FTIR-based insight into nickel binding mechanisms in soil containing polystyrene microplastics

The interaction between microplastics (MPs) and heavy metals in soil is governed by complex surface processes that are still insufficiently characterized at the molecular level. This study investigated surface chemical transformations of polystyrene (PS) MPs in soil and their role in nickel (Ni) binding using Fourier-transform infrared spectroscopy (FTIR). Soil was spiked with 1% (w/w) PS and Ni at 50 and 500 mg kg^-1 under controlled conditions. Infrared spectra of PS recovered from soil revealed pronounced surface modifications compared to pristine PS. The appearance and intensification of bands associated with Al–O (914 cm^-1) and Fe–O/Mn–O vibrations (<650 cm^-1), together with broadening of the 1026 cm^-1 band overlapping with Si–O–Si stretching vibrations, indicated the formation of PS–soil associations. These findings suggested that PS particles interact with mineral components, forming composite aggregates enriched in reactive oxygen-containing functional groups. With increasing Ni concentrations, additional spectral changes were observed. Enhanced intensity of mineral-associated bands indicated that Ni mainly interacts with oxygen-containing mineral sites (Si–O, Al–O, and Fe/Mn–O). Moreover, lower but consistent modifications in PS-specific aromatic vibrations (1601, 1492, 1451, and 695 cm^-1) demonstrated that Ni interacts not only with soil mineral phases but also partially with PS surfaces. Negatively charged PS surfaces may promote electrostatic attraction of positively charged Ni species, while π–metal interactions between Ni ions and aromatic rings further contribute to localized Ni accumulation. Therefore, partial Ni association with PS may induce competition for binding sites and facilitate subsequent desorption processes. Such interactions may contribute to enhanced Ni mobility in soil systems containing PS-MPs. Spectroscopic findings obtained in this study provide mechanistic evidence that MPs modify metal binding pathways in soil and should be considered active geochemical agents rather than inert particles in contaminated terrestrial environments.