Improvement of the microplastics separation methodology in freshwater systems based on reused zinc chloride

, Rodrigues Mariana Oliveira, Gonçalves Ana Marta Mendes, Gonçalves Fernando José Mendes, Abrantes Nelson.

Microplastics (plastic particles with ¡ 5 mm; MPs) are considered emerging aquatic pollutants since they can derive from a variety of sources (primary and/or secondary) reaching high densities, they are ubiquitous and persistent and they can interact negatively with wildlife and human beings, depending on their shape and polymer type/specific density. These properties have led to an increasing societal and scientific concern. Nevertheless, the scientific community has not yet stablished a standard method/protocol for separating MPs from aquatic systems resulting in data that cannot be compared. Density separation is one of the most adopted methodologies using high concentrated or saturated salt solutions such as sodium chloride, zinc chloride (ZnCl2) or sodium iodide (NaI) to increase the water density and to make MPs float. Currently, ZnCl2 is reported as the most cost-efficiency method (density of 1.6 – 1.8 g/cm3) for separating MPs of low and high density, however it also has disadvantages such as being very hazardous, corrosive and requiring large amounts of product. Moreover, there is an immense lack of information relative to this substance when compared to others (e.g. NaI). Hence, the present study aimed to overcome these disadvantages by presenting an improved methodology based on the reuse of the ZnCl2 solution while keeping its efficiency. For that, artificial samples were prepared and subjected to the ZnCl2 methodology up to 5 times. These samples included MPs of polyethylene terephthalate and polyvinyl chloride, two of the densest polymers found in freshwater systems. Vacuum filtration and visual inspection, using a stereomicroscope, followed the density separation. This study showed that the ZnCl2 solution can be reused at least five times maintaining an efficiency above 95%.

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