Time-Resolved Infrared Reflectance Studies of the Dehydration-Induced Transformation of Uranyl Nitrate Hexahydrate to the Trihydrate Form

Abstract

Uranyl nitrate is a key species in the nuclear fuel cycle. However, this species is known to exist in different states of hydration, including the hexahydrate ([UO2(NO3)2(H2O)6] often called UNH), the trihydrate [UO2(NO3)2(H2O)3 or UNT], and in very dry environments the dihydrate form [UO2(NO3)2(H2O)2]. Their relative stabilities depend on both water vapor pressure and temperature. In the 1950s and 1960s, the different phases were studied by infrared transmission spectroscopy but were limited both by instrumental resolution and by the ability to prepare the samples for transmission. We have revisited this problem using time-resolved reflectance spectroscopy, which requires no sample preparation and allows dynamic analysis while the sample is exposed to a flow of N2 gas. Samples of known hydration state were prepared and confirmed via X-ray diffraction patterns of known species. In reflectance mode the hexahydrate UO2(NO3)2(H2O)6 has a distinct uranyl asymmetric stretch band at 949.0 cm(-1) that shifts to shorter wavelengths and broadens as the sample desiccates and recrystallizes to the trihydrate, first as a shoulder growing in on the blue edge but ultimately results in a doublet band with reflectance peaks at 966 and 957 cm(-1). The data are consistent with transformation from UNH to UNT as UNT has two inequivalent UO2(2+) sites. The dehydration of UO2(NO3)2(H2O)6 to UO2(NO3)2(H2O)3 is both a structural and morphological change that has the lustrous lime green UO2(NO3)2(H2O)6 crystals changing to the matte greenish yellow of the trihydrate solid. The phase transformation and crystal structures were confirmed by density functional theory calculations and optical microscopy methods, both of which showed a transformation with two distinct sites for the uranyl cation in the trihydrate, with only one in the hexahydrate.

Hyperspectral imaging of minerals in the longwave infrared: the use of laboratory directional-hemispherical reference measurements for field exploration data

“…In fact, the ðNH 4 Þ 2 HPO 4 spectrum displays some distinct features that can be used for identification, namely the NH 4 þ cation peak at 970 cm −1 , as well as the 1060, 1100 cm −1 doublet that we have previously associated as being two of the phosphate anion band modes. 27 As to the strong reflectance peaks associated with symmetric cations 46,[63][64][65] such as NH 4 þ or UO 2 2þ as well as anions 27,44 such as PO 4 3− or SO 4 2− , these peaks arise from the first surface scattering of reststrahlen bands. As we have previously reported in studies that quantified reflection versus wavelength as a function of particle size, 44 the amplitude of the other bands relative to the reststrahlen bands varies significantly with particle size.…”

“…1) can rotate such that light entering the sphere can be directed downward to a 1.9-cm-diameter sample port in the bottom of the sphere, back out the entrance port, or upward to a 3.2-cm-diameter reference port that also serves as a specular exclusion port for the specular component reflected from the sample. [45][46][47] When the mirror is pointing down, the angle between the incident IR beam and the normal to the sample surface is 14.8 deg. The A562 sphere has a matte gold interior surface, i.e., a gold overcoat on a roughened surface, typical for such IR devices.…”

“…The sphere has a purge gas connection to provide a constant dry nitrogen purge during measurement. 46 For the mid-IR measurements reported here, the standard SiC source and Ge/KBr beamsplitter were used. The sphere's liquid nitrogen-cooled mercury cadmium telluride (MCT) detector has a 2 × 2 mm element with a 60-deg field of view.…”

A new methodology for determination of water of crystallization for uranyl nitrate samples using scanning electron microscope, energy dispersive X-ray, and MCPN-5

“…This form con tains a small amount of im pu rity such as alkali there fore, it may re quire sev eral cy cles of pu ri fica tion. It is pre pared by dis solv ing ura nium ox ides or hy drox ides in an in or ganic sol vent (ni tric acid) [2].…”

Model representations of kerogen structures: An insight from density functional theory calculations and spectroscopic measurements

“…Direct comparison between vibrational properties measured by neutron, Raman or infrared spectroscopies and spectra simulated from classical or quantum mechanical methods has proven to be a powerful tool for model validation 5 , 8 . In this study, infrared signatures for a variety of generic kerogen models proposed recently 4 , 5 were calculated within the framework of density functional perturbation theory (DFPT) and compared to Fourier transform infrared (FTIR) spectra collected from kerogen samples with different types and maturity.…”