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

•Mn(VII)/CaSO3 is better than Mn(VII)/Na2SO3 for abating organics at pH ≥7.0.•Much less residual Mn(II) was achieved by Mn(VII)/CaSO3 than by Mn(VII)/Na2SO3.•Both radicals and Mn intermediates are involved in Mn(VII)/CaSO3 system.•Excess S(IV) could quench radicals but did not greatly consume reactive Mn species.•Slow release of S(IV) from CaSO3 alleviated the S(IV) quenching effect on radicals. 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 (