Sorption of tetravalent thorium on muscovite

Adsorption of tetravalent thorium to the (0 0 1) basal surface of the phyllosilicate muscovite from an aqueous solution (1 × 10−4 mol/L Th(IV) in 1 × 10−1 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 × 10−6-4.88 × 10−3 mol/L and 1 × 10−1 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 Kapp = 2 × 104 L/mol up to [Th]tot = 1.02 × 10−3 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 (ΔGhyd = −5815 kJ/mol (Marcus, 1991)) for the small and highly-charged Th4+ cation is a controlling parameter in its surface speciation. The surface occupancy (0.4 Th per unit cell area, AUC) measured by RAXR exceeds the expected level for surface charge compensation by tetravalent Th (0.25 Th/AUC). However, the radiometric uptake measurements show smaller occupancies (0.21 Th/AUC) after rinsing by deionized water, indicating a partial removability of sorbed thorium. Thorium oligomerization was observed at total Th concentrations [Th]tot ⩾ 2.0 × 10−3 mol/L in presence of the surface, although solubility studies suggest that Th is soluble under these solution conditions.