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Electrochimica Acta 56 (2011) 5060–5070

Contents lists available at ScienceDirect

Electrochimica Acta

journa l homepage: www.e lsev ier .com/ locate /e lec tac ta

emoval of arsenic from drinking water by the electrocoagulation using Fe and Allectrodes

. Kobyaa,∗, U. Gebologlua, F. Ulua, S. Oncela, E. Demirbasb

Gebze Institute of Technology, Department of Environmental Engineering, 41400 Gebze, TurkeyGebze Institute of Technology, Department of Chemistry, 41400 Gebze, Turkey

r t i c l e i n f o

rticle history:eceived 3 December 2010eceived in revised form 21 March 2011ccepted 22 March 2011vailable online 30 March 2011

eywords:rsenic

a b s t r a c t

A novel technique of electrocoagulation (EC) was attempted in the present investigation to remove arsenicfrom drinking waters. Experiments were carried out in a batch electrochemical reactor using Al and Feelectrodes with monopolar parallel electrode connection mode to assess their efficiency. The effectsof several operating parameters on arsenic removal such as pH (4–9), current density (2.5–7.5 A m−2),initial concentration (75–500 �g L−1) and operating time (0–15 min) were examined. Optimum operatingconditions were determined as an operating time of 12.5 min and pH 6.5 for Fe electrode (93.5%) and15 min and pH 7 for Al electrode (95.7%) at 2.5 A m−2, respectively. Arsenic removal obtained was highest

−3

lectrocoagulationrinking water

ron electrodeluminum electrode

with Al electrodes. Operating costs at the optimum conditions were calculated as 0.020D m for Feand 0.017D m−3 for Al electrodes. EC was able to bring down aqueous phase arsenic concentration toless than 10 �g L−1 with Fe and Al electrodes. The adsorption of arsenic over electrochemically producedhydroxides and metal oxide complexes was found to follow pseudo second-order adsorption model.Scanning electron microscopy was also used to analyze surface topography of the solid particles at Fe/Al

proce

electrodes during the EC

. Introduction

The presence of arsenic in ground and surface waters is one ofhe major environmental problems as many people are exposedo excessive arsenic amounts through contaminated drinkingater [1,2]. The most serious problems being encountered inany regions of the world such as Argentina, Bangladesh, Chile,

ndia, Mexico, Mongolia, Myanmar, Nepal, New Zealand, Thailand,aiwan, Turkey, and Vietnam when arsenic concentration in groundnd surface water exceeds national and international drinkingater standards (permissible limit of 10 �g L−1) [1,3]. Naturalater sources especially in the west regions of Turkey such asutahya, Izmir and Afyon contain much higher levels of arseniconcentrations (10–900 �g L−1) than the allowed maximum levelf contamination [4,5].

Arsenic contaminated natural waters are a significant problemince this compound is known as toxic, mutagenic and car-inogenic. Chronic health effects of arsenic include developmentf various skin lesions such as hyperpigmentation (dark spots),

ypopigmentation (white spots), and keratosis of hands and feet.ong-term exposure to arsenic can also cause cancer of the blad-er, lungs, skin, kidney, liver and prostate [6,7]. Due to its high

∗ Corresponding author. Tel.: +90 262 6053214; fax: +90 262 6538490.E-mail address: kobya@gyte.edu.tr (M. Kobya).

013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved.oi:10.1016/j.electacta.2011.03.086

ss.© 2011 Elsevier Ltd. All rights reserved.

toxic effects on human health, the USEPA and WHO have loweredthe maximum contaminant level for arsenic in drinking water from50 to 10 �g L−1 [3,8]. The lowering of this maximum contaminantlevel makes it necessary to find novel technologies to meet theregulation.

The major arsenic species present in natural and ground watersare arsenate ions: H3AsO4, H2AsO4

−, HAsO42− and AsO4

3− (oxi-dation state V) and arsenite ions, H3AsO3, H2AsO3

− and HAsO32−

(oxidation state III). However, As(V) ions are most prevalent in oxy-genated water while As(III) is found in anaerobic conditions like inwell water or in groundwater Fe/Al gets dissolved from the anodegenerating corresponding metal ions, which almost immediatelyhydrolyze to polymeric iron or aluminum oxyhydroxides. Thesepolymeric oxyhydroxides are excellent coagulating agents [9].

The task of removing arsenic from water has received exten-sive attention. Major treatment approaches include treatment withlime, alum or iron coagulation and chemical oxidation [10,11] andadsorption, reverse osmosis, membrane filtration and ion exchange[12–14]. These treatment technologies take considerable time;require an extensive set-up and they are not economically appli-cable in small community systems. Moreover, the entire treatmentrequires several pH adjustments as well as the addition of acid,

coagulants such as alum, ferric sulphate and chloride, lime, causticor polymeric flocculants. These processes generate a consider-able quantity of secondary pollutants (chloride, sulphate in thecoagulation-precipitation) and large volumes of sludge or waste

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M. Kobya et al. / Electrochi

hich pose serious environmental problems. Therefore, there is aeed for developing, not require chemical additions, a low cost andechnically effective arsenic removal technology.

In recent years, electrocoagulation (EC) shows the greatestotential for application in the treatment of drinking water andastewaters [15–19]. The advantages of EC over conventional

echnologies include high removal efficiency, compact treatmentacility, and possibility of complete automation. The arsenic cane removed successfully from industrial wastewater, surface andround waters by the EC process and removal efficiency could bebtained as high as 99% [20–38]. However, most of the studies onrsenic removal by the EC process in literature are in solutions con-aining high arsenic concentrations prepared by deionised waternd industrial wastewater; consequently, their results can differith respect to those obtained with real water (groundwater andotable water) because a great number of species are present ineal water that interfere with the arsenic removal process.

Recently, some household filters and micro-alloyed aluminumomposite are used for the arsenic removal from aqueous solu-ions [39] and treatment of metal finishing industry wastewaterontaining copper and zinc in spontaneous reduction-coagulationrocess [40]. The filtrates for the arsenic removal do not fulfill theecommended guideline value in removing pathogens and otherhemicals like fluoride present in drinking water, but the EC pro-ess is able to reduce arsenic concentration lower than 10 �g L−1 inrinking and ground waters.

The purpose of this work is to achieve lower concentration ofrsenic than the permissible level of arsenic using Fe and Al elec-rodes by the EC process and to reduce the energy and electrodeonsumption by shortening the treatment time for As removalrom drinking water. Influence of operation parameters that affecthe EC process; initial pH, current density, operating time, initialrsenic concentration and electrode type was determined for betteremoval efficiency. Operating costs based on energy and electrodeonsumptions were also calculated.

. Arsenic removal mechanism with EC

EC consists of an in situ generation of coagulants by an electricalissolution of Fe or Al electrodes. The generation of metallic cationsakes place at the anode, whereas at the cathode, typically a H2roduction occurs together with OH− release. Ferric or aluminum

ons generated by electrochemical oxidation of Fe/Al electrode mayorm monomeric species and polymeric hydroxyl metallic com-lexes depending on the pH of the aqueous medium, which havetrong affinity for dispersed particles as well as counter ions toause coagulation [15–19]. In an EC process with Fe or Al electrode;he main anode (Eqs. (1)–(5)), cathode (Eqs. (6) and (7)), hydroly-is (Eqs. (11)–(20)), co-precipitation and adsorption reactions (Eqs.21)–(25)) are shown in the following equations:

.1. Anodic reactions for Fe or Al electrodes

In two-step process where iron is firstly oxidized to ferrous ionhich depending on anode potential, then oxidizes to ferric ion

20–38]:

e(s) → Fe2+ + 2e− (1)

e2+ → Fe3+ + e− (2)

e(s) → Fe3+ + 3e− (3)

l(s) → Al3+ + 3e− (4)

cta 56 (2011) 5060–5070 5061

When the current is applied, the electrodissolution of the anodeis accompanied with the oxidation of water

2H2O → O2 + 4H+ + 4e− (5)

2.2. Cathodic reactions for Fe or Al electrodes

The second step would take place at the anode. The general cath-ode reaction takes place at the cathode and results in the liberationof hydrogen. This is represented as:

2H2O + 2e− → H2(g) + 2OH− (Fe electrode) (6)

3H2O + 3e− → (3/2)H2(g) + 3OH− (Al electrode) (7)

When introducing air (or oxygen) to the process, Fe2+ is oxidizedrapidly:

O2(g) + 4Fe2+ + 2H2O → 4Fe3+ + 4OH− (in bulk solution) (8)

Generally, the Al3+ or Fe3+ ions released from anode are grad-ually hydrolyzed and formed the Al(OH)3(s) and Fe(OH)3(s) if thereis no other reactive species in solution. For Fe electrodes, the rateof the oxidation depends on the availability of dissolved oxygen.Typically at the cathode the solution becomes alkaline with time.The applied current forces OH− ion migration towards the cathodeand combine with hydroxide ions (Eqs. (9) and (10)):

Fe3+ + 3OH− → Fe(OH)3(s) (9)

Al3+ + 3OH− → Al(OH)3(s) (10)

2.3. Hydrolysis reactions in the solution

At pH 4 < pH < 7, iron undergoes hydrolysis according to reac-tions (1)–(3) [38,41]

Fe + 6H2O → Fe(H2O)4(OH)2(aq) + 2H+ + 2e (11)

Fe + 6H2O → Fe(H2O)3(OH)3(aq) + 3H+ + 3e (12)

Fe3+ hydroxide begins to precipitate floc with yellowish color

Fe(H2O)3(OH)3(aq) → Fe(H2O)3(OH)3(s) (13)

Fe causes the evolution of H2 from cathodic reaction (Eq. (6)).Rust may also be formed.

2Fe(H2O)3(OH)3(s) → Fe2O3(H2O)6 (14)

At pH 6 < pH < 9, precipitation of Fe3+ hydroxide (Eq. (14)) con-tinues. The minimum soluble iron concentration Fe(OH)3 solubilityoccurs over the pH range of 7–10 and Fe2+ hydroxide precipitationalso occurs presenting a dark green floc

2Fe(H2O)4(OH)2(aq) → Fe(H2O)4(OH)2(s) (15)

The pH for the minimum solubility of Fe(OH)n is in the range of7–8. EC floc is formed due to the polymerization of iron oxyhydrox-ides. Formation of rust occurs as shown in the following reactions[41,42]:

2Fe(OH)3 → Fe2O3 + 3H2O (hematite, maghemite) (16)

Fe(OH)2 → FeO + H2O (17)

2Fe(OH)3 → Fe(OH)2 + Fe3O4 + 4H2O (magnetite) (18)

Fe(OH)3 → FeO(OH) + H2O (goethite, lepidocrocite) (19)

Hematite, maghemite, rust, magnetite (Fe3O4), lepidocrociteand goehite were identified by XRD in the literature [9,38,41,42].

The compositon of pollutants in water or wastewater affects thegeneration and the type of green rust. The oxidation of Fe2+ ionsin green rust results in the formation of goethite, lepidocrocite,hematite, maghemite or magnetite in oxygen depleted systems. As

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062 M. Kobya et al. / Electrochi

emoval with EC using Fe electrodes was formation of a dark greenoc. In general, hydrogen gas and green rust were formed at theathode as shown in Eq. (15) [41,42]

Fe + (12 + x)H2O → (1/2)(12 − x)H2(g)

+ xFe(OH)3 · (6 − x)Fe(OH)2(s) (20)

.4. Co-precipitation and adsorption reactions

The arsenic removal occurs by ligand exchange, arsenate dis-laces a hydroxyl group of FeOOH giving rise to an insoluble surfaceomplex [22,23]:

FeOOH(s) + H2AsO4− → (FeO)2HAsO4

− + H2O + OH− (21)

FeOOH(s) + HAsO42− → (FeO)3AsO4(s)

− + H2O + 2OH− (22)

Al(aq)3+ + (3m − n)OH−

+ nHAsO4(aq)2− → Alm(OH)(3m−n)(HAsO4)n(s) (23)

Al–OH(s) + HAsO4(aq)2− → ≡ Al–OAs(O)2(OH)(s)

−

+ OH(aq)− (24)

l(OH)3(s) + AsO4(aq)3− → [Al(OH)3 ∗ AsO4

3−](s) (25)

here the surface symbols ≡ is used to denote the bonds of theations with the surface of the solid [9,37].

. Experimental

.1. Materials

Stock arsenic solutions of 1000 mg As L−1 were prepared accord-ng to the EPA standard method by dissolving As2O3 in drinking

ater containing 20% (v/v) KOH and then neutralizing by 20% (v/v)2SO4 to a phenolphthalein end point and then diluting to 1000 mLith 1% (v/v) H2SO4 and was stored at 4 ◦C in the refrigerator. The

est solutions containing of 75–500 �g L−1 of arsenic were preparedy diluting of stock solution with drinking water before use.

.2. Experimental set-up

The EC experiments were carried out in a batch mode using750 mL Plexiglas reactor (81 mm × 81 mm × 126 mm in dimen-

ion) using vertically positioned iron electrodes spaced by 13 mmnd dipped in the arsenic solution. Two anodes and two cathodesith dimensions of 50 mm × 73 mm × 2 mm made of iron and alu-inum plate (Fe: 99.5% purity, Al: 99.3% purity), were connected

o a digital DC power supply (Agilent 6675A model; 120 V, 18 A) inonopolar parallel connection mode and equipped with galvanos-

atic operational options [16,19]. The total effective electrode areaas 219 cm2 and the constant current was adjusted according to aesirable value (1.75–7.5 A m−2).

pH and conductivity of solutions before and after the EC wereeasured by a pH meter (Mettler Toledo 2050e) and a conductiv-

ty meter (Mettler Toledo 7100e). pH of the solutions was adjustedy adding either 0.1 N NaOH or 0.1 N H2SO4. The solution wasonstantly stirred at 200 rpm (Heidolp 3600) to reduce the massransport over potential of the EC reactor.

cta 56 (2011) 5060–5070

3.3. Experimental procedure

In each run, 560 mL arsenic solution (75–500 �g L−1) was placedinto the EC reactor. The anodes and cathodes were connected tothe positive and negative outlets of a DC power supply. Organicimpurities and oxide layer on electrode surfaces were removed bydipping for 2 min in a solution freshly prepared by mixing HCl solu-tion (35%) and hexamethylenetetramine aqueous solution (2.80%)[16,19]. Current and cell potential were held constant at desiredvalues for each run and the experiment was started. The samplesat the different operating times taken from the EC reactor were fil-tered using a 45 �m Millipore membrane and arsenic concentrationwas measured. At the end of the run, the electrodes were washedthoroughly with water to remove any solid residues on the surfaces,dried and reweighed. In addition, sludge after the EC experimentwas dried at 105 ◦C temperature for sludge analysis.

3.4. Analytical procedure

Based on the standard method suggested by APHA et al. [43],an atomic absorption spectrometer (PerkinElmer SIMAA 6000 AAS)equipped with a manual hydride generator at 188.9 nm wavelengthwas employed to determine the arsenic concentration in the sam-ples. The detection limit for this study was 0.1 �g L−1 of arsenic andanalysis of the duplicates was within 2% of errors.

Laboratory scale experiments were carried out at room tem-perature. All the chemical reagents used were of analytical grade.The chemical analysis of the drinking water (pH, alkalinity, andpresence of arsenic, iron, sulphate and phosphate) was carriedout according to Standard methods [43]. It was found that pHof water, bicarbonate alkalinity were varied in the range 7.1–7.8and 45–50 mg L−1 CaCO3, respectively. The dissolved iron, man-ganese, magnesium, phosphate and arsenic concentration were notin detectable range in drinking water. Characterizations of drink-ing water were 85 mg L−1 of Cl−, 18 mg L−1 of SO4

2−, 85 mg L−1 ofHCO3

−, conductivity of 1.7 mS cm−1, 10 mg L−1 of NO3−, pH 7.6,

6.2 mg L−1 of dissolved O2 and 20 mg CaCO3 L−1 of total hardnessbefore the EC process, and 75 mg L−1 of Cl−, 10 mg L−1 of SO4

2−,60 mg L−1 of HCO3

−, conductivity of 1.2 mS cm−1, 8 mg L−1 of NO3−,

pH 6.5–8.2, 4.6 mg L−1 of dissolved O2 and 12 mg CaCO3 L−1 of totalhardness after the EC process, respectively.

4. Results and discussion

4.1. Effect of initial pH on arsenic removal

pH of the medium changes generally during the EC processwhich depends on type of the electrode material and initial pH.The variation of arsenic concentration with operating time at dif-ferent initial pH for Fe and Al electrodes is shown in Fig. 1. The rateof arsenic removal was sharp at the beginning of the process andlater approached a constant value. This could be explained by thefact that the arsenic ions were more abundant at the beginning ofthe EC; hence the rate of reaction was high, while on the other handthere was reduced concentration of arsenic ions at the end of theprocess, resulting in a slower reaction rate.

For Fe electrodes, removal of arsenic was increased as initialpHi was changed from 4.5 to 8.5 with respect to operating time.As seen in Fig. 1(a), the allowed permissible limit, 10 �g L−1, forarsenic removal was obtained in the pHi range 4.5–8.5 as 9.5 �g L−1

in 15 min for pHi 4.5, 8.1 �g L−1 in 10 min for pHi 5.5, 4.9 �g L−1 in

10 min for pHi 6.5, 6.5 �g L−1 in 8 min for pHi 7.5 and 6.1 �g L−1

in 8 min for pHi 8.5, respectively. As pHi was ≥6.5, the operatingtime was decreased which led to increase effectively in arsenicremoval. The oxidation of Fe2+ was slow at pHi 6.5 and hydroxides

M. Kobya et al. / Electrochimica Acta 56 (2011) 5060–5070 5063

Fig. 1. Effect of initial pH for (a) Fe electrode and (b) Al electrode on the residuala(

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dfwTop

dosage rate and bubble production rate, size and growth of flocs

rsenic concentration as a function of the EC treatment time from drinking waterCD = 2.5 A m−2).

roduced at the cathode were not used up which resulted in anncrease in pHi. At 6.5 ≥ pH ≤ 8.5, Fe(OH)3(s) led to a pH decrease.he Fe2+ produced during the EC underwent complete oxidationo form Fe(OH)3(s) at pHi ≥ 6.5. Therefore, removal rate of arsenicas better as pHi ≥ 6.5. Lakshmanan et al. [22,28] indicated that atH 8.5, Fe2+ was completely oxidized and resulted in Fe(OH)3(s).s Fe(OH)3(s) is amphoteric, some Fe(OH)4

− formation was likelyt pH 8.5 since OH− ions were consumed to form iron hydroxideomplexes.

For Al electrodes, removal of arsenic was increased as initial pHias changed from 5.0 to 9.0 with respect to operating time. As

een in Fig. 1(b), the allowed permissible limit for arsenic removalas obtained in the pHi range 5.0–9.0 as 9.5 �g L−1 for pHi 5.0,

.7 �g L−1 for pHi 6.0, 6.5 �g L−1 for pHi 7.0 and, 8.8 �g L−1 for pHi

.0 in 15 min, respectively. Removal efficiencies of 93.5% for Fe and5.7% for Al electrodes were achieved with a residual arsenic con-entration of 10 �g L−1 or less for drinking water in the EC process.

A slight increase in the pH was observed with operating timeuring the EC process (Fig. 2). The effluent pHfin were changedrom 6.4 to 9.5 for Fe electrodes and from 6.8 to 9.8 Al electrodeshen the experiment was operated at pHi in the range 4.5–9.0.

his slight increase of pH value was associated to the formationf hydrogen gas at the cathode [19,37] and hydroxyl ions (OH−)roduced at the cathode electrode according to Eqs. (5) and (6)

Fig. 2. Changes of pH before and after the EC treatment of arsenic from drinkingwater for (a) Fe electrode and (b) Al electrode (Ci = 150 �g L−1, CD = 2.5 A m−2).

in the EC process. Thus, the electrolytic reactor was capable ofproducing enough OH− ions to compensate the acid-buffer andmake the solution alkaline. These ions were also mainly consumedin the ferric/aluminum oxyhydroxides formation which explainedthe slight pH increase after the EC treatment [44,45]. The Fe3+

ions may undergo hydration and depending on the pH of solu-tion, Fe(OH)2+, Fe(OH)2

+, Fe(OH)3, Fe(OH)6−, Fe(OH)4

− species mayalso be formed. The following aluminum species were also foundin aqueous solution with respect to pH: Al3+, Al(OH)2

+, Al(OH)3,Al(OH)4

−, Al13(OH)345+ and Al13(OH)32

7+ [16,18,19,42]. In the ECprocess, a slight increase observed with time in the pH was withinthe regulatory drinking water standards. Kobya et al. [19] observedthe same effect during the EC process and also reported that EC canact as pH neutralization step (Table 1).

4.2. Effects of current density on arsenic removal

It is well known that current density determines coagulant

which can influence removal efficiency of the pollutions in the ECprocess. Results in Fig. 3 showed an important drop in the arsenicconcentration with an increase of the operating time for both elec-

5064 M. Kobya et al. / Electrochimica Acta 56 (2011) 5060–5070

Table 1The effect of pH on the removal of arsenic by Fe and Al electrodes and operating cost parameters in the EC process.

Electrode pHi Ci (�g L−1) CEC (�g L−1) Re (%) tEC (min) Cenergy (kWh m−3) Celec, exp (kg m−3) Wsludge (kg m−3) OC (D m−3)

4.5 500 9.5 98.1 15 0.0148 0.0168 0.012 0.01555.5 500 8.1 98.4 10 0.0164 0.0155 0.014 0.0152

Fe 6.5 500 4.9 99.0 10 0.0140 0.0153 0.015 0.01467.5 500 6.5 98.7 8 0.0131 0.0148 0.018 0.01278.5 500 6.1 98.8 8 0.0131 0.0135 0.013 0.0115

5.0 150 9.5 93.7 15 0.0324 0.0077 0.018 0.01546.0 150 4.7 96.9 15 0.0324 0.0083 0.017 0.0165

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Al 7.0 150 3.5 97.7 158.0 150 8.8 94.1 159.0 150 48.5 67.6 >15

rodes at Ci = 150 �g L−1, pHi 6.5 for Fe and 7.0 for Al electrodes asurrent density varied in the range 2.5–7.5 A m−2. The rate of reduc-ion in the arsenic concentration was almost sharp in the earlytages of the process and decreased to gradual reduction in laterart of electrolysis (Fig. 3). The arsenic removal efficiencies werehanged in the range 93.5–94.1% for Fe and 93.7–96.5% Al elec-rodes at current density of 2.5–7.5 A m−2 (Table 2). In addition,he results satisfied for all cases where arsenic concentration per-

issible level in drinking water was obtained lower than 10 �g L−1

nd this concentration was marked in blue in Fig. 3. An importantecrease of the arsenic concentration appeared in the treated drink-

ng water solution just after 5 min for Fe and 2.5 min of operationor Al electrodes, respectively. This can be attributed at high cur-ent densities; the extent of anodic dissolution (Faraday’s law, Eq.26)) increased which increased the hydroxide cationic complexesesulting in increased arsenic removal [16,19,21].

elec,theo = itECMw

zFv(26)

here Celec, theo (kg Al or Fe electrode m−3 treated drinking water)s the theoretical amount of ion produced by current i (A) passed

or a duration of operating time tEC (s), z is the number of elec-rons involved in the oxidation/reduction reaction; for Fe, zFe = 2nd for Al, zAl = 3. Mw is the atomic weight of anode materialMw,Fe = 55.85 g mol−1, Mw,Al = 26.98 g mol−1), F is the Faraday’s

able 2he effect of current density on the removal of arsenic by Fe and Al electrodes and results

Parameters Current density (A m−2)

Fe electrode

Current density, j (A m−2) 2.5 5.0Current intensity imposed, i (A) 0.05 0.11Operating time, tEC (min)a 12.5 7.5Initial arsenic concentration, Ci (�g L−1) 150 150Effluent arsenic concentration, CEC (�g L−1)b 9.8 8.9Arsenic removal efficiency, Re (%) 93.5 94.1Average voltage between electrodes, U (V) 0.8 1.2Initial pHi 6.5 6.5Final pHf 7.3 7.8Coulomb passed (C) 37.5 49.5Charge loading (Faradays m−3) 0.694 0.9161Theoretically produced metal (mol) 1.94 × 10−4 2.57 × 10−4

Arsenic removed (mol) 1.57 × 10−6 1.87 × 10−6

Metal/arsenic ratio, Me/As (mol/mol) 123.8 137.4Removal arsenic/Coulomb passed (mg C−1) 0.0038 0.0029Energy consumption, Cenergy (kWh m−3) 0.0149 0.0276Electrode consumption, Celec, exp (kg m−3)c 0.0211 0.0272Electrode consumption, Celec, theo (kg m−3)d 0.0194 0.0257Current efficiency, CE (%) 109 106Sludge production, Wsludge (kg m−3) 0.031 0.034Operating cost, OC (D m−3) 0.0196 0.0255

a Minimum operating time for WHO recommended level of maximum arsenic concentb Data show values of arsenic concentrations under WHO recommended as 10 �g L−1.c Experimental calculations.d Theoretical calculations.

0.0324 0.0087 0.020 0.01720.0324 0.0085 0.020 0.01670.0324 0.0061 0.013 0.0125

constant (96,485 C mol−1) and v is the volume (m3) of the drink-ing water in the EC reactor. According to Faraday’s law, the chargepassed to the solution was directly proportional to amount of elec-trode (Fe or Al) dissolved. This implied that the arsenic removalby EC may be governed by the formation of metal-hydrous fer-ric or aluminum oxide complexes [9,33]. An operating time of12.5 min for Fe and 15 min for Al electrodes at 2.5 A m−2 wasselected to achieve a residual arsenic concentration of 10 �g L−1

or less for drinking water in the EC process which resulted inremoval efficiencies of 93.5% for Fe and 95.7% for Al electrodes andreducing of the operating cost (Table 2). This was associated withthe higher dissolution of Fe2+ or Al3+ ions when current densitywas increased (anode and cathode reaction Eqs. (1)–(10)), and thearsenic removal increased consequently. This was consistent withthe results reported by several authors [24,25].

More sludge was also produced from Fe and Al electrodes athigher current density due to elevated dissolution rate of anode.Arsenic species removed by metal hydroxide flocs were formed inthe vicinity of the anode surface either by surface complexationor electrostatic attraction. Therefore, when high current densi-ties were applied, the coagulation and removal time of arsenic

improved. However, these parameters should be kept at low levelto achieve a low-cost treatment. For that a compromise of the cur-rent density and electrolysis time was necessary to optimize thetreatment efficiency with the lowest cost. Considering this operat-

of detailed experimental parameters in the EC process.

Al electrode

7.5 2.5 5.0 7.50.16 0.05 0.11 0.165 15 10 4150 150 150 1509.2 6.5 9.5 5.293.9 95.7 93.7 96.51.6 1.5 1.9 2.36.5 7.0 7.0 7.08.1 7.5 7.5 7.648.0 45.0 66.0 38.40.9161 0.833 1.22 0.7112.49 × 10−4 1.55 × 10−4 2.28 × 10−4 1.33 × 10−4

1.88 × 10−6 1.90 × 10−6 1.90 × 10−6 1.90 × 10−6

132.5 81.3 121.7 68.70.0029 0.0032 0.0021 0.00380.0375 0.0324 0.0635 0.04720.0268 0.0087 0.012 0.00690.0249 0.0075 0.011 0.0064108 116 109 1080.058 0.020 0.019 0.01350.0261 0.0172 0.0252 0.1485

ration (10 �g L−1) in the drinking water.

M. Kobya et al. / Electrochimica Acta 56 (2011) 5060–5070 5065

Fu(

id

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ig. 3. Effect of current density for (a) Fe electrode and (b) Al electrode on the resid-al arsenic concentration as a function of the EC treatment time from drinking water150 �g L−1, pHi 6.5 for Fe and 7.0 for Al electrodes).

ng cost factor, all further experiments were carried out at currentensity values of 2.5 A m−2 for both electrodes.

In the EC process, the coagulant Al3+ or Fe2+ were producedy electrochemical sacrifice of the aluminum or iron anodes. Itsosages were determined by charge loading. The quality of EC efflu-nt depended on the amount of coagulant produced (mg) or appliedharge loading. Theoretically, according to Faraday’s law, wheneverFaraday of charge passes through the circuit, 9.0 g of aluminum

r 28 g of iron are dissolved at each anode of a different connec-ion mode EC unit. When the charge loading of the EC was low,he coagulant dosages (Al3+ of Fe2+) were not sufficient to removalll arsenic ions, and thus the arsenic removal efficiency was notigh. Fig. 4a showed the arsenic removal as a function of charge

oading. The charge loading (Q = i tEC/Fv) was calculated as thepplied current multiplied by the residence time per unit of vol-me [34,45]. The arsenic removal was proportional to the charge

oading in range of 0.1–0.9 F m−3 (Fig. 4a). The curve for the arsenicemoval flattened above approximately 0.69 and 0.83 F m−3 chargeoading values for Fe and Al electrodes. This was as expected sinceess amount of arsenic was remaining. So, relatively more Fe3+ or

l3+ had to be added at this low concentration to remove rest of thersenic. Mass transfer could be the limiting step for this concen-ration and it could be hindered by the increased gas productionhen applying higher current. Another reason could be that the

Fig. 4. Effects of (a) charge loading and (b) energy consumption on removal ofarsenic from drinking water in the EC process.

stress on the lower edges of two electrodes together with relativemuch higher electric field around them would make them corrodemuch faster than other parts of the electrodes. When consideringthat the only reaction at the anodes were Fe2+ and Al3+, then thetheoretical amount of produced Fe2+ and Al3+ could be calculatedby the total electrical charge passed through the EC system (Eq.(26)). The overall arsenic removal as a function of applied electricalcharge in Table 2 also showed that the EC process was working at anacceptable level. The arsenic removal rates for Fe for Al electrodesat 2.5 A m−2 were 0.0038 and 0.0032 mg Coulomb−1, respectively.

It was clear that a technically efficient process must also be fea-sible economically. The major operating cost of EC was associatedwith electrical energy consumption during process [16,21,27]. Theelectrical energy required to arsenic removal for Al and Fe elec-trodes was calculated in terms of kWh m−3 using the equation givenas follows:

Cenergy = UitEC

v(27)

where U is cell potential (V) in the EC reactor. Energy consump-tion versus operating time for both electrodes in the EC processwas shown in Fig. 4b and linear dependences were observed. The

5 mica Acta 56 (2011) 5060–5070

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nergy consumptions for both electrodes were required 0.8 V fore and 1.5 V for Al electrodes (Table 2). Therefore, longer operat-ng times may be used to obtain higher arsenic removal efficienciessing less electric energy. The energy consumptions for Fe elec-rode (0.015 kWh m−3) were found to be lower than that for Allectrode (0.032 kWh m−3). This result was probably due to theigher potential required to achieve a certain current density.

According to Faraday’s law, it was clear that Al3+ or Fe2+ doseeleased from anode depended on the electrolysis time and current.o in the EC process, current density and operating time were theost important parameters affecting the arsenic removal efficiency

nd controlling the reaction rate in the EC reactor. As seen in Table 2,xperimental electrode consumption values at the 2.5 A m−2 were.021 kg m−3 for Fe and 0.0087 kg m−3 for Al electrodes while theheoretical electrode consumption reached its highest value of.019 kg m−3 for Fe and 0.0075 kg m−3 for Al electrodes as min-

mum operating time needed to meet the WHO recommendedevel of maximum arsenic concentration in the drinking water. It

as clear that electrode consumptions were higher with Fe elec-rode material based on a molecular weight. By considering atomiceights of Al and Fe, the consumptions on molar basis were veryifferent; the calculated values were 0.074 mol g m−3 for Fe and.093 mol g m−3 for Al electrodes at a given current density of.5 A m−2.

It was also an important parameter for the EC process becauset affected lifetime of the electrode [46]. The Faradic yield or cur-ent efficiency (CE) is defined as the ratio of the actual electrodeonsumption to the theoretical value. The CE calculation was basedn the comparison of experimental weight loss of aluminum orron electrodes (Celec, exp) during the EC process with theoreticalmount of aluminum or iron dissolution (Celec, theo) according toq. (28) by the electrode consumption difference before and afterhe EC process. The current efficiency of the arsenic removal exper-ments with electrode connection modes was calculated using theollowing equation:

E(%) = Celec,exp

Celec,theo× 100 (28)

In Table 2, the results showed that the current efficiencies foroth electrodes were in the range 106–116%. This difference in massay be explained by the “corrosion pitting” phenomenon which

aused holes and led practically to a metallic metal (Fe or Al) lossn the electrode surface [47]. This mass was erroneously calculateds the metal dissolved by the EC process.

Amount of electrochemically dissolved Fe or Al electrodes forrsenic removal methods in the EC are a major cost item thats directly proportional to removed arsenic concentration in therinking water. However, effectiveness of the EC process in theifferent process conditions was very sensitive to the Me (Fe orl):As ratio, and this parameter can vary by several orders of mag-itude. The ratio metal (Me:Fe or Al; theoretically produced metal)o arsenic (removed from the EC process) at 10 mg L−1 in mol mol−1

as calculated and shown in Table 2. It was seen from the table thathe lowest ratio was obtained for 123.8 mol Fe mol−1 arsenic and8.7 mol Al mol−1 arsenic at the 2.5 A m−2. This meant less metalecessary to dissolve in order to adsorb or co-precipitate arsenic

ons for both electrodes. In addition, this low ratio assured only amall amount of final solid waste product in comparison to con-entional treatment methods. Furthermore, it could be seen thato residual iron or aluminum was present in the outlet solution,

nsuring a high arsenic removal efficiency of the produced ferric orluminum ions. This could indicate that OH− ions was produced

t the cathode and was promoting the arsenic, hydrolysis, pre-ipitation or co-precipitation (Eqs. (11)–(25)). These rations waslso compared to the literature values, where Me:As ratios for con-entional precipitation with FeCl3/Al(OH)3 addition fluctuate was

Fig. 5. Effects of initial concentration on the residual arsenic concentration as afunction of the EC treatment time from drinking water (a) Fe electrode and (b) Alelectrode (CD = 2.5 A m−2, pHi 6.5 for Fe and 7.0 for Al electrodes).

between 5 and 191 [34,37]. The Fe:As ratio had to be high to reducethe concentration of arsenic below 10 mg L−1 around 20–28 using abatch EC process [9,21], while the airlift reactor carrying out EC ona 100 mg L−1 solution was around 14 mol mol−1. The higher Me:Asratio with 100 mg L−1 could be due to mass transfer being the lim-iting step. For the efficient removal of arsenic from drinking water,the ratio typically was larger than 10 [23,37]. Our results were inagreement with the literature values (Table 2).

4.3. Effects of arsenic concentration on arsenic removal

The effect of initial arsenic concentration from 75 to 500 mg L−1

was examined with drinking water at 2.5 A m−2, pH 6.5 for Fe andpH 7.0 Al electrodes. Fig. 5a and b illustrated the effect of the initialarsenic concentration as a function of operating time. As can be seenin Fig. 5a and b, the initial arsenic concentration increased from 75

to 500 mg L−1, the operating time and removal efficiency for achiev-ing the arsenic concentration of 10 mg L−1 increased from 7.5 to15 min and from 88.5 to 98.1% for Fe and from 4.0 to 15.0 min andfrom 86.9 to 98.7% for Al electrodes, respectively. In the EC process,

M. Kobya et al. / Electrochimica Acta 56 (2011) 5060–5070 5067

Table 3The effect of initial concentration on the removal of arsenic by Fe and Al electrodes and operating cost parameters in the EC process.

Electrode Ci (�g L−1) CEC (�g L−1) Re (%) tEC (min) Cenergy (kWh m−3) Celec, exp (kg m−3) Wsludge (kg m−3) OC (D m−3)

75 8.6 88.5 7.5 0.0076 0.0415 0.013 0.0358Fe 150 9.8 93.5 12.5 0.0149 0.0211 0.031 0.0196

500 9.5 98.1 15.0 0.0206 0.1055 0.075 0.09120.000.030.03

tcaibtAfroh[sawci

4

c

F2

the initial pH and initial arsenic concentration in the EC process at10 mg L−1 were 0.012–0.018 kg m−3 and 0.013–0.075 kg m−3 for Feand 0.013–0.020 kg m−3 and 0.013–0.025 kg m−3 for Al electrodes

75 9.8 86.9 4.0Al 150 9.5 93.7 15.0

500 6.5 98.7 15.0

he rate of arsenic removal was proportional to the initial arseniconcentration. Higher arsenic removal efficiency at higher initialrsenic concentration can be achieved by a proportionate increasen current density and electrode surface area. This can potentiallye explained by three reasons. First, at higher arsenic concentra-ions arsenic ions readily electrocoagulated in the vicinity of Fe orl electrodes; however, with progress of the removal, increased dif-

usional resistance to movement to the surface of the anode acted toeduce the rate of EC. Second, one of the most important pathwaysf arsenic removal by the EC process was adsorption onto metallicydroxide flocs, and the adsorption capacity of flocs was limited37]. Third, since our experiments were all performed under theame operating conditions, the release rate of the hydroxyl ions waslmost constant. It was found that the amount of sludge producedas nearly same for all the experiments performed in initial arsenic

oncentrations range studied (Table 3). As expected, operating costncreased with increasing of operating time.

.4. Characteristics of EC sludge

The sludge production was another important parameter inharacterizing the EC process. The EC sludge production was pro-

ig. 6. (a) SEM micrograph and (b) EDS spectra of sludge and imaged magnified000×.

90 0.0056 0.013 0.009624 0.0087 0.020 0.017208 0.0246 0.025 0.0416

portional to characteristics of wastewater, settable solids andmatter destabilized by coagulation and concentration flocculentand was also proportional to current density and residence time[9,36,38]. Sludge contained arsenic and metals (Al or Fe) ions inthe EC reactor. In the EC process, arsenic was separated from andfloated on the surface of the wastewater in the form of sludge. Whenvalues of sludge for each electrode were compared with respectmolar electrode consumptions based on a molar basis, it was seenthat aluminum hydroxide flocs bound more water, chemically orphysically, than iron hydroxide flocs did. Values of the sludge in

Fig. 7. Pseudo second-order plot for arsenic adsorption (a) Fe electrode and (b) Alelectrode at different current densities in the EC process.

5068 M. Kobya et al. / Electrochimica Acta 56 (2011) 5060–5070

Table 4The effect of current density and initial concentration on the adsorption kinetics of arsenic by Fe and Al electrodes in the EC process.

Parameters Fe electrode Al electrode

k2 (g mg−1 min−1) qe (mg g−1) r2 k2 (g mg−1 min−1) qe (mg g−1) r2

CD (A m−2)2.5 0.1431 5.2 0.99 0.0232 17.5 0.955.0 0.1322 2.6 0.94 0.0447 7.9 0.947.5 0.1810 1.7 0.95 0.0584 5.5 0.93

Ci (�g L−1)0.0.0.

(0Gwc

TC

O

75 0.2608 3.4150 0.1431 5.2500 0.0790 12.8

Tables 1 and 3). The sludge values at the optimum conditions were

.031 kg m−3 for Fe and 0.020 kg m−3 for Al electrodes (Table 2).enerally, more amount of sludge was formed when Fe electrodeas used. From operating cost point of view, Al electrode was

learly more economic material type than Fe electrode. Surface

able 5omparisons of arsenic removals with different electrodes and operating conditions in th

As content and water type Type of electrode Type of EC reactor Re (%)

Drinking water(C0 = 150 �g L−1 As(III),pHi = 6.5)

Fe plate Batch 93.5

Drinking water(C0 = 150 �g L−1 As (III),pHi = 7.0)

Al plate Batch 95.7

Potable water(C0 = 100–1000 �g L−1

As(III) and As(V)pHi = 5–8.0)

Rod-shaped Fe Batch air fed >90–99

Synthetic solution(C0 = 50 �g L−1 As(V),pHi = 7.5 and 6.5c)

Rod-shaped Fe Continuous(0.245 L min−1) andbatch

>75–85

Synthetic solution(C0 = 500 �g L−1 As(V),pHi = 7.0)

Al plate Batch 98.4

Model water(C0 = ∼190 �g L−1 As(V),pHi = 8.07)

Carbon steel plate AISI1018

Continuous air fed(3 L min−1)

>95

Ground water(C0 = 133 �g L−1

(122.5 �g L−1 As(V) and10.5 �g L−1 As(III))pHi = 8.06)

Carbon steel plate: AISI1018

Continuous air fed(0.875 L min−1)

99

Synthetic solution(C0 = 100 mg L−1 As(V),pHi = 7.0)

Mild steel plate Batch 94

Ground water (Mexico)(C0 = 131 �g L−1 As,pHi = 7.2)

Low carbon steel plateASTM 1018

Bipolar batch air fed >93

Synthetic solution(C0 = 100 mg L−1 As(V),pHi = 7.0)

Fe cylinder Batch airlift >98

Synthetic solution(C0 = 100 mg L−1 As(V),pHi = 7.0)

Fe cylinder Continuous airlift(0.06–0.24 L min−1)

88–50

Synthetic solution(C0 = 100 mg L−1 As(V))

Steel plate: A37-24ES Continuous air fed(3 L h−1)

>98

Groundwater (Mexico)(C0 = 40 �g L−1 Astotal,pHi = 7.0)

Carbon steel plate Lamar Mobil Pilot Plant(30 L min−1)

>99

Synthetic solution(C0 = 2 mg L−1 As(III) andAs (V), pHi = 6–8)

Fe plate Batch >99

C: operating cost (including costs of energy and electrode consumption), Re: As removaa 100–1000 �g L−1 for As(III).b 100–1000 �g L−1 for As(V).c Initial pH for batch process.d The steady state operation mode, the residence time.e Steady state time.f Energy cost only.

96 0.0242 12.8 0.9899 0.0223 17.5 0.9595 0.0026 46.2 0.94

topography of the sludge was analyzed using SEM (SEM, Philips

XL30S-FEG). The SEM image in Fig. 6a indicated the presenceof mostly amorphous structure which had an aggregate size of20–50 �m. The sludge at 2000× magnification appeared to be arelatively uniform cake (showing cracks due to drying). Energy dis-

e EC process.

Current density(A m−2)

tEC (min) OC ($ m−3) Reference

2.50 12.5 0.020 This study

2.50 15 0.017 This study

0.19 30–45a5–45b – [22]

0.25 2 – [24]

20.00 30 – [25]

45.00 0.08 – [27]

45.00 0.75 – [33]

150.00 50 – [32]

3.00 0.34 – [29]

120.00 60 – [34]

180.00 16.7–4.2 – [34]

120.00 9d (90e) – [37]

4.60 1.5 0.002f [39]

1.53 5 – [40]

l efficiency, tEC: operating time, j: current density.

M. Kobya et al. / Electrochimica Acta 56 (2011) 5060–5070 5069

Table 6Comparison of the conventional processes with EC process for As removal [38,48].

Process type Process conditions Advantages Disadvantages

Precipitation with alum pH ≤ 6.5, ERS = lower as well ashigher As0, AEAC ≤ 20 �g L−1,Re = 20–90%, OC = medium

Well established; suitable forhome use

Use of chemicals; high arseniccontaminated sludge; dose of oxidizingchemicals highly influence on theremoval efficiency

Precipitation with iron pH 6–8, ERS = lower as well ashigher As0, AEAC ≤ 20 �g L−1,Re = 60–90%, OC = medium

Proven and reliable Use of chemical; high arseniccontaminated sludge; dose of oxidizingchemicals highly influence on theremoval efficiency

Precipitation with Fe/Mn pH > 7, ERS = lower as well ashigher As0,AEAC ≤ 10 �g L−1,Re = 40–90%,OC = medium

Proven and reliable Higher and lower pH reducesefficiency; use of chemical; higharsenic contaminated sludge; dose ofoxidizing chemicals highly influenceon the removal efficiency

Lime softening pH ≥ 10.5, ERS = lower As0,AEAC ≤ 10 �g L−1, Re = 80–90%,OC = high

Proven and reliable; reducescorrosion

Sulphate ions influence efficiency;secondary treatment is required; use ofchemicals

Reverse osmosis ERS = lower As0, AEAC ≤ 2�g L−1,Re ≥ 90%, OC = high

Highest water quality; treats widerange of dissolved salts, minerals;turbidity

Expensive to install and operation;frequent membrane monitoring; pH,temperature and pressure control tomeet membrane tolerance

Electro dialysis pH 7–9, ERS = lower As0,AEAC ≤ 3 �g L−1, Re ≥ 95%,OC = high

Pure quality water Less proven; costly; needs oxidizingagents

Ion exchanges pH 7.5, ERS = lower As0,AEAC ≤ 2 �g L−1, Re ≥ 90%, OC = high

Can produce treated water with Asconcentration less than 2 �g L−1

Efficiency affected by sulphate,nitrates, fluorides ions, TDS, selenium,etc.

Adsorption in activatedalumina

pH 5.5–6.0, ERS = lower As0,AEAC ≤ 1 �g L−1, Re ≥ 90%, OC = low

Well established; suitable for homeuse; typically inexpensive withsimple Replacement requirements;improves test and odour

Careful monitoring; effectiveness isbased on contaminant type;concentration and rate of water usage;bacteria may grow on alumina surface

Adsorption on activated carbon pH 2–9, ERS = lower As0,AEAC ≤ 7 �g L−1, R = 30–90%,

Typically inexpensive with simplereplacement requirements;im

Efficiency depends on the ash contentin the carbon and on the metal

A nge o

pf(si

4

oaoss

q

(

(acttcagtwFu

e

OC = low

EAC: attainable effluent As concentration; Re: removal efficiency; ERS: effective ra

ersive spectrum (EDS) of the sludge at the optimum conditionsor both electrodes in Fig. 6b showed the presence of As removed0.8 at%) from the sample solution. Other elements detected in theludge were Au, iron and aluminum corresponding to the gold coat-ng used for the SEM process and electrodes, respectively.

.5. Adsorption kinetics of EC-arsenic removal

The kinetics give information about the arsenic adsorptionn the solid adsorbent phase [30,37]. The adsorption kinetics ofrsenic on amount of electrochemically dissolved metal (ferricr aluminum oxyhydroxides) were determined from the pseudoecond-order kinetic model equation. The linearized pseudoecond-order kinetic model was expressed as [20,28]:

t = (Ci − Cf)vCi

(29)

t

qt

)= 1

k2 q2e

+ 1qe

t (30)

The slope and intercept were determined from the plot oft/qt) versus t. In the equation, qt (mg g−1) is the amount ofrsenic species adsorbed at time t (min), Ci and Cf are arseniconcentrations before and after the EC, k2 (g mg−1 min−1) ishe rate constant of the pseudo second-order adsorption, qe ishe amount of arsenic adsorbed at equilibrium (the adsorptionapacity), and hi = k2q2

e (mg g−1 min−1) is the initial arsenicdsorption rate, respectively. This kinetic model could provide aood fit to the experimental data for the treatment times inves-

igated and the correlation coefficients (r2) for the linear plotsere higher than 0.92 for all the experimental data (Table 4 and

ig. 7). The arsenic removal depended on the amount of coag-lant generated in the EC since the applied charge was directly

proves test and odour concentration; not proven

f separation; OC: operating cost.

proportional to the amount of coagulant generated. The amountof both dissolved electrodes was increased 3.2 times more withincreasing of the current density. In this case, values of qt wouldbe decreased as the amount of dissolved electrode increased. Itcan also be observed that the adsorbent capacity also increasedwith the increase in the initial arsenic concentration (Table 4).The adsorbent capacities and rate constants at 2.5–7.5 A m−2

and 75–500 �g L−1 in the EC were obtained in the range5.2–1.7 mg g−1, 3.4–12.8 mg g−1 and 0.1431–0.1810 g mg−1 min−1,0.2608–0.0790 g mg−1 min−1 for Fe and 17.5–5.5 mg g−1,12.6–46.2 mg g−1 and 0.0232–0.0584 g mg−1 min−1,0.0242–0.0026 g mg−1 min−1 for Al electrodes, respectively(Table 4). The error limits for the kinetic values were ±0.1. Similarkinetic results in the literature were observed in arsenic removal bythe EC process from underground and drinking water [20,28,37].

4.6. Operating cost of arsenic removal

One of the most important parameters that affect the applicationof any method of water and wastewater treatment greatly is theoperating cost (OC). The operating cost included material (mainlyelectrodes) cost, utility (mainly electrical energy) cost, as well aslabor, maintenance and other fixed costs. The latter costs itemswere largely independent of the type of the electrode material.Thus; energy, electrode material and chemicals costs were takeninto account as major cost items, in the calculation of the operatingcost as D m−3 of the drinking water treated [16,21]:

operating cost (OC,D m−3) = aCenergy + bCelectrode + cCchemicals

(31)

where Cenergy (Eq. (27), consumption kWh m−3), Celectrode (Eq.(26), consumption kg electrode m−3) and Cchemicals (consumption

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070 M. Kobya et al. / Electrochi

g chemicals m−3) of water treated. Unit prices, a, b and c given forhe Turkish Market, November 2010 were as follows:

a is electrical energy prices (0.072 D kWh−1),b is electrode material price for Al (1.65 D kg−1) and Fe(0.85 D kg−1),c is chemical prices for NaOH and H2SO4 (0.40 D kg−1 and0.15 D kg−1).

The operating costs were calculated for the removal of arsenicrom drinking water at 10 mg L−1 or lower. The operating costst pH 4–9 and 75–500 mg L−1 were 0.012–0.016 D m−3 and.04–0.09 D m−3 for Fe and 0.015–0.017 D m−3 and 0.01–0.04 D m−3

or Al electrodes (Tables 1 and 3). The lower initial arsenic con-entration yielded lower operating cost for removal efficiency ofrsenic under the present experimental conditions. On the otherand, the operating cost was increased for both electrodes as theurrent density was increased (Table 2). As a result, the operatingosts at optimum operating conditions (pH 6.50 for Fe and 7.0 forl, CD = 2.5 A m−2 and Ci = 150 mg L−1) were calculated 0.020 D m−3

t 12.5 min for Fe and 0.017 D m−3 at 15.0 min for Al electrodes.nit cost of the process for Al (0.0012 D m−3 min) was found to beheaper than that of Fe (0.0016 D m−3 min−1).

Comparisons of arsenic removals with different electrode mate-ials and operating conditions in the EC process were illustratedn Table 5 [48]. As removal efficiencies in EC process with currentensities of 0.19–180 A/m2 and operating time 0.08–90 min werebtained in the range of 50–>99%. It was difficult to compare theperating costs of EC process since some of cost items were notiven with details in most of the published reports in the litera-ure such as energy, electrode, chemicals, etc. In this study, it wasbtained for better rate of As removal efficiency of 93.5% for Felectrode and 95.7% for Al electrode, respectively (Table 5).

Table 6 was produced to show comparison of conventional tech-iques with EC process including process conditions, advantagesnd disadvantages. As seen in Table 6, the EC process was superioro the other techniques since conventional techniques had someimitations such as use of chemical, large volume of high arsenicontaminated sludge, need of secondary treatment in some cases,onger operating time, difficulty in monitoring, high installationnd operating cost and lower efficiency in many cases.

. Conclusions

Batch EC studies were performed to evaluate the influence ofarious experimental parameters such as initial pH, electrolysisime, initial concentration and current density on the removal ofrsenic in drinking water. The EC process was able to decreasehe residual arsenic concentration to less than 10 �g L−1 (belowhe limit set by the WHO). Optimum values of pH and currentensity for arsenic removal were found to be 6.5 for Fe and 7.0or Al and 2.5 A m−2, respectively. The optimum operating timeor arsenic removal was 12.5 min for Fe and 15 min for Al elec-rodes. The operating costs at the optimum operating conditionsere calculated as 0.020 D m−3 for Fe and 0.017 D m−3 for Al

lectrodes. The surface of sludge was analyzed by SEM and themage was mainly composed of irregular and porous particles hav-ng amorphous structure. The adsorption kinetics of arsenic onmount of electrochemically dissolved metal (ferric or aluminumydroxides) based on Faraday’s laws was followed to the pseudoecond-order adsorption kinetic model. The arsenic adsorption

apacities at 2.5–7.5 Am−2 and 75–500 �gL−1 for the treatmentrsenic from the drinking water in the EC process were in the range.2–1.7 mg g−1 and 3.4–12.8 mg g−1 for Fe and 17.5–5.5–mg g−1

nd 12.8–46.2 mg g−1 for Al electrodes. The removal mechanism of

[[[

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cta 56 (2011) 5060–5070

arsenic by the EC process might be co-precipitation and adsorptionwith metal hydroxides generated in the process.

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