The mechanism and efficiency of MnO₂ activated persulfate process coupled with electrolysis

Pure three-dimensional manganese oxides (MnO₂) were successfully synthesized by a simple one-step hydrothermal process. The obtained particles were characterized via XRD, BET, SEM, XPS and FTIR techniques. To enhance the efficiency of heterogeneous catalytic process, a facile and effective electrochemical method was introduced. The degradation of C. I. Acid Orange 7 (AO7) as the target pollutant in aqueous solution by an oxidation system involving MnO₂ activated peroxydisulfate (PDS) coupled with electrochemical method is reported herein. Influences of some key reaction parameters such as initial pH (pH₀), current density, initial AO7 concentration, dosage of MnO₂ and anions (Cl^-, NO₃^-, HCO₃^- and H₂PO₄^-) were investigated. The cyclic voltammetry (CV) was performed to investigate the charge transfer process occurred at the surface of catalyst. LC-MS/MS analysis was applied to identify degradation intermediates and a plausible degradation mechanism is proposed accordingly. Activated sludge inhibition tests were carried out to evaluate the change of toxicity of the dye solution in the oxidation process. The inorganic by-products such as NO₂^-, NO₃^-, and NH₄⁺ along with AO7 degradation were also identified. The stability of MnO₂ catalyst was evaluated by recycling experiments and the electrical energy consumption was also investigated. Radical quenching tests with several scavengers (methanol, tert-butyl alcohol, 1,4-benzoquinone and phenol) were performed to clarify the dominating reactive species participating in this oxidation process and the underlying mechanisms involving the generation of radical from the proposed electro-assisted heterogeneous activated PDS system were identified.