Kinetics and mechanism of 2,5-dichlorophenol removal by ferrate(VI): A combined experimental and computational investigation

Abstract

The kinetics and mechanism of 2,5-dichlorophenol (2,5-DCP) removal using ferrate(VI) (Fe(VI)) were studied by a combined experimental and computational method. Reaction kinetics of Fe(VI) with 2,5-DCP was experimentally measured by the ABTS method. The reaction exhibited second-order kinetics, being first order in each reactant. The reaction rate constants decreased as the pH increased, i.e., 421.18 ± 16.89 M⁻¹ s⁻¹ at pH 7.0 to 5.91 ± 0.39 M⁻¹ s⁻¹ at pH 10.5. Based on the relationship between temperature and the measured rate constants, kinetic parameters such as the activation enthalpy, activation entropy, and activation energy were calculated as 34.51 ± 1.87 kJ·mol⁻¹, −99.87 ± 6.28 J·mol⁻¹·K⁻¹, and 36.99 ± 1.87 kJ·mol⁻¹, respectively. The species-specific second-order rate constants were determined by a least-squares calculation, and the reactivity of ${\text{HFeO}}_4^{‐}$ with unprotonated 2,5-DCP was highest among parallel reactions between 2,5-DCP and Fe(VI) species. The presence of Cl⁻ and humic acid had adverse effects on 2,5-DCP removal kinetics and efficiency. Additionally, eight degradation intermediates of 2,5-DCP were identified by mass spectrometry, and reaction types and sites were determined based on DFT studies, then detailed degradation pathways involving substitution, dechlorination, and ring cleavage were proposed. The experimental results were further rationalized by the DFT-based theoretical calculation. Efficient removal of 2,5-DCP from real waters utilizing Fe(VI) technology provided valuable and insightful information for the environmental elimination of endocrine-disrupting compounds (EDCs).