remediation of soil contaminated with arsenic, zinc, and nickel by pilot-scale soil washing

Remediation of Soil Contaminatedwith Arsenic, Zinc, and Nickel byPilot-Scale Soil WashingIlwon Ko,a Cheol-Hyo Lee,a Kwang-Pyo Lee,a Sang-Woo Lee,b and Kyoung-Woong Kimb

a Environmental Research Center, Oikos Co., Ltd., Gasan-dong, Geumcheon-gu, Seoul, 153-775, Korea; [email protected] (forcorrespondence)b Arsenic Geoenvironment Laboratory (NRL), Department of Environmental Science and Engineering, Gwangju Institute of Scienceand Technology (GIST), Oryong-dong, Puk-gu, Gwangju, 500-712, Korea

Published online 9 September 2005 in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ep.10101

The use of soil washing has successfully demon-strated bench-scale and field applications for the reme-diation of oxyanions and cationic heavy metals. How-ever, it is not easy to simultaneously remove bothoxyanions and cations using one wash solution in thesame extraction. Also contaminated soils from metalmining areas present other difficulties to remove con-taminants because of their strong binding properties.Correspondingly, the efficiency of soil washing can beinhibited by the limited extraction of contaminants.The objective of this study was to find the optimalprocess conditions for the enhanced washing of recal-citrant contaminants. This study focused on the effec-tiveness of engineering-scale washing of field soil usingthree types of inorganic acids (HCl, H2SO4, andH3PO4). In this study, soil contaminated with arsenic,nickel, and zinc was obtained from a closed iron/serpentine mining area. The H2SO4 and H3PO4 washsolutions, including the competitive oxyanion, en-hanced the removal of arsenic. The acid attack onarsenic binding in the Fe/Mn oxide and organic/sul-fides fractions may be a possible removal mechanismin addition to anionic competitive desorption. The lowefficiency of soil washing results from the strong bind-ing associated with minerals and the enrichment ofcontaminants in the highly reactive fine particles. Forthis soil used, the physical separation of particles couldenhance the overall effectiveness of soil washing. There-fore, for the soils contaminated with recalcitrant con-taminants in metal mining areas, the physical separa-

tion of fine particles can be significant in addition tothe chemical extraction. © 2005 American Institute ofChemical Engineers Environ Prog, 25: 39–48, 2006

Keywords: soil washing, arsenic, zinc, nickel, fineparticles

INTRODUCTIONSoil washing has been used for the remediation of

heavy metal–contaminated soils [1]. A water-based soiltreatment process relies on traditional physical andchemical extraction and separation processes for re-moving a broad range of organic and inorganic con-taminants. Soil is a natural mixture of mineral andorganic particles and their weathered derivatives. Atypical particle-size distribution reveals three specificfractions that are important to soil washing: (1) theoversize fraction; (2) the sand fraction, which is largerthan approximately silt size (74 �m); and (3) the finesconsisting of materials smaller than silt size [2]. In mostcases, the contaminants will reside and be concentratedin the fine particle fraction, whereas lower concentra-tions of the main contaminants often exist in the sandsand the oversize fractions. To evaluate the feasibility ofsoil washing of particular contaminants, representativesoil can be sampled, a sieving size classification per-formed, and the variation of concentration in soil par-ticle fractions monitored. This information, coupledwith background information about the site, can pro-vide significant insights into the possible treatment de-sign as well as the full-scale configuration of a soilwashing system.

Extraction of heavy metals from contaminated soil isa process that is mainly controlled by the dissolution of© 2005 American Institute of Chemical Engineers

Environmental Progress (Vol.25, No.1) April 2006 39

the metal–mineral bond followed by the dispersion ofthe contaminants in the washing solution as an emul-sion, complex, or suspension. Organic acids such asEDTA (ethylenediaminetetraacetic acid) and DTPA (di-ethylenetriaminepentaacetate) [3–5] removed cationicheavy metals from the contaminated soil better thaninorganic acids [6]. Generally, the solubility of cationicmetals decreases as the solution pH increases, but thesolubilities of arsenic, selenium, phosphate, and mo-lybdenum increase as the pH increases. It is not easy todetermine the optimal conditions of leaching or immo-bilization of both oxyanions and cations. The washingof soil to remove arsenic has been effected using var-ious chemicals such as sodium hydroxide [7], phos-phate [8], and acid [9].

Although there are many sites contaminated witharsenic and heavy metals, there have been only a fewattempts to test their simultaneous removal. The opti-mal conditions of chemical extraction of both arsenicand heavy metals can be significantly affected by thespecific nature of their binding strengths [10]. For thesimultaneous removal of arsenic and other heavy met-als, the optimal conditions for the efficient extractionshould be properly determined.

This study investigated the effectiveness of engi-neering-scale washing of field soil contaminated simul-taneously with both oxyanion (arsenic) and heavy met-als (zinc and nickel) using three different inorganicacids (HCl, H2SO4, and H3PO4). This soil was obtainedfrom a closed iron/serpentine mining area. After theestablishment of the feasibility of acid washing, theextraction of the chemicals was monitored by thechemical binding form in the sequential extractionanalysis and the variation of concentration in eachparticle fraction. In addition, the condition for the phys-ical separation of fine particles was optimized to en-hance overall effectiveness. This procedure would al-

low the design of an appropriate full-scale washingstrategy.

MATERIALS AND METHODS

Preparation of Contaminated SoilThe Dalcheon iron mining area is located in the

southeastern part of Seoul, Korea [11]. Iron has beenmined there from 1906 to 1993 [12]. Serpentine rockwas also extracted from 1966 until quite recently. Soilfrom the site was prepared near the mining area,blended using an excavator and processed by screen-ing using 10 mm sized sieve. Bulk samples were driedat 50° C and sieved to obtain particle sizes � 10 mm.Soil particles of �10 mm were used for the soil washingtests to compare with the results of the pilot-scaleexperiments. The coarser grains such as gravel (�2mm) were, in general, identified as noncontaminatedfraction as opposed to the primary minerals and sec-ondary weathered fine fractions including contami-nants. The bulk soil was a chemically contaminated soiland partially a mixture from the loss of mining materi-als, that is, serpentine, magnetite, and pyrite.

Soil Washing ExperimentsThe pilot-scale soil washing equipment used in this

study consisted of five units: soil washing scrubber(drum-type), vibrating screen, screw feeder, high-pres-sure air supply, and ceramic filter system, as shown inFigure 1. All parts of the soil washing equipment weremade of stainless steel. The screw feeder was used totransfer contaminated soil into the washing scrubbersimultaneously with mixing wash solution. The wash-ing scrubber had a drum-type cylinder with an innerscrew blade to facilitate soils moving forward. In thetreatment process, soils and washing solution were

Figure 1. Schematic diagram of pilot-scale soil washing equipment.

40 April 2006 Environmental Progress (Vol.25, No.1)

shaken. After both the treated soils and the wash solu-tion passed through the washing scrubber, they wereseparated and screened using the vibrating screen with74 �m (200 mesh) sieve size. In the ceramic filtersystem, the waste wash solution was treated. The fineparticles and the dissolved inorganics were removedand the washing water was recycled as new washingsolution after the addition of acid solution.

For each batch experiment, a 40 kg soil sample wasplaced in the screw feeder for 5 min, and the dilutedacid solution was added at a 1:10 of soil:acid solutionratio. The wash solution was kept in the range of pH2–3 in the front compartment of the washing scrubber;a washing time of 10 min was used. The acid-washedsoils were screened in the vibrating screen, and thensoil particles of a size � 74 �m were discharged asremediated soil. All batch experiments were duplicatedand bulk soil samples were prepared for total concen-tration and sequential extraction analysis. In addition,the remediated soils were collected every batch andinstantly sieved into five fractions (0.074–0.149, 0.149–0.250, 0.250–0.420, 0.420–0.841, and 0.841–10 mm).Both bulk and five soil fractions were extracted withaqua regia (HCl:HNO3 � 3:1) and their chemical formswere determined using the modified BCR method [13].All solutions were stored at 4° C for later analysis.During the soil washing process, the amount of fine soilparticles was also gravimetrically measured.

Chemical AnalysisA group in a European Community Bureau of Ref-

erence (BCR) project proposed a three-step extractionprocedure for the analysis of sediment, soil, sludge,and mine waste. This process is also useful for deter-mining the chemical forms of metals during soil wash-ing to evaluate the contaminant extraction efficiency. Inthis study, we used the BCR three-step sequential ex-traction procedure as developed by Quevauviller et al.[13] as slightly modified by Whalley and Grant [14]. InStep 1 of the extraction, 0.11 mol/L acetic acid at pH 2.8was used. Metals present in ionic form that are boundto carbonates and the exchangeable fraction were re-leased. In Step 2 of the extraction, 0.1 mol/L hydrox-ylamine hydrochloride at pH 2 was used. Metals boundto amorphous Fe and Mn (hydro)oxides were leached.In Step 3 of the oxidation process in an acid-stabilizedsystem with 30% hydrogen peroxide was subjected toextraction using 1 mol/L ammonium acetate at pH 2,adjusted with nitric acid. Metals bound to organic mat-ter and sulfides were removed. For an internal checkon the sequential extraction procedure, the residualfrom Step 3 was digested (Step 4) in hydrochloric acidand nitric acid (aqua regia). The concentrations of met-als in solutions from four steps were summed andcompared with their total concentration of the originalsample. In each analytical step, standard and blanksamples were also analyzed and duplicate sampleswere prepared. Results of extraction steps are reportedon a dry mass basis. Reagents were prepared accordingto the procedures described by Quevauviller et al. [13].

The total concentrations of contaminants in the orig-inal and acid-washed samples were determined in du-plicate samples. Zinc and nickel were determined by

induced coupled plasma atomic emission spectrometry(ICP-AES, Thermo Jarrel Ash). The concentration oftotal dissolved arsenic was determined by hydride gen-erator–atomic absorption spectrometry (HG-AAS, Per-kin–Elmer ZL 5100). The arsenic analysis method fol-lowed the procedure given in the Standard Methods[15]. The sodium borohydride solution (Sigma–AldrichChemical Co.) was supplemented with 0.1 M sodiumhydroxide and 10 wt % HCl solution (Merck) was usedfor carrier.

Soil particle size analysis was performed by sievingand weighing the air-dried soil with five fractions(0.074–0.149, 0.149–0.250, 0.250–0.420, 0.420–0.841,and 0.841–10 mm). Approximately 5 kg of bulk soil wassieved and then each fraction was gravimetrically mea-sured. Then, the measured fractions were normalizedwith total weight of bulk soil (wt/wt %). Additionally,the weight of fine soil particles was also determinedafter oven drying at 105° C. Potentiometric titrationwith a microtitrator (702SM, Metrohm) was performedto measure the acidity of 1.0 g soil (�10 mm) using 1.0M HCl under N2 gas purging to prevent carbonateinterference. The soil pH was determined by placing10 g of air-dried and sieved (�10 mm) soil in a beakerand adding 100 mL deionized water and stirring for 30min. The pH values of the samples were determinedusing an Orion Ion Analyzer (Orion Research Inc.)equipped with a pH electrode. The organic matter wasmeasured after furnace drying at 450° C. Mineral iden-tification in the solid samples was performed by X-raydiffraction (XRD) using a Rigaku X-ray diffractometer(Model D/Max-3C) with a Cu tube.

RESULTS AND DISCUSSION

Characterization of Contaminated SoilThe bulk soil used in this study highly contaminated

with heavy metals having mean concentrations of 43,340, and 68 mg/kg for arsenic, zinc, and nickel, respec-tively (Table 1). Based on a mineralogical analysis ofthe contaminated soil, it can be hypothesized thatnickel can be strongly incorporated in serpentine min-eral, and arsenic and zinc may be partially incorporatedin the pyrite as a contaminant source. Therefore, thisfield soil consisted of recalcitrant contaminants of ar-senic, zinc, and nickel, which may need a strong chem-ical attack for chemical extraction. The total amount ofsilt and clay in this materials was 2.4 wt/wt % based onthe dry sieving analysis of the field soil. Its sandytexture seems to be favorable for soil washing. Char-acteristically, the soil pH was 7.7, alkaline because ofthe presence of calcite minerals.

Acid Soil WashingSeveral experiments were performed using three

different inorganic acids: HCl, H2SO4, and H3PO4. Asoil pH of 2 was preferred for the extraction process tosolubilize both oxyanion and cationic metals becauseoxyanionic arsenic can be desorbed from mineral sur-faces in extremely acidic conditions (pH � 2) or alka-line conditions (pH � 10). The amount of the acidconsumed was 0.55 mol H�/kg to reduce the soil pH 2.


The fine particles (pH 7.9) were more alkaline than thecoarse particles (pH 7.6); these fine particles (�74 �m)consumed much more acid than the coarse particles(�74 �m), as seen in Table 1. The presence of fineparticles is critical for the extraction of metals as a resultof the highly reactive surfaces.

Soil washing yielded various removal efficiencies fordifferent initial concentrations of arsenic, zinc, andnickel (Figure 2). For arsenic, H3PO4 and H2SO4 had aneffective removal efficiency of 75 and 70%, respec-tively, compared with 63% with HCl, as shown in Fig-ure 2a. However, HCl, H2SO4, and H3PO4 removed 59,58, and 60% of the total zinc, respectively. The removalefficiencies were similar for each acid solution (seeFigure 2b). These results indicate that H3PO4 andH2SO4 dissociate into competing oxyanions, such asphosphate and sulfate, with arsenic. Adsorbed arsenicmay then be desorbed and efficiently extracted into anacid solution [16]. Generally, during acid treatment,arsenate predominantly occurs because of arsenite’sstrong and fast oxidation reaction. Although both ar-senite and arsenate can be also extracted from soil,phosphate is able to desorb both arsenite and arsenate.It is known that both arsenite and arsenate can bestrongly desorbed with phosphate as a result of anioniccompetitive desorption from the adsorption affinity ofphosphate � arsenate � sulfate � arsenite [10]. Sulfatecan also extract arsenite more strongly than arsenate.Correspondingly, arsenic washing is efficient usingH3PO4 and H2SO4.

In addition, despite a high nickel removal efficiencyusing H3PO4, the overall removal efficiencies were

�50%, as shown in Figure 2c. The foregoing likelyoccurred because nickel is strongly incorporated intoserpentine mineral. As a result, acid extraction waslimited. The removal of zinc and nickel was less sen-sitive to each inorganic acid because of simple acidattack. Organic acids are reported to efficiently washcationic metals by acid attack and aqueous organiccomplexation; however, arsenic forms a weak organiccomplex [17, 18]. Arsenic may be desorbed by thecompetitive anions and extracted by acid attack. Thus,inorganic acids with competing anions can be suitablefor the simultaneous extraction of oxyanion and cation.On the other hand, the overall removal efficiencieswere relatively low because of mineral incorporation ofthe contaminants. Therefore, acid washing seemed tobe also partially inhibited as a result of the strong bondsin minerals.

In this study, the acid solution changed the proper-ties of the soil during acid washing. The treated soilafter acid washing led to the loss of organic matter andbecame acidic (Table 1). After acid washing, the pH 3.3of the fine particles was less than that (pH 5.7) of thecoarse particles. In addition, the acid-buffer capacity infine particles was large enough to maintain a low soilpH of treated soil. Correspondingly, a suitable post-treatment such as neutralization may be needed, par-ticularly for fine particles. Acid washing also led to theremoval of considerable amounts of organic matter.The organic matter in the fine particle fraction de-creased from 2.8 to 1.1%; the organic matter in thecoarse fraction also decreased from 0.5 to 0.2%. The

Table 1. Physicochemical characterization of treated soil used in the soil washing experiment.

Analysis Unit

Value

NoteBefore acid washing After acid washing

Concentration of As, mg/kg 43 (36–51)* As — Aqua regiaZn, and Ni 340 (310–364) Zn — digestion

68 (44–88) Ni —Mineralogical analysis — Magnetite, hematite, serpentine, calcite, pyrite,

quartz, feldspar, and kaoliniteXRD analysis

Acid consumption mol H�/kg 0.55 (bulk soil) — Potentiometric0.28 (�74 �m) — titration0.67 (�74 �m) —

Water content % 4.6 15.6 (13.1–25) Drying at 100° CCEC meq/100 g 9.2 5.1Soil particle size

distribution— Sandy soil 2.4% smaller

than 74 �m (silt andclay)

Production of 5.5 wt %fine particle (�74�m)

Dry or wetsieving

Bulk density g/cm3 1.64 (bulk soil) 1.77 (�74 �m)1.6 (�74 �m)

Organic matter % 1.2 (bulk soil) — Ignition at 450° C0.5 (�74 �m) 0.2 (0.1–0.4) (�74 �m)2.8 (�74 �m) 1.1 (1.0–1.3) (�74 �m)

Soil pH — 7.7 (bulk soil) — pH in water7.6 (�74 �m) 5.7 (5.4–6.3) (�74 �m)7.9 (�74 �m) 3.3 (3.1–3.5) (�74 �m)

*Values in parentheses means the range for measured value of each property.


Figure 2. Removal efficiencies of (a) As, (b) Zn, and (c) Ni using various acids in the washing process.


fine particles have more organic matter than the coarseparticles as a result of a large sorption capacity.

Chemical Speciation of Arsenic, Zinc, and NickelA sequential extraction was performed to investigate

the change of the chemical binding form during theacid washing. The various inorganic acid types show asimilarity of the extraction pattern in each chemicalfraction, but each inorganic contaminant has a slightlydifferent removal efficiency (Figure 3). More than 80%of the total arsenic in each fraction was removed fromall chemical fractions except for the residual fraction(�30%). This result indicates that, in addition to an-ionic competitive desorption, the acid attack of arsenicbinding in the Fe/Mn oxide and organic/sulfides frac-tion may also be a possible mechanism. Oxyanionicdesorption of arsenic by phosphate and sulfate can berestrained in a strong arsenic binding and, correspond-ingly, for an effective extraction, is likely to be accom-panied by a breakdown of a strong arsenic binding.Despite the low removal efficiency in the Fe/Mn oxideand organic/sulfides fraction, zinc’s concentration inthe exchangeable fraction is high; the residual fractionstill yields an extremely low removal efficiency. Theamount of nickel removal is relatively small comparedwith its total content; the removal efficiency is alsoextremely low in the residual fraction. It is suspected

that nickel may be incorporated in a recalcitrant min-eral form, which is related to serpentine minerals fromthe mineralogical analysis (Table 1). Therefore, in thewashing of the contaminated soil, the acid-extractablefraction included the predominant exchangeableFe/Mn oxide and organic/sulfides fractions; the resid-ual fraction was partially acid extractable. This resultimplies that bioavailable chemical forms of arsenic,zinc, and nickel can be removed together with a partialremoval of recalcitrant forms.

Variation of Contaminant Concentration in SoilParticle Fraction

Besides the chemical binding strength, chemical ex-traction of contaminants also relies on the soil particlesize because the fine particles with large surface areaare much more reactive than coarse particles [2, 19]. Asa significant monitoring factor, Figure 4 shows thevariation of concentration in soil particle size distribu-tion after acid washing. In the contaminated soil, con-centrations of arsenic and zinc in the particle size frac-tion increase from the coarse to the fine fractions.However, the concentration of nickel is enriched inboth the coarse and fine fractions, which indicates thepresence of serpentine minerals containing nickel inthe coarse particle fraction. Three kinds of inorganicacids more efficiently extracted contaminants from the

Figure 3. (a) Exchangeable fraction, (b) Fe/Mn oxide fraction, (c) organic/sulfides fraction, and (d) residualfraction of As, Zn, and Ni in the contaminated soil and washed soil using various acids (each number abovebar diagram represents the removal efficiencies in each chemical fraction).


Figure 4. Concentration variation of (a) As, (b) Zn, and (c) Ni with respect to soil particle sizes after acidwashing.


coarse particle fraction than from the fine particle frac-tion; these results were compared with the concentra-tions of the contaminated soil (open circles). Althoughthe fine particle fraction had a higher concentration ofcontaminants than the contaminated soil, H3PO4 re-duced the concentration of arsenic in the fine fraction(Figure 4a). Therefore, phosphate seems to competi-tively desorb arsenic during acid washing. The en-hanced concentration in the fine particle fraction isexpected to come from the readsorption of the ex-tracted aqueous form of the contaminants. Fine parti-cles have a high surface area, and thus are reactive withaqueous adsorbates such as inorganic contaminants.During acid washing, aqueous arsenic in the acid con-dition predominantly occurs as arsenate. In the finalsection of the washing scrubber, the pH of the washsolution slightly increased toward pH 3–4. Consider-able amounts of aqueous arsenic may be adsorbedagain onto the reactive surfaces of the fine particles.The concentration of nickel in the fine fraction wasmuch higher than the concentration of arsenic and zinc(Figures 4b and 4c). This result occurred because of thehighly recalcitrant mineral incorporation of nickel inaddition to its strong adsorption into fine soil particles.Arsenic and zinc may be also strongly adsorbed intothe fine particles with less acid-extractable properties.

Physical Separation of Fine ParticlesAs shown above, serpentine as a mining material

and the fine particles enriched with contaminants cansignificantly affect the overall removal efficiency of soilwashing. In particular, the fine particles are signifi-cantly enriched with contaminants and recalcitrantmining materials and are critical in optimizing thewashing process. To measure the amount of fine par-ticles, their production was monitored as a function ofthe cumulative batch washing step (13 batches contain-ing a total of 520 kg soil). The concentration of the

suspended fine particles slightly increased from 12.4(4.9 wt/wt %) to 16.3 g/L (6.5 wt/wt %) for the cumu-lative washing steps (Figure 5). In this process, fineparticles might be suspended only within limits as aresult of the saturation of the suspended particles andtheir precipitation in the washing solution. After acidwashing of 520 kg soil, the concentration of fine parti-cles was higher (5.5 wt/wt %) than 2.4 wt/wt % fromthe dry sieving analysis. Generally, fine particles areproduced from the silt and clay fractions and partiallyfine sand [1, 2]. Because aggregated particles are dis-persed in a water-based solution, fine particles are alsosuspended in larger amounts.

Fine particles are highly enriched with recalcitrantcontaminants; thus, their separation from treated soilcan reduce the concentration of contaminants. In ad-dition to removal efficiency by chemical extraction, theoverall removal efficiencies can be calculated from thereductions in the amounts of contaminants that areremoved by physical separation. The bulk concentra-tions of arsenic, zinc, and nickel were obtained fromeach mass ratio of particle size fraction using the fol-lowing equation:

Cj � �i

mi � Ci � m1 � C1 � m2 � C2

� · · · � mi � Ci

(1)

where Cj is the adjusted bulk concentration of thecontaminant, Ci is the measured concentration fromeach soil particle fraction, and mi is the mass ratio ofeach soil particle fraction. The removal efficiencieswere computed from the selected size fraction exceptfor the particle fraction less than the specific separationsize (Table 2). From the calculated results, the overallremoval efficiencies are enhanced as the size for phys-ical separation increases (Table 2). Even in the recalci-

Figure 5. Production of the cumulative amount of fine soil particles during a 13-batch washing of 520 kg offield soil.


trant case of nickel washing, removal efficiencies ofapproximately 90% can be effected for the separationsize of 420 �m.

For a full-scale design, the performance of a soilwashing system is also typically measured by the vol-ume reduction attained [2]. This reduction can be cal-culated by weighing the separated fine particles andfeed soil (see the following equation):

Performance by volume reduction �%�

� 1 � �Si � Sc

Si� � 1 �

Sf

Si

(2)

where Si, Sc, and Sf represent the weight of the initialfeed soil, the clean soil, and the fine particle fraction forthe physical separation, respectively. The soil volumereduction is proportional to the treatment cost of fineparticles (Table 2). The low amount of fine particles forphysical separation reduces their treatment cost. Theobjective of the process is to treat the entire volume ofcontaminated soil. Thus, the preferred size of fine par-ticles is 74 or 149 �m, given their high removal results,such as 92.5 or 97.6%, respectively [20]. Oversize andsand fractions separated from the fines will be treatedwith the main full-scale washing scrubber and returnedto the site as clean backfill [21]. Concurrently, the fineswill be separated and either dewatered into a sludgecake or further treated using other extraction methods.

CONCLUSIONSIn this study, acid washing of soil that included

recalcitrant mining minerals and inorganic contami-nants was investigated. The efficiency of removal ofarsenic, zinc, and nickel was significantly influenced bythe types of acid solution, the chemical forms of thecontaminants, and the soil particle size distribution.The following conclusions can be made from this studyon the simultaneous removal of oxyanionic arsenic andcationic zinc and nickel using a pilot-scale acid wash-ing technique:1. The three types of inorganic acids showed the dif-

ferent characteristics of soil washing in the bulk andvarious particle size fractions. Using the sameamount of solution, both H2SO4 and H3PO4 effi-

ciently removed arsenic by competitive oxyanionicdesorption in addition to the acid extraction. More-over, even in the fine particle fraction, the enrich-ment of arsenic was slightly reduced by H3PO4washing. However, the cationic zinc and nickelwere less sensitive to the inorganic acid type andsimply affected by the acid attack. In addition, theoverall efficiency of metal removal was likely to below because of the strong binding of mineral incor-poration of the contaminants.

2. The acid extraction of arsenic, zinc, and nickel wasstrongly affected by the chemical binding form dur-ing the soil washing. The acid-extractable fractionswere the exchangeable, Fe/Mn oxide, and organic/sulfides fractions; the residual fraction was partiallyacid extractable. In particular, the removal of nickelwas recalcitrant because of its incorporation in ser-pentine mineral.

3. The effectiveness of acid washing also depended onthe enrichment of contaminants in the particle sizefraction. The concentration of nickel in the fineparticle fraction was much greater than that of ar-senic and zinc because of the highly recalcitrantmineral incorporation of nickel and its strong ad-sorption onto fine soil particles. The overall removalefficiencies were enhanced by an increased size offine particles. However, for the treatment of theseparated fine particles, the overall separation offine particles should be determined by using thehigh performance by volume reduction.

ACKNOWLEDGMENTSThis research was financed and supported by Envi-

ronmental Research Center, Oikos Co., Ltd. through thesoil washing project on a metal-contaminated soil. Thework was also supported by a grant from the KoreaInstitute of Science and Technology Evaluation andPlanning (KISTEP) to the Arsenic Geoenvironment Lab-oratory (NRL) at Gwangju Institute of Science andTechnology (GIST), Korea.

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Table 2. Relationship between physical size separation and effectiveness of soil washing.

Size of fineparticles forseparation(�m)

Averageremovalof fine

particles(%)

Washingperformance

by volumereduction

(%)

Cost fortreatment of

fineparticles

(US$)

Removalefficiency(%) (HCl)

Removalefficiency

(%) (H2SO4)

Removalefficiency

(%) (H3PO4)

As Zn Ni As Zn Ni As Zn Ni

74 2.4 97.6 6,940* 63 59 38 70 58 42 75 60 45149 7.5 92.5 21,543 72 67 49 82 67 54 82 68 54250 20 79.5 59,221 87 76 62 91 79 69 93 81 69420 49 51.1 141,463 93 86 82 94 87 87 96 89 89

*The amount of contaminated soil is 16,424 m3, density of fine particles is 1.6 ton/m3 and cost is 11 $/ton.


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remediation of soil contaminated with arsenic, zinc, and nickel by pilot-scale soil washing

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