Unrecognized role of bisulfite as Mn(III) stabilizing agent in activating permanganate (Mn(VII)) for enhanced degradation of organic contaminants

Very recently, it has been reported that bisulfite can activate permanganate to enhance the oxidation of selected organic contaminates at acid and neutral pH, where aquo and hydroxo Mn(III) formed via the fast reaction of permanganate with bisulfite is proposed to be responsible. In this work, to gain further insight into the role of bisulfite, we investigated the degradation of an emerging endocrine disruptor bisphenol S (BPS) by the permanganate/bisulfite system. We found that permanganate showed very sluggish reactivity toward BPS, while the permanganate/bisulfite system rapidly oxidized BPS at pH 4–7 with rates (∼2.8–85.2 s⁻¹) that were 6–7 orders of magnitude faster than those for permanganate alone (∼10⁻⁶ s⁻¹). Both rate and extent of BPS loss in the permanganate/bisulfite system maximized at the [bisulfite]:[permanganate] ratio of 5:1–10:1, appreciably higher than the stoichiometric ratio of 2:1. Similarly, thiosulfate and hydrosulfite also showed activating effects, where the loss of BPS exhibited dependency on the [thiosulfate]:[permanganate] or [hydrosulfite]:[permanganate] ratio as well. Comparatively, arsenite showed negligible activating effect, while the combination of arsenite and bisulfite resulted in appreciable degradation of BPS by permanganate even though arsenite outcompeted bisulfite for permanganate. In addition, it was found that several model organic ligands appreciably inhibited the degradation of BPS in the permanganate/bisulfite system while Fe(III) showed negligible effect. These results were rationalized by the fact that reduced sulfur inorganics acted as complexing agents to stabilize Mn(III), and the formation of reactive Mn(III)-sulfur complexes responsible for BPS degradation was at near diffusion-controlled rate (∼10⁹ M⁻¹ s⁻¹) and thus not a rate-limiting step in the permanganate/bisulfite process.