XAFS analysis of coprecipitation of zinc by sulfide ions in an acidic solution

The fluorescence XAFS (X-ray absorption fine structure) technique using synchrotron radiation was applied to characterize zinc in the Hg-Zn, Cd-Zn, and Bi-Zn coprecipitates, and to elucidate the reaction mechanism of the coprecipitation of zinc from a strong acidic solution. Hg LII-, Cd K-, and Bi LIII-edge XAFS spectra suggested that the respective host materials of the coprecipitates listed above are metacinnabar (HgS), greenockite (CdS), and bismuthinite (Bi2S3) and that existence of zinc has not affected the local structure of the host metal sulfides in each system. On the other hand, the Zn K-edge XAFS spectra of each coprecipitate indicated that the chemical forms of zinc compounds are controlled by the crystal structure of the host sulfides.The shapes of the Zn K-XAFS spectra of the Hg-Zn and Cd-Zn coprecipitates showed a strong resemblance to those of crystalline standards ZnS, wurtzite and spharelite. It was suggested that the two coprecipitated phases (HgS, ZnS) and (CdS, ZnS) may form a solid solution in the Hg-Zn and Cd-Zn coprecipitates. The local structure around the zinc(II) ion in the Bi-Zn coprecipitate is the same as that around hexaaqua-zinc(II) ions, and adsorption of soluble ions or mechanical occlusion from the mother liquor is regarded as a driving force of coprecipitation in the Bi-Zn system. (Copyright (c) 1999 Elsevier Science B.V., Amsterdam. All rights reserved.)