The UREX Simple Feed simulant was subjected to Raman spectroscopic measurements. The Raman spectrum in Fig. 4.1 (blue spectrum) shows two strong bands due to UO22+ (870 cm-1) and NO3- (1047 cm-1). The UREX simple feed stimulant was contacted with 30% TBP/n-dodecane, resulting in an organic phase loaded with UO2(NO3)2. The Raman spectrum of the loaded solvent shown in Fig. 4.1 (red spectrum) contains bands due to dodecane, TBP, uranyl, and nitrate were observed. The UO22+ (859 cm-1) and NO3- (1029 cm-1) bands were both shifted to lower energy in the

organic phase spectrum compared to the aqueous phase spectrum due to the UO2(TBP)2(NO3)2 complex formation. The spectrum of the loaded solvent was compared to the spectrum of water-washed TBP/n-dodecane, and it was found that the bands due to the solvent do not interfere with the uranyl or nitrate Raman bands. It was demonstrated that the depletion of uranyl ions from the aqueous phase upon extraction can be easily followed using Raman spectroscopy. The intensity of the uranyl band (870 cm-1) decreased from the initial spectrum to the final spectrum, indicating a reduced concentration of UO22+ in the aqueous phase after extraction.

In order to evaluate the detection limit for U(VI) and nitrate in the UREX Simple Feed system, a series of feed simulant solutions containing 0.0003-1.31 M UO2(NO3)2 in 0.8 M HNO3 were prepared and subjected to Raman spectroscopic measurements. The spectral overlay obtained is shown in Fig. 4.2 (left). A linear relationship was established between the Raman response of the respective UO22+ (870 cm-1) and NO3- (1047 cm-1) bands and the concentration of UO2(NO3)2 and nitrate in the 0.8 M HNO3 solution. A treatment recommended by the International Union of Pure and Applied Chemistry (IUPAC) was used for the evaluation of the detec­tion limit (Long 1983). In this treatment, the detection limit is calculated using equation 4.1,

DL = kSb 4.1

m + tSm

where DL is the detection limit, k is a numerical coefficient, m is the slope, Sb and Sm are the standard errors for the intercept and slope of a calibration plot, respectively, and t is Student’s value for (n — 2) degrees of freedom at the chosen confidence level. In accord with IUPAC recommendations, a k value of 3 was applied, which in turn calls for a 99.87% confidence level. This confidence level was used in the linear regression analysis, and the denominator in equation 4.1 was taken as the upper 99.87% value of the
slope. This treatment yielded the detection limit of 3.1 mM for UO22+ and 2.6 mM for NO3- under the applied measurement conditions. The analogous Raman calibration measurements using extraction solvent containing 30 vol% TBP/n-dodecane loaded with UO2(NO3)2 afforded detection limits of

1.9 and 21 mM for UO22+ and nitrate, respectively. The Raman spectra of variable UO2(NO3)2 extracted into TBP/n-dodecane solvent is shown in Fig.

4.2 (right). To account for variable baseline shifts in the organic solvent system, the intensity of the UO22+ and nitrate bands (858.9 cm-1 and 1029 cm-1, respectively) were normalized to the intensity of the dodecane solvent band at 1300.9 cm-1. The 1300.9 cm-1 Raman band in dodecane is a strong vibrational band ascribed to the -(CH2)n — in-phase twist character­istic of n-alkanes (Lin-Vien 1991). Due to its constant concentration as the diluent, this dodecane band is used as an internal standard.

Initial chemometric analysis of the Raman spectral data was undertaken using PLS analysis of the di-component uranyl nitrate — nitric acid solutions. Predictive models based on PLS analysis of Raman spectral data (contain­ing variable UO22+/total nitrate/proton concentrations) showed linear response over the 0-1.3 M and 0-3.5 M range for uranyl and nitrate species, respectively. Results of the PLS modeling based on Raman UO22+, NO3-, and H+ measurements are depicted in Fig. 4.3.