Homogeneous versus heterogeneous Mn(II) oxidation in peroxymonosulfate assisting chlorination: Synergistic role for enhanced Mn(II) oxidation in water treatment.

Removal of Mn(II) is an essential step for addressing water discoloration in water treatment utilities worldwide. However, conventional chlorination suffers from poor oxidation of Mn(II) due to its low homogeneous oxidation kinetics. This study explored the oxidation capability of a new chemical dosing strategy employing peroxymonosulfate (PMS) to assist the chlorination process (PMS@Cl ₂ ) for effective Mn(II) oxidation. The study comprehensively explored both oxidation kinetics and underlying mechanisms associated with homogeneous and heterogeneous oxidation within the PMS@Cl ₂ system. At an [Mn(II)] ₀ of 1 mg/L, chlorination demonstrated inability in oxidizing Mn(II), with <10 % oxidation even at an elevated [Cl ₂ ] of 150 μM (∼10 mg/L). By contrast, PMS completely oxidized 100 % Mn(II) within a 30-minute reaction at a much lower [PMS] of 60 μM (k _obs = 0.07 min ^-1 and t _1/2 = 9 min), demonstrating its superior Mn(II) oxidation kinetics (over one order of magnitude faster than conventional chlorine). PMS@Cl ₂ exhibited an interesting synergistic benefit when combining a lower dose PMS with a higher routine dose Cl ₂ (loPMS@hiCl ₂ ), e.g. [PMS]:[Cl ₂ ] at 15:30 or 30:30 μM. Both conditions achieved 100 % Mn(II) oxidation, with even better values of k _obs and t _1/2 (0.16-0.17 min ^-1 and ∼4 min) relative to PMS alone at 60 µM. The synergic benefit of PMS@Cl ₂ was attributed to distinct functions played by PMS and Cl ₂ in both homogeneous and heterogeneous oxidation processes. Reactive species identification excluded the possible involvement of SO ₄ ^•- , OH ^• , or chlorine radicals in the homogeneous oxidation of the PMS@Cl ₂ system. Instead, the dominant species was O ₂ ^•- radical generated during the reaction of Mn(II) and PMS. Furthermore, the heterogeneous oxidation emphasized the important role of combining Cl ₂ dosing, which demonstrated an increased reactivity and electron transfer with the Mn-O-Mn complex, surpassing PMS. Overall, heterogeneous oxidation accelerated the oxidation kinetics of the PMS@Cl ₂ system by 1.1-2 orders of magnitude relative to the homogeneous oxidation of Cl ₂ alone. We here demonstrated that PMS@Cl ₂ could offer a more efficient mean of soluble Mn(II) mitigation, achieved with a relatively low routine dose of oxidant in a short reaction period. The outcomes of this study would address the existing limitations of traditional chlorine oxidation, minimizing the trade-offs associated with high residual chlorine levels after treatments for soluble manganese-containing water.
Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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