Radioactively contaminated land is present at several nuclear facilities across the world. Uranium and strontium are two priority radionuclides requiring remediation as they are commonly found in elevated concentrations in groundwaters and soils at nuclear sites such as uranium fuel fabrication plants (e.g. Portsmouth Gaseous Diffusion Plant, Ohio, U.S.) and waste storage facilities (e.g. the Hanford Site, Washington, U.S.) respectively. There are many disadvantages associated with traditional approaches to treating land impacted by radioactive contamination. Remediation strategies such as âdig and dumpâ and âpump and treatâ are expensive and building the necessary infrastructure on-site may present logistical challenges at congested nuclear facilities. Further, they generate large quantities of radioactive waste that may increase radiation exposure risks. Alternative methods to sustainably immobilise radionuclide contaminants in-situ must be considered, as well as the long-term fate of these remediated radionuclide phases. Uranium remediation is primarily achieved by reducing mobile U(VI) to largely insoluble U(IV), whilst strontium remediation generally aims to coprecipitate Sr2+ with calcium minerals that are stable under environmental conditions. Phosphate complexation has the capacity to effectively co-remediate uranium and strontium. In this project, novel abiotic and microbial treatments for remediating aqueous uranium and strontium contamination were investigated using sediments and groundwater representative of the subsurface at the Sellafield site. In a series of sediment microcosms, novel Fe0-based nanomaterials and glycerol phosphate displayed the highest proficiencies for uranium and strontium removal respectively, through reductive precipitation of U(IV) and strontium-phosphate mineralisation. Further, the radionuclide endpoints produced were recalcitrant to dissolution and subsequent radionuclide remobilisation under oxidising conditions. One of the Fe0-based treatments is a novel composite material called Carbo-iron, which is composed of Fe0 nanoparticles anchored on activated carbon colloids. Adding phosphate-functionality to the surface of Carbo-iron using a bioavailable phosphate source was highly effective at removing uranium from solution by abiotic U(VI) phosphate precipitation, however, the modified nanocomposite material did not enhance strontium remediation compared to Sr2+ adsorption to sediments. Instead, separate sediment microcosms amended with urea successfully remediated aqueous Sr2+ through coprecipitation with calcium carbonate biominerals. Finally, Carbo-iron was coated in a sodium carboxymethyl cellulose solution and injected into columns containing Sellafield-representative sediments, prior to the continuous flow of U(VI)-amended artificial Sellafield groundwater. The migration of uranium in sediments columns unamended with Carbo-iron was significantly impeded through sorption processes. The amount of uranium entering the Carbo-iron sediment column was limited due to compreignacite precipitation in the tubing attached to the base of the column. Migration of the coating from the sediment core likely caused compreignacite formation in the influent tubing. Nonetheless, uranium transport was restricted under dynamic conditions that are more representative of the natural subsurface. Overall, the results presented in this thesis display the great promise of novel approaches and materials for in-situ radionuclide immobilisation under environmentally-relevant scenarios. This includes amendments of raw and phosphate-functionalised composite materials containing Fe0 nanoparticles for uranium removal and urea for strontium remediation, as well as glycerol phosphate for cotreating uranium and strontium contamination.
Radionuclide remediation via functionalised nanoparticles
White-Pettigrew, M. (Author). 1 Aug 2024
Student thesis: Phd