Adsorption and structural environment of Co(II) at the zinc oxide- and zinc sulfide-aqueous solution interfaces

Abstract

Adsorption of Co(II) at the aqueous interfaces of ZnO and ZnS (wurtzite) has been studied by classical wet-chemical methods, and the structure and composition of Co adsorption complexes at these interfaces have been determined by in-situ X-ray absorption fine structure (XAFS) spectroscopy. On both adsorbents, adsorption as a function of pH shows the normal behavior, i.e., a pronounced adsorption within a relatively narrow pH range. There is no evidence for ion exchange between Zn and Co on either of the solids, indicating that uptake must be due to surface complexation or surface precipitation. By use of log(IAP) (IAP = alpha(Co(II))alpha(OH)(-2)) vs pH diagrams, the maximum surface complex binding capacities of Co(II) on ZnO and ZnS were estimated to be 2.5 and 2.0 mu mol/m(2), respectively. At the molecular level, the XAFS results show the presence of both four- and six-coordinated oxygen-ligated Co(II) on ZnO, corresponding to Co-O distances of 1.95 and 2.10 Angstrom. On ZnS, two Co-O distances are also indicated and a Co-S correlation is observed at 2.36 Angstrom. These spectroscopic results, together with the wet-chemical data, show that the Co-S correlation is due to surface complexation of Co(II) to sulfide sites. Higher shell EXAFS analysis indicates that Co(II) adsorbs to defect structural positions where Zn atoms are missing at the surface. On ZnO these Co(II) surface complexes are slightly distorted as compared to the ZnO bulk structure, while almost no distortions are observed for Co(II) surface complexes on ZnS. At increased Co(II) coverages, a structurally disordered Co(OH)(2) phase is formed on both adsorbents. The disorder is shown to be of a short- to medium-range nature, i.e., disorder within 4-5 Angstrom radius of the central Co atoms as compared to crystalline Co(OH)(2), and is observed even at the high surface coverage of 42.1 mu mol/m(2) on ZnS. (Less)