Desalination 251 (2010) 12–19

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Desalination

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Investigation of process parameters for the removal of polyvinyl alcohol fromaqueous solution by iron electrocoagulation

Wei-Lung Chou a,⁎, Chih-Ta Wang b, Kai-Yu Huang a

a Department of Safety, Health and Environmental Engineering, Hungkuang University, Sha-Lu, Taichung 433, Taiwanb Department of Safety Health and Environmental Engineering, Chung Hwa University of Medical Technology, Tainan Hsien 717, Taiwan

⁎ Corresponding author. Tel.: +886 4 26318652x400E-mail address: wlchou@sunrise.hk.edu.tw (W.-L. Ch

0011-9164/$ – see front matter © 2009 Elsevier B.V. Adoi:10.1016/j.desal.2009.10.008

a b s t r a c t

a r t i c l e i n f o

Article history:Received 21 July 2009Received in revised form 1 October 2009Accepted 10 October 2009Available online 3 November 2009

Keywords:ElectrocoagulationPolyvinyl alcohol (PVA)Specific energy consumptionKineticsActivation energy

The feasibility of polyvinyl alcohol (PVA) removal from aqueous solution was investigated using iron electro-coagulation. Several parameters, including type of electrode pair, applied voltage, supporting electrolytes,and solution temperature were investigated. The effects of applied voltage, supporting electrolyte, and solu-tion temperature on PVA removal efficiency and specific energy consumption were also investigated. Exper-imental results indicated that a Fe/Al electrode pair was the most efficient choice of the four electrode pairstested in terms of PVA removal efficiency. The optimum applied voltage, supporting electrolyte concentra-tion, and solution temperature were found to be 10 V, 100mg L−1 NaCl, and 298 K, respectively. A pseudo-second-order kineticmodel provided a goodfit to experimental results at various applied voltages and solutiontemperatures. In addition, the activation energy was calculated to be 49.45 J mol−1 based on pseudo-second-order rate constants from the Arrhenius equation, indicating that PVA precipitation in aqueous solution wasattributable to the electrocoagulation process.

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© 2009 Elsevier B.V. All rights reserved.

1. Introduction

In many industrial fields, wastewaters contain recalcitrant conta-minants that are hazardous and barely degradable by biological treat-ment. An example of a recalcitrant pollutant is polyvinyl alcohol (PVA),a well-known water-soluble polymer, which is widely used in warp-sizing agents, paper-coating agents, emulsion paints, adhesives, anddetergent based industries [1]. PVA is also used as a sizing agent and anophthalmic lubricant in the textile and pharmaceutical industries,respectively. The global production of PVA is estimated to be nearly650,000tons/year. The large amount of PVAdischarged from industrialeffluents is harmful to human health and the environment [2]. It isdifficult to convert wastewaters containing PVA to harmless end pro-ducts like water and carbon dioxide; thus, PVA adversely affects theecosystem and accumulates in the human body via the food chain [3].Furthermore, PVA also creates environmental issues due to its abilityto mobilize heavy metals from sediments in lakes and water streams[4]. Conventional biological technologies do not effectively breakdown PVA because the degrading capacity of most microorganismsfor PVA is extremely restricted and specific [5]. Using mixed culturesacclimatized to PVA solution, one study found that only approximately40% of the PVA was mineralized after 48 days of incubation [6]. Theformation of foam in biological equipment for PVA wastewater treat-ment makes achieving stable operation and acceptable results very

difficult [7]. Therefore, there is a growing interest in the developmentof an efficient method for the removal of PVA from aqueous solution.

A considerable amount of scientific work on the degradation ofPVA has been carried out, most of which has focused on photochem-ically initiated degradation processes [2,8–10]. Other physico-chem-ical studies related to the degradation of PVA used methods such asultrasonic techniques [11], direct oxidation by KMnO4 [12], radiation-induced degradation [13], adsorption by variousmaterials, and extrac-tion resins [14–17]. However, to date there is scarce research on thedevelopment of an electrochemical treatment for the removal of PVA.

Over the past ten years, electrochemical techniques have beendeveloped for wastewater remediation and environmental pollutionabatement [18–20]. These methods can prevent pollution problemsfrom industrial effluents because of their versatility and environmen-tal suitability; their main reagent, the electron, is a clean one [21].Other advantages include high energy efficiency, adaptability of auto-mation, and safety. A traditional physico-chemical treatment for thedecontamination of wastewaters before discharge is coagulation; theelectrocoagulation method produces similar results. An applied cur-rent (or voltage) is used to dissolve iron or aluminumsacrificial anodesimmersed in the polluted water, producing the corresponding metalions and hydroxide ions, which yield species such as hydrous ferricoxides and hydroxides of aluminum. These electrochemically gener-ated metallic ions can hydrolyze near the anode to form a series ofmetallic hydroxides capable of destabilizing dispersed particles. Thisprocess takes advantage of the combined effects of charge neutrali-zation, surface complexation and adsorption onto the in-situ formedmetal hydroxides produced from the oxidation of corrodible anode

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materials. The coagulant species is believed to be responsible for theaggregation and precipitation of suspended particles, and the adsorp-tion of dissolved pollutants. Pollutants can then be concentrated bysedimentation and removed as slurry. In recent years, electrocoagu-lation has received increasing attention as a water treatment tech-nique that offers higher removal efficiency compared to conventionalmethods. It is an emerging technique in treating potable water [22],urban wastewater [23], metal laden wastewater [24,25], restaurantwastewater [26], colored water [27], mechanical cutting oil [28],wastewater containing phosphates [29],fluoride [30], arsenic [31], anddispersed fine particles from chemical mechanical polishing [32,33].However, few studies have been conducted on treating wastewatercontaining PVA by electrocoagulation.

In the present study, parameters such as applied voltage, support-ing electrolyte concentration, and solution temperature were inves-tigated in terms of the PVA precipitation rate. A technically effectiveprocess must be economically feasible with regard to its electricalenergy consumption, and practically applicable to environmentalproblems. The effects of the three operational parameters (appliedvoltage, supporting electrolyte, and solution temperature) on specificenergy consumption under the optimum conditionswere evaluated inthis study. In addition, the kinetic constants for the degradation of PVAin aqueous solution at various applied voltages and solution tem-peratures were determined. The activation energy of the electro-coagulation process was also calculated using a pseudo-second-orderkinetic model.

2. Experimental

2.1. Materials and apparatus

Polyvinyl alcohol (PVA, molecular weight in the range of 13,000to 23,000 gmol−1) was purchased from Sigma-Aldrich (Saint Louis,MO 63103, USA) with a hydrolysis degree ranging from 98 to 99%.An aqueous solution containing polyvinyl alcohol was prepared indeionized water at 363 K by stirring. The concentration of the sup-porting electrolyte in the aqueous solutions was adjusted by theaddition of NaCl (Tedia Company, USA). Potassium iodide (KI) wasobtained from Union Chemical Work Ltd. (Hsin-Chu, Taiwan) andiodine (I) was obtained from Toyobo Co. Ltd. (Osaka, Japan). Boricacid (H3BO3) was purchased from Merck (Darmstadt, Germany). Allchemical reagents were prepared by dilution with deionized waterto the desired concentrations. Fig. 1 shows a schematic diagram of theexperimental apparatus and the electrode assembly for the electro-coagulation system used in this work. The electrolytic cell was a 1.0 L

Fig. 1. Schematic diagram of the electrocoagulation equipment.

Pyrex® glass vessel equipped with a water jacket and a magneticstirrer. The temperature of the electrolytic cell was controlled bycontinuously circulating water through the water jacket from arefrigerated circulating bath (Model BL-720, Taiwan). A magneticstirrer bar (Suntex, SH-301, Taiwan) was spun at the center of thebottom of the reactor. Cast iron (Fe) and aluminum (Al) plates (8 cm×6 cm×0.2 cm) were used in four combinations as the anode/cathodepair. The electrode pair was immersed in synthetic wastewater con-taining polyvinyl alcohol to a depth of 4.5 cm, with the electrodesapproximately 2 cm apart. The effective area of the electrode pairwas 27 cm2. Electrical voltage was provided by a manually control-lable DC power supply (Fann-chern, GC50-20D, Taichung, Taiwan)operating in constant–voltage mode (range: 0–50 V). Characteristicsof the polyvinyl alcohol aqueous solution, such as pH (Sartorius, Pro-fessionalMeter PP-20, Germany) and conductivity (Euteoh, Cyberscan510, Singapore), were determined using ROC EPA standard methods[34].

2.2. Experimental methods and analysis

Before each experiment, the electrodes were polished with sand-paper to remove scale, then dipped in 3 M HCl to a depth of 6 cm for10 min, and then cleaned with deionized water. For each test run, acircular container with 0.5 L of synthetic wastewater containing poly-vinyl alcohol was used as the reactor. The magnetic stirrer was turnedon and set at 300 rpm. The stirrer speed was fast enough to providegood mixing in the electrolytic cell and yet slow enough to not breakup the flocs formed during the treatment process. A fixed amount ofNaCl, between 50 and 300 mg L−1, was added to the synthetic waste-water to increase the solution conductivity and facilitate the electro-coagulation treatment. The reaction was timed, starting from whenthe D.C. power supply was switched on. During electrocoagulationwith an iron electrode, an oxide film formed at the anode. In order toovercome electrode passivation at the anode, the electrodes wererinsed in diluted HCl solution after each experiment. One electro-coagulation test run lasted 120 min in all experiments. The aqueoussolution of PVA had a very clear color prepared by dilution with de-ionized water before electrocoagulation. However, after electrocoa-gulation, particulates of colloidal ferric hydroxides were produced,which led to the yellow-brown color of the aqueous solution. Sampleswere periodically taken from the reactor and deposited for 6 h in a10-ml Pyrex glass column. After the electrocoagulation treatment, theconductivity and pH of synthetic wastewater containing polyvinylalcoholweremeasuredwith amulti-meter andpHmeter, respectively.In a 50 ml volumetric flask, 2 ml of PVA solution was added 5 ml ofboric acid (4%) and 1 ml of 0.1 M iodine solution prepared in KI and,the solution was finally made up to 25 ml. Quantitative determinationof polyvinyl alcohol concentration in syntheticwastewaterwas carriedout using a UV–Vis spectrophotometer (Hach, DR-2800, USA) afteraddition of boric acid and iodine solutions according to the proceduredescribed by Finley [35]. This method is based on the blue color gen-erated by reaction of PVA with iodine in the presence of boric acid. Acalibration curve was obtained by plotting the absorbance value at680 nm as a function of the polyvinyl alcohol concentration. The cal-culation of the polyvinyl alcohol removal efficiency after the electro-coagulation treatment was performed using this formula:

Rð%Þ = C0V0−CtVt

C0V0× 100 ð1Þ

where C0 is the initial concentration in mg L−1, Ct is the concentra-tion value at time t in mg L−1, V0 is the initial volume of the treatedwastewater in liters, and Vt is the volume of the treated wastewaterat time t in liters. All samples were measured in duplicate to ensuredata reproducibility, and an additional measurement was carried outif necessary.

Fig. 2. Effect of the type of electrode pair on the removal efficiency of PVA (initial pH 6.5,C0=100mg L−1, t=120 min, V=10 V, T=298 K, d=2 cm, NaCl=100mg L−1,agitation speed=300 rpm).

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2.3. Iron electrocoagulation mechanism of PVA

Electrocoagulation requires only simple equipment. Its advantagesinclude ease of operation, a brief reactive retention period, and adecreased amount of sludge. If iron electrodes are used, the Fe ionsgenerated immediately undergo further spontaneous reactions toproduce the corresponding hydroxides and/or polyhydroxides. Ferricions are commonly produced at the anode during the dissolution ofiron, while OH− ions are generated at the cathode. Mixing the solutionproduces metal hydroxides that remove pollutants (e.g., dyes andcations) by adsorption and co-precipitation. In the present study, thepH values for all cases were measured in the range of 6 to 9 during theelectrocoagulation progress. A mechanism for the production of themetal hydroxides using iron anodes in the range of pH 4 to pH 9 hasbeen proposed [20,36]:

Anode : 2FeðsÞ þ 12H2OðlÞ→2FeðH2OÞ3ðOHÞ3ðaqÞ þ 6Hþ þ 6e

− ð2Þ

Bulk solution : 2FeðH2OÞ3ðOHÞ3ðaqÞ→2FeðH2OÞ3ðOHÞ3ðsÞ ð3Þ

Bulk solution : 2FeðH2OÞ3ðOHÞ3ðsÞ→Fe2O3ðH2OÞ6ðsÞ þ 3H2OðlÞ ð4Þ

Cathode : 6Hþ þ 6e

−→3H2ðgÞ ð5Þ

Overall : 2FeðsÞ þ 9H2OðlÞ→Fe2O3ðH2OÞ6ðsÞ þ 3H2ðgÞ ð6Þ

The Fe (OH)m (where m=2 or 3) formed remains in the aqueoussolution as a gelatinous suspension,which can remove pollutants fromwastewater either by electrostatic attraction or by complexation fol-lowedbycoagulation. Ferric ions generatedby theelectrochemical oxi-dation of the iron electrode may form monomeric ions, Fe (OH)3, andhydroxyl complexeswith hydroxide ions and polymeric species, namely,Fe(H2O)63+, Fe(H2O)5(OH)2+, Fe(H2O)4(OH)2+, Fe2(H2O)8(OH)24+, and Fe2(H2O)6(OH)44+. The formation of these complexes depends strongly onthe solution pH [36].

PVA is a water-soluble synthetic polymer. Water, which acts as aplasticizer, will then reduce its tensile strength, but increase its elon-gation and tear strength. PVA aqueous solution is a colloidal disper-sion in which PVA constitutes the dispersed phase and water actsas the continuous phase. PVA has both hydrophilic and hydrophobicfunctional groups. The anionic head groups on the PVA prevent ag-gregation and coagulation of the PVA droplets via electrostatic repul-sion. During the electrocoagulation procedure, the sacrificial ironanode is oxidized to polymeric ionic species. With progressive elec-trolysis, the ionic strength of the solution increases. Ionic polymericiron species can neutralize the surface charge of PVA molecules. ThepH of the solution increases during this electrochemical process dueto the generation of hydroxides at the cathode, while ferric ions areproduced at the anodeduring the dissolutionof iron. Therefore,mixingthe solution produces metal hydroxides, which can destabilize thecolloidal PVA species in the solution and then remove the colloidal PVAspecies by adsorption and co-precipitation.

3. Results and discussion

3.1. Type of electrode pair

A number of electrocoagulation studies have been carried out onvarious types of wastewater using iron or aluminum as the electrodes[22–33]. The type of electrode pair is regarded as a significant factoraffecting the performance of the electrocoagulation process [37].Therefore, appropriate selection of the electrode pair is important.Four combinations of iron and aluminum plates were investigated inthis study to determine the optimum electrode pair. Fig. 2 shows theeffect of the electrode pair on the removal efficiency of PVA. As shownin this figure, the iron anode and aluminum cathode had the highest

removal efficiency after 120 min of electrolysis, followed by the Fe/Fe,Al/Al, and Al/Fe anode/cathode pairs. The PVA removal efficienciesachieved were approximately 77.1%, 55.8%, 44.1%, and 36.7% for theFe/Al, Fe/Fe, Al/Al, and Al/Fe anode/cathode pairs, respectively. ThePVA removal efficiencies for Fe/Al and Fe/Fe pairs using Fe as the anodewere greater than those for Al/Al and Al/Fe pairs using Al as the anode.This can be explained by the chemical reactions that take place at thealuminum anode and the iron anode.

For the Al anode:

Al→Al3þ þ 3e

− ð7Þ

For the Fe anode:

Fe→Fe2þ þ 2e

− ð8Þ

The nascent aluminum and iron ions are very efficient coagulantsfor particulate flocculation. From the above two reactions, we cancalculate the electrochemical equivalent mass for Al and Fe. The elec-trochemical equivalentmole for aluminum is 12.43mmol (Ah)−1, andthat for iron is 18.59 mmol (Ah)−1. Therefore, more coagulants aretheoretically produced from iron anodes than from aluminum whenthe same electric charge is passed through them. All subsequent elec-trocoagulation experiments were conducted using the Fe/Al electrodecombination.

3.2. Effect of applied voltage

In electrochemical processes, the applied voltage strongly affectsthe performance of electrocoagulation [37]. The effect of appliedvoltage on the PVA abatement rate from wastewater was studiedat 5 V, 10 V, and 20 V. A series of experiments was conducted with100 mg L−1 initial PVA concentration, 100 mg L−1 NaCl concentra-tion, and a 300-rpmagitation speed. Fig. 3 displays the effect of appliedvoltage on the PVA abatement rates in normalized form for variousdurations of electrolysis. As the time of electrolysis was increased,comparable increases in the PVA abatement rate were observed for allapplied voltages. After 120 min of electrolysis, it can be seen fromFig. 3that 46%, 77%, and 95% of the original PVA were removed for appliedvoltages of 5 V, 10 V, and 20 V, respectively. As the applied voltagewasincreased, the PVA removal rate also increased. This was likely due tothe PVApresent in aqueous solution that reactedwith iron ions (Fe (II)or Fe (III)) to form insoluble compounds and was thus mostly re-moved. The required treatment times to reach over 40% abatement inPVA were 18 min, 45 min, and 95 min for 20 V, 10 V, and 5 V, respec-tively. As the applied voltage was increased, the required time for theelectrocoagulation process decreased. A sufficient voltage through thesolution caused the metal ions generated by the dissolution of the

Fig. 4. Effect of applied voltage on the removal efficiency of PVA and specific energyconsumption (initial pH 6.5, C0=100mg L−1, t=120 min, T=298 K, d=2 cm, NaCl=100mg L−1, agitation speed=300 rpm).

Fig. 3. Effect of applied voltage on normalized PVA concentration (initial pH 6.5, C0=100mg L−1, t=120 min, T=298 K, d=2 cm, NaCl=100mg L−1, agitation speed=300 rpm).

Fig. 5. Effect of supporting electrolyte concentration on normalized PVA concentration(initial pH 6.5, C0=100mg L−1, V=10 V, t=120 min, T=298 K, d=2 cm, agitationspeed=300 rpm).

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sacrificial electrode to hydrolyze, forming a series of metallic hydrox-ide species. These species neutralized the electrostatic charges on thedispersed particles to reduce the electrostatic interparticle repulsionenough for the van der Waals attraction to predominate, thus facili-tating agglomeration [38]. It took a longer time (about 95 min) toreach over 40% PVA removal for an applied voltage of 5 V. This isbecause the applied voltage of 5 V was unable to completely desta-bilize the suspended oxide particles in the solution. Therefore, 10 Vand 20 V are suitable operating voltages for electrocoagulation.

3.2.1. Effect of applied voltage on removal efficiency and specific energyconsumption

The electrical energy consumption of wastewater treatment wasevaluated to determine whether electrocoagulation is economicallyviable for PVA removal from aqueous solution. Once the requiredvoltages and the corresponding currents were obtained from the elec-trocoagulation experimental tests, the amount of energy consumedwas estimated. The specific energy consumption (SEC) function wascalculated on the basis of per-kg PVA removal during electrocoagula-tion (kWh kg−1) at a constant voltage using the following equation:

SEC =∫I × Udt

ðC0V0−CtVtÞ × 3:6=

U∫IdtðC0V0−CtVtÞ × 3:6

ð9Þ

where U, Iavg, and△t are the applied voltage (V), average current (A),and electrolysis time (s), respectively. In addition, C0 is the initialconcentration in milligrams per liter, Ct is the concentration value attime t in milligrams per liter, V0 is the initial volume of the treatedwastewater in liters, and Vt is the volume of the treated wastewater attime t in liters. A reasonable removal efficiency and relatively lowenergy consumption were determined below.

PVA solutionswere treated using iron electrocoagulation at appliedvoltages in the range of 5 V to 20 V to determine the optimal removalefficiency and specific energy consumption. The effect of the appliedvoltage on removal efficiencyand specific energy consumption is shownin Fig. 4. Here, it can be seen that an increase in the applied voltagefrom 5 V to 20 V led to a dramatic increase in the PVA removal effi-ciency, from 46.1% to 95.5%. When the applied voltage was increasedfrom 5 V to 10 V, the PVA removal efficiency increased appreciably,from 46.1% to 77.1%, whereas the corresponding specific energyconsumption increased only slightly. However, when the appliedvoltage was increased from 10 V to 20 V, the PVA removal efficiencyincreased only slightly, from 77.1% to 95.5%, whereas the corre-sponding specific energy consumption increased significantly, from15.7kWh kg−1 to 75.8kWh kg−1. Consequently, when consideringremoval efficiency and specific energy consumption, an applied volt-age of 10 V provides the optimum performance for the present elec-trocoagulation with reasonable removal efficiency and relatively lowspecific energy consumption.

3.3. Effect of supporting electrolyte concentration

Solution conductivity affects the current efficiency, applied volt-age, and consumption of electrical energy in electrolytic systemsbecause the current passing through the circuit is a function of theconductivity under a certain applied voltage. Therefore, the conduc-tivity of the solution is a significant parameter in an electrochemicalcell. The most commonly used method to overcome this drawbackis the addition of a small amount of electrolyte, which increases theelectrical conductivity of the solution. The conductivity of a solutionincreases as the supporting electrolyte concentration increases, socurrent passing through the circuit increases in potentiostatic mode[39]. In this study, sodium chloride (NaCl) was used as the support-ing electrolyte for increasing the conductivity of the aqueous solu-tion. The effect of the supporting electrolyte concentration on PVAabatement rate in normalized form as a function of treatment time isdemonstrated in Fig. 5. With increasing electrocoagulation time, sig-nificant increases in the PVA abatement rate were observed regard-less of the supporting electrolyte concentration. This suggests that thepresence of the supporting electrolyte improves PVA removal effi-ciency. As the concentration of the supporting electrolyte increased,the PVA abatement rate in aqueous solution also increased. After120 min of electrolysis, 46.9%, 77.1%, 94.4%, and 95.8% of the originalPVA were removed for electrolyte concentrations of 50 mg L−1,100 mg L−1, 200 mg L−1, and 300 mg L−1, respectively. The treatmenttimes required to reach 40% PVA removal efficiency were around15 min, 27 min, 48 min, and 81 min for 300 mg L−1, 200 mg L−1,100 mg L−1, and 50 mg L−1, respectively. This was likely due tothe destruction of the passivation layer by Cl− anions, catalyzing the

Fig. 7. Effect of temperature on normalized PVA concentration (initial pH 6.5, C0=100mg L−1,V=10 V, t=120min, NaCl=100mg L−1,d=2 cm, agitation speed=300 rpm).

16 W.-L. Chou et al. / Desalination 251 (2010) 12–19

dissolution of the electrode material via pitting corrosion, which is atype of localized corrosion caused by a high chloride concentration inthe solution. Localized corrosion of iron, aluminum, and other metalstakes place through (i) the adsorption of aggressive anions such as Cl−

on the oxide layer due to ion–ion interaction forces; (ii) the chemicalreaction of the adsorbed Cl− with ions in the oxide layer; (iii) the dis-solution or thinning of the layer; and (iv) the direct attack of theexposed metal, which starts intense localized dissolution (localizedcorrosion) [40]. The de-passivation effect was more significant whenmore Cl− anions were added to the solution. Therefore, it was ex-pected that electrocoagulation in the presence of NaCl might improvethe removal efficiency of PVA by increasing the available metal co-agulant in the solution due to the reduction of the oxide layer and theenhancement of anodic dissolution of the electrode material. In addi-tion, the drawback of electrode passivation was partially solved whenNaCl was used as the supporting electrolyte.

3.3.1. Effect of supporting electrolyte on removal efficiency and specificenergy consumption

The concentration of the supporting electrolyte was adjusted tothe desired levels by adding a suitable amount of NaCl to the aqueoussolution containing PVA. Fig. 6 indicates the effect of the supportingelectrolyte on the removal efficiency and the specific energy consump-tion during the electrocoagulation process. Increasing the concentra-tion of the supporting electrolyte from 50 mg L−1 to 300 mg L−1 ledto an increase in PVA removal efficiency from 46.9% to 95.8%. Fig. 6also indicates that the specific energy consumption decreased from17.49kWh kg−1 to 15.68kWh kg−1 when the concentration of thesupporting electrolyte was increased from 50 mg L−1 to 100 mg L−1.However, there was a significant upward trend for the specific energyconsumption when the concentration of the supporting electrolytewas above 100 mg L−1. When the concentration of the supportingelectrolyte was increased from 100 mg L−1 to 200 mg L−1 and300 mg L− 1, the specific energy consumption increased from15.68kWhkg−1 to 30.21kWhkg−1 and 46.47kWhkg−1, respectively.Consequently, when considering the removal efficiency and specificenergy consumption, 100 mg L−1 NaCl provides the optimum balancewith reasonable removal efficiency at relatively low specific energyconsumption.

3.4. Effect of solution temperature

The effect of temperature on electrocoagulation has rarely been in-vestigated, although this technologyhas beenknown for over 100 years.In this study, the effects of solution temperature on the PVA abate-ment rate of aqueous solutionwere studied at 288 K, 298 K, 308 K, and318 K. Fig. 7 shows the effect of solution temperature on PVA abate-ment rate in normalized form as a function of treatment time. As the

Fig. 6. Effect of supporting electrolyte concentration on the removal efficiency of PVAand specific energy consumption (initial pH 6.5, C0=100mg L−1, V=10 V, t=120min,T=298 K, d=2 cm, agitation speed=300 rpm).

time of electrolysis increased, comparable increases in the PVA abate-ment rate were observed regardless of the solution temperature. After120 min of electrolysis, it can be seen from Fig. 7 that 48%, 77%, 82%,and 85% of the original PVA concentration were removed for tem-peratures of 288 K, 298 K, 308 K, and 318 K, respectively. The treat-ment times required to reach 40% PVA removal were around 21 min,35 min, 50 min, and 93 min for temperatures of 318 K, 308 K, 298 K,and 288 K, respectively. The influence of solution temperature can beattributed to the increased destruction of the iron oxide film on theanode surface and the increased rate of all reactions involved in theprocess according to the Arrehenius equation [20]. Increased temper-ature promoted the generation of hydroxyl radicals in the bulk solu-tion, which led to higher mass transfer and more frequent collisionsof species, resulting in an increased reaction rate of the radicals withpollutants [41]. However, little improvement in the PVA abatementrate after 120 min of electrolysis was observedwhen the solution tem-peratureswere increased from 308 K to 318 K. This could be explainedby the opposing effects of higher temperature, possibly causing anincrease in the solubility of the precipitates or the generation of un-suitable flocs [42]. For solution temperatures higher than 308 K, thebeneficial effects are balanced by the adverse effects.

3.4.1. Effect of temperature on removal efficiency and specific energyconsumption

In order to evaluate the effect of solution temperature on the spe-cific energy consumption and PVA removal efficiency, a number ofexperiments were performed after 120 min of electrolysis with theFe/Al electrode pair, 100 mg L−1 initial PVA, 100 mg L−1 NaCl, 10 Vapplied voltage, and 300 rpm agitation speed. The solution tempera-ture of the PVA aqueous solution was controlled to the desired levelby a water jacket from a refrigerated circulating bath. Fig. 8 shows the

Fig. 8. Effect of temperature on the removal efficiency of PVA and specific energy con-sumption (initial pH 6.5, C0=100mg L−1, V=10 V, t=120 min, NaCl=100mg L−1,d=2 cm, agitation speed=300 rpm).

17W.-L. Chou et al. / Desalination 251 (2010) 12–19

effect of solution temperature on the performance of PVA removalefficiency and specific energy consumption after 120 min of electrol-ysis using iron electrocoagulation. The specific energy consumptiondecreased from 25.43kWh kg−1 to 15.68kWh kg−1 when the solu-tion temperatures were increased from 288 K to 298 K, whereas thecorresponding PVA removal efficiency increased from 48% to 77%.However, beyond a solution temperature of 298 K there was an up-ward tendency for the specific energy consumption. When the solu-tion temperatures were increased from 298 K to 308 K and 318 K, thespecific energy consumption increased from 15.68kWh kg−1 to25.14kWh kg−1 and 28.99kWh kg−1, respectively, whereas thecorresponding PVA removal efficiency increased slightly from 77%to 82% and 84%, respectively. Consequently, when considering boththe specific energy consumption and the PVA removal efficiency, atemperature of 298 K offers the best overall performance with a rea-sonable PVA removal efficiency and relatively low specific energyconsumption.

3.5. PVA removal kinetics of electrocoagulation

The overall electrocoagulation process in PVA removal kineticsis described by a pseudo-kinetic model in which the rate constantdepends on the applied voltage and temperature. This model providespreliminarydata for evaluating the reaction rate constants. The pseudo-kinetic rate equation for representing the removal rate from theaqueous solution in PVA concentration is described by the follow-ing mth order reaction kinetics:

dCdt

= −kCm ð10Þ

Fig. 9. Pseudo-first-order kinetic model plots of different (a) applied voltages and(b) temperatures.

where C represents the PVA concentration, m is the order of reaction,k is the reaction rate constant, and t is the time. For a pseudo-first-order reaction, the above Eq. (13) becomes:

lnCt

C0

� �= −k1t: ð11Þ

The slope of the plot of ln(Ct/C0) versus time gives the value of therate constant k1 (min−1). Here, C0 is the initial concentration in mil-ligrams per liter, Ct is the concentration value in milligrams per literat time t, and t is the time in minutes. Fig. 9(a) and (b) shows thekinetics of the PVA removal as the pseudo-first-order model fitting forvarious applied voltages and temperatures, respectively [see Eq. (11)].

For a pseudo-second-order reaction, the above Eq. (13) becomes:

1Ct

− 1C0

= k2t: ð12Þ

The slope of the plot of 1/Ct versus time gives the value of the rateconstant k2 (L mg−1min−1). Fig. 10(a) and (b) shows the kinetics ofthe PVA removal as the pseudo-second-ordermodel fitting for variousapplied voltages and temperatures, respectively [see Eq. (12)]. The val-ues of kwith pseudo-first-order and pseudo-second-order models forPVA removal for various applied voltages and temperatures were de-termined graphically; they are shown in Tables 1 and 2, respectively.The conformity between experimental data and the model values wasevaluated using correlation values (R2). As shown in Table 1, regard-less of the applied voltage, the R2 value for the pseudo-second-ordermodel was relatively higher than that for the pseudo-first-ordermodel. The second-order kinetic model fits well with the observeddata for applied voltage in the electrocoagulation process. When the

Fig. 10. Pseudo-second-order kinetic model plots of different (a) applied voltages and(b) temperatures.

Table 2Kinetic rate constants with pseudo-first-order and pseudo-second-order models forpolyvinyl alcohol removal at various temperatures.

Pseudo-first-order model Pseudo-second-order model

Temperature (K) k1×103 (min−1) R2 k2×103 (L mg−1 min−1) R2

288 4.4 0.94 0.1 0.98298 9.4 0.95 0.4 0.99308 13.2 0.95 0.8 0.99318 13.3 0.95 0.9 0.99

Fig. 11. Pseudo-second-order kinetic rate constant versus reciprocal of temperature inabsolute scale (initial pH 6.5, C0=100mg L−1, V=10 V, t=120min, NaCl=100mg L−1,d=2 cm, agitation speed=300 rpm).

Table 1Kinetic rate constants with pseudo-first-order and pseudo-second-order models forpolyvinyl alcohol removal at various applied voltages.

Pseudo-first-order model Pseudo-second-order model

Applied voltages k1×103 (min−1) R2 k2×103 (L mg−1 min−1) R2

5 V 4.7 0.95 0.1 0.9810 V 12.4 0.95 0.4 0.9820 V 27.5 0.92 3.8 0.99

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applied voltage was increased from 5 V to 20 V, the pseudo-second-order rate constant increased significantly. As shown in Table 2, re-gardless of the solution temperature, the correlation value (R2) for thepseudo-second-order model was relatively higher than that for thepseudo-first-order model. When the solution temperatures wereincreased from 288 K to 308 K, the pseudo-second-order rate constantincreased significantly. However, beyond a solution temperature of308 K, there was no significant improvement in the kinetic rate con-stant. This suggests that the PVA removal from aqueous solution byiron electrocoagulation follows a pseudo-second-order kinetic modelat various applied voltages and solution temperatures.

The rate of a reaction depends on temperature. To understandthe relationship between temperature and the rate of a reaction, weassume that the rate constant depends on the temperature of the re-action. The pseudo-second-order rate constant is expressed by theArrhenius equation [43].

k = A exp−EaRT

� �: ð13Þ

Where A is the proportionality constant of the reaction, Ea is theactivation energy (J mol−1), R is the gas constant (8.314 Jmol−1K−1),and T is the temperature (K). The Arrhenius equation can be used todetermine the activation energy of a reaction. We start by taking thenatural logarithm of both sides of Eq. (14):

ln k = lnA− EaRT

: ð14Þ

According to this equation, a plot of ln k versus 1/T should producea straight line with a slope of−Ea/R, as shown in Fig. 11. An activationenergy of 49.45 J mol−1 was calculated using the slope of the fittedequation (least-squares correlation coefficient=0.9532).

4. Conclusion

The electrochemical removal of PVA from aqueous solution wasinvestigated in a batch electrocoagulation system. The removal effi-ciencies of PVA were approximately 77.1%, 55.8%, 44.1%, and 36.7% forthe Fe/Al, Fe/Fe, Al/Al, and Al/Fe anode/cathode pairs, respectively.The Fe/Al electrode pair was determined to be the best choice ofthe four electrode combinations tested in this study. The addition of100 mg L−1 NaCl as a supporting electrolyte was regarded as the opti-mum value, providing reasonable removal efficiency at relatively lowspecific energy consumption; with the same considerations, an ap-plied voltage of 10 V and a solution temperature of 298 K were found

be the optimum values for the present electrocoagulation. The ex-perimental results indicate that the kinetics of PVA removal can bedescribed by the pseudo-second-order model. Values of the kineticrate constants for PVA removal at various applied voltages and solutiontemperatures were calculated. The kinetic results show that a pseudo-second-order kinetic model fits the experimental data. In addition, thevalue of activation energywas calculated to be 49.45 Jmol−1, based onpseudo-second-order rate constants from the Arrhenius equation,indicating that PVA abatement from aqueous solution is attributable tothe electrocoagulation process.

Acknowledgement

The authors would like to thank the National Science Councilof Taiwan, ROC for financially supporting this study under contractnumber NSC97-2221-E-241-004.

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