treatment of arsenic-contaminated soils. ii: treatability study and remediation

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TREATMENT OF ARSENIC-CONTAMINATED SOILS.II: TREATABILITY STUDY AND REMEDIATION

By Joel Miller,1 Member, ASCE, Humayoun Akhter,2 Frank K. Cartledge,3

and Mary McLearn4

ABSTRACT: Treatment of sandy soils contaminated with arsenic was investigated at a bench scale and carriedthrough to remediation in the field. The initial treatability study looked at many combinations of cement bindersand reagents. Salts of iron, barium, manganese, and magnesium were generally effective at reducing arsenicleachability. The most consistently low potential for leaching [toxicity characteristic leaching procedure (TCLP)and a modified version of the American Nuclear Society’s ANS16.1] was observed when the soils were treatedwith a mixture of Type I portland cement and ferrous sulfate. For instance, the average of arsenic concentrationsin TCLP leachates in many treated soil samples from four sites was 0.26 mg/L. Better protection against leachingwas observed when the soil was pretreated with FeSO4 ?7H2O, then with portland cement. In addition to chemicalcontainment, the mixture should prevent ground-water leaching by physical entrapment, because the perme-abilities of the treated soils were in the range of 1029–10210 m/s. Scanning electron microscope micrographsshowed a dense mass with minimal void space, and a combination of X-ray diffraction, thermal analysis, andsolid-state nuclear magnetic resonance (NMR) spectroscopy indicated the formation of a normal hydrated cementmatrix, with some excess ettringite being present due to the extra sulfate being added to the formulation. Resultsfrom the bench-scale treatability study were reproduced very faithfully in the field, with permeabilities andcompressive strengths being similar to those observed in the laboratory and TCLP leachability being even lowerthan predicted by the laboratory study.

INTRODUCTION

The objective of the current work is to determine conditionsfor optimization of cement-based solidification/stabilization(S/S) for soils contaminated with arsenic and to demonstratethe capability of transferring that optimization from the labo-ratory to field application. A companion paper (Akhter et al.2000) described soil characterization at four industrial siteswith soils containing arsenic. The contamination ranges up to2,000 ppm of As, mainly in the upper 4 ft of soil, due totopical applications of arsenic(III) oxide as a herbicide over along period. This paper describes soil treatment using S/S,including a preliminary treatability study, optimization on alarger scale, then actual remediation and subsequent testing.

There has been much interest in application of S/S to in-organic arsenic wastes, and variable results have been reported(Conner 1990). The chemistry of arsenic as it applies to S/S,particularly including an extensive discussion of the solubili-ties of arsenic salts, has been discussed in a recent report (Tay-lor and Fuessle 1994). A common strategy employed to opti-mize S/S is to identify the toxic species of greatest interest inthe material, then arrange for the presence of appropriate coun-terions in the stabilized product that will minimize the solu-bility of the toxic species.

Early work on S/S of arsenic-containing media has beensummarized by Conner (1990), who reported leachate resultsfrom a variety of As-containing materials subjected to S/S witha number of binders. When the As concentrations in the ma-

1Proj. Engr., Southern Company Services, 42 Inverness Ctr. Pkwy.,Birmingham, AL 35242.

2Sr. Res. Sci., Avlon, Inc., 5401 West 65th St., Bedford Park, IL 60638.3Prof., Dept. of Chem., Louisiana State Univ., Baton Rouge, LA

70803-1804 (corresponding author). E-mail: [email protected]

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 0733-9372/00/0011-1004–1012/$8.00 1 $.50 per page. Paper No. 21754.

URNAL OF ENVIRONMENTAL ENGINEERING / NOVEMBER 20

J. Environ. Eng. 200

terial are high (>635 mg/kg), leachate concentrations often canbe brought below 5 mg/L but usually not below drinking waterlimits, 0.05 mg/L. Treatment sometimes actually increases theleachability, presumably because of different speciation underbasic conditions. Similar results have been reported for othertoxicity characteristic leaching procedure (TCLP) leachingstudies, and column and long-term equilibrium batch leachingexperiments indicate that As leaches at an elevated rate com-pared to most heavy cations (Cote et al. 1985; Cote and Con-stable 1987; Bricka et al. 1988; Erickson 1992).

Previous work on contaminated soils (Akhter et al. 1990)and reports of immobilization of various arsenic-containingmaterials [S. R. Thompson, U.S. Patent No. 3,980,558 (1976);D. A. Young, ‘‘Landfill material,’’ U.S. Patent No. 4,142,912(1979); Kyle and Lunt 1991] have shown the potential fortreatment of arsenic with portland cement. Work with a varietyof binders, including portland cement, fly ash, lime and blastfurnace slag, has shown leachability to be consistently lowestwith portland cement as the binder. Furthermore, prior workhas shown that the presence of arsenic compounds promoteslong-term changes in pozzolanic matrices containing fly ashand that these changes are accompanied by increased arsenicleachability (Akhter et al. 1997). Consequently, the treatabilitystudies in the present study have concentrated almost exclu-sively on use of portland cement as the binder. The significantexception to that is due to several reports of good performanceof silicates in stabilization of arsenic [U.S. Environmental Pro-tection Agency (USEPA); Chu et al. 1991]. Hence some for-mulations containing soluble silicates added to cement wereincluded in the current treatability studies.

Iron salts have a strong affinity for arsenic. They are used,for instance, in processes to remove As from water (Manzioneet al. 1990; Maeda et al. 1992). Calcium forms a very insol-uble salt with arsenate, as do other alkaline earth metal cations(magnesium and barium) and manganese (Mn21). Indeed, alu-minum, calcium, iron, and manganese mineral components ofsoils are the species that are usually responsible for the oftenobserved strong binding of arsenic to soils (Bhumbla and Kee-fer 1994; Yan-Chu 1994). These cations have been proposedfor use in various arsenic treatment schemes [D. A. Young,U.S. Patent No. 4,142,912 (1979); L. Zvolanek et al., ‘‘Neu-tralization and treatment of arsenic-containing waste,’’ Czech-

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oslovakia Patent No. 275,668 (1992); R. R. Stanforth andA. K. Chowdhury, ‘‘In situ method for decreasing heavy metalleaching from soil or waste,’’ U.S. Patent No. 5,202,033(1993); J. N. Stark, ‘‘Cementitious encapsulation of waste ma-terials and/or contaminated soils containing heavy metals torender them immobile,’’ U.S. Patent No. 5,276,255 (1994);Taylor and Fuessle 1994]. Taylor and Fuessle (1994) includeda discussion of the relative merits of Fe(III) and Fe(II) as sta-bilizing agents for arsenate. They noted that the extremely lowsolubility of Fe(OH)3 means that in basic media a ferric ar-senate should be expected to be solubilized in favor of pre-cipitation of ferric hydroxide. The greater solubility of ferroushydroxide compared to ferric hydroxide should not create asgreat a problem in the case of precipitation of the arsenate saltwith Fe(II). Indeed, large-scale site remediations involving ar-senic soil and ground-water contamination have been carriedout with ferrous sulfate (Paulson and Schnettgoecke 1992;USEPA 1998).

The work in the present paper is in three stages:

1. A treatability study was carried out using small samplesto select the optimum set of binding agents for S/S. Alarge number of samples were subjected to TCLP leach-ing and the results compared.

2. The second stage used larger samples to determine thesuccess of scale-up and the effects of mixing conditionson the leachability results. The larger-scale samples werecharacterized in a more thorough manner to determinevarious parameters that predict the success of the S/Streatment in limiting leachability and producing a dura-ble solid product. A major concern is that the cementhydration reactions have proceeded in the expected man-ner to produce a dense and durable matrix. Hence, anumber of techniques that identify the products of ce-ment hydration were included.

3. The third stage was full-scale site remediation to deter-mine how well the laboratory results were replicated inthe field.

EXPERIMENTAL

Materials and Sample Preparation

The cements used were Type I portland supplied by RiverCement Co., St. Louis, Mo., and Type V from Lehigh CementCo., Waco, Tex. The sodium silicate was Type N from PQCorp., Valley Forge, Pa. Attapulgite clay was supplied by Flor-idin Co., Quincy, Fla. Other materials were reagent gradechemicals. The following hydrated salts were used, althoughthe tables contain only the unhydrated formula: FeSO4 ?7H2O,Fe2(SO4)3 ?5H2O, FeCl3 ?6H2O, FeCl2 ?4H2O, MnCl2 ?4H2O,BaCl2 ?2H2O, MgCl2 ?6H2O, and MgSO4 ?7H2O.

The treatability study was carried out in two stages. Initially,a large number of samples were prepared using a variety ofreagents along with Type I portland cement. To handle thelarge number, individual sample size was kept small. Sampleswere prepared using 10.0 g of soil in a 20-mL glass screw-cap vial and mixing by hand with a glass stirring rod. In thesecond stage of the treatability study, preparation of solidifiedsamples on a larger scale was carried out, followed by testingof a range of properties. The samples were prepared in akitchen-sized Hobart mixer under stirring at low speed. Ap-proximately 3 kg of soil was used in each run, and the mixedmaterial was transferred to molds appropriate for the varioustest protocols. In these preparations, water was added in suf-ficient quantities to obtain a slurry with plastic properties thatled to good mixing, as judged visually. The soil/water ratio islower for these samples than for the small-scale ones in thefirst stage of the treatability study. Using the minimum amount

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J. Environ. Eng. 20

of water yields more desirable matrix properties, particularlyincreased density and lower permeability. In both stages of thestudy, each mix was prepared in duplicate. When a reagent-to-arsenic molar ratio is quoted in the tables, that molar ratiowas calculated based on the soil sample that had the highestarsenic concentration. Thus, when an Fe/As molar ratio of 2is quoted, that is the molar ratio for the S1,1—6 to 12 sample.(The sample identifications have the following meaning: S1–S4 refer to Sites 1–4, the next number identifies the locationon the site; and the final numbers after the dash give the depthin inches from which the sample was taken.) The remainingsamples using other soils employed the same soil-to-FeSO4

weight ratio; hence, the Fe/As molar ratio varies somewhat asthe As content varies.

TCLP

The TCLP was carried out as described in USEPA (1996),with the exception that the small samples were extracted witha correspondingly reduced volume of leaching fluid. The datareported in Table 1 include the arsenic concentration in theTCLP leachates from the two samples and the average of thetwo duplicates. The quantitation limit for the instrument beingused (inductively coupled plasma spectrometer) is 0.035 mg/L. The percent retention of arsenic in the formulation wascalculated as 100% 2 [(average As in TCLP of treated sample/average As in TCLP of untreated soil) 100%]. The USEPAprocedures for TCLP specify the use of two different extrac-tion fluids, with the choice based on sample alkalinity. Ordi-narily (and always in the present study) the soils will be ex-tracted with Fluid 1 (a buffered solution of acetic acid andsodium acetate), whereas cement-solidified samples will be ex-tracted with Fluid 2 (a nonbuffered acetic acid solution). Tomake a fair comparison, the percent retention data are calcu-lated using data obtained with the same extraction fluid,namely, the one appropriate for the solidified samples (Fluid2). Percent retention of arsenic calculated in this manner is afair representation of the performance of the treated sample.However, it is common in S/S practice to compare the totalamount of arsenic leached in a TCLP determination to the totalamount of arsenic in the soil before treatment. Such a calcu-lation will give a considerably higher value for the percentretention. No manner of calculating a percent retention is com-pletely meaningful. As noted below, a portion of the As in theleachates comes from the reagents used for solidification. Inthese calculations, that portion is included in what is leachedfrom the sample. Once the sample is prepared, it is not pos-sible, of course, to distinguish the As in the leachate that camefrom the soil from that which came from the binders. But thepercent retention calculation is based only on total As leachedversus As leached from untreated soil.

Chemical analysis of the extracts was carried out by induc-tively coupled plasma atomic emission spectroscopy or atomicabsorption, as described in Akhter et al. (2000).

Other Physical Tests

In addition to the TCLP leach testing, a ‘‘dynamic’’ leachingtest was carried out on the larger-scale solidified samples. Thetest is a modification of the American Nuclear Society’sANS16.1 leaching protocol. The modification is that recom-mended by Environment Canada, Burlington, Ontario (Stege-mann 1991), and the purpose of the modification is to allowa better chance for analytical detection of hazardous constit-uents of interest. The analytical portion of the ANS16.1 pro-tocol uses radioactivity for detection purposes, and it is pos-sible to quantify radioactivity to much lower detection limitsthan is possible for individual chemical compounds in a po-tentially complex mixture of hazardous constituents. In the dy-

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namic leach test, a 4.5-cm-diameter 3 7.4-cm-high cylinderof the solidified material is suspended in 1,400 mL of distilled,deionized water, which acts as the leachant. At specified in-tervals over a period of 9 days, the leachant is removed andanalyzed and fresh leachant is added. Two leachant renewalschedules are specified in the protocol, and both scheduleswere followed in the present work. Some of the results fordynamic leaching, as well as those for hydraulic conductivity,reported here were obtained by Environment Canada’s Waste-water Technology Centre, Burlington, Ontario.

Acid neutralization capacity (the number of equivalents ofmineral acid required to bring the final pH of the crushedmatrix below pH 9), wet/dry weathering (weight loss throughup to a dozen cycles of wetting, then drying under vacuum),freeze/thaw weathering (weight loss through up to a dozencycles of freezing, then thawing), and hydraulic conductivitywere tested according to the protocols of Environment Canada(Stegemann 1991).

Spectroscopy and Microscopy

A JEOL (Akishima, Japan) 860 scanning electron micro-scope was used in this study. The loose soils were sprinkledon 10-mm-diameter aluminum stubs. The S/S products werebroken into small millimeter-size fragments with a hammerand mounted on aluminum stubs. All samples were coated

1006 / JOURNAL OF ENVIRONMENTAL ENGINEERING / NOVEMBER 20


with gold. Observations were made at 20 keV. The micro-graphs shown in this report were acquired digitally.

The X-ray diffractometry runs were made with a ScintagInc. (Cupertino, Calif.) PAD-V automated diffractometer. Theruns were made from 27 to 707 2u with a step width of 0.027using CuKa radiation. The excitation voltage was 45 kV at 35mA. Counting time was 3 s.

Specimens were characterized using 27Al and 29Si high res-olution solid-state magic angle spinning NMR. The solidifiedsample was crushed using a mortar and pestle and then sievedthrough a 100-mesh sieve. The 29Si solid-state NMR studieswere performed on a 200-MHz (4.7-Tesla) Bruker Inc. (Bille-rica, Mass.) MSL-200 wide-bore NMR spectrometer. A 5-ms907 pulse with a relaxation delay of 5 s was used to acquirethe spectra at a resonance frequency of 39.7 MHz. The spectrawere obtained using about 300 mg of the sample in a zirconiarotor spinning at 5 KHz and are reported relative to externaltetramethylsilane. Enough scans were acquired to obtain a sig-nal-to-noise ratio of 40. The 27Al spectra were obtained on a200-MHz Bruker AC-200 spectrometer using a ChemagneticsInc. (Fort Collins, Colo.) solid-state probe. At 3-ms 307 pulsewith a relaxation delay of 0.02 s was used to acquire the spec-tra at a resonance frequency of 52.1 MHz. The spectra wereobtained using about 150 mg of the sample in Torlon rotorsspinning at 7 KHz and are reported relative to external aque-ous AlCl3. In both Si and Al spectra, the peaks were integrated

TABLE 1. TCLP Leachability of Stabilized Soil Samples

Soil/ordinary portlandcement (OPC)

weight ratio(1)

Additive(2)

Additive/Asmole ratio

(3)

As in TCLP(mg/L, average 6 sn)

(4)

Retentiona

(%)(5)

Leachate(average pH)

(6)

10 None — 0.188 6 0.0015 91.2 5.45 None — 0.172 6 0.013 91.9 11.44 None — 0.200 6 0.0025 90.6 11.5

10 Na2SiO3 20b 0.550 6 0.068 74.2 5.710 K2SiO3 20b 0.253 6 0.0045 88.1 5.310 H2O2 3 0.212 6 0.012 90.0 5.4

5 H2O2 30 0.265 6 0.002 87.6 10.610 FeSO4 2 0.202 6 0.0035 90.5 5.3

5 FeSO4 2 0.253 6 0.011 88.1 11.310 FeSO4 5 0.219 6 0.0035 89.7 5.2

5 FeSO4 5 0.231 6 0.022 89.1 10.910 FeSO4 20 0.674 6 0.054 68.3 5.2

5 FeSO4 20 0.188 6 0.025 91.2 7.010 FeSO4 25 0.655 6 0.06 69.2 4.8

5 FeSO4 25 0.187 6 0.024 91.2 5.710 FeCl2 5 0.186 6 0.012 91.3 5.3

5 FeCl2 5 0.191 6 0.022 91.0 11.010 FeCl2 20 0.181 6 0.014 91.5 5.110 Fe2(SO4)3 5 0.280 6 0.016 86.8 5.2

5 Fe2(SO4)3 5 0.152 6 0.01 92.9 11.210 Fe2(SO4)3 10 0.964 6 0.68 54.7 5.1

5 Fe2(SO4)3 10 0.255 6 0.084 88.0 8.610 Fe2(SO4)3 20 0.440 6 0.024 79.3 4.8

5 Fe2(SO4)3 20 0.145 6 0.015 93.2 7.610 FeCl3 5 0.184 6 0.0015 91.4 5.2

5 FeCl3 5 0.192 6 0.034 91.0 10.710 MnCl2 5 0.219 6 0.014 89.7 5.5

5 MnCl2 5 0.181 6 0.011 91.5 8.310 BaCl2 5 0.199 6 0.001 90.7 5.3

5 BaCl2 5 0.179 6 0.021 91.6 11.310 BaCl2 20 0.220 6 0.006 89.7 5.210 Attapulgite 20b 0.190 6 0.004 91.1 5.510 Rice husk ash 20b 0.211 6 0 90.1 5.310c None — 0.248 6 0.004 88.4 5.5

5c None — 0.161 6 0.002 92.4 11.110c Bentonite 20b 0.218 6 0.001 89.8 5.2

Note: All samples prepared from S3,1—24 to 36 and cured for 60 days.aComparing TCLP of treated soil with TCLP of untreated soil using same extraction fluid (TCLP Fluid 2).bSoil/additive ratio by weight.cType V, sulfate-resisting, low aluminate cement.

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TABLE 2. Arsenic in TCLP (Average of Two Runs)—Comparison of Several Formulations

Mixa

(1)S1,1—6 to 12

(2)S1,1—24 to 36

(3)S1,2—6 to 12

(4)S1,3—12 to 24

(5)S3,1—24 to 36

(6)S3,3—6 to 18

(7)S4,1—6 to 12

(8)

OPC alone 0.719 0.467 0.351 1.018 0.259 0.662 1.768FeSO4 (2) 0.365 0.342 0.293 0.469 0.247 0.513 0.896FeSO4 (5) 0.525 0.275 0.290 0.364 0.265 0.407 0.382Fe2(SO4)3 (5) 0.488 0.367 0.227 0.246 0.207 0.556 0.439

Note: All formulations use 10/1 soil/OPC ratio and 7 days of cure.aNumber in parentheses gives additive/arsenic ratio (see text).

using the window integration method and Bruker software.From many determinations using duplicate and triplicate sam-ples prepared in separate vials, integrations were reproducibleto 64%.

RESULTS AND DISCUSSION

Small-Scale Treatability Study

A large number of samples, each prepared from 10 g of soil,were cured for 7 to 60 days, then crushed and extracted ac-cording to TCLP. The arsenic leachabilities and leachate pHvalues are reported in full in the Electric Power Research In-stitute’s (EPRI’s) report referred to in the acknowledgment,and representative data are shown in Tables 1 and 2. The var-iables investigated were the nature of the binder, soil-to-binderratio, nature of the stabilizing additive, and additive-to-arsenicmole ratio. The soil-to-added-water ratio was kept constant at0.3, to the extent possible. However, when larger amounts ofsolid additive were included in the formulation, the soil-to-water ratio had to be increased, in some cases up to 0.6, toallow satisfactory mixing.

The principal aim of the initial studies was to optimize per-formance with respect to leachability, because that is the mat-ter of most environmental concern and is the variable that isgenerally hardest to control in S/S practice. The binder usedin most of the small-scale studies was Type I portland cement,for reasons noted above. For many of the sample preparations,two proportions of binder were used: 10:1 and 5:1 soil-to-cement ratios. There were differences in the leachabilities inmany cases between the samples with higher and lower cementproportions, but there was not consistency that either propor-tion gave better performance. In many cases, the two propor-tions gave very similar TCLP leachability. Because the lowerbinder-to-soil ratio gave satisfactory strength, the larger-scalestudies carried out in the second round used the 10:1 soil-to-cement ratio.

Several binders in addition to Type I portland cement alonewere investigated. This category includes Type V (low alu-minate, sulfate-resisting) portland cement, and sodium and po-tassium silicates, which alter the cement matrix. These bindermixes show poorer performance at 60 days of cure than port-land cement alone. Silicates have been recommended by sev-eral research groups investigating immobilization (Chu et al.1991; Bates et al. 1992), but these results show reduced per-formance compared to portland cement alone. Actually, sili-cates vary substantially in structure, and much of the literatureabout their use does not specify crucial variables, such as thesodium-to-silicon ratio of a soluble silicate. Hence, it is per-haps not surprising that silicates might show good perfor-mance in one study but not in another. Adsorptive additiveslike bentonite and attapulgite clays, Zeomix (a commercial ze-olite-based adsorbent), and rice husk ash show some promise,particularly the clays. Of the two clays, attapulgite gives moreconsistent improvement of performance compared to portlandcement alone than does bentonite.

Because the soils all show an oxidizing redox potential(Akhter et al. 2000), it is to be expected that the arsenic will

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J. Environ. Eng. 20

be almost exclusively in the 15 oxidation state (i.e., arsenatenot arsenite). Two sets of samples were prepared using hydro-gen peroxide as an oxidizing additive. The 60-day samples,after pretreatment with hydrogen peroxide and cement solidi-fication, showed much the same leachability with and withoutperoxide addition, or sometimes greater leachability with per-oxide. Given these results, oxidizing additives were not inves-tigated further.

Salts of barium, iron, magnesium, and manganese, eitheralone or in combinations, all showed the ability to decreaseleachability compared to treatment with cement alone, but notconsistently in every formulation. The results varied with thesoil/cement and additive/arsenic ratios. From the results ob-tained, there does not seem to be a compelling reason tochoose barium, magnesium, or manganese salts over the morereadily available and cheaper iron salts. As noted above, bothFe21 and Fe31 salts have been used in arsenic treatmentschemes. Both show some effectiveness in the present studies,but Fe21 salts show more consistent performance as soil/ce-ment and additive/arsenic ratios are varied. This conclusion isbeing drawn from the data generated at all four of the inves-tigated sites where the soils are very similar in character. Table2 shows a representative comparison of formulations fromSites 1, 3, and 4. Both ferrous and ferric sulfate perform con-sistently better than portland cement alone in the present work.Furthermore, there is a significant body of information fromother bench-scale studies, as well as from arsenic treatment inthe field, in which these salts have been used. Ferrous sulfatewas chosen over ferric sulfate for further larger-scale studies,because FeSO4 showed results indicating less variation inleachability as a function of the soil/cement and additive/ar-senic ratios. Thus, ferrous sulfate should be more reliable inthe field, where there is less precise control over those ratios.

The average of two pH determinations on the two leachatesalso is reported in the tables. The two pH measurements or-dinarily differed by no more than 0.2-pH units, but an occa-sional pair of duplicates differed by as much as 1.6-pH units.The leachabilities of the solidified samples do not show thekind of correlation that is familiar from S/S treatment of cat-ionic heavy metals, where basic pH is correlated with lowleachability. It was a premise of this work from the outset thatcontrolling leachate pH would not be sufficient for immobili-zation of arsenic. It is desirable to have low solubility for saltsthat do not show substantial pH dependence. Hence, for leach-ates with low As concentration, the pH is sometimes high andsometimes low, mostly depending, of course, on the proportionof cement in the mix.

Most of the initial set of formulations in the 7-day leachingtests were tested also after 60 days of cure, because effectsassociated with curing time are important in S/S practice. Al-though it may take years to approach nearly complete hydra-tion, the 60-day samples should represent a considerably fur-ther advanced hydration, as compared to 7 days. There are,indeed, some differences in leachabilities in the 60-day sam-ples compared with the 7-day samples, but generally thoseformulations that did well at 7 days also did well at 60 days.Although there are exceptions, it is commonly observed in thiswork and in S/S practice in general that increasing cure time


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results in decreasing leachability. For instance, the followingarsenic concentrations in TCLP leachates have been observedfor soil S1,1—6 to 12 treated with portland cement alone ata 10:1 soil/cement ratio: 7-day cure, 1.175 mg/L; 28-day cure,0.677 mg/L; 60-day cure, 0.417 mg/L; and 120-day cure,0.377 mg/L. It is reasonable to expect such an observation,because cement hydration reactions are slow, and in a typicalcement paste the reactions are only about 65%–75% completeafter 28 days of cure. The increasing degree of hydration in aportland cement generally is associated with increasingstrength and density, and decreasing permeability, and thesefactors should produce more effective stabilization.

Larger-Scale Treatability Study

The second round of the treatability study concentrated oniron salts (but included a few other combinations for compar-ison). Again, the Fe31 salts showed less consistent performancethan Fe21 salts. For instance, in one case using the S1,1—24to 36 soil, leachability actually increased in the treated soilcompared to the untreated. The BaCl2 addition showed con-sistently good performance, as did FeCl2. Using FeSO4, treat-ment of the soil with the iron salt for a period of 5–7 daysprior to addition of the portland cement consistently improved



performance. This phenomenon was investigated further withlarger-scale samples, as shown in Table 3 for the three Site 1soils showing the greatest leachability. Treatment with FeSO4

alone, then leaching after 7 days, showed a significant decreasein leachability compared to the soil alone. But addition of port-land cement further decreases the leachability.

Table 4 presents data on the larger S/S samples preparedwith portland cement and ferrous sulfate using soils from Site3 (data from the other three sites are available in the reportnoted in ‘‘Acknowledgments’’). In the discussion that follows,references to maximum, minimum, and average values of thevarious properties refer to all of the samples, not just thoserepresentative ones shown in Table 4. The TCLP leachabilityresults for arsenic were determined after 7 and 28 days of cure.Almost all of the mixes show improvements at 28 days com-pared to 7 days, in line with expectations from prior work onthese and other soil samples. For the 26 samples from foursites for which 28-day TCLP leachabilities were determined,the average is 0.255 mg/L. The highest single value is 0.501mg/L at S2,2—24 to 36, and that is probably due to the Run1 outlier measuring 0.614 mg/L. On average, about half of theAs is apparently coming from the binding agents, because the‘‘reagent blank’’ shows 0.124 mg/L compared to the averageleachability of 0.255 mg/L. Hence, with any cement-based

TABLE 4. Properties of Stabilized Site 3 Samples Using Iron(II) Sulfate, Fe/As = 2

Soil sample(As in soil

TCLP)(1)

7 Day TCLPa

Leachate(pH)(2)

As inTCLP(mg/L)

(3)

28 Day TCLPb

Leachate(pH)(4)

As inTCLP(mg/L)

(5)

28-Dayunconfined

compressivestrength(MPa)

(6)

28-daybulk

density(g/cm3)

(7)

Acidneutral’ncapacityc

(8)

Leach-abilityindex

(9)

Hydraulicconductivityd

(m/s)(10)

Wet/drye

(%)(11)

Freeze/thawe

(%)(12)

S3,1—6 to 18 (2.7 mg/kg) 5.5 0.242 5.6 0.208 4.8 1.97 — —f — — —S3,1—24 to 36 (1.5 mg/kg) 5.5 0.222 5.8 0.183 6.0 1.94 4.0 12.5d 1.6 3 1029 0.6 0.8S3,2—6 to 18 (3.0 mg/kg) 5.5 0.252 5.7 0.228 5.1 1.90 4.0 —f — 0.5 0.9S3,2—24 to 36 (3.6 mg/kg) 5.5 0.260 5.6 0.236 5.9 1.92 — —f — — —S3,3—6 to 18 (3.7 mg/kg) 5.4 0.403 5.6 0.292 5.6 1.97 — —f 1.2 3 1029 — —S3,3—24 to 36 (1.4 mg/kg) 5.4 0.222 5.9 0.226 6.4 1.98 — —f — — —

Note: Samples were prepared using 3,000 g of soil, 300 g of OPC, 16.5 g of FeSO4 ?7H20, and 400 mL of water. Iron/arsenic mole ratio is approximately2 for S3,3—6 to 18 soil. Same iron/soil ratio was used throughout; hence, iron/arsenic ratio varies. Iron was mixed with soil 5 days before cement andwater were added. Reported values of pH, As concentrations, unconfined compressive strength, density, and hydraulic conductivities are averages of tworuns.

a‘‘Reagent blank’’ from TCLP carried out with combined reagents, but not including soil was 0.124 mg/L.bTwenty-eight day samples were prepared by mixing soil with Fe salt and water, adding OPC 5 days later, then allowing 28 days for cure. ‘‘Reagent

blank’’ from TCLP carried out with combined reagents, but not including soil was 0.104 mg/L.cReported as equivalents of acid required to bring pH of solidified sample <9.0.dLeachability index and hydraulic conductivities were measured at Environment Canada’s Wastewater Technology Centre.eWet/dry and freeze/thaw results are reported as percent relative weight loss after 12 cycles.fUsing ANS16.1 procedure, multiple samples gave As concentration below detection limit of 0.035 mg/L; hence, no valid leachability index can be

derived.

TABLE 3. Properties of Stabilized Site 1 Samples Using Iron(II) Sulfate

Soil sample(soil TCLP As)

(1)

7-Day TCLPNo OPCa

Water/soil(weightratio)(2)

Leachate(pH)(3)

As in TCLP(mg/L)

(4)

7-Day TCLPFe, Then OPCb


Leachate(pH)(6)

As in TCLP(mg/L)

(7)

7-Day TCLPFe 1 OPCc


Leachate(pH)(9)

As inTCLP(mg/L)(10)

S1,1—6 to 12 (1.9 mg/kg) 0.069 3.1 1.160 0.069 5.8 0.365 0.069 5.4 0.719S1,1—24 to 36 (2.7 mg/kg) 0.069 3.1 1.659 0.069 6.9 0.342 0.079 5.5 0.467S1,2—6 to 12 (3.6 mg/kg) 0.069 3.0 0.950 0.069 5.6 0.293 0.063 5.3 0.351

Note: Samples were prepared using 2,520 g of soil, 252 g of OPC, 13.9 g of FeSO4 ?7H2O, and variable amount of water. Iron/arsenic mole ratio is2.0 for S1,1—6 to 12 soil. Same iron/soil weight ratio was used throughout; hence, iron/arsenic ratio varies as soil As concentration varies. Reportedvalues of pH and As concentration are averages of two runs.

aNo cement added. ‘‘Reagent blank’’ from TCLP carried out with combined reagents, but not including soil was 0.113 mg/L.bFe mixed with soil, OPC added 5 days later, then cured 7 days. ‘‘Reagent blank’’ from TCLP carried out with combined reagents, but not including

soil was 0.123 mg/L.cFe and OPC added at same time, then cured for 7 days. ‘‘Reagent blank’’ from TCLP carried out with combined reagents, but not including soil was

0.224 mg/L.

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S/S system, it seems unlikely that one will be able to do con-sistently better than that observed with the FeSO4 /cement sam-ples. These TCLP results are on average about 20 times lowerthan the USEPA TCLP characteristic 5.0-mg/L limit for As.Given the fact that the binders leach As under TCLP condi-tions, it is unlikely that leachate concentrations lower than thedrinking water limit for As (0.05 mg/L) can be achieved. How-ever, long curing times may approach that limit more closely.Indeed, some examples have already been observed in whichthe As concentration in the TCLP leachate for the treated sam-ple is lower than the corresponding reagent blank; i.e., the soil/binder system shows lower leachability than either the soil orthe binder separately.

As described in the EPRI report referred to in ‘‘Acknowl-edgments,’’ many large-scale samples were prepared usingType I portland cement with and without FeSO4. The TCLPresults are clearly better for the samples containing FeSO4,both at 7 and 28 days. The differences are more significant forthose soils containing the higher concentrations of arsenic. Thedata also show a consistent trend of decreasing leachabilitywith increasing cure time. For the soils with lower arsenicconcentrations, both binder systems work well, particularly af-ter 28 days, when the arsenic concentrations in the TCLPleachates are approximately equal to the expected level of ar-senic coming from the binder alone.

Physical Properties

A dynamic leach test was used to allow the calculation ofan effective diffusion coefficient for hazardous constituentswithin the solidified sample. The leachability index, which isthe negative logarithm of the effective diffusion coefficient De

is reported in Table 4. In the present work De could not becalculated in most cases from the data obtained, because mul-tiple leaching solution samples during the course of the testshowed arsenic concentrations below the detection limit of0.035 mg/L. Counting all outliers, no single leaching solutionfrom any of the samples showed an arsenic concentration >0.1mg/L. For the great majority of test runs, most of the leachingsolution samples were below detection limits, with the highestconcentration being <0.05 mg/L. Four of the solidified sampleswere sent to Environment Canada’s Wastewater TechnologyCentre, where there is instrumentation capable of measuringarsenic concentrations to lower detection limits. The leach-ability indexes determined in those four cases were: S1,1—6to 12, 11.4; S3,1—24 to 36, 12.5; S4,1—6 to 12, 11.7; andS2,2—6 to 15, 12.8. These are quite high numbers for arsenic-containing solids and predict very low leachability. For in-stance, for solidified sample S1,1—6 to 12 with a leachabilityindex of 11.4, the fraction leached after 100 years from a bar-rel-sized solid 90-cm long by 55-cm diameter (volume/surfacearea ratio = 10.5) would be 0.01. The larger leachability in-dexes for the other three samples predict a lower fractionleached.

The volume changes that can be expected during remedia-tion can be judged from a comparison of the wet density ofthe soil and bulk density of the solidified sample. In all thesoil/binder systems tested from all of the sites, the largest pre-dicted volume increase was 12%, and in a few cases a volumedecrease of up to 6% was observed. These samples were pre-pared with relatively small proportions of binding agents, andthere is a quite substantial increase in density on treatment.Thus, volume changes are relatively small. Densities obtainedin cements and concretes, and also solidified materials, aredependent upon mixing efficiencies. If mixing is less efficientin the field, then the densities of the treated materials may notbe as high as those noted here and volume increases would begreater. The volume increase estimated in the field was 20%.

The acid neutralization capacity noted in Table 4 was mea-

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J. Environ. Eng. 20

sured according to specifications of the Environment Canadareport (Stegemann 1991). To quote the report, ‘‘Solidifiedwastes are very resistant to attack by an acidic leachant (suchas acidic landfill leachant) provided that they remain mono-lithic and have a high acid neutralization capacity.’’ The ANCis reported as the number of equivalents (eq) of a mineral acidrequired to bring the final pH of the crushed matrix below pH9. The Environment Canada protocol suggests that S/S prod-ucts going into a sanitary landfill, where they will be exposedto a relatively large amount of acidic leachant from biodegra-dation of garbage, should have an ANC of 3 eq/kg. All of thesolidified samples in the present study exceed 3 eq/kg.

Compressive strengths at 28 days are quite substantial forall of the solidified samples, ranging from a low of 1.8 MPa(260 psi) for S4,1—18 to 24 to a high of 9.6 MPa (1,390 psi)for S2,3—30 to 36. These are much greater strengths than thebenchmark value of 0.34 MPa (50 psi) that the USEPA hasoften utilized to indicate that cementing reactions have takenplace during the curing of the solidified samples. Indeed, thereis ample evidence from the spectroscopic measurements to becited later that normal cement hydration has taken place inthese solidified samples. The compressive strengths are suffi-ciently high to give the expectation that the solidified mass inthe field will have great durability.

Permeability is a matter of obvious concern in S/S practice,because leachability will be limited only if ground-water flowtakes place around, not through, the waste form. Hydraulicconductivity is noted in Table 4 for two treated samples butwas measured for a total of eight such samples. Among allthese samples the range of observed values was 2 3 1029 to8 3 10210 m/s. These are much lower values than those seenfor sandy soils. A value of 5 3 1025 m/s was measured for anuntreated soil at Site 2.

In engineering practice, two major factors limiting the du-rability of concrete structures are the stresses exerted by re-peated cycles of freeze/thaw or wet/dry. The EnvironmentCanada test protocol measures weight loss after each cycle,and loss of physical integrity is defined as a 30% weight lossin either of these two tests. The maximum weight loss ob-served after 12 freeze/thaw cycles was 9.5% from S4,1—6 to12, but the next highest weight loss was 3.5%, and most ofthe samples showed 1% or less. All of the samples subjectedto wet/dry cycling showed <1% weight loss after 12 cycles.This excellent weathering resistance is in agreement with ex-pectations from the relatively high unconfined compressivestrengths.

Microscopy and Spectroscopy

Fig. 1 shows a low magnification scanning electron micro-graph of the solid prepared from soil S1,4—24 to 36 andportland cement only. This is the same soil that is shown inFig. 1 of the preceding paper. Individual sand grains can stillbe recognized by their rounded morphology, but cement ma-trix has filled the interstices between the sand grains. Micro-structurally the S/S products with and without ferrous sulfatedid not appear very different. Hydration products of portlandcement were common. Massive calcium hydroxide crystalswere common to both, and ettringite crystals could be identi-fied, particularly in those samples containing ferrous sulfate.

Fig. 2 shows the X-ray powder diffraction analyses patternsof the S1,4—24 to 36 soil sample and the same soil solidified/stabilized by portland cement only and by portland cementplus ferrous sulfate. The patterns are not shown to full scale,as all other peaks than those of quartz are very weak. Apartfrom quartz (all the tall peaks belong to this phase), mont-morillonite is an important constituent of the soil. When port-land cement was added to the soil, two important cement hy-dration products, calcium hydroxide (CH) and monosulfate,


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FIG. 1. Scanning Electron Micrograph of Treated Soil S1,4—24 to 36 Using Type I Portland Cement Alone (Magnification1603)

can be seen in the final product, along with ettringite (AFt)when extra sulfate is added.

Solid-state NMR spectroscopy also can give informationabout cement setting reactions, and both Si and Al spectrawere obtained for soils before treatment and products aftertreatment. In the soils before treatment, the Si spectra showhigh proportions of chain-branched (Q3) and cross-linked (Q4)



polysilicates at 292 and 2110 ppm, respectively. The Q4 frac-tions (presumably mainly quartz) for the five soils measuredranged from 29 to 56% of the silicates. The correspondingNMR data for the portland cement/ferrous sulfate solidifiedsoils that have been cured for at least 28 days still show theQ3 and Q4 fractions, as expected, because they will not takepart in the cement hydration reactions, at least not after rela-tively short cure times. But the Si spectra now show largeramounts of Q0, Q1, and Q2 fractions, the silicates that are char-acteristic of portland cement. The Q0 fraction represents or-thosilicate in unhydrated portland cement clinker, and Q1 andQ2 are the chain end and chain lengthening units, respectively,in the silicate oligomers of calcium silicate hydrate gel (C-S-H). As expected for normal cement hydration reactions, theproportions of Q1 and Q2 are greater than those for Q0, mean-ing that hydration has occurred to a very appreciable extent.

X-ray diffraction showed that the treated soils contained cal-cium hydroxide (CH) and ettringite (AFt) in addition to thequartz and montmorillonite peaks from the soils. The X-raypowder diffraction analyses data showed AFm (monosulfate)peaks when portland cement alone was used as the S/S agent.The addition of ferrous sulfate to the portland cement resultedin the presence of AFt in these cured samples. In portlandcement, where the SO3 /Al2O3 ratio is typically 0.6, AFm isthe more stable phase (Taylor 1990). The addition of the fer-rous sulfate will alter this ratio and can make AFt more stablein the solidified product.

The treated soils can be considered to be basically mechan-ical mixtures of the respective soils and portland cement. The

FIG. 2. X-Ray Diffraction Patterns of S1,4—24 to 36 Soil and Treated Soils Using Portland Cement Alone and Portland Cement PlusFerrous Sulfate

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FIG. 3. Mixing Equipment Used in Site Remediations, Show-ing Mixer Head to Left, Backhoe to Right, and Flexible Pipingalong Boom for Delivery of Reagents

mixtures, however, are tight, dense ones, as can be seen fromcomparison of Fig. 1 in this and the companion paper. Thedense structure correlates very well with the observation oflow permeabilities for the treated soils. The phases expectedfrom hydrated cement, namely, calcium hydroxide and ettrin-gite, were always present in the solidified soils. Their presencesuggests that the arsenic-containing soil did not significantlyaffect the hydration of the cement component. The same con-clusion can be reached from the NMR data, which show asuperposition of the peaks observed in the soils on the silicateand aluminate peaks that are typical for hydrated portland ce-ment.

SITE REMEDIATIONS

Site Mixing Operations

Sites 3 and 4 were chosen for remediation, and because bothwere active industrial sites, normal operations had to be sus-pended before treatment. Both had a shallow gravel andcrushed shell layer on the surface, which was removed andtransported off-site. At Site 3 the depth to the clay confininglayer varied from about 1.8 to 2.4 m below the surface andtreatment was carried out to a depth of 2.1–2.7 m to penetratea short distance into the clay layer. The treatment zone encom-passed approximately 770 m3. Site 4 did not have a clay layernear the surface, and treatment was performed to a depth ofapproximately 1.2–4.9 m, based on the surveyed extent ofcontamination. At Site 4 approximately 2,500 m3 of soil weretreated. Both sites had equipment on concrete pads. The padswere not disturbed as S/S treatment was performed around thesides of the pads. No treatment was performed underneath thepads, but the soils left untreated by this procedure were es-sentially encapsulated by the pad itself and the surroundingtreated subsurface soils.

The mixing equipment consisted of a backhoe equippedwith a rotating mixer head studded with metal tines, as shownin Fig. 3. Prepared slurries of the treatment mix, consisting of

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ferrous sulfate, cement, and water, were pumped through hosesmounted on the boom into the mixer head. The slurries for thetwo sites had different compositions because of differences insoil-water content but were intended to closely correspond tothe proportions used in the final round of the treatability study.Site 3 was relatively dry, whereas the soils at Site 4 weresaturated. At Site 3, slurry mix No. 1 (1:1 by weight waterand ferrous sulfate) was mixed into the soil using the propor-tions 2:180 by weight. After 24 h, slurry mix No. 2 (1 partType I portland cement to 1.25 parts water) was mixed withthe soil using the proportions 2.25 parts slurry to 10 parts soil.At Site 4 less water was used in the slurries. Slurry mix No.1 was 1 part FeSO4 to 0.8 part water, and this was mixed withthe soil in the proportion of 1.8 parts slurry to 180 parts soil.Again after 24 h, slurry mix No. 2 (2 parts Type I portland to1 part water) was mixed with the soil in the proportion of 1.5parts slurry to 10 parts soil. The contractor planned the mixingsequence and divided the sites into several sections for treat-ment. The treatment proportions did not result in any free-standing surface water at Site 3, but there was free-standingwater at the completion of daily mixing operations at Site 4.This water had either evaporated or reacted by the next morn-ing. After treatment, the contractor estimated an approximately20% volume increase at Site 4, but a smaller increase at thedrier Site 3.

Quality Control Sampling Program and Test Results

Three rounds of sampling were carried out for the treatedsoils. At the time of treatment (Sampling Event 1), treated soilslurries were placed in 7.6-cm-diameter cylinders, tamped toeliminate voids, then stored in sealed containers for later test-ing at 28 and 90 days of cure. In Sampling Events 2 (90 daysafter treatment) and 3 (6 months after treatment), a drill rigwas used to core 0.08-m-diameter 3 1.5-m-long samples fromthe treated soils, which were then sealed and transported tothe testing laboratory. All the posttreatment analyses were car-ried out by an independent testing laboratory.

Representative posttreatment test data are shown in Table 5and are in very good agreement with the results from the treat-ability study. The majority of the samples showed arsenic con-centrations in the TCLP leachates at the nondetect level (0.05mg/L) but ranged up to a high of 0.17 mg/L. The latter islower than the average observed in the treatability study (0.26mg/L). One conclusion from the treatability study was that thelower limit of As leachability could be determined by the Ascontent of the cement used and the latter could be quite var-iable. A single cement batch was used in the treatability study,which was different from that used in the actual remediation.The ANS16.1 leaching procedure was carried out on six sam-ples, but all of the leaching solutions showed As as nonde-tectable. The observed unconfined compressive strengths atSite 4 were comparable to those observed in the treatabilitystudy, but the strengths observed at Site 3 were higher than atSite 4 and also higher than in the treatability study. The soilsat Site 4 were more nearly saturated with water and had ahigher organic content, and these factors may account for the

TABLE 5. Minima and Maxima in Posttreatment Test Data

Site(1)

Test(units)

(2)Untreated soil

(3)

Treated soilin lab study

(4)

Posttreatment Samples

28-day cure(5)

90-day cure(6)

180-day cure(7)

3 TCLP for arsenic (mg/L) 1.42–3.70 0.18–0.29 ND–0.11 ND–0.08 NDPermeability (m/s) 1 3 1025 1.0–1.7 3 1029 0.5–0.6 3 1029 0.5–1.9 3 1029 —

4 TCLP arsenic (mg/L) 0.15–3.50 0.22–0.38 ND–0.12 ND–0.17 NDPermeability (m/s) 5 3 1025 0.7–1.9 3 1029 0.5–2.2 3 1029 0.5–1.6 3 1029 —

Note: ND = not detected with detection limit of 0.05 mg/L.


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strength differences at the two sites. The permeabilities ob-served were all in the 1029–10210 m/s range. Neither the per-meabilities nor the compressive strengths varied significantlyafter 90 days, as compared to the 28-day cure period.

CONCLUSIONS

Treatment of sandy soils contaminated with arsenic was in-vestigated at a bench scale and carried through to remediationin the field. Leachability testing (TCLP and a modified versionof ANS16.1) indicated low potential for ground-water contam-ination when the soils were treated with a mixture of Type Iportland cement and ferrous sulfate. The arsenic concentrationin TCLP leachates averaged over all of the treated soil samplesfrom four sites was 0.26 mg/L. Leaching was reduced whenthe soil was pretreated first with FeSO4, then with portlandcement. In addition to chemical containment, the mixtureshould prevent ground-water leaching by physical encapsula-tion as well, because the permeabilities of the treated soilswere in the range of 1029–10210 m/s. The scanning electronmicroscope micrographs showed a dense mass with minimalvoid space, and a combination of X-ray diffraction, thermalanalysis, and solid-state NMR spectroscopy indicated the for-mation of a normal hydrated cement matrix, with some excessettringite being present due to the extra sulfate being added tothe formulation.

The optimized treatment recipe from the bench-scale studywas transferred to the field in two site remediations involvingapproximately 3,300 m3 of contaminated soil. Using a two-stage mixing process, first with a ferrous sulfate slurry, thenwith a portland cement slurry 24 h later, the soils were treatedusing a backhoe-mounted mixer head. Sampling was carriedout in the field 3 and 6 months after treatment at depths of0.15–<3 m. Permeability, unconfined compressive strength,and arsenic leachability in TCLP were determined by standardlaboratory testing procedures. Results from the bench-scaletreatability study were reproduced very faithfully in the field,with TCLP leachability being even lower than predicted. Themajority of the samples, and all those taken after 6 months ofcure, showed arsenic at nondetectable levels (i.e., <0.05mg/L).

ACKNOWLEDGMENTS

The writers wish to thank the EPRI for partial support of this work.A fuller description of this work is available in EPRI report TR-106700(9049-01). The writers gratefully acknowledge the permeability and dy-namic leaching data provided by Julia Stegemann and Robert J. Caldwellof the Wastewater Technology Centre, and the thermal analysis data byAmitava Roy of Louisiana State University, Baton Rouge, La.

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J. Environ. Eng. 20

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