## 4 Discussion on Mechanisms  
### Adsorption Mechanisms on Biosorbents  
The MB adsorption over unmodified or natural biosorbents is influenced by the textural properties of biosorbents, surface chemistry of biosorbents, and specific interactions between the biosorbent and MB molecules [109; 114]. As with many other adsorption processes, the adsorption mechanisms of MB dye over biosorbents can be categorised under three steps as illustrated in Figure 10:  
* Film diffusion or interphase diffusion: MB molecules diffuse from the bulk phase to the external surface of the biosorbent;
* Pore diffusion or intraparticle diffusion: MB molecules diffuse from the pore mouth through the biosorbents pores into the immediate proximity of the internal active surface, which is the point where the MB adsorption occurs; and
* Adsorption of MB molecules on the inner/external surface of biosorbents [110; 115; 116] based on the combination mechanism of electrostatic interaction, hydrogen bonding, functional group interaction, and van der Waals forces [48]. In discussing the sorption mechanisms of MB onto these biosorbents, the thermodynamic, kinetic, and isotherm results provide a deep understanding. The thermodynamic analysis revealed that adsorption onto LD and RS is exothermic with negative enthalpy changes, indicative of physisorption, which aligns with the negative Gibbs free energy values, confirming the spontaneous nature of the process. Contrastingly, the endothermic adsorption observed for CH and HH suggests a different interaction, possibly hinting at a more complex mechanism that involves energy absorption, potentially leading to a different adsorption capacity and effectiveness for these biosorbents. While the pseudo-second-order kinetic model, which typically signifies chemisorption due to its assumption of chemical bond formation as the rate-limiting step, fits our kinetic \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Adsorbents**} & \multicolumn{4}{c}{**Langmuir Model**} & \multicolumn{4}{c}{**Freundlich Model**} \\  & **Qo (mg/g)** & **\(b\)** & **R2** & \(\varepsilon\) * (\%)** & **\(K_{F}\)** & **\(n\)** & **R2** & \(\varepsilon\) * (\%)** \\ \hline _L. digitata_ (LD) & 500.0 & 0.028 & 0.997 & 0.7 & 16.66 & 1.24 & 0.996 & 3.1 \\ C. Husk (CH) & 137.0 & 0.185 & 0.994 & \textgreater{}10 & 43.43 & 3.95 & 0.999 & 0.37 \\ H. Husk (HH) & 117.7 & 0.178 & 0.999 & 7.35 & 34.67 & 3.58 & 0.813 & \textgreater{}10 \\ Rapeseed (RS) & 84.8 & 0.118 & 0.998 & \textgreater{}10 & 28.44 & 4.47 & 0.988 & 2.6 \\ \hline \hline \end{tabular}  
* \(\varepsilon\) is the mean absolute percentage error (MAPE). The unit of \(\text{K}_{F}\) is (mg/g/(mg/L)\({}^{n}\).  
\end{table} 
Table 4: Adsorption isotherm parameters in Langmuir and Freundlich models. Figure 10: The primary adsorption steps of MB over biosorbents. Image adapted from [PERSON] et al. [100] with additional data from [115; 116; 117]. data well, the low enthalpy values for all biosorbents suggest a process initially dominated by physisorption. This assertion is supported by isotherm studies, where the Langmuir model's applicability to LD and HH indicates a homogenous adsorption surface and monolayer coverage, whereas the Freundlich isotherm fit for RS and CH biosorbents points to multi-layer adsorption, a characteristic of physisorption. These findings collectively suggest a multi-step adsorption process. Initially, rapid physisorption likely occurs, driven by the favourable thermodynamics, followed by a slower chemisorption, as the kinetics suggest, possibly within a monolayer as indicated by the Langmuir isotherm.