treatment of arsenic-contaminated soils. i: soil characterization
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TREATMENT OF ARSENIC-CONTAMINATED SOILS.I: SOIL CHARACTERIZATION
By Humayoun Akhter,1 Frank K. Cartledge,2 Joel Miller,3 Member, ASCE,and Mary McLearn4
ABSTRACT: Industrial sites frequently have arsenic-contaminated soils as a result of repeated applications ofarsenic herbicides. Four such sites were investigated to determine the suitability of cement-based solidification/stabilization (S/S) for in situ soil treatment. Arsenic concentrations ranged up to about 2,000 ppm in the soil,although leachability was relatively low. No toxicity characteristic leaching procedure leachates showed Asconcentrations as high as 5 mg/L. The low leachability appears to be due, at least in part, to iron present in thesoil. Although soils with higher As concentrations generally showed greater leachability, a somewhat strongerrelationship existed between the percentage of As in the soil that was leached and the iron concentration in thesoil. Another factor working in favor of the success of S/S in the present cases is the sandy character of thesoils with little clay or organic content. Thus, the quartz sand will serve as an aggregate and should not offerany interferences to cement hydration. A third favorable circumstance is afforded by the oxidizing character ofthe soils. The weathered arsenic present in the soils should be in the form of As(V), and arsenate salts presenta wider range of possibilities for precipitation of insoluble arsenic species than arsenite salts. A significantvariable with the potential to affect S/S is the soil moisture content, which varied greatly among the four sitesdue to differing water table depth.
INTRODUCTION
For many years various companies in the United States usedarsenic trioxide as a topical herbicide to control weeds, andsoil arsenic concentrations, particularly near the surface, areoften substantial. The environmental impact of such soil con-tamination sites vary widely, but some form of remediation isoften a wise course. Among the potentially applicable reme-diation technologies is in situ solidification/stabilization (S/S).At least potentially, it is effective at contaminant immobili-zation, relatively inexpensive to implement, and can be carriedout with a facility downtime measured in days. The papers inthis series describe the application of in situ S/S to such sites,from initial site selection and soil characterization, through atreatability study to optimize arsenic immobilization, to fieldapplication at two sites and posttreatment sampling and test-ing.
Also known as chemical fixation or encapsulation, S/S iswidely applied to waste streams and contaminated soils. Themost common form of the technology uses a cement or poz-zolanic binder to convert the waste to a solid (if necessary)and, depending on the constituents of the waste stream andthe binder, the treatment may reduce toxicity and/or water sol-ubility of hazardous materials and may create a monolithicwaste form that limits contaminant mobility due to its lowpermeability and small surface area. The process is most com-monly applied in cases where the contaminants of concern areheavy metals in cationic forms (e.g., Cd21, Cr31, and Pb21).However, applicability to a wide variety of waste materials,including As wastes, has been proposed.
1Sr. Res. Sci., Avlon, Inc., 5401 West 65th St., Bedford Park, IL 60638.2Prof., Dept. of Chem., Louisiana State Univ., Baton Rouge, LA
70803-1804 (corresponding author). E-mail: [email protected]
3Proj. Engr., Southern Company Services, 42 Inverness Ctr. Pkwy.,Birmingham, AL 35242.
4Mgr., Transmission and Distribution Soil and Water Issues, ElectricPower Research Inst., 3412 Hillview Ave., Palo Alto, CA 94304.
Note. Associate Editor: Susan E. Powers. Discussion open until April1, 2001. Separate discussions should be submitted for the individual pa-pers in this symposium. To extend the closing date one month, a writtenrequest must be filed with the ASCE Manager of Journals. The manuscriptfor this paper was submitted for review and possible publication on Au-gust 30, 1999. This paper is part of the Journal of Environmental En-gineering, Vol. 126, No. 11, November, 2000. qASCE, ISSN 0733-9372/00/0011-0999–1003/$8.00 1 $.50 per page. Paper No. 21753.
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Arsenic wastes are grouped together for discussion of treat-ment standards in the land disposal regulations promulgatedby the U.S. Environmental Protection Agency (USEPA) (Fed-eral 1990). Considerable discussion is allocated to S/S becauseit was considered by the USEPA to be a ‘‘potentially appli-cable technology.’’ However, S/S is not currently consideredthe best demonstrated available technology for any arsenicwaste or wastewater. The following quote indicates the rea-sons:
EPA has relatively inconclusive performance data for sta-bilization of arsenic in three different wastes using ninedifferent binders. Analysis of these data indicates that theeffectiveness of any particular stabilization binder appearsto be highly dependent upon the waste types. This result iswhat might be expected giving (sic) the chemical nature ofarsenic and the relative sensitivity of the effectiveness ofstabilization processes with respect to the presence of or-ganics and organo-metallics (Federal 1990).
The current treatment standard for nonwastewaters containingarsenic is 5.0 mg/L in extraction procedure or toxicity char-acteristic leaching procedure (TCLP) leachates (Code 1997;USEPA 1997).
In specific language, the USEPA does not preclude the useof S/S for treatment of As (particularly inorganic As) wastesbut recommends that its use be determined on a case-by-casebasis. Given the wide range of chemical characteristics of Aswastes, such a position is quite reasonable. Nevertheless, as aresult of these misgivings, there has not been (at least to thewriters’ knowledge) a large-scale demonstration or remedia-tion in which As concentrations in the wastes have been high.Treatment of As-containing wastes by S/S techniques for dis-posal in controlled landfill environments is being practiced(Conner and Lear 1992), and general interest in the technologyas applied to As is high, as evidenced by presentations anddiscussion at a USEPA workshop (1992).
Effective application of in situ S/S requires thorough char-acterization of the site hydrogeology and the soil and its con-tamination. The S/S procedure has the goals of producing awaste form that is less permeable than the soil surrounding itand converting hazardous constituents into forms that havelower toxicity and lower water solubility. Ease of mixing withthe binder as well as binder setting reactions, pore structure,and permeability are affected by water content in the zone to
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TABLE 1. Site Characteristics
Site number(1)
Setting(2)
In active use(3)
Water table depth(m)(4)
Shallow clayconfining layer
(5)
Variable soilconsistency
(6)
Location of highAs concentration
(7)Accessibility
(8)
1 Urban No 0.9–1.5 Yes No Shallow Difficult2 Urban No 1.1 Unknown No to 2 m Probably shallow Difficult3 Rural Yes >2.5 Yes No Variable Easy4 Rural Yes 0.3 No Somewhat Shallow, above 3 m Fairly easy
be remediated and clay/sand organic matter content of the soil.The presence of specific chemical species has major effects onbinder chemistry. For instance, there is voluminous literatureon cement accelerators and retarders. Redox potential of thesoil affects the chemical species of the waste present and maydetermine the choice of binder and additives.
In the present study, four sites, all containing sandy soil withlittle organic content, were selected for study and potentialremediation. A general description of the characteristics of thefour sites under consideration is given in Table 1. All the sitesare located in areas where the soils are generally sandy withlittle clay or organic content. This paper describes details ofthe site characterization. Because arsenic is the element of con-cern at these sites, particular attention has been placed uponfactors that could have effects on arsenic chemistry, and henceeventual leachability, such as redox potential and iron content.Most reported studies of soil remediation give few details ofsoil characterization; hence, the effects of soil characteristicson the success of S/S cannot be judged.
EXPERIMENTAL
Because preliminary investigations have shown that theprincipal contamination was present at relatively shallowdepths, soil sampling was done to a depth of 1 1/2–2 m, usinga hand auger. The soil samples were transferred to plastic-linedcanvas bags containing approximately 20 kg of soil each. Eachsampling, which spanned a depth of 0.15–0.3 m in the soilhorizon, consisted of approximately 40 kg of soil. For in-stance, in the data tables that follow, samplings are given codenames such as S1,1—6 to 12. In the code names, S1–S4refers to Sites 1–4, the next number refers to the location onthe site, and the final numbers after the dash give the depth ininches from which each sample was taken. Thus, samplingS1,1—6 to 12 refers to the two bags taken from Site 1 atLocation 1 and at a depth of 12–18 in. (0.15–0.3 m). Within72 h of sampling, the two bags from each sampling were com-bined and homogenized using ASTM procedures (D 2217-85).The homogenization was performed by four passes through aGeotest Instrument Corp. (Evanston, Ill.) 16-chute samplesplitter. The homogenized soils were stored at room tempera-ture (22 6 27C) in polyethylene containers with seal-tight plas-tic lids.
Standard methods used for soil characterization were pH(USEPA Method 9045B, revision 2, November 1992), mois-ture content (ASTM D 4959-89), percent organic matter(ASTM D 2974-87), specific gravity (ASTM D 854-92), andgrain-size distribution (ASTM D 422). Redox potential for thesoil was determined according to the method of Faulkner etal. (1989) using a Pt electrode and Ag/AgCl reference. Totalconstituent analysis of the soil was performed after acid di-gestion using USEPA Method 3050A followed by elementalanalysis (except for Hg and Se) using a Jarrell-Ash AtomCompdirect-reading inductively coupled argon plasma spectrometer.Selenium was determined by atomic absorption and mercurywas determined by cold vapor atomic absorption accordingto standard methods [American Public Health Association(APHA) 1989]. All USEPA methods referred to herein arecontained in USEPA (1997).
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J. Environ. Eng. 20
The TCLP was carried out in accordance with EPA Method1311, with the following exception: To carry out TCLP onlarge numbers of samples, one routinely uses 10 g of the sam-ple rather than 100 g and a correspondingly reduced quantityof leaching solution. The TCLP can use either of two extrac-tion fluids, depending on the alkalinity of the sample. For thesoils themselves, Fluid 1 is appropriate for determination ofwhether the soils could be considered hazardous waste. Be-cause the treated samples to be produced in this work willhave high alkalinity, Fluid 2 is appropriate. For direct com-parisons to be made between treated and untreated samples,TCLP on the soil samples was carried out using TCLP Fluid2 as well as Fluid 1.
Selected data from this testing is given in the ‘‘Results’’section, but each determination was carried out in duplicateon separate soil samples or separately prepared solidified sam-ples. The full data on these samples is available in an ElectricPower Research Institute (EPRI) report [TR-106700 (9049-01)] or from the writers. Under the conditions described, evenwith the reduced size of the TCLP samples, reproducibilitywas rarely worse than 65%.
The X-ray powder diffraction analyses were performed witha Scintag Inc. (Cupertino, Calif.) PADV automated X-ray dif-fractometer. The samples were crushed to a fine powder withporcelain and agate mortars and sieved through a 200-meshsieve. The X-ray powder diffraction analyses scan was madewith copper Ka radiation from 37 to 707 2u with 0.027 stepwidth, excitation voltage of 45 kV at 35 mA, and 1–3 s count-ing time.
RESULTS
Site 1 covers approximately 810 m2 and has a surficial aq-uifer consisting of a slightly silty sand to a depth of 3–5 mbelow the surface. A clay confining layer exists below the sandaquifer. The water table varies from 1 to 2 m below the groundsurface over a 12-month observation period. Preliminary soilsampling and analysis showed the total As concentrations tobe highest in the upper 0.3–0.6 m, ranging from 320 to 2,000mg/kg. At depths of 1.5–3.4 m below ground surface, the soilAs concentrations range from below detection limits (0.7 mg/kg) to 30 mg/kg. The clay confining layer (aquitard) has afines content from 50 to 90% (passes No. 200 sieve) and apermeability of approximately 1 3 10210 m/s.
Site 2 covers approximately 1,000 m2 and, to a 2.1-m depth,consists of fine-to-medium sand with small amounts of silt andlittle clay. The water table is located at about 1 m below thesurface. The soil As concentrations in the upper 1.4 m rangedfrom 180 to 700 mg/kg with no obvious trend of decreasingconcentration with depth.
Site 3 covers <400 m2 and consists of slightly silty fine-to-medium grain sand for a depth of 2.4–3.4 m below the sur-face, with a clay confining unit below this. The water table isconfined to the top of the clay layer and thus is at least 2.4 mbelow the ground surface. The permeability of the clay layeris approximately 5 3 10210 m/s. In the upper sandy soil zone,the As concentrations range from 100 to 1,800 mg/kg.
Site 4 covers approximately 1,000 m2 and is located be-tween a wooded area and a swamp. The soils are predomi-nantly silty sands with some organic matter found in the upper
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TABLE 2. Representative Characterization Data
Soil sample(1)
pH(2)
Total As(mg/kg)
(3)
Total Fe(mg/kg)
(4)
TCLP As(mg/kg)
(5)
Silt or clay(%)(6)
Moisture(%)(7)
Dry density(g/cm3)
(8)
S1,1—6 to 12 5.7 1,000 3,400 1.9 8 14 1.6S1,2—6 to 12 7.6 790 3,700 3.6 7 17 1.5S1,4—24 to 36 6.2 81 4,000 0.2 4 10 1.5S2,2—24 to 30 6.7 670 2,800 4.2 8 11 1.6S2,3—12 to 18 5.4 610 3,200 2.6 12 12 1.6S3,1—6 to 18 7.3 390 1,100 2.7 9 8 1.5S3,1—24 to 36 7.3 160 890 1.5 9 7 1.5S3,2—6 to 18 7.3 420 1,200 3.0 9 9 1.5S3,2—24 to 36 7.2 460 1,300 3.6 5 8 1.5S3,3—6 to 18 6.4 690 1,300 3.7 8 8 1.5S3,3—24 to 36 5.1 280 1,300 1.4 5 1 1.5S4,1—6 to 12 5.8 740 1,500 3.1 4 17 1.4S4,1—18 to 24 6.1 400 1,100 1.6 2 8 1.4S4,2—6 to 12 5.8 520 650 1.5 4 15 1.4S4,3—6 to 12 7.5 360 1,300 3.5 2 12 1.4
4.6 m and occasional clay-rich sands at depths of 8–12 mbelow ground surface. The water table is quite shallow, rang-ing from 0.3 to 0.6 m below ground surface. The total Asconcentrations are in the range of 500–1,900 mg/kg in theupper 0.6 m of the soil, diminishing to 10 mg/kg or less at4.6 m. The permeability of the sands is approximately 5 31025 m/s.
For the purposes of the treatability study to be describedlater, a total of 28 soil samplings were made at various loca-tions and several depths down to 1 m below the surface at thefour sites: 7 at Site 1, 7 at Site 2, 6 at Site 3, and 8 at Site 4.Representative data on the soil characteristics are given in Ta-ble 2.
The soil samples from the four sites are rather uniform ingeneral soil characteristics. They are all sandy soils with aminor clay fraction. The Site 2 soils had the largest silt/clayfraction, with samples ranging up to 14%. Site 4 soils had thesmallest clay fraction, with a high of only 4%. Full tabulationof grain-size distributions is available from the writers. As ex-pected from the small clay fraction, all of the soils were non-plastic. The total organic content was also low for the soils atall four sites. A maximum of 3% organic content was observedat Site 1, and the typical content was 1–2%. From the pointof view of cement-based S/S, these are favorable characteris-tics. The low clay content generally means that efficient mix-ing can be carried out and also means that the strong waterabsorbing characteristics of clays will not affect the wateravailable for cement hydration reactions. Furthermore, sand isa good aggregate for cement formulations. Organics com-monly show set-retarding influences on cement hydration, andthat complication should be absent here.
Moisture content in the soils was quite variable, being low-est at Site 3, where it ranged from 1 to 9%. The highest valueswere noted at Site 4, where the moisture content ranged from9 to 17%. Water-to-cement ratio has a major influence on theproperties of cured portland cement, notably on pore structureand permeability. For greatest strength and lowest permeabil-ity, the lowest practical water-to-cement ratio should ordinarilybe used. Because the binder will be added to the cement as aslurry, the water content of the slurry should be different fromsite to site.
The soils are all well-aerated top soils; hence, the redoxconditions were all oxidizing, with potentials in the 1400- to1500-mV range. That redox potential should ensure that thearsenic present in the soils will be in the As(V) oxidation state,not As(III). Arsenic(V) compounds are generally less toxicthan As(III), and the former has a wider range of insolublesalts. It is particularly noteworthy that calcium arsenate haslow water solubility, because the cement treatment adds large
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FIG. 1. Scanning Electron Micrograph of Soil S1,4—24 to 36,Magnification 253
quantities of calcium ion to the soils. Soil pH varied fromslightly basic (high of 7.8) to slightly acidic (low of 4.9), withmore samples being acidic than basic. Soil pH can certainlybe influential in determining the mobility of contaminants inwater. However, once remediation has been carried out withportland cement, the alkalinity of the hydrated cement willoverwhelm any soil acidity.
Arsenic was known to be present at all of the sites, but itwas not known whether other metals were presented in concen-trations that would affect the S/S strategy. In the soils used forthe treatability study, the highest concentrations observed in anysample in the total analyses were Ag (130 mg/kg), As (1,000mg/kg), Ba (65 mg/kg), Cd (11 mg/kg), Cr (14 mg/kg), Fe(7,100 mg/kg), Hg (0.2 mg/kg), Ni (5 mg/kg), Pb (100 mg/kg),and Se (<0.07 mg/kg). Preliminary small-scale sampling hadshown hot spots with arsenic concentrations as high as 2,000mg/kg. When the sampling was done for the treatability study,approximately 50-kg samples were obtained and homogenized.In these homogenized materials, the highest As concentrationwas 1,000 mg/kg. The TCLP leachates showed no examples ofmetal concentrations approaching the USEPA treatment stan-dards for hazardous wastes, except in the case of As. Even forarsenic, despite relatively high total arsenic concentrations,TCLP leachate concentrations (using extraction Fluid 1 in eachcase) did not exceed the limit of 5 mg/L. The observed TCLParsenic concentrations ranged from 0.16 to 3.6 mg/L at Site 1,from 0.8 to 4.2 mg/L at Site 2, from 1.4 to 3.7 mg/L at Site 3,and from 0.15 to 3.5 mg/L at Site 4. It is clear that some soil
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FIG. 3. Percent of Total Arsenic Extracted in TCLP versus IronContent of Soil
FIG. 2. Arsenic in TCLP Extracts Versus Total Arsenic in Soil
characteristics (see discussion of iron content below) result inlow arsenic leachability. Because of high As concentrations inthe soils and TCLP leachability that is in some cases onlyslightly below the TCLP limit, remediation of the soils is de-sirable; even minor changes in soil characteristics could resultin leachability above the TCLP limit.
Scanning electron microscopy was used to examine five ofthe soil samples prior to any treatment. All of the samplesexamined were very similar in appearance, as exemplified bythe micrograph shown in Fig. 1. Fig. 1 shows the soil referredto in Table 2 as S1,4—24 to 36 at low magnifications (253).Basically, the soil consists of coarse sand grains that are notclosely consolidated. At higher magnification the sand grainscan be seen to be coated with a fine layer of clay minerals,which can be identified by their platy nature. The sandy char-acteristic should make the soil a reasonable aggregate in thecement solidification. These pretreatment micrographs will becompared with posttreatment ones to give a visual estimate ofthe extent of consolidation, and hence lowered permeability,that results from treatment.
ARSENIC LEACHABILITY AS FUNCTION OF SOILIRON CONTENT
Arsenic is ubiquitous at low concentration in soils world-wide but is usually not readily available for leaching by water.This is due to a variety of factors, particularly the strong ad-sorption of As to aluminum, iron, and manganese componentsin soils (O’Neill 1990; Mok and Wai 1994). Fig. 2 shows aplot of the As concentration in the TCLP leachates versus theAs concentration in the soil, and a rough correlation is ob-served. The linear least-squares regression line is shown onthe plot, and the correlation coefficient r is 0.651. There is noreason to expect a simple relationship to exist between leach-ability and soil concentration, because the species to which Asis strongly sorbed are quite variable in concentration. One ofthe important sorbing elements has been examined in moredetail. Fig. 3 shows a plot of the percent of the total As in thesoil that is extracted in TCLP plotted against soil iron concen-tration. Fig. 3 shows data for Sites 1–3, and there is a generalrelationship that increasing iron concentration results in re-duced As leachability (r = 0.768). There is obvious scatter inthe plot, and the correlation coefficient is not high. But a high
degree of correlation cannot be expected, because the relation-ship between leachability and soil composition will certainlybe complex. Iron precipitated on sand grains will have a dif-ferent chemical composition depending upon the conditionsunder which precipitation took place, particularly pH and thepresence of other chemical species. The various iron oxidesand oxyhydroxides that may be formed under different con-ditions (hematite, maghemite, magnetite, goethite, lepidocro-cite, ferrihydrite, feroxyhite, and akaganeite) differ in iron ox-idation state, crystalline structure, and specific surface areaand, thus, differ greatly in sorptive capacity. In addition, thereare no doubt variations in aluminum and manganese concen-trations and mineral forms in the soils as well. Thus, it isperhaps surprising that the correlation observed here betweentotal iron concentration and leachability is as good as it is.Indeed, in the case of Site 4, the relationship between ironconcentration and As leachability is much less clear, with thepercent of As leached ranging from 0.1 to 1.0. The Site 4 soilshave lower iron concentrations than those from Sites 1 and 2,and the iron concentrations are more variable than for the soilsfrom Site 3.
CONCLUSIONS
The soils on all four sites represent a favorable case forS/S. Although arsenic concentrations range up to 2,000 ppmin the soil, leachability is relatively low. No TCLP leachatesshowed As concentrations as high as 5 mg/L, the USEPAbenchmark value that would characterize the material as a haz-ardous waste. The low leachability appears to be due, at leastin part, to the soil iron concentrations, and a rough relationshipexists between the percentage of As in the soil that is leachedand the iron concentration in the soil. Another factor workingin favor of the success of S/S is the sandy character of thesoils with little clay or organic content. Thus, the soil will actas a reasonable aggregate and should offer minimal interfer-ence to the in situ mixing process and subsequent cement hy-dration. A third favorable circumstance is afforded by the ox-idizing character of the soils. The weathered arsenic presentin the soils should be in the form of As(V), and arsenate saltspresent a wider range of possibilities for precipitation of in-soluble arsenic species than arsenite salts.
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All four sites contain arsenic contamination at comparableconcentration levels, confined mainly to the top 1.2 m of thesoil. Although soil arsenic concentrations are relatively high,leachability is low, and iron content in the soils appears to bean important factor in determining leachability. All the siteshave sandy soils with little clay or organic matter. A significantvariable is the depth to the water table and hence the moisturecontent. The eventual decision was to carry out remediationsat Sites 3 and 4, one with a near-surface water table and theother considerably lower and therefore drier. The two sitesvaried also in that Site 3 had a clay confining layer at a depthof approximately 4 m, whereas Site 4 had only occasionalclay-rich sands down to 12 m. Both sites were in rural settingsthat provided easy accessibility. The decision on which sitesto remediate was not made, however, until after the treatabilitystudy phase of the work was nearly completed; thus, soils fromall four sites were included in that study.
ACKNOWLEDGMENTSThe writers wish to thank the EPRI for partial support of this work.
A more complete description of the work is available in EPRI report TR-106700 (9049-01).
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APPENDIX. REFERENCES
American Public Health Association (APHA). 1989. Standard methodsfor the examination of water and wastewater, 17th Ed., Washington,D.C., 3-29–3-31.
Code of federal regulations. (1997). 40 CFR 268.40, July 2.Conner, J. R., and Lear, P. R. (1992). ‘‘Treatment of landban-varianced
arsenic wastes at TSDFs.’’ Proc., Workshop on Removal, Recovery,Treatment and Disposal of Arsenic and Mercury, 51–55.
Faulkner, S. P., Patrick, W. H., Jr., and Gambrell, R. P. (1989). ‘‘Fieldtechnique for measuring wetland soil parameters.’’ Soil Sci. Soc. Am.J., 53, 883–890.
Federal Register. (1990). 55(No. 106; June 1), 22556–22561.Mok, W. M., and Wai, C. M. (1994). ‘‘Mobilization of arsenic in contam-
inated river waters.’’ Arsenic in the environment, Part I, J. R. Nriagu,ed., Wiley, New York, 99–117.
O’Neill, P. (1990). ‘‘Arsenic.’’ Heavy metals in soils, B. J. Alloway, ed.,Halsted Press, New York.
U.S. Environmental Protection Agency (USEPA). (1992). Proc., Work-shop on Removal, Recovery, Treatment and Disposal of Arsenic andMercury., USEPA Rep., EPA/600/R-92/105, USEPA, Washington, D.C.
U.S. Environmental Protection Agency (USEPA). (1997). Test methodsfor evaluating solid wastes, physical/chemical methods (SW846), 3rdEd., Washington, D.C.
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