Immobilization of hexavalent uranium U(VI) by hydroxyapatite under oxic conditions.

• Dissolved U is quantitatively retained (> 99.99 %) by hydroxyapatite (HAP). • At acid pH, the high solubility of HAP favors the formation of a uranyl phosphate. • The phosphate phase is a solid solution rich in Na meta-autunite. • At neutral to basic pH, surface complexation prevails at low surface loadings. • Non-stoichiometric dissolution of HAP explains pH-dependent surface complexation. The immobilization of hexavalent uranium U(VI) by hydroxyapatite (HAP) was investigated under oxic conditions as a function of pH and surface loading ([U(VI)] 0 /[HAP] 0) to evaluate the applicability of HAP as migration barriers or waste forms in nuclear waste repositories. This study focused on the retention mechanisms, which were sought by integrating the solid-phase characterization (e.g., X-ray diffraction, scanning electron microscopy with energy dispersive spectroscopy, and U L III -edge X-ray absorption spectroscopy) with the solution-phase analysis of dissolved Ca, P, and U. At acidic pH, due to the increasing solubility of HAP, U(VI) retention is mainly controlled by the formation of a uranyl phosphate following HAP dissolution. Importantly, this phosphate is in the form of a solid solution with Na meta-autunite being the principal component, which considerably expands the stable domain of uranyl phosphates compared to the pure phases (e.g., chernikovite, Na meta-autunite, and meta-autunite). At neutral to basic pH, however, the uranyl phosphate formation does not prevail till [U(VI)] 0 /[HAP] 0 = 25 mg/g, below which the formation of U(VI)-phosphate ternary surface complexes mainly accounts for U(VI) retention. At basic pH, the surface substitution of PO 4 3− for CO 3 2− and the dominance of uranyl carbonate species make surface complexation less favorable under this pH condition. In this study, the initially present U(VI) is near completely immobilized (> 99.99 %) by HAP, with the greatest retention noted at circumneutral pH, thus demonstrating the effectiveness of HAP for the containment of U(VI) under ambient geochemical conditions. [Display omitted] [ABSTRACT FROM AUTHOR]