Radionuclide uptake during iron (oxyhydr)oxide formation : application to the Enhanced Actinide Removal Plant (EARP) process

The Enhanced Actinide Removal Plant (EARP) located at Sellafield, is a key facility for processing nuclear effluents in the UK. The EARP process decontaminates radioactive waste effluent by inducing the coprecipitation of Fe(III) along with any radionuclides in solution. The resulting radioactive solid phase is then separated from the decontaminated aqueous phase by ultrafiltration processes over several weeks. In the future the EARP facility's role will be expanded to treat effluents produced from nuclear decommissioning activities. However there remains a limited understanding around the mechanisms of radionuclide removal from solution and subsequent sequestration within the solid phase under conditions relevant to such industrial effluent treatment facilities. In this work, the fate of U(VI) and Th(IV) were investigated during the EARP process. XRD and TEM analyses revealed that the solid product from the EARP coprecipitation process was 2-line ferrihydrite which transformed over time to form hematite, with some goethite formation observed in the Th containing system. During this coprecipitation process U was initially removed from solution by adsorption to ferrihydrite as a bidentate, edge-sharing surface complex associated with ternary carbonate complexes. As the U containing system was aged, a maximum range of 61 - 75% U became consistently incorporated within the newly formed hematite phase for a wide range of systems containing 6 orders of magnitude total U:Fe molar ratio. Such a constant proportion of incorporated U suggests that this incorporation occurs during a particle mediated mechanism of hematite growth, such as oriented attachment. Interestingly, Th was removed from solution during the EARP coprecipitation process by a combination of both adsorption and occlusion mechanisms. EXAFS analyses revealed that the local coordination environment of Th associated with the solid phase altered considerably with increased aging time, and correspondingly Th became increasingly recalcitrant to remobilisation over time suggesting the formation of further occluded or even incorporated Th species within the transforming iron (oxyhydr)oxide phases. This thesis progresses the fundamental understanding of radionuclide interactions with iron (oxyhydr)oxide phases during coprecipitation and aging processes under conditions relevant to industrial waste treatment processes such as EARP.