Polyethylene terephthalate (PET) microplastics as radionuclide (U-232) carriers: Surface alteration matters the most

Polyethylene terephthalate (PET) plastics find widespread use in various aspects of our daily lives but often end up in the environment as (micro)plastic waste. In this study, the adsorption efficiency of PET microplastics for U-232 has been investigated prior and after surface alteration (e.g. oxidation (PET-ox), MnO₂-coating (PET/MnO₂) and biofilm-formation (PET/Biofilm)) in the laboratory (at pH 4, 7 and 9) and seawater samples under ambient conditions and as a function of temperature. The results revealed a significant increase in the adsorption efficiency upon surface alteration, particularly after biofilm development on the MP's surface. Specifically, the K_d values evaluated for the adsorption of U-232 by PET, PET-ox, PET/MnO₂ and PET/Biofilm are 12, 27, 73 and 363, respectively, at pH 7 and under ambient conditions. The significantly higher adsorption efficiency of the altered and particularly biofilm-coated PET, emphasizes the significance of surface alteration, which may occur under environmental conditions. In addition, according to the thermodynamic investigations the adsorption of U-232 by PET-MPs (both non-treated and modified), the adsorption is an endothermic and entropy-driven reaction. A similar behavior has been also observed using seawater solutions and assumes that surface alteration is expected to enhance the radionuclide, stability, mobility and bioavailability in environmental water systems.