Nanoplastics enhance the stability of kaolinite and affect the sorption of Pb2+ in aquatic environments

Abstract

The heteroaggregation of nanoplastics with minerals has been widely investigated. However, few studies have focused on the interaction between similar negatively charged clay minerals and nanoplastics, especially how their interactions affect the migration of heavy metals. This study investigated heteroaggregation of kaolinite (a model of natural mineral) with polystyrene nanoplastics (PSNPs) and the effect of heteroaggregates on lead (Pb2+) sorption. Dynamic light scattering (DLS) measurement highlighted that PSNPs owned higher colloidal stability than kaolinite in aquatic environments. Due to the surface charge heterogeneity of kaolinite, the heteroaggregation behavior between kaolinite and PSNPs was highly pH-dependent. At pH 3.0 and 6.0, kaolinite particles with positively charged edge surfaces favored the association of PSNPs. PSNPs were found to be primarily attached on the kaolinite edge (010) surface, followed by Al-terminated (001) and Si-terminated (00 1¯) surfaces based on density functional theory (DFT) calculations. At pH 10.0, they had negligible interaction due to the strong electrostatic repulsion. Interestingly, kaolinite carried more negative charges after heteroaggregation with PSNPs, resulting in slower edge-to-face homoaggregation and higher dispersibility, and thus increasing the sorption capacity for Pb2+. This investigation provided a new understanding of nanoplastics interactions with negatively charged minerals and highlighted the vital role of nanoplastics in mineral stability and contaminant migration.