Resolving the nanoscale mechanisms of calcite growth and dissolution from nonstoichiometric and microbial solutions

Abstract

The relative rates of calcite (CaCO3) precipitation and dissolution largely determine the preservation and subsequent accumulation of carbonate in the geologic record and are fundamental parameters for predicting the fate of fossil fuel carbon dioxide as well as the sequestration of several co-precipitated trace elements. Here we use the surface techniques, atomic force microscopy (AFM) and vertical scanning interferometry (VSI) to elucidate the nanoscale mechanisms of calcite growth and dissolution from nonstoichiometric and microbial solutions. Our results clearly demonstrate that the Ca 2+/CO32- ratio of carbonate solutions, at constant saturation, determines both the kinetics and anisotropy of step advancement. Anisotropic step velocities, in turn, alter step generation rates at screw dislocations, thereby significantly affecting the overall growth and dissolution rates of calcite surfaces. These results reflect different mechanistic roles for the cation and anion during both growth and dissolution and suggest limitations on the application of concentration-based rate laws in solutions of varying ionic ratios. Further, this study offers clear demonstration that the crystal surface exerts a primary control on growth and dissolution rates through step-specific and defect-directed interactions, producing differences in rate that could not be predicted from considerations of bulk chemistry alone. Further insight into calcite dissolution in natural systems was achieved by investigating the effect of Shewanella oneidensis MR-1 surface colonization on the dissolution rates of calcite (CaCO3) and dolomite (CaMg(CO 3)2). By quantifying and comparing the significant processes occurring at the microbe---mineral interface, a mechanistic understanding of the way in which microbes alter the dissolution rates of carbonate minerals was achieved. MR-1 attachment under aerobic conditions was found to influence carbonate dissolution through two distinct mechanistic pathways: (1) inhibition through interference with etch pit development and (2) catalytic removal of carbonate material at the cell---mineral interface during irreversible attachment to the mineral surface. The relative importance of these two competing effects was found to vary with the solubility of the carbonate mineral studied. This study demonstrates the dynamic and competitive relationship between microbial surface colonization and mineral dissolution that may be expected to occur in natural environments.