X-ray standing wave investigation of the surface structure of selenite anions adsorbed on calcite

The adsorption of selenite ions (SeO32-) From a dilute aqueous solution onto a freshly-cleaved calcite (10 (1) over bar 4 surface was studied with the X-ray standing wave (XSW) technique. The complex ion SeO32- is found to selectively adsorb at the CO32- site via ionic exchange, forming a two-dimensional solid-solution of the form Ca(SeO3)(x)(CO3)(1-x) at the interface. The calcite (10 (1) over bar 4), (0006) and (11 (2) over bar 0) Bragg reflections were used to triangulate the Se position with respect to the calcite lattice. The local surface structure al the SeO32- adsorbate site, derived From the XSW results, is consistent with a model in which the base of the SeO3(2-) trigonal pyramid aligns with (and replaces) the CO32- equilateral triangular group. The SeO32- adsorption saturated at a coverage of 0.02 monolayers. Under identical chemical conditions, selenate (SeO42-) adsorption was inhibited. (C) 1997 Elsevier Science B.V.