surface. This is a general problem for traditional wet chemis-Wet samples of Th(IV) sorbed on colloidal amorphous silica try investigations of sorption phenomena. Whereas potentioparticles (Aerosil OX 200) have been studied with extended Xmetric, spectrophotometric, liquid-liquid extraction and ray absorption fine structure spectroscopy (EXAFS) at the Th L III other techniques have enabled a fairly complete and unamedge (16.3 keV). The samples were collected with sample surface biguous modelling of coordination chemistry in solution (see coverages ranging from 3.8 to 230%. No fundamental changes in e.g. 11), these techniques, although valuable, give a much the structural environment of Th in terms of the types and disless complete picture of sorption processes. This is mainly tances of nearest neighbors with increasing surface loading were because they are only able to give indirect information redetected, but the results indicate an increasing degree of structural garding the state of the solid/solution interface. Hence, many disorder with increasing surface coverage. The O shell around Th shows a distorted configuration with two O atoms at 2.33 A ˚disstructural and mechanistic aspects of sorption processes retance and four to six O atoms at 2.55 A ˚. Only one Si shell is main unclear. observed, with two Si atoms at a distance of 3.8-3.9 A ˚. These Until recently, there has been a lack of experimental methresults have been interpreted with a double corner-sharing surface ods that can give the necessary direct information on the complex of Th on silica, where the Th atom shares one O atom surface structure under the conditions of interest. Lately, with each of two coordinated SiO 4 tetrahedra, accounting for the however, techniques such as X-ray absorption fine structure first O coordination shell detected as well as the Th-Si distances. (XAFS) (12) and Rutherford backscattering (RBS) (2) The longer Th-O distances result from coordinated water molehave yielded valuable new insights regarding adsorption.
cules from solution. No Th-Th interactions could be detected in
There is clearly still a need for further studies of different the EXAFS spectra.