RSC Advances

PAPER

Environmental Technology Division, C

Research-Central Leather Research Institute

Tamil Nadu, India. E-mail: ganesansekar

Tel: +91-44-24911386 ext. 7141

† Electronic supplementary informa10.1039/c3ra45199b

Cite this: RSC Adv., 2014, 4, 9971

Received 17th September 2013Accepted 16th December 2013

DOI: 10.1039/c3ra45199b

www.rsc.org/advances

This journal is © The Royal Society of C

Electrochemical treatment of reverse osmosisconcentrate generated by the leather industryusing a Cu–graphite electrode†

R. Boopathy and G. Sekaran*

Electrochemical treatment of reverse osmosis concentrate (ROC) generated by the leather industry was

evaluated using a copper coated graphite (Cu–graphite) electrode for the elimination of refractory

organic pollutants. The effect of dilution of the ROC and applied current density on the performance of

electrochemical oxidation was investigated. The presence of chloride ions in the ROC favoured the

complete elimination of TKN and COD removal by 98% at optimum conditions: current density, 100 mA

cm�2 and electrolysis period, 6 h. The increase in ROC dilution slightly decreased the percentage

removal of COD from 98% to 96%. The effect of current density on the removal of COD and TKN in raw

ROC was found to be lower than in more diluted ROC. It was concluded that electrochemical treatment

was more favourable for ROC samples without dilution. The generation of trihalomethanes during the

electrochemical oxidation of ROC was effectively removed in an activated carbon packed bed column.

Introduction

Advanced membrane technology has become increasinglyattractive for the reclamation of wastewater discharged fromvarious industries compared with conventional technologies.Membrane processes are highly efficient, economical, easy tooperate and automate, do not require the intensive use ofchemicals used in pre-treatment operations and occupy lessspace.1–4 Moreover, membrane separation techniques combineprocess stability with an excellent effluent quality. Reverseosmosis treatment imparts water with very high quality forreuse.5–10However, the generation of reverse osmosis concentrate(ROC) in signicant quantities and the cost of its disposal ortreatment was considered to be one of the main disadvan-tages.11,12 The ROC is characterised by a high concentration ofinorganic salts, organic compounds and biological constituents.The concentration of chromium in ROC may vary from 0.84 mgL�1 to 1.16 mg L�1 in the ROC generated by the leather industry.The salinity of ROC generated from industrial origin was lessthan that of the ROC discharged from desalination plants. Solleyet al.13 reported that the contaminants in ROC could be seventimesmore concentrated than in the feed water. As ROC containsanthropogenic organics,14 its release into the environment needsto be managed carefully.15 Several methods, such as coagulation

ouncil of Scientic & Industrial

(CSIR-CLRI), Adyar, Chennai, 600 020,

an@gmail.com; Fax: +91-44-24452941;

tion (ESI) available. See DOI:

hemistry 2014

and activated carbon adsorption,16 ozonation,14 combined O3

with biological activated carbon,15 photocatalysis and electro-chemical oxidation17,12 have been investigated for the manage-ment of ROC.16,14 Among these, electrochemical oxidation hascertain advantages for ROC treatment that include effective androbust control of reaction conditions, in situ generation ofoxidants and operation at ambient conditions (temperature andpressure). Furthermore, the high salinity of ROC allows a highelectrical conductivity, thereby lowering ohmic losses in anelectrochemical system and decreasing energy consumption.There are reports on the treatment of ROC by electrochemicaloxidation using different electrode materials.12,16,17

Electrochemical oxidation can occur both via direct andindirect pathways. Direct oxidation involves only electrontransfer at the anodic surface. Typically, the direct oxidationrate exhibits slow kinetics18 at higher current densities withdiffusion limitations. Therefore, indirect oxidation processesmediated by electro-generated oxidising agents (OCl� and OH�)from wastewater, and with inorganic mediators (e.g. chloride,bromide) were oen targeted to enhance the oxidation perfor-mance. Besides process conditions, the choice of anode mate-rial is of great importance for the electrochemical oxidation as itaffects the performance and selectivity of the process.

Apart from the well-known but expensive boron-doped dia-mond (BDD) electrode, a number of different anode materialsincluding thin lm oxides (e.g. PbO2, SnO2), noble metals (Pt,Pd), and dimensionally stable anodes (DSA) such as Ti-basedmetal coated with metal oxides (e.g. RhOx, RuO2, IrO2) or mixedmetal oxides (MMOs) such as Ru–IrO2 and Pt–IrO2 have beeninvestigated in recent years for the treatment of tannerywastewater, landll leachate, petroleum wastewater and other

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bio-refractory organic waste streams.18,19 High chloride contentin the ROC led to in situ generation of active chlorine species(i.e. Cl2, HOCl and OCl�) for an effective indirect oxidation oforganics.20,21 However, these potential oxidants led to theformation of chlorinated by-products such as trihalomethanes(THMs) and/or haloacetic acids (HAAs). The possible formationof chlorite, chlorate, and perchlorate is of particular concernbecause of their toxic effects on living organisms.22–24 Hence, theeffective operating conditions for the degradation of persistentpollutants at controlled by-products generation is important.

A recent review specied that fulvic like material such ashydrophobically charged fractions were removed by membraneseparation, whose uorescence intensity was reduced by anadvanced treatment method.25–27 Finally, it was shown thatoxidants, such as ozone and chlorine could increase or decreasethe uorescence intensity of the effluent.28,29 Hence, in thisstudy, the uorescence spectra of ROC before and aer elec-trochemical oxidation was recorded to identify the effectivenessof the electrochemical oxidation.

In this study Cu–graphite was used as an electrode for thetreatment of ROC generated from secondary biologically treatedtannery wastewater. The reason for the selection of Cu–graphiteas the electrode material was for its dimensional stability in thetreatment of ROC and the cost of the material is lower thanother oxide coated electrodes. The degree of dilution andcurrent density were varied to evaluate the controlling step forthe electrochemical oxidation of organic pollutants in ROC.

Materials and methodsReverse osmosis concentrate (ROC)

Tannery ROC was collected from a commercial CETP in Rani-pet, Tamil Nadu, India and transported to the laboratory andstored at 25 �C until any further use. The ROC was characterisedby following the methodology of APHA (Table 1).30

Experimental set-up for the electrolysis of ROC

A rectangular shaped electrochemical cell (length, 15 cm; width,3.5 cm and height, 15 cm) with a triangular hopper bottom was

Table 1 Physico-chemical characteristics of ROC generated by theleather industry

Sample no. Parameters Mean valuea

1 pH 6.422 Conductivity (mS cm�1) 34.863 Total dissolved solids 152404 Chemical oxygen

demand640

5 Total organic carbon 2206 Total Kjeldahl nitrogen 467 Ammonia nitrogen 258 Calcium 11249 Magnesium 13.810 Chloride 999611 Sulphate 3020

a All the values are expressed in mg L�1, except pH and conductivity.

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fabricated using acrylic plastic. Two copper coated cylindricalgraphite (diameter, 1 cm and length, 15 cm) rods were used aselectrodes (preparation of copper coated graphite is presentedin the ESI†); and positioned horizontally with a 1 cm innerelectrode spacing between the rods as shown in Fig. 1. Thesurface area of each electrode was 47 cm2. ROC of volume200mL was taken into the electrochemical cell and re-circulatedthrough a peristaltic pump, operated at ow rate of 3.3 cm3

min�1. The electrochemically oxidized solution was thenapplied to a mesoporous activated carbon (surface area, 220 m2

g�1) packed bed column. Aliquots of samples were withdrawnfrom the reactor for the characterisation of parameters such asCOD, TOC and TKN.

Analytical methods

Dissolved organic carbon was measured in a Shimadzu TOC-Vanalyser (Shimadzu, Japan). The samples were ltered throughglass microber lter paper of pore size 0.45 mm. COD wasdetermined in accordance with the procedure illustrated byVyrides et al., to overcome the chloride interference.31 Theconcentration of TKN, ammonium–nitrogen and chromium(VI)were determined as per the APHA method.

Total free chlorine generation during the electrolysis processwas analysed through color development with DPD (N,N diethyl-p-phenlenediamine) followed by absorbance measurement in aspectrophotometer at l550 nm. And the concentration ofhydrogen peroxide was determined by following the method-ology of APHA.30 The generation of total halomethane (THM)during electrolysis was estimated by eluting it by solventextraction and detected by gas chromatographic techniques.

The Excitation Emission Matrixes (EEMs) were recordedusing a Cary Eclipse uorescence spectrophotometer. Thesystem comprised of a Xenon arc lamp as a radiation source,excitation and emission gratings, and a sample chamber to holdquartz cuvettes (10 � 10 mm) and a red-sensitive photo-multi-plier tube was used as the detector. The EEMs were recorded inthe excitation wavelength range of lex 200–500 nm with a stepwidth of 5 nm and in the emission wavelength range of lem350–600 nm with a step width of 5 nm.

Results and discussionCyclic voltammetry

The electrochemical behaviour of the graphite and Cu–graphiterods in ROC was studied separately using cyclic voltammetry.Fig. 2 shows the voltammograms obtained in the potentialrange between 0 and 2 V with a scan rate of 10 mV s�1. Ananodic current peak was observed at 1.1 V for the graphite andCu-coated graphite electrodes. The observed anodic oxidationpeak was lower than the reported value of 1.6 V as with a BDDelectrode.32 The decrease in electrode activity with the BDDelectrode was due to the formation of oxygen containing func-tional groups, which act as a barrier for electron transfer.33,34

But in the graphite electrode system, the electrode surface wasregenerated by a quasi-reversible reaction. This is conrmed bythe appearance of cathodic current peaks at 0.93 V and 0.85 V

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Fig. 1 Experimental setup for the electrochemical oxidation of ROC generated by the leather industry.

Fig. 2 Cyclic voltammetry spectra of graphite and Cu–graphite rodsas anodes in the electrochemical oxidation of ROC; inner plot showsthe anodic current peak at 1.1 V.

Paper RSC Advances

for the graphite and Cu–graphite systems respectively. Theresult suggested that, the selection of a copper coated graphiteelectrode was technically feasible for the treatment of ROCgenerated by the leather industry.

Effect of ROC dilution

The effect of ROC dilution was studied by performing theelectrochemical degradation of ROC at different dilutions suchas 1 : 0, 1 : 1, 1 : 2 and 1 : 4 (v/v) (volume of ROC : volume ofdistilled water). Fig. 3 presents the variation in COD and TKN ofROC with respect to electrolysis time. A regular linear decreasein parameters of ROC with time of electrolysis was observed.Thus, the electrochemical degradation of ROC was controlledby applied current density. The removal of COD and TKNincreased with dilution and complete removal was attained atminimum electrolysis time. Complete COD removal wasobtained for the dilutions 1 : 1, 1 : 2 and 1 : 4 at 120, 60 and30min of electrolysis time respectively. Similarly, complete TKN

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removal was obtained at 30, 15, 10 and 10 min for 1 : 0, 1 : 1,1 : 2 and 1 : 4 dilutions of ROC. It is known that the oxidationrate is independent of the organic concentration in kineticallycontrolled reactions. But, in this study, the change in organicconcentration was done by the increase in dilution whichcaused a signicant change in COD and TKN removal (Fig. 3).The removal of COD from ROC at an electrolysis period of30 min for all the dilutions is illustrated in Fig. 3a. Thepercentage of COD removal was found to have decreased from96.8% to 92.5% with an increase in ROC dilution from 1 : 0 to1 : 4 respectively. This may be due to the decrease in chlorideion concentration in the ROC by dilution, because the chlorideions were responsible for the removal of TKN and COD in ROCby indirect oxidation through the generation of secondaryoxidizing agents (chlorite and hypochlorite ions).35

The generation of hydrogen peroxide during the electrolysisof ROC was probably due to reducing oxygen at the cathodesurface. Thus, the generation of hydrogen peroxide in the Cu–graphite/Cu–graphite system may be attributed to a high activesurface area of the electrodes.36 The solution pH aer electrol-ysis was found to have decreased from 6.4 to 3.2. The generationof hydrogen peroxide under acidic conditions liberates thehydroxyl radicals, which is responsible for the indirect oxida-tion of organic pollutants in ROC. The concentration of H2O2 inthe bulk ROC (1 : 0) was found to be 108 mg L�1 and it was 92,60 and 22 mg L�1 for the dilutions of 1 : 1, 1 : 2, 1 : 4 respec-tively. These values suggest that the concentration of hydrogenperoxide decreased with increasing dilution of ROC. This maybe attributed to the presence of chloride ions, which eased thediffusion of ions for electrolysis and thereby increased thehydrogen peroxide concentration. The appreciable reduction inCOD and TKN in diluted ROC that was observed, may beattributed to the generation of secondary oxidizing agents suchas hypochlorous acid and OCl� ions during electrolysis. Thegeneration of hypochlorous acid and ionization thereof to OCl�

ions were responsible for the elimination of TKN content fromthe ROC, probably by converting it into chloramines and theninto nitrogen.37–39

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Fig. 3 Electrochemical degradation profiles of organic pollutants by Cu–graphite/Cu–graphite system in ROC of different dilutions (a) removalof COD, (b) removal of TKN.

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The generation of hypochlorous acid (HOCl) and hypochlo-rite ions in the bulk solution from the chlorine gas generated atthe anodic surface is indicated in eqn (2) and (3). The algebraicsum of dissolved chlorine gas, hypochlorous acid and hypo-chlorite is termed as free chlorine. The hypochlorite ion is themajor component of the free chlorine in the normal pH range ofwastewater.

2Cl� / Cl2 + 2e� (1)

Cl2 + H2O / HOCl + H+ + Cl� (2)

HOCl / H+ + OCl� (3)

The maximum evolution of free chlorine gas was observed as9, 3.5, 2 and 1 mg L�1 for the ROC dilutions 1 : 0, 1 : 1, 1 : 2 and1 : 4 respectively (Fig. 4b) at optimum conditions: currentdensity, 50 mA cm�2 and electrolysis time, 120 min. As expec-ted, the increase in dilution decreased the formation of freechlorine in the bulk solution due to the decrease in concen-tration of chloride ions in ROC.

The results presented in Fig. 4a indicate that the electro-chemical oxidation was limited by mass transfer control indiluted ROC. To verify this phenomenon, ROC of dilution 1 : 2was selected and operated at different current densities andtheir critical COD (CODcr) was determined. According to basicprinciples of electrochemistry, if the limiting step in the elec-trochemical oxidation process is the organic pollutant transferfrom the bulk to the anode surface, the change in the currentdensity would not affect the COD and TKN removal rate. Thus,current density between 10 and 100 mA cm�2 was applied for anelectrolysis time of 60 min and the results are shown in Fig. 4a.COD removal became constant for the current densities higherthan 50 mA cm�2, which means that electrochemical oxidationin ROC of 1 : 2 dilution and higher dilutions was affected by themass transport limiting mechanism. The application of themodel proposed for the evaluation of mass transport limita-tions is represented in eqn (4), where A represents the geometricarea of the electrode in m2. The data obtained at currentdensities of 50 and 100 mA cm�2 were used to determine the

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mass transport coefficient (1.68 � 10�5 m s�1) using thefollowing mathematical expression

½COD�t½COD�0

¼ exp

���Akm

V

�t

�(4)

The mass transport coefficient obtained from eqn (4) wasused to calculate the critical COD (CODcr).

CODcr ¼ 1

4AFkm(5)

CODcr may be regarded as the COD below which organicspresent in the solution are oxidised with mass transfer limita-tion. The calculated CODcr was found to be 104 mg L�1 at50 mA cm�2 at a dilution of 1 : 4 which is mainly under masstransport control in the electrochemical oxidation of ROC.

The limiting current density, jlim predicts the stoichiometricrequirement of current density for the electrochemical oxida-tion of ROC and it was calculated using the following equation

jlim ¼ 4FkmCOD0 (6)

the calculated jlim, for ROC of dilution 1 : 4 (initial COD,80mg L�1), was found to be 13 mA cm�2; which was much lowerthan the optimum applied current density (50 mA cm�2) in thepresent investigation. Hence, it is justied that, a higheramount of applied current density was necessary for effectiveremoval of COD.

The specic energy consumption, Esp, in W h (g COD)�1 forthe electrochemical oxidation of ROC was calculated usingeqn (7)

Esp ¼ UIDt

3600VDCOD(7)

where U is the cell voltage, in V; I the applied current density,expressed in A; Dt is the duration of electrolysis, in s; V isvolume of solution, in L and DCOD is the removed COD, in mgL�1. Fig. 4c shows that the increased dilution of ROC andcurrent density signicantly increased the energy consumption.The specic energy consumption for ROC at dilution 1 : 4 & at100mA cm�2; and at dilution 1 : 0 & at 5 mA cm�2 were found to

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Fig. 4 (a) Influence of applied current density on COD/COD0 at ROC dilution of 1 : 2 for an electrolysis time of 60min, (b) generation of total freechlorine at 100 mA cm�2 (c) specific energy consumption on electrochemical oxidation of ROC at different dilutions.

Paper RSC Advances

be 2.15 � 10�3 and 2.8 � 10�5 W h (g COD)�1 respectively. Theratio of Esp value to the lowest Esp value (the lowest Esp value wasobtained for ROC of dilution 1 : 4) signies an index to char-acterise the energy efficiency of electrochemical oxidation. Inthe present study it was observed that Esp at 1 : 0 dilution/Esp at1 : 4 dilution was 76.78, which indicates that the electro-chemical oxidation of ROC at 1 : 0 dilution was more energyefficient over other dilutions. This was expected, that

Fig. 5 Effect of current density on electrochemical oxidation of ROCremoval of COD, (b) removal of TKN.

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consumption of energy was minimal for the process operatedunder kinetic control.

Effect of current density

The effect of current density on COD and TKN removal wascarried out by varying the current density from 5 to 100 mAcm�2. Fig. 5a and b show the kinetic data of COD and TKNremoval for the selected current density values. In all cases, a

at 1 : 0 dilution using Cu–graphite/Cu–graphite electrode system (a)

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Fig. 6 Total halomethane concentration profile in electrochemicaloxidation of ROC at different current densities.

Fig. 7 Excitation and emission matrix of (a) ROC and (b) ROC after elec

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decrease in COD and TKN was observed with an increase inapplied current density in electrochemical oxidation. Themaximum COD and TKN reduction of 98.1% and 100%respectively at 100 mA cm�2 was achieved with an electrolysistime of 6 h. The COD was removed by 96.8, 90.6, 78.8, 62.5%with an applied current density of 50, 20, 10, 5 mA cm�2

respectively. An applied current density of 100 mA cm�2

eliminated TKN by 100% aer 20 min of electrolysis time,while 100% removal of TKN was achieved at current densitiesof 50, 20, 10, 5 mA cm�2 aer 30, 40, 90, 180 min of electrolysistime respectively. This may be due to the generation ofsecondary oxidizing agents such as HOCl and OCl� which wereresponsible for the oxidation of TKN. This corroborates withthe observations recorded by Vijayaraghavan et al. using agraphite electrode.40 The generation of hydrogen peroxide wasalso responsible for the removal of organic content fromthe ROC.

trochemical oxidation.

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Paper RSC Advances

Generation of by-products during electrochemical treatmentof ROC

During the electrochemical oxidation of ROC, the formationof chloro and bromo organic compounds was more probabledue to the generated free chlorine gas.41,42 This was conrmedfrom the selective elution of the electrolysed ROC solution(dialysed) aer adsorption onto a C18 column using n-hexaneand ethanol as eluting solvents. The compounds extractedwith solvents (before and aer electrochemical oxidation)were analysed by gas chromatography. The results (Fig. 6)depict that low or nil formation of organic halide was iden-tied at low applied current densities (5, 10 and 20 mA cm�2).But, there was a considerable amount of total halomethane(THM) concentration (Fig. 6) detected at applied currentdensities above 50 mA cm�2. Still, the detected concentrationof THMs was within the permissible concentrationstandard for drinking water (100 mg L�1). Hence, it is veryimportant to optimize the current density to be applied inorder to increase the process efficiency and reduce theformation of THMs.

The formed halogenated organic compounds could beeffectively removed by adsorption using activated carbon.43

Hence, a mesoporous activated carbon packed bed column wasused to remove the halogenated organic compounds aerelectrochemical oxidation at current densities beyond 50 mAcm�2. The results suggest that the concentration of THMs wasfound to be below a detectable limit for the electrochemicallyoxidized solution aer being passed through a packedbed adsorption column. Further, the electrochemically oxidisedROC contained Cr(VI) in the range of 0.1–0.2 mg L�1. Theconcentration of chromium in the nal solution aeradsorption in a mesoporous activated carbon column wasfound to be below a detectable limit. Hence, the integrationof electrochemical oxidation and an activated carbon packedbed column would be an effective system for the treatmentof ROC.

Fluorescence spectroscopy analysis

The number, composition and structure of uorophores in theorganic matter of ROC samples are variable and essentiallyunknown. Researchers have devised simple techniques forsummarising EEMs, to identify uorescence peaks ineffluents.44,45

The EEMs of the ROC and aer electrochemical oxidationwere recorded to identify the removal of organic pollutants.Fig. 7 shows peaks at l 220–320/385–415 nm indicating thepresence of humic like substances in the ROC. A blue shi isassociated with the fragmentation of aromatic organiccompounds into smaller compounds possibly by reducingwith p-electron systems by electrolysis. A blue shi is asso-ciated with either a decrease in the number of aromatic rings,fewer conjugated bonds in a chain structure, the conversionof a linear system to a non-linear systems or elimination ofparticular functional groups including carbonyl, hydroxyl,and amine groups.46,47 The uorescence intensitydecreased remarkably aer electrochemical oxidation as

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shown in Fig. 7b. This concludes that the organic content wasremoved signicantly from the ROC by electrochemicaloxidation.

Conclusions

Treatment of a reverse osmosis concentrate stream generatedby the leather processing industry was investigated by electro-chemical oxidation using a Cu–graphite/Cu–graphite system.The electrochemical oxidation of COD and TKN in ROC waskinetically monitored and an increase in the dilution of ROCshied the process to being controlled by mass transport ofoxidizing agents. The increase in dilution increased the CODremoval efficiency, and increased the energy cost too. Further,the generation of halogenated organic compounds wascontrolled by decreasing the applied current density and periodof electrolysis. The generated toxic halogenated organiccompounds could be effectively removed by passing through anactivated carbon adsorption column.

Abbreviations

ROC

Reverse osmosis concentrate DSA Dimensionally stable anode MMO Mixed metal oxide THM Total halomethane CETP Common effluent treatment plant APHA American public health association TOC Total organic carbon COD Chemical oxygen demand TKN Total Kjeldahl nitrogen EEM Excitation and emission matrix km Mass transport coefficient (m s�1) Esp Specic energy consumption (W h g�1 of COD)

Acknowledgements

We (Authors) thank the Council of Scientic and IndustrialResearch (CSIR), India for the nancial assistance to carry outthe work.

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40 K. Vijayaraghavan, D. Ahmad and T. S. Fadzin, In situhypochlorous acid generation for the treatment of brackish

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RSC Adv., 2014, 4, 9971–9979 | 9979