Study on Electrochemical Degradation of Nicosulfuron by IrO2-Based DSA Electrodes: Performance, Kinetics, and Degradation Mechanism

The widely used sulfonylurea herbicides have caused negative effects on the environment and human beings. Electrochemical degradation has attracted much attention in the treatment of refractory organic compounds due to its advantage of producing no secondary pollution. Three kinds of IrO2-based dimensionally stable anodes (DSAs) were used to degrade nicosulfuron by a batch electrochemical process. The results showed that a well-distributed crack network was formed on the Ti/Ta2O5-IrO2 electrode and Ti/Ta2O5-SnO2-IrO2 electrode due to the different coefficients of thermal expansion between the Ti substrate and oxide coatings. The oxygen evolution potential (OEP) increased according to the order of Ti/RuO2-IrO2 < Ti/Ta2O5-SnO2-IrO2 < Ti/Ta2O5-IrO2. Among the three electrodes, the Ti/Ta2O5-IrO2 electrode showed the highest efficiency and was chosen as the experimental electrode. Single factor experiments were carried out to obtain the optimum electrolysis condition, shown as follows: currency intensity 0.8 A; electrode spacing 3 cm, electrolyte pH 3. Under the optimum conditions, the degradation of nicosulfuron followed first-order kinetics and was mainly due to indirect electrochemical oxidation. It was a typical diffusion-controlled electrochemical process. On the basis of the intermediate identified by high performance liquid chromatograph-mass spectrometry (HPLC-MS), two possible degradation routes were proposed.


Introduction
Sulfonylurea herbicides have been considered as "new formulation pesticides" because of their high selectivity and low persistence in the environment. They have been widely used to control a variety of broad-leafed weeds and grasses [1,2]. In recent years, with high toxicity and high residue herbicides fading out of the market, the production and application of sulfonylurea herbicides have been developing more rapidly. Their sales have accounted for more than 11% of the global herbicide market [3,4]. Sulfonylurea herbicides are highly soluble in water and have moderate to high mobility. Some of the commonly used sulfonylurea herbicides are able to remain in natural environmental medium such as soil and water [5,6]. The residual sulfonylurea herbicides can be transmitted through drinking water, the atmosphere, and plants, causing negative effects to the environment and human beings [7][8][9]. With China's accession to the World Trade Organization, a large number of agricultural products have been exported to the United States, the European Union, and other regions. However, due to pesticide residues, many products are not able to meet the export standards. Therefore, how to remove the residues of sulfonylurea herbicides has become an urgent problem in China [10].

Experimental Materials
Nicosulfuron was obtained from Jingbo Agrochemicals Technology Co., LTD., Shandong, China, and used directly without any further purification. Its structural formula and general characteristics are shown in Table 1. The methanol and acetic acid used in HPLC were of chromatographic grade. The other chemicals were of analytical grade and used without further purification. Solution was prepared using deionized water with the resistivity near to 18.3 M⋅cm.

Analytical Methods
The surface morphology and composition of the electrodes were characterized by a SEM (Regulus8220, Hitachi, Japan) and x-ray energy dispersive spectrometer (EDS, QUANTAX, Bruker, Germany). The electrochemical performance of the IrO2-based electrodes was executed using LSV and CV at room temperature. A computer-controlled electrochemical workstation (CHI 660C, Shanghai Chenhua, China) with a conventional three-electrode cell system was used. The fabricated electrode (5 mm × 5 mm) was used as the working electrode, and a platinum sheet (5 mm × 5 mm) was used as the counter electrode. The saturated calomel electrode (SCE) was employed as the reference electrode. Before each experiment, it was cleaned by flame annealing and quenching with pure water [33].
The concentrations of nicosulfuron were measured with a HPLC (LC-10ATvp, SHIMADZU Corporation, Japan), using a mobile phase of methanol/water/acetic acid of 60:40:2 (V/V/V). Before the analysis, the mobile phase was sonicated for 30 min to remove the dissolved gas. The separation was performed using an InertSustain C18 column (150 mm × 4.6 mm × 5 μm, SHIMADZU Corporation, Tokyo, Japan) at the flow rate of 0.8 mL/min and column temperature of 303 K. The detection wavelength was 254 nm.
The intermediate products of nicosulfuron degradation were identified by HPLC-MS (Agilent 6520B, Agilent Technologies, Santa Clara, CA, USA), equipped with an electrospray ionization source (ESI). As for the HPLC condition, methanol/water/acetic acid (60:40:2) was used as mobile phase and flow rate was set to 0.4 mL/min with a Shim-pack VP-ODS liquid chromatography column (150 mm × 4.6 mm × 5 μm, SHIMADZU Corporation, Tokyo, Japan). The injection volume was 1 μL and the column temperature was 303 K. The MS conditions were in positive ion detection mode and mass The methanol and acetic acid used in HPLC were of chromatographic grade. The other chemicals were of analytical grade and used without further purification. Solution was prepared using deionized water with the resistivity near to 18.3 MΩ·cm.

Analytical Methods
The surface morphology and composition of the electrodes were characterized by a SEM (Regulus8220, Hitachi, Japan) and x-ray energy dispersive spectrometer (EDS, QUANTAX, Bruker, Germany). The electrochemical performance of the IrO 2 -based electrodes was executed using LSV and CV at room temperature. A computer-controlled electrochemical workstation (CHI 660C, Shanghai Chenhua, China) with a conventional three-electrode cell system was used. The fabricated electrode (5 mm × 5 mm) was used as the working electrode, and a platinum sheet (5 mm × 5 mm) was used as the counter electrode. The saturated calomel electrode (SCE) was employed as the reference electrode. Before each experiment, it was cleaned by flame annealing and quenching with pure water [33].
The concentrations of nicosulfuron were measured with a HPLC (LC-10ATvp, SHIMADZU Corporation, Japan), using a mobile phase of methanol/water/acetic acid of 60:40:2 (V/V/V). Before the analysis, the mobile phase was sonicated for 30 min to remove the dissolved gas. The separation was performed using an InertSustain C18 column (150 mm × 4.6 mm × 5 µm, SHIMADZU Corporation, Tokyo, Japan) at the flow rate of 0.8 mL/min and column temperature of 303 K. The detection wavelength was 254 nm.
The intermediate products of nicosulfuron degradation were identified by HPLC-MS (Agilent 6520B, Agilent Technologies, Santa Clara, CA, USA), equipped with an electrospray ionization source (ESI). As for the HPLC condition, methanol/water/acetic acid (60:40:2) was used as mobile phase and flow rate was set to 0.4 mL/min with a Shim-pack VP-ODS liquid chromatography column (150 mm × 4.6 mm × 5 µm, SHIMADZU Corporation, Tokyo, Japan). The injection volume was 1 µL and the column temperature was 303 K. The MS conditions were in positive ion detection mode and mass range was between 50 m/z and 1600 m/z. The following parameters were used: ion source temperature 373 K, dry gas flow rate 10 L/min, atomizer pressure 35 psig, capillary voltage 3500 V, capillary outlet voltage 120 V, cone voltage 65 V, and eight-pole RF voltage 750 V.

Electrolysis Experiment
Electrolysis experiments were performed by batch processes under galvanostatic condition to study the electrocatalytic performance of the electrodes (Figure 1). The IrO 2 -based electrodes were used as the anode accompanied with a Ti electrode of the same size as the cathode. To examine the influencing parameters, a volume of 400 mL nicosulfuron solution was used to conduct the electrolysis experiments. The reaction conditions included current intensity (0.3 A, 0.5 A, 0.8 A, and 1.0 A), pH value (1.0, 3.0, 6.0, and 9.0), and electrode spacing (1.5 cm, 3 cm, 4.5 cm, and 6 cm). For the supporting electrode, 0.1 M Na 2 SO 4 was used. During the experiments, samples were drawn from the reactor at certain time intervals. The concentration of nicosulfuron was quantitatively analyzed by HPLC. Each experiment was conducted three times. The results used the average values and the relative standard deviations (RSDs) were less than 5%. range was between 50 m/z and 1600 m/z. The following parameters were used: ion source temperature 373 K, dry gas flow rate 10 L/min, atomizer pressure 35 psig, capillary voltage 3500 V, capillary outlet voltage 120 V, cone voltage 65 V, and eight-pole RF voltage 750 V.

Electrolysis Experiment
Electrolysis experiments were performed by batch processes under galvanostatic condition to study the electrocatalytic performance of the electrodes (Figure 1). The IrO2-based electrodes were used as the anode accompanied with a Ti electrode of the same size as the cathode. To examine the influencing parameters, a volume of 400 mL nicosulfuron solution was used to conduct the electrolysis experiments. The reaction conditions included current intensity (0.3 A, 0.5 A, 0.8 A, and 1.0 A), pH value (1.0, 3.0, 6.0, and 9.0), and electrode spacing (1.5 cm, 3 cm, 4.5 cm, and 6 cm). For the supporting electrode, 0.1 M Na2SO4 was used. During the experiments, samples were drawn from the reactor at certain time intervals. The concentration of nicosulfuron was quantitatively analyzed by HPLC. Each experiment was conducted three times. The results used the average values and the relative standard deviations (RSDs) were less than 5%.

Structure and Morphology of the Electrode Coatings
The SEM micrographs of the IrO2-based electrodes as well as their EDS elemental analysis are shown in Figure 2. The three kinds of IrO2-based electrodes all showed "cracked" structures, which was a typical feature of oxide electrodes formed during heat-expansion and cold-contraction processes. The formation of the crack was due to the different coefficients of thermal expansion between the Ti substrate and oxide coatings [37,38]. The cracks connected with each other as networks. Especially, the networks were well-regulated and well-distributed on the surface of the Ti/Ta2O5-IrO2 electrode and Ti/Ta2O5-SnO2-IrO2 electrode, which helped to increase the real surface area of the electrode. Their corresponding EDS spectra displayed clear oxygen and iridium peaks for each electrode. The distinct additive metal peak was present for each of the binary oxide coatings. Further quantitative analysis showed that the metal oxide for each binary composition was present in approximately 20 %wt.

Structure and Morphology of the Electrode Coatings
The SEM micrographs of the IrO 2 -based electrodes as well as their EDS elemental analysis are shown in Figure 2. The three kinds of IrO 2 -based electrodes all showed "cracked" structures, which was a typical feature of oxide electrodes formed during heat-expansion and cold-contraction processes. The formation of the crack was due to the different coefficients of thermal expansion between the Ti substrate and oxide coatings [37,38]. The cracks connected with each other as networks. Especially, the networks were well-regulated and well-distributed on the surface of the Ti/Ta 2 O 5 -IrO 2 electrode and Ti/Ta 2 O 5 -SnO 2 -IrO 2 electrode, which helped to increase the real surface area of the electrode. Their corresponding EDS spectra displayed clear oxygen and iridium peaks for each electrode. The distinct additive metal peak was present for each of the binary oxide coatings. Further quantitative analysis showed that the metal oxide for each binary composition was present in approximately 20% wt.

Linear Sweep Voltammetry Measurement (LSV)
The oxygen evolution potential (OEP) can characterize the difficulty of the oxygen evolution reaction on the electrodes. A high OEP is advantageous for restraining the occurrence of side reactions, enhancing the organics degradation ability and weakening the influence of oxygen to peel the catalyst layer. Therefore, the OEP is positively correlated with the organics degradation ability of the anodes [39]. The performance of the electrodes varied with the baseplate composition and preparation method of the oxide film. Higher oxygen evolution potential can be obtained by improving the material and structure of electrodes [40,41]. In this paper, LSV curves were used to examine the OEPs of the three electrodes. As shown in Figure 3, the OEPs increased according to the order of Ti/RuO2-IrO2 < Ti/Ta2O5-SnO2-IrO2 < Ti/Ta2O5-IrO2. The Ti/Ta2O5-IrO2 electrode with higher OEP should have better degradation performance and lower power consumption during the electrochemical degradation process.

Linear Sweep Voltammetry Measurement (LSV)
The oxygen evolution potential (OEP) can characterize the difficulty of the oxygen evolution reaction on the electrodes. A high OEP is advantageous for restraining the occurrence of side reactions, enhancing the organics degradation ability and weakening the influence of oxygen to peel the catalyst layer. Therefore, the OEP is positively correlated with the organics degradation ability of the anodes [39]. The performance of the electrodes varied with the baseplate composition and preparation method of the oxide film. Higher oxygen evolution potential can be obtained by improving the material and structure of electrodes [40,41]. In this paper, LSV curves were used to examine the OEPs of the three electrodes. As shown in Figure 3 Figure 4 shows typical CV curves of three IrO2-based electrodes in the long potential range between −1.0 V and +1.8 V at a scan rate of 50 mV/s. The shape indicates that their initial OEP was about +1.3 V. When the applied potential was higher than +1.3 V, the current increased with increasing electrode potential. Thus, the CV curves were measured between −0.1 V and +1.0 V in the short potential region. The electrochemical effective surface areas of the three IrO2-based electrodes were estimated based on the CV curves recorded in the short potential, revealing that the electrochemical active surface areas reduced in the following order: Ti/Ta2O5-IrO2 > Ti/Ta2O5-SnO2-IrO2 > Ti/RuO2-IrO2. This was consistent with the SEM images shown in Figure 2.   Figure 4 shows typical CV curves of three IrO 2 -based electrodes in the long potential range between −1.0 V and +1.8 V at a scan rate of 50 mV/s. The shape indicates that their initial OEP was about +1.3 V. When the applied potential was higher than +1.3 V, the current increased with increasing electrode potential. Thus, the CV curves were measured between −0.1 V and +1.0 V in the short potential region. The electrochemical effective surface areas of the three IrO 2 -based electrodes were estimated based on the CV curves recorded in the short potential, revealing that the electrochemical active surface areas reduced in the following order: Ti/ Ta Figure 4 shows typical CV curves of three IrO2-based electrodes in the long potential range between −1.0 V and +1.8 V at a scan rate of 50 mV/s. The shape indicates that their initial OEP was about +1.3 V. When the applied potential was higher than +1.3 V, the current increased with increasing electrode potential. Thus, the CV curves were measured between −0.1 V and +1.0 V in the short potential region. The electrochemical effective surface areas of the three IrO2-based electrodes were estimated based on the CV curves recorded in the short potential, revealing that the electrochemical active surface areas reduced in the following order: Ti/Ta2O5-IrO2 > Ti/Ta2O5-SnO2-IrO2 > Ti/RuO2-IrO2. This was consistent with the SEM images shown in Figure 2.

The Effect of Electrode Material
Electrode materials are the most important part of electrochemical oxidation reactors. Therefore, the development of electrodes with good performance and high stability has become a major research focus [42]. Three kinds of IrO 2 -based electrodes were used as anodes to degrade nicosulfuron under the condition of current intensity 0.3 A, electrodes spacing 3 cm, and pH 6. The effects of different electrodes on the removal rate are shown in Figure 5.

The Effect of Electrode Material
Electrode materials are the most important part of electrochemical oxidation reactors. Therefore, the development of electrodes with good performance and high stability has become a major research focus [42]. Three kinds of IrO2-based electrodes were used as anodes to degrade nicosulfuron under the condition of current intensity 0.3 A, electrodes spacing 3 cm, and pH 6. The effects of different electrodes on the removal rate are shown in Figure 5. Ti electrodes coated with transient metal oxides have been studied extensively due to their excellent electrocatalytic activities [43]. Electrodes with different coatings exhibit different electrocatalytic properties. As shown in the Figure 5, the catalytic performance of the Ti/Ta2O5-IrO2 electrode was better than that of the two other electrodes. The reason might be that the Ti/Ta2O5-IrO2 electrode could provide a larger surface area and higher OEP for the electrochemical reaction. In the following experiment, the Ti/Ta2O5-IrO2 electrode was chosen as the experimental electrode.

The Effect of Current Intensity
Current intensity is an important experimental parameter that affects the electrochemical oxidation process, as it determines the production capacity of hydroxyl radicals (·OH s) on the electrode, which decides the removal rate of organic matter in aqueous solutions [44]. However, the higher the current intensity is, the greater the power consumption is. Thus, it is extremely essential to choose an appropriate current intensity to achieve a high electrolysis efficiency along with a low energy consumption. A suitable current intensity is essential for achieving high electrolysis efficiency with low power consumption. Ti electrodes coated with transient metal oxides have been studied extensively due to their excellent electrocatalytic activities [43]. Electrodes with different coatings exhibit different electrocatalytic properties. As shown in the Figure 5, the catalytic performance of the Ti/Ta 2 O 5 -IrO 2 electrode was better than that of the two other electrodes. The reason might be that the Ti/Ta 2 O 5 -IrO 2 electrode could provide a larger surface area and higher OEP for the electrochemical reaction. In the following experiment, the Ti/Ta 2 O 5 -IrO 2 electrode was chosen as the experimental electrode.

The Effect of Current Intensity
Current intensity is an important experimental parameter that affects the electrochemical oxidation process, as it determines the production capacity of hydroxyl radicals (·OH s) on the electrode, which decides the removal rate of organic matter in aqueous solutions [44]. However, the higher the current intensity is, the greater the power consumption is. Thus, it is extremely essential to choose an appropriate current intensity to achieve a high electrolysis efficiency along with a low energy consumption. A suitable current intensity is essential for achieving high electrolysis efficiency with low power consumption.
The effect of current intensity on nicosulfuron electrolysis is shown in Figure 6. The increase of current intensity had a positive influence on the degradation of nicosulfuron in the range of 0.3-1.0 A. When the current intensity was 0.8 A, nicosulfuron removal rate reached 83.3% after electrolysis for 7 h, which was almost the same as that of 1.0 A. The increase of current intensity had dual effects on electrolysis. The high current intensity helps to speed up the production rate of ·OH and then accelerate the oxidation rate of organic matter on the electrode surface [45]. However, the excessive current intensity was conducive to the occurrence of side reactions, consuming the formed ·OH and reducing its availability. The current intensity was too low to meet the energy requirement for electrolysis. The selected optimal current intensity in this experiment was 0. The effect of current intensity on nicosulfuron electrolysis is shown in Figure 6. The increase of current intensity had a positive influence on the degradation of nicosulfuron in the range of 0.3-1.0 A. When the current intensity was 0.8 A, nicosulfuron removal rate reached 83.3% after electrolysis for 7 hours, which was almost the same as that of 1.0 A. The increase of current intensity had dual effects on electrolysis. The high current intensity helps to speed up the production rate of ·OH and then accelerate the oxidation rate of organic matter on the electrode surface [45]. However, the excessive current intensity was conducive to the occurrence of side reactions, consuming the formed ·OH and reducing its availability. The current intensity was too low to meet the energy requirement for electrolysis. The selected optimal current intensity in this experiment was 0.8 A.

The Effect of Electrode Spacing
Electrolytic experiments were performed at different spacings to study the influence on nicosulfuron degradation and results are shown in Figure 7. When the electrode spacing was 1.5 cm, the highest removal rate was obtained. In the same electrolyte system, the smaller the electrode spacing was, the shorter the distance the electrons were required to transfer, which was advantageous in terms of increasing energy transfer during the electrolysis process. Therefore, the short distance was helpful for increasing the electrolysis efficiency [19]. However, the excessively small electrode spacing would lead to a low treating volume between the electrodes, which was not beneficial to improving treatment efficiency. Herein, the electrode spacing of 3 cm was chosen for the rest of the experiments.

The Effect of Electrode Spacing
Electrolytic experiments were performed at different spacings to study the influence on nicosulfuron degradation and results are shown in Figure 7. When the electrode spacing was 1.5 cm, the highest removal rate was obtained. In the same electrolyte system, the smaller the electrode spacing was, the shorter the distance the electrons were required to transfer, which was advantageous in terms of increasing energy transfer during the electrolysis process. Therefore, the short distance was helpful for increasing the electrolysis efficiency [19]. However, the excessively small electrode spacing would lead to a low treating volume between the electrodes, which was not beneficial to improving treatment efficiency. Herein, the electrode spacing of 3 cm was chosen for the rest of the experiments.

The Effect of Electrolyte pH
Solution pH is one of the most important factors that affects the performance of electrochemical processes. The effect of pH values ranging from 1 to 9 on the nicosulfuron degradation was investigated, and the results are presented in Figure 8. The removal rate of nicosulfuron was higher in acid conditions or in alkaline conditions than in neutral conditions. The maximum removal rates were obtained at pH 3, which was 86.6% at 7 hours. The optimum reaction pH was 3.0 for

The Effect of Electrolyte pH
Solution pH is one of the most important factors that affects the performance of electrochemical processes. The effect of pH values ranging from 1 to 9 on the nicosulfuron degradation was investigated, and the results are presented in Figure 8. The removal rate of nicosulfuron was higher in acid conditions or in alkaline conditions than in neutral conditions. The maximum removal rates were obtained at pH 3, which was 86.6% at 7 h. The optimum reaction pH was 3.0 for nicosulfuron degradation.

The Effect of Electrolyte pH
Solution pH is one of the most important factors that affects the performance of electrochemical processes. The effect of pH values ranging from 1 to 9 on the nicosulfuron degradation was investigated, and the results are presented in Figure 8. The removal rate of nicosulfuron was higher in acid conditions or in alkaline conditions than in neutral conditions. The maximum removal rates were obtained at pH 3, which was 86.6% at 7 hours. The optimum reaction pH was 3.0 for nicosulfuron degradation.
Zhao et al. indicated that nicosulfuron could be degraded to some extent via chemical hydrolysis under natural conditions, especially under acidic pH conditions [46]. The low pH inhibited the oxygen evolution reaction and resulted in the improvement of nicosulfuron degradation efficiency [47]. This result was in agreement with that of Ruan et al. [48], who proved that the degradation rates of sulfonylurea herbicides were affected by pH and that acidic pH was beneficial to the degradation process.  Zhao et al. indicated that nicosulfuron could be degraded to some extent via chemical hydrolysis under natural conditions, especially under acidic pH conditions [46]. The low pH inhibited the oxygen evolution reaction and resulted in the improvement of nicosulfuron degradation efficiency [47]. This result was in agreement with that of Ruan et al. [48], who proved that the degradation rates of sulfonylurea herbicides were affected by pH and that acidic pH was beneficial to the degradation process.

Optimization Experiment
According to the results of the single factor experiments conducted above, the optimum conditions were obtained as follows: current intensity 0.8 A, electrode spacing 3 cm, and pH 3. An electrolysis experiment was conducted under the optimum conditions, and the variation of nicosulfuron concentration with time is shown in Figure 9. A kinetic equation was obtained by the least-square method, which is shown in Equation (1).
where C 0 was the initial concentration of nicosulfuron, and C t was the concentration of nicosulfuron at a given time (t). The equation indicated that the degradation of nicosulfuron under optimum conditions followed first-order kinetics.
concentration levels of 5-500 mg/L in 6 days [46]. Qi et al. suggested that B. subtilis could degrade approximately 80% of nicosulfuron at the initial concentration of 400 mg/L within 3 days [49]. Rapid degradation of nicosulfuron has been considered to be an urgent issue for controlling nicosulfuron contamination. The electrochemical method, as an advanced oxidation process, could provide suitable conditions for the rapid degradation of nicosulfuron.  Under the optimum conditions, the removal rate of nicosulfuron at 100 mg/L could exceed 80% by 7 h electrolysis, which indicated that electrochemical processes could complete the degradation process in a relatively short time. In comparison, biological treatments usually have taken several days to achieve more than an 80% removal rate. Zhao et al indicated that the strain ZWS11 adapted to the initial concentrations as high as 500 mg/L and degraded about 80% at initial concentration levels of 5-500 mg/L in 6 days [46]. Qi et al. suggested that B. subtilis could degrade approximately 80% of nicosulfuron at the initial concentration of 400 mg/L within 3 days [49]. Rapid degradation of nicosulfuron has been considered to be an urgent issue for controlling nicosulfuron contamination. The electrochemical method, as an advanced oxidation process, could provide suitable conditions for the rapid degradation of nicosulfuron.

Electrochemical Mechanism
The Ti/Ta 2 O 5 -IrO 2 electrode was characterized by CV tests in 0.1 M Na 2 SO 4 solution in the absence and in the presence of 100 mg/L nicosulfuron. Compared with that in the blank solution ( Figure 10), no additional peak was found in the 100 mg/L nicosulfuron solution, which indicated that the direct electron transfer did not occur on the Ti/Ta 2 O 5 -IrO 2 electrode. The electrochemical oxidation of organics on the surface of the anode was divided into direct oxidation and indirect oxidation. If there was no new anodic current peak during the positive sweep process, it meant that oxidization was not occurring directly on the surface of the anodes and the degradation of organics was mainly carried out by indirect oxidation. Thus, in this experiment, nicosulfuron was not oxidized directly on the surface of the anode but was decomposed by the oxidizing agent that existed in the solution, such as the hydroxyl radical (·OH). Equations (2) and (3) indicate the indirect oxidation process. MO x represents the metal oxide coating of Ti/Ta 2 O 5 -IrO 2 electrode.
capacitive behavior, redox peaks were seen in the CV curves [51]. The redox couple in all CV curves exhibited an increased current response with the increase of the scan rate from 10 to 150 mV/s. It showed that the peak currents at different scan rates were directly proportional to the square root of the scan rates (v½) (Figure 11b), indicating that the redox reactions on the Ti/Ta2O5-IrO2 electrode were a typical diffusion-controlled electrochemical process [52,53].  The voltammetric response on the Ti/Ta 2 O 5 -IrO 2 electrode at different scan rates is shown in Figure 11a. The CV curves at all scan rates remained closed, indicating that surface charge had enough time to access the entire solution at both high and low scan rates [50]. Due to pseudo-capacitive behavior, redox peaks were seen in the CV curves [51]. The redox couple in all CV curves exhibited an increased current response with the increase of the scan rate from 10 to 150 mV/s. It showed that the peak currents at different scan rates were directly proportional to the square root of the scan rates (v 1 2 ) (Figure 11b), indicating that the redox reactions on the Ti/Ta 2 O 5 -IrO 2 electrode were a typical diffusion-controlled electrochemical process [52,53].

Electrochemical Degradation Mechanism of Nicosulfuron
Many studies have proved that the new intermediate products, formed during the degradation process of nicosulfuron, might be more toxic and stable than the precursor herbicides [1,54,55]. Thus, it is necessary to identify the intermediate products and investigate their toxicity to the environment.
Nicosulfuron was electrolyzed under the optimum condition of current intensity 0.8 A, electrode spacing 3 cm, and pH 3, and the sample solution with the electrolysis removal rate of about 50% was subjected to HPLC-MS detection. Three peaks were obtained by LC separation, and their retention times were 4.767~4.850 min, 6.217~6.367 min, and 7.900~8.000 min, respectively.

Electrochemical Degradation Mechanism of Nicosulfuron
Many studies have proved that the new intermediate products, formed during the degradation process of nicosulfuron, might be more toxic and stable than the precursor herbicides [1,54,55]. Thus, it is necessary to identify the intermediate products and investigate their toxicity to the environment.
Nicosulfuron was electrolyzed under the optimum condition of current intensity 0.8 A, electrode spacing 3 cm, and pH 3, and the sample solution with the electrolysis removal rate of about 50% was subjected to HPLC-MS detection. Three peaks were obtained by LC separation, and their retention times were 4.767~4.850 min, 6.217~6.367 min, and 7.900~8.000 min, respectively.
According to the chemical structure and reported degradation pathway of nicosulfuron, the detected fragments were deduced and shown in Figures 12-14. The ESI analysis showed the presence of the pseudo-molecular ion at m/z 411 ([M+H] + ), and the speculative substance was nicosulfuron (C 15 H 18 N 6 O 6 S). The signal at m/z 843 was attributed to the sodium adduct of the dimeric form of the pseudo-molecular ion ([2M+Na] + ) ( Figure 12).
(A) (B) Figure 11. CV curves of the Ti/Ta2O5-IrO2 electrode at different scan rates. (A) CV curves at different scan rates; (B) The plots of peak current vs. the square root of scan rate.

Electrochemical Degradation Mechanism of Nicosulfuron
Many studies have proved that the new intermediate products, formed during the degradation process of nicosulfuron, might be more toxic and stable than the precursor herbicides [1,54,55]. Thus, it is necessary to identify the intermediate products and investigate their toxicity to the environment.
Nicosulfuron was electrolyzed under the optimum condition of current intensity 0.8 A, electrode spacing 3 cm, and pH 3, and the sample solution with the electrolysis removal rate of about 50% was subjected to HPLC-MS detection. Three peaks were obtained by LC separation, and their retention times were 4.767~4.850 min, 6.217~6.367 min, and 7.900~8.000 min, respectively.

Electrochemical Degradation Mechanism of Nicosulfuron
Many studies have proved that the new intermediate products, formed during the degradation process of nicosulfuron, might be more toxic and stable than the precursor herbicides [1,54,55]. Thus, it is necessary to identify the intermediate products and investigate their toxicity to the environment.
Nicosulfuron was electrolyzed under the optimum condition of current intensity 0.8 A, electrode spacing 3 cm, and pH 3, and the sample solution with the electrolysis removal rate of about 50% was subjected to HPLC-MS detection. Three peaks were obtained by LC separation, and their retention times were 4.767~4.850 min, 6.217~6.367 min, and 7.900~8.000 min, respectively.    Figure 13) and was considered to be the main product of nicosulfuron electrolysis.
The intermediate product with retention time from 7.192 min to 7.309 min gave a fragment ion at m/z 156, which was an indication of ([M+H] + ). The speculative substance was identified as 4,5-dimethoxypyrimidin-2-amine (C 6 H 9 N 3 O 2 ) ( Figure 14). The other fragment at m/z 340 was derived from the opening of the pyridinic ring. The m/z 182 fragment ( Figure 12) was derived from the cleavage of the C-N bond of the sulfonylureic bridge.
From the speculated degradation product information, two degradation pathways of nicosulfuron under the electrolysis conditions were proposed, as shown in Figure 15. Route 1 was the cleavage of the C-N bond of the sulfonylureic bridge and produced M1 and M2, which were the two most important intermediates during the nicosulfuron synthesis process. M1 and M2 have also been reported as degradation products by microorganisms [10,22,44]. Sabadie reported that the hydrolysis of nicosulfuron under acidic conditions was also carried out in accordance with route 1 [19]. Route 2 was the opening of the pyridinic ring and formed M3, which was also found by Benzi et al. during the photoinduced gradation process using a xenon lamp [1]. M4 was produced through the further cleavage of the C-N bond in the side chain of M3. The different results were due to the different reactive conditions, indicating that the reactive condition would influence the degradation pathway of nicosulfuron [19].

Conclusions
The electrochemical method was chosen to degrade nicosulfuron due to its controllability and environmental friendliness. Three kinds of IrO2-based DSA electrodes were used to electrolyze nicosulfuron by a batch process. A well-distributed crack network was formed on the Ti/Ta2O5-IrO2 electrode and Ti/Ta2O5-SnO2-IrO2 electrode. Their OEPs increased according to the order of Ti/RuO2-IrO2 < Ti/Ta2O5-SnO2-IrO2 < Ti/Ta2O5-IrO2. Nicosulfuron was used as the model pollutant to choose the electrode with the highest performance. The Ti/Ta2O5-IrO2 electrode showed the highest efficiency and was chosen for use as the anode in the electrochemical experiment. The obtained optimum electrolysis conditions were: currency intensity 0.8 A, electrode spacing 3 cm, electrolyte pH 3. Under the optimum conditions, the degradation of nicosulfuron followed first-order kinetics and was mainly due to indirect oxidation. The redox reaction on the Ti/Ta2O5-IrO2 electrode was a typical diffusion-controlled electrochemical process. On the basis of the intermediates identified by HPLC-MS, two possible degradation routes of nicosulfuron were proposed, including the cleavage of the C-N bond of the sulfonylureic bridge and the opening of the pyridinic ring.

Conclusions
The electrochemical method was chosen to degrade nicosulfuron due to its controllability and environmental friendliness. Three kinds of IrO 2 -based DSA electrodes were used to electrolyze nicosulfuron by a batch process. A well-distributed crack network was formed on the Ti/Ta 2 O 5 -IrO 2 electrode and Ti/Ta 2 O 5 -SnO 2 -IrO 2 electrode. Their OEPs increased according to the order of Ti/RuO 2 -IrO 2 < Ti/Ta 2 O 5 -SnO 2 -IrO 2 < Ti/Ta 2 O 5 -IrO 2 . Nicosulfuron was used as the model pollutant to choose the electrode with the highest performance. The Ti/Ta 2 O 5 -IrO 2 electrode showed the highest efficiency and was chosen for use as the anode in the electrochemical experiment. The obtained optimum electrolysis conditions were: currency intensity 0.8 A, electrode spacing 3 cm, electrolyte pH 3. Under the optimum conditions, the degradation of nicosulfuron followed first-order kinetics and was mainly due to indirect oxidation. The redox reaction on the Ti/Ta 2 O 5 -IrO 2 electrode was a typical diffusion-controlled electrochemical process. On the basis of the intermediates identified by