Mechanistic insights into chlorine-mediated heterogeneous activation of peroxymonosulfate for selective ammonia oxidation to nitrogen gas in biologically treated landfill leachate effluent

Background Sulfate radical−based advanced oxidation processes (SR-AOPs) show promising prospects for the pretreatment and advanced treatment of ammonia nitrogen in landfill leachate. However, in high chloride ion environments, SR-AOPs often lack selectivity, leading to the over-oxidation of ammonia nitrogen to undesirable nitrate NO 3 − and NO 2 − rather than the desired N 2 , which remains a major challenge. Methods In this study, CoFe 2 O 4 was prepared by a gel-calcination method and the effects of chloride ions (Cl − ) and other parameters on the performance of CoFe 2 O 4 -activated peroxymonosulfate (PMS) oxidation system for the removal of ammonia nitrogen and the selectivity of oxidation products were systematically investigated. Significant findings Cl − played a crucial role in determining the ammonia removal efficiency and nitrogen selectivity. Under the optimum conditions of 30 mM of PMS, 0.1 g/L of catalyst and 20 mM of Cl − , the ammonia removal efficiency of the system was nearly 100 %, and the nitrogen selectivity reached 88.1 %. The main species in the activation process of the PMS was Co(II)-OH, and the formation of this complex was promoted by Fe(III). SO 4 · − , HO · , and chlorine species (Cl · , ClO · , and HClO) were generated in the system. Among them, SO 4 · − and HO · indirectly contributed to ammonia oxidation by reacting with Cl − to produce reactive chlorine species, which subsequently oxidized ammonia nitrogen.