Controlling Small Particles for Two-Step Density Sorting of Simulated Microplastics: Overcoming Surface Tension Effects with Surfactants

Infrared spectrometers are commonly recommended for analyzing microplastics (MPs) in sediment samples. However, these instruments are costly and time consuming, limiting the scope of surveys and our understanding of the distribution and long-term variation of MPs. Although it is challenging to determine MPs by floatation sorting, it is possible to estimate the ratio of MPs that float and sink in seawater. The study employed floatation sorting to confirm whether MPs with densities lower than the liquid float and those with densities higher sink, even for MPs smaller than 1 mm. As expected, large MPs (1 to 4.75 mm in size) with densities higher than that of the liquid sank. Unexpectedly, small MPs (212 μm to 1 mm) with densities higher than the liquid’s density also floated. Assuming the unexpected floating was due to surface tension, we added a surfactant to lower it, causing MPs with densities higher than the liquid’s to either sink as expected or accelerate sinking. Thus, with the use of a surfactant, even small MPs can be sorted by density if a heavy liquid is used after water.