Why does dissolved oxygen govern Mn(III) formation and micro-pollutant abatement in the permanganate/bisulfite process?

Permanganate/bisulfite (PM/BS) process is a novel advanced oxidation process for degradation of micro-pollutants in water/wastewater treatment. The abatements of iopamidol (IPM), metribuzin (MET) and phenol by the PM/BS process after 15 s were 65%, >99% and >99%, respectively, while no obvious abatements were observed with PM or BS alone. These accelerated degradations by the PM/BS process were attributed to the formations of Mn(III), sulfur radical (SO4•−) and hydroxyl radical (HO•). Interestingly, it was found that dissolved oxygen (DO) governed the abatement of micro-pollutants in PM/BS process. When DO was around 0.1 mg L−1, the abatements of all tested micro-pollutants were <5%, significantly lower than those under 9.1-mg L−1 DO (65%−99%). Moreover, the formation of Mn(III) in the PM/BS process exhibited a positive dependence on DO consumption. Via electron paramagnetic resonance and quenching experiments, it was further found that the reduction of PM by BS generated SO3•−, which could be oxidized by DO to SO5•−. The Mn(III) formation stoichiometry of [PM]:[BS]:[DO] in the PM/BS process was determined to be 1:5:1.5 ratio. SO5•− promoted the formations of not only Mn(III), but also SO4•− and HO•, among which Mn(III) and SO4•− mainly contributed to the degradation of micro-pollutants. Especially, this study illustrates that the formation pathway of Mn(III) in PM/BS process is not the direct reduction of Mn(VII), but the oxidation of Mn(II) by SO5•−. Meanwhile, maintaining enough DO is indispensable for the degradation of micro-pollutants by the PM/BS process.