## 3 Results
### Mono-Metal Adsorption Capacity  
The adsorption capacity of almond shell biochar and solution pH as a function of the initial metal ion concentration, in the case of mono-metallic adsorption experiments, are summarized in Table 3. It is clearly shown that biochar removed with great efficiency Sr\({}^{2+}\), Mn\({}^{2+}\) and Pb\({}^{2+}\) at the lower initial concentrations between 5 mg/L and 25 mg/L, reaching maximum values of 99.1%, 97.8% and 99.9%, respectively. At higher initial concentrations the uptake of metals by the solid material decreased, pointing to the progressive saturation of active sites on its surface. Nevertheless, for Pb\({}^{2+}\) adsorption efficiency was remarkable even at the higher concentrations of 100 mg/L, attaining a value of 98.7%. A critical parameter governing the surface charge of the adsorbent and the ionization of the metals in the solution is the pH. Table 3 shows that pH was alkaline (between 8.2 and 10.5) throughout the adsorption process of all three metal cations Sr, Mn and Pb, favoring their electrostatic attraction on biochar surface, or their complexation with negatively charged functional groups on the surface. Also, the alkaline pH could enhance deprotonation at the solid surface, creating more negative sites for increased adsorption of cations through surface complexation. These findings agree with previous reports [25]. Additionally, the small drop in pH with increasing initial concentration suggests that precipitation of hydroxyl species was less favored [26]. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multicolumn{2}{c}{**Initial Ion**} & \multicolumn{4}{c}{**Removal Efficiency (\%)**} & \multicolumn{4}{c}{**pH After Adsorption**} \\ \cline{2-9}
**Concentration (mg/L)** & **Sr\({}^{2+}\)** & **Mn\({}^{2+}\)** & **Pb\({}^{2+}\)** & **As\({}^{5+}\)** & **Sr\({}^{2+}\)** & **Mn\({}^{2+}\)** & **Pb\({}^{2+}\)** & **As\({}^{5+}\)** \\ \hline
1 & 98.9 & 97.5 & 99.9 & - & 9.8 & 10.5 & 9.7 & 8.2 \\
5 & 93.2 & 90.9 & 99.7 & 2.2 & 9.6 & 10.4 & 9.5 & 8.0 \\
10 & 99.1 & 97.8 & 99.2 & 29.3 & 9.5 & 10.0 & 9.4 & 7.9 \\
25 & 68.8 & 97.8 & 99.3 & 24.4 & 9.3 & 9.6 & 9.3 & 7.9 \\
50 & 58.5 & 97.5 & 99.1 & 41.2 & 9.1 & 8.9 & 9.3 & 8.0 \\
75 & 57.2 & 80.1 & 98.7 & 45.2 & 8.9 & 8.6 & 9.3 & 8.0 \\
100 & 49.0 & 53.4 & 98.7 & 49.4 & 8.6 & 8.2 & 9.2 & 8.0 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Removal efficiency of heavy metals during mono-metal adsorption. In order to investigate the relationship between equilibrium concentration and sorption capacity of biochar, the [PERSON] and Freundlich models were employed. Adsorption isotherms, as previously defined by Equations (3) and (4), are indicated in Figure 2 and model parameters are included in Table 4. These results show that the [PERSON] model fitted accurately the experimental data of Sr\({}^{2+}\) and Mn\({}^{2+}\) with correlation coefficients 0.945 and 0.977, respectively, implying a monolayer adsorption mechanism of these metals on the adsorbent surface. On the other hand, for Pb\({}^{2+}\) the best model to fit the experimental data, suggesting a multilayer adsorption mechanism, was that of Freundlich with correlation coefficient 0.984. The maximum adsorption capacity of almond shell biochar for Sr\({}^{2+}\) was 12.3 mg/g, for Mn\({}^{2+}\) was 23.8 mg/g and for Pb\({}^{2+}\) was 24.7 mg/g. These values are comparable to those obtained from similar biochar materials [3; 7; 8; 15; 16; 17; 18]. The behavior of the As\({}^{5+}\) ion was different to those of Sr\({}^{2+}\), Mn\({}^{2+}\) and Pb\({}^{2+}\), because As\({}^{5+}\) is an anionic heavy metal. From Table 3 it can be observed that adsorption of As\({}^{5+}\) by the biochar was quite poor, particularly at the lower initial concentrations. Removal efficiency increased gradually with As\({}^{5+}\) concentration gradient, reaching a maximum value of 49.4% at 100 mg/L.