Cobalt incorporation in calcite: Thermochemistry of (Ca,Co)CO 3 solid solutions from density functional theory simulations

Abstract

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. 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. We calculate the solvus and spinodal lines in the phase diagram using a sub-regular solution model, 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. We also calculate solid/aqueous distribution coefficients to evaluate the effect of the strong non-ideality of mixing on the equilibrium with aqueous solution, showing that the thermodynamically-driven incorporation of cobalt in calcite (and of calcium in spherocobaltite) is always very low, regardless of the Co/Ca ratio of the aqueous environment.

Crystallization of nanostructured cobalt hydroxide carbonate at ambient conditions: a key precursor of Co3O4

“…Recently, the incorporation of cobalt into the crystal structure of calcite has been proposed as a possible technique to immobilize this toxic metal in the environment (Katsikopoulos et al, 2008). However, even in equilibrium with a very Co-rich aqueous solution, calcite is predicted to have a very low level of substitutional cobalt impurities, so calcite is not an effective way to immobilize Co cations (González-López et al, 2014). At ambient temperature, the precipitation of crystalline cobalt hydroxide carbonates from aqueous solutions of Co 2+ and CO 2À 3 is prevented by the formation of Co 2+ aqueous complexes.…”

Bromate incorporation in calcite and aragonite

“…The use of relatively large supercells is needed to maximise the distance between each impurity and its periodic images (at least ~15 Å apart), as well as to explore the configurational space of relative positions of cation and anion impurities. The symmetrically inequivalent substitution configurations were obtained using the methodology implemented in the Site Occupancy Disorder program (SOD) (Grau-Crespo et al, 2007), which has been useful before in the investigation of impurity distribution in carbonate minerals (González-López et al, 2014;Ruiz-Hernandez et al, 2010;Wang et al, 2011).…”

Theoretical analysis of uranium-doped thorium dioxide: Introduction of a thoria force field with explicit polarization

“…Successful applications of SOD include a study of the thermophysical properties of carbonates, investigations into the mixing thermodynamics of oxides and a simulation of cation distribution and thermodynamics in sulfides. [24][25][26][27][28][29][30][31] This program generates the complete configurational space for each composition in a supercell of the structure, before extracting the subspace of symmetrically inequivalent configurations, for which energies and other properties are evaluated. In this case a 2x2x2 thoria supercell ( Figure 1) was used as the base simulation cell, and we have worked with uranium concentrations up to x = 0.16.…”