## 4 Conclusions  
Our study showed the adsorption efficiency of laboratory-prepared micro-scale fixed-bed columns using 1 g of avocado seed and peel tissues to remove Cd, Pb, Hg, and Ni, and this bio-adsorbent was pretreated with ethanol and ethylene glycol solutions before the retention process. Favorable adsorption with high retention capacity was found for Pb+2 for the ethanol pretreatment process at higher concentrations (120 mg/L). Even though lower removal percentages were found for Cd+2, Hg+2, and Ni+2, the adsorption equilibrium data obtained best fit the Langmuir isotherm model for the four metals under study, with regression values (R2) over 0.98. Ni showed the lowest adsorption capacities and negative R\({}_{\text{L}}\) values, suggesting deficient adsorbent development. Regeneration testing of the bio-adsorbent showed the highest recovery when using 1 mol/ L HCl solution. However, testing is needed to determine adsorption-desorption efficiencies and maximum regeneration and reuse since, nowadays, it is important to use eco-friendly and cost-effective adsorbents for the remediation of contaminants. As far as we know, this is the first study carried out using fixed bed columns with avocado seed and peel wastes for the removal of metal contamination in aqueous solutions. Nevertheless, more experimental factors are still needed, including temperature, adsorbent dose, particle size, or even other material pretreatment processes, to evaluate the adsorption capacity of this bio-adsorbent. The following supporting information can be downloaded at [[https://www.mdpi.com/article/10.3390/app142310851/s1](https://www.mdpi.com/article/10.3390/app142310851/s1)]([https://www.mdpi.com/article/10.3390/app142310851/s1](https://www.mdpi.com/article/10.3390/app142310851/s1)). Table S1: Results of Cd, Hg, Ni, and Pb adsorption isotherm tests using microscale fixed bed columns with pretreated avocado seeds and peel. Conceptualization: [PERSON] and [PERSON]; Methodology: [PERSON]-N., [PERSON] and P.Y.V.-T.; Software: [PERSON]-N., [PERSON]-N. and P.Y.V.-T.; Validation: [PERSON]-N., [PERSON]N. and P.Y.V.-T.; Formal Analysis: [PERSON]-N., [PERSON] and P.Y.V.-T.; Investigation: [PERSON], [PERSON] and [PERSON]; Resources: [PERSON], [PERSON] and H.N. ; Data Curation: [PERSON], [PERSON], [PERSON], [PERSON] and [PERSON]; Writing--Original Draft: [PERSON], [PERSON], [PERSON] and [PERSON]; Writing--Review and Editing: [PERSON], [PERSON], [PERSON] and H.N. ; Visualization: [PERSON], [PERSON] and [PERSON]; Supervision, [PERSON]; Project Administration: [PERSON]; Funding Acquisition, [PERSON], [PERSON] and H.N. All authors have read and agreed to the published version of the manuscript. This research received no external funding. The authors also thank PURE for the financial support to carry out the experiments. Not applicable. The raw data supporting the conclusions of this article will be made available by the authors on request. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Metal**} & \multirow{2}{*}{**Activator**} & \multicolumn{2}{c}{**Best**} & \multicolumn{2}{c}{**Desorption Percentage [\%]**} \\ \cline{3-6}  & & **Adsorption** & \multirow{2}{*}{**H\({}_{2}\)O**} & **HCl** & **HCl** \\  & & **Ci [mg/L]** & & **0.01 mol/L** & **1 mol/L** \\ \hline \multirow{3}{*}{Cd} & Ethanol & 20 & 21.6 & 43.9 & 89.6 \\  & Ethylene glycol & 15 & 23.2 & 44.1 & 92.1 \\ \hline \multirow{3}{*}{Ni} & Ethanol & 12 & 39.3 & 96.9 & 53.9 \\  & Ethylene glycol & 6 & 41.3 & 89.8 & 64.3 \\ \hline \multirow{3}{*}{Pb} & Ethanol & 90 & 4.0 & 28.1 & 95.1 \\  & Ethylene glycol & 60 & 1.6 & 39.1 & 97.7 \\ \hline \multirow{3}{*}{Hg} & Ethanol & 10 & 3.2 & 12.8 & 42.3 \\  & Ethylene glycol & 5 & 4.1 & 13.6 & 45.9 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Desorption test developed with water, HCl 0,01 mol/L, and HCl 1 mol/L. **Acknowledgments:** The authors thank Mira Avocado Oil for donating the ground avocado peel and seed waste used in this research.