_Applied sciences_  
_Article_  
Exploring the Utilisation of Natural Biosorbents for Effective Methylene Blue Removal  
**[PERSON] 1,2**(r), [PERSON] 2,*(r), [PERSON] 3(r), [PERSON] 4(r), [PERSON] 1 and [PERSON] 2(r)  
1 Low Carbon Energy and Resources Technologies Group, Faculty of Engineering, University of Nottingham, Nottingham NG7 2 TU, UK; [EMAIL_ADDRESS] (F.G.); [EMAIL_ADDRESS] (L.A.S.)  
2 Advanced Materials Research Group, Faculty of Engineering, University of Nottingham, Nottingham NG7 2 RD, UK; [EMAIL_ADDRESS]  
3 Department of Mechanical Engineering, University of Sheffield, Sheffield S3 7 RD, UK  
4 Advanced Centre of Biochemical Engineering, Bernard Katz Building, University College London, Gower Street, London WC1H 6 BT, UK; [EMAIL_ADDRESS]  
*Correspondence: [EMAIL_ADDRESS]; Tel: +44-115-951-951  
This paper presents a comprehensive analysis of the adsorbent capacity of five distinctly different biosorbents derived from untreated biomasses. The optimal adsorption capacity of seaweed (_Laminaria digitata_), horse chestnut husk, hazelnut husk, rapeseed residue, and whitewood to remove methylene blue (MB) dye was assessed by analysing the effects of particle size, pH, temperature, and initial dye concentrations. Furthermore, the adsorption kinetics, isotherms, and adsorption thermodynamics were investigated. The results showed that relatively high MB adsorption capacity was achieved by _Laminaria digitata_ (\(\sim\)180 mg/g), in addition to a reasonable MB adsorption capacity of horse chestnut husk (\(\sim\)130 mg/g), hazelnut husk (\(\sim\)110 mg/g), and rapeseed residue (\(\sim\)80 mg/g). However, whitewood provides a relatively low adsorption capacity of below 20 mg/g. The best fit with experimental results regardless of biosorbent type was a pseudo-second-order kinetic model with the lowest mean absolute percentage error (\(\epsilon\), MAPE \(<\) 2.5%) and the highest correlation coefficients (R\({}^{2}\)\(>\) 0.99). Although the pseudo-second-order kinetic model is often associated with chemisorption, the low enthalpy values (\(<\)29.30 kJ/mol) typically suggest that the adsorption process is more characteristic of physisorption, which involves weaker van der Waals forces rather than the stronger covalent bonds of chemisorption. This proposed a multi-step adsorption process involving both physisorption and chemisorption. The adsorption isotherm of Langmuir showed superior fitting results for _Laminaria digitata_ and hazelnut husk. In contrast, rapeseed residue and horse chestnut husk fit better with the Freundlich adsorption isotherm. The Langmuir adsorption isotherms showed a maximum adsorption capacity of \(\sim\)500 mg/g for _Laminaria digitata_, followed by horse chestnut husk (\(\sim\)137 mg/g), hazelnut husk (\(\sim\)120 mg/g), and rapeseed residue (\(\sim\)85 mg/g). The Gibbs free energy was negative for _Laminaria digitata_ \(<\) horse chestnut husk \(<\) hazelnut husk \(<\) 0, which suggests that the removal of MB is thermodynamically favourable, as the adsorption process occurs spontaneously. The results of the study indicate that MB dye removal using untreated biomasses has the potential to be a low-cost valorisation option in the holistic whole life cycle valorisation pathway for _Laminaria digitata_, horse chestnut husk, and hazelnut husk.  
biosorbents; methylene blue; wastewater treatment; adsorption; surface chemistry