The role of anaerobic respiration in the immobilization of uranium through biomineralization of phosphate minerals

Although bioreduction of uranyl ions (U(VI)) and biomineralization of U(VI)–phosphate minerals are both able to immobilize uranium in contaminated sediments, the competition between these processes and the role of anaerobic respiration in the biomineralization of U(VI)–phosphate minerals has yet to be investigated. In this study, contaminated sediments incubated anaerobically in static microcosms at pH 5.5 and 7.0 were amended with the organophosphate glycerol-2-phosphate (G2P) as sole phosphorus and external carbon source and iron oxides, sulfate, or nitrate as terminal electron acceptors to determine the most favorable geochemical conditions to these two processes. While sulfate reduction was not observed even in the presence of G2P at both pHs, iron reduction was more significant at circumneutral pH irrespective of the addition of G2P. In turn, nitrate reduction was stimulated by G2P at both pH 5.5 and 7.0, suggesting nitrate-reducing bacteria provided the main source of inorganic phosphate in these sediments. U(VI) was rapidly removed from solution in all treatments but was not reduced as determined by X-ray absorption near edge structure (XANES) spectroscopy. Simultaneously, wet chemical extractions and extended X-ray absorption fine structure (EXAFS) spectroscopy of these sediments indicated the presence of U–P species in reactors amended with G2P at both pHs. The rapid removal of dissolved U(VI), the simultaneous production of inorganic phosphate, and the existence of U–P species in the solid phase indicate that uranium was precipitated as U(VI)–phosphate minerals in sediments amended with G2P. Thus, under reducing conditions and in the presence of G2P, bioreduction of U(VI) was outcompeted by the biomineralization of U(VI)–phosphate minerals and U(VI) sorption at both pHs.