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

Although permanganate activation by sodium sulfite (Mn(VII)/Na 2 SO 3 ) 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 (CaSO 3 ) was employed as a slow-release source of SO 3 2− /HSO 3 − (S(IV)) to activate Mn(VII) in this study. It was found that the application of CaSO 3 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 MnO 2 with the presence of Ca 2+ , very low Mn residual (<0.05 mg/L) was achieved in Mn(VII)/CaSO 3 system. Mn(VII)/CaSO 3 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)), SO 4 •− , and HO • were produced in the first stage. However, the second stage was governed by the interaction between MnO 2 and S(IV), with SO 4 •− 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.