## 3 Results and Discussions
### Stability and Pb Adsorption of the Activated Pyrolysis Plastic Char in Water  
The behavior in terms of stability, e.g., plausible leaching of organic matter and metals, of the activated char in the aqueous solution is very important due to the given potential application, i.e., as an adsorbent of contaminants in water. Accordingly, special attention was paid to the possible lixiviation of metals and organic and inorganic carbon. As the pH of the aqueous media is highly relevant to adsorption processes, the leaching effect was assessed at 3 different pH values, i.e., 3, 7, and 11. The organic content released in water was considerably diminished after thermal chemical activation, see Figure 4. The original char from the pyrolysis of the plastics performed the highest TOC release, over 120 mg\(\cdot\)L\({}^{-1}\), which supposes 12% of the mass content. However, the HCl stabilization considerably reduced this effect, although still high for an application as an adsorbent; in this SC sample, an important effect on the pH was observed, with a higher TOC release when increasing the pH. The thermal and chemical activation dramatically diminished the TOC leached, being minimal, below ca. 1 mg\(\cdot\)L\({}^{-1}\). The release of IC was also diminished with HCl stabilization and chemical activation, leaching IC below 3 mg\(\cdot\)L\({}^{-1}\). Generally, a proportional increase of IC with pH was observed due to the formation of carbonates. However, NaOH was an exception with the IC lixiviated, approx. 22 mg\(\cdot\)L\({}^{-1}\) independently of the pH. \begin{table}
\begin{tabular}{c c c c c c c} \hline
**Element** & **C** & **SC** & **SC-N2** & **SC-CO2** & **SC-NaOH** & **SC-KOH** \\ \hline Li & 14 & 16 & 29 & 28 & 10 & 8 \\ \hline Ti & 43,249 & 59,395 & 55,978 & 106,635 & 48,744 & 49,696 \\ \hline V & 17 & 18 & 26 & 37 & 13 & 66 \\ \hline Cr & 281 & 555  
The release of the leached metals during the contact of the solid samples was analyzed by measuring the metals in the aqueous solution normalized by the maximum content of metals in the solid samples. The results are represented in Figure 5. Among the metals with the highest contribution, as the previously provided analysis of the composition of the samples, Ti and Zn were not detected in the aqueous solution, probably due to the stability of their oxides, in all the pH tested [54,55]. Sb and Ba appeared in moderate concentration in the solids, and less than 10% was released in the original char, the stabilized, and the thermal treated with N\({}_{2}\) and CO\({}_{2}\). The chemical activation with NaOH and KOH led to samples that were more stable in this sense. From the rest of the metals detected in the solid, Cu and Ni, which appeared in the solids at minor concentrations, were not detected in the leaching tests. Figure 5 shows important leaching, in terms of percentage, of Mo and W; however, it should be considered that the concentration of these metals is low in the solids, range of ppb. The most hazardous metals detected in the solid samples, Cd, Hg, and Pb, were at very low concentrations. The release of Cd was undetectable in the samples with potential application as adsorbents, e.g., SC-NaOH and SC-KOH. Aqueous Hg and Pb were not registered in any sample. pH is one of the most important variables to study in adsorption processes due to the high influence it may have during the application of this technology. The behavior of the solid regarding the pH in an aqueous solution was evaluated by the determination of the pH\({}_{\mathrm{pzc}}\). Figure 6 shows the results obtained for each sample. In general terms, the samples displayed a basic character, except for SC and SC-N\({}_{2}\). The sample SC represents the char after HCl stabilization, which explains the acidic value for pH\({}_{\mathrm{pzc}}\) below 2. However, the sample SC-N\({}_{2}\) can be assumed as neutral with no significant change in pH observed in the tests, so the surface could be considered barely neutral.