## 3 Results and Discussion
### Effect of the Addition of Biochar Materials on Selenium Mobility in Soil  
The distribution of Se concentrations in soil and _Pileum_ plants at different concentrations and proportions (including the control) is shown in Figure 1. Th initial Se concentration in non-treated soil with no addition of biochar materials was 0.854 mg/kg. After artificially contaminating the soil with three doses of Se (1 mg/kg, 2 mg/kg, and 3 mg/kg), an average Se value of 1.221 mg/kg was registered. Then, after adding biochar for 3 mg/kg-dose treatments, the Se level decreased in soil from 1.985 mg/kg to 1.628 mg/kg, and this concentration continued to decrease when adding activated biochar to reach 1.554 mg/kg in presence of 50 g AB and 1.336 mg/kg using 10 g AB (Figure 1). Thus, for Se bioremediation in soil, the biochar addition contributed to Se immobilization, but the activated biochar was more efficient for Se immobilization due to its decreased level in the soil. This proves the efficiency of activated biochar in enhancing Se bioremediation in soil compared to biochar owing to its improved physical structure and better performance for Se sequestration. Biochar has a rudimentary porous structure, relatively neutral pH values, aromatic components, and active surface functional groups acting as heavy metals immobilizers in soil via different mechanisms such as physical adsorption, ion exchange, electrostatic interaction, complexation, and precipitation [30]. On the other hand, activated biochar demonstrated better performance than pristine biochar such as increased specific surface area, surface energy, and pore volume which enhanced the physical adsorption as well as the van der Waals interactions with heavy metals on the surface of the activated biochar [31]. Also, other mechanisms such as the increase in cation exchange capacity related to acidic oxygen functional groups such as -COOH and -OH when soil pH is lower than biochar pH at point of zero charges (6.6 and 9.0 for biochar and activated biochar, respectively) enhances the interaction of biochar with metals. [PERSON] et al. [32] declared that the ion exchange process is more effective than physical adsorption. Moreover, metals can be immobilized by forming stable complexes with oxygen functional groups such as -COOH, -COH, and -OH present at the surface of biochar [33]. Complexation can be generated between the positively charged metal cation and the C=O ligand of the oxygen functional group [34]. Another possible mechanism of metal immobilization in the soil is its co-precipitation with alkaline biochar to form insoluble phosphates and carbonates in the soil [35]. Finally, biochar, when highly negatively charged, improves electrostatic  
\begin{table}
\begin{tabular}{c c c} \hline \hline  & **B** & **AB** \\ \hline \hline \multicolumn{3}{c}{Textural properties} \\ \hline \hline \multicolumn{3}{c}{SBET (m\({}^{2}\) g\({}^{-1}\))} & 177 & 590 \\ \hline \hline \multicolumn{3}{c}{Vt (cm\({}^{3}\) g\({}^{-1}\))} & - & 0.34 \\ \hline \hline \multicolumn{3}{c}{Vt (cm\({}^{3}\) g\({}^{-1}\))} & 0.11 & 0.23 \\ \hline \multicolumn{3}{c}{Vm (cm\({}^{3}\) g\({}^{-1}\))} & - & 0.11 \\ \hline \hline \multicolumn{3}{c}{Physicochemical properties} \\ \hline \hline pH & 5.9 & 8.6 \\ \hline pHPZC & 6.6 & 9.0 \\ \hline C (\%) & 75.4 & 89.1 \\ \hline H (\%) & 3.5 & 0.6 \\ \hline N (\%) & 0.9 & 0.2 \\ \hline S (\%) & 0.5 & 0.0 \\ \hline O (\%) & 19.7 & 10.1 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Textural and physicochemical properties of biochar and activated biochar prepared as a soil amendment.  
attraction with metal cations depending on the soil pH, valence, and ionic radius of metals and the biochar point of zero charges [36; 37].