Quantum Chemical Investigation of the Selenite Incorporation into the Calcite (10-14) Surface

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$_3^{2-}$) into calcite (CaCO$_3$).

We carried out a detailed quantum chemical study of the incorporation of
SeO$_3^{2-}$ into the calcite surface and the subsurface layers.

As the main result we present the structural changes upon incorporation of
selenite (SeO$_3^{-2}$) into the dry and hydrated calcite ($10\overline{1}4$) surface. For
the dry surface we add results for the incorporation into the subsurface
layers. This results are complemented by energetic considerations and in turn
used to estimate the thermodynamic partition coefficient $\mathbf D$ for
SeO$_3^{-2}$ incorporation into the
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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.