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| Titel |
Cobalt incorporation in calcite: thermochemistry of (Ca,Co)CO3 solid solutions from density functional theory simulations |
| VerfasserIn |
Jorge González-López, Sergio E. Ruiz-Hernández, Ángeles Fernández-González, Amalia Jimenez, Nora H. de Leeuw, Ricardo Grau-Crespo |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250113132
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| Publikation (Nr.) |
 EGU/EGU2015-13327.pdf |
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| Zusammenfassung |
| The incorporation of cobalt in mixed metal carbonates is a possible route to the
immobilization of this toxic element in the environment. However, the thermodynamics of
(Ca,Co)CO3 solid solutions are still unclear due to conflicting data from experiment and from
the observation of natural ocurrences. Atomistic computer simulations, which allow the
evaluation of thermodynamic properties without the interference of unknown kinetic factors,
have been increasingly used in recent years for the investigation of the thermodynamics of
mixing and impurity incorporation in carbonate. We report here the results of a computer
simulation study of the mixing of calcite (CaCO3) and spherocobaltite (CoCO3), using
density functional theory calculations.
Our simulations suggest that previously proposed thermodynamic models, based only
on observed compositions, significantly overestimate the solubility between the
two solids and therefore underestimate the extension of the miscibility gap under
ambient conditions. The enthalpy of mixing of the disordered solid solution is strongly
positive and moderately asymmetric: calcium incorporation in spherocobaltite is
more endothermic than cobalt incorporation in calcite. Ordering of the impurities in
(0001) layers is energetically favourable with respect to the disordered solid solution
at low temperatures and intermediate compositions, but the ordered phase is still
unstable to demixing. The solvus and spinodal lines in the phase diagram using
a sub-regular solution model has been also calculated, and conclude that many
Ca1-xCoxCO3 mineral solid solutions (with observed compositions of up to x=0.027,
and above x=0.93) are metastable with respect to phase separation. The strong
non-ideality of this solid solution has an important effect on the solid solution /
aqueous solution thermodynamic partitioning: the equilibrium level of substitutional
impurities in the endmember solids is always low, regardless of the composition
of the aqueous solution. Thus, even in equilibrium with a very Co-rich aqueous
solution, calcite is predicted to have a very low level of cobalt impurities, which is
unfortunate as it means that Co/Ca substitution in calcite is not an effective way to
immobilize Co(II) cations in solution. On the other hand, our results also mean that if
spherocobaltite could be formed (which is admittedly difficult due to competition from other
cobalt-bearing phases), almost perfect immobilization of the Co2+ ions would be achieved
as there would be negligible Ca2+/Co2+ ion exchange with aqueous solutions. |
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