## 3 Results and Discussions
### Stability and Pb Adsorption of the Activated Pyrolysis Plastic Char in Water
The activation with CO\({}_{2}\), NaOH, and KOH led in all the cases to activated carbonaceous materials with a strong basic character describing an order SC-NaOH > SC-KOH > SC-CO\({}_{2}\). The behavior of the SC-NaOH was remarkably outstanding, able to raise the pH of the aqueous medium to pH\(\sim\)11, whatever the initial pH was. Figure 4: Organic and inorganic leaching of the char from plastic pyrolysis before and after activation at different pH values. Experimental conditions: C\({}_{\mathrm{solid}}\) = 1 g\(\cdot\)L\({}^{-1}\); time contact, 24 h.  
The adsorption isotherms were obtained for each sample, taking samples at 3 different times, 1 h, 2 h, and 24 h, i.e., equilibrium conditions. The results are displayed in Figure 7. If the adsorption performances are compared, the following adsorption capacity order is  
Figure 5: Metal leaching in the char from pyrolysis before and after activation at different pH. Figure 6: Determination of the pH\({}_{\rm{pxc}}\) by the drift method of the char from plastic pyrolysis before and after activation. observed: SC-NaOH > SC-KOH > C > SC-CO\({}_{2}\) > SC-N\({}_{2}\) > SC. The original char displayed a moderate adsorption capacity; however, its direct application lacks interest, and a stabilization step via washing is required to remove the highly easily released organic carbon. The stabilized sample, SC, led to the lowest adsorption performance, but the treatment was efficient in removing the release of undesirable organic matter. The activation with CO\({}_{2}\) and N\({}_{2}\) raised the adsorption ability concerning the SC sample; nevertheless, it was still insufficient to reach the results obtained with the native char. The chemical activation with NaOH and KOH considerably improved the adsorption results, leading to the solids with the highest adsorption capacities, especially in the case of NaOH activation. Nonetheless, the pH evolution during these experiments was a crucial variable since over pH = 6.0, according to the acid-base equilibria, the Pb\({}^{2+}\) ions start to precipitate as Pb(OH)\({}_{2}\)[56], resulting in the adsorption results initially registered being overestimated. This phenomenon has been reported in the literature with adsorbents that display a strong basic character, contributing to a precipitation mechanism of Pb rather than removal by adsorption of the ions onto the surface of the solid [57]. Figure 8 shows the variation of the pH for each solid depending on the initial Pb concentration tested. As depicted in this figure, the samples that were more affected by the pH rise and, therefore, by the precipitation phenomena were those with a strong basic character, i.e., SC-CO\({}_{2}\), SC-NaOH, and SC-KOH, with SC-NaOH the sample with the highest contribution of precipitation as the final pH was not lower than 6.9 with the maximum Pb concentration tested. As previously shown in Figure 6, the sample SC-NaOH had a very strong basic acid character. Accordingly, a blank test to assess the precipitation contribution was carried out in the absence of solid, but adjusting the pH to the values registered during the equilibrium adsorption for each concentration tested. The removal monitored by precipitation was used to correct the adsorption capacities isotherms, as shown in Table 3. Low lead concentrations were highly affected by the precipitation effect specially in adsorption tests performed by SC-N\({}_{2}\), SC-CO\({}_{2}\), SC-NaOH, and SC-KOH. Only the SC sample did not show precipitation of lead since the pH of the liquid-solid suspension was between 4 and 5 for all concentrations tested. Figure 7: Adsorption isotherms of Pb onto the char from plastic pyrolysis before and after activation. The dashed lines represent the fitting to the [PERSON] isotherm model.