Molecular scale of characterization of sorption products at oxide water interfaces
- Stanford Univ., Stanford, CA (United States)
Molecular-scale characterization of sorption products is essential for developing predictive models of surface complexation and reactive transport. Such information can be obtained for many systems of environmental interest using x-ray absorption fine structure (XAFS) spectroscopy. We have used XAFS to study the mechanism(s) of sorption and the structures and compositions of sorption complexes of Cr(VI) on Fe{sub 3}O{sub 4} and of Co(II) and Pb(II) on {alpha}-Al{sub 2}O{sub 3}, {alpha}-SiO{sub 2}, and TiO{sub 2} (rutile) in situ, i.e., with bulk water present, at surface coverages ranging from less than a monolayer (< 1 {mu}M/m{sup 2}) to several monolayers. Using both powder XAFS and grazing-incidence (GI) XAFS from single crystal sorbents, we have been able to distinguish between inner- vs. outer-sphere surface complexation and to detect precipitates. We have also been able to study the reduction of Cr(VI) to Cr(III) on magnetite surfaces. Our GI-XAFS results for Co(II) and Pb(II) on the (0001) and (10-12) surfaces of {alpha}-Al{sub 2}O{sub 3} have shown that both aqueous cations bond to the (10-12) surface in an inner-sphere, tridentate fashion, whereas on the (0001) surface, Pb(II) forms an outer-sphere complex while Co(II) forms an inner-sphere complex. These differences in sorption can be understood using simple valence bond models of the sorbent surface and sorption complex.
- OSTI ID:
- 370158
- Report Number(s):
- CONF-960376--
- Country of Publication:
- United States
- Language:
- English
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