## 3 Results and Discussion  
In Figure 1 one can see the SEM microphotographs of the AP and PAP samples before and after the sorption process. As can be noticed, the materials are characterized by the presence of various pores and diversified surfaces. The PAP compared to AP has a cavernous structure due to the changes during the temperature decomposition of organic compounds. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Model** & **Equation** & \\ \hline \multirow{2}{*}{Dubinin–Radushkevich} & \(q_{e}=q_{d}\text{exp}(-K_{ad}t^{2})\) & (6) \\ \cline{2-3}  & \(\epsilon=RTln\left(1+\frac{1}{C_{r}}\right)\) & (7) \\ \hline \end{tabular} Where \(C_{r}\)—metal ion concentration in equilibrium (mg/dm\({}^{3}\)); \(q_{e}\)—sorption capacity in equilibrium (mg/g); \(q_{\text{max}}\)—maximum sorption capacity (mg/g); \(b\)—Langmuir constant (dm\({}^{3}\)/mg); \(K_{r}\)—Fecundlich constant (mg\({}^{1-(1/n)}\)(dm\({}^{3}\))\({}^{1/n}\)g\({}^{-1}\)); \(n\)—heterogeneity coefficient; \(K_{r}\)—constant binding equilibrium responsible for the maximum binding energy (dm\({}^{3}\)/g); \(B\)—constant associated with the sorbent heat (J/mol); \(R\)—gas constant (8.314) mol/(K); \(T\)—temperature (K); \(b\)—Temkin isotherm constant; \(qa\)—theoretical isotherm saturation capacity (mg/g); \(K_{ad}\)—Dubinin-Radushkevich isotherm constant (mol\({}^{2}\)/kJ\({}^{2}\)); \(t\)—D-R isotherm constant.  
\end{table} 
Table 1: _Cont._ \begin{table}
\begin{tabular}{l c} \hline \hline
**Model** & **Equation** \\ \hline Pseudo first-rate order & \(log(q_{e}-q_{t})=log_{ge}-\frac{k_{1}}{2303}t\) & (8) \\ \hline Pseudo second-rate order & \(\frac{t}{q_{t}}=\frac{1}{k_{2}q_{t}^{2}}+\frac{t}{q_{e}}\) & (9) \\ \hline Weber–Morris & \(q_{t}\)—\(K_{id}t^{0.5}+I\) & (10) \\ \hline \end{tabular} Where \(k_{1}\)—pseudo-first-order kinetics constant (1/min); \(k_{2}\)—pseudo-second-order kinetics constant; \(K_{id}\)—the intra-particle diffusion rate constant (mg/g min\({}^{0.5}\)); \(I\)—intercept of the line in the Weber–Morris model.  
\end{table} 
Table 2: Linearized equation of sorption kinetic models. Studying the EDS (energy dispersive spectroscopy) analysis, the main occurring elements in the materials are oxygen and carbon, with potassium, silicon and phosphorus are also present. Additionally, an analysis of the materials after the sorption process shows the presence of chromium (Figure 1A-C) and cadmium ions (Figure 1E,F) on the material surface. This result is proof of adsorption occurrence on both used materials and ions. Providing the CHN (carbon, hydrogen, nitrogen) analysis, it was possible to calculate the percentage of the occurring main elements. Using the dry state of the sample, the results for the unmodified avocado peel are presented in Table 3.  
As can be seen, the carbonization process led to an increase in the mass fraction of carbon and a decrease in the mass fraction of oxygen in the sample, which is in accordance with literature data [8; 17; 18]. To gain the information connected with the specific surface area, a BET (Brunauer-Emmett-Teller isotherm) analysis was performed. The surface area of the avocado peel was equal to 4.57 m\({}^{2}\)/g and 9.70 m\({}^{2}\)/g for the pyrolyzed material. Due to the pyrolysis time, there is a possibility that the present micropores widened up, leading to the low value of the specific surface area, which is in accordance with the microporous structure of the PAP [19]. Figure 1: SEM-EDS microphotographs of AP (**A–C**) and PAP (**D–F**). The FTIR (Fourier transform infrared spectroscopy) spectrum provided in Figure 2 of PA prior and after metal ion sorption exhibits various characteristic peaks of the cellulous structure that are absent in the PAP version of the material, suggesting that the pyrolysis process transformed avocado peel into biochar. Additionally, changes in the peaks suggest the occurrence of bonding between the material surface and the metal ions after the adsorption process.