PNNL

Abstract

The thermodynamic and kinetic factors controlling the competitive heterogeneous nucleation and growth of ubiquitous metal carbonate and hydroxide phases are poorly understood. In this work, calcite (CaCO3) and magnesite (MgCO3) powders were reacted with NiCl2 (0-600 µM) for 7 days at 22 °C and 5 °C. The reacted powders were analyzed with X-ray photoelectron spectroscopy (XPS), scanning electron microscopy, and energy-dispersive X-ray spectroscopy to characterize the Ni surface precipitates formed. Evidence from these techniques pointed to the formation of mixed Ni carbonate-Ni hydroxide amorphous surface precipitates. On calcite, XPS detected primarily Ni(OH)2 despite the initial solutions being more supersaturated with respect to NiCO3 than to Ni(OH)2 by a factor of 17–18. In contrast, NiCO3 was the dominant component on magnesite in the same conditions. Decreasing the temperature had the effect of increasing the proportion of NiCO3 to the detriment of Ni(OH)2. The experimental observations were consistent with the lower lattice mismatch of NiCO3 with respect to magnesite compared to calcite favoring NiCO3 nucleation and temperature most influencing Ni(OH)2 nucleation. Comparison to previous work on Co-reacted powders indicated the differences in lattice mismatch and/or water exchange rate impacted the composition of the surface precipitates more than the relative thermodynamic stabilities of the competing minerals. Overall, this work sheds light on the factors that control the competition between surface precipitates when mineral surfaces are in contact with aqueous solutions supersaturated with respect to multiple mineral phases.