Zero-valent iron boosts nitrate-to-ammonia bioconversion via extracellular electron donation and reduction pathway complementation

Microbial nitrate reduction to ammonia (NRA) presents a promising route to recover wastewater nitrogen resources, but its practical application is currently challenged by limited bacterial activity and reaction selectivity. Here, we propose a facile strategy to boost microbial NRA by using zero-valent iron (ZVI) as an environmentally-benign augment. Unlike the previously reported hybrid systems that rely mainly on mediated electron transfer between ZVI and bacteria for enhanced denitrification, we revealed a combined pathway of direct and mediated electron transfer from ZVI to bacteria, along with a complementation between biological and abiotic nitrogen conversion processes, to promote the NRA process. The bio-hybrid exhibited over 13-fold higher NO₃⁻ reduction activity than the individual bacteria or ZVI groups, nearly 100% NRA selectivity, and good stability for treating real wastewater. Our work provides an efficient and scalable route to combine ammonia production with wastewater valorization, which may be readily incorporated into various wastewater treatment processes to maximize resource recovery.