Impact of UV aging on the fluorescence characteristic of dissolved organic matter derived from microplastics and biochar

Microplastics (MPs) and biochar (BC) are ubiquitous in the environment, and their coexistence will inevitably affect their aging process and the physicochemical characteristics of the released dissolved organic matter (DOM). In this study, six types of MPs and BC prepared from rice straw were selected. Fluorescence features (such as fluorescent components, sources, and humification degree) of DOM derived from MPs, BC, and MPs-BC composite systems under UV aging were characterized, as well as the initial and dark conditions. The results showed that under initial and dark conditions, the MPs and BC systems were dominated by protein-like (the proportion ranging from 75% to 91%) and humic-like substances (the proportion ranging from 84% to 91%), respectively. Dark conditions are most conducive to the release of the humic-like substances for the BC system, while MPs could promote the discharge of fluorescent DOM in the composite systems. After UV aging, the humic-like substances in the MPs system increased significantly, while the humic substances in the BC system underwent mineralization and degradation, with the protein-like substances slightly increased. Except for the composite systems of polystyrene and poly (ethylene terephthalate) (where the humic-like substances increased significantly), the characteristics of the source, humic-like substances, and humification degree of DOM in the MPs-BC composite systems were more similar to those of the BC system. The MPs-BC composite system accelerated the terrestrial humic-like substances released under the dark conditions. The impact of UV aging on the release of humic-like substances in different MPs-BC composite systems depends on the type of MPs. Results from the principal component analysis showed that fluorescence characteristics could be the indicator for the application of BC in the environment and the degradation of different MPs under UV aging. This study could provide data for the release characteristics of DOM in environments where MPs and BC co-exist, as well as the impact on the environmental behavior of pollutants.