Unlocking Tetravalent Iron Reactivity at Mildly Acidic pH through Decay Kinetics Technique.

Tetravalent iron (FeaqIVO2+) is a short-lived, high-valent intermediate of broad interest in physical chemistry due to its strong oxidizing potential, tunable redox properties, and pH-dependent speciation. These features make FeaqIVO2+ not only a subject of fundamental interest in physical chemistry but also a promising oxidant in wastewater treatment for the degradation of trace organic contaminants (TrOCs). However, previous kinetic studies have been restricted to highly acidic conditions (pH ≤ 1.0) to mitigate Fe(III) absorbance interference at 320 nm, limiting understanding under environmentally relevant pH regimes. Here, we developed a modified decay-kinetics method by monitoring FeaqIVO2+ at 425 nm, thereby eliminating Fe(III) interference and enabling direct determination of second-order rate constants (kFeaqIVO2+,TrOC) at pH 3.0. The measured kFeaqIVO2+,TrOC values reveal selective reactivity toward sulfonamides (4.17-8.24 × 105 M-1 s-1) and phenols (0.22-5.49 × 105 M-1 s-1) and support quantitative structure-activity analysis of phenol oxidation, advancing the mechanistic basis for selective oxidation in both catalysis and wastewater treatment.