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Selection of a density separation solution to study microplastics in tropical riverine sediment

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Abstract

Microplastics (MPs) are small (< 5 mm) plastic particles that are widely found in marine, freshwater, terrestrial and atmospheric environments. Due to their prevalence and persistence, MPs are considered an emerging contaminant of environmental concern. The separation and quantitation of MPs from freshwater sediments is a challenging and critical issue. It is necessary to identify the fate and sources of MPs in the environment, minimise their release and adverse effects. Compared to marine sediments, standardised methods for extracting and estimating the amount of MPs in freshwater sediments are relatively limited. The present study focuses on MP recovery efficiency of four commonly used salt solutions (NaCl, NaI, CaCl2 and ZnCl2) for isolating MPs during the density separation step from freshwater sediment. Known combinations of artificial MP particles (PS, PE, PVC, PET, PP and HDPE) were spiked into standard river sediment. Extraction using NaI, ZnCl2 and NaCl solutions resulted in higher recovery rates from 37 to 97% compared to the CaCl2 solution (28–83%) and varied between polymer types. Low-density MPs (PE, HDPE, PP and PS) were more effectively recovered (> 87%) than the denser polymers (PET and PVC: 37 to 88.8%) using NaCl, NaI and ZnCl2 solutions. However, the effective flotation of ZnCl2 and NaI solutions is relatively expensive and unsafe to the environment, especially in the context of developing countries. Therefore, considering the efficiency, cost and environmental criteria, NaCl solution was selected. The protocol was then tested by extracting MPs from nine riverine sediment samples from the Red River Delta. Sediments collected from urban rivers were highly polluted by MPs (26,000 MPs items·kg−1 DW) compared to sediments located downstream. Using a NaCl solution was found to be effective in this case study and might also be used in long-term and large-scale MP monitoring programmes in Vietnam.

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The main part of the research data is included in the article. Other data can be made available from the corresponding author upon request.

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Funding

This research was funded by the Vietnam National Foundation for Science and Technology Development (NAFOSTED) under grant number 11/2020/TN. The authors would like to thank the COMPOSE project, funded by the French MEAE and conducted by the IRD and French Embassy in Vietnam for providing research facilities. The authors also thank many individuals for their help in collecting the samples in the field.

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Correspondence to Thi Thuy Duong.

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Duong, T.T., Le, P.T., Nguyen, T.N.H. et al. Selection of a density separation solution to study microplastics in tropical riverine sediment. Environ Monit Assess 194, 65 (2022). https://doi.org/10.1007/s10661-021-09664-0

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  • DOI: https://doi.org/10.1007/s10661-021-09664-0

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