X-ray absorption spectroscopy of cobalt(II) multinuclear surface complexes and surface precipitates on kaolinite
- Stanford Univ., CA (United States). Dept. of Geological and Environmental Sciences
The authors have examined the local atomic structure of multinuclear complexes and surface precipitates of Co(II) sorbed to kaolinite (Al[sub 2]Si[sub 2]O[sub 5](OH)[sub 4]), a common clay mineral, using extended X-ray absorption fine structure (EXAFS) spectroscopy. Structural information from quantitative analysis of EXAFS spectra is used to examine differences, on a molecular level, among Co species sorbed on kaolinite at different coverages from solutions under- and over-saturated with respect to solid Co(OH)[sub 2] phases, and between sorbed species and Co hydroxide solids precipitated from solutions in the absence of kaolinite. The EXAFS results indicate formation of oxide or hydroxide-bridged multinuclear surface complexes on kaolinite at surface coverages well below a statistical monolayer from undersaturated solutions. As Co surface loading on kaolinite is increased, the number of CO second-neighbor atoms at 3.10--3.12 A increases from [approx]2 to 6, the number of second-neighbor Co backscatterers in crystalline Co(OH)2(s), and interatomic distances determined by EXAFS remain constant. Long-term equilibration of oversaturated sorption samples (21 or 45 days) resulted in a decrease in backscattering amplitudes for both single and multiple-scattering paths among neighboring Co atoms relative to short-term (24--36 h) equilibration. The contraction in interatomic distances and disruption of long-range structure indicated by EXAFS for sorption samples can be accounted for by small particle sizes of the new surface phase, structural misregistry on an atomic scale between a Co hydroxide phase and kaolinite, or dissolution of the kaolinite substrate and substitution of minor impurities into a Co hydroxide phase. The apparent reduced solubilities of the sorption products relative to bulk solubilities of Co(OH)[sub 2](s) phases may be related to Ostwald-step processes, the formation of an impure Co hydroxide phase, or a lower near-surface solvent dielectric constant.
- OSTI ID:
- 7163783
- Journal Information:
- Journal of Colloid and Interface Science; (United States), Journal Name: Journal of Colloid and Interface Science; (United States) Vol. 165:2; ISSN 0021-9797; ISSN JCISA5
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
540210* -- Environment
Terrestrial-- Basic Studies-- (1990-)
ALUMINIUM COMPOUNDS
ALUMINIUM SILICATES
CHEMISTRY
COBALT COMPOUNDS
COBALT HYDROXIDES
ENVIRONMENTAL TRANSPORT
HYDROGEN COMPOUNDS
HYDROXIDES
KAOLINITE
MASS TRANSFER
MINERALS
OXYGEN COMPOUNDS
PHASE STUDIES
PRECIPITATION
SEPARATION PROCESSES
SILICATE MINERALS
SILICATES
SILICON COMPOUNDS
SOIL CHEMISTRY
SORPTION
SORPTIVE PROPERTIES
SPECTROSCOPY
SURFACE PROPERTIES
TRANSITION ELEMENT COMPOUNDS
X-RAY SPECTROSCOPY