## 2 Materials and Methods
### X-Ray Tomography  
To determine the spatial distribution of uranophane relative to the solution flow in the column and to the sediments extracted in the URP**6** and URP**12** cores (see Section Characterization of Packed Columns), X-ray tomography was performed on the Nikon(r) XT H 225 tomograph, Nikon Corporation, Tokyo, Japan, with a 0.1 mm copper filter (current 78 \(\upmu\)A and voltage 93 kV). X-ray tomography, both industrial and synchrotron, is widely used for the 3D characterization of natural porous media, and the results of particle and pore distribution resulting from fluid transport [48]. Data was collected using Inspect-X software version XT4.4.4, and volume generation was performed using CTPro 3D software version 3.1. The density of the uranium ore in the CT scans was calibrated to the uranophane ore used in these experiments (Appendix B.3, Figure B8). \begin{table}
\begin{tabular}{c c} \hline
**Sediment Sample** & **ID Name** \\ \hline Coarse silt & CS \\ Fine silt & FS \\ Clay from CSC & C\({}_{1}\) \\ Clay from FSC & C\({}_{2}\) \\ \hline \end{tabular}
\end{table} 
Table 2: _Cont._Photographs were taken with short-wave UV light (254 nm) using a Spectroline lamp model ENF-240C New York, NY, USA, and pictures with visible light from the laboratory. X-Ray Absorption Fine Structure (XAFS)  
XAFS analysis has frequently been applied to analyze uranium in the environment [11; 12; 14; 49; 50; 51; 52] since it allows the local environment of U to be characterized, including its speciation and interatomic distances. This information will enable the identification of the mineral species of U present in the samples, as well as the nature of the adsorption of the uranyl ion in the sediments. The XAFS measurement consists of obtaining the absorption coefficient as a function of the energy of the X-rays, \(\mu\)(E). XAFS is an interference effect between photoelectron waves emitted upon absorption of X-rays and waves reflected by neighboring atoms. In the region of near-edge X-ray absorption spectroscopy (XANES), high resolution is employed to determine the incident energy on the sample. In the region of extended X-ray absorption fine structure (EXAFS), the energy dependence of the coefficient \(\mu\)(E) is transformed into the function \(\mu\)(k) of the wavenumber k of the absorbed X-ray beam. Mathematical procedures are applied to the experimental \(\mu\)(k) function to identify the fingerprints of the short-range structure from the EXAFS spectra for comparison with experimental and theoretical model compounds. For measurement of XAFS, sediment samples were prepared on cellulose discs of 6 mm diameter. The measurements were performed at beamline I20-scanning of the Diamond Light Source, Didcot, UK, in fluorescence mode with individual scans resultants of averaging 11 channels of a Ge fluorescence detector in a cryostat at liquid nitrogen temperature, with Si (111) double crystal monochromator. The energy was calibrated by measuring the K-edge of a Yttrium sheet [53]. Similarly, a measurement of a \(\beta\)-uranophane from the Pena Blanca sample was carried out [54] as a model compound of XAFS at beamline B18 of the Diamond Light Source, UK [55]. To identify the component species in the study samples through the normalized \(\mu\)(E) spectrum, the linear combination analysis of the model spectra measurements under similar conditions in pure substances can be applied. Alternatively, theoretical spectra can be fitted from the structure of these substances. In the latter case, after obtaining the experimental \(\mu\)(k) function, it is \"amplified\" by normalizing it to a unit absorption \(\chi\)(k) and multiplying it by k\({}^{3}\). The Fourier transform is applied to this weighted function k\({}^{3}\chi\)(k), and \(\mid\chi\)(R)\(\mid\) is obtained. The \(\mid\chi\)(R)\(\mid\) reveals the characteristics of the absorption coefficient as a function of the distance, or radial distribution, between the X-ray-absorbing uranium atom and the atoms of neighboring elements.