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.