Experimental and theoretical insights into the RCS-Involved electro-catalytic transformation of 4-nitrophenol

The important role of reactive chlorine species (RCS) in electrochemical system has been widely concerned for water disinfection recently. In this study, we built an electrochemical system using carbon nanotube as cathode and oxide precursor (Ti/SnO₂-Sb₂O₅-IrO₂) as anode, where RCS was produced from Cl^-. This system was used to degrade nitrogen contaminants, i.e. NO₃^- and 4-nitrophenol. Optimization of the reaction conditions was carried out by a treatment of inorganic nitrogen contaminant NO₃^- and the optimal condition of the electrochemical system was determined at U = 5.5 V, and pH = 10 with a Cl^- concentration of 2000 mg L^-1, and the removal efficiency of NO₃^- can reach up to 60.6% in 150 min. Under the optimal condition, a common nitrogenous organic pollutant, 4-nitrophenol was treated and a removal efficiency of nearly 100% in 90 min. To investigate the detailed degradation mechanism in the applied electrochemical system, a combined method of products identification and density functional theory (DFT) calculation was employed. It concluded that Cl radicals' generation was stimulated was stimulated by the OH radicals after adding Cl^- into the electrochemical system. These two radicals jointly promoted the transformation of 4-nitrophenol resulting in the formation of more toxic organic and inorganic substances. In addition, a conversion of organic nitro group to amino group leading to the formation of 4-aminophenol was found and explained by the indirect reduction theory.