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Insights into the photoaging behavior of biodegradable and nondegradable microplastics: Spectroscopic and molecular characteristics of dissolved organic matter release
Summary
Researchers compared how biodegradable and conventional microplastics break down under ultraviolet light and what dissolved substances they release. They found that biodegradable PLA microplastics released more protein-like organic matter during UV exposure than conventional polystyrene, and this matter was more readily used by microorganisms. The study suggests that biodegradable plastics, while designed to be better for the environment, may introduce different ecological risks as they break down.
Biodegradable plastics are increasingly used as a potential alternative to nondegradable plastics to tackle plastic pollution. However, recent studies have raised concerns about the ecological risks posed by biodegradable microplastics (MPs), which mainly focused on the risks generated by MPs themselves, neglecting the risks associated with the MPs derived dissolved organic matter (DOM). Therefore, this study selected polylactic acid (PLA) MPs with 50 µm particle size and polystyrene (PS) MPs with 50 µm and 500 nm particle sizes as representatives of biodegradable and nondegradable MPs, respectively, to comparative investigate their photoaging behavior, particularly the differences in DOM release. The results showed that both PLA-MPs and PS-MPs exhibited considerable photoaging under ultraviolet irradiation, accompanied by different color changes (PS turned yellow and PLA turned grayish brown), which were attributed to the different functional groups produced on their surfaces after photoaging (PS-MPs: CO, PLA-MPs: terminal -COOH). Additionally, excitation-emission matrix characterization combined with parallel factor analysis revealed that 50 µm PLA-MPs (16-23 %) released more protein-like low molecular weight DOM during photoaging than that of both 50 µm PS-MPs (7-13 %) and 500 nm PS-MPs (8-18 %). Fourier transform-ion cyclotron resonance-mass spectrometry (FT-ICR-MS) further confirmed that PLA-MPs (41.4 %) produced more unstable DOM easily utilized by microorganisms than that of 50 µm PS-MPs (6.3 %) and 500 nm PS-MPs (7.9 %). These results together suggested that biodegradable MPs with small particle size derived DOM may have a greater impact on microbial activity and carbon cycle than that of nondegradable MPs.