Multinuclearity of aqueous copper and zinc bisulfide complexes: An EXAFS investigation

Abstract

Listen

Translate article

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 Å (coord. no. 3 ± 1 ). Second shell Cu atoms were present at 2.72-2.75 Å ( 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 Cu 4(RS) 2− 6 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 Å 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.