Geochemical fate of microplastics (MPs): XPS characterization of goethite binding to primary and secondary MPs and their surface modification

Environmental context Plastic pollution is a growing crisis, with tiny microplastic (MP) particles spreading through our environment and potentially affecting ecosystems in ways we do not fully understand. This study found that thermoplastic-bead MPs and polyethylene terephthalate MPs can chemically interact with naturally occurring minerals like goethite, altering the surfaces of the MPs and potentially influencing how pollutants and microbes interact with the altered particles. Understanding these surface changes is crucial for predicting how MPs behave in nature and assessing their long-term environmental impact. Rationale This study investigated how goethite (GT) affects the surface of thermoplastic-bead microplastics (TPB-MPs) and polyethylene terephthalate MPs (PET-MPs) under simulated natural conditions. Chosen for its prevalence and roles in environmental geochemistry, GT interacts with MPs and influences their behavior and pollutant interactions. The hypothesis was that GT adsorption alters MP surface chemistry, affecting environmental biogeochemistry. Methodology MP surfaces were examined before and after GT treatment using X-ray photoelectron microscopy (XPS). Prior Fourier–transform infrared (FT-IR) analysis identified TPB-MPs as PET-based materials, indicating compositional similarity. XPS revealed the chemical compositions and electron binding energies in PET-MPs and TPB-MPs before and following GT adsorption. Results GT adsorption decreased surface carbon and increased oxygen content more in PET-MPs than TPB-MPs. PET-MPs showed stronger charge transfer and hydrogen bonding with GT, whereas TPB-MPs interactions were weaker and dominated by Van der Waals forces. Variations in peak intensity indicated enhanced C–O and O–C═O bonds and masking of C–C/C═C bonds in PET-MP. TPB-MP’s interactions with GT were weaker. Shifts in Fe2p doublet suggested chemical changes from GT adsorption. Discussion The results show that GT changes the surface chemistry of PET-MPs, enhancing their environmental transformation and reactivity. Binding energy shifts indicate surface hydrogen bonding and potential oxidation and charge transfer, highlighting GT’s role in mediating MP–mineral interactions. TPB-MPs exhibit weaker GT adsorption and fewer chemical changes, influencing their persistence and interactions with pollutants. Future research should explore oxidative transformation and microbial responses to MPs with mineral coatings. GT adsorption alters surface composition, electron scattering, and peak intensities mainly through physical interactions, with chemical effects needing further study.