Enhanced peroxymonosulfate activation via complexed Mn(II): A novel non-radical oxidation mechanism involving manganese intermediates

In recent years, the activation of persulfates (peroxydisulfate (PDS) and peroxymonosulfate (PMS)) via transition metal ions for contaminants degradation has received extensive attention in water treatment. There has been growing interest on the mechanism (radical versus non-radical pathway) of activation processes. Interestingly, in contrast to copper, iron or cobalt ions regarded as effective activators for persulfates, manganese ion (Mn(II)) is inefficient for persulfates activation. Inspired by the enhanced stability of manganese species by ligands, this study for the first time systematically investigated the Mn(II)/persulfates with different ligands as a novel oxidation technology. UV-vis spectrometry, chemical probing method and mass spectrometry were used to explore the reactive intermediate (free radical versus high-valent manganese species) therein. It was surprisingly found that the oxidation efficiency of Mn(II)/ligand/persulfates system was highly dependent on the nature of persulfates and ligands. Mn(II) chelated by amino ligands such as ethylenediaminetetraacetic acid (EDTA) and nitrilotriacetate (NTA) could efficiently trigger the oxidation of contaminants (e.g., recalcitrant compounds nitrophenol, benzoic acid and atrazine) by PMS, suggesting a promising Mn(II)/ligand/PMS technology for environmental decontamination especially under manganese-rich conditions. High-valent Mn species (Mn(V)) but not free radicals was demonstrated to be the dominant reactive intermediate, where Mn(III) species played a vital role in Mn(V) generation. The formation of Mn(III) species was found to be affected by the reactivity of persulfates and the type of ligands, thus influencing its further oxidation to Mn(V) species. This study presents a new oxidation process based on the combination of PMS and Mn(II) complex and broadens the knowledge of persulfates activation as well as manganese chemistry for decontamination in water treatment.