Progress on the photo aging mechanism of microplastics and related impact factors in water environment

As an emerging pollutant, microplastics (MPs, plastic particles with size 5 mm) pollution in water has acquired extensive attention in recent years due to their small particle size and stable properties, which can act as a carrier for pollutants in water environment and have toxic hazards to aquatic organisms. After entering into water environment, MPs are readily undergone varied types of weathering processes (i.e., hydrolysis, photo oxidation, mechanical abrasion, and biodegradation), leading to shifted surface morphology (i.e., surface roughness, specific surface area, and size) and physicochemical properties (i.e., melting temperature, surface charge, molecular weight, crystallinity, and hydrophobicity). Among different weathering processes of MPs in water, sunlight induced photo aging process has been considered as primary contributor, which will further impact the MPs behaviors, such as aggregation and deposition in water column, adsorption to organic and inorganic pollutants, leaching of additives, as well as toxicity to organisms in water. From the perspective of MPs aging mechanism, photo aging process of MPs in water environment is a free radical associated chain reaction process and readily to be affected by polymer types, plastic additives, halogen ions, and natural organic matters. These components are generally photochemical activated and prone to involved in the direct or free radical associated indirect photo aging process processed. However, limited information has referred the impact of plastic additives and water constitutes on the long term weathering process for MPs in water, which will pose a great difference between laboratory-accelerated weathering and long-term natural weathering. Studying the photo aging mechanism of MPs in water environment is of great significance for evaluating their formation process and potential ecological risks. Based on the above considerations, we systematically reviewed the photo aging and fragmentation processes of MPs in water environment. The content of this work contains: (1) Photo aging mechanism and changes of MPs physicochemical properties during aging process in water. Research results revealed that photo aging process could affect the molecular arrangement and mechanical properties of MPs, and further fragment into smaller size secondary MPs even in nano-scale particles, which is important to explain the formation of MPs in current water environment. (2) Critical effect of plastic properties, such as polymer type, size, and plastic additives on the aging of MPs in water; for the impact of plastic additives on the aging process of MPs, results in this work suggested that photo aging mechanism of MPs could be modified due to the presence of plastic additive (i.e., antioxidant and colorant). (3) Role of water constitutes (i.e., Cl−, Br, and NOM) on the photo aging process of MPs in water. The results emphasized the multiple impacts of water constitutes (halogen ions and natural organic matter) in the photo aging process of MPs in the water environment, illustrating that MPs may have different aging time and mechanisms in different water matrices. Moreover, impact of microorganism induced biodegradation for MPs in water is also proposed. In further study, more concentration should be concerned on the combined effects of plastic additives and water quality conditions on the aging process of MPs, which is helpful to shorten the distance between laboratory accelerated weathering and long-term aging process of MPs in natural waters. Besides, results of this paper can also provide theoretical support for comprehensively and systematically assessing the environmental behavior, toxicological effects and ecological risks of aged MPs in natural water environment.