Over the past six decades, nuclear fuel cycle activities, uranium mining and reprocessing operations have resulted in radioactively contaminated land at several nuclear mega sites, including Sellafield, UK, Hanford and Rifle, USA and Mayak, Russia. Groundwaters and sediments at these sites have become contaminated with several radionuclides (e.g. 90Sr, U, 99Tc and Pu), and their continued migration through the subsurface poses a substantial environmental hazard as natural attenuation processes alone may not provide long-term sequestration. Contaminated nuclear sites may require the deployment of remediation technologies to sequester radionuclides from groundwaters, limiting subsurface migration of contaminants. In-situ remediation strategies, which typically use borehole or injection arrays, treat contaminants within the subsurface and often have significant advantages over conventional technologies (e.g., pump and treat), which typically require large operational footprints and in some cases, elevated worker doses. In situ injections of chemical amendment solutions can facilitate favourable alterations to subsurface geochemical conditions (e.g., redox and pH) and precipitation of insoluble mineral phases, e.g. phosphate and carbonates, which can sequester radionuclides through adsorption and incorporation. Additionally, bacteria within the subsurface can mediate the degradation of injected amendment solutions, slowly forming biominerals (e.g. Ca-phosphates), preventing injection well clogging and increasing the dispersion of amendments within the treatment zone. In situ phosphate mineralisation was used to remediate radioactively contaminated land at the Hanford Site USA. Here, microbially mediated phosphate biomineralisation sequestered 90Sr and U through adsorption and precipitation to amorphous Ca-phosphate biominerals. Over time, these phases recrystallise to form highly stable crystalline phases (e.g. hydroxyapatite and autunite), with 90Sr and U incorporated and immobilized within the crystal lattice. Phosphate minerals have been suggested as radionuclide sinks due to their low solubility under environmental conditions and ability to incorporate a range of radionuclides. Despite significant development of in situ phosphate mineralisation technologies at US nuclear sites, their efficacy has yet to be assessed under deployment conditions relevant to UK nuclear sites. This thesis investigated the applicability of in situ phosphate mineralisation technologies to remediate Sr and U contaminated land under various biogeochemical conditions relevant to nuclear sites, including Sellafield. Batch microcosms and flowing column experiments were set up under oxic conditions and using Sellafield-relevant groundwaters and sediments. A multi-technique approach was utilised to monitor aqueous geochemistry and characterise sediment endpoints, including inductively coupled plasma atomic emission spectroscopy (ICP-AES), mass spectrometry (ICP-MS), scanning electron microscopy (SEM), 16S microbial community analysis and x-ray absorption and fluorescence spectroscopy (XAS and XRF). In the first study, the fate of Sr during in situ phosphate mineralisation was explored in batch experiments using three prominent phosphate-generating amendment solutions, Ca-citrate/Na-phosphate, glycerol phosphate and polyphosphate. The results showed significant Sr removal occurred with all three amendments when compared to sorption to sediment alone. Characterisation of sediment endpoints with XAS and SEM showed Sr was sequestered through adsorption and partial incorporation into poorly ordered hydroxyapatite. For the second study, U behaviour during phosphate mineralisation was investigated. Aerobic microcosm experiments amended with Ca-citrate/Na-phosphate and glycerol phosphate were conducted, with XAS analysis carried out post-treatment to determine U speciation. This study showed that U was removed through highly insoluble uranyl orthophosphate precipitation in both Ca-citrate/Na-phosphate and glycerol phosphate amended systems. In the final study, flowing column experiments were conducted to assess the ability of Ca-citrate/Na-phosphate and glycerol phosphate amendments to remediate Sr under flowing conditions relevant to the Sellafield subsurface. In addition to stable non-active Sr columns, parallel columns with ultra-trace 90Sr were set up to compare Sr behaviour across orders of magnitude in concentration. Here, columns pre-treated with the phosphate-generating amendments showed enhanced Sr retention compared to sorption alone. Micro-focus XRF identified Sr hot spots in both pre-treated and sorption control columns. SEM and EDX analysis identified they were likely due to natural mineral phases. This demonstrates the complexity of understanding Sr behaviour in complex environmental scenarios where stable Sr and 90Sr are present and highlights the importance of the multi-technique approach to Sr analyses. Trace 90Sr columns showed the behaviour of Sr is consistent across orders of magnitude in concentration. These studies confirm the potential for in situ phosphate mineralisation technologies to remediate 90Sr and U contaminated land and widen the possible deployment window for these approaches to several nuclear sites including Sellafield.
Management of Radionuclide Contamination in Groundwaters at Sellafield: In situ Phosphate Mineralisation Approaches
Robinson, C. (Author). 10 Dec 2023
Student thesis: Phd