Method Validation: Extraction of Microplastics from Organic Fertilisers

It has been demonstrated that organic fertilisers could be a source of microplastics (MPs) in agricultural soils. These organic fertilisers comprise a diverse array of matrices including organic waste and by-products. Currently, there is no established methodology for the extraction of MP from these matrices. The present article aims to validate a standardised protocol for the extraction of MPs from a diverse range of complex, organic-rich samples. The protocol has been developed to ensure a high recovery of MPs, to preserve their integrity, and to eliminate organic particles that interfere with FTIR analyses. Spiked MPs sized 315–5000 µm were subjected to a two-step process involving chemical digestion (H2O2, 30% (w/v), 53 °C) and density separation (NaI, >1.60 g·cm−3). This resulted in a mean extraction rate exceeding 95%, with undigested matter remaining below 5%. No evidence of fragmentation was observed. Furthermore, the chemical nature of spiked microplastics is still perfectly interpretable from the FTIR spectra despite the different chemical treatments undergone. These findings thus validate the method for the microplastic range 315–5000 µm. However, a new method for reanalysing the project’s data produced contrasting results, suggesting a significant drop in recovery rates for size ranges below 250 µm. This reanalysis approach constitutes the second innovation of this protocol, and enables a more critical analysis of the results obtained in publications on microplastics.