Developing a Reliable Method for Separating Microplastics from a Variety of Beach Sediments

Recent interest in the contamination of the marine environment with microplastics has resulted in a number of published methods for separating the microplastic fibers and particles from beach sediments. These sediments are composed of mineral and rock fragments (including shells) and varying amounts of organic matter. Generally, this involves a density separation, as plastics have a much lower density than the rock and mineral fraction of sand and silt deposits. Some methods also include the removal of organic matter using hydrogen peroxide. Using sand samples from the Staten Island shoreline, we tested some of these procedures. The simplest involved only a density separation using saturated NaCl solution. However, on filtration of the supernatant, the filter was easily clogged with very fine mineral material and organic matter. Other published methods overcame this problem by using a 300 µm mesh instead of a filter. This obviously loses finer material, including microplastics, and was something we wanted to avoid. Removing organic matter from the sediment using hydrogen peroxide (H 2 O 2 ) prior to the density separation reduced the amount of clogging, but did not solve the problem completely. Best results were obtained by separating the samples into different sediment size fractions by dry sieving, before completing the density separation. The 1 – 5 mm fraction only required the density separation, with visual observation and removal of organic matter and the occasional flat shell. The 300 µm – 1 mm fraction was treated with H 2 O 2 to remove organic matter prior to the density separation. Similarly, the < 300 µm fraction had organic matter removed before the density separation. The main drawback with the smaller size fraction is the oxidation of some iron in the sediment, that results in an orange iron oxide deposit on the filter. However, this does not inhibit the visual identification of the microplastic fibers and particles. Interestingly, all size fraction separates yielded small microplastic fibers, suggesting that these fibers stick to the surface of minerals, until they are agitated in the salt water solution. We will continue to test this method on a greater range of sediment types to ensure that it works with a variety of sediments (e.g. different grain sizes, organic matter concentrations).