## 3 Results and Discussion
### Adsorption Isotherms  
The effect of the initial concentration on the removal of Cd\({}^{2+}\) was evaluated by varying the initial concentration in intervals of 25 and 100 mg/L at pH 6 in order to understand the driving forces behind the adsorption process. Figure 7 shows the adjustment of the experimental data to [PERSON], [PERSON] and Dubinin-Radushkevich's models after 24 h of contact in a batch system; the adjustment parameters of the models are shown in Table 5.  
The R\({}^{2}\) parameter close to 1 for all the biomasses in Langmuir and Freundlich models suggested that the materials are well described by these isotherms; however, the sum of squares provides additional information to consider. For YP and OBP biomasses, the SS was lower for Langmuir model, while for CH, CP and CCR biomasses, the Freundlich model best fits the data. Hence, the YP and OBP follows a monolayer adsorption and the CH, CP y CCR a multilayer process [71].  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Kinetic Model**} & \multirow{2}{*}{**Parameters**} & \multicolumn{5}{c}{**Values**} \\ \cline{3-7}  & & **YP** & **OPB** & **CH** & **CP** & **CCR** \\ \hline \multirow{4}{*}{PFO} & q\({}_{\text{e1}}\) & 17.00 & 16.52 & 3.51 & 13.31 & 10.09 \\  & k\({}_{1}\) & 0.14 & 0.28 & 0.04 & 0.14 & 0.06 \\  & SS & 10.43 & 0.86 & 1.92 & 9.17 & 5.08 \\  & R\({}^{2}\) & 0.90 & 0.99 & 0.76 & 0.90 & 0.67 \\ \hline \multirow{4}{*}{PSO} & k\({}_{2}\) & \(1.2\times 10^{3}\) & \(2.8\times 10^{3}\) & 3.58 & 13.98 & 10.58 \\  & q\({}_{\text{e2}}\) & 18.087 & 16.99 & 4.27 & 617.52 & 136.38 \\  & SS & 4.70 & 0.06 & 1.92 & 9.17 & 5.07 \\  & R\({}^{2}\) & 0.97 & 0.99 & 0.80 & 0.96 & 0.85 \\ \hline \multirow{4}{*}{Elovich} & \(\beta\) & 0.51 & 1.13 & 2.94 & 0.89 & 0.93 \\  & \(\alpha\) & 90.81 & \(1.2\times 10^{4}\) & 16.69 & 702.92 & 36.71 \\ \cline{1-1}  & SS & 1.27 & 1.74 & 2.82 & 10.52 & 6.89 \\ \cline{1-1}  & R\({}^{2}\) & 0.99 & 0.99 & 0.86 & 0.98 & 0.75 \\ \hline \multirow{4}{*}{Intraparticle Diffusion} & k\({}_{3}\) & 15.79 & 15.98 & 2.70 & 12.28 & 7.82 \\ \cline{1-1}  & SS & 5.04 & 3.52 & 6.58 & 2.03 & 4.25 \\ \cline{1-1}  & R\({}^{2}\) & 0.78 & 0.94 & 0.54 & 0.75 & 0.68 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Adjustment parameters of the Pseudo-first order, Pseudo-second order, Elovich and Intraparticle difussion’s models of Cd\({}^{2+}\) adsorption onto YP, OPB, CH, CP and CCR.  
According to the values of n, the adsorption process on Cd\({}^{2+}\) is favorable on YP, CH, CP and CCR, and that bioadsorbents have a good affinity for ion removal [5; 76]. The values lower than 8 kJ/mol reported for the mean energy of adsorption of ions by sorbate (E) calculated for the Dubinin-Radushkevich model suggest that the process is controlled mostly by the fission absorption mechanism with strong interactions between active centers and Cd\({}^{2+}\) ions [77]; This model presents a good fit, since the calculated maximum adsorption capacity (qDR) values are the closest to the experimental ones with R\({}^{2}\geq 0\).The above assumes that the bioadsorbents under study have a heterogeneous structure, which is required by the Dubinin-Radushkevich isotherm [78].  
Figure 7: Fitting the [PERSON], [PERSON]’s models of Cd\({}^{2+}\) adsorption ions on (**a**) YP, (**b**) OPB, (**c**) CH (**d**) CP and (**e**) CCR.