Chemical oxidants, such as permanganate (Mn(VII)), ferrate (Fe(VI)), hydroxyl radical (HO●), and sulfate radical (SO4●−), have been widely applied for the elimination of contaminants in water and wastewater treatment. However, traditional oxidants have low oxidation capacity and SO4●−/HO●-based advanced oxidation processes (AOPs) are non-selective toward substrates and thereby sensitive to the nontarget matrix constituent. Therefore, we developed the sulfite-coupling Mn(VII) (PM/BS) process which could oxidize organic contaminants at extraordinarily high rates. Many lines of evidences indicated that both radicals, including HO● and SO4●−, and reactive manganese species (RMnS), including Mn(VI), Mn(V), and possible Mn(III), contribute to the degradation of contaminants in the PM/BS process. The contributions of the reactive oxidative species (ROS) were strongly dependent on the type of contaminants, pH levels, and the molar ratio of sulfite to Mn(VII). Furthermore, we found that the sulfite-coupling Fe(VI) (Fe(VI)/sulfite) process exhibited efficient oxidation of various organic contaminants and the ROS in this process changed from Fe(V) to SO4●−/HO● with increasing molar ratio of sulfite to Fe(VI). In these two processes, the reaction between sulfite and Mn(VII)/Fe(VI) is initiated by one-electron transfer to form SO3●− and Mn(VI)/Fe(V). Then the generated SO3●− reacts rapidly with O2 to form SO5●−, which in turn leads to the formation of SO4●− and HO●. In addition, we confirmed the generation of Cr(V) and SO4●− and proposed a new mechanism for the decomposition of coexisting organic contaminants during Cr(VI) reduction by sulfite. We also found that Mn(II) could be effectively oxidized by O2 to MnO2 with the addition of sulfite under environmentally relevant conditions and sulfur-centered radicals played a critical role in the bromate reduction by sulfite. This work advances the fundamental understanding of the contaminants degradation in the sulfite-mediated AOPs.

Mechanisms of sulfite-mediated advanced oxidation processes

ACS Fall 2021

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