Molecular insights into facet-specific interaction mechanism underlying humic acid adsorption on rutile TiO2

The distribution and mobility of rutile nanoparticles in natural environments are profoundly affected by the ubiquitous presence of humic acid (HA), and their interaction promotes the co-transport of heavy metals and organic pollutants, thereby increasing their environmental risk and potential exposure. A quantitative understanding of facet-specific adsorption mechanisms of HA on rutile surfaces is of vital significance but remains elusive. Herein, we quantified the intermolecular interactions between HA and different rutile crystal facets at nano- and atomic scales. HA adsorption exhibited a significant facet-dependent trend: (001) > (110) > (100). Based on atomic force microscope force measurements, the anisotropic adsorption was indicated by HA-rutile interactions, with adhesion energies of ~0.302 mJ/m 2 for (100), ~0.573 mJ/m 2 for (110), and ~1.942 mJ/m 2 for (001). Molecular dynamics further evidenced HA exhibited the strongest Ti-O coordination and hydrogen bond-mediated adsorption on (001) facet, forming a dense interfacial layer with the lowest diffusivity. Moreover, Ti-O coordination energies for representative bidentate ligands were calculated as −154.4, −132.8, and −118.7 kcal/mol on (001), (110), and (100) facets, respectively, based on density functional theory, much higher than hydrogen bonding. Therefore, HA adsorption anisotropy is mainly governed by Ti-O complexation, with hydrogen bonding playing a complementary role. This work provides nano/atom-scale insights into quantitative interactions between HA and rutile crystal facets, advancing the understanding of NOM-mineral interfacial processes in aqueous environment.