Photoaging alters the aggregation behavior of functionalized nanoplastics differently: effects of leached organic matter and surface properties changes

Abstract

The aggregation behavior of nanoplastics (NPs) is largely controlled by the photoaging process. However, it remains unclear how the release of nanoplastic-derived dissolved organic matter (NPDOM) induced by photoaging affects the aggregation behavior of NPs with different surface functional groups. Herein, the aggregation behavior was studied for three types of polystyrene NPs, namely, pristine NPs (PS-Bare), NPs with amino (PS-NH₂) and NPs with carboxyl (PS-COOH), in monovalent and divalent counterion solutions. The dominating mechanisms were addressed using various characterization methods and calculation by extended Derjaguin–Landau–Verwey–Overbeek (XDLVO) theories. The results showed that the photoaging process inhibited the aggregation for PS-Bare and PS-COOH in a NaCl solution due to the increased steric repulsion arising from the adsorption of the NPDOM but promoted their aggregation in a CaCl₂ solution due to the bridging effect between Ca²⁺ and the newly generated O-containing functional groups. In contrast, the aggregation of PS-NH₂ was facilitated by photoaging both in mono- and divalent counterions, primarily attributed to reduced electrostatic repulsion and diminished hydration repulsion. These findings provide new insights into understanding and predicting the transport and fate of NPs with different surface chemistry properties in natural environments.