Localised solution environments drive radionuclide fractionation in uraninite

Highlights: • Release of radionuclides from uraninite is highly dependent on solution composition. • U and Th show highest recovery in sulfate versus chloride and carbonate solutions. • Comparatively, Ra, Po and Pb show highest recovery in carbonate solutions. • Pore pressure has no prominent effect on radionuclide recovery. • Nanoscale characterisation reveals RN disequilibrium and fractionation. We explore the role of various solution environments – chloride brines, acid mine drainage (sulfate) and groundwater (carbonate), as well as pore pressure in producing secular disequilibrium among the various radionuclides (RN) in the U-decay series upon leaching of uraninite – the most abundant U-ore and a widespread accessory mineral in U-rich rocks. We observed that the end products of the U-decay chain, ²⁰⁶Pb and ²⁰⁷Pb, exist primarily at the surface/edges of grains or within large pores in the uraninite. In contrast, the intermediate daughters ²²⁶Ra, ²¹⁰Pb, ²¹⁰Po, and ^234/230Th, exist primarily within the bulk of uraninite, requiring breakdown by leaching for subsequent mobility to occur. Overall, pore pressure had little effect on RN mobility, with solution environment being the primary factor in creating significant mobility and disequilibrium among the RN, as it drives the initial breakdown of uraninite and influences the subsequent differential solubility of individual RNs. This was particularly the case for carbonate-bearing fluids, leading to significant fractionation of the various daughter RN arising from variable complexation and sorption phenomena. Understanding the geochemical behaviour of the RN in the U-decay series is important for predicting and managing the risks associated with RN in both environmental (acid-mine drainage) and engineered (metallurgical extraction) processes. Effective modelling of long-term RN behaviour should incorporate this strong relative fractionation caused by contrasting geochemical behaviour of individual RN during and after their release into the water from uraninite and subsequent interaction with the surrounding aquifer host rocks.