Enhanced diclofenac abatement by ferrate in the presence of montmorillonite: Trapping in-situ formed Fe(III)

Abstract

Ferrate (Fe(VI)) is an environmentally-friendly chemical oxidant employed in water and wastewater treatment. The effect of montmorillonite (MMT) on the abatement of diclofenac (DCF) by Fe(VI) was studied. Results indicate that the presence of 200 mg/L MMT enhanced the second-order rate constant for the reaction between DCF and Fe(VI) from 117 to 219 M-1 s-1 at pH 7.6, whereas no significant enhancement was observed for silicon dioxide and kaolinite. In-situ formed Fe(III) from Fe(VI) decay played important roles on the enhanced oxidation of DCF by Fe(VI). In addition, no enhancement was observed in phosphate buffer since phosphate could sequester Fe(III). A similar enhancement of DCF abatement by Fe(VI) in the presence of MMT can be observed using preformed Fe-MMT. Adsorbed Fe(III) which is present in the form of FeOOH can increase the layer spacing of MMT from 11.4 to 15.0 Å. In-situ formed Fe-MMT increases the oxidation efficiency of methylphenyl sulfoxide, a probe compound. Addition of 50 mg/L MMT during Fe(VI) oxidation can increase the yield of reactive high-valent iron intermediates (e.g., Fe(IV) and Fe(V)) by 43%. This work provides new insights into Fe(VI) application in pre-oxidation for the abatement of micropollutants in water treatment processes.