Adsorption of uranyl species onto the rutile (110) surface: A periodic DFT study

To model the structures of dissolved uranium contaminants adsorbed on mineral surfaces and further understand their interaction with geological surfaces in nature, we have performed periodic density funtional theory (DFT) calculations on the sorption of uranyl species onto the TiO ₂ rutile (110) surface. Two kinds of surfaces, an ideal dry surface and a partially hydrated surface, were considered in this study. The uranyl dication was simulated as penta- or hexa-coordinated in the equatorial plane. Two bonds are contributed by surface bridging oxygen atoms and the remaining equatorial coordination is satisfied by H ₂O, OH ^-, and CO ₃ ^2- ligands; this is known to be the most stable sorption structure. Experimental structural parameters of the surface-[UO ₂ (H ₂O) ₃] ²⁺ system were well reproduced by our calculations. With respect to adsorbates, [UO ₂(L1) _x(L2) _y(L3) _z] ⁿ (L1=H ₂O, L2= OH ^-, L3=CO ₃ ^2-, x≤3, y≤3, z≤2, x+y+2z≤4), on the ideal surface, the variation of ligands from H ₂O to OH ^- and CO ₃ ^2- lengthens the U-O _surf and U-Ti distances. As a result, the uranyl-surface interaction decreases, as is evident from the calculated sorption energies. Our calculations support the experimental observation that the sorptive capacity of TiO ₂ decreases in the presence of carbonate ions. The stronger equatorial hydroxide and carbonate ligands around uranyl also result in U= O distances that are longer than those of aquouranyl species by 0.1-0.3 Å. Compared with the ideal surface, the hydrated surface introduces greater hydrogen bonding. This results in longer U=O bond lengths, shorter uranyl-surface separations in most cases, and stronger sorption interactions.