Studies of the electrodegradation of methyl orange

8/10/2019 Studies of the electrodegradation of methyl orange

1/7

Journal of Hazardous Materials 149 (2007) 492498

Studies on degradation of Methyl Orange wastewater bycombined electrochemical process

Hongzhu Ma, Bo Wang , Xiaoyan Luo Institute of Energy Chemistry, College of Chemistry and Materials Science, Shaanxi Normal University, XiAn 710062, China

Received 14 September 2006; received in revised form 1 February 2007; accepted 6 April 2007Available online 8 April 2007

Abstract

The combined process of electro-catalytic oxidation and de-colorizationof wastewatercontained Methyl Orange (MO)in a double-anode system,with iron plate and graphite plate as anodes and graphite plate as cathode assisted by Co 2O3CuOPO 4 3 modied kaolin, was investigatedsystematically. The effects of pH, current density and electrolyte on de-colorization efciency were also studied. Chemical oxygen demand (COD)was selected as another parameter to evaluate the efciency of this combined degradation method on treatment of MO wastewater and the resultsrevealed that when initial pH was 5.0, current density was 30 mA cm 2 , NaCl as electrolyte and its concentration was 2.5g dm 3 , the color removalefciency and COD removal can reach 100% and 89.7%, respectively. Meanwhile, the kinetics and the possible mechanism were also discussed. 2007 Elsevier B.V. All rights reserved.

Keywords: Electro-catalytic oxidation; Double-anode system; Methyl Orange; De-colorization

1. Introduction

Dyewastewaterusually consists of a numberof contaminantsincluding acids, bases, dissolved solids, toxic compounds, andcolored materials which are noticeable even at very low con-centrations and must be removed before the wastewater can bedischarged. It is reported that approximately 5 tonne of dye dis-charge from dyes and coloration industries every year [1]. Dueto environmental requirements in recent years, different tech-niques have been used, e.g. adsorption, oxidation, reduction,and electrochemical reactions. They have individual advantages,but also have some constrains causing inadequate effectivenesswhen applied individually [2]. For example, adsorption on acti-vated carbon is only successful for hydrophobic dyes, chemicalcoagulation can produce huge volume of hazardous sludge andposes a problem of sludge disposal and cannot bring any effectfor dissolved dyestuffs. Fentons reagent is effective only whenpH


2/7

H. Ma et al. / Journal of Hazardous Materials 149 (2007) 492498 493

Fig. 1. Methyl Orange structure.

to identify electro-catalytic degradation of MO wastewater by

electro-catalytic-oxidation and electro-coagulation combinedprocess and the conjunctional effect when integrating the cat-alyst (modied kaolin) into the electro-coagulation system.Additionally, a brief discussion on the kinetic study and pos-sible mechanism has also been proposed. Moreover, the optimalparameters (such as initial pH, current density and the con-centration of electrolyte) in treating the wastewater containingMO in this combined process were also studied. The last sec-tion compares the electrochemical degradation process withH2O2catalyst system.

2. Materials and methods

2.1. Reagents

All chemicals used in the experiment are of analytical gradeand used without further purication. Methyl Orange ( Fig. 1) isobtained from Ciba Co. (Switzerland).

Kaolin, provided by Shanghai Reagent Co., China, are com-posed of Al 4[Si4O10](OH) 8 (surface area: 20 m 2 g 1 and porevolume: 0.5 cm 3 g 1).

The concentration of Methyl Orange in wastewater used inthe experiments was 400mg dm 3 . NaCl (0.15 mol dm 3) orNa2SO4 (0.15mol dm 3) was used as electrolyte, and NaOH orHCl in proper amounts was used to get the suitable pH value.

2.2. Catalyst preparation and characteristics

CuOCo 2O3PO43 modied kaolin was prepared byimpregnation method as our early paper [21]. The catalyst has

the BET surface of 38m 2 g 1 compared with the neat kaolin of 20 m2 g 1 BET surface. By the modication process, the previ-ouskaolinite structurehas disappearedanda new poredstructurehas been formed.

2.3. Set-up of wastewater treatment

Theschematic diagram of experimentalset-up of ECcell withcatalyst is shownin Fig.2. Doubleanodeandsinglecathodewerepositioned vertically and parallel to each other with an inter gapof 0.5 cm. The materials used for the anode are ion plate andgraphite plate, and that used for the cathode is graphite plate.The supercial surface of working electrode (3.4cm 6cm)was 20.4 cm 2 . Ten grams of smashed catalyst was introducedin the solution to form a multi-phase electrochemical oxidationreactor, with a working volume of 0.15 dm 3. The solution wasconstantly stirred at 100 rpm using a magnetic stirrer in orderto maintain uniform concentration of the electrolyte solution.The electric power was supplied with regulated DC power sup-ply, WYK302B2, XiAn, China. The current and voltage wereadjustable between 02.5 A and 035 V, respectively.

2.4. Analysis techniques

All the experiments runs were conducted in duplicate andanalysis of each parameter was done triplicate for each run. Thecoefcient of variation obtained was not more than 5% for thethree determinations and for the experiment runs conducted induplicate. TheinitialpHof thesolutionwas testedusingan Orion290 pH meter. COD during the electrolysis was determined byan open reux, dichromate titrimetric method as described in

standard methods [22]. All parameters are measured accordingto standard methods for examination of water and wastewater[23]. During the degradation tests, dye concentration was deter-mined by double beam UVvis spectrophotometer (UV-7504,China). The maximum absorbency visible wavelength (470 nm)

Fig. 2. Schematic diagram of experimental set-up of EC cell.


3/7

494 H. Ma et al. / Journal of Hazardous Materials 149 (2007) 492498

was detected and used as color removal index. Color removalratio was calculated as our previous paper [24].

3. Results and discussion

3.1. Effect of initial pH

In general, MO has two chemical structures dependent onpH value of solution. Quinoid structure is a main form at lowpH value while azo structure at high pH. It is expected thatpH value of solution possibly affected electro-catalytic reactionon electrode surface. Therefore, the effect of initial pH of MOwastewater on de-colorizationefciency was investigated in thiswork. Apparently, appropriate pH is benet to the cleavage of the azo bond.

As can be seen from Fig. 3, a signicant difference on de-colorizationwasnotedwith NaCl as electrolyte at initial pH 2, 5,9 and 11, respectively. It was observed that MO molecules werereadily decomposed when initial pH was 5.0. The efciency

of decomposition was, however, considerably effectively in theappropriate acidic solution (the de-colorization efciency canreach 84%) than that of in the alkaline solution when initial pHwas 11 (the de-colorization efciency was only 31%), indicatedthat the catalytic activity of the modied kaolin in acid solutionsis higher than that of in the basic media.

In this combined electrochemical degradation process,electro-coagulation process was combined with catalytic oxi-dation process. The pollutant was mainly degraded byelectro-catalytic oxidation, because modied kaolin has verystrongadsorption andcatalytic property. When operated at mod-erate pH, the sweep ocs Fe(OH) 3 and other hydroxyl species,

produced on the sacricial anode, coagulated with modiedkaolin, which promoted the degradation process and made thesuspended particles to deposit. As Fig. 3 shows, the best initialpH was 5 andcouldmeanwhileachieve95% de-colorizationef-ciency of MO after 60 min, which can be explained that whenin this pH region, the degradation process can occur effectively

Fig. 3. Effect of initial pH on de-colorization efciency of MO (room temper-

ature; current density: 30mA cm 2

; NaCl: 2.5 g dm 3

; MO: 400mg dm 3

).

by both catalytic-oxidation and chemical coagulation. Fe 3+ ionsmay undergo hydration to form Fe(OH) 2+ , Fe(OH) 2+, Fe(OH) 3and other species under moderate acid conditions. A lowest de-colorization efciency was observed at pH above 11, whichis probably due to less Fe(OH) 3 formation and low catalyticactivity.

3.2. Effect of supporting electrolyte

Fig. 4 shows two supporting electrolytes (Na 2SO4 andNaCl) and their concentration on de-colorization efciency.As can be seen, when Na 2SO4 was substituted by NaCl, thede-colorization efciency increased from 68.8% to 95% after60 min, which fully indicated that introduction of NaCl aselectrolyte can enhance the degradation efciency and shortenreaction time, maybe due to the reaction between the gener-ated chlorine/hypochlorite and dye molecule. The pollutantswere oxidized by chlorite/hypochlorite that were then reducedto chlorite ion.

Anodereaction : 2Cl Cl2 + 2e (1)

Cathodereaction : 2H 2O + 2e H2 + 2OH

(2)

Bulksolutionreaction : Cl 2 + H2O HOCl + HCl (3)

HOCl H+ + OCl (4)

This was classied as indirect oxidation and also promotedelectro-catalytic oxidation of pollutant. Moreover, the increasedNaCl can result in a rise of current density at the same voltage.Therefore, the effect of NaCl dosage in terms of de-colorizationefciency hasbeenstudied.Increasingthe concentrationof NaCl

upto2.5gdm 3

accelerated the removalrate,enablingcompletemineralization of the MO to an extent of 99% de-colorizationefciency in 60 min [21]. A further increase in NaCl concen-tration (up to 5 g dm 3) did not bring about any improvementin de-colorization efciency, so the optimal NaCl concentrationused in the successive experiments was 2.5 g dm 3.

Fig. 4. Effect of different supporting electrolyte and their concentration on dyeremoval (room temperature; initial pH: 5.0; current density: 30mA cm 2 ; MO:

400mgdm 3

).


4/7


5/7


Fig. 9. UVvis spectra during the electrochemical degradation of MO (ini-tial pH: 5.0; NaCl: 2.5 g dm 3 , 10 g smashed catalyst, room temperature, MO:400mgdm 3).

3.5. Electrolysis time on COD and color removal

Effect of time on COD and color removal under the optimalconditions (initial pH:5.0; room temperature;NaCl: 2.5g dm 3;MO:400mgdm 3) wasinvestigated. Ascanbe seen from Fig.8 ,COD and color removal increased as a function of time elapsed.After 60 min of the treatment, COD and color removal reachedthe maximum of 89.6% and 95%, respectively.

3.6. UVvis spectral changes during the degradation process of Methyl Orange wastewater

A typical UVvis spectrum of MO as a function of electrol-ysis time (60min) was inspected ( Fig. 9). A peak observed at272 nm was assigned to azo structure of dye. The visible peak at 470 nm was attributed to aromatic rings in dye molecules.However, these two absorbency peaks decrease evidently afterthe treatment, it may be concluded that the conjugated bonds of MO must be destroyed and the molecules are broken into smallones.

4. Proposed mechanism

To further investigate the de-colorization process of MO, itis necessary to study the possible mechanism. Initially, it iswell-known that the removal of dye is due to the cleavage of chromophore bond in the dye molecule. The whole degradationinvolves two main processes: catalytic-oxidation and electro-coagulation.

4.1. Kinetic studies

An exhaustive kinetic study has been carried out on thecombined degradation of MO. Before addition of catalyst tothe solution of MO, no remarkable change on the color was

observed.However, whenthe catalystwasintroduced, the oxida-

tive reaction was started and a gradual loss of the absorbance of MO was observed as Fig. 10 shown.

The kinetic measurements were conducted under pseudo-rst-order condition. The observed rate constant, k 0 , wasdetermined from rst-order kinetics plot, ln At = ln A0 k 0w t ,as shown in Fig. 10, where At is the absorbance at time t = t , A0is the absorbance at t =0, and w is the amount of the catalystused. The k 0 value per g of dry catalyst (pgdr) was determinedfor the catalyst at optimal temperature and used to evaluate theactivationparameters. It is well-known that thereactionof higherrate constant is associated with lower activation energy, whichindicates that MO was easilyattacked by oxidizingagent such ashydrogen peroxide (H 2O2), OH radicals when the catalyst wasintroduced. The activity of the catalyst in H 2O2 decompositionwas found to follow the same order [24,25] .

4.2. Proposed mechanism

The degradation of pollutants by combined electrochem-

istry was concluded due to OH radicals produced by catalyticoxidation of modied kaolin and adsorption process of electro-generated Fe(OH) 3 sweep ocs. The catalytic oxidationprocess involve the following several steps: (i) mass transfer of O2 formed from anode toward the surface of cathode, slight OHradicals also generated form the anode, (ii) in moderate acidicconditions, H 2O2 is continuously produced by two-electronreduction of oxygen on the cathode surface.

O2 + 2H+ + 2e H2O2 (5)

H2O2 2 OH (6)

H2O OH + H+ + e (with MOx modied kaolin) (7)

(iii) H2O2 were decomposed and also produced highly reac-tive hydroxyl radicals in the presence of heterogeneous catalyst,(iv) the reaction between OH radicals and organic pollutantsadsorbed on the surface of catalyst. The role of the catalyst isnotonly to participate incatalyzing theformation andthe decom-position of H 2O2 but also to adsorb the pollutants making the

Fig. 10. First-order plot for the reaction of 400mg dm 3 MO at =470nm on

de-colorization.


6/7

H. Ma et al. / Journal of Hazardous Materials 149 (2007) 492498 497

Fig. 11. Schematic diagram of the proposed mechanism.

further degradation readily occur on its surface [8,9,21] . On theother hand, electro-coagulation process involves the generationof coagulants in situ by dissolving electrically iron ions from ionanode [13]. The generation of iron ions takes place at the anode.

Fe 2e Fe2+ (8)

Fe2+ reacted with H 2O2 in the acidic conditions to form OHradicals:

Fe2+ + H2O2 + H+ Fe3+ + OH + H2O (9)

The excess of Fe 3+ formed precipitates as Fe(OH) 3 sweepocs have large surface area which are benecial for a rapidadsorption of soluble pollutants and trapping of colloidal par-ticles like suspended kaolin particles [15]. These ocs arepolymerized as:

n Fe(OH) 3 Fen (OH)3n (10)

and are easily removed from aqueous medium by sedi-mentation. Secondary anodic reactions occur also duringelectro-coagulation. For example, in acidic conditions, withNaCl as electrolyte, native and free chlorine and hypochloriteare formed. Contaminants are then expected to be removed bythe combined action of both their homogeneous degradationwith OH radicals produced following Eq. (6) and coagulationwith the Fe(OH) 3 precipitate. Furthermore, for all the param-eter effect tested, the rate of degradation almost varies in theinitial 20 min, this should ascribe to the great porous structureof the fresh catalyst, when the catalyst was introduced, the pol-lutants was immediately absorbed and concentrated, thus therate of degradation enhanced. In other words, electro-catalyticoxidation played very important role in electrolysis processes.The main reaction mechanism of degradation of pollutant issummarized in Fig. 11.

4.3. Comparison with H 2O2 catalyst system

To prove the possible mechanism proposed, another exper-

iment has been conducted with H 2O2catalyst system to

Fig. 12. Comparison of de-colorization rate under two different systems.

compare with that of combined electrochemicalprocess. The de-colorization under two different systems is presented in Fig. 12.Twomillilitresof30%H 2O2 was addeddropwise into thesystemper10 min; it maybe observedobviously that thede-colorizationrate could barely reach 72% after 60 min with H 2O2catalystsystem, which should be ascribed to the decompositions of H2O2 in the presence of catalyst. However, it can also be seenfrom Fig. 12 that the de-colorization under H 2O2catalyst sys-tem was slower than that of electrochemical combined process,which demonstrated that reactions occurred in electrochemicalcombined system was not only the decomposition of H 2O2 gen-erated by electrochemical reaction but also by other reactions(e.g. direct oxidation on electrodes).

5. Conclusions

The electro-catalytic oxidation treatment of Methyl Orangewastewaterassistedby CuOCo 2O3PO43 modiedkaolininadouble-anode system with iron plate and graphite plate as anodeandgraphiteplate ascathode were investigated.The resultof thisstudy revealed the following: When the process was operated atpH of 5, room temperature andsupporting electrolyte NaCl con-centration of 2.5 g dm 3, it can be achieved 100% color removaland 89.7% COD removal. The whole degradation of MO in ourcombined electrochemical reactor involved two eminent pro-

cesses: catalytic oxidation and electro-coagulation, which isdifferent from the path that proceeds in H 2O2kaolin system.It may also be concluded that the electrochemical degradationassisted by kaolin adsorption effect can improve the whole pro-cess of dyeing wastewater treatment.

References

[1] D. Brown, Effects of colorants in the aquatic environment, Ecotoxicol.Environ. Safety 13 (1987) 139147.

[2] O.J. Hao, H. Kim, P.C. Chiang, De-colorization of wastewater, Crit. Rev.Environ. Sci. Technol. 30 (4) (2000) 449505.

[3] W.G. Kuo, Decolorizing dye wastewater with Fentons reagent, Water Res.

26 (7) (1992) 881886.


7/7


[4] E.G. Solozhenko, N.M. Soboleva, V.V. Goncharuk, De-colorization of azodye solutions by Fentons reagent, Water Res. 29 (9) (1995) 22062210.

[5] S.H. Lin, C.M. Lin, Treatment of textile waste efuent by ozonation andchemical coagulation, Water Res. 27 (12) (1993) 17431748.

[6] M.F. Sevimli, H.Z. Sarikaya, Ozone treatment of textile efuents and dyes:effect of applied azone dose, pH and dye concentration, J. Chem. Technol.Biotechnol. 77 (7) (2002) 842850.

[7] J. Sarasa, M.P. Roche, M.P. Ormad, E. Gimeno, A. Puig, J.L.Ovelleiro, Treatment of wastewater resulting from dyes manufacturingwith ozone and chemical coagulation, Water Res. 32 (9) (1998) 27212727.

[8] N. Daneshvar, H.A. Sorkhabi, M. Kobya, Decolorization of reactive dyesolution containing Acid Red 14 by electrocoagulation with a compara-tive investigation of different electrode connections, J. Hazard. Mater. 112(2004) 5562.

[9] J.-S. Do, M.-L. Chen, Decolorization of dye-containing solutions byelectro-coagulation, J. Appl. Electrochem. 24 (1994) 785790.

[10] M.C. Gutierrez, M. Crespi, A review of electrochemical treatments forcolor elimination, J. Soc. Dyers Colour. 115 (1999) 342345.

[11] A.G. Vlyssides, M. Loisidou, P.K. Karlis, A.A. Zorbas, D. Papaioannou,Electrochemical treatmentof a textile dyewastewaterusing Pt/Ti electrode,J. Hazard. Mater. B70 (1999) 4152.

[12] J.P. Lorimer, T.J. Manson, M. Plates, S.S. Phull, Dye efuent decolorisa-tion using ultrasonically assisted electro-oxidation, Ultrason. Sonochem. 7(2000) 237342.

[13] X.M. Zhan, J.L. Wang, X.H. Wen, Y. Qian, Indirect electrochemicaltreatment of saline dyestuff wastewater, Environ. Technol. 22 (2001)11051111.

[14] L. Szpyrkowicz, C. Juzzolino, S.N. Kaul, S. Daniele, M.D. de, Faveri,Electrochemical oxidationof dyeing baths bearing disperse dyes, Ind. Eng.Chem. Res. 39 (2000) 32413248.

[15] M.A. Sanroman, M. Pazos, M.T. Ricart, C. Cameselle, Electrochemicaldecolourisation of structurally different dyes, Chemosphere 57 (2004)233239.

[16] M.C. Rivera, M.M.D. Jimenez, M.P.E. Gonzalez,Degradationof the textiledyes Basic yellow 28 and Reactive black 5 using diamond and metal alloyselectrodes, Chemosphere 55 (2004) 110.

[17] X. Chen, G. Chen, P.L. Yue, Anodic oxidation of dyes at novel Ti/B-diamond electrodes, Chem. Eng. Sci. 58 (2003) 9951001.

[18] A. Fernandes, A. Morao, M. Magrinho, I. Goncalves, Electrochemicaldegradation of C.I. Acid Orange 7, Dyes Pigments 61 (2004) 287296.

[19] O.T. Can, M. Bayramoglu, M. Kobya, Decolorization of reactive dye solu-tions by electro-coagulation using aluminum electrodes, Ind. Eng. Chem.Res. 42 (2003) 33913396.

[20] N. Daneshvar, H.A. Sorkhabi, M.B. Kasiri, Decolorization of dye solutioncontaining Acid Red 14 by electrocoagulation with a comparative inves-tigation of different electrode connections, J. Hazard. Mater. 112 (2004)5562.

[21] L. Gu, B. Wang,H. Ma, W. Kong, Catalyticoxidation of anionic surfactantsby electrochemical oxidation with CuOCo 2O3PO43 modied kaolin,J. Hazard. Mater. B137 (2) (2006) 842848.

[22] Standard Methods for the Examination of Water and Wastewater (APHA,AWWA, WPCF), 20th ed., Washington, DC, 1998.

[23] American Public Health Association, American Water Works Associa-tion and Water Pollution Control Federation, Standard Methods for theExamination of Water and Wastewater, 19th ed. American Public HealthAssociation, Washington, DC, 1995.

[24] Q. Zhuo, H. Ma,B. Wang, L. Gu,Catalytic decolorization of azo-stuff withelectro coagulation method assisted by cobalt phosphomolybdate modiedkaolin, J. Hazard. Mater. 142 (2007) 8187.

[25] P. Canizares, F. Martianez, C. Jimeanez, J. Lobato, M.A. Rodrigo, Coag-ulation and electrocoagulation of wastes polluted with dyes, Environ. Sci.Technol. 40 (2006) 6418.

Documents

Studies of the electrodegradation of methyl orange