Importance of High-Valent Iron Complex and Reactive Radicals in Organic Contaminants' Abatement by the Fe-TAML/Free Chlorine System

This study demonstrated the formation of high-valent iron complex as well as free radicals in the reaction of iron(III)-tetraamidomacrocyclic ligand (Fe^III-TAML) with free chlorine. Fe^IV(O)-TAML was generated at pH 5.0-10.0, while Fe^IV(O)-TAML can be further transformed into Fe^V(O)-TAML by free chlorine at pH 5.0-7.0. The one-electron reduction of free chlorine formed Cl^•. The performance of the Fe^III-TAML/free chlorine system toward organic contaminants was evaluated by investigating the degradation of three target organic contaminants with different functional groups. The pseudo-firstorder rate constant (k_obs) of organic contaminants' degradation by the Fe^IIITAML/ free chlorine system was about 400-2000-fold greater than that by the Fe^III-TAML/hydrogen peroxide (H₂O₂) system at pH 7.0. Fe^V(O)-TAML and Fe^IV(O)-TAML were proposed as the major high-valent iron oxidants responsible for organic contaminants' degradation at pH 5.0-7.0 and at pH 8.0-10.0, respectively. Additionally, Cl^• and secondary free radicals (e.g., Cl₂^•- and HO^•) were responsible for organic contaminants' degradation. The Fe^III-TAML/free chlorine system might be a promising oxidation process because both selective oxidants (high-valent iron complex) and nonselective oxidants (free radicals) contribute to decomposition of organic contaminants in this process.