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+ 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 μM 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+. The apparent second-order rate constant (kapp) for the oxidation of ABTS by Fe(VI) at pH 7.0 was determined to be 1.1 × 106 M−1 s−1, which was 3–5 orders of magnitude higher than those for the reactions of ABTS+ with DCF (kapp,ABTS+-DCF = 2.8 × 103 M−1 s−1) and Fe(VI) with DCF (kapp,Fe(VI)-DCF = 17.7 M−1 s−1). Both the kapp,Fe(VI)-ABTS and kapp,Fe(VI)-DCF decreased obviously with increasing pH, while the kapp,ABTS+-DCF 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).