Guiwei Li

Stringent control of Mn is critical for drinking water safety. However, conventional rapid sand filters exhibit slow biological maturation for Mn(II) removal, limiting their performance in engineered Mn(II) removal. This study examined the effectiveness of partially replacing sand with natural manganese ore (NMO) to enhance Mn(II) removal and elucidated the underlying mechanisms. The results showed that replacing only 1/8 of sand with NMO enabled consistently effective Mn(II) removal (residual x-mediated chemical processes rapidly diminished, after which MnO_x-enhanced biological processes promptly took over. Compared with sand columns, 1/8 NMO columns biologically matured nearly 100 days earlier, and biomass on the rough, porous MnO_x surface was 1-2 orders of magnitude higher than that on sand. Although NMO partly altered microbial communities, the relative abundance of Mn(II)-oxidizing bacteria did not increase; the larger absolute biofilm biomass primarily drove sustained efficient Mn(II) removal. These findings highlighted the important but previously overlooked role of MnO_x in promoting biofilm accumulation and accelerating the maturation of biological Mn(II) removal systems and provided a practical and cost-effective upgrade for conventional sand filtration.

Efficient manganese removal at water treatment plants is becoming increasingly significant due to growing health and aesthetic concerns associated with Mn in drinking water. Recently, biological granular activated carbon (GAC) filtration has demonstrated effectiveness in removing dissolved Mn(II) biologically. However, most water treatment plants continue to rely on conventional sand filtration. To enhance biological Mn(II) removal on the basis of sand filtration remains a challenge. This study aimed to explore whether partially integrating GAC into sand filters could significantly enhance Mn(II) removal. Results showed that even a 25% replacement of sand with GAC enabled rapid filter maturation and Mn(II) removal efficiency that was statistically comparable to the 100% GAC filter (p > 0.05), with marked improvement over the 100% sand filter. Solid analysis revealed that GAC media facilitated biomass accumulation that was 25 times greater than that observed with sand, and biogenic MnO_x also adhered more strongly to GAC surfaces. This caused the GAC portion of the GAC–sand filter to play a dominant role in Mn(II) removal. Additionally, the GAC media fostered both biomass growth and MnO_x accumulation in the underlying sand layer, enabling the sand portion to remove more Mn(II). This proof-of-concept study highlighted a promising strategy for addressing Mn issues by integrating GAC into sand filtration systems without requiring substantial infrastructure modifications.

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.