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

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 Cl-2(center dot-) in seawater which could react with HO2 center dot and prevent the formation of O-2(center dot-) , 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.

Automated summary

This study found that the aging rate of polypropylene microplastics (PP MPs) is significantly reduced in estuary water and seawater compared to ultrapure water, leading to longer resistance time in marine environment. Additionally, an inhibitory effect on the aging process of PP MPs in seawater was found with increased Cl- concentration, due to the formation of Cl-2(center dot-) which reacts with HO2(center dot) and prevents the formation of O-2(center dot-). This study highlights the impact of water matrices on the photo aging rate of MPs in natural water.