## 3 Results  
### Characterization of Sawdust  
An ash and dry matter content of 1.552% was obtained, which guarantees that the biomass obtained is of good quality and does not contain heavy metals in its structure [28]. This ash content may be due to the presence of aluminum oxides, silica, calcium, magnesium, and iron in its structure, which could function as active centers of adsorption [29]. SEM microphotographs of eucalyptus sawdust at 20 and 50 \(\upmu\)m magnification, taken to show the variation in shape and superficial texture of the adsorbent [19], are shown in Figure 1. A widely heterogeneous structure is observed with the presence of fiber fragments, blocks with appreciable and irregular circular indentations, heterogeneous spirals, fragments embedded square grids, heterogeneous spirals, and very pronounced pores forming porous structures, which contribute to augment the area of contact. The fiber fragments shown are due to the presence of cellulose, hemicellulose, and lignin in the sawdust's structure, which benefits the adsorptive capacities of the material due to the presence of COOH, NH\({}_{\text{R}}\), and OH groups, which have the capacity to retain Pb (II) ions with the formation of the complex [30]; The presence of pore formation, which is a mechanism of Pb (II) adsorption, was also observed. The FTIR analysis of the eucalyptus sawdust before and after the adsorption (Figure 2) was performed to know the surface chemistry of the biomass and to establish the functional groups present in the structure of the biomaterial and involved in the uptake of the \begin{table}
\begin{tabular}{c c c} \hline \hline
**Model** & **Equation** & **Parameters** \\ \hline \multirow{6}{*}{Langmuir} & \multirow{6}{*}{\(\text{q}_{\text{z}}=\text{q}_{\text{max}}\frac{\text{K}_{\text{c}}\text{C}_{\text{c}}}{ \text{1+K}_{\text{c}}\text{C}_{\text{f}}}\)} & \multirow{6}{*}{\
\begin{tabular}{c} \(\text{q}\text{t}\text{ (mg/g): is the amount of metal adsorbed on the bioadsorbent. \\ Cf (mg/L): residual metal concentration in solution. \\ qmax (mg/g): maximum adsorption capacity of the Langmuir model. \\ L: is Langmuir’s constant and can be correlated with the variation of the adsorption area and the porosity of the adsorbent. \\ \hline \multirow{6}{*}{[PERSON]} & \multirow{2}{*}{\(\text{q}_{\text{z}}=\text{k}_{\text{c}}\text{C}_{\text{e}}^{\text{1/n}}\)} & \multirow{2}{*}{
\begin{tabular}{c} \(\text{n}\text{ (mg/g): represents the adsorption intensity and \\ indicates the heterogeneity of the active sites. \\ qt (mg/g): the amount of metal adsorbed at equilibrium. \\ Ce (mg/L): residual concentration of the metal in solution. \\ \end{tabular} } \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Isothermal models. Figure 1: Scanning electron microscopy (SEM) microphotographs of eucalyptus sawdust. metal [31]. In the spectrum of unsaturated eucalyptus a high intensity peak at 3391.94 cm\({}^{-1}\) was identified as related to the stretching of the O-H- group present in the phenolic and carboxylic structure [27]; the widening and symmetry of this band indicates the presence of strong hydrogen bonds, corresponding to the pectin functional groups [32]. The bands 2918.85 cm\({}^{-1}\), 2847.49 cm\({}^{-1}\), and 2515.04 cm\({}^{-1}\) are related to the stretching of the C-H bond present in aromatic rings, corresponding to the functional groups of cellulose and lignin [33]; the signals 1651.76 cm\({}^{-1}\), and 1514.51 cm\({}^{-1}\) correspond to the stress C=C, present in aromatic rings corresponding to the lignin functional groups [34]. The high-intensity peaks at 1454.35 cm\({}^{-1}\) and 1424.18 cm\({}^{-1}\) show the C-H deformation corresponding to the hemicellulose functional groups [35].