Qi Zhou, Wangmi Chen, Xiuwei Ao, Jianlong Wang, Zifu Li
Understanding of the influence of coexisting Mn(II) on solid-phase denitrification (SPD) systems in treating nitrate-contaminated groundwater remains limited. This study investigated the impact of different Mn(II) concentrations on the performance and mechanisms of sawdust-based SPD systems for treating nitrate-contaminated groundwater within 106 days. The SPD systems still maintained high nitrate removal efficiency (>99 % within 48 h) with Mn(II) addition. Surprisingly, 10 mg/L Mn(II) improved denitrification rates, and enhanced PO43--P and DOC removal while Mn(II) itself was not eliminated. Kinetic modeling confirmed that first-order and Gompertz models effectively (R2 > 0.91) predicted nitrate removal under varying Mn(II) concentrations. XPS confirmed the coexistence of Mn(II), Mn(III), and Mn(IV), supporting an active Mn(II) redox process. Microbial community shifts under high Mn(II) favored Firmicutes over Proteobacteria. 10 mg/L Mn(II) promoted enrichment of hydrolysis/fermentative bacteria (Bacteroidetes_vadinHA17), spore-forming/Mn-oxidizing bacteria (unclassified_f__Sporomusaceae), and Mn-reducing bacteria (Paludibaculum and Anaerovorax), revealing that the dual role of high Mn(II) in promoting supply of biomass carbon sources and enhancing Mn redox cycle. Differential functional gene analysis revealed that 10 mg/L Mn(II) stimulated the TCA cycle, electron transfer, and denitrifying enzymes for enhancing denitrification. Mn(II) oxidation was primarily mediated by Fe-Mn superoxide dismutase (Fe-MnSOD) within spore-forming bacteria, whereas MnOx reduction was driven by flagellar and pili and electron shuttles (riboflavin and heme). Based on these findings, we proposed a mechanism driven by 10 mg/L Mn(II), beyond which Mn cycling continuously facilitates N removal. These insights offer a new insight into the Mn redox cycle, contributing to the optimization of sawdust-based SPD systems for groundwater remediation.