Three Kinetic Patterns for the Oxidation of Emerging Organic Contaminants by Fe(VI): The Critical Roles of Fe(V) and Fe(IV)

For the first time, this study showed that the apparent second-order rate constants (k_app) of six selected emerging organic contaminants (EOCs) oxidation by Fe­(VI) increased, remained constant, or declined with time, depending on [EOC]₀/[Fe­(VI)]₀, pH, and EOCs species. Employing excess caffeine as the quenching reagent for Fe­(V) and Fe­(IV), it was found that Fe­(V)/Fe­(IV) contributed to 20–30% of phenol and bisphenol F degradation by Fe­(VI), and the contributions of Fe­(V)/Fe­(IV) remained nearly constant with time under all the tested conditions. However, the contributions of Fe­(V)/Fe­(IV) accounted for over 50% during the oxidation of sulfamethoxazole, bisphenol S, and iohexol by Fe­(VI), and the variation trends of k_app of their degradation by Fe­(VI) with time displayed three different patterns, which coincided with those of the contributions of Fe­(V)/Fe­(IV) to their decomposition with time. Results of the quenching experiments were validated by simulating the oxidation kinetic data of methyl phenyl sulfoxide by Fe­(VI), which revealed that the variation trends of k_app with time were significantly determined by the change in the molar ratio of Fe­(V) to Fe­(VI) with time, highlighting the key role of Fe­(V) in the oxidative process. This study provides comprehensive and insightful information on the roles of Fe­(V)/Fe­(IV) during EOC oxidation by Fe­(VI).