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, CO, COO–, C–O–C, CO and OH. 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 CH in pristine PP-MPs and OH in aged PP-MPs were most affected by EPS, respectively. XPS further explained the oxidation mechanism of PP-MPs by EPS: the initial CC firstly formed CO bond, and was finally transformed into O–C=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.