Phosphate (Bio)mineralization Remediation of ⁹⁰Sr-Contaminated Groundwaters

Abstract

Historical operations at nuclear mega-facilities such as Hanford, USA, and Sellafield, UK have led to a legacy of radioactivity-contaminated land. Calcium phosphate phases (e.g., hydroxyapatite) can adsorb and/or incorporate radionuclides, including 90Sr. Past work has shown that aqueous injection of Ca-phosphate-generating solutions into the contaminated ground on both laboratory and field scales can reduce the amount of aqueous 90Sr in the systems. Here, two microbially mediated phosphate amendment techniques which precipitated Ca-phosphate, (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. All three amendments led to enhanced Sr removal from the 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 that Sr-containing, Ca-phosphate phases were deposited on sediment grains, and XAS analyses of the sediments amended with Ca-citrate/Na-phosphate and glycerol phosphate confirmed Sr incorporation into Ca-phosphates occurred. Overall, Ca-phosphate-generating treatments have the potential to be applied in a range of nuclear sites and are a key option within the toolkit for 90Sr groundwater remediation.

Enhanced Strontium Removal through Microbially Induced Carbonate Precipitation by Indigenous Ureolytic Bacteria

“…Sediment microcosm experiments were incubated for 25 days in both open (air equilibrated) and closed systems to investigate whether microbial communities indigenous to various sediments, representative of the Sellafield subsurface, were capable of enhancing strontium removal via MICP, initiated by the breakdown of urea under open and closed conditions (Table ). Sediment microcosms were constructed with a 1:10 ratio of sediment to synthetic groundwater–water representative of the Sellafield region . Prior to sediment microcosm incubations, 100 ppm of Sr was added as strontium dichloride hexahydrate (SrCl 2 ·6H 2 O) to artificial groundwater (AGW) of the following composition (g/L): MgSO 4 ·7H 2 O, 0.05; CaSO 4 , 0.008; KCl, 0.01; NaCl, 0.012; CaCl 2 ·2H 2 O, 0.092; NaNO 3 , 0.028; NaHCO 3 , 0.08.…”

“…Sediment microcosms were constructed with a 1:10 ratio of sediment to synthetic groundwater–water representative of the Sellafield region. 140 Prior to sediment microcosm incubations, 100 ppm of Sr was added as strontium dichloride hexahydrate (SrCl 2 ·6H 2 O) to artificial groundwater (AGW) of the following composition (g/L): MgSO 4 ·7H 2 O, 0.05; CaSO 4 , 0.008; KCl, 0.01; NaCl, 0.012; CaCl 2 ·2H 2 O, 0.092; NaNO 3 , 0.028; NaHCO 3 , 0.08. After 7 days, the microcosms were spiked with 88 ppm of Sr 2+ and 80 ppm of Ca 2+ to instigate further Sr 2+ coprecipitation with calcium carbonate in the urea-bearing systems and display differences in Sr/Ca removal rates in the urea-free controls and urea-amended systems more clearly.…”