CHEM. RES. CHINESE UNIVERSITIES 2010, 26(4), 630—635

Electrochemical Oxidation of Chlorimuron-ethyl on Ti/SnO2-Sb2O5/PbO2 Anode for Waste Water Treatment YU Shi-jun1,2, XUE Bin2, WANG Jian-ya2*, SUN Jian2 and SHEN Zhi-qiu1 1. School of Chemical Engineering, Dalian University of Technology, Dalian 116012, P. R. China; 2. School of Petrochemical Engineering, Shenyang University of Technology, Liaoyang 111003, P. R. China Abstract The electrochemical oxidation of chlorimuron-ethyl on Ti/SnO2-Sb2O5/PbO2 electrode was studied by cyclic voltammetry. The electrochemical behaviour of the electrode in a sodium sulfate solution and in the mixture solution of sodium sulfate and chlorimuron-ethyl was studied. The experimental results of cyclic voltammetry show that the acidic medium was suitable for the efficient electrochemical oxidation of chlorimuron-ethyl. Some electro-generated reagent was formed in the electrolysis process and chlorimuron-ethyl could be oxidized by the electro-generated reagent. A Ti/SnO2-Sb2O5/PbO2 electrode was used as the anode and the electrolysis experiment was carried out under the optimized conditions. The electrolysis process was monitored by UV-Vis spectrometry and high performance liquid chromatography(HPLC), and the chemical oxygen demand(COD) was determined by the potassium dichromate method. The mechanism of chlorimuron-ethyl to be oxided was studied primarily by the cyclic voltammetry and UV-Vis spectrometry. The results of electrolysis experiment demonstrate the possibility of the electrode to be used as an anode for the electrochemical treatment of chlorimuron-ethyl contained in waste water. Keywords Ti/SnO2-Sb2O5/PbO2 electrode; Chlorimuron-ethyl; Cyclic voltammetry; Electrochemical oxidation; Waste water treatment Article ID 1005-9040(2010)-04-630-06

1

Introduction

The use of direct and indirect electrochemical oxidation for the treatment of aqueous wastes has undergone rapid development in recent years[1―3]. This technology can be successfully applied in the treatment of waste water containing non-biodegradable organics such as herbicides, phenols, anilines[4―8]. The characteristics of waste water have a marked influence on the rate and efficiency of the electrochemical process because the organic compounds contained in the waste water may be oxidized in different forms. Moreover, the pH of the waste water greatly influences the current efficiency. In addition to the characteristics of waste water, operating conditions, such as current density and temperature play an important role in the electrochemical oxidation of organic waste water. The electrochemical activity of each organic compound depends on the anode material used. In fact, the spontaneous oxidation reaction by oxygen transfer is characterized by low rate constants when

carried out on a electrode of traditional materials such as Au, Pt and C[9―11]. It is now belived that the step preceding the transfer of an oxygen atom in the oxidation mechanism of organic substances in aqueous solution is the discharge of the water molecule, leading to an adsorbed hydroxyl radical[12,13]. The use of an anode material with a high oxygen evolution potential favors the oxygen transfer. Lead dioxide fulfills this criterium. Ti/PbO2 is a kind of material of metal oxide anode, which has been used in electrolysis industry as a dimension stable anode(DSA) due to its good characters of stability, higher oxygen evolution potential, conductivity, corrosion resistance and longer service life[14]. There may be crack between the electrodeposited PbO2 layer and the Ti substrate in the Ti/PbO2 electrode where PbO2 is electrodeposited on the Ti substrate directly, and the crack may cause the PbO2 layer to fall off. In recent years, the improvement for Ti/PbO2 electrode has been reported[15―19]. The researchers reported that the middle layers of SnO2SbOx between the PbO2 layer and the Ti substrate, that

——————————— *Corresponding author. E-mail: [email protected] Received June 29, 2009; accepted September 22, 2009. Supported by the Science and Technology Foundation of the Education Department of Liaoning Province, China (No.2009A557).

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is SnO2+Sb2O3 or SnO2+Sb2O5, made the electrode improved obviously. The electrodes with the middle layers of SnO2+Sb2O3 or SnO2+Sb2O5 are superior to the electrodes made by electrodepositing PbO2 on the Ti substrate directly, and so the surface crystal of PbO2 is fine and the specific surface area is aggrandized. The results of scanning electron microscope (SEM) measurement of the cross section of the electrode demonstrate that the middle layers of SnO2-SbOx were compact and united closely with the Ti substrate and the electrodeposited PbO2 layer. Therefore, the electrocatalytic activity, the electrochemical stability and the service life of the electrodes were improved evidently. In recent years, herbicides are widely used in agriculture all over the world because they can be selectively used and can be degraded easily in soil. Owing to their low solubility in water, herbicides may appear as pollutants in underground water, surface water and drinking water, and they could do great harm to the human and natural environment. The use of electrochemical oxidation for the treatment of herbicides in waste water has undergone rapid development in recent years because it only needs simple equipment and can not produce secondary pollution[20―22]. Chlorimuron-ethyl or ethyl 2-(4-chloro-6methoxypyrimidin-2-ylcarbamoylsulfamoyl) benzoate is one of the highly efficient and widely used herbicides, whose formula structure(C15H15ClN4O6S) is shown in Fig.1.

Fig.1

Structure of chlorimuron-ethyl

The goal of the present work is aimed at studying the electrochemical oxidation of chlorimuron-ethyl on Ti/SnO2-Sb2O5/PbO2 anode in order to evaluate the potential application of this electrode to the electrochemical treatment of chlorimuron-ethyl contained in waste water.

2 2.1

Experimental Preparation of Ti/SnO2-Sb2O5/PbO2 Electrode The Ti/SnO2-Sb2O5/PbO2 electrode was prepared

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via depositing the middle layer of SnO2-Sb2O5 by spray pyrolysis technique on the Ti substrate under high temperature oxidation, and then the surface layer of PbO2 was electrodeposited on the surface of the middle layer[15,23,24]. 2.2

Cyclic Voltammetry Experiments

The cyclic voltammetry experiments were carried out with a three-electrode cell(50 mL) at room temperature. Meshy Ti/SnO2-Sb2O5/PbO2 electrode of 2 cm2 was used as working electrode, a platinum wire electrode was used as the counter electrode and a saturated calomel electrode(SCE) was used as the reference electrode. To increase the reproducibility of the polarization measurements, the electrode was electrochemically preconditioned before use and in polarizing the electrodes the preconditioning was conducted for 30 min in a 1.0 mol/L H2 SO4 solution at an anodic current density of 50 mA/cm2. The cyclic voltammograms were determined and recorded with a CHI 620 Electrochemical Workstation(CH Instruments, USA) controlled by a computer. A 0.1 mol/L Na2SO4 solution was used as supporting electrolyte for the cyclic voltammetric study, and the pH was adjusted by adding an appropriate amount of a H2SO4 or NaOH solution. 2.3

Electrolysis

A DJS 292 potentiostat was used for the electrolysis experiments. These electrolysis experiments were carried out in a one-compartment cell with 100 cm3 of electrolyte solution under constant current density condition. Meshy Ti/SnO2-Sb2O5/PbO2 electrode of 6.0 cm2 was used as the anode, and a stainless steel electrode with the same area was used as the cathode. Preconditioning of the anode was conducted by polarizing the electrodes for 30 min in 1 mol/L H2SO4 at an anodic current density of 50 mA/cm2. The potential was constant during each electrolysis, indicating that neither appreciable deterioration of the electrode nor passivation phenomena took place. A 0.02 mol/L Na2SO4 solution was used as supporting electrolyte for the electrolysis studies, and the pH was adjusted by adding an appropriate amount of a H2SO4 or NaOH standard solution. 2.4

Analytical Procedures UV-Vis spectra were measured and recorded on a

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TU-1900 UV-Vis spectrophotometer. The concentration of chlorimuron-ethyl was determined by HPLC[25] on a HPLC serries 200(Perkin-Elmer company). Chemical oxygen demand(COD) was determined by the potassium dichromate method. All the chemicals were analytical-reagent grade and all the solutions were prepared with distilled water.

3 3.1

Results and Discussion Cyclic Voltammetry

The cyclic voltammetric experiments were performed at a sweep rate of 100 mV/s and the scanning potential range was from 0.0 to 1.8 V. The cyclic voltammograms obtained in a 0.1 mol/L Na2SO4 solution and in a 2.2×10–6 mol/L chlorimuron-ethyl+0.1 mol/L Na2 SO4 solution are shown in Fig.2.

in Fig.2, we can see that the anodic current in the potential range of more than 1.0 V is larger in the chlorimuron-ethyl solution than that in blank solution. It can be resulted that chlorimuron-ethyl can not only be oxidized directly on the anode surface but can also be oxidized by the electro-generated reagent in the substrate solution under the experimental conditions. The transfer rate of the electro-generated reagent from the anode surface to the substrate is accelerated by the reaction of chlorimuron-ethyl oxidized by the electro-generated reagent, and the anodic current of the electro-generated reagent forming is therefore enlarged. Under the experimental conditions, the oxygen evolution potential starts from about 1.5 V. In the potential range of oxygen evolution, the anodic current in the chlorimuron-ethyl solution is enlarged faster than that in blank solution. It is reasonable to assume that chlorimuron-ethyl can be oxidized fast on the electrode surface by the electro-generated reagent and the evolution oxygen, and the oxidizing reaction accelerates the mass transfer rate of the electrogenerated reagent, such as peroxodisulphate and oxygen, from the electrode surface to the substrate solution. 3.2

Fig.2

Comparasion of cyclic voltammograms of blank(a) and chlorimuron-ethyl solution(b) c(Chlorimuron-ethyl)=5.0×10–6 mol/L. c(Na2SO4)=0.1 mol/L, sweep rate: 100 mV/s; pH=4.0.

It can be observed from the curve a in Fig.2 that there is an anodic current peak in the cyclic voltammogram recorded during the potential sweeping in the blank solution. According to the reference report[6], the anodic current peak was derived from the forming of electro-generated reversible redox reagent, such as peroxodisulphate. The experimental results show that sulphate ions in the solution could be oxidized on the surface of the Ti/SnO2-Sb2O5/PbO2 working electrode, and the electro-generated redox reagent could be formed in the lower potential range than the oxygen evolution potential. The reference reported that the electro-generated reagent played an important role in the global oxidation rate, but the electro-generated reagent did not influence the reaction mechanism of the pollutants being oxidized on the anode surface, and later it acted as the intermediary for shuttling electrons between the pollutant substrate and the electrode. Comparing the cyclic voltammograms a and b

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Influence of pH on the Oxidizing Reaction

The influence of pH on the reaction of chlorimuron-ethyl being oxidized on the Ti/SnO2-Sb2O5/PbO2 anode was investigated within pH range from 2.0 to 9.0. The cyclic voltammetric experiments were performed at a sweep rate of 100 mV/s in a 0.1 mol/L Na2SO4 solution and 2.2×10–6 mol/L chlorimuronethyl, within a potential range from 0.0 to 1.8 V. The voltammetric curves obtained are shown in Fig.3. It can be seen from Fig.3 that in a concentrated acidic medium, the anodic current peak is higher than that in

Fig.3

Cyclic voltammograms under different pH conditions in chlorimuron-ethyl solution

pH: a. 2.0; b. 4.0; c. 7.0; d. 9.0. c(Chlorimuron-ethyl)=5.0×10–6 mol/L; c(Na2SO4)=0.1 mol/L; sweep rate: 100 mV/s.

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a less acidic medium(Fig.3 curves a, b and c). It can be resulted that the electro-generated reagent is formed easily in a more concentrated acidic medium, and chlorimuron-ethyl can be oxidized easily in a concentrated acidic medium. In the less acidic media, the rates of forming the electro-generated reagent and that of the chlorimuron-ethyl oxidized by the electro-generated reagent, as well as that of the chlorimuron-ethyl being oxidized directly on the electrode surface decreased, therefore, the anodic current formed by the electro-generated reagent decreased. In a basic medium, the potential of oxygen evolution was lower than that in the acidic medium, so the pH condition was not suitable for the oxidation of chlorimuron-ethyl. Thus, it seems reasonable to assume that concentrated acidic media are good conditions for chlorimuron-ethyl being oxidized directly on the anode, the forming of electro-generated reagent and the oxidizing of chlorimuron-ethyl by the electro-generated reagent in the substrate solutions. 3.3 Influence of Chlorimuron-ethyl Concentration on Oxidizing Reaction The influence of the chlorimuron-ethyl concentration on the oxidizing reaction on the Ti/SnO2Sb2O5/PbO2 anode was investigated in a concentration range from 0 to 2.16×10–5 mol/L chlorimuron-ethyl. The cyclic voltammetric experiments were performed at a sweep rate of 100 mV/s in a 0.1 mol/L Na2SO4 solution at pH=4.0 and within a scanning potential range from 0.0 to 1.8 V. The cyclic voltammetric curves obtained are shown in Fig.4.

Fig.4

Cyclic voltammograms in chlorimuron-ethyl solutions with different concentrations c(Na2SO4)=0.1 mol/L, sweep rate: 100 mV/s, pH=4.0. c(Chlorimuron-ethyl)/(mol·L–1): a. 0; b. 2.2×10–6; c. 1.08× 10–5; d. 2.16×10–5.

The reactions of chlorimuron-ethyl being oxidized on the anode surface and being oxidized by the electro-generated reagent in the substrate solution are actually very complex processes, involving several

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oxidation stages, the mechanism of the fouling formation and the nature of substrate solution all depend, to a large extent, on the experimental conditions, such as chlorimuron-ethyl concentration, pH value, polarization potential, polarization time. It can be seen from Fig.4 that in a potential range from 1.0 V to 1.5 V, the anodic current increased with the increasing of the chlorimuron-ethyl concentration in a lower concentration range of chlorimuron-ethyl, the result is shown by curves a and b in the inset in Fig.4. In the same potential range, the anodic current decreased with the increasing of the chlorimuron-ethyl concentration in a higher concentration range of chlorimuron-ethyl, the result is shown by curves b, c and d in the inset in Fig.4. In the potential range of oxygen evolution(the potential values more than about 1.5 V), the anodic current increased with the increasing of the chlorimuron-ethyl concentration in a lower concentration range of chlorimuron-ethyl, the result is shown by curves a, b and c in Fig.4. In the same potential range, the anodic current decreased with the increasing of the chlorimuron-ethyl concentration in a higher concentration range of chlorimuron-ethyl, the result is shown by curves c and d in Fig.4. It is reasonable to assume that the results derived from the chlorimuron-ethyl being oxidized directly on the electrode surface and the fouling formation by the production of incomplete oxidization of chlorimuron-ethyl. It can be resulted that chlorimuron-ethyl not only can be oxidized by the electro-generated reagent in the substrate solution but can also be oxidized directly on the anode surface under the experimental conditions. The transfer rate of the electro-generated reagent from the anode surface to the reaction substrate can be accelerated by the reaction of chlorimuron-ethyl oxidized by the electro-generated reagent but can be reduced by the fouling formed. Fortunately, the fouling can be oxidized completely under a higher potential condition, such as under the oxygen evolution potential condition, and the further oxidation reaction can make chlorimuron-ethyl mineralized completely, which can be illuminated by the changing rule of the anodic current. 3.4 Electrolysis for the Degradation of Chlorimuron-ethyl To test the possibility of using Ti/SnO2-Sb2O5/ PbO2 anode for the electrochemical incineration of

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chlorimuron-ethyl, electrolysis experiments under constant current conditions were carried out. A Ti/SnO2-Sb2O5/PbO2 electrode with an area of 6.0 cm2 was used as the anode, and a stainless steel with the same area as that of the anode was used as the cathode. The factors which influence the oxidation of chlorimuron-ethyl, such as current density, electrolyte, pH value, space between the anode and cathode were optimized, and the optimum electrolyte condition was adopted in the electrolysis experiments. The space between the anode and cathode was 10 mm, the concentration of the support electrolyte, Na2SO4, was 0.02 mol/L, the pH was 4.5, the current density was 40 mA/cm2, and the initial concentration of chlorimuron-ethyl was 3.6×10–5 mol/L. The concentration was monitored real timely by HPLC and the result is shown in Fig.5. In Fig.5, c0 is the initial concentration of chlorimuron-ethyl, and c is the monitored concentration of chlorimuron-ethyl at the corresponding electrolysis time.

Fig.5

Degradation curve of chlorimuron-ethyl in electrolysis

The electrochemical oxidation of chlorimuronethyl was a complex process, the UV-Vis spectra were recorded in the electrolysis of chlorimuron-ethyl and they are shown in Fig.6. It can be seen from Fig.5 and Fig.6 that chlorimuron-ethyl was rapidly degraded by

Fig.6

UV-Vis spectra of chlorimuron-ethyl at different electrolysis time Electrolysis time/min: a. 0; b. 1.0; c. 2.0; d. 5.0; e. 15.0; f. 45; g. 100; h. 160; i. 300.

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electrochemical oxidation, and some intermediate products were formed in the process of the electrolysis, and finally, the intermediate products were destroyed almost completely. The chemical oxygen demand(COD) was monitored with potassium dichromate method in the process of chlorimuron-ethyl degradation and the result was shown in Fig.7. It can be seen from Fig.7 that the COD decreased nearly directly with the electrolysis time during the first 30 min. With further electrolysis, the rate of COD being removed was gradually slow with the increasing of electrolysis time. These results indicate that the oxidation reaction was a kinetic-controlled in the first 30 min and then was a diffusion-controlled when the organic pollutant was in a small concentration in the electrolysis time which was longer than about 30 min. The curve of COD being removed shows that under the above experimental conditions, chlorimuron-ethyl could be almost completely degraded and was fully removed.

Fig.7

4

COD removed during the electrolysis

Conclusions

The research of the electrochemical behavior of chlorimuron-ethyl on the Ti/SnO2-Sb2O5/PbO2 anode shows that chlorimuron-ethyl could be oxidized directly on the surface of the electrode, and the acidic medium is suitable for the efficient electrochemical oxidation of chlorimuron-ethyl, and sulfate is a suitable medium for the forming of electro-generated reagent on the surface of the anode. chlorimuron-ethyl could be oxidized by the electro-generated reagent not only in the substrate solution but also on the anode surface under the experimental conditions. The fouling formed by the production of incomplete oxidization of chlorimuron-ethyl on the surface of the electrode could be oxidized completely at a higher potential, and the further oxidation reaction could make chlorimuron-ethyl mineralized completely. The electrolysis experimental results demonstrate that it is

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possible to use Ti/SnO2-Sb2O5/PbO2 as an anode for electrochemical treatment of waste water containing chlorimuron-ethyl.

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