## 3 Results
### Mechanisms of Mono-Metal/Multi-Metal Adsorption  
Some of the complex mechanisms, which are responsible for the adsorption of heavy metals by biochars are ion exchange, electrostatic attraction, chemical complexation on biochar surface, surface precipitation with inorganic species and electron coordination between metal and adsorbent [6; 10; 14; 15; 18; 22]. In order to investigate the mechanisms controlling the sorption of Sr\({}^{2+}\), Mn\({}^{2+}\), Pb\({}^{2+}\) and As\({}^{5+}\), FTIR and XRD analyses were performed before and after metals adsorption. FTIR spectra are represented in Figure 4. Prior to adsorption, Figure 4a shows small peaks at wave numbers 834 cm\({}^{-1}\) and 876 cm\({}^{-1}\), which are attributed to the C-H bending vibration of aromatic compounds and carbonates CO\({}_{3}\)\({}^{2-}\)[9]. Peaks at 1006 cm\({}^{-1}\) and 1226 cm\({}^{-1}\) are associated with C=C stretching of alkene groups and ethers, respectively. The intense band at 1402 cm\({}^{-1}\) represents the C-O asymmetric stretching vibration in the -COO\({}^{-}\) and CO\({}_{3}\)\({}^{2-}\) group, or O-H stretching of alcohols. The peaks at 1574 cm\({}^{-1}\) and 2356 cm\({}^{-1}\) indicate vibrational stretching of C=C from cycloalkenes and O=C=O, respectively [27]. These data show that almond shell biochar was rich in oxygen and negatively charged functional groups, which play a significant role in the adsorption of cationic metals. In the case of mono-metal adsorption, when Sr\({}^{2+}\) was adsorbed on the biochar surface, from Figure 4b it can be observed that all peaks were shifted to higher wave numbers, revealing that CO\({}_{3}\)\({}^{2-}\), C-O, C=C and O=C=O functional groups were involved in the sorption of Sr\({}^{2+}\). Furthermore, the disappearance of the 1402 cm\({}^{-1}\) band after Sr\({}^{2+}\) adsorption implies some interaction between C-O or O-H groups with Sr\({}^{2+}\). The new peaks appearing at 1144 cm\({}^{-1}\) and 1464 cm\({}^{-1}\) are related to the C-O stretching of alcohols/aliphatic ethers and the C-H stretching of alkanes, respectively. The FTIR spectrum obtained after Mn\({}^{2+}\) adsorption, in Figure 4c, is similar to the one representing Sr\({}^{2+}\) adsorption. The differences are the new broad band at 1170 cm\({}^{-1}\), assigned to C-O stretching of alcohols or aromatic esters, as well as the disappearance not only of the 1402 cm\({}^{-1}\) band, but also of the 1226 cm\({}^{-1}\) band, indicating additional interactions between C-O functional groups on biochar surface and Mn\({}^{2+}\). From Figure 4d, which presents the FTIR spectra of the material after adsorption of Pb\({}^{2+}\), the shift of the peaks to higher wave numbers is also noticeable. Once again, the 1402 cm\({}^{-1}\) band did not appear, whereas new peaks at 1144 cm\({}^{-1}\) and 1438 cm\({}^{-1}\) correspond to C-O stretching of alcohols/aliphatic ethers and O-H groups from carboxylic acids. These findings suggest that Pb\({}^{2+}\) was adsorbed mainly to C-O and O-H groups from alcohols and carboxylic compounds. In the case of arsenate adsorption, Figure 4e illustrates a new peak centered at 920 cm\({}^{-1}\), which is characteristic of the stretching vibration of As-O bonds [15], confirming the adsorption of As on the biochar surface. New peaks were also observed at 1150 cm\({}^{-1}\) and 1450 cm\({}^{-1}\), attributed to vibrational stretching of C-O from alcohols/aliphatic ethers and C-H from alkanes, respectively. Due to the anionic character of arsenate, the FTIR spectrum was quite different than those of Sr, Mn and Pb cations adsorption. Several peaks assigned to bonds from stretching of aromatic compounds (C-H), carboxylic compounds (COOO\({}^{2-}\), C-O), alkenes (C=C) and alcohols (O-H) disappeared, while others assigned to bonds from the stretching of ethers (C-O), cycloalkenes (C=C), or carbon dioxide (O=C=O) were shifted to higher wave numbers.