# Biochars as a Solution for Silver Removal and Antimicrobial Activity in Aqueous Systems
## 3 Results and Discussion
### Adsorption Studies
#### 3.4.5 XPS Study of Ag Adsorption  
The chemical composition of the biochars surface after Ag\({}^{+}\) adsorption was analyzed using X-ray photoelectron spectroscopy (XPS). The XPS spectra confirmed the presence of C, N, O, and Ag in all the biochar samples, indicating that these elements are key components of the biochar surface. The elemental composition data reveal that the ARHB sample has a higher percentage of nitrogen and oxygen compared to the other biochars (see Table 6). This suggests a greater presence of functional groups, such as amines, hydroxyl, and carboxyl groups, which are known to play a critical role in adsorption processes. The increased number of these functional groups likely enhances ARHB's ability to adsorb silver ions, resulting in its higher adsorption capacity. This is further supported by the fact that ARHB shows the highest amount of silver, reinforcing the idea that its surface functionalization contributes to a more efficient adsorption process compared to the other samples.  
The detection of the Ag 3d peak in the XPS spectra is a significant observation, as it confirms the successful adsorption of silver ions onto the biochar surfaces. The binding energies associated with the Ag 3d\({}_{5/2}\) and Ag 3d\({}_{3/2}\) orbitals, shown in Figure 10, serve as reliable indicators of silver's presence [48; 49].  
However, accurately determining the oxidation state of silver can be challenging due to the minimal chemical shifts observed in XPS spectra. Differences in binding energy reflect changes in electron density due to the oxidation state, with Ag\({}^{+}\) showing slightly higher binding energy due to the reduced electron density around the silver nucleus.  
Specifically, the binding energies for the RHB and ARHB samples are observed at approximately 368.1 eV (Ag 3d\({}_{5/2}\)) and 374.0 eV (Ag 3d\({}_{3/2}\)). In contrast, the CB sample exhibits slightly shifted peaks, with binding energies near 368.8 eV (Ag 3d\({}_{5/2}\)) and 374.9 eV (Ag 3d\({}_{3/2}\)). This shift suggests a difference in the chemical environment of the adsorbed  
\begin{table}
\begin{tabular}{l l l l l} \hline \hline
**Sample** & **C, at.\%** & **O, at.\%** & **N, at.\%** & **Ag, at.\%** \\ \hline CB/Ag & 94.86 & 4.01 & 0.88 & 0.25 \\ \hline RHB/Ag & 83.00 & 13.82 & 1.01 & 0.16 \\ \hline ARHB/Ag & 80.78 & 14.40 & 4.47 & 0.35 \\ \hline \hline \end{tabular}
\end{table}
Table 6: The atomic surface elemental content in the biochars.  
silver, likely due to the phenomenon of \"charge transfer states\", as discussed in previous studies [50]. These values are more likely consistent with the presence of ionic silver (Ag\({}^{+}\)). For the RHB and ARHB samples, both the peak separation (6.0 eV) and the binding energy positions suggest the presence of metallic silver (Ag\({}^{0}\)) [51]. These observations highlight the complex nature of Ag\({}^{+}\) ion adsorption on the biochars, affirming the formation of metallic Ag clusters on the surfaces of RHB and ARHB.  
The findings establish a basis for further research into the antibacterial properties and effectiveness of the biochars. Preliminary studies focused on the ARHB material, which showed the highest adsorption capacity for silver ions.