ADSORPTION OF THORIUM ON AMORPHOUS SILICA - AN EXAFS STUDY

Wet samples of Th(IV) sorbed on colloidal amorphous silica particles ( Aerosil OX 200) have been studied with extended X-ray absorption fine structure spectroscopy (EXAFS) at the Th L-III edge (16.3 keV), The sa mples were collected with sample surface coverages ranging from 3.8 to 230%. No fundamental changes in the structural environment of Th in t erms of the types and distances of nearest neighbors with increasing s urface loading were detected, but the results indicate an increasing d egree of structural disorder with increasing surface coverage, The O s hell around Th shows a distorted configuration with two O atoms at 2.3 3 Angstrom distance and four to six O atoms at 2.55 Angstrom. Only one Si shell is observed, with two Si atoms at a distance of 3.8-3.9 Angs trom. These results have been interpreted with a double corner-sharing surface complex of Th on silica, where the Th atom shares one O atom with each of two coordinated SiO4 tetrahedra, accounting for the first O coordination shell detected as well as the Th-Si distances. The lon ger Th-O distances result from coordinated water molecules from soluti on, No Th-Th interactions could be detected in the EXAFS spectra. (C) 1997 Academic Press.