Revealing the complex oxidation behavior of extracellular polymeric substances interacted with pristine and aged polypropylene microplastics

This study explored the interacting mechanism between pristine and aged polypropylene microplastics (PP-MPs) and extracellular polymeric substances (EPS) in the natural water environment. Three-dimensional excitation- emission matrix (3D-EEM) spectra displayed that the change in the fluorescence components of EPS were induced by high PP-MPs concentrations. Two-dimensional correlation spectroscopy (2D-COS) revealed that the order of EPS structural change was the phosphate groups, C--O, -- O, COO-, C-O-C, C-O - O and O-H. - H. From the scanning electron microscopy (SEM) images, it was observed that the surfaces of aged PP-MPs were rougher after interaction with EPS. The carbonyl index (CI) of FT-IR and the O/C ratio of X-ray photoelectron spectroscopy (XPS) showed that the addition of EPS increased the oxygen-containing groups and accelerated the surface oxidation of MPs. 2D-COS showed that C-H - H in pristine PP-MPs and O-H - H in aged PP-MPs were most affected by EPS, respectively. XPS further explained the oxidation mechanism of PP-MPs by EPS: the initial C-C - C firstly formed C--O -- O bond, and was finally transformed into O-C=O. =O. The aged PP-MPs interacted more strongly with EPS and there was a higher affinity between them. This work will contribute to better understand the interaction between MPs and EPS and to assess the ecological risks to MPs.