## 3 Results and Discussions
### Stability and Pb Adsorption of the Activated Pyrolysis Plastic Char in Water
Experimental conditions: C\({}_{\text{Pb0}}\) = 10–500 mg L\({}^{-1}\); C\({}_{\text{adsorbent}}\) = 0.5 g.L\({}^{-1}\); T = 20 \({}^{\circ}\)C.  
Different models frequently used in liquid phase adsorption were checked, i.e., the two-parameter models of [PERSON] [58] and [PERSON] [59] and the three-parameter Sips' model [60]. Table 4 summarizes the fitting results of each model for the case of equilibria conditions after subtracting the contribution of precipitation. [PERSON]'s model produced, in general terms, the worst fitting results in those samples in which the saturation level was close to being reached. The Freundlich equation considers that the energy of adsorption on a homogeneous surface is independent of surface coverage, making it impossible to predict the maximum adsorption capacity [61]. [PERSON]'s model, which implies the ideal assumptions of preferential monolayer coverage and energetic homogeneity of adsorption sites, fitted quite well to almost all the experimental series. The [PERSON] model provided, in contrast, the best fitting results in all the samples. [PERSON] model is the result of a combination of both the [PERSON] and [PERSON] models, which is deduced to predict the heterogeneity of the adsorption systems as well as to circumvent the limitations associated with the increased concentrations of the adsorbate of the [PERSON] model [62]. In turn, [PERSON] leads  
Figure 8: Evolution of the adsorption equilibria pH with the initial concentration of Pb. \begin{table}
\begin{tabular}{l l c c c c c c} \hline \hline \multirow{2}{*}{**Sample**} & \multirow{2}{*}{**q\({}_{\text{e}}\) (mg\(\cdot\)g\(-\)1)**} & \multicolumn{6}{c}{**C\({}_{\text{i}}\) (mg\(\cdot\)L\(-\)1)**} \\ \cline{3-8}  & & **10** & **20** & **50** & **100** & **200** & **500** \\ \hline \multirow{3}{*}{C} & Experimental & 18.78 & 34.00 & 82.56 & 163.2 & 248.8 & 348.0 \\ \cline{2-8}  & Corrected & 13.95 & 27.02 & 82.56 & 163.2 & 248.8 & 348.0 \\ \hline \multirow{3}{*}{SC} & Experimental & 0.602 & 2.801 & 11.41 & 21.61 & 44.80 & 94.05 \\ \cline{2-8}  & Corrected & 0.602 & 2.801 & 11.41 & 21.61 & 44.80 & 94.05 \\ \hline \multirow{3}{*}{SC-N\({}_{2}\)} & Experimental & 15.64 & 36.56 & 73.60 & 117.2 & 124.1 & 154.0 \\ \cline{2-8}  & Corrected & 0.537 & 1.427 & 16.92 & 117.2 & 124.1 & 154.0 \\ \hline \multirow{3}{*}{SC-CO\({}_{2}\)} & Experimental & 19.26 & 34.42 & 85.88 & 171.5 & 247.6 & 294.0 \\ \cline{2-8}  & Corrected & 0.709 & 1.298 & 3.758 & 22.03 & 243.2 & 294.0 \\ \hline \multirow{3}{*}{SC-NaOH} & Experimental & 9.001 & 31.38 & 85.60 & 170.3 & 357.6 & 862.4 \\ \cline{2-8}  & Corrected & 0.373 & 1.129 & 3.859 & 31.19 & 94.64 & 315.8 \\ \hline \multirow{3}{*}{SC-KOH} & Experimental & 20.32 & 33.72 & 98.98 & 193.4 & 371.8 & 701.0 \\ \cline{2-8}  & Corrected & 0.011 & 0.215 & 2.187 & 35.30 & 82.51 & 701.0 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Experimental adsorption capacities and corrected adsorption capacities after precipitation. to the production of an expression that allows for predicting the saturation capacity at high concentrations, i.e., the q\({}_{\text{S}}\) value shown in Table 4.  
In addition to extensively studied materials in the fields of wastewater treatment [22; 23; 24; 25], several researchers have utilized the char of pyrolysis as a raw material to produce activated carbon and use it as an adsorbent of lead from aqueous solutions. Table 5 recompiles some of the published works. The production of activated carbon as an adsorbent of lead from aqueous solutions has been explored mainly from biomass wastes, with differences in the properties and lead adsorption capacities of produced activated carbons. The variability can be attributed mainly to differences in the composition of the raw materials but also the differences in pyrolysis and activation conditions. Although biomass is the main precursor to producing activated carbons from waste, the production of these adsorbents from tire waste or mixtures of biomass and plastics has also been reported in the literature.