## 3 Results
### Modeling Adsorption from Uranium in Solution  
#### 3.2.1 Batcth Experiment  
In the experiment performed at pH = 5, the adsorption capacities of the sediments increased rapidly as the concentration of U(VI) was increased in the solution. The fitted graph for the Langmuir and Freundlich isotherm models is presented in Figure 5A, while the parameters corresponding to each isotherm are reported in Table 4. The Langmuir adsorption model is based on several key assumptions: (a) the surface is homogeneous, (b) there's a specific number of adsorption sites, leading to saturation when all are occupied, (c) the heat of adsorption remains constant regardless of surface coverage, and (d) all sites are equivalent, with energy unaffected by other molecules. However, a limitation of this model is its assumption of uniform heat of adsorption and monolayer formation on a homogeneous surface. Most surfaces are heterogeneous, with multiple adsorption sites that lead to variations in adsorption energy [60]. Figure 5: Adsorption isotherms for U(VI) at pH = 5 (**A**) and pH = 7 (**B**). \begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Sample**} & \multicolumn{8}{c}{**Mineral Phase (\%)**} \\ \cline{2-13}  & **Quartz** & **Calcite** & **Albite** & **Anorthite** & **Sanidine** & **Hematite** & **Natrolite** & **Kaolinite** & **Montmorillonite** & **Uranophane** & **Weeksite** \\ \hline Sand a & 42.2 & 7.6 & - & 13.5 & 25 & - & 7.1 & 4.6 & LOD & - & - \\ Silt b & 36 & - & 16 & - & 44.3 & 2 & - & 2.1 & LOD & - & - \\ Clay b & 14.6 & - & 20 & - & 24.1 & 6.3 & - & 34.4 & \textless{}1 & - & - \\ Uranium & & & & & & & & & & 84 & \\ mineral b & & & & & & & & & & & \\ \hline \hline \end{tabular}  
* a [31] b present work, LOD = below detection limit.  
\end{table} 
Table 3: Phase analysis of the mineral based on data obtained by XRD. The Langmuir model's correlation factor R\({}^{2}\) > 0.90 was observed, while the Freundlich model's R\({}^{2}\) < 0.6. Therefore, it was concluded that the adsorption of U(VI) from the uranyl nitrate solution at pH = 5 was well described by the Langmuir model. In the case of the experiment carried out at pH = 7, an erratic behavior of the adsorption capacities of the sediments was obtained as the concentration of U(VI) in the solution increased. It was then deduced that other mechanisms must be predominating over the surface adsorption phenomenon. The uranium concentration values of the experiment at pH = 7 were attempted to be fitted with the [PERSON] and Freundlich isotherm models; the parameters corresponding to each isotherm are shown in Table 4. The fitting quality parameters are very low and are shown in italics. The null fit of both isotherm models corroborates that uranium at neutral pH is being removed from the aqueous solution by other mechanisms. What can be established is that at pH = 5, there was greater removal of U(VI) with Q\({}_{\rm e}\) values \(\sim\) 40 \(\times\) 10\({}^{3}\) mg kg\({}^{-1}\), while for adsorption at pH = 7, values of Q\({}_{\rm e}\) barely exceeded 30 \(\times\) 10\({}^{3}\) mg kg\({}^{-1}\). The Langmuir model predicts a maximum Q value of 44.8 \(\times\) 10\({}^{3}\) mg kg\({}^{-1}\) for adsorption at pH = 5 (see Table 4). The phase composition of the sediments is shown in Table 3. In the batch adsorption of U(VI) on feldspars, a dependence on pH has been reported in the range 5 to 7, where the maximum removal is reached, while the presence of dissolved CO\({}_{2}\)(g) promotes the precipitation of uranium above pH = 7 [61]. In general, the adsorption of U(VI) in sediments containing a significant amount of clays occurs through two pathways: by cation exchange and by strong adhesion at the edges of the oxides in the minerals, forming surface complexes with ionizable hydroxyl groups [62]. The cation exchange process is favored at low pH and ionic strength by the formation of an external sphere complex between uranium and the surface with fixed charges.