Validation of sample preparation methods for small microplastics (≤10 µm) in wastewater effluents

The interest to monitor microplastics in various environmental matrices has grown substantially in recent years. However, monitoring microplastics remains a challenge due to interactions with complex environmental matrices, especially those rich in natural organic matter (NOM) like wastewater effluents. Therefore, sample preparation methods are needed to remove NOM and ensure minimal interference during analysis. So far, there have been only a few attempts to standardize and validate the effects of sample preparation methods on microplastics. However, no efforts exist that validated the impacts of sample preparation methods on small microplastic particles (≤10 µm). Those smaller particles might be more susceptible to adverse effects after chemical digestion as a result of the increased surface area to mass ratio, compared to their larger counterparts. In this study, pellets from six polymers were successfully tagged with a fluorescent dye and subsequently fractured using ultrasound treatment to produce small particles of tagged microplastic. The generated microplastics (≤10 µm) were then subjected to two sample preparation methods that were previously validated for wastewater effluents, namely Fenton and hydrogen peroxide oxidation. The effects on microplastics were assessed using size distribution changes, which were measured via fluorescence microscopy. Results revealed some large changes in the size distribution of microplastics (≤10 µm) after applying Fenton and hydrogen peroxide. However, these changes were largely reduced when excluding particles < 1 µm. The results indicated that, for the most part, both sample preparation methods can be used for small microplastic particles (1–10 µm), whereas hydrogen peroxide is more suitable if nanoplastics (<1 µm) are to be investigated.