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

Abstract

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 (k_obs) 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 CO₃^2- restrained it. Besides, SO₄^2-, 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. Graphical abstract ᅟ.

Keywords: Anionic effect; By-products; Cationic effect; Diclofenac; Ferrate.