A workflow for improving estimates of microplastic contamination in marine waters: A case study from North-Western Australia

Abstract

Plastic pollution is ubiquitous throughout the marine environment, with microplastic (i.e. <5 mm) contamination a global issue of emerging concern. The lack of universally accepted methods for quantifying microplastic contamination, including consistent application of microscopy, photography, an spectroscopy and photography, may result in unrealistic contamination estimates. Here, we present and apply an analysis workflow tailored to quantifying microplastic contamination in marine waters, incorporating stereomicroscopic visual sorting, microscopic photography and attenuated total reflectance (ATR) Fourier transform infrared (FTIR) spectroscopy. The workflow outlines step-by-step processing and associated decision making, thereby reducing bias in plastic identification and improving confidence in contamination estimates. Specific processing steps include (i) the use of a commercial algorithm-based comparison of particle spectra against an extensive commercially curated spectral library, followed by spectral interpretation to establish the chemical composition, (ii) a comparison against a customised contaminant spectral library to eliminate procedural contaminants, and (iii) final assignment of particles as either natural- or anthropogenic-derived materials, based on chemical type, a compare analysis of each particle against other particle spectra, and physical characteristics of particles. Applying this workflow to 54 tow samples collected in marine waters of North-Western Australia visually identified 248 potential anthropogenic particles. Subsequent ATR-FTIR spectroscopy, chemical assignment and visual re-inspection of photographs established 144 (58%) particles to be of anthropogenic origin. Of the original 248 particles, 97 (39%) were ultimately confirmed to be plastics, with 85 of these (34%) classified as microplastics, demonstrating that over 60% of particles may be misidentified as plastics if visual identification is not complemented by spectroscopy. Combined, this tailored analysis workflow outlines a consistent and sequential process to quantify contamination by microplastics and other anthropogenic microparticles in marine waters. Importantly, its application will contribute to more realistic estimates of microplastic contamination in marine waters, informing both ecological risk assessments and experimental concentrations in effect studies.

Spatial distribution of microplastics in sediments and surface waters of the southern North Sea

“…Particles with a match of at least 700 (out of 1000) were counted as safely identified. If the match ranged between 600 and 700 the spectra were manually compared to database spectra and evaluated based on expert knowledge, as suggested by other studies (Hanke et al, 2013;Kroon et al, 2018;Lusher et al, 2013).…”

“…It has been shown that visual identification alone is insufficient (Kroon et al, 2018;Song et al, 2015) imaging are the most commonly used (Hanke et al, 2013;Käppler et al, 2016;Silva et al, 2018). Generally, there is a requirement within the research field to establish the usage of polymer characterization methods that omit the need for pre-sorting.…”

“…According to Kroon et al (2018) a merely visual identification of the potential MP particles is not sufficient. More than 60% of the particles might be misassigned (Hidalgo-Ruz et al, 2012;Kroon…”

Evaluating wastewater effluent as a source of microplastics in environmental samples

“…The majority of them (14) employed no chemical confirmation to guarantee plastic as the material and relied on visual observation alone. The smaller the particle, the less accurate the plastic identification by visual techniques, with particles below 1.5 mm having a significant increase in error of identification (Löder et al, 2017;Kroon et al, 2018). Raman and FTIR spectroscopy are complimentary techniques for determining microplastic polymer types, as they are able to characterize extremely small microplastics (with a minimum size of 1 µm and 20 µm, respectively) (Käppler et al, 2016).…”

New toxic emerging contaminants: beyond the toxicological effects

“…Air, water, and soil are continuously being exposed to these intruders and, as a direct consequence, human beings, and all living organisms on the planet as well. One of the most mediatic contaminants during the recent years among the aforementioned are plastics, microplastics, and nanoplastics, being a huge problem in our oceans and river waters (Zhu et al 2018;Alimi et al 2018;Kroon et al 2018).…”