4.4.1 Nature of Solid-associated Uranium
Electron microscopy has confirmed the hypothesis that uranium can be adsorbed on mineral surfaces, present as a surface-precipitated phase, or precipitated in discrete uranium-containing particles. The distribution of uranium among these three types of solid-associated phases is a function of the equilibration time of goethite with uranium and the degree to which the dissolved phase is supersaturated with respect to a uranium- containing solid phase. Observations of discrete uranium-containing particles in electron micrographs and uranium clustering in element maps are consistent with observations of X-ray diffraction peaks for schoepite (reported in Chapter 3). Furthermore, the low crystallinity uranium surface precipitate observed with TEM (Fig. 4.7) is consistent with the absence of a signal for schoepite in the X-ray diffraction pattern (Table 4.1) and the homogenous distribution of uranium in the SEM-EDX element map (Fig. 4.3). Although air-dried samples were stored for as long as 16 months before SEM and TEM analyses, the consistency of SEM, TEM (3 months longer storage for TEM than for SEM), and XRD measurements (generally performed within a week of sample collection) suggests that no significant redistribution of uranium occurred in the dried samples during the storage period.
4-16 1. Adsorption:
Rapid uptake of uranium from solution as monomeric or polymeric surface complexes. Dissolved Uranium: (UO2)3(OH)5+, UO22+ UO2 UO2 UO2 UO2 (UO2)3(OH)5 (UO2)3(OH)5 (UO2)3(OH)5 2. Surface Precipitation: Continuing uptake of uranium from solution as oligomeric surface clusters form. Dissolved Uranium: (UO2)3(OH)5+, UO22+ 3. Precipitation: Discrete uranium- containing phases form by ripening and detach into solution.
UO2 UO2
(UO2)3(OH)5
Figure 4.8: Schematic representation of the growth of uranium-containing precipitates following adsorption on goethite.
A three-step mechanism is suggested for the growth of discrete uranium-
containing particles at low degrees of supersaturation (Fig. 4.8). In the first step, uranium adsorbs to the goethite surface as monomeric or polymeric surface complexes on the short time-scales discussed in Chapter 3. In the second step, oligomeric surface clusters form on the goethite surface, possibly nucleating at sites of initial uranium sorption. In the third step, the oligomeric surface clusters ripen with uranium migrating from smaller to larger particles, until the particles are sufficiently large to be stable in solution. The close association of goethite particles with the larger uranium-containing particles in
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TEM images suggests that the initial coordination of uranium to the goethite surface may persist even once discrete uranium-containing particles have formed.
The transformation from monomeric surface complexes to schoepite-like polymeric surface complexes was recently observed spectroscopically as the loading of uranium on montmorillonite increased (Chisholm-Brause et al., 2001). According to the mechanism illustrated in Figure 4.8, the uranium concentration at the goethite surface should actually decrease as oligomeric surface clusters ripen into larger discrete uranium- containing particles that detach from the surface. This phenomenon was observed with quantitative EDX analysis of uranium sorbed on goethite particles in material from the Koongarra ore deposit. The solid-associated uranium concentration on goethite increased with the total uranium content until the precipitation of a uranyl oxyhydroxide solid, and then the solid-associated concentration decreased dramatically (Lumpkin et al., 1999).
4.4.2 Utility of Electron Microscopy
Both scanning and transmission electron microscopy were useful in determining the distribution of uranium in goethite suspensions as a function of uranium concentration and equilibration time. Although SEM did not provide the high resolution of TEM, its advantages were the ease of sample preparation, high sample throughput (about two samples per hour), and collection of element maps (though this can also be done on many TEM systems). Further, SEM analysis served as a screening tool for selecting samples for TEM analysis. TEM analysis was more time consuming, but ultimately offered higher resolution images and the opportunity for phase identification with electron diffraction.
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4.4.3 Need for Additional Characterization
It would be naïve to suggest that complete characterization of uranium-
contaminated solids can be accomplished through the use of any single technique. In the current study, information about the crystalline structure of the solid phases was gained through X-ray diffraction measurements, which complemented imaging and element composition information achieved with SEM and TEM. In addition to electron microscopy, other techniques are available for determining the spatial distribution of a contaminant. Spatially resolved elemental composition data for plutonium on Yucca Mountain zeolitic tuff were acquired by electron microprobe analysis (EMPA) and micro-synchrotron X-ray fluorescence (SXRF) (Duff et al., 2001). SXRF has also been used to examine FEMP soils (Bertsch et al., 1994) and evaporation basin sediments (Duff et al., 1997; Duff et al., 2000).
Various spectroscopic techniques can probe the bonding environment of uranium, providing additional information for characterization of the uranium-goethite system. Infrared spectroscopy was used to identify ternary uranyl-carbonate sorption complexes on hematite (Bargar et al., 1999). Raman spectroscopy was used in conjunction with SEM and TEM to identify uranium-containing phases at the FEMP (Morris et al., 1996), and was also used to characterize different types of surface complexes on
montmorillonite (Morris et al., 1994). Different montmorillonite surface complexes were also characterized with luminescence spectroscopy (Chisholm-Brause et al., 2001), a technique also used in identifying sorption complexes and uranium-containing
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absorption fine structure spectroscopy (EXAFS) has been used to determine the structure of surface complexes and precipitates by identifying the nearest neighbors to sorbed uranium atoms. The structures of uranium surface complexes on goethite (Moyes et al., 2000), hematite (Bargar et al., 1999; Bargar et al., 2000), montmorillonite (Chisholm- Brause et al., 1994; Dent et al., 1992; Sylwester et al., 2000), vermiculite and
hydrobiotite (Hudson et al., 1999), kaolinite (Thompson et al., 1998), and silica (Dent et al., 1992; Sylwester et al., 2000) have been determined using EXAFS.
4.4.4 Conclusions
Electron microscopy was useful in distinguishing among sorbed uranium, surface- precipitated uranium, and discrete uranium-containing precipitates. Imaging and element mapping with SEM-EDX showed the clustering of uranium with both increasing uranium content and equilibration time. Higher magnification imaging with TEM was used to observe both surface precipitates and discrete uranium-containing particles. The SEM- EDX and TEM measurements are consistent with each other and also with solution chemistry and XRD measurements. Electron microscopy can be an important component of a suite of techniques used in the characterization of uranium in contaminated
environmental media. Determination of the solid phase speciation of uranium can yield useful information for predicting uranium mobility in the environment.