Sustainable removal of methylene blue using minimally modified hydrochar from durian peels with experimental adsorption and density functional theory studies

This study investigated the use of hydrochar (HC) derived from durian peels as an adsorbent for removing methylene blue (MB) from an aqueous environment. HC was synthesized from durian peels using hydrothermal carbonization under varying temperature (160 – 200°C) and time (2 – 6 h) conditions. The optimal condition 180°C for 2 h (HC-180-2) was identified. HC-180-2 was evaluated in MB adsorption experiments and adsorbent characterization. It achieved a maximum MB adsorption capacity ( q ) of 51.6 mg/g at room temperature, reaching equilibrium within 150 min, and the q value increased to 59.2 mg/g at 65°C. The adsorption followed pseudo-second-order kinetics (R 2 = 0.996) and Langmuir isothermal behavior (R 2 = 0.996), indicating chemisorption on energetically uniform adsorption sites. Thermodynamic analysis yielded Gibbs free energy values ranging from -43.0 to -55.3 kJ/mol and an enthalpy change of 48.5 kJ/mol, which further confirmed the spontaneous and endothermic nature of the chemisorption process. The surface area of HC-180-2 increased from 3.04 to 6.36 m²/g compared to the biomass, confirming the chemisorption and dependence on chemical functionality rather than physical surface area. Structural characterizations revealed enhanced aromatization and functional group formation, including sulfone and ester groups. Density functional theory calculations revealed two possible HC-MB conformation with adsorption mechanisms involving hydrogen bonding, π-π stacking and π-sulfur interactions. The chemisorption nature was also confirmed through Quantum Theory of Atoms in Molecules electron density pathway analysis. While the adsorption capacity was moderate compared to chemically modified adsorbents, the minimally processed durian peels HC positioned itself as a promising green alternative for MB removal.