Fig. 3.

A) Sketch showing the custom-made conical spiral conveyor, which improves the extraction efficiency of MP in a density separation despite being buried by particulate matter. The spiral conveyor is operated within a separation funnel (Squibb form). The particles of the sample are transported upwards and released in a fountain-like manner into the density separation solution above, where they can freely separate. A video (picture sequence over 2 hours) visualising the efficiency of the density separation process by means of the conical spiral conveyor is provided as supplementary material to the paper. Approximately 0.5 g of test MP i.e. fluorescent green polyethylene microspheres (125–150 μm, ρ = 1025 kg m⁻³) were added initially in the lower part of the sediment fraction (glass particles, 40–70 μm). SPT (ρ = 1800 kg m⁻³) is added as the density separation solution. When starting the spiral conveyor, it can be observed that the test MP are being transported upwards and partially rise to the top. Some test MP cannot be freed in the first cycle and are covered again by sediments. After several cycles (after ∼2 h) all particles are extracted and float on the top of the density solution (supernatant). B) The technical drawing of the large spiral conveyor (ø 35 mm) with details on the physical dimensions. Other sizes can be up or down-scaled accordingly.