High Salinity Promotes the Aging of Polystyrene Microplastics with Humic Acid in Seawater

The aging of microplastics (MPs) in the sea may be influenced by their interaction with dissolved organic matter (DOM), while the contribution of salinity was previously overlooked. Hence, this study investigated the influence of salinity on the aging of polystyrene microplastics (PS MPs) with humic acid (HA) in simulated and real seawater. Results showed that HA promoted the aging of PS MPs both under low (5 PSU) and high salinity (35 PSU), reflected in the formation of fragments and the production of oxygen-containing functional groups (O-C=O, C-OH, and C-O) of PS MPs. Moreover, high salinity promoted the aging of PS MPs with HA more significantly, as evidenced by the oxygen/carbon ratio values in the order of 0.15 (L-HA-High) > 0.1 (L-HA-Low) > 0.08 (D-HA-High) > 0.06 (D-HA-Low) > 0.02 (Unaged). This was associated with the adsorption of HA and the generation of •OH on PS MPs. Duo to the reduction of electrostatic repulsion between HA and PS MPs, HA was readily adsorbed on the surface of PS MPs (0.39 mg HA/g) under high salinity.Furthermore, the adsorbed HA generated more •OH under high salinity, thus promoting the fragmentation and oxidation of PS MPs in the aging process. Collectively, our findings highlight the importance of salinity influencing the aging of MPs with DOM, which help to assess the environmental risk of MPs pollution in the sea.