Generation of Reactive Oxidants during Nitrate Electrolysis: An Approach to the Simultaneous Removal of Nitrate and Organic Contaminants

This study demonstrated that organic contaminants are effectively oxidized by reactive oxidants generated through multiple pathways during nitrate (NO₃^-) electrolysis under acidic conditions. The electrochemical degradation of benzoic acid (BA) was 3.6 times faster in the NO₃^- electrolyte than in the inert perchlorate (ClO₄^-) electrolyte, suggesting that BA degradation is facilitated by specific reactions with intermediates formed during NO₃^- electrolysis. A series of electrolytic experiments and analyses confirmed that BA degradation during NO₃^- electrolysis occurs through three reaction pathways: (i) hydroxylamine (HA) produced by cathodic reduction of NO₃^- anodically produces peroxynitrous acid (ONOOH), which oxidizes BA via a hydroxyl radical (cathodic/anodic coupled pathway), (ii) the cathodic reduction of NO₃^- yields ONOOH through the reaction of nitrous acid (HNO₂) and hydrogen peroxide (cathodic pathway), and (iii) the anodic oxidation of NO₃^- forms a nitrate radical that degrades BA (anodic pathway). The prolonged electrolytic reaction of BA and NO₃^- resulted in a simultaneous reduction in total carbon and nitrogen concentrations. Furthermore, the enhanced electrochemical degradation of various organic compounds in the NO₃^- electrolyte was consistently observed. These findings provide mechanistic insights into the chemistry of NO₃^- electrolysis, indicating that the electrochemical system could be an effective alternative for the simultaneous removal of NO₃^- nitrogen and organic carbon in numerous nitric acid-based industrial wastewaters.