The mineral phase calcite (CaCO3) is a main component in the sediments of biologically productive freshwater lakes. The growth of calcite crystals involves the adsorption of calcium and carbonate ions to the crystal surface. Stable surface complexes are most likely to be formed at surface irregularities such as kinks and steps. The presence of other ions may lead to the incorporation of these ions into the crystal lattice as in the case of manganese and copper, or to the inhibition of the calcite crystal growth (e.g. magnesium). Both processes involve substitution of calcium by these ions at the surface complexation sites (Stumm, 1992).
In this study we use continuous wave (CW) and pulsed EPR methods in order to obtain information about the coordination structure of metal ions adsorbed on calcite surfaces (Möhl et al., 1990). As a first step the complexation of Cu(II) with 13C labelled carbonate and bicarbonate ligands in aqueous solution has been investigated. As expected from simulations, different metal-carbonate species are found, depending on the pH and the composition of the solution. The hyperfine couplings between the unpaired electron of the Cu(II) ion and the surrounding 13C and proton nuclei provide strucural and electronic information about the complexes: monodentate bonding with the bicarbonate ligands shows only weak isotropic and dipolar coupling to the 13C nuclei and can take place both at equatorial and axial sites of the distorted octahedral coordination structure. The carbonate ions are found to form bidentate complexes in the equatorial plane; the larger coupling constants reflect the delocalization of spin density from the Cu(II) ion over the s and p bonds to the 13C nuclei.
The Cu(II)-carbonate complexes in solution serve as model compounds for the investigation of the adsorption of Cu(II) on the calcite crystal surface. The speciation of the carbonate surface depends on pH and solution composition and the Cu(II) ions are expected to form different complexes , depending on the available carbonate surface ligands (Van Cappellen et al., 1993). Structural characterization of these surface complexes by EPR will give insight on a molecular level into the mechanisms and kinetics of crystal growth and growth inhibition.
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Stumm, W., Chemistry of the Solid-Water Interface (Wiley, New York, 1992).
Van Cappellen, P., Charlet, L., Stumm, W. & Wersin, P., Geochim. Cosmochim. Acta 57, 3505-3518 (1993).