## 3 Results and Discussion
Analyzing the FTIR scheme provided in Figure 2, it can be seen that there is a presence of a band in 3300-3500 cm\({}^{-1}\) (AP) that is related to the stretching vibrations of hydroxyl (OH) groups, which are common in the carbohydrates and cellulose found in plant material. The band in the range 2800-3000 cm\({}^{-1}\) is associated with the stretching vibrations of aliphatic C-H bonds, which are present in lipids and carbohydrates. Region 1700-1750 cm\({}^{-1}\) corresponds to the stretching vibrations of carbonyl (C=O) groups, often \begin{table}
\begin{tabular}{l c c c} \hline \hline  & \multicolumn{3}{c}{AP (\%)} \\ \hline C & H & N & O \\ \hline
10.5 & 2.8 & 1.1 & 85.6 \\ \hline  & \multicolumn{3}{c}{PAP (\%)} \\ \hline C & H & N & O \\ \hline
76.2 & 1.8 & 3.4 & 18.6 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Percentage of Carbon, Hydrogen, Nitrogen, and Oxygen occurring in the AP and PAP. Figure 2: Fourier transform infrared (FTIR) spectra of materials prior and after the sorption process. found in esters, ketones, and aldehydes. C-O stretch (1000-1200 cm\({}^{-1}\)): this region includes stretching vibrations of C-O bonds in various functional groups like alcohols, ethers, and esters, common in plant polysaccharides. Cellulose peaks (around 1200-1100 cm\({}^{-1}\)): specific peaks in this region are indicative of cellulose, a major component of plant cell walls. The FTIR spectrum of avocado peel after pyrolysis shows changes in the above-mentioned bands due to the thermal degradation and conversion of organic matter into char. Decrease in O-H and C-H bands: the broad O-H and C-H bands decrease in intensity or shift due to the removal of volatile organic compounds during pyrolysis. Increase in C=O bands: An increase in the intensity of C=O bands is observed due to the formation of carbonyl-containing compounds during pyrolysis. The peaks associated with cellulose decreased in intensity, indicating the breakdown of cellulose during the pyrolysis process. Studying the FTIR scheme, it can be observed that after the sorption process of both Cd and Cr ions, there are significant differences in peak size and shifting, showing that in the process of removal, chemical bonding took place. The chemical composition of used biomass is the main factor by which pyrolysis is affected. The thermal analysis of avocado peel was conducted using TGA (Figure 3). The TGA and DTG curves of AP show two main stages of weight loss: The first one starts in the 15 th minute of the process, which corresponds to the temperature range between 300 and 400 \({}^{\circ}\)C. The second one starts after 20 min, which equals 450-500 \({}^{\circ}\)C and corresponds to the oxidation of solid carbon material and residual volatile matter. These results are similar to those obtained by [PERSON] et al. In their manuscript, the authors used _Citrus unshiu_ peel for the thermogravimetric analysis [20]. [PERSON] et al. reported that the pectin and hemicellulose decomposition is in the range between 250 and 350 \({}^{\circ}\)C, which shows similarities to the results obtained in this manuscript [21]. Figure 3: TG analysis of avocado peel (AP). To acquire information about the pH for highest sorption capacity and metal ion removal, the PZC (point of zero charge) was computed for both AP and PAP material. The point of zero charge (PZC) is defined as the pH value at which the surface charge of the adsorbent becomes neutral. To determine the PZC, a graph is created by plotting the pH variations in the solution against the initial pH. The PZC is identified as the pH at which this plot intersects the pH value of 0. In this case, we found that the point of zero charge for AP was around 6.81 and 9.84 for PAP (Figure 4). These results show that there were basic groups resulting in the presence of a positive charge on the biochar surface. When the pH of the solution is below the PZC, the material's surface carries a positive charge resulting from the protonation of acidic groups.