Activation of oxygen with sulfite for enhanced Removal of Mn(II): The involvement of SO 4 • .

Taking advantage of the active oxidants generated in the process of Mn(II)-catalyzed sulfite oxidation by oxygen, this study sought to enhance Mn(II) removal from water by activating oxygen with sulfite. The results revealed that Mn(II) can be effectively oxidized by oxygen to MnO ₂ with the addition of sulfite under environmentally relevant conditions, and the performance of this process is dependent on the dosage of sulfite and the initial pH. Mn K-edge XANES analysis indicates that Mn(II) removal is primarily due to the transformation of Mn(II) to MnO ₂ and, secondarily, to the adsorption of Mn(II) on generated MnO ₂ . Co-existing NaCl and CaCl ₂ negatively affect Mn(II) removal, while the presence of Fe(II) considerably enhances Mn(II) removal by improving both Mn(II) oxidation and Mn(II) adsorption on the generated solids. Consequently, Mn(II) removal is as high as 98% in the presence of 1.0 mg/L of Fe(II) and both the residual Mn (<0.1 mg/L Mn) and Fe (<0.3 mg/L Fe) can meet China's drinking water standard. The experiments with real water samples also demonstrate the effectiveness of the sulfite-promoted Mn(II) removal process, especially in the presence of Fe(II). The enhancing effect of sulfite on Mn(II) oxidation by oxygen is mainly associated with the generation of HSO ₅ ^- , and the critical step for generating HSO ₅ ^- is the rapid oxidation of SO ₃ ^•- by oxygen. EPR and radical scavenging studies demonstrate that SO ₄ ^•- radical is the key reactive oxygen species responsible for Mn(II) oxidation by HSO ₅ ^- .
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