsulation
of radioactive, concentrated salts and basin sludges with low levels of mercury ranging from 1.3
to 9.2 ppm (Burbank and Weingardt, 1996). Carter et al. discusses the difficulties encountered with
the microencapsulation of high-level arsenic wastes due to the high volatility of arsenic trioxide.
In general, there is little performance data available on the effectiveness of polyethylene
encapsulation of mercury-containing wastes. Key performance data from these studies is
summarized in Table 2-3.
Faucette et al. used polyethylene for macroencapsulation and microencapsulation of a variety of
mixed waste streams from the DOE Rocky Flats Plant in Colorado. The purpose of the
macroencapsulation demonstration was to compare two containment methods including physical
contact (e.g., on-site pouring of the waste form) versus pre-manufactured inserts. The objectives
of the microencapsulation demonstration were to identify optimal processing equipment, test
various additives to reduce the teachability of metals in surrogate wastes, and complete a
treatability study for actual salt wastes. Key performance data from this study are included in
Table 2-3.
Supplemental
Heating Tapes
Waste Form
Container
Off-Gas
Treatment
(Bag House)
Figure 2-3. Polyethylene Macroencapsulation
Faucette et al. demonstrated the macroencapsulation of low-level, mixed wastestreams identified
as combustibles (e.g., paper, cloth, plastics), laboratory glassware, scrap metals (e.g., pipe,
valves, hand tools), and lead (e.g., sheet, bricks, tape). Two different approaches were used to
create a 1-inch-thick polyethylene barrier around the wastes including physical contact and
18
-------
Table 2-3. Key Performance Data for Polyethylene Encapsulation
Author/
Vendor
Faucette et al.
(1994)
Faucette et al.
(1994)
Burbank and
Weingardt
(1996)
Burbank and
Weingardt
(1996)
Carter et al.
(1995)
Kalb et al.
(1996)
Type
MA
Ml
Ml
Ml
Ml
Ml
Material
LDPE
LDPE
LDPE
LDPE
HOPE
LDPE
Scale
BP/F
BP/F
BP
BP
BP
BP
Waste Type
Combustibles,
laboratory
glassware,
scrap metals,
and lead (e.g.,
sheet, bricks,
tape).
F006 Waste
Code: Nitrate
Salts with Cd,
Cr, Pb, Ni, and
Ag
Ammonium
sulfate/solar
basin sludge
Solar basin
sludge
As2O3
Off-gas scrub
solution
Waste
Form
Size
5 to 10
gal
NR
1.25
gal
1.25
gal
NR
NR
Waste
Loading
(wt%)
NR
50
40 to 50
40 to 50
(20 vol%)
50 to 70
Compressive
Strength
(psi)
NR
NR
1,088 to
2,465
1,088 to
2,465
NR
1,950
to
2,180
Density
(g/cm3)
NR
NR
NR
NR
NR
1.21
to
1.45
Before
Hg
TCLP
(mg/L)
NA
NA
0.46
[9.2
ppm](a)
0.065
[1.3
ppm](a)
NA
0.14
After
Hg
TCLP
(mg/L)
NA
NA
0.442
to 1.07
0.107
to
0.122
NA
<0.009
(a) Untreated waste TCLP not reported, estimated by total Hg level in waste divided by 20.
-------
pre-manufactured inserts. Polyethylene was used with a melt index of 50 to 200 grams/10
minutes per American Society for Testing and Materials (ASTM) D1238-90b. It was determined
that LDPE experienced less cratering and cracking than HDPE and had a lower expansion
coefficient (e.g., shrank less upon cooling). The basket holding the waste material in place also
had to be flexible and yield as the polymer cooled and contracted. During scale-up to a 5-gallon
container, Faucette et al. found that it was necessary to modify the process by using a "crock
pot" to heat the waste form to control the temperature and viscosity of the polyethylene during
the pour. Faucette et al. also stated that the basket required to hold the waste could cause
potential leak paths. Macroencapsulation with a pre-manufactured polyethylene insert also was
demonstrated.
The insert consisted of an open top, thick-walled polyethylene liner, which was placed into a 5-
gallon metal container. The insert then was filled with waste and capped with molten
polyethylene. The pre-manufactured insert resulted in a waste form of known thickness and only
the cap needed to be poured on site.
One limitation of polyethylene encapsulation is that wastes must be dewatered prior to
processing. Faucette et al. tested four different types of drying units for pretreatment of waste
materials including a spray dryer, a horizontal thin film evaporator, a vertical thin film
evaporator, and a horizontal rotary/blender dryer. The drying units were tested for their ability to
concentrate a nitrate salt aqueous waste stream contaminated with various metals and high
chlorides and sulfates. The horizontal thin film unit was chosen because it produced salts with
the largest particle size and the highest bulk density. Several additives to the polyethylene were
tested for their ability to reduce the teachability of cadmium, chromium, lead, nickel, and silver.
The addition of surfactant (0.5 wt% sodium stearate) was found to improve the wetting of the
salts by the polyethylene and reduce the teachability of cadmium and chromium. Calcium oxide
and magnesium oxide also significantly reduced the TCLP results for cadmium and chromium.
Carbon, alumina, diatomite, and class C fly ash were found to reduce chromium teachability by
93 to 98%, but cadmium was unaffected. It also was determined that excess water (e.g., >2 wt%)
caused the salts to clump together, resulting in highly variable feed characteristics and a
heterogeneous product.
Burbank and Weingardt explored the use of polyethylene for the microencapsulation of mixed
wastestreams at the DOE site in Hanford, WA. Two wastes contained detectable levels of
mercury along with other metals, including ammonium sulfate cake wastes with 9.2 ppm of
mercury and solar evaporation basin sludge with 1.3 ppm of mercury. These wastes were
incorporated into polyethylene at a 40 to 50 wt% loading. Prior to encapsulation, calcium oxide
was added to the wastes to help reduce the teachability of metals. Based on TCLP results, the
amendment of the wastes with calcium oxide did not reduce mercury teachability.
Microencapsulation of the ammonium sulfate cake waste with polyethylene resulted in a mercury
TCLP of 0.442 mg/L. With the addition of calcium oxide, this same wastestream had a mercury
TCLP of 1.07 mg/L. It is clear from these results that, even at relatively low levels in waste,
polyethylene encapsulation alone cannot adequately reduce the availability of mercury, and
chemical stabilization (e.g., transformation to HgS) is necessary prior to the encapsulation of
such wastes with polyethylene. Also, due to the high processing temperatures of polyethylene
encapsulation, it is likely that a large fraction of mercury in these wastes will be volatilized
20
-------
unless it has been chemically fixed. Key performance data from this study are included in Table
20
-3.
Carter et al. used HDPE with a melting point of 130 °C (266 °F) and an operating temperature of
180 - 210 °C to microencapsulate powdered arsenic trioxide (As2C>3). It was found that at a 20
volume percent (vol%) loading of this compound, the viscosity of the HDPE increased
dramatically and the mixture became unworkable. Scanning electron microscope (SEM)
micrographs showed that the arsenic trioxide had sublimed and recrystallized. When arsenic
trioxide was stabilized to calcium oxide, the volatility decreased, but achievable waste loadings
in HDPE remained low. Mercury and its compounds are also highly volatile compared to other
metals (e.g., mercuric chloride sublimes at 300 °C or 572 °F), so the results of this study could
provide some insight into the challenge of using polyethylene to process wastes containing high
levels of arsenic and mercury. Key performance data from this study are summarized in Table
2O
-3.
There are several vendors that provide macroencapsulation services with pre-manufactured
HDPE containers including Chemical Waste Management, Boh Environmental, and Ultra-Tech,
International. These macroencapsulation methods are allowed under the alternative debris
standards (40 CFR 268.45) because the definition of macroencapsulation for debris does not
preclude the use of materials that meet the definition of tank or container (40 CFR 260.10).
Chemical Waste Management provides /^-inch-thick HDPE vaults measuring 21 feet by 7 feet
for the disposal of hazardous waste debris. A 3-inch-thick soil liner is used in the vault to
provide a physical cushion between the bottom of the vault and the debris. Soil or sand is
typically used to fill any void spaces around the debris. Once the vault is full, the lid is secured
to the vault with adhesives and screws. The vault then is placed in a subtitle C landfill.
Chemical Waste Management also provides 225-millimeter HDPE-lined roll-off boxes for
hazardous waste debris disposal. Boh Environmental's Arrow-Pak™ technology consists of
compacting 55-gallon drums filled with mixed/hazardous waste debris into 12-inch-thick pucks.
The compacted drums are loaded into an 85-gallon metal overpack drum and then into a 1-inch-
thick HDPE tube about 21 feet in length and 30 inches in diameter. Each tube fits the equivalent
of 21 55-gallon drums. Both ORNL and DOE's Hanford have used this technology for the
macroencapsulation of mixed waste debris. This technology achieves a mixed waste debris
volume that is typically one-fourth that of on-site macroencapsulation with polyethylene (INEL,
2002). Ultra-Tech, International offers a series of pre-manufactured, medium-density
polyethylene containers for macroencapsulation. The containers can be custom-made in any
size, but have been manufactured to over-pack one 55-gallon drum to containers 52 inches in
diameter and 20 feet in length. A resistance wire system is embedded in the lid of each
container. Once the debris waste is in place, an electrical current is applied to the wires, heating
them up to melt the polyethylene, and creating an effective seal around the top of the container.
This technology is currently being tested by DOE's Mixed Waste Focus Area Program (Ultra-
Tech, International, 2002).
The following is a list of advantages and limitations associated with the use of polyethylene for
the encapsulation of hazardous wastes:
21
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Advantages
a Polyethylene has a high mechanical strength, flexibility, and chemical resistance.
a Polyethylene is highly resistant to biological degradation.
a Polyethylene allows higher waste loadings (up to 70 wt%) compared to conventional
Portland cement.
a Polyethylene is readily available in postconsumer recycled forms.
a Equipment is commercially available and the process can be automated, so the operator
input requires only drum placement.
a Pre-manufactured vaults and containers can be used, which provide a final waste form of
known barrier thickness and integrity.
a Because HDPE is used in landfill liners, extensive studies have been performed to
document the chemical resistance and long-term durability of HDPE.
Limitations
a External heating is required and the process occurs at a higher temperature than the SPC
and CBPC methods.
a Polyethylene does not chemically incorporate the waste, and with mercury-containing
wastes volatilization may be a significant concern.
a Chemical stabilization of mercury-contaminated wastes prior to encapsulation may be
necessary to meet TCLP requirements.
a The encapsulation of high-level arsenic wastes with polyethylene is problematic due to
the sublimation of arsenic compounds at high temperature (>200 °C).
a Small quantities of secondary waste are generated.
a For large-scale or on-site pouring, LDPE is preferred because HDPE is prone to
cratering, cracking, and excessive shrinking.
a LDPE is intolerant of the presence of free liquids and organics.
a Wastes must be pretreated to remove moisture.
a Molten polyethylene can cause severe burns, so extra safety precautions are necessary.
a Small quantities of secondary waste are generated.
2.4 Other Encapsulation Materials
Several other materials have been developed and demonstrated for the encapsulation of mercury-
containing hazardous wastes including asphalt, polyester and epoxy resins, synthetic elastomers,
polysiloxane, sol-gels (e.g., polycerams), and Dolocrete™. Key performance data are
summarized in Table 2-4. In addition, a variety of materials currently available for the
encapsulation of other metal-containing wastes are discussed.
2.4.1 Asphalt
Asphalt or bitumen has been used to microencapsulate soil contaminated with low-levels of
heavy metals (Smith et al., 1995 and Hubbard et al., 1990). Radian corporation reported using
cold-mix asphalt to microencapsulate soil contaminated with mercury (78 mg/kg). The material
had a compressive strength of 176 psi. Hot-mix asphalt was deemed to be inappropriate because
the elevated temperatures could promote the volatilization of mercury (SAIC, 1998). Kalb et al.
22
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(1996) discusses the microencapsulation of up to 60 wt% of a mixed waste incinerator off-gas
scrub solution with asphalt. The mercury TCLP in the untreated wastes was 0.14 mg/L versus
<0.009 mg/L in the asphalt microencapsulated waste. Compressive strengths averaged 570 psi in
the final waste forms.
2.4.2 Polyester and Epoxy Resins
Polyester is an example of a thermosetting resin or a cross-linked polymer that undergoes a
chemical reaction to solidify. Several thermosetting resins have been tested for the encapsulation
of salt-containing mixed wastes including orthophthalic polyester, isophthalic polyester, vinyl
ester, and a water-extendible polyester. These wastes contained metals, including mercury, at
the 1,000 ppm level. With polyester resins, waste loadings of 50 wt% were achieved for
unconcentrated spent off-gas scrub solutions and 70 wt% for nitrate/chloride salts. In addition,
compressive strengths ranged from 5,100 to 6,200 psi. Mixed waste, salt surrogate TCLP tests
for mercury ranged from <0.01 to 0.2 mg/L (DOE, 1999b). Orebaugh (1993) reported using
several epoxy resins (e.g., Sty cast 2651 and Thermoset 300) to macroencapsulate mixed waste,
lead billets. The waste forms were subjected to 6-foot drop tests to gauge their stability and
mechanical strength.
2.4.3 Synthetic Elastomers
Synthetic elastomers are materials having properties similar to natural rubber and have been used
in the microencapsulation and stabilization of metal-contaminated wastes. Carter et al. explored
the use of styrene-butadiene rubber (Solprene 1204) for the encapsulation of powdered arsenic
trioxide (As2C>3). Up to 64 wt% of arsenic trioxide was incorporated into the rubber, but beyond
this level the rubber became unworkable. Meng et al. (1998) reports using tire rubber for the
immobilization of mercury-contaminated soils. A clay-loam soil was spiked with mercuric oxide
and mercuric chloride at 300 mg/kg. Acetic acid leachate tests showed a reduction from 3.5
mg/L in the untreated soil to 0.034 mg/L in the soil mixed with tire rubber. The used tire rubber
contained approximately 2 to 4% sulfur and less than 32% carbon black.
2.4.4 Polysiloxane
Polysiloxane or ceramic silicon foam (CSF) consists of 50 wt% vinyl-polydimethyl-siloxane, 20
wt% quartz, 25 wt% proprietary ingredients, and less than 5 wt% water. The use of this material
for encapsulation is patented by Orbit Technologies. The material sets at room temperatures (30
°C or 86 °F) and is resistant to extreme temperatures, pressures, and chemical exposure. The
polysiloxane technology was demonstrated on salt waste surrogates, which were spiked with
lead, mercury, cadmium, and chromium at 1,000 ppm levels. Up to 50 wt% waste loading was
demonstrated. The final waste form had a compressive strength of 600 psi at 40 wt% loading.
For high chloride salt wastes, the mercury TCLP was 0.01 mg/L and for high nitrate salt wastes
the mercury TCLP was 0.06 mg/L. The final waste forms for both waste types did not pass for
chromium. The authors recommend pretreatment for the chemical stabilization of wastes with
metals at levels greater than 500 ppm (DOE, 1999c). In addition, Miller et al. (2000) reports on
the use of silicone foam to encapsulate a DOE surrogate waste containing high levels of
chromium. Salt waste loadings of up to 48 wt% were achieved in this study.
23
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Table 2-4. Key Performance Data for Various Encapsulation Materials
Author/
Vendor
Kalb et al.
(1996)
Radian(b)
DOE(1999b)
Orebaugh
(1993)
Carter et al.
(1995)
Meng et al.
(1998)
DOE(1999c)
DOE(1999d)
Dolomatrix
(2001)
Type
Ml
Ml
Ml
MA
Ml
Ml
Ml
Ml
Ml
Material
Asphalt
Asphalt
Polyester
Epoxy
Styrene-
butadiene
rubber
Tire
Rubber
Poly-
siloxane
Sol-Gels
Dolocrete
TM
Scale
BP
F
BP
BP
BP
BP
BP
BP
F
Waste Type
Off-gas scrub
solution
Soil
(Hg 78 mg/kg)
Salt-containing
mixed wastes
Mixed waste,
lead billets
As2O3
Soil
(Hg 300
mg/kg)
Salt-containing
mixed wastes
Salt-containing
mixed wastes
Hg-waste at
15,300 mg/kg
Waste
Form
Size
NR
NA
NR
5 gal
NR
(100
g)
NR
NR
NR
Waste
Loading
(wt%)
30 to 60
NR
50
NR
64
(4 g rubber
/100 g soil)
50
60 to 70
NR
Compressive
Strength
(psi)
540-610
176
5, 100 to
6,200
NR
NR
NR
420 to 637
1,050 to
1,513
145
Density
(g/cm3)
1.08 to
1.42
NR
NR
1.43 to 1.5
(Resin
Only)
1.7
(Rubber
Only)
NR
NR
NR
NR
Before
Hg
TCLP
(mg/L)
0.14
NR
50(a)
NA
NA
(3.5
acetic
acid)
50(a)
50(a)
765(a)
After
Hg
TCLP
(mg/L)
O.009
NR
<0.01
to 0.2
NA
NA
(0.034
acetic
acid)
0.01 to
0.06
0.044
to 0.23
<0.1
to
(a) Untreated waste TCLP not reported, estimated by total Hg level in waste divided by 20.
(b) SAIC(1998).
-------
2.4.5 Sol-Gels
Sol-gels or polycerams are a hybrid material derived from the chemical combination of organic
polymers and inorganic ceramics. A DOE study (DOE, 1999d) explored the use of a polyceram
consisting of a polybutadiene-based polymer combined with silicon dioxide for the stabilization
of high salt wastes. The salt waste surrogates contained lead, chromium, mercury, cadmium, and
nickel at 1,000 ppm levels. The polymer and silicon dioxide are combined first and then mixed
with the waste and then solidified to encapsulate the waste. The setting of the waste form takes
place at temperatures ranging from 66 to 70 °C (151 to 158 °F). Waste loadings from 30 to 70
wt% were demonstrated. Compressive strengths of the final waste forms ranged from 137 to
1,513 psi. The initial waste forms in the demonstration had a high open porosity and did not pass
the TCLP test for mercury. Another set of waste forms were fabricated and subjected to a
secondary infiltration of polyceram solution after initial drying. The second set of tests was able
to demonstrate a decrease in the mercury TCLP to 0.044 mg/L.
2.4.6 Dolocrete™
Dolocrete™ is a proprietary calcined dolomitic binder material that can be used for the
microencapsulation of inorganic, organic, and low-level radioactive waste. Dolocrete™ is
reported to successfully encapsulate wastes containing aluminum, antimony, arsenic, bismuth,
cadmium, chromium, copper, iron, lead, mercury, nickel, tin, and zinc. The encapsulation of
mining waste with up to 590,000 mg/kg of arsenic resulted in a TCLP of 3.9 mg/L, which meets
the current arsenic TCLP limit of 5 mg/L. Mercury-contaminated wastes with up to 15,200
mg/kg were treated to reach a TCLP level of <0.1 mg/L. Compressive strengths of the final
waste forms often exceed 145 psi (Dolomatrix, 2001).
2.4.7 Materials Used With Other Metals
For the stabilization/solidification of other hazardous metal wastes, cement, Pozzolan, and lime
are the most commonly used encapsulation materials; however, research continues into the use of
other binders and additives to enhance performance of the final waste form and to reduce project
costs (Conner and Hoeffner, 1998). Figure 2-4 lists binders and additives that have been found
to decrease metal teachability including fly ash, clays, slags, iron compounds, activated carbon,
and other materials. In addition to conventional cement, Pozzolan, and lime-based binders,
another encapsulation material reported in the literature is proprietary silicate binders. Several
vendors and authors report using silicate-based materials for the encapsulation of metal-
contaminated wastes including Chemfix Technologies, Inc.; Silicate Technology Co. (STC),
Mitchell et al. (2001); and Evangelou (2000).
25
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Other
•Type I, II, V Portland cements
•Class F fly ash
•Cement kiln dust
•Lime kiln dust
•Slag
•Sodium silicate and proprietary polysilicate mixtures
•Dolocrete™
•Calcium carbonate (limestone)
•Calcium sulfate (gypsum)
•Iron oxide (hematite)
•Calcium phosphate (apatite)
•Organophillic clay with additives
pH Control
•Sulfuric acid
•Phosphoric acid
•Buffer solution
Other Pretreatment Additives
•Potassium permanganate (oxidation)
•Hydrogen peroxide (oxidation)
•Calcium hypochlorite (oxidation)
•Potassium or sodium persulfate (oxidation)
•Ferric or calcium chloride (precipitation)
•Ferric or ferrous sulfate (precipitation)
•Magnesium oxide (adsorbent)
•Activated carbon (adsorbent)
Note: Based on information from Wickramanayake et al., 2001; Conner and Hoeffner, 1998; Sun
et al., 2001; Dutre and Vandecasteele, 1995; and Rha et al., 2000.
Figure 2-4. Materials and Additives for Stabilization/Solidification of Other Metals
Chemfix Technologies, Inc., has developed a stabilization/solidification process using
proprietary additives of soluble silicates and calcium-containing reagents. This process was
tested under the U.S. EPA's Superfund Innovative Technology Evaluation (SITE) program in
March of 1989. The Chemfix™ process was most successful in reducing the teachability of
cadmium, copper, chromium, lead, nickel, and zinc. However, some difficulty was experienced
in the treatment of arsenic and mercury-containing wastes. Before treatment, lead TCLP levels
ranged from 390 to 890 mg/L in contaminated soil. After treatment, lead TCLP levels ranged
from <0.5 to 47.0 mg/L or a 94% to >99% reduction in lead teachability. Initial copper TCLP
26
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levels in contaminated soil ranged from 12 to 120 mg/L, whereas after treatment levels were
reduced to 0.54 to 0.60 mg/L or a 96% to >99% reduction. The compressive strength of the final
waste form was 90 psi (U.S. EPA, 1991).
STC developed a stabilization/solidification process, which relies upon the use of a proprietary
silicate-mineral reagent that binds the metals into a layered alumino-silicate structure. The STC
process was tested in the U.S. EPA SITE program in November of 1990. The SITE program
involved the testing of the STC process on soils contaminated with both inorganic constituents
(e.g., arsenic, chromium, and copper) and organic constituents (e.g., pentachlorophenol). Before
treatment, arsenic TCLP levels ranged from 1.1 to 3.3 mg/L. After treatment, arsenic TCLP
levels ranged from 0.09 to 0.88 mg/L or a 35% to 92% reduction. Chromium TCLP levels
actually increased as a result of treatment from <0.05 to 0.27 mg/L before treatment to 0.19 to
0.32 mg/L after treatment. Initial copper TCLP levels ranged from 1.4 to 9.4 mg/L and were
reduced by 90% to 99% to 0.06 to 0.10 mg/L. The compressive strength of the final waste forms
ranged from 760 to 1,400 psi (U.S. EPA, 1992).
Mitchell et al. reports using silica microencapsulation for the treatment of aqueous acid rock
drainage (ARD), which contained elevated levels of aluminum, arsenic, copper, iron, nickel, and
sulfate. The process employs a proprietary mixture of chemicals referred to as KB-1™, which
includes a silica-based reagent to chemisorb the metals from the aqueous phase into a solid
matrix. The sludge generated from this water treatment process was able to meet TCLP limits,
which eliminated the need to dispose of the material as a hazardous waste. Evangelou (2000)
also discusses the treatment of ARD through the microencapsulation of pyrite with iron phospate
or silicate binders. Silicate materials were found to reduce the leaching of sulfate from pyrite
wastes relative to the control treatments with limestone and phosphate.
27
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3.0 Cost and Vendor Information
Table 3-1 includes a summary of typical cost data, along with vendor information, for several
materials used for the macroencapsulation/microencapsulation of hazardous wastes. Table 3-1
also includes typical costs for competing technologies for mercury-contaminated hazardous
wastes including thermal recovery, acid leaching, and vitrification. However, it should be noted
that both thermal recovery and acid leaching will generate highly concentrated, secondary waste
streams that will ultimately have to be immobilized prior to disposal. In general, vitrification is
best suited to low volatility metals as opposed to mercury and arsenic. The cost data presented in
Table 3-1 are meant to provide an order-of-magnitude cost range for each technology. True
technology costs will be specific to the waste type, waste chemical and physical properties, and
the levels of contaminants in the waste.
28
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Table 3-1. Summary of Cost and Vendor Information for Encapsulation and Other Treatment Technologies
Technology
SPSS
CBPC
Polyethylene (on-site pour)
Arrow-Pak (HOPE)
Ultra-Macroencapsulation
System
Polyester Resin
Synthetic Elastomer
Polysiloxane
Sol-Gels
Developer/Vendor
Brookhaven National Laboratory, NY
Argonne National Laboratory, IL
Envirocare, UT
Boh Environmental, New Orleans, LA
Ultra-Tech, International, FL
SGN Eurisys Services Co., Richland,
WA
No vendor information available
Orbit Technologies, Carlsbad, CA
Pacific Northwest National Laboratory,
Richland, WA
Estimated Full-Scale Costs
$2.88/kgor$1.31 per Ibs
$15.45 per kg or
$7.00 per Ibs
$90 to $100 per ft3
880 drums for $1,100,000
30" dia, 40" height $480 to
$700
6' x 6' x 20' $20,000
$11.52 per kg or
$5.22 per Ibs
$25 per ton of used tire
rubber, 4 wt% in treated soil
$1,900 per ft3
NA
Reference
Morris et al. (2002)
DOE(1999a)
DOE (1998)
(Hanford, 2002)
(Ultra-Tech, 2002)
(DOE, 1999b)
Mengetal. (1998)
(DOE, 1999c)
(DOE, 1999d)
Other Technologies
Acid Extraction
Cement-Based
Stabilization/Solidification
DeHg®
Thermal Recovery
X-Trax™ Thermal Desorption
Vitrification
Environmental Technologies
International, Wyomissing, PA
Various
Nuclear Fuel Services, TN
Mercury Recovery Services, New
Brighton, PA
Remediation Technologies, Tuscon, AZ
Westinghouse Science and Technology
Center, Pittsburgh, PA
$100 to $250 per ton
$16.37 per kg or $7.42 per
Ibs
$5, 000 per ft3
$8.48 per kg or $3.85 per Ibs
$650 to $1,000 per ton
$100 to $600 per ton
$400 to $870 per ton
Mulligan etal. (2001)
DOE(1999a)
DOE(1999c)
Morris et al. (2002)
Mulligan etal. (2001)
Mulligan etal. (2001)
Mulligan etal. (2001),
(U.S. EPA, 1997)
to
VO
Note: The cost data presented above are meant to provide an order-of-magnitude
specific to the waste type, waste chemical and physical properties, and the levels
cost range for each technology.
of contaminants in the waste.
True technology costs will be
-------
4.0 Future Development and Research Needs
A large body of literature exists regarding the research and development of alternative materials
to conventional Portland cement for the encapsulation of hazardous metal-containing wastes.
SPC, CBPC, and polyethylene are the most established materials, and each has its advantages
and disadvantages for use in the macroencapsulation or microencapsulation of mercury-
containing hazardous wastes.
Although several studies were noted which demonstrated the successful encapsulation of high-
level, mercury-containing wastes with SPC and CBPC, the body of evidence for competent
polyethylene encapsulation is limited. The higher temperatures of the polyethylene process may
pose some difficulty in effective encapsulation of these wastes due to the volatile nature of
mercury compounds. A better understanding of the long-term stability of final waste forms may
be needed for some binder materials. In general, the long-term stability of materials
encapsulated with SPC or CBPC have not been addressed, except for encapsulated mixed wastes,
which are extensively tested under NRC protocols. Improving the understanding of the kinetics
of low-temperature processes such as SPC or CBPC could help in scale-up and process
optimization. Also, a better understanding is needed regarding the role of excess sulfides in
increasing mercury teachability. In addition, the performance objectives or acceptance criteria
for macroencapsulated wastes could be standardized to provide guidance regarding the minimum
layer thickness of the barrier, the expected long-term leaching performance of the final waste
form, the target compressive strength, and the tolerance for void spaces in the final waste form.
Currently, few full-scale commercial applications of encapsulation technologies are available;
however, further commercialization and technology transfer may occur if the demand for
macroencapsulation increases as a result of changes in regulatory requirements. Both SPC and
CBPC processes have been patented, but licensing of the technologies has generally been limited
to one or two companies and application of these processes at the industrial-scale is limited. The
Envirocare facility in Utah does have a full-scale system in place for polyethylene encapsulation.
In addition, the use of pre-manufactured HDPE containers for macroencapsulation, as allowed
under the U.S. EPA alternative debris standards, appears to offer a cost-effective solution to the
disposal of hazardous waste debris. The use of other materials such as synthetic elastomers,
polyester resins, polysiloxane, or sol-gels appears somewhat promising, but relatively few
studies have been completed to date. With several of these materials, including polysiloxane and
sol-gels, it appears that an additional chemical stabilization step may be needed when elevated
levels of metals are present, since the TCLP criteria for mercury and chromium were not met in
initial trails. In addition, the use of asphalt for encapsulation is most likely limited to
contaminated soils with only low levels of mercury or other metals.
30
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Because of the varied nature of industrial wastes, site-specific treatability tests will most likely
be required for the selection of the most appropriate encapsulation material. The selection
criteria should include chemical compatibility of the waste and binder materials, final waste form
performance, technology implementability (e.g., the availability of processing equipment and
vendor experience), safety and health issues, and project-specific estimated costs.
31
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Rha, C.Y., S.K. Kang, and C.E. Kim. 2000. Investigation of the Stability of Hardened Slag
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