Effect of dissolved oxygen on the peroxymonosulfate activation pathway in an electrochemical Co/P/CA cathode system

Although dissolved oxygen plays an important role in electro-Fenton-like processes, few investigations have revealed its underlying effects in such processes. Herein, the effect of dissolved oxygen on peroxide activation in an electro-Fenton-like system comprising electrochemical cells and peroxymonosulfate (PMS) was investigated. Cobalt phosphide-modified carbon aerogel (Co/P/CA) was used as the cathode material owing to the high conductivity and catalytic activity of Co/P/CA. Several free radicals and their effects on organic pollutant removal were observed using electron paramagnetic resonance spectrometry and quenching experiments, respectively. The observations revealed that in the presence of O₂, hydroxyl radical (·OH), superoxide (O₂^-·), and singlet oxygen (¹O₂) served as the primary active species in the PMS activation process, while in the presence of N₂, ·OH and sulfate radical (SO₄^-·) served as the dominant active species in this process. The factor responsible for the difference in the PMS activation pathways available under O₂ and N₂ conditions was investigated using rotating disk electrode tests and free energy calculations. The tests indicated that O₂ facilitates PMS activation to form ·OH instead of SO₄^-·. The dissolved oxygen subsequently underwent a single-electron-reduction reaction and was converted into O₂^-·, which could serve as a source of ¹O₂. When N₂ was introduced, Co species, particularly Co(II), played a key role in activating PMS. The free radicals ·OH and SO₄^-· were generated during the PMS activation process. This study clearly demonstrates the mediating catalysis role of dissolved oxygen in electro-Fenton-like system through experimental data and theoretical calculations, thereby positively contributing to future studies regarding the continuous activation of peroxides in composite systems and improvement of the efficiency of waterbody remediation.