The role of active manganese species and free radicals in permanganate/bisulfite process

Bisulfite-activated permanganate (PM/BS) process has shown extremely great potential for the rapid degradation of organic contaminants while the active oxidants involved in this process have been under disputation. In this work, the active oxidants in PM/BS process were re-identified and the contributions of different active oxidants to the degradation of various organic contaminants were investigated with a competition kinetics method. Many lines of evidence indicated that both radical species, including hydroxyl radical (HO^[rad]) and sulfate radical (SO₄^[rad]−), and reactive manganese species (RMnS), including Mn(VI), Mn(V), and possible Mn(III), contribute to the degradation of contaminants in PM/BS process. SO₄^[rad]− contributed much more than HO^[rad] to the degradation of all contaminants of interest except nitrobenzene in PM/BS process. RMnS were very selective and showed high reactivity to phenolic and sulfoxide compounds as well as carbamazepine. With the increase of initial pH and the [bisulfite]/[permanganate] molar ratio, a mechanistic changeover from RMnS to radicals occurred during phenol degradation in PM/BS process. The generation of RMnS and radical species, especially SO₄^[rad]−, could be well explained by the proposed mechanism consisting of radical chain reactions and manganese reduction reactions via both electron transfer and oxygen transfer steps.