Cobalt hydroxide–cobalt carbonate competitive growth on carbonate surfaces

Abstract

The presence of mineral surfaces can affect the outcome of geochemical reactions by providing alternative nucleation pathways, but not in ways that can yet be reliably quantified. In this work, the reaction of Co(II) with calcite (CaCO3) and magnesite (MgCO3) powders at room temperature was used to quantify the effects of the nature of the mineral substrate and of solution chemistry on the competitive heterogeneous growth between cobalt carbonate (CoCO3) and cobalt hydroxide (Co(OH)2). Experiments were first performed to determine the appropriate solubility product constants of CoCO3 and Co(OH)2, for which several values have been reported in the literature. X-ray photoelectron spectroscopy measurements were then performed to quantify the relative proportion of each phase in surface precipitates as a function of the nature of the substrate, initial saturation level, and pH. Scanning electron microscopy and energy-dispersive X-ray spectroscopy were also used to characterize the morphology and composition of surface precipitates. On calcite powders, Co(OH)2 formed predominantly despite the initial solutions being more supersaturated with respect to CoCO3 than to Co(OH)2, indicating that the kinetics of heterogenous growth were faster for Co(OH)2 than for CoCO3. In contrast, magnesite powders were much more favorable to the growth of CoCO3 because of the low lattice mismatch between the two phases, which allowed for heteroepitaxial growth. However, the proportion of CoCO3 in surface precipitates decreased with increasing initial supersaturation, likely due to the resulting decrease of the free energy barrier for Co(OH)2 nucleation and the rapid kinetics of Co(OH)2 growth. Lowering pH increased the proportion of CoCO3 on both substrates. These findings highlight how the interplay between lattice mismatch, heterogeneous nucleation barriers, and rates of heterogenous growth can influence competitive heterogeneous growth and dramatically affect the outcome of geochemical reactions.