Hydrous Ferric Oxide Incorporated Diatomite for Arsenic

8/11/2019 Hydrous Ferric Oxide Incorporated Diatomite for Arsenic

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Hydrous Ferric Oxide IncorporatedDiatomite for Remediation of Arsenic Contaminated GroundwaterM I N J A N G , * , S O O - H O N G M I N ,

J A E K W A N G P A R K , A N DE R I C J . T L A C H A C

Department of Civil and Environmental Engineering,University of Wisconsin s Madison, 1415 Engineering Drive, Madison, Wisconsin 53706, and Natural Resource Technology,Incorporated, 23713 West Paul Road, Suite D,Pewaukee, Wisconsin 53072

Two reactive media [zerovalent iron (ZVI, Fisher Fe0) andamorphous hydrous ferric oxide (HFO)-incorporatedporous, naturally occurring aluminum silicate diatomite[designated as Fe (25%)-diatomite]], were tested for batchkinetic, pH-controlled differential column batch reactors(DCBRs), in small- and large-scale column tests (about 50and 900 mL of bed volume) with groundwater from ahazardous waste site containing high concentrations ofarsenic (both organic and inorganic species), as well asother toxic or carcinogenic volatile and semivolatile organiccompounds (VOC/SVOCs). Granular activated carbon(GAC) was also included as a reactive media since apermeable reactive barrier (PRB) at the subject site wouldneed to address the hazardous VOC/SVOC contaminationas well as arsenic. The groundwater contained an extremelyhigh arsenic concentration (341 mg L - 1) and the resultsof ion chromatography and inductively coupled plasma massspectrometry (IC - ICP- MS) analysis showed that thedominant arsenic species were arsenite (45.1%) andmonomethyl arsenic acid (MMAA, 22.7%), while dimethylarsenic acid (DMAA) and arsenate were only 2.4 and 1.3%,respectively. Based on these proportions of arsenicspecies and the initial As-to-Fe molar ratio (0.15 molAsmolFe- 1), batch kinetic tests revealed that the sorptiondensity (0.076 molAs molFe- 1) for Fe (25%)-diatomite seems to be less than the expected value (0.086 molAs molFe- 1)calculated from thesorptiondensity data reported by Laffertyand Loeppert (Environ. Sci. Technol. 2005 , 39 , 2120-2127), implying that natural organic matters (NOMs) mightplay a significant role in reducing arsenic removalefficiency. The results of pH-controlled DCBR tests usingdifferent synthetic species of arsenic solution showed that the humic acid inhibited the MMAA removal of Fe (25%)-diatomite more than arsenite. The mixed system of GACand Fe (25%)-diatomite increased the arsenic sorption speed to more than that of either individual media alone. Thisincrease might be deduced by the fact that the addition ofGAC could enhance arsenic removal performance of Fe(25%)-diatomite through removing comparably high portions

ofNOMs.Small- and large-scalecolumn studies demonstrated that the empty bed contact time (EBCT) significantlyaffected sorpton capacities at breakthrough ( C ) 0.5 C 0)fortheFe0 /sand(50/50, w/w)mixture,butnot forGACpreloadedFe (25%)-diatomite. In the large-scale column tests withactual groundwater conditions, the GAC preloaded Fe (25%)-diatomite effectively reduced arsenic to below 50 g L- 1

for 44 days; additionally, most species of VOC/SVOCs werealso simultaneously attenuated to levels below detection.

IntroductionHuman activities and natural phenomena have causedreleases of arsenic into groundwater and surface water,creating potentially serious environmental problems forhumans and other living organisms. Since arsenic contami-nation is a health risk for many countries around the world,there is an urgentdemandfor a highlyeffective, reliable, andeconomical technique for the removal of arsenic fromgroundwater.

Permeable reactive barrier (PRB) technology may be a

practical and economical alternative to conventional groundwater remediationsystems for treatingarsenic contaminatedgroundwater. PRBs involve the placement or formation of areactive treatment zone in the path of a dissolved contami-nant plume with the objective of passively removing targetcontaminants or altering them by physical, chemical, and/orbiologicalprocesses toreduce their toxicityand/or mobility in the subsurface. PRBs have been successfully applied tomany organic compounds such as chlorinated ethenes,including trichloroethylene and tetrachloroethylene, andinorganiccontaminantssuchas chromium, uranium, arsenic,andother dissolved metals in groundwater ( 1- 5 ). PRBshavebecome increasingly popular because the operation andmaintenance costs are significantly less expensive than thetraditional pump and treat method. Many materials havebeenused as a reactivemediumin pilot-scale PRBs, including recycled foundry waste, zerovalent iron (Fe 0), activatedalumina, and ferric oxide. Among them, Fe 0 has beenextensively utilized as a reactive medium in PRBs becauseit hasbeen shown to be effective in removing anionic metals,such asCr(VI), Cr(III),As(V),andAs(III),as wellas halogenatedorganic compounds by means of coprecipitation and reduc-tive dehalogenation ( 6 - 8 ). Several forms of Fe 0 with goodstructural and hydrodynamic properties are also com-mercially available. However, Fe 0 can potentially elevate pHand Fe(II) concentration in groundwater due to a corrosionreaction with dissolved oxygen. This reaction will changegroundwaterconditions, resulting in potentiallyundesirablechemical reactions and a reduction in oxidation potential.In addition, due to relatively slow adsorption kinetics andcompetition effects of other anions such as silicate or

phosphate, it is necessary to design a PRB with an extrathickness of Fe 0 materials ( 9 ). To overcome these drawbacksof Fe 0, an effective and economical adsorptive medium hasbeen developed in our previous study for the removal of arsenic from water ( 10 ).

Previous research has shown that amorphous hydrousferric oxide (HFO) incorporated into porous, naturally occurring aluminum silicate diatomite [designated as Fe(25%)-diatomite] was more efficient than the conventionalmedium, AAFS-50, for arsenite and arsenate removal, andthat the increase in efficiency was particularly pronounced with respect to arsenite ( 10 ). Diatomite (or diatomaceousearth) is a lightweightsedimentaryrockcomposedprincipally

* Corresponding author phone: + 82-2-3702-6592; fax: + 82-2-3702-6609; e-mail: [email protected]; current address: Soil Reme-diation Team, Korea Mine Reclamation Corporation, Coal Center,80-6 Susong-dong, Jongno-gu, Seoul, 110-727, Korea, Republic of.

University of Wisconsin -- Madison. Natural Resource Technology, Incorporated.

Environ. Sci. Technol. 2007 , 41, 3322 - 3328

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of silica microfossils of aquatic unicellular algae having avarietyof pore structures with upto 80 - 90%voids. Diatomitehas not only been approved as a food-grade material by theFood and Drug Administration (FDA), but it is also stable inthe liquid phase since it originated and is produced from asea or lake. These properties will result in good stability ina saturated environment, such as a PRB, as well as envi-ronmental friendliness. Meanwhile, amorphous HFO hasbeen widely studied as a promising adsorptive material forremoving both arsenate and arsenite ( 11) due to its highselectivity for arsenic species. An incipient wetness impreg-

nation method using a vortexing device was developed todisperse and incorporate an HFO precursor homogeneously on the pore surfaces of diatomite. Since a large volume of precursor solution is not needed or wasted, this techniqueis simple, economical, and environmentally friendly.

In this study, two reactive media [Fe 0 and Fe(25%)-diatomite] were tested with groundwater from a hazardous waste site in northern Wisconsin containing high concentra-tions of both organic and inorganic species of arsenic, as well as other toxic or carcinogenic VOC/SVOCs. Granularactivated carbon (GAC) was also included in this study as areactive media since a PRB at the subject site would need toaddress the hazardous VOC/SVOC contaminations as wellas arsenic. Adsorption with GAC is a common, proventechnology for removal of VOC/SVOC compounds from

water.The VOC/SVOCremoval wascharacterizedin the large-scale columntestsas describedbelowto determine theeffectof simultaneous arsenic adsorption with both media. In anattempt to discover another purpose for the use of GAC, wetried to find some synergetic effects of GAC on the arsenicremoval performance of Fe (25%)-diatomite by removing natural organic matters(NOMs) in groundwater, whichhaveubiquitous presence in natural aquatic systems and signifi-cant effects on the arsenic mobility due to their redox andcomplexation capabilities. Since NOMs have negatively charged functional groups such as carboxylic, phenolic,quinone, amino, sulfhydryl,nitroso, and hydroxyl functionalgroups( 12 ) ata neutralpH, theycouldgivenot onlysignificantinhibitory effects on the arsenic removal performance of Fe(25%)-diatomite, but also desorption effects that are com-parative to phosphate ( 13). Fisher Fe 0, Fe (25%)-diatomite,and GAC were first subjected to kinetics tests to determinearsenic adsorption rates and capacities with actual groundwater and synthetic solutions. Fisher Fe 0 showed not only the highest arsenic removal speeds, but also the highestarsenic sorption capacitiesamong the othercommercializedFe 0 products ( 7 ). Then, the Fe 0/sand (50/50, w/w) mixtureandGAC preloaded Fe (25%)-diatomitewere testedin small-scale column tests with a fast flow rate. Finally, these tworeactive media were tested in large-scale columns to deter-minebreakthroughswith actualgroundwater flowconditionsat the subject site. All testing utilized groundwater from thesubject site.The objectives of thisstudy were(1) to investigatethe arsenic removal efficiences of Fe (25%)-diatomite com-pared with Fe 0, (2) to evaluate the enhancement of arsenicremoval performance with the addition of GAC, and (3) tofind out the applicabilities of dual media [GAC preloaded Fe(25%)-diatomite] for actual high concentrations of organicand inorganic arsenic, as well as VOC/SVOC contaminatedgroundwaters through different sizes of column tests.

Materials and MethodsSample Collection and Characterization. The bulk groundwater sample was collected from a monitoring well at thesubject site.This particularlocationwas selected baseduponanalytical datafrom previouscharacterizationwork.The bulk sample was collected utilizing low-flow methods ( 14) tominimize aeration of the groundwater during sample col-lection. This was done to minimize changes in arsenic

speciation and loss of VOC/SVOC compounds which canoccur during sample aeration. The bulk sample was filteredduring collection with a 0.45- m inline disposable groundwater filter to remove suspended solids that could interfere with laboratory chemical analysis and testing. Following filtration, the bulk sample was preserved in a refrigerator (4 C) during long-term storage and handling, then pumpeddirectly into 19 L Tedlar and/or Teflon bags to minimizeaeration for column tests. The arsenite oxidation processesand microbial arsenic transformations considered for theseprocedures were negligible.

The total arsenic concentrations of groundwater orsynthetic solutions wereanalyzedwith an inductivelycoupledplasma atomic emissionspectrometer (ICP - AES, Jobin YyonInc., Edison, NJ) for samples having arsenic concentrationslarger than 1 mg L - 1. An atomic absorption spectropho-tometer (AAS, Varian AA-975) and a GTA-95 graphite tubeatomizer with a programmable sample dispenser (Palo Alto,California) were also used for samples having arsenicconcentrations lower than 1 mg L - 1. Inthiscase,a50mgL - 1nickel solution was used as a matrix modifier. The detectionlimits of ICP - AES and AAS-graphite were 0.033 and 0.0006 mol L - 1 for arsenic, respectively. Ion chromatography andinductively coupled plasma mass spectrometry (IC - ICP -MS) were used to quantify arsenic species in groundwatersamples. The chromatographic separationof arsenic species

was achieved using gradient elution with dilute sodiumhydroxide on a high-capacity anion exchange column. A DX-500 ion chromatograph (Dionex) and an Elan 6000 ICP - MS(Perkin-Elmer) wereused forarsenic speciationanalysis. Thegroundwater sample also contained 2 - 3 mg L - 1 volatileorganic compounds (VOCs) and 3 - 4 mg L - 1 semi-volatileorganic compounds (SVOCs). VOC/SVOC compounds wereidentified and quantified by gas chromatography and massspectrometry (GC - S) following USEPA methods SW846-8260B (VOCs) and 8270B (SVOCs). Analytical testing forchemical composition and VOC/SVOC compounds of thegroundwater was conducted at TestAmerica Analytical Test-ing Corporation (Watertown, WI). Preparation of Fe (25%)-diatomite is described in the Supporting Information.

Batch Kinetic Study. A groundwater sample of 300 mL

was utilized for each kinetic of Fe (25%)-diatomite, Fe0

, GAC,ormixed media [GACandFe (25%)-diatomite].Thesesamples were purged with nitrogen gas for 24 h to remove the VOC/SVOC compoundspriorto kinetics.Except Fe 0 (halfvolume),masses of media were predetermined to have the samevolume (Table S3 in the SupportingInformation).Most casesof the column studies showed the mixture of Fe 0 with sandat thecondition of 50/50 (w/w)to avoid a significant clogging effect. An aliquot of 1.5 mL of suspension was withdrawn at10- 60 min intervals and centrifuged at a speed of 10 000rpm for 5 min. A 1-mL sample of the suspension was diluted with 19 mL of 1% HNO 3 solution prepared with deionized water. These samples were then analyzed for total arsenicutilizing an ICP - AES (Jobin Yyon Inc., Edison, NJ).

pH-Controlled Differential Column Batch Reactors(DCBRs). The pH-controlled DCBR tests were conductednot only to estimate the removal kinetics of different arsenicspecies on Fe (25%)-diatomite, but also to find the NOMadditioneffecton arseniteand MMAA thatwere thedominantarsenic species of groundwater. Figure S1 in the Supporting Information shows the schematics of pH-controlledDCBRs.The most important reason to use pH-controlled DCBR is tocontrolthe pH condition of feeding arsenic solutionbecausepH is one of the most significant parameters in affecting arsenic species and active surface sites of metal oxide ( 10 ).FortheevaluationoftheeffectofNOMonthearsenicremovalfor Fe (25%)-diatomite, humic acid (sodium salt, Sigma- Aldrich) was used as a representative of NOM species ingroundwater ( 15 , 16 ). Arsenite standard solution for ICP

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(arsenic trioxide, Sigma-Aldrich), sodium arsenate heptahy-drate(Na 2HAsO47H 2O),monosodiumacidmethane arsonate(Na 2HAsNaO 3, Chem Service),and cacodylicacid (C 2H7 AsO2,Sigma-Aldrich) were used to prepare 1000 mg L - 1 stock solutions of arsenite, arsenate, MMAA, and DMAA, respec-tively. The pH-controlled DCBR tests were run with thefollowing conditions: media mass (1 g),pH (6.8), empty bedcontact time (EBCT)(3.3min), arsenic (10 mgas AsL - 1),andhumic acid (100 mg L - 1, 50.3- 65.3 mg C L - 1). For arsenickinetic results, a pseudo second-order kinetic equation wasfound to fit well for many chemisorption processes using

heterogeneous materials ( 17 ). Therefore, all of the kineticdata from our experiments were fit with a pseudo second-order kinetic model in estimating the rate constants, initialsorption rates, and adsorption capacities for total arsenic.Ho et al. described the pseudo second-order kinetic rateequation ( 18 , 19 ). The pseudo-second-order kinetic modelcan be solved with the following equations. The kinetic rateequation is expressed as follows ( 17 - 20 ):

where q eq is the sorption capacity at equilibrium, and q isthesolid-phase loading of arsenic. The k 2 (g mmol - 1 min - 1) isthepseudo-second-orderrate constantfor thekinetic model.By integrating eq 1 with the boundary conditions of q ) 0(at t ) 0) and q ) q t (at t ) t ), the following linear equationcan be obtained:

In this equation, v 0 (mmol g - 1 min - 1) is the initial sorptionrate. Therefore, the v 0 and q eq values of kinetic tests can bedetermined experimentally by plotting t versus t / q t.

Column Studies. The 15-mm diameter and 150-mmheight glass columns (Bio-Rad Laboratories, Hercules, CA) were used for the small-scalecolumn experiments.The setupof media and operational parameters is shown in Table 1.For column B, 4 g of GAC was preloaded to 10 g of Fe (25%)-diatomite. The total mass of Fe 0/sand (50/50, w/w, 90 g) incolumn A was 6.4 times higher than that of GAC preloadedFe (25%)-diatomite. The groundwater sample was purged with N 2 gasfor 24 h to removeVOC/SVOCcompounds. Afterabout 200 mL of distilled water were passed through thecolumns, the groundwater sample was pumped into thebottom of the columns at a flowrate of approximately 6 mLhr - 1. Effluent samples were collected and diluted with 1%HNO 3 solution prior to total arsenic analysis as described inthe previous section.

The large-scale column tests were conducted for about3 months at a flowrate of 0.088 - 0.094 m day - 1 which

approximated groundwater flow conditions at the subjectsite.The VOC/SVOCremovalwas characterizedduring large-scale columin testing, which also allowed for analysis of theeffect of VOC/SVOCcompounds onarsenicadsorption.Table1 summarizes the experimental setup of the large-scalecolumn tests.Predeterminedamounts of mediawere packedinto 50-mm diameter by 500-mm high glass columns (AceGlass). In column B, GAC (40 g) was preloaded to Fe (25%)-diatomite(200 g).The total mass of GACpreloadedFe (25%)-diatomite(240g) was7.5timeslessthanthe Fe 0/sandmixture(1800 g, column A) at the similar bed volume. Bags,

connections, and transfer tubing made of Teflon were usedto prevent aeration and/or VOC/SVOC loss during columnoperation. Effluent samples were collected at 2 - 3 day intervals, and diluted with 1% HNO 3 solution prior to totalarsenic analysis and VOC/SVOCanalysis usingthe analyticalmethods described in the previous section.

Results and DiscussionSample Characteristics and Arsenic Speciations. Table S1shows thechemicalanalysis of thegroundwater sample using various analysis methods( 21- 23). Thesample hadvery higharsenic concentrations of 341 mg L - 1 (4.55 mmol L - 1) that were 6820 times higher than 50 g L- 1. The extremely higharsenic concentration represents arsenic contaminationcaused by human activities such as arsenic-based herbicide

or pesticide production at the subject site. Meanwhile,naturally occurring arsenic has been found in the groundwater of most aquifers of Wisconsin. Arsenic contaminationis especially prevalent in the sedimentary bedrock of northeastern Wisconsin ( 24). The primary cause for thisarsenic contamination is the oxidationof sulfide mineralizedzones containing arsenic. About 3.5% of private drinking water wells in Outagamie and Winnebago counties in Wisconsin were reported to have an arsenic concentrationhigher than 50 g L- 1. Along with arsenic, the groundwatercontains high concentrations of other anions such as sulfate(1600 mg L - 1 or 16.7 mmol L - 1) and chloride (300 mg L - 1 or8.5 mmol L - 1),as well ascationssuch assodium (440mg L - 1or 19.1 mmol L - 1), calcium (690 mg L - 1 or 17.3 mmol L - 1),and magnesium (49 mg L - 1 or 2.0 mmol L - 1). Since typicalgroundwater contains much lower concentrations, it wasthought that various salts (CaSO 4, NaCl, MgCl 2, etc), inaddition to arsenic, leaked into the groundwater from thehazardouswaste sites.Comparedto typical groundwater (1 -5 0 m g C L - 1), the groundwater contains a high TOCconcentration (72 mg C L - 1), representing potentially highNOMs or organic matters derived from the hazardous wastesite.

Table S2 in the Supporting Information shows theretention time, formulas, dissociation constants, concentra-tions, and percentages of known arsenic species (arsenite,arsenate, MMAA, and DMAA) and several unknown arsenicspeciesdetectedby IC - ICP- MSanalysis forthe groundwatersample. Dominant known arsenic species detected were

TABLE 1. Small-Scale and Large-Scale Column Tests Setup and Operational Conditions

columnID

flowrate(mL h- 1)

SLVa (m d- 1)

EBCTb (day) media setup

bedvolume

(mL)

totalmass ofmedia

small-scale column testsA 6.46 0.88 0.3 Fe 0 /sand (50/50, w/w) 46.9 90B 6.76 0.92 0.310 4 g of GAC preloaded 10 g of Fe (25%)-diatomite 50.2 14

large-scale column testsA 7.33 0.088 5.1 Fe 0 /sand (50/50, w/w) 886.9 1,800B 7.79 0.094 4.7 40 g of GAC preloaded 200 g of Fe (25%)-diatomite 937.1 240

a SLV ) Surface loading velocity. b EBCT is the time required for the liquid in an adsorption bed to pass through the column assuming thatall liquid passes through at the same velocity. It is equal to the volume of the empty bed divided by the flow rate.

dq t dt

) k 2 (q eq - q )2 (1)

t q

) 1v 0

+ 1q eq

t (2)

v 0 ) k 2 q eq 2 (3)

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arsenite (143 880 g L- 1, 45.1%) and MMAA (72 350 g L- 1,22.7%), but DMAA and arsenate were only 7600 (2.4%) and4010 g L- 1 (1.3%),respectively.Sincearsenite is highlytoxicand less removable for most metal oxides in subsurfaceenvironments, there is an urgent need to remediate groundwater in this area with an effective method. Methylatedarsenic species have been known to have less toxicity thaninorganic arsenic; however, they have not only highermobilities in subsurface environments, but also possibilitiesfor the demethylation process. The total arsenic concentra-tion of all species determined by IC - ICP - MS was about

94% of the total arsenic determined by ICP - AES, showing a reliable measurement of arsenic analysis. Along withmethyl- and demethylation of arsenic species, the unknownspecies might be formed through chemical or microbialtransformations of known arsenic species. In the previousstudy ( 25 ), the major portion of arsenic found within thegroundwaterat the site was in theorganic form even thoughlarge variations of speciation existed depending on specificsites. Accordingly, in order to set an efficient strategy of remediation, it is critical to speciate arsenic for eachcontamination site because the predominant organic formsof arsenic (MMAA or DMAA) and inorganic arsenite couldnot be easily adsorbed in minerals and are transported inthe dissolved form due to their high acid dissociationconstants ( 26 ).

BatchKineticStudy(AdsorptionCharacteristics). Kinetictests were conducted to determine several adsorptionparameters such as initial sorption rates, pseudo second-order rate constants, and equilibrium sorption capacities.Table S3 in the Supporting Information shows the kineticdata obtained from the pseudo second-order kinetic modelfor the sorption of total arsenic on Fe (25%)-diatomite, Fe 0,and GAC, respectively. All data in Figure S2 show reliablefittingconditions sincedeterminationcoefficients wereabove0.99 (Table S3). Fe (25%)-diatomite (20.5 mg g - 1) exhibitedabout 3 or 6 times higher arsenic sorption capacities thanFe 0 (6.6 mg g - 1) or GAC (3.8 mg g - 1), respectively. The initialsorption rates (0.24 mg g - 1 min - 1) of total arsenic removalfor Fe (25%)-diatomite were also 1.2 - 1.8 times greater thanthoseofFe 0 (0.19 mgg - 1 min - 1)andGAC(0.11mgg - 1 min - 1).

The sorption capability of GAC might be due to organicarsenic contents that have hydrophobic characteristics. Fe 0showed 0.0049 mol As mol Fe - 1 of sorption density. Otherliterature showed the range of 0.00035 - 0.0056 mol As mol Fe - 1of arsenic sorption capacities for Fe 0 in batch and columntests, in which higher sorption capacities were shown forhigher arsenic concentrations and arsenite species ( 6 , 7 , 9 , 27 - 29 ). Surface precipitation and/or adsorption by ferrousand/or ferric oxyhydroxides continuously produced by corrosion of Fe 0 have beenknownto be themain mechanismsof inorganicarsenic removalfor Fe 0 (7 , 29 ). Fe (25%)-diatomitehad 0.076 mol As mol Fe - 1 of sorption density at 0.15 of As-to-Fe molar ratio. In other studies using HFO, Raven et al.(30 ) showed sorption maxima of about 0.6 (or 0.58)and 0.25(or0.16)mol Asmol Fe- 1 wereachieved for arseniteand arsenateat pH 4.6(or pH 9.2), representinga higher sorption capacity for arsenite than arsenate at higher arsenic concentrations.Dixit and Hering ( 11) showed 0.31 and 0.24 mol As mol Fe - 1 forarsenite and arsenate at 0.03 - 0.3 of As-to-Fe molar ratio,respectively.Lafferty andLoeppert( 31) also reported a highersorption maxima of arsenite for HFO than other arsenicspecies.The calculatedsorptionmaxima ofarsenite, arsenate,MMAA,and DMAA on ferrihydrite were 0.2, 0.105,0.094, and0.075 mol As mol Fe - 1 with 0 - 0.22 of As-to-Fe molar ratio atpH 7.0. Thus, based on the above observations, the sorptiondensities of arsenite for HFO were close to the sorptionmaxima at < 0.3 of As-to-Fe molar ratio. Through X-ray diffraction(XRD) analysis, arsenic adsorption envelopes(pHeffect), BET surface areas, and surface complexation mod-

elings, ourprevious study identified ironoxide incorporatedintodiatomiteasHFO( 10 ).Withthe dataof sorptiondensitiesfor each arsenic species in the research of Lafferty and

Loeppert ( 31) and the proportions of arsenite (45.1%) andMMAA (22.7%) among total arsenic species, the sorptiondensity (0.076 mol As mol Fe - 1) seems to be less for Fe (25%)-diatomite than the expected values (0.086 mol As mol Fe - 1), in which sorption densities of only arsenite and MMAA werecalculated. As shown in other studies, the lower sorptiondensity observed with Fe (25%)-diatomite may partly be aresult of different arsenic species and content of NOMs ( 13,16 , 29 , 31, 32 ). Generally, NOMs have ubiquitous presencein natural aquatic systems and significant effects on thearsenic mobility due to their redox and complexationcapabilities.Since NOMshavenegativelycharged functionalgroups such as carboxylic, phenolic, quinone, amino, sulf-hydryl, nitroso, and hydroxyl functional groups ( 12 ) at aneutral pH, they could behave competitively toward thearsenic removal performance of media. Thus, we tried toestimate theeffect of NOMon the removal ofdifferentarsenicspecies by Fe (25%)-diatomite by conducting pH-controlledDCBR tests with synthetic solutions.

pH-ControlledDCBR Tests(Effectsof Arsenic Speciationand Humic Acid on Sorption Capabilities of Fe (25%)-Diatomite). Figure 1 shows the kinetic results of differentspecies of arsenic for Fe (25%)-diatomite in pH-controlledDCBR and the humic acid effects on arsenite and MMAA adsorptions. All data fit very well with the pseudo second-order kinetic model, showing > 0.97 of determination coef-ficients ( R 2)(Table S4). Fe (25%)-diatomite showed similarinitial sorptionrates (0.015 - 0.02mgg - 1 min - 1)and q eq (9.3 -10 mg g - 1) for arsenate, arsenite, and MMAA, while muchless for DMAA that had 0.005 mg g - 1 min - 1 (v 0) and 2.7 mg g - 1 (q eq ). Fe (25%)-diatomite had about 100% of arsenicsorption densities close to sorption maxima (0.037 mol Asmol Fe - 1) for arsenate, arsenite, and MMAA, while only 25%(0.01 mol As mol Fe - 1) for DMAA. These sorptionbehaviorsarequite similar to those reported by Lafferty and Loeppert(31), in which they studied the sorption behaviors of in-organicandmethyl-arseniccompounds withferrihydriteandgoethite. Although spectroscopic analysis is needed to findout the surface complexes geometry, the plausible reasonfor the similar adsorption trends for arsenite, arsenate, andMMAA is the fact that they have two hydroxyl groups whichcould complex with the surface of HFO. The lower capacity for DMAA might not only be due to the additional methylgroup of molecular geometry instead of oxygens, but also

FIGURE 1. Kinetic results using pH-controlled DCBR tests: media(1 g L- 1), pH 6.8, EBCT 3.3 min, arsenic (10 mg L- 1), and humic acid(100 mg L- 1)



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becauseof theelectrondonatingcharacteristicsof themethylgroups that could weaken the Fe - O- As bond ( 31).

In the presence of humic acid, concentrations of sorbedarsenite and MMAA decreased, even though the decreasetrends were different.The adsorptiontrend of arsenite in thepresence of humic acid wasinitially similar to that ofarsenite without humic acid for about 3 h, but gradually increasedand equilibrated at about 10 h with about 50% of sorbedarsenic concentration without humic acid. In the case of MMAA, humic acid significantly inhibited the MMAA ad-sorption with Fe (25%)-diatomite. Only about 10% of theadsorptioncapacity ofMMAA wasobtainedwith humic acid.Bowell ( 32 ) also showed that the methyl-arsenic adsorptioncapacities of several ferric oxides (goethite, hematite, orlepidocrocite) in the presence of fulvic acid were much lessthan those of inorganic arsenic at a neutralpH, representing that the competitioneffect of organic acids is greater towardMMAA than arsenite. This higher competition for MMAA might be dueto thefact that thehydrophobic characteristicsof MMAA could create a similar sorption mechanism with

humic acid for the sorption sites of HFO, and negatively charged MMAA species could be more easily complexed by metals and deprotonated functional groups within humicacid. More scientific investigations are needed for thecompetition effect between organic acids and inorganic/organic arsenic species.

Batch Kinetic Study [GAC Addition on Arsenic RemovalPerformance of Fe (25%)-Diatomite]. Additional kineticstudies were conducted to determine the synergetic effect of the addition of GAC to Fe (25%)-diatomite in groundwater.Figure 2 shows the kinetic results of GAC and Fe (25%)-diatomite individually and mixed together for groundwater.The mixed system exhibited an initial sorption rate of approximately 0.577 mg g - 1 min - 1, which is about 2.5 timeshigher than that of the individual system consisting of Fe(25%)-diatomite.Thus,these increasesof initialsorption ratesmight be the result of the additionof GAC, deducingthat theaddition of GAC could enhance the arsenic removal per-formance of Fe (25%)-diatomite by removing comparably high portions of NOMs that can compete for or block thesorption sites of Fe (25%)-diatomite ( 32 ).

ColumnStudies. Figure3Ashowstheresultsofthesmall-scale column test, which compared column A [Fe 0/sandmixture (50/50, w/w)] and column B (GAC preloaded Fe(25%)-diatomite). Column B demonstrated better arsenicremoval capacity than column A. The column containing Fe(25%)-diatomite could reduce the arsenic concentration inthe effluent to less than 1 mg L - 1 from the influent arsenicconcentration of 341 mg L - 1 for 7.5 BV, while column A did

not reduce the arsenic concentration to less than 1 mg L - 1after 0.8 BV. Arsenic concentrations in the effluent fromcolumnA increasedsharply to170 mgL - 1 until approximately 8 BVs had passed through the column, then continued toincrease steadily after that point, but not as quickly. Thebreakthroughs ( C ) 0.5 C 0) ofcolumn A and B were 11.3and18 BV, at which arsenic adsorption capacities (or sorptiondensities) for column A and B were 2.6 mg (g of Fe 0)- 1 (or0.002 mol As mol Fe - 1) and 26.8 mg [g of Fe (25%)-diatomite] - 1(or 0.099 mol As mol Fe - 1), respectively.

Figure 3B shows arsenic removal observed for about 3months during the large-scale column tests, which wereconducted at conditions which approximated anticipatedPRB conditions in the field (e.g., flowrate, anaerobic envi-ronment, presence of VOC/SVOC compounds). Column B[GACpreloaded Fe (25%)-diatomite] showedgreater arsenicremoval capabilities than column A (Fe 0/sand mixture) didfor groundwater, with a reduction in effluent arsenicconcentrations to less than 1 mg L - 1 for 44 days (8.9 L or 9.5BV), obtained by ICP - AES analysis. The large-scale Fe 0/sandmixture did not reduce the arsenic concentration in thecolumn effluent below 1 mg L - 1, even though the EBCT (5.1d) of large-scale column tests was 16 times longer than thatof small-scalecolumntests (0.3 d).Based onthebreakthroughof 1 mg L - 1, this result shows 42 L kg - 1 of the normalizedvolume of treatedgroundwater to media mass, which is about8% higher than the results from the small-scale columntests(39Lkg - 1 forcolumn B, Figure3A). Effluent samplesof large-

FIGURE 2. Kinetic results using the groundwater sample, batch test using the single mediumof GAC (11.3g L- 1) or Fe (25%)-diatomite(8.7 g L- 1), and the mixed media of GAC (11.3 g L- 1) and Fe (25%)-diatomite (8.7 g L- 1)

FIGURE 3. (A)Small-scale column tests, column A: Fe0 /sand mixture(50/50, w/w) (90 g) and column B: GAC (4 g) preloaded Fe (25%)-diatomite (10 g), and (B) large-scale column tests, column A: Fe0 / sand (50/50, w/w) (1800 g) and column B: GAC (40 g) preloaded Fe(25%)-diatomite (200 g).

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scalecolumnB wereagain analyzedwith AAS-graphitewhichhad higher sensitivity than ICP - AES. AAS analysis showsarsenic was treated to less than 50 g L- 1 for 8.4 L (9 BV),eventhougharsenicconcentrationswere initiallyhigherthan50 g L- 1 due to an unstable initial operational condition.Large-scale column B showed 20.8 mg [g of Fe (25%)-diatomite] - 1 of sorption capacity (or 0.078 mol As mol Fe - 1), while column A had 4.4 mg (g of Fe 0)- 1 (or 0.0033 mol Asmol Fe - 1) at breakthrough ( C ) 0.5 C 0). Thus, compared tosmall-scale column tests, large-scale column A showed abouta 1.7 times higher sorption capacity or density, while large-

scale column B had about 78% of arsenic sorption capacity.Different results of the normalized volume of treatedgroundwater to media mass and sorption capacities atbreakthrough might be mainly a result of different flowratesand media compositions. However, based on those observa-tions, results between the small- andlarge-scalecolumn testsfor the GAC preloaded Fe (25%)-diatomite indicated thatarsenic removal performance is independent of flowrate inthis range during the test period, compared to the Fe 0/sandmixture that showed flowrate-dependent characteristics onarsenic removals. The increase of arsenic removal perfor-mance in the large-scale column test could be explained by the fact that the corrosion and hydrolysis process related toFe 0 is the rate-limiting factor of arsenic removal since thearsenic removal step by HFO could be very fast, as shown

in the column containing Fe (25%)-diatomite ( 9 ).Exceptfor some VOC/SVOCcompounds that were at highconcentrations in influents, the GAC preloaded Fe (25%)-diatomite removed most species below the detection levelsat both 7 and 15.6 BV (Table S5). However, the Fe 0/sandmixture provided limited attenuation of halogenated VOCs,BETX (benzene, ethylbenzene, toluene, and xylenes), andnaphthalene, butdid notaffectthe concentrations ofacetone,phenol, or 4-methyl 2-pentanone (MIBK).In thecase of GACpreloaded Fe (25%)-diatomite, trichloroethene, cis -1,2-dichloroethene, and vinyl chloride were observed to beattenuated during the large-scale column test. Chloroben-zene, 1,2-dichlorobenzene, 1,4-dichlorobenzene, andmethylene chloride were attenuated significantly(Table S5).Degradation of halogenated VOCs, particularly chlorinated

ethenes, by corrosion reactions catalyzed by Fe0

has been well-documented ( 33- 36 ). However, degradation with Fe 0hasbeen eithernotdemonstratedor notevaluatedin previousstudies ( 8 ) for these halogenated VOCs. In addition, limitedattenuation of BETX and naphthalene was observed in theFe 0/sand column. No relative decrease in concentrationwasobserved for phenol, acetone, or MIBK either. In fact, theconcentration of acetone in the effluent from the column of the Fe 0/sand mixture was actually higher than the influentconcentration during column operation.

Implications for PRB Application. Large-scale columnstudies showed that GAC preloaded Fe (25%)-diatomite ispromising in the PRB application because high arsenicconcentrations were effectively reduced to below 50 g L- 1,and most species of hazardous VOC/SVOCs were alsosimultaneously attenuated below the detection levels withtheactualgroundwatercondition.The independencyof EBCT(or flowrate) in terms of sorption capacities at breakthrough(C ) 0.5 C 0) or normalized volume of treated groundwaterto media mass at 1 mg L - 1 breakthrough in treating arsenicmust be a beneficial characteristic because this system canbe effectively applied to groundwater having a wide rangeof flowrate. Along with high sorption speeds, the GACpreloaded Fe (25%)-diatomite could be easier to handle andinstall for PRB applications since their bulk density is 6 - 8times lighter than theFe 0/sandmixture. During the 3-monthoperation of large-scale column tests, Fe (25%)-diatomitedid not break off, indicating good hydraulic stabilities. Withspectroscopic analysis, more abiotic studies are underway

to find the competition effects of NOMs and removalmechanisms of different arsenic specieswith different ratiosof GAC and Fe (25%)-diatomite.

AcknowledgmentsThe present study was supported by the owner/operator of the subject site, who wished to remain anonymous in thispublication. The authors also acknowledge the work of Dr.Dirk Wallschlaeger at Trent University in Peterborough,Ontario, Canada for the identification and quantification of organic and inorganic species of arsenic in the groundwatersamples used in this study.

Supporting Information AvailableMethodology of Fe (25%)-diatomite preparation, schematicof pH-controlled DCBR (Figure S1), characteristics of thegroundwater sample (Table S1), arsenic speciation results(TableS2), conditionsandresultsof batch kineticstests (TableS3 and Figure S1), results of the pH-controlled DCBR tests(Table S4),and the results of VOC/SVOC analysisfor influentsand effluents of each column in large-scale column tests(Table S5). This material is available free of charge via theInternet at http://pubs.acs.org.

Literature Cited(1) Ebert, M.;Kober,R.; Parbs, A.;Plagentz, V.;Schafer, D.;Dahmke,

A. Assessing degradation rates of chlorinated ethylenes incolumn experiments with commercial iron materials used inpermeable reactivebarriers. Environ. Sci.Technol. 2006 , 40 (6),2004 - 2010.

(2) Wilkin, R. T.; Su, C.; Ford, R. G.; Paul, C. J. Chromium-removalprocesses during groundwaterremediationby a zerovalent ironpermeable reactive barrier. Environ. Sci. Technol. 2005 , 39 ,4599 - 4605.

(3) Morrison,S. J.;Mushovic,P. S.;Niesen,P. L.Earlybreakthroughof molybdenum and uranium in a permeable reactive barrier.Environ. Sci. Technol. 2006 , 40 (6), 2018 - 2024.

(4) Weisener, C. G.; Sale, K. S.; Smyth, D. J. A.; Blowes, D. W. Fieldcolumn study using zerovalent iron for mercury removal fromcontaminatedgroundwater. Environ.Sci. Technol. 2005 , 39 (16),6306 - 6312.

(5) Szecsody, J. E.; Fruchter, J. S.; Williams, M. D.; Vermeul, V. R.;Sklarew, D. In situ chemical reduction of aquifer sediments:Enhancement of reactive iron phases and TCE dechlorination.Environ. Sci. Technol. 2004 , 38 (17), 4656 - 4663.

(6) Nikolaidis, N. P.; Dobbs, G. M.; Lackovic, J. A. Arsenic Removalby Zero-valent Iron: Field Laboratory and Modeling Studies.Water Res. 2003 , 37 , 1417- 1425.

(7) Su,C.; Puls,R. W.Arsenateand ArseniteRemoval by ZerovalentIron: Kinetics, Redox Transformation, and Implications for InSitu Groundwater Remediation. Environ. Sci. Technol. 2001 ,35 , 1487- 1492.

(8) USEPA. Permeable Reactive Barrier Technologies for Contami-nant Remediation ; EPA/600/R-98/125; Office of Research andDevelopment, Office of Solid Waste and Emergency Response,USEPA: Washington, DC, 1998.

(9) Su, C.; Puls, R. W. In Situ Remediation of Arsenic in SimulatedGroundwater UsingZerovalent Iron: LaboratoryColumn Testson Combined Effects of Phosphate and Silicate. Environ. Sci.Technol. 2003 , 37 , 2582- 2587.

(10) Jang, M.; Min, S. H.; Kim, T. H.; Park, J. K. Removal of Arseniteand Arsenate Using Metal Oxide Incorporated Naturally-Occurring Porous Diatomite. Environ. Sci. Technol. 2006 , 40 (5), 1636 - 1643.

(11) Dixit, S.; Hering, J. G. Comparison of arsenic (V) and arsenic(III) sorption ontoiron oxideminerals: implicationsfor arsenicmobility. Environ. Sci. Technol. 2003 , 37 , 4182- 4189.

(12) Redman, A. D.; Macalady, D. L.; Ahmann, D. Natural organicmatter affects arsenic speciation and sorption onto hematite.Environ. Sci. Technol. 2002 , 36 (13), 2889 - 2896.

(13) Ghosh, A.; Saez, A. E.; Ela, W. Effect of pH, competitive anionsand NOM on the leaching of arsenic from solid residuals. Sci.Total Environ. 2006 , 363, (1- 3), 46- 59.

(14) USEPA. Low-flow(minimaldrawdown)ground-water sampling procedures ; Office ofResearchand Development,Officeof Solid Wasteand EmergencyResponse,USEPA: Washington,DC, 1996.



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