Towards a Consensus Method for the Isolation of Microplastics from Freshwater Sediments

Environmental pollution caused by plastic waste is of global concern. There is growing interest in the study of microplastics in freshwater systems. However, the lack of harmonized analytical methodology makes it difficult to compare results obtained by different laboratories. This work compared methods for the recovery of microplastics from freshwater sediments based on density separation by flotation followed by digestion of organic matter. Simulated sediment was spiked with virgin polypropylene, polystyrene, polyamide (PA), polyethylene terephthalate (PET), and polyvinyl fluoride (PVF) pellets, and post-consumer polytetrafluoroethylene (PTFE) fragments. Density separation was carried out using distilled water and NaCl, CaCl2, ZnCl2, and NaI solutions, both for intact pellets/fragments and following grinding and sieving to three size fractions (<1 mm, 1-2 mm, and >2 mm). Digestions with HNO3, NaOH, and Fenton's reagent were compared. Only NaI quantitatively recovered all types of polymers. However, CaCl2 and ZnCl2 recovered all but PVF and PTFE. Different flotation patterns were observed for different size fractions of the same polymer, highlighting the fact that density is not the only factor affecting recovery. Digestion efficiencies were 6-78% in HNO3, 4-45% in NaOH, and 49-80% in Fenton's reagent. Overall, CaCl2 is recommended for density separation and Fenton's reagent for organic matter removal.