Ultrafiltration separation of nanoscale Am(VI)-polyoxometalate clusters from lanthanides

Abstract

Partitioning of americium from lanthanides (Ln) present in used nuclear fuel plays a key role in the sustainable development of nuclear energy. This task is challenging because of the chemical similarities between Am(III) and Ln(III). Oxidization of Am(III) to Am(VI) has the potential to facilitate separations in principle, but rapid reduction of Am(VI) back to Am(III) by radiolysis products and organic reagents required for the traditional solvent extraction/solid extraction processes hampers practical redox-based separations. Herein, we report a nanoscale polyoxometalate (POM) cluster with a vacancy site compatible with the selective coordination of hexavalent actinides (238U, 237Np, 242Pu, and 243Am) over trivalent lanthanides in nitric acid media. This cluster is the most stable Am(VI) species in aqueous media observed to date. Ultrafiltration-based separation of nanoscale Am(VI)-POM clusters from hydrated lanthanide ions by commercially-available, fine-pored membranes enables the development of a once-through americium/lanthanide separation strategy that is highly efficient, rapid, environmentally benign, and requires minimal energy input.