Uptake of Cu2+ by the calcium carbonates vaterite and calcite as studied by continuous wave (CW) and pulse electron paramagnetic resonance

The investigation of the Cu2+ uptake by the calcium carbonate minerals vate rite and calcite with continuous wave and purse electron paramagnetic reson ance (EPR) yields information on a molecular scale about the relevant compl exation reactions at the mineral-water interface. The structural assignment is based on changes in the coordination geometry of the copper complexes. Magnetic interactions of the unpaired Cu2+ electron with nuclei of C-13 lab eled carbonate ligands and protons from water or hydroxyl ligands in the fi rst and second coordination spheres of the cation are detected by pulse EPR techniques. Our results show that the Cu2+ ions are rapidly dehydrated upo n adsorption on the mineral surface. The strong surface binding is due to m onodentate coordination to three or four carbonate surface ions, comparable to chelate complexation in solution. The formation of square-planar or squ are-pyramidal copper complexes at exposed surface sites like kinks and step s yields a convincing explanation for the inhibition of calcium carbonate g rowth and dissolution. Upon recrystallization the Cu2+ ions are integrated into the calcite lattice where they exhibit a dynamic Jahn-Teller effect. T he resulting local lattice distortions are expected to destabilize the CuxC a(1-x)CO3(s) solid solution. Our results support the concept of a dynamic c alcium carbonate surface, covered by a thin, structured surface layer. The detailed structural information obtained for Cu2+ provides a better underst anding of the interaction of other metal ions with calcium carbonate minera ls. Copyright (C) 1999 Elsevier Science Ltd.