Enhanced microplastic retrieval efficiency from cultivated soil samples through optimized pre-treatment in density-based extraction

Abstract

Accurate assessment of microplastic contamination levels in soil is critical to understanding the potential risks they pose to ecosystem health and for developing effective mitigation strategies. However, efficient methods for extracting microplastics from soil samples are still lacking. In this study, we optimized the existing extraction methods through the pre-treatment of soil samples. The pre-treatment-based protocol included several phases. Firstly, the soil samples were treated with anhydrous ethanol to remove biofilm and dirt adhering to the surface of microplastics. Subsequently, the samples were subjected to washing with a metal sieve under pressurized liquid flow to remove small sand particles trapped within the microplastic pores. To further enhance the extraction efficiency, a short pre-digestion step with 30% hydrogen peroxide was introduced to remove organic matter adhered to the microplastics’ surface, with the aim of restoring the density of environmental microplastics to their polymer components. Following the pre-treatment steps, a rapid and efficient microplastic flotation process was conducted using a 1.5 g/cm3 ZnCl2 solution. Spiking experiments using microplastics demonstrated this method could process a large sample volume (100 g dry weight) and achieve high extraction efficiencies ranging from 88 % to 96 % for different microplastic types. The extracted microplastics were found to be virtually free of impurities, facilitating subsequent qualitative and quantitative analyses. Validation experiments further confirmed the superiority of the optimized method compared to traditional density-based approaches. The optimized method yielded 50.7% more microplastics than traditional density-based methods, it exhibited a significant advantage (p < 0.05) in the extraction of polyethylene, with an extraction rate increase of 88.6 %. The high efficiency of this method in extracting microplastics was thoroughly demonstrated. It represents a simple, low-cost, efficient, and low-toxicity approach for extracting microplastics from soil samples. Consequently, it holds significant potential for inclusion in future standardization efforts regarding microplastic extraction from soil samples.