# Ecotoxicological Evaluation of Methiocarb Electrochemical Oxidation
## 3 Results and Discussion  
Figure 1 presents the normalized variation of MC and TOC concentrations with applied charge during the EO assays accomplished with different supporting electrolyte and applied current densities. When the influence of the supporting electrolyte was analyzed, the MC and TOC removal rates were higher for the solutions containing NaCl. In fact, in the presence of chloride, MC was completely degraded during the initial period of the assays for both \(j\) whereas in the presence of Na\({}_{2}\)SO\({}_{4}\), the MC removal rate was lower and decreased along the assays. This faster degradation in the presence of chloride has been described in the literature for other contaminants and, according to the authors, is due to the additional indirect oxidation in the bulk of the solution by active chlorine species generated from the oxidation of chloride ions at the BDD [33; 39]. To elucidate the reaction kinetics in both electrolytes, the decay in time of the MC concentration determined by HPLC for the first hour of the assays was analyzed. The MC degradation showed a first-order reaction mechanism with the following kinetic constants: k = \(0.28\times 10^{-3}\) s\({}^{-1}\) (Na\({}_{2}\)SO\({}_{4}\)\(\_\)100 A m\({}^{-2}\)); k = \(0.77\times 10^{-3}\) s\({}^{-1}\) (Na\({}_{2}\)SO\({}_{4}\)\(\_\)300 A m\({}^{-2}\)); k = \(1.6\times 10^{-3}\) s\({}^{-1}\) (NaCl\(\_\)100 A m\({}^{-2}\)); k = \(1.7\times 10^{-3}\) s\({}^{-1}\) (NaCl\(\_\)300 A m\({}^{-2}\)). This showed that, in the chloride medium, the oxidation happened mainly in the bulk of the solution and, because the MC concentration was low, even at the lowest applied current density the chlorine active species formed were enough to oxidize MC at a very good reaction rate. Regarding oxidation in the presence of  
Figure 1: Variation with applied charge of the normalized (**a**) methiocarb (MC) concentration and (**b**) total organic carbon (TOC) for the electrochemical oxidation (EO) experiments performed at 100 and 300 A m\({}^{-2}\) using Na\({}_{2}\)SO\({}_{4}\) or NaCl as the supporting electrolyte. sulfate, the oxidation mechanism must be either by hydroxyl or sulfate radicals with a lower lifetime, thus giving more importance to MC diffusion towards the anode's surface. Regarding the TOC, for the highest \(j\), the difference between the removal rates when NaCl or Na\({}_{2}\)SO\({}_{4}\) were used was more pronounced than at 100 A m\({}^{-2}\). Regardless of that, MC and TOC removals were energetically more efficient at the lowest \(j\). These results indicated that at 300 A m\({}^{-2}\), MC and by-product oxidation through hydroxyl radicals were reduced and the oxidation by active chlorine species was enhanced, which was due to the higher amount of active chlorine species available at this \(j\) and to the lower amount of hydroxyl radicals available, caused by the augmentation of secondary reactions such as the oxidation of the electrolyte or oxygen and hydrogen evolution [40]. Although the mentioned reactions were always present, their rate increased with applied current densities. The different degradation mechanisms occurring when NaCl or Na\({}_{2}\)SO\({}_{4}\) were used were also evidenced in pH variation along the assays (Figure 2a). When NaCl was used, an increase in pH during the initial period of the assays was found, followed by a decrease to below the initial value and ending in a plateau. The initial increase in pH observed was probably due to the chloride oxidation/water reduction and the subsequent decrease could be a result of the formation of by-products such as short chain carboxylic acids [17]. In the presence of Na\({}_{2}\)SO\({}_{4}\), a decrease in pH during the initial period of the assays was observed with a subsequent tendency to a plateau. As an MC molecule has nitrogen in its structure, TN variation during the assays was also assessed. A normalized TN variation with an applied charge, presented in Figure 2b, showed that in the presence of Na\({}_{2}\)SO\({}_{4}\) no nitrogen removal occurred.