Magnetism-Assisted Density Gradient Separation of Microplastics

A versatile method for the efficient separation of different types of microplastics from particle mixtures is presented. Magnetism-assisted density gradient separation (Mag-DG-Sep) relies on a bespoke separation cell connected to a gradient pump and located between two like-pole-facing neodymium magnets. In Mag-DG-Sep, particle mixtures initially sunk in water are subjected to a gradient of increasing concentration of MnCl₂, enabling the sequential suspension and collection of particles with different densities. The suspension process is assisted by the paramagnetism of the MnCl₂ solution placed between the two magnets, which contributes to focusing the ascending particles from the bottom of the separation cell to the outlet, thus enhancing the resolution of the separation process. To demonstrate the concept, a mixture of polyethylene (PE) polymer particles with a similar size range (180-212 μm) but different densities (ca. 0.98, 1.025, 1.08, and 1.35 g cm^-3) was selectively separated in a single Mag-DG-Sep run. These particles were also efficiently separated when mixed with other types of particles, such as glass or soil. A generic linear MnCl₂ gradient can be directly applied for sample screening covering a broad range of densities (0.98-2.20 g cm^-3), while steps can be introduced in the gradient, increasing the separation resolution of particles with close densities (1.025-1.08 g cm^-3). As a proof-of-concept application, Mag-DG-Sep facilitated sample preparation of microplastics present in a soil sample prior to their examination by attenuated total reflection Fourier-transform infrared spectroscopy.