AN EXAFS STUDY OF URANYL-ION IN SOLUTION AND SORBED ONTO SILICA AND MONTMORILLONITE CLAY COLLOIDS

The mechanisms of sorption which control the uptake of ionic species from solution at mineral surfaces are of fundamental importance in colloid science, but are poorly understood because of a lack of knowledge of the structure of the surface species that form. EXAFS spectroscopy provides a means of studying the structure of sorbed species directly. The technique has been applied here to investigate uranyl species in solution and sorbed onto a montmorillonite clay and silica colloids. In solution at low pH (<3) the X-ray absorption fine structure (EXAFS) data are consistent with a monomeric aquo- complex containing five or six aquo- ligands (U-Oeq) in the equatorial plane of the uranyl ion. At higher pH (4-5) and degree of hydrolysis a reduction in EXAFS amplitude of the equatorial oxygen shell is observed and can be ascribed to increased structural disorder. A uranium neighbor shell at 3.82 Å is observed for a 0.01 mole dm-3 solution at pH 4.3, confirming the formation of polynuclear species. EXAFS indicates that the structure of the uranyl ion is retained on sorption at a clay/water and an oxide/water interface. The EXAFS of uranyl species sorbed onto montmorillonite at pH 5 closely resembles those in hydrolyzed solutions and are consistent with the uptake of solution species by an outer-sphere, ion-exchange mechanism. The EXAFS of uranyl species sorbed onto silica from solutions at pH 3 and 5 are similar, but quite different from those sorbed on the clay, and resemble those of the uranyl hydrous oxide. © 1992.