Molecular mechanisms of methylene blue removal by hydrochar: Insights for sustainable water treatment design

Water contamination by synthetic dyes presents serious environmental and public health challenges, underscoring the urgent need for efficient and sustainable remediation technologies. This study aims to employ comprehensive molecular dynamics simulations to unravel the molecular-scale mechanisms governing the adsorption of methylene blue (MEB) onto hydrochar adsorbents with systematically tuned oxygen functionalization. Three distinct hydrochar architectures are investigated: a mixed-functional-group model representative of typical experimental hydrochar, featuring balanced hydroxyl, carboxyl, and carbonyl groups (O/C = 0.33); a minimally functionalized carbon-rich framework (O/C = 0.03); and a hydroxyl-enriched surface with high oxygen content (O/C = 0.55). Our results show that all systems exhibit high adsorption efficiency, achieving maximum capacities ranging from 349.8 to 381.8 mg/g, consistent with experimental literature values. Strong Langmuir isotherm behavior (R 2 0.93) confirms monolayer adsorption mechanisms for all systems. Molecular-level analysis reveals van der Waals interactions and - stacking between aromatic dye molecules and hydrochar surfaces as the primary adsorption driving force, complemented by hydrogen bonding networks. The results show that hydroxyl functionalization enhances capacity by 12% compared to carbon-only surfaces, while mixed functionalization optimizes binding affinity and robustness across variable concentration regimes. These findings demonstrate that these molecular insights enable rational design of sustainable hydrochar adsorbents tailored for specific water treatment applications. • Molecular dynamics simulations reveal van der Waals interactions and - stacking as primary ad- sorption mechanisms for methylene blue removal by hydrochar. • Hydroxyl-rich functionalization increases maximum adsorption capacity by 12% compared to carbon- only surfaces, achieving 381.8 mg/g capacity. • Mixed functional group hydrochar (O/C = 0.33) provides optimal balance with highest binding affinity and 88.3% efficiency across variable concentrations. • Strong Langmuir isotherm behavior (R 2 0.93) and molecular-level insights enable rational design of sustainable hydrochar adsorbents for water treatment applications.