Enhanced Oxidation of Organic Contaminants by Mn(VII)/CaSO₃ Under Environmentally Relevant Conditions: Performance and Mechanisms

Although permanganate activation by sodium sulfite (Mn(VII)/Na₂SO₃) has shown great potential for rapid abatement of organic contaminants, the limited reactivity under alkaline conditions and undesirable Mn residual may prevent its widespread application. To solve these challenges, calcium sulfite (CaSO₃) was employed as a slow-release source of SO₃²⁻/HSO₃⁻ (S(IV)) to activate Mn(VII) in this study. It was found that the application of CaSO₃ solid could extend the effective working pH range of Mn(VII)/S(IV) from ≤7.0 to ≤9.0. Moreover, due to the enhanced precipitation of MnO₂ with the presence of Ca²⁺, very low Mn residual (<0.05 mg/L) was achieved in Mn(VII)/CaSO₃ system. Mn(VII)/CaSO₃ system is a unique two-stage oxidation process in terms of reaction kinetics and reactive oxidants. Specifically, Mn(VII) was rapidly consumed and reactive Mn intermediates (e.g., Mn(VI), Mn(V)), SO₄^•−, and HO^• were produced in the first stage. However, the second stage was governed by the interaction between MnO₂ and S(IV), with SO₄^•− and HO^• serving as the dominant reactive oxidants. Taking advantage of an automatic titrator, excess S(IV) was found to greatly quench the generated radicals, whereas it did not cause a significant consumption of reactive Mn species. All these results improved our understanding of the Mn(VII)/S(IV) process and could thus facilitate its application.