Promoted oxidation of diclofenac with ferrate (Fe(VI)): Role of ABTS as the electron shuttle

Reaction of Fe(VI) with 2,2'-azino-bis-(3-ethylbenzothiazoline-6-sulfonate) (ABTS) is widely adopted to determine aqueous ferrate (Fe(VI)) concentration based on ABTS(center dot+) formation. Interestingly, this study found that the addition of ABTS could accelerate the oxidation of diclofenac (DCF) by Fe(VI) significantly. Observed first-order rate constant of DCF in the presence of 30 mu M ABTS was found to be 36.2 folds of that without ABTS, with values of 3.08 and 0.085 min(-1), respectively. It was partly attributed to the formation of ABTS(center dot+). The apparent second-order rate constant (k(app)) for the oxidation of ABTS(center dot+) by Fe(VI) at pH 7.0 was determined to be 1.1 x 10(6)M(-1) s(-1), which was 3-5 orders of magnitude higher than those for the reactions of ABTS(center dot+) with DCF(k(app),(center dot+)(ABTS)(-DCF) =2.8 x 10(3) M-1 s(-1)) and Fe(VI) with DCF(k(app),(Fe) ((VI)-DCF) =17.7 M-1 s(-1)). Both the k(app),(Fe(VI)-ABTS) and k(app,Fe(VI)-DCF) decreased obviously with increasing pH, while the k(app,ABTS)(-DCF)(center dot+) exhibited little pH dependency. By acting as the electron shuttle, ABTS could enhance the removal efficiency of DCF over wide pH and natural organic matter concentration ranges. This study provides new insights to reconsider the role of organic matter during Fe(VI) oxidation and highlights the potential for increasing the reactivity of Fe(VI). (C) 2017 Elsevier B.V. All rights reserved.