Fluorescence spectroscopy of U(VI)-silicates and U(VI)-contaminated Hanford sediment

Abstract

Time-resolved U(VI) laser fluorescence spectra (TRLFS) were recorded for a series of natural uranium-silicate minerals including boltwoodite, uranophane, soddyite, kasolite, sklodowskite, cuprosklodowskite, haiweeite, and weeksite, a synthetic boltwoodite, and four U(VI)-contaminated Hanford vadose zone sediments. Lowering the sample temperature from RT to ϳ 5.5 K significantly enhanced the fluorescence intensity and spectral resolution of both the minerals and sediments, offering improved possibilities for identifying uranyl species in environmental samples. At 5.5 K, all of the uranyl silicates showed unique, well-resolved fluorescence spectra. The symmetric O ϭ U ϭ O stretching frequency, as determined from the peak spacing of the vibronic bands in the emission spectra, were between 705 to 823 cm Ϫ1 for the uranyl silicates. These were lower than those reported for uranyl phosphate, carbonate, or oxy-hydroxides. The fluorescence emission spectra of all four sediment samples were similar to each other. Their spectra shifted minimally at different time delays or upon contact with basic Na/Ca-carbonate electrolyte solutions that dissolved up to 60% of the precipitated U(VI) pool. The well-resolved vibronic peaks in the fluorescence spectra of the sediments indicated that the major fluorescence species was a crystalline uranyl mineral phase, while the peak spacing of the vibronic bands pointed to the likely presence of uranyl silicate. Although an exact match was not found between the U(VI) fluorescence spectra of the sediments with that of any individual uranyl silicates, the major spectral characteristics indicated that the sediment U(VI) was a uranophane-type solid (uranophane, boltwoodite) or soddyite, as was concluded from microprobe, EXAFS, and solubility analyses.

An integrated study of uranyl mineral dissolution processes: etch pit formation, effects of cations in solution, and secondary precipitation

“…(1) The coatings formed in Mg-HCl and Ca-HCl solutions are structurally and chemically similar to the silicagel-type-coatings (H 4 SiO 4 (H 2 O) n ) on the surface of feldspars [59] and (2) The hillocks observed three days after treatment in a SrCl 2 -HCl solution of pH 2 have a composition of (H 2 SiO 3 or Si 2 O 2 (OH) 4 ).…”

“…Potential phases could be phases structurally related to cejkaite, Na 4 4− , which is the dominant uranyl aqueous-species at high pH [22]. The clusters are usually connected by interstitial cations (Na, Ca, K), anions (F-, (SO 4 ) 2− and H 2 O groups.…”

“…These minerals are important in controlling the mobility of uranium in the contaminated subsurface. For example, the uranyl silicate, boltwoodite, and the uranyl phosphate, metatorbernite, have formed in contaminated sediments of the Hanford site [2][3][4], uranyl phosphates occur in contaminated soils of the Fernald site [5], and uranyl sulfates are common in altered mine tailings [6]. The interactions of these minerals with aqueous systems, and the details of how they dissolve and precipitate, are integral to understanding and predicting the mobility of uranium.…”

Spatially Resolved U(VI) Partitioning and Speciation: Implications for Plume Scale Behavior of Contaminant U in the Hanford Vadose Zone

“…Direct characterizations of the core sediment from borehole 299-E33-45 have provided valuable information about the waste plume (2), and selected results (discussed further in later sections) are shown in Figure 2. Microscopic and spectroscopic studies of selected samples from this borehole identified uranyl silicate (uranophane) as the dominant U species, residing within micro fractures of feldspar grains (9)(10)(11)(12)(13). Column and batch experiments using contaminated sediments from this borehole indicated that slow dissolution kinetics of the U(VI) silicates and intragranular mass transfer are expected to keep rates of U(VI) release into pore waters low (14).…”

Characterization of Direct-Push Vadose Zone Sediments from the 241-B and 241-BX Tank Farms

“…At liquid helium temperature, all three sediment samples (B1JWW6C fine, B1JWW6C coarse, and B1JWW6A) displayed the strong fluorescence spectra (Figure 4.27) characteristic of uranyl compounds, with a set of almost evenly spaced vibronic bands ranging from 480 to 600 nm (Wang et al 2005a). The spectra collected at steady-state conditions at 0 second delay with a gate width of 10 ms showed a broader pattern, especially for B1JWW6C fine and coarse sediments, than time-delayed spectra (1.2-ms delayed 4.50 spectra in Figure 4.27).…”