CHEMICAL-STATE OF CD IN APATITE PHOSPHATE ORES AS DETERMINED BY EXAFSSPECTROSCOPY

Natural apatites used in fertilizer industries often contain trace amo unts of Cd, which may reach concentrations of several tens to a few hu ndred parts per million. Cd is not eliminated during the production of phosphate fertilizers, and its concentration in the final product can exceed environmental norms. Knowledge of the chemical state of Cd in apatite ores is a prerequisite for the design of technical processes o f extraction. In the present study, Cd K-edge EXAFS spectroscopy was u sed to investigate the structural environment of Cd present in sedimen tary apatite ores from West Africa. These apatites are fluorinated and contain goethite, quartz, and crandallite as ancillary phases detecte d by X-ray diffraction or EXAFS spectroscopy. Cd K-edge EXAFS spectra for two natural samples were analyzed and compared with those for Cd-c ontaining reference minerals, including hydroxylapatite, goethite, ota vite, and crandallite. A good spectral resemblance was observed betwee n natural products and synthetic apatite containing small amounts of C d. This spectral likeness indicates that the majority of Cd atoms are diluted in the apatitic framework and do not form Cd-10(PO4)(6)(OH,F)( 2) clusters. This finding was confirmed by quantitative analysis of th e EXAFS spectra, which indicated that Cd atoms are surrounded by neare st O atoms at 2.33 Angstrom, next-nearest P atoms at similar to 3.53 A ngstrom, and a third-nearest shell of Ca atoms at similar to 4.02 Angs trom. A comparison of these data with those obtained for synthetic apa tites allowed us to assess that Cd occupies both Ca crystallographic s ites with a slight preference for the Ca2 site.