There is an urgent need to reliably quantify and qualify microplastics in the various environmental matrices. However, such a task involves distinguishing microplastic particles from natural particles such as sand, plant and animal debris. Even with the employment of spectroscopic identification methods like Fourier- Transformation Infrared spectroscopy (FTIR) and Raman spectroscopy, the interference caused by the natural particles can significantly impede the detection of microplastics. Hence, microplastics need to be separated from their natural matrices. Organics have a similar density to microplastics and need to be removed via chemical digestion methods, such as oxidation, alkali, acids, and enzymatic reactions. These reactions may inadvertently alter the microplastics being investigated.
The goal of this study was to optimize and validate a standardized sample preparation method to remove organic matter from wastewater samples, without altering the microplastics. This was tested on several microplastics (PS, PE, PP, PET, PVC, PA and PLA) by first optimizing and validating the sample preparation methods for larger microplastics fragments (80 – 330 μm) and then re-validating the two resulting methods (Hydrogen peroxide and Fenton protocols) for smaller microplastic fragments (< 10 μm).
Fenton reaction was further utilized during a field sampling campaign in cooperation with the Institut für Energie- und Umwelttechnik (IUTA), where effluents of wastewater treatment plants (WWTPs) were sampled to determine the´microplastic retention efficiency of a tertiary sand filter. The sampling system consisted of a sealed system made entirely out of metal to avoid plastic contamination. The system consisted of a high-volume pump and three modular cartridges, where a set of steel mesh candle filters (100 μm, 50 μm, 10 μm) were placed in a cascade. Additionally, the effluent of the 10 μm filter was also sampled in order to account for smaller particles (< 10 μm). Sampled volumes were 5000 - 7000 Liters for 100 μm & 50 μm filters, 200 Liters for 10 μm filter, as well as 2.5 Liters for the fraction < 10 μm.