SPECIATION OF URANYL SORBED AT MULTIPLE BINDING-SITES ON MONTMORILLONITE

We have investigated the structures of U (VI) complexes as uranyl moieties sorbed onto a reference montmorillonite, SAz-1, using X-ray absorption fine structure spectroscopy (XAFS). The uranyl-loaded clays were prepared from aqueous solutions of uranyl nitrate in the pH range from 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 clay. For all samples, XAFS results indicate that there are two axial oxygen atoms at 1.78-1.80 Angstrom, as expected for the uranyl moiety. The average numbers and distances of equatorial oxygen atoms about uranyl sorbed on the clay vary significantly as a function of surface coverage. At high coverage (34.6% CEC), the average number and distance of equatorial oxygen atoms are near those found for the fully hydrated uranyl 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 number and distance of equatorial oxygen atoms are intermediate between those at higher and lower coverage. These changes suggest that sorbing U is reacting with at least three different sites on the clay as U concentration increases. The existence of multiple surface sites and sorption complexes which are structurally distinct from solution species need to be considered for rigorous modeling of sorption processes.