Advances in encapsulation technologies for the management of mercury-contaminated hazardous wastes

Journal of Hazardous Materials B114 (2004) 211–223

Advances in encapsulation technologies for the management ofmercury-contaminated hazardous wastes�

Paul Randalla,∗, Sandip Chattopadhyayb,1

a National Risk Management Research Laboratory, Land Remediation and Pollution Control Division, U.S. Environmental Protection Agency,Office of Research and Development, 26 W. Martin Luther King Drive, Cincinnati, OH 45268, USA

b Environmental Restoration Department, Battelle Memorial Institute, 505 King Avenue, Columbus, OH 43201, USA

Received 20 January 2004; received in revised form 4 August 2004; accepted 18 August 2004Available online 30 September 2004

Abstract

Although industrial and commercial uses of mercury have been curtailed in recent times, there is a demonstrated need for the developmentof reliable hazardous waste management techniques because of historic operations that have led to significant contamination and ongoingh l alternativest of encap-s ntaminatedw lications form er stabiliza-t in the liter-a Dolocretea ent contactw pretreatmento from thet eeds are alsodP

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azardous waste generation. This study was performed to evaluate whether the U.S. EPA could propose treatment and disposao the current land disposal restriction (LDR) treatment standards for mercury. The focus of this article is on the current stateulation technologies that can be used to immobilize elemental mercury, mercury-contaminated debris, and other mercury-coastes, soils, sediments, or sludges. The range of encapsulation materials used in bench-scale, pilot-scale, and full-scale appercury-contaminated wastes are summarized. Several studies have been completed regarding the application of sulfur polym

ion/solidification, chemically bonded phosphate ceramic encapsulation, and polyethylene encapsulation. Other materials reportedture as under development for encapsulation use include asphalt, polyester resins, synthetic elastomers, polysiloxane, sol–gels,TM,nd carbon/cement mixtures. The primary objective of these encapsulation methods is to physically immobilize the wastes to previth leaching agents such as water. However, when used for mercury-contaminated wastes, several of these methods require ar stabilization step to chemically fix mercury into a highly insoluble form prior to encapsulation. Performance data is summarized

esting and evaluation of various encapsulated, mercury-contaminated wastes. Future technology development and research niscussed.ublished by Elsevier B.V.

eywords:Mercury; Encapsulation; Stabilization; Solidification; Hazardous wastes

� This paper has not been subjected to the Agency’s review and thereforeoes not necessarily reflect the views of the Agency, and no official endorse-ent should be inferred. Mention of trade names or commercial productsoes not constitute endorsement or recommendation for use. Though the dataited in this paper were collected from published literature, no attempt haseen made to verify the quality of data collected from the various sources.

∗ Corresponding author. Tel.: +1 513 569 7673; fax: +1 513 569 7620.E-mail addresses:[email protected] (P. Randall), chattopad-

[email protected] (S. Chattopadhyay).1 Tel.: +1 614 424 3661; fax: +1 614 424 3667.

1. Introduction

The development of effective treatment optionsmercury-contaminated wastes is a significant technicapractical challenge. There is little to no economic benderived from mercury recovery and recycling. In additeffective treatment is often challenging due to the highicity, volatility, and environmental mobility of mercury athe varied nature and composition of industrial waste pucts. Principal industrial sources of mercury-contaminwastes include chlor-alkali manufacturing, weapons protion, copper and zinc smelting, gold mining, paint appltions, and other processes[1]. Although industrial and com

304-3894/$ – see front matter. Published by Elsevier B.V.oi:10.1016/j.jhazmat.2004.08.010

212 P. Randall, S. Chattopadhyay / Journal of Hazardous Materials B114 (2004) 211–223

mercial uses of mercury have been curtailed in recent times,there is a demonstrated need for the development of reliablehazardous waste management techniques because of historicoperations that have led to significant contamination and on-going hazardous waste generation. This article focuses on thecurrent state of encapsulation materials being used to immo-bilize elemental mercury, mercury-contaminated debris, andmercury-contaminated wastes, soils, sediments, and sludges.A technology overview is provided, along with a summaryof the regulatory drivers and testing and evaluation criteriainvolved with implementation of these treatment approaches.The various encapsulation materials under investigation arediscussed and future technology development and researchneeds are provided.

2. Technology overview

As an inorganic element, mercury cannot be destroyed,but it can be converted into less soluble or leachable formsto inhibit migration into the environment after disposal. En-capsulation technologies are based primarily on solidificationprocesses that act to “substantially reduce surface exposureto potential leaching media” (40 Code of Federal Register[CFR] 268.42). Encapsulation technologies can also involvea tiona tion,o ch iss

twow oen-c en-c psu-l ounda ifiedb examp lationo theg priort ap-s thee

val-u en-c n thep cury-c chal-l andt tabi-l ono ucesa ally< als.H nt-b bilityh also

difficult to stabilize with cement-based processes due to theformation of anionic species that are soluble at high pH[4,5].Therefore, a significant amount of research has gone into thedevelopment of other encapsulation materials that can be usedas alternatives to the cement-based process. Sulfur polymerstabilization/solidification (SPSS), chemically bonded phos-phate ceramic (CBPC) encapsulation, and polyethylene en-capsulation are just three of the techniques that are currentlybeing tested and used to improve the long-term stability ofhazardous wastes. Each encapsulation material identified inthe literature will be reviewed in terms of the key features ofthe encapsulation process, current applications and technol-ogy status, and available performance data. The advantagesand disadvantages associated with each material will also bediscussed.

3. Regulatory background

The management and ultimate disposal of mercury-contaminated hazardous waste is controlled by U.S. EPA reg-ulations known as the Land Disposal Restrictions (LDRs) (40CFR Part 268). Under the current LDR program, the U.S. EPAhas established thermal recovery (e.g., roasting/retorting) ast reat-m cury.F er-c r im-m on)m ra-d verya iliza-t forhT tiont rcuryt very,e

pro-g landd mayb pro-c d mayn pti-m m-p fide( lfate( eh -i ro-gs atedd terms hu-m

combination of physical entrapment through solidificand chemical stabilization through precipitation, adsorpr other interactions. This combined treatment approaometimes referred to as stabilization/solidification.

Hazardous waste materials can be encapsulated inays: microencapsulation or macroencapsulation. Micrapsulation involves mixing the waste together with theasing material before solidification occurs. Macroencaation involves pouring the encasing material over and ar

larger mass of waste, thereby enclosing it in a solidlock. Sometimes these processes are combined. Forle, Singh et al. (1998) demonstrated the microencapsuf mercury-contaminated crushed light bulbs in whichlass was crushed and mixed with the encasing material

o solidification[2]. Mattus (1998) reported the macroenculation of mercury-contaminated lead pipes by pouringncasing material over and around the lead pipes[3].

There are a wide variety of materials currently being eated in the scientific community and in industry for theapsulation of hazardous wastes. This review focuses oerformance data related to the encapsulation of merontaminated wastes, which has proven to be especially

enging given mercury’s chemical and physical propertieshe varied nature of industrial wastes. Conventional sization/solidification methods typically include the fixatif metals using Portland cement and fly ash. This prodn impermeable, solid waste form at a high pH (typic10) that limits the solubility and leachability of most metowever, it is very difficult to stabilize mercury with cemeased processes because it does not form a low-soluydroxide solid. Arsenic and hexavalent chromium are

-

he best demonstrated available technology (BDAT) for tent of wastes containing greater than 260 mg/kg of meror treatment of wastes with less than 260 mg/kg of mury, other extraction technologies (e.g., acid leaching) oobilization technologies (e.g., stabilization/solidificatiay be considered[4]. Because mercury contained inioactive or mixed waste is not suitable for thermal recond recycling, the U.S. EPA also recognizes that stab

ion/solidification may be an appropriate treatment optioneavily contaminated mercury mixed wastes or debris[6].his review provides details on the variety of encapsula

echnologies available, but does not focus on other mereatment approaches (e.g. acid leaching, thermal recotc.).

The U.S. EPA is considering changes to the LDRram to require a macroencapsulation step prior to theisposal of stabilized mercury wastes. Mercury wastese stabilized using sulfide or other chemical fixationesses, but the stabilization process is pH dependent anot permanently immobilize mercury for disposal. The oal pH range is 4–8 for chemical fixation of mercury coounds to the highly insoluble solid form, mercuric sulHgS). At high pH, the more soluble solids mercurous suHg2SO4), mercuric sulfate (HgSO4), and mercury sulfidydrogen sulfide complex (HgS[H2S]2) are formed depend

ng on oxidizing or reducing conditions. At a low pH, hyden sulfide gas may escape from the waste[7,8]. Combiningtabilization with macroencapsulation to prevent pH-relegradation of the treated waste may improve its long-tability and therefore minimize any potential threats toan health and the environment.

P. Randall, S. Chattopadhyay / Journal of Hazardous Materials B114 (2004) 211–223 213

4. Testing and evaluation background

Performance data for encapsulated wastes typically in-clude both physical data (e.g., strength, density, and per-meability) and/or chemical data (e.g., leachability). Formacroencapsulated waste, the most important evaluation cri-teria are the compressive strength, the waste form density,the presence of void spaces, and the barrier thickness. Theprimary focus during macroencapsulation is to create an inertsurface coating or jacket around the waste that substantiallyreduces the potential for exposure to leaching media[3]. Formicroencapsulated waste, the toxicity characteristic leachingprocedure (TCLP) in the EPA publication SW-846, plays animportant role in determining whether or not the materialcan be accepted by a landfill. According to the LDR rules,mercury hazardous waste is defined as any waste that hasa TCLP value greater than 0.2 mg/L. Mercury-contaminatedwastes that exceed this value generally must be treated tomeet the Universal Treatment Standard (UTS) of 0.025 mg/Lor less prior to disposal in a landfill. In addition, some statesmay set criteria that define hazardous wastes given the to-tal metal concentration such as California’s Total ThresholdLimit Concentration (TTLC) of 20 mg/kg for mercury. TheU.S. Nuclear Regulatory Commission (NRC) has also de-veloped its own waste form acceptance criteria for mixed( stingp entalf iono nd ir-r hingb

5

re-h cap-s

6

com-p ne-o upont t.%e en-t ial isr PC),a t ap-p la-t con-c atelyd uitedt cor-r

Fig. 1. Sulfur polymer stabilization/solidification process.

Fig. 1 provides a simplified block-diagram for the SPSSencapsulation process[10]. For macroencapsulation, moltenSPC is poured over and around the waste or debris and isthen allowed to set into a monolithic waste form. The rec-ommended mixing temperature for SPC is between 260 and280◦F. For microencapsulation of liquid, elemental mercury,a two-stage process is followed that has been patented by Kalbet al. of Brookhaven National Laboratory (BNL) under U.S.Patent No. 6,399,849. First, the elemental mercury is mixedin a heated reaction vessel at 104◦F with powdered SPC.Other chemical stabilization agents such as sodium sulfideand triisobutyl phosphine sulfide can also be added duringthis initial step. The heated reaction vessel helps to acceler-ate the reaction between mercury, SPC, and the additives toform HgS. An inert gas atmosphere is also used in the vesselto prevent the formation of mercuric oxide. Next, additionalSPC is added and the mixture is heated to 266◦F to form ahomogenous molten liquid. The liquid is then poured into amold and allowed to set into a monolithic waste form. Thistwo-step process minimizes both the oxidation of mercury tomercuric oxide and the amount of unreacted mercury. BNLrecently licensed the SPSS technology to Newmont MiningCorporation for the encapsulation of liquid elemental mer-cury generated as a byproduct of gold mining operations.Newmont and BNL are currently working on scaling-up thet

se ofSp

nch-s . Thes f thet curyv the

e.g. radioactive) wastes. In general, NRC waste form terocedures examine the influence of various environm

actors including the effect of thermal cycling and immersn compressive strength, the impact of biodegradation aadiation on waste form stability, and the long-term leacehavior.

. Encapsulation materials review

The following materials were identified during a compensive literature review as having been used for the enulation of mercury-contaminated wastes.

. Sulfur polymer stabilization/solidification

The SPSS process can be used to convert mercuryounds into the highly insoluble HgS form and to simultausly encapsulate the waste. The SPSS process relies

he use of a thermoplastic material which contains 95 wlemental sulfur and 5 wt.% of organic modifiers, dicyclop

adiene and oligomers of cyclopentadiene. This matereferred to in the literature as sulfur polymer cement (Slthough it is not a cementitious material. SPC melts aroximately 235◦F and sets rapidly upon cooling. It is re

ively impermeable to water compared to conventionalrete and has a high mechanical strength at approximouble that of conventional concrete. SPC is also well s

o harsh environments with high levels of mineral acids,osive electrolytes, or salt solutions[9].

echnology for industrial use[11,12].Several studies have been completed regarding the u

PC for heavy metal-contaminated wastes[13,3,14,9]. Keyerformance data from these studies are provided inTable 1.

Fuhrmann et al. (2002) presents the results from a becale SPSS treatment of radioactive elemental mercurytudy explored three issues including the leachability oreated waste, the formation of mercuric sulfide, and meraporization during processing. Microencapsulation of


Table 1Key performance data for sulfur polymer stabilization/solidification

Author/vendor

Type Scale Waste type Waste formsize

Waste loading(wt.%)

Compressivestrength (psi)

Density(g/cm3)

Hg leachateuntreated (mg/L)

Hg leachatetreated (mg/L)

Mattus(1998)

MA BP Mixed waste cadmiumsheets

5 gallons 15.8–28.6 NR NR NA NA

Mattus(1998)

MA BP Mixed waste leadpipes/glovescontaminated with Hg

5 gallons 31.3–38.8 NR NR NA NA

Fuhrmannet al.(2002)

MI BP Radioactive Hg0 5 gallons 33.3 NR NR 2.64 0.020 to >0.40


MI BP Radioactive Hg0 with3 wt.% triisobutylphosphine sulfideadditive to SPC

5 gallons 33.3 NR NR 2.64 >0.40


MI BP Radioactive Hg0 with3 wt.% Na2S·9H2O ad-ditive to SPC

5 gallons 33.3 NR NR 2.64 0.0013–0.050

Darnell(1996)a

MI BP/F Metal oxides includingHg, Pb, Ag, As, Ba,and Cr at 5 wt.% each

NR 40 4000 NR 250b <0.2

Kalb et al.(1996)

MI BP Mixed waste off-gasscrub solution

NR 25–45 3850–8160 1.86–1.94 0.14 <0.009

PildyshTech-nologies,Inc.

MI BP Mine tailings <100�m ∼40% NR NR 413 <0.1

BP: bench-scale/pilot-scale; F: full-scale; MA: macroencapsulation; MI: microencapsulation; NA: not applicable; NR: not reported; TCLP: toxicity characteristicleaching procedure.

a Sodium sulfide nonahydrate was added to reduce metal leachability.b Untreated waste TCLP not reported, so estimated based on total Hg level in waste divided by 20.

elemental mercury with SPC alone resulted in TCLPs rang-ing from 20 to >400�g/L. Treatment using a 3 wt.% sodiumsulfide nonahydrate additive resulted in TCLPs ranging from1.3 to 50�g/L. The authors used x-ray diffraction studies todetermine that elemental mercury and SPC reacted to formprimarily the more soluble, meta-cinnabar form of mercuricsulfide. However, elemental mercury and sodium sulfide non-ahydrate formed primarily cinnabar, which explains the im-proved leaching behavior in those tests. The results of furtherstudies also demonstrated that mercury volatilization was re-duced through the treatment with sodium sulfide. Headspacemeasurements for elemental mercury alone ranged from 9.2to 12.7 mg/m3 in vapor, ranged from 0.41 to 4.5 mg/m3 withjust SPC, and 0.20 to 1.3 mg/m3 with the addition of sodiumsulfide. These results suggest that, for adequate retention ofthe mercury during processing, the use of additives such assodium sulfide may be necessary[13].

Oak Ridge National Laboratory (ORNL) completed atreatability test to scale-up the SPC process for the macroen-capsulation of mixed waste debris, contaminated with mer-cury and other metals[3,15]. The ORNL treatability studyobjectives included scaled-up equipment selection, determi-nation of the size and shape of the final waste form, and pro-cess parameter monitoring and optimization. The treatabilitystudy was performed using two mixed waste streams gener-a 8 lb

of lead pipes contaminated with mercury. It was found thatpreheating the debris to 284–302◦F for 6 h helped to preventfast cooling of the SPC at the waste–binder interface duringthe pour. Preheating also helped to reduce the formation ofair pockets. Vibrating the container during and after the SPCpour also improved setting of the waste form. Heating tapeswere used to maintain a target temperature of 320◦F at thetop portion of the container. This allowed air bubbles from thesetting to escape. The optimal additional heating time was de-termined to be 10 h after the pour had ended. During testing,examination of the waste form cross sections revealed goodcontact between the debris pieces and SPC and no identifi-able interface between pour layers. No H2S or SO2 off-gasseswere detected during the tests. The investigators were able toincorporate up to 28.6 wt.% of the cadmium sheets and upto 38.8 wt.% of the mercury-contaminated lead pipes in thefinal waste forms[3,15].

Darnell (1996) demonstrated the use of SPC for the mi-croencapsulation of up to 5 wt.% of metal oxides includingmercury, lead, silver, arsenic, barium, and chromium. Darnellmicroencapsulated a variety of wastes including dehydratedboric acid salts, incinerator hearth ash, mixed waste fly ash,and dehydrated sodium sulfate salts. Darnell also found thatan additional chemical stabilization step was needed to treatmercury to meet TCLP limits. A 7 wt.% sodium sulfide non-a on-
ted at ORNL: (i) 457 lb of cadmium sheets and (ii) 44 hydrate (Na2S·9H2O) was added to the SPC mixture to c


vert metal oxides to more leach-resistant metal sulfides. TheU.S. EPA TCLP limits were achieved for all metals[9].

Based on the information reviewed from the sources pre-viously identified, some of the advantages of the SPSS pro-cess are as follows: (i) high concentration mercury wastescan be effectively treated, including elemental mercury; (ii)relatively low temperature process (260–280◦F); (iii) su-perior water tightness (e.g., low permeability and porosity)compared to Portland cement; (iv) high resistance to corro-sive environments (e.g., acids and salts); (v) high mechan-ical strength; (vi) simple to implement because mixing andpouring equipment is readily available; (vii) easier to usethan other thermoplastics, like polyethylene, because of itslow viscosity and low-melt temperature; (viii) SPC can beremelted and reformulated.

Pildysh Technologies, Inc. has developed a proprietary,thermoplastic sulfur-based technology, called TerraBondTM,for encapsulating and stabilizing hazardous wastes. The pri-mary physical encapsulation is induced by allotropic sulfurcrystal conversions. A hydrophobic sealant (secondary en-capsulation) applied to the pellet surface provides a barrieragainst contaminant leaching.

One must also be aware of the limitations of the above-mentioned processes. Some of them are as follows: (i) volatilelosses of mercury may occur and engineering controls aren pro-c ock-e ithl t thef al-k ic orc lingr siblei era-t ands

7e

wells ma-t nge.S e tob . Al-t ds,so epa ha dt esiumos otas-

Fig. 2. Chemically bonded phosphate ceramic process.

sium phosphate hydrate as shown in the reaction below:

MgO + KH2PO4 + 5H2O → MgKPO4 · 6H2O (MKP)

Iron oxide phosphates can also be used to form a low-temperature ceramic, but research into the use of this materialis limited [16].

Fig. 2provides a simplified block-diagram for the CBPCencapsulation process. First, enough water is added to thewaste in the disposal drum to reach the stoichiometric wa-ter content. Next, calcined magnesium oxide and monopotas-sium phosphate binders are ground to a powder and blended ina one-to-one molar ratio. Additional ingredients (e.g., fly ashor K2S for mercury fixation) also are added to the binders. Thewater, binders, additional ingredients, and waste are mixedfor about 30 min. Under most conditions, heat from the reac-tion causes the waste matrix to reach a maximum temperatureof approximately 176◦F. After mixing is stopped, the wasteform typically sets in about 2 h and cures in about 2 weeks.Mixing can be completed in a 55-gallon disposal drum with aplanetary type mixer. The waste, water, binder, and additivescan be charged to the drum using hoppers, feeding chutes, andpiping as needed. Argonne National Laboratory (ANL) hassix patents covering the use of this material for the encapsu-lation of hazardous wastes. The technology has been licensedto Wangtec, Inc., for the treatment of incinerator ashes fromp

mon-s andms

n off de-b ury-c nch-

eeded; (ii) aqueous wastes must be dewatered prior toessing; (iii) SPC can develop an excess of voids or air pts if cooled too quickly; (iv) metal debris or pieces w

arge thermal mass may require preheating to prevenormation of air pockets; (v) not compatible with strongaline solutions (>10%), strong oxidizing agents, aromathlorinated solvents, or expanding clays; (vi) SPC handequires the use of engineering controls to mitigate posgnition and explosion hazards; (vii) if excessive tempures are created, SPC will emit hydrogen sulfide gasulfur vapor.

. Chemically bonded phosphate ceramicncapsulation

Chemically bonded phosphate ceramics (CBPCs) areuited for encapsulation because the solidification of thiserial occurs at low temperatures and within a wide pH raimilar to SPSS, successful treatment with CBPC is duoth chemical stabilization and physical encapsulation

hough mercury will form low-solubility phosphate solitabilization with a small amount of sodium sulfide (Na2S)r potassium sulfide (K2S) to form HgS greatly improves therformance of the final CBPC waste form. Hg3(PO4)2 hassolubility product of 7.9× 10−46, compared to HgS witsolubility product of 2.0× 10−49 [7]. CBPCs are forme

hrough an acid–base reaction between calcined magnxide (MgO) and monopotassium phosphate (KH2PO4) inolution to from a hard, dense ceramic of magnesium p

ower plants in Taiwan[17].Several detailed studies have been completed to de

trate the use of CBPCs for both macroencapsulationicroencapsulation of hazardous wastes[2,7,17,18]. Table 2

ummarizes key performance data from these studies.Singh et al. (1998) demonstrated the encapsulatio

our waste streams with CBPC including cyrofracturedris, lead bricks, lead-lined plastic gloves, and mercontaminated crushed light bulbs. The study was a be


Table 2Key performance data for chemically bonded phosphate ceramic encapsulation

Author/vendor Type Scale Waste type Waste formsize

Waste loading(wt.%)


Density(g/cm3)



Singh et al.(1998)

MA BP Cyrofractured debris 1.2–3 gallons 35 5000–7000 1.81 NA NA

Singh et al.(1998)

MA BP Lead bricks NR NR 5000–7000 1.8 NA NA

Sing et al.(1998)

MA BP Lead-lined gloves 5 gallons NR 5000–7000 1.8 NA NA

Singh et al.(1998)

MI BP Hg-contaminatedcrushed light bulbs

5 gallons 40 5000–7000 1.8 0.200–0.202 <0.00004–0.00005

DOE (1999a)a MI BP DOE surrogate wastesof nitrate salts andoff-gas scrub solution

NR 58–70 1400–1900 1.7–2.0 540–650 <0.00004–<0.00005

Wagh et al.(2000)a

MI BP DOE ash (HgCl2 at0.5 wt.%)

100 g NR NR NR 40 <0.00085

Wagh et al.(2000)a

MI BP Delphi DETOX (with0.5 wt.% each HgCl2,Ce2O3, Pb(NO3)2)

100 g NR NR NR 138–189 <0.00002–0.01

Wagh et al.(2000)a

MI BP Soil (HgCl2 at0.5 wt.%)

100 g NR NR NR 2.27 <0.00015

Wagh andJeong(2001)b

MI BP DETOX Wastestream(HgCl2 at 0.5 wt.%)

162–500 g 60–78 NR NR 250c 0.0047–0.0151

Wagh andJeong(2001)b

MI BP DETOX Wastestream(Hg at 0.5 wt.%)

162–500 g 60–78 NR NR 250c 0.00719–0.00764

a Potassium sulfide was added to reduce metal leachability.b Sodium sulfide nonahydrate was added to reduce metal leachability.c Untreated waste TCLP not reported, so estimated based on total Hg level in waste divided by 20.

scale project with waste form sizes ranging from 1.2 to 5gallons and consequently some material handling and sizereduction (e.g., shredding) was necessary. The CBPC fabri-cation process was approximately the same for each wastewith the exception of minor formula changes in the wt.% ofwater, ash, or binders and the addition of potassium sulfide(K2S) in the mixture for the mercury-contaminated crushedlight bulbs. The mercury-contaminated crushed light bulbswere pretreated by mixing with a potassium sulfide solutionfor approximately 1 h. The glass was then set into CBPCwith a formulation of 40 wt.% ash, 40 wt.% binder (MgO andKH2PO4 powders mixed in 1:1 molar ratio) and 20 wt.% wa-ter. Mercury levels in the glass waste were around 200 partsper million (ppm). The crushed glass ranged in size from 2 to3 cm long× 1 to 2 cm wide. During the mixing of the wastewith the binder, the glass was crushed down to sizes less than60 mm and a waste loading of approximately 40 wt.% wasachieved. Each waste form was allowed to cure for about2 weeks prior to performance testing. The cross sections ofthe final waste forms were observed to be very homogenous,dense, and free of air pockets. A complete, intact coating withcontinuous adhesion was observed around the wastes and nogaps were present at waste–binder interfaces. TCLP tests onthe mercury-contaminated wastes showed 200–202�g/L inthe untreated wastes compared to <0.04–0.05�g/L for thet

tedt t of

salt-containing, mercury-contaminated mixed wastes[17].A significant proportion of DOE mixed wastes containgreater than 15 wt.% salts and these wastes are very difficultto treat with conventional methods. Salts are soluble, easilyhydrated, and cause deterioration of stability of the mineralmicrostructure over time by substitution reactions. As aconsequence of these properties, salts adversely impactconventional cement matrices by causing a decrease incompressive strength and an increase in metal leachability.The waste streams used in this study included saturatedsalt solutions (NaNO3 and NaCl), activated carbon, ionexchange resins, spent incinerator off-gas scrub solution, andNa2CO3. These surrogate wastes were spiked with hazardousconstituents including lead, chromium, mercury, cadmium,nickel, and trichloroethylene (TCE) at levels up to 1000 ppm.Waste loadings in CBPC of up to 70 wt.% (40 wt.% salt) wereachieved during the study. Several performance tests werecompleted on the CBPC-encapsulated wastes, includingcompressive strength, U.S. EPA TCLP tests, and salt anionleaching tests. The CBPC binder was amended with K2S,which successfully stabilized mercury to meet the TCLPlimit in these wastes. However, wastes containing relativelyhigh concentrations of salts (>42 wt.% salt loading) mayleach Resource Conservation and Recovery Act (RCRA)hazardous metals, and could therefore compromise thel

scales sur-

reated wastes[2].A U.S. Department of Energy study was comple

o test the effectiveness of CBPCs in the treatmen

ong-term stability of the encapsulated materials.Wagh et al. (2000) discusses the results of bench-

tudies for the encapsulation of mercury-contaminated


rogate wastes including DOE ash waste, secondary wastestreams from the DETOXSM wet oxidation process, andcontaminated topsoil[7]. The surrogate waste streams weredosed with mercuric chloride (HgCl2) at 0.1–0.5 wt.% andalso with other metals including lead and cesium. Initial testsshowed that encapsulation with CBPC alone caused mercuryleaching to decrease by a factor of three to five times. How-ever, for adequate mercury stabilization, Wagh et al. deter-mined that a small amount of Na2S or K2S should be used inthe binder. For use with CBPC, the K2S formulation was ini-tially deemed to be the most appropriate because the CBPCbinder is a potassium-based material. Other potential addi-tives for mercury stabilization referenced by the author in-clude H2S or NaHS. In this study, K2S was mixed directlywith MgO and KH2PO4 powders to form one binder pow-der. The optimal range of K2S in the binder powder wasfound to be 0.5 wt.% and it was also established that lev-els significantly above this dose resulted in the formationof Hg2SO4, which has a much higher solubility than HgS(Hg2SO4 has a solubility product of 7.99× 10−7 versus HgSwith a solubility product of 2.0× 10−49). All of the surro-gate wastes were successfully treated to levels below the U.S.EPA TCLP criteria for mercury. Long-term (90-day) leach-ing tests indicated that the diffusion of mercury through theCBPC matrix is 10 orders of magnitude lower than in cements

thee -t -twf ry tos ntoH theC tureo ludingaaT withs -l derw TheC d tot tedHr %w andt scales acid(

gesa mer-c cess( (iii)n tread re-

Fig. 3. Polyethylene macroencapsulation.

quires no additional heat input; (v) superior water tightnessand chemical resistance compared to Portland cement; (vi)simple to implement since mixing and pouring equipment isreadily available; (vii) nonflammable materials and stable andsafe with oxidizing salts; (viii) does not generate secondarywastes or potentially hazardous off-gasses[2,7,18].

Based on the information reviewed from the sources pre-viously identified, the following are the limitations of theprocess: (i) pretreatment with K2S or other compounds isneeded for chemical stabilization of mercury; CBPC alone isnot enough; (ii) excess sulfide will increase the leachabilityof mercury, so careful processing is needed; (iii) some wasteconstituents (e.g., hematite) may accelerate setting times anddecrease workability of the CBPC slurry; (iv) only limiteddata is available to support the long-term effectives and dura-bility of CBPC waste forms; (v) the leaching of salt anionsover time could deteriorate the integrity of the waste for highsalt wastes.

The relative merits of the CBPC process compared to theSPSS process are discussed in the future development andresearch needs section.

8. Polyethylene encapsulation

oss-l cifict n-c icallya e as ap thy-l d inc emi-c andC tters r tem-p withw

ystems.Wagh and Jeong (2001) continued work related to

ncapsulation of DETOXSM wastes[18]. The study invesigated with the effect of hematite (Fe2O3) on the fabricaion and setting of the CBPC waste form. The DETOXSM

astes contained approximately 95 wt.% Fe2O3, which wasound to be highly reactive and caused the CBPC sluret too quickly before mercury could be effectively fixed igS. Additional tests were conducted in order to modifyBPC fabrication process to account for the reactive naf these wastes. Two surrogate wastes were created incwaste stream with 0.5 wt.% HgCl2 and 94.32 wt.% Fe2O3

nd a waste stream with 0.5 wt.% Hg0 and 95 wt.% Fe2O3.wo samples of each surrogate waste were pretreatedodium sulfide nonahydrate (Na2S·9H2O) for 2 h, which alowed sufficient time for the mercury to form HgS. The binas then added and the slurry was mixed until it set.BPC samples were cured for 3 weeks and subjecte

he U.S. EPA TCLP test. Final TCLP results for the treagCl2 waste ranged from 4.7 to 15.1�g/L and the Hg0 wastes

anged from 7.19 to 7.64�g/L. Waste loadings of 60–78 wt.ere achieved. Setting times were rapid (10–18 min)

he authors suggested that it may be possible in large-ystems to slow down the reaction by adding boricat <1 wt.%).

The following is a list of some of the reported advantassociated with the use of CBPC: (i) high concentrationury wastes can be treated; (ii) lower temperature pro∼176◦F) than SPSS and polyethylene encapsulation;o water removal is necessary as CBPC can be used tory solids and sludges/liquids; (iv) unlike SPC, CBPC

t

Polyethylene is a thermoplastic material or a noncrinked linear polymer that melts and liquefies at a speransition temperature (248◦F). Polyethylene physically eapsulates the waste and does not interact with or chemlter the waste materials. Polyethylene is readily availablost-consumer recycled material (e.g., low-density polye

ene [LDPE] and high-density polyethylene [HDPE] useommercial packaging/containers). It also has good chal resistance and is water insoluble. According to Kalbolombo (1997) the physical properties of LDPE are beuited to encapsulation because HDPE requires greateeratures and pressures during processing and mixingastes[19].


Fig. 3 provides a simplified block-diagram for thepolyethylene macroencapsulation process. The key equip-ment used in this process typically includes a polymer ex-truder and feed hoppers. Kinetic mixers have also been usedfor polyethylene encapsulation[20]. Polyethylene macroen-capsulation typically involves the use of a basket placed in-side a drum to allow at least a 1 in. barrier around the wastematerial. Molten polyethylene is then poured from an ex-truder over and around the waste in the drum. In addition,an alternative to on-site pouring is the use of premanufac-tured containers as discussed below. Polyethylene microen-capsulation typically involves directly mixing the waste ma-terial and polyethylene at an elevated temperature (typically248–302◦F) in an extruder. The mixture of waste materialand polyethylene is then poured into a drum and allowedto set. Microencapsulation may require several pretreatmentsteps, including drying of wet wastes and physical separationto resize or improve the particle distribution of the waste.In addition, off-gas treatment is needed for any water va-por, volatile organic compounds (VOCs), or volatile metals(e.g., arsenic and mercury) in the waste[21]. Both polyethy-lene microencapsulation and macroencapsulation servicesare commercially available. In 1998, the Envirocare facilityin Utah installed and permitted a single screw extruder sys-tem that can process up to 5 tonnes of waste per day. The finalw andh in.[

ethy-l on ofh ialv iron-m cap-s ners.O enem atedw ced cap-s -r en-c curyw

lenef DOEs t hadr herh ined9 con-t ratedi psu-l ding( ow-e actu-a lfatec had am m

oxide, this same waste stream had a higher mercury TCLP of1.07 mg/L. It is clear from these results that polyethylene en-capsulation alone cannot adequately reduce the availability orleachability of mercury, even at relatively low concentrations[23].

Also, due to the high processing temperatures of polyethy-lene encapsulation, it is likely that a large fraction of mercurywould be volatilized unless it was pretreated or chemicallyfixed. This issue is highlighted by work completed by Carteret al. (1995) with arsenic, which is also a highly volatilemetal. Carter et al. (1995) used HDPE with a melting pointof 266◦F and an operating range at (356–410◦F) to mi-croencapsulate powdered arsenic trioxide (As2O3). It wasfound that at a 20 vol.% loading of this compound, the vis-cosity of the HDPE increased dramatically and the mixturebecame unworkable. Scanning electron microscope (SEM)micrographs showed that the arsenic trioxide had sublimedand recrystallized. When arsenic trioxide was stabilized withcalcium carbonate, the volatility decreased, but achievablewaste loadings in HDPE remained low. Mercury and its com-pounds are also highly volatile (e.g., mercuric chloride sub-limes at 572◦F), so the results of this study provide someinsight into the challenge of using polyethylene to processwastes containing high levels of either arsenic or mercury[24].

ven-d pre-m asteM rna-t notc turedc natedd t 40C in ar ucha lene[

roll-o t.7 ands -s az-a ctedd ruma ter-n nsityp madei e lido elec-t meltt d thet

oci-a uslyi nicals hy-

aste forms are typically set in 30- to 55-gallon drumsave a minimum exterior surface coating of LDPE of 1–2

20].Several studies have been carried out using poly

ene for both macroencapsulation and microencapsulatiazardous wastes[21–24]. In addition, several commercendors (e.g., Chemical Waste Management, Boh Envental, and Ultra-Tech, International) provide macroen

ulation services using premanufactured HDPE containly one study was found which dealt with the polyethylicroencapsulation of two types of mercury-contaminastes[23]. In general, there is little to no performanata available on the effectiveness of polyethylene enulation for mercury-contaminated wastes.Table 3summaizes key performance data from several polyethyleneapsulation case studies (both with and without merastes).Burbank and Weingardt explored the use of polyethy

or the microencapsulation of mixed wastestreams at theite in Hanford, Washington. Two wastes were tested thaelatively low concentrations of mercury, along with oteavy metals. An ammonium sulfate cake waste conta.2 ppm of mercury and a solar evaporation basin sludge

ained 1.3 ppm of mercury. These wastes were incorponto polyethylene at a 40–50 wt.% loading. Prior to encaation, calcium oxide was added to the higher waste loa50 wt.%) specimens to help reduce metal leachability. Hver, the calcium oxide amendment did not reduce, butlly increased mercury leachability. The ammonium suake waste microencapsulated with polyethylene aloneercury TCLP of 0.442 mg/L. With the addition of calciu

In addition to on-site processing, there are severalors that provide macroencapsulation services withanufactured HDPE containers including Chemical Wanagement, Boh Environmental, and Ultra-Tech, Inte

ional. In general, the use of a tank or container isonsidered macroencapsulation. However, premanufacontainers can be used and are allowed for contamiebris under the LDR alternative debris standards aFR 268.45. Premanufactured containers can result

eduction of the overall waste form volume by as ms one-fourth compared to an on-site pour of polyethy

25].Chemical Waste Management provides HDPE-lined

ff boxes and 1/2-in.-thick HDPE vaults measuring 21 f×ft. The vaults have a lid that is secured with adhesivescrews. Boh Environmental’s Arrow-PakTM technology conists of compacting 55-gallon drums filled with mixed or hrdous waste debris into 12-in.-thick pucks. The comparums are loaded into an 85-gallon metal overpack dnd then into a 1-in.-thick HDPE tube. Ultra-Tech, Inational offers a series of premanufactured, medium-deolyethylene containers. The containers can be custom-

n any size. A resistance wire system is embedded in thf each container. Once the debris waste is in place, an

rical current is applied to the wires, heating them up tohe polyethylene, and creating an effective seal arounop [26].

The following is a list of some of the advantages assted with the use of polyethylene as reported by previo

dentified researchers: (i) polyethylene has a high mechatrength, flexibility, and chemical resistance; (ii) polyet


Table 3Key performance data for polyethylene encapsulation

Author/vendor Type Material Scale Waste type Waste formsize

Waste loading(wt.%)


Density(g/cm3)



Faucette et al.(1994)

MA LDPE BP/F Combustibles,laboratoryglassware,scrap metals,and lead (e.g.,sheet, bricks,tape)

5–10 gallons NR NR NR NA NA

Faucette et al.(1994)

MI LDPE BP/F F006 wastecode: nitratesalts with Cd,Cr, Pb, Ni, andAg

NR 50 NR NR NA NA

Burbank andWeingardt(1996)

MI LDPE BP Ammoniumsulfate/solarbasin sludge

1.25 gallons 40–50a 1088–2465 NR 0.46 [9.2ppm]b

0.442–1.07

Burbank andWeingardt(1996)

MI LDPE BP Solar basinsludge

1.25 gallons 40–50 1088–2465 NR 0.065 [1.3ppm]b

0.107–0.122

Carter et al.(1995)

MI HDPE BP As2O3 withand withoutCaCO3

NR 20 vol.% NR NR NA NA

Kalb et al.(1996)

MI LDPE BP Off-gas scrubsolution

NR 50–70 1950–2180 1.21–1.45 0.14 <0.009

a Prior to encapsulation, calcium oxide was added.b Untreated waste TCLP not reported, estimated by total Hg level in waste divided by 20.

lene is readily available in post-consumer recycled forms;(iii) encapsulation equipment is commercially available andthe process can be automated; (iv) premanufactured vaultsand containers can be used under some circumstances; (v)polyethylene is used in landfill liners and extensive stud-ies document its chemical resistance and long-term durabil-ity.

The following are some of the limitations of the process:(i) external heating is required and the process occurs at ahigher temperature than the SPC and CBPC methods; (ii)the high temperatures make effective polyethylene encap-sulation of mercury and arsenic problematic; (iii) polyethy-lene does not chemically incorporate the waste and mer-cury volatilization and leachability are a significant concern;(iv) wastes will typically have to be preprocessed to removemoisture and/or to achieve adequate particle size distribu-tions.

The relative merits of the polyethylene encapsulation pro-cess compared to the SPSS and CBPC processes are dis-cussed in the future development and research needs sec-tion.

9. Other encapsulation materials

mon-s haz-a sins,s olyc-

erams), DolocreteTM, and carbon/cement mixtures. Keyperformance data from these studies are summarized inTable 4.

9.1. Asphalt

Asphalt or bitumen has been used to microencapsu-late soil contaminated with low-levels of heavy metals[27,28]. Radian Corporation reported using cold-mix as-phalt to microencapsulate soil contaminated with mercury(at 78 mg/kg). Hot-mix asphalt was deemed to be inap-propriate because the elevated temperatures could promotethe volatilization of mercury[29]. Kalb et al. (1996) dis-cusses the microencapsulation of up to 60 wt.% of a mixedwaste incinerator off-gas scrub solution with asphalt. Themercury TCLP in the untreated wastes was 0.14 mg/L ver-sus <0.009 mg/L in the asphalt microencapsulated waste[14].

9.2. Polyester and epoxy resins

Polyester is an example of a thermosetting resin or across-linked polymer that undergoes a chemical reactionto solidify. Several thermosetting resins have been testedfor the encapsulation of salt-containing mixed wastes in-c inyle con-t ithp eved

Several other materials have been developed and detrated for the encapsulation of mercury-contaminatedrdous wastes including asphalt, polyester and epoxy reynthetic elastomers, polysiloxane, sol–gels (e.g., p

luding orthophthalic polyester, isophthalic polyester, vster, and a water-extendible polyester. These wastes

ained metals, including mercury, at the 1000 ppm level. Wolyester resins, waste loadings of 50 wt.% were achi


Table 4Key performance data for various encapsulation materials

Author/vendor Type Material Scale Waste type Waste formsize

Waste loading(wt.%)


Density (g/cm3) Hg leachateuntreated(mg/L)


Kalb et al.(1996)

MI Asphalt BP Off-gas scrubsolution

NR 30–60 540–610 1.08–1.42 0.14 <0.009

Radiana MI Asphalt F Soil (Hg78 mg/kg)

NA NR 176 NR NR NR

DOE (1999b) MI Polyester BP Salt-containingmixed wastes

NR 50 5100–6200 NR 50b <0.01–0.2

Orebaugh(1993)

MA Epoxy BP Mixed waste,lead billets

5 gallons NR NR 1.43–1.5 (resinonly)

NA NA

Carter et al.(1995)

MI Styrene-butadienerubber

BP As2O3 NR 64 NR 1.7 (rubber only) NA NA

Meng et al.(1998)

MI Tire rubber BP Soil (Hg300 mg/kg)

100 g (4 g rubber/100 g soil)

NR NR 3.5 0.034

DOE (1999b) MI Polyesterresins

BP Salt-containingmixed wastes

NR 50 5100–6120 1.03–1.09(polyester resinonly)

NR <0.01–0.2

DOE (1999c) MI Poly-siloxane BP Salt-containingmixed wastes

NR 50 420–637 NR 50b 0.01–0.06

DOE (1999d) MI Sol–gels BP Salt-containingmixed wastes

NR 30–70 150–1500 NR 50b 0.044–0.23

Dolomatrix(2001)

MI DolocreteTM F Hg-waste at15,300 mg/kg

NR NR 145 NR 765b <0.1

Zhang andBishop(2002)

MI Powderreactivatedcarbon andcement

BP Hg-contaminatedsand up to1000 mg/kg

NR 50 NR NR ∼0.10 to 10 ∼0.010 to0.090

a SAIC (1998).b Untreated waste TCLP not reported, estimated by total Hg level in waste divided by 20.

for unconcentrated spent off-gas scrub solutions and 70 wt.%for nitrate/chloride salts. Mixed waste, salt surrogate TCLPtests for mercury ranged from <0.01 to 0.2 mg/L[30].In addition, Orebaugh (1993) has reported using severalepoxy resins (e.g., Stycast 2651 and Thermoset 300) tomacroencapsulate mixed waste, lead billets. However, nodata on mercury encapsulation with epoxy resins was noted[31].

9.3. Synthetic elastomers

Synthetic elastomers are materials having properties sim-ilar to natural rubber and have been used in the microen-capsulation and stabilization of metal-contaminated wastes.Meng et al. (1998) reports using tire rubber for the treat-ment of mercury-contaminated soils[32]. A clay-loam soilwas spiked with mercuric oxide and mercuric chlorideat 300 mg/kg. Acetic acid leachate tests showed a reduc-tion from 3.5 mg/L in the untreated soil to 0.034 mg/Lin the soil mixed with tire rubber. The used tire rub-ber contained approximately 2–4% sulfur and less than32% carbon black. Other researchers have reported us-ing styrene-butadiene rubber (Solprene 1204) for the en-

capsulation of powdered arsenic trioxide (As2O3). Up to64 wt.% of arsenic trioxide was incorporated into the rub-ber, but beyond this level the rubber became unworkable[24].

9.4. Polysiloxane

Polysiloxane or ceramic silicon foam (CSF) consistsof 50 wt.% vinyl-polydimethyl-siloxane, 20 wt.% quartz,25 wt.% proprietary ingredients, and less than 5 wt.% wa-ter. The use of this material for encapsulation is patentedby Orbit Technologies. The material sets at room temper-atures (30◦C or 86◦F) and is resistant to extreme temper-atures, pressures, and chemical exposure. The polysiloxanetechnology was demonstrated on salt waste surrogates, whichwere spiked with lead, mercury, cadmium, and chromium at1000 ppm levels. Up to 50 wt.% waste loading was demon-strated. For high chloride salt wastes, the mercury TCLP was0.01 mg/L and for high nitrate salt wastes the mercury TCLPwas 0.06 mg/L. The final waste forms for both waste typesdid not pass for chromium. The authors recommend pretreat-ment for the chemical stabilization of wastes with metals atlevels greater than 500 ppm[33].


9.5. Sol–gels

Sol–gels or polycerams are a hybrid material derived fromthe chemical combination of organic polymers and inorganicceramics. A DOE study explored the use of a polyceram con-sisting of a polybutadiene-based polymer combined with sil-icon dioxide for the stabilization of high salt wastes. The saltwaste surrogates contained lead, chromium, mercury, cad-mium, and nickel at 1000 ppm levels. The polymer and sili-con dioxide are combined first and then mixed with the wasteand then solidified to encapsulate the waste. The setting ofthe waste form takes place at temperatures ranging from 151to 158◦F. Waste loadings from 30 to 70 wt.% were demon-strated. The initial waste forms in the demonstration had ahigh open porosity and did not pass the TCLP test for mer-cury. Another set of waste forms were fabricated and sub-jected to a secondary infiltration of polyceram solution afterinitial drying. The second set of tests was able to demonstratea decrease in the mercury TCLP to 0.044 mg/L[34].

9.6. DolocreteTM

DolocreteTM is a proprietary calcined dolomitic bindermaterial that can be used for the microencapsulation of inor-ganic, organic, and low-level radioactive waste. DolocreteTM

i g alu-m um,c ry-c n re-p

9

acti-v ess-f gatew sings ithP stesw imitf theP uryb ons.T anera om-p

1

archa onalP etal-c re them es and roen-

capsulation of mercury-contaminated hazardous wastes asdiscussed below.

Although several studies were noted which demonstratedthe successful encapsulation of high-level, mercury-contaminated wastes with SPC and CBPC, the body ofevidence for competent polyethylene encapsulation islimited. The higher temperatures of the polyethylene processmay pose some difficulty in effective encapsulation of thesewastes due to the volatile nature of mercury compounds. Itappears that SPC and CBPC are the only commercially viableencapsulation technologies for high concentration and/or ele-mental mercury wastes. Both SPC and CBPC processes havebeen patented, but licensing of the technologies has generallybeen limited to one or two companies and application of theseprocesses at the industrial-scale is limited. Polyethyleneencapsulation is likely limited to the handling of debris andother wastes containing only minimal or low-levels of mer-cury. The Envirocare facility in Utah does have a full-scalesystem in place for polyethylene encapsulation. In addition,the use of premanufactured HDPE containers for macroen-capsulation, as allowed under the U.S. EPA alternative debrisstandards, appears to offer a cost-effective disposal solution.

There are several technical issues related to the imple-mentation of SPC and CBPC encapsulation that need to beresolved. A better understanding of the long-term stability oft -terms s notb motedb vedu turep ationc C isp (e.g.h ma-t relyu is an asingm iveso couldb muml hingp ssives astef

las-t atp d tod anea iliza-t ls arep iumw haltf oilsw

re-q ation

s reported to successfully encapsulate wastes containininum, antimony, arsenic, bismuth, cadmium, chromi

opper, iron, lead, mercury, nickel, tin, and zinc. Mercuontaminated wastes with up to 15,300 mg/kg have beeorted to reach a final TCLP level of <0.1 mg/L[35].

.7. Carbon and cement mixtures

Zhang and Bishop (2002) report using powdered reated carbon (PAC), along with Portland cement, to succully encapsulate mercury-contaminated wastes. Surroastes were created with up to 1000 mg/kg of mercury uand, water, and Hg(NO3)2. These wastes were mixed wAC and then solidified with Portland cement. The waere successfully treated to below the U.S. EPA TCLP l

or mercury. In addition, it was determined that pretreatingAC with CS2 increased its adsorption capacity for mercy a factor of 10–100 times depending upon pH conditihe authors report that this approach is a potentially clend more effective means of stabilizing mercury wastes cared to sulfide precipitation[36].

0. Future development and research needs

A large body of literature exists regarding the resend development of alternative materials to conventiortland cement for the encapsulation of hazardous montaminated wastes. SPC, CBPC, and polyethylene aost established materials, and each has its advantagisadvantages for use in the macroencapsulation or mic

d

he final waste forms may be needed. In general, the longtability of materials encapsulated with SPC or CBPC haeen addressed and waste form degradation may be proy high salt loadings in wastes and other factors. An impronderstanding is needed of the kinetics of low-temperarocesses such as SPC or CBPC. This additional informould help in scale-up and process optimization. CBProne to rapid setting in the presence of reactive wastesematite) and rapid cooling with SPC can lead to the for

ion of undesirable air pockets. For all technologies thatpon fixing mercury into its mercuric sulfide form, thereeed to further assess the role of excess sulfides in increercury leachability. In addition, the performance objectr acceptance criteria for macroencapsulated wastese standardized to provide guidance regarding the mini

ayer thickness of the barrier, the expected long-term leacerformance of the final waste form, the target compretrength, and the tolerance for void spaces in the final worm.

The use of other innovative materials (e.g. synthetic eomers, polyester resin, DolocreteTM, etc.) appears somewhromising, but relatively few studies have been completeate. With several of these materials, including polysiloxnd sol–gels, it appears that an additional chemical stab

ion step may be needed when elevated levels of metaresent, since the TCLP criteria for mercury and chromere not met in initial trails. In addition, the use of asp

or encapsulation is most likely limited to contaminated sith only low-levels of mercury or other metals.It is clear that waste specific treatability tests will be

uired for the selection of the most appropriate encapsul


material for a given industrial waste stream. The selection cri-teria should include chemical compatibility of the waste andbinder materials, final waste form performance, technologyimplementability (e.g., the availability of processing equip-ment and vendor experience), safety and health issues, andproject-specific estimated costs.

Acknowledgements

The U.S. Environmental Protection Agency through itsOffice of Research and Development funded the research de-scribed here under contract number GS-10F-0275K. The au-thors wish to acknowledge Wendy Condit for assistance withthis paper. This paper is a summary of a more detailed EPA re-port titled: “Technical Report — Advances in EncapsulationTechnologies for the Management of Mercury-ContaminatedHazardous Wastes.”

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Advances in encapsulation technologies for the management of mercury-contaminated hazardous wastes

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