SPECIATION OF URANYL SORBED AT MULTIPLE BINDING-SITES ON MONTMORILLONITE

We have investigated the structures of U (VI) complexes as uranyl moie ties sorbed onto a reference montmorillonite, SAz-1, using X-ray absor ption fine structure spectroscopy (XAFS). The uranyl-loaded clays were prepared from aqueous solutions of uranyl nitrate in the pH range fro m 3.0 to 3.5. The U concentrations on the clay ranged from 1.7 to 34.6 % of the reported cation exchange capacity (CEC = 1.2 meq/g) of the cl ay. For all samples, XAFS results indicate that there are two axial ox ygen atoms at 1.78-1.80 Angstrom, as expected for the uranyl moiety. T he average numbers and distances of equatorial oxygen atoms about uran yl sorbed on the clay vary significantly as a function of surface cove rage. At high coverage (34.6% CEC), the average number and distance of equatorial oxygen atoms are near those found for the fully hydrated u ranyl species in aqueous solution. However, there are fewer equatorial oxygen atoms at a shorter average distance about uranyl sorbed at low coverage (1.7% CEC). At moderate coverage (7.3% CEC), the average num ber and distance of equatorial oxygen atoms are intermediate between t hose at higher and lower coverage. These changes suggest that sorbing U is reacting with at least three different sites on the clay as U con centration increases. The existence of multiple surface sites and sorp tion complexes which are structurally distinct from solution species n eed to be considered for rigorous modeling of sorption processes.