Kinetics and mechanism of diclofenac removal using ferrate(VI): roles of Fe3+, Fe2+, and Mn2+

In this study, the effect of Fe3+, Fe2+, and Mn2+ dose, solution pH, reaction temperature, background water matrix (i.e., inorganic anions, cations, and natural organic matters (NOM)), and the kinetics and mechanism for the reaction system of Fe(VI)/Fe3+, Fe(VI)/Fe2+, and Fe(VI)/Mn2+ were investigated systematically. Traces of Fe3+, Fe2+, and Mn2+ promoted the DCF removal by Fe(VI) significantly. The pseudo-first-order rate constant (kobs) of DCF increased with decreasing pH (9–6) and increasing temperature (10–30 °C) due to the gradually reduced stability and enhanced reactivity of Fe(VI). Cu2+ and Zn2+ ions evidently improved the DCF removal, while CO32− restrained it. Besides, SO42−, Cl−, NO3−, Mg2+, and Ca2+ almost had no influence on the degradation of DCF by Fe(VI)/Fe3+, Fe(VI)/Fe2+, and Fe(VI)/Mn2+ within the tested concentration. The addition of 5 or 20 mg L−1 NOM decreased the removal efficiency of DCF. Moreover, Fe2O3 and Fe(OH)3, the by-products of Fe(VI), slightly inhibited the DCF removal, while α-FeOOH, another by-product of Fe(VI), showed no influence at pH 7. In addition, MnO2 and MnO4−, the by-products of Mn2+, enhanced the DCF degradation due to catalysis and superposition of oxidation capacity, respectively. This study indicates that Fe3+ and Fe2+ promoted the DCF removal mainly via the self-catalysis for Fe(VI), and meanwhile, the catalysis of Mn2+ and the effect of its by-products (i.e., MnO2 and MnO4−) contributed synchronously for DCF degradation.