Adsorption of Uranyl Species onto the Rutile (110) Surface: A Periodic DFT Study

• Published in: Chemistry : a European journal Vol. 18; no. 5; pp. 1458 - 1466
• Main Authors: Pan, Qing-Jiang, Odoh, Samuel O., Asaduzzaman, Abu Md, Schreckenbach, Georg
• Format: Journal Article
• Language: English
• Published: Weinheim WILEY-VCH Verlag 27.01.2012; WILEY‐VCH Verlag; Wiley Subscription Services, Inc
• Subjects: Chemistry; contaminant adsorption; density functional calculations; Ligands; TiO2 rutile; uranium
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201101320

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 TiO2 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 H2O, OH−, and CO32− ligands; this is known to be the most stable sorption structure. Experimental structural parameters of the surface–[UO2(H2O)3]2+ system were well reproduced by our calculations. With respect to adsorbates, [UO2(L1)x(L2)y(L3)z]n (L1=H2O, L2=OH−, L3=CO32−, x≤3, y≤3, z≤2, x+y+2z≤4), on the ideal surface, the variation of ligands from H2O to OH− and CO32− lengthens the UOsurf 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 TiO2 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. Contaminant adsorption: The sorption of uranyl species onto the TiO2 rutile (110) surface was explored in periodic DFT calculations. Adsorbates SA–SC, with H2O, OH−, and CO32− ligands (see figure), represent the most important UVI species in natural aqueous systems. Variations in geometry parameters of surface complexes were rationalized by the coordination competition of equatorial groups. The present calculations indicate an energetic preference for the sorption of aquouranyl species versus other adsorbates.