An All-glass Method for Microplastic Flotation and Isolation from Sediment

Extracting sparse microplastics from soils and sediments is challenging. We developed a flotation method to process 250 g (180 ml) of sediment, which uses only glass containers up to contact with the filter and may be scaled to larger volumes. Processing larger volumes of sediment leads to the recovery of greater quantities of microplastics and a decrease in the relative impact of contamination. The novel aspect of the approach is using a glass cylinder to separate low-density from higher-density particles. The cylinder (8 cm diameter, 500 ml volume) with ground ends was placed in a glass tray (32x22x8 cm). 125 g of sediment was placed in the cylinder, plastic standards were added, then 125 g of additional sediment. Standards were 5-PS, 5-PP, and 5-PET cut pieces of (~1mm) and 5-PET spheres (~115 um). Sediments and standards were mixed dry in the cylinder with a glass rod. ZnCl 2 (1.55 g/cc) solution was added, alternating between the outside and inside of the cylinder until the liquid level was 2 cm above the sediment. While holding the cylinder, the sediment was stirred for 30 s in a zig-zag motion. Sediment inside the cylinder was allowed to settle for 10 minutes. The cylinder was lifted about one-half cm and slowly moved laterally, allowing the heavier material to fall from the bottom. The cylinder was then placed on a submerged glass plate, and the plate, captured liquid, and particles were transferred to a smaller glass tray. The transferred material was filtered through 1.2 um polycarbonate filters over a stainless steel screen and rinsed with water. Remaining ZnCl 2 solution was filtered and recovered for reuse. Three characterized sand types with microplastic standards were processed in duplicate as well as one lakeshore sample. Recovery across all sand types and standards was 92% (129/140 standard particles recovered). No sand or plastic type showed systematically poorer recovery. This method avoids clogging at separatory funnel valves and loss on insides of suction tubes. Parts are simple, all-glass, and easy to rinse into the filter. With longer settling, even clays separate from less dense particles. The method can be scaled to larger sediment volumes. We developed this method for difficult-to-process soils that contain variable amounts of clays, organics, and low bulk density barnacle shells at the shore of a saline lake.