Molecular Dynamics Simulation of Natural Organic Matter-TiO2 Nanoparticle Interaction in Aqueous Environment: Effects of Ca2+ and Na+ Ions

This work aimed to explore the dynamic properties and mechanisms between natural organic matter and engineered nanomaterials (NOM-ENMs) interactions in aqueous solutions with the existence of monovalent and divalent cations. Due to the limitation of experimental method characterizing the interactions at the atomic level, the molecular dynamics simulation method was used in the study. Comparative interaction processes between NOM and TiO2 nanoparticles in the aqueous solutions with or without Ca2+ and Na+ ions were simulated. Results showed that the Temple-Northeastern-Birmingham (TNB) encircled the TiO2 nanoparticles and formed a horseshoe-style binding by two hydrogen bonds in the absence of cations. However, in the presence of cations, strong attraction between the mediating divalent cations and the negatively charged COO- groups induced the OCOO- atoms to move away from the surface, which resulted in a hinge-style binding. The less TNB fragment encircling the nTiO(2) might decrease the steric and electrostatic repulsion between TiO2 nanoparticles coated by humic acid. That is, the Ca2+ might destabilize the TiO2 nanoparticles in the presence of NOM. The mediating effect of Na+ cation on NOM-ENMs interaction is insignificant, whereas the adsorbed Na+ on the surface of the nTiO(2) might change the surface charge and potentially affect the environmental behavior of the nTiO(2). This study facilitates to understand the fate and potential toxicity of ENMs in the aquatic environments.