This article was downloaded by: [Kocaeli Universitesi]On: 23 April 2012, At: 12:15Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House,37-41 Mortimer Street, London W1T 3JH, UK

Environmental TechnologyPublication details, including instructions for authors and subscription information:http://www.tandfonline.com/loi/tent20

Decolourization of melanoidins by a electrocoagulationprocess using aluminium electrodesM. Kobya a & E. Gengec ba Gebze Institute of Technology, Department of Environmental Engineering, Cayırova,Gebze, Kocaeli, Turkeyb Kocaeli University, Department of Environmental Protection, Arslanbey, Kartepe, Kocaeli,Turkey

Available online: 01 Mar 2012

To cite this article: M. Kobya & E. Gengec (2012): Decolourization of melanoidins by a electrocoagulation process usingaluminium electrodes, Environmental Technology, DOI:10.1080/09593330.2012.671371

To link to this article: http://dx.doi.org/10.1080/09593330.2012.671371

PLEASE SCROLL DOWN FOR ARTICLE

Full terms and conditions of use: http://www.tandfonline.com/page/terms-and-conditions

This article may be used for research, teaching, and private study purposes. Any substantial or systematicreproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form toanyone is expressly forbidden.

The publisher does not give any warranty express or implied or make any representation that the contentswill be complete or accurate or up to date. The accuracy of any instructions, formulae, and drug doses shouldbe independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims,proceedings, demand, or costs or damages whatsoever or howsoever caused arising directly or indirectly inconnection with or arising out of the use of this material.

Environmental TechnologyiFirst, 2012, 1–10

Decolourization of melanoidins by a electrocoagulation process using aluminium electrodes

M. Kobyaa∗ and E. Gengecb

aGebze Institute of Technology, Department of Environmental Engineering, Cayırova, Gebze, Kocaeli, Turkey; bKocaeli University,Department of Environmental Protection, Arslanbey, Kartepe, Kocaeli, Turkey

(Received 29 November 2012; final version received 24 February 2012 )

The decolourization of melanoidins was studied with a batch electrocoagulation (EC) process using aluminium electrodes. Theeffects of conductivity (κ = 500–3000 μS/cm), initial pHi (4.2–8.2), current density (j = 2.5–7.5 A/m2), initial melanoidinconcentration (C0 = 100–800 mg/L) and operating time (tEC = 0–60 min) were investigated on the decolourization effi-ciency. The results obtained from the EC process were extremely efficient and able to achieve a decolourization efficiencyof >98% at pHi = 4.2, j = 5 A/m2, κ = 2500 μS/cm, C0 = 100 mg/L and tEC = 10 min. The decolourization perfor-mance was dependent on pHi value since the lower pH values led to faster reactions and higher decolourization efficiency.Melanoidins in the EC process were removed by precipitation and charge neutralization at pH < 6.5, and both adsorptionand sweep coagulation by amorphous Al(OH)3(s) occurred at pH > 6.5. The operating cost was calculated as 0.0096 ¤/m3.

Keywords: melanoidins; decolourization; electrocoagulation; operating cost

1. IntroductionMelanoidins are found in some wastewaters such as distil-leries and fermentation plants that use molasses [1]. Infor-mation on the structural composition of melanoidins can befound in the literature [2]. Melanoidins are high molecularweight nitrogenous brown polymers and are formed dur-ing the non-enzymatic browning reaction (Millard reaction)between amino compounds and carbohydrates [3]. The exis-tence of melanoidins in water causes a dark brown colour.When untreated, melanoidins prevent the penetration ofsunlight when discharged into a water resource, thus affect-ing the dissolved oxygen concentration and photosyntheticactivity of marine plants, and creating anaerobic conditionsthat kill most of the aerobic marine plants and animals.Hence, melanoidins in wastewater require treatment beforeits safe disposal into the environment [4].

Removal of wastewaters containing melanoidins hasbeen studied by several treatment methods such as oxi-dation of ozonation, ultraviolet (UV)/H2O2 and electro-chemical process [5–11], adsorption [12] and coagulation-flocculation [3,13–16]. Conventional biological processesare effective in removing the biochemical oxygen demandfrom wastewaters containing melanoidins. However, thebrown colour remains or even darkens in the biologi-cally treated effluent due to repolymerization of pigments[14,15]. Moreover, the biological processes have drawbackssuch as longer treatment time, requirements of additionalnutrients and pure cultures. Some of the above methods are

∗Corresponding author. Email: kobya@gyte.edu.tr

not successful due to lower efficiencies of colour removal.These methods also require high reagent dosages, highoperating costs and generate a large amount of sludge [3].Therefore, the electrochemical treatment methods such aselectrocoagulation (EC) may be used effectively for theremoval of organic pollutants in terms of lower operatingtime and costs.

Recently, the EC process has been widely and suc-cessfully used to treat numerous wastewaters includingthose from tannery [16], textile and dying [17–19], dis-tillery and fermentation [7,8,10,11], metal plating [20,21],pulp and paper mill [22], poultry slaughterhouse [23] andolive mill [24] processes and so on. Recently, the ECprocess has also been used for the removal of residualcolour and chemical oxygen demand (COD) from wastew-aters containing natural melanoidins such as molasses,alcohol distillery and yeast processes [8,10,11] but nostudies have focused on the removal mechanism associ-ated with EC from synthetic melanoidins. The purposeof this study was to conduct experimental investigationsinto the decolourization of synthetic melanoidins in theEC process. The effects of several operating parame-ters such as initial pH (pHi), current density, conductiv-ity, initial melanoidin concentration and operating timeon the decolourization efficiency were investigated, andremoval mechanisms were discussed based on experimen-tal results. The operating costs in the EC process were alsocalculated.

ISSN 0959-3330 print/ISSN 1479-487X online© 2012 Taylor & Francishttp://dx.doi.org/10.1080/09593330.2012.671371http://www.tandfonline.com

Dow

nloa

ded

by [

Koc

aeli

Uni

vers

itesi

] at

12:

15 2

3 A

pril

2012

2 M. Kobya and E. Gengec

2. Description of the EC processThe EC process is based on the in situ formation of thecoagulant as the sacrificial anode corrodes due to appliedcurrent, while the simultaneous evolution of hydrogen atthe cathode allows for pollutant removal by flotation. Threemain processes occur during the EC: (i) electrolytic reac-tions at the electrode surfaces, (ii) formation of coagulantsin the aqueous phase and (iii) adsorption of soluble orcolloidal pollutants on coagulants, and removal by sedi-mentation or flotation. Therefore, the mechanisms of theEC are highly dependent on the chemistry of the aqueousmedium [25,26]. When aluminium is used as an electrodematerial, the reactions are as follows.At the anode:

Al −→ Al3+ + 3e− (1)

Al3+ and hydroxyl ions are generated by the electrode reac-tions in Equations (1) and (3) to form various monomericspecies such as Al(OH)2+, Al(OH)+2 , Al(OH)−4 at lowpH values, and polymeric species such as Al2(OH)4+

2 ,Al6(OH)3+

15 , Al7(OH)4+17 , Al8(OH)4+

20 , Al13O4(OH)7+24 and

Al13(OH)5+34 are transformed initially into Al(OH)3(s) and

finally polymerized to Aln(OH)3n (Equations (2) and 93))in the solution [18,27]:

nAl(OH)3 −→ Aln(OH)3n (2)

Al3+ + 3H2O −→ Al(OH)3(s) + 3H+ (3)

At the cathode:

3H2O + 3e− −→ 3/2H2 + 3OH− (4)

On the other hand, the cathode may be chemically attackedby hydroxyl ions generated during H2(g) evolution at highpH values [17,28]:

2Al + 6H2O + 2OH− −→ 2Al(OH)−4 + 3H2 (5)

The formation rates of the different species in the EC processalso play an important role in the decolourization pro-cess. Several interaction mechanisms are possible betweenorganic molecules such as humic substances or melanoidinsand hydrolysis by-products and the rates of these depend onthe pH of the aqueous medium and types of ions present.There are four removal mechanisms of humic substancesin the literature which are precipitation, charge neutraliza-tion of negatively charged colloids by cationic polymerichydrolysis products, adsorption on amorphous Al(OH)3(s)and sweep coagulation [3,8,13–15].

3. Materials and methods3.1. Preparation of synthetic melanoidinsSynthetic melanoidins were prepared following the reportgiven in the literature [29]. 1 M glucose (99% purity,Merck), 1 M glycine (99% purity, Merck) and 0.5 M

Na2CO3 (99% purity, Merck) were dissolved in a litre ofdistilled water. Then the solution was autoclaved at 121◦Cfor 3 h and following this the resulting brown solutionwas dialyzed against distilled and deionized water for 2weeks by a membrane (Cellu Sep H1, Membrane Filtra-tion Products, Inc, USA) having 10 kDa molecular weightcut-off. The nondialysable fraction was dried at 60◦C for24 h. The synthesis of melanoidins was performed and theyield was around 10%. Low molecular substances from thepowder product were removed by a Soxhlet extractor withether. Solutions of known concentrations were prepared bydissolving the powdered melanoidins in distilled water.

3.2. Experimental set-up and procedureThe EC set-up and experimental procedure can be found inour previous study [8]. The synthetic melanoidin solutionswere prepared using distilled water. The pH and conduc-tivity were adjusted to desired values. In each run, a 0.85 Lmelanoidin solution was placed into the EC reactor and thecurrent density was adjusted to the desired value by a digitalDC power supply (TDK-Lambda Genesys model; 50 V–30 A) operated in galvanostatic mode and the experimentaloperation was started.

3.3. Instruments and analysis proceduresDecolourization of melanoidins was measured using aUV–Visible spectrophotometer (Perkin–Elmer 550 SE) at475 nm [6].

The samples were analysed for COD and total organiccarbon (TOC) according to Standard Methods [30]. CODwas measured by the closed reflux titrimetric method andTOC levels were determined through combustion of thesamples at 680◦C using a non-dispersive infrared (IR)source (Tekmar Dohrmann Apollo 9000).

The CHN elemental composition of nondialysablemelanoidins was determined using a model Thermo Finni-gan Flash EA 1112 Series. The compositions of syntheticmelanoidins were: 42.7 ± 1.6% C, 5.6 ± 0.3% H, 6.6 ±0.3% N and 6.47 ± 0.2% C/N.

The IR spectra of the samples were recorded in the range4000–650 cm−1 on a PerkinElmer Spectro 100 T spec-trophotometer using a pellet technique. Fifty scans werecollected for each sample at a resolution of ±3 cm−1.

A Malvern zeta potential and size analyser (MalvernInstruments, Zetasizer Nano Series, UK) was used to deter-mine the zeta potential of the solution during the ECprocess.

The conductivity measurement was carried out using aconductivity meter (Elmetron CX-401 model). The conduc-tivity of the solutions was adjusted by adding NaCl (99.5%,Merck). The pH of the solutions was measured by using apH meter (WTW inolab pH 720 model) and adjusted byeither 0.1 M NaOH or 0.1 M H2SO4 (Merck).

Dow

nloa

ded

by [

Koc

aeli

Uni

vers

itesi

] at

12:

15 2

3 A

pril

2012

Environmental Technology 3

4. Results and discussionThe effects of the parameters on the decolourization effi-ciency of melanoidins by the EC process were studied todetermine the optimum operating conditions. The decolour-ization mechanism was discussed with the experimentalresults.

4.1. Effect of initial pH on decolourizationThe pH is an important parameter influencing the removalof melanoidins in the EC process. The water solubility ofmelanoidins can be increased by altering the pH of thesolution and the colour intensity of melanoidin changes.In general, the brown colour intensifies at a high pH,and the bleaching of melanoidins can be observed at lowpH values [3]. Thus, melanoidin solutions with pHi 2.0–12.0 without the EC process were adjusted to examinecolour (absorbance) changes at 475 nm. The polymerizationdegree of the melanoidin solution changed with pHi. Thedecolourization efficiency at pH 2.0–4.2 decreased from72.0% to 5.0% (Figure 1) due to the precipitation of themelanoidins (depolymerization or decolourization).

The studies in the literature revealed that the isoelectricpoint of melanoidins was 2.5. At pHi 2.5, melanoidins havea net zero charge [3]. However, as the pH was increased, thenet electrical charge of melanoidins became more negative.At such pHi, melanoidins may acquire a net zero chargeconsequently leading to their precipitation from the solu-tion. These results may be attributed to be the predominanceof an undissociated form of melanoidin at a pHi < pKa ofthe melanoidins, which is 3.5 [3]. The pKa generally servesas the starting point of the melanoidin precipitation fromthe solution. The decolourization efficiency at pHi 8–12decreased from 0.0 to −5.0% (polymerization or colour-ization). Increasing the solution pHi enhanced the degree

Figure 1. The absorbance and colour removal of melanoidinsolutions with pHi 2.0–12.0.

Figure 2. (a) Effect of pHi on decolourization and (b) changeof final pHf during the EC process (conditions: j = 5A/m2,C0 = 100 mg/L, κ = 2500 μS/cm).

of polymerization. There was a stable region between pHi4.2 and 8.0 for the decolourization.

The EC experiments were performed in the pHi rangeof 4.2–8.2 at j = 5 A/m2, κ = 2500 μS/cm and C0 =100 mg/L. As seen in Figure 2(a), the decolourization effi-ciency depended on both the operating time and pHi. ThepHi values affected the removal of melanoidins. For exam-ple, over 98% of the decolourization efficiency was realizedat 10, 21, 25 and 30 minutes with pHi 4.2, 5.2, 6.2 and8.2, respectively. The results indicated that the decolouriza-tion performance was dependent on pHi values, because thelower pH values led to faster reactions and higher decolour-ization efficiency. Almost complete removal of melanoidinswas observed after 10 min of the operating time. The finalpH (pHf ) of the solution was around 6.5, close enough toneutral for discharge to the environment. The decolouriza-tion efficiency at pHi 4.2 was faster than that of the rest ofpHi values (Figure 2(a)). When pHi was fixed at 4.2, 5.2,6.2 and 8.2 for the decolourization of melanoidins, effluentpHf was observed to be 6.5 at 10 min, 7.9 at 21 min, 8.3 at

Dow

nloa

ded

by [

Koc

aeli

Uni

vers

itesi

] at

12:

15 2

3 A

pril

2012

4 M. Kobya and E. Gengec

25 min and 8.9 at 30 min, respectively (Figure 2(b)). On theother hand, the amounts of aluminium ions generated withinthe EC reactor according to Faraday’s law at these optimumoperating times were 3.95 × 10−3 kg/m3 for initial pHi 4.2,8.29 × 10−3 kg/m3 for pHi 5.2, 9.87 × 10−3 kg/m3 for pHi6.2 and 11.84 × 10−3 kg/m3 for pHi 8.2, respectively. Asseen in Figure 2(b) for pHi 4.2, the pHf of the decolourizedsolution varied between 4.2 and 7.5. As an example, pHfwas 6.4 at 10 min.

4.1.1. Decolourization mechanism of melanoidinmolecules

Melanoidin molecules (MMs) resemble humic substances,being acidic, polymeric and highly dispersed colloids,which are negatively charged due to the dissociation of car-boxylic, hydroxyl and phenolic groups. Melanoidins consistof both hydrophobic (neutral and acidic) and hydrophiliccomponents such as −OH, −NH2, > C=O, −COOH and−NH−CH=O [31–33]. MMs are connected with hydrogenbonds between functional groups. The most active func-tional groups are carboxyl and phenolic hydroxyl groups,and dissociation of H+ relates to the pH of the solu-tion. When pH is acidic, carboxyl and hydroxyl ions existin −COOH and −OH forms, respectively. When the pHis alkaline, they can exist in −COO− and −O− forms[12,33]. At this pH MMs take more negative charge andthey consequently need more positive charge to neutral-ize the negative charge. Melanoidins are soluble in dilutealkaline solution but precipitate from an acidified solution(pHi < 2.5). The acidic nature of melanoidins is usu-ally attributed to the ionization behaviour of the −COOHand phenolic −OH groups. Hydrophobic groups of MMswere primarily adsorbed on Al-hydroxides flocs in theEC process. Thus, it was anticipated that formation ofless polar and more hydrophobic molecules would leadto enhanced adsorption on Al-hydroxides. Adsorption atinterfaces, depending on the adsorbate and the interface,was influenced by hydrophilic, electrostatic, hydration andhydrophobic interactions [7,14].

On the other hand, formation rates of the differentspecies played an important role in the decolourization pro-cess of melanoidins in the EC process. Several interactionmechanisms, which are discussed in detail by Migo et al.[3], were possible between MMs and the hydrolysis prod-uct as the rates of these depend on the pH of the mediumand the types of ions present. Monomeric/polymeric Al-species and Al(OH)3(s) between pH 4 and 9 and Al(OH)−4in pH > 9 were formed during the EC process. The amor-phous Al(OH)3(s) flocs (sweep flocs) having a large specificsurface area can absorb some soluble organic compoundssuch as MMs onto their surface [13,15,34]. AmorphousAl(OH)3(s) has a minimum solubility within the pH rangeof 6.5–7.8 [26]. The monomeric and polymeric Al-speciesare explained in Equations (6)–(10) with respect to changein pH values:

Charge neutralization and precipitation:

MMs + Monomeric − Al

−→ (MMs − Monomeric − Al)(s) (pH4–5) (6)

MMs + Polymeric − Al

−→ (MMs − Polymeric − Al)(s) (pH5–6.5) (7)

Adsorption and sweep coagulation (pH > 6.5):

MMs + Al(OH)3(s)

−→ (MMs − Al(OH)3(s)) + Al(OH)3(s) (8)

MMs + Al(OH)3(s)

−→ (MMs − Polymeric − Al)(s) + Al(OH)3(s) (9)

MMs + Al(OH)3(s) −→ MMs − Polymeric

− Al − Al(OH)3(s) + Al(OH)3(s) (10)

The decolourization of MMs and the pHf of the solutionwithin 10 min of operating time rose to 98.4% and 6.4 whenpHi was 4.2. The decolourization results in Figure 2 showedthat the MMs with the species were effectively removedby mechanisms of precipitation and charge neutralizationbetween pHi of 4.2 and 6.5. Charge neutralization wasachieved when the net negative charge of MMs was neu-tralized by the positively charged Al3+ ions, thus allowingtheir agglomeration and settling to take effect. The removalmechanism was controlled by adsorption and sweep coag-ulation due to the amorphous Al(OH)3(s) flocs that formedafter the effluent pH = 6.4 and >10 min of operating time(Figure 2). The decolourization efficiencies were lower atpHi values (5.2–8.2) until the 15 min of operating time sincefunctional groups in MMs became more negative and therewere not enough positive polymeric species present forcharge neutralization and precipitation. Therefore no pre-cipitation was observed at these pH ranges. After 20 min ofoperating time, the decolourization efficiencies increasedto >98% which enhanced floc formations and the efflu-ent pH increased to over 7.7. The removal mechanism wascontrolled at these pH ranges by adsorption and sweepcoagulation.

4.1.2. Zeta potential measurementsColloids are maintained in suspension by electrostaticrepulsion between particles. The zeta potential providesan effective measurement of the charge on a particle. Theaddition of aluminium coagulant can suppress the electricdouble layer around the colloidal particle, thus encouragingthe pollutant [15,35].

As seen in Figure 3(a), the values of the zeta potentialfirstly decreased sharply until 4 min of operating time. Thepolymeric Al-species and amorphous Al(OH)3(s) (the theo-retical amount of electrochemically dissolved aluminiumat the anode is 4.74 mg/L at 12 min) were formed by

Dow

nloa

ded

by [

Koc

aeli

Uni

vers

itesi

] at

12:

15 2

3 A

pril

2012

Environmental Technology 5

Figure 3. Zeta potential measurements: (a) melanoidin solutionsduring the EC process and (b) melanoidin solutions at different pHvalues after the EC process (conditions: C0 = 100 mg/L, pHi 4.2,κ = 2500 μS/cm, j = 2.5 A/m2).

OH− ions after 4 min of operating time. This was proba-bly due to an increase in negatively charged ions formedat the cathode. For this reason the zeta potential started toincrease, so that the MMs were governed by precipitationand charge neutralization from polymeric Al-species andfinally were adsorbed on Al(OH)3(s). Moreover, carboxylor amino groups of the MMs interacted with the hydroxylions generated at the cathode during the EC process andnegatively charged −COO− was formed, causing the low-ering of the zeta potential, especially in the first stage ofthe EC process. When a high enough amount of coagu-lant was produced at the anode, the coagulant interactedwith the negatively charged fragments of the MMs whichled to an increase in the zeta potential. After the EC, thezeta potential of the treated wastewater decreased with anincrease in the pHf (Figure 3(b)). This pH dependent phe-nomenon was probably due to the deprotonation (−COO−)of acidic organic functional groups such as carboxylic acid(−COOH) in MMs and the formation of Al(OH)−4 . Theresults of zeta potential measurements also supported theproposed mechanisms.

Figure 4. (a) Fourier transform IR analysis of sludge and (b)ratios of absorbance before and after the EC process.

4.1.3. Fourier transform IR spectrum analysisThe infrared spectra of sludge from the melanoidins wererecorded in the 4000–650 cm−1 range (Figure 4(a)). Thechanged and shifted functional groups were extremelyimportant to the understanding of the changing of themelanoidin structure before and after the EC process.

For the structural characterization, the samples wereanalysed. The stretching frequencies at 3426, 2940, 1721,1658, 1641, 1566 and 1408 cm−1 were corresponding to thepresence of an alcoholic (−OH), −C−H stretching, ketonic(=C=O), aldehydic (−COH), carboxylic (−COOH),carbon–carbon double bound (−C=C−) and asymmetric−NO2 group, respectively (Table 1). Figure 4(b) showed theratios of absorbances before and after the EC process ver-sus the wavenumber. The peak at 1739 cm−1 was attributedto the conjugated C−C double bonds corresponding to thedeep brown colour.

4.2. Effect of current densityCurrent density and operating time are important opera-tional parameters for the decolourization and cost of theremoval processes [11]. The decolourization efficiency asa function of the operating time was studied at current den-sity values in the range of 2.5–7.5 A/m2 (Figure 5). As seenin Figure 5, a steep increase in melanoidin removal at only>2.5 A/m2 was observed and then became gradual. The

Dow

nloa

ded

by [

Koc

aeli

Uni

vers

itesi

] at

12:

15 2

3 A

pril

2012

6 M. Kobya and E. Gengec

Table 1. Fourier transform IR analysis of melanoidin and sludgeproduced during the EC process.

Wavenumber(cm−1) Functional group

3400–3200 OH stretch form −COOH and −COH2800–3100 C–H stretch from −CH, −CH2, and −CH31739 C=C stretching1641 H−O−H1595 C=O stretching1375, 1301 O−H stretching1217 C−N stretching1063 Al−O972 Al−O−HO1870 C−O−C asymmetric stretching

Figure 5. Effect of current density on decolourization efficiencyof melanoidin (conditions: C0 = 100 mg/L, pHi 4.2, 100 mg/L,κ = 2500 μS/cm).

maximum decolourization efficiencies at 14 min of oper-ating time and current densities of 2.5, 5.0 and 7.5 A/m2

were 96.9%, 99.1% and 99.2% respectively. However, asthe current density was increased from 2.5 to 7.5 A/m2, theoptimum operating time was decreased from 10 to 4 min.

This can be attributed to high current densities; theextent of anodic dissolution (Faraday’s law, Equation (12))increased positively charged polymeric Al-species resultingin increased melanoidin removal:

ELC = itECMw

zFv(11)

where ELC is the electrode consumption (kg/m3), tEC (s)is the operating time, z is the number of electrons involvedin the oxidation/reduction reaction for Al, z = 3. Mw is theatomic weight of the anode material (Mw = 26.98 g/mol),F is Faraday’s constant (96485 C/mol) and v is the volume(m3) of the solution in the EC reactor.

According to Faraday’s law, the charge passed to thesolution was directly proportional to the amount of elec-trode dissolved. The energy consumptions (ENC) given inEquation (12) are also an extremely important parameter in

the EC process as in all other electrolytic processes. ENCs(kWh/m3) were calculated using the following equation:

ENC = UitEC

v(12)

where U is the cell voltage (V).Values of ENC and ELC, charge loading (Q), current

efficiency and removal efficiency (Rm) with respect to dif-ferent current densities and operating times are illustratedin Table 2. 97% or more of the decolourization efficien-cies were obtained at 2.5, 5.0 and 7.5 A/m2 for 10, 5 and4 min, respectively (Table 2). The values of ELC and ENCincreased as the current density was increased (Table 2).

In the EC process, the number of aluminium ionsproduced was proportional to the charge loading. Theincrease in the decolourization efficiency of melanoidinswas expected as the charge loading (F/m3) increased sinceit affected the decolourization efficiency:

Q = itEC

Fv(13)

Experiments in the EC process may provide informationon the optimal conditions to generate the amounts of Al3+

ions and Al hydroxides needed for the decolourization ofmelanoidins. Table 2 showed the effect of charge loading onthe decolourization efficiency and on the loss of aluminiummass via dissolution in the EC process. The increase ofcharge loading increased the amount of Al electrode dis-solved in the EC process and improved the decolourizationefficiency. Greater than 98% of decolourization efficiencyat 2.5, 5.0 and 7.5 A/m2 was achieved at the 10, 5 and 4 minoperating times. Charge loading at the optimum operatingconditions was found to be ≥ 0.439 F/m3. The decolour-ization efficiency and loss of Al electrode mass were ≥98%and 0.0044 kg/m3.

The Faradic yield or current efficiency (CE) is defined asthe ratio of the actual electrode consumption to the theoreti-cal value. The CE calculation was based on the comparisonof experimental weight loss of the aluminium electrodes(Ce) and the theoretical amounts of aluminium dissolu-tion (Ct) according to Equation (11) through the electrodeconsumption difference before and after the EC process.CE at different current densities was calculated using thefollowing equation:

CE(%) = Ce

Ct× 100 (14)

CE at different current densities and operating times waschanged in the range of 103–115% (Table 2). The differ-ence in mass may be explained by the ‘corrosion pitting’phenomenon which caused holes and practically led to ametallic Al loss on the electrode surface [36].

Dow

nloa

ded

by [

Koc

aeli

Uni

vers

itesi

] at

12:

15 2

3 A

pril

2012

Environmental Technology 7

Table 2. Decolourization efficiency of melanoidin with different current densities.

j (A/m2) i (A) U (V) tEC (min) ENC (kWh/m3) ELC (kg/m3) Q (F/m3) CE (%) Rm (%)

2.5 0.06 1.44 10 0.0169 0.0043 0.439 108 96.5∗5.0 0.12 2.26 10 0.0532 0.0086 0.878 109 98.67.5 0.18 2.95 10 0.1041 0.0124 1.317 104 98.82.5 0.06 1.40 5 0.0082 0.0021 0.219 106 6.55.0 0.12 2.30 5 0.0271 0.0046 0.439 115 96.8∗7.5 0.18 2.95 5 0.0521 0.0062 0.658 103 97.92.5 0.06 1.40 4 0.0066 0.0017 0.176 106 5.85.0 0.12 2.30 4 0.0216 0.0033 0.351 103 74.47.5 0.18 2.95 4 0.0416 0.0051 0.527 106 97.5∗

∗Optimum decolourization conditions

4.3. Effect of conductivityThe conductivity of melanoidin solutions affects the volt-age between the electrodes, the energy and the electrodeconsumptions. The solution resistance is reduced by thedecreasing distance between the electrodes which leads todecrease in the electrode potential [17,25,26]. NaCl wasused to obtain the desired conductivity in the EC pro-cess. Increasing solution conductivity using NaCl couldsignificantly reduce the adverse effects of bicarbonate andsulphate anions. Figure 6 shows the effect of conductivityon the decolourization efficiency of melanoidins between500 and 3000 μS/cm. From Figure 6(a), it can be seenthat the decolourization efficiency increased from 95.4%to 98.2% with an increase in conductivity of melanoidinsafter 5 min. In addition, the energy consumption decreasedfrom 0.0377 to 0.0261 kWh/m3 with increasing conductiv-ity (Figure 6(b)). Voltages decreased from 3.2 to 2.9, 2.5, 2.3and 2.2 with increasing conductivity. Energy consumptionconsiderably reduced to almost a third with changes inthe conductivity. The electrode consumption varied from0.0127 to 0.0038 kg/m3 in the conductivity range of 500–3000 μS/cm. The higher decolourization efficiency withlower energy and electrode consumptions was obtained formelanoidin solutions at about κ = 2500 μS/cm. Our resultsshowed a good agreement with literature studies [8,17,25].2500 μS/cm was selected as the optimum conductivityvalue and this value was used in the experiments.

4.4. Effect of initial melanoidin concentrationThe melanoidin solutions with different initial melanoidinconcentrations (100–800 mg/L) were treated in the ECprocess. The decolourization efficiency was decreasedfrom 98.9% to 94.8% with an increase in concentra-tion of melanoidins from 100 to 800 mg/L at 55 min.When initial melanoidin concentrations were 100, 400 and800 mg/L, values of COD for melanoidins were 128, 464and 924 mg/L which had some relation with the value ofC0 as CCOD is equal to approximately 1.20 × C0 (Figure 7).The removal efficiencies of colour and COD for differentinitial concentrations and minimum operating times were

Figure 6. Effect of conductivity on decolourization efficiency(conditions: C0 = 100 mg/L, pHi 4.2, 5 min; j = 5A/m2).

98.4% and 97% for 100 mg/L at 10 min, 98.1% and 91%for 400 mg/L at 40 min and 94.8% and 90% for 800 mg/L at60 min, respectively. Moreover, the electrode consumptionsincreased from 0.0096 to 0.0742 kg/m3 in the concentrationrange of 100–800 mg/L at optimum operating times.

It is quite clear that under the optimum experimen-tal conditions, lower melanoidin concentrations and loweroperating times resulted in a higher percentage of thedecolourization efficiency. This was likely due to theformation of insufficient polymeric Al-species producedby the electrode to coagulate the greater number of MMs

Dow

nloa

ded

by [

Koc

aeli

Uni

vers

itesi

] at

12:

15 2

3 A

pril

2012

8 M. Kobya and E. Gengec

Figure 7. Effect of initial melanoidin concentration on decolour-ization and COD efficiencies (conditions: κ = 2500 μS/cm, pHi4.2, j = 5A/m2).

at higher concentrations, which led to a decrease in thedecolourization and COD efficiencies. In addition, at higherinitial melanoidin concentrations, Al(OH)3(s) formed nearthe surface, fouling the electrodes, and intermediate prod-ucts formed in the solution, which blocked the electrodeactive sites. Both of these factors gave rise to a decrease inthe decolourization and COD efficiencies. Briefly, increas-ing concentration of melanoidins resulted in the increase ofanionic charges and coagulant demand in the EC process tomaintain effective removal of MMs.

4.5. Operating cost of melanoidin decolourizationOperating cost (OC) is an important economical parameterin the EC process. The OC includes material cost (mainlyelectrodes), utility cost (mainly electrical energy), as wellas labour, maintenance and other fixed costs. In this study,

energy, electrode material and chemicals costs for decolour-ization of the melanoidins were taken into account as majorcost items in the calculation of the OC in ¤/m3 [23]:

OC = aENC + bELC + cCC (15)

Costs for ELC (kg/m3) in Equation (11) and ENC(kWh/m3) in Equation (12) were calculated. a, b and c givenfor the Turkish market in June 2011 were electrical energyprice (0.072¤/kWh), electrode material price (1.65¤/kg Al)and chemical costs (CC, 0.73¤/kg for NaOH, 0.29¤/kg forH2SO4 and 0.15¤/kg for NaCl), respectively.

Table 3 shows the OC value under optimum operatingconditions for the decolourization efficiency of melanoidinsIn the EC process, total OC increased with increasing cur-rent density and operating time since energy and electrodeconsumptions were related to these parameters. OCs at 2.5–7.5 A/m2 changed from 0.0084 to 0.0115 ¤/m3. OCs at500–3000 μS/cm decreased from 0.0238 to 0.0083 ¤/m3

since energy and electrode consumptions decreased at con-stant current density. OCs also increased from 0.0096 to0.0742 ¤/m3 with increasing initial concentration (100–800 mg/L) due to increase of the operating time for higherdecolourization efficiency and electrode dissolution in theEC process. As a result, the OC for the decolourization effi-ciency of melanoidins under the optimum conditions was0.0096 ¤/m3.

5. ConclusionsThe optimum operating conditions for the decolouriza-tion of melanoidins were pHi = 4.2, j = 5 A/m2, κ =2500 μS/cm, C0 = 100 mg/L and tEC = 10 min. Thedecolourization efficiencies were lower at initial pH val-ues until 10 min of operating time since functional groupsin MMs became more negative and there were not enoughpositive polymeric Al-species present for charge neutral-ization and precipitation. After 20 min of operating time,the values of the decolourization efficiency were increased

Table 3. Operating cost values for decolourization efficiency of melanoidin at optimum conditions.

Operating parameters Results

C0 Conductivity pHi j tEC Rm ENC ELC OC(mg/L) (μS/cm) (-) (A/m2) (min) (%) (kWh/m3) (kg/m3) (¤/m3)

100 2500 4.2 2.5 10 96.5 0.0169 0.0043 0.0084100 2500 4.2 5.0 5 96.8 0.0266 0.0046 0.0096100 2500 4.2 7.5 4 97.5 0.0417 0.0051 0.0115100 500 4.2 5.0 5 95.2 0.0377 0.0127 0.0238100 1000 4.2 5.0 5 95.8 0.0341 0.0114 0.0214100 1500 4.2 5.0 5 96.2 0.0283 0.0078 0.0150100 2500 4.2 5.0 5 96.8 0.0266 0.0046 0.0096100 3000 4.2 5.0 5 96.8 0.0261 0.0038 0.0083100 2500 4.2 5.0 5 96.8 0.0266 0.0046 0.0096400 2500 4.2 5.0 30 95.6 0.0268 0.0239 0.0415800 2500 4.2 5.0 55 94.2 0.0271 0.0437 0.0742

Dow

nloa

ded

by [

Koc

aeli

Uni

vers

itesi

] at

12:

15 2

3 A

pril

2012

Environmental Technology 9

to >98% which enhanced floc formations and the removalmechanism was controlled at these pH ranges by adsorptionand sweep coagulation. OC at the optimum conditions was0.0096 ¤/m3.

References[1] P. Kumar and R. Chandra, Decolourisation and detoxifica-

tion of synthetic molasses melanoidins by individual andmixed cultures of Bacillus spp., Bioresource Technol. 97(2006), pp. 2096–2102.

[2] B. Cammerer, V. Jalyschkov, and L.W. Kroh, Carbohydratestructures as part of the melanoidin skeleton, Int. CongressSeries 1245 (2002), pp. 269–273.

[3] V.P. Migo, E.J. Del Rosario, and M. Matsumura, Floccula-tion of melanoidins induced by inorganic ions, J. Ferment.Bioeng. 83 (1997), pp. 287–291.

[4] S. Mohana, C. Desai, and D. Madamwar, Biodegradingand decolourization of anaerobically treated distillery spentwash by a novel bacterial consortium, Biores. Technol. 98(2007), pp. 333–339.

[5] M. Coca, M. Pena, and G. Gonzalez, Variables affectingefficiency of molasses fermentation wastewater ozonation,Chemosphere 60 (2005), pp. 1408–1415.

[6] J. Dwyer, L. Kavanagh, and P. Lant, The degradation ofdissolved organic nitrogen associated with melanoidin usinga UV /H2O2 AO. Chemosphere, 71 (2008), pp. 1745–1753.

[7] Y. Satyawali, W. Verstraete, and M. Balakrishnan, Integratedelectrolytic treatment and adsorption for the removal ofmelanoidins, Clean 38 (2010), pp. 409–412.

[8] E. Gengec, M. Kobya, E. Demirbas, A. Akyol, and K. Oktor,Optimization of baker’s yeast wastewater using responsesurface methodology by electrocoagulation, Desalination286 (2012), pp. 200–209.

[9] P. Canizares, M. Hernández-Ortega, M.A. Rodrigo, C.E.Barrera-Diaz, G. Roa-Morales, and C. Sáez, A compari-son between conductive-diamond electrochemical oxidationand other advanced oxidation processes for the treatmentof synthetic melanoidins, J. Hazard. Mater. 164 (2009),pp. 120–125.

[10] Y. Yavuz, EC and EF processes for the treatment of alco-hol distillery wastewater, Sep. Purif. Technol. 53 (2007),pp. 135–140.

[11] M. Kobya and S. Delipinar, Treatment of the baker’s yeastwastewater by electrocoagulation, J. Hazard. Mater. 154(2008), pp. 1133–1140.

[12] A. Simaratanamongkol and P. Thiravetyan, Decolorizationof melanoidin by activated carbon obtained from bagassebottom ash, J. Food Eng. 96 (2010), pp. 14–17.

[13] Z. Liang, Y. Wang, Y. Zhou, H. Liu, and Z. Wu, Stoichiomet-ric relationship in the coagulation of melanoidins-dominatedmolasses wastewater, Desalination 250 (2010), pp. 42–48.

[14] Z. Liang, Y. Wang, Y. Zhou, H. Liu, and Z. Wu, Variablesaffecting melanoidins removal from molasses wastewaterby coagulation/flocculation, Sep. Purif. Technol. 68 (2009),pp. 382–389.

[15] Y. Zhou, Z. Liang, and Y. Wang, Decolorization and CODremoval of secondary yeast wastewater effluents by coag-ulation using aluminum sulfat, Desalination 225 (2008),pp. 301–312.

[16] G.B. Raju, S. Prabhakar, S.S. Rao, and K. Gopalakr-ishna, Pilot-scale studies on the treatment of tannery efflu-ents by electroflotation, Int. J. Environ. Pollut. 30 (2007),pp. 332–344.

[17] M. Kobya, E. Demirbas, O.T. Can, and M. Bayramoglu,Treatment of levafix orange textile dye solution by electro-coagulation, J. Hazard. Mater. 132 (2006), pp. 183–188.

[18] M. Kobya, O.T. Can, and M. Bayramoglu, Treatmentof textile wastewaters by electrocoagulation using ironand aluminum electrodes, J. Hazard. Mater. 100 (2003),pp. 163–178.

[19] I. Arslan-Alaton, I. Kabdasli, V. Burcu, and O. Tunay, Elec-trocoagulation of simulated reactive dyebath effluent withaluminum and stainless steel electrodes, J. Hazard. Mater.164 (2009), pp. 1586–1594.

[20] I. Kabdasli, T. Arslan, I. Arslan-Alaton, T. Olmez-Hanci, andO. Tunay, Organic matter and heavy metal removals fromcomplexed metal plating effluent by the combined electroco-agulation/Fenton process, Water Sci. Technol. 61 (2010),pp. 2617–2624.

[21] M. Kobya, E. Demirbas, N.U. Parlak, and S. Yigit,Treatment of cadmium and nickel electroplating rinsewater by electrocoagulation, Environ. Technol. 31 (2010),pp. 1471–1481.

[22] M. Vepsalainen, J. Selin, P. Rantala, M. Pulliainen, H.Sarkka, K. Kuhmonen, A. Bhatnagar, and M. Sillanpaa,Precipitation of dissolved sulphide in pulp and paper millwastewater by electrocoagulation, Environ. Technol. 32(2011), pp. 1393–1400.

[23] M. Kobya, E. Senturk, and M. Bayramoglu, Treatment ofpoultry slaughterhouse wastewaters by electrocoagulation,J. Hazard. Mater. 133 (2006), pp. 172–176.

[24] T. Coskun, F. Ýlhan, N.U. Demir, E. Debik, and U. Kurt,Optimization of energy costs in the pretreatment of olivemill wastewaters by electrocoagulation, Environ. Technol.33 (2012), pp. 801–807.

[25] O.T. Can, M. Bayramoglu, and M. Kobya, Decolorization ofreactive dye solutions by electrocoagulation using aluminumelectrodes, Ind. Eng. Chem. Res. 42 (2003), pp. 3391–3396.

[26] M.Y.A. Mollah, R. Schennach, J.R. Parga, and D.L. Cocke,Electrocoagulation (EC)-science and applications, J. Haz-ard. Mater. 84 (2001), pp. 29–41.

[27] A.S. Koparal, Y.S. Yildiz, B. Keskinler, and N. Demircioglu,Effect of initial pH on the removal of humic substances fromwastewater by electrocoagulation, Sep. Purif. Technol. 59(2008), pp. 175–182.

[28] H. Liu, C. Hu, H. Zhao, and J. Qu, Coagulation of humic acidby PACl with high content of Al13: The role of aluminumspeciation, Sep. Purif. Technol. 70 (2009), pp. 225–230.

[29] J. Dahiya, D. Singh, and P. Nigam, Decolorization of syn-thetic and spentwash melanoidins using the white-rot fungusPhanerochaete chrysosporium JAG–40, Biores. Technol. 78(2001), pp. 95–98.

[30] APHA/AWWA/WPCF, Standard Methods for the Exam-ination of Water and Wastewater, Washington, DC, USA,1998.

[31] Y. Shi, H. Liu, X. Zhou, A. Xie, and C.Y. Hu, Mechanism onimpact of internal electrolysis pretreatment on biodegrad-ability of yeast wastewater, Chinese Sci. Bull. 54 (2009), pp.2124–2130.

[32] X. Zhou, H. Liu, Y. Q. Liang, and M. Zuo, The main com-ponents of color and dissolved organic matter from yeastindustry effluent, 2nd Conference on Environmental Scienceand Information Application Technology, 17–18 July 2010,China, Vol. 1, pp. 833–836.

[33] D. Ghernaout, B. Ghernaout, A. Saiba, A. Boucherit,and A. Kellil, Removal of humic acids by continuouselectromagnetic treatment followed by electrocoagulation inbatch using aluminium electrodes, Desalination 239 (2009),pp. 295–308.

Dow

nloa

ded

by [

Koc

aeli

Uni

vers

itesi

] at

12:

15 2

3 A

pril

2012

10 M. Kobya and E. Gengec

[34] M. Rebhun and M. Lurie, Control of organic matter by coag-ulation and floc separation, Water Sci. Technol. 27 (1993),pp. 1–20.

[35] M. Jang, H. Lee, and Y. Shim, Rapid removal of fineparticles from mine water using sequential processes of

coagulation and flocculation, Environ. Technol. 31 (2010),pp. 423–432.

[36] X. Chen, G. Chen, and P.L. Yue, Separation of pollutantsfrom restaurant wastewater by electrocoagulation, Sep.Purif. Technol. 19 (2000), pp. 65–76.

Dow

nloa

ded

by [

Koc

aeli

Uni

vers

itesi

] at

12:

15 2

3 A

pril

2012