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

Heteroaggregation between nanoplastics (NPs) and titanium dioxide nanoparticles (TiO₂NPs) determines their environmental fates and ecological risks in aquatic environments. However, the co-photoaging scenario of NPs and TiO₂NPs, interaction mechanisms of TiO₂NPs with (aged) NPs, as well as the dependence of their heteroaggregation on TiO₂NPs facets remain elusive. We found the critical coagulation concentration (CCC) of polystyrene nanoplastics (PSNPs) with coexisting RTiO₂NPs was 1.9 - 2.2 times larger than that with coexisting ATiO₂NPs, suggesting a better suspension stability of PSNPs+RTiO₂NPs. In addition, CCC of TiO₂NPs 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 TiO₂NPs promoted oxidation of PSNPs with the action of HO· and O₂^·- under UV light, leading to inhibited heteroaggregation. Moreover, Van der Waals and Lewis-acid interaction dominated the formation of primary heteroaggregates of PSNPs-TiO₂NPs (E_SE = ‒2.20 ∼ ‒2.78 eV) and APSNPs-TiO₂NPs (E_SE = ‒3.29 ∼ ‒3.67 eV), respectively. The findings provide a mechanistic insight into the environmental process of NPs and TiO₂NPs, and are significant for better understanding their environmental risks in aquatic environments.