Adsorption of tetravalent thorium to the (0 0 1) basal surface of the phyllosilicate muscovite from an aqueous solution (1 Â 10 À4 mol/L Th(IV) in 1 Â 10 À1 mol/L NaCl, pH = 3.2) was studied by crystal truncation rod (CTR) and resonant anomalous X-ray reflectivity (RAXR) measurements. Th uptake to the muscovite surface from solutions with total Th concentrations [Th] tot = 1 Â 10 À6 -4.88 Â 10 À3 mol/L and 1 Â 10 À1 mol/L NaCl, pH = 3.2 was quantified by alpha-spectrometry. The uptake measurements showed that Th adsorption to the muscovite surface follows a Langmuir isotherm with an apparent adsorption constant K app = 2 Â 10 4 L/mol up to [Th] tot = 1.02 Â 10 À3 mol/L. The CTR and RAXR results identified one dominant Th species with a very broad distribution centered $10 Å above the surface, in agreement with strongly hydrated extended outer sphere sorption. The findings indicate that the large energy of hydration (DG hyd = À5815 kJ/mol (Marcus, 1991)) for the small and highly-charged Th 4+ cation is a controlling parameter in its surface speciation. The surface occupancy (0.4 Th per unit cell area, A UC ) measured by RAXR exceeds the expected level for surface charge compensation by tetravalent Th (0.25 Th/A UC ). However, the radiometric uptake measurements show smaller occupancies (0.21 Th/A UC ) after rinsing by deionized water, indicating a partial removability of sorbed thorium. Thorium oligomerization was observed at total Th concentrations [Th] tot P 2.0 Â 10 À3 mol/L in presence of the surface, although solubility studies suggest that Th is soluble under these solution conditions.

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