Performance and mechanism of anaerobic granular sludge enhancing uranium immobilization via extracellular polymeric substances in column reactors and batch experiments

Column reactors and batch experiments were conducted to investigate the performance and mechanism of uranium (U) immobilization enhanced by extracellular polymeric substances (EPS) from anaerobic granular sludge (ANGS). Successful remediation of anoxic U-contaminated groundwater was achieved using ANGS-inoculated column reactors (all higher than 98% of U removal rate for the three columns at hydraulic retention time of 72-h), and parameters (hydraulic retention time, nitrate content, the type and content of the external carbon sources) influenced the reactor performance. 306-day column studies implied the enhanced U immobilization by EPS, where EPS acted as energy source, electron donors and U reservoir with a contribution of 3.5–6.8% to total U removal by ANGS. Results in 26-h batch experiments further demonstrated that U immobilization performance of ANGS could be promoted (6.4–22%) by EPS through adsorption and reduction. EPS induced the precipitation of U(VI) and U(IV) particulates, and amino and carboxyl groups were the main functional groups for EPS binding with U. Complexation was confirmed between humic acids in EPS and U. The interaction mechanisms between U and EPS were directly evidenced to involve complexation, induced particulate formation and reduction.