Influence of Hydrogen Bonding on the Structure of the (001) Corundum–Water Interface. Density Functional Theory Calculations and Monte Carlo Simulations

Density functional theory calculations and classical Monte Carlo simulations are applied to study the behavior of water in contact with a hydroxylated corundum (001) surface. Using DFT with periodic boundary conditions at <i>T</i> = 0 K, we systematically study the influence of the number of water molecules on the surface geometry and on the structure of the contact water layer. Only little effect of the thickness of the water layer on the geometry of the surface hydroxyl groups is observed. On the other hand, the molecules in the second layer have strong influence on the arrangement of water molecules in direct contact with the solid surface. In order to mimic macroscopic systems at room temperature, we perform inhomogeneous MC simulations of model corundum surface in contact with the water phase modeled by SPC/E model. The water molecules are classified according to their hydrogen-bonded partners into several groups. It is found that the preferential orientation of interfacial water molecules is primarily determined by the type of their hydrogen bonding. The hydroxyl groups at the corundum surface can serve as hydrogen bond donor or acceptor, depending on their orientation. No surface hydroxyls are found to coordinate two water molecules at the same time. On the other hand, water molecules coordinated by two different surface groups appear in MC simulations

Uranium Redox Transformations after U(VI) Coprecipitation with Magnetite Nanoparticles

Uranium redox states and speciation in magnetite nanoparticles coprecipitated with U­(VI) for uranium loadings varying from 1000 to 10 000 ppm are investigated by X-ray absorption spectroscopy (XAS). It is demonstrated that the U M₄ high energy resolution X-ray absorption near edge structure (HR-XANES) method is capable to clearly characterize U­(IV), U­(V), and U­(VI) existing simultaneously in the same sample. The contributions of the three different uranium redox states are quantified with the iterative transformation factor analysis (ITFA) method. U L₃ XAS and transmission electron microscopy (TEM) reveal that initially sorbed U­(VI) species recrystallize to nonstoichiometric UO_2+<i>x</i> nanoparticles within 147 days when stored under anoxic conditions. These U­(IV) species oxidize again when exposed to air. U M₄ HR-XANES data demonstrate strong contribution of U­(V) at day 10 and that U­(V) remains stable over 142 days under ambient conditions as shown for magnetite nanoparticles containing 1000 ppm U. U L₃ XAS indicates that this U­(V) species is protected from oxidation likely incorporated into octahedral magnetite sites. XAS results are supported by density functional theory (DFT) calculations. Further characterization of the samples include powder X-ray diffraction (pXRD), scanning electron microscopy (SEM) and Fe 2p X-ray photoelectron spectroscopy (XPS)