Enhanced Pore Structure of Powdered Activated Carbon Can Boost Its Performance in Catalyzing Mn(II) Oxidation by Chlorine

Mn(II) oxidation by chlorine under neutral pH conditions is inherently slow. Previous studies have demonstrated that a few milligrams per liter of powdered activated carbon (PAC) can dramatically accelerate this process, but catalytic efficiency varies widely across different PAC types. In this study, various chemical and physical modifications were applied to coal- and straw-based PAC to enhance their catalytic efficiency and identify the key characteristics decisively influencing catalysis. The results showed that chemical treatments with HNO3 or NaOH had no notable impact on the PAC-catalyzed Mn(II) oxidation. In contrast, physical modifications, steam-reactivation, and ultrasonication substantially enhanced PAC's catalytic performance, increasing Mn(II) oxidation rates by 50-300%. Characterization revealed that alterations in surface functional groups, induced by chemical treatments, had no noticeable impact on the Mn(II) oxidation. Instead, enhancements in pore structure achieved through physical modifications remarkably accelerated the Mn(II) oxidation process. The specific surface area of PAC exhibited a strong positive correlation with both the Mn(II) oxidation and chlorine decay rates. Mn(II) oxidation by chlorine retained by PAC (not chlorine in water) demonstrated the occurrence of Mn(II) and chlorine reactions in PAC pores. The increased local concentrations of the reactants and confined catalysis in nanoscale pores were proposed to interpret the rapid Mn(II) oxidation.