A comparative study of reactive manganese species and electron transfer pathway in oxidation efficiency and environmental impact: Which activation route for potassium permanganate is optimal?

Various methods have been explored to activate potassium permanganate (Mn(VII)) for the elimination of organic compounds, typically by generating highly-reactive manganese species (RMnS) or mediated by electron transfer process (ETP). However, the oxidation selectivity, transformation pathways, toxicity byproduct potential, and efficacy in complicated water matrices associated with RMnS and ETP have not been comprehensively evaluated and compared, which is important for selecting a fit-of-purpose mechanism for water remediation. This study selected Mn(VII)/graphite process and ultraviolet (UV)/Mn(VII) process as the model ETP-dominated system and RMnS-dominated system, respectively. RMnS demonstrated significantly higher degradation efficiency for bromophenols, with oxidation rate constants 2.69–6.28 times higher than ETP. The oxidation efficiency of RMnS could be enhance under alkaline condition, whereas the degradation efficiency of ETP is dependent on the combined effects of solution pH and pKa of compounds. Furthermore, RMnS exhibited a stronger dehalogenation capacity, enabling the almost complete release of bromide ions from bromophenols with the formation of non-brominated organic product. Correspondingly, the RMnS process obviously reduced the brominated disinfection byproducts formation potential (DBPFPs). Mass spectrometry results revealed that the ETP process tended to form more polymeric brominated dimer products during the oxidation of bromophenol, leading to more DBPFPs production. ETP process showed superior degradation efficiency in real water backgrounds due to robustness against complicated water matrices, and displayed lower energy and oxidant consumption. Findings of this study elucidated the efficiency and mechanistic differences between RMnS and ETP, providing guidance for selecting activation methods to enhance KMnO₄-based water treatment process.