# Ecotoxicological Evaluation of Methiocarb Electrochemical Oxidation
## 3 Results and Discussion
However, when NaCl was used, TN concentration decreased by more than 50%. According to the literature, organic nitrogen is converted to NH\({}_{4}\)\({}^{+}\) in the EO using a BDD anode, which in turn is oxidized mainly to nitrate and nitrogen gas as NH\({}_{4}\)\({}^{+}\) oxidation takes place by indirect oxidation through active chlorine species [41,42]. It was also observed that, for the solutions containing chloride, TN removal was energetically more efficient at the highest current density. These results agreed with the enhancement of the electrolyte oxidation reaction at a higher \(j\) because more active chlorine species available lead to a higher TN removal rate. As MC is a highly toxic compound, the ecotoxicological evaluation of the treatment methods applied to its degradation is mandatory. Thus, the ecotoxicity of the treated samples obtained in this study as well of the initial samples was evaluated towards _Daphnia magna_. As it can be perceived from Figure 3 and Figure S1 (Supplementary Material), the electrochemical oxidation treatment of methiocarb solutions led to a remarkable decrease in the acute toxicity towards _Daphnia magna_. In both MC solutions containing Na\({}_{2}\)SO\({}_{4}\) or NaCl, the initial toxicity was very high although solutions prepared with NaCl were slightly less toxic (EC\({}_{50}\) = 0.21%, 470.4 TUs against EC\({}_{50}\) = 0.27%, 370.9 TUs for solutions prepared  
Figure 2: Variation with applied charge of the (**a**) pH and (**b**) normalized total nitrogen (TN) for the EO experiments performed at 100 and 300 A m\({}^{-2}\) using Na\({}_{2}\)SO\({}_{4}\) or NaCl as the supporting electrolyte. with Na\({}_{2}\)SO\({}_{4}\) and NaCl, respectively). Methiocarb is an acetylcholinesterase-inhibitor pesticide widely used in agriculture as a molluscicide, acaricide and avicide and its toxicity towards aquatic invertebrates has already been reported [43]. Figure 3a presents the EC\({}_{50}\) results after 48 h of exposure for the different experimental conditions studied. The higher EC\({}_{50}\) values were attained for the experiments performed at 100 A m\({}^{-2}\) using NaCl as the supporting electrolyte. At these experimental conditions, an increase in EC\({}_{50}\) from 0.27% to 62% was achieved after a 5 h assay (1.8 kC). However, when the assay duration was extended for one more hour to 6 h (2.16 kC) in total, the EC\({}_{50}\) decreased to 50%. In solutions containing chloride, the increase in toxicity during the electrochemical treatment is usually attributed to the formation of chlorinated compounds [44; 45], which exhibit a high octanol/water coefficient (Kow), thus affecting biological membranes and presenting higher toxicity for living organisms [37]. Furthermore, the formation of perchlorate at the final stage of the electrochemical process when using a BDD anode is well described in the literature [46; 47; 48]. According to [PERSON] et al. [45], the hydroxyl radicals formed in large quantities during the electrolysis of aqueous solutions when a BDD anode is used in the presence of chlorides can be oxidized successively to different oxochlorinated compounds according to Equations (1)-(4). \[\mathrm{Cl}^{-}+\mathrm{OH}^{\bullet}\rightarrow\mathrm{ClO}^{-}+\mathrm{H}^{+ }+\mathrm{e}^{-}, \tag{1}\]  
\[\mathrm{ClO}^{-}+\mathrm{OH}^{\bullet}\rightarrow\mathrm{ClO}_{2}^{-}+ \mathrm{H}^{+}+\mathrm{e}^{-} \tag{2}\]  
\[\mathrm{ClO}_{2}^{-}+\mathrm{OH}^{\bullet}\rightarrow\mathrm{ClO}_{3}^{-}+ \mathrm{H}^{+}+\mathrm{e}^{-} \tag{3}\]  
\[\mathrm{ClO}_{3}^{-}+\mathrm{OH}^{\bullet}\rightarrow\mathrm{ClO}_{4}^{-}+ \mathrm{H}^{+}+\mathrm{e}^{-} \tag{4}\]  
Attending that, in the last hour of the assay, the organic load in the solution is low and its oxidation is under mass transport control, the occurrence of the reactions described by Equations (1)-(4) will be enhanced and the perchlorate concentration in the solution, as the final oxidation product, will increase, explaining the increase in ecotoxicity found.