Development of cost-effective methodologies to identify and quantify microplastics in seawater samples

The accumulation of plastic debris in marine environments and their potential negative effects on marine organisms have become issues of high priority for environmental policy. Microplastics (MPs; 5 mm - 1 µm) are reported to be within the most abundant pieces of litter found in the marine environment accumulating in the water column, on beaches, in sediments, in biota and in the air. Despite recent improvements on the procedures to detect and identify MPs, there is still a need to standardise methods and to produce guidelines for a cost-effective detection, identification and quantification of MPs. The standardisation and establishment of cost-effective methodologies is essential to facilitate comparison among studies, to enable a robust risk-exposure assessment of organisms and humans to plastic particles, and to support the establishment of long-term monitoring programmes. The goal of this work is to create low-cost procedures for the identification of MPs (over 50 µm) in seawater samples, which could be applied in laboratories, in-situ and in citizen-science projects. We have tested and optimised the digestion of seawater samples with a heavy load of organic matter using Fenton’s Reagent. Collected MPs were dyed using Nile Red, observed under blue and ultraviolet light filters, allowing detection and visualization of the particles under a fluorescence microscope. A classification tree model was constructed to distinguish between polymer-based particles and other (cellulose, mineral, etc) particles. To discern particles from being plastic or not, we developed a classification tree model with the C4.5 algorithm. We validated our methodology using a quality-control sub-sample set, where all the particles were further analysed using micro Fourier Transform Infrared (µFTIR) Spectroscopy. Preliminary results show that the classification tree model has an 82 % probability of a correct classification of MPs. We anticipate that our results will enable a cost-effective and standardized methodology for identification of MPs in seawater samples, supporting the establishment of long-term monitoring programmes.