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.