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Uranium (VI) adsorbate structures on portlandite [Ca(OH)2] type surfaces determined by computational modelling and X-ray absorption spectroscopy

DOI: 10.3390/min11111241 DOI Help

Authors: Christopher A. Lee (National Nuclear Laboratory Limited) , Arjen Van Veelen (Los Alamos National Laboratory) , Katherine Morris (University of Manchester) , J. Fred W. Mosselmans (Diamond Light Source) , Roy A. Wogelius (University of Manchester) , Neil A. Burton (University of Manchester)
Co-authored by industrial partner: No

Type: Journal Paper
Journal: Minerals , VOL 11

State: Published (Approved)
Published: November 2021

Open Access Open Access

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.

Journal Keywords: portlandite; uranyl; adsorption; hyperalkaline; extended X-ray absorption fine structure (EXAFS); potential of mean force (PMF); molecular dynamics (MD); density functional theory (DFT)

Subject Areas: Chemistry, Earth Science

Instruments: I18-Microfocus Spectroscopy

Added On: 11/11/2021 09:26


Discipline Tags:

Earth Sciences & Environment Radioactive Materials Mineralogy Chemistry Materials Science Nuclear Waste Inorganic Chemistry Geology Geochemistry

Technical Tags:

Spectroscopy X-ray Absorption Spectroscopy (XAS) Extended X-ray Absorption Fine Structure (EXAFS)