## 3 Results and Discussion
### Packed Bed Column Performance  
Table 10 shows the results obtained for the study of the influence of temperature and bed height on the removal efficiency of Cr (VI) and Ni (II) on PSR and YSR; for this experiment, a concentration of 100 mg/L was taken as the initial condition. The adsorption percentage was calculated using Equation (2). For the removal of Cr (VI) on PSR, a positive effect of the increase in temperature was evidenced, which can be caused by the increase in the diffusion rate of the ions in the mesopores of the adsorbent due to the formation of bonds with the groups; likewise, the increase in the bed height benefits the removal due to the greater availability of the active centers and the residence time; consequently, the metal has more time to diffuse in the solid phase [73]. Thus, the best conditions for removing Cr (VI) using PSR are a bed height of 8.14 cm and a temperature of 68 \({}^{\circ}\)C. When using YSR to remove Cr (VI), it is evident that the temperature and bed height conditions do not seem to influence the process since percentages >90% were obtained in almost all cases. Therefore, it can be concluded that the lower bed height (1.5 cm) and the ambient temperature (30 \({}^{\circ}\)C) are sufficient to favor the adsorbent-adsorbate contact area. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & & \multicolumn{2}{c}{**PSR**} & \multicolumn{2}{c}{**YSR**} \\ \hline
**Model** & **Parameter** & **Cr (VI)** & **Ni (II)** & **Cr (VI)** & **Ni (II)** \\ \hline \multirow{3}{*}{\
\begin{tabular}{c} Pseudo-first-order \\ \end{tabular} } & \(q_{c1}\) (mg/g) & 9.65 & 18.54 & 75.8 & 105.3 \\  & \(k_{1}\) (min\({}^{-1}\)) & 4.92 & 0.12 & \(1.7\times 10^{12}\) & \(8.2\times 10^{12}\) \\  & R\({}^{2}\) & 0.99 & 0.96 & 0.78 & 0.97 \\ \hline \multirow{3}{*}{
\begin{tabular}{c} Pseudo-second-order \\ \end{tabular} } & \(q_{c2}\) (mg/g) & 9.73 & 19.24 & 94.6 & 112.9 \\  & \(k_{2}\) (g/mg min) & 2.99 & 0.012 & 3.49 & 2.24 \\ \cline{1-1}  & R\({}^{2}\) & 0.99 & 0.98 & 0.98 & 0.99 \\ \hline \hline \end{tabular}
\end{table} 
Table 8: Kinetic model fitting parameters. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & & \multicolumn{2}{c}{**PSR**} & \multicolumn{2}{c}{**YSR**} \\ \hline
**Model** & **Parameter** & **Cr (VI)** & **Ni (II)** & **Cr (VI)** & **Ni (II)** \\ \hline \multirow{3}{*}{
\begin{tabular}{c} Langmuir \\ \end{tabular} } & \(q_{max}\) (mg/g) & 44,021.46 & 27,775.59 & 77.5 & \(3.86\times 10^{8}\) \\  & \(b\) (L/mg) & \(3.67\times 10^{-6}\) & \(1.6256\times 10^{5}\) & 2.9 & \(3.9\times 10^{38}\) \\  & R\({}^{2}\) & 0.94 & 0.99 & 0.92 & 0.08 \\ \hline \multirow{3}{*}{Freundlich} & \(k_{F}\) (mg/g) & 0.013 & 0.14 & 54.2 & \(8.7\times 10^{-15}\) \\  & \(n\) & 0.63 & 0.99 & 12.8 & 0.49 \\ \cline{1-1}  & R\({}^{2}\) & 0.97 & 0.99 & 0.96 & 0.99 \\ \hline \hline \end{tabular}
\end{table} 
Table 9: Fitting parameters of isotherm models. For Ni (II) removal, removal efficiencies >80% were obtained at the different conditions evaluated using the two bio adsorbents; all obtained values were within the calibration curve and above the detection limit. The lowest adsorption values were obtained using YSR at 40 \({}^{\circ}\)C/3 cm and 55 \({}^{\circ}\)C/1.5 cm, respectively, with an efficiency of 81%. It is also evident that the bed height has a direct influence, thus favoring the adsorption efficiency of Ni (II) present in the solution; this is because the greater the amount of adsorbent in the column, the more binding sites are available, thus expanding the mass transfer zone [74]. When the bed height is low, axial dispersion phenomena predominate during mass transfer, which reduces the residence time and the rate of diffusion of ions from the solution to the adsorbent [73]. Thus, it was found that the optimum conditions for removing Ni (II) on PSR were a bed height of 11.4 cm and a temperature of 33 \({}^{\circ}\)C, while for YSR, they were: 43 \({}^{\circ}\)C and 9 cm for temperature and bed height respectively.