No abstract

Crystals of Co2CO3(OH)2 have been synthesized under ambient conditions, in contrast to hydrothermal methods reported previously. We have developed a simple but efficient methodology to obtain an initial amorphous phase that evolves to a crystalline cobalt hydroxide carbonate after one week of maturation. X-ray diffraction analysis indicates that this phase crystallizes in the space group P21/a (a = 12.886(6), b = 9.346(3), c = 3.156(1) Å, β = 110.358(6)°). The platelet morphology of Co2CO3(OH)2 agrees with its lamellar crystal structure. High-resolution transmission electron microscopy (HRTEM) reveals that each individual platelet is comprised of nanodomains disoriented with respect to their neighbours. The kinetics and the activation energy (Ea = 6.26 kJ mol–1) of the transformation process have been estimated using the rate constant method. The precipitation of solids leads to a decrease in the cobalt concentration in the solution (∼88%) reaching values of ∼150 ppm, which can be considered a successful reduction from the perspective of water quality. The calcination in air of the synthetized platelets produced exclusively Co3O4. The thermo-X-ray difraction results confirm that Co2CO3(OH)2 is transformed over a small range of temperatures (225–235°C) into pure Co3O4. HRTEM images show that the lamellar nanomorphology is preserved in the Co3O4 phase. Therefore, understanding the crystallization behaviour of Co2CO3(OH)2 can help to minimize environmental problems caused by cobalt pollution and may facilitate the management of methods to obtain phases with specific nanomorphologies used widely in material sciences.

No abstract

The crystal structure of cobalt carbonate hydroxide Co2CO3(OH)2, a solid important in materials and environmental science, is investigated using density functional theory (DFT) simulations and powder X-ray diffraction (PXRD) measurements.

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.