Magnetic labelling and extraction of micrometer-sized microplastics from soil

Ubiquitous microplastics (MP) have emerged as a global environmental concern, recently also in soils. However, limited attention has been given to the behaviour of small-sized MP (< 10 μm) due to the challenges associated with separating and quantifying MP from an exceedingly complex matrix. 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 polystyrene spheres), to induce surface melting 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 ± 7% in water. The same MP were then added to a sandy soil suspension to assess and optimize labelling and extraction efficiency of the MP from the soil. The following parameters were optimized: 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 94 ± 12%. To the best of our knowledge, this research represents the first successful quantitative extraction of MP < 10 μm from soil and opens new possibilities for fate assessing of small MP and cleaning the environment.