LASER AND X-RAY SPECTROSCOPIC STUDIES OF URANIUM-CALCITE INTERFACE PHENOMENA

Calcite (CaCO3) was treated with uranyl solutions of different concent rations in perchlorate medium at an initial pH of 6. X-ray photoelectr on spectroscopy (XPS) measurements showed that there is no physi- or c hemisorption of uranyl perchlorate molecules at the mineral surfaces a nd no diffusion of ClO4- anions into the mineral grains. We conclude t his, because no chlorine atoms could be detected by XPS measurements. A significant amount of uranyl hydroxides is formed at the mineral sur faces. In addition, up to 20% of the calcium ions are exchanged by ura nyl ions at the calcite surface. The inter-atomic uranium-oxygen dista nces of the reaction products at the mineral surface were determined u sing extended X-ray absorption fine structure (EXAFS) spectroscopy. In addition, the contacting solutions were studied by time-resolved lase r-induced fluorescence spectroscopy (TRLFS). A new uranyl species was found in solution, which is also present in seepage waters from mine-t ailing piles. (C) 1997 Elsevier Science B.V.