## 3 Results and Discussion
### Bio-adsorption Percentage  
The initial contaminant metals' content was evaluated in the raw material before each pretreatment process. Results were below the quantification limit for Cd and Hg, and 0.51 \(\pm\) 0.02 and 0.39 \(\pm\) 0.02 mg/kg for Pb and Ni, respectively. After the pretreatments, the contaminant metal content of the biomaterials was below the quantification limits for all tested metals. Thus, these pretreatments with ethanol and ethylene glycol not only help \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Range of Wavelength (cm\({}^{-1}\))** & **Bond** & **Functional Group** &
\begin{tabular}{c} **Wavelength Band** \\ **Identified (cm\({}^{-1}\))** \\ \end{tabular} \\ \hline
3650–3200 & O-H & Hydroxyl & 3299.15 \\ \hline
2950–2800 & C-H, C-H2 & Alkane & 2926.02 \\ \hline
1700–1740 & C=O & Carbonyl & 1743.07 \\ \hline
1640–1610 & C=C & Alkene & 1623.06 \\ \hline
1450–1375 & C-C & Alkane & 1457.96 \\ \hline
1300–1000 & C-O-C & Cellulose & 1079.15 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The main bands of functional groups identified by FTIR-ATR on both pretreated avocado biomasses. to open the surface structure of the bio-adsorbent, allowing it to receive the contaminant cations, but also to remove the cations that were bonded before. Multipoint calibration curves at different concentrations were used for quantification in both FAAS and CVAAS techniques. Linear regression coefficients (R\({}^{2}\)) were evaluated (above 0.99) (Table 2). Unlike previous studies where the percentage of removal increased with the increasing concentration of the analyte [76; 100], in our study, metal adsorption decreased when metal concentration increased for both pretreatment processes (Figure 3). For Pb, it was observed that the biomaterial had higher remotion percentages at concentrations up to 125 mg/L than the other three metals tested (Cd, Hg, and Ni), in which the optimal adsorption occurs at values lower than 35 mg/L (Figure 3). As metal concentration increased, lower retention \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Parameter** & **Cd** & **Hg** & **Pb** & **Ni** \\ \hline Equation & y = 0.4031x\(-\) 0.004 & y = 0.0009x\(-\) 0.0011 & y = 0.05104x\(-\) 0.01172 & Y=0.0118x\(-\) 0.00001 \\ R\({}^{2}\) & 0.9999 & 0.9980 & 0.9993 & 0.9999 \\ LOD (mg/L) & 0.01 & 0.005 & 0.1 & 0.1 \\ LOQ (mg/L) & 0.03 & 0.010 & 0.3 & 0.3 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Calibration curve parameters for the determination of metals using FAAS for Cd, Ni, Pb, and CVAAS for Hg. Figure 2: Photographs taken on a Nikon TU Plan Fluor 5X0.15 A/0 EPI D contrast microscope Nikon (Tokyo, Japan) of the bio-adsorbents (**a**) untreated and with the pretreatments (**b**) ethanol and (**c**) ethylene glycol. by the material was observed due to the reduction in, and an eventual competition for, the available active binding sites on the adsorbent surface [43; 107]. Furthermore, a relatively high concentration of metal ions in the solution, compared to the possible adsorption sites available, saturates the surface, leading to a decrease or even desorption of the metal. The avocado bio-adsorbent showed the highest retention capacity for Pb in ethanol pretreatment. The maximum adsorption percentage was 99.50% at an initial concentration of 85 mg/L. This decreased slightly to 95.00% at 110 mg/L, followed by a considerable decrease of up to 70.77% at 130 mg/L. The maximum removal percentage using the ethylene glycol pretreatment was 98.73% at a concentration of 52 mg/L; however, retention drastically decreased to 76.36% at 90 mg/L. Other studies have found a maximum removal percentage of 98% for Pb using pretreated biomass of avocado seeds treated with 85% phosphoric acid [73; 108], while 83% removal has been achieved with the same non-pretreated bio-adsorbent [108] when using Pb solutions with a concentration of 30 mg/L. Cd showed stable and promising removal percentages when using ethanol pretreatment. However, concentrations of the metal were not higher than 20 mg/L.