MULTINUCLEARITY OF AQUEOUS COPPER AND ZINC BISULFIDE COMPLEXES - AN EXAFS INVESTIGATION

Copper and zinc bisulfide complexes in NaHS solutions were investigated by Extended X-ray Absorption Fine-Structure spectroscopy (EXAFS). A goal was to investigate the possibility that bisulfide complexes are multinuclear. Copper and zinc K-edge absorption spectra were recorded in fluorescence mode at room temperature and at 80 K. Lowering temperature improved signal to noise ratios, but did not significantly change measured metal-sulfur distances or coordination numbers. In equilibrium with S-absent assemblages in the Cu-S system, dissolved copper consisted of colorless Cu(I) species having Cu-S interatomic distances of 2.30-2.33 angstrom (coord. no. 3 +/- 1). Second shell Cu atoms were present at 2.72-2.75 angstrom (coord. no. 1.1). The presence of second shell Cu atoms, which was confirmed by duplicate experiments using different starting materials, demonstrates that the predominant complexes in these solutions are multinuclear. The Cu-Cu distance is shorter than in most known Cu-S cluster complexes and Cu-S minerals, but is in good agreement with known Cu-Cu distances in Cu4(RS)62- clusters. In equilibrium with a CuS + S assemblage, shorter Cu-S distances and no evidence of an ordered second shell were observed. In these polysulfide-rich solutions, different Cu complexes apparently predominate. Solutions saturated with sphalerite or amorphous ZnS precipitate yielded Zn-S and Zn-Zn distances similar to those in solid sphalerite, although the Zn-Zn coordination number was lower and second shell S was also present. In the Zn case, but not the Cu case, it is possible that the species being characterized by EXAFS were <200 angstrom particles not in reversible equilibrium with the solution phase. EXAFS is a promising tool for determining molecular structures of complexes of geochemical interest, but at present its application is limited to solutions containing somewhat more than 1 mM of the target element.