## 3 Results and Discussion
### Adsorption Studies
#### 3.2.7 XPS--Analysis  
With surface-sensitive techniques such as XPS, it is useful to examine the carbon XPS spectra of various materials to determine the type and relative amount of chemical groups at their surfaces. In most organic XPS spectra, we would expect to find four functional groups, and the presence of more of one relative to another would be evidence of a chemical change at the surface.  
The surfaces of the studied biomaterials were analyzed both before and after adsorption of Cu(II) ions. Evidence for these modifications is apparent from changes observed in the C1s photoelectron spectra. To gain insight into the concentration of functional groups present before and after Cu adsorption, a curve-fitting procedure was performed on the C1s  
Figure 12: FTIR spectra of LA and waste material L and after Cu(II) sorption: LAS and LS, respectively. In the spectra of L, LAS, and LS, only the bands with frequency changes in comparison with LA are explicitly indicated with the corresponding numbers. The dash-dot lines note the bands that undergo a change in both frequency and intensity.  
peak for each material under investigation. The results of this fitting procedure revealed the following:  
* The major C1 peak, located at approximately \(\sim\)285 eV, corresponds to the C-C bond.
* The second C2 peak, observed at \(\sim\)286.5 eV, is associated with the C-OH, C-O-C, or C-N bonds.
* The C3 peak, situated at \(\sim\)288.0 eV, corresponds to the O-C=O bond (Figure 13 for the samples LA and LAS as representative for all plant materials).  
These peaks can be attributed to C atoms in the forms of C-C, C-O (alcoholic or ether), C-O-C (ether), and O-C=O (carboxylate groups or ester groups), respectively.  
The quantitative findings are summarized in Table 5. The amount of alcoholic, ether, carboxylate, and ester groups changed after copper adsorption, indicating their involvement in the Cu(II) adsorption process. These findings suggest the formation of surface complexes.  
The results from Table 6 confirm the presence of copper on the studied plant materials after the adsorption.  
In summary, the findings are consistent with IR analysis, confirming the involvement of hydroxyl, carboxylate, ether, and ester groups in the adsorption of copper(II) ions onto the biosorbent surfaces. Similar group participation has been reported by other researchers for copper biosorption on plant materials and bacteria [42; 43; 44]. The biosorption of copper ions on the studied plant materials is a complex process, likely involving a combination of surface complexation, electrostatic attraction, and ion exchange.  
Figure 13: C1s photoelectron spectra/binding energies (B.E.) of LA (**a**) and LAS (**b**).  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Sorbent** & \begin{tabular}{c} **C1** \\ **C-C, \%** \\ \end{tabular} & \begin{tabular}{c} **C2** \\ **C-O-C, C-OH, C-N, \%** \\ \end{tabular} & \begin{tabular}{c} **C-C=O, \%** \\ \end{tabular} & \
\begin{tabular}{c} **O1** \\ **C-O, \%** \\ \end{tabular} &
\begin{tabular}{c} **O2** \\ **C-O, \%** \\ \end{tabular} \\ \hline MO & 87.1 & 10.5 & 2.4 & 34.6 & 65.4 \\ MOS & 84.2 & 10.8 & 5.0 & 62.3 & 37.7 \\ M & 84.3 & 9.9 & 5.8 & 63.3 & 36.7 \\ MS & 81.6 & 12.3 & 6.1 & 46.7 & 53.3 \\ LA & 83.6 & 13.9 & 2.5 & 48.2 & 51.8 \\ LAS & 83.9 & 10.6 & 5.5 & 51.3 & 48.7 \\ L & 87.9 & 8.9 & 3.2 & 61.4 & 38.6 \\ LS & 84.4 & 10.9 & 4.7 & 61.7 & 38.3 \\ \hline \hline \end{tabular} \% from total carbon and oxygen.  
\end{table}
Table 5: Calculated relative amounts of carbon functional groups present on the surface of MO and MOS, M and MS, LA and LAS, and L and LS.