Chemical characterization of weathered polyethylene microplastics in marine coastal waters using OPLS-DA technique and ATR FT-IR spectra

: Predicting the fate of microplastics (MPs) in the environment and assessing their potential environmental hazard is crucial in defining effective mitigation strategies. This paper presents a novel approach for a comprehensive understanding of stressing factors controlling the distribution of MPs, their degradation and physicochemical transformation, by coupling spectroscopic techniques with multivariate analyses. Samples of floating MPs were collected from three transects of four coastal-port areas with different hydrodynamic characteristics and level of anthropogenic pressures (one industrial port, one commercial port and two tourist port areas). Polyethylene (PE) MPs fragments were analysed by Attenuated total reflectance-Fourier transform Infrared Spectroscopy (ATR-FTIR), then Orthogonal Partial Least Square-Discriminant Analysis (OPLS-DA) was applied on the registered spectra. This approach revealed specific spectral differences between PE samples collected near industrial/commercial ports and those collected near tourist ports. The spectral regions most responsible for that distinction were the vinyl (1200-900 cm-1) and carbonyl (1800-1600 cm-1) regions, and to a less extent those used to calculate the crystallinity index (729 cm-1 and 719 cm-1). In contrast, the hydroxyl region (3500-3000 cm-1) was somewhat less effective for discrimination. Samples from tourist port areas exhibited higher chemical index values, suggesting greater exposure to photodegradation processes due to prolonged UV light exposure. Conversely, PE MPs originating from industrial port areas exhibited lower vinyl and carbonyl indices and a less pronounced crystalline phase, despite being more polluted: we hypothesize that distinct transport pathways might have favoured prolonged residence time in water, and/or significant fouling resulting in both cases in mitigated photo-oxidation phenomena.