Engineered Halloysite–CoFeO ₄ –Au Nanohybrids for Selective Adsorption of Aromatic Pollutants: A Molecular-Level Study of Surface Interactions

The persistent presence of aromatic amines (e.g., benzidine and p-phenylenediamine) and triazole-based herbicides (e.g., amitrole) in aquatic environments poses serious environmental and public health concerns, highlighting the urgent need for high-performance adsorbent materials. In this work, we present a halloysite-based nanohybrid, CoFeO₄@HNTs-AuNPs, engineered through hierarchical surface functionalization with cobalt ferrite and gold nanoparticles. Using atomistic molecular dynamics simulations combined with well-tempered metadynamics, we systematically investigate the adsorption mechanisms, interfacial structure, and dynamic behavior of target pollutants on pristine and modified halloysite nanotubes. The functionalized nanohybrid exhibits substantially enhanced interaction energies (e.g., -539.4 kJ·mol⁻¹ for benzidine) and stronger molecular confinement, arising from synergistic van der Waals interactions, π-π stacking with AuNPs, and hydrogen bonding with surface functional groups. To further quantify the thermodynamic driving forces of adsorption, molecular mechanics poisson-Boltzmann surface area (MM-PBSA) calculations were employed as a complementary energetic analysis. The MM-PBSA results reveal a marked increase in adsorption free energy upon surface functionalization, providing a thermodynamic rationale for the enhanced adsorption stability observed on CoFeO₄@HNTs-AuNPs. Structural and dynamic descriptors, including radial distribution functions, contact number analysis, mean squared displacement, solvent-accessible surface area, and hydrogen bond analysis, consistently confirm the superior adsorption stability and selectivity of the modified surface. Free energy landscapes reconstructed from metadynamics further demonstrate the thermodynamic favorability and accessibility of adsorption on the nanohybrid. Overall, this study establishes a molecular-level framework for the rational design of multifunctional halloysite-based nanoadsorbents and advances the development of next-generation materials for the targeted removal of persistent organic pollutants from aqueous environments.