Co-photoaging inhibited the heteroaggregation between polystyrene nanoplastics and different titanium dioxide nanoparticles

Heteroaggregation between nanoplastics (NPs) and titanium dioxide nanoparticles (TiONPs) determines their environmental fates and ecological risks in aquatic environments. However, the co-photoaging scenario of NPs and TiONPs, interaction mechanisms of TiONPs with (aged) NPs, as well as the dependence of their heteroaggregation on TiONPs facets remain elusive. We found the critical coagulation concentration (CCC) of polystyrene nanoplastics (PSNPs) with coexisting RTiONPs was 1.9 - 2.2 times larger than that with coexisting ATiONPs, suggesting a better suspension stability of PSNPs+RTiONPs. In addition, CCC of TiONPs with coexisting photoaged PSNPs (APSNPs) was larger 1.7 - 2.2 times than that with PSNPs coexisting, indicating photoaging inhibited their heteroaggregation due to increasing electrostatic repulsion derived from increased negative charges on APSNPs and the polymer-derived dissolved organic carbon. Coexisted TiONPs promoted oxidation of PSNPs with the action of HO· and O under UV light, leading to inhibited heteroaggregation. Moreover, Van der Waals and Lewis-acid interaction dominated the formation of primary heteroaggregates of PSNPs-TiONPs (E = ‒2.20 ∼ ‒2.78 eV) and APSNPs-TiONPs (E = ‒3.29 ∼ ‒3.67 eV), respectively. The findings provide a mechanistic insight into the environmental process of NPs and TiONPs, and are significant for better understanding their environmental risks in aquatic environments.