Comparative Performance of Magnetic Iron Oxide Nanoparticles in Removing Polyethylene Micro- and Nanoplastics from Model Water Systems

Abstract In this study, magnetic iron oxide nanoparticles (MIONs) were synthesized using three different. methods: coprecipitation, microwave-assisted synthesis, and hydrothermal synthesis. The resulting nanoparticles were further functionalized with citric acid (CA) and cetyltrimethylammonium bromide (CTAB) to enhance their physicochemical properties and adsorption capabilities. Comprehensive characterization was carried out using thermogravimetric analysis (TGA), Fourier Transform Infrared spectroscopy (FTIR), X-ray powder diffraction (XRD), electrokinetic measurements of zeta potential (ZP), transmission electron microscopy (TEM), and magnetic measurements. The synthesized MIONs were applied for the removal of polyethylene (PE) micro- and nanoplastics (MPs and NPs) from aqueous solutions. TGA and FTIR analyses confirmed successful functionalization, particularly with CA, where up to 45% mass loss was attributed to the decomposition of the coating agent. Zeta potential measurements revealed a significant shift in the isoelectric point (IEP), further confirming surface modification. The removal efficiencies varied between 33.56% and 93.39%, with the highest efficiency achieved using unmodified hydrothermally synthesized MIONs. Interestingly, CTAB-functionalized MIONs synthesized via coprecipitation also exhibited high adsorption capacities, reaching up to 118.2 mg/g. The superior performance of the bare hydrothermal particles and the good performance of the CTAB-coated coprecipitated particles may reflect differences in particle properties and surface chemistry that favor interactions with MPs and NPs. Post-removal characterization indicated partial adsorption of PE on the MIONs surface, as shown by increased mass loss in TGA curves and subtle changes in FTIR spectra. Magnetic measurements demonstrated a decrease in magnetization after functionalization, as expected, but the MIONs retained sufficient magnetic responsiveness for separation processes. Overall, CTAB-functionalized MIONs synthesized via the coprecipitation and unmodified hydrothermally synthesized MIONs were identified as the most promising candidates for PE MPs and NPs removal. The results further demonstrated that the effect of surface functionalization strongly depends on the synthesis route and nanoparticle properties, as functionalization did not universally improve removal efficiency. This study highlights the importance of synthesis route and surface chemistry in the design of magnetic nanomaterials for PE MPs and NPs removal from model aqueous systems.