Transformation of Dissolved Organic Matter by Aqueous Fe(IV) under Acidic Conditions: Bulk-to-Molecular Insights into Reaction Kinetics and Mechanisms

Abstract Aqueous Fe(IV) has emerged as a significant nonradical oxidant in iron-based oxidative water treatment, yet its reaction kinetics and mechanisms with dissolved organic matter (DOM) remain largely unexplored. Herein, second-order rate constants (k) for Fe(IV) reactions with DOM and DOM-relevant model compounds were determined under acidic conditions using a competition kinetics method performed on self-assembled quenched-flow apparatuses. At pH 3.0, k for 15 DOM isolates ranged from (2.1 ± 0.2) × 104 MC–1 s–1 to (11.6 ± 0.9) × 104 MC–1 s–1 and correlated strongly with the electron-donating capacity of DOM. Similarly, k for nine para-substituted phenolic compounds correlated linearly with the Hammett constant σ+, EHOMO, and vertical ionization potential, supporting Fe(IV) selectivity toward electron-rich substrates. Across pH 1.0–3.5, k for DOM increased monotonically from the 104 to 105 MC–1 s–1 scale. In contrast, k for 11 model compounds representing major DOM moieties spanned 101–107 M–1 s–1 and exhibited moiety-specific pH dependences, indicating that individual DOM components contribute differently to overall Fe(IV)-DOM reactivity. Fluorescence spectroscopy revealed broad-spectrum oxidation of DOM fluorophores, whereas high-resolution mass spectrometry showed that Fe(IV) preferentially transformed unsaturated moieties, oxidized nitrogen/sulfur-containing functionalities, and fragmented moderate-molecular-weight molecules. These findings provide a bulk-to-molecular kinetic and mechanistic basis for predicting Fe(IV) fate in DOM-containing waters and optimizing Fe(IV)-based advanced oxidation processes.