Efficient generation of highly reactive FeIVO2+ via the Fe(II)/Cl2 reaction for water purification.

The coexistence of ferrous iron (Fe(II)) and chlorine species (e.g., molecular chlorine (Cl2), HOCl, and OCl-) in aqueous systems can initiate the Fe(II)/chlorine reaction, yet its underlying mechanisms remain largely unexplored. Here, we report for the first time that Fe(II) predominantly reacts with Cl2 at acidic pH and high Cl- concentrations, with a second-order rate constant of 2.6 × 108 M-1 s-1, to generate highly reactive ferryl ions (FeIVO2+). In contrast, Fe(II) primarily interacts with HOCl at near-neutral pH, producing chlorine atoms (Cl•) via single electron transfer. The generated FeIVO2+and Cl• can subsequently produce hydroxyl radical (HO•) through hydrolysis. Compared to the Fe(II)/H2O2 system, the Fe(II)/chlorine system demonstrated selective oxidation of structurally diverse micropollutants and exhibited 1.7 to 6.1 times higher rate constants of nucleic acid component elimination in aqueous environments. Notably, the enhanced water purification efficiency was observed at acidic pH and high Cl- concentrations, attributed to the increased generation of FeIVO2+ via the reaction between Fe(II) and Cl2. Given that the Fe(II)/chlorine scenario frequently occurs in water treatment and groundwater remediation using chlorine, this study provides valuable insights into the efficient generation of FeIVO2+ via the Fe(II)/Cl2 reaction.