Mechanisms of Metal Ion Sorption on Calcite: Composition Mapping by Lateral Force Microscopy

We show that lateral force microscopy (also known as frictional force microscopy) can differentiate between substrate and overlayer phases during an inorganic surface reaction. A calcite substrate is imaged in situ, while immersed in aqueous solutions of pH ∼ 6−9 containing metal ions (Cd 2+, Sr 2+, and La 3+ ) at concentrations of 10 - 5 to 10 - 3 M. Cd 2+ and Sr 2+ passivate surface steps, initiating overgrowth only in solutions already supersaturated relative to their respective carbonates. In contrast, La 3+ initiates overgrowth even in undersaturated conditions and carries the reaction to completion by scavenging carbonate anions directly from the dissolving calcite surface. Monomolecular surface steps play a central role, serving as both dissolution sites for the substrate and nucleation sites for the overgrowth.