comparison of density separation and oil-based extraction
3.3 Comparison of the two extraction methods
In terms of recovery, the two extraction methods were comparable, however, the density separation method yielded better recovery rate for 20 PET MPs than the oil-based extraction. For extraction of 10 PET MPs, the oil-based extraction was slightly better than density separation method, as shown in Figure 4. The variability of measurements (shown as SD values on the graph) were lower for density separation method for both, 10 and 20 PET MPs, making density separation a more replicable method.
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Figure 4. Oil extraction and density separation from alluvial soil recovery rates (%) of 10 and 20 PET MPs (N=6).
4. Discussion
Both extraction methods have proven advantages and disadvantages in terms of cost, time effectiveness and material safety.
Oil-based extraction is a safe and cheap method, which does not use any hazardous saline solution. It is highly applicative to different polymer, such as PE, PU, PS, etc. and soil types, such as compost and agricultural soil [10]. On the other hand, it is time consuming due to the overnight freezing process and subsequent thawing. It also requires a custom-made equipment (PTFE cylinder and a piston). An oil layer frequently formed on the cylinder’s wall, where the MP particles got trapped and had to be washed off the inner walls. Moreover, the PET MP particles also tended to stick to the outside of the frozen sample cylinder and had to be, therefore, scraped from the surface of the frozen sample. Furthermore, this method, albeit in small quantities, uses foodstuff. Before further analysis, removal of oil traces with hexane is required [10]. Oil-based extraction method efficiency and accuracy vary due to different properties of environmental samples. Lipophilic properties of plastic polymers may also be modified, due to contaminants adhered to the surface of MPs [14].
Density separation method is a simple method and easily applicable in the laboratory. Saline solutions with sufficiently high density can cover a wide range of different MP polymers, from low to high-density polymers, and is, therefore, highly applicable to different polymer and soil types [14,15]. The density of ZnCl2 solution has not changed after the first filtration, therefore, ZnCl2 solution can be recycled to be used in further filtration processes. However, the preparation of saturated saline solutionrequires large quantities of a chosen salt, which makes density separation method expensive. In our study, 1 kg of ZnCl2 salt was used to prepare 1.2 L of saturated solution with density of1.6 g cm- 3. Saline solutions for density separation method (NaI, ZnCl2, ZnBr2) are toxic and corrosive salts, hazardous to the environment, and what is more, highly reactive. Möller et al. (2020)[3] reported that ZnCl2
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solution can react with natural components of the sediment, especially with carbonates (resulting in bubbling and foam), which can hamper the procedure, however, no such thing had occurred in our samples when ZnCl2 was used. Literature reports that low-density MPs can be more easily extracted than high-density MPs [14], however, PET is at the high-end of density of plastic polymers (1.38 g cm- 3), and the recoveries obtained in this study show otherwise.
Both methods require an additional step to remove or decrease organic content from a sample, especially for soil samples rich in organic matter (agricultural soil, compost and soil-compost mixtures). This step was not required in our study since alluvial soil is poor in organic matter content (2.30 ± 0.13 %).
Recovery obtained with both extraction methods in this study was better or in the similar range as the one reported in the literature. It should be noted, however, that experimental designs (e.g., number and size of MPs used, environmental matrices used) was different in different studies, therefore, comparison of recovery rates is not straightforward.
Currently, oil extraction method appears to be safer for the environment, however, density separation method is more time efficient, is easier to handle and ZnCl2 solution can be recycled and reused again. Density separation method is able to achieve slightly higher recovery rates and lower variability. By optimising and testing both methods on a wide range of different polymer types, concentrations and sizes of MPs and different matrices with varying organic matter content in comparable experimental designs, we will be able to determine which method is more suitable and yields better recoveries.
Further work will focus on quantification and identification of microplastics in real environmental samples, such as agricultural soil, bio-waste compost, sludge from wastewater treatment plants and others.
Acknowledgements
This work was supported by the Slovenian Research Agency, research core funding No. P3-0388 (Mechanisms of Health Maintenance), and research project No. Z2-2643 (Agri-iMPact:
Microplastic Prevalence and Impact in Agricultural Fields).
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