Inorganic arsenic sorption by drinking-water treatment residual-amended sandy soil: effect of soil solution chemistry

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1 Int. J. Environ. Sci. Technol. (13) 1:1 1 DOI 1.7/s y ORIGINAL PAPER Inorgnic rsenic sorption y drinking-wter tretment residul-mended sndy soil: effect of soil solution chemistry R. Ngr D. Srkr K. C. Mkris R. Dtt Received: 6 Ferury 12 / Revised: 26 Mrch 12 / Accepted: 29 July 12 / Pulished online: 8 Septemer 12 Ó CEERS, IAU 12 Astrct Previous studies in our lortory hve demonstrted tht drinking-wter tretment residuls re effective sorents of rsenic V. However, the effect of soil solution chemistry on rsenic V sorption y drinking-wter tretment residuls-mended soils remins to e explored. The current study uses tch incution experimentl set up to evlute the effect of soil solution, competing lignds, nd complexing metl on rsenic V sorption y sndy soil (Immoklee series) mended with two rtes (25 nd 5 g kg -1 )of luminum nd iron-sed drinking-wter tretment residuls. Experiments were conducted t three initil rsenic lods (125, 1,875, 3,75 mg kg -1 ) nd constnt solid: solution rtio of g L -1. An optimum equilirtion time of 8 dys, otined from kinetic studies, ws utilized for sorption experiments with oth luminum nd iron drinking-wter tretment residul-mended soil. Presence of phosphte decresed rsenic V sorption y oth luminum nd iron drinking-wter tretment residul mended soils, with strong dependence on, drinking-wter tretment residul R. Ngr Weiss Assocites, 2 Powell Street, Suite 925, Emeryville, CA 948, USA D. Srkr Deprtment of Erth nd Environmentl Studies, Montclir Stte University, Montclir, NJ, USA K. C. Mkris Cyprus Interntionl Institute for the Environment nd Pulic Helth in Assocition with the Hrvrd School of Pulic Helth, 5 Iroon Street, 115 Nicosi, Cyprus R. Dtt (&) Deprtment of Biologicl Sciences, Michign Technologicl University, 1 Townsend Drive, Houghton, MI 49931, USA e-mil: rupdtt@mtu.edu types, drinking-wter tretment residul ppliction rtes, nd phosphte concentrtions. Addition of sulfte hd no effect on rsenic V sorption y luminum or iron drinkingwter tretment residul-mended soil. A complementing effect of clcium on rsenic V sorption ws oserved t higher. Results elucidting the effect of soil solution chemistry on the rsenic V sorption will e helpful in clirting drinking-wter tretment residul s sorent for remedition of rsenic-contminted soils. Keywords Arsenic Drinking-wter tretment residuls Btch incution Remedition Sorption Introduction Given the widespred occurrence of rsenic (As) in soils nd the potentil humn helth risk posed y this crcinogen, it is impertive to develop cost-effective remedition strtegies to tret soil As. The utiliztion of wste yproduct generted from the drinking-wter tretment process (i.e. the drinking-wter tretment residul or WTR) hs een suggested s cost effective method for rsenic V [As(V)] immoiliztion. Addition of luminum (Al) or iron (Fe) slts to rw wter to remove colloids, color nd sediment genertes WTR tht contins very high concentrtions of Al- nd/or Fe-hydroxides (Elliott nd Dempsey 1991; Mkris et l. 5). Erlier soil incution studies in our lortory demonstrted the high As(V) sorption cpcity of WTR in As-contminted soils nd low in Al nd Fe oxyhydroxides (i.e., Immoklee nd Millhoper series soils) (Srkr et l. 7; Ngr et l. 9). The WTR-mended soils significntly (p \.1) incresed the overll mount of As(V)-sored compred to tht of unmended controls. The X-ry sorption spectroscopy

2 2 Int. J. Environ. Sci. Technol. (13) 1:1 1 studies y Mkris et l. (7) further supported the stility of sored As(V) y WTR, forming inner-sphere mononucler identte complexes with WTR surfce hydroxyls. However, thorough understnding on effect of soil solution chemistry on As(V) sorption y WTR-mended soils re crucil efore further field trils of WTR ppliction in As-contminted soils. Previous studies y Srkr et l. (7) showed the significnt effect of solid solution rtio (SSR) nd WTR ppliction rtes on As(V) sorption y two Florid soils mended with Al- or Fe-WTR. Arsenic(V) sored y WTRmended soils significntly incresed y incresed WTR ppliction rte from 2.5 to 1 %. Recent studies demonstrted tht Al- nd Fe-WTR performed well in removing As(V) from the queous solution tht contins vrile nd plethor of interfering ions such s phosphte, sulfte, nd clcium (Ngr et l. 1). It ws lso reported tht Al-WTR ws more effective thn Fe-WTR in removing As(V) under vriety of solution conditions, hence the effect of nd phosphte competition ws more pronounced for Fe-WTR, in comprison to Al-WTR. Severl other studies hve shown tht sorption of As(V) on the soil depends on the vrile chrge developed on the soil surfce, which is function of soil solution (Golderg 2; Goh nd Lim 4). One of the most significnt competing ions phosphte, n nlog of As(V), is often used in fertilizers in griculturl res where As(V) my hve een pplied s pesticide or hericide (Willims et l. 3). The presence of phosphte ws reported to decrese As(V) sorption y Al/Fe hydroxides nd soils (Mnning nd Golderg 1996; Jin nd Loeppert, Goh nd Lim 4). Other nions such s chloride, nitrte, sulfte, nd chromte hrdly ffect As(V) sorption y oxides nd hydroxides in soil (Livesey nd Hung 1981). Furthermore, sorption of As(V) on soil minerls depends on the residence time, which hs een ttriuted to different sites of rectivity, surfce nucletion precipittion or diffusion into micropores of the sorent (Sprks 1999). Severl kinetic studies hve shown tht the sorption rte of As(V) on Fe/Al oxide minerls nd WTR ws initilly rpid, ut ws followed y slower phse (Rven et l. 1998; O Reilly et l. 1; Mkris et l. 6). Srkr et l. (7) oserved liner As(V) sorption y WTR (without soil) for different As(V) lods (rnging from 225 to 7,5 mg As kg -1 WTR) ut proceeded slower therefter, reching % fter 48 h. There were numerous studies conducted on the effects of soil/solution chemistry on As(V) sorption y pure hydroxides, soils, nd WTR (no soil) systems; to the est of our knowledge, no work hs een conducted to investigte the effect of soil solution chemistry on As(V) sorption in WTR-mended soils. The ojectives of this study were to (1) evlute As(V) sorption s function of soil solution prmeters, nmely, contct time nd t different initil As(V) doses t two WTR ppliction rtes, nd (2) determine effect of competing lignds (phosphte nd sulfte) nd complexing metl (clcium) on As(V) sorption in WTR-mended soil t n optimum SSR nd two WTR ppliction rtes. Mterils nd methods Soil nd WTR collection nd soil mendment Immoklee series soil, low in Fe Al oxyhydroxides, hving low As(V) retention cpcity, ws used for the study. Surfce ( 15 cm) soil smples of Immoklee were collected from Southwest Florid Reserch nd Eduction Center, Immoklee, FL, USA. The Fe- nd Al-sed WTRs were otined from the drinking-wter tretment plnts in Tmp, FL nd Brdenton, FL, USA, respectively. Soil nd WTR smples were llowed to ir-dry nd were susequently pssed through 2-mm sieve efore eing sujected to chrcteriztion nd sorption experiments. WTR (Fe- or Al-sed) ws thoroughly mixed with the soil t 25 nd 5 g kg -1 rtes, the moisture content ws djusted to 7 % of wter holding cpcity nd equilirted for 7 dys (7d). The ppliction rtes of WTR were chosen sed on prcticl ppliction rtes of iosolids in griculturl fields (Elliott et l. 2). Previous incution studies hve shown tht Immoklee soil possesses negligile As(V) retention cpcity (Srkr et l. 7); therefore, control ( % WTR) tretment ws not included in the sorption experiments. WTR nd soil chrcteriztion WTR nd soil smples were chrcterized for severl physicochemicl properties s discussed in Ngr et l. (9). In rief, solution, electric conductivity, nd orgnic mtter contents were mesured using stndrd protocols (Ben-Dor nd Bnin 1989; Hnlon et l. 1997, ). Oxlte-extrctle Fe nd Al concentrtions were determined using Tmm s regent (Loeppert nd Inskeep 1996). An inductively coupled plsm mss spectrometer (ICP-MS; Perkin Elmer Eln 9 model) ws used for elementl determintions. Adequte lnks, duplictes nd mtrix spikes were used to meet qulity ssurnce nd qulity control requirements. Experimentl design Arsenic(V) sorption kinetics Kinetic experiments were conducted t selected time intervls (1, 2, 4, 8, nd 16 dys) to determine the effect of

3 Int. J. Environ. Sci. Technol. (13) 1:1 1 3 contct time on As(V) sorption y WTR-mended soil. Smples were rected with As(V) solutions to ttin initil As lods of 125, 1,85 nd 3,75 mg kg -1. This rnge of As lods ws selected sed on the previous studies on WTRmended soils (Srkr et l. 7). Stock As(V) solutions were prepred in.1 M KCl using sodium hydrogen rsente (NH 2 AsO 4 7H 2 O, KR Grde, Aldrich, USA), nd SSR of g L -1 (Srkr et l. 7) ws used. All smples were shken t 1 rpm on reciprocl shker, nd smples were collected t fixed definite time intervls, centrifuged t 4,g for 25 min, filtered nd nlyzed for totl solule As with grphite furnce tomic sorption spectrometer (GFAAS). No control ws imposed during kinetic experiments ut the of the smples ws mesured efore nd fter shking. Optimum contct time otined from this experiment ws utilized for further sorption experiments. Arsenic(V) sorption in sence of competing lignds nd complexing metl The effect of on As(V) sorption ws studied y determining the mount of As(V) sored within the rnge of 3 9. After initil soil-wtr equilirtion, representtive smples were rected with three initil As lods (125, 1,85, nd 3,75 mg kg -1 ) t g L -1 SSR. Arsenic(V) sorption envelope ws otined y djusting series of levels (3 9) with predetermined mount of 1 M HCl or NOH for ech tretment. Arsenic(V) sorption in presence of competing lignds nd complexing metl Sorption envelopes for the rection of As(V) with WTRmended soil were otined in the presence or sence of competing lignds (phosphte nd sulfte) nd complexing metl (clcium) t n initil As concentrtion of 125 mg kg -1 (lowest As lod). The molr rtios of As to competing/complexing lignds were 1:1, 1:2, nd 1:5. Stock solutions of phosphte nd sulfte were prepred in.1 M KCl using sodium phosphte monosic monohydrte (NH 2 PO 4 H 2 O, Regent Grde, Fisher, USA) nd sodium sulfte dechydrte (N 2 SO 4 1H 2 O, Regent Grde, Fisher, USA), respectively. The source of clcium ws clcium nitrte tetrhydrte (C(NO 3 ) 2 4H 2 O, Regent grde, Fisher, USA). The solution ws dded to the soil smples t g L -1 SSR, nd ws djusted to desired levels (3 9). Sttisticl nlysis Sttisticl nlysis ws performed using JMP IN version 5.1 (Sll et l. 5). Two-wy nlysis of vrince ws performed to exmine the effect of, contct time, initil As lods, competing lignds (phosphte nd sulfte), nd complexing metl (clcium) on As(V) sorption y WTRmended soil. Tukey Krmer honest significnt difference test ws used to evlute differences mong tretment mens. Tretment differences were deemed significnt t p B.1. Results nd discussion Soil nd WTR chrcteriztion Generl physicochemicl properties of the Immoklee soil nd WTR hve een discussed elsewhere (Ngr et l. 9). In rief, the soil nd WTR smples were cidic ( \ 5.4) in nture. Immoklee soil, which ws sndy in texture hd very low oxlte-extrctle Fe nd Al contents (.4 nd.2 g kg -1 ) compred to Fe-WTR (Fe content 78.7 g kg -1 ) nd Al-WTR (Al content 82. g kg -1 ), suggesting the very low oxynion sorption cpcity of soil. Effect of contct time on rsenic(v) sorption y WTR-mended soil At the lowest initil As concentrtion (125 mg kg -1 ), n pprent equilirium stte ws reched in 1 dy for Al-WTRmended soils (Fig. 1). Incresing initil As lod resulted in incresed contct time to rech equilirium (*4 dys), nd significnt (p \.1) decreses in As(V) sorption. Srkr et l. (7) oserved similr trends of As(V) sorption in WTR-mended soil with incresing initil As lods. Mkris et l. (6) reported tht in sence of soil, As(V) sorption in Al-WTR (* % of As lod of 7,5 mg kg -1 ) reched n equilirium in 2 dys suggesting limited inding sites in Al- WTR-mended soils compred to Al-WTR (no soil). At 25 g kg -1 ppliction rte, 45 % of totl As(V) ws sored in the tretment with initil As lod of 1,875 mg kg -1, wheres 23 % sorption ws oserved in the tretment with initil As lod of 3,75 mg kg -1 (Fig. 1). Incresing ppliction rte to 5 g Al-WTR kg -1 resulted in significnt (p \.1) increse in overll As(V) sorption (Fig. 1). Mximum As(V) sorption t equilirium ws 58 nd 47 % for 1,875 nd 3,75 mg kg -1, respectively. Similr to Al-WTR-mended soil, mximum As(V) sorption for Fe-WTR-mended soil ws chieved in 1 dy for the lower initil As lod (125 mg kg -1 ) t oth ppliction rtes (Fig. 2, ). With increse in the initil As lod, however, significnt (p \.1) decrese in As(V) sorption nd n increse in equilirtion time up to 8 dys ws oserved. Longer equilirtion time compred to Al-WTR-mended soil (4 dys) is consistent with oservtions in previous studies (Mkris et l. 6; Srkr et l. 7; Ngr et l. 1), nd suggested tht Al-WTR hs greter

4 4 Int. J. Environ. Sci. Technol. (13) 1: mgas/kg 1875 mgas/kg 375 mgas/kg mgas/kg 1875 mgas/kg 375 mgas/kg Contct time (Dys) mgas/kg 1875 mgas/kg 375 mgas/kg Contct time (Dys) Fig. 1 Arsente sorption y Al-WTR-mended soil, t 25 g kg -1 () nd 5 g kg -1 () ppliction rtes, s function of contct time nd initil rsenic lods. Dt re the men of three replictes ± one stndrd devition As(V) sorption cpcity compred to the Fe-WTR. Arsenic(V) sorption kinetics y Fe-WTR (without soil) hs shown tht 2 dys were required to rech n equilirium stte (Mkris et l. 6), which incresed to 8 dys in the present study in the presence of soil with miniml As(V) retention cpcity. Similr to Al-WTR-mended soil, Fe-WTRmended soil showed significnt (p \.1) interction etween rte nd initil As concentrtions (dt not shown). Even though mximum As(V) sorption onto Al-WTRmended soil ws oserved fter 4 dys, further experiments for oth the WTR-mended soil were conducted t n optimum rection time of 8 dys for consistency. Effect of on rsenic(v) sorption y WTR-mended soil For Al-WTR-mended soil (25 g kg -1 ppliction rte), mximum As(V) sorption (* %) for the lowest As lod (125 mg kg -1 ) ws oserved t rnge 3. 6., which grdully decresed to 84 % with further increse in [ 6 (Fig. 3). This trend is similr to sorption Contct time (Dys) mgas/kg 1875 mgas/kg 375 mgas/kg Contct time (Dys) Fig. 2 Arsente sorption y Fe-WTR-mended soil, t 25 g kg -1 () nd 5 g kg -1 () ppliction rtes, s function of contct time nd initil rsenic lods. Dt re the men of three replictes ± one stndrd devition envelope for As(V) sorption y Al-WTR (no soil) (Ngr et l. 1). However, the effect of is more pronounced in the present study due to decrese in sorption sites in the presence of soil. With increse in As concentrtions in solution, ville sorption sites re sturted, resulting in more pronounced effect of (Fig. 3). Mximum As(V) sorption for higher As lods (1,875 nd 3,75 mg kg -1 ) ws 76 nd 66 %, respectively, t 3, which decresed to 42 nd 19 % when incresed to 9. (Fig. 3). With n increse in ppliction rte (5 g kg -1 ), there ws significnt (p \.1) increse in As(V) sorption, nd the effect of ecme less pronounced (Fig. 3). This could e explined y n increse in ville sites y dditionl mounts of Fe/ Al hydroxides t higher ppliction rte of Al-WTR. The effect of the incresed ppliction rte ecme more pronounced t higher initil As lod due to significnt (p \.1) interction etween rte nd initil As lods (dt not shown). For higher As lods (1,875 nd 3,75 mg kg -1 ), mximum As(V) sorption ws 93 nd 73 %,

5 Int. J. Environ. Sci. Technol. (13) 1:1 1 5 respectively, which decresed to 49 nd 23 % with n increse in to 8. (Fig. 3). Similr to Al-WTR-mended soil, Fe-WTR-mended soil experienced significnt (p \.1) decrese in As(V) sorption with n increse in, nd this effect ecme more pronounced with incresing As lod (Fig. 3). Overll, As(V) sorption y Fe-WTR-mended soil ws significntly (p \.1) lower thn Al-WTR-mended soil, similr to trends oserved in previous studies (Mkris et l. 6; Srkr et l. 7; Ngr et l. 1). Mximum As(V) sorption (t 3) in Fe-WTR-mended soil (25 g kg -1 ppliction rte) for the lowest As lod ws * %, which decresed to 57 % with incresing initil As concentrtion to 3,75 mg kg -1. Arsenic(V) sorption further decresed to 3 %, with n increse in to 7.5 for 3,75 mg kg -1 As lod (Fig. 3c). In the present study, there ws n interesting pttern of incresing As(V) sorption fter 7. nd this effect ws more prominent t higher ppliction rte (5 g kg -1 )offe- WTR (Fig. 3d). For 5 g kg -1 ppliction rte, As(V) sorption decresed to 67 nd 23 % for 125 nd 3,75 mg kg -1 lods of As, respectively, t 7 which incresed to 7 nd 3% with n increse in [ 7(Fig. 3d). This increse could e contriuted y reductive dissolution of Fe-WTR t higher (Ngr et l. 1). The results were in ccordnce with our previous studies on As(V) sorption envelops y Fe-WTR (no soil) (Ngr et l. 1). Effect of competing lignds on As(V) sorption y WTR-mended soil Phosphte resulted in significnt decrese in As(V) sorption for Al-WTR-mended soils with strong dependence mgas kg mgas kg mgas kg mgas kg mgas kg mgas kg c mgas kg mgas kg mgas kg -1 d mgas kg mgas kg mgas kg Fig. 3 Arsenic sorption y Al-WTR-mended soil t 25 g kg -1 () nd 5 g kg -1 () ppliction rtes nd y Fe-WTR-mended soil t 25 g kg -1 (c) nd 5 g kg -1 (d) ppliction rtes s function of, nd initil rsenic lods. Contct time ws 8 dys nd SSR ws 1:5. Dt re the men of three replictes ± one stndrd devition

6 6 Int. J. Environ. Sci. Technol. (13) 1:1 1 on (Fig. 4, ). At n equimolr concentrtion of phosphorus (P) nd As, As(V) sorption ws lmost % t 3 7 for 25 g kg -1 ppliction rte (Fig. 4). Arsenic(V) sorption significntly decresed to,, nd % with incresing vlues to 7, 8, nd 9, respectively, compred to the control (without P) (Fig. 4). This trend suggested tht t lower, sufficient sorption sites were present on the WTRs for phosphte nd As(V) to sor simultneously. Incresed, however, resulted in n incresed negtive chrge on the WTR surfce nd more competition for potentil inding sites (Jin nd Loeppert ). Similr -dependent competition for As(V) sorption in the presence of phosphte hs een reported in previous studies on oxide minerls with WTR (Mnning nd Golderg 1996; Jin nd Loeppert ; Ngr et l. 1). With increse in the As:P molr rtio to 1:5, lrge drop in As(V) sorption ws oserved over the entire rnge, with only % sorption t 8 (Fig. 4). An increse in the Al- WTR ppliction rte to 5 g kg -1 incresed potentil inding sites for As(V) sorption resulting in significnt (p \.1) decrese in the competitive effect of P (Fig. 4). Al-WTR-mended soil (5 g kg -1 ) experienced no significnt decrese in As(V) sorption with n increse in the As:P molr rtio to 1:5 t \ 6. However, t [ 6, As(V) sorption significntly (p \.1) decresed to 82, 65, nd % t As:P molr rtio of 1:1, 1:2, nd 1:5, respectively (Fig. 4). For Fe-WTR-mended soil, similr -dependent competition effect of phosphte ws oserved (Fig. 4). The effect of phosphte competition nd, however, were 1 Control (No P) As:P- 1:1 As:P- 1:2 As:P- 1:5 1 Control (No P) As:P- 1:1 As:P- 1:2 As:P- 1: c 1 Control (No P) As:P- 1:1 As:P- 1:2 As:P- 1:5 d 1 Control (No P) As:P- 1:1 As:P- 1:2 As:P- 1: Fig. 4 Effect of competing ion (phosphte) on rsente sorption y Al-WTR-mended soil t 25 g kg -1 () nd 5 g kg -1 () ppliction rtes nd y Fe-WTR-mended soil t 25 g kg -1 (c) nd 5 g kg -1 (d) ppliction rtes s function of. Contct time ws 8 dys nd SSR ws 1:5. Initil rsenic lod ws 125 mg kg -1. Dt re the men of three replictes ± one stndrd devition

7 Int. J. Environ. Sci. Technol. (13) 1: Control (No S) As:S-1:1 As:S-1:2 As:S-1:5 1 Control (No S) As:S-1:1 As:S-1:2 As:S-1: Finl Finl c 1 Control (No S) As:S-1:1 As:S-1:2 As:S-1:5 d 1 Control (No S) As:S-1:1 As:S-1:2 As:S-1: Finl Finl Fig. 5 Effect of competing ion (sulfte) on rsente sorption y Al- WTR-mended soil t 25 g kg -1 () nd 5 g kg -1 () ppliction rtes nd y Fe-WTR-mended soil t 25 g kg -1 (c) nd 5 g kg -1 (d) ppliction rtes s function of. Contct time ws 8 dys nd SSR ws 1:5. Initil rsenic lod ws 125 mg kg -1. Dt re the men of three replictes ± one stndrd devition more pronounced compred to Al-WTR-mended soil due to comprtively smller externl nd internl specific surfce re of Fe-WTR for nion sorption (Mkris et l. 4). Arsenic(V) sorption (t 25 g kg -1 ppliction rte of Fe WTR) decresed to 97, 84, nd % t 3 nd As:P rtios of 1:1, 1:2, nd 1:5, respectively, which further decresed to 76, 57, nd 55 % fter incresing the to 8, compred to the control (without P) tretment (Fig. 4c). As result of incresed ppliction rte to 5 g kg -1, the effect of phosphte competition nd ecme less pronounced. Mximum As(V) sorption t the lowest P lod (As:P 1:1) ws 99 % t 3, which decresed to 77 % with n increse in the As:P rtio to 1:5 (Fig. 4d). An increse in the to 7.5 resulted in further decrese in As(V) sorption to 67 nd 23 % for the lowest nd highest P lods, respectively. The present study showed n pproximte 5 7 % increse in As(V) sorption ove 7 (Fig. 4d). As discussed erlier, this increse in sorption might e the result of incresed surfce re due to the reductive dissolution of Fe-WTR t higher (Ngr et l. 1). There ws no significnt effect of sulfte on As(V) sorption y Al- or Fe-WTR-mended soils t oth the ppliction rtes (25 nd 5 g kg -1 ) with vrying (Fig. 5). These results re in greement with the previous finding of As(V) sorption y Al- nd Fe-WTR (no soil), where ddition of sulfte ( mmol L -1 ) hd n insignificnt effect on sorption t n entire rnge (Ngr et l. 1). The unffected sorption ehvior of As(V) y sulfte could e explined y strong specific inding sites for As(V) on WTR surfces (Yng et l. 6; Mkris et l. 7), while sulfte my form outer sphere complexes. These results were lso in ccordnce with previous studies where reserchers found no influence of sulfte on

8 8 Int. J. Environ. Sci. Technol. (13) 1:1 1 1 Control (No C) As:C- 1:1 As:C- 1:2 As:C- 1:5 1 Control (No C) As:C- 1:1 As:C- 1:2 As:C- 1: c 1 Control (No C) As:C- 1:1 As:C- 1:2 As:C- 1:5 d 1 Control (No C) As:C- 1:1 As:C- 1:2 As:C- 1: Fig. 6 Effect of complexing ion (clcium) on rsente sorption y Al-WTR-mended soil t 25 g kg -1 () nd 5 g kg -1 () ppliction rtes nd y Fe-WTR-mended soil t 25 g kg -1 (c) nd 5 g kg -1 (d) ppliction rtes s function of. Contct time ws 8 dys nd SSR ws 1:5. Initil rsenic lod ws 125 mg kg -1. Dt re the men of three replictes ± one stndrd devition As(V) sorption on ferrihydrite in the entire rnge (Jin nd Loepprt ; Mnful et l. 1989). Effect of complexing metl on As(V) sorption y WTR-mended soil The overll sorption of As(V) ws enhnced in the presence of complexing ion, clcium (C), in the soil solution (Fig. 6). This effect ws not significnt (p \.1), however, for oth ppliction rtes (25 nd 5 g kg -1 ) of Al-WTR, ecuse most of the As(V) (* %) ws lredy sored (Fig. 6, ). Conversely, Fe-WTR-mended soil experienced significnt (p \.1) increse in As(V) sorption in the presence of C, nd this effect ws more significnt t [ 7 (Fig. 6c, d). At 25 g kg -1 ppliction rte of Fe-WTR, As(V) sorption (without C) t 7 nd 8 ws 5 nd 48 %, respectively, which incresed to 65 nd %, respectively, fter the ddition of C (As:C 1:2 rtio) into the system (Fig. 6c). When the As:C rtio incresed to 1:5, As(V) sorption incresed to 9 % t [ 7 (Fig. 6c). A similr trend ws oserved t 5 g kg -1 ppliction rte of Fe-WTR, where As(V) sorption incresed to lmost % t the As: C rtio 1:5 compred to the control (without C) tretment (Fig. 6d). Precipittion of clcium rsente could e the possile mechnism ehind incresed As(V) sorption t higher. Similr C-induced increses in As(V) sorption hve een reported for Fe-WTR without soil (Ngr et l. 1). The study showed tht precipittion of clcium rsente could e the possile mechnism ehind incresed As(V) sorption t higher, s clculted using ACT2 progrm of the Geochemist Workench 6. softwre.

9 Int. J. Environ. Sci. Technol. (13) 1:1 1 9 Conclusion In our previous studies, WTR showed promising results in removing As(V) from queous solution with vrying solution chemistry (Ngr et l. 1). The present study demonstrted the effectiveness of WTRs in removing As(V) from low Fe/Al hydroxide contining sndy soil under vrying set of soil solution chemistry. The Al- WTR-mended soil hd greter As(V) sorption cpcity thn tht of Fe-WTR-mended soil, consistent with previous experiments (Mkris et l. 4; Ngr et l. 1). For lowest As lod (125 mg kg -1 ), no -dependent effect ws oserved on As(V) sorption y Al- or Fe-WTRmended soil t oth ppliction rtes (25 nd 5 g kg -1 ). With increse in As lod, As(V) sorption significntly decresed with incresing, eing more prominent for lower ppliction rte of WTR. Presence of phosphte decresed As(V) sorption y oth Al- nd Fe-WTRmended soils with strong dependence on nd ppliction rtes. For Al-WTR-mended soils, there ws no significnt effect of phosphte ddition t lower (3 6) t oth ppliction rtes, ut with increse in eyond 7, As(V) sorption significntly decresed. The effect of phosphte competition nd, however, were more pronounced for the Fe-WTR-mended soil. Arsenic(V) sorption y oth Al- nd Fe-WTR-mended soil (t oth ppliction rtes) ws unffected y the ddition of sulfte. Addition of clcium resulted in incresed As(V) sorption y Fe-WTR-mended soils, more prominently for the higher ppliction rte of WTR. No pprent effect of clcium ddition ws oserved in Al-WTR-mended soil, ecuse most of the As(V) (* %) ws lredy sored y WTR surfces. The present study provides n in-depth understnding of As(V) sorption y soil mended with Alor Fe-WTR under vrying soil solution chemistry, which is necessry to demonstrte the effectiveness of WTR ppliction in the nturl soil environment. Field-sed experiments to vlidte the current results re necessry to further clirte the ppliction of WTRs in remediting As-contminted soils. Acknowledgments The uthors grtefully cknowledge NIH- SCORE, SALSI, nd USEPA-STAR progrms for funding this study. References Ben-Dor E, Bnin A (1989) Determintion of orgnic mtter content in rid-zone soils using simple loss-on-ignition method. Commun Soil Sci Plnt Anl : Elliott HA, Dempsey BA (1991) Agronomic effects of lnd ppliction of wter tretment sludges. J. AWWA 83: Elliott HA, O Connor GA, Brinton S (2) Phosphorus leching from iosolids-mended sndy soils. J Environ Qul 31: Goh K, Lim T (4) Geochemistry of inorgnic rsenic nd selenium in tropicl soil: effect of rection time, nd competitive nions on rsenic nd selenium dsorption. Chemosphere 55: Golderg S (2) Competitive dsorption of rsente nd rsenite on oxides nd cly minerls. Soil Sci Soc Am J 66: Hnlon EA, Gonzlez JS, Brtos JM (1997) Soil (1:2v/v). 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