Pathway-oriented regulation of biological nitrogen biotransformations by iron-based magnetic materials: mechanisms, pathway competition, and engineering implications.

The global pursuit of carbon-neutral wastewater treatment has shifted attention from maximizing nitrogen removal to controlling nitrogen fate, whether as innocuous N2, recoverable NH4+, or undesired intermediates. Iron-based magnetic materials are increasingly proposed for this purpose. However, the mechanistic basis for pathway selectivity has not been systematically compared across material classes. This review synthesizes evidence for three representative magnetic interventions: magnetite (Fe3O4), nanoscale zero-valent iron (nZVI), and neodymium-iron-boron (NdFeB) magnetic field-providing materials across denitrification, dissimilatory nitrate reduction to ammonium (DNRA), and anaerobic ammonium oxidation (anammox) to establish a mechanistic framework for material selection. These materials regulate nitrogen biotransformation through electron transfer, electron donation, microbial carriers, and magnetic field stimulation. Fe3O4 primarily promotes biomass retention, iron bioavailability, and interfacial electron transfer. Although nZVI increases electron availability and reducing capacity, excessive input may favor DNRA over denitrification in nitrate-reducing systems and weaken canonical anammox by promoting iron-coupled nitrogen transformations. NdFeB indirectly modulates nitrogen pathways by altering mass transfer, extracellular polymeric substances (EPS) production and composition, sludge aggregation, and microbial metabolism. Cross-pathway comparison shows that denitrification relies on taxonomically diverse but functionally recurring communities, whereas DNRA is governed mainly by competition for electrons and substrates. In contrast, anammox maintains more distinct core functional populations but is vulnerable to excessive reduction. These mechanistic differences explain why effective dosages and field intensities are process-specific rather than universal. Because the DNRA comparison includes only two study-condition observations, its apparent intervention range and ammonium-recovery potential remain indicative rather than established engineering guidance. Future research should therefore move beyond empirical performance enhancement and focus on pathway-oriented material design, including process-specific intervention ranges, combined use of reactive particles and magnetic fields, material recovery and reuse, and mechanism-guided strategies for controlling nitrogen fate.