Zinc oxide nanoparticles dissolution and toxicity enhancement by polystyrene microplastics under sunlight irradiation

Zinc oxide nanoparticles (ZnO NPs) dissolution is a critical process for the transformation and toxicity of ZnO NPs in aquatic environments. However, the effect of microplastics (MPs) on dissolution and toxicity of ZnO NPs under sunlight irradiation is still lacking. Herein, the dramatic increase in sunlight-induced ZnO NPs dissolution by polystyrene (PS) MPs was proven, which was attributed to the increase in h⁺-dependent and proton-dependent ZnO NPs dissolution by PS MPs, yielding ¹O₂ generation inhibition and acid release, respectively. The sizes, functional groups and aging status of PS MPs and pH were characteristic ZnO NPs dissolution through modifying ¹O₂, •OH and O₂^•- generation and acid release. Furthermore, the ZnO NPs dissolution affected by PS MPs also occurred in three realistic water samples, which were mainly governed by dissolved organic matter (DOM) and CO₃^2-, rather than Cl^- or SO₄^2-. The PS MPs (1 μg/mL) dramatically altered the Zn²⁺:ZnO ratio in ZnO NPs suspension after 96 h of sunlight irradiation and presented vehicle effects on Zn²⁺, which in turn significantly increased the ion-related toxicity of ZnO NPs to Daphnia magna. Based on the PS MPs enhanced dissolution and toxicity of ZnO NPs, the effects of PS MPs on the environmental risk assessment of ZnO NPs should be seriously considered in freshwater environments under sunlight irradiation.