ADSORPTION AND STRUCTURAL ENVIRONMENT OF CO(II) AT THE ZINC OXIDE-AQUEOUS AND ZINC SULFIDE-AQUEOUS SOLUTION INTERFACES

Adsorption of Co(II) at the aqueous interfaces of ZnO and ZnS (wurtzit e) has been studied by classical wet-chemical methods, and the structu re and composition of Co adsorption complexes at these interfaces have been determined by in-situ X-ray absorption fine structure (XAFS) spe ctroscopy. On both adsorbents, adsorption as a function of pH shows th e normal behavior, i.e., a pronounced adsorption within a relatively n arrow pH range. There is no evidence for ion exchange between Zn and C o on either of the solids, indicating that uptake must be due to surfa ce complexation or surface precipitation. By use of log(IAP) (IAP = al pha(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 a nd 2.0 mu mol/m(2), respectively. At the molecular level, the XAFS res ults show the presence of both four- and six-coordinated oxygen-ligate d Co(II) on ZnO, corresponding to Co-O distances of 1.95 and 2.10 Angs trom. On ZnS, two Co-O distances are also indicated and a Co-S correla tion is observed at 2.36 Angstrom. These spectroscopic results, togeth er with the wet-chemical data, show that the Co-S correlation is due t o 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 structur e, while almost no distortions are observed for Co(II) surface complex es on ZnS. At increased Co(II) coverages, a structurally disordered Co (OH)(2) phase is formed on both adsorbents. The disorder is shown to b e of a short- to medium-range nature, i.e., disorder within 4-5 Angstr om 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.