Nanoplastics (NPs) have been successively detected in different environmental matrixes and have aroused great concern worldwide. However, the fate of NPs in real environments such as seawater remains unclear, impeding their environmental risk assessment. Herein, multiple techniques were employed to monitor the particle number concentration, size, and morphology evolution of polystyrene NPs in seawater under simulated sunlight over a time course of 29 days. Aggregation was found to be a continuous process that occurred constantly and was markedly promoted by light irradiation. Moreover, the occurrence of NP swelling, fragmentation, and polymer leaching was evidenced by both transmission electron microscopy and scanning electron microscopy techniques. The statistical results of different transformation types suggested that swelling induces fragmentation and polymer leakage and that light irradiation plays a positive but not decisive role in this transformation. The observation of fragmentation and polymer leakage of poly(methyl methacrylate) and poly(vinyl chloride) NPs suggests that these transformation processes are general for NPs of different polymer types. Facilitated by the increase of surface functional groups, the ions in seawater could penetrate into NPs and then stretch the polymer structure, leading to the swelling phenomenon and other transformations.
Keywords: fragmentation; nanoplastics; polymer leakage; seawater; swelling.