Innovations in analytical methods to assess the occurrence of microplastics in soil

The occurrence of microplastics (MPs, particles <5 mm) in the environment has raised concerns globally due to their extensive use, slow degradation, low recycling rates, and potential risks to the ecosystem and human health. In the last decade, research on MPs in soil ecosystems has increased but is relatively limited compared to studies on MPs in aquatic environments. The soil matrix complexity and the ubiquitous presence of MPs in the environment make the analytical method development, validation, and Quality Assurance/ Quality Control (QA/QC) procedures challenging to analyze MPs in soil. This review paper comprehensively summarizes the emerging techniques and state-of-the-art methods for extracting, analyzing, and characterizing MPs in soil. The choice in analytical technique ultimately relies on the research objectives, whether polymer type, morphological information, quantity, or the presence of MPs in soil is of interest. Among the reviewed studies, globally, a broad range of concentrations (up to 6.9 × 105 particles/kg) of MPs was observed. However, the data was mainly limited to China. Lastly, the validation of analytical methods, QA/QC procedures, and recommendations for future research are addressed. There are no standardized methods to analyze MPs in soil and validate analytical processes, making it difficult to compare methods and results among studies, resulting in large differences in reported results between laboratories participating in interlaboratory studies. This indicates a need to improve and further develop more robust and efficient analytical techniques for analyzing MPs in soil, conduct more interlaboratory studies, manufacture and use certified reference materials, and promote large-scale monitoring research globally.