# Biochars as a Solution for Silver Removal and Antimicrobial Activity in Aqueous Systems
## 3 Results and Discussion
### Adsorption Studies
#### 3.4.4 Effect of Temperature  
Silver adsorption experiments were performed at temperatures of 298 K, 313 K, and 333 K to investigate the effect of temperature (Figure 9). At higher temperatures, the amounts adsorbed for three biochars decreased, indicating that adsorption is exothermic and energy in the form of heat is liberated during the adsorption process. Adsorption thermodynamics and calculating the corresponding thermodynamic parameters (\(\Delta\)G\({}^{0}\), \(\Delta\)H\({}^{0}\), and \(\Delta\)S\({}^{0}\)) is an integral part of the study of adsorption and plays an important role in estimating the adsorption mechanism. The equilibrium distribution constant (K) characterizes the balance between silver (Ag) in the solid and liquid phases [46,47].  
\[\mathrm{K_{d}=Q_{e}/C_{e}} \tag{10}\]  
\[\Delta\mathrm{G^{0}=-RT\ lnKd} \tag{11}\]  
\[\mathrm{lnK_{d}=\Delta S^{0}/R-\Delta H^{0}/RT} \tag{12}\]  
where \(\mathrm{K_{d}}\) is the equilibrium constant, R is the gas constant (J mol\({}^{-1}\) K\({}^{-1}\)), and T is the temperature (K). The enthalpy change, \(\Delta\)H\({}^{0}\), and the entropy change, \(\Delta\)S\({}^{0}\), are determined from the slope and intercept of the plot of lnKd versus 1/T, respectively (Figure 9). The values of \(\Delta\)G\({}^{0}\), \(\Delta\)H\({}^{0}\), and \(\Delta\)S\({}^{0}\) parameters are summarized in Table 5.  
\(\Delta\)G\({}^{0}\) signifies the feasibility of the process, while \(\Delta\)S\({}^{0}\) reflects the disorderliness at the solid-liquid interface during adsorption. The negative \(\Delta\)G\({}^{0}\) values observed for the adsorption of silver ions indicate that the process occurs spontaneously in all studied biochars,  
\begin{table}
\begin{tabular}{c c c} \hline
**Adsorbent** & **Adsorption Capacity, mg g\({}^{-1}\)** & **References** \\ \hline Biochar from biosolids & 48.31 & [41] \\ \hline Biochar from wood waste & 17.8 & [42] \\ \hline Biochar from sunflower husks & 22.3 & [42] \\ \hline Biochars from sunflower husks + & 14.0 & [42] \\ rapeseed pomace & 18.6 & [43] \\ \hline Rice husks & 1.6 & [44] \\ \hline Biomaterials based on _Populus_ & 2.94.2 & [45] \\ _alba_ L. and _Robinia pseudoacacia_ L. & 23.1 & Present study \\ \hline \end{tabular}
\end{table}
Table 4: Adsorption capacity of various biosorbents for silver ions.  
Figure 9: Variation in the equilibrium constants as a function of temperature.  
underscoring its thermodynamic favourability and suggesting potential applications across various fields.  
The dominance of physisorption is evident from the \(\Delta\)G\({}^{0}\) values, which fall within the range of 0 to 20 kJ mol\({}^{-1}\), indicating that physical adsorption is the prevailing mechanism. The negative \(\Delta\)H\({}^{0}\) values confirm the exothermic nature of Ag\({}^{+}\) adsorption, further supporting its physical character. As temperatures increase, the weakening of hydrogen bonds and van der Waals interactions diminish the physical affinity between the sorbent's active sites and silver ions, resulting in a decrease in removal efficiency.