A review in Fe(0)/Fe(Ⅱ) mediated autotrophic denitrification for low C/N wastewater treatment

Fe-mediated autotrophic denitrification has emerged as a promising technology for low carbon-to-nitrogen (C/N) wastewater treatment due to its cost-effectiveness, operational safety, and independence from organic carbon. While several reviews have addressed this process, there is still a lack of comprehensive analyses that connect iron materials, denitrification performance, and microbial communities. The practical applicability of solid-phase iron has received little attention, and the key controversial issue - Fe(Ⅱ) oxidation pathway - has not been specifically examined. This review systematically examines both liquid- and solid-phase iron sources, with particular emphasis on the practical applicability of solid-phase iron, and further provides an integrated discussion of denitrification performance and associated functional microbes. In addition, this review summarizes a range of iron-oxidizing denitrifiers and highlights several key genera in detail. It also provides an in-depth analysis of Fe(Ⅱ) oxidation pathways, with particular attention to the ongoing debate regarding the involvement of enzymatic mechanisms. Moreover, the latest advancements in both natural and engineered applications are reviewed. Operational parameters such as temperature (T), pH, dissolved oxygen (DO), Fe/N ratio, and other influencing factors are discussed. Finally, several critical challenges that fundamentally affect Fe-mediated autotrophic denitrification are highlighted. This review aims to support the practical implementation of Fe-mediated autotrophic denitrification in low C/N wastewater treatment and contribute to the sustainable development of environmentally friendly biotechnologies for advanced nitrogen control.