Co-crystallization of Co(II) with calcite: Implications for the mobility of cobalt in aqueous environments

Abstract

Two different experimental processes (precipitation and crystallization in silica gel) have been carried out to investigate the degree of non-ideality and the crystallization behavior of the calcite–sphaerocobaltite solid solution from aqueous solutions. Solid phases of different compositions were precipitated at 25 °C by mixing a Na 2CO 3 aqueous solution with solutions with different ratios of CaCl 2 and CoCl 2. The precipitates were characterized by X-ray powder diffraction (XRD) and scanning electron microscopy and microanalyses (SEM-EDS). The precipitation behavior is quite complex and involves formation of Co-bearing calcite, Co-bearing aragonite, and a Co-rich low crystallinity phase that was identified as CoCO 3 · nH 2O by Fourier Transform Infrared Spectroscopy (FTIR). The precipitates identified as Co-bearing calcites consist of virtually homogeneous (Co,Ca)CO 3 solid-solution crystals with a calcite-type structure and cobalt mole fractions ( X sph) up to 0.33. Within this range, the evolution of the unit cell parameters with composition deviates markedly from the straight line connecting the parameters of the end-members. The excess volume of mixing ( V E) of these Co-bearing calcites shows a marked positive deviation. Calcite-type solid-solution crystals grown in gels at a lower supersaturation develop in general homogeneously, with a maximum cobalt content X sph = 0.16. Occasionally, some crystals exhibit compositional zoning with a core richer in Co, but X sph never exceeds the value of 0.16. The positive excess volume of mixing and the limited incorporation of cobalt in gel-grown crystals suggest the presence of a miscibility gap and consequently a non-ideal solid solution. During precipitation, the effective distribution coefficients of cobalt between the fluid and the Co-bearing calcites vary within the range from 11 to 17, which indicates a significant preferential partitioning of cobalt towards the solid phase in spite of the high supersaturation attained in these experiments. On these bases, the degree of non-ideality of the solid solution and the equilibrium partitioning of cobalt are discussed. The study suggests that a rigorous assessment of the Co(II) behavior in aqueous environments would require a re-evaluation of the available data on both the solubility product of sphaerocobaltite and the critical stability constants of the Co-bearing aqueous species.