## 3 Results and Discussion
### Determination of Bio-adsorption Isotherms  
The Langmuir and Freundlich isotherm models were applied in the present study (Table 3). For the Langmuir model, adsorbents with the highest possible qm and a high value of K\({}_{\text{L}}\) are the most desirable for their use in metal sequestration [83]. The adsorption isotherms for Cd, Hg, Pb, and Ni ions on the avocado adsorbent are plotted in Figure 4. The experimental data fitted the Langmuir model better than the Freundlich model in all cases. Our results showed that the avocado bio-adsorbent performed differently for each metal adsorption, regardless of pretreatment. From Langmuir model calculations, Cd and Ni showed the lowest adsorption capacities (qm) and negative R\({}_{\text{L}}\) values, which suggest an inefficient development of the adsorbent to these metals' retention. Aside from the qm values for Hg being low, the R\({}_{\text{L}}\) values demonstrate a favorable adsorption process [111]. Pb had the highest qm values, and its R\({}_{\text{L}}\) was in the favorable range for adsorption. Regression values (R\({}^{2}\)) indicate that the adsorption process for the four metals studied fits the Langmuir model more accurately. (Table 3 and Figure 4), in which all R\({}^{2}\) values were above 0.99. The higher R\({}^{2}\) values in the Langmuir isotherm model show the monolayer adsorption of these metals onto the surface with a finite number of identical sites [87]. Therefore, it can be assumed that a single metal molecule is adsorbed at each site, and each site might fit a single adsorbed molecule [82]. This kind of adsorption process has homogeneous and constant energy [112, 113]. Contrary to our results, [PERSON] et al. [76] showed significantly higher adsorption capacities for Cd and Pb with the Langmuir model when using unmodified and acid-modified powdered avocado seed. Results were 162.58-177.99 mg/g and 802.23-932.08 mg/g for Cd and Pb, respectively. Other composite mineral adsorbents composed of clinoptilolite, feldspars, and Portland cement have been used for the removal of Cd and Pb from contaminated water, showing maximum adsorption capacities of 932.08 mg/g for Pb and 177.99 mg/g for Cd [87]. The Langmuir isotherm model is based on the hypothesis that all adsorption active sites are equivalent and that the ability of a molecule to bind to the surface is independent of whether there are nearby occupied positions. Furthermore, adsorption is restricted to a monolayer, and there are no lateral interactions between the adsorbate molecules [114]. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Metal**} & \multirow{2}{*}{**Pretreatment**} & \multicolumn{4}{c}{**Langmuir**} & \multicolumn{4}{c}{**Freundlich**} \\ \cline{3-8}  & & **qm [mg/g]** & **K\({}_{\text{L}}\) [L/mg]** & **R\({}^{2}\)** & **K\({}_{\text{F}}\)** & **1/n** & **R\({}^{2}\)** \\ \hline \multirow{3}{*}{Cadmium} & Ethanol & 0.332 & \(-\)5.889 & 0.996 & 0.255 & 0.253 & 0.590 \\  & Ethylene glycol & 0.176 & \(-\)0.818 & 0.929 & 0.125 & 0.454 & 0.443 \\ \cline{1-1}  & Ethanol & 0.043 & 15.194 & 0.999 & 0.030 & 0.267 & 0.887 \\ \cline{1-1}  & Ethylene glycol & 0.036 & 13.653 & 0.988 & 0.024 & 0.282 & 0.906 \\ \cline{1-1}  & Ethanol & 2.573 & 11.501 & 0.999 & 2.216 & 0.066 & 0.712 \\ \cline{1-1}  & Ethylene glycol & 1.711 & 2.270 & 0.999 & 1.354 & 0.075 & 0.970 \\ \cline{1-1} Nickel & Ethanol & 0.209 & \(-\)1.280 & 0.989 & 0.307 & \(-\)0.091 & 0.532 \\ \cline{1-1}  & Ethylene glycol & 0.097 & \(-\)1.981 & 0.992 & 0.150 & \(-\)0.131 & 0.579 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Calculated maximum adsorption capacity (qm) values for Cd, Hg, Pb, and Ni ions and parameters of Langmuir and Freundlich models using the peel and seed avocado adsorbent. In the analysis of Freundlich model calculations, 1/n values demonstrated probably favorable adsorptions for all metals except for Ni. Other studies using avocado seed adsorptions are also reported in the literature.