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

Abstract

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 (Cd2+, Sr2+, and La3+) at concentrations of 10-5 to 10-3 M. Cd2+ and Sr2+ passivate surface steps, initiating overgrowth only in solutions already supersaturated relative to their respective carbonates. In contrast, La3+ 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.