Electrochemical degradation of sinapinic acid on a BDD anode

Desalination 272 (2011) 148–153

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Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l

Electrochemical degradation of sinapinic acid on a BDD anode Sourour Chaâbane Elaoud a, Marco Panizza b,⁎, Giacomo Cerisola b, Tahar Mhiri a a b

Laboratoire de chimie physique, département de chimie, Faculté des Sciences de Sfax, 3000, Université de Sfax, Sfax, Tunisie Department of Chemical and Process Engineering, University of Genoa, P.le J.F. Kennedy 1, 16129 Genova, Italy

a r t i c l e

i n f o

Article history: Received 19 November 2010 Received in revised form 30 December 2010 Accepted 4 January 2011 Available online 26 January 2011 Keywords: Sinapinic acid Polyphenol Wastewater Electrochemical degradation Boron-doped diamond

a b s t r a c t The electrochemical oxidation of sinapinic acid (4-hydroxy-3,5-dimethoxy-cinnamic acid), one of the most representative polyphenolic type compounds present in olive oil mill wastewater, was studied by galvanostatic electrolysis using boron-doped diamond (BDD) as anode. The influence of several operating parameters, such as applied current density, initial sinapinic acid concentration, temperature, flow rate and initial pH value, was investigated. UV spectroscopy and chemical oxygen demand measurements were conducted to study the reaction kinetics of sinapinic acid mineralization. The experimental results showed that the electrochemical process was suitable for almost completely removing COD, due to the production of hydroxyl radicals on the diamond surface. In particular, the COD removal follows a pseudo first-order kinetics and the apparent rate constant increased with flow rate and temperature, while it is almost unaffected by applied current and pH. Under optimal experimental conditions of flow rates (i.e. 300 L h­1), temperature (T = 50 °C) and current density (i.e. 10 mA cm­2), 97% of COD was removed in 3 h electrolysis, with 17 kWh m­3 energy consumption. © 2011 Elsevier B.V. All rights reserved.

1. Introduction The expansion of the olive oil industry, with increased production of noxious waste known as olive mill wastewater (OMW), presented a serious environmental concern in the main olive- producing countries of the Mediterranean region such as Italy, Spain, Greece, Tunisia, Turkey and Algeria. OMW has a high polluting organic load, due to a high content of organic substances, including sugars, phenolic-like substances (tannins, polyphenols), polyalcohols, pectins and lipids [1]. As regards , low-molecular mass phenolic compounds (hydroxytyrosol, tyrosol, p-hydroxyphenyl acetic acid, p-coumaric acid, sinapic acid, vanillic acid and caffeic acid) are usually present in OMW. It is known that phenolic compounds are major contributors to the toxicity and the antibacterial activity of OMW [2], which limits its microbial degradability [3]. Thus alternative methods are required to achieve the abatement of phenols prior any biological treatment. Most physical and physicochemical methods, such as precipitation, flocculation/clarification, coagulation, filtration and evaporation in open ponds [2] give only partial solution to the problem. On the contrary, the advanced oxidation processes (AOPs) in which the highly oxidizing species hydroxyl radicals (•OH) is produced by photocatalysis, Fenton's reagent, ozone or hydrogen peroxide with ultraviolet light [4,5], could provide a solution for such environmental problem when used for pre-treatment of OMW because enable the

⁎ Corresponding author. Tel.: +39 010 3536032; fax: +39 010 3536028. E-mail address: [email protected] (M. Panizza). 0011-9164/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2011.01.011

complete oxidation of toxic pollutants without formation of dangerous by-products. Recently, a new AOP induced by electrochemistry was proposed as an alternative method for organic removal [6,7]. It is based on the production of large amount of hydroxyl radicals on the anode surface from water discharge [8,9]: •

H2 O→ OH þ e þ H

þ

ð1Þ

or in the bulk of the solution taking place from Fenton's reagent, where hydrogen peroxide is generated in situ from the two-electron reduction of O2 at RVC, carbon-felt or gas-diffusion cathodes [10–12]: þ

O2 þ 2H þ 2e →H2 O2 2þ

Fe

2þ

þ H2 O2 →FeðOHÞ

ð2Þ •

þ OH

ð3Þ

In the anodic oxidation, a wide variety of electrode materials such as noble metals (platinum) [13], dimensionally stable anodes [14], lead dioxide (PbO2) [15] and tin dioxide (SnO2) [16] have been investigated ; however, the highest efficiency has been obtained with boron-doped diamond (BDD) [17–19]. In fact, it has been demonstrated that many biorefractory compounds such as phenols [20,21], dyes [22–25] and pesticides [26,27] can be completely mineralised with high current efficiency, even close to 100%, using BDD anodes. Sinapinic acid (SA, 4-hydroxy-3,5-dimethoxy-cinnamic acid) is one of the most representative polyphenolic type compounds present in olive oil mill wastewater. Many paper reported on the treatment of this compound by anaerobic digestion ; however, high amounts of

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biorefractory and oxidation-refractory organics remain at the treatment. [28,29]. For example, it was reported that SA was only partially removed by Pseudomonas mira [30], Rhodotorula glutinis [31] or Stichococcus bacillaris [32]. Given the bactericide factor, the inhibitor character and the anti bacteriological activity of this compound, the traditional biological digestion cannot be applied and therefore new technologies have to be taken into consideration to avoid its accumulation in the environment. Therefore, in this paper we report the results about the anodic oxidation of SA at BDD electrode, under galvanostatic conditions. The influence of several process parameters such as applied current density, initial concentration of SA, temperature, recirculation flow rate and pH, in order to study the kinetics of sinapinic acid degradation and verify its complete mineralization, has been evaluated. 2. Experimental 2.1. Chemicals

149

2.4. Analysis The degradation of sinapinic acid was monitored by spectrophotometric measurements using a JASCO V 630 spectrophotometer. The chemical oxygen demand (COD) of the initial and electrolyzed samples was determined with a Dr. Lange LASA50 system. The current efficiency (CE) of the electrolysis, which was an average value, was calculated from the values of the COD using the relationships: CEð%Þ =

ðCOD0 −CODt Þ FV × 100 8It

ð4Þ

where COD0 and CODt are the chemical oxygen demands at times t = 0 and t (in gO2 L­1), respectively, and I is the current (A), F was the Faraday constant (96487 C mol­1), V is the volume of the electrolyte (L). The specific energy consumption (Ec, in kWh m­3) was obtained as follows: Ucell It V3600

ð5Þ

The synthetic solution was prepared by dissolving different amount of SA (C11H12O5) (Sigma Aldrich, 98% purity) used without further purification, in bidistilled water, in 50 mM Na2SO4 (Sigma Aldrich). Na2SO4 was chosen as the supporting electrolyte, because it was one of the most common salts used for the electrochemical degradation process. The molecular structure of SA is shown in Fig. 1. Analytical grade NaOH and H2SO4 (Sigma Aldrich) were used to adjust the initial pH of the solution.

EC =

2.2. Electrode materials The boron-doped diamond thin-film electrode was supplied by CSEM Centre Swiss d'Electronique et de Microtechnique of Neuchâtel. It was synthesised by the hot filament chemical vapour deposition technique (HF CVD) on single crystal p-type Si b1 0 0N wafers. The doping level of boron in the diamond layer expressed as B/C ratio was about 3500 ppm. The obtained diamond film thickness was about 1 μm with a resistivity of 10–30 mΩ cm. In order to stabilize the electrode surface and to obtain reproducible results, the diamond electrode was pre-treated at 25 °C by anodic polarization in 1 M HClO4 at 10 mA cm­2 during 30 min using stainless steel as the counter electrode. This treatment made the surface hydrophilic.

In order to obtain qualitative information on the changes in the molecule of SA, representative UV-vis spectra evolution during the electrolysis of 1 mM SA at j = 10 mA cm­2 were recorded and the results are reported in Fig. 2. The UV-vis spectra of SA before the treatment presents two welldefined absorption bands around 230 and 312 nm can be observed and they can be attributed to π →π* transition in aromatic rings. During the electrolysis, the two absorbance bands observed around 230 and 312 nm continuously and simultaneously decrease and they disappear after 2 hours of treatment (Fig. 2, inset). Total disappearance of the two bands suggests that no more aromatic intermediates exist in the water. These results show that total degradation of SA and its aromatic intermediates has been achieved by oxidation with BDD anode.

2.3. Electrochemical measurements

3.2. Effect of the current density

The bulk electrolyses were performed in a one-compartment electrolytic flow cell with parallel plate electrodes applying a constant current, using an AMEL 2055 potentiostat/galvanostat. BDD was used as the anode and stainless steel AISI 304 as the cathode. Both electrodes were circular with a geometric area of 50 cm2 each and an interelectrode gap of 1 cm. The electrolyte was stored in a 0.35 L thermoregulated glass reservoir (20 °C) and circulated through the electrolytic cell by a centrifugal pump with different recirculation flow rates.

The effect of applied current density on the COD decay and current efficiency during the oxidation of 1 mM SA was investigated by electrolysis at different current densities, i.e., 10, 20 and 30 mA cm­2.

3. Results and discussion 3.1. UV-vis spectra changes during electrolyses

1

3 A/A0 (-)

0.8

2.5

Absorbance (a.u.)

COOH

where Ucell is the average cell voltage (V), I is the current (A), t is the electrolysis time (s) and V is the volume of the treated solution (L).

2

0.6 0.4 0.2 0 0

1.5

50

100

150

200

Time (min)

1 0.5 0 200

OCH3

H3CO OH

Fig. 1. Molecular structure of the sinapinic acid.

250

300

350

400

450

500

Wavelength (nm) Fig. 2. Evolution of the UV spectra with time during galvanostatic electrolysis of sinapinic acid 1 mM. Flow rate 300 L h­1; pH = 4; T = 20 °C; j = 10 mA cm­2. The inset shows the time evolution of the normalised absorbance peak (ratio absorbance/initial absorbance: A/A°) at (Δ) 230 nm and (□) 312 nm.

150

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3

80

2.5

40

0

200

0

50

100 150 Time (min)

200

100

2 1.5 1 0.5 0 0

0 0

50

100

150

As shown in Fig. 3, for all the applied current densities, the degradation of SA continues practically until the almost complete COD removal. Furthermore, only a slight effect of the applied current was observed in the COD removal and only a negligible improvement in the oxidation rate was observed when the current density varied from 10 to 30 mA cm­2. This behavior indicates that in these experimental conditions, the oxidation of SA is completely under mass transport control and an increase of the applied current favors only the secondary reaction of oxygen evolution: þ

ð6Þ

This was confirmed by the fact that the current efficiency (Fig. 3, inset) decreased with the current density. The decay of COD concentration exhibits an exponential behavior with all the applied current indicating a first-order reaction kinetics for the oxidation reaction. Working in galvanostatic condition, the concentration of •OH can be approximated in a steady state and therefore, the oxidation rate expression can be written as follows: d½COD = k½• OH½COD = kapp ½COD dt

ð7Þ

which can be integrated to give the following expression: COD0 = kapp ⋅t CODt

40

60

80

100

120

140

Time (min)

Fig. 3. Influence of applied current density on COD removal and current efficiency (inset) during oxidation on BDD anode. Conditions: sinapinic acid 1 mM; flow rate 300 L h­1; pH = 4; T = 20 °C; applied current density:(◊) 10 mA cm­2; (□) 20 mA cm­2; (Δ) 30 mA cm­2.

2H2 O→O2 þ 4H þ 4e

20

200

Time (min)

Ln

Ln (COD0/COD)

300

COD / (mg L-1)

120

C.E. (%)

400

ð8Þ

Fig. 4. Kinetic analysis for pseudo first-order removal during the electrolysis of 1 mM sinapinic acid. Conditions: T = 20 °C; pH = 4; flow rate 300 L h­1; applied current density: (◊) 10 mA cm­2; (□) 20 mA cm­2; (Δ) 30 mA cm­2.

increased with initial SA concentration due to the presence of a greater amount of organics in the medium. At a given time, the current efficiency (Fig. 5, inset) always increases with rising polyphenol concentration, confirming that the oxidation was under mass transport control. For all the concentrations the COD removal follows a pseudo first-order kinetics and the apparent rate constants were 0.023, 0.0208 and 0.016 min­1 for the SA concentrations of 0.5, 1 and 1.5 mM, respectively.

3.4. Effect of temperature Fig. 6 shows the effect of temperature on the variation of COD during the electrolysis of 0.5 mM M SA at 10 mA cm­2. The higher was the temperature, the faster was the COD removal. In fact, when the temperature increased from 20 to 50 °C the time for the mineralization of the SA decreased from 180 to 120 min. Since the reaction between the electrogenerated •OH and the SA is a fast reaction and it is only slightly affected by temperature, this behavior can be explained in term of increase of mass transfer rate of organics from solution to the electrode surface [33]. In fact, as shown in Fig. 6, inset, the COD removal follows a pseudo first-order kinetics and the apparent rate constant increased with temperature, being 0.0205, 0.0225 and 0.0311 min­1 at 20, 35 and 50 °C, respectively. The apparent activation energy for the electrochemical oxidation of SA at the BDD electrode was found to be 20 kJ mol­1, which is close for a diffusion-controlled homogeneous reaction (typically less than 40 kJ mol­1 [34].

600

3.3. Effect of the sinapinic acid initial concentration Fig. 5 shows the effect of the initial concentration of SA on the trend of COD and CE during galvanostatic electrolysis at 20 °C, applying a current density of 10 mA cm­2. Overall SA oxidation was achieved in all cases but the time for the complete COD removal

100 80

C.E. (%)

500

COD (mg L-1)

where COD0 and CODt are the COD of the solution at the beginning and at time t respectively, and kapp is the apparent observed pseudo first-order rate constant. Apparent rate constants determined by 0 plotting the Ln COD CODt against time at different applied current (Fig. 4), were 0.0214, 0.0197 and 0.0228 min­1 for the applied current of 10, 20 and 30 mA cm­2 respectively. The relative absorbance of the SA solution at 230 and 312 nm also decreases as a function of electrolysis time, but again no significant effect on the process was observed when the current density varied from 10 to 30 mA cm­2 (data not shown).

400 300

60 40 20 0

0

50

100

150

200

250

Time (min)

200 100 0 0

50

100

150

200

250

Time (min) Fig. 5. Influence of sinapinic acid concentration on COD removal and current efficiency (inset) during oxidation on BDD anode. Conditions: current density 10 mA cm­2; flow rate 300 L h­1; T = 20 °C; pH = 4. SA concentration: (◊) 0.5 mM; (Δ) 1 mM (□) 1.5 mM.

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400 Ln(COD0/COD)

5

3 2 1 0

200

mass transport control. In all the conditions the decay of sinapinic acid concentration can be well described by a pseudo first-order kinetics (Fig. 7, inset). Apparent rate constants for the degradation of SA increased with flow rates and their values were 0.0072, 0.0162 and 0.0205 min­1 for flow rates of 100, 180 and 300 L h­1, respectively.

4

0

50

100

150

200

Time (min)

3.6. Effect of pH

100

0

50

100

150

200

Time (min) Fig. 6. Influence of temperature on the decay of COD during electro-oxidation of 1 mM sinapinic acid on BDD anode. Conditions: flow rate 300 L h­1; current density 10 mA cm­2; pH= 4. T: (Δ) 20 °C; (◊) 35 °C; (□) 50 °C. The inset shows the kinetic analysis for pseudofirst-order reaction.

In addition, the increase of COD removal rate with temperature can also be due to the electrogeneration of inorganic oxidizing agents. In fact, it has been demonstrated [35] that some peroxodisulphates can be formed in solutions containing sulphates during electrolysis with BDD electrodes (reaction 9): 2

2

2SO4 →S2 O8 þ 2e

ð9Þ

These reagents are known to be very powerful oxidants and they can act as a mediator for organic oxidation and the reaction rate between peroxodisulphates ions and organic compounds increases with temperature. 3.5. Effect of flow rate To confirm that the oxidation is under mass transport control, several experiments were performed at different flow rates of the solution into the electrochemical cell. As shown in Fig. 7, the variation from 100 to 300 L h­1 causes an increase of the COD removal rate indicating that, applying a current density of 10 mA cm­2, the electrochemical incineration with BDD is controlled by the rate at which organic molecules are transported from the bulk liquid to the electrode surface rather than by the rate at which electrons are delivered at the anode. In these conditions the electrolysis is carried out at a current higher than the limiting one and the process is under

300

400 2

300

1 0

100

200

300

400

Time (min)

100

100

100

3

0

200

00

For large industrial application it is also very important to estimate the treatment costs, and thus Fig. 9, reports the variation of specific energy consumption, calculated from Eq. (5), as function of COD removal, in the best operating conditions found previously (i.e. flow rate 300 L h­1; pH = 4; T = 50 °C; j = 10 mA cm­2). For low COD removal, the specific energy consumption increases almost linearly and later it has an exponential decrease. This behavior can be probably explained by the formation of more refractory products, such as carboxylic acids which are hardly oxidizable intermediates [40,41], and by the decrease of organic content in the solution. Although BDD oxidation is very effective, its energy consumption is relatively high for practical application as the only treatment process, but it could be used as refining technology in a two step process consisting in an electrochemical oxidation with BDD anode giving an effluent with a highly detoxified organic load, that may be treated by a post biological treatment.

4

Ln(COD0/COD)

COD (mg L-1)

400

3.7. Energy consumption

C.E. (%)

0

The effect of pH on the electrochemical oxidation of organics has been previously investigated by many authors [25,27,36]. Some authors reported that the oxidation process is more favorable in alkaline media [36,37]. In contrast, others indicated that the efficiency of the process was increased in acidic media [38,39]. According to this literature, it can be concluded that the effect of pH strongly depends on the nature of the investigated organics and of the supporting electrolyte. Therefore, the effect of pH on the degradation rate of sinapinic acid was studied at large pH range from acidic to basic. Aqueous solutions of SA (1 mM) were electrolysed at pH values of 2, 4, and 9 (Fig. 8). As can be seen from this figure, the pH of the medium slightly affects the degradation kinetics of SA and this indicates that the degradation of SA can be performed at any pH value between 2 and 9 without any significant loss in oxidation efficiency of the system (Fig. 8, inset). For this reason the electrochemical oxidation is a viable treatment for real effluents where adjustment of effluent's pH are not recommended.

200

300

400

Time (min) Fig. 7. Influence of flow rate on the decay of COD during electro-oxidation of 1 mM Sinapinic acid on BDD anode. Conditions: current density 10 mA cm­2; pH = 4; T = 20 °C. Flow rate: (Δ) 100 L h­1; (◊) 180 L h­1; (□) 300 L h­1. The inset shows the kinetic analysis for pseudo-first-order reaction.

COD (mg L-1)

COD (mg L-1)

300

151

80 60 40 20 0 0

200

50

100

150

200

Time (min)

100

0

0

50

100

150

200

Time (min) Fig. 8. Influence of the solution pH on the evolution of COD and CE (inset) during the electrolysis of 1 mM sinapinic acid. Conditions: current density 10 mA cm­2;. T = 20 °C. Flow rate: 300 L h­1. pH: (Δ) 2; (◊) 4; (□) 9.

152

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20

Ec (kWh m-3)

15

10

5

0

0

20

40

60

80

100

COD removal (%) Fig. 9. Evolution of specific energy consumption with COD removal during oxidation of 1 mM SA. Conditions: flow rate 300 L h­1; pH = 4; T = 50 °C; j = 10 mA cm­2.

4. Conclusion The electrochemical degradation of SA (from 0.5 to 1.5 mM) has been investigated using BDD anodes and a stainless steel cathode under all conditions tested involving, applied current density from 10 to 30 mA cm­2, liquid flow rate between 100 and 300 L h­1, temperature varying from 20 to 50 °C and for different pH values between 2 and 9. The experimental results allowed us to draw the following conclusions: − complete degradation of SA and COD removals higher than 97% were obtained within the investigated range regardless of current density, flow rate, temperature, pH and initial organic concentration, due to the formation of hydroxyl radicals from the water discharge; − in our experimental conditions, the oxidation was under mass transport control and the COD removal was well described by a pseudo-first-order kinetic; − the oxidation rate and the current efficiency were strongly affected by recirculation flow rate and temperature, while they are unaffected by solution pH; − an increase of the applied current favors slightly the COD removal, but it also results in a decrease of CE due to the enhancement of secondary reactions. This preliminary study suggest that anodic oxidation with BDD electrode constitutes an excellent method for the treatment of effluents contaminated with SA and related polyphenols. Acknowledgements The financial support of the MIUR project “Study on the anodic materials for the electrochemical production of oxidants for water disinfection” is gratefully acknowledged. References [1] S. Sayadi, R. Ellouze, Roles of Lignin Peroxidase and Manganese Peroxidase from Phanerochaete chrysosporium in the Decolorization of Olive Mill Wastewaters, Appl. Environ. Microbiol. 61 (1995) 1098–1103. [2] R. Capasso, A. Evidente, L. Schivo, G. Orru, M.A. Marcialis, G. Cristinzio, Antibacterial polyphenols from olive oil mill waste waters, J. Appl. Bacteriol. 79 (1995) 393–398. [3] R. Borja, S.E. Garrido, L. Martínez, A. Ramos-Cormenzana, A. Martín, Kinetic study of anaerobic digestion of olive mill wastewater previously fermented with Aspergillus terreus, Process Biochem. 28 (1993) 397–404. [4] J. Beltran De Heredia, J. Torregrosa, J.R. Dominguez, J.A. Peres, Kinetic model for phenolic compound oxidation by Fenton's reagent, Chemosphere 45 (2001) 85–90. [5] J.J. Pignatello, E. Oliveros, A. MacKay, Advanced oxidation processes for organic contaminant destruction based on the Fenton reaction and related chemistry. Critical Rev. Environ. Sci. Technol. 36 (2006) 1–84.

[6] E. Brillas, I. Sirés, M.A. Oturan, Electro-Fenton process and related electrochemical technologies based on Fenton's reaction chemistry, Chem. Rev. 109 (2009) 6570–6631. [7] M. Panizza, G. Cerisola, Direct and mediated anodic oxidation of organic pollutants, Chem. Rev. 109 (2009) 6541–6569. [8] C.A. Martinez-Huitle, E. Brillas, Decontamination of wastewaters containing synthetic organic dyes by electrochemical methods: a general review, Appl. Catal. B Environ. 87 (2009) 105–145. [9] P. Canizares, C. Saez, F. Martinez, M.A. Rodrigo, The role of the characteristics of p-Si BDD anodes on the efficiency of wastewater electro-oxidation processes, Electrochem. Solid State Lett. 11 (2008) E15–E19. [10] I. Sires, E. Guivarch, N. Oturan, M.A. Oturan, Efficient removal of triphenylmethane dyes from aqueous medium by in situ electrogenerated Fenton's reagent at carbon-felt cathode, Chemosphere 72 (2008) 592–600. [11] M.A. Oturan, E. Guivarch, N. Oturan, I. Sires, Oxidation pathways of malachite green by Fe3+-catalyzed electro-Fenton process, Appl. Catal. B Environ. 82 (2008) 244–254. [12] M. Panizza, G. Cerisola, Electro-Fenton degradation of synthetic dyes, Water Res. 43 (2009) 339–344. [13] A.G. Vlyssides, M. Loizidou, P.K. Karlis, A.A. Zorpas, D. Papaioannou, Electrochemical oxidation of a textile dye wastewater using a Pt/Ti electrode, J. Hazard. Mater. 70 (1999) 41–52. [14] O. Simond, V. Schaller, C. Comninellis, Theoretical model for the anodic oxidation of organics on metal oxide electrodes, Electrochim. Acta 42 (1997) 2009–2012. [15] Y. Samet, S.C. Elaoud, S. Ammar, R. Abdelhedi, Electrochemical degradation of 4-chloroguaiacol for wastewater treatment using PbO2 anodes, J. Hazard. Mater. 138 (2006) 614–619. [16] R. Cossu, A.M. Polcaro, M.C. Lavagnolo, M. Mascia, S. Palmas, F. Renoldi, Electrochemical treatment of landfill leachate: oxidation at Ti/PbO2 and Ti/SnO2 anodes, Environ. Sci. Technol. 32 (1998) 3570–3573. [17] M. Panizza, G. Cerisola, Application of diamond electrodes to electrochemical processes, Electrochim. Acta 51 (2005) 191–199. [18] M. Panizza, G. Cerisola, Electrocatalytic materials for the electrochemical oxidation of synthetic dyes, Appl. Catal. B Environ. 75 (2007) 95–101. [19] E. Hmani, S. Chaabane Elaoud, Y. Samet, R. Abdelhedi, Electrochemical degradation of waters containing O-Toluidine on PbO2 and BDD anodes, J. Hazard. Mater. 170 (2009) 928–933. [20] P. Canizares, J. Garcia-Gomez, C. Saez, M.A. Rodrigo, Electrochemical oxidation of several chlorophenols on diamond electrodes. Part I. Reaction mechanism, J. Appl. Electrochem. 33 (2003) 917–927. [21] L. Gherardini, P.A. Michaud, M. Panizza, C. Comninellis, N. Vatistas, Electrochemical oxidation of 4-chlorophenol for wastewater treatment. Definition of normalized current efficiency, J. Electrochem. Soc. 148 (2001) D78–D82. [22] M. Panizza, G. Cerisola, Electrochemical degradation of methyl red using BDD and PbO2 anodes, Ind. Eng. Chem. Res. 47 (2008) 6816–6820. [23] J. Rodriguez, M.A. Rodrigo, M. Panizza, G. Cerisola, Electrochemical oxidation of Acid Yellow 1 using diamond anode, J. Appl. Electrochem. 39 (2009) 2285–2289. [24] C. Saez, M. Panizza, M.A. Rodrigo, G. Cerisola, Electrochemical incineration of dyes using a boron-doped diamond anode, J. Chem. Technol. Biotechnol. 82 (2007) 575–581. [25] A.M. Polcaro, A. Vacca, M. Mascia, S. Palmas, Oxidation at boron doped diamond electrodes: an effective method to mineralise triazines, Electrochim. Acta 50 (2005) 1841–1847. [26] A. Ozcan, Y. Sahin, A.S. Koparal, M.A. Oturan, Propham mineralization in aqueous medium by anodic oxidation using boron-doped diamond anode: influence of experimental parameters on degradation kinetics and mineralization efficiency, Water Res. 42 (2008) 2889–2898. [27] E. Brillas, I. Sires, C. Arias, P.L. Cabot, F. Centellas, R.M. Rodriguez, J.A. Garrido, Mineralization of paracetamol in aqueous medium by anodic oxidation with a boron-doped diamond electrode, Chemosphere 58 (2005) 399–406. [28] M. Beccari, F. Bonemazzi, M. Majone, C. Riccardi, Interaction between acidogenesis and methanogenesis in the anaerobic treatment of olive oil mill effluents, Water Res. 30 (1996) 183–189. [29] R. Capasso, G. Cristinzio, A. Evidente, F. Scognamiglio, Isolation, spectroscopy and selective phytotoxic effects of polyphenols from vegetable waste waters, Phytochemistry 31 (1992) 4125–4128. [30] M. Jurkova, M. Wurst, Biodegradation of aromatic carboxylic acids by Pseudomonas mira, FEMS Microbiol. Lett. 111 (1993) 245–250. [31] J.K. Gupta, C.J. Jebsen, H. Kneifel, Sinapic acid degradation by the yeast Rhodotorula glutinis, J. Gen. Microbiol. 132 (1986) 2793–2799. [32] M. DellaGreca, G. Pinto, A. Pollio, L. Previtera, F. Temussi, Biotransformation of sinapic acid by the green algae Stichococcus bacillaris 155LTAP and Ankistrodesmus braunii C202.7a, Tetrahedron Lett. 44 (2003) 2779–2780. [33] X. Chen, G. Chen, Anodic oxidation of Orange II on Ti/BDD electrode: variable effects, Sep. Purif. Technol. 48 (2006) 45–49. [34] N.B. Tahar, A. Savall, Mechanistic aspects of phenol electrochemical degradation by oxidation on a Ta/PbO2 anode, J. Electrochem. Soc. 145 (1998) 3427–3434. [35] M. Panizza, P.A. Michaud, G. Cerisola, C. Comninellis, Anodic oxidation of 2naphthol at boron-doped diamond electrodes, J. Electroanal. Chem. 507 (2001) 206–214. [36] P. Canizares, J. Lobato, R. Paz, M.A. Rodrigo, C. Saez, Electrochemical oxidation of phenolic wastes with boron-doped diamond anodes, Water Res. 39 (2005) 2687–2703.

S.C. Elaoud et al. / Desalination 272 (2011) 148–153 [37] G. Lissens, J. Pieters, M. Verhaege, L. Pinoy, W. Verstraete, Electrochemical degradation of surfactants by intermediates of water discharge at carbon-based electrodes, Electrochim. Acta 48 (2003) 1655–1663. [38] C.A. Martinez-Huitle, S. Ferro, A. De Battisti, Electrochemical incineration in the presence of halides, Electrochem. Solid State Lett. 8 (2005) D35–D39. [39] O. Scialdone, A. Galia, C. Guarisco, S. Randazzo, G. Filardo, Electrochemical incineration of oxalic acid at boron doped diamond anodes: role of operative parameters, Electrochim. Acta 53 (2008) 2095–2108.

153

[40] C. Flox, C. Arias, E. Brillas, A. Savall, K. Groenen-Serrano, Electrochemical incineration of cresols: a comparative study between PbO2 and boron-doped diamond anodes, Chemosphere 74 (2009) 1340–1347. [41] I. Sires, E. Brillas, G. Cerisola, M. Panizza, Comparative depollution of mecoprop aqueous solutions by electrochemical incineration using BDD and PbO2 as high oxidation power anodes, J. Electroanal. Chem. 613 (2008) 151–159.