Heteroaggregation of nanoplastics with oppositely charged minerals in aquatic environment: Experimental and theoretical calculation study

The transport, fate, and toxic effects of nanoplastics (NPs) would be affected by various environmental factors. In this study, the aggregation behaviors of polystyrene nanoplastics (PSNPs) with or without the presence of oppositely charged minerals (Mg/Al layered double hydroxides (LDH) and kaolin) in aqueous phase were investigated through settling experiment, and the influencing factors including pH and ionic strength (IS) were also analyzed systematically. Our results revealed that pH had a negligible effect on the fate of individual PSNPs under natural aquatic conditions, while high IS would cause homoaggregation. The positively charged LDH decreased the stability of PSNPs, while negatively charged kaolin had a weak effect on it, suggesting that electrostatic interaction was directly related to the stability of PSNPs in aquatic environment. Further experiments of PSNPs with other metal oxides with different surface charges (Al2O3 and SiO2) confirmed the importance of electrostatic interaction for PSNPs-minerals heteroaggregation. Transmission electron microscopic (TEM), Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, and Density function theory (DFT) calculations were combined for the first time to explain the heteroaggregation mechanism, demonstrating that there were expected to exist hydrogen bond and van der Waals interaction in addition to the dominant force of electrostatic interaction. Our findings are expected to shed light on the environmental behavior of PSNPs in a complex aquatic environment.