The structure of monomeric and dimeric uranyl adsorption complexes on gibbsite: A combined DFT and EXAFS study

We investigated the structure of uranyl sorption complexes on gibbsite (pH 5.6-9.7) by two independent methods, density functional theory (DFT) calculations and extended X-ray absorption. ne structure (EXAFS) spectroscopy at the U-L-III edge. To model the gibbsite surface with DFT, we tested two Al(hydr)oxide clusters, a dimer and a hexamer. Based on polarization, structure, and relaxation energies during geometry optimization, the hexamer cluster was found to be the more appropriate model. An additional advantage of the hexamer model is that it represents both edges and basal faces of gibbsite. The DFT calculations of (monomeric) uranyl sorption complexes show an energetic preference for the corner-sharing versus the edge-sharing configuration on gibbsite edges. The energy difference is so small, however, that possibly both surface species may coexist. In contrast to the edge sites, sorption to basal sites was energetically not favorable. EXAFS spectroscopy revealed in all investigated samples the same interatomic distances of the uranyl coordination environment (RU-Oax approximate to 1.80 angstrom, RU-Oeq approximate to 2.40 angstrom), and towards the gibbsite surface (RU-O approximate to 2.87 angstrom, RU-Al approximate to 3.38 angstrom). In addition, two U-U distances were observed, 3.92 angstrom at pH 9.7 and 4.30 angstrom at pH 5.6, both with coordination numbers of similar to 1. The short U-U distance is close to that of the aqueous uranyl hydroxo dimer, UO2(OH)(2), reported as 3.875 angstrom in the literature, but significantly longer than that of aqueous trimers (3.81-3.82 angstrom), suggesting sorption of uranyl dimers at alkaline pH. The longer U-U distance (4.30 angstrom) at acidic pH, however, is not in line with known aqueous uranyl polymer complexes. Based on the EXAFS findings we further refined dimeric surface complexes with DFT. We propose two structural models: in the acidic region, the observed long U-U distance can be explained with a distortion of the uranyl dimer to form both a corner-sharing and an edge-sharing linkage to neighboring Al octahedra, leading to RU-U = 4.150 angstrom. In the alkaline region, a corner-sharing uranyl dimer complex is the most favorable. The U-O path at similar to 2.87 angstrom in the EXAFS spectra arises from the oxygen atom linking two Al cations in corner-sharing arrangement. The adsorption structures obtained by DFT calculations are in good agreement with the structural parameters from EXAFS analysis: U-Al (3.394 angstrom), U-U (3.949 angstrom), and U-O (2.823 angstrom) for the alkaline pH model, and U-Al (3.279 angstrom), U-U (4.150 angstrom), and U-O (2.743 angstrom) for the acidic pH model. This work shows that by combining EXAFS and DFT, consistent structural models for uranyl sorption complexes can be obtained, which are relevant to predict the migration behavior of uranium at nuclear facilities. (C) 2009 Elsevier Ltd. All rights reserved.