Enhanced biofilter startup using pine biochar for efficient Mn(II) removal from acidic wastewater: Mechanisms and performance

Manganese contamination in acidic wastewater poses significant environmental and health risks, but conventional biological treatments often fail under low-pH conditions. To address this challenge, we developed a biochar-amended biofiltration system by first isolating an acid-tolerant manganese-oxidizing bacterium, Bacillus megaterium strain M3, and systematically evaluating its synergy with rice straw-derived, cow dung-derived, and pine waste-derived biochar. Batch studies demonstrated that pine waste-derived biochar significantly enhanced microbial Mn(II) oxidation, with biochar-MnOB systems showing 22.55 % higher oxidation rates than biochar-free controls. In continuous biofilters, this synergistic effect reduced startup time by 20–33 % compared to quartz sand systems, achieving compliance with the < 2 mg/L Mn discharge standard within 30 days, even at a pH of 4.0. Advanced characterization revealed biochar functions through multiple mechanisms: its porous and inert structure provided microbial habitat while buffering localized pH, its presence was associated with increased secretion of protein-rich extracellular polymeric substances, and correlated with the formation of higher-valency manganese oxides through enhanced enzyme activity. By bridging agricultural waste recycling with wastewater treatment, this study offers a cost-effective, sustainable solution for acidic Mn-laden wastewater remediation, emphasizing the untapped potential of pine waste-derived biochar in environmental biotechnology. • Acid-tolerant Bacillus megaterium strain M3 was isolated for wastewater treatment. • Pine biochar enhanced Mn(II) oxidation rates by 22.55 % in batch systems. • Biochar-amended biofilters reduced startup time by 20–33 % versus quartz sand. • Manganese oxidase activity and protein-rich EPS were enhanced by biochar. • Pine biochar converted forestry waste into a high-value filtration material.