Comparison of pre-treatment methods and heavy density liquids to optimize microplastic extraction from natural marine sediments

Abstract

The ubiquitous occurrence of anthropogenic particles, including microplastics in the marine environment, has, over the last years, gained worldwide attention. As a result, many methods have been developed to estimate the amount and type of microplastics in the marine environment. However, there are still no standardized protocols for how different marine matrices should be sampled or how to extract and identify these particles, making meaningful data comparison hard. Buoyant microplastics are influenced by winds and currents, and concentrations could hence be expected to be highly variable over time. However, since both high density and most of the initially buoyant microplastics are known to eventually sink and settle on the seafloor, marine sediments are proposed as a suitable matrix for microplastics monitoring. Several principles, apparatuses, and protocols for extracting microplastics from marine sediments have been presented, but extensive comparison of the different steps in the protocols using real environmental samples is lacking. Thus, in this study, different pre-treatment and subsequent density separation protocols for extraction of microplastics from replicate samples of marine sediment were compared. Two pre-treatment methods, one using inorganic chemicals (NaClO + KOH + Na4P2O7) and one using porcine pancreatic enzymes, as well as one with no pre-treatment of the sediment, were compared in combination with two commonly used high-density saline solutions used for density separation, sodium chloride (NaCl) and zinc chloride (ZnCl2). Both pre-treatment methods effectively removed organic matter, and both saline solutions extracted lighter plastic particles such as polyethylene (PE) and polypropylene (PP). The most efficient combination, chemical pre-treatment and density separation with ZnCl2, was found to extract > 15 times more particles (≥ 100 µm) from the sediment than other treatment combinations, which could largely be explained by the high presence and efficient extraction of PVC particles.

Methods for the detection and characterization of boat paint microplastics in the marine environment

“…However, it turned out that the trawl samples were not degraded as efficiently due to the high presence of plant seeds, and other terrestrial organic matter. Therefore, the degradation treatment was also supplemented by a density separation where the sample was treated with a heavy liquid (zinc chloride solution) that causes the natural organic material to float while the very heavy paint flakes and small seashell residues sink (Mattsson et al, 2022). Then the trawl filtration samples were filtered on a new 100 µm mesh filter for further microscope analysis.…”

Unraveling the Marine Microplastic Cycle: The First Simultaneous Data Set for Air, Sea Surface Microlayer, and Underlying Water

“…38 Additionally, PE and PP were the most commonly identified polymers in previous microspectroscopy studies in the area, although previously limited to larger size fractions (>50 or >300 μm). 34,35,38,39 Hence, since this study sampled with lower size limits (>10 μm) albeit with lower sampling volumes and the fact that Py-GC/MS and spectroscopy may have different and complementary detection capabilities, the difference in detected polymer clusters is not surprising in general. 41…”

“…The westerly facing zones showed higher concentrations of MP compared to surrounding, urban influenced coasts with less exposure to the North Sea like urban area of Uddevalla . Recent studies already emphasized the occurrence, fate, and transport of MP in these western fjord systems for water, beaches, and sediment samples. − The general presence of MP in this particular environment made it ideal for the presented investigation.…”

Preservation, storage, and sample preparation methods for freshwater microplastics – a comprehensive review

“…57 Density separation methods are based on the oatation of lighter particles on the surface of high-density salt solutions (varies from 1.2 to 1.8 g cm −3 ). 158 With all this considered, based on the density of the solution used for density separation, different types of MPs with various densities are extracted from the sediment samples. 42 Different solutions and liquids are used in the density separation stage, the most frequently used of which is NaCl solution.…”