Unravelling the complex adsorption behavior of extracellular polymeric substances onto pristine and UV-aged microplastics using two-dimensional correlation spectroscopy

In this study, we examined the preferential adsorption behavior of extracellular polymeric substances (EPS) on two commonly used microplastics (MPs), including polyethylene (PE) and polypropylene (PP), through adsorption kinetics and isotherm experiments. The effects of UV aging on EPS adsorption were also explored. The results showed that EPS adsorption was greater on UV-aged MPs than on pristine MPs, with an adsorption capacity of approximately 27.0 mg-C/m² for aged PP-MPs and 23.5 mg-C/m² for pristine PP-MPs, and 16.4 mg-C/m² for aged PE-MPs and 12.5 mg-C/m² for pristine PE-MPs. Fourier transform infrared (FTIR) spectra revealed that EPS adsorption was governed by hydrogen bonds between EPS molecules and methylene groups in pristine MPs and C[dbnd]O, C–O groups in UV-aged MPs, as evidenced by the addition and the reduction of the related peaks shown in MP surfaces and residual EPS after adsorption, respectively. Fluorescence spectroscopy revealed that protein-like components exhibited higher Freundlich adsorption affinity (i.e., K_f), ranging from 0.34 to 1.29 (RU⋅L m⁻²) (RU⋅L⁻¹)ⁿ than humic-like components (0.01 to 0.12 (RU⋅L m⁻²) (RU⋅L⁻¹)ⁿ). The EPS molecular distribution was consistent with the FTIR results, showing preferable adsorption of biopolymer versus humic substances fraction. Two-dimensional correlation spectroscopy combined with FTIR (2D-COS-IR) revealed that polysaccharides in EPS were more preferentially adsorbed onto MPs and the order of the sequence was polysaccharides > proteins > humic-like substances for all MPs tested. This study provides new insight into the complex adsorption behavior of heterogeneous EPS on MPs and the associated alterations in the potential environmental fate of MPs.