## 3 Results
### Multi-Metal Adsorption Capacity  
In order to investigate the competition between ions, when the four metals coexisted in the solution and how this could affect the sorption of each metal by the biochar material studied, multi-metal adsorption experiments were carried out, as described in the experimental section. Table 5 compares the removal efficiency of each metal and solution pH, as a function of initial ion concentration and adsorbent dose. These data clearly show that the uptake of each metal increased in the presence of the other ions (see Table 3) and the removal efficiency became higher as the total ion flux density increased. For an adsorbent dose of 4 g/L, even at the highest initial concentration of 100 mg/L efficiency values ranged from 97% to 99.6%. The ratio of multimetal-to-monontal adsorption efficiency in this case was greater than unity, specifically 1.97, 1.86, 1.02 and 1.97 for Sr2+, Mn2+, Pb2+ and As5+, respectively. This behavior reveals that there was no competition between these ions for sorption sites on the solid phase and that functional groups on the biochar surface were sufficient for a high adsorption capacity. The stronger affinity of biochar for one metal ion over another may be attributed to the higher atomic weight, ionic radius, electronegativity and hydrolysis constant of this metal [1,12]; however, the adsorption sequence does not follow always the same order as these properties, which is also supported by other studies [1]. The majority of previous investigations reported a reduction in adsorption capacity under multi-metal conditions, due to competitive adsorption [1,10,12,13], and only a few reported the opposite trend for binary systems [6]. The effect of pH on multi-metal adsorption efficiency was the same as in the case of mono-metal adsorption. Table 5 shows that pH was alkaline throughout the process (ranging between 7.9 and 9.2), favoring the chemical complexation and/or electrostatic attraction of the metals on the biochar surface. Moreover, it can be observed that when the adsorbent dose was lowered, the removal efficiency remained practically unchanged for Sr, Mn and Pb ions, whereas it was improved in the case of arsenate. The adsorption isotherms, derived from the application of Langmuir and Freundlich models to the uptake of metals by the adsorbent material under multi-metal conditions,  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline  & \multicolumn{6}{c}{**Adsorbent Dose 4 g/L**} \\ \cline{2-9}
**Initial Ion** & \multicolumn{6}{c}{**Removal Efficiency (\%)**} & \multicolumn{6}{c}{**pH After Adsorption**} \\ \cline{2-9}
**Concentration (mg/L)** & **Sr2+** & **Mn2+** & **Pb2+** & **As5+** & **Sr2+** & **Mn2+** & **Pb2+** & **As5+** \\ \hline
5 & 97.9 & 99.9 & 100 & 88.4 & 9.1 & 9.1 & 9.1 & 9.1 \\
10 & 98.0 & 99.9 & 100 & 90.8 & 8.2 & 8.2 & 8.2 & 8.2 \\
25 & 98.3 & 99.9 & 99.9 & 95.0 & 8.2 & 8.2 & 8.2 & 8.2 \\
50 & 97.6 & 99.8 & 99.9 & 96.8 & 8.0 & 8.0 & 8.0 & 8.0 \\
75 & 97.4 & 99.7 & 99.9 & 97.4 & 7.9 & 7.9 & 7.9 & 7.9 \\
100 & 97.0 & 99.6 & 99.9 & 97.7 & 7.8 & 7.8 & 7.8 & 7.8 \\ \hline \multicolumn{9}{c}{**Adsorbent Dose 2 g/L**} \\ \hline
5 & 98.9 & 99.9 & 100 & 99.9 & 9.2 & 9.2 & 9.2 & 9.2 \\
10 & 94.1 & 99.9 & 100 & 99.9 & 8.2 & 8.2 & 8.2 \\
25 & 98.4 & 99.8 & 99.9 & 99.9 & 8.3 & 8.3 & 8.3 \\
50 & 94.5 & 99.2 & 99.9 & 99.9 & 8.1 & 8.1 & 8 are illustrated in Figure 3, whereas simulated parameters are compared in Table 6. The adsorption mechanism of Mn\({}^{2+}\) and As\({}^{5+}\) was the same as the one corresponding to the mono-metal system, describing a monolayer adsorption according to the Langmuir model for Mn\({}^{2+}\) and a multilayer adsorption according to the Freundlich model for As\({}^{5+}\). However, the adsorption mechanism of Sr\({}^{2+}\) and Pb\({}^{2+}\) changed to multilayer and monolayer, respectively, for the multi-metal system. Experimental equilibrium data were fitted quite accurately by the two models, with best-fitting correlation coefficients varying between 0.85 and 0.99.