A critical review of biochar-enhanced Feammox: Multilevel mechanisms and sustainable applications for nitrogen removal

Abstract

As a novel anaerobic nitrogen removal pathway, ammonium oxidation coupled with iron reduction (Feammox) demonstrates significant advantages in low C/N ratio environments. However, its practical application is limited by bottlenecks such as low electron transfer efficiency, unsustainable iron cycling, and pH sensitivity. Biochar provides an innovative solution for enhancing Feammox due to its unique porous structure, redox activity, and surface functional groups. This review systematically elucidates the quadruple mechanisms of biochar in enhancing Feammox performance: (i) serving as an electron shuttle to accelerate electron transfer between microbes and Fe(III); (ii) facilitating Fe(III)/Fe(II) cycling regeneration to address the challenge of sustained iron supply; (iii) optimizing microbial community structure by enriching Feammox bacteria and activating the expression of electron transfer-related genes; and (iv) regulating microenvironmental pH and extracellular polymeric substance (EPS) secretion to improve system resilience against environmental stressors. Key factors influencing biochar efficacy are summarized to guide the design of tailored biochar materials for specific Feammox applications. Furthermore, we propose a biochar-driven Feammox-Anammox-NDFO coupled process to achieve synchronous nitrogen-iron-electron cycling and NH₄⁺ /NO₃^- co-removal, offering a novel paradigm for low-carbon wastewater treatment. Future research should focus on the targeted design and optimization of biochar to achieve highly efficient Feammox enhancement, thereby advancing global nitrogen cycling and sustainable wastewater nitrogen removal technologies.