Photoaging enhanced heteroaggregation between nanoplastics and Al2O3 in aquatic environment: Experiments and DLVO theory calculation

Nanoplastics experience complex aging processes when exposed to the environment, leading to substantial modifications in their physicochemical properties. These transformations significantly influence their interaction with mineral colloids, consequently altering their migration and fate. This study systematically investigated the homoaggregation of pristine and UV irradiation aged polystyrene nanoplastics (PSNP), as well as the heteroaggregation with alumina (Al 2 O 3 ) minerals under different water chemical conditions. The results showed that photoaging not only altered the surface morphology of PSNP, but also increased their surface negative charge and generated oxygen-containing functional groups, thereby improving the stability of the aged PSNP. Al₂O₃ heteroaggregated with PSNP, leading to a decrease in stability. The interaction between Al₂O₃ and aged PSNP was more pronounced compared to pristine PSNP. The pH value and ionic strength significantly influenced heteroaggregation, with acidic conditions and low ionic strength promoting heteroaggregation through stronger electrostatic attraction. Bivalent electrolytes (Ca 2+ ) were more effective than monovalent electrolytes (Na + ) in inducing PSNP heteroaggregation, as more carboxyl groups of PSNP were generated after photoaging, which combined with Ca 2+ through the bridging effect and enhanced the heteroaggregation. Furthermore, the heteroaggregation behaviors of both pristine/aged PSNP and Al 2 O 3 followed the DLVO theory. The findings revealed the important controlling effect and mechanism of UV irradiation and Al 2 O 3 on the fate of nanoplastics in aquatic environments.