Micro- and nanoplastics in soil : Analytical methods and environmental fate

Nhu Phan Le, Quynh and Halsall, Crispin and Quinton, John and Surridge, Ben and Ashton, Lorna (2025) Micro- and nanoplastics in soil : Analytical methods and environmental fate. PhD thesis, Lancaster Environment Centre.

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Abstract

Microplastics (MPs) and nanoplastics (NPs) are emerging pollutants in various environments, with soil identified as their largest reservoir. However, their sources, environmental fate, transport mechanisms, and impacts remain poorly understood, primarily due to challenges in analysing MPs and NPs within complex soil matrices. While several analytical approaches exist for microplastic analysis in soil, standardized approaches are lacking, and research on nanoplastic is extremely limited despite their higher toxicity potential. To address these gaps, this PhD study first developed and optimized a quick and efficient extraction method for MPs from soil, followed by quantification with Nile red staining-fluorescence microscopy for its speed, cost-effectiveness, and high sample throughput (Chapter 2). This method was then compared with digital microscopy, Fourier-transformed infrared and Raman micro-spectroscopies, pyrolysis gas chromatography coupled with mass spectrometry, and quantitative proton nuclear magnetic resonance spectroscopy, each employing tailored extraction protocols (Chapter 3). Testing with spiked MPs of various types, sizes, and soil types (clayey, loamy and sandy) revealed significant impacts of extraction and analytical methods on recovery rates. Fluorescence microscopy was particularly effective for detecting small conventional plastics, while proton nuclear magnetic resonance spectroscopy excelled in analysing biodegradable MPs. Organic matter and clay in the soil matrix were identified as key complicating factors. Fluorescence microscopy, combined with Raman and Fourier-transformed infrared micro-spectroscopies for chemical identification, was further applied to investigate agricultural soil organic amendments as a major source of soil plastics (Chapter 4 and 5). This included investigation of MPs in sewage sludge and anaerobic digestate from biogas plants, as well as soils treated with these materials. Microplastics as small as 25 μm were detected, with concentrations reaching 3650 MPs/g in sewage sludge and 1050 MPs/g in anaerobic digestate. Amended soils exhibited significantly higher MPs concentrations than control fields, with detailed analyses confirming the transfer of plastics by type, size, and shape. Additionally, this study developed a novel nanoplastic extraction method coupled with thermal desorption proton transfer reaction mass spectrometry for highly sensitive NPs analysis (Chapter 6). Applying this approach to Antarctic soils revealed NPs concentrations of up to 300 ng/g, with atmospheric source modelling indicating contributions from both local and long-range deposition, alongside clear seasonal patterns. This PhD research marks a significant advancement in analytical methods for microplastics and nanoplastics in soil, provides critical evidence of agricultural practices and atmospheric transport as plastic contamincation sources, and ultimately delivers essential data for risk assessment and policy development to tackle plastic pollution—one of the most pressing environmental challenges of our time.