Understanding Two Variation Patterns of Organic Contaminant Degradation with pH in the FeVI System under Acidic Conditions

Recently, the application of FeVI in water decontamination has sparked much attention, while owing to the instability of FeVI, there is limited information to clarify its behavior in removing organic contaminants (OCs) under acidic conditions. This work discovered that altering the reaction pH from 7.0 to 3.0 caused two patterns of performance variation in the FeVI system during the elimination of nine representative OCs. Specifically, the best removal of OCs containing electron-donating moieties was observed at pH 6.0, while that of other OCs with electron-withdrawing moieties was presented at pH 3.0. Mechanism research indicated that during FeVI oxidation, high-valent Fe species were active oxidants at pH above 6.0 and •OH as a secondary active species would be derived from the Fenton process at pH below 6.0. Due to the pH-dependent activity of Fenton chemistry, •OH formation exhibited a clear pH dependence. The relative contributions of high-valent Fe species and •OH in oxidizing OCs highly relied on the substrate-specific reactivity. Despite the presence of different active species at pH 3.0–7.0, the FeVI system still effectively immunized the most common water matrices and favored the detoxification of OCs. These results greatly enrich the fundamental knowledge of FeVI oxidation behavior while pointing to considerable potential for designing more effective and rapid FeVI oxidation processes.