## 4 Conclusions  
The biochar derived from the avocado peel effectively removed both Cr(VI) and Cd(II) metal ions from the aqueous solutions. This process is best described by the pseudo-second-order kinetic model for both used materials and ions. The equilibrium adsorption isotherms with the highest fit are Freundlich for Cr(VI) ions and Dubinin-Radushkevich for Cr(II) ions. The process was very rapid at the beginning and most of the metal ions were removed in the first minutes of the process for both AP and PAP. The maximum sorption capacity \(q_{e}\) values for Cr(VI) and Cd(II) were equal to 6 mg/g; 9.7 mg/g for AP and 7.1 mg/g; 10.28 mg/g for PAP, showing a slightly higher capacity of biochar compared to untreated peel. According to the kinetics, the mechanism adsorption of both used metal ions and materials might involve chemisorption and C\(\pi\)-cation interaction. The values obtained from  
\begin{table}
\begin{tabular}{l l l} \hline \hline \multirow{2}{*}{**Biochar Material**} & \multicolumn{2}{c}{**Sorption Capacity (mg/g)**} \\ \cline{2-3}  & **Cr(VI)** & **Cd(II)** \\ \hline Sawdust & 3.2 [27] & \\ \hline Modified date seed biochar & 27 [28] & \\ \hline Coconut tree sawdust & 3.46 [29] & \\ \hline Oakwood char & 0.37 [30] \\ \hline _Miscanthus sacchariflorus_ & 11 [31] \\ \hline Buffalo weed & 11.63 [32] \\ \hline Pig manure biochar & 107 [33] \\ \hline \hline \end{tabular}
\end{table}
Table 6: _Cont._  
\begin{table}
\begin{tabular}{l l l l l l l} \hline \hline \multicolumn{6}{c}{**Elution [\%]**} \\ \hline \hline \multirow{2}{*}{Eluent} & \multicolumn{2}{c}{Cycle 1} & \multicolumn{2}{c}{Cycle 2} & \multicolumn{2}{c}{Cycle 3} \\ \hline \hline \multicolumn{6}{c}{AP} \\ \hline \hline  & Cr(VI) & Cd(II) & Cr(VI) & Cd(II) & Cr(VI) & Cd(II) \\ \hline Citric acid & 75 & 81 & 79 & 74 & 71 & 78 \\ \hline Acetic acid & 52 & 64 & 61 & 66 & 55 & 53 \\ \hline Sodium chloride & 24 & 31 & 27 & 29 & 21 & 26 \\ \hline Water & 0.4 & 0.3 & 0.1 & 0.2 & 0.1 & 0.1 \\ \hline \multicolumn{6}{c}{PAP} \\ \hline \hline Citric acid & 77 & 79 & 75 & 82 & 81 & 72 \\ \hline Acetic acid & 55 & 58 & 57 & 63 & 61 & 55 \\ \hline Sodium chloride & 20 & 19 & 22 & 21 & 27 & 19 \\ \hline Water & 0.2 & 0.1 & 0.3 & 0.2 & 0.2 & 0.3 \\ \hline \hline \end{tabular}
\end{table}
Table 7: Percentage of Cr(VI) and Cd(II) elution from AP and PAP with the use of different eleuents, respectively.  
sorption/desorption studies show that this material can be used multiple times without significant loss in its sorption capacity. We conclude that this material both before and after the pyrolysis process can be used as an effective, low-cost, and environmentally friendly sorbent for heavy metal removal from aqueous solutions.  
Conceptualization, J.C.; methodology, J.C. and P.S.; validation, J.C.; investigation, J.C. and P.S.; data curation, J.C. and P.S.; writing--original draft, J.C.; writing--review and editing, J.C.; visualization, P.S.; supervision, J.C. and P.S. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.  
The data presented in this study are available in the article.  
The authors declare no conflict of interest.