Assessing ligand selectivity for uranium over vanadium ions to aid in the discovery of superior adsorbents for extraction of UO₂²⁺ from seawater

Uranium is used as the basic fuel for nuclear power plants, which generate significant amounts of electricity and have life cycle carbon emissions that are as low as renewable energy sources. However, the extraction of this valuable energy commodity from the ground remains controversial, mainly because of environmental and health impacts. Alternatively, seawater offers an enormous uranium resource that may be tapped at minimal environmental cost. Nowadays, amidoxime polymers are the most widely utilized sorbent materials for large-scale extraction of uranium from seawater, but they are not perfectly selective for uranyl, UO₂²⁺. In particular, the competition between UO₂²⁺ and VO²⁺/VO₂⁺ cations poses a significant challenge to the efficient mining of UO₂²⁺. Thus, screening and rational design of more selective ligands must be accomplished. One of the key components in achieving this goal is the establishment of computational techniques capable of assessing ligand selectivity trends. Here, we report an approach based on quantum chemical calculations that achieves high accuracy in reproducing experimental aqueous stability constants for VO²⁺/VO₂⁺ complexes with ten different oxygen donor ligands. The predictive power of the developed computational protocol is demonstrated for amidoxime-type ligands, providing greater insights into new design strategies for the development of the next generation of adsorbents with high selectivity toward UO₂²⁺ over VO²⁺/VO₂⁺ ions. Importantly, the results of calculations suggest that alkylation of amidoxime moieties present in poly(acrylamidoxime) sorbents can be a potential route to better discrimination between the uranyl and competing vanadium ions in seawater.