Gradient columns to measure the density of microplastics

Density gradient columns are an established industrial method for measuring the density of plastics, but have rarely been applied to environmental plastics. In this study 14 density gradient columns were used to measure the density of 150 environmental plastics particles from an urban beach, plus 100 microplastics of known identity, representing what is believed to be the most extensive density dataset for environmental plastic debris available in scientific literature. In total, 92 % of investigated particles had their density measured, with the remainder falling outside of the range of the density columns: 800-1418 kg·m^-3. Error values for individual plastic particles were conservatively estimated as ≤0.27 kg·m^-3, equating to the density difference associated with a distance of 1 mm in the density gradient column. Moreover, error values for plastics of known identity, based on the standard deviation of five different particles of the same polymer type, were generally low, ≤± 1.78 kg·m^-3 for 75 % of polymers. The most notable exception was crumb rubber from used tyres, with a density of 1204.84 ± 105.87 kg·m^-3, reflecting a heterogenous material. The majority of environmental plastics were polyethylene pellets, with densities from 823.47 to 1143.47 kg·m^-3, a much wider range than reported in literature for this polymer. The densest environmental pellet was biologically attached to a stone-like particle. Otherwise, there was no evidence that environmental processing, in the form of biofilm growth or weathering, was driving variability in density. Most pellets with extremely high or low density were coloured, indicating that additives or impurities introduced during manufacturing altered the density of the virgin resin. Overall, density gradient columns show great promise for improving our knowledge of microplastic density. They represent an accurate and efficient high-throughput method, which can measure the density of ∼40 microplastics simultaneously over relatively short time periods.