Electrochemical Treatment of Simulated Textile Effluent

By N. Mohan, N. Balasubramanian, and V. Subramanian*

Electrooxidation of simulated Acid Blue 113 dye effluent has been carried out using a RuO2/Ti electrode. The influence of theinitial concentration of dye, supporting electrolyte, current densities and pH on COD reduction has been critically studied.Electrochemical analysis such as Instantaneous Current Efficiency (ICE) and Electrochemical Oxygen Demand (EOD) wereused.

1 Introduction

Textile industries are one of the most polluting industries interms of the volume and complexity of its effluent discharge.The dyeing and finishing operations in textile industriescontribute a major share to wastewater generation. In a typicaltextile industry approximately 1000 liters of water is con-sumed per ton of cloths processed. The water employed in thedyeing and finishing processes eventually end up as waste-water. Considerable variations are expected in the textilewastewater, as the dyeing processes in these industries arebatch processes. Textile wastewater is characterized by highBOD, COD and TOC and strong color, which should betreated before discharging it into the environment.

Conventionally textile wastewater is treated throughBiological, Physical and Chemical Methods. Biological treat-ments have gained attention in recent years, due to their eco-friendly nature. These methods are time consuming andcannot be employed where the effluents are biologicalresistant. Textile wastewater contains unused dyestuffs whichare highly structured polymers and difficult to decomposebiologically [1±3].

Traditionally activated carbon has been widely used for thetreatment of textile wastewater. However these methods arevery expensive and the adsorbents are difficult to regenerate[4,5]. Chemical treatment involves the addition of high valuechemicals resulting in un-reacted chemicals being present inthe treated water, which is also unacceptable. In addition, thismethod generates a considerable amount of sludge leading tosoil contamination [6,7].

The drawbacks associated with conventional treatmentmethods forced the environmental scientist/researchers tolook for eco-friendly treatment techniques to degrade thepollutants completely without generating any secondarypollutants. An electrochemical treatment method would besuch an alternative, as it would not generate any pollutants andwould give complete degradation of the pollutants present inthe effluents [8±11].

There is a wide selection of literature available onelectrochemical oxidation of wastewater containing organicpollutants. However, there are only a few reports on

electrochemical treatment of textile wastewater [12±15]. It isattempted in the present investigation to study the electro-chemical oxidation of dye wastewater using metal oxidecoated electrodes, covering a wide range of operatingconditions.

2 Experimental

Synthetic effluents of Acid Blue 113 have been chosen forthe present investigation. The basis for the selection of acidblue 113 dye is the toxicity impact of azo dye to theenvironment. It has generally been observed that the dyeexhaustion has been 93% with M: L ratio of 1:30 for a dyeshade of 0.5 %. The effluents come out of the dye-processingunit containing 0.15 to 0.33g L±1 unused dye. Accordingly, inthe present investigation effluents are prepared syntheticallyhaving concentrations; 0.333 g L±1 which corresponds to thedye bath wash water, and 0.16 g L±1 which corresponds to thecombined wastewater (dye bath, scouring, bleaching etc.). Thestructure of the dye is gives below:

The schematic diagram of the experimental setup is shownin Fig. 1 and consists of a glass beaker of 100ml capacity with alid as the electrolytic cell.

Chem. Eng. Technol. 24 (2001) 7, Ó WILEY-VCH Verlag GmbH, D-69469 Weinheim, 2001 0930-7516/01/0707-0749 $ 17.50+.50/0 749

±

[*] N. Mohan (author to whom correspondence should be addressed: e-mail:mohan29@yahoo.com); V. Subramanian, Department of Textile Technol-ogy, AC Tech, Anna University, Madras-600 025, India, N. Balasubrama-nian (e-mail: nbsbala@yahoo.com ), Department of Pollution Control,Central Electrochemical Research Institute, Karaikudi-630 006, India.

0930-7516/01/0707-0749 $ 17.50+.50/0

Figure 1. Experimental setup; schematic diagram: 1) DC power supply,2) saturated calomel electrode, 3) anode, 4) cathode, 5) magnetic stirrer.

Full Paper

Proper provisions are made in the lid for fixing the anode,cathode, and salt bridge of the reference electrode system,thermometer, and an opening for periodic sampling for CODestimation. Commercially available RuO2/Ti and StainlessSteel [SS] have been used as anode and cathode respectively.Sodium chlorides of 0.58 g L±1 and 1.74 g L±1 were used as thesupporting electrolyte. The electrolysis was carried out undergalvanostatic conditions covering a wide range of operatingconditions. Samples were collected at regular time intervalsfor COD estimation. The COD was estimated using standard adigestion technique [16].

3 Analysis of the Electro-oxidation Process

The electro-oxidation of wastewater in the presence of asupporting electrolyte is influenced by various operatingparameters. The progress of the destruction of the organicpollutant has been monitored by COD estimation. Thepotentials required for oxidation of organic pollutants aregenerally high and the production of oxygen from theelectrolysis of water molecules may determine the reactionyield.Theinstantaneouscurrentefficiency(ICE)maydefinedas

�COD ��Decrease in COD��volume of the solution�

mass of oxygenequivalent eletricity(1)

�COD �2F�COD v

16q

� �(2)

wheregCOD instantaneous current efficiency (ICE)F Faraday constantq ampere hour

If gCOD is plotted with electrolysis time the area under thecurve gives the value of the Electrochemical Oxidation Index[EOI]1), i.e.

EOI �R �

0�COD dt

�(3)

Where s is the total time of electrolysis. The value of EOIgives an idea about the nature of the oxidation process. Ifthe EOI value is high, the species can be oxidized veryeasily. On the other hand, it is very difficult to oxidizespecies having low EOI values. The ElectrochemicalOxygen Demand is defined as

EOD � I�F�gorg

8 � EOI (4)

The degree of oxidation (X) of an organic pollutant isdefined as the ratio of the amount of oxygen required for theoxidation of one gram of organic to its oxidation products (theamount is the EOD) to the amount of oxygen initially requiredfor complete oxidation to CO2. i.e.

X � EODCOD�

(5)

and

COD� � COD�gorg

L

� �initial

� �ÿ1

(6)

The above analysis (Eqs. (1) to (6)) gives the performance ofthe electro-oxidation of organic compounds. This analysis hasbeen extensively investigated for phenolic compounds [17]. Itis attempted in the present investigation to extend the analysisfor electro-oxidation of textile wastewater.

4 Theory

In electrochemical anodic oxidation processes, the pollu-tants are either directly oxidized at the surface of the electrode[direct electrolysis] or the oxidation occurs with the help ofoxidizing species generated electrochemically [indirect elec-trolysis]. The energy supplied to an electrochemical reactorplays an important role in any electrochemical process. Theenergy supplied to an electrode undergoes the following stepsduring the process [18]:1. The electro active particle is transferred to the electrode

surface from the bulk solution.2. The electro active particle is adsorbed on to the surface of

the electrode.3. Electron transfer occurs between the bulk and the

electrode.4. The reacted particle is either transported to the bulk

solution (desorption) or deposited at the electrode surface.From the above, the transfer of electrons between the

solution and electrode surface plays an important role in theelectrochemical process as the electrical energy is convertedinto chemical energy at the interface of the electrode. Ageneralized scheme for direct and indirect electro-oxidationprocesses can be seen in Fig. 2 [19].

Figure 2. Schematic representation of direct and indirect electro-oxidationprocess.

The electrochemical oxidation (conversion/and combus-tion) of organic compounds is theoretically possible beforeoxygen evolution (due to discharge of water molecule), but inpractice the oxidation reaction is very slow due to kineticrather than thermodynamic limitation. The following reac-tions are taking place at the anode and cathode:

750 Ó WILEY-VCH Verlag GmbH, D-69469 Weinheim, 2001 0930-7516/01/0707-0750 $ 17.50+.50/0 Chem. Eng. Technol. 24 (2001) 7

±

1) List of symbols at the end of the paper.

Full Paper

Anode

2Cl± ® Cl2 + 2e± (7)

Cathode

2H2O +2e± ®H2 + 2 OH± (8)

Bulk solution

Cl2 +H2O ®HOCl + Cl± +H+ (9)

HOCl ® OCl± + H+ (10)

The over all desired reaction is [20]:

Dye + OCl± ® intermediates ® CO2 + Cl±+ H2O (11)

Thus the organic species are finally converted into CO2. Thehypochlorite ions and free chlorine generated act as the mainoxidizing agents. As in any electrochemical process thereactions occur at the electrode/electrolyte interface so it isessential for the reactive species to be available at sufficientconcentration at this interface, in order to have good reactionefficiency. In other words the organic should move to theelectrode surface where it is oxidized, this is achieved throughthe effective agitation of the electrolyte thus the oxidation ofthe organic species is dependent on the active area of theelectrode and pollutant concentration. The reaction rate maybe written as [21]

dC/dt =k [C] (12)

Where k is rate constant. The slope of a plot of ln C/Ci versustime of electrolysis gives the value of the reaction rateconstant k. The reaction rate constant for the presentexperiments were estimated and the influence of supportingelectrolyte, dye concentration, pH and the current densitieswere critically examined.

5 Results and Discussions

The variation of COD with electrolysis time is presented inFig. 3. It can be ascertained from this figure that the CODdecreases with increasing electrolysis time. The rate ofreduction of COD is very sharp at the beginning of theprocess and approaches an almost constant value at the end ofthe process. The trend of COD reduction with electrolysistime remains the same for all the current densities employed inthe present investigation. It can also be noted from Fig. 3 thatthe rate of COD reduction increases with increasing currentdensity. The influence of dye concentration on COD removalis presented in Fig. 4.

The COD reduction decreases with increasing dye concen-tration. This may be explained since the ratio of dye

concentration to OCl± radical increased with an increase inthe dye concentration and hence the reduction of COD.

The concentration of supporting electrolyte plays animportant role in the electro-oxidation process. An increasein the concentration of the supporting electrolyte increasesthe cell conductivity and the OCl± radicals. The increase inOCl± radicals enhances the destruction of organics present inthe wastewater. While the energy consumption decreases withthe increasing conductivity of the electrolytic cell, Tab. 1 andFig. 5.

Table 1. The energy consumption decreases with the increasing conductivity ofthe electrolytic cell.

Dye con.g L-1

NaCl con.g L-1

NaCl: dye ratio % CODreduction

ICE Energyconsumption

KWh/kgof COD

0.330.330.160.16

0.581.740.581.74

1.7575.2373.625

10.875

51.1953.3278.1480.21

0.2230.2320.3460.356

131.091.895.661.3

Chem. Eng. Technol. 24 (2001) 7, Ó WILEY-VCH Verlag GmbH, D-69469 Weinheim, 2001 0930-7516/01/0707-0751 $ 17.50+.50/0 751

Figure 3. Variation of COD with electrolysis time, current densities;n: 1 A/dm2;u: 2A/dm2;s: 3 A/dm2;l: 4 A/dm2;&: 5 A/dm2; experimental conditions: AcidBlue 113, pH: 4; dye concentration: 0.33 g L±1, supporting electrolyteconcentration 0.58 g L±1.

Figure 4. Effect of initial dye concentration on COD removal: dye concentra-tion:n: 0.16 g L±1;u: 0.33 g L±1; experimental conditions: Acid Blue 113, pH = 9;supporting electrolyte concentration 0.58 g L±1, current density: 1 A/dm2.

Full Paper

Figure 5. Effect of supporting electrolyte concentration on COD reduction;supporting electrolyte concentration u: 0.16 g L±1 ; n: 0.33 g L±1; experimentalconditions: Acid Blue 113; dye concentration 0.16 g L±1; pH = 9.

It is very clear from the data that an increase in the ratio ofNaCl to dye concentration results in a considerable reductionin energy consumption, Tab. 1.

It can be seen from the results presented in the above tablethat by increasing the ratio of supporting electrolyte to dyeconcentration from 1.757 to 5.237, the energy consumption isreduced to 91 kWh/Kg of COD to 131 KWh/Kg of COD.While the percentage of COD reduction is increasedconsiderably.

5.1 Effect of pH

Experiments were conducted under three pH conditionscovering acid, alkaline and neutral. It was observed during thepresent investigation that the COD reduction was good underalkaline conditions, Fig.6.

Figure 6. Effect of pH on COD reduction: pH: u :4; n: 7; s: 12; experimentalconditions: Acid Blue 113; dye concentration 0.16 g L±1; supporting electrolyteconcentration 0.16 g L±1.

This may be due to the fact that the OCl± radicals are morestable under alkaline conditions.

The rate constant, k, was estimated from a plot of lnC/Ci

versus electrolysis time. It has been observed that the rateconstant, k, is influenced by the initial concentration oforganic present in the wastewater [Fig.7a].

Figure 7a. Variation of reaction rate constant k with initial dye concentration;dye concentration: u: 0.16 g L±1; n: 0.33 g L±1; experimental conditions: AcidBlue 113; supporting electrolyte concentration 1.75 g L±1, pH = 9.

Figure 7b. Variation of reaction rate constant k with pH; current densities:n: 5 A/dm2; u: 4 A/dm2; s: 3 A/dm2; l: 2 A/dm2; &: 1 A/dm2; experimentalconditions: Acid Blue 113; supporting electrolyte concentration 0.58 g L±1, dyeconcentration: 0.33 g L±1.

It may also be noted from the figure that the rate constant ismarginally influenced by the current density. The temperatureof the electrolytic solution was increased slightly with anincrease in current density. The electrolytic solutions temper-ature was increased to 37�C from room temperature of 33 �C.This increase in temperature may have been reflected by amarginal increase in the rate constant. The pH of theelectrolytic solution does not have a significant impact onthe rate constant �k', Fig 7b.

Instantaneous Current Efficiency and ElectrochemicalOxygen Demand were estimated using Eqs. (1)±(6). Instanta-neous Current Efficiency was estimated through the CODestimation technique. The variation of ICE with currentdensities is shown in Fig. 8.

It can be ascertained from the figure that the ICE isindependent of current density. However ICE and EOD arestronglydependentonthechargesuppliedforelectrolysis,Fig.9.

These observations are in close agreement with theobservation of Comninellis et al. [22].

6 Conclusion

The experiments were carried out in a batch electrochem-ical cell for simultaneous color removal and COD reduction inthe dye effluent. The following conclusions can be made:

752 Ó WILEY-VCH Verlag GmbH, D-69469 Weinheim, 2001 0930-7516/01/0707-0752 $ 17.50+.50/0 Chem. Eng. Technol. 24 (2001) 7

Full Paper

1. It is observed from the present investigation that theorganic pollutant present in the wastewater can be oxidizedby an electrochemical technique.

2. The COD reduction is significantly effected by the initialpollutant concentration, supporting electrolyte concentra-tion and pH.

3. Instantaneous Current Efficiency and ElectrochemicalOxygen Demand are independent of current density

4. The reaction was assumed to follow pseudo first orderkinetics and the reaction rate constants were estimated.

Acknowledgement

The first author wishes to thank: CSIR, India for sponsoringSRF fellowship to carry out the research programme, M/SShidimo Interaux(P) Limited Surat India for supplying thedye stuff, the director, CECRI, India for extending thefacilities to carry out the experiments at CECRI.

Received: July 12, 2000 [CET 1275]

Symbols used

A [dm2] area of the electrodeC [ppm] dye concentration at time t

Ci [ppm] initial dye concentrationDCOD [±] reduction in CODEC [KWh/Kg] energy consumption, KWh/Kg of CODF [±] Faraday constantgCOD [±] gram of organic present in the effluenti [A] currentk [min±1] reaction rate constantt [h] electrolysis timev [mL3] volume of the electrolytes [h] total electrolysis timeQ [Ah/dm3] charge supplied to the systemq [Ah] ampere hour

References

[1] Kamal, M. M.; Magda, M. K.; Yoseef, B. M.; Waly, A, Adsorption ofDirect Dyes by Cellulose Derivatives, Am. Dye Rep. 80 (1991) pp. 34.

[2] O'Neil, C.; Hawks, F. R.; Santra, R. R.; Esteves, P.; Haws, D. L.; Wilcock,S. J., Anaerobic and Aaerobic Treatment of Simulated Textile Effluent,J. Chem. Tech. Biotech. 74 (1999) pp.993±999.

[3] Razo-Flores, E.; Luijten, M.; Donlon, B.; Lettinga, G.; Field, J.,Biodegradation of Selected Azo Dyes under Methanogenic Conditions,Wat. Sci. Tech. 36 (1997) pp. 65.

[4] Poots, V. J. P.; Mckay, G.; Healy, J. J., The Removal of Acid dye fromEffluent Using Natural Adsorbents ± I, Water Research 10a (1976) pp.1061±1066.

[5] Poots, V. J. P.; Mckay, G.; Healy, J. J., The Removal of Acid Dye fromEffluent Using Natural Adsorbents ± II, Water Research 10b (1976) pp.1067±1070.

[6] Wilking, A, Dietrich Frahne Textile Effluent Treatment Method for the90's, Melliand Textilberchit, E 32, 1995.

[7] Kermer, W.-D.; Steenken-Richter, I., Decolourization of Dye HouseWaste Water by Ion Pairs Extraction, Melliand Textilbericht 6 E116, 1995.

[8] Kennedy, M., Electrochemical Wastewater Treatment Technology forTextile, Am. Dye Rep 9 (1991) pp. 26.

[9] Sequeira, C. A. C., Electrochemical Oxidation of Organic Pollutants forWastewater Treatment, in: Environmental Oriented Electrochemistry,Elsevier Sci., Amsterdam 1994.

[10] Kotz, R.; Stucki, S.; Carcer, B., Electrochemical Wastewater TreatmentUsing High over Voltage Anodes, Part I: Physical and ElectrochemicalProperties of SnO2 Anodes, J. Appl. Electrochem. 21 (1991) pp. 14±20.

[11] Stucki, S.; Kotz, R.; Carcer, B.; Suter, W., Electrochemical WastewaterTreatment Using High over Voltage Anodes, Part II: Anode Perfor-mance and Applications, J. Appl. Electrochem. 21 (1991) pp. 99±104.

[12] Wilcock, A.; Brewster, M.; Tincher, W., Using ElectrochemicalTechnology to Treat Textile Wastewater: Three Case Studies, Am. DyeRep. 8 (1992) pp. 15±22.

[13] Mohan, N., Studies on Electrochemical Oxidation of Acid Dye Effluent,Ph. D. Thesis, Anna University, Madras, March 2000.

[14] Neumczyk, J; Szpyrkowicz, L.; Zilio Grandi, F., ElectrochemicalTreatment of Textile Wastewater, Wat. Sci. Tech. 34 (1996) pp. 17±24.

[15] Lin, S. H.; Peng, Chi-F., Treatment of Textile Wastewater by Electro-chemical Method, Wat. Res. 28 (1994) No. 2, pp. 277±282.

[16] Ramesh, R.; Anbu, M., Chemical Methods for Environmental Analysis:Water and Sediments, Macmillan India Limited, India, (1996) pp. 41±42.

[17] Comninellis, C. H.; Pulgarin, C., Electrochemical Oxidation of Phenolfor Wastewater Treatment, J. Appl. Electrochem. 21 (1991) pp. 703±708.

[18] Goodridge, F.; Scott, K., Electrochemical Engineering: A Guide toDesign of Electrolytic Plant, Plenum Press, New York 1995, pp. 104.

[19] Rejeswar, K.; Ibanesz, J. G., Environmental Electrochemistry: Funda-mentals and Application in Pollution Abatement, Academic Press Inc.New York 1997.

[20] Comninellis, C. H.; Pulgarin, C., Electrochemical Oxidation of Phenolfor Wastewater Treatment Using SnO2 Anodes, J. Appl. Electrochem. 23(1993) pp. 108±112.

[21] Allan, S. J.; Kadar, K. Y. H.; Bino, M., Electrooxidation of Dye Stuff inWastewater, J. Chem. Technol. 62 (1995) pp. 111±117.

[22] Comninellis, C. H.; Nerini, A., Anodic Oxidation of Phenol in thePresence of NaCl for Wastewater Treatment, J. Appl. Electrochem. 25(1995) pp. 23±28.

Chem. Eng. Technol. 24 (2001) 7, Ó WILEY-VCH Verlag GmbH, D-69469 Weinheim, 2001 0930-7516/01/0707-0753 $ 17.50+.50/0 753

Figure 8. Influence of current densities on instantaneous current efficiency(IEC); experimental conditions: Acid Blue 113, dye concentration: 0.16 g L±1,supporting electrolyte concentration: 1.75 g L±1; pH = 4.

Figure 9. Effect of dye concentration on ICE (l and*)and EOD (l and&) forthe dye concentration of 0.33 and 0.16 g L±1 respectively; experimentalconditions: Acid Blue 113, supporting electrolyte concentration 1.75 g L±1,pH = 9, current density 1 A/dm2.

Full Paper