Computer Science
Scientific paper
Jul 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996gecoa..60.2515o&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 60, Issue 14, pp.2515-2532
Computer Science
6
Scientific paper
The local molecular structure of Co(II) surface complexes sorbed to two pure mineral substrates, quartz ( -SiO 2 ) and rutile (TiO 2 ), was examined with X-ray absorption spectroscopy (XAS) and extended X-ray absorption fine structure (EXAFS) analysis. Absorption spectra were collected for samples equilibrated with Co solutions under- and over-saturated with respect to solid Co-hydroxide phases (0.15-3.00 mM) at Co surface coverages of 0.63-9.51 mol m -2 and for equilibration times of 23 h to 21 days. Quantitative analysis of the EXAFS spectra indicates no significant structural differences in the local atomic environment around Co sorbed to quartz, regardless of the surface coverage or the equilibration time. For all Co/quartz samples, the local Co environment is similar (but not identical) to that of Co in solid, crystalline cobalt hydroxide (Co(OH) 2 (S)), Cobalt is octahedrally coordinated by six O atoms at 2.06-2.11 Å; second-neighbor Co backscatterers are found at 3.11-3.12 Å, slightly contracted relative to the Co---Co distance in Co(OH) 2 (s)(3.173 Å). These results and the analysis of Co multiple-scattering suggest the formation of, primarily, large, multinuclear Co complexes or disordered hydroxide-like precipitates for Co sorbed to quartz. For Co sorption on rutile, the EXAFS spectra vary with changes in Co surface coverage and are distinctly different from Co/quartz spectra at all surface coverages examined. At low Co coverages, Co apparently sorbs directly to the rutile surface as mononuclear or small multinuclear complexes. This is indicated by two to three second-neighbor Ti atoms at two distinct distances, 2.95-2.99 Å and 3.60-3.63 Å, around sorbed Co. These Co---Ti distances are similar to Ti---Ti distances in the bulk rutile structure. The small number of Ti backscatterers indicates that Co has not diffused into the rutile structure but rather that Co occupies Ti-equivalent sites at the rutile surface. For high surface coverage samples equilibrated for eleven days, the average number of Co second-neighbor backscattering atoms increases, indicating larger-sized multinuclear complexes, but the EXAFS spectra do not resemble those of either the Co/quartz sorption samples or Co(OH) 2 (s). Analysis of these high coverage Co/rutile samples indicates Ti backscatterers at distances near those expected for anatase, a TiO 2 polymorph, suggesting a local geometry for Co and Ti octahedra at the rutile/water interface similar to that of the cation octahedra in anatase. We suggest that differences in Co sorption on quartz and rutile may be attributed to the availability of reactive surface sites on quartz and rutile which are structurally favorable for the sorption of octahedrally-coordinated Co 2+ .
Brown Gordon E. Jr.
Chisholm-Brause Catherine J.
O'Day Peggy A.
Parks George A.
Towle Steven N.
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