Abstract: Photoaging is an important transformation process of microplastics (MPs) in the environment. However, the photoaging process of MPs derived from commonly used commercial plastics is unclear. Herein, pure polyethylene (PE) polymer (PE_s), PE plastic bag (PE_b) and agricultural plastic film (PE_m) were used as examples to examine changes of physical and chemical properties during photoaging process of MPs. PE_b and PE_m MPs were prepared by mechanical crushing. The results showed that small pieces of MPs were peeled off from original MPs with the irradiation. Rough surface of PE MPs gradually changed to smooth surface. After irradiation, crystallinity of MPs increased by 6.1%-12.14%. The X-ray photoelectron spectroscopy and Fourier Transform Infrared Spectrometer characterization showed that C-O content increased by 0.08-0.125, -OH content increased by 0.016-0.098, carbonyl index increased by 0.06-0.153, and conjugated ketones increased by 0.005-0.081. MPs showed different degrees of photoaging: PE_s>PE_b>PE_m. The photoaging was mainly induced by the light absorption of original chromophores, which led to bond breakage and photooxidation reaction, and then produced oxidation products such as ketones, carboxylic acids, and esters. The presence of additives caused differences in concentrations of hydroperoxide products, which are important intermediates in the photoaging process, resulting in different photoaging rates. Our results revealed the differences in photoaging process of PE MPs from different sources, which provided important information for evaluating ecological risks of PE MPs.

Key words: microplastics, photoaging mechanism, commercial polyethylene plastics, photooxidation product, oxygen-containing functional group