Molecular dynamics reveals functional group control of Rhodamine B hydrochar adsorption

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This study elucidates the molecular mechanisms of Rhodamine B (RHB) dye removal from aqueous solution using hydrochar adsorbents through comprehensive molecular dynamics simulations. Three model hydrochars were evaluated across a wide range of concentrations: mixed oxygen-containing functional groups (HC1), pure aromatic carbon (HC2), and hydroxyl-enriched (HC3). HC1 achieved the highest adsorption efficiency (94.5%) and maximum capacity (24.8–620.3 mg g ⁻¹ ), outperforming HC2 (86.1%, 25.4–475.1 mg g ⁻¹ ) and HC3 (92.6%, 20.9–459.5 mg g ⁻¹ ). Both Langmuir and Freundlich isotherms describe the adsorption behaviour, indicating favorable monolayer adsorption on heterogeneous surfaces. Molecular-level analysis identifies van der Waals contacts and π – π stacking as the primary driving forces, with hydrogen bonding providing secondary stabilization. These results demonstrate that a diverse distribution of oxygen-containing functional groups, rather than pure hydroxyl enrichment, optimises hydrochar for cationic dye removal, offering molecular design guidelines for high-performance biomass-derived adsorbents in wastewater treatment.