Uranium coordination on clay surface at nanoscale: Integration of EXAFS data and Ab initio molecular dynamics

Understanding the interactions between uranium and clay minerals at the molecular level is crucial for addressing environmental contamination and nuclear waste management. In this work we are focusing on the determination the molecular-level structure of uranium (VI) surface species on smectite using a combination of EXAFS spectroscopy and ab initio molecular dynamics simulations (AIMD). A set of samples was prepared with uranyl cation adsorbed on bentonite clay of three deposits at various pH and uranium concentrations. To elucidate the structure of the adsorbed species at nanoscale, AIMD simulations were carried out for uranyl coordination models at basal surface and on edge sites of clay mineral. Linear combination fitting of experimental EXAFS spectra with those calculated from AIMD trajectories allowed us to specify the fractions of each adsorbed complex configuration present at pH 3, 5 and 8. At low pH, the speciation of the adsorbed uranyl cation is mainly comprised of outer-sphere basal complex. When pH is increased to 5, edge complexes at (110) surface become predominant, and at pH 8 uranyl cation is mainly localized at (010) surface of clay mineral. With the implementation of advanced molecular dynamics simulations and spectroscopic techniques, the obtained results for the first time reveal the structure of the adsorbed uranyl complexes on clay mineral surfaces at nanoscale.