Activities at the Sellafield, UK nuclear licensed site over the last 70+ years have led to discharges of radionuclides to the sub-surface. Resulting in a significant legacy of radioactively contaminated land. Strontium-90 (90Sr) is one of the more challenging radionuclides in the sub-surface due to its relatively high mobility in the Sellafield sub-surface. Developing remediation strategies for 90Sr is therefore important in maintaining the stewardship of the site in the medium to long term. Calcium phosphate minerals such as hydroxyapatite (HAp) have been suggested as promising radionuclide (including 90Sr) sinks, as they can incorporate a range of radionuclides within their structure. The formation of HAp can be achieved in situ through both biotic and abiotic approaches. In previous studies conducted at the Hanford nuclear site, USA, aqueous injection of HAp-generating solutions have reduced the amount of mobile 90Sr within contaminated groundwater on both laboratory and field scales. In this study, prominent biotic (calcium citrate / sodium phosphate, glycerol phosphate) and abiotic (polyphosphate) phosphate amendment techniques were tested using microcosm experiments with the aim of developing a targeted remediation toolkit for 90Sr remediation at Sellafield. Aqueous geochemical results suggest all three amendment solutions removed Sr from solution at an enhanced rate when compared to the sediment sorption control. Here, the calcium citrate / sodium phosphate amendment was the most effective at Sr removal with the polyphosphate least effective. Scanning electron microscopy in conjunction with energy dispersive x-ray spectroscopy conducted on sediments after treatment with calcium citrate / sodium phosphate and glycerol phosphate amendments showed the presence of strontium containing calcium phosphate phases deposited on the surface of larger sediment grains. Extended x-ray absorption fine structure analysis of these treated sediments is consistent with Sr incorporation into hydroxyapatite. Demonstrating the potential for these techniques to provide remediation strategies at nuclear sites.

Approaches to groundwater radionuclide remediation at Sellafield - in situ phosphate mineralisation

ACS Spring 2022

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