Quantum Chemical Investigation of the Selenite Incorporation into the Calcite (101̅4) Surface

Abstract

Selenium is a common pollutant in soils and aquifers. The radioisotope 79 Se, an abundant fission product of 235 U, is of particular concern in the context of nuclear waste disposal safety due to its long half-life and its expected high mobility in the multi-barrier system around potential nuclear waste disposal sites. Oxidized selenium species are relatively soluble and show only weak adsorption at common mineral surfaces. However, a possible sorption mechanism for selenium in the geosphere is the structural incorporation of selenium(IV) (selenite, SeO 2− 3 ) into calcite (CaCO 3 ).We carried out a detailed quantum chemical study of the incorporation of SeO 2− 3 into the calcite surface and the subsurface layers.As the main result we present the structural changes upon incorporation of selenite (SeO −2 3 ) into the dry and hydrated calcite (1014) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 coefficient D for SeO −2 3 incorporation into the surface and subsurface layers. The results corroborate the recently proposed entrapment model for selenium(IV) coprecipitation with calcite and show that equilibrium incorporation of selenite into calcite may occur in the surface layer, but is practically impossible in subsurface layers or the bulk.2

Mechanism of wettability alteration of the calcite {101̄4} surface

“…The water molecules close to CO 3 2À groups point with their hydrogen atoms toward the interface, giving rise to the second hydration layer. This pattern is similar to results from force-field 41 and quantum-chemical 42 simulations.…”

Structure and Surface Complexation at the Calcite(104)–Water Interface

“…The occupancy for this water site is 67 ± 15%. This water structure is in agreement with recent 3D CTR studies on calcite(104) ,, and simulation results. ,, …”

“…The model for the description of CTR data includes a fixed bulk calcite structure from the literature and an interfacial calcite region with adjustable structural parameters, including four calcite monolayers (ML), similar to previous CTR studies. , Carbonate ions in the interfacial region are treated as rigid bodies, with three translational and three rotational degrees of freedom. Two adsorbed water molecules are explicitly included in the structural model above the calcite structure, i.e., H 2 O-1 above the surface calcium atom and H 2 O-2 above the outwardly oriented oxygen atom of the surface carbonate group, as suggested by previous SXRD studies ,,− and simulation results. ,,,− Water molecules have to be approximated as oxygen atoms because hydrogen does not contribute to the CTR signal due to its low X-ray scattering cross-section. Beyond the adsorbed water, a semi-infinite, laterally structureless, water profile was included in the model to account for scattering from bulk water.…”

Radioactive selenium: origin and environmental dispersion scenarios

“…For instance, some studies suggest that 79 Se may become immobilized when interacting with uranyl phases present as low concentration impurities in the nuclear fuel waste due to substitution of Si by Se in solid structures of, for example, α-uranophane -(Ca(UO 2 ) 2 (SiO 3 OH) 2 • 5H 2 O) and boltwoodite -HK(UO 2 )(SiO 4 ) • 1.5(H 2 O) (Chen et al, 1999). Others suggest favoured structural incorporation of Se(IV) into calcite, particularly on the surface and potentially in deeper layers upon precipitation at highly supersaturated conditions (Heberling et al, 2014;Polly et al, 2017). The most effective process regarding mineral inclusion could be Se incorporation to Fe-bearing minerals, present as container corrosion products or constituent of host rocks and in bentonite backfills.…”