## 3 Results and Discussion
### Adsorption Kinetics  
Figure 6 shows the fit of the kinetic data to the PFO, PSO, Elovich and intraparticle diffusion models, in order to identify the stages that control the process. It was found at best experimental conditions presented at 55 \({}^{\circ}\)C, particle size 0.5 mm and 0.03 g adsorbent, the saturation time of the bioadsorbents evaluated is at 60 min and that equilibrium is reached after 120 min, indicating that the availability of active sites in the biomass is reduced due to the occupation of these by the metal ion [67; 68; 69]. The values of the fitting parameters of the \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Temperature (\({}^{\circ}\)C)**} & \multirow{2}{*}{**Particle Size (mm)**} & \multirow{2}{*}{**Adsorbent Dose (g)**} & \multicolumn{4}{c}{**Adsorption Capacity (mg/g)**} \\ \cline{3-6}  & & & **YP** & **OPB** & **CH** & **CP** & **CCR** \\ \hline
40 & 1.00 & 0.15 & 57.46 & 42.10 & 33.31 & 29.88 & 23.89 \\
70 & 1.00 & 0.15 & 58.33 & 47.50 & 33.16 & 28.07 & 22.20 \\
80 & 0.50 & 0.33 & 18.26 & 23.12 & 13.15 & 14.24 & 11.49 \\
29.8 & 0.50 & 0.33 & 22.33 & 13.93 & 14.33 & 13.77 & 11.83 \\
55 & 0.50 & 0.03 & 123.66 & 197.95 & 160.83 & 76.86 & 105.40 \\
55 & 1.22 & 0.33 & 29.80 & 9.26 & 13.00 & 12.57 & 11.60 \\
70 & 1.00 & 0.50 & 10.71 & 19.93 & 9.95 & 8.16 & 8.05 \\
55 & 0.14 & 0.33 & 14.57 & 25.71 & 15.29 & 12.33 & 12.25 \\
70 & 0.36 & 0.15 & 56.21 & 47.76 & 33.08 & 26.02 & 21.66 \\
40 & 0.36 & 0.15 & 40.07 & 29.54 & 33.31 & 29.98 & 24.55 \\
40 & 0.36 & 0.50 & 18.92 & 14.93 & 9.99 & 8.99 & 8.07 \\
55 & 0.50 & 0.62 & 15.12 & 14.08 & 8.04 & 7.03 & 7.14 \\
55 & 0.50 & 0.33 & 22.34 & 24.99 & 15.42 & 12.03 & 12.29 \\
40 & 1.00 & 0.50 & 18.92 & 18.64 & 9.99 & 8.98 & 8.59 \\
70 & 0.36 & 0.50 & 18.64 & 19.00 & 9.95 & 8.60 & 8.24 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Cd\({}^{2+}\) adsorption capacity by varying temperature, particle size and adsorbent dose. \begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{**Adsorbent**} & \multirow{2}{*}{**Adsorption Capacity**} & \multirow{2}{*}{**Conditions**} & \multirow{2}{*}{**Reference**} \\ \cline{3-3}  & & & & \\ \hline Barley husk biomass & 41.25 & pH = 7, dose = 3 g/L and temp = 30 \(\pm\) 2 \({}^{\circ}\)C & [PERSON] et al. [64] \\ Canola biomass & 25.16 & pH = 7, Co = 100 mg/L & [PERSON] et al. [65] \\ Oil palm residual biomass & 17.40 & pH = 6, dose = 0.5 g, temp = 28 \({}^{\circ}\)C & [PERSON] et al. [44] \\ Cork biomass & 14.77 & pH = 6, temp = 40 \({}^{\circ}\) & [PERSON] et al. [5] \\ Rice husk & 21.28 & pH = 6, temp = 25 \({}^{\circ}\)C, Co = 100 mg/L & [PERSON] et al. [66] \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Comparative adsorption capacities reported in previous works for different biomasses. evaluated models and their \(\text{R}^{2}\) are recorded in Table 4. Taking into account the calculated parameters, it was found that the PSO and Elovich models are the best fitting models for the experimental Cd\({}^{2+}\) adsorption data on YP and CP, this is also observed in Figure 6a,d. According to this result, the process happens by chemisorption in the system, in which the adsorption rate is limited by valence forces given by the exchange of electrons between the adsorbate and the adsorbent due to the non-homogeneous surface of the solid [70; 71; 72]. Figure 6: Fitting to Pseudo-first order, Pseudo-second order, Elovich and intraparticle diffusion adsorption kinetic models adjustment for removal of Cd\({}^{2+}\) ions on (**a**) YP, (**b**) OPB, (**c**) CH, (**d**) CP and (**e**) CCR. Figure 6b depicts that the ion removal data on OPB biomass present a good fit to all the models suggesting that the rate of adsorption depends on a mechanism that acts on the active sites on the surface of the biomass by chemisorption [57].