Uranium (VI) Adsorbate Structures on Portlandite [Ca(OH)2] Type Surfaces Determined by Computational Modelling and X-ray Absorption Spectroscopy

Abstract

Portlandite [Ca(OH)2] is a potentially dominant solid phase in the high pH fluids expected within the cementitious engineered barriers of Geological Disposal Facilities (GDF). This study combined X-ray Absorption Spectroscopy with computational modelling in order to provide atomic-scale data which improves our understanding of how a critically important radionuclide (U) will be adsorbed onto this phase under conditions relevant to a GDF environment. Such data are fundamental for predicting radionuclide mass transfer. Surface coordination chemistry and speciation of uranium with portlandite [Ca(OH)2] under alkaline groundwater conditions (ca. pH 12) were determined by both in situ and ex situ grazing incidence extended X-ray absorption fine structure analysis (EXAFS) and by computational modelling at the atomic level. Free energies of sorption of aqueous uranyl hydroxides, [UO2(OH)n]2–n (n = 0–5) with the (001), (100) and (203) or (101) surfaces of portlandite are predicted from the potential of mean force using classical molecular umbrella sampling simulation methods and the structural interactions are further explored using fully periodic density functional theory computations. Although uranyl is predicted to only weakly adsorb to the (001) and (100) clean surfaces, there should be significantly stronger interactions with the (203/101) surface or at hydroxyl vacancies, both prevalent under groundwater conditions. The uranyl surface complex is typically found to include four equatorially coordinated hydroxyl ligands, forming an inner-sphere sorbate by direct interaction of a uranyl oxygen with surface calcium ions in both the (001) and (203/101) cases. In contrast, on the (100) surface, uranyl is sorbed with its axis more parallel to the surface plane. The EXAFS data are largely consistent with a surface structural layer or film similar to calcium uranate, but also show distinct uranyl characteristics, with the uranyl ion exhibiting the classic dioxygenyl oxygens at 1.8 Å and between four and five equatorial oxygen atoms at distances between 2.28 and 2.35 Å from the central U absorber. These experimental data are wholly consistent with the adsorbate configuration predicted by the computational models. These findings suggest that, under the strongly alkaline conditions of a cementitious backfill engineered barrier, there would be significant uptake of uranyl by portlandite to inhibit the mobility of U(VI) from the near field of a geological disposal facility.

In Situ EXAFS Study of Sr Adsorption on TiO2(110) under High Ionic Strength Wastewater Conditions

“…The F-test and dimensional Hamilton test was performed to justify each path included in the fit [33]. Finally, shells were chosen that gave the best fit, e.g., split oxygen shell, and that were chemically reasonable based on the chemistry of each system (see Supplementary Material SS1) [28][29][30][34][35][36][37][38][39].…”

Identification and Decomposition of Uranium Oxychloride Phases in Oxygen-Exposed UCl3 Salt Compositions

“…The uranium oxides are well studied with the U­(IV) oxidation state favoring UO 2 at lower oxygen pressures trending up in oxidation states as the oxygen pressure increases, resulting in a U 3 O 8 with an average oxidation state (AOS) of 5.33 and eventually in UO 3 with a U­(VI) oxidation state. − The oxychloride region is dominated by a UOCl 2 when the oxidation state is U­(IV), and while under a higher oxygen presence, UO 2 Cl 2 with a U­(VI) oxidation state is favored. , Deeper investigation into the U­(VI) oxidation state shows that the number of chlorine atoms bonded seems to be a function of chlorinity of the media and everything from UO 2 Cl to UO 2 Cl 5 can be formed with a similar uranyl base. − Many of these are synthesized and studied within aquo-choro or acidic media. In these cases, it was identified that the typical bonds observed by extended X-ray absorption fine structure (EXAFS) measurements are a shortest-bond-length U-O ax axial bond, signature of the UO 2 , a medium-bond-length U-O eq binding H 2 O or OH – equatorially to the uranium, and some varying number of equatorial longer U–Cl bonds. − …”