Reactive species conversion into 1O2 promotes substantial inhibition of chlorinated byproduct formation during electrooxidation of phenols in Cl−-laden wastewater

Abstract

The generation of chlorinated byproducts during the electrochemical oxidation (EO) of Cl−-laden wastewater is a significant concern. We aim to propose a concept of converting reactive species (e.g., reactive chlorines and HO• resulting from electrolysis) into 1O2 via the addition of H2O2, which substantially alleviates chlorinated organic formation. When phenol was used as a model organic compound, the results showed that the H2O2-involving EO system outperformed the H2O2-absent system in terms of higher rate constants (5.95 × 10−2 min−1vs. 2.97 × 10−2 min−1) and a much lower accumulation of total organic chlorinated products (1.42 mg L−1vs. 8.18 mg L−1) during a 60 min operation. The rate constants of disappearance of a variety of phenolic compounds were positively correlated with the Hammett constants (σ), suggesting that the reactive species preferred oxidizing phenols with electron-rich groups. After the identification of 1O2 that was abundant in the bulk solution with the use of electron paramagnetic resonance and computational kinetic simulation, the routes of 1O2 generation were revealed. Despite the consensus as to the contribution of reaction between H2O2 and ClO− to 1O2 formation, we conclude that the predominant pathway is through H2O2 reaction with electrogenerated HO• or chlorine radicals (Cl• and Cl2•−) to produce O2•−, followed by self-combination. Density functional theory calculations theoretically showed the difficulty in forming chlorinated byproducts for the 1O2-initiated phenol oxidation in the presence of Cl−, which, by contrast, easily occurred for the Cl•-or HO•-initiated phenol reaction. The experiments run with real coking wastewater containing high-concentration phenols further demonstrated the superiority of the H2O2-involving EO system. The findings imply that this unique method for treating Cl−-laden organic wastewater is expected to be widely adopted for generalizing EO technology for environmental applications.