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Journal of Hazardous Materials 176 (2010) 735–740

Contents lists available at ScienceDirect

Journal of Hazardous Materials

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

comparative study of electrocoagulation and coagulation of aqueoususpensions of kaolinite powders

ehtap Gülsün Kılıc ∗, Cetin Hostenepartment of Mining Engineering, Middle East Technical University, 06531, Ankara, Turkey

r t i c l e i n f o

rticle history:eceived 25 September 2009eceived in revised form7 November 2009ccepted 18 November 2009

a b s t r a c t

Removal of kaolinite particles from their synthetically prepared suspensions was studied by electroco-agulation and coagulation to investigate the effect of such operating parameters as initial pH, coagulantdosage, applied voltage, current density, and time. Coagulation was more effective in a wider pH range(pH 5–8) than electrocoagulation which yielded optimum effectiveness in a relatively narrower pH rangearound 9, where, in both methods, these pH values corresponded to near-zero zeta potentials of kaolinite

vailable online 22 November 2009

eywords:lectrocoagulationoagulationeta potential

particles. The mechanism for both coagulation methods was aggregation through charge neutralizationand/or enmeshment in aluminum hydroxide precipitates. The kinetics of electrocoagulation was veryfast (<10 min) in approaching a residual turbidity, which could be modeled with a second-order rateequation.

© 2009 Elsevier B.V. All rights reserved.

aolinite suspensionurbidity

. Introduction

The drainage of slurry streams containing a considerablemount of ultrafine particles, such as clay particles, is becomingmportant due to the growing demand for high density disposal of

ining slurries and tailings. They are generally discharged into tail-ng dams as slurries due to their high water content (e.g. >75 wt%).he removal of pollutants from these slurries is usually slow andncomplete because colloidal particles are extremely small [1].n addition, the surfaces of these colloids usually have the sameharge, which causes repulsion forces and prevents aggregation2]. Kaolinite particles, which are one of the predominant mineralsn tailings, are commonly generated in mineral industry at neu-ral to high pH and they are always charged negative in processater. Kaolinite particles tend to form stable dispersions with poorewatering characteristics [3].

Coagulation is the most common and practical method ofemoving colloidal solids from wastewater. This is a processf destabilizing colloids, aggregating them, and binding themogether for ease of sedimentation [4]. Destabilization involves

ither an increase in ionic strength or neutralization of the parti-les’ surface charge by the addition of chemicals called coagulantsr flocculants. One type of the commonly used coagulants isydrolyzing metal coagulants. These coagulants are mostly based

∗ Corresponding author. Tel.: +90 312 2105820; fax: +90 312 2105822.E-mail address: mgulsum@hotmail.com (M.G. Kılıc).

304-3894/$ – see front matter © 2009 Elsevier B.V. All rights reserved.oi:10.1016/j.jhazmat.2009.11.097

on aluminum or ferric salts, such as aluminum sulfate and ferricchloride [5]. Coagulants can be added directly, as in conventionalcoagulation, or produced by electrolysis, as in electrocoagulation.In the electrocoagulation process, the coagulant is produced in situby electrooxidation of an appropriate anode, usually aluminum oriron. Electrodissolution of the sacrificial anode into the suspensionleads to the formation of hydrolysis products that are effective inthe destabilization of colloidal particles [2].

When aluminum is the anode material, Al3+ is added to themedium and some ionic monomeric hydrolysis species can beformed, depending on the pH of the solution. The hydrolysis ofAl forming monomeric (contains one metal ion or in this case,one Al3+) species is shown in Eqs. (1)–(4). One of the importantaspects of the reactions is that H3O+ or H+ is produced, resultingin a decrease in pH or increased acidity. The magnitude of the pHdecrease depends on the Al concentration in the solution [6].

[Al(H2O)6]3+ + H2O ↔ [Al(OH)(H2O)5]2+ + H3O+ (1)

[Al(OH)(H2O)5]2+ + H2O ↔ [Al(OH)2(H2O)4]+ + H3O+ (2)

[Al(OH)2(H2O)4]+ + H2O ↔ [Al(OH)3(H2O)3] + H3O+ (3)

[Al(OH)3(H2O)3] + H2O ↔ [Al(OH)4(H2O)2]− + H3O+ (4)

Besides these, polymeric Al species such as Al2(OH)24+ and

Al3(OH)45+ are also formed by hydrolysis reactions in aqueous solu-

tions. The hydrolysis of Al forming polymeric Al species is presentedin Eq. (5) [6].

xAl3+ + yH2O → Alx(OH)y(3x−y)+ + yH+ (5)

7 azardous Materials 176 (2010) 735–740

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36 M.G. Kılıc, C. Hosten / Journal of H

here Alx(OH)y(3x−y)+ represents positively charged Al–OH poly-

ers, such as [Al13(OH)30]9+, [Al24(OH)60]12+, and [Al54(OH)144]18+

6].However, these are not likely to be significant given the low

oncentration of the metals usually used in coagulation. In practicenly the monomeric forms and the hydroxide precipitate are likelyo be important [7].

The adsorption of monomeric and polymeric hydrolysis speciesn particle surfaces leads to electrical double layer compressionnd surface charge neutralization. In addition, another coagula-ion mechanism, i.e. sweep flocculation, is observed at sufficientlyigh electrolyte dosage [8]. During the removing of hydroxide pre-ipitate, impurity particles are enmeshed, and they are effectivelyemoved from the suspension. In sweep flocculation, particles arewept out of the medium by an amorphous hydroxide precipitate7]. Also, interactions between the colloidal particles and the pre-ipitated polymeric hydroxides become significant.

In this paper, the effects of the key parameters, involving initialH, coagulant dosage, applied voltage, current density, and timeere studied on the turbidity removal efficiency for synthetic aque-

us suspensions of kaolinite. Furthermore, studies on pure kaoliniteere included to understand the coagulation mechanism in elec-

rocoagulation and coagulation.

. Materials and methods

Electrocoagulation and coagulation tests were conducted withqueous suspensions of kaolinite (Al4(Si4O10)(OH)8) powdersbtained from Eczacıbası Esan, Turkey. The particle density andET surface area were 2.60 g/cm3 and 16.42 m2/g, respectively.he median particle diameter was found to be 1.82 �m by laseriffraction. Cation exchange capacity of kaolinite was calculated.5 meq/100 g clay.

Elemental composition of the kaolinite was determined withpectro IQ Benchtop X-ray Florescence Spectrometer (Table 1). Theesults showed that amount of silicon dioxide and aluminum oxideas 48.15% and 38.36%, respectively, which was in compliance with

he expected kaolinite structure. Mineral composition of the kaoli-ite sample was determined using X-ray diffraction analysis. XRDatterns were taken by using Rigaku Miniflex Diffractometer withu K( (30 kV, 10 mA, � ( 1.54050 Å) radiation. In XRD trace of kaolin-

te, kaolinite sample used in the study was found to be almost purend a small amount of quartz was detected.

Kaolinite suspensions used for electrocoagulation and coagula-ion were prepared by mixing appropriate amounts of powdered

aterial with 250 mL of distilled water so that a solid concentra-ion of 0.20 g/L was obtained. Before and after electrocoagulationnd coagulation the turbidity of suspensions was measured with

Table 1Elemental composition (%) of kaolinite.

Element %

SiO2 48.15Al2O3 38.36Na2O 0.133MgO <0.023P2O5 <0.00069Cl 0.02427SO3 0.1984K2O 0.5033CaO 0.0856TiO2 0.4996MnO 0.00376Fe2O3 0.4323V2O5 0.0172Cr2O3 0.0632LOI 11.50

Fig. 1. Electrocoagulation experimental setup (1) power supply, (2) magnetic stir-rer, (3) conductivity meter, (4) voltmeter, (5) ampermeter, (6) pH meter, (7)turbidimeter, (8) electrocoagulation cell.

a Lamotte Model 2008 Turbidimeter. Results were expressed innephelometric turbidity units (NTU). The zeta potential of the kaoli-nite particles was measured with a Zetasizer Nano-Z meter inaqueous suspensions having pH values from 2 to 10. The valuesvaried in the range of −10 mV to −40 mV.

The batch electrocoagulation cell used in the experimen-tal study was constructed of plexiglas with the dimensions of65 mm × 65 mm × 110 mm (Fig. 1). The pH of suspension wasadjusted to the desired values using 0.1 M H2SO4 and 0.1 M NaOHsolutions and measuring the pH with a pH meter (Hanna pH 211).Electrical conductivity of suspensions was measured using a con-ductivity meter (Lovibond SensoDirect Con2000). The conductivityof the wastewater is adjusted by adding an appropriate amount ofNaCl.

Conventional coagulation tests were conducted using standardjar testing technique (Fig. 2) with the addition of aluminum sul-fate solutions (10−2 to 10−6 mol/L) at room temperature (nominally21 ◦C) and a constant stirring speed of 300 rpm.

Detailed information about experimental devices (Figs. 1 and 2)and procedures were given in a previous study [9]. Experimentalvariables of electrocoagulation and conventional coagulation stud-ies are presented in Table 2.

The efficiency of turbidity removal, R (%), was calculated usingthe formula given below;

R (%) = T0 − Tt

T0× 100 (6)

where T0 is the initial turbidity of the suspension just before elec-trocoagulation and coagulation, and Tt is the turbidity at the end ofa predetermined settling time (t) after the electrocoagulation andcoagulation run.

Fig. 2. Coagulation experimental setup (1) magnetic stirrer, (2) pH meter, (3) con-ductivity meter, (4) turbidimeter.

M.G. Kılıc, C. Hosten / Journal of Hazardous Materials 176 (2010) 735–740 737

Table 2Experimental variables for electrocoagulation and coagulation experiments.

ElectrocoagulationpH 2–11Electrical potential 0–60 VCurrent density 10–200 A/m2

Electrocoagulation time 1–60 min

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Fe

CoagulationpH 2–11Aluminum dosage 0–100 mg Al/L

The amount of aluminum dissolved in an electrocoagulationxperiment was theoretically calculated using Faraday’s Law. Fara-ay’s law can be written simply between current density and themount of substance dissolved.

= itM

nF(7)

here w, aluminum dissolved (g Al/cm2); i, current densityA/cm2); t, time (s); M, molecular weight of Al (M = 27); n, numberf electrons involved in the oxidation reaction (n = 3); F, Faraday’sonstant, 96,500 C/mol.

. Results and discussion

.1. The effect of initial pH on electrocoagulation and coagulation

pH is an important factor influencing the performance oflectrocoagulation and coagulation processes [10–15]. In electro-oagulation, the highest turbidity removal efficiency (about 90%)ccurred near pH 9, where the zeta potential approached the iso-lectric point of the kaolinite particles (Fig. 3). pHs below 9 led toositive values of the zeta potential, whereas higher values of pHaused negative zeta potentials. This behavior could be explained inerms of the charge of the kaolinite particles, which were positiveue to the adsorption of cations such as Al3+, Al(OH)2

+, Al(OH)2+

nd negative due to the adsorption of anions (Al(OH)4−).

In the coagulation case, the effect of initial pH on the turbid-ty removal efficiency is shown in Fig. 4. It is seen from the figurehat the turbidity removal efficiency was only 55% at pH 2.5 andncreased steadily with increasing pH up to 93% at pH 6.5 and thenradually decreased to 50% at pH 11. Zeta potential measurements

f kaolinite particles are also presented in Fig. 4. The zeta potentialurve went through two isoelectric points at pH values of 3.5 and.5. The zeta potential remained at its near-zero values within theide pH range of 5–8 where the turbidity removal efficiencies were

bout 90%.

ig. 3. Turbidity removal efficiency and zeta potential after electrocoagulationxperiments at different initial pHs (40 V; electrocoagulation time: 10 min).

Fig. 4. Turbidity removal efficiency and zeta potential after coagulation experi-ments at different initial pHs (15 mg Al/L; coagulation time: 10 min).

Regarding the effect of initial pH on electrocoagulation andcoagulation, cationic monomeric species Al3+ and Al(OH)2

+ pre-dominate at low pH (2–4). When pH is between 5 and 8, variousmonomeric species such as Al(OH)2

+ Al(OH)2+2, and polymeric

species such as Al6(OH)153+, Al7(OH)17

4+, Al13(OH)345+ predom-

inate, which transform finally into Al(OH)3 [15–17]. When pHis higher than 9, the monomeric Al(OH)4

− anion concentrationincreases.

The charge neutralization and/or the enmeshment of the parti-cles into a precipitate are the main coagulation mechanisms in thetreatment of kaolinite from suspension. The charge neutralizationcan be achieved by the adsorption of cationic aluminum species orby the precipitation of charged aluminum hydroxide precipitatesonto the surface of kaolinite particles. The generation of a grow-ing aluminum hydroxide precipitate takes place in the system athigh concentrations of aluminum and pH values close to neutrality(sweep flocculation) [18].

3.2. The effect of coagulant dosage on electrocoagulation andcoagulation

In order to compare the electrocoagulation and coagulation pro-cesses on the basis of the amount of aluminum, either releasedfrom the electrodes into the suspension or added as aluminum sul-fate, experiments were conducted under the optimum conditionsof each process. Fig. 5 presents the experimental conditions andthe effects of aluminum dosages on the turbidity removal efficien-cies obtained with electrocoagulation and coagulation in kaolinitesuspensions. These data suggest that, as long as the operating con-ditions are optimized for each process, the source of aluminum doesnot matter in relation to the turbidity removal. Equal dosages ofaluminum in the suspensions lead to similarly effective results inremoving the suspension turbidity.

3.3. The effect of applied voltage on electrocoagulation

In order to examine the effect of applied electrolysis voltageon turbidity removal efficiency and current density in the elec-trocoagulation process (Fig. 6), experiments were carried out ina wide range of applied voltage from 0 V to 60 V at fixed val-ues of pH, suspension concentration, and electrocoagulation time.

As the applied voltage increased, the removal efficiency and thecurrent passing through the solution increased. The latter causeddissolution of aluminum from the sacrificial electrode formingaluminum hydroxide species. These species neutralized the elec-trostatic charges on dispersed particles to reduce the electrostatic

738 M.G. Kılıc, C. Hosten / Journal of Hazardous Materials 176 (2010) 735–740

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ig. 5. The effect of aluminum dosage (mg/L) on kaolinite removal by electrocoag-lation (pH 9; 40 V; 10 min) and coagulation (pH 6; 10 min).

nterparticle repulsion so that the van der Waals attraction predom-nates, thus enhancing agglomeration [19]. At low voltages (<20 V),mount of aluminum dissolved from anodes stays insufficient. Thisrevents effective compression of the double layer and destabiliza-ion of the suspended particles. In kaolinite suspension, optimumoltage was found to be 40 V. The current density at the optimumurbidity removal efficiency was found to be 21.6 A/m2 after 10 minf electrocoagulation conducted with suspensions having about8–32 �S/cm conductivity.

.4. The effect of current density and time on electrocoagulation

Previously, it was shown that current density can influ-nce the treatment efficiency of the electrocoagulation process13,15,20,21]. The effect of increasing current density on turbid-ty removal efficiency (Fig. 7) was studied at constant voltagey the adding appropriate amounts of NaCl to kaolinite suspen-ions. Table 3 shows the amounts of NaCl added and the resultingurrent densities and conductivities in 40-V electrocoagulationxperiments. The optimum turbidity removal efficiency was about1% which was accompanied with the current density of about

7 A/m2. With increasing current density removal efficiency did nothange much, 89% efficiency was observed at 173–176 A/m2. Thencrease in turbidity removal efficiency with current density cane explained by the formation of higher amount of destabilized

ig. 6. The effect of applied voltage on current density and turbidity removal effi-iency in the kaolinite suspension (pH 9; electrocoagulation time: 10 min).

Fig. 7. The effect of current density on turbidity removal efficiency (pH 9; 40 V;electrocoagulation time: 10 min).

particles due to charge neutralization. Since excessive amountsof aluminum were released from the electrodes at higher currentdensities, the most likely explanation for the decrease in turbidityremoval can be regarded as the formation of slow-settling, low-density flocs within the more aluminum hydroxide precipitates.

It appears that conductivity had some effect on the removalefficiency of kaolinite in suspension in the investigated range(15–300 �S/cm). In previous studies, Chen et al. [22] found thatconductivity had little effect on the separation of pollutants fromrestaurant wastewater in the investigated range from 443 �S/cmto 2850 �S/cm. Kobya et al. [11] studied the effect of wastewaterconductivity on the performance of the electrocoagulation processusing aluminum and iron electrodes. They found that the turbidityremoval efficiency remained almost unchanged in the conductivityrange of 1000–4000 �S/cm for both electrode materials. But, it wasin contrast to that given by Lin and Peng [23] for electrocoagulationof textile wastewater using iron electrodes.

Fig. 8 displays the effect of time on turbidity removal efficiencyat 87 A/m2. The turbidity removal efficiency increased from 52% to72% within 3 min and reached up to 90% at 10 min. It did not changesignificantly after 10 min of electrocoagulation. Merzouk et al. [15]stated that the treatment time was shortened with high current.The degree of anodic dissolution of aluminum increases at highcurrent density. This forms a greater amount of precipitate for theremoval of pollutants.

The effect of current density and conductivity on the theoreticalamount of aluminum released from the electrodes and the energyconsumption for kaolinite suspensions are shown graphically in

2 2

Fig. 9. When current density was changed from 4 A/m to 176 A/m ,the amount of aluminum was increased from 0.24 g Al/m2 to10.8 g Al/m2 and the energy requirement was increased from0.3 kWh/m3 to 11 kWh/m3 at the same voltage (40 V) for kaolinitesuspension.

Table 3Concentration of NaCl added (g/L) and the resultant changes in current density andconductivity in kaolinite suspension.

NaCl addition (g/L) Current density (A/m2) Conductivity (�S/cm)

0.0005 4.348 150.0015 6.957 170.0025 9.565 190.0180 34.783 500.0555 71.739 1250.0730 89.130 1600.1430 176.087 300

M.G. Kılıc, C. Hosten / Journal of Hazardous Materials 176 (2010) 735–740 739

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ig. 8. The effect of time on turbidity removal efficiency of kaolinite suspension at7 A/m2 current density (pH 9; 40 V).

In summary, turbidity removal increased with increasing cur-ent density and conductivity due to formation of higher amountf Al3+ ion and the resultant increase in formation rate of Al(OH)3.ll the results obtained were consistent with the previous studies

11,13].Turbidity removal efficiencies and kaolinite concentrations as a

unction of electrocoagulation time at constant electrolysis voltage40 V) are shown in Fig. 10. Turbidity removal efficiency increasedith increasing electrocoagulation time up to 10 min which was

ufficient for good removal efficiencies. The same trend of evolutionf removal efficiency with electrocoagulation time was reported byayramoglu et al. [24]. The process can be divided into two steps:primary rapid step and a secondary slow step, resembling the

dsorption rate of Pb2+ ions onto the microparticles in a previoustudy by Huang et al. [25].

Kaolinite concentration in suspension decreased rather sharplyithin first 10 min of electrocoagulation, approaching a residual

evel of concentration which did not change appreciably at pro-onged electrocoagulation. The form of the concentration-vs.-timerofile shown in Fig. 10 suggested fitting a second-order rate equa-ion for the reduction of kaolinite concentration in suspension in

ig. 9. The effect of current density (A/m2) and conductivity (�S/cm) on energyonsumption and total dissolved aluminum (g Al/m2) in kaolinite suspension (40 V;lectrocoagulation time: 10 min).

Fig. 10. The effect of electrocoagulation time on the turbidity removal efficiencyof kaolinite from suspension and kaolinite concentration (g/L) in the suspension at20 A/m2 current density (pH 9; 40 V).

the following form:

dC

dt= −k(C − Cr)

2 (8)

where C is the concentration of kaolinite particles in suspensionat any time t, Cr is the residual value of the kaolinite concentra-tion at prolonged times, and k is the second-order rate constant ofelectrocoagulation. The integrated rate equation takes the form

C = C0 + Cr(C0 − Cr)kt

1 + (C0 − Cr)kt(9)

The predicted values of the model parameters Cr and k froma nonlinear regression analysis were found to be 0.025 g/L and8.601 min−1, respectively, with a corrected sum of squares valueof 0.918 in regression.

4. Conclusions

The effectiveness of both electrocoagulation and conventionalcoagulation depends on the initial pH of the suspension. In theelectrocoagulation case, the optimum pH for the removal of kaoli-nite from the suspension was found to be 9. The highest turbidityremoval efficiency for electrocoagulation was 87% at pH 9. Inthe coagulation case, highest turbidity removal efficiencies were88–93% which occurred within the pH range 5–8.

In the electrocoagulation and coagulation experiments, suspen-sion pHs for optimum turbidity removal were around the isoelectricpoint of the kaolinite particles having adsorbed aluminum species.

The coagulation mechanism for the removal of kaolinite fromsuspension was the charge neutralization of the kaolinite byadsorption of aluminum cations and enmeshment of kaolinite par-ticles in a growing precipitate (for high aluminum concentrationsand pH values close to neutral).

Electrocoagulation and coagulation removed the turbidityequally well when the same amount of aluminum was introducedinto the suspensions, provided that all other operating parameterswere optimized specifically for each process.

In electrocoagulation, the optimal applied voltage was deter-mined to be 40 V, at which the removal efficiency increased toabout 85% as the higher amount of Al3+ via anodic metal disso-lution caused coagulation. When the voltage was increased to 50 V,

no further improvement occurred in the removal efficiency.

Turbidity removal efficiency was initially directly proportionalto the current density. However, further increase in current densitycaused decrease in removal efficiency. Optimum current density forkaolinite was 87 A/m2.

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The turbidity removal efficiencies for kaolinite suspensionsncreased sharply during 10 min of electrocoagulation time, beyond

hich no significant improvement was observed. The rate of elec-rocoagulation was very fast, leading to an optimum removal ofurbidity within 10 min. The kinetics of electrocoagulation coulde modeled by a second-order rate equation.

cknowledgements

The authors gratefully acknowledge Dr. Sahinde Demirci andr. Ali Ihsan Arol for their helpful discussions. The financial sup-ort provided by the Research Fund of the Middle East Technicalniversity in the scope of BAP-2006-07-02-00-01 project is greatlyppreciated. The authors are also thankful to the METU-Centralaboratory for the measurement of particle size distributions.

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