Molecular properties and biotoxicity of dissolved organic matter leached from microplastic (MP-DOM) during typical hydrothermal treatment of sewage sludge

Microplastic-derived dissolved organic matter (MP-DOM) is crucial for assessing ecological and environmental impact of microplastics. However, the factors that influence the ecological effects of MP-DOM are yet to be determined. This study investigated the influence of plastic type and leaching conditions (thermal hydrolysis, TH; hydrothermal carbonization, HTC) on the molecular properties and toxicity of MP-DOM using spectroscopy and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS). Results revealed that plastic type was the main factor affecting the chemodiversity of MP-DOM, compared to leaching conditions. Polyamide 6 (PA6) dissolved the largest number of DOM due to the presence of heteroatoms, followed by polypropylene (PP) and polyethylene (PE). From TH to HTC processes, the molecular composition of PA-DOM remained constant, with CHNO compounds being dominant, and labile compounds (lipids-like, and protein/amino sugar-like compounds) accounting for >90 % of the total compounds. In polyolefin-sourced DOM, CHO compounds were dominant, and the relative concentration of labile compounds decreased dramatically, resulting in the higher degree of unsaturation and humification than PA-DOM. The mass difference network analysis showed that the main reaction in PA-DOM and PE-DOM was oxidation while in PP-DOM, it was the carboxylic acid reaction. However, plastic type and leaching conditions jointly influenced the toxic effects of MP-DOM. PA-DOM was bioavailable, while polyolefin-sourced DOM leached under HTC treatment exhibited toxicity, with lignin/CRAM-like compounds being the dominant toxic compounds. Notably, the 2-fold higher relative intensity of the toxic compounds and the 6-fold higher abundance of highly unsaturated and phenolic-like compounds in PP-DOM resulted in the higher inhibition rate than PE-DOM. Toxic molecules in PE-DOM mainly directly dissolved from PE polymers, while almost 20 % of toxic molecules in PP-DOM resulted from molecular transformation, where dehydration (-HO) was the core reaction. These findings offer advanced insights into the management and treatment of MPs in sludge.