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@Cl₂) for effective Mn(II) oxidation. The study comprehensively explored both oxidation kinetics and underlying mechanisms associated with homogeneous and heterogeneous oxidation within the PMS@Cl₂ system. At an [Mn(II)]₀ of 1 mg/L, chlorination demonstrated inability in oxidizing Mn(II), with <10 % oxidation even at an elevated [Cl₂] 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@Cl₂ exhibited an interesting synergistic benefit when combining a lower dose PMS with a higher routine dose Cl₂ (loPMS@hiCl₂), e.g. [PMS]:[Cl₂] 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@Cl₂ was attributed to distinct functions played by PMS and Cl₂ in both homogeneous and heterogeneous oxidation processes. Reactive species identification excluded the possible involvement of SO₄^•-, OH^•, or chlorine radicals in the homogeneous oxidation of the PMS@Cl₂ system. Instead, the dominant species was O₂^•- radical generated during the reaction of Mn(II) and PMS. Furthermore, the heterogeneous oxidation emphasized the important role of combining Cl₂ 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@Cl₂ system by 1.1-2 orders of magnitude relative to the homogeneous oxidation of Cl₂ alone. We here demonstrated that PMS@Cl₂ 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.