Mechanism of enhancing nitrogen removal by iron-modified biochar in anammox treatment of ammonia nitrogen wastewater from rare earth mines

Anaerobic ammonium oxidation (anammox) presents a potential solution for treating ammonium salt leachate from rare earth ore (REO) tailings due to its unique advantages. However, the performance of anammox mediated nitrogen removal significantly fluctuates under different environmental conditions. Therefore, it is crucial to enhance the tolerance of anammox processes when treating REO wastewater. In this work, iron modified biochar (FeBC) was prepared using sugarcane bagasse to investigate the critical role of iron in microbiology and its enhancement effects and mechanisms on the anammox system. Characterization confirmed Fe loading on the sugarcane bagasse surface in the form of micro-nano crystalline particles, with FeBC exhibiting significantly increased specific surface area and porosity compared to unmodified biochar. Following 70 d recovery period, the addition of FeBC showed the best performance. Compared to the blank control group, the NH 4 ⁺-N removal rate was improved by 21.64%, the EPS content was increased by 71.5 mg/g VSS, and the relative abundance of the nitrogen removal functional bacterial unclassified_f_Brocadiaceae increased from 5.84% to 16.85%. Based on relevance analysis, unclassified_f_Brocadiaceae indicated strong correlations with FeBC and PN content. Microorganisms exhibited high affinity for Fe released from FeBC, forming a reversible Fe-N redox cycle that optimize the electron transfer network to alleviate inhibitory effects of adverse environments on anammox system. These findings provide new insights for the efficient treatment of ammonia nitrogen wastewater from REO and the stabilization and upgrading of anammox processes.