Assigning EXAFS results for uranyl adsorption on minerals via formal charges of bonding oxygen centers

EXAFS is a common tool for studying the speciation of adsorbed actinides on mineral surfaces as it provides information on structural features of the latter. Yet, the interpretation of the resolved distances can be a challenge. We present a qualitative correlation between the formal charges of equatorially coordinated oxygen centers of uranyl(VI) complexes and the corresponding equatorial bond lengths U-O, involving oxygen centers either of ligands or surfaces of phyllosilicates. This correlation is established computationally with density functional results for uranyl and uranyl monohydroxide species adsorbed on basal and edge surfaces of kaolinite, pyrophyllite, and model 2:1 clay minerals with charged layers. The correlation is independent of the specific mineral and its surface orientation. It enables one to extend the current interpretation of EXAFS data for equatorial U-O distances, suggesting assignments to specific surface groups.