Novel integrated workflow for microplastics extraction, quantification, and characterization in organic fertilizing residuals using micro-Fourier transform infrared spectroscopy (μ-FTIR)

Organic fertilizing residuals (OFRs) enhance soil fertility and support sustainable agriculture due to their rich nutrient and organic matter content. However, these materials are increasingly recognized as a significant source of microplastics (MPs) in agricultural soils, raising concerns about the safety of agroecosystems. Therefore, there is an urgent need to develop a reliable workflow for MP analysis in diverse OFRs, given the challenges of extracting small MPs from such organic matter-rich matrices. This study presents an oxidative-alkaline tandem digestion method that achieves an average organic matter (OM) removal efficiency of 93 % across various OFRs. In addition, density centrifugation with NaCl and ZnCl₂ brines was utilized to recover six microplastic polymers (PP, PVC, PET, PS, PE, and HDPE), achieving a recovery rate of over 95 % for large MPs (600 μm-4.75 mm) and over 83 % for small MP-PE beads (38-45 μm). Micro-Fourier transform infrared spectroscopy (μ-FTIR) analysis confirmed that digestion and separation steps did not affect MPs' spectral integrity and chemical identification. To validate the workflow, we applied it to analyze MPs in various OFRs from Québec, allowing for the successful detection of 19 MP polymers with sizes down to 10-50 μm. This workflow can be applied to multiple OFRs to extract, quantify, and characterize MPs. Ultimately, this workflow will facilitate efficient MPs analysis across diverse OFRs, providing essential data for robust risk assessment and better environmental management to mitigate MP pollution from OFR applications in agricultural soils.