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Study of arsenic(III) and arsenic(V) removal fromwaters using ferric hydroxide supported on silica gelprepared at low pHTülin Deniz Çiftçi a , Onur Yayayürük a & Emür Henden aa Department of Chemistry, Faculty of Science, Ege University, Bornova, İzmir, TurkeyPublished online: 26 Mar 2011.

To cite this article: Tülin Deniz Çiftçi , Onur Yayayürük & Emür Henden (2011) Study of arsenic(III) and arsenic(V) removalfrom waters using ferric hydroxide supported on silica gel prepared at low pH, Environmental Technology, 32:3, 341-351, DOI:10.1080/09593330.2010.499546

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Environmental Technology

Vol. 32, No. 3, February 2011, 341–351

ISSN 0959-3330 print/ISSN 1479-487X online© 2011 Taylor & FrancisDOI: 10.1080/09593330.2010.499546http://www.informaworld.com

Study of arsenic(III) and arsenic(V) removal from waters using ferric hydroxide supported on silica gel prepared at low pH

Tülin Deniz Çiftçi, Onur Yayayürük and Emür Henden*

Department of Chemistry, Faculty of Science, Ege University, Bornova,

[Idot ]

zmir, Turkey

Taylor and Francis

(

Received 4 March 2010; Accepted 1 June 2010

)

10.1080/09593330.2010.499546

Removal of As(III) and As(V) species using ferric hydroxide supported on silica gel was studied. Laboratory reagentquality silica gel was used as to avoid uncertainties that may be caused by impurities. Ferric hydroxide precipitationwas realized at various pH values and a relatively low pH 6.0 was chosen because, at this pH, the highest arsenicremoval capacity and removal efficiency were obtained and clear supernatant solution was observed. It was alsoshown by arsenic speciation analysis at trace level that As(III) is adsorbed onto ferric hydroxide partly withoutoxidation to As(V); this has been a controversial point in the literature. The effects on arsenic removal of someparameters such as pH, flow rate and matrix ions were investigated. In the batch method, initial pH change of thesolution did not significantly affect the arsenic removal efficiencies for As(III) and As(V) in the pH range of 3.1–9.7.This was attributed to the decreases of the initial pH values to around 5 at equilibrium. The column capacities of1.32 mg As(III)/g sorbent and 1.21 mg As(V)/g sorbent were found for initial concentration of 1.00 mg/L arsenic.Batch capacities were 16.2 mg As(III)/g sorbent and 17.7 mg As(V)/g sorbent for initial arsenic concentration of 100mg/L. The method was applied successfully to the removal of As(III) and As(V) from drinking water, geothermalwater and mineral water.

Keywords:

arsenic removal; drinking water; geothermal water; mineral water; ferric hydroxide

1. Introduction

Arsenic is a ubiquitous element in the Earth’s crust andranks 20th among the most abundant elements [1].Many factors such as anthropogenic activities, biologi-cal actions, and geochemical reactions help to mobilizearsenic into groundwater. Mining activities, combustionof fossil fuels and use of arsenic pesticides create addi-tional impacts [2]. Arsenic exists in natural watermainly as As(III) in arsenite and As(V) in arsenateforms. While As(III) is dominant in groundwater,As(V) is dominant in surface water because of theoxidizing conditions [3,4]. It is toxic to man and otherliving organisms [5], and its toxicity depends on theoxidation state [6]. Biologically, As(III) is about 60times more toxic than As(V), and inorganic arseniccompounds are about 100 times more toxic than organicarsenic compounds [7,8]. Therefore, in order to deter-mine the toxicity of water and design an arsenicremoval procedure, arsenic speciation analysis needs tobe done.

The problem of arsenic contamination in groundwa-ter of West Bengal has been claimed as the biggestcalamity in the world [9], and several other countriessuch as Bangladesh, New Zealand, USA, Pakistan,

Taiwan, Japan, etc., have also been impacted byarsenic-contaminated water [10].

The World Health Organization published 10

µ

g/Las the drinking water guideline for arsenic in 1993 [10].According to this arsenic limit, many drinking watersources all over the world have became unacceptable.Therefore, research interest in arsenic removal fromdrinking water using low-cost and simple methods hasgreatly increased. Many treatment technologies such asion exchange, precipitation, coagulation and filtration,reverse osmosis, electro dialysis, lime softening andoxidation-filtration have been published for the removalof arsenic from water. Low removal capacity, toxicresidual and high costs are the main limiting factors forarsenic removal techniques. Moreover, for the removalof As(III), pre-oxidation to As(V) is necessary withmost arsenic removal techniques. However, technolo-gies based on adsorption remain attractive and promis-ing because of their simplicity, ease of operation andhandling, and sludge-free operation. An adsorptivematerial needs have properties such as low-cost,removal ability of both As(III) and As(V), selectivity,high physical strength, and practical usage for small andlarge water treatment facilities. Methods of removal of

*Corresponding author. Email: emur.henden@ege.edu.tr

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arsenic from water by adsorption on granular ferrichydroxide (GFH) [11], activated carbon [12], zero-valent iron [13], synthetic zeolites [14], activated redmud [15], and iron oxide-coated sand [16] have beenreported.

The most widely used sorbents are iron(III) oxides,which are produced by a range of methods. Generally,ferric salts are precipitated at high pH, usually withNaOH, and the products vary in chemical structure,composition, and physical characteristics with the meth-ods of precipitation. Most of these sorbents can removeboth As(III) and As(V) [3,17–20] and, therefore, havecommonly been preferred for arsenic removal becauseof their cost effectiveness and high adsorption capacities[22]. As(V) is adsorbed as ionic species; controversialreports on the mechanism of As(III) adsorption havebeen published, some reports stating the oxidation ofAs(III) to As(V) by the ferric oxides before adsorption[22–25] and the others reporting As(III) adsorption asthe molecular species of H

3

AsO

3

[26,27]. X-ray absorp-tion near edge structure (XANES) analysis indicatedthat As(III) remained stable on the goethite surfacetoward heterogenous oxidation to As(V) [28]. X-rayabsorption spectroscopy (XAS) studies by Farquhar

et al.

[29] confirmed that the adsorbed As(III) andAs(V) remained in original oxidation states on thesurfaces of goethite and lepidocrocite. However, suchinstrumental methods are usually limited due to theircomparatively high detection limits [30].

Amorphous Fe-O-OH was found to have the highestadsorption capability since it has the highest surface area[2,31]. Most of the iron oxides are available only as finepowders or are generated

in situ

as gels or suspensionsin aqueous solution. These forms of iron oxide retaintheir strong affinities to As(III) and As(V) but are limitedto reactor configurations incorporating large sedimenta-tion and filtration units which cause difficulty in solid/liquid separation [32]. Thus, they are not suitable forcolumn adsorption in water and wastewater treatment.Furthermore, iron(III) oxide alone is not suitable as filtermedium due to its low hydraulic conductivity [33]. Daus

et al.

[34] studied As(III) and As(V) adsorption onto fivedifferent sorbents: activated carbon(AC), zirconiumloaded AC (Zr-AC), zero-valent iron (Fe

0

), granulatediron hydroxide (GIH), and a sorption medium with thetrade name ‘Absorptionsmittel3’(AM3). The highestlevel of removal of As(V) was obtained by Zr-AC andof As(III) by AC. Streat

et al.

[35] have reported thatGFH in water treatment was distinctly advantageoussince the process involves little maintenance ormanpower, and there is no pre-treatment required otherthan chlorination. A method was also described toremove arsenic by oxidation of Fe(II) to iron(III)hydroxide by aeration [18]. Application of Fe(II) insteadof Fe(III) was reported to be advantageous, because

partial oxidation of As(III) to As(V) by aeration occursat the same time. However, another study has reportedthat Fe(III) is more effective and economical than Fe(II)due to lower required coagulant dose and pH [36]. Iron-coated zeolite (ICZ) and iron-coated sand were also usedas the adsorbent. The capacity of iron ICZ for As(V) was15 times higher than that of iron-coated sand [37]. Theadsorption capacity of iron oxide impregnated onto acti-vated alumina was also reported to be higher than thevalues for iron oxide-coated sand and ferrihydrite [38].

The purpose of this study was to investigate As(III)and As(V) removal by using ferric hydroxide precipi-tated at relatively low pH onto silica gel as support(FHSS). Laboratory quality silica gel was used in orderto eliminate possible effects of impurities in the mate-rial. The pH values of the ferric oxide precipitation andarsenic adsorption, adsorbent dose and column flow ratewere optimized. Arsenic removal efficiencies andadsorption capacities were determined. An analyticalprocedure based on species analysis of arsenic at tracelevel was designed to clarify if As(III) is oxidized toAs(V) before adsorption onto FHSS. The FHSS devel-oped in this study was found to have advantages overtraditionally used iron oxides, such as sorption of bothAs(III) and As(V) with high removal efficiencies andrelatively high capacities, elimination of the elaboratefiltration step in the batch method of arsenic removal,ease of preparation and suitability for column use. Theproposed sorbent could be used for arsenic removalfrom drinking water, mineral water and geothermalwater at high temperature.

2. Materials and methods

2.1. Apparatus

For arsenic determination, a GBC 904 PBT atomicabsorption spectrometer coupled with a quartz tubeatomizer and GBC HG-3000 continuous flow hydridegeneration system (HGAAS) were used. However,when interferences are present, particularly in thesample applications and for the determination of As

3+

and As

5+

species in admixtures, measurements weremade by using a laboratory-made batch-type hydridegeneration system [39], and interferences were elimi-nated by masking with EDTA and KI [40]. The hollowcathode lamp current was set at 10 mA, and slit widthof 0.5 nm and a wavelength of 193.7 nm were used. Adeuterium lamp was used for background correctionand nitrogen was the carrier gas.

Determination of total arsenic in mineral watersamples was made using an HP-4500 inductivelycoupled plasma mass spectrometer. A Nüve ST402model shaker with a thermostated water bath was usedfor adsorption studies. A MultiLine P4 model pH meterwas used for pH measurements. Fe(III) analysis was

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made by using a Schimadzu 160B UV-Vis. spectropho-tometer. Na

+

and K

+

were determined by using aJenway PFP7 model Flame photometer, and SO

42

−

wasdetermined by using a Scientific. Inc. Micro 100 modelturbidimeter. The characterization of prepared FHSSwas performed by using a scanning electronic micro-scope combined with an X-ray energy dispersive spec-trometer (SEM-EDX) (Philips XL-30S FEG).

2.2. Reagents

All the reagents were of analytical reagent grade, andglass distilled water was used throughout. Arsenic(III)and arsenic(V) stock standard solutions at 1000 mg/Lwere prepared by dissolving As

2

O

3

(Merck) andNa

2

HAsO

4

.7H

2

O (Merck), respectively, in concentratedHCl and diluted with distilled water. As(III) and As(V)stock solution contained 2 M HCl. More dilute standardsolutions were prepared daily by dilution of the stocksolutions. 8.3% KI solution was prepared from KI(Merck). Sodium tetrahydroborate(III) solution wasprepared by dissolving NaBH

4

(Merck) pellets in 0.01M NaOH. 0.01 M EDTA (Merck) was used for maskinginterferences in arsenic determination. For arsenicremoval studies, 2 M Fe(III) solution was prepared bydissolving 54.06 g FeCl

3

.6H

2

O (Merck) in 0.01 M HCland diluting to 100 mL with distilled water. For thepreparation of 2 M CH

3

COOH/CH

3

COO

−

buffer,CH

3

COONa.3H

2

O was dissolved in distilled water andthe pH values of the solutions were adjusted to therequired values by adding 2 M HCl. 0.1 M NaOH solu-tion and 1 M NH

4+

/NH

3

buffer solution were used as theother solutions for the preparation of FHSS to comparewith CH

3

COOH/CH

3

COO

−

buffer. NH

4+

/NH

3

buffersolution was prepared by adding HCl solution to NH

3

(Merck) solution. The silica gel (Acros Organics) usedwas 0.2–0.5 mm in diameter with a pore diameter ofca. 4 nm.

2.3. Procedure for arsenic determination

For the determination of total arsenic the continuousflow HGAAS system was used. In this system thesample (8 mL/min), 10.2 M HCl solution (2 mL/min)and 0.6% NaBH

4

solution (2 mL/min) are continuouslypumped with a peristaltic pump into a mixing coilwhere hydrides are formed and then separated from theliquid phase in the gas–liquid separator and swept intothe quartz tube atomizer with a nitrogen flow for atomicabsorption measurements. Conversion of As(V) toarsine was not quantitative under these conditions.Therefore, it was necessary to reduce As(V) to As(III)using 8.3% KI in 1 M HCl prior to reduction withsodium tetrahydroborate(III) to AsH

3

. The relative stan-dard deviation for the determination of 20

µ

g/L arsenic

was 1.4 % (

n

= 7). The limit of detection for arsenic was0.5

µ

g/L.As(III) and As(V) species were determined in

admixtures by using batch-type HGAAS [39]. Avolume of 1 mL of the sample or standard solution ofarsenic in 0.1 M HCl was injected into the reactionvessel of 20 mL volume containing 1 mL of 4% NaBH

4

solution. The generated arsine was swept by a nitrogenflow (134 mL/min) through a CaCl

2

and CaSO

4

-containing drying tube into the quartz tube atomizer.Arsenic absorption at 193.7 nm was measured. The rela-tive standard deviation for the determination of 30

µ

g/LAs(III) was found to be 4.76% (

n

= 7). The limit ofdetection was found to be 1.2

µ

g/L. For the determina-tion of As(V), it was reduced to As(III) with KI prior toborohydride reaction as above. Analytical performancefor As(V) was similar to that obtained for As(III).

2.4. Adsorption studies

2.4.1. Procedure for the preparation of FHSS

1.0 g silica gel was weighed in a 100 mL conical flask.25 mL of 2 M Fe(III) in 0.01 M HCl was added ontosilica gel and shaken for 24 h at 25

°

C. After decantingthe solution, Fe(III) ions on the silica gel were precipi-tated by the addition of 25 mL 2 M CH

3

COOH/CH

3

COO

−

buffer (pH 6.0) and shaken for another 24 h.The supernatant liquid was decanted. FHSS particleswere washed with distilled water by decantation repeat-edly until the aqueous washings, tested with acidicNH

4

SCN solution, became iron-free.

2.4.2. Effect of the precipitation conditions on the capacity and the removal efficiency of FHSS

0.1 M NaOH solution, 1 M NH

4+

/NH

3

buffer of pH 9.0,and 2 M CH

3

COOH/CH

3

COO

−

buffers of pH 4.0, 5.0,and 6.0 were used for the preparation of the FHSS. Forthe arsenic adsorption capacity measurements, 25 mL of100 mg/L As(III) or As(V) solutions at pH 6.0 wereadded onto 0.125 g FHSS and the mixture was shakenfor 24 h. Unadsorbed arsenic in the solutions wasmeasured. For the arsenic removal efficiency studies,25 mL of 200

µ

g/L As(III) and As(V) solutions at pH6.0 were added onto 0.625 g FHSS and shaken for 24 hand unadsorbed arsenic was measured using HGAAS.Iron content of all the adsorbents were determined usingflame atomic absorption spectroscopy.

2.4.3. Characterization of FHSS

25 mL volumes of 800 mg/L As(III) and As(V) wereadded onto separate portions of 0.25 g FHSS andshaken for 72 h. The particles were washed withdistilled water and dried at 40

°

C for 90 min. Parallel

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experiments were also carried out using silica gel with-out iron. The surface morphology and chemical compo-sition of the FHSS were determined by SEM-EDXanalysis.

2.4.4. Controlling the presence of acetate ions in FHSS

It is reported that basic iron(III) acetate may alsoprecipitate along with iron(III) hydroxide precipitationwhen iron(III) is precipitated with acetate buffer [41].Therefore, presence of acetate in the precipitate wascontrolled. For this purpose, ferric hydroxide on thesilica gel was dissolved with 2 M HCl and Fe(III) in thesolution was removed by passing through a columncontaining Amberlite IR-120 cation exchange resin inorder to avoid its possible interference on acetic aciddetermination. Acetic acid in the iron-free solution wasthen determined by conductometric and potentiometrictitration with 0.1 M NaOH.

2.4.5. Investigation of pH dependence of As(III) and As(V) removal

The uptake of As(III) and As(V) by the adsorbent atvarious initial pH levels was studied in order to deter-mine the optimum pH for arsenic removal. A volume of25 mL of 100

µ

g/L As(III) or As(V) solution with pHin the range of 3.1–11.2 was added onto 0.5 g of FHSSand shaken for 24 h. Unadsorbed arsenic in each solu-tion was determined by HGAAS and arsenic removalefficiencies were calculated.

2.4.6. Effect of adsorbent dose

Various amounts of silica gel were weighed and theadsorbents were prepared as described earlier. 25 mL of200

µ

g/L As(III) or As(V) solutions was added onto theadsorbents and shaken for 24 h at 25

°

C. The effect ofthe adsorbent dose on the removal of As(III) and As(V)was determined by following the unadsorbed arsenic inthe solutions.

2.4.7. Column capacity studies

FHSS particles were packed into a 1.0 cm i.d. glasscolumn so as to have a 10 mL packing volume, withglass wool fitted at the bottom and top. 1000

µ

g/LAs(III) or As(V) (pH 6.0) solutions were passed throughthe column at a constant speed (5.0 mL/min) at roomtemperature using a peristaltic pump. The same proce-dure was also applied for 750 mg/L As(III) and As(V)solutions. The adsorption capacities were determinedby evaluating breakthrough curves.

2.4.8. Study of As(III) sorption mechanism

A series of experiments was carried out in order to clarifywhether As(III) is adsorbed onto FHSS as As(III) speciesor oxidized to As(V) by Fe(III) in the FHSS beforeadsorption. For this purpose, 30 mL of 400 mg/L As(III)solution was passed through the column containing 2 mLadsorbent at a flow rate of 7 mL/min. The column waswashed well with distilled water, column fillings werethen transferred into a beaker and iron hydroxide in theFHSS was dissolved with 20 mL of 2 M HCl. Iron andarsenic ions were passed into the solution. The solutionwas transferred to a volumetric flask and the silicagel was washed with water until a white colour wasobtained. The washing solutions were also added into thevolumetric flask and diluted to 100 mL with distilledwater. 1 mL of the solution was diluted to 50 mL andwas then passed through a 15 mL cation exchange resin(IR-120)-containing column. Iron(III) in the solutionwas held by the resin and, therefore, an arsenic-containing colourless eluate was obtained. 5 mL of theeluate was diluted to 100 mL with distilled water. As(III)in this solution was measured using EDTA as the inter-ference masking agent with batch-type HGAAS. Thepresence of As(V) in the eluate was controlled afterreducing As(V) to As(III) with KI and ascorbic acid.Parallel experiments were carried out throughout theprocedure using As(V) in order to check if it was reducedto As(III) by any means before the determination step.

For the batch method, 25 mL of 5 mg/L As(III) solu-tion was added onto 0.625 g adsorbent and shaken for24 h at 25

°

C. Sorbed arsenic was determined asdescribed above.

2.5. Applications to real samples

Commercially bottled drinking and mineral water wereused as the water samples. Geothermal water sampleswere collected from areas near Izmir. In order to under-stand the effect of the arsenic removal procedure on theion content of the sample water, Na

+

and K

+

were deter-mined using a Flame photometer. Ca

2+

and Mg

2+

weredetermined by titration with EDTA in the presence ofEBT indicator. For the determination of SO

42

−

concen-tration, the turbidimetric method was used.

3. Results and discussion

3.1. Batch studies

3.1.1. Effect of precipitation pH on the adsorption capacity and the removal efficiency of FHSS

It is known that when the precipitated iron(III) hydroxidecontains colloids, arsenic removal efficiency seriouslydecreases. In such cases, an elaborate microfiltrationstep is added to the system in order to remove the

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arsenic-containing colloids, thus improving the effi-ciency. It is again well known that the pH of the precip-itation solution seriously affects the particle size of thehydrated iron oxide precipitates. Therefore, we havestudied the effect of the precipitation pH on the totalarsenic removal capacity.

The amount of adsorbed arsenic was calculated bythe difference of the initial and residual amounts ofarsenic in solution. The results are shown in Figure 1.Total batch capacity of FHSS was the highest at pH 6.0for both As(III) and As(V) (17.7 mg/g and 16.2 mg/g,respectively), possibly because it had also the highestiron content.

Figure 1. Effect of the precipitation pH on the total batch capacity for As and Fe. Iron values were decreased 10 times (initial As concentration: 100 mg/L, adsorbent dose: 4 g/L).

When CH

3

COOH/CH

3

COO

−

buffers with pH 4.0 and5.0 were used for the precipitation, the colours of theadsorbent were yellowish and orange, respectively.When the adsorbent was prepared by using pH 6.0 buffer,the colour was dark red. When NaOH and NH

4+

/NH

3

buffers were used for the precipitation, cloudy suspen-sions were formed, whereas CH

3

COOH/CH

3

COO

−

buffer formed a clear supernatant solution.As shown in Figure 2, ferric oxide precipitation pH

did not significantly affect the removal efficiency ofAs(V). However, for the effective removal of As(III),FHSS had to be prepared by precipitating ferric hydrox-ide at pH 5.0–6.5. This may be the reason for thecontroversial results reported in the literature regardingthe As(III) sorption efficiency of ferric oxide [2].

Figure 2. Effect of the precipitation pH on the removal efficiency (initial As concentration: 200

µ

g/L, adsorbent dose: 20 g/L).

3.1.2. Characterization of FHSS

SEM and EDX data showed that all the FHSS materialsare robust and granular, not aggregated. Silica gel alone

contained no detectable iron, while iron on FHSS couldclearly be seen. After the adsorption of As(III) andAs(V), the arsenic peak was determined on the FHSS.However, the arsenic peak was not seen on silica gelalone. The experimental data have proved that silica gelwithout iron does not sorb arsenic significantly. SEM-EDX images are shown in Figure 3.

Figure 3. SEM-EDX images of the materials: (a) FHSS, (b) silica gel, (c) As(III) adsorbed on FHSS, (d) As(V) adsorbed on FHSS.

3.1.3. Controlling the presence of acetate ions in the FHSS particles

Acetic acid in the iron-free solution, obtained bydissolving FHSS in dilute HCl and removing Fe(III)with a cation exchange resin, was determined byconductometric and potentiometric titration. It wasfound that basic iron(III) acetate as stated in the litera-ture [41] was not formed in measurable amounts duringferric hydroxide precipitation by CH

3

COOH/CH

3

COO

−

buffer of pH 6.0 under the experimental conditionsused.

3.1.4. Investigation of the adsorption pH dependence of the As(III) and As(V) removal efficiencies

Unbuffered As(III) or As(V) solutions with initial pH inthe range of 3.1–11.2 were added onto FHSS, and themixture was shaken for 24 h. Arsenic removal efficien-cies were calculated by measuring unadsorbed arsenicin the solution. The obtained values are shown inFigure 4. In the batch method, initial pH did not signif-icantly affect the arsenic removal efficiencies for As(III)and As(V) in the pH range 3.1–9.7. The removal

Figure 1. Effect of the precipitation pH on the total batch capacity for As and Fe. Iron values were decreased 10 times (initialAs concentration: 100 mg/L, adsorbent dose: 4 g/L).

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efficiencies varied in the range 96.0–99.8 % for As(III)and 95.6–99.9 % for As(V). It is clear that there is adecrease in both As(III) and As(V) removal efficienciesat pH 11.2. Haque

et al.

[42] reported an initial pH rangeof 6–7 for maximum adsorption of As(V). Similarly, adecrease of As(V) adsorption above pH 7 on goethite

was also reported [43]. Our results showed that As(V)could quantitatively be adsorbed up to initial pH 9.7.

Figure 4. Effect of the initial pH of the solution on As(III) and As(V) removal (initial arsenic concentration: 100

µ

g/L, adsorbent dose: 20 g/L, sorption time: 24 h).

It was observed that pH of the sorption mediumdecreased by time. Equilibrium pH values measured after24 h against the initial pH values are shown in Figure 5.Under the experimental conditions used, initial pH values

Figure 2. Effect of the precipitation pH on the removal efficiency (initial As concentration: 200 µg/L, adsorbent dose: 20 g/L).

Figure 3. SEM-EDX images of the materials: (a) FHSS, (b) silica gel, (c) As(III) adsorbed on FHSS, (d) As(V) adsorbed onFHSS.

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above 6 decreased to around 5 at equilibrium, and thisdecrease of the pH values at equilibrium explains whythe variations in the initial solution pH did not signifi-cantly affect arsenic removal efficiencies (Figure 4).

Figure 5. Variation of the equilibrium pH with the initial pH of the solution (initial arsenic concentration: 100

µ

g/L, adsorbent dose: 20 g/L, sorption time: 24 h).

The pH of the solution with initial pH 11.2decreased to pH 10.6 at equilibrium. When the pH isabove 9.2, the negatively charged arsenic speciesbecomes predominant [3] and the adsorbent surface alsobecomes negatively charged. Thus, electrostatic repul-sion between FHSS and arsenic anions results in adecrease of arsenic adsorption.

3.1.5. Effect of adsorbent dose

The effects of the adsorbent dose on the removal effi-ciency of As(III) and As(V) are shown in Figure 6.Even at the lowest adsorbent dose used, very higharsenic removal efficiencies were observed. Removalefficiencies of 97.3% and 98.7% were obtained forAs(III) and As(V), respectively, at 2.5 g/L adsorbentdose. Beyond 5.0 g/L adsorbent dose the removal effi-ciencies of both As(III) and As(V) were more than99.5% for the initial As(III) and As(V) concentration of200

µ

g/L.Figure 6. Arsenic removal efficiencies of FHSS depending on the adsorbent dose (25 mL 200 µg/L of As(III) or As(V) solutions were added onto the adsorbents and shaken for 24 h at 25 °C).

Figure 4. Effect of the initial pH of the solution on As(III) and As(V) removal (initial arsenic concentration: 100 µg/L, adsorbentdose: 20 g/L, sorption time: 24 h).

Figure 5. Variation of the equilibrium pH with the initial pH of the solution (initial arsenic concentration: 100 µg/L, adsorbentdose: 20 g/L, sorption time: 24 h).

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The adsorbent was also analysed for hydrated ferrichydroxide content by investigating the differencebetween FHSS and silica gel dried at the same temper-ature, 40°C. The results showed that 20.0% (w/w) of theadsorbent was ferric hydroxide (n = 9, RSD = 4.6%).The average content of iron on the dry silica gel was alsodetermined using Flame AAS as 306 mg Fe/g silica gel.

3.2. Column studies

3.2.1. Study of As(III) sorption mechanism

Sorption of As(V) onto hydrated iron(III) oxides fromnatural waters is mainly due to the ionic adsorption ofH2AsO4

− and HAsO42−, the main species existing at

natural water pH. However, there are controversialapproaches in the literature for explaining the mecha-nism of adsorption of As(III) onto hydrated iron oxide.In one approach, it is stated that As(III) is oxidized toAs(V) by Fe(III) in the sorbent before adsorption [22–25]. In another approach, physical adsorption is causedmainly by Van der Waals forces and electrostatic forcesbetween adsorbate molecules, and the atoms whichcomprise the adsorbent surface [26,27]. However, suchexplanations have been based on very high arsenicconcentrations because of the detection limits of thespectroscopic techniques utilized.

We have, therefore, carried out studies in order toexperimentally identify the arsenic species adsorbed byFHSS. FHSS after As(III) adsorption at pH 6.0 wasdissolved in 2 M HCl, considering that at high acidconcentration oxidation of As(III) to As(V) in solutionwould be more difficult. EDTA was added to maskFe(III) interferences in the hydride generation reaction,and arsenic in the solutions was determined using

batch-type HGAAS. More than 99% of the arsenic inthe solution was found to be still in the As(III) oxidationstate when sorption was realized in the column; a 7 mL/min solution flow rate and relatively high arsenicconcentration (400 mg/L) was used. However, when amore dilute As(III) solution (500 µg/L) was passedthrough the column slowly (1.5 mL/min), 62% ofAs(III) was oxidized to As(V).

Arsenic adsorption experiments were repeated withthe batch method. Due to the longer contact time of theadsorbent and As(III) in the batch method, As(III) couldbe oxidized to As(V). 5 mg/L As(III) in 25 mL solutionwas adsorbed by 0.625 g of FHSS for 24 h. The sorbentwas then dissolved in 2 M HCl, and concentrations ofarsenic species in the solution were determined as above.Some 30–40% of As(III) was found to be oxidized.

The above results show that As(III) can be adsorbedeven at trace level without oxidation to As(V) by FHSS.Part of the As(III) was found to be oxidized either byFHSS or Fe(III) passed into the solution after dissolvingFHSS in HCl. The mechanism of As(III) sorption onferric oxide appears to be dependent on the experimen-tal conditions and arsenic concentration. Therefore,further studies for the identification of As(III) sorptionmechanism at the µg/L level is required.

3.2.2. Estimation of column capacity for As(III) and As(V)

In this study, 1000 µg/L As(III) and As(V) (pH 6.0)solutions were passed through the column containing10 mL sorbent. Arsenic in the eluent was determined byHGAAS. Breakthrough curves obtained are shown inFigure 7.

Figure 6. Arsenic removal efficiencies of FHSS depending on the adsorbent dose (25 mL 200 µg/L of As(III) or As(V) solutionswere added onto the adsorbents and shaken for 24 h at 25°C).

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Environmental Technology 349

Figure 7. Breakthrough curve for As(III) and As(V) (initial arsenic concentration: 1000 µg/L, volume of column filling: 10 mL, flow rate: 5 mL/min).Columns started leaking As(III) after 300 bedvolume and As(V) after 326 bed volume. Total capaci-ties of the column were 1.32 mg As(III)/g adsorbent and1.21 mg As(V)/g adsorbent. The capacities in differentunits were calculated and are shown in Table 1.

The same procedure was applied with an initialconcentration of As(III) and As(V) of 500 mg/L.Solutions were passed through the column under thesame conditions. Total capacities were calculated as5.78 mg As(III)/g adsorbent and 5.61 mg As(V)/gadsorbent. The capacities of the adsorbent varied withthe initial arsenic concentration. Both batch and columncapacities found were relatively high compared with theother adsorbents reported in the literature [44].

3.2.3. Applications to real samples

The adsorption column method was applied in order toremove arsenic in drinking, mineral and geothermalwaters. In order to see the effect of the adsorption onFHSS on the composition of water, concentrations of

As(III), As(V), Na+, K+, Ca2++ Mg2+, SO42− and HCO3

−

were determined in the original samples, and after pass-ing through the column. Column tests were carried outin water samples spiked with As(III) and As(V) toinvestigate the removal efficiencies of FHSS. Resultsare shown in Table 2 and Table 3.

A decrease in the concentration of HCO3− in the

effluents of the mineral water can be explained by theloss of CO2 gas as the pressure is released and partialadsorption by the sorbent. The concentration of theother ions did not show important changes while pass-ing through the column. Therefore, it can be concludedthat As(III) and As(V) are removed by FHSS particleswithout changing the water composition.

The percentage of arsenic removal from drinkingwater by silica-containing iron(III) oxide adsorbent wasreported to be 89.6% for As(III) in the literature [45]. Itcan be seen in Table 3 that both As(III) and As(V) were

Figure 7. Breakthrough curve for As(III) and As(V) (initial arsenic concentration: 1000 µg/L, volume of column filling: 10 mL,flow rate: 5 mL/min).

Table 1. Total sorption capacities of the sorbent in thecolumn for As(III) and As(V) (initial concentration of arsenic:1000 µg/L, volume of column filling: 10 mL, flow rate: 5 mL/min).

Sorption capacities of FHSS

As(III) (mg) As(V) (mg)

1 g sorbent 1.32 1.211 mL sorbent 0.91 0.831 g iron 4.31 3.96

Table 2. Water sample parameters before and after passingthrough the column packed with FHSS.

Drinking water Mineral water

Parameters Influent Effluent Influent Effluent

pH 7.60 7.10 6.53 6.32Na+ (mg/L) 1.87 1.68 19.56 17.93K+ (mg/L) 1.37 1.21 9.55 7.96Ca2+ + Mg2+

(mmol/L)17.07 17.03 315 285

HCO3− (mg/L)* – – 466 311

SO42− (mg/L) 32.80 31.40 402 390

*For mineral water.

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350 T.D. Ciftci et al.

removed quantitatively (> 97.4%) from drinking andmineral water, and geothermal water at high tempera-tures (85–100°C) by FHSS particles under the condi-tions used. The effect of the presence of sulphide onarsenic removal was also studied by adding Na2S ontothe sample, and it was found that in the presence of500 µg/L S2− in geothermal water, As(III) and As(V)could be removed with high efficiencies (> 91.8%).

4. Conclusion

It was found that ferric oxide which was precipitatedonto silica gel as support at only pH 5.0–6.5 couldadsorb both As(III) and As(V) effectively. Therefore,pH 6.0 was chosen as the precipitation pH. Ferrichydroxide precipitated at pH 6.0 on silica gel andpacked in a column can be used to remove As(III) andAs(V) with no oxidation step from drinking water,mineral water and geothermal water at high temperaturewithout causing major changes in the water composi-tion. Since granular ferric oxide was obtained by precip-itating at relatively low pH, no colloid formation tookplace, and therefore there is no need for a microfiltra-tion step to obtain high removal efficiencies. The phys-ical strength of the system was increased by using silicagel as support material. The capacity of the column ishigh, and preparation of the material is easy. Adsorptionof As(III) at trace level by FHSS was shown to be real-ized without oxidation to As(V) by means of arsenicspeciation analysis.

AcknowledgementsThis project was supported by the Research Fund of EgeUniversity. The authors thank Filiz Yi[gbreve] it for her contributionsto this project.

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