Insights into the photosensitivity and photobleaching of dissolved organic matter from microplastics: Structure-activity relationship and transformation mechanism

Revealing the structure-activity relationship between physicochemical properties and photoactivities of microplastic dissolved organic matter (MPDOM) is significant for understanding the environmental fate of MPs. Here, we systematically analyzed the physicochemical properties and molecular composition of DOM derived from MPs including polystyrene (PS), polyethylene glycol terephthalate (PET), polyadipate/butylene terephthalate (PBAT), polylactic acid (PLA), polypropylene (PP), and compared their photosensitivity and photobleaching behaviors. Results indicated that PSDOM and PETDOM had more similar properties and compositions, and showed stronger photosensitivity and photobleaching effects than PBATDOM, PLADOM and PPDOM. The [³DOM^∗]_SS and [¹O₂]_SS varied in the range of 0.31-13.03 × 10^-14 and 1.71-5.49 × 10^-13 M, respectively, which were within the reported range of DOM from other sources. The SUVA₂₅₄, HIX, AI_modwa, Xc_wa and lignin/CRAM-like component showed positive correlation with the [³DOM^∗]_SS, [¹O₂]_SS and Φ_³DOM*. The negative correlation between E₂/E₃ and [³DOM^∗]_SS was due to the higher proportion of low-molecular weight components in MPDOM. The lignin/CRAM-like component was identified to be the crucial photobleaching-component. The lignin/CRAM-like in PSDOM showed a deepened oxidation degree, while its change trend in PETDOM was from unsaturated to saturated. These findings provide new insights into the relevant photochemical fate of MPDOM.