Microplastic Extraction From Wastewater: Comparison And Proposal Of Methods

Abstract: Research on the extraction of microplastics from wastewater often overlooks small microplastics and fails to agree on a universal method that effectively reduces organic matter content while preserving microplastics in a timely and cost-effective manner. We compare common methods and propose an alternative method not previously tested on wastewater. Introduction and Background Microplastics (MPs), defined as solid particles of synthetic polymers smaller than 5 mm, are an emerging contaminant ubiquitously occurring in the environment [1-3]. Water resource recovery facilities (WRRFs) are an important pathway for MPs to enter the environment [4-6] via biosolids application [7] and effluent discharge [8] . Despite high removal rates, facilities can discharge a wide magnitude of MPs daily into the environment [9], [10]. There persists a lack of consensus on an accepted universal recovery method for a given matrix, with most MP extraction studies having been performed on aquatic sediments [11-13]. Additionally, many studies study polymers much larger than those typically found in wastewater-150-330 µm for primary MPs [14]-which can hinder the applicability of their findings. Removing most or all organic matter while maintaining polymer integrity is challenging, leading to the rise of a plethora of extraction methods published in literature. This study was conducted to investigate the potential errors associated with widely used procedures. The results will showcase the reliability of analytical procedures of microplastics analysis, as well as inform future method development. Methodology Here we present a summary of the work (Table 1), which is fully documented in the paper and presentation. After comparing six protocols from four common extraction methods that are inexpensive, rapid, and easy to perform, we used the least harsh organic matter digestion method with oil extraction, the latter of which is a method that has not been widely tested on MPs in wastewater and relies on the hydrophobic properties of MPs rather than polymer density. This study uses three size ranges of four polymers. The size range was selected to demonstrate the effect of size on extraction efficiency, while polymer types were selected for their abundance in wastewater [15], and their range in density-both higher and lower than that of water-was used to demonstrate the efficacy of oil extraction regardless of density. Results and Discussion Inorganic Digestion Methods All extraction methods had average recovery rates above 96% across polymer types and sizes, while the range of recovery varied considerably between methods. F1 had a narrow recovery range (92%-100%), indicating that it is suitable across polymer types. Iron impurities formed during the production of KOH [16] and the iron catalyst used in Fenton's reagent can result in precipitation of ferric solids. Of the three Fenton's reagent methods, F2 experienced the greatest amount of iron precipitation, containing 17.3 mg on average. Mass retention of MPs tended to decrease with smaller size, although PP samples had a relatively stable recovery. Statistical analysis via one-way ANOVA tests (α = 0.05) with controlled polymer type and size (degrees of freedom = 5, F-critical = 3.106) found that extraction methods had statistically significant recovery rates for several data groups. Additionally, all tests that did not find a difference between recovery methods were statistically insignificant. However, a larger sample size could have resulted in more statistically significant results. Small MPs were evaluated using scanning electron microscopy (SEM) to compare changes in surface morphology before and after treatment. Images for virgin polymers and each extraction method on small PS spheres are shown below (Fig. 2). HCl deforms the entire shape, while KOH, H2O2, and F1 create flaking and possible minor precipitation. F2 covers much of the surface in iron precipitate, while F3 creases the surface. Oil Extraction Methods The oil extraction procedure (OEP) with oleic acid and canola oil showed average recoveries of 91% and 87%, respectively, from primary influent, and 95% and 92%, respectively, from tertiary effluent. In tertiary effluent, oleic acid outperforms canola oil for all polymer types and sizes except medium LDPE (Fig. 3). Overall, there is no significant difference in removal between MP sizes. Similar results (90-100%) can be found for other studies that performed OEP on solid and aqueous matrices [17], [18]. However, these two studies used a lower MP size limit of 200 and 300 µm, respectively, which does not account for most MPs found in wastewater [19], [20]. Further, few if any studies on MP extraction via OEP are evaluated on wastewater, whose composition differs considerably from surface water. The full paper includes an expanded discussion of the role of commercial oil grades on the variability and repeatability of the experiments. Conclusion

*The best method is F1 due to low precipitation and relatively high recovery stability.

*Combining F1 with OEP may further enhance this method by increasing non-MP organic matter removal while minimizing MP degradation.

*In testing polymers that are representative of those found within wastewater, these results can be applied directly to wastewater-microplastic analysis.