## 3 Results
### Mechanisms of Mono-Metal/Multi-Metal Adsorption
These results are consistent with the degree of graphitization of biochar, offering a larger number of \(\pi\) donors [10] for electron coordination with As\({}^{5+}\) ion. Furthermore, they suggest chemical complexation between As\({}^{5+}\) and functional groups of the adsorbent, such as CO\({}_{3}\)\({}^{2-}\), C-O and O-H. Also, the small increase in solution pH values with adsorption efficiency of As\({}^{5+}\), previously presented, reveals that As\({}^{5+}\) ions could have interacted with O-H groups of the solid surface through ligand exchange. As concerns multi-metal adsorption, Figure 4f shows that there was a great difference between the FTIR spectra before and after simultaneous adsorption of Sr, Mn, Pb and As metals. The intensity of transmittance was enhanced, peaks attributed to bonds C-H, C=C, C-O and O-H from vibrational stretching of aromatic, alkene, ether, alcohol and carboxylic compounds disappeared, while two new peaks were formed at wave numbers 1150 cm\({}^{-1}\) and 1444 cm\({}^{-1}\), representing C-O and C-H stretching of alcohols/aliphatic ethers and alkanes, respectively. These changes in the post-sorption spectra indicate that the multi-metal adsorption process occurred via complexation of the heavy metals with ionized C-H, C=C groups, or oxygen-containing functional groups C-O, O-H and possibly electron coordination between the metals and the biomass matrix. These observations are in agreement with past investigations on various metals adsorption by biochars of different origins [6; 10; 13; 14; 17]. A comparison of the XRD spectra of biochar before and after mono-metal or multi-metal adsorption is made in Figure A1. Although the interpretation of such spectra is difficult, due to the amorphous carbon of biochars, several important inorganic species could be identified. Original biochar (Figure A1a) was enriched in Ca and K mineral phases,  
Figure 4: FTIR spectra of (**a**) biochar before adsorption, (**b**) after Sr\({}^{2+}\) adsorption, (**c**) after Mn\({}^{2+}\) adsorption, (**d**) after Pb\({}^{2+}\) adsorption, (**e**) after As\({}^{5+}\) adsorption, (**f**) after multi-metal adsorption. appearing in the forms of calcite and arcanite. Minerals hydroxplapatite, niter, quartz, kalsilite and magnesioferrite were identified in smaller amounts. After adsorption of each heavy metal Sr, Mn, Pb and As separately, or simultaneously in the multi-metal system, it can be observed that new diffraction peaks appeared, corresponding to mineral phases strontium (SrCO\({}_{3}\)), rhodochrosite (MgCO\({}_{3}\)), groutite (MnOOH), cerussite (PbCO\({}_{3}\)) and As-hydroxylapatite (Ca\({}_{5}\)(P\({}_{2.27}\)As\({}_{0.73}\)O\({}_{4}\))\({}_{3}\)OH. This indicates that anions CO\({}_{3}\)\({}^{2-}\), PO\({}_{4}\)\({}^{3-}\) and OH\({}^{-}\) released from the biochar reacted with these heavy metals, forming mineral precipitates onto the surface. Carbonates of lead precipitates have been reported by previous studies on mono/competitive adsorption of lead with other metals [13; 14]. From the above analysis, it can be speculated that the immobilization mechanisms of Sr\({}^{2+}\), Pb\({}^{2+}\), Mn\({}^{2+}\) and As\({}^{5+}\) were chemical complexation between metal ions and functional groups on the biochar surface, electron coordination between metals and adsorbent and mineral precipitation onto the surface. The sorption mechanisms are schematically shown in Figure 5.