ELECTROCOAGULACION [email protected]

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Research ArticleReceived: 13 November 2010 Revised: 6 February 2011 Accepted: 3 March 2011 Published online in Wiley Online Library: 11 April 2011

(wileyonlinelibrary.com) DOI 10.1002/jctb.2625

Treatment of laundry waste-waterby electrocoagulationFatemeh Janpoor, Ali Torabian and Vahid Khatibikamal∗

Abstract

BACKGROUND: The present study describes an electrocoagulation process for treating laundry waste-water using aluminumplates. The effect of various parameters such pH, voltage, hydraulic retention time (HRT), and number of aluminum platesbetween the anode and cathode on efficiency of treatment are investigated.

RESULTS: Experimental results showed that by increasing HRT, treatment efficiency increases but beyond 45 min changes arenegligible. Among the results for chemical oxygen demand (COD), phosphorus, detergent, colour and turbidity, the lowestdecrease was found for phosphorus. The larger the HRT, the greater the electrical current needed to achieve constant voltageand temperature in the system. The pH of the influent is a very significant variable which affects the treatment of laundrywaste-water considerably, the optimal range being 6.0–8.0. In addition, it was found that the pH increases from 8.3 to morethan 10 over the first hour of treatment after which the pH remains relatively constant. Finally, kinetic analysis indicates thatthe adsorption system obeys a second-order kinetic model.

CONCLUSION: The aluminum hydroxide generated in the cell decreases the concentration of pollutants in laundry waste-waterto a permissible level. It is concluded that, compared with other treatment processes, electrocoagulation is more effective intreating laundry waste-water under appropriate conditions.c© 2011 Society of Chemical Industry

Keywords: electrocoagulation; laundry waste-water; aluminum plates; hydraulic retention time; adsorption kinetics

INTRODUCTIONWaste-water from all laundry sources accounts for approximately10% of municipal sewer discharges. In addition to high suspendedsolids and biological oxygen demand (BOD) loading, the levels ofoil and grease, heavy metals and other organics exceed municipaldischarge standards. Commonly, laundry effluents contain morethan 1000 ppm suspended solids, 5000 ppm chemical oxygendemand (COD), 1100 ppm fats, oil and grease (FOG), and 1300 ppmBOD, in addition to metals and organic solvents such as toluene,benzene, and perchlorethylene.1

The most widely used methods, such as traditional coagulation,flotation, adsorption, and chemical oxidation or a combination ofthese are insufficient for laundry waste-water treatment, especiallyfor the simultaneous removal of high content of suspended solids,surfactants and phosphate, so a new method of treatment is nec-essary. Electrochemical methods for the treatment of waste-waterhave recently attracted attention due to their safe and environmen-tally friendly nature. They are effective in treating waste-waterscontaining several organic and inorganic compounds, includingphenol, dyes, metal ions, cyanide, etc., because various degra-dation and removal mechanisms may exist simultaneously in anelectrochemical reactor.2 – 4 In the past, electrochemical oxidationwas usually employed to degrade surfactants in waste-water.5,6

Leu et al.7 reported that linear alkyl sulfonates (LAS) and alkylben-zene sulfonates (ABS) could be completely removed by indirectelectrochemical oxidation in conjunction with chemical coagu-lation. Lissen et al.5 used boron doped diamond and graphiteelectrodes to degrade two surfactants in dilute solution and found

that the process was not diffusion-controlled. Although surfactantscan be effectively removed by electrochemical oxidation, the in-stantaneous current efficiencies are very low, varying from 5–12%.Among the electrochemical technologies, electrocoagulation andelectroflotation may be effective substitutes for conventional co-agulation and flotation6. Also electrocoagulation/electroflotationhas been reported to successfully treat different kinds of waste-water, containing oil,8 fluoride,9 arsenic,10 dyes,11 – 16 suspendedparticles,17 chromium ions18 and phosphate.19,20

The removal of arsenic by electrocoagulation using a combinedAl–Fe electrode system was investigated by Jewel et al.21

Observation of the substitution of Fe3+ ions by Al3+ ions in the solidsurface indicates an alternative mechanism of arsenic removalby these metal hydroxides and oxyhydroxides by providinga larger surface area for arsenic adsorption by retarding theformation of crystalline iron oxides. Research shows that owingto their amorphous nature the presence of polymeric aluminumhydroxides would provide significantly larger surface areas forarsenic species adsorption. Merzouk et al.22 reported that twomain mechanisms are generally considered in electrocoagulation:precipitation at pH values lower than 4 and adsorption at higher

∗ Correspondence to: Vahid Khatibikamal, Department of Civil and Environmen-tal Engineering, Graduate Faculty of Environment, University of Tehran, Iran.E-mail: [email protected]

Department of Civil and Environmental Engineering, Graduate Faculty ofEnvironment, University of Tehran, Iran

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www.soci.org F Janpoor, A Torabian, V Khatibikamal

pH values. Adsorption may take place on Al(OH)3 or on themonomeric Al(OH)4 –anion depending on the chemical structureof the pollutant.

Recently several researchers have investigated the electrolyticreduction of laundry waste-water. For example, Ge et al.2 stud-ied the performance of an electrocoagulation–electroflotationprocess in a single reactor using three aluminum plates placedbetween two titanium electrodes of opposite charge and inves-tigated the change of pH during the process, and the effect ofhydraulic retention time (HRT) and electrical current on the effi-ciency of COD removal. Wang et al.23 evaluated the removal ofCOD from a simulated laundry waste-water using electrocoagula-tion–electroflotation technology and suggested that the rate ofCOD removal was significantly influenced by applying ultrasoundto the electrocoagulation cell. Furthermore, My et al.24 and Chen25

found that there are few advantages for the treatment of laundrywaste-water by electrocoagulation compared with other methods.Energy consumption could be decreased owing to better conduc-tivity due to the presence of salts, and the reaction conditionscould be easily controlled by changing the cell current or voltage.The fine bubbles and poly-nuclear hydroxy complexes producedby electrocoagulation were effective in floating and coagulatingthe pollutants. As a result, the electrocoagulation system is usuallydesigned to operate at high voltage, usually higher than 10 V, tobreak down the inhibiting layer. The results in the above articleswere restricted to evaluation of COD removal efficiencies, andeffects on other pollutants such as detergent, colour and turbiditywere not investigated.

The purpose of this work was to investigate the removalefficiency of COD, phosphorous, detergent, colour and turbiditywith a high cell voltage and with different numbers of solubleelectrodes (aluminum electrodes). Then, the effect of differentoperating conditions including initial pH, pH changes during theprocess, and charge loading were evaluated. Finally, the adsorptionkinetics of the process were analyzed.

MATERIAL AND METHODSExperimental deviceThe laboratory scale reactor consisted of an undivided plasticelectrocoagulation cell (20 cm × 10 cm × 15 cm) with an anode,cathode and variable number of plates in parallel. All the electrodesand additional plates were aluminum sheets (20 cm × 7.5 cm ×2 mm). The gap between plates was varied between 15 and 30 mm.Magnetic stirring (400 rpm) was applied in all tests to provide ahomogenous solution in the batch reactor containing 1.5 L ofwaste-water. A DC stabilized power source was used to supplyconstant current (0–2 A) at variable voltage (0–30 V) (constantcurrent source).

SamplesIn this investigation, samples of waste-water from a laundry centerlocated in Tehran, Iran were used. The typical composition of thewaste-water is shown in Table 1.

ExperimentationExperiments were conducted in a bipolar batch reactor withtwo aluminum electrodes and a variable number of aluminumplates. At the beginning of the experiment, the electrocoagulationcell was thoroughly washed and rinsed with de-ionized waterfollowed by rinsing with the sample solution, and then a 1.5 Lsample was placed in the electrocoagulation cell. At the endof each experiment, the solution was filtered through a 0.2 µmmembrane filter before determination of the pH, COD, colour,turbidity, detergent and phosphorous content. All experimentswere repeated twice and the experimental error was below 3%;average data are reported.

AnalysisCOD, detergent and phosphorus were measured according toStandard Methods.26 Turbidity was recorded on a 2100N ISTurbidimeter (Hach). The pH of the waste-water was measuredwith a 720A pH meter (Orion). Colour was determined by dilutionand the UV-vis absorption scan (190–350 nm) was obtained witha UV-vis spectrophotometer (Shimadzu, Japan).

Mechanism of electrocoagulationElectrocoagulation involves the processes of electrochemistry,coagulation and hydrodynamics 27 and involves the creation ofmetallic hydroxide flocs in the waste-water by electrodissolutionof soluble anodes, usually iron or aluminum. The electrical currentcauses dissolution of the sacrificial metal anode and at appropriatepH values the resulting metal ions form a wide range of coagulatedspecies and metal hydroxides that destabilize and aggregatethe suspended particles or precipitate and adsorb dissolvedcontaminants28 thus purifying the polluted water and generatinggases, mainly hydrogen at the cathode.

In the case of aluminum, the main reactions are:oxidation at the anode:

Al-3e −−−→ Al3+ (1)

reduction at the cathode:

3H2O + 3e− −−−→ 3/2H2 + 3OH− (2)

Then the generated Al3+ and OH− ions react to form Al(OH)3

Al3+ + 3H2O −−−→ Al(OH)3 + 3H+ (3)

Al(OH)3 + OH− −−−→ Al(OH)−4 (4)

The hydrolysis and polymerization of aluminum hydroxidespecies under appropriate pH conditions subsequently give riseto the formation of various monomeric species such as Al(OH)2+ ,Al(OH)2

+, Al2(OH)24+, Al(OH)4

− and various polymeric speciessuch as Al6(OH)15

3+, Al7(OH)174+, Al8(OH)20

4+, Al13O4(OH)247+,

Al13(OH)345+, which can effectively remove pollutants by adsorp-

tion resulting in charge neutralization, and by enmeshment in aprecipitate.

Table 1. Laundry waste-water quality

ParameterCOD

(mg L−1)Detergent(mg L−1) Colour

Turbidity(NTU)

Phosphorous(mg L−1)

Suspended solids(mg L−1)

Lead(mg L−1)

Zinc(mg L−1)

Concentration 4155 463 1430 245 27.6 987 4.35 3.2

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Treatment of laundry waste-water by electrocoagulation www.soci.org

The polymeric species have reactive groups that bind to specificsites on the surface of the colloidal particles with the remainder ofthe long-chain molecule extending into the waste-water. Once theextended portion of the polymer becomes attached to anothercolloidal particle, they can be bridged by the polymer. If no otherparticle is available or if there is an excess of polymer, the freeextended portions of the polymer molecule can wrap around theoriginal particle, effectively restabilizing the colloid. Restabilizationcan also occur by aggressive mixing or extended agitation, whichmay break the interparticle bridging and allow the freed polymericsections to enclose the original particles.

This type of coagulation is called sweep coagulation and therate of precipitation is influenced by:

1. oversaturation: to obtain fast precipitation and efficient sweepcoagulation, high concentrations of Al(OH)3 are required;

2. presence of anions: the rate is improved by the presence ofvarious anions, the most effective of which are sulfates;

3. concentration of colloids: the rate is also improved by highconcentrations of colloidal particles. The reason for this is thatthe colloids themselves can act as nuclei for the formation ofprecipitates.

Other reactions can be observed at the cathode surface that cancause the precipitation of carbonate salts.

HCO−3 + OH− −−−→ CO2−

3 + H2O (5)

Ca2+ + CO2−3 −−−→ CaCO3 (6)

Mg2+ + CO2−3 −−−→ MgCO3 (7)

During electrocoagulation metal hydroxide flocs are formedwhich have a large surface area beneficial for rapid adsorptionof soluble organic compounds and trapping of colloidal particles.These flocs are finally easily removed from aqueous medium bysedimentation or flotation.

Because of the characteristics of the laundry waste-water,molecular chlorine is generated during electrolysis from thepresence of chloride salts:

2Cl− −−−→ Cl2 + 2e− (8)

The generated molecular chlorine can then be hydrolyzed toform hypochlorous acid and hypochlorite ions:

Cl2 + H2O −−−→ HOCl + H+ + Cl− (9)

HClO −−−→ ClO− + H+ (10)

These species, because of their high oxidative potentials, candegrade organic compounds. In addition if the anode potential ishigh enough, direct oxidation of organic compounds may occurat the anode.

RESULTS AND DISCUSSIONEffect of initial pH on the performance of theelectrocoagulation processAn important factor affecting the performance of the electro-chemical process is the initial pH of the electrolyte, which affectsprocess performance, and the final pH, which affects the solubil-ity of the Al hydroxides. To examine the effect of initial pH, thelaundry waste-water was adjusted to the desired pH (in the range3–10) with diluted aqueous sodium hydroxide or sulfuric acid.

Figure 1. Removal efficiencies of COD, phosphorus, detergent, turbidityand colour as a function of initial pH (HRT, 15 min; voltage, 30 V; two extraplates; distance between plates, 15 mm).

The results (Fig. 1) demonstrate the removal efficiencies of COD,turbidity, detergent, colour and phosphorus as a function of theinfluent pH with the optimum pH around 6–8. However, phos-phorus removal drops dramatically at pH values <6 or >8. A slightdrop in COD, turbidity and colour removal is observed at pH >9.These results can be explained by the distribution of aluminumionic species. In the pH range 5–9 hydrolysis and polymerizationof Al3+ give rise to species such as Al(OH)2+ , Al2(OH)2

4+, Al(OH)3

and charged hydroxo cationic complexes such as Al13(OH)327+,

which are efficient for coagulation.2 When pH is greater than 10,the main hydrolysis product is Al(OH)4− , which does not favour theformation of an anodized aluminum surface and the adsorption ofdispersed solids. At low pH where only Al3+ is present adsorptionis insignificant.29

Effect of HRT on pH changesThe pH of a solution is one of the most important parameters, butcontrolling the pH is very difficult because the reactions involvedchange the pH. These changes were measured during the testat different times and with different numbers of plates (Fig. 2).It is obvious that up to 1 h pH increases from 8.3 to >10, afterwhich pH remains relatively constant. Thus, pH adjustment maybe needed before the process effluent is discharged. The mainreason for the changes in pH is the production of excess OH−

from the waste-water due to H2 and O2 bubble ‘purge’, causingan increase in pH. In addition, chemical dissolution of Al (reaction1) will consume H+ and increase the pH. Moreover, reaction 3 willshift towards the left again resulting in a pH increase. At high pHvalues, reactions 5–7 will proceed readily and reaction 4 may alsotake place, both leading to a decrease in pH. The pH neutralizationeffect makes this process effective over a much wider pH range,which makes it superior to traditional chemical coagulation, whichis highly sensitive to pH change and effective coagulation is onlyachieved at pH 6–7.30

Effect of HRT on removal of COD, turbidity, detergent, colourand phosphorusExperiments were carried out in a cell equipped with two Alelectrodes and two Al plates located between the electrodes ata fixed potential of 30 V, 15 mm distance between plates, andat different electrolysis times. Figure 3 shows that COD removalincreases during the first 15 min at a relatively high rate, thenslows, reaching a plateau after 40 min reaction time. This is a

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Figure 2. Variation of pH in bipolar electrocoagulation reactor (voltage,30 V; distance between plates, 15 mm).

Figure 3. Removal efficiencies of COD, phosphorus, detergent, colour andturbidity with electrolysis time (voltage, 30 V; two extra plates; distancebetween plates, 15 mm).

very attractive feature for densely populated areas where thecompactness of the treatment facility is particularly important.The mechanism of the electrochemical process in aqueoussystem is quite complex.31 However, it can be suggested thatduring the electrocoagulation process the aluminum electrodesare mainly responsible for electrocoagulation. The removalof COD by electrocoagulation could be due to the removalof suspended solids and precipitation of dissolved COD byelectrocoagulation, electroflotation, direct anodic oxidation, andindirect oxidation by chloride ions. Moreover, COD removalmay also involve electrochemical oxidation and adsorption byelectrostatic attraction and physical entrapment.

Phosphorus present in the laundry waste-water is removedby adsorption on metal hydroxides produced from the respectivecoagulants. Electrochemically produced aluminum ions have beenfound to be more efficient at removing phosphorus than thesame amount of aluminum contained in an aluminum sulfatesolution.32 The main disadvantage of this process is that thepresence of anions like chloride and sulfate reduce the removalefficiency and increase the total dissolved solids (TDS) in thetreated waste-water. So to overcome these difficulties, in thepresent investigation, aluminum plates are used for the anode (forthe generation of the coagulants) and cathode. Electrochemical iongeneration has several distinct advantages, coagulants introducedwithout corresponding sulfate or chloride ions are more efficient at

removing contaminants and by eliminating competing anions andusing a pure coagulant source, lower metal residuals are obtainedand less sludge is produced than when metal salts are used.This is an important advantage because sludge managementcosts 40–50% of the total treatment costs. A contaminant-freeion source allows maximum adsorptive removal of the variousdissolved metal species requiring treatment. Contaminants inindustrial grade aluminum salts would end up in either the treatedeffluent or sludge cake. Fluctuating flow rates or contaminantloads that cause difficulties in the operation of chemical treatmentsystems do not affect the electrochemical process. Duringaluminum electrolysis, hydroxide micro-flocs are formed rapidlyby anodic dissolution. After electrolysis the water is gently stirredfor few minutes to agglomerate micro-flocs into larger easilysettleable flocs. All kinds of microparticles and negatively chargedions, including phosphate, are attached to the flocs by electrostaticattachment. As shown, phosphorous removal in the first 15 minincreases sharply and continues to rise slowly up to 45 min whenit reaches a plateau after which further removal amounts to about7% and can be neglected. Therefore, the optimum HRT of ECprocess for phosphorous removal in this study is 45 min.

The process for detergent removal may be assumed toinvolve initial adsorption onto the particulate surface, making ithydrophobic, and thus forcing the particulate onto the surfaceof rising bubbles. Finally the detergent–particulate–bubblecomposite is floated to the surface. Figure 3 shows that thedetergent concentration decreases fairly rapidly in the first 15 minand then the rate decreases and reaches a plateau after 60 min.

The colour and turbidity of the waste-water also decrease duringthe process and this decrease may involve physical adsorption byhighly charged polynuclear hydroxy aluminum complexes, suchas Al2(OH)2

4+, Al7(OH)174+, Al13(OH)34

5+, Al3(OH)45+, Al(OH)6

3−,Al(OH)7

4− and AlO2−, contained in the water.33

Effect of voltage on COD, phosphorus, detergent, colour andturbidity removalOperating voltage and electric current are critical in batchelectrocoagulation. According to Faraday’s law, the amount ofaluminum dissolved electrochemically is proportional to chargeloadings. The passage of 1 F (26.8 Ah) of current evolves0.0224 Nm3 hydrogen gas, which is much greater than the volumeof gas released in traditional dissolved air flotation. Consequently,increasing current density will increase the charge loading leadingto increased removal of pollutants. Furthermore, better collectionefficiencies can be obtained during electroflotation by generationof smaller bubbles with increasing current density.34

Voltage is the only operating parameter that can be controlleddirectly because, considering reactions 4–6, current will changewith time. In addition the build up of sediments like carbonatesalts on the aluminum plate can affect the electrical current.

In this system electrode spacing is fixed and voltage is acontinuous supply. The voltage directly determines both thecoagulant dose and bubble generation rate, as well as stronglyinfluencing both mixing of solution and mass transfer at theelectrodes. Thus a set of experiments was carried out to quantifythe impact of operating voltage on reactor performance. Theremoval efficiencies of pollutants in Table 2 show that the removalof COD, phosphorus, detergent, turbidity and colour increasedwith increasing voltage. In evaluating batch electrocoagulation asa technology to provide a low cost, low maintenance local waste-water treatment, this set of experiments clearly demonstrates twoimportant results. First, that operating current density is the key


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Table 2. Removal efficiencies of COD, phosphorus, detergent, colour and turbidity as a function of voltage (two extra plates; distance betweenplates, 1.5 cm)

COD removal (%)Phosphorus removal

(%)Detergent removal

(%) Colour removal (%) Turbidity removal (%)

Time (min) 10 V 20 V 30 V 10 V 20 V 30 V 10 V 20 V 30 V 10 V 20 V 30 V 10 V 20 V 30 V

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

15 41.4 47.1 79.7 39.1 43.5 53.6 34.3 47.1 59.8 33.1 49.4 69.1 26.7 47.3 88.4

30 52.7 67.2 81.6 55.1 54.7 66.3 44.7 67.2 75.8 42.4 56.6 73 45.7 63.4 91.4

45 56.1 73.2 88.6 69.2 73.6 83.3 51.2 73.2 89.6 48.1 71.2 85.2 51.4 69.1 92.8

60 61.4 79.5 89.3 79 84.1 88.4 55.9 79.5 93.3 55.4 78.9 88 57.2 70.8 94.4

75 66 82.1 89.8 76.8 87.3 90.6 60.7 82.1 95 61.4 80.1 93.1 62.1 74.1 96.7

90 67.3 82.7 89.9 81.2 88 90.9 62 82.7 95.5 65.3 81 94 65.4 75 97

operational parameter, affecting not only the system’s responsetime, but also strongly influencing the dominant mode of pollutantseparation. Second, these results indicate that running the reactorat the highest allowable current density may not be the mostefficient mode of operation. For any specific application, theoptimal current density will invariably involve a trade-off betweenoperating costs and efficient use of the introduced coagulant.Moreover, changes in electrical current with fixed voltage wereevaluated and with voltages of 5, 10, 20 and 30 V, the currentwould be 0.15, 0.49, 0.81 and 1.32 A, respectively.

Effect of distance between anode, cathode and extra platesThe effect of distance between anode, cathode and extra platesis presented in Table 3. The results show that, with all otherparameters constant, removal efficiencies of all pollutants increasewhen the distance between the plates is decreased. The mainreason for this is that resistance between plates at constantvoltage is decreased so the current increases thus increasing theconcentration of coagulants and bubbles.

Effect of extra plates between cathode and anode onperformance of electrocoagulation processThe variation of COD, phosphorus, detergent, colour and turbiditywith electrolysis time and the effect of increasing the numberof plates between electrode plates is shown in Fig. 4. Increasingthe number of plates increases the removal efficiencies of thepollutants. When two and four extra plates are located betweenanode and cathode, the performance of the electrocoagulationprocess is enhanced because the amount of coagulants risesand so the removal efficiencies increase. It is clear that thisincrease in efficiency in a bipolar reactor is better than those foundwith a monopolar reactor, and is connected to concentration ofcoagulants produced in both cells. Increasing the number of platesin a bipolar reactor has a negative influence on removal efficiencybecause the extra plates increase the resistance of the systemand consequently the current and concentration of coagulantsdecrease. Thus, in each investigation, determination of optimumnumber of extra plates is essential.

Adsorption kineticsContrary to conventional batch adsorption processes in which theadsorption capacity and pollution concentration reach a maximumsimultaneously, in an electrocoagulation process the amount ofinsoluble Al(OH)3 particles starts from zero. As stated earlier, in

electrocoagulation, two distinct processes take place: generationof flocs (electro-dissolution) and adsorption of pollutant on thegenerated flocs (physical adsorption). The removal of pollutant byadsorption onto flocs is very similar to conventional adsorptionexcept for the generation of flocs. The electrode consumption andconcentration of generated flocs can be estimated according toFaraday’s Law. Since the amount of coagulant can be determinedfor a given time, the pollutant removal can be modeled byadsorption phenomenon. Experimental isotherms provide a usefultool to describe the adsorption capacity of a specific adsorbent andmoreover, play a vital role for the analysis and design of adsorptionsystems and for modeling and simulation of adsorption processes.Many theoretical models have been developed to describe theexperimental data corresponding to adsorption isotherms, butone of these, the Lagergren model, has been widely used todescribe the behaviour of adsorbent–adsorbate. So, in this studythe adsorption kinetic data for COD, phosphorus and detergentare analyzed using the Lagergren rate equation. The first-orderLagergren model is:35

dq

dt= k1(qe − q) (11)

where qe and q are the adsorption capacities at equilibrium andat time t (min), respectively; k1 (min−1) is the rate constant offirst-order adsorption. The integrated form of the above equationis:

log(qe − q) = log(qe) − k1t

2.303(12)

The values of qe and k1 were calculated from the slope of theplots of log (qe − q) versus time (t). A straight line was obtainedsuggesting the applicability of this kinetic model. However, it wasfound that the calculated qe values were not compatible with theexperimental values (data not shown), so the adsorption does notobey first-order kinetics adsorption.36

The linearized second-order kinetic model is expressed as:

dq

dt= k2(qe − q)2 (13)

where k2 is the rate constant for second-order adsorption. Theintegrated form of Equation (13) is

1

qe − q= 1

qe+ k2t (14)

t

q= 1

k2q2e

+ t

qe(15)


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Table 3. Effect of distance between anode, cathode and extra plates on performance of electrocoagulation (two extra plates; voltage, 30 V)

COD removal (%)Phosphorus removal

(%)Detergent removal

(%) Colour removal (%)Turbidity removal

(%)

Time(min) 1.5 cm 3 cm 1.5 cm 3 cm 1.5 cm 3 cm 1.5 cm 3 cm 1.5 cm 3 cm

0 0 0 0 0 0 0 0 0 0 0

15 79.7 61.3 53.6 50.4 59.8 38.3 69.1 46.8 88.4 84.8

30 81.6 79.6 66.3 64.3 75.8 55.9 73 57.1 91.4 90.3

45 88.6 85.6 83.3 79.3 89.6 67.6 85.2 72.8 92.8 91.3

60 89.3 87.3 88.4 83.3 93.3 71.5 88 78.6 94.4 92.7

75 89.8 87.6 90.6 89.5 95 72.0 93.1 78.9 96.7 95.0

90 89.9 87.6 90.9 89.2 95.5 72.3 94 79.2 97 95.5

Figure 4. Effect of varying number of extra plates on COD (a), phosphorus (b), detergent (c), colour (d), and turbidity (e) removal (voltage, 30 V; distancebetween plates, 15 mm).


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Figure 5. Second-order kinetic model of COD (a), phosphorus (b), detergent (c) removal with electrolysis time (voltage, 30 V; distance betweenplates, 15 mm).

Table 4. Comparison between calculated qe and K2 for COD, phosphorus and detergent in second-order adsorption isotherm (voltage, 30 V;distance between plates, 15 mm)

Parameters Kind of reactor qe calculated (mg) K2 (g mg−1 min−1 R2

CO D Monopolar 1282 4.504 × 10−5 0.994

Bipolar (2 extra plates) 3846 8.78 × 10−5 0.999


Phosphorus Monopolar 18.2 2.189 × 10−3 0.969

Bipolar (2 extra plates) 30.3 2.051 × 10−3 0.993


Detergent Monopolar 139.7 5.451 × 10−4 0.995



The plots of t/q versus time (t) (Fig. 5) are straight lines and thevalues of qe and k2 were calculated from the slope and interceptof these plots. Correlation coefficients for the second-order kineticmodel obtained in bipolar reactor studies were >0.96. Table 4shows the computed results obtained and these indicate that theadsorption system follows a second-order kinetic model. Also, it isobvious that the kinetic of electrocoagulation is fast, so coagulantgeneration and the adsorption of pollutants are started rapidly.

CONCLUSIONIn this study the performance of a parallel-plate electrocoagulationprocess with aluminum electrodes for the treatment of laundrywaste-water were investigated. The effect of variation of pH, volt-age, hydraulic retention time, distance and number of aluminumplates between the anode and cathode were studied in detail.

The experimental results show that COD, phosphorus, detergent,colour, and turbidity removal efficiencies were enhanced by in-creasing voltage, HRT, and the addition of extra aluminum platesbetween the anode and cathode. After treatment, the removalpercentages of COD, phosphorus, detergent, colour, and turbiditywere 93.2%, 96.7%, 93.5%, 90.1% and 95.9%, respectively, frominitial concentrations of 4155 mg L−1, 27.6 mg L−1, 463 mg L−1,1430 mg L−1, and 245 mg L−1, respectively. The adsorption ki-netic data analyzed using the Lagergren rate equation showedthat the adsorption follows second-order kinetics.

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