Formation of a ternary neptunyl(V) biscarbonato inner-sphere sorption complex inhibits calcite growth rate

Neptunyl, Np(V)O(2)(+), along with the other actinyl ions U(VI)O(2)(2+) and Pu(V,VI)O(2)((+,2+)), is considered to be highly mobile in the geosphere, while interaction with mineral surfaces (inner- or outer-sphere adsorption, ion-exchange, and coprecipitation/structural incorporation) may retard its migration. Detailed information about the exact interaction mechanisms including the structure and stoichiometry of the adsorption complexes is crucial to predict the retention behavior in diverse geochemical environments. Here, we investigated the structure of the neptunyl adsorption complex at the calcite-water interface at pH 8.3 in equilibrium with air by means of low-temperature (15K) EXAFS spectroscopy at the Np-L(III) edge. The coordination environment of neptunyl consists of two axial oxygen atoms at 1.87(±0.01)Å, and an equatorial oxygen shell of six atoms at 2.51(±0.01)Å. Two oxygen backscatterers at 3.50(±0.04)Å along with calcium backscatterers at 3.95(±0.03)Å suggest that neptunyl is linked to the calcite surface through two monodentate bonds towards carbonate groups of the calcite surface. Two additional carbon backscatterers at 2.94(±0.02)Å are attributed to two carbonate ions in bidentate coordination. This structural environment is conclusively interpreted as a ternary surface complex, where a neptunyl biscarbonato complex sorbs through two monodentate carbonate bonds to steps at the calcite (104) face, while the two bidentately coordinated carbonate groups point away from the surface. This structural information is further supported by Mixed Flow Reactor (MFR) experiments. They show a significant decrease of the calcite growth rate in the presence of neptunyl(V), in line with blockage of the most active crystal growth sites, step and kink sites, by adsorption of neptunyl. Formation of this sorption complex constitutes an important retention mechanism for neptunyl in calcite-rich environments.