Interfacial interactions of humic acids with polystyrene nano-plastics in aqueous/ionic environments: a molecular dynamics exploration

Plastics pose a serious threat to both marine and freshwater life after being discarded and broken down into smaller particles such as micro and nano-plastic particles. The nano-sized plastic particles are small enough in size and similar to surfaces of biological molecules, thereby potentially altering the intra-cellular interactions and their biological fate. In order to increase our understanding of the interactions among the molecules of natural organic matter and carboxylate functionalized polystyrene nano-plastics, we have performed a modelling study of the aquatic environment at different pH conditions using molecular dynamics (MD) simulations at an atomistic scale. Humic acids (HAs) are the most common constituents of natural organic matter, usually found in soil, water, and its sediments. As a proxy for humic acids, we have used Temple-Northeastern-Birmingham (TNB), an equivalent molecule to HA in its composition. We show that TNB molecules exhibit strong interactions with polystyrene (PS) nano-plastic particles at pH-4 but weak interaction at pH-7, whereas moderate interaction at pH-9 both in fresh and saltwater. The interaction between carboxylated polystyrene particles and TNB molecules is found to be counter-ion mediated and is enhanced in the presence of saltwater, i.e., at 0.5 M NaCl electrolyte. An enhanced condensation of Na+ counter-ions onto the surface of nano-plastics brings more water molecules to its interface, hence, enriching the hydrophilicity of nano-plastics. An ordered network structure of water molecules has also been observed at the interface of the PS-slab with an increase in pH of the aquatic environment, leading to a preferential alignment of water molecules, resulting in a strong hydration layer. This strong hydration layer also keeps the TNB molecules away from the vicinity of the PS-slab interface. The surface potential trends obtained from the MD simulations are in agreement with the measured zeta potential values showing that the surface charge density of PS nano-plastics increases with an increase in the pH of aquatic solutions. Hence, our simulations provide molecular-level insights into the phenomenon associated with the adsorption/accumulation of molecules of natural organic matter towards the nano-plastics and are helpful in understanding the formation of eco-corona on plastic nanoparticles.