electrochemical oxidation of 2-nitrophenol in aqueous medium by

0 downloads 0 Views 144KB Size Report
H2O2. (2). This reaction can take place at different cathodes such as mercury pool [21], reticulated vitreous carbon. [26,27], activated carbon fiber [28], carbon felt.
J. Environ. Eng. Manage., 17(2), 89-96 (2007)

ELECTROCHEMICAL OXIDATION OF 2-NITROPHENOL IN AQUEOUS MEDIUM BY ELECTRO-FENTON TECHNOLOGY Salah Ammar,1 Nihal Oturan2 and Mehmet A. Oturan2,* 1

Département de Chimie, Faculté des Sciences de Gabes Université de Gabes 6072 Gabes, Tunisia 2 Laboratoire Géomatériaux et Géologie de l'Ingénieur Université Paris - Est 77454 Marne-la-Vallée Cedex 02, France

Key Words: 2-Nitophenol, water treatment, electro-Fenton process, carbon felt cathode, mineralization ABSTRACT The oxidative degradation of aqueous 2-nitrophenol (2-NP) solutions at pH 3 has been studied in a non divided electrochemical cell by indirect electrochemical oxidation technique using electroFenton process. Hydroxyl radical, a very powerful oxidizing agent, was produced from Fenton's reagent (Fe2+ + H2O2) in a catalytic way under current controlled electrolyses conditions on a carbon felt electrode as cathode. This environmentally friendly treatment technique allows to efficiently decontaminate aqueous solutions polluted by 2-NP. In situ generated hydroxyl radicals lead to the degradation of 2-NP until its total mineralization. The degradation rate and mineralization efficiency increase with applied current. The reaction kinetics follow a pseudo-first order reaction. Catechol, resorcinol, 1,2,4-trihydroxybenzene, hydroquinone and benzoquinone were detected as aromatic reaction intermediates. Malonic, glyoxilic and oxalic acids were identified as ultimate end-products before complete mineralization. The initial organic nitrogen was converted into NO3- and NH4+ ions. INTRODUCTION Nitrophenols have been widely used as reagents for production of pesticides, herbicides, explosives and as intermediates in the synthesis of dyes [1,2]. They are typical bio-refractory organic compounds and considered to be one of the primary toxic pollutants by U.S. Environmental Protection Agency (USEPA). 2-Nitrophenol (2-NP) is among the most toxic and persistent organic pollutant of industrial wastewater. The USEPA has listed 2-NP as “priority pollutant” [3] and recommended the restriction of its concentration in natural waters to less than 4.8 μg L-1. 2-NP exhibits high toxicity and/or mutagenecity for many living organisms either directly or through some of its catabolic metabolites [4]. The increasing discharge of nitrophenols into the environment results in the pollution of the natural water resources leading to many environmental problems with serious consequences to aquatic fauna and flora as well as the water treatment for human needs. These compounds can not be effectively treated by traditional technologies such as biological degradation, *Corresponding author Email: [email protected]

physicochemical treatment (coagulation-floculation), classical filtration or adsorption [4-6]. Direct photochemistry is not efficient either [7]; while ozonolysis is quite slow [8]. In contrast, the advanced oxidation processes (AOPs) such as: H2O2 photolysis [9,10], Fenton’s reagent [9,11-13], photo-Fenton [12-14], and heterogeneous photocatalysis [15,16] were shown to be able to degrade efficiently the nitrophenols. These processes are based on the oxidation power of hydroxyl radical (·OH) which is a non selective powerful oxidant (E0(·OH/OH-) = 2.80 V/SHE (standard hydrogen electrode) in acidic medium) and a very reactive species. In situ generated hydroxyl radicals react with organic pollutants leading to their mineralization, i.e., their transformation into CO2, H2O and inorganic ions. Recently, more efficient processes for the destruction of organic micropollutants have been described. In these environmentally friendly techniques, hydroxyl radicals are generated by electrochemistry. In direct electro-oxidation, they are produced at the electrode surface by anodic oxidation of water using a high O2-overvoltage anode such as PbO2 or boron-

J. Environ. Eng. Manage., 17(2), 89-96 (2007)

90

doped diamond (BDD) [17-20]: H2O

·OH(ads) + H+ + e-

2e

(1)

This reaction can take place at different cathodes such as mercury pool [21], reticulated vitreous carbon [26,27], activated carbon fiber [28], carbon felt [23,24,29,30], and O2-diffusion [22,25,31] cathodes. The oxidation power of electrogenerated weak oxidant H2O2 is enhanced in the presence of Fe2+ ions [21-25]: H2O2 + Fe2+ + H+ Fe3+ + ·OH + H2O

(3)

3+

Fe ions formed by Eq. 3 are reduced at the cathode at the potential of oxygen reduction : (4) Fe3+ + eFe2+ Equations 2 and 4 occur simultaneously at the cathode, continuously supplying H2O2 and Fe2+ to Eq. 3 in order to produce hydroxyl radicals in a catalytic way. When the anode is Pt, the O2 needed for Eq. 2 to produce H2O2 is formed on the anode by oxidation of water [32]: 2 H 2O

O2 + 4 H+ + 4 e-

+

1/2 O2

H2 O 2

1/2 O2 + 2 H

e

-

+

2e

Fe3+

OH

. OH

-

-

H2 O H+

Fig. 1. Schematic representation of the electrocatalytic production of Fenton’s reagent in order to produce hydroxyl radicals by the electro-Fenton process [32].

(HPLC) analyses. The effect of the applied current was studied as the main parameter to determine optimal experimental conditions. The rate and efficiency of mineralization of electrolyzed solutions were determined by TOC measurements. We investigated also the evolution of inorganic ions concentrations such as NH4+ and NO3 released during mineralization of organic nitrogen of the initial pollutant. Taking into account the identification of reaction intermediates, their evolution during treatment and mineralization measurements, a general mineralization reaction pathway for 2-NP was proposed.

(5)

Thus, the sum of Eqs. 2-5 constitutes an overall catalytic system. Two catalytic cycles taking place during this system for continuous regeneration of Fe2+ and H2O2 in order to produce hydroxyl radicals via Fenton’s reaction (Eq. 3) are schematized in Fig. 1. The formed hydroxyl radicals react with the pollutants present in medium:

pollutant + ·OH oxidation products

O2 + 2 H

Fe2+

In the case of the indirect advanced electrooxidation process (electro-Fenton), hydroxyl radicals are produced via Fenton’s reaction in homogeneous medium via electrogenerated H2O2 in the presence of ferrous ions. H2O2 is formed in acidic medium by the two-electron reduction of dissolved oxygen [21-25]: O2 + 2 H+ + 2 eH2O2 (2)

-

(6)

The efficiency of the electro-Fenton process can be enhanced by using a high O2-overvoltage anode such as BDD. In this case, hydroxyl radicals are formed in the medium from Fenton’s reaction (Eq. 3), as well as at the BDD anode surface from Eq. 1 leading to a significant rise of their formation rate and consequently the acceleration of the oxidation rate of organic pollutants [33-35]. In this study we report the oxidative degradation of 0.2 mM 2-NP aqueous solutions (equivalent to 14.4 mg L-1 of total organic carbon (TOC)) by electroFenton method using a carbon felt cathode/Pt anode system. The 2-NP decay and the evolution of its aromatic intermediates as well as carboxylic acids were followed by high-performance liquid chromatography

EXPERIMENTAL DETAILS 1. Chemicals

2-NP, hydroquinone, benzoquinone, catechol, resorcinol and 1,2,4-trihydroxybenzene were either reagent or analytical grade supplied by Sigma-Aldrich, Fluka and Acros Organics and were utilized without further purification. Sulfuric acid, anhydrous sodium sulfate, heptahydrated ferrous sulfate and ferric sulfate of analytical grade were purchased from Fluka and Acros Organics. All solutions were prepared with ultra-pure water obtained from a Millipore Milli-Q system with resistivity > 18 MΩ cm-1 at room temperature. 2. Electrochemical Apparatus and Procedures

Experiments were carried out at room temperature in a 0.25 L undivided cylindrical glass cell of 6 cm diameter inside equipped with two electrodes: a 50 cm2 piece of carbon-felt as working electrode (cathode) and a 4.5 cm2 Pt sheet as auxiliary electrode (anode). Electrolyses were carried out under current controlled electrolysis conditions and monitored by an EG&G Princeton Applied Research 273A potentiostat/galvanostat. The applied current was 60 mA for

Ammar et al.: 2-NP Oxidation by Electro-Fenton

3. Analysis Procedures

0.2

Concentration (mM)

kinetic studies, 100 mA for experiments concerning intermediates analyses and 300 mA for the mineralization trials. Solutions are stirred magnetically in a rate of 400 rpm. Compressed air was bubbled for 5 min through the solution prior to the electrolyses. A catalytic quantity of ferric iron (0.2 mM) was added to solutions before starting the electrolysis. The current remained constant during electrolysis and samples were withdrawn at regular coulometric charges for chromatographic and/or TOC analyses.

0.16

0.12

0.08

0.04

0 0

The decrease of 2-NP concentration during the treatment and the evolution of chemical composition of treated solutions were monitored by a MerckHitachi Lachrom HPLC equipped with a diode array UV-Vis detector (model L-7455) and fitted with a reverse phase Purospher RP-18, 5 μm, 4.6 × 250 mm column purchased from Merck. The column was thermostated at 40 °C and eluted at isocratic mode for all experiments with a mobile phase composed of water/acetonitril/H3PO4 (69/30/1, v/v) at a flow rate of 1.0 mL min-1. Detection was performed at 220 nm. Generated carboxylic acids were identified and quantified by ion-exclusion HPLC with a Merck Lachrom liquid chromatograph fitted with a Supelco Supelcogel H 9 μm, 25 cm × 4.6 mm column set at 40 °C, and coupled with a L-2400 UV detector selected at λ = 210 nm. In both HPLC techniques 20 μL samples were injected and measurements were controlled through an EZChrom Elite 3.1 software. Inorganic ions were analysed by ion chromatography using a Dionex ICS-1000 Basic Ion Chromatography system fitted with an IonPac AS4A-SC, 25 cm × 4 mm (i.d.) anion-exchange column. These measurements were conducted by injecting 25 μL samples and using a mobile phase composed of 1.8 mM of sodium carbonate and 1.7 mM of sodium bicarbonate at the flow rate of 0.8 mL min-1. The TOC of initial solutions and its evolution during electrolysis was determined by catalytic oxidation with a Shimadzu VCSH TOC analyser. RESULTS AND DISCUSSION 1. Degradation Kinetics

Degradations of 0.2 mM aqueous 2-NP solutions of pH 3 were carried out at room temperature by electro-Fenton under current controlled electrolysis conditions. In the case of a undivided electrolysis cell, the pH of the medium remains about constant between 2.8 and 3.0 because of the H+ consumed by Eq. 2 are compensated by their production in Eq. 5. HPLC analysis of 2-NP concentration during the treatment allowed us to monitor its evolution as a function of the electrolysis time. Applied current (I) was the main experimental parameter studied; the other parameters

91

5

10

15

20

25

30

Time (min)

Fig. 2. Effect of applied current on degradation kinetics of 0.2 mM 2-NP aqueous solution of pH 3 (volume = 250 mL) containing 0.1 mM Fe3+ as catalyst using a carbon felt/Pt electrolytic cell: (◆) 60 mA, (●) 100 mA, (▲) 300 mA.

being fixed to their optimal values: solution pH to 3 and catalyst (Fe3+ ions) concentration to 0.1 mM [36,37]. Figure 2 shows the significant role played by applied current on the degradation rate of organic pollutants. A very fast concentration decay with increasing current intensity was observed. At the highest current of 300 mA, the complete degradation of 0.20 mM 2-NP was reached in only 10 min. However, this time becomes gradually longer when applied current decreases. The decrease of 2-NP concentration vs. time was exponential in all cases highlighting that 2-NP degradation follows a pseudo-first-order kinetics. Indeed, the degradation of 2-NP by hydroxyl radicals (Eq. 6) can be described by second order kinetics: d [2 - NP] = kabs [2-NP] [·OH] (7) dt Assuming a quasi steady-state value for ·OH concentration during treatment (because of its very high reactivity and non accumulation in the medium), Eq. 7 becomes: d [2 - NP] = kapp [2-NP] (8) dt with kapp = kabs [⋅OH] , kapp (in unit of s-1) and kabs (in unit of M-1 s-1) being the apparent and absolute rate constants respectively. In fact, the degradation rate of 2-NP is controlled by the operational parameters such as applied current, pH of the medium, concentration of the catalyst, etc. The effect of these parameters appears on the level of the [·OH], because its formation rate and thus its steady state concentration are determined by these experimental parameters. Under this condition, the absolute rate constant of the reaction between 2-NP and ·OH can be evaluated by the method of competition kinetics with a compound for which absolute rate constant is well established [29]. In order to determine the kabs of 2-NP, competitive ki-

J. Environ. Eng. Manage., 17(2), 89-96 (2007)

92

5

6

y = 0.1321x R2 = 0.9975

2-NP

5

3

4-HBA 2

y = 0.0848x R2 = 0.9959

1

0 0

5

10

15

20

25

30

35

40

Time (min)

Fig. 3. Kinetic analysis for the competitive pseudo-firstorder reactions of („) 2-NP and (▲) 4-HBA (as reference compound) with hydroxyl radical in order to determine the absolute rate constant (kabs) for the hydroxylation reaction of 2-NP under the following experimental conditions: V = 250 mL, [2-NP]0 = [4-HBA]0 = 0.2 mM, [Fe3+] = 0.1 mM, current I = 100 mA.

netic experiments were performed in the presence of equal concentrations of 2-NP and 4-HBA (4hydroxybenzoic acid, kabs(4-HBA) = 1.63 × 109 M-1s-1) [38]: d [4 - HBA] = kapp(4-HBA) [4-HBA] (9) dt Assuming pseudo first order kinetics for both 2-NP and 4-HBA reactions with hydroxyl radicals and no side reactions, apparent rate constants are given by the slope of the corresponding straight lines of ln (C0/C) = f(t) plot (Fig. 3). kapp(2-NP) and kapp(4-HBA) were found to be 0.132 min-1 and 0.085 min-1, respectively. Combination of integrated equations from (8) and (9) permits to calculate the absolute rate constant for 2-NP as follow: k app ( 2- NP ) kabs(2-NP) = kabs(4-HBA) × k app ( 4-HBA ) = 3.4 × 109 M-1 s-1. This value is in agreement with the absolute rate constant of 4-nitrophenol hydroxylation reaction: kabs(4-NP) = 3.8 × 109 M-1 s-1 [39]. 2. Identification and Evolution Degradation Products

of

Aromatic

The HPLC analysis allowed to monitor the change in chemical composition of the electro-Fenton treated solutions as a function of electrolysis time or total electrical charge consumed. The fast decrease in 2-NP concentration (Fig. 2) during the electro-Fenton treatment was accompanied by the appearance of aromatic intermediates in the early stages of the electrolysis (Fig. 4). The complete degradation of 2-NP was achieved in 16 min under the present experimental conditions. Catechol, resorcinol, hydroquinone benzoquinone and 1,2,4-trihydroxybenzene were identified by their retention time compared with those of

Concentration (µM) .

Ln(C0/Ct) .

4

4

3

2

1

0 0

20

40

60

80

100

Time (min)

Fig. 4. Time-course of 2-NP aromatic derivatives formed during electro-Fenton treatment of a 0.2 mM 2-NP aqueous solution: 1,2,4-Trihydroxybenzene (◆), hydroquinone (■), benzoquinone (▲), resorcinol (∗) and catechol (●). Experimental conditions: V = 250 mL, pH = 3, [Fe3+] = 0.1 mM, I = 60 mA.

the authentic compounds. These intermediates show that hydroxyl radical addition on aromatic ring of 2NP constitutes the main reaction mode in the oxidation of 2-NP. The concentrations of intermediates reached a maximum steady state value at around 20 min for an aqueous solution containing initially 0.2 mM of 2-NP, and then decreases until total disappearance. Complete degradation of 2-NP and its hydroxylated aromatic derivatives occurred in less than 2 h. The early appearance of 1,2,4-trihydroxybenzene may seem surprising but can be explained by the rapid hydroxylation of catechol and hydroquinone formed in the first step which are more reactive with hydroxyl radicals than 2-NP. The fact that its degradation rate is slower than the other intermediates consolidates this hypothesis. Hydroquinone is also oxidized by hydroxyl radicals into benzoquinone. After 90 min of electrolysis, the 2-NP and its degradation intermediates have completely disappeared except 1,2,4trihydroxybenzene which remained in the solution up to 110 min. 3. Identification and Evolution of Carboxylic Acids

The primary derivatives react with hydroxyl radicals to give polyhydroxylated compounds and quinones before undergoing oxidative ring opening reactions leading to the formation of short chain carboxylic acids [24,29,34-37]. In order to identify and quantify the carboxylic acids formed, a 0.2 mM 2-NP aqueous solution of pH 3 was treated by electroFenton under I = 100 mA constant current. Glyoxilic, oxalic and malonic acids were identified and their evolution were followed by ion-exclusion HPLC

Ammar et al.: 2-NP Oxidation by Electro-Fenton

93

0.4

0.3

-1

TOC (mg L )

Concentration (mM)

0.5

0.2

0.1

0 0

50

100

150

200

250

300

350

400

Time (min)

chromatography. Figure 5 shows their evolution as a function of electrolysis time during electro-Fenton treatment. Maleic and glycolic acids were detected in low concentrations. They were generated as soon as the electrolysis is started with a large formation rate for the glycolic and oxalic acids. Transformation of glycolic acid into oxalic acid by electrochemical advanced oxidation methods was already reported [34]. Because of the poor reaction rate with hydroxyl radicals, carboxylic acids remained in the medium even after 7 h of treatment. 4. Degradation Conditions

of

2-NP

under

Electro-Fenton

Evolution of organic carbon removal of a 0.2 mM aqueous 2-NP solution of pH 3 was monitored during electro-Fenton treatment at I = 300 mA in the presence of 0.1 mM Fe3+ as catalyst. Mineralization of aqueous 2-NP solution was then monitored by measuring the TOC values during treatment. The decay of the TOC as a function of electrolysis time is shown in Fig. 6. The overall mineralization was attained after 8 h showing that electrogenerated hydroxyl radicals efficiently destroy aromatic hydroxylated intermediates and all final carboxylic acids as end-products. The mineralization of 2-NP aqueous solution can be proved by a supplementary analysis which consolidates the TOC measurements. Organic nitrogen of 2NP converted to mineral ions such as NH4+ and NO3during its mineralization by electro-Fenton treatment was followed by ion chromatographic analyses. The concentrations of NH4+ and NO3- were 0.09 and 0.11 mM, respectively, after 45 min of electrolysis (Fig. 7) under experimental conditions of Fig. 6. The sum of concentrations of these two ions is equivalent to that of organic nitrogen contained in the starting molecule. This result proves the total transformation of organic nitrogen into mineral ions. In longer electrolysis times,

Time (h)

Fig. 6. TOC removal with time during electro-Fenton treatment of 250 mL of 0.2 mM 2-NP solution containing 0.05 M Na2SO4. Experimental conditions: pH = 3.0, I = 300 mA constant current.

0.16

Concentration (mM)

Fig. 5. Evolution of carboxylic acids concentration during electro-Fenton treatment of 250 mL of 0.2 mM 2-NP aqueous solution: Oxalic (◆), malonic (●) and glyoxalic (▲) acids. [Fe3+] = 0.1 mM, [Na2SO4] = 0.05 M, pH = 3, I = 100 mA.

0.12

0.08

0.04

0 0

50

100

150

200

250

300

350

400

Time (min)

Fig. 7. Accumulation of nitrate (■) and ammonium (▲) ions released by mineralization of organic nitrogen of 2-NP by electro-Fenton treatment of 250 ml of 0.2 mM aqueous solution of pH 3. [Fe3+] = 0.1 mM, [Na2SO4] = 0.05 M and I = 300 mA .

nitrate ions concentration decreased in favour of NH4+ ions. This could be due to direct electrochemical reduction of NO3- ions on the carbon felt cathode. The identification and the evolution behavior of the aromatic reaction intermediates and carboxylic acids as well as the evolution of TOC removal and mineral ions allowed us to suggest a mineralization mechanism pathway for 2-NP oxidative degradation during electro-Fenton treatment (Fig. 8). Oxidation reactions are initiated by hydroxyl radical addition on the aromatic moiety of starting pollutant. Monohydroxylated derivatives undergo nitro group elimination reactions to form polyhydroxylated aromatics which oxidized to quinones. Carboxylic acids are formed by ring opening reactions of polyhydroxylated derivatives and/or quinones. These end-products are finally converted to CO2 and water (mineralization) by oxidative reaction of hydroxyl radicals.

J. Environ. Eng. Manage., 17(2), 89-96 (2007)

94

OH

OH NO 2

OH NO 2

+ · OH

· OH / O2 - NO2

OH

OH OH

+

+ OH

OH

OH

· OH / O2

· OH / O2

- H2 O O

OH OH

OH O O

OH

O

+

HO

O OH

+

CH2OH

OH

oxidative ring opening

COOH

· OH / O2

CO2 + H2O

Fig. 8. Proposed reaction sequence for the mineralization of 2-NP in aqueous acid medium by electrogenerated hydroxyl radicals.

CONCLUSIONS In this study we demonstrated that a toxic and persistent organic pollutant, 2-NP can be efficiently destroyed in aqueous medium by electro-Fenton process. Complete degradation of 250 mL 0.2 mM 2-NP aqueous solution was achieved in less than 10 min when applied current was 300 mA. Absolute rate constant of hydroxylation reaction of 2-NP was kabs(2-NP) = 3.4 ×109 M-1 s-1. An aqueous solution containing initially 27.8 mg L-1 2-NP (0.2 mM) was completely mineralized in 8 h. The treated solutions contain no organic matter. Degradation and mineralization reactions of 2-NP with hydroxyl radical obey a pseudofirst-order kinetics. Aromatic and aliphatic reaction intermediates formed during electro-Fenton treatment were also mineralized. The treated final solutions contain only inorganic ions such as NO3- and NH4+. The process appears as an ecologically friendly water treatment method because the reagents (H2O2 and Fe3+ ions) required for in situ hydroxyl radicals production are generated catalytically by electrochemistry. As hydroxyl radicals are non selective oxidation agents, the process can be extended to the treatment of wastewater contaminated by other toxic and persistent organic pollutants. REFERENCES 1. Alif, A. and P. Boule, Photochemistry and environment. Part XIV. Phototransformation of nitrophenols induced by excitation of nitrite and nitrate ions. J. Photoch. Photobio. A, 59(3), 357367 (1991). 2. Takahashi, N., T. Nakai, Y. Satoh and Y. Katoh,

Variation of biodegradability of nitrogenous organic compounds by ozonation. Water Res., 28(7), 1563-1570 (1994). 3. U.S. Environmental Protection Agency, Health and Environmental Effects, Ambient Water Quality Criteria for Nitrophenols. EPA 440/5-80-063, Washington, DC (1980). 4. Roldan, M.D., R. Blasco and F.J. Castillo, Degradation of p-nitrophenol by the phototrophic bacterium Rhodobacter capsulatus. Arch. Microbiol., 169(1), 36-42 (1997). 5. Haghighi-Podeh, M.R. and S.K. Bhattacharya, Fate and toxic effects of nitrophenols on anaerobic treatment systems. Water Sci. Technol., 34(5), 345-350 (1996). 6. Karimi-Jashmi, A. and R.M. Narkaitz, Impact of pH on the adsorption and desorption kinetics of 2nitrophenol on activated carbons. Water Res., 31(12), 3039-3044 (1997). 7. Chow, Y.L., Photochemistry of nitro and nitroso compounds. In S. Patai (Ed). The Chemistry of Functional Group, Supplement F. John Wiley & Sons, Chichester, UK, pp. 181-291 (1982). 8. Beltran, F.J., V. Gomez-Serrano and A. Duran, Degradation kinetics of para-nitrophenol ozonation in water. Water Res., 26(1), 9-17 (1992). 9. Goi, A. and M. Trapido, Hydrogen peroxide photolysis, Fenton reagent and photo-Fenton for the degradation of nitopenols: A comparative study. Chemosphere, 46(6), 913-922 (2002) 10. Chen, B., C. Yang and N.K. Goh, Photodegradation of nitroaromatic compounds in aqueous solutions in the UV/H2O2 process. J.

Ammar et al.: 2-NP Oxidation by Electro-Fenton

Environ. Sci.-China, 17(6), 886-893 (2005). 11. Lipczynska-Kochany, E., Degradation of aqueous nitrophenols and nitrobenzene by means of the Fenton reaction. Chemosphere, 22(5-6), 529-536 (1991). 12. Kavitha, V. and K. Palanivelu, Degradation of nitrophenols by Fenton and photo-Fenton processes. J. Photoch. Photobiol. A, 170(1), 83-95 (2005). 13. Kiwi, J., C. Pulgarin and P. Peringer, Effect of Fenton and photo-Fenton reactions on the degradation and biodegradability of 2- and 4nitrophenols in water treatment. Appl. Catal. BEnviron., 3(4), 335-350 (1994). 14. Sun, Y. and J.J. Pignatello, Photochemicalreactions involved in the total mineralization of 2,4-D by Fe3+/H2O2/UV. Environ. Sci. Technol., 27(2), 304-310 (1993). 15. San, N., A. Hatipoglu, G. Koçtürk and Z. Çınar, Photocatalytic degradation of 4-nitrophenol in aqueous TiO2 suspensions: Theoretical prediction of the intermediates. J. Photoch. Photobio. A, 146(3), 189-197 (2002). 16. Lee, G.D., S.K. Jung, Y.J. Jeong, J.H. Park, K.T. Lim, B.H. Ahn and S.S. Hong, Photocatalytic decomposition of 4-nitrophenol over titanium silicalite (TS-1) catalysts. Appl. Catal. A-Gen., 239(1-2), 197-208 (2003). 17. Comninellis, C., Electrocatalysis in the electrochemical conversion/combustion of organic pollutants for wastewater treatment. Electrochim. Acta, 39(11-12), 1857-1862 (1994). 18. Tahar, N.B. and A. Savall, Electrochemical degradation of phenol in aqueous solution on bismuth doped lead dioxide: A comparison of the activities of various electrode formulations. J. Appl. Electrochem., 29(3), 277-283 (1998). 19.Fryda, M., D. Hermann, L. Schäfer, C.P. Klages, A. Perret., W. Haenni, C. Comninellis and D. Gandini, Properties of diamond electrodes for wastewater treatment. New Diamond Front. Carbon Technol., 9(3), 229-240 (1999). 20. Kraft, A., M. Stadelmann and M. Blaschke, Anodic oxidation with doped diamond electrodes: A new advanced oxidation process. J. Hazard. Mater., 103(3), 247-261 (2003). 21. Oturan, M.A. and J. Pinson, Hydroxylation by electrochemically generated ·OH radicals. Monoand polyhydroxylation of benzoic acid: Product and isomers’ distribution. J. Phys. Chem., 99(38), 13948-13954 (1995). 22.Brillas, E., E. Mur, R. Sauleda, L. Sanchez, J. Peral, X. Domènech and J. Casado, Aniline

95

mineralization by AOP’s: Anodic oxidation, photocatalysis, electro-Fenton and photoelectroFenton processes. Appl. Catal. B-Environ. 16(1), 31-42 (1998). 23. Oturan, M.A., An ecologically effective water treatment technique using electrochemically generated hydroxyl radicals for in-situ destruction of organic pollutants: Application to herbicide 2, 4-D. J. Appl. Electrochem., 30(4), 475-482 (2000). 24. Oturan, M.A., J. Peiroten, P. Chartrin and A.J. Acher, Complete destruction of p-nitrophenol in aqueous medium by electro-Fenton method. Environ. Sci. Technol., 34(16), 3474-3479 (2000). 25. Boye, B., M.M. Dieng and E. Brillas, Degradation of herbicide 4-chlorophenoxyacetic acid by advanced electrochemical oxidation methods. Environ Sci. Technol., 36(13), 3030-3035 (2002). 26. Alvarez-Gallegos, A. and D. Pletcher, The removal of low level organics via hydrogen peroxide formed in a reticulated vitreous carbon cathode cell. Part 2: The removal of phenols and related compounds from aqueous effluents. Electrochim. Acta, 44(14), 2483-2492 (1999). 27. Xie, Y.B. and X.Z. Li, Interactive oxidation of photoelectrocatalysis and electro-Fenton for azo dye degradation using TiO2-Ti mesh and reticulated vitreous carbon electrodes. Mater. Chem. Phys., 95(1), 39-50 (2006). 28. Wang, A., J. Qu, J. Ru, H. Liu and J. Ge, Mineralization of an azo dye Acid Red 14 by electro-Fenton’s reagent using an activated carbon fiber cathode, Dyes Pigments, 65(3), 227-233 (2005). 29. Hanna, K., S. Chiron and M.A. Oturan, Coupling enhanced water solubilization with cyclodextrin to indirect electrochemical treatment for pentachlorophenol contaminated soil remediation, Water Res., 39(12), 2763-2773 (2005). 30. Aaron, J.J. and M.A. Oturan, New photochemical and electrochemical methods for the degradation of pesticides in aqueous media. Environmental applications, Turk. J. Chem., 25(4), 509-520 (2001). 31. Brillas, E., B. Boye, I. Sirès, J.A. Garrido, R.M. Rodriguez, C. Arias, P.L. Cabot and C. Comninellis, Electrochemical destruction of chlorophenoxy herbicides by anodic oxidation and electro-Fenton using a boron-doped diamond electrode, Electrochim. Acta., 49(25), 4487-4496 (2004). 32. Oturan, M.A., N. Oturan, C. Lahitte and S. Trevin, Production of hydroxyl radicals by electrochemically assisted Fenton’s reagent. Application to the mineralization of an organic micropollutant, pentachlorophenol. J. Electroanal.

96

J. Environ. Eng. Manage., 17(2), 89-96 (2007)

Chem., 507(1-2), 96-102 (2001). 33. Flox, C., P.L. Cabot, F. Centellas, J.A Garrido, R.M. Rodríguez, C. Arias and E. Brillas, Electrochemical combustion of herbicide mecoprop in aqueous medium using a flow reactor with a boron-doped diamond anode. Chemosphere, 64(6), 892-902 (2006). 34. Boye, B., M.M. Dieng and E. Brillas, Degradation of herbicide 4-chlorophenoxyacetic acid by advanced electrochemical oxidation methods. Environ. Sci. Technol., 36(13), 3030-3035 (2002). 35.Sirés, I., J.A. Garrido, R.M. Rodríguez, P.L. Cabot, F. Centellas, C. Arias and E. Brillas, Electrochemical degradation of paracetamol from water by catalytic action of Fe2+, Cu2+, and UVA light on electrogenerated hydrogen peroxide. J. Electrochem. Soc., 153(1), D1-D9 (2006). 36. Diagne, M., N. Oturan and M.A. Oturan, Removal of methyl parathion from water by electrochemically generated Fenton’s reagent. Chemosphere, 66(5), 841-848 (2007). 37. Sirés, I., J.A. Garrido, R.M. Rodríguez, E. Brillas,

N. Oturan and M.A. Oturan, Catalytic behavior of the Fe3+/Fe2+ system in the electro-Fenton degradation of the antimicrobial chlorophene. Appl. Catal. B-Environ., 72(3-4), 382-394 (2007). 38. De Heredia, J.B., J. Torregrosa, J.R. Dominguez and J.A. Peres, Kinetic model for phenolic compound oxidation by Fenton’s reagent. Chemosphere, 45(1), 85-90 (2001). 39. Buxton, G.V., C.L. Greenstock, W.P. Helman and A.B. Ross, Critical review of rate constants for reactions of hydrated electrons, hydrogen atoms and hydroxyl radicals (.OH۟/.O-۟ ) in aqueous solutions. J. Phys. Chem. Ref. Data, 17(2), 513-886 (1988). Discussions of this paper may appear in the discussion section of a future issue. All discussions should be submitted to the Editor-in-Chief within six months of publication. Manuscript Received: December 27, 2006 Revision Received: February 17, 2007 and Accepted: February 26, 2007