Photo aging of polypropylene microplastics in estuary water and coastal seawater: Important role of chlorine ion

•Photo aging of PP MPs was inhibited in seawater than in ultrapure water.•Cl- was the primary contributor to the inhibitory effect of PP MPs in seawater.•O2•- rather than •OH was responsible for the photo aging of PP MPs in seawater.•Light produced Cl2•- in seawater could quench HO2• and impend PP aging process. Photo aging of microplastics (MPs) in water environment are relevant to free radical associated polymer chain reaction, and various photo chemical reactive constitutes (i.e., Cl−, Br−, NO3−, CO32−, and natural organic matters) would affect the reaction, leading to a great difference in the photo aging mechanism of MPs between freshwater and seawater system. This study investigated light induced photo aging process of polypropylene (PP) MPs in ultrapure water, estuary water, and seawater. Results revealed that the aging rate of PP MPs was significantly decreased in estuary water and seawater compared with that in ultrapure water, leading to a longer resistance time after emission in marine environment. Besides, lower carbonyl index was found with the increased aqueous Cl− concentration, highlighting the important role of Cl− in the inhibitory effect for PP MPs aging process in seawater. This is due to the formation of Cl2•− in seawater which could react with HO2• and prevent the formation of O2•−, thus inhibit the photo aging process of PP MPs under light irradiation. The finding in this study clearly indicates the impact of the water matrices on the photo aging rate of MPs in natural water. [Display omitted]