Removal of 3,4-dichlorophenol from water utilizing ferrate(vi): kinetic and mechanistic investigations and effects of coexisting anions†

3,4-Dichlorophenol (3,4-DCP) has been detected in industrial and domestic effluents, posing adverse effects on human health. Herein, the ferrate (Fe(VI)) oxidation of 3,4-DCP was systemically and comprehensively investigated by determining its kinetics, mechanisms, and effect of anions. The pH dependence of the reaction kinetics was found to be strong and the second-order reaction rate constant k varied nonlinearly from 342.82 ± 21.81 M⁻¹ s⁻¹ to 8.21 ± 0.27 M⁻¹ s⁻¹ with the increase in pH from 6.5 to 10.5, whereas its temperature dependence was quite weak. The individual species-specific second-order rate constants for the reaction were obtained using a least-squares regression approach. Protonated Fe(VI) has higher reactivity than its unprotonated species, while 3,4-DCP was just the opposite, leading to the reaction of HFeO₄⁻ with 3,4-DCP⁻ occurring fastest among the four parallel reactions. Significantly, 3,4-DCP removal kinetics was inhibited by coexisting anions. Meanwhile, the efficiency of 3,4-DCP removal by Fe(VI) at different Fe(VI) concentrations, solution pH, and coexisting anions was determined. Furthermore, eight intermediates of 3,4-DCP oxidation were identified, and a detailed mechanism involving hydroxylation, substitution, dechlorination, and ring-opening steps of the Fe(VI)/3,4-DCP reaction was proposed. The removal of 3,4-DCP was enhanced in actual water samples, demonstrating that Fe(VI) technology could be regarded as an efficient approach for removing 3,4-DCP from water.