The structure of uranium (VI) sorption complexes on silica, alumina, and montmorillonite

We have investigated the adsorption of the uranyl ion (UO22+) in contact with amorphous silica (SiO2) gamma-alumina (Al2O3), and montmorillonite surfaces in the pH range of 3.1-6.5, using X-ray absorption fine structure (XAFS) spectroscopy to observe the local structure around the uranium atom. Analysis shows that in all samples the uranyl ion structure is preserved, with two axial oxygen atoms detected at ca. 1.8 Angstrom. For the montmorillonite samples at low pH a single equatorial oxygen shell is observed at ca. 2.4 Angstrom, with a coordination number of 6 +/- 1. At near-neutral pH (6.41) and high ion concentration (0.1 M NaCl), two separate equatorial shells are observed with bond lengths of ca. 2.30 and 2.48 Angstrom. The samples of uranyl on silica and gamma-alumina are all observed to have two separate equatorial shells with bond lengths of ca. 2.30 Angstrom and 2.49 Angstrom. A uranium shell at ca. 4.0 Angstrom is observed in the near-neutral pH (similar to 6.5) samples of uranyl on silica and on gamma-alumina. A silicon shell at ca. 3.10 Angstrom is observed in the sample of uranyl on silica at pH 6.5. These results suggest that adsorption of the uranyl ion onto montmorillonite at low pH occurs via ion exchange, leaving the inner-sphere uranyl aquo-ion structure intact. At near-neutral pH and in the presence of a competing cation, inner-sphere complexation with the surface predominates. Adsorption of the uranyl onto the silica and gamma-alumina surfaces appears to occur via an inner-sphere, bidentate complexation with the surface, with the formation of polynuclear surface complexes occurring at near-neutral pH. Copyright (C) 2000 Elsevier Science Ltd.