Structure and reactivity of the calcite–water interface

The zetapotential of calcite in contact with aqueous solutions of varying composition is determined for pre-equilibrated suspensions by means of electrophoretic measurements and for non-equilibrium solutions by means of streaming potential measurements. Carbonate and calcium are identified as charge determining ions. Studies of the equilibrium solutions show a shift of isoelectric point with changing CO2 partial pressure. Changes in pH have only a weak effect in non-equilibrium solutions. The surface structure of (1 0 4)-faces of single crystal calcite in contact to solutions corresponding to those of the zetapotential investigations is determined from surface diffraction measurements. The results reveal no direct indication of calcium or carbonate inner-sphere surface species. The surface ions are found to relax only slightly from their bulk positions; the most significant relaxation is a ∼4° tilt of the surface carbonate ions towards the surface. Two ordered layers of water molecules are identified, the first at 2.35 ± 0.05 Å above surface calcium ions and the second layer at 3.24 ± 0.06 Å above the surface associated with surface carbonate ions. A Basic-Stern surface complexation model is developed to model observed zetapotentials, while only considering outer-sphere complexes of ions other than protons and hydroxide. The Basic-Stern SCM successfully reproduces the zetapotential data and gives reasonable values for the inner Helmholtz capacitance, which are in line with the Stern layer thickness estimated from surface diffraction results.

A basic stern surface complexation model for calcite is developed to relate measured calcite zetapotentials to the molecular structure of the calcite–water interface observed by in situ surface diffraction measurements.Figure optionsDownload high-quality image (113 K)Download as PowerPoint slideResearch highlights
► Two layers of water molecules, 2.35 ± 0.05 Å and 3.24 ± 0.06 Å above the calcite(1 0 4)-face, are identified by X-ray surface diffraction at all solution conditions investigated.
► X-ray surface diffraction indicates that Ca2+ and CO32- do not adsorb as inner-sphere complexes on terrace planes at the calcite(1 0 4)-face.
► Negative calcite zetapotential measured between pH 5.5 and 11 in Ca2+ free solutions; Ca2+ and CO32- identified as potential determining ions.
► A new basic stern surface complexation model for calcite is presented, using consistent values for Helmholtz capacitance and Stern layer thickness.