Development of an extraction and determination method for microplastics in compost and soil matrices

Plastic pollution and its degradation into microplastics (MPs) represent a critical environmental challenge. Initially studied in aquatic ecosystems, MPs are now recognized as ubiquitous emerging contaminants, detected in atmosphere, water, soils, compost and soft tissues of living organisms. This widespread occurrence raises increasing concern about their environmental fate and potential impacts on ecosystems and human health, particularly in terrestrial matrices such as compost and agricultural soils. Our goal was to develop and evaluate a method for extracting and determination MPs in these complex matrices, given their significance for agricultural sustainability. A methodology was developed based on reported techniques, adapted to overcome the inherent heterogeneity of the samples. The protocol included physical separation and analysis. Characterization involved Fourier-transform infrared spectroscopy (FTIR) for polymer identification, optical microscopy for morphometric analysis, and statistical methods to compare samples. Extraction was performed via sequential density fractionation (NaCl: 1.2 g·mL⁻¹; ZnCl₂: 1.6 g·mL⁻¹; NaI: 1.8 g·mL⁻¹) and oxidative digestion with H₂O₂, followed by filtration and stereomicroscope counting. We analysed three compost samples with different C/N ratios and one reference sample, four paddy soil samples from surface and subsurface horizons, flood sediment, soil amended with pelleted compost, and an undisturbed forest soil as a negative control. Process blanks and air controls (with/without air conditioning) were included to assess laboratory air quality. Results confirmed the presence of MPs in all samples, with maximum extraction in the intermediate-density solution (1.6 g·mL⁻¹). In compost, concentrations ranged from 1,793 to 8,736 microparticles per kilogram (MP kg⁻¹). In soils, surface horizons contained higher MP abundance (≥ 5 × 10³ MP kg⁻¹) than subsurface horizons (≤ 10³ MP kg⁻¹), with amended soils and sediment showing intermediate levels. Statistical analysis revealed significant differences between samples and a positive association between MP abundance and organic matter content. Air controls indicated airborne contamination, exacerbated by air conditioning use. Although FTIR could not conclusively identify polymers due to fouling, detailed analysis of particle shape, size, and colour was achieved via spectroscopic microscopy. MPs in the reference compost were attributed to cross-contamination, highlighting the challenge of avoiding it. In summary, the method successfully extracted MPs, showing a predominance of mid-density plastics isolated with ZnCl₂. Our findings also emphasize the need for strict anti-contamination measures, especially in non-specialized labs. This preliminary study underscores the urgency of developing efficient and reproducible protocols for accurate polymer identification and confirms MP pollution as a priority issue in compost and agricultural soil research.