Simultaneous removal of critical rare earth elements and chalcogen oxyanions by anaerobic granular sludge

Conventional mining of economically and strategically important critical rare earth elements (REEs) (such as neodymium, lanthanum and dysprosium), and chalcogens (such as selenium and tellurium) are associated with a huge economic and environmental cost. Therefore, the need to recover REEs as well as chalcogens from different waste streams including wastewaters is becoming urgent. Batch assays on synthetic chalcogen/REE-laden wastewater showed that the presence of REEs significantly improved the tellurite removal rate (> 80%) and enhanced selenate removal by 66% ± 10%. Three 3.9 L continuous upflow anaerobic granular sludge bed (UASB) reactors were operated at a hydraulic retention time of 24 h and 30 °C. Selenate reduction was achieved with a removal efficiency of ~ 98% with an influent pH of 4.0 for more than 28 days. The effect of REEs on tellurite removal in the UASB bioreactor could not be clearly established since a soluble tellurium removal efficiency of more than 98% was observed already before the addition of REEs at elevated tellurite concentrations. The complete REE removal in both batch assays and UASB reactors at higher pH (7.0 ± 0.5) was attributed to precipitation, whereas chalcogen oxyanions removal was due to microbial reduction. However, at acidic pH, biosorption was responsible for REE’s removal, and the Se-enriched sludge exhibited a superior REE’s removal efficiency than the non-enriched and Te-enriched sludge.