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Journal of Hazardous Materials 169 (2009) 1127–1133

Contents lists available at ScienceDirect

Journal of Hazardous Materials

journa l homepage: www.e lsev ier .com/ locate / jhazmat

ffect of pH and chloride concentration on the removal of hexavalent chromiumn a batch electrocoagulation reactor

.G. Arroyo, V. Pérez-Herranz ∗, M.T. Montanés, J. García-Antón, J.L. Guinónepartamento de Ingeniería Química y Nuclear, Universidad Politécnica de Valencia, Camino de Vera s/n, 46022 Valencia, Spain

r t i c l e i n f o

rticle history:eceived 4 December 2008eceived in revised form 17 April 2009

a b s t r a c t

In this work, the effect of pH and chloride ions concentration on the removal of Cr(VI) from wastewaterby batch electrocoagulation using iron plate electrodes has been investigated. The initial solution pHwas adjusted with different concentrations of H2SO4. The presence of chloride ions enhances the anode

ccepted 20 April 2009vailable online 3 May 2009

eywords:exavalent chromiumlectrocoagulation

dissolution due to pitting corrosion. Fe2+ ions formed during the anode dissolution cause the reductionof Cr(VI) to form Cr(III), which are co-precipitated with Fe3+ ions at relatively low pH.

The reduction degree of Cr(VI) to Cr(III) and the solubility of metal hydroxide species (both chromic andiron hydroxides) depend on pH. At higher concentrations of H2SO4, the reduction of Cr(VI) to Cr(III) byFe2+ ions is preferred, but the coagulation of Fe3+ and Cr(III) is favoured at the lower H2SO4 concentrations.

Hhloride ions

. Introduction

Chromium salts are used in many industries such as leatheranning, electroplating, wood preservation, manufacture of dyes,aper, and fertilizers, industries that employ Cr(VI) as corrosion

nhibitor and industrial operations involving the use of ferrous andonferrous metals. These industries generate residues in whichhromium can appear in solution as Cr(III) and Cr(VI) compounds.he characteristic of these two species differs considerably: com-ounds of Cr(VI) have been related as very toxic, mainly due toheir confirmed carcinogenic action, whereas Cr(III) is essential forhe maintenance of the metabolism of lipids, glucose and proteins1].

Due to the high toxicity of hexavalent chromium, its concen-ration in industrial effluents is strictly controlled before beingiscarded into sewages or rivers, which is a major environmental

ssue, affecting every industrialized country in the world [2]. Dis-harge consents are, however, site specific and are dependent uponhe dilution capabilities of the water body into which the effluents discharged [3].

To meet environmental regulations, effluents or water contami-ated with hexavalent chromium must be treated before discharge.

he elimination of chromium from residual wastewaters is a chal-enge for the control of the contamination. Conventional chromiumemoval methods include adsorption [4–6], chemical precipitation7–9], biological degradation [10], ion exchange [11], solvent extrac-

∗ Corresponding author. Tel.: +34 96 3877632; fax: +34 96 3877639.E-mail address: vperez@iqn.upv.es (V. Pérez-Herranz).

304-3894/$ – see front matter © 2009 Elsevier B.V. All rights reserved.oi:10.1016/j.jhazmat.2009.04.089

© 2009 Elsevier B.V. All rights reserved.

tion [12], biosorption [13–15], membrane separation [16–18] andelectrochemical methods [3,19–22]. Electrochemical based meth-ods allow controlled and rapid reactions, smaller systems becomeviable and, instead of using chemicals and micro-organisms, thesystems employ only electrons to facilitate water treatment. Amongthe electrochemical methods, electrocoagulation (EC) is a simple,efficient and economical method for wastewater treatment, as itoffers the possibility to be easily distributed, and require minimumamount and number of chemicals, thus reducing the amount ofsludge which must be disposed [23]. In this process, robust andcompact instrumentation is easily achievable, and hence, it willhave the potential to replace sophisticated processes that requirelarge volumes and number of chemicals [24].

Electrocoagulation has been applied for the treatment ofwastewater containing heavy metals [25–29], including hexava-lent chromium [30–32]. However, only a few authors have focusedon the variables that are crucial to the improvement of the perfor-mance of this application. In this work, the effect of pH and chlorideions concentration on the electrocoagulation efficiency in removinghexavalent chromium from the chromium plating industries usingiron electrodes has been studied.

2. Background

Electrocoagulation is a process consisting of creating metallic

hydroxide flocks within the wastewater by electrodissolution of sol-uble anodes, usually made of iron or aluminum [33]. The generationof metallic cations takes place at the anode, due to the electro-chemical oxidation of the iron or aluminum, whereas at the cathodethe production of H2 typically occurs. The generated gas helps the

1 ardous Materials 169 (2009) 1127–1133

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otation of flocculated particles, and therefore the process some-imes is named as electroflocculation [34].

EC involves three successive stages [24]:

Formation of coagulants by electrolytic oxidation of the anode.Destabilization of the contaminants, particulate suspension, andbreaking of emulsions.Aggregation of the destabilized phases to form flocks.

In the case of the treatment by EC of hexavalent chromium solu-ions, when a potential difference is applied to the electrodes of theC reactor from an external power source, the following electro-hemical reactions can take place:

If iron is employed as anode material, its electrochemical oxida-tion generates Fe2+ ions:

Fe → Fe2+ + 2e− (1)

On the other hand, if the anode potential is sufficiently high,secondary reactions may occur at the anode surface, such as theoxidation of H2O and Cl− ions:

2H2O → 4H+ + O2 + 4e− (2)

2Cl− → Cl2 + 2e− (3)

Chloride ions can be added to the solution to increase its con-ductivity for minimizing the energy consumption and to preventthe passivation of the iron anode by hexavalent chromium, asCl− ions catalyze the dissolution of iron by pitting corrosion [35].Therefore the presence of Cl− ions can improve the removal effi-ciency of Cr(VI).In the cathode, the simultaneous reduction of water and hexava-lent chromium, in the form of dichromate, can take place:

H2O + 2e− → H2 + 2OH− (4)

r2O72− + 14H+ + 6e− → 2Cr3+ + 7H2O (5)

In addition to the electrochemical reactions mentioned previ-usly, Fe2+ ions produced in the anode, can directly reduce Cr(VI)o Cr(III) according to the following reaction:

r2O72− + 14H+ + 6Fe2+ → 2Cr3+ + 6Fe3+ + 7H2O (6)

Depending on the solution pH, the generated Fe3+ ionsill immediately undergo further spontaneous reactions toroduce various monomeric and/or polymeric metal hydrox-

des complexes, such as Fe(OH)3, Fe(OH)4−, Fe(H2O)3(OH)3,

e(H2O)63+, Fe(H2O)5(OH)2+, Fe(H2O)4(OH)2

+, Fe2(H2O)8(OH)24+

nd Fe2(H2O)6(OH)42+ [36]. These compounds remain in the aque-

us stream as a gelatinous suspension, which can remove theollutants from wastewater either by complexation or by electro-tatic attraction, followed by coagulation. On the other hand, thencrease of the solution pH due to hydroxyl ions which are pro-uced at the cathode in reaction (5) causes the co-precipitation ofr(III) and Fe(III) as CrxFe1−x(OH)3 between pH 2 and 6 [37], actingynergistically to remove the pollutants from water. At higher pH,r3+ ions can precipitate as Cr(OH)3.

In addition, the following physiochemical reactions may alsoake place in the EC cell [24]:

Cathodic reduction of impurities present in wastewater.Discharge and coagulation of colloidal particles.

Electrophoretic migration of the ions in solution.Electroflotation of the coagulated particles by O2 and H2 bubblesproduced at the electrodes.Reduction of metal ions at the cathode.Other electrochemical and chemical processes.

Fig. 1. Scheme of the experimental arrangement. (1) Power supply, (2) voltmeter,(3) electrocoagulation reactor, (4) anode and (5) cathode.

Therefore, pH and chloride ions concentration can have a greatinfluence on the removal efficiency of hexavalent chromium byelectrocoagulation because the reduction of Cr(VI) to Cr(III) by Fe2+

ions is preferred to occur in acidic conditions, but the coagulation ofFe3+ and Cr(III) is favourable in alkali conditions. On the other hand,the presence of chloride ions improves the dissolution of iron by pit-ting corrosion and avoids the electrode passivation. Then the effectof these two variables on the removal of hexavalent chromium byelectrocoagulation must be optimized.

3. Experimental

Fig. 1 shows a scheme of the experimental arrangement.Batch electrocoagulation experiments were carried out in a 1 Lreactor using vertically positioned electrodes dipped in the solu-tion. The anode was made of flat plate iron with dimensions of140 mm × 8 mm × 3 mm, while the cathode was a flat plate AISI 316stainless steel with dimensions of 140 mm × 8 mm × 1 mm, bothelectrodes having a submerged surface area of 80 cm2. The elec-trode spacing was 18 mm. Electrodes were connected to a DC powersupply (GRELCO model GVD 305). Electrocoagulation experimentswere performed at a constant current of 0.5 A. A digital voltmeterwas used to measure the cell voltage.

Electrocoagulation experiments were run for 5 h. Prior to eachexperiment, the electrodes were abraded with sand-paper toremove scale, then treated with a solution of HNO3 10% in orderto reject any effect due to the different prehistory of the elec-trodes, washed with distilled water, dried and then, the anodewas weighted. Samples were drawn periodically from the solu-tion. pH was measured using a CRISON micro pH 2000 pHmeter, conductivity was measured using a CRISON micro CM2200 conductivity meter. Total chromium and iron concentra-tions were determined by atomic absorption spectrophotometryon a PerkinElmer model Aanalyst 100 atomic absorption spec-trophotometer with air–acetylene flame at 428.9 and 372.0 nmwavelengths, respectively, 0.2 nm spectral bandwidth and operat-ing current of 10 mA. The concentration of Cr(VI) was determinedby titration with a Fe2+ standard solution. The Cr(III) concentrationwas then calculated from the difference between the total Cr andCr(VI) concentrations.

After each experiment the anode was cleaned with diluted HNO3and distilled water in order to remove the oxide layer formed onthe electrode surface during the electrocoagulation process andweighted. Then, the cleaning solution was analysed by atomicabsorption spectrophotometry as described previously in order

rdous Materials 169 (2009) 1127–1133 1129

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trations. In the absence of chloride ions, the reduction of chromiumand its removal are less than 20% due to the partial iron anode passi-vation, which prevents the generation of Fe2+ and Fe3+, which cause,respectively, the reduction of Cr(VI) to Cr(III) and the coagulation.

Fig. 3. Progress of the fraction of total Cr removed by precipitation for differentconcentrations of H2SO4 and NaCl.

M.G. Arroyo et al. / Journal of Haza

o determine the amount of iron adhered to the electrode sur-ace.

Wastewater used in the experiments was prepared syntheticallyy dissolving analytical grade reagents provided from PANREACn distilled water. Electrocoagulation experiments were performed

ith 0.05 M solutions of Cr(VI) prepared by dissolving the propermount of CrO3 in distilled water. In order to increase the conductiv-ty of the solution, and to prevent the passivation of the electrodes,aCl in concentrations of 0, 0.1, 0.2, 0.5, 0.7, 0.9 and 1.0 g/L wasdded to the solution. On the other hand, H2SO4 in concentrationsf 0, 0.001, 0.01, 0.1 M was used to adjust the initial pH of the solu-ion. All the experiments were carried out at room temperatureithout stirring.

. Results and discussion

During EC, many processes of Cr(VI) removal can take placeimultaneously: chemical reduction of Cr(VI) to Cr(III) and subse-uent precipitation of the trivalent chromium hydroxide, cathodiceduction and adsorption, sweep coagulation, co-precipitation asCrxFex−1](OH)3. Cathodic reduction contributes to remove a frac-ion of Cr(VI) from solution mainly at the start of the experiment.n this case, charge loading not only determines the amount of ironoagulant produced in the process, but also affects the reductioneaction of Cr(VI) to Cr(III). In addition, in the EC reaction unit, theron electrodes dissolve into solution as ferrous cations that causehe chemical reduction of Cr(VI) to produce Cr3+ and Fe3+ accordingo reaction (6). Thus, chemical reduction of hexavalent chromium isoupled to EC due to the presence of Fe2+ ions, which play a doubleole as a catalyst and a coagulant.

Also, it has been established that the influent pH and theresence of chloride ions as supporting electrolyte are importantperating factors influencing the performance of EC processes, ashe presence of chloride ions improve the dissolution of iron byitting corrosion and avoids the anode passivation, while the reduc-ion degree of Cr(VI) and the solubility of metal hydroxide speciesboth chromic and iron hydroxides) depend on pH, since the reduc-ion of Cr(VI) to Cr(III) by Fe2+ ions is preferred to occur in acidiconditions, but the coagulation of Fe3+ and Cr3+ is favourable inlkali conditions.

Therefore, the effect of pH and supporting electrolyte on theeduction of Cr(VI) to Cr(III) and its removal by EC using iron platelectrodes was investigated simultaneously. The variation of theraction of Cr(VI) reduced to Cr(III), XCr(VI), and the fraction of totalr removed by precipitation, XtotalCr, versus time are presented inigs. 2 and 3, respectively, for different concentrations of H2SO4 andaCl.

For a given current density applied to the EC reactor, the reduc-ion of Cr(VI) to Cr(III) and its removal by EC are dependent on H2SO4nd Cl− concentrations. As can be seen in Fig. 2, for all concentra-ions of sulphuric acid and chloride ions, the fraction of hexavalenthromium reduced to Cr(III), increases with time. After 5 h of EC,Cr(VI) varies from 15% in absence of chloride ions and 0.001 M2SO4 to 85% at the higher concentrations of Cl− and H2SO4. How-ver, as shown in Fig. 3, the fraction of chromium removed byrecipitation, XtotalCr, remains almost constant over time except for.001 M of sulphuric acid and 1 g/L of NaCl, in which case, after theecond hour of operation, XtotalCr is growing rapidly over time due tohe co-precipitation of iron and chromium as a result of the increasen the pH of the solution. In these last conditions, around 60% of the

nitial Cr(VI) present in the solution is removed after approximatelyh.

The values of XCr(VI) and XtotalCr obtained after 5 h of operationre presented in Figs. 4 and 5, respectively, for different concentra-ions of H2SO4 and NaCl. As can be seen in these figures, supporting

Fig. 2. Progress of the fraction of Cr(VI) reduced to Cr+3 for different concentrationsof H2SO4 and NaCl.

electrolyte concentration is an effective parameter on the treatmentefficiency of both processes, the reduction of hexavalent chromiumand its removal by precipitation, especially at low H2SO4 concen-

Fig. 4. Effect of H2SO4 and NaCl concentrations on the fraction of Cr(VI) reduced toCr3+.

1130 M.G. Arroyo et al. / Journal of Hazardous Materials 169 (2009) 1127–1133

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tively. The iron concentration in the solution increases over time,reach a maximum, which depends on the initial H2SO4 and NaClconcentrations, and falls when the solution pH is higher than 3 dueto the co-precipitation of Fe3+/Cr3+ hydroxides between pH 2 and

Fig. 7. Effect of supporting electrolyte on the evolution of pH with time.[H2SO4] = 0.001 M.

ig. 5. Effect of H2SO4 and NaCl concentrations on the fraction of Cr(VI) removed byrecipitation.

owever, when chloride ions are present in the solution both theeduction of Cr(VI) to Cr(III) and their removal are favoured.

For a given NaCl concentration, XCr(VI) increases from 60% for theower H2SO4 concentration of 0.001 M to 90% for the higher H2SO4oncentration of 0.1 M, while XtotalCr decreases from 60% for theower H2SO4 concentration of 0.001 M to 5% for the higher H2SO4oncentration of 0.1 M. Thus, an increase in H2SO4 concentrationroduces an increase in the fraction of Cr(VI) reduced to Cr(III)Fig. 4) and a decrease in the fraction of chromium removed (Fig. 5),s the chemical reduction of Cr(VI) by Fe2+ ions is favoured at lowH and the precipitation of Cr3+/Fe3+ hydroxides is favoured at highH. At the higher concentrations of H2SO4, although Cr(VI) ions areeduced to Cr(III), they remain dissolved in water.

Finally, it can be mentioned that in the absence of H2SO4 andaCl the removal of Cr(VI) by EC is not possible, since in these con-itions the anode is passivated, there is no generation of Fe2+ ions,nd the reduction of Cr(VI) to Cr(III) and the precipitation of ther3+/Fe3+ hydroxides do not take place.

The addition of NaCl increases the solution conductivity and pre-ents the anode passivation. Chloride ions catalyze the dissolutionf the electrode material by the pitting corrosion phenomenon,hich is a type of localized corrosion caused by a high chloride

oncentration in the solution [35]. Therefore the presence of NaClmproves the removal efficiency of Cr(VI) by increasing the avail-ble metal coagulant in solution. However, for NaCl concentrationsreater than 0.7 g/L, the removal efficiency decreases (Fig. 5) ashloride ions increase the solubility of chromium tri-hydroxide andts effect is stronger than the passivating action of chloride on thelectrode [28].

A typical progress of pH change for various initial concentra-ions of H2SO4 and NaCl can be seen in Figs. 6 and 7. Initial pHalues in the range 1–1.5 were obtained. With progress of EC, theolution pH increases to final values between 3 and 6, depending onhe initial concentrations of H2SO4 and NaCl. In this pH range, theo-precipitation of Cr(III) and Fe(III) as CrxFe1−x(OH)3 takes place37]. As can be seen in Fig. 6, for a given concentration of NaCl, theolution pH reached at the end of the EC process increases with theecrease of the initial concentration of H2SO4, so the higher Cr(VI)emoval efficiencies are obtained at low concentrations of H2SO4s shown in Fig. 5. On the other hand, as shown in Fig. 7, for a givenoncentration of H2SO4, the solution pH does not vary significantlyith the concentration of NaCl, except in the absence of NaCl, in

hich case, the solution pH remains almost constant with time,

ince in these conditions the anode is passivated and the reactionshat lead to the removal of chromium are inhibited. When chlorideons are added to the solution, the anode dissolution is enhanced,

Fig. 6. Effect of H2SO4 on the evolution of pH with time. [NaCl] = 1 g/L.

and ferric ions generate various monomeric and polymeric metalhydroxides which cause an increase of the solution pH [27].

Figs. 8 and 9 show the changing concentration of iron in solutionduring EC for different concentrations of H2SO4 and NaCl, respec-

Fig. 8. Effect of H2SO4 on the evolution of the residual iron concentration with time.[NaCl] = 1 g/L.

M.G. Arroyo et al. / Journal of Hazardous

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ig. 9. Effect of supporting electrolyte on the evolution of the residual iron concen-ration with time. [H2SO4] = 0.001 M.

[37]. For a given H2SO4 concentration, residual iron in solution isigher with the concentration of NaCl as shown in Fig. 9, and thisesult in a higher solution pH, which helps in precipitating Cr(III)nd Fe3+ as insoluble hydroxides, which is a favourable factor inmproving the fraction of Cr(VI) removed.

At the end of the EC process relatively low residual Fe concen-ration was obtained, especially at the higher NaCl concentrationnd the lower H2SO4 concentration, as a consequence of the higherH reached in these conditions.

The residual concentration of Fe ions in solution can be esti-ated from the speciation diagram of Fe(III) shown in Fig. 10 [38].

his figure shows the concentration of Fe species in soluble form,n equilibrium with the amorphous hydroxide. Although the mosttable solid form is goethite for iron, this reaches equilibrium verylowly, and it is usual to consider amorphous precipitates, since thisorm quite rapidly and is much more relevant in practical applica-ions of coagulants. As can be seen in Fig. 10, the dominant speciesn solution changes from Fe3+ to Fe(OH)4

− over a range of more+

han 8 pH units, and intermediate species, such as Fe(OH)2 and

e(OH)3, can represent more than 90% of soluble forms at interme-iate pH values. These calculations can be affected by the presencef chloride and sulphate anions, which form complexes with Fe.owever, it has been established that the addition of chloride and

Fig. 10. Speciation diagram of Fe(III) as a function of pH.

Materials 169 (2009) 1127–1133 1131

sulphate ions to the solution makes only a slight difference to thesolubility equilibrium of Fe, and the magnitude of the increase ofsolubility due to the formation of complexes is very small com-pared with the change of pH [39]. As well as the simple monomerichydrolysis products considered above, Fe3+ ions can form a rangeof polynuclear species although they do not greatly affect solutioncompositions, such as those shown in Fig. 10 [40]. From the stand-point of coagulation, the total amount of soluble metal species isimportant.

The initial increase in the iron concentration over time, shownin Figs. 8 and 9, is because the pH is initially very low, and theiron solubility in these conditions is the highest, as can be seenin the speciation diagram of Fig. 10. The maximum concentrationof iron in the solution is reached when this concentration coin-cides with the solubility limit at a given pH. From this point, thesolution pH increases and the concentration of iron decreases. Themaximum concentration of iron in the solution, reached at pH 3,is around 0.7 mg/L (1.25 × 10−5 M), as shown in Figs. 8 and 9; thisvalue approximately coincides with the solubility of iron at pH 3shown in Fig. 10.

Once the evolution of pH and the concentration of iron in thesolution have been studied, a possible mechanism for electroco-agulation of Cr(III) can be proposed. Although the mode of actionof metal coagulants is broadly understood, there are still someuncertainties, with regard to the nature of the active species andthe role of dissolved salts in water, specially in the case of ironwhere there are redox as well as hydrolytic reactions to considerand the subject becomes quite complex. The mechanisms of actionof prehydrolyzed forms are also by no means fully understood, butdepending on the coagulant dosage, the following situations cantake place [38]:

• At very low coagulant dosage particles remain stable.• Sufficient dosage of coagulant gives charge neutralisation and

hence coagulation.• Higher dosage gives charge neutralisation and restabilisation.• Still higher dosage gives hydroxide precipitate and sweep floccu-

lation.

At the pH reached in the electrocoagulation reactor, between2 and 5.5, the speciation diagram of Fig. 10 shows that the domi-nant species in solution are positively charged, and then it is notpossible a simple charge neutralization mechanism to explain theelectrocoagulation of chromium, as Cr3+ ions will be repelled bythe positively charged iron complexes. Therefore, are the coagu-lant dosage and the composition of the solution, which justifiesthat chromium can be eliminated by EC. Solution chemistry hasconsiderable influence on coagulation by hydrolysing metal ions.The presence of highly charged anions, such as sulphate, can havea large effect on hydroxide precipitation. Sulphate can reduce thepositive charge of the precipitate in the acid region, so that largeflocs are formed over a wide pH range [41]. This could facilitate theadsorption of Cr(III) ions on the amorphous metal hydroxide pre-cipitate. On the other hand, the great amount of coagulant formedby electrooxidation of the iron anode would facilitate the process ofsweep flocculation due to extensive hydroxide precipitation. It haslong been recognized that, in many cases, optimal removal of par-ticles from water is achieved under conditions of rapid hydroxideprecipitation. Although precise mechanism is still not fully under-stood, it is clear that impurity particles are enmeshed in the growingprecipitate and hence can be removed from water by sedimenta-

tion. This process has become known as sweep flocculation, sinceparticles are “swept out” of water by an amorphous hydroxideprecipitate. This process generally gives considerably improved par-ticle removal than when particles are destabilized just by chargeneutralization.

1132 M.G. Arroyo et al. / Journal of Hazardous Materials 169 (2009) 1127–1133

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ig. 11. Effect of NaCl on the fraction of the oxidized iron present in the sludge andn the solution. [H2SO4] = 0.001 M.

Figs. 11 and 12 present the effect of NaCl on the distributionf the fraction of Fe in sludge and in solution at the end of theC process for 0.001 M H2SO4 and 0.1 M H2SO4, respectively. Forhe lowest H2SO4 concentration of 0.001 M (Fig. 11), the fraction ofotal Fe in sludge and in solution remain almost constant with theoncentration of NaCl, while for the highest H2SO4 concentrationf 0.1 M (Fig. 12), the fraction of total Fe in sludge increases and theraction of total Fe in solution decreases with the increase of theaCl concentration. On the other hand, for a given concentration ofaCl, the fraction of total Fe in sludge at the end of the EC process

ncreases and the fraction of total Fe in solution decreases withhe decrease of the H2SO4 concentration, since higher final pH iseached at the lower initial concentration of H2SO4. Finally, it can beentioned that a fraction of the iron anode oxidized, to complete

he 100%, remains adhered on the electrode as iron oxides. Thisraction of the oxidized iron adhered to the electrode surface isbout 20% for 0.001 M H2SO4 and 5% for 0.1 M H2SO4, and doesot contribute to the EC process.

Fig. 13 shows the effect of H2SO4 and NaCl concentrations on theurrent efficiency obtained at the end of the EC experiments for thelectrochemical dissolution of the iron anode. As can be seen in thisgure, total iron dissolution found from experimentations is higher

han that predicted from Faraday’s law, and current efficiency val-es higher than 100% are obtained. Current efficiency relates thectual amounts of anodic metal dissolution to that expected theo-

ig. 12. Effect of NaCl on the fraction of the oxidized iron present in the sludge andn the solution. [H2SO4] = 0.1 M.

Fig. 13. Effect of H2SO4 and NaCl concentrations on current efficiency for the elec-trochemical dissolution of the iron anode.

retically from Faraday’s law:

� = mzF

MIt(7)

where � is the current efficiency, m is the electrode mass lost at theend of the EC process, z is the number of electrons transferred inthe electrode reaction, here for Fe, z = 2, F is the Faraday’s constant(96,486 C/mol), M is the atomic weight of iron, I is the current passedand t is the duration time of the EC experiment.

These high values of current efficiency can be due to the com-bined effect of protons and chloride ions which act synergisticallycatalyzing the anode dissolution. In acidic pH the electrode isattacked by H+ and enhances Fe dissolution:

Fe + 2H+ → Fe2+ + H2 (8)

Also chloride ions catalyze the dissolution of the electrode mate-rial by pitting corrosion as commented previously. Only in absenceof chloride ions and low H2SO4 concentrations, the current effi-ciency was lower than 100%.

5. Conclusions

In this work, the effect of pH and chloride ions concentra-tion on the removal of hexavalent chromium from wastewaterby batch electrocoagulation using iron plate electrodes has beeninvestigated. The initial solution pH was adjusted with differentconcentrations of H2SO4. The removal of hexavalent chromium byelectrocoagulation involves two stages: the reduction of Cr(VI) toCr(III) in the cathode or by the Fe2+ ions generated from the oxida-tion of the iron anode and the subsequent co-precipitation of theFe3+/Cr3+ hydroxides.

At high concentrations of H2SO4 the reduction of Cr(VI) to Cr(III)is favoured, but in these conditions there is no precipitation ofchromium due to the low pH values reached in the solution. Theprecipitation of Fe3+/Cr3+ hydroxides takes place at pH higher than3, these pH values being reached at the lower initial concentrationsof H2SO4.

Chloride ions that enhance the anode dissolution by pitting cor-rosion, which favours the reduction of Cr(VI) to Cr(III) and thesubsequent precipitation of Fe3+/Cr3+ hydroxides, especially at lowconcentrations of H2SO4.

High current efficiencies, greater than 100%, are obtained for

the anode oxidation due to the synergistic effect of chloride ionsand protons, which catalyze the anode dissolution, especially atthe higher concentrations of H2SO4. However, part of the oxidizedanode remains adhered on the electrode surface in the form ofiron oxides, which reduces the fraction of hexavalent chromium

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emoved, as part of the Fe2+ ions generated do not pass to the solu-ion. This phenomenon occurs mainly at the lower concentrationsf H2SO4.

cknowledgements

M.G. Arroyo is grateful to the CONACYT (México) for a postgradu-te grant (Ref.: 206331). This work was supported by the Ministerioe Ciencia e Innovación, convention no. PET2007 0197 02.

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