LEAD ADSORPTION AT THE CALCITE-WATER INTERFACE - SYNCHROTRON X-RAY STANDING-WAVE AND X-RAY REFLECTIVITY STUDIES

By combining synchrotron X-ray standing wave (XSW) measurements with s ynchrotron Xray reflectivity measurements, we have determined: (1) the precise three-dimensional location within the calcite unit cell of su bmonolayer Pb ions adsorbed at the calcite (104) surface from dilute a queous solutions, and (2) the precise one-dimensional location of thes e unit cells relative to the calcite surface. Our XSW measurements, us ing three separate calcite Bragg reflections for triangulation, show t hat most adsorbed Pb ions occupy Ca sites in the calcite lattice with an ordered coverage of 0.05 equivalent monolayers, while the remaining Pb ions are disordered with a coverage of 0.03 equivalent monolayers. Our X-ray reflectivity measurements show that the ordered Pb ions occ ur primarily (>70%) in the surface atomic layer of calcite. Atomic for ce microscopy (AFM) was used to characterize the topography of the cal cite (104) surface under conditions similar to the X-ray experiments. The quantitative morphological information obtained by AFM was used to develop realistic models of the calcite surface. The calculated X-ray reflectivities for these model surfaces were compared with the measur ed X-ray reflectivities. The new combined X-ray method that we have de veloped can be used to determine the atomic-scale structure of other m etals adsorbed at mineral-water interfaces. Such high-resolution struc tural determinations are essential before detailed conceptual and theo retical models can be further developed to understand and predict the behavior of dissolved metals in mineral-water systems. Copyright (C) 1 997 Elsevier Science Ltd.