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Mineralization of chlorobenzene in aqueous medium by anodic oxidation and electro-Fenton processes using Pt or BDD anode and carbon felt cathode
Mineralization of chlorobenzene in aqueous medium by anodic oxidation and electro-Fenton processes using Pt or BDD anode and carbon felt cathode Hicham Zazou, Nihal Oturan, Mutlu S¨onmez-C ¸ elebi, Mohamed Hamdani, Mehmet A. Oturan PII: DOI: Reference:
S1572-6657(16)30215-6 doi: 10.1016/j.jelechem.2016.04.051 JEAC 2629
To appear in:
Journal of Electroanalytical Chemistry
Received date: Revised date: Accepted date:
16 March 2016 27 April 2016 28 April 2016
Please cite this article as: Hicham Zazou, Nihal Oturan, Mutlu S¨ onmez-C ¸ elebi, Mohamed Hamdani, Mehmet A. Oturan, Mineralization of chlorobenzene in aqueous medium by anodic oxidation and electro-Fenton processes using Pt or BDD anode and carbon felt cathode, Journal of Electroanalytical Chemistry (2016), doi: 10.1016/j.jelechem.2016.04.051
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ACCEPTED MANUSCRIPT Mineralization of chlorobenzene in aqueous medium by anodic oxidation and electro-Fenton processes using Pt or BDD anode
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and carbon felt cathode
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Hicham Zazou1, 2, Nihal Oturan1, Mutlu Sönmez-Çelebi1,3, Mohamed Hamdani2, Mehmet A. Oturan1
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Université Ibn Zohr d'Agadir, Faculté des Sciences, BP 8106, Cité Dakhla, Agadir Morocco
University of Ordu, Faculty of Science and Arts, Department of Chemistry, 52200 Ordu, Turkey
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Université Paris-Est, Laboratoire Géomatériaux et Environnement (LGE), EA 4506, UPEM, 77454 Marne-la-Vallée, France.
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Paper submitted to Journal of Electroanalytical Chemistry for publication
* Corresponding author: Email: [email protected] (Mehmet A. Oturan) Phone: +33 149 32 90 65
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ACCEPTED MANUSCRIPT
ACCEPTED MANUSCRIPT Abstract This study focuses on the in situ destruction of pesticide chlorobenzene (CB) by using two
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electrochemical advanced oxidation processes, namely anodic oxidation and electro-Fenton.
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Influence of several operating parameters such as applied current, catalyst concentration and
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supporting electrolytes was assessed to optimize oxidation and mineralization of CB. Kinetics study showed that CB is quickly oxidized by •OH following a pseudo first-order reaction kinetics. The rate constant of oxidative degradation of CB by hydroxyl radicals was
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determined by competition kinetics method and found to be 4.35 × 109 M-1 s-1. The quasi
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complete mineralization (95% TOC removal) of 0.1 mM CB aqueous solution was achieved at 4 h treatment. Formation and evolution of aromatic and aliphatic (short-chain carboxylic acids) intermediates during treatment were monitored by HPLC analysis and a mineralization
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pathway was proposed. The results obtained highlight the great efficiency of electro-Fenton process in effective destruction of a very persistent pollutant such as CB that can be
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extrapolated to other toxic/persistent organic pollutants.
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Keywords: Chlorobenzene; Electro-Fenton; Anodic Oxidation; Hydroxyl radicals; Kinetics, Mineralization; TOC removal
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ACCEPTED MANUSCRIPT 1. Introduction The intensive use of pesticides in the environment constitutes a threat to living beings [1–3].
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Chlorobenzene (CB) is one of the most common organic pollutants in industrial wastewaters
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[4] since it is involved in the synthesis of many chemicals, particularly in the field of
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pesticides [5]. Therefore, CB is considered as a hazardous waste and a prior toxic pollutant by the U.S. Environmental Protection Agency [4]. Chlorinated aromatic compounds are
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considered as the most problematic categories of environmental pollutants because they are mostly non-biodegradable or very slowly degradable by microorganisms [6]. It is therefore
methods to remove them from water.
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important to assess the fate of these compounds in the environment and develop effective
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Recently, there is great interest in the remediation of waters containing toxic and organic
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pollutants with different methods such as chemical, photochemical, photocatalytical and
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electrochemical processes [7–11]. In this work we focus on electrochemical advanced oxidation processes (EAOPs) like electro-Fenton (EF) and anodic oxidation (AO) for removal of CB from water. These processes enable efficient degradation of persistent organic
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pollutants in aqueous medium by in situ generating highly reactive oxidizing agents such as hydroxyl radicals (•OH), which are able to oxidize efficiently almost all organic contaminants [11–17]. This radical is the second most strong oxidant known, after fluorine, having a very high standard potential (E°(•OH/H2O) = 2.80 V/ SHE) that makes it able to non-selectively react with organics to give hydroxylated or dehydrogenated derivatives until their mineralization (transformation into CO2, water and inorganic ions) [18–22]. One of the most popular EAOPs is the EF process based on the continuous supply of H2O2 from reaction (1) [23-25]. Formation of homogeneous •OH starts via Fenton reaction (2) once a catalytic amount of ferrous iron salt added to the solution [2, 23, 26]. The process is electrocatalytic since Fe2+ ions is electro-regenerated according to the reaction (3) from ferric iron produced 3
ACCEPTED MANUSCRIPT by reaction (2) [20, 27, 28]: (1)
H2O2 + Fe2+ → Fe3+ + HO– + •OH
(2)
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O2 + 2H+ + 2e– → H2O2
(3)
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Fe3+ + e– → Fe2+
Other popular EAOP is the AO process in which organics pollutants in a contaminated
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solution are oxidized by direct charge transfer on the anode (M) from heterogeneous hydroxyl radicals (M(•OH)) formed from oxidation of water (reaction (4)) [29-31]. The availability and
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efficiency of these radicals is depending to the anode material (M) [30, 31]. The boron-doped diamond (BDD) anode appears as the most effective anode because of the high O2 evolution
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overpotential and physisorption on the surface [30-34]. M + H2O → M(•OH) + H+ + e-
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(4)
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The use of a high overpotential anode material like BDD in EF process enhances strongly the efficiency of the process since this operation is consisting of a coupling between EF and AO
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processes: •OH and BDD(•OH) are generated simultaneously [35]. Therefore the oxidative degradation of CB was carried out by EF comparatively with AO. The electrolyses were carried out with both anode materials: Pt and BDD. A three-dimensional electrode material, carbon felt, was used as the cathode in all experiments [35, 36]. The influence of the applied current and the kind of supporting electrolytes on the effectiveness of the treatment were evaluated. The degradation kinetics was followed by high performance liquid chromatography (HPLC). The mineralization efficiency was studied for the two processes following the evolution of the total organic carbon (TOC) content. Identification of aromatic intermediates, short-chain carboxylic acids and TOC results enabled us to propose a plausible mineralization pathway for mineralization of CB by hydroxyl radicals. Although the oxidative degradation of
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ACCEPTED MANUSCRIPT CB has been carried out already by Liu et al. [37] and Hsiao and Kobe [38], the present work provides new data on the degradation and mineralization of CB. Hsiao and Kobe reported, in
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a pioneer work the oxidation of CB in a packed bed flow reactor by electrochemically
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generated Fenton's reagent with a low degradation (40% in 40 min) rate. More recently Liu et
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al. [37] gave a more interesting work investigating the electro-oxidation of CB by AO using Pt and BDD anodes. A degradation kinetic model was developed and identification of number aromatic intermediates was carried out. The present study aims to complete the work of Liu et
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al. and Hsiao and Kobe investigating the oxidation and mineralization of CB by electro-
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Fenton process and elucidating the mineralization kinetics through TOC removal measurements as a function of electrode material, current applied. And supporting electrolyte
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used. Results obtained in this study were then compared with those reported in [37] and [38].
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2.1 Chemicals
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2. Experimental
Chlorobenzene (CB, C6H5Cl) was obtained from sigma-Aldrich. Iron (II) sulfate heptahydrate
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used as Fe2+ (catalyst) source was analytical grade from Fluka. Hydroxybenzoic acid was obtained from Merck. Acetic, maleic, oxalic, fumaric, formic, succinic and malonic acids were of analytical grade and supplied by Fluka. Anhydrous sodium sulfate and sodium chlorate used as supporting electrolytes were obtained by Across. The pH was adjusted with analytical grade sulfuric acid purchased from ACS reagent Across. Sodium nitrate (Merck), ammonium nitrate and sodium phosphate (Aldrich) were used as standards for ionic chromatography (IC) analysis. Organic solvents and other chemicals used were either of HPLC or analytical grade purchased from Merck and Fluka. 2.2. Procedures and equipment
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ACCEPTED MANUSCRIPT All electrolyses were conducted in an open and undivided cylindrical glass cell of 250 mL capacity, equipped with two electrodes. The working electrode was a 70 cm2 (14 cm x 5 cm)
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piece of carbon-felt (from Mersen, France), placed on the inner wall of the cell covering the
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internal perimeter. The counter electrode was cylindrical Pt grid or BDD plate (6 cm x 4 cm)
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(from CONDIAS GmbH, Germany) placed at the center of the cell. Prior to the electrolysis, compressed air was bubbled through the aqueous solutions, which were stirred continuously by magnetic stirrer (500 rpm). A catalytic quantity of ferrous ion (0.1 mM) was introduced
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into the solution before the beginning of electrolysis. The supporting electrolytes were
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Na2SO4 (50 mM) and NaCl (100 mM). Different currents intensities were applied in the degradation and the mineralization experiments, these currents were measured and displayed continuously throughout electrolysis using a DC power supply (HAMEG Instruments, HM
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8040-3). Before starting electro-oxidation treatment, the initial solutions were adjusted to pH
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3.0 with analytical grade 1 M sulfuric acid.
2.3. Instruments and product analysis procedures
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The solution pH was measured with a glass electrode calibrated with standard buffers at pH values of 4, 7 and 10. The concentration decay of CB was monitored by HPLC using a Merck-Hitachi Lachrom chromatograph equipped with a diode array UV-Vis detector (model L-7455) fitted with a reverse phase purospher RP-18, 5µm, 4,6 × 250 mm column purchased from Merck (France). The column was eluted at isocratic mode for all experiments with a mobile phase composed of 70:30 (v/v) methanol/water at a flow rate of 0.8 mL min-1. The column was thermostated at 40 °C. Detection was performed at 230 nm for kinetic studies. The short-chain carboxylic acids were identified and quantified using an ion-exclusion column (Supelcogel H column; ɸ = 7, 8 mm × 300 mm) with a mobile phase of 4 mM H2SO4 at 210 nm. Inorganic ions were analyzed by ionic chromatography from Thermo-Fisher
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ACCEPTED MANUSCRIPT (Dionex-100 equipped with a conductivity detector). An anionic exchanger column (IonPac AS14-Dionex) was used for Cl- analysis. The TOC content of initial CB solutions and its
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evolution during electrolysis was performed at pH 3. The CB samples were filtered (0.22 µm)
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and acidified with HCl (1% HCl 2 mM). The injection volumes were 50 µL. TOC values were
efficiency (MCE) was estimated as follows [23, 36]: 𝑛 𝐹 𝑉𝑆 ∆(𝑇𝑂𝐶)𝑒𝑥𝑝 4.32 × 107 𝑚 𝐼 𝑡
x 100
(5)
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MCE (%) =
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determined with Shimadzu VCSH TOC analyzer. From these data, the mineralization current
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where F is the Faraday constant (96487 C mol-1), VS is the solution volume, ∆(TOC)exp is the experimental TOC change at a given time (mg L-1), 4.32×107 is an homogenization factor
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(3600 s h-1 ×12000 mg mol-1), m is the number of carbon atoms of CB (6 atoms), I is the
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applied current (A) and t is the electrolysis time (h). The number of electrons (n) consumed per mole of CB was taken as 28 considering total mineralization according to the following
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reaction [2, 18, 37]:
(6)
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C6H5Cl + 12H2O → 6CO2 + 29H+ + Cl- + 28e-
3. Results and discussion
3.1. Effect of experimental parameters on oxidative degradation of MC The performance of electrode pairs Pt/carbon felt and BDD/carbon felt was tested for EF and AO processes. The efficiency of the anodes was analyzed and compared in the same conditions in order to optimize the degradation of MC solutions. All experiments were carried out with 230 mL of 0.1 mM MC solutions at pH 3.0. The decay kinetics of MC concentration with the generated oxidant (•OH)/M(•OH) was studied at different current values from 50 to
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ACCEPTED MANUSCRIPT 500 mA, using 50 mM of Na2SO4 or 100 mM of NaCl and 0.1 mM of catalyst (Fe2+)
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Figure 1: Effect of current intensity on the decay of CB concentration (C0=0.1 mM) in 230 mL aqueous solution with 50 mM Na2SO4 or 100 mM NaCl (and 0.1 mM Fe
2+
for EF), using
Pt/carbon felt and BDD/carbon cells. Currents intensity: (□): 50 mA; (♦): 100 mA; (Δ): 300 mA; (●): 500 mA. Method: (a) EF-Pt-Na2SO4; (b) EF-BDD-Na2SO4; (c) EF-Pt-NaCl; (d) EF-BDDNaCl; (e) AO-Pt-Na2SO4; (f) AO-BDD-Na2SO4; (g) AO-Pt-NaCl; (h) AO-BDD-NaCl. The inset panels show the corresponding kinetic analysis assuming that CB follows a pseudo-first-order reaction.
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ACCEPTED MANUSCRIPT Figure 1 shows the effect of the current applied to the profile of the kinetic curves of the oxidation of CB by •OH/M(•OH). The electrolysis time required for complete disappearance
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of CB is becoming shorter when the intensity of current applied is higher. These results can be
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explained by the increased rate of electrochemical reactions (1), (3) and (4) favoring
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production of high amount of •OH/M(•OH) leading to the quick oxidation of CB. Figures 1-(a) to 1-(d) show the efficiency of the oxidative degradation of CB by EF process with Pt and BDD anodes using sodium sulfate (Na2SO4) or sodium chloride (NaCl) salts as supporting
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electrolyte. In the case of Na2SO4 the complete disappearance of CB is attained after 10 and
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12 min with Pt and BDD anode, respectively, at I = 500 mA. However this time is as slow as 30 min for both anodes in the case of NaCl under same operating conditions. On the other hand, it is noticed that the positive effect of current rise becomes non-significant for currents
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above 300 mA. This phenomenon can be explained by the progressive enhancement of the rate of side reactions such as the reduction of H2O2 and evolution of H2 at the cathode [18, 23]
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and evolution of O2 at the anode. The decay of CB concentration exhibits almost identical shapes with both anodes but with significantly longer electrolysis times. Figures 1(e) to 1(h)
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show the degradation of the CB by AO process. The kinetic of concentration decay is slower than EF process with a better efficiency for BDD anode in Na2SO4 medium. Similarly, degradation kinetics is accelerated by increasing the current applied from 50 to 500 mA.
3.2. Evaluation of apparent and absolute rate constants The values of apparent rate constants for oxidative degradation of CB (0.1 mM, 230 mL) by EF and AO treatment under different operating conditions( (nature of anode material, current intensity applied and supporting electrolyte) are given in Table.2. These values are determined following pseudo-first order reaction kinetics (shown in insert panels of Figure 1). This reaction order is justified by the very short life time of hydroxyl radicals allowing the
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ACCEPTED MANUSCRIPT application of steady state approximation for the concentration of hydroxyl radicals and the exponential decay of CB concentration [18, 23].
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Table 1: Pseudo-first-order rate constants for oxidative degradation CB (C0 = 0.1 mM) by
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electrochemical treatment with different supporting electrolytes (50 mM Na2SO4 or 100 mM
Electrode
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Method
1.84
0.99
100
2.39
0.99
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3.27
0.99
500
3.78
0.99
NaCl
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0.88
0.97
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NaCl) at pH 3.0 and room temperature under different current intensities.
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1.08
0.96
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1.50
0.98
500
1.75
0.97
50
1.67
0.99
100
2.26
0.99
300
3.26
0.99
500
3.51
0.99
50
0.51
0.99
100
0.93
0.99
300
1.49
0.99
500
1.89
0.99
50
0.39
0.99
100
0.59
0.99
300
0.95
0.99
500
1.05
0.98
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0.08
0.99
Supporting electrolyte
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50
Na2SO4
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BDD
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Pt
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Na2SO4
NaCl
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kapp×10-1
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I (mA)
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300
0.13
0.97
500
0.14
0.97
50
0.91
0.99
100
1.25
0.99
1.67
0.99
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2.03
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0.59
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0.96
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0.96
0.99
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1.18
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Comparing these kapp values for oxidative degradation of CB by both processes, it can be concluded that the oxidation efficiency is also affected by the supporting electrolyte and the
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nature of the anode material used. BDD and Pt anodes provide almost similar oxidation efficiencies in both processes while the former anode supplies better rate values in AO
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process. The kapp are significantly low in the case of AO when using NaCl as supporting electrolyte (Table 1). This behavior can be explained by oxidation of chloride ion to Cl2 and chlorinated ions (reactions 7-11) as reported by Rocha et al [39]. These reactions consume hydroxyl radicals hinder degradation efficiency of CB. 2Cl− → Cl2 + 2e−
(7)
Cl− + •OH → ClO− + H+ + e−
(8)
ClO−+ •OH → ClO2− + H+ + e−
(9)
ClO2−+ •OH → ClO3− + H+ + e−
(10)
ClO3− + •OH → ClO4− + H+ + e−
(11)
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Na2SO4 as supporting electrolyte. Finally the general efficiency trend is as following: EF-Pt-
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Na2SO4 > EF-BDD-Na2SO4 > AO-BDD-Na2SO4 > EF-Pt-NaCl > EF-BDD-NaCl > AO-
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BDD-NaCl > AO-Pt-Na2SO4 > AO-Pt-NaCl. This trend is in agreement with those reported already [13, 23, 24].
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The absolute (or second order) rate constant of reaction between CB and hydroxyl radicals was determined by competition kinetics method [23, 40, 41] using para-hydroxybenzoic acid
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(p-HBA) as standard competitor for which the absolute rate constant of reaction with hydroxyl radicals (kp-HBA) is known as 2.19 × 109 M-1 s-1 [42]. For this purpose, competitive
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kinetic experiments were performed in the presence of equal amounts of CB and p-HBA
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assuming pseudo first order kinetic for reactions of both CB and p-HBA with hydroxyl
equation (12). [CB]0 [CB]t
)=k
kCB
p−HBA
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Ln (
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radicals under experimental conditions of Figure 2 and kCB was determined according to
[p−HBA]0
Ln (
[p−HBA]t
)
(12)
where [CB]0 and [CB]t denote the concentration of CB at time zero and at any time of the electrolysis, respectively, and [p-HBA]0 and [p-HBA]t denote the concentration of p-HBA at time zero and at any time of the electrolysis, respectively. These concentrations were monitored by HPLC analysis during constant current electrolysis. The absolute rate constant kCB was then determined from the slope of the straight line obtained by plotting ln ([CB]0/[CB]t) = f (ln ([p-HBA]0/[p-HBA]t) [43] and found to be 4.35 × 109 M-1 s-1. This value is close to that reported by Buxton et al. [44] (4.5 × 109 M-1 s-1) determined from pulse radiolysis method.
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Figure 2: The ln C0/Ct values of CB and p-HBA as a function of time for the pseudo-first order degradation kinetics of CB and p-HBA in EF-Pt process under following operating conditions: 2+
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[CB]0 = [p-HBA]0 = 0.1 mM; V = 230 mL; [Fe ] = 0.1 mM; pH = 3; [Na2SO4] = 50 mM and I = 50
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3.3. Mineralization of CB aqueous solution
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All the CB solutions treated by EF and AO process with Pt and BDD anodes underwent progressive mineralization with increasing electrolysis time. Reproducible TOC removal values were obtained under all experimental conductions. Figure 3 shows the comparative TOC decay kinetics for the treatment of 0.2 mM CB solution (corresponding to 14.5 mg L-1 initial TOC value) containing 50 mM Na2SO4 or 100 mM NaCl as supporting electrolyte at pH 3.0 and room temperature for 4 h. The pH remained practically unchanged during these trials reaching a final value close to 2.8 for EF process. As can be seen, a more rapid and continuous TOC removal was achieved with EF compared to AO treatment. For instance, an almost complete mineralization (>95% TOC removal) was obtained with BDD anode at 500 mA after 4 h treatment (Figure 3b). Pt anode
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ACCEPTED MANUSCRIPT reached 80% TOC removal efficiency under same conditions (Figure 3a). Conversely a poor mineralization degree was attained in AO with Pt anode while a significantly better mineralization efficiency (89.5) was reached with AO-BDD-Na2SO4. The lower
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mineralization ability of Pt anode can be explained by the small Pt(•OH) concentration formed
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on the Pt surface (in relation with the low O2 evolution overpotential) from reaction (4) and their chemisorption on the Pt surface inversely to the BDD(OH) that are physisorbed [23, 40]. In contrast, related to the surface properties, BDD has a much higher oxidative ability. At a
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current intensity greater than 500 mA, No improvement was observed for mineralization
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efficiency for current intensity greater than 500 mA (Figure 3).
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Figure 3: TOC removal with electrolysis time for 230 mL 0.2 mM CB solution (TOC0 = 14.5 mg L )) containing 50 mM Na2SO4 or 100 mM NaCl (and 0.1 mM Fe
2+
for EF), using Pt/carbon felt or ..
..
BDD/carbon felt cells. Currents intensity: (-♦-): 100 mA; (-Δ-): 300 mA; (-●-): 500 mA and ( ): 700 mA. Method: (a): EF-Pt-Na2SO4, (b): EF-BDD-Na2SO4, (c): EF-Pt-NaCl, (d): EF-BDD-NaCl, (e): AO-Pt-Na2SO4, (f): AO-BDD-Na2SO4; (g): AO-Pt-NaCl and (h): AO-BDD-NaCl.
In general, the TOC removal rate is high during the first 2 h (Figure 3) due to the relatively easy oxidative degradation of CB and its aromatic intermediates from M(OH) in AO and
OH/M(OH) in EF processes [18] leading to the formation of aliphatic compounds
(carboxylic acids) that are resistant to mineralization by these radicals. Therefore TOC removal rate becomes poor after 2 h treatment in both processes except in the case of AO-
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ACCEPTED MANUSCRIPT BDD-Na2SO4 (Figure 3f) since BDD(OH) has relatively high oxidation power compared to
OH and accordingly is able to mineralize more efficiently short-chain carboxylic acids [45].
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The results of Figure 3 show that the treatments with Na2SO4 are significantly more efficient
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compared to NaCl which is a rather limited system (Table 2). The formation of the
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peroxodisulfate ion, a relatively strong oxidant, has been suggested by Serrano et al. [46] in a sulfate medium (reaction 13). The low mineralization efficiency of EF and AO treatments in
NU
chloride medium can be explained by oxidation of this ion to chlorine (reaction 7) and then the related side reactions (14-15) playing a negative role on the mineralization efficiency [2,
MA
23, 30, 38]. Therefore, the amount of current consumed for oxidation of chloride ion (reaction 7) and the wasting reactions of chlorinated species taking place in the vicinity of anode
D
surface that consume OH (reactions 8-11) hamper the mineralization efficiency [40, 46]. This
TE
effect is prominent in the case of AO with Pt anode Fig. 3g) since its oxidation power is
CE P
significantly lower than BDD anode.
(13)
Cl2 + H2O → HOCl + H+ + Cl−
(14)
AC
2SO42- → S2O82- + 2e−
HOCl → H+ + ClO−
(15)
On the other hand, in the case of EF-Pt-Na2SO4 and EF-BDD-Na2SO4, the MCE values were obtained for the lowest current. This setting is dropped with the electrolysis time (Figure 4).
17
ACCEPTED MANUSCRIPT
Table 2. TOC removal (mineralization) degree and evolution of MCE after 1 and 4 h of treatment
BDD
Na2SO4
Na2SO4
MCE
100
15.5
29.0
51.8
4.1
300
40.0
7.7
75.0
0.7
500
52.0
3.2
80.0
0.3
700
49.0
2.5
75.0
0.3
55.0
15.0
90.0
0.9
66.0
3.9
92.0
0.2
500
75.0
1.7
95.0
0.1
700
63.0
1.8
93.0
0.1
300
12.0
10.0
27.0
2.0
500
21.0
5.0
33.0
1.1
100
28.8
24.5
58.7
3.6
300
43.0
6.5
68.2
0.9
500
45.0
3.5
72.2
0.4
700
43.4
2.8
71.6
0.3
100
-
-
-
-
300
17.3
4.3
21.6
1.0
500
25.3
2.3
38.0
0.5
700
29.8
1.6
41.8
0.3
300
8.7
10.2
16.6
2.3
500
11.6
5.9
20
1.3
100
38.8
9.4
78.7
0.8
300
44.7
2.9
85.0
0.2
500
47.2
1.6
89.2
0.2
700
42.7
1.3
84.8
0.2
100
31.6
10.6
54.6
1.8
300
44.4
2.9
62.4
0.3
500
48.6
1.5
64.7
0.3
100
Pt
Na2SO4
AC
AO
NaCl
CE P
BDD
NaCl
BDD
BDD
D
TE
NaCl
Na2SO4
NaCl
After 4h of treatment
% TOC removal
300
Pt
After 1h of treatment
IP
I (mA)
SC R
Pt
Supporting electrolyte
NU
EF
Electrode
MA
Method
T
with EF and AO processes under the experimental conditions described in Figure 3.
18
% TOC removal
MCE
ACCEPTED MANUSCRIPT 700
38.3
1.4
59.8
0.2
T
The influence of operating parameters such as the nature of anode used, current intensity
IP
applied and kind of supporting electrolyte is depicted in Table 2. This table presents %TOC
SC R
removal and the MCE values calculated from data of Figure 3 according to the equation (5) after 1 h and 4 h treatment. Better MCE values are obtained from EF-Pt-Na2SO4 and EF-
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BDD-Na2SO4 cells in agreement with results discussed above. In all cases, %MCE gradually decreases with increasing current intensity and electrolysis time. This trend may be related to
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the gradual decrease in the concentration of CB and accumulation of intermediate products more difficult to oxidize such as carboxylic acids [47]. The acceleration of the rate of side
D
reactions (H2 evolution at cathode, O2 evolution at anode and destruction of H2O2 at both
TE
electrodes) constitutes another reason of poor %MCE values on longer treatment times. The results obtained highlight the great mineralization efficiency (75% and 95%) of the EF-BDD-
CE P
Na2SO4 cell due to the simultaneous generation of •OH (in the bulk) and BDD(•OH) on the anode surface. Concerning MCE better results are obtained at lower current intensity and
AC
shorter electrolysis times in agreement with already reported studies [18, 23, 30, 48].
3.4. Identification of reaction intermediates and mineralization pathway The degradation of the toxic/persistent organic compounds by advanced oxidation processes can produce aromatic intermediates more toxic than initial compounds with negative effect on the environment [40, 49]. Thus, the identification of reaction intermediates during electrochemical advanced oxidation processes is important. This identification is also useful in order to be able to propose a mineralization pathway by the attack of hydroxyl radicals on target pollutant. Therefore we attempted to identify and quantify intermediate products formed during degradation of CB by hydroxyl radicals generated in EF process with Pt anode.
19
ACCEPTED MANUSCRIPT This identification was performed by HPLC analysis by comparison of retention times (tR) of formed intermediates with those of authentic products. Experiments were conducted at a low
T
current (60 mA) in order to favor the accumulation of hydroxylation reaction intermediates.
IP
Even under this low-current condition, the total oxidation of CB and its aromatic by products
SC R
occurs in less than 120 min (Figure 4).
0.25
(a)
NU MA
0.15 0.1
0
D
0.05
TE
Concentration / mM
0.2
10
20
30
40
50
60
Time / min
CE P
0
AC
Concentration / mM
0.012
(b)
0.01
0.008 0.006 0.004 0.002 0 0
20
40
60
80
100
120
Time / min
Figure 4: Concentration decay of CB (a) and formation and evolution of the oxidation reaction intermediates (b) generated during EF-Pt process. Intermediates: CB (×); Hydroquinone (■); Benzoquinone (□); Catechol (●); Phenol (◊); 2-Chlorophenol (▲) and 4-Chlorophenol (). I = 2+
60 mA; [Na2SO4] = 50 mM; [Fe ] = 0.1 mM; V = 230 mL; pH=3.
20
ACCEPTED MANUSCRIPT All the intermediates are formed from the beginning of the treatment. Most of these intermediates are also reported by Liu et al. [37] except the phenol, 2-chlorophenol and 4-
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chlorophenol that were identified in this study. Besides the 1,3-dihydroxybenzene was not
IP
detected in this the present study. In addition, Hsiao and Kobe [39] have already identified the
SC R
formation of 4-chlorophenol.
The main reaction is the electrophilic addition of OH on the benzene ring leading to the
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formation of 2-chlorophenol and 4-chlorophenol. The phenol is formed by ipso attack of OH on the position of chlorine atom conducting to its substitution. The two chlorophenols
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undertake same substitution reaction to form catechol and hydroquinone. The latter intermediate is then oxidized into benzoquinone. This reaction can take place whether by
D
electron transfer at the anode or by OH in the bulk of solution. The total disappearance of the
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CB and intermediates is achieved after 120 min of electrolysis.
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It is well known that polyhydroxylated products and quinones are not stable toward the attack of OH/M(OH) and undertake oxidative ring opening reactions to lead to the formation of
AC
short-chain carboxylic acids [18, 23, 38, 40]. Formation and evolution of carboxylic acids generated during the mineralization of CB during EF treatment with Pt and BDD anodes were depicted in Figure 5. Four main carboxylic acids (oxalic, malonic, acetic and formic) are formed from the beginning of the electrolysis. Formic and fumaric acids are detected in trace level; they are not accumulated or formed at low concentration. The comparison between Pt and BDD anodes is very noticeable: Carboxylic acids are mineralized quickly with BDD anode (Figure 5b) while they are very slowly oxidized in the case of Pt anode leading to their accumulation. This situation was already highlighted by Liu et al. [38] for formic, maleic (not detected in this study) and oxalic acids. All of four carboxylic acids are remained at the end of 4 h treatment in the latter case (Figure 5a) whereas only oxalic and acetic acids are present at
21
ACCEPTED MANUSCRIPT low concentrations in the former case. These results are in agreement with data concerning the mineralization efficiency (in terms of TOC removal percentage) (Figure 3) and data presented
T
in Table 2. Thus the relatively lower TOC removal efficiency with Pt anode can be explained
IP
by its lower oxidation power against carboxylic acids leading their accumulation in the
AC
CE P
TE
D
MA
NU
SC R
medium.
Figure 5: Formation and evolution of main carboxylic acids generated during the electrolysis of 0.2 mM CB solution at pH 3.0, 300 mA and room temperature. Methods: (a) EF-Pt-Na2SO4, (b) EF-BDD-Na2SO4, Acids: (●) oxalic, (■) malonic, (♦) acetic and (▲) formic.
The mineralization of organics having heteroatoms in their initial structure is accompanied to the release of inorganic ions. Therefore Cl– ions are formed during EF treatment of CB. The
22
ACCEPTED MANUSCRIPT evolution of chloride Cl– during EF treatment with Pt and BDD anodes was monitored by ion chromatography and presented in Figure 6. The time-course of Cl– concentration exhibits a
T
very different behavior depending on anode materials. A stoichiometric Cl– concentration is
IP
released in the case of EF-Pt cell. The release is complete at 1 h and the concentration of 1.2
SC R
mM remains constant along treatment. The time course profile is very different in the case of BDD anode. The maximum concentration of 0.11 mM is attained more quickly (at about 30 min) and then it is gradually decreased to reach 0.05 mM at 3 h electrolysis. This trend was
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already highlighted by Liu et al [37]. This result is in agreement with previous reports and can
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be explained by the oxidation of Cl– to Cl2 (reaction 7) and/or to ClO- or to further oxidized species according to the reactions (8-11) on the BDD anode while the Pt anode is not powerful enough to oxidize this ion. The same result is already reported by Rocha et al. [39]
TE
D
for the degradation of Novacron Yellow on BDD and Pt anodes with and without NaCl in solution. On the other hand, the fast release of chloride ions indicates that the dechlorination
50].
CE P
of CB occurs before the ring opening reactions of aromatic moiety of the molecule [23, 41,
AC
The complete mineralization of organic chlorine of CB to Cl– constitutes another evidence for mineralization of CB.
23
ACCEPTED MANUSCRIPT
T SC R
IP
0.08
0.04
0 0
0.5
1
NU
Concentration / mM
0.12
1.5
2
2.5
3
3.5
MA
Time / h
Figure. 6. Evolution of chloride ions during electrolysis of 0.12 mM CB solution at pH 3.0 and room temperature by EF process with Pt (□) and BDD (Δ) anode. [Na2SO4] = 50 mM; [Fe ] =
TE
D
0.1 mM; I = 300 mM.
2+
Taking into account the identified aromatic intermediates, short-chain carboxylic acids and
CE P
total release of chloride ions combined to TOC removal results we can propose a plausible
AC
reaction pathway for mineralization of CB by hydroxyl radicals (Figure 7).
24
ACCEPTED MANUSCRIPT Cl
.
Cl
IP
T
OH
Cl
OH
.
SC R
OH
. OH
OH
OH
OH
-Cl
-Cl
NU
OH OH
OH
O
OH
O
- H2O
OH
OH
- H2O
MA
.
.
.
O
D
O
CE P
TE
.
AC
O
OH
Ring opening reactions
O
OH
O
O
OH
OH
HO
OH
O
.
OH
O
OH
CO 2 + H2O
Figure 7. Proposed reaction sequence for mineralization of CB by OH generated during EF process.
4. Conclusions
From the results presented above the following main conclusions can be summarized as following:
25
ACCEPTED MANUSCRIPT - The MCE values show that the treatment of CB is more advantageous with an applied current of 300 mA. Current values exceeding 300 mA lead to more energy consumption
T
without bringing significant enhancement in mineralization degree.
IP
- The nature of supporting electrolyte has a great influence on the efficiency of
SC R
electrochemical oxidation of CB. The treatment with Na2SO4 appears to be more efficient. - Oxidative degradation of CB follows pseudo-first order reaction kinetics. The rate constant
NU
for oxidation of CB by hydroxyl radicals was determined by competition kinetics method and found to be 4.35 × 109 M-1 s-1.
MA
- EF process with BDD anode seems to be more effective electrochemical advanced oxidation
mA. Oxidative
degradation
of
TE
-
D
process achieving 92% TOC removal at 4 h electrolysis period at a current intensity of 300
leads
intermediates
to
(phenol,
the
formation
catechol,
of
several
2-chlorophenol
4-
CE P
hydroxylated/polyhydroxylated
CB
chlorophenol, hydroquinone and benzoquinone). Mineralization of these aromatics results in
AC
formation of short-chain linear carboxylic acids as ultimate end-products before complete mineralization.
- A plausible mineralization reaction pathway was proposed taking into account identified reaction intermediates and TOC removal results.
Acknowledgement This work is done in the framework of the Moroccan-French integrated action program VOLUBILIS (PHC No. ma/11/250) and under the MESRSFC / SCAC program. The authors are grateful for their financial support. M. Sönmez-Çelebi acknowledges financial support from Turkish Council of Higher Education.
26
AC
CE P
TE
D
MA
NU
SC R
IP
T
ACCEPTED MANUSCRIPT
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SC R
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ACCEPTED MANUSCRIPT Highlights Generation of strong oxidant OH by electrochemical advanced oxidation.
T
Oxidative degradation of MCB by anodic oxidation and electro-Fenton
IP
Determination of the rate constant for oxidation of MCB by OH
SC R
Quasi complete mineralization of 0.1 mM MCB solution at 4 h treatment Identification of aromatic/aliphatic intermediates and inorganic end-products
AC
CE P
TE
D
MA
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Proposal of a plausible mineralization pathway
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S1572-6657(16)30215-6 doi: 10.1016/j.jelechem.2016.04.051 JEAC 2629
To appear in:
Journal of Electroanalytical Chemistry
Received date: Revised date: Accepted date:
16 March 2016 27 April 2016 28 April 2016
Please cite this article as: Hicham Zazou, Nihal Oturan, Mutlu S¨ onmez-C ¸ elebi, Mohamed Hamdani, Mehmet A. Oturan, Mineralization of chlorobenzene in aqueous medium by anodic oxidation and electro-Fenton processes using Pt or BDD anode and carbon felt cathode, Journal of Electroanalytical Chemistry (2016), doi: 10.1016/j.jelechem.2016.04.051
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ACCEPTED MANUSCRIPT Mineralization of chlorobenzene in aqueous medium by anodic oxidation and electro-Fenton processes using Pt or BDD anode
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and carbon felt cathode
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Hicham Zazou1, 2, Nihal Oturan1, Mutlu Sönmez-Çelebi1,3, Mohamed Hamdani2, Mehmet A. Oturan1
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Université Ibn Zohr d'Agadir, Faculté des Sciences, BP 8106, Cité Dakhla, Agadir Morocco
University of Ordu, Faculty of Science and Arts, Department of Chemistry, 52200 Ordu, Turkey
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Université Paris-Est, Laboratoire Géomatériaux et Environnement (LGE), EA 4506, UPEM, 77454 Marne-la-Vallée, France.
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Paper submitted to Journal of Electroanalytical Chemistry for publication
* Corresponding author: Email: [email protected] (Mehmet A. Oturan) Phone: +33 149 32 90 65
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ACCEPTED MANUSCRIPT
ACCEPTED MANUSCRIPT Abstract This study focuses on the in situ destruction of pesticide chlorobenzene (CB) by using two
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electrochemical advanced oxidation processes, namely anodic oxidation and electro-Fenton.
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Influence of several operating parameters such as applied current, catalyst concentration and
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supporting electrolytes was assessed to optimize oxidation and mineralization of CB. Kinetics study showed that CB is quickly oxidized by •OH following a pseudo first-order reaction kinetics. The rate constant of oxidative degradation of CB by hydroxyl radicals was
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determined by competition kinetics method and found to be 4.35 × 109 M-1 s-1. The quasi
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complete mineralization (95% TOC removal) of 0.1 mM CB aqueous solution was achieved at 4 h treatment. Formation and evolution of aromatic and aliphatic (short-chain carboxylic acids) intermediates during treatment were monitored by HPLC analysis and a mineralization
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pathway was proposed. The results obtained highlight the great efficiency of electro-Fenton process in effective destruction of a very persistent pollutant such as CB that can be
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extrapolated to other toxic/persistent organic pollutants.
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Keywords: Chlorobenzene; Electro-Fenton; Anodic Oxidation; Hydroxyl radicals; Kinetics, Mineralization; TOC removal
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ACCEPTED MANUSCRIPT 1. Introduction The intensive use of pesticides in the environment constitutes a threat to living beings [1–3].
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Chlorobenzene (CB) is one of the most common organic pollutants in industrial wastewaters
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[4] since it is involved in the synthesis of many chemicals, particularly in the field of
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pesticides [5]. Therefore, CB is considered as a hazardous waste and a prior toxic pollutant by the U.S. Environmental Protection Agency [4]. Chlorinated aromatic compounds are
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considered as the most problematic categories of environmental pollutants because they are mostly non-biodegradable or very slowly degradable by microorganisms [6]. It is therefore
methods to remove them from water.
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important to assess the fate of these compounds in the environment and develop effective
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Recently, there is great interest in the remediation of waters containing toxic and organic
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pollutants with different methods such as chemical, photochemical, photocatalytical and
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electrochemical processes [7–11]. In this work we focus on electrochemical advanced oxidation processes (EAOPs) like electro-Fenton (EF) and anodic oxidation (AO) for removal of CB from water. These processes enable efficient degradation of persistent organic
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pollutants in aqueous medium by in situ generating highly reactive oxidizing agents such as hydroxyl radicals (•OH), which are able to oxidize efficiently almost all organic contaminants [11–17]. This radical is the second most strong oxidant known, after fluorine, having a very high standard potential (E°(•OH/H2O) = 2.80 V/ SHE) that makes it able to non-selectively react with organics to give hydroxylated or dehydrogenated derivatives until their mineralization (transformation into CO2, water and inorganic ions) [18–22]. One of the most popular EAOPs is the EF process based on the continuous supply of H2O2 from reaction (1) [23-25]. Formation of homogeneous •OH starts via Fenton reaction (2) once a catalytic amount of ferrous iron salt added to the solution [2, 23, 26]. The process is electrocatalytic since Fe2+ ions is electro-regenerated according to the reaction (3) from ferric iron produced 3
ACCEPTED MANUSCRIPT by reaction (2) [20, 27, 28]: (1)
H2O2 + Fe2+ → Fe3+ + HO– + •OH
(2)
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O2 + 2H+ + 2e– → H2O2
(3)
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Fe3+ + e– → Fe2+
Other popular EAOP is the AO process in which organics pollutants in a contaminated
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solution are oxidized by direct charge transfer on the anode (M) from heterogeneous hydroxyl radicals (M(•OH)) formed from oxidation of water (reaction (4)) [29-31]. The availability and
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efficiency of these radicals is depending to the anode material (M) [30, 31]. The boron-doped diamond (BDD) anode appears as the most effective anode because of the high O2 evolution
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overpotential and physisorption on the surface [30-34]. M + H2O → M(•OH) + H+ + e-
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(4)
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The use of a high overpotential anode material like BDD in EF process enhances strongly the efficiency of the process since this operation is consisting of a coupling between EF and AO
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processes: •OH and BDD(•OH) are generated simultaneously [35]. Therefore the oxidative degradation of CB was carried out by EF comparatively with AO. The electrolyses were carried out with both anode materials: Pt and BDD. A three-dimensional electrode material, carbon felt, was used as the cathode in all experiments [35, 36]. The influence of the applied current and the kind of supporting electrolytes on the effectiveness of the treatment were evaluated. The degradation kinetics was followed by high performance liquid chromatography (HPLC). The mineralization efficiency was studied for the two processes following the evolution of the total organic carbon (TOC) content. Identification of aromatic intermediates, short-chain carboxylic acids and TOC results enabled us to propose a plausible mineralization pathway for mineralization of CB by hydroxyl radicals. Although the oxidative degradation of
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ACCEPTED MANUSCRIPT CB has been carried out already by Liu et al. [37] and Hsiao and Kobe [38], the present work provides new data on the degradation and mineralization of CB. Hsiao and Kobe reported, in
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a pioneer work the oxidation of CB in a packed bed flow reactor by electrochemically
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generated Fenton's reagent with a low degradation (40% in 40 min) rate. More recently Liu et
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al. [37] gave a more interesting work investigating the electro-oxidation of CB by AO using Pt and BDD anodes. A degradation kinetic model was developed and identification of number aromatic intermediates was carried out. The present study aims to complete the work of Liu et
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al. and Hsiao and Kobe investigating the oxidation and mineralization of CB by electro-
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Fenton process and elucidating the mineralization kinetics through TOC removal measurements as a function of electrode material, current applied. And supporting electrolyte
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used. Results obtained in this study were then compared with those reported in [37] and [38].
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2.1 Chemicals
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2. Experimental
Chlorobenzene (CB, C6H5Cl) was obtained from sigma-Aldrich. Iron (II) sulfate heptahydrate
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used as Fe2+ (catalyst) source was analytical grade from Fluka. Hydroxybenzoic acid was obtained from Merck. Acetic, maleic, oxalic, fumaric, formic, succinic and malonic acids were of analytical grade and supplied by Fluka. Anhydrous sodium sulfate and sodium chlorate used as supporting electrolytes were obtained by Across. The pH was adjusted with analytical grade sulfuric acid purchased from ACS reagent Across. Sodium nitrate (Merck), ammonium nitrate and sodium phosphate (Aldrich) were used as standards for ionic chromatography (IC) analysis. Organic solvents and other chemicals used were either of HPLC or analytical grade purchased from Merck and Fluka. 2.2. Procedures and equipment
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ACCEPTED MANUSCRIPT All electrolyses were conducted in an open and undivided cylindrical glass cell of 250 mL capacity, equipped with two electrodes. The working electrode was a 70 cm2 (14 cm x 5 cm)
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piece of carbon-felt (from Mersen, France), placed on the inner wall of the cell covering the
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internal perimeter. The counter electrode was cylindrical Pt grid or BDD plate (6 cm x 4 cm)
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(from CONDIAS GmbH, Germany) placed at the center of the cell. Prior to the electrolysis, compressed air was bubbled through the aqueous solutions, which were stirred continuously by magnetic stirrer (500 rpm). A catalytic quantity of ferrous ion (0.1 mM) was introduced
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into the solution before the beginning of electrolysis. The supporting electrolytes were
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Na2SO4 (50 mM) and NaCl (100 mM). Different currents intensities were applied in the degradation and the mineralization experiments, these currents were measured and displayed continuously throughout electrolysis using a DC power supply (HAMEG Instruments, HM
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8040-3). Before starting electro-oxidation treatment, the initial solutions were adjusted to pH
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3.0 with analytical grade 1 M sulfuric acid.
2.3. Instruments and product analysis procedures
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The solution pH was measured with a glass electrode calibrated with standard buffers at pH values of 4, 7 and 10. The concentration decay of CB was monitored by HPLC using a Merck-Hitachi Lachrom chromatograph equipped with a diode array UV-Vis detector (model L-7455) fitted with a reverse phase purospher RP-18, 5µm, 4,6 × 250 mm column purchased from Merck (France). The column was eluted at isocratic mode for all experiments with a mobile phase composed of 70:30 (v/v) methanol/water at a flow rate of 0.8 mL min-1. The column was thermostated at 40 °C. Detection was performed at 230 nm for kinetic studies. The short-chain carboxylic acids were identified and quantified using an ion-exclusion column (Supelcogel H column; ɸ = 7, 8 mm × 300 mm) with a mobile phase of 4 mM H2SO4 at 210 nm. Inorganic ions were analyzed by ionic chromatography from Thermo-Fisher
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ACCEPTED MANUSCRIPT (Dionex-100 equipped with a conductivity detector). An anionic exchanger column (IonPac AS14-Dionex) was used for Cl- analysis. The TOC content of initial CB solutions and its
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evolution during electrolysis was performed at pH 3. The CB samples were filtered (0.22 µm)
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and acidified with HCl (1% HCl 2 mM). The injection volumes were 50 µL. TOC values were
efficiency (MCE) was estimated as follows [23, 36]: 𝑛 𝐹 𝑉𝑆 ∆(𝑇𝑂𝐶)𝑒𝑥𝑝 4.32 × 107 𝑚 𝐼 𝑡
x 100
(5)
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MCE (%) =
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determined with Shimadzu VCSH TOC analyzer. From these data, the mineralization current
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where F is the Faraday constant (96487 C mol-1), VS is the solution volume, ∆(TOC)exp is the experimental TOC change at a given time (mg L-1), 4.32×107 is an homogenization factor
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(3600 s h-1 ×12000 mg mol-1), m is the number of carbon atoms of CB (6 atoms), I is the
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applied current (A) and t is the electrolysis time (h). The number of electrons (n) consumed per mole of CB was taken as 28 considering total mineralization according to the following
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reaction [2, 18, 37]:
(6)
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C6H5Cl + 12H2O → 6CO2 + 29H+ + Cl- + 28e-
3. Results and discussion
3.1. Effect of experimental parameters on oxidative degradation of MC The performance of electrode pairs Pt/carbon felt and BDD/carbon felt was tested for EF and AO processes. The efficiency of the anodes was analyzed and compared in the same conditions in order to optimize the degradation of MC solutions. All experiments were carried out with 230 mL of 0.1 mM MC solutions at pH 3.0. The decay kinetics of MC concentration with the generated oxidant (•OH)/M(•OH) was studied at different current values from 50 to
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ACCEPTED MANUSCRIPT 500 mA, using 50 mM of Na2SO4 or 100 mM of NaCl and 0.1 mM of catalyst (Fe2+)
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Figure 1: Effect of current intensity on the decay of CB concentration (C0=0.1 mM) in 230 mL aqueous solution with 50 mM Na2SO4 or 100 mM NaCl (and 0.1 mM Fe
2+
for EF), using
Pt/carbon felt and BDD/carbon cells. Currents intensity: (□): 50 mA; (♦): 100 mA; (Δ): 300 mA; (●): 500 mA. Method: (a) EF-Pt-Na2SO4; (b) EF-BDD-Na2SO4; (c) EF-Pt-NaCl; (d) EF-BDDNaCl; (e) AO-Pt-Na2SO4; (f) AO-BDD-Na2SO4; (g) AO-Pt-NaCl; (h) AO-BDD-NaCl. The inset panels show the corresponding kinetic analysis assuming that CB follows a pseudo-first-order reaction.
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ACCEPTED MANUSCRIPT Figure 1 shows the effect of the current applied to the profile of the kinetic curves of the oxidation of CB by •OH/M(•OH). The electrolysis time required for complete disappearance
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of CB is becoming shorter when the intensity of current applied is higher. These results can be
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explained by the increased rate of electrochemical reactions (1), (3) and (4) favoring
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production of high amount of •OH/M(•OH) leading to the quick oxidation of CB. Figures 1-(a) to 1-(d) show the efficiency of the oxidative degradation of CB by EF process with Pt and BDD anodes using sodium sulfate (Na2SO4) or sodium chloride (NaCl) salts as supporting
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electrolyte. In the case of Na2SO4 the complete disappearance of CB is attained after 10 and
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12 min with Pt and BDD anode, respectively, at I = 500 mA. However this time is as slow as 30 min for both anodes in the case of NaCl under same operating conditions. On the other hand, it is noticed that the positive effect of current rise becomes non-significant for currents
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above 300 mA. This phenomenon can be explained by the progressive enhancement of the rate of side reactions such as the reduction of H2O2 and evolution of H2 at the cathode [18, 23]
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and evolution of O2 at the anode. The decay of CB concentration exhibits almost identical shapes with both anodes but with significantly longer electrolysis times. Figures 1(e) to 1(h)
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show the degradation of the CB by AO process. The kinetic of concentration decay is slower than EF process with a better efficiency for BDD anode in Na2SO4 medium. Similarly, degradation kinetics is accelerated by increasing the current applied from 50 to 500 mA.
3.2. Evaluation of apparent and absolute rate constants The values of apparent rate constants for oxidative degradation of CB (0.1 mM, 230 mL) by EF and AO treatment under different operating conditions( (nature of anode material, current intensity applied and supporting electrolyte) are given in Table.2. These values are determined following pseudo-first order reaction kinetics (shown in insert panels of Figure 1). This reaction order is justified by the very short life time of hydroxyl radicals allowing the
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ACCEPTED MANUSCRIPT application of steady state approximation for the concentration of hydroxyl radicals and the exponential decay of CB concentration [18, 23].
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Table 1: Pseudo-first-order rate constants for oxidative degradation CB (C0 = 0.1 mM) by
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electrochemical treatment with different supporting electrolytes (50 mM Na2SO4 or 100 mM
Electrode
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Method
1.84
0.99
100
2.39
0.99
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3.27
0.99
500
3.78
0.99
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0.88
0.97
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NaCl) at pH 3.0 and room temperature under different current intensities.
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1.08
0.96
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1.50
0.98
500
1.75
0.97
50
1.67
0.99
100
2.26
0.99
300
3.26
0.99
500
3.51
0.99
50
0.51
0.99
100
0.93
0.99
300
1.49
0.99
500
1.89
0.99
50
0.39
0.99
100
0.59
0.99
300
0.95
0.99
500
1.05
0.98
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0.08
0.99
Supporting electrolyte
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50
Na2SO4
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BDD
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Pt
Na2SO4
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Na2SO4
NaCl
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kapp×10-1
R2
(s-1)
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I (mA)
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300
0.13
0.97
500
0.14
0.97
50
0.91
0.99
100
1.25
0.99
1.67
0.99
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Na2SO4
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BDD
100
2.03
0.99
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0.59
0.99
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0.96
0.99
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0.96
0.99
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1.18
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NaCl
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Comparing these kapp values for oxidative degradation of CB by both processes, it can be concluded that the oxidation efficiency is also affected by the supporting electrolyte and the
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nature of the anode material used. BDD and Pt anodes provide almost similar oxidation efficiencies in both processes while the former anode supplies better rate values in AO
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process. The kapp are significantly low in the case of AO when using NaCl as supporting electrolyte (Table 1). This behavior can be explained by oxidation of chloride ion to Cl2 and chlorinated ions (reactions 7-11) as reported by Rocha et al [39]. These reactions consume hydroxyl radicals hinder degradation efficiency of CB. 2Cl− → Cl2 + 2e−
(7)
Cl− + •OH → ClO− + H+ + e−
(8)
ClO−+ •OH → ClO2− + H+ + e−
(9)
ClO2−+ •OH → ClO3− + H+ + e−
(10)
ClO3− + •OH → ClO4− + H+ + e−
(11)
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Na2SO4 as supporting electrolyte. Finally the general efficiency trend is as following: EF-Pt-
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Na2SO4 > EF-BDD-Na2SO4 > AO-BDD-Na2SO4 > EF-Pt-NaCl > EF-BDD-NaCl > AO-
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BDD-NaCl > AO-Pt-Na2SO4 > AO-Pt-NaCl. This trend is in agreement with those reported already [13, 23, 24].
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The absolute (or second order) rate constant of reaction between CB and hydroxyl radicals was determined by competition kinetics method [23, 40, 41] using para-hydroxybenzoic acid
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(p-HBA) as standard competitor for which the absolute rate constant of reaction with hydroxyl radicals (kp-HBA) is known as 2.19 × 109 M-1 s-1 [42]. For this purpose, competitive
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kinetic experiments were performed in the presence of equal amounts of CB and p-HBA
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assuming pseudo first order kinetic for reactions of both CB and p-HBA with hydroxyl
equation (12). [CB]0 [CB]t
)=k
kCB
p−HBA
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Ln (
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radicals under experimental conditions of Figure 2 and kCB was determined according to
[p−HBA]0
Ln (
[p−HBA]t
)
(12)
where [CB]0 and [CB]t denote the concentration of CB at time zero and at any time of the electrolysis, respectively, and [p-HBA]0 and [p-HBA]t denote the concentration of p-HBA at time zero and at any time of the electrolysis, respectively. These concentrations were monitored by HPLC analysis during constant current electrolysis. The absolute rate constant kCB was then determined from the slope of the straight line obtained by plotting ln ([CB]0/[CB]t) = f (ln ([p-HBA]0/[p-HBA]t) [43] and found to be 4.35 × 109 M-1 s-1. This value is close to that reported by Buxton et al. [44] (4.5 × 109 M-1 s-1) determined from pulse radiolysis method.
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Figure 2: The ln C0/Ct values of CB and p-HBA as a function of time for the pseudo-first order degradation kinetics of CB and p-HBA in EF-Pt process under following operating conditions: 2+
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[CB]0 = [p-HBA]0 = 0.1 mM; V = 230 mL; [Fe ] = 0.1 mM; pH = 3; [Na2SO4] = 50 mM and I = 50
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3.3. Mineralization of CB aqueous solution
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All the CB solutions treated by EF and AO process with Pt and BDD anodes underwent progressive mineralization with increasing electrolysis time. Reproducible TOC removal values were obtained under all experimental conductions. Figure 3 shows the comparative TOC decay kinetics for the treatment of 0.2 mM CB solution (corresponding to 14.5 mg L-1 initial TOC value) containing 50 mM Na2SO4 or 100 mM NaCl as supporting electrolyte at pH 3.0 and room temperature for 4 h. The pH remained practically unchanged during these trials reaching a final value close to 2.8 for EF process. As can be seen, a more rapid and continuous TOC removal was achieved with EF compared to AO treatment. For instance, an almost complete mineralization (>95% TOC removal) was obtained with BDD anode at 500 mA after 4 h treatment (Figure 3b). Pt anode
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ACCEPTED MANUSCRIPT reached 80% TOC removal efficiency under same conditions (Figure 3a). Conversely a poor mineralization degree was attained in AO with Pt anode while a significantly better mineralization efficiency (89.5) was reached with AO-BDD-Na2SO4. The lower
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mineralization ability of Pt anode can be explained by the small Pt(•OH) concentration formed
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on the Pt surface (in relation with the low O2 evolution overpotential) from reaction (4) and their chemisorption on the Pt surface inversely to the BDD(OH) that are physisorbed [23, 40]. In contrast, related to the surface properties, BDD has a much higher oxidative ability. At a
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current intensity greater than 500 mA, No improvement was observed for mineralization
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efficiency for current intensity greater than 500 mA (Figure 3).
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TOC / mg L-1
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Figure 3: TOC removal with electrolysis time for 230 mL 0.2 mM CB solution (TOC0 = 14.5 mg L )) containing 50 mM Na2SO4 or 100 mM NaCl (and 0.1 mM Fe
2+
for EF), using Pt/carbon felt or ..
..
BDD/carbon felt cells. Currents intensity: (-♦-): 100 mA; (-Δ-): 300 mA; (-●-): 500 mA and ( ): 700 mA. Method: (a): EF-Pt-Na2SO4, (b): EF-BDD-Na2SO4, (c): EF-Pt-NaCl, (d): EF-BDD-NaCl, (e): AO-Pt-Na2SO4, (f): AO-BDD-Na2SO4; (g): AO-Pt-NaCl and (h): AO-BDD-NaCl.
In general, the TOC removal rate is high during the first 2 h (Figure 3) due to the relatively easy oxidative degradation of CB and its aromatic intermediates from M(OH) in AO and
OH/M(OH) in EF processes [18] leading to the formation of aliphatic compounds
(carboxylic acids) that are resistant to mineralization by these radicals. Therefore TOC removal rate becomes poor after 2 h treatment in both processes except in the case of AO-
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ACCEPTED MANUSCRIPT BDD-Na2SO4 (Figure 3f) since BDD(OH) has relatively high oxidation power compared to
OH and accordingly is able to mineralize more efficiently short-chain carboxylic acids [45].
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The results of Figure 3 show that the treatments with Na2SO4 are significantly more efficient
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compared to NaCl which is a rather limited system (Table 2). The formation of the
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peroxodisulfate ion, a relatively strong oxidant, has been suggested by Serrano et al. [46] in a sulfate medium (reaction 13). The low mineralization efficiency of EF and AO treatments in
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chloride medium can be explained by oxidation of this ion to chlorine (reaction 7) and then the related side reactions (14-15) playing a negative role on the mineralization efficiency [2,
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23, 30, 38]. Therefore, the amount of current consumed for oxidation of chloride ion (reaction 7) and the wasting reactions of chlorinated species taking place in the vicinity of anode
D
surface that consume OH (reactions 8-11) hamper the mineralization efficiency [40, 46]. This
TE
effect is prominent in the case of AO with Pt anode Fig. 3g) since its oxidation power is
CE P
significantly lower than BDD anode.
(13)
Cl2 + H2O → HOCl + H+ + Cl−
(14)
AC
2SO42- → S2O82- + 2e−
HOCl → H+ + ClO−
(15)
On the other hand, in the case of EF-Pt-Na2SO4 and EF-BDD-Na2SO4, the MCE values were obtained for the lowest current. This setting is dropped with the electrolysis time (Figure 4).
17
ACCEPTED MANUSCRIPT
Table 2. TOC removal (mineralization) degree and evolution of MCE after 1 and 4 h of treatment
BDD
Na2SO4
Na2SO4
MCE
100
15.5
29.0
51.8
4.1
300
40.0
7.7
75.0
0.7
500
52.0
3.2
80.0
0.3
700
49.0
2.5
75.0
0.3
55.0
15.0
90.0
0.9
66.0
3.9
92.0
0.2
500
75.0
1.7
95.0
0.1
700
63.0
1.8
93.0
0.1
300
12.0
10.0
27.0
2.0
500
21.0
5.0
33.0
1.1
100
28.8
24.5
58.7
3.6
300
43.0
6.5
68.2
0.9
500
45.0
3.5
72.2
0.4
700
43.4
2.8
71.6
0.3
100
-
-
-
-
300
17.3
4.3
21.6
1.0
500
25.3
2.3
38.0
0.5
700
29.8
1.6
41.8
0.3
300
8.7
10.2
16.6
2.3
500
11.6
5.9
20
1.3
100
38.8
9.4
78.7
0.8
300
44.7
2.9
85.0
0.2
500
47.2
1.6
89.2
0.2
700
42.7
1.3
84.8
0.2
100
31.6
10.6
54.6
1.8
300
44.4
2.9
62.4
0.3
500
48.6
1.5
64.7
0.3
100
Pt
Na2SO4
AC
AO
NaCl
CE P
BDD
NaCl
BDD
BDD
D
TE
NaCl
Na2SO4
NaCl
After 4h of treatment
% TOC removal
300
Pt
After 1h of treatment
IP
I (mA)
SC R
Pt
Supporting electrolyte
NU
EF
Electrode
MA
Method
T
with EF and AO processes under the experimental conditions described in Figure 3.
18
% TOC removal
MCE
ACCEPTED MANUSCRIPT 700
38.3
1.4
59.8
0.2
T
The influence of operating parameters such as the nature of anode used, current intensity
IP
applied and kind of supporting electrolyte is depicted in Table 2. This table presents %TOC
SC R
removal and the MCE values calculated from data of Figure 3 according to the equation (5) after 1 h and 4 h treatment. Better MCE values are obtained from EF-Pt-Na2SO4 and EF-
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BDD-Na2SO4 cells in agreement with results discussed above. In all cases, %MCE gradually decreases with increasing current intensity and electrolysis time. This trend may be related to
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the gradual decrease in the concentration of CB and accumulation of intermediate products more difficult to oxidize such as carboxylic acids [47]. The acceleration of the rate of side
D
reactions (H2 evolution at cathode, O2 evolution at anode and destruction of H2O2 at both
TE
electrodes) constitutes another reason of poor %MCE values on longer treatment times. The results obtained highlight the great mineralization efficiency (75% and 95%) of the EF-BDD-
CE P
Na2SO4 cell due to the simultaneous generation of •OH (in the bulk) and BDD(•OH) on the anode surface. Concerning MCE better results are obtained at lower current intensity and
AC
shorter electrolysis times in agreement with already reported studies [18, 23, 30, 48].
3.4. Identification of reaction intermediates and mineralization pathway The degradation of the toxic/persistent organic compounds by advanced oxidation processes can produce aromatic intermediates more toxic than initial compounds with negative effect on the environment [40, 49]. Thus, the identification of reaction intermediates during electrochemical advanced oxidation processes is important. This identification is also useful in order to be able to propose a mineralization pathway by the attack of hydroxyl radicals on target pollutant. Therefore we attempted to identify and quantify intermediate products formed during degradation of CB by hydroxyl radicals generated in EF process with Pt anode.
19
ACCEPTED MANUSCRIPT This identification was performed by HPLC analysis by comparison of retention times (tR) of formed intermediates with those of authentic products. Experiments were conducted at a low
T
current (60 mA) in order to favor the accumulation of hydroxylation reaction intermediates.
IP
Even under this low-current condition, the total oxidation of CB and its aromatic by products
SC R
occurs in less than 120 min (Figure 4).
0.25
(a)
NU MA
0.15 0.1
0
D
0.05
TE
Concentration / mM
0.2
10
20
30
40
50
60
Time / min
CE P
0
AC
Concentration / mM
0.012
(b)
0.01
0.008 0.006 0.004 0.002 0 0
20
40
60
80
100
120
Time / min
Figure 4: Concentration decay of CB (a) and formation and evolution of the oxidation reaction intermediates (b) generated during EF-Pt process. Intermediates: CB (×); Hydroquinone (■); Benzoquinone (□); Catechol (●); Phenol (◊); 2-Chlorophenol (▲) and 4-Chlorophenol (). I = 2+
60 mA; [Na2SO4] = 50 mM; [Fe ] = 0.1 mM; V = 230 mL; pH=3.
20
ACCEPTED MANUSCRIPT All the intermediates are formed from the beginning of the treatment. Most of these intermediates are also reported by Liu et al. [37] except the phenol, 2-chlorophenol and 4-
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chlorophenol that were identified in this study. Besides the 1,3-dihydroxybenzene was not
IP
detected in this the present study. In addition, Hsiao and Kobe [39] have already identified the
SC R
formation of 4-chlorophenol.
The main reaction is the electrophilic addition of OH on the benzene ring leading to the
NU
formation of 2-chlorophenol and 4-chlorophenol. The phenol is formed by ipso attack of OH on the position of chlorine atom conducting to its substitution. The two chlorophenols
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undertake same substitution reaction to form catechol and hydroquinone. The latter intermediate is then oxidized into benzoquinone. This reaction can take place whether by
D
electron transfer at the anode or by OH in the bulk of solution. The total disappearance of the
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CB and intermediates is achieved after 120 min of electrolysis.
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It is well known that polyhydroxylated products and quinones are not stable toward the attack of OH/M(OH) and undertake oxidative ring opening reactions to lead to the formation of
AC
short-chain carboxylic acids [18, 23, 38, 40]. Formation and evolution of carboxylic acids generated during the mineralization of CB during EF treatment with Pt and BDD anodes were depicted in Figure 5. Four main carboxylic acids (oxalic, malonic, acetic and formic) are formed from the beginning of the electrolysis. Formic and fumaric acids are detected in trace level; they are not accumulated or formed at low concentration. The comparison between Pt and BDD anodes is very noticeable: Carboxylic acids are mineralized quickly with BDD anode (Figure 5b) while they are very slowly oxidized in the case of Pt anode leading to their accumulation. This situation was already highlighted by Liu et al. [38] for formic, maleic (not detected in this study) and oxalic acids. All of four carboxylic acids are remained at the end of 4 h treatment in the latter case (Figure 5a) whereas only oxalic and acetic acids are present at
21
ACCEPTED MANUSCRIPT low concentrations in the former case. These results are in agreement with data concerning the mineralization efficiency (in terms of TOC removal percentage) (Figure 3) and data presented
T
in Table 2. Thus the relatively lower TOC removal efficiency with Pt anode can be explained
IP
by its lower oxidation power against carboxylic acids leading their accumulation in the
AC
CE P
TE
D
MA
NU
SC R
medium.
Figure 5: Formation and evolution of main carboxylic acids generated during the electrolysis of 0.2 mM CB solution at pH 3.0, 300 mA and room temperature. Methods: (a) EF-Pt-Na2SO4, (b) EF-BDD-Na2SO4, Acids: (●) oxalic, (■) malonic, (♦) acetic and (▲) formic.
The mineralization of organics having heteroatoms in their initial structure is accompanied to the release of inorganic ions. Therefore Cl– ions are formed during EF treatment of CB. The
22
ACCEPTED MANUSCRIPT evolution of chloride Cl– during EF treatment with Pt and BDD anodes was monitored by ion chromatography and presented in Figure 6. The time-course of Cl– concentration exhibits a
T
very different behavior depending on anode materials. A stoichiometric Cl– concentration is
IP
released in the case of EF-Pt cell. The release is complete at 1 h and the concentration of 1.2
SC R
mM remains constant along treatment. The time course profile is very different in the case of BDD anode. The maximum concentration of 0.11 mM is attained more quickly (at about 30 min) and then it is gradually decreased to reach 0.05 mM at 3 h electrolysis. This trend was
NU
already highlighted by Liu et al [37]. This result is in agreement with previous reports and can
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be explained by the oxidation of Cl– to Cl2 (reaction 7) and/or to ClO- or to further oxidized species according to the reactions (8-11) on the BDD anode while the Pt anode is not powerful enough to oxidize this ion. The same result is already reported by Rocha et al. [39]
TE
D
for the degradation of Novacron Yellow on BDD and Pt anodes with and without NaCl in solution. On the other hand, the fast release of chloride ions indicates that the dechlorination
50].
CE P
of CB occurs before the ring opening reactions of aromatic moiety of the molecule [23, 41,
AC
The complete mineralization of organic chlorine of CB to Cl– constitutes another evidence for mineralization of CB.
23
ACCEPTED MANUSCRIPT
T SC R
IP
0.08
0.04
0 0
0.5
1
NU
Concentration / mM
0.12
1.5
2
2.5
3
3.5
MA
Time / h
Figure. 6. Evolution of chloride ions during electrolysis of 0.12 mM CB solution at pH 3.0 and room temperature by EF process with Pt (□) and BDD (Δ) anode. [Na2SO4] = 50 mM; [Fe ] =
TE
D
0.1 mM; I = 300 mM.
2+
Taking into account the identified aromatic intermediates, short-chain carboxylic acids and
CE P
total release of chloride ions combined to TOC removal results we can propose a plausible
AC
reaction pathway for mineralization of CB by hydroxyl radicals (Figure 7).
24
ACCEPTED MANUSCRIPT Cl
.
Cl
IP
T
OH
Cl
OH
.
SC R
OH
. OH
OH
OH
OH
-Cl
-Cl
NU
OH OH
OH
O
OH
O
- H2O
OH
OH
- H2O
MA
.
.
.
O
D
O
CE P
TE
.
AC
O
OH
Ring opening reactions
O
OH
O
O
OH
OH
HO
OH
O
.
OH
O
OH
CO 2 + H2O
Figure 7. Proposed reaction sequence for mineralization of CB by OH generated during EF process.
4. Conclusions
From the results presented above the following main conclusions can be summarized as following:
25
ACCEPTED MANUSCRIPT - The MCE values show that the treatment of CB is more advantageous with an applied current of 300 mA. Current values exceeding 300 mA lead to more energy consumption
T
without bringing significant enhancement in mineralization degree.
IP
- The nature of supporting electrolyte has a great influence on the efficiency of
SC R
electrochemical oxidation of CB. The treatment with Na2SO4 appears to be more efficient. - Oxidative degradation of CB follows pseudo-first order reaction kinetics. The rate constant
NU
for oxidation of CB by hydroxyl radicals was determined by competition kinetics method and found to be 4.35 × 109 M-1 s-1.
MA
- EF process with BDD anode seems to be more effective electrochemical advanced oxidation
mA. Oxidative
degradation
of
TE
-
D
process achieving 92% TOC removal at 4 h electrolysis period at a current intensity of 300
leads
intermediates
to
(phenol,
the
formation
catechol,
of
several
2-chlorophenol
4-
CE P
hydroxylated/polyhydroxylated
CB
chlorophenol, hydroquinone and benzoquinone). Mineralization of these aromatics results in
AC
formation of short-chain linear carboxylic acids as ultimate end-products before complete mineralization.
- A plausible mineralization reaction pathway was proposed taking into account identified reaction intermediates and TOC removal results.
Acknowledgement This work is done in the framework of the Moroccan-French integrated action program VOLUBILIS (PHC No. ma/11/250) and under the MESRSFC / SCAC program. The authors are grateful for their financial support. M. Sönmez-Çelebi acknowledges financial support from Turkish Council of Higher Education.
26
AC
CE P
TE
D
MA
NU
SC R
IP
T
ACCEPTED MANUSCRIPT
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SC R
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ACCEPTED MANUSCRIPT Highlights Generation of strong oxidant OH by electrochemical advanced oxidation.
T
Oxidative degradation of MCB by anodic oxidation and electro-Fenton
IP
Determination of the rate constant for oxidation of MCB by OH
SC R
Quasi complete mineralization of 0.1 mM MCB solution at 4 h treatment Identification of aromatic/aliphatic intermediates and inorganic end-products
AC
CE P
TE
D
MA
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Proposal of a plausible mineralization pathway
33