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

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 1 O 2 via the addition of H 2 O 2 , which substantially alleviates chlorinated organic formation. When phenol was used as a model organic compound, the results showed that the H 2 O 2 -involving EO system outperformed the H 2 O 2 -absent system in terms of higher rate constants (5.95 × 10 −2 min −1 vs. 2.97 × 10 −2 min −1 ) and a much lower accumulation of total organic chlorinated products (1.42 mg L −1 vs. 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 1 O 2 that was abundant in the bulk solution with the use of electron paramagnetic resonance and computational kinetic simulation, the routes of 1 O 2 generation were revealed. Despite the consensus as to the contribution of reaction between H 2 O 2 and ClO − to 1 O 2 formation, we conclude that the predominant pathway is through H 2 O 2 reaction with electrogenerated HO • or chlorine radicals (Cl • and Cl 2 • − ) to produce O 2 • − , followed by self-combination. Density functional theory calculations theoretically showed the difficulty in forming chlorinated byproducts for the 1 O 2 -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 H 2 O 2 -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.