Mixed Oxygen Functional Groups Drive Methyl Orange Adsorption on Hydrochar: An All-Atom Molecular Dynamics Study

All-atom molecular dynamics simulations of Methyl Orange (MO) adsorption on three hydrochar models show that mixed oxygen-containing functional groups (HC1, O/C = 0.33) and hydroxyl-enriched surfaces (HC3, O/C = 0.55) both achieve high adsorption efficiencies, whereas the oxygen-lean aromatic carbon framework (HC2, O/C = 0.03) is less efficient. Average adsorption efficiencies (mean ± standard deviation across the 14 simulated loadings) are 96.0 ± 3.7% for HC1, 95.5 ± 4.4% for HC3, and 86.8 ± 4.9% for HC2. Freundlich isotherms fit the equilibrium data best (R2 = 0.83–0.96), reflecting energetically heterogeneous binding driven by van der Waals contacts, π-π stacking, and hydrogen bonding. The simulation capacities fall within the experimental range reported for biochar and hydrochar adsorbents; within the finite, non-porous, idealized cluster models investigated, the results support a molecular-scale design hypothesis that moderate oxygen incorporation distributed among mixed functional groups favors MO adsorption.