## 3 Results and Discussion
### Adsorption Thermodynamics  
In adsorption thermodynamics, the equilibrium relation is between the liquid phase and the adsorbed phase. The thermodynamic properties are thus described by the enthalpy and entropy of the system. From the slope and intercept of the Van't Hoff plot (Figure 6),  
Figure 5: Impact of initial MB concentrations (50–200 ppm) on (**a**) removal efficiency at 30 \({}^{\circ}\)C for 24 h. Adsorption capacity isotherms at the initial MB concentrations of 200 ppm (**b**), 100 ppm (**c**), and 50 ppm (**d**). the standard enthalpy (\(\Delta\)H\({}^{\circ}\)/R) and standard entropy (\(\Delta\)S\({}^{\circ}\)/R) of the kinetic data can be determined [107]. The enthalpy of adsorption provides a measure of the heat released or adsorbed during the adsorption process. A change of entropy during the adsorption process indicates the affinity of the adsorbent to the adsorbate [108]. The thermodynamic parameters for the biosorbents used in this study (Table 2) were obtained from the slope of the Van't Hoff plots in Figure 6. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Adsorbents** & \(\Delta\)**H\({}^{\circ}\)** & \(\Delta\)**S\({}^{\circ}\)** & \(\Delta\)**G\({}^{\circ}\)**293 K** & \(\Delta\)**G\({}^{\circ}\)**308 K** & \(\Delta\)**G\({}^{\circ}\)**313 K** \\  & **k/mol** & **k/mol K** & **k/mol** & **k/mol** & **k/mol** \\ \hline _L. digitala_ (LD) & \(-\)7.36 & \(-\)0.004 & \(-\)6.157 & \(-\)6.115 & \(-\)6.074 \\ Rapeseed (RS) & \(-\)3.61 & \(-\)0.014 & 0.666 & 0.812 & 0.959 \\ C. Husk (CH) & 29.30 & 0.096 & 1.059 & 0.094 & \(-\)0.870 \\ H. Husk (HH) & 15.48 & 0.057 & \(-\)1.306 & \(-\)1.879 & \(-\)2.453 \\ Whitewood (WW) & 6.33 & 0.006 & 4.482 & 4.419 & 4.356 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Gibbs free energy change \(\Delta\)G\({}^{\circ}\), standard enthalpy \(\Delta\)H\({}^{\circ}\), and standard entropy \(\Delta\)S\({}^{\circ}\) for RS, LD, WW, and biochars. Figure 6: Van’t Hoff plot for MB adsorption on (**a**) LD, (**b**) CH, (**c**) HH, (**d**) RS, and (**e**) WW in a 200 pm of MB solution at 20, 30, 40 \({}^{\circ}\)C.  
LD and RS showed a negative \(\Delta\)H\({}^{\circ}\) (\(-\)3.61 kJ/mol and \(-\)7.36 kJ/mol). This indicates their adsorption processes are exothermic [6]. However, CH, HH, and WW showed positive \(\Delta\)H\({}^{\circ}\) (29.30 kJ/mol, 15.48 kJ/mol, and 6.33 kJ/mol) values, indicating an endothermic adsorption [69]. By analysing the level of \(\Delta\)H\({}^{\circ}\), further information regarding the adsorption mechanisms can be determined. Chemical adsorption is the primary adsorption mechanism when \(\Delta\)H\({}^{\circ}\) is between 40 and 200 kJ/mol [109]. When \(\Delta\)H\({}^{\circ}\) is below 25 kJ/mol, the acting forces are van der Waals forces, which can be attributed to physical adsorption [109]. The electrostatic interaction between biosorbents and dyes results in non-covalent bonds with an enthalpy range of 20-80 kJ/mole. Based on the \(\Delta\)H\({}^{\circ}\) levels in Table 2, the MB adsorption onto LD, RS, HH, and WW was driven by physical adsorption [95]. In contrast, CH \(\Delta\)H\({}^{\circ}\) levels are inbetween physisorption and chemisorption. Thus the process for CH could be presented as a physical adsorption enhanced by a non-covalent bonding between dye and biosorbents. A negative \(\Delta\)S\({}^{\circ}\) for RS (\(-\)0.014 kJ/mol K) and LD (\(-\)0.004 kJ/mol K) were observed while the \(\Delta\)S\({}^{\circ}\) values were positive for the biosorbents CH (0.096 kJ/mol K), HH (0.057 kJ/mol K), and WW (0.006 kJ/mol K). The positive \(\Delta\)S\({}^{\circ}\) of the system indicates the affinity of adsorbent for MB [69], as well as an increase in the randomness at the interface adsorbent/adsorbate during adsorption [7, 66]. A positive \(\Delta\)G\({}^{\circ}\) value was observed for all samples (0 \(<\) RS \(<\) CH \(<\) WW) at 20 \({}^{\circ}\)C. With increasing adsorption temperature, the \(\Delta\)G\({}^{\circ}\) ranking changed to CH \(<\) 0 \(<\) RS \(<\) WW.