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

Kathleen R Salome,Stefan J Green,Melanie J Beazley,Samuel M Webb,Joel E Kostka,Martial Taillefert

doi:10.1016/j.gca.2012.12.037

Abstract

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.

Highlights

The United States Department of Energy (DOE) currently manages 120 nuclear legacy waste sites spread over 36 states contaminated with heavy metals and radionuclides, such as uranium (U) (DOE, 1997; NABIR, 2003)
In all treatments without addition of external terminal electron acceptors (TEAs), background nitrate and sulfate concentrations exchanged from the original sediment after 3 hours of equilibration averaged 450 μM and 800 μM, irrespective of the pH of the incubations (Figure 1A-D)
Nitrate was removed in the sulfate-amended reactors, only ephemeral accumulation of nitrite was observed, while traces of nitrite were observed in the G2P-amended reactors with no external TEA (Figure 1C-D)

Summary

Introduction

In oxic groundwater where U(VI) is the dominant oxidation state, uranium usually occurs as the 15 highly mobile uranyl ion UO22+ (Langmuir, 1997). The dominant aqueous forms of uranyl in the environment include the free uranyl ion at low pH and positively charged hydroxyl complexes at circumneutral pH (5 ≤ pH ≤ 6.5) (Langmuir, 1997). At pH 5.0, aqueous U(VI) adsorbs strongly to manganese oxides given their low pHzpc (Han et al., 2007) and even to ferric oxides despite the net positive charge of both uranyl hydroxide complexes and metal oxides (Han et al, 2007; Hsi and Langmuir, 1985; Waite et al., 1994). Ferric oxides represent one of the most important U(VI) sorbents in soils of the

Methods

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Discussion

Conclusion