Enhancing the Abatement of Permanganate-inert Micropollutants: Multiple Roles of Nascent Manganese Dioxide in Permanganate Oxidation.

Permanganate (Mn(VII)) is widely used as an oxidant in water treatment and usually reduced to nascent manganese dioxide (MnO2), which could promote Mn(VII) oxidation for the Mn(VII)-reactive compounds such as phenols and anilines. However, the removal of micropollutants containing diverse functional groups and the underlying mechanisms remain largely unexplored. This study reveals that Mn(VII)/nascent MnO2 was effective for the degradation of Mn(VII)-inert micropollutants, including sulfonamide antibiotics, β-blockers and trimethoprim, with observed first-order rate constants (k'obs) of 0.126 ∼ 9 min-1 at pH 4.0. The synergetic effect of Mn(VII) and nascent MnO2 on the degradation of Mn(VII)-inert micropollutants decreased significantly when pH increased from 4.0 to 9.5. MnO2 played multiple roles in micropollutant degradation, which acted as a catalyst to promote the Mn(VII) oxidation of trimethoprim and propranolol, as well as an oxidant in propranolol degradation. Besides, Mn(III) oxidation accounted for 58% of the overall degradation of propranolol, but was not important for trimethoprim oxidation. Hydroxylated products were common products formed in Mn(VII)/MnO2. Differently, trimethoprim tended to form single-ring products via MnO2-catalyzed Mn(VII) oxidation, while propranolol preferentially formed dimers via in situ formed MnO2 oxidation. This study is the first to report that MnO2 enhances the abatement of Mn(VII)-inert micropollutants during Mn(VII)-based water treatment and unravels the multiple roles of MnO2 in micropollutant degradation by Mn(VII)/MnO2.