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

Abstract

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 30μM ABTS was found to be 36.2 folds of that without ABTS, with values of 3.08 and 0.085min, respectively. It was partly attributed to the formation of ABTS. The apparent second-order rate constant (k) for the oxidation of ABTS by Fe(VI) at pH7.0 was determined to be 1.1×10Ms, which was 3-5 orders of magnitude higher than those for the reactions of ABTS with DCF (k=2.8×10Ms) and Fe(VI) with DCF (k=17.7Ms). Both the k and k decreased obviously with increasing pH, while the k 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).

Revelation of Fe(V)/Fe(IV) Involvement in the Fe(VI)–ABTS System: Kinetic Modeling and Product Analysis

“…At [ABTS] 0 /[Fe(VI)] 0 < 1.0, Fe(IV) is more likely to dominate the substrate degradation in the long run. In Dong' study, 28 the addition of a small amount of ABTS ([ABTS] 0 /[Fe(VI)] 0 < 0.2) could accelerate Fe(VI) oxidation of diclofenac in the course of 600 s at pH 8.0, which was explained by the possible involvement of more powerful oxidants, Fe(V) and ABTS •+ , generated from the reaction of Fe(VI) and ABTS. However, the kinetic simulation by the model in this study suggested a different explanation.…”

“…Meanwhile, Dong's study 28 reported that [ABTS] 0 /[Fe-(VI)] 0 > 10 was recommended for using the ABTS method for Fe(VI) determination in the presence of organic substrates (e.g., diclofenac) because an excess amount of ABTS could maintain the stability of ABTS •+ by inhibiting it from possible reaction with the substrate. This dilemma may render difficult application of the ABTS method for determination of Fe(VI) in the presence of substrates (e.g., amino acids, phenol, and alcohol) 46 susceptible to ABTS •+ oxidation, unless a new observed stoichiometry between Fe(VI) and ABTS •+ (i.e., >1) was applied when [ABTS] 0 /[Fe(VI)] 0 was greater than 10.…”

“…27 Later, ABTS was used as an activator to enhance the degradation of diclofenac by Fe(VI) ([ABTS] 0 /[Fe(VI)] 0 < 0.2), and the enhancement was attributed to the faster oxidation by ABTS •+ formed in situ. 28 Recently, the Fe(VI)− ABTS system at [ABTS] 0 /[Fe(VI)] 0 = 1.0 was proposed to be an "Fe(V)-only" system based on the assumption that there were negligible reactions between Fe(V)/Fe(IV) and ABTS •+ . 29 However, the kinetic information of reactive intermediate species (i.e., Fe(V), Fe(IV), ABTS •+ , and ABTS 2+ ) in the Fe(VI)−ABTS system was still missing.…”

Application of Fe(VI) in abating contaminants in water: State of art and knowledge gaps

“…The relevant reaction mechanism and the type of the dominating oxidants of these reaction systems warrant in-depth investigation. Dong et al (2017) found that ABTS, acting as the electron shuttle, accelerated the reaction of DCF with Fe(VI) over a wide pH range. It was proposed that Fe(VI) can oxidize ABTS to ABTS + , which was responsible for the enhanced oxidation of DCF (Dong et al, 2017).…”

“…Dong et al (2017) found that ABTS, acting as the electron shuttle, accelerated the reaction of DCF with Fe(VI) over a wide pH range. It was proposed that Fe(VI) can oxidize ABTS to ABTS + , which was responsible for the enhanced oxidation of DCF (Dong et al, 2017). However, the potential promoting effects of the in situ generated Fe(V)/ Fe(IV) were not considered.…”