Insight into electrochemical degradation of Cartap (in Padan 95SP) by boron-doped diamond electrode: kinetic and effect of water matrices

In this work, the kinetic electrochemical degradation of Cartap (CT) (in Padan 95 SP) at boron-doped diamond (BDD) electrode was investigated. This study indicated that the degradation of CT underwent both direct and indirect oxidations. Water matrices can either accelerate or inhibit the removal efficiency of CT: adding 15 mM Cl− improved kCT from 0.039 min−1 to 0.054 min−1 (increased by 38%), while kCT decreased by 61.5% and 64% when increasing the concentration of HCO3− and humic acid (HA) to 15 mM and 15 mg L−1, respectively. CT degradation was inhibited in the presence of methanol (MeOH) and tert-butanol (TBA) due to the scavenging effect of those chemicals toward reactive species. The contribution of reactive oxidants was calculated as: DET (direct electron transfer) accounted for 15%; •OH accounted for 61.5%; SO4•− accounted for 12.8%; ROS (the other reactive oxygen species) accounted for 8.5%. The transformation pathways of major reactive species were established.


O
). In addition, other related electrolyte species also contribute to the organic decomposition [8]. The description of the interaction between oxidizing species and target compounds for both mechanism ways (direct and indirect oxidation) can be simplified in Eq. (1): BDD is incapable of absorbing organics (see Text S1 and Figure S1 for more details) and a nonactive anode, which can react with H 2 O to produce the physisorbed •OH (BDD(•OH)) (Eq. (2)), rending the organics degradation (Eq. (3)). The SO 4 •-(Eq. (4)) also contributes to the degradation of organics.  •-- 4 4 SO SO + e � ® (4) Herein, this work reported the degradation of CT (in Padan 95SP) on BDD electrode. The BDD electrode was selected in this study because it performs very good electrochemical capability (high stability, resistance to corrosion, high overpotential for oxygen evolution) [9- [10]11]. Padan 95SP was selected for the degradation in EO process because of its widespread use in pest control in Vietnam.
The kinetic degradation of CT can be expressed as: where k CT represents the first-order rate constant of CT in the EO process, min -1 ; The purpose of this study was to: (1) investigate the direct and indirect oxidation of CT in EO process; (2) study the effect of water matrices on CT removal efficiency; (3) insight into the contribution of radicals to the degradation of CT.

BDD characterization
The BDD electrode was characterized using scanning electron microscopy (SEM, JSM-IT200, Japan), equipped with energydispersive X-ray spectroscopy (EDX) to analysis the elemental composition. X-ray diffraction (XRD, D8 ADVANCE ECO, Germany) with Cu Kα radiation (0.154 nm) was also used to determine the crystal structure of BDD.

Electrolysis
The electrolysis was carried out at ambient temperature (22 °C) in undivided cell of 400 mL. A BDD was used as the working electrode, supplied from Neocoat (Switzerland). The exposed surface area of BDD was 3.8 cm 2 , the diamond layer was about 2.5-3 µm. In all cases, a Platinum foil with the surface area of one side of 2 cm 2 and Ag/AgCl (saturated KCl) were used as counter and reference electrodes, respectively. To stir 250 mL electrolyte solution continuously during the process, a magnetic bar was implied. Before starting experiment the working and counter electrodes were washed in ultrasonic bath for 10 min to remove contaminants, and then washed again with ultrapure water. The pH values of solution was controlled by 1 M H 2 SO 4 or 1 M NaOH using a pH meter. Each experiment was duplicated for verification.
Electrolysis experiments were conducted under galvanostatic control at the applied current density ranged from 10 to 40 mA cm -2 using IviumStat (5 A current compliance/10 V power supply). Linear sweep voltammetry (LSV) test was performed in the potential range of 0-2 V at scan rate of 50 and 100 mV s -1 in 0.05 M Na 2 SO 4 . For CVs tests, the BDD electrode was characterized electrochemically in 0.05 M Na 2 SO 4 solution in the absence and in the presence of Padan 95 SP. The cyclic voltammetry (CV) curves for the BDD electrode were recorded between -2.0 V to 2.0 V (E 0 vs. SCE) at different scan rates, sample interval was 0.01 V. The LSVs and CVs tests were conducted using Metrohm Autolab installed by Nova 2.1.3 software for electrochemical interface. Electrochemical impedance spectroscopy (EIS) was determine using Metrohm Autolab to investigate the conductivity of BDD in 0.2 M H 2 SO 4 under the frequency from 1 × 10 5 to 1 × 10 −2 Hz at open circuit potential.

Analysis methods
The solution samples were withdrawn at the time intervals and immediately measured using DTNB procedure, which can be clearly described elsewhere [12,13]. This technique was based on the generated yellow anions (3-carboxy-4-nitrophenylthiolate anion), which were then determined at the maximum wavelength of 412 nm in a UV-Vis spectrophotometer (V730, Japan). The spectrum for CT determination can be seen in Text S2 and Figure S2.
The degradation efficiency of CT is calculated according to Eq. (6): where C is the remaining content of CT at a given electrolytic time and C 0 is the initial concentration. The theoretical mineralization of CT is proposed in Eq. (7) The concentrations of BA and NB were measured by HPLC aligent 1200 (Germany) with C18 column (250 mm × 4.6 mm, 5 μm) coupled with a UV detector. The detection of BA and NB was performed at 227 and 270 nm, respectively, at a flow rate of 1 mL min -1 . The mobile phase of methanol/water (65:35) (v/v) contained 1 % phosphoric acid. The column temperature was 30 ºC. Additionally, the concentration of BA and NB can be also calculated using UV-Vis (V730, Japan), because the formed byproducts and the presence of SO 4 2did not cause any interferes for the detection of BA and NB at 224 nm and 270 nm, respectively (The HPLC/UV spectrum of BA/NB and the degradation of BA/NB during the EO process can be seen in Figure S3).

BDD characterization and electrochemical properties
Due to the lack of information about this commercial BDD electrode, BDD was again characterized using SEM, XRD and EIS. As can be seen in Figure 1a, the BDD layer consists of the grains with the medium size of 200 nm. The orient of the grains was randomly grown on the substrate Si. According to EDX spectrum, some elements were detected, including C (71.5%), B (15.1%), O (11.8%), and Si (11.6%) ( Figure 1b). Moreover, the phase of BDD can be proved by the diffraction peak at 2θ = 70.35°, which can be coincided with crystal planes of the hexoctahedral phase of diamond (diamond cubic) ( Figure 1c). The roman spectrum and XPS of BDD can be clearly seen in our previous publication [10].
The impedance spectrum of BDD and its comparison with Ti and Pt are shown in Figure 2. The arc diameter of BDD was much smaller than Ti and higher than Pt electrodes, indicating that charge transfer resistance of those electrodes followed the order: Ti (35 kΩ) > BDD (92.6 Ω) > Pt (13 Ω) (use the electrochemical circle fit command in NOVA 2.1.3 software to calculate the resistance). The above result indicates that the BDD can be considered a good conductive electrode thanks to the BDD layer on Si substrate.

Cyclic voltammetry curve in absence and presence of Padan 95 SP
The electrochemical character of BDD electrode with SO 4 2anion is an important factor to initially study the CT degradation mechanism in Na 2 SO 4 . The CV curves of BDD electrode under different conditions are depicted in Figure 3. Similar to previous study [14], we also found the oxidation peak P1 occurring at the potential between 1 V and 1.3 V (Figure 3a), suggesting that the direct oxidation of CT can occur due to the electron transfer on the BDD surface. Generally, the oxidation peak potential must be less than the oxygen evolution potential (Eq. (8)), as also confirmed by M. Panizza and G. Cerisola [8]: When increasing the scan rate to 100 mV s -1 , the direct oxidation of CT was enhanced, as a result of the promotion of electron exchange at BDD surface. This is because the high scan rates provides high current density for shorter time frame, thereby increasing the oxidation current peak [15]. Therefore, the degradation of CT underwent both mechanisms: (1) direct oxidation via electron transfer at the surface of BDD; (2) the indirect oxidation by reactive radicals. The contribution of direct oxidation and indirect oxidation (via •OH, SO 4 •-, etc.) were discussed in subsections 3.4 and 3.5. In the negative potential, the reduction peak P2 at -1.2 V is probably associated to the evolution of H 2 from H 2 O and/or the formation of persulfate (Eqs. (9) and (10)). When increasing the scan rate to 100 mV s -1 , the intensity of the reduction peak increased.
The further investigation of the scan rates at different concentrations of CT were illustrated in Figures 3b and 3c and there are no significant differences between two ranges of concentration, indicating that the degradation through direct oxidation is unchanged at high CT concentration.
In addition, the effect of current density on O 2 evolution and CT degradation can be found in detail in Texts S3 and S4, Figures

2-
, Cl -, Fe 2+ , NO 3 -, etc.) and humic acid (HA) can be found in natural water. In this study, HCO 3 and Clwere selected because they are common anions in water. HA was chosen because it represents the organic matter in natural water. Their presence affects the degradation efficiency of organics during the process. The formation of radicals and their transformation in the presence of water matrices can be listed in Table S2. Therefore, the effects of HCO 3 -, Cland HA on CT removal were investigated, as shown in Figure 4.
Effect of HCO 3 -: k CT (the first-order rate constant for CT) decreased from 0.039 min -1 to 0.014 min -1 (decreased by 74%) when increasing HCO 3 concentration from 0 to 15 mM ( Figure 4a). This result could be attributed to the scavenging effect of bicarbonate toward •OH and SO 4 •-(Reactions (82) and (83), Table S2). This scavenging effect leads to the formation of CO 3 •with weaker oxidation capability (their reaction rate constant toward organics is in the range of 10 -6 -10 -7 M -1 s -1 , Table S2), thereby reducing the removal efficiency of process. As a result, CT removal decreased from 68% to 32% after 30 min when increasing HCO 3 concentration from 0 to 15 mM ( Figure 4b). Effect of Cl -: k CT increased from 0.039 min -1 to 0.054 min -1 (increased by 1.4 folds) when increasing Clconcentration from 0 to 15 mM (Figure 4c). The removal efficiency increased from 68% to 81% after 30 min as increasing Clconcentration to 15 mM (Figure 4d). The presence of chloride ion can enhance the CT removal due to some aspects: (1) Clcould be oxidized by electrolysis to produce active chlorine and/or chloride radicals (ClO -/Cl•) [16,17]. The oxidation capability of Cl• toward organics is comparable to •OH (in the range of 10 9 -10 10 M -1 s -1 . Table S2), thereby contributing to the degradation of CT; (2) Clreacts with •OH and SO 4 •to produce reactive chlorine species (RCS: Cl•, Cl 2 Table S2) which could also enhance the removal efficiency of CT in the electrolysis process. Despite the fact that the scavenging effect of Cllead to decreased concentration of SO 4 •-, the significant formation of RCS at high concentration of Clmight compensate the fade of SO 4 •-, leading to the improvement of process. The same results can be observed in other AOPs [18][19][20][21], where the kinetic formation of radicals can be similarly observed.
Effect of HA: HA acts as scavenger of oxidizing species, leading to a reduction in the degradation efficiency. As can be seen in Figure 4e, increasing the concentration of HA to 15 mg L -1 reduced k CT from 0.039 min -1 to 0.014 min -1 (decreased by 64%), and the removal efficiency decreased from 68% to 23% after 30 min. This result can be explained by several mechanisms [16]: (1) HA competed with CT for the adsorption at the Pt cathode and BDD anode, thus, less CT was degraded at the surface of electrodes; (2) HA competed with sulfate ion on the BDD anode, leading to a reduction in the formation of SO 4 •-(Eq. (4)); (3) HA caused the scavenging effect toward •OH and SO 4 •-(Reactions (101) and (102), Table S2).

Determination of reactive species
Generally, •OH and SO 4 •were found to be major radicals in EO process [22]. To determine the role of these radicals to CT degradation, TBA used as a scavenger for •OH ( , , , etc.
) [16]. The reaction between SO 4 •and TBA can be ignored ( , , , etc. ) [16]. MeOH was used as probe for •OH (9.7 × 40 8 M -1 s -1 ) and SO 4 •-(1.0 ×10 7 M -1 s -1 ). As seen in Figure 5a, k CT decreased by 35% and 43% when adding 100 mM TBA and MeOH, respectively. CT removal efficiency decreased from 68% to 37% and to 29% after 30 min at 100 mM TBA and MeOH, respectively ( Figure  5b). The significant decrease in k CT , and k CT (in TBA) > k CT (in MeOH) suggested that •OH and SO 4 •were the major radicals contributing to CT degradation. In addition, the other ROS ( , , , etc.   Table S2) could also form in the electrolysis process as the reaction chains in the solution. However, their role on CT degradation was assumed to be negligible due to their low concentration as suggested by [16].

The relative contribution of reactive species to CT degradation
In EO process, the oxidation of CT can be taken placed by several oxidizing factors as displayed in Eq. (5) ) completely, then k DET was calculated to be 0.006 min -1 , accounted for 15% of CT degradation ( Figure 6). The change in k CT with addition of scavengers can be seen in Figure 6. •OH and SO 4 •could be generated by electrolysis alone at the surface of anode (Eqs. (2) and (4)) and/or from the reactions chains in solution (Reactions 1-29, Table S2). The relative contribution of •OH and SO 4 •was calculated according to Eqs. (11) and (12) [22]. As a result, •OH and SO 4 •accounted for 61.5% and 12.5%, respectively.
where k CT represents the degradation rate constant of CT without addition of scavenger, min -1 ; k TBA represents the degradation rate constant of CT with addition of TBA (2.2 mM), min -1 ; k MeOH is the degradation rate constant with addition of MeOH (5 mM Figure   S3). By solving the reactions (15) and (16)

Transformation and interaction mechanism of radicals
It is known that the degradation of organics was performed mainly by •OH radicals, so the mechanism of generating hydroxyl radicals on the anode surface is written as follows: After losing electrons at the surface of BDD the H 2 O molecules generate •OH radical (Eq. (17)). Hydroxyl radical is a strong oxidant, which can attack CT rapidly to generate radical CT•. As a low stable radical, CT• reacts with hydroxyl radical to further generate new intermediates and the end products (i.e. CO 2 and H 2 O). Besides, the formation of SO 4 •can take place through electron-transfer on the surface of BDD and/or from •OH. SO 4 •then attacks CT to reproduce SO 4 2and cationic radical +• CT , as described in Eqs. (20) and (21). SO 4 •is considered as very active oxidant (E 0 = 2.6 V), can further destroy this organic radical to the end products, or generate smaller molecules. HSO could be oxidized to form SO 4 •-(Eq. (20)), which can recombine (Eq. (23)) [27,28] or further react with sulfate and/or hydrosulfate ions to form persulfate S 2 O 8 2-(Eqs. (26) and (27)) [29,30], as confirmed by the work of F. Zhang et al. [14] using ESR spectrum. Furthermore, the evolution of O 2 takes place via a recombination of •OH as in Eq. (28) or from the hydrolysis of persulfate (Eq. (29)): (29) It is worth nothing that the electrochemical degradation of organics is a complex of various mechanisms, in which the oxidizing radicals could be produced by different ways, involving in the degradation of organics as well as their none-used disappearance. Briefly, we can see the whole process of oxidizing CT in Figure 7. Additionally, the degradation pathway of CT in EO process can be found in our previous study [17].

Conclusion
The kinetic electrochemical degradation of CT (in Padan 95 SP) was investigated. The degradation of CT underwent two mechanisms: direct oxidation by electron transfer and indirect oxidation by reactive generated species. The removal efficiency of CT depended on the presence of water matrices: 15 mM Climproved k CT by 38%, while k CT was reduced by 61.5% and 64% when adding 15 mM HCO 3 and 15 mg L -1 HA, respectively. CT removal efficiency was reduced in the presence of MeOH and TBA, indicating that reactive species play an important role in CT degradation. The contribution of reactive oxidants was established: DFT accounted for 15%; •OH accounted for 61.5%; SO 4 •accounted for 12.8%; ROS accounted for 8.5%. As a small part of study, the effect of applied current on CT degradation was investigated, indicating the O 2 evolution at higher current inhibited the CT removal. In addition, the possible transformation pathways of reactive species was suggested.

Conflict of interest
The author declare no competing financial interest.

Acknowledgments
The author thanks Dr. Bui Dinh Nhi (Viet Tri University of Industry, Phu Tho, Viet Nam) for his SEM, XRD experimental support and helpful discussion, and Dr. Vo Thang Nguyen (University of Science and Education -The University of Danang) for her experimental support and helpful discussion.    1 Supporting information Text S1: adsorptive possibility of BDD toward organic compounds To check the change in concentration of CT and organic compounds in the presence of BDD without applied potential, different types of organic compounds were used. Those chosen compounds are dye compounds, which can be adsorbed high by some adsorbent (i.e. active carbon). However, there was no any reduction in concentration of CT and dye compounds ( Figure S1), indicating that BDD is a nonadsorptive electrode.

Text S2: spectrum for Cartap detection
The absorbance spectrum of standard CT (in Padan 95SP) in the ranges of interested concentration were recorded in Figure  S2 to establish the calibration curve for assessing the degradation of CT. It is also noted that the absorbance spectrum from CT decomposition during electrochemical process did not differ with the presence of other species (i.e. intermediates, electrolytes and pH). Condition for this measurement: 0.2 mL each trail was mixed well with 0.8 mL DTNB solution (1 g L -1 DTNB in methanol) and then with 4 mL buffer solution, as can be seen in detail elsewhere for preparing the sample for UV-Vis measurement [1,2].
Text S3: linear polarization curve As a side reaction in electrochemical degradation of CT, the oxygen formation at BDD was assessed via linear sweep voltammetry (LSV). Thus, the linear polarization curves of BDD electrode was tested at two different scan rates of 50 and 100 mV s -1 in 0.05 M Na 2 SO 4 electrolyte solution, as shown in Figure S4. It is argued that the oxygen evolution reaction (OER) occurs since the current passing the electrode suddenly increases. Figure S4 shows that the higher OER (1.6 V) can be achieved at slower scan rate (50 mV s -1 ), indicating the dependence of OER on scan rate. Additionally, the oxygen evolution overvoltage of both cases is pretty lower than that we expected from BDD. It should be kept in mind that the potential for oxygen evolution at anode is just a relative value and it might also differ under the experimental conditions. For example, Costa et al. [3] have pointed out that OER even occurred at the potential less than 2 V vs. SCE at 0.01 M H 2 SO 4 . They also showed a strong dependence of OER on the organic target, which contributes to a decrease in the potential of oxygen evolution when increasing its concentration. Similarly, O. Davila et al. figured out that the onset potentials for OER in electrolysis of BDD depends on the mixtures with dibenzothiophene (67 mg L -1 at scan rate: 10 mV s -1 ) [4]. Some authors argued that the oxygen evolution rate decreased when adding organic compounds due to their competition with •OH [5,6]. In our condition, it is not an exceptional when we observed the low EOR. This might be due to the low concentration of supporting electrode (here: 0.05 M Na 2 SO 4 ), dominating the oxygen evolution instead of oxidizing sulfate ions to other species. However, the anodic current is low within the potential interval from 1.50 to 2 V, indicating that not much oxygen was generated. The oxygen evolution at some BDD anodes are depicted in Table S1 below:

Text S4: effect of applied current density
The effect of the applied current density (j) on the CT degradation in the undivided cell is displayed in Figure S5. As shown in Figure S5, the CT degradation rate increased at higher applied current density. More clearly, this result shows about 70% CT removed after 30 min at j = 40 mA cm -2 , meanwhile only 57% at j = 10 mA cm -2 . The result shows that this process fitted well the pseudo-first order kinetic, which is described by the Eq. (S1): where C t and C 0 are the concentration of CT at the interval time (t) and the beginning time, respectively. k CT is the apparent rate constant of CT. We perform the degradation kinetic of CT in Figure S5b to determine the change in the k CT versus current density. Obviously, k CT increased from 0.022 min -1 to 0.039 min -1 (increased by 1.7-folds) when increasing the current density by 4 folds, suggesting that a production of·•OH radicals becomes inefficiently at higher current due to the competing reactions, here is oxygen evolution [10] (see Eq. (S2)). The fact can be demonstrated by mineralization current efficiency (MCE) in our previous paper [11]. This bubble gas can also deactivate surface of anode, thereby reducing the removal efficiency.
Additionally, based on the calculation results for k CT from Figures S5 and S6, and the suggestion of reference [12], we propose the relationship between the degradation rate constant k CT and the applied current density j as the quadratic function, as shown in Figure S6.This function fits well our experimental results ranging from 10 mA cm -2 to 40 mA cm -2 with R-square of 0.978. Thus, we propose this relationship in Eq. (S3).