Microplastics’ Aging Processes in the Aquatic Environment: Aging Mechanisms, Altered Environmental Behaviors and Ecotoxicity

Microplastics(MPs) have been one of the most concerned environmental issues since the 21st century. MPs are plastic particles with diameters less than 5 mm, which are widely sourced in the aquatic environment. After being released into the aquatic environment, MPs would experience complicated physical, chemical and biological aging processes driven by mechanical forces, photooxidation, and biological degradation, which would result in changes in MPs’ physicochemical properties, such as the particle size, specific surface area, hydrophobicity, crystallinity, etc. In this review, we have illustrated the current study status on MPs’ environmental aging processes and pointed out the knowledge gaps. MPs’ aging processes would consequentially influence MPs’ interaction with the co-existing organic and inorganic pollutants in the aquatic environment as well as lead to the leaching of chemical additives involved in plastic production, including plasticizers and flame retardants, which can trigger high ecotoxicity to the aquatic environment. Furthermore, aged MPs would cause ecotoxicity to aquatic organisms via bioaccumulation, biomagnification, and indirect carrier pathways. The review would provide an in-depth overview of MPs’ environmental aging processes and consequential ecotoxicity implications, as well as propose future research needs.