Multidimensional mechanisms of biochar in mitigating Fe(III) stress in anammox consortia

The stress of high-concentration Fe(III) severely restricts the anaerobic ammonium oxidation (anammox) engineering application. Herein, we systematically investigated the mitigating mechanisms of biochar-mediated anammox system against Fe(III) stress by constructing a batch reaction system with a gradient Fe(III) concentration (0-100 mg/L). The results showed that biochar notably mitigated Fe(III) toxicity by synergizing multiple pathways, such as physical adsorption, chemical reduction, and biological sheltering. The systematic nitrogen removal efficiency (NRE) was stabilized at 84.7 ± 1.40 % at 50 mg/L Fe(III) and the protein (PN) secretion from the extracellular polymeric substances (EPS) was remarkable increased (34.90 mg/g-VSS) to form a dynamic protective barrier; the mineral phase analysis showed that the biochar drove the conversion of Fe(III) to Fe(II) through surface redox-active functional groups to low-toxicity Fe₃O₄/FeO(OH) conversion, reducing free Fe(III) bioavailability. Microbial community resolution showed that biochar selectively protected the functional bacterium Candidatus Brocadia, whose relative abundance was elevated by 11.5 % in the 50 mg/L Fe(III) group. Additionally, significant enrichment of genera typical of nitrate-dependent Fe(II) oxidation (NDFO) (Acinetobacter and Thermomonas) in reactors with high Fe(III) concentrations promoted the nitrogen conversion process and Fe(II) utilization. This research reveals the core mechanism of biochar to enhance Fe(III) tolerance in the anammox system by regulating EPS secretion, iron morphology transformation, and microbial functional response. The study fills the knowledge gap in mitigating the inhibitory effects of high iron concentrations on anammox bacteria, thereby providing a theoretical basis and engineering application potential for the optimization of low-carbon nitrogen removal technology for high-concentration heavy metal wastewater.