Extraction of microplastics from marine environmental matrices: density separation protocol validation

Microplastics (MPs) differently concentrate within marine sediments depending on transport and accumulation agents as well as chemical-physical properties of plastic compounds (Chaukura et al., 2021). When MPs occur within marine sediments they can be utilized by agglutinated organisms, as Sabellariid polychaets, to build arenaceous reefs, thus MPs can also accumulate in bioconstructed substrates. Heterogeneity of MPs accumulated in different environmental matrices, rising interest in the last years but the lack of standardized guidelines for extraction brought us to a widespread inconsistency of data due to different recovery and quantification protocols. Size, shape, floating behavior and density of plastic particles are handful information to correctly recognized true MPs from other grains in environmental matrices. Particularly, density of plastic materials, ranging from ~0.8 to -1.4 g cm*3, makes MPs lighter then sedimentary grains made of lithic fragments, minerals and biogenic remains (Ruggero et al., 2020) allowing a density separation. The aim of this work is to validate the extraction protocol using Sodium Iodide (NaI) brine solution (Kedzierski et al., 2017) applied to two types of environmental matrices: coastal marine sediments (mainly sands) and arenaceous bioconstructions built by the honeycomb worm Sabellaria spinulosa (Leuckart 1849). Extraction efficiency of NaI brine solution was compared with the extraction efficiency of sodium chloride (NaCl). The comparison was applied to both sediment and bioconstruction samples also using two different quantities (10g and 40g). Moreover, each sample was previously added with microplastics of known quantity and type: plastic particles with sizes from 250μm to 500μm, made of plastic compounds commonly produced by industry in a wide range of densities, such as polyethylene terephthalate PET (1.38 g cm-3), polypropylene PP (0.855–0.946 g cm-3), and polyvinyl chloride PVC (1.1–1.35 g cm-3). Manipulated samples were put in 200 ml solution of NaI and NaCl, mixed and sonicated. After 6-12 hours, the obtained solutions were filtered through a 0.23 μm membrane for MPs isolation. Identification and quantification of MPs on the filters were performed under a stereomicroscope. Results documented a higher quantity of recovered MPs for the samples treated with NaI, confirming the higher extractive capability of this salt. On other hand, there were no statistically significant differences in tested sample quantities (40g vs 10 g) nor in the number of extracted MPs with respect to the type of analyzed matrix (bioconstruction and sediment), indicating that the proposed protocol correctly works for different environmental matrices also using very small quantity of material. Chaukura N. et al. (2021). Microplastics in the Aquatic Environment—The Occurrence, Sources, EcologicalImpacts, Fate, and Remediation Challenges. Pollutants. 1(2):95-118.https://doi.org/10.3390/pollutants1020009 Kedzierski M., et al. (2017). Efficient microplastics extraction from sand. A cost effective methodology basedon sodium iodide recycling. Marine pollution bulletin, 115(1-2), 120-129.https://doi.org/10.1016/j.marpolbul.2016.12.002 Ruggero F., et al. (2020). Methodologies for microplastics recovery and identification in heterogeneous solidmatrices: a review. J. Polym. Environ. 28, 739–748. https://doi.org/10.1007/s10924-019-01644-3.