LAUSR

Abstract The crystallisation behaviour of calcium carbonate phases in the system Ca 2 + -Co 2 + -CO 3 2 − -H 2 O was examined in batch reactors. Experiments with different initial ratios of Co2+/Ca2+ (from 0 to 1.0) in the aqueous solution were performed to examine precipitation and mineral transformations under ambient conditions. The solids recovered from the aqueous solution after different ageing periods (ranging from 5 min to 2 months after precipitation) were characterised by X-ray diffraction, transmission and scanning electron microscopy and EDX microanalyses. The evolution of the aqueous solutions was also followed by measuring their pH and their chemical composition by UVA–visible spectroscopy. Our experimental data indicate that there are two sequences of solvent-mediated transformations in aged carbonates, one involving Co-rich phases and another involving calcium carbonates. Both sequences are characterised by the initial formation of amorphous or poorly crystalline hydrated phases, which transform into crystalline and anhydrous phases with ageing via a dissolution and reprecipitation mechanism. The stability and solubility of the phases are concerned with the incorporation of Co in Ca-bearing phases and Ca in the Co-bearing phases. Co-rich carbonates evolve from a hydrated amorphous hydroxycarbonate (Co2CO3(OH)2 ⋅H2O) to the crystalline phase Co2CO3(OH)2. The transformation from the amorphous to crystalline phase (Co2CO3(OH)2) was increasingly delayed as the initial Co/Ca ratio lowered. The sequence of Ca carbonates is affected by the amounts of Co2+ in the initial solution, and instead of the sequence ACC (amorphous calcium carbonate) → vaterite (Vtr) → calcite (Cal) obtained for control experiments, we observed ACC → monohydrocalcite → aragonite (Arg). Moreover, the Co2+/Ca2+ ratio controls the earliest calcium carbonate phases to appear and the subsequent polymorph transformation reactions. In terms of cobalt incorporation into the solids, monohydrocalcite and Arg incorporate a certain amount of cobalt into their crystalline structures. The thermodynamic solubility products of cobalt hydroxide carbonates (amorphous and crystalline) were determined. This experimental study advances the progress of understanding carbonate precipitation-dissolution-recrystallisation reactions under ambient conditions.