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

Abstract

This study demonstrated the formation of high-valent iron complex as well as free radicals in the reaction of iron(III)-tetraamidomacrocyclic ligand (FeIII-TAML) with free chlorine. FeIV(O)-TAML was generated at pH 5.0–10.0, while FeIV(O)-TAML can be further transformed into FeV(O)-TAML by free chlorine at pH 5.0–7.0. The one-electron reduction of free chlorine formed Cl•. The performance of the FeIII-TAML/free chlorine system toward organic contaminants was evaluated by investigating the degradation of three target organic contaminants with different functional groups. The pseudo-first-order rate constant (kobs) of organic contaminants’ degradation by the FeIII-TAML/free chlorine system was about 400–2000-fold greater than that by the FeIII-TAML/hydrogen peroxide (H2O2) system at pH 7.0. FeV(O)-TAML and FeIV(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 FeIII-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.