Evaluating Best Practices in Raman Spectral Analysis for Uranium Speciation and Relative Abundance in Aqueous Solutions

Abstract

Raman spectroscopy is emerging as a powerful tool for identifying hexavalent uranium speciation in situ; however, there is no straightforward protocol for identifying uranyl species in solution. Herein, uranyl samples are evaluated using Raman spectroscopy, and speciation is monitored at various solution pH values and anion compositions. Spectral quality is evaluated using two Raman excitation wavelengths (532 and 785 nm) as these are critical for maximizing signal-to-noise and minimizing background from fluorescent uranyl species. The Raman vibrational frequency of uranyl shifts according to the identity of the coordinating ions within the equatorial plane and/or solution pH; therefore, spectral barcode analysis and rigorous peak fitting methods are developed that allow accurate and routine uranium species identification. All in all, this user's guide is expected to provide a user-friendly, straightforward approach for uranium species identification using Raman spectroscopy.

Detection and identification of solids, surfaces, and solutions of uranium using vibrational spectroscopy

“…The Raman-active bands can be fit to a Gaussian function, and the full width at half-maximum (Γ) of these bands is typically between 13 and 15 cm −1 [48]. Vibrational band frequencies are observed for monomeric, dimeric, and trimeric hydrolysis products.…”

“…For instance, a ~20–30 cm −1 shift in vibrational frequency of the symmetric stretch has been reported as oligomeric species are generated by hydrolysis [56]. This shift is also accompanied by slight broadening of the bands (ΔΓ = 5 cm −1 ) due to the formation of these larger soluble clusters [48]. …”

“…Typically, this is done by assuming that the cross section is the same across similar species. This was demonstrated vide supra with the uranyl carbonate system [48], but advanced chemometric methods can also be used for quantifying relative amounts of various species in more complicated systems. Chemometrics [155] successfully facilitated the identification and quantification of oligomeric uranyl species (U total = 0.1 M) present in solutions with pH ranging from 1.55 to 4.20 using both Raman and FT-IR spectroscopies.…”

“…Increases in bandwidth upon adsorption arise from the formation of multiple uranyl species, varying uranyl orientations, and/or activation of multiple vibrational modes all of which can overlap spectrally [214]. For example, the bandwidth (FWHM) of a ν 1 Raman mode for an adsorbed species is ~57 cm −1 , which is broader [224] than a variety of uranium species in solution (FWHM = 13–20 cm −1 ) [48]. Multiple uranyl species can arise from original surface speciation or change upon adsorption with or without coordinated ligand displacement [221,224,236–238].…”

“…An excellent review of IR vibrational frequencies of uranyl (VI) mineral compounds was also provided by Cejka in 1999, but the reported uses of vibrational spectroscopy for uranium chemistry has grown in number and significant advances in sample preparation and data analysis have increased in both breadth and depth; therefore, there is a critical need to provide an update on this important chemical technique [39]. We have also published a study investigating the best practices for collecting and analyzing Raman spectra in aqueous solutions, but determined that additional information was lacking in regards to the analysis of solid state samples and surfaces [48]. Building on the previous work by Cejka [39], the current review focuses on how both IR and Raman spectroscopies are used to characterize solid state uranium compounds; identify U adsorbed on surfaces; and provide speciation information within aqueous solutions, organic solvents, and ionic liquids.…”