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Effect of Adsorption and Passivation Phenomena on the Electrochemical Oxidation of Phenol and 2Chlorophenol at Carbon Black Diamond Composite Electrode Mohammed Abdulridha Ajeel, Mohamed Kheireddine Aroua, Wan Mohd Ashri Wan Daud, and Shaukat Ali Mazari Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b03422 • Publication Date (Web): 06 Jan 2017 Downloaded from http://pubs.acs.org on January 8, 2017

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Industrial & Engineering Chemistry Research

Effect of Adsorption and Passivation Phenomena on the Electrochemical Oxidation of Phenol and 2-Chlorophenol at Carbon Black Diamond Composite Electrode

Mohammed A. Ajeela, Mohamed Kheireddine Arouab, Wan Mohd Ashri Wan Daudb, Shaukat Ali Mazaric a

Department of Chemistry, AL-Karkh University of Science, Baghdad 10066, Iraq.

b

Department of Chemical Engineering, Faculty of Engineering, University of Malaya, Kuala Lumpur 50603, Malaysia c

Department of Chemical Engineering, Dawood University of Engineering and Technology, Karachi 74800, Pakistan. Email: [email protected]

Abstract This work reports the importance of adsorption and passivation phenomena during electro-oxidation of phenolic mixture. A case study of anodic oxidation of phenol and 2chlorophenol is conducted in this study. A carbon black diamond electrode with 20% carbon black (20CBD) was used as the anode. The anodic oxidation behavior of 100 mg/L phenol and 2-chlorophenol each on 20CBD electrode was investigated using cyclic voltammetry in aqueous solutions of 0.25 M Na2SO4. Electrochemical impedance technique was used to investigate the effect of electrode passivation and adsorption of phenol and 2-chlorophenol through electrochemical degradation process. Results show that the 2-chlorophenol oxidation was easier than that of phenol oxidation. Even in the mixture of 1:1 of 2-chlorophenol and phenol, each 100 mg/L, the removal rate of 2chlorophenol was higher than phenol. After 6 hours, 2-chlorophenol degraded upto 94% whereas phenol degraded only 20% in the same time. The mass transfer resistance of phenol was higher up to ten folds than that of 2-chlorophenol. Moreover, the passivation resistance generated on the electrode surface by phenol oxidation was also higher than that of generated from 2-chlorophenol oxidation. Keywords

ACS Paragon Plus Environment


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Phenol,

2-Chlorophenol,

Electrochemical

degradation,

Passivation,

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Adsorption,

Impedance Introduction: Phenolic compounds represent the most widespread pollutants in industrial wastewater, with high toxicity and strong ability to resist the biological degradation; promulgating a serious threat to the environment

1, 2

. Phenol and 2-Chlorophenol are listed as priority

pollutants by the Environmental Protection Agency while 2-chlorophenol is also characterized as a carcinogen

3, 4

. Various uses of phenolic substrates are also reported

such as in oil refineries, pharmaceuticals, cellulose, herbicides, pesticides, and dyes

1, 5

.

Given the strong toxic nature and resistivity of these compounds to biodegradation, research efforts have been undertaken to develop suitable techniques that can mineralize these substrates. Anodic oxidation of phenolic compounds has gained the attention of researchers; however, most of the research is focused on the effect of anodic materials, such as SnO2, Pt, PbO2 , BDD and IrO2 , where the selection criteria is based on stability and activity of electrodes towards anodic oxidation 6-9. In the anodic oxidation of organic substrates, the rate determining step usually includes oxidized surface sites formed by a reaction with previously adsorbed water molecules pollutant molecules toward

10

. Moreover, the adsorption of

the anode surface has a significant effect on the

electrochemical oxidation treatment

11

. On the other hand, anodic oxidation of phenolic

compounds produces a passive adherent film on the electrode surface and leads to rapid current decline and electrode fouling

12, 13

. So, in phenolic compounds incineration by

electrochemical oxidation, the passivation of electrode surface could be a control step of the anodic oxidation process

14

. In addition to the electrode material, many parameters

have a significant effect on passivation phenomena, such as specific phenolic substrates, their concentration, and other operational parameters

10

. Phenol and 2-chlorophenol

degradation on 20CBD was investigated in our laboratory

15, 16

. It was found that the

removal of 2-chlorophenol was 4.5 times faster than that of phenol. This interesting finding motivated us to continue our investigation to understand the reasons behind this difference in degradation rates. To the best of our knowledge, anodic degradation of mixtures of phenol and 2-chlorophenol is rarely studied and reported in open literature.


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Moreover, this work uses a novel approach to investigate the adsorption and passivation of phenolic compounds on electro-degradation by using impedance techniques. In this study, the phenolic compounds have been investigated through cyclic voltammetry and the phenolic mixtures through electrolysis process. The adsorption of 2-chlorophenol and phenol substrates and the passivation of the electrode by phenol and 2-chlorophenol were studied by electrochemical impedance techniques.

2. Experimental 2.1. Electrode preparation A 20CBD electrode was prepared with a geometric surface area of 1.0 and 2.27 cm2. The detonation diamond powder with 6 nanometers average particle size (98.3% purity, Sigma-Aldrich) was mixed carefully with 20% of carbon black, which had a specific surface area of 550 m2/g s and 13 nanometers was average particle size (99% purity, Alfa Aesar). The mixture was mixed in the suspension of polytetrafluoroethylene (60 wt.%) (Sigma-Aldrich) in water and 1,3- propanediol (98% purity, Sigma-Aldrich) and then dried as described in previous work 15. 2.2. Electrode Characterization Cyclic voltammetry and electrochemical impedance tests were performed in a single glass cell compartment of 100 mL at 25 °C to study the 20CBD electrode properties. An electrolyte of 0.5 M H2 SO4 (98% Sigma-Aldrich) was used to study the electrode oxidation power. The active area of the 20CBD electrode was obtained by Chronoamperometry technique using an aqueous solution of 0.1 M KH2PO4 containing 5 mM K4Fe(CN)6. Electrochemical active area of the electrode was estimated using the Cottrell equation (Eq. 1)17. Cyclic voltammetry technique and Randles-Sevick equation (Eq. 2) was used to validate the active area of the 20CBD electrode. The voltammetry solution was 5 M H2SO4 containing 5 mM K4Fe(CN)6 and the scan rate was 100 mV. I=

𝑛 𝐹 𝐴 𝐷 1⁄2 𝐶0 𝜋 1⁄2 𝑡 1⁄2

(1)



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ip = 0.4463 𝑛 𝐹 𝐶 𝐴 √

𝑛𝐹𝑣𝐷

(2)

𝑅𝑇

where I is the current (A), ip is the peak current value (A), A is the working electrode electrochemical area (cm2), Co is the bulk concentration of K4Fe(CN)6 (mol/cm3), n is number of electrons, D is the diffusion coefficient (6.20 x 10-6 cm2/s) of K4Fe(CN)6; while the remaining parameters have their usual meanings. 2.3. Electrochemical oxidation Voltammetric experiments were conducted in one compartment of a 100 mL glass cell at 25 °C to investigate the electrochemical oxidation behavior of phenol and 2-chlorophenol on the 20CBD electrodes. A solution of 0.5 M H2 SO4 (98% Sigma-Aldrich) as a blank solution, and two aqueous solutions of 100 mg/L 2-chlorophenol (99.5% Sigma-Aldrich) and another solution of 100 mg/L phenol (99.5% Merk) were prepared. Solutions were prepared using double distilled water. Ag/AgCl was used as the reference electrode and platinum wire was used as the counter electrode. The electrochemical impedance experiments were performed with a potential amplitude of the AC signal, which was kept at 10 mV and the measured frequency range was 0.01–105 Hz. The electrochemical experiments on 20CBD electrode were performed by Autolab Metrohm potentiostat with NOVA 1.10 software. The electrolysis of mixed solution (100 phenol + 100 mg/L 2chlorophenol) and 0.25 M Na2SO4 as the supporting electrolyte was conducted over the 20CBD and platinum anodes for anodic oxidation. The experiments were conducted at applied current densities of 30 mA/cm2 and pH 3 at 25 °C. Solution stirring was performed using a C-MAG HS 7 magnetic stirrer. 2.4. Analytical technique The degradation of a mixture of phenol and 2-chorophenol was observed using high performance liquid chromatography (HPLC). The separation column was C18 (4.6 mm × 250 mm × 5μm ), used at 20℃. The eluent used was the mixture of 60% acetonitrile + 39.9% water + 0.1% H2PO4 by volume. The injection volume was 20 μL and eluent flow rate was 1 mL/min. The detection wavelength was set as 280 nm for phenol, 2chlorophenol and aromatic intermediates. Carboxylic acids such as maleic, fumaric,


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oxalic and formic acid oxidation products were detected using the mixed solution of 25% methanol + 10 mM NaH2 PO4 as mobile phase at pH of 2.7 with a flowrate of 0.7 ml/min. The injection volume was 10μL and the samples were filtered through a 0.25 μm membrane filter. Phenol and 2-chlorophenol diffusivity coefficient was estimated by Warburg impedance which is given in Eq. (2) 17 as: 𝑅𝑇

W = 1.414 𝐴(𝑛𝐹)2 𝐶

(1)

𝑜 √𝐷

where, W is Warburg impedance, R is the gas constant (8.314 J/ K. mol ), T is the absolute temperature (K), A is the electrode surface area, n is the number of transferred electrons, D is the diffusion coefficient (cm2/s), and Co is the bulk concentration (mol/cm3) of the phenol and 2-chlorophenol species.

3. Results and discussion 3.1. Electrode characterization Fig. 1(a) shows the background cyclic voltammetry curves for 20CBD electrodes in 0.5 M H2SO4. It depicts that the 20CBD electrode background current was low and featureless in between -0.65 and 2.2 V. The values of working potential window and oxidation potential of 20CBD electrode are 2.85 and 2.2 V vs. Ag/AgCl respectively. The 20CBD electrode oxidation potential was within 2.2 V vs. Ag/AgCl, which is considered high and suitable for the anodic oxidation, which is also commonly reported for BDD electrodes 18, 19. However, the electrode potential window for 20CBD is similar to that of a low quality polycrystalline BDD electrode, which have a potential window comparable to that of carbonaceous electrodes

20, 21

. Literature shows that 20CBD electrode potential

window was less than that of BBD electrode, and analogous to that of Ti/PbO2 and Ti/SnO2 -Sb2 O5

22

. The electrochemical active areas of the 20CBD electrode are

estimated by Chronoamperometry technique using an aqueous solution of 0.1 MKH2PO4, containing 5 mM K4Fe(CN)6. Electrochemical active area of electrode was obtained



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using Cottrell equation

17

. Fig. 1(b) shows the chronoamperometry measurements at a

20CBD electrode, which depicts the experimental plots of I verses t-1/2, which uses the best fits. Furthermore, the value of slope of the straight line was used to estimate the electrochemical active area by Cottrell equation (Eq.1). The electrochemical active area of 20CBD electrode was found to be equal to 23.1 cm2. The active area is greater than surface area by 20 times, which enhances the electrode reactivity and reduces the electrode electrical resistance. Fig. 1(c) shows the voltammogram of 20CBD electrode with 0.5 mM of [Fe(CN)6]4−/3− in 0.5 M H2SO4 as supporting electrolyte. The active area of 20CBD electrode was estimated by Randles-Sevick equation (Eq. 2) using the current peak (ip) measured from Fig. 1(c). The value of 20CBD active area estimated by RandlesSevick method was 21.36 cm2 and it validate that estimated using chronoamperometry technique and Cottrell equation. The increase in the electrochemical active area is attributed to the porosity of the electrode and it improved with the increase in the porosity of electrode. This shows the significance of 20CBD electrode to that of other inert electrodes like BDD, PbO2, and SnO2. 3.2. Electro-oxidation behavior of phenol and 2-chlorophenol over 20CBD. Prior to the electrolysis investigation, the anodic oxidation behaviors of phenol and 2chlorophenol were investigated over 20CBD electrode using cyclic voltammetry. Experiments were performed in an aqueous solution of 0.25 M Na2SO4 (pH 3) and in the presence and absence of 100 mg/L (1.04 mM) of phenol and 100 mg/L (0.78 mM) 2chlorophenol at a sweep rate of 100 mV/s. Fig. 2 shows the voltammogram of phenol and 2-chlorophenol on the 20CBD electrode, the anodic peak potentials were situated at 1.25 and 1.4 V vs. Ag/AgCl respectively. It is obvious that the anodic oxidation of 2chlorophenol on 20CBD was higher than that of phenol. The anodic peak current in the 0.25 M Na2SO4 (pH 3) solution containing 2-chlorophenol was higher than that obtained in the same solution containing phenol. Although, the molar concentration of phenol was higher than that of 2-chlorophenol for the same mass concentration (100 mg/L). Moreover, it is well known that the current peak value of current peak increases with concentration. However, the 2-chlorophenol current peak was higher than that of phenol, which suggests that the anodic oxidation of 2-chlorophenol on the 20CBD electrode was


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more active as compared to phenol. Furthermore, no cathodic peak appeared in phenol and 2-chlorophenol solutions, which indicates that the anodic oxidation of both solutions on the 20CBD electrode is irreversible. Fig. 3 elucidates the voltammogram of 100 mg/L 2-chlorophenol and 100 mg/L phenol in aqueous solution of 0.25 M Na2SO4 using different sweep rates of 25, 50, 100, 200, and 500 mVs-1 on 20CBD anode. The potential oxidation peaks of 2-chlorophenol and phenol, as well as the corresponding current peaks, increased with the increase in sweep rates, which shows that 2-chlorophenol and phenol anodic oxidation process are irreversible systems23, 24. Moreover, the plots of the square roots of the sweep rates against the current peaks showed a straight line. Thus, it can be assumed that on 20CBD anode the anodic oxidation of 2-chlorophenol and phenol is a diffusion controlled process. 3.2. Mixture of phenol and 2-chlorophenol electrochemical degradation Fig. 4(a) shows the anodic oxidation trend of mixture of 2-chlorophenol and phenol in 0.25 M Na2SO4 (pH 3) as the supporting electrolyte on the 20CBD electrode at 30 mA /cm2 and 25 °C. This figure elucidates the 2-chlorophenol oxidizing on 20CBD electrode was clearly higher than that for phenol. After 6 hours of electrolysis, the removal of phenol and 2-chlorophenol on the 20CBD were 21% and 94%, respectively. To investigate the effect of initial concentration of components on electro-degradation, the electrolysis of mixed solution of 200 mgL-1 2-chlorophenol + 300 mgL-1 phenol was conducted. As expected, the increase of initial concentration enhanced the degradation rate of 2-chlorophenol and phenol, as depicted in Fig. 4(b). The increase in degradation is attributed to the mass transport of chemicals from bulk to electrode surface due to higher diffusion rate. Moreover, this leads to the increased current efficiency of the electrode. Despite the higher initial phenol concentration than that of 2-chlorophenol the degradation behavior of both components remain unchanged, where after 6 hours, 2chlorophenol and phenol degraded up to 96% and 23% respectively. Fig. 5 shows the electro-degradation reaction products of 300 mgL-1 phenol + 200 mgL-1 2-chlorophenol. Benzoquinone and hydroquinone as aromatic chemicals and a mixture of fumaric acid, maleic acid, formic acid, and oxalic acid as carboxylic acid chemicals are observed as the main reaction products identified during electrolysis process. The



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hydroquinone and benzoquinone were the significant products during the initial hours of reaction, however, the amount of these products reduced later and the carboxylic acid substrates became the main products. The decay of hydroquinone and benzoquinone during reaction was attributed to the electro-oxidation of these substrates on electrode surface and produced carboxylic acid substrates. These results reveal that the aromatic intermediates were transformed into aliphatic carboxylic acids, because they reacted rapidly with hydroxyl radicals produced on 20CBD electrode surface 25. The anodic oxidation of mixture of 100 mgL-1 2-chlorophenol + 100 mgL-1 phenol was also investigated on platinum electrode with the same parameters that were used for 20CBD electrode. Fig. 6 shows the removal trend of the phenolic mixture on platinum electrode, which is similar to that of 20CBD anode. After 6 hour of reaction, the removal rate of 2-chlorophenol was higher than that of phenol with a quantitative removal of 75% and 41% respectively. The comparison between 20CBD electrode and platinum electrode shows that the removal rate of 2-chlorophenol on the 20CBD electrode was higher than that on the platinum electrode. On the other hand, the removal rate of phenol was higher on platinum anode. The analysis of electrochemical oxidation reaction products of phenolic mixture on 20CBD and platinum anodes brought to light that more aromatic intermediates accumulated in case of platinum anode, particularly benzoquinone which are considered more toxic than phenolic compounds

26

. In contrast, the use of 20CBD

anode led to produce the mixture of intermediates with less amounts of benzoquinone compared to carboxylic acids as depicted in Fig 5. This behavior can be attributed to the high oxidation power of 20CBD electrode as that of platinum electrode. Moreover, the molar balance of reacted phenol and 2-chlorophenol atoms on 20CBD electrode with detected intermediate atoms after 10 hours of reaction disclose that more than 70% of reacted substrates were converted into CO2. The highest detected intermediate was formic acid with 15% and then oxalic acid with 6.9%. Whereas, the lowest detected intermediate percent was quinone with 1.2% and then the benzoquinone with 1.8%. In contrast, on platinum electrode the highest detected intermediate was the benzoquinone with 28%. Three most important factors play a significant role in the anodic oxidation of phenolic compound. The first of them is the adsorption of molecules from bulk to the electrode


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surface, second is the passivation phenomenon, and third is reaction mechanism. It is well-known that during phenol oxidation reaction, a side reaction occurs, which produces the dimers, which later react together and generate oligomers tiny black particles

27, 28

.

However, the main oxidation reaction of phenol produces quinone then benzoquinone and later carboxylic acid as depicted in Fig 7(b). The dimer and oligomer formation reaction lead to consume high amount of current and reduces the phenol oxidation reaction on the electrode surface then causing fouling of electrode. On the other hand, most of the reported chlorophenol reaction mechanisms suggest that the reaction initiated by chloride radicals and the production of dimer and oligomer are scarce compared to phenol reaction as shown in Fig 7(a) 25, 29-31. 3.3 Adsorption of Phenol and 2-Chlorophenol on 20CBD electrode Electrochemical impedance technique offers the ability to investigate the adsorption of molecules, passivation phenomenon on the electrode surface and electrode/electrolyte interface for phenol and 2-chlorophenol aqueous solutions. To evaluate adsorption of phenol and 2-chlorophenol on 20CBD electrode, impedance with a potential of 0.35 V vs. Ag/AgCl was used. However, no significant cathodic current was observed as shown in Fig. 2, and frequency ranged from 0.01 to 105 Hz. Aqueous solutions of 100 mgL-1 phenol in 0.25 M Na2SO4 and 100 mgL-1 2-chlorophenol in 0.25 M Na2SO4 were used. Fig. 8(a) shows the Nyquist plot for 20CBD electrodes in phenol and 2-chlorophenol solutions, in which a semicircle was observed at high frequencies, whereas a straight line with a unit slope was found for low frequencies, which showed the kinetic and the mass transfer control regimes respectively

32

. Randles equivalent circuit was the best fit for

impedance of 20CBD anode, as depicted in the inset of Fig. 8(a). The fitting parameters consisted of the solution resistance (Rs), which was parallel with a combination of the impedance of the faradic and non-faradic reaction. The faradic reaction impedance consisted of Warburg resistance (W) together with charge transfer resistance (Rct). Other than that, the non-faradic impedance part was the double layer capacitance (Cdl)

33-35

.

Warburg impedance is mass transfer resistance for phenol or 2-chlorophenol and their reaction intermediates during diffusion from and to the electrode surface 36. It is obvious from Fig. 8(a) that the value of Warburg resistance in phenol solution is 53.2 Ohm/s0.5,



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which is higher than that in 2-chlorophenol (5.5 Ohm/s0.5) solution by almost ten folds. Warburg coefficient describes more precisely the practical diffusion behavior, and it is given in Eq. (2). According to the values of Warburg impedance and Eq. (2), the diffusion coefficients of 2-chlorophenol and phenol were 3.2×10-4 cm2/s and 1.79×10-6 cm2/s respectively. It indicates that the migration of phenol molecules from bulk solution to the electrode surface is difficult compared to the migration of 2-chlorophenol molecules. Furthermore, the value of double layer capacitance in 2-chlorophenol solution was 0.643µF, which is higher than that of phenol solution, which is 0.32 µF, showing the high adsorption of 2-chlorophenol. Maximum capacitance was fitted fairly well with the minimum value of the adsorption resistance parameter 37. Meanwhile, the charge transfer resistance in 2-chlorophenol solution was 22.41 Ohm, which is lower than that of phenol solution, which is 25.15 Ohm. A platinum electrode was selected as a conventional electrode to investigate the effect of phenolic molecules adsorption on electro-oxidation process using the same technique, which was used for 20CBD electrode. Fig 8(b) demonstrates the Nyquist plot for platinum electrode in phenol and 2-chlorophenol solutions. The impedance of platinum electrode was same that of 20CBD electrode, in which a semicircle was observed at high frequencies, and a straight line with a unit slope was found for low frequencies. The diffusion coefficients of 2-chlorophenol and phenol were estimated according to the Warburg impedance of platinum electrode and Eq. (2). The 2-chlorophenol diffusion coefficient was higher than that of phenol, the values were as 1.799×10-8 cm2/s and 7.1× 10-9 cm2/s respectively. Although, this result is congruent with that of 20CBD electrode, however the difference between 2-chlorophenol and phenol diffusion coefficients on platinum electrode is less than that on 20CBD electrode. This may be the reason for the removal rate variation between 2-chlrophenol and phenol on platinum electrode, which was lower than that occurred on 20CBD electrode. The increase in diffusivity coefficient and enhanced adsorption of 2-chlorophenol on 20CBD electrode surface compared to phenol is the existence of chlorine atom. The increase in chlorine atoms in phenol molecule causes the increase in adsorption rate of molecule

38-40

. Moreover, this phenomenon may be explained by the solubility of

phenolic substrates in water. Less solubility in water, higher the adsorption rate due to intramolecular hydrogen bonding

39

. Generally, solubility of phenol in water is higher


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than that of 2-chlorophenol, which enhances the adsorption of 2-chlorophenol compared to phenol. The specific interactions between phenolic molecules and water play a significant role in their diffusion coefficients. The water molecules favor, if located around hydroxyl group of phenol. Whereas, chlorine atom of 2-chlorophenol is nearest, which reduces the adsorption rate of phenol molecule 40. According to the mechanism of electrochemical oxidation, the diffusion of organic chemicals to the electrode surface is a key step, both for direct and indirect electro-degradation process 41. 3.4 Electrode passivation in phenol and 2-chlrorphenol solutions Electro-oxidation and passivation of Phenol and 2-chlorophenol on 20CBD electrode were studied through electrochemical impedance with 600 and 2000 mV using 0.25 M Na2SO4 containing 100 mgL-1 phenol and 100mgL-1 2-chlorophenol. The impedance spectrum was investigated in the frequency region of 10 to 105 Hz. The non-stationary of the system at high applied potentials due to passivation phenomenon during phenol and 2-chlorophenol electro-oxidation reduced the frequency range of impedance experiments, particularly in the low frequency region 42, 43. Fig. 9 evidences impedance plots of phenol and 2-chlorophenol oxidation. While the value of capacitive loop increased with applied potential value until it reached to a maximum value of 1300 mV for phenol oxidation and 1400 mV for 2-chlorophenol, which later was reduced with applied potential. The potential of the highest spectra value represents the oxidation potential. Meanwhile, the formation of the intermediate compounds during the anodic oxidation caused the blocking of the electrode surface and maximized the size of the spectra

44, 45

. These

results confirm the result of cyclic voltammetry. Fig. 10(a) illustrates the value of charge transfer resistance at oxidation potential in 2-chlorophenol solution is 52.4 Ohm, which is less than that in phenol solution (87.54 Ohm). According to which, electron transfer between electrode and electrolyte interface was faster in 2-chlorophenol solution, and this fact has a significant factor in the electrolysis process. Moreover, Fig 10(b) shows the spectra of consecutive scans at oxidation potentials of phenol and 2-chlorophenol. There was an increase in the spectra sizes in both electrolytes with successive scans, revealing the effect of electrode surface blockage on the charge transfer resistance values. However, charge transfer resistance (passivation resistance) of phenol was 125 Ohm, and



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Page 12 of 26

it is higher than that of 2-chlorophenol which is 80.75 Ohm. It is obvious that the passivation of the electrode during anodic oxidation of phenol is greater than that of 2chlorophenol. This is also significant reason confirming that the degradation of 2chlorophenol is more active on 20CBD electrode compared to phenol. Conclusion Cyclic voltammetry results show that the electro-oxidation of 2-chlorophenol over 20CBD electrode was more active than electro-oxidation of phenol. The oxidation potential of 2-chlorophenol and phenol was at 1400 mV and 1300 mV respectively. Meanwhile, the degradation rate of 2-chlorophenol was higher than that of phenol. After 6 hours of reaction, 94% of 2-chlorophenol was degraded whereas, phenol degraded only up to 20%. The electro-oxidation of the phenolic mixture on platinum anode has the same reaction trend as that of 20CBD. On the other hand, the mass transfer resistance values for 20CBD and platinum electrodes in phenol electrolyte were higher than that in 2chlorophenol electrolyte. Moreover, the charge transfer resistance at oxidation potential in phenol electrolyte was higher than that in 2-chlorophenol. Also, the 20CBD electrode passivation in phenol solution was greater than in 2-chlorophenol solution. These results demonstrate that the adsorption of 2-chlorophenol was higher than for phenol. On contrary, the passivation of the 20CBD electrode in phenol solution was higher than 2chlorophenol, which justifies the high degradation rate of 2-chlorophenol compared to phenol on the 20CBD electrode. Acknowledgement This work was carried at the Center for Separation Science and Technology (CSST) and was

financed

through

the

High

Impact

Research

UM.C/HIR/MOE/ENG/43.

References


Grant

Project

No.

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(19) Martin, H. B.; Argoitia, A.; Landau, U.; Anderson, A. B.; Angus, J. C., Hydrogen and oxygen evolution on boron‐doped diamond electrodes. Journal of the Electrochemical Society 1996, 143, (6), L133-L136. (20) Fujishima, A., Diamond electrochemistry. Elsevier: 2005. (21) Panizza, M.; Cerisola, G., Application of diamond electrodes to electrochemical processes. Electrochimica Acta 2005, 51, (2), 191-199. (22) Compton, R. G., Electrode Kinetics: Reactions: Reactions. Elsevier: 1987. (23) Mafatle, T.; Nyokong, T., Use of cobalt(II) phthalocyanine to improve the sensitivity and stability of glassy carbon electrodes for the detection of cresols, chlorophenols and phenol. Analytica Chimica Acta 1997, 354, (1–3), 307-314. (24) Xavier, J.; Ortega, E.; Ferreira, J.; Bernardes, A.; Pérez-Herranz, V., An electrochemical study of phenol oxidation in acidic medium. Int. J. Electrochem. Sci 2011, 6, 622-636. (25) Wang, H.; Wang, J. L., The cooperative electrochemical oxidation of chlorophenols in anode–cathode compartments. Journal of hazardous materials 2008, 154, (1), 44-50. (26) Polcaro, A. M.; Palmas, S., Electrochemical oxidation of chlorophenols. Industrial & engineering chemistry research 1997, 36, (5), 1791-1798. (27) Li, X.-y.; Cui, Y.-h.; Feng, Y.-j.; Xie, Z.-m.; Gu, J.-D., Reaction pathways and mechanisms of the electrochemical degradation of phenol on different electrodes. Water Research 2005, 39, (10), 1972-1981. (28) Tahar, N. B.; Savall, A., Mechanistic aspects of phenol electrochemical degradation by oxidation on a Ta/PbO2 anode. Journal of the electrochemical society 1998, 145, (10), 34273434. (29) Canizares, P.; Garcia-Gomez, J.; Saez, C.; Rodrigo, M., Electrochemical oxidation of several chlorophenols on diamond electrodes Part I. Reaction mechanism. Journal of applied electrochemistry 2003, 33, (10), 917-927. (30) Song, S.; Zhan, L.; He, Z.; Lin, L.; Tu, J.; Zhang, Z.; Chen, J.; Xu, L., Mechanism of the anodic oxidation of 4-chloro-3-methyl phenol in aqueous solution using Ti/SnO 2–Sb/PbO 2 electrodes. Journal of hazardous materials 2010, 175, (1), 614-621. (31) Coteiro, R.; De Andrade, A., Electrochemical oxidation of 4-chlorophenol and its byproducts using Ti/Ru0. 3M0. 7O2 (M= Ti or Sn) anodes: preparation route versus degradation efficiency. Journal of Applied Electrochemistry 2007, 37, (6), 691-698. (32) Yuan, X.-Z. R.; Song, C.; Wang, H.; Zhang, J., Electrochemical impedance spectroscopy in PEM fuel cells: fundamentals and applications. Springer Science & Business Media: 2009. (33) Sakharova, A.; Nyikost, L.; Pleskov, Y., Adsorption and partial charge transfer at diamond electrodes—I. Phenomenology: an impedance study. Electrochimica acta 1992, 37, (5), 973-978. (34) Hernando, J.; Lud, S. Q.; Bruno, P.; Gruen, D. M.; Stutzmann, M.; Garrido, J. A., Electrochemical impedance spectroscopy of oxidized and hydrogen-terminated nitrogeninduced conductive ultrananocrystalline diamond. Electrochimica Acta 2009, 54, (6), 1909-1915. (35) Bo, Z.; Wen, Z.; Kim, H.; Lu, G.; Yu, K.; Chen, J., One-step fabrication and capacitive behavior of electrochemical double layer capacitor electrodes using vertically-oriented graphene directly grown on metal. Carbon 2012, 50, (12), 4379-4387. (36) Hwang, S.; Lee, B. S.; Chi, Y. S.; Kwak, J.; Choi, I. S.; Lee, S.-g., Faradaic impedance titration and control of electron transfer of 1-(12-mercaptododecyl) imidazole monolayer on a gold electrode. Electrochimica Acta 2008, 53, (5), 2630-2636. (37) Zielinska, D.; Pierozynski, B., Electrooxidation of quercetin at glassy carbon electrode studied by ac impedance spectroscopy. Journal of Electroanalytical Chemistry 2009, 625, (2), 149-155.


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Figures

Figure 1. (a) Cyclic voltammogram of 20CBD electrode in an aqueous solution of 0.5 M H2 SO4 . (b) Chronoamperomtry result of 20CBD electrode in aqueous solution of 0.1M KH2PO4 containing 5mM K4Fe(CN)6. Inste is the plot of current vs. t-1/2. (c) Cyclic voltammograme curve of 20CBD electrode in aqueous solution of 0.5 M H2SO4 continuing 5mM K4Fe(CN)6. Scan rate 100 mV/s and 25℃ temperature


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Figure 2. Cyclic voltammetry of 20CBD electrode in blank aqueous solution of 0.25 M Na2SO4, aqueous solutions of 0.25 M Na2SO4 (pH 3) containing 100 mg/L phenol and 0.25 M Na2SO4 (pH 3) solution containing 100 mg/L 2-chlorophenol. Scan rate 100 mV/s and 25℃ temperature.



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Figure 3. Cyclic voltammetry of 20CBD electrode in (a) aqueous solutions of 0.25 M Na2SO4 (pH 3) containing 100 mg/L 2-chlorophenol (b) 0.25 M Na2SO4 (pH 3) solution containing 100 mg/L . Scan rate 20, 50, 100, 200 and 500 mV/s and 25℃ temperature.


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Figure 4. The electrochemical degradation of (a)100 mg/L 2-chlorophenol + 100 mg/L phenol and (b) 200 mg/L 2-chlorophenol + 300 mg/L phenol over 20CBD electrode with time (the pH: 3; the constant current density: 30 mA/cm2; volume: 100 mL; supporting electrolyte (Na2SO4) concentration: 0.25 M; temperature 25℃ ).



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Figure 5. Intermediates substrates of electro-degradation of 200 mg/L phenol + 300 mg/L 2-chlorophenol.


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Figure 6. The electrochemical degradation of 100 mg/L 2-chlorophenol + 100 mg/L phenol over platinume electrode with time (the pH: 3; the constant current density: 30 mA/cm2; volume: 100 mL; supporting electrolyte (Na2SO4) concentration: 0.25 M; temperature 25℃ ).



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Figure 7. Proposed reaction mechanism for the anodic degradation of (a) 2-chlorophenol and (b) phenol over 20CBD electrode.


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Figure 8. Nyquist plot at (a) 20CBD and (b) platinum electrodes in aqueous solution of 0.25 M Na2SO4 (pH 3) containing 100 mg/L phenol and another solution containing 100 mg/L 2-chlorophenol . Inset is equivalent circuit of the reaction.



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Figure 9. Nyquist plot of 20CBD electrode in aqueous solution of 0.25 M Na2SO4 ( pH 3) containing (a) 100 mg/L 2-clorophenol (b) 100 mg/L phenol at different applied potential.


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Figure 10. Nyquist plot of 20CBD electrode in aqueous solution of 0.25 M Na2SO4 (pH 3) containing 100 mg/L phenol and 100mg/L 2-chlorophenol at (a) oxidation potential, (b) for consecutive runs (run number 10) at oxidation potential.



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The TOC graphic.


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Effect of Adsorption and Passivation Phenomena ... - ACS Publications

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