Molecular interaction of pristine and photoaged polylactic acid microplastics with extracellular polymeric substances from Microcystis aeruginosa

Microplastics (MPs) existence in aquatic environments may cause changes in properties of water bodies and associated sediments. In freshwater systems, most of the photosynthetic products of phytoplankton are released in the form of extracellular polymeric substances (EPS). EPS often interact with some charged particles through specific functional groups in their macromolecules. An in-depth understanding of EPS-MPs interactions is essential to assess the environmental effects of MPs on freshwater and marine systems. In this study, the mechanism behind the interactions of pristine and photoaged polylactic acid microplastics (PLA-MPs) with EPS was systematically investigated at the molecular scale. Results show that the addition of EPS remarkably enhanced the fluorescence intensity of humic- and fulvic-like substances within pristine and aged PLA as revealed by fluorescence excitation-emission matrix. Fourier transform infrared spectroscopy, Raman spectra, and atomic force microscopy coupled with infrared spectroscopy (AFM-IR) further demonstrated that interactions between EPS and pristine PLA were mainly via hydrogen bonding-induced local chain rearrangement or breakage. Meanwhile, aging of PLA led to the stronger interaction with EPS. Compared with pristine PLA, aged PLA exhibited more pronounced surface alteration and molecular weight reduction after exposure to EPS under the applied experimental conditions, suggesting a greater susceptibility to hydrolytic transformation. Moreover, the aged PLA displayed pronounced cracks and grooves, especially after reaction with EPS. EPS addition also increased the negative surface charge of PLA, especially in aged PLA. This study highlights the complexity of PLA-EPS interaction mechanisms, and offers a possible view on predicting the environmental behavior of MPs and regulating their biodegradation process.