The role of nitrite in electrocatalytic oxidation of phenol: An unexpected nitration process relevant to groundwater remediation with boron-doped diamond electrode.

Using boron-doped diamond (BDD) to mineralize recalcitrant organics has been one of the hottest areas of research interest in the field of water treatment. Here we report for the first time that, in the presence of nitrite ions (NO2-), the anodic oxidation of phenol with BDD electrode will lead to the formation of nitrated by-products of phenol. These by-products include 2-nitrophenol, 4-nitrophenol, 2,4-dinitrophenol, 2,6-dinitrophenol, 2,4,6-trinitrophenol, 2,3,4,6-tetranitrophenol, 2,3,4,5,6-pentanitrophenol, as well as a large number of dimers and trimers of nitrophenols. Increasing the concentration of NO2- will not only greatly affects the degradation and mineralization of phenol, but also enhances the formation of nitrophenols. The nitrated by-products are mainly generated via electrophilic substitution reactions mediated by nitrogen dioxide radicals and hydroxyl radicals, as well as via coupling reactions of phenol. In addition, it is found that several simple nitrophenols may also be formed in nitrate media. As a whole, formation of nitrated by-products is a novel phenomenon in anodic oxidation processes. Since nitrated aromatics are well known for their persistence in the environment, their formations in BDD anode cells should be carefully scrutinized before such technology is applied to groundwater remediation.