## 5 Conclusions  
This paper presents a comparative analysis of the adsorption process interdependencies and how these are influenced the adsorption properties of five distinctly different biomass feedstocks: seaweed (_Laminaria digitata_), horse chestnut husk, rapeseed residue, hazelnut husk, and whitewood. Based on the comparative evaluation of natural biosorbents in methylene blue removal:  
* Particle size should be as small as possible for all-natural biosorbents to ensure maximum MB uptake. For this study, <106 \(\upmu\)m was found to be the optimal particle size for adsorption. * Highly acidic environments (pH < 2) resulted in low uptakes of MB for all the natural biosorbents. A pH near neutral (pH = 6) was found to give the optimal balance of MB uptake for pH adjustment of the MB solution. * The surface structure of the natural biosorbents (surface area and pore volume) had a lower impact on MB uptake than surface functional groups. * LD exhibited the greatest potential to be used as an adsorbent for MB dye, with an uptake of ~180 mg/g. Furthermore, waste biomasses HH and CH showed relatively high levels of MB removal (~127 mg/g for CH and ~110 mg/g for HH), and RS, as an agricultural waste, also had a reasonable MB removal (~80 mg/g). However, WW demonstrated the lowest MB removal (<20 mg/g). * Based on the thermodynamic analysis, the adsorption of MB on LD and RS provided a negative \(\Delta\)H\({}^{\circ}\) (~3.61 kJ/mol and ~7.36 kJ/mol), indicating an exothermic process. However, the adsorption on CH and HH showed a positive \(\Delta\)H\({}^{\circ}\) (29.30 kJ/mol, 15.48 kJ/mol), indicating an endothermic process. The MB removal by LD, HH, and CH can be a thermodynamically favourable process thanks to the negative Gibbs free \begin{table}
\begin{tabular}{l l l l l} \hline
**Biosorbents** & **Capacity (mg/g)** & **Ref.** & **Biosorbents** & **Capacity (mg/g)** & **Ref.** \\ \hline Orange peel & 13.9–50.0 & [8,17,111] & Coffee husk & 72.9–111 & [122] \\ Guava leaf & 295.0 & [123] & Tea waste & 85.1 & [22] \\ Spent coffee grounds & 18.7 & [112] & Coconut husk & 99 & [3] \\ Wheat shells & 16.5–21.5 & [69] & Phosphate rock & 101.1 & [124] \\ Chitosan nanocomp. & 20.5, 111 & [4,113] & Algal waste & 104 & [41] \\ Hazelnut shell & 38.2, 41.3 & [112,125] & Dehyd. peanut hull & 108.6 & [126] \\ Oak sawdust & 38.4 & [68] & Pinecone biomass & 109.9 & [127] \\ Yellow passion fruit & 44.7 & [128] & Modified lychee seeds & 124.5 & [29] \\ Rice husk & 40.6, 40.5 & [70,129] & Caster seed shell & 158.7 & [3] \\ Coconut bunch waste & 70.9 & [130] & Lathyrus sativus husk & 98.3 & [92] \\ \hline _L. digitata_ (LD) & 176.2 * (500) & \multirow{2}{*}{This study} & C. Husk (CH) & 127.9 * (137) & \multirow{2}{*}{This study} \\ Rapeseed (RS) & 80.5 * (85) & & H. Husk (HH) & 109.6 * (117) & \\ \hline \end{tabular}  
* Adsorption capacities in a concentration of 200 ppm of MB solution at 30 \({}^{\circ}\)C for 60 min. The values in brackets are the maximum adsorption capacities determined by the Langmuir model.  
\end{table} 
Table 5: List of maximum MB adsorption capacities of various untreated biomasses from this study and the literature. energy (\(\Delta\)G\({}^{\circ}\)), which indicates MB adsorption on these biosorbents is spontaneous and physical in nature. * The adsorption of MB onto these biosorbents aligns strongly with the pseudo-second-order kinetic model, as shown by an R\({}^{2}\) greater than 0.99 and a MAPE between 0.1 and 2.5%. Despite this model typically suggesting chemisorption, the low enthalpy values shows at a process dominated by physisorption, leading to a probable sequential mechanism starting with physisorption and transitioning to chemisorption. * The adsorption process was described well by the Langmuir isotherms for LD and HH biosorbents and by Freundlich the adsorption isotherm for RS and CH biosorbents. Langmuir adsorption isotherms showed relatively high maximum adsorption capacities (Qo); \(\sim\)500 mg/g for LD, \(\sim\)137 mg/g for CH, and \(\sim\)117 mg/g for HH.