Citrate Enhanced Adsorption of U(VI) on Goethite: Characterization of Uranyl Speciation Using EXAFS
Citric acid promotes the adsorption of uranyl (U(VI) as UO22+) on goethite (a-FeOOH) at pH=5. Enhanced adsorption does not appear to follow a simple stoichiometric relationship between total citrate and total uranyl. An excess of citric acid relative to uranyl, or a high concentration of surface-bound citrate is required. An extended X-ray absorption fine structure spectroscopy (EXAFS) study was conducted to help understand the surface interactions between citrate, UO22+, and goethite between pH 3.5 and pH 5.5. Two principal surface species were found to be necessary and sufficient to describe the uranyl EXAFS spectra for the range of adsorbed citrate concentrations studied. (The possibility of additional minor surface species has not been eliminated, but inherent uncertainties in the data do not permit their clear identification.) One species is identified as an inner-sphere, uranyl–goethite complex, which exists at pH 5.5 in the absence of citrate. A second species is interpreted to be an adsorbed uranyl + citrate complex, which displaces the binary uranyl–goethite complex as the concentration of adsorbed citrate increases. The EXAFS spectra from samples with intermediate adsorbed citrate concentrations were reproduced using linear combinations of the spectra from these two end-member species. The uranyl + citrate surface complex appears to dominate adsorbed uranyl speciation at ligand : metal ratios of 10 : 1, or when the total surface-bound citrate approaches saturation. Although the exact structural configuration for the uranyl + citrate surface complex cannot be identified at this time, coordination between uranyl and one or more citrate molecules appears to involve both a bidentate bond (four-membered ring structure) and a putative eight-membered ring structure. The distribution and type of surface species should provide useful constraints on the development of model simulations of surface complexation behavior in this system.
- Research Organization:
- Idaho National Lab. (INL), Idaho Falls, ID (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- DE-AC07-99ID-13727
- OSTI ID:
- 912121
- Report Number(s):
- INEEL/JOU-00-00980; JCISA5; TRN: US0800279
- Journal Information:
- Journal of Colloid and Interface Science, Vol. 244, Issue 1; ISSN 0021-9797
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
ABSORPTION
ADSORPTION
CITRATES
CITRIC ACID
CONFIGURATION
DISTRIBUTION
FINE STRUCTURE
GOETHITE
SATURATION
SPECTRA
SPECTROSCOPY
adsorption
citrate
organic ligand
surface complexation
uranium
uranyl
x-ray absorption spectroscopy