Microplastic analysis in soils: A comparative assessment

Microplastic (MiP) contamination poses environmental risks, but harmonizing data from different quantification methods and sample matrices remains challenging. We compared analytical protocols for MiP quantification in soil, consisting of Digital, Fluorescence, Fourier-transform infrared (FTIR), and Raman Microscopy as well as quantitative Pyrolysis-Gas Chromatography-Mass Spectroscopy (Py-GC-MS) and 1-proton nuclear magnetic resonance (¹H NMR) spectroscopy as detection techniques. Each technique was coupled with a specific extraction procedure and evaluated for three soils with different textures and organic carbon contents, amended with eight types of large MiPs (0.5-1 mm) - high- and low-density polyethylene (HDPE and LDPE), polypropylene (PP), polystyrene (PS), polyamide (PA), polyethylene terephthalate (PET), polyvinyl chloride (PVC), and a biodegradable mulch film product composed of polybutylene adipate-co-terephthalate/ polylactic acid (PBAT/ PLA). In addition, we included two types of small MiPs (20-250 µm) composed of either LDPE or PBAT/ PLA in the tests. The results showed that protocols for Digital, Fluorescence, and ATR-FTIR microscopy recovered 74-98 % of the large MiPs, with fluorescence yielding the highest recoveries. Raman spectroscopy was most sensitive to soil organic matter residues, requiring more sophisticated sample pretreatment. Fluorescence staining with subsequent Fluorescence microscopy detection effectively recovered most small-sized LDPE-MiP but missed 56-93 % of small PBAT/ PLA particles. For the latter, reliable quantification was achieved only using Soxhlet extraction combined with ¹H NMR spectroscopic quantification. Pyrolysis-GC-MS showed intermediate results, displaying low sensitivity to plastic type and lower recoveries as soil clay content increased. We conclude that different methods have different sensitivities for different MiP materials in different soils, i.e. comparisons of MiP loads and threshold settings for MiP loads across methodologies require careful consideration. Yet, our data indicate that adding stained large MiP as an internal standard could enhance extraction control, while Soxhlet-extraction with subsequent ¹H NMR analysis is most powerful for controlling future thresholds of small MiP from biodegradable materials.