Magnetic labelling and extraction of micrometer-sized microplastics from sandy soil

Ubiquitous microplastics (MP) have emerged as a global environmental concern. However, limited attention has been given to the migration and distribution of small-sized MP (< 10 μm) due to the challenges associated with separating MP. Here, we show that magnetic labelling of MP greatly increases the efficiency of MP extraction from soil using a magnetic field. Magnetic labelling was achieved by exploiting the glass transition of polystyrene MP sphere. By heating MP (4 µm), to induce a surface transition to rubbery state in a suspension containing Fe₃O₄ magnetic nanoparticles (MNS), the MNS were adsorbed onto the MP surface. Subsequent cooling to room temperature, led to fixation of the MNS into the MP surface layer enabling MP extraction using a magnet. Incubating MP and MNS at 90°C for 2.5 h gave the highest MP recovery rate of 92 % in water. The same MP were added to sandy soil to optimize labelling and extraction. Optimized parameters included dispersant type, organic matter digestion, and MNS size, concentration, and storage time. Compared to conventional MP detection methods, the MP recovery using magnetic extraction improved from 26 % to 93 %. To the best of our knowledge, this research represents the first successful quantitative extraction of MP < 10 μm from soil.