HN-AD biofilm reactor couples nitrogen removal with rare earth element enrichment under oxygen and carbon-source regulation

Groundwater in rare earth mines commonly exhibits combined NH 4 + -N, and NO 3 − -N, primarily stemming from the extensive use of (NH 4 ) 2 SO 4 as a leaching agent. It is characterized by high acidity, low dissolved oxygen levels, and the coexistence of rare earth elements (REEs) and heavy metals (HMs), which poses significant challenges for biological nitrogen removal. Existing research primarily relies on simulated wastewater systems, leaving the feasibility and mechanisms for practical application in rare earth mine groundwater environments unclear. This study established a continuous-flow biofilm reactor column dominated by heterotrophic nitrification-aerobic denitrification (HN-AD) microbial communities under real rare-earth mine groundwater conditions. The reactor column continuously achieved TN removal rates above 90% at a C/N ratio of 13 and an aeration rate of 1.2 L/min. The system showed notable enrichment and removal capabilities for 16 REEs and HMs, with removal rates of 99% and 95%, respectively. Extracellular polymers played a crucial role in buffering metal toxicity and maintaining biofilm stability. Functional microbial communities represented by Hydrogenophaga , Azoarcus , Paracoccus , and Flavobacterium formed a synergistic nitrogen transformation network. This study elucidates the mechanism by which the HN-AD biofilm system functions in groundwater remediation within rare earth mining areas, providing a new theoretical foundation and engineering reference for this in-situ biological remediation technology. • HN-AD biofilm column achieves >90% total nitrogen removal. • Simultaneously achieve removal of rare earth elements (>99%) and heavy metals (>95%). • Under acidic conditions, EPS buffers metal toxicity and stabilizes biofilms. • Microbial communities achieve nitrogen transformation under metal stress conditions.