Phosphate (bio)mineralisation remediation of 90Sr contaminated groundwaters

Activities at globally significant nuclear facilities such as Hanford, USA and Sellafield, UK have led to a substantial legacy of radioactivity contaminated land. Calcium phosphate minerals such as hydroxyapatite (HAp) can adsorb and incorporate radionuclides, including the high yield fission product 90Sr, within their structure. Past work has shown that aqueous injection of HAp-generating solutions into contaminated sediments and groundwaters on both a laboratory and field scale can reduce the amount of mobile 90Sr in the systems. In this study, two microbially mediated phosphate amendment techniques designed to precipitate HAp, (i) Ca-citrate / Na-phosphate and (ii) glycerol phosphate were tested in batch experiments alongside an abiotic treatment ((iii) polyphosphate), using stable Sr and site relevant groundwaters and sediments. Geochemical results suggest all three amendments led to removal of Sr from solution compared to the sediment only control. The Ca-citrate / Na-phosphate treatment removed 97%, glycerol phosphate 60% and polyphosphate 55% of the initial Sr. At experimental end points, scanning electron microscopy showed Sr-containing, Ca-PO4 phases were deposited on the surface of sediment grains. XAS analyses of the sediment end points amended with Ca-citrate / Na-phosphate and glycerol phosphate amendments confirmed Sr incorporation into Ca-PO4 phases was occurring. Overall, the data show that these Ca-PO4 treatments have the potential to be broadly applied at a range of nuclear sites and are a key option within the toolkit for 90Sr groundwater remediation.