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Arsenic removal from water using naturaliron mineral–quartz sand columns

Huaming Guo a,⁎, Doris Stüben b, Zolt Berner b

a Department of Water Resources and Environment, China University of Geosciences, Beijing 100083, PR Chinab Institute of Mineralogy and Geochemistry, University Karlsruhe (TH), Karlsruhe 76128, Germany

Received 22 November 2006; received in revised form 30 January 2007; accepted 4 February 2007Available online 23 March 2007

Abstract

The study has investigated the feasibility of using siderite-coated quartz sand and/or hematite-coated quartz sand columns forremoving As from water. Arsenic-spiked tap water and synthetic As solution with As concentrations from 200 to 500 μg/L wereused for the experiments. Since three coating methods employed to prepare siderite-coated quartz sand and hematite-coated quartzsand had no significant impact on As adsorption in batch tests, the column fillings were produced by means of the simplest oneinvolving mechanically mixing the Fe mineral with quartz sand. Fixed bed tests show that the combination of siderite-coated quartzsand and hematite-coated quartz sand greatly promoted the column performance in removing As and the presence of As(III) in theinfluent improved the removal efficiency of the column. The relatively low capacity in treating As-spiked tap water arose from thesuppression of FeCO3 dissolution in the presence of high HCO3

− concentration (333 mg/L), which consequently limited theformation of fresh Fe(III) oxides. However, the H2O2-conditioning greatly increased As adsorption capacity of the column forremediating As-spiked tap water. The Toxicity Characteristic Leaching Procedure (TCLP) test shows that the spent adsorbents werenot hazardous and could be safely disposed of to landfill.© 2007 Elsevier B.V. All rights reserved.

Keywords: Adsorption; As species; Siderite-coated sand; Hematite-coated sand; Batch tests; TCLP

1. Introduction

High As concentrations have been found in naturalgroundwater sources which provide drinking water formillions of people in parts of Bangladesh, West Bengal,Argentina, China, Mexico, USA, Chile and Japan(Smedley and Kinniburgh, 2002). As a worldwideenvironmental problem, arsenic (As) contamination of

drinking water is of great concern, at both governmentaland scientific levels, because of the chronic effects of Aspoisoning on those drinking that water for a long time.The chronic effects commonly include skin diseases(pigmentation, dermal hyperkeratosis, skin cancer), manyother cardiovascular, neurological, hematological, renaland respiratory diseases, as well as lung, bladder, liver,kidney and prostate cancers (Morton and Dunette, 1994).

Unfortunately, there is no known cure for Aspoisoning and therefore providing As free drinkingwater is the only way to eliminate its adverse healtheffects (Genç-Fuhrman et al., 2005a). Research is in-tensive on improving the established, and on developing

Science of the Total Environment 377 (2007) 142–151www.elsevier.com/locate/scitotenv

⁎ Corresponding author. Tel.: +86 10 8232 0679; fax: +86 10 82321081.

E-mail address: hmguo@cugb.edu.cn (H. Guo).

0048-9697/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.scitotenv.2007.02.001

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novel, treatment technologies for removing As fromdrinking water (Nikolaidis et al., 2003; Ahn et al., 2003;Saitfúa et al., 2005; Zhang and Itoh, 2005). The majorcurrent methods include coagulation, precipitation, ionexchange, and adsorption using a variety of Fe-contain-ing solids as adsorbents. Hsia et al. (1994) and WilkieandHering (1996) have studied hydrous Fe oxide, Ravenet al. (1998) and Jessen et al. (2005) ferrihydrite, Sun andDoner (1998) goethite, Nikolaidis et al. (2003) zero-valent Fe, Zhang and Itoh (2005) Fe oxide-loaded slag,Zeng (2003) silica-containing Fe oxide, Zouboulis andKatsoyiannis (2002) Fe oxide-loaded alginate beads, andZhang et al. (2003) Ce(IV)-doped Fe oxide for Asretention. These synthetic Fe-containing adsorbents hadgood affinities for both As(III) and As(V) due to theirhigh specific surface area.

Most people affected by As contamination world-wide are poor, live in small communities and rely ontube wells as a source of drinking water (Genç-Fuhrmanet al., 2005b). The process utilizing natural geomaterialsas adsorbents is considered as a promising technologyfor As removal because it is cost-effective in installationand maintenance, easy to operate and readily availablefor small scale treatment plants and household systems,and may provide largely sludge-free operation and havea regeneration capability (Bajpai and Chaudhuri, 1999).Those geomaterials included natural zeolite and volca-nic stone (Elizalde-González et al., 2001), natural ironores (Zhang et al., 2004), oxisol (Ladeira and Ciminelli,2004), and ferruginous manganese ore (Chakravartyet al., 2002).

Natural siderite and natural hematite have previouslybeen investigated for As removal from water solution.These minerals with a grain size of 0.25–0.50 mm wereeffective in removing As species when packed incolumn filters (Guo et al., accepted for publication).Though primary results obtained in batch experimentsshow that As removal by natural siderite with a grainsize of 0.04–0.08 mm was up to 895 μg/g, much higherthan that with a grain size of 0.25–0.50 mm (68.1 μg/g),the fine grain size fractions, simultaneously generated inthe process of mineral crushing, were not well utilized.The principal reason was that fine particles wouldinduce a significant hydraulic obstruction when waterflows through the column accommodating them.

Consequently, this study was carried out to investi-gate the possibility of using the fine particles of sideriteand hematite as coating materials onto matrix of quartzsand or feldspar under continuous flow conditions forAs removal. The main objectives are to (i) develop newadsorbents from the fine particles of natural siderite andnatural hematite that are suitable for the column

application; (ii) investigate As adsorption using thenew adsorbents in batch tests; (iii) characterize Asuptake of the columns packed with the new adsorbentsunder different operating conditions. The present paperalso addressed the adsorption performance of the newadsorbents in a packed-bed column in contact with As-spiked tap water. The existence of multi-component inthe solution, as usually found in real-world systems, isof particular importance when practical applicationsinvolving the column filters are considered, where com-petition between different components for the availableadsorption sites occurs (Deschamps et al., 2005).

2. Materials and methods

2.1. Materials

In this study, natural Fe minerals, including twosiderites (SIO3 and SIO4) and one hematite (HIO1), weretaken from Mineral Collection Centre of Freiberg,Germany. Siderite contents are 78 and 96% in SIO3 andSIO4, respectively. SampleHIO1 is a hematite-dominatedmineral with hematite content of 86%. Relativelyconsiderable goethite is presented in SIO3 (19%). Guoet al. (accepted for publication) provided more detaileddescriptions of mineralogical and chemical compositionsof those samples. The mineral samples were ground andsieved to produce various particle size fractions. Particlesize fractions, b0.04, 0.04–0.08, 0.08–0.1, 0.1–0.25 and0.25–0.50 mm, were used in the study.

All reagents used, including sodium chloride (NaCl)and hydrogen peroxide (H2O2), were of analyticalgrade. Stock As solutions (100 mg/L) were preparedfor As(III) and As(V) from sodium arsenite (NaAsO2,Fluka Chemical) and sodium arsenate (Na2HAsO4·7H2O, Fluka Chemical), respectively, using Milli-Qwater. All glassware and sample bottles were washedwith a detergent solution, rinsed with tap water, soakedwith 1% sub-boiled HNO3 for at least 12 h, and finallyrinsed with Milli-Q water three times.

2.2. Analytical methods

Total As concentration was analyzed by standardmethod using graphite furnace atomic absorptionspectrometry (GF-AAS, Model 4110ZL, PerkinElmer). A flow injection hydride generation system(FIAS 200, Perkin Elmer) coupled with atomicabsorption spectrometry (AAS 4001, Perkin Elmer)was used for As speciation. The detailed analyticalprocedure was described by Rüde and Puchelt (1994).The detection limits of total As, As(III) and As(V) were

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1 μg/L, 0.2 μg/L and 0.3 μg/L, respectively. Major anionswere determined by ion chromatography (IC, DX-100,Dionex) coupled with a Dionex As40 automated sampler,major cations and Mn by flame atomic absorptionspectrometry (Flame-AAS, Model 1100B, PerkinElmer). The detection limits of Cl−, NO3

−, SO42−, HPO4

2−,K, Na, Ca, Mg and Mn were 0.01 mg/L. Iron and traceelements were determined by high resolution inductivelycoupled plasma mass spectrometry (HR-ICP-MS, Axiom,VG Elemental). The detection limits of Fe, Cr, Ni, Zn andPb were 0.4 μg/L, 0.2 μg/L, 0.1 μg/L, 0.4 μg/L and0.1 μg/L, respectively.

The solution pH was monitored by a WTW pH probe(pH-Electrode SenTix 43-1) and meter (Model # pH330), dissolved oxygen (DO) by a WTW meter (Model# MultiLine P4, coupled with an oxygen sensor ofOxiCal-SL), and electrical conductivity (EC) by porta-ble WTW EC meter (Model # LF330, coupled with anEC probe of TetraCon 325).

2.3. Preparation of mineral-coated materials

Minerals with grain size range of 0.04–0.08 mmwere used as coating agents, and feldspar or quartz sandwith grain size range of 0.125–0.25 mm as matrix to becoated. Before coating, feldspar and quartz sand wereimmersed in 1% sub-boiled HNO3 for 24 h, rinsed withMilli-Q water three times, and dried in an oven at 50 °Covernight. After cooling off to room temperature in adesiccator, they were stored in capped bottles.

2.3.1. Mechanical methodsMechanical coating was carried out in two different

ways. One (mechanical method #1) involved moisteningthe acid-rinsed feldspar or quartz sand using a spraybottle that sprayed a very fine mist, sprinkling 20%mineral (by weight) over uniformly damp feldspar orquartz sand, and homogeneously mixing them. Thecoated feldspar or quartz sand was spread in a thin layer(1–2 cm) on a tray and dried in an oven at 105 °C for 4 h(Genç-Fuhrman et al., 2005b). The dried coatedmaterials were stored in capped bottles. The other(mechanical method #2) only involved mixing 20%mineral (by weight) with the acid-rinsed feldspar orquartz sand homogeneously. The coating methods andcorresponding materials are listed in Table 1.

2.3.2. Chemical methodIn the first step of chemical coating, 20% mineral (by

weight) was fully mixed with acid-rinsed feldspar orquartz sand. The reactive solutionwith 0.5MofH2O2wascontinuously added to themixture until the ratio of solid tosolution approximated to 400 g/L. After 24 h of reaction,the supernatant was decanted and the residual was dried inan oven at 105 °C for 12 h. The coated materials preparedby chemical method are listed in Table 1.

2.4. Batch experiments

Batch experiments were carried out with pristinematerials to investigate the effect of grain size range, and

Table 1Preparation of mineral-coated quartz sand/feldspar in batch experiments

Sample no. Coatingmethod

Acid-rinsed feldsparor quartz

Mineralused

Wetting thefeldspar/quartz

Mechanicallymixing

Adding H2O2

solutionDrying inthe oven

CM1 Mechanicalmethod #1

Feldspar – a – – – –CM2 Feldspar SIO3 × b × – ×CM3 Feldspar HIO1 × × – ×CM4 Quartz sand – – – – –CM5 Quartz sand SIO3 × × – ×CM6 Quartz sand HIO1 × × – ×CM7 Mechanical

method #2Feldspar – – – – –

CM8 Feldspar SIO3 – × – –CM9 Feldspar HIO1 – × – –CM10 Quartz sand – – – – –CM11 Quartz sand SIO3 – × – –CM12 Quartz sand HIO1 – × – –CM13 Chemical

methodFeldspar – – – – –

CM14 Feldspar SIO3 – – × ×CM15 Feldspar HIO1 – – × ×CM16 Quartz sand – – – – –CM17 Quartz sand SIO3 – – × ×CM18 Quartz sand HIO1 – – × ×a Indicates that the related procedure has not been performed.b Indicates that the related procedure has been performed.

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with coated materials to test the effect of coating methodson As adsorption. The procedure for the pristine materialtest included adding 50mL of a synthetic As solutionwithAs(V) concentration of 2000 μg/L in a 100 mL inertpolyethylene (PE) bottle, which contained 0.1 g of pristineFe mineral. With regard to mineral-coated materials,initial As(V) concentration was set to 1000 μg/L and thedosage of adsorbent was 10 g/L. The bottles were thencapped tightly and manually shaken for 2 min every 8 h.The experiments were conducted at a room temperature of20±2 °C for 24 h. After pseudo-equilibriumwas reached,the supernatant was decanted and filtered through0.45 μm cellulose acetate filter. The filtered solutionwas analyzed for total As. In order to maintain a relativelyconstant ionic strength, all synthetic As solutionscontained 0.01 M NaCl as the background electrolyte.

2.5. Column experiments

Batch experiments show that preparation methodshad no significant impact on As removal by Fe mineral-

coated materials and quartz sand as a matrix was betterthan feldspar to be coated in terms of As adsorption.Herein, mechanical method #2 was employed to coatquartz sand with the investigated minerals with the grainsize range of 0.04–0.08 mm, which produced coatedmaterials as column fillings. The filling materials ofcolumns are shown in Table 2.

Plexiglass columns with an inner diameter of 30 mmand a height of 150 mm were used in the column study,which acted as fixed-bed up-flow reactors. The workingvolume of the column is 100 mL. Synthetic solutioncontaining 500 μg/L total As or As-spiked tap watercontaining 200 μg/L total As (Table 2), as feedwater, waspumped through the column filter with a peristaltic pump(Model # 205S, Watson Marlow Company, Germany).The chemical compositions of the tap water are shown inTable 3. Effluent solutions were collected at regularintervals and analyzed for residual As species. In order toactivate mineral-coated quartz sand, one column filter(C13 in Table 2) was in-situ pre-conditioned by 0.5 MH2O2 solution at an upward flow rate of 0.51 mL/min for

Table 2Adsorbent fillings and operating conditions of the column filters

Column no. Filling materials a Flow rate(mL/min)

Feedwater Was itconditionedby H2O2?

C5 SIO3-coated quartz sand/HIO1-coated quartzsand in the lower/upper half of the column

1.48 500 μg/L As(V) and 0.01M NaCl in Milli-Q water

No

C6 SIO4-coated quartz sand/HIO1-coated quartzsand in the lower/upper half of the column

1.48 500 μg/L As(V) and 0.01M NaCl in Milli-Q water

No

C9 SIO3-coated quartz sand/quartz sand inthe lower/upper half of the column

1.48 500 μg/L As(V) and 0.01M NaCl in Milli-Q water

No

C10 HIO1-coated quartz sand (20% in weight) 1.48 500 μg/L As(V) and 0.01M NaCl in Milli-Q water

No

C11 SIO4-coated quartz sand/HIO1-coated quartzsand in the lower/upper half of the column

1.48 250 μg/L As(V), 250 μg/L As(III)and 0.01 M NaCl in Milli-Q water

No

C12 SIO4-coated quartz sand/HIO1-coated quartzsand in the lower/upper half of the column

2.15 100 μg/L As(V) and 100 μg/L As(III)in tap water b

No

C13 SIO4-coated quartz sand/HIO1-coated quartzsand in the lower/upper half of the column

2.15 100 μg/L As(V) and 100 μg/L As(III)in tap water b

Yes c

a The coating of filling materials adopted mechanical methods #2 as shown in Table 1, SIO3, SIO4 and HIO1 used have a grain size fraction of0.04–0.08 mm, and feldspar or quartz sand has a grain size range of 0.125–0.25 mm.b Chemistry of tap water is shown in Table 3.c Prior to column adsorption, Column C13 was conditioned by 0.5 M H2O2 solution for 50 h at an upward flow rate of 0.51 mL/min.

Table 3Chemical compositions of tap water used in the column experiments

pH EC a HCO3− Cl− NO3

− SO42− HPO4

2− K+ Na+ Ca2+ Mg2+ Cr Mn Fe Co Ni Cu Zn As Rb Sr Mo Cd Sb Ba PbμS/cm

mg/L

mg/L

mg/L

mg/L

mg/L

mg/L

mg/L

mg/L

mg/L

μg/L

μg/L

μg/L

μg/L

μg/L

μg/L

μg/L

μg/L

μg/L

μg/L

μg/L

μg/L

μg/L

μg/L

μg/L

7.31 683 333 21.0 4.94 76.2 b0.01 1.83 10.7 113 12.5 0.13 4.35 8.40 0.09 1.50 58.3 1437 0.35 0.77 357 0.46 1.95 0.37 74.2 2.73

a EC means electrical conductivity.

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50 h. Columns with different fillings and operatingconditions (Table 2) were set up to evaluate the effect oftheir combinations on As removal.

2.6. The Toxicity Characteristic Leaching Procedure(TCLP) test

Although the TCLP is not accepted worldwide, it isusually used to determine solid materials as inert orhazardous in terms of leachability of chemical contami-nants (US EPA, 1999). The TCLP was applied to the As-loaded adsorbents used in the column adsorption experi-ments. Herein, the spent adsorbent was extracted withextraction fluid (5.7 mL of glacial CH3COOH added to500 mL of Milli-Q water, plus 64.3 mL of 1 N NaOH anddiluted to 1 L, pH 4.93), with a liquid/solid ratio of 20. Theextraction was achieved by agitating the specimens for18 h in a shaker, after which the liquid phasewas separatedoff using 0.45 μm cellulose acetate filter and analyzed fortotal As by GF-AAS.

3. Results and discussion

3.1. Batch tests

3.1.1. Effect of grain size on As(V) adsorptionDifferent grain size ranges (b0.04, 0.04–0.08, 0.08–

0.10, 0.1–0.25, and 0.25–0.50 mm) of samples SIO3,SIO4, and HIO1 were used to test the effect of grain sizeon As(V) removal from aqueous solution. The exper-imental results show a significant difference in As(V)

removal over all of the investigated grain size ranges(Fig. 1). Generally, as expected, the As removaldecreased with the increase of grain size ranges. Onsample SIO3, the adsorbed As of 1060 μg/g with thegrain size range of b0.04 mm decreased to 238 μg/gwith the fraction of 0.25–0.50 mm. Guo et al. (acceptedfor publication) suggested that, with SIO3, SIO4, andHIO1, As adsorption generally depended on the surfacearea of the adsorbent.

Over these grain size ranges, sample SIO3 removed thegreatest amount of As from water, followed by samplesSIO4 and HIO1. With a grain size range of 0.25–0.50 mm, sample HIO1 almost had the same adsorptioncapacity as samples SIO3 and SIO4. It's a little differentfrom the previous results which show that, with a grainsize range of 0.25–0.50 mm, HIO1 was the best materialfor removal of As species among SIO3 and SIO4 (Guoet al., accepted for publication). The reason for thedifference is that the reaction condition of the previousstudy was fixed to initial As concentration of 1000 μg/Land adsorbent dosage of 10 g/L, both of which affectedAsadsorption. In comparison with the hematite, the sideritesincreased As adsorption more sharply with a decrease inthe particle size ranges between 0.25–0.50 and 0.08–0.10 mm (Fig. 1). It could be speculated that the sideriteremarkably developed the high reactive sites when it wascrushed to fine particles. Though those materials with thegreat capacity for As adsorption would not be solely usedas column fillings due to the significant hydraulicobstruction, theymay be promising materials to be coatedon the coarse particles suitable for column application.

Fig. 1. Effect of the grain size fraction on As(V) adsorption on SIO3,SIO4 and HIO1 (initial As=2000 μg/L; dosage=2 g/L; ionicstrength=0.01 M NaCl; T=20±2 °C; contact time=24 h).

Fig. 2. As(V) adsorption on feldspar/quartz sand and mineral-coatedfeldspar/quartz sand by means of different coating methods (Table 1)(initial As=1000 μg/L; dosage=10 g/L; ionic strength=0.01 M NaCl;T=20±2 °C; contact time=24 h).

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3.1.2. Effect of coating methods on As(V) removalThe effect of variation in the coating method on As(V)

adsorption is illustrated in Fig. 2, showing that mineral-coated materials generally removed a marked amount ofAs(V). Perusal of Fig. 2 reveals that with all coatingmethods, mineral-coated quartz sand adsorbed muchmore As(V) than mineral-coated feldspar, indicating thatquartz sandwas a goodmatrix for Femineral coating. Ironoxide-coated quartz sand was also found to be anemerging adsorbent for As removal by Thirunavukkarasuet al. (2003) and Gupta et al. (2005). Furthermore, with allcoating methods, the average removal by SIO3-coatedquartz sand was 97.7 μg/g, which is greater than that byHIO1-coated quartz sand (51.9 μg/g).

As compared with other coatingmethods, the chemicalcoating generally made the coated quartz sand the best inremovingAs(V), which arose from the activation of the Fe

minerals by H2O2 solution. In general, the coatingmethods investigated had no significant impact on Asadsorption. Because mechanical method #2 only involvedmechanically mixing Fe minerals and feldspar/quartzsand, it is simple, cost-effective and therefore very suitablefor practical use in small systems. Hence mechanicalmethod #2 was employed to generate mineral-coatedquartz sand in the following column experiments.

3.2. Column studies

3.2.1. Effect of adsorbent fillings on As removalThe effect of adsorbent fillings on As removal was

investigated at the same operating conditions with initialAs(V) of 500 μg/L, flow rate of 1.48 mL/min, and noH2O2 pre-conditioning. Columns C5, C6, C9 and C10with different adsorbent fillings (Table 2) were setup to

Fig. 3. Development of As species in the effluents from Columns C10 (a), C9 (b), C6 (c), and C5 (d), with 500 μg/L As(V) in the influent and flowrate of 1.48 mL/min. Columns C10, C9, C6 and C5 were packed with HIO1-coated quartz sand, SIO3-coated quartz sand/quartz sand, SIO4-coatedquartz sand/HIO1-coated quartz sand, and SIO3-coated quartz sand/HIO1-coated quartz sand, respectively.

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remove As(V). Breakthrough curves obtained are shownin Fig. 3.

As depicted in Fig. 3a, the concentration develop-ment demonstrates that As was not efficiently eliminat-ed, and the column was broken through rapidly. Theadsorption capacity of Column C10 appeared to beapproximately equal to that of packed HIO1 calculatedfrom the results of the batch test.

In comparison with Column C10, Columns C5, C6and C9 had a much longer breakthrough time. The As-drinking water standard of 10 μg/L was exceeded aftermore than a total throughput of 3600 pore volumes(PV). The high adsorption capacity of the columnssuggests siderites with the fine grain size may formaggregates in the columns that enhanced solid-statediffusion (Lopez et al., 1998), or serve as Fe(II) source

for formation of fresh Fe(III)-oxide coating on quartzsand or siderite particles. The combination of HIO1-coated quartz sand and SIO3-coated quartz sand inColumn C5 showed a good adsorption performance. Ifwe simply separated the adsorption role of SIO3-coatedquartz sand section from that of HIO1-coated section inColumn C5, and the adsorption role of SIO3-coatedquartz sand section from that of sand section in ColumnC9, the contribution of HIO1-coated quartz sand wouldtreat more than 1500 PV of artificial solution with Asconcentration of 500 μg/L meeting the WHO drinkingwater guideline, which was much greater than theamount by Column C10 solely packed with HIO1-coated quartz sand. This simple calculation demon-strates that the combination of siderite-coated quartzsand and hematite-coated quartz sand greatly promoted

Fig. 4. Development of As species in the effluents from Columns C11 (a), C6 (b), C12 (c), and C13 (d) packed with SIO4-coated quartz sand/HIO1-coated quartz sand. Columns C11 and C6 treated synthetic solution containing 250 μg/L As(III) and 250 μg/L As(V), and synthetic solutioncontaining 500 μg/L As(V), respectively, at the flow rate of 1.48 mL/min, while Columns C12 and C13 treated As-spiked tap water containing100 μg/L As(III) and 100 μg/L As(V) at the flow rate of 2.15 mL/min. Column C13 was pre-conditioned by 0.5 M H2O2 solution.

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the column performance in removing As. Due to thehigher adsorption capacity of SIO3 in comparison withSIO4, Column C5 scavenged more As than Column C6.

3.2.2. Effect of influent As species on As removalColumn C11 with the same adsorbent fillings as

Column C6 was setup to investigate the effect ofinfluent As species on As removal. The results presentedin Fig. 4a and b show that the variation in inorganic Asspecies had an impact on As removal. The filter treatedAs solution with 500 μg/L As(V) in meeting the WHOdrinking water guideline up to 3600 PV total throughput(Column C6, Fig. 4b), while with 250 μg/L As(III) and250 μg/L As(V) up to 4100 PV total throughput(Column 11, Fig. 4a). It seems that the adsorptionefficiency was improved by the presence of As(III). Theeffluent Mn from Column C11 was a little higher thanthat from Column C6 (not shown), which possibly arosefrom the oxidation of As(III) in the presence of Mnoxides (Oscarson et al., 1981; Nesbitt et al., 1998). TheAs(III) oxidation caused a surface alteration, creatingfresh reaction sites for As adsorption on mineralsurfaces (Manning et al., 2002). In comparison withmany adsorbents being more effective in removing As(V) than As(III) (Gupta and Chen, 1978; Elizalde-González et al., 2001; Chakravarty et al., 2002;Thirunavukkarasu et al., 2003), the materials studiedcan be used for remediating As-affected groundwaterusually containing substantial As(III) without pre-oxidation of As(III) to As(V).

The results also show that As speciation in ColumnC6 effluent was generally the same as that in ColumnC11 effluent, with the ratio of As(V)/total As of around0.80. The presence of As(III) in Column C6 effluentindicates that As(V) was partially reduced in thecolumn.

3.2.3. Arsenic removal from As-spiked tap waterAfter having clarified the high adsorption capacity of

the mineral-coated quartz sand columns from syntheticAs solution with respect to As(III) and As(V), anothercolumn (C12), with the same adsorbent filling as Col-umn C6, was utilized to evaluate the filter performanceusing multi-component As-spiked tap water. Thebreakthrough curve of Column C12 obtained with tapwater, spiked with 100 μg/L As(III) and 100 μg/L As(V), is shown in Fig. 4c.

Fig. 4c shows that removal efficiency of Column C12was relatively low. The drinkingwater standard of 10μg/Lwas exceeded after 400 PV total throughputs. Thedrinking water standard of 50 μg/L, as adopted in somedeveloping countries, was surpassed after 1400 PV.

Adsorption of As from As-spiked tap water in ColumnC12 was much lower than that from synthetic As solutionin Column C6. The Fe(III) oxides with a red colour wereobserved presenting on the siderite-coated sand ofColumn C6 after it was run for 10 days, while the fillingmaterials in Column C12 kept relatively stable inappearance during the whole experiment operation. Theformation of the fresh Fe(III) oxides was believed tocontribute to the high removal capacity of the column. Itmeans that the limited fresh Fe oxides developing inColumn C12 caused the low adsorption efficiency.

Fig. 5. Development of concentrations of Mn (a) and Fe (b) in theeffluents from Columns C6 and C12 packed with SIO4-coated quartz/HIO1-coated quartz. Columns C6 and C12 treated synthetic Assolution and As-spiked tap water, respectively.

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In the SIO4-coated quartz sand section presenting thered Fe oxides, siderite dissolution (Eq. (1)) must haveoccurred to provide Fe2+, which possibly is a prereq-uisite to the formation of the fresh Fe oxides.

FeCO3 þ H2O↔Fe2þ þ HCO−3 þ OH− ð1Þ

Because the As-spiked tap water contained highHCO3

−, the concentration of which was 333 mg/L, itwould restrain the dissolution of FeCO3 and suppress theproduction of solution Fe2+ (Jensen et al., 2002). Thedissolved Fe2+ in ambient water is readily oxidized andchanged into Fe(III) oxides/oxyhydroxides. Therefore,high HCO3

− indirectly limits the formation of fresh Fe(III)oxides. The low dissolution of the siderite in Column C12could also be verified by much lower concentrations ofMn and Fe in the effluents, in comparisonwith those in theeffluents of Column C6 (Fig. 5). On the other hand,competition for adsorption sites possibly accounted forthe low As adsorption due to the presence of anions(mainly HCO3

− and SO42−) in the influent. However,

neither HCO3− nor SO4

2− were strong suppressers at theconcentrations found in natural waters because of the highabundance of available adsorption sites in the packedcolumn (Genç-Fuhrman and Tjell, 2003; Genç-Fuhrmanet al., 2005b; Guo et al., accepted for publication).

In order to investigate the possibility of using H2O2

solution as a reaction agent to improve the columnperformance, a H2O2-conditioned column (C13), withthe same adsorbent filling as Column C12, wasemployed to remove As from As-spiked tap water.The results are presented in Fig. 4d, where it can be seenthat the in-situ conditioning resulted in a laterbreakthrough and higher adsorption efficiency. Thiseffect was probably due to the development of fresh Fe(III) oxides in the process of H2O2 conditioning. Itseems that H2O2 conditioning was an efficient way toachieve the high retention capacity of the columnapplied to treat the real-world water, especially highHCO3

− water.

3.3. Classification of the spent adsorbent

The evaluation of the leaching potential of spentadsorbents is important in assessing their possibleenvironmental impacts. The TCLP method wasemployed to classify the spent adsorbents as hazardousor inert waste. The results show that the As released wasbelow 400 μg/L. This finding suggests that either theadsorbed As was bound tightly by surface charges or itwas coprecipitated by fresh oxyhydroxide minerals as astructural component. More importantly, the established

U.S. EPA standard of 5 mg/L was not exceeded,indicating that these spent adsorbents were inert andcould be landfilled.

4. Conclusions

Both siderite and hematite with a grain size of 0.04–0.08 mm having high As adsorption capacity could becoated onto quartz sand particles by means of bothmechanical mixing and chemical coating. However,coating methods investigated had no significant impacton As adsorption of the coated materials in batch tests.The column fillings produced by means of mechanicalmixing were effective in removing inorganic As fromsynthetic solution. The combination of siderite-coatedquartz sand and hematite-coated quartz sand greatlypromoted the column performance. With the porevolume of 33 mL in the column, the filter packed withSIO3-coated quartz sand and HIO1-coated quartz sandtreated 231 L (7000 PV) of water with 500 μg/L of As(V) in meeting the WHO guideline value of 10 μg/L As.

The presence of As(III) in the influent improved theadsorption efficiency of the siderite-coated quartz sand andhematite-coated quartz sand column. The higher adsorptioncapacity may arise fromAs(III) oxidation causing a surfacealteration and creating fresh reaction sites for As adsorptionon mineral surfaces. Since As-enriched groundwater isgenerally dominated byAs(III), up to 96% of total As (Guoet al., 2003; Wagner et al., 2005), the adsorption propertymakes them promising materials for As removal fromgroundwater without pre-oxidation of As(III) to As(V).

With As-spiked tap water, the removal efficiency ofthe SIO4-coated quartz sand and HIO1-coated quartzsand column was relatively low. The reason for the lowefficiency was that high HCO3

− of the tap waterrestrained the dissolution of FeCO3 and consequentlylimited the formation of fresh Fe(III) oxides. However,the H2O2-conditioning greatly increased As adsorptionefficiency of the SIO4-coated quartz sand and HIO1-coated quartz sand column in remediating As-spiked tapwater, which was considered as an efficient way toachieve the high retention capacity of the columnapplied to treat the real-world water, especially highHCO3

− water. Importantly, the TCLP test shows that thespent adsorbents were classified as an inert waste andcould be safely disposed of to landfill.

Acknowledgements

H.M.G. is grateful to the Alexander von HumboldtFoundation, Germany, for providing an Alexander vonHumboldt Research Fellowship to carry out this

150 H. Guo et al. / Science of the Total Environment 377 (2007) 142–151

Autho

r's

pers

onal

co

py

research. Funding for this research has also beenprovided by the Natural Science Foundation of China(No. 40572145). The authors would like to express theirappreciations for technical supports of our colleagues C.Moessner (HR-ICP-MS, IC), T. Neumann (FI-HG-AAS), and G. Preuss (GF-AAS, Flame-AAS).

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