Potential-driven balance of extracellular electron transfer activity and microstructure stability in Shewanella biofilm for uranium recovery.

Bioelectrochemistry enables uranium recovery from uranium-contaminated water. Within microbial electrolysis cells (MECs), electroactive biofilm growth is critical, but the impact of electrode potential on biofilm structure and function remains unknown. This study explored variations in micro-structure, extracellular electron transfer, and uranium recovery of Shewanella oneidensis MR-1 biofilms cultivated under different potential shocks from -0.2 to 0.6 V (vs Ag/AgCl) in MECs at -0.2 V. Across this range, uranium recovery exhibited a clear non-monotonic trend, peaking at 0.2 V and exhibiting the most unreliable performance at 0.6 V. At 0.2 V (low potential, LP), the biofilm electrode exhibited 99.72 % uranium recovery with 42.19 % U(IV) reduction. Additionally, transmission electron microscopy revealed needle-like mineral within the inner extracellular polymeric substances (EPS), attributed to moderate c‑type cytochrome (c-Cyts) density (104.89 % of open circuit potential, OCP), suitable charge transfer resistance (Rct, 68.34 % of OCP), and a dense EPS network (bulk density 138.07 % of OCP). At 0.6 V, the biofilm exhibited higher electrochemical activity (c-Cyts 113.15 % and Rct 48.41 % of OCP), but poorer structural stability (EPS bulk density 101.38 % of OCP), causing fluctuating recovery (99.39 % to 94.10 %) and a lower U(IV) proportion (33.90 %). These results indicate that balancing biofilm activity and structural stability is essential for efficient and sustainable recovery of uranium. The LP biofilm electrode achieved 98.09 % uranium recovery with a surface distribution coefficient (Kd,s) of 1.414 L/cm2 in real mining groundwater. This finding presents a balance model linking biofilm electrode activity and stability, highlighting the necessity of regulating cultivation potential shock.