U(VI) sorption during ferrihydrite formation: Underpinning radioactive effluent treatment

Iron (oxyhydr)oxide nanoparticles are known to sorb metals, including radionuclides, from solution in various environmental and industrial systems. Effluent treatment processes including the Enhanced Actinide Removal Plant (EARP) (Sellafield, UK) use a neutralisation process to induce the precipitation of iron (oxyhydr)oxides to remove radionuclides from solution. There is a paucity of information on mechanism(s) of U(VI) removal under conditions relevant to such industrial processes. Here, we investigated removal of U(VI) from simulated effluents containing 7.16 mM Fe(III) with 4.2 × 10^-4-1.05 mM U(VI), during the base induced hydrolysis of Fe(III). The solid product was ferrihydrite under all conditions. Acid dissolutions, Fourier Transform infrared spectroscopy and thermodynamic modelling indicated that U(VI) was removed from solution by adsorption to the ferrihydrite. The sorption mechanism was supported by X-ray Absorption Spectroscopy which showed U(VI) was adsorbed to ferrihydrite via a bidentate edge-sharing inner-sphere species with carbonate forming a ternary surface complex. At concentrations ≤0.42 mM U(VI) was removed entirely via adsorption, however at 1.05 mM U(VI) there was also evidence for precipitation of a discrete U(VI) phase. Overall these results confirm that U(VI) sequestered via adsorption to ferrihydrite over a concentration range from 4.2 × 10^-4-0.42 mM confirming a remarkably consistent removal mechanism in this industrially relevant system.