Analysing the role of surface aging on the impact of microplastic pollution in soil matrices

Savage, Dom and Surridge, Ben (2024) Analysing the role of surface aging on the impact of microplastic pollution in soil matrices. PhD thesis, Lancaster University.

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Abstract

Plastics are modern-day super materials, capable of being tailored to virtually every material application imaginable. However, their low cost, resilient nature, and meteoric rise into prolific production has contributed significantly to their widespread pollution of the environment. In recent years citizens and scientists alike have recognised this as an emerging ecological and environmental crisis, and as such keen concern has followed, and significant interest to understand and address this crisis has emerged. Once present in the environment plastics present a number of novel hazards. Whilst they are resilient to complete mineralisation, they readily break down and fragment into smaller particles known as microplastics and nanoplastics. These tiny particles are promptly dispersed throughout the natural world, making their retrieval and cleanup all but impossible. As a result, understanding what happens to these materials as they age within the environment, and assessing what impact they will have, is essential. This study is naturally an uphill battle for a number of reasons: these materials are highly diverse, owing to their ability to be synthesised readily from a wide variety of monomers in a wide variety of arrangements, their ability to be further adapted with chemical additives, and their uncontrolled degradation in environmental matrices which changes their properties further still. Because of this, existing research has often focused on the use of so-called ‘pristine’ materials; outof-the-factory polymers which are easy to obtain, characterise, and categorise. However, whilst this approach is a sensible first step, it leaves a lot of critical understanding out of the picture. More recently, research has progressed to test hypotheses with ‘aged’ materials, as they are more representative of pollution found in the environment. There are a number of common methods for approaching this, but they have their own limitations, and are generally quite slow, making rapid progress on this issue challenging. In very recent studies, new potential tools have been discussed to evaluate hypotheses related to material aging on much faster timescales, such as the use of plasmas. In this thesis, plasma degradation was pioneered as a proxy for environmental aging in microplastics research. Plasma surface treatment is not a new technology, and indeed polymers have been routinely exposed to plasma in other fields of research to achieve a variety of outcomes. However, its application as a potential aging method to rapidly simulate environmental photodegradation is all but missing. Herein an in-depth analysis and optimisation of plasma degradation for this application was made, with recommendations for sensible implementation of this technique. Overall, it was deemed a rapid alternative method that can significantly accelerate the pace of aging-focused studies, whilst remaining a plausible abstraction of photodegradation within appropriately designed methodologies. This technique was demonstrated to have enormous potential in its application to microplastics research. The optimised aging protocol was then carried forth into two research chapters: the first an analysis of environmental sorption dynamics of common soil nutrients to aged plastic surfaces, using XPS as a method to determine surface adhesion. Results indicated material-specific and aging-specific sorption occurs, however it was concluded that further research was needed to ascertain if nutrient immobilisation is a real concern. The second was a soil incubation study, investigating the impact that the environmental aging of microplastic pollution will have on soil health. These results concluded that even in a low (but representative) concentration of 0.25% w/w microplastics have a small but notable and significant impact on soil health, with some treatments being able to influence phosphorus and ammonium availability, affect soil aggregation, as well as alter pH and EC values within the soil. There was evidence of aging-specific variation, however these data indicated material-specific features were more relevant. These studies demonstrated that plasma surface treatment can be used as an effective and sensible aging protocol for microplastics research in the environment.