Challenges in optimising methods for extraction of microplastics from complex environmental matrices

Abstract

Microplastics are a widespread contaminant of concern globally. Society, researchers and policy makers are interested in the quantities of microplastics present in the environment and the harm they pose to habitats, organisms and humans. Due to the rapid rise of research published finding microplastics in new complex environments, there has also been an increase in the development of complicated, multi-staged microplastic extraction methods. Successful and impactful results often rely on efficient and reliable methods. Researchers in this field face challenges when developing methods for these new, complex environments, such as standardisation, validation and producing accessible methods that consider monetary cost and environmental impact.
Validation of methods is rarely undertaken, but when it is, it is achieved by performing a recovery rate study. These studies involve ‘spiking’ the environmental matrix with known amounts of microplastics, running the method and counting the recovered microplastics. To investigate published validation studies, a meta-analysis of 71 recovery rate studies, was carried out. Recovery rates were found to vary dependant on the environment studied, but were highest from plant material, whole organisms and excrement (>88% recovery). The average recovery rates provided an indication that microplastics in the environment could be underestimated by approximately 14% depending on the methods used. Methods were then replicated and developed to extract microplastics from fishmeal, which is a complex environmental medium found to contain microplastics. Fishmeal is a foodstuff that is of high importance for global food security and the microplastic contaminants have potential to enter food chains. Fishmeal properties were measured to investigate their influence on recovery of microplastics. An environmentally benign, generally safe and affordable method using calcium chloride, a dispersant and potassium hydroxide was found to produce the highest recovery rate (66.3%). Positive correlations were found between recovery rate and protein content of fishmeal, whereas negative correlations were found between recovery rate and organic content. To fully optimise methods for assessment of microplastics it is important to consider emerging analytical methods in the field and to test these with environmentally relevant material. The use of solvent extractions and thermal desorption GC-MS were investigated for their effectiveness at identifying chemical indicator markers of virgin microplastics, artificially weathered microplastics and naturally weathered microplastics. Chemical indicator markers were more commonly present in direct thermal desorption samples than the solvent extraction samples, and in some cases the most appropriate markers were present for weathered microplastics, but not for the virgin microplastics.
The findings of this thesis highlight the importance of validating methods when trying to achieve standardisation, and the implications of underestimating true microplastic abundance due to low recovery rates. Development of affordable, safe and environmentally conscious methods is possible, however there may be a trade-off between this and effectiveness of the method. Moreover, this thesis has demonstrated how the recovery of microplastics can vary dependant on environmental and microplastic properties. Thus microplastics should not be considered a single contaminant, but more like a mixture of chemicals and additives with different histories in various matrices. With the prospect of standardised, validated, affordable, safe, environmentally benign and realistic methods, research in this field would become more accessible to many people worldwide, which is needed when studying such a ubiquitous group of pollutants.

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Identifiers

ORCID for Chloe Jane Way: orcid.org/0000-0002-0940-3995

ORCID for Ian Williams: orcid.org/0000-0002-0121-1219

ORCID for Graham Langley: orcid.org/0000-0002-8323-7235

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