Abstract:

 The behavior of calcite surfaces in the presence of Cd2+ and Pb2+ is investigated by in situ fluid cell Atomic Force
Microscopy (AFM) to determine the differences between Cd/calcite and Pb/calcite interactions and the growth mode of surfaceprecipitate. Experiments are conducted on {101 _4} cleavage faces exposed to Cd- and Pb-bearing solutions that are undersaturatedwith respect to calcite, otavite, and cerussite. Two observations are made: (1) The presence of Cd2+ does not affect the relative stepkinetics along <4_41> steps (i.e.v s+>v s-) during calcite dissolution, whereas Pb2+ strongly retards the retreat of the obtuse <441>+steps to reverse the relative step kinetics; (2) The initial dissolution is followed by a development of surface precipitation afterthe flow-through input is turned off to allow the surfaces to equilibrate with the solutions. The surface precipitate grows in a layerspreading mechanism showing clearly defined directions in the presence of Cd but not Pb.
　　The kinetic effects of cadmium and lead on calcite dissolution are suggested to be related to a selective cation adsorptionin accordance with the cation size, the geometry of adsorption complexes, and the crystallographic direction dependent stepgeometry. The Cd2+ ions, having a slightly smaller ionic radius than Ca2+, are thought to have a stronger presence along the acute<4_41>- steps that are geometrically more confined. In contrast, the much larger Pb2+ ions are believed to form distorted octahedralsurface complexes to adsorb predominantly at the more open <4_41>+ steps. The structural incompatibility between cerussite andcalcite is speculated to account for the lack of step directions for the layer growth on calcite surfaces in the presence of Pb2+. Theoccurrence of layer-by-layer growth indicates that epitaxial growth plays an important role in the formation of (Ca, Cd)CO3 and (Ca,Pb)CO3 solid solutions.

Key words: uptake of divalent metals, solid solution, adsorption, dissolution, layered growth, calcite surface, AFM