## 3 Results
### Modeling Adsorption from Uranium in Solution
#### XAFS  
To explain the origin of high concentrations of uranium in fine sediment segments, XAFS was performed to identify the mineral components. The XANES spectra, in the case  
Figure 7: Computer tomography images, showing the raw image (left side) and the probability of the material’s occurrence as a function of density for the distribution distance in the column profile (right side). (**A**) In sample URP6 FSD, the highest concentration of particles is observed between 10 and 55 mm, showing a coincidence with the uranophane horizon. (**B**) For sample URP12 FSD, the particle concentration is shown with a greater dispersion, up to 110 mm, which indicates the mobility of the particulate mineral of uranophane in the direction of the flow of distilled water (solution). In raw images, colors are according to the particle size; fine particles are in red, and coarse particles are in green and black colors. UV and visible light photography corroborate the particle-dragging behavior (see Appendix B.3, Figure B9). of the U-L\({}_{3}\) edge, allow us to differentiate the exact energy of the edge in the spectrum \(\mu\)(E), and thus to distinguish whether the species is U(IV) or U(VI). Figure 8 shows the normalized U-L\({}_{3}\) XANES spectra of UN6 FSC, UN6 CSC, URP6 FSC, URP6 CSC, and UO\({}_{2}\). The energy of the absorption edge was chosen through the first derivative at the edge of each spectrum. The positions of the maximum resonance (white line WL) are 17,177.5, 17,176.6 eV, 17,177.5 eV, 17,176.8 eV, and 17,177.4 eV, respectively. The spectra show a \"shoulder\" on the side of the WL, a distinctive feature of the oxidation state of U(VI) [49; 65; 66], in our case, uranyl ions. An EXAFS spectrum is typically acquired over thousands of eV beyond the absorption edge. The XAFS spectra of samples UN6 CSC and UN6 FSC showed low count statistics, making it impossible to interpret the EXAFS region of these spectra. EXAFS spectra were obtained for the fine sediment samples from the URP columns, and analyses were performed using both LCA and theoretical modeling. Figure 9 shows the EXAFS spectra obtained from the U-L\({}_{3}\) edge for the URP6 CSC and URP6 FSC samples. Items (A) and (C) correspond to the Fourier transform in radial distance and (B) and (D) to the weighted function in k\({}^{3}\). Items (C) and (F) show the spectra of the LCA interpretation. Table 9 presents the numerical results of the modeling analysis with theoretical standards. The experimental EXAFS spectra were fitted using a theoretical model of \(\alpha\)-uranophane [67]. In Figure 9A,B, the main features of the radial distribution function for sample URP6 CSC, within the radial range of 1.30-1.805 A, are associated with axial oxygen atoms. Distances between 2.00 and 2.43 A correspond to equatorial oxygen atoms, while the range from 2.5 to 3.2 A is attributed to U-Si bonding. Similar results were observed for the EXAFS spectra of the URP6 FSC sample (Figure 9C,D). The best fits are shown in Figure 9, and the corresponding fitting parameters are provided in Table 9. [37; 49] have reported that EXAFS spectra for different groups of uranium-bearing minerals exhibit characteristic spectral features. The features observed in Figure 9B,D occur at energies consistent with those reported for uranophane group minerals by [49]. These authors reported axial oxygen distances of 1.818 A, equatorial oxygen distances of 2.292-2.480 A, and a U-Si bond distance of 3.140 A for uranophane. Figure 8: 5 XANES spectra for UO\({}_{2}\) (gray line), UN6 CSC (red line), URP6 FSC (blue line), URP6 CSC (green line), and UN6 FSC (wine line). Figure 9: Left: (**A**) the Fourier transforms in radial distribution function, (**B**) k\({}^{3}\)-weighted spectra of experimental (black solid line) and fitted (blue solid line with crossed) for U L\({}_{3}\) EXAFS, and (**C**) linear combination fitting for URP6 CSC.