Side-chain chlorine controls interfacial photoreacivity of aliphatic hydrocarbon microplastics by modulating production of persistent free radicals and reactive oxygen species

Microplastics actively participate in the sorption and transformation of organic pollutants, while the effect of microplastic-contained heteroatoms on the transformation of sorbed pollutants remains unknown. This study deeply reveals the role and mechanism of side-chain chlorine in aliphatic microplastics for modulating their interfacial photoreactivity using N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD) as a model pollutant. The phototransformation of 6PPD in aliphatic microplastics was enhanced by the side-chain chlorine, with rates 1.15-2.47 times higher than that of individual 6PPD. However, the enhancement became reversed with a higher chlorine content. The abnormal phenomenon was mainly because side-chain chlorine accelerated the production of persistent free radicals (PFRs), but synchronously consumed the reactive oxygen species (ROS) (•OH and O). In-situ EPR detected the formation of chlorine radical (Cl•) due to the reactions between •OH and Cl, where Cl was generated from the attack of 6PPD-derived ROS on carbon-chlorine bond of microplastics. The formed Cl• further transferred electrons to polymers to yield abundant chlorine-containing PFRs and Cl to finally decrease the amounts of ROS for attacking 6PPD. The findings highlight the important role of side-chain chlorine in modulating the interfacial behavior and combined risk of aliphatic microplastics, which exhibit a highly active behavior and risk compared to microplastics without heteroatoms in natural environment.