Simple Generation of Suspensible Secondary Microplastic Reference Particles via Ultrasound Treatment

Abstract

In the environment the weathering of plastic debris is one of the main sources of secondary microplastic (MP). It is distinct from primary MP, as it is not intentionally engineered, and presents a highly heterogeneous analyte composed of plastic fragments in the size range of 1 µm−1 mm. To detect secondary MP, methods must be developed with appropriate reference materials. These should share the characteristics of environmental MP which are a broad size range, multitude of shapes (fragments, spheres, films, fibers), suspensibility in water, and modified particle surfaces through aging (additional OH, C=O, and COOH). To produce such a material, we bring forward a rapid sonication-based fragmentation method for polystyrene (PS), polyethylene terephthalate (PET), and polylactic acid (PLA), which yields up to 10 5 /15 mL dispersible, high purity MP particles in aqueous media. To satisfy the claim of a reference material, the key properties-composition and size distribution to ensure the homogeneity of the samples, as well as shape, suspensibility, and aging -were analyzed in replicates (N = 3) to ensure a robust production procedure. The procedure yields fragments in the range of 100 nm−1 mm (<20 µm, 54.5 ± 11.3% of all particles). Fragments in the size range 10 µm−1 mm were quantitatively characterized via Raman microspectroscopy (particles = 500-1,000) and reflectance micro Fourier transform infrared analysis (particles = 10). Smaller particles 100 nm−20 µm were qualitatively characterized by scanning electron microcopy (SEM). The optical microscopy and SEM analysis showed that fragments are the predominant shape for all polymers, but fibers are also present. Furthermore, the suspensibility and sedimentation in pure MilliQ water was investigated using ultraviolet-visible spectroscopy and revealed that the produced fragments sediment according to their density and that the attachment to glass is avoided. Finally, a comparison of the infrared spectra from the fragments produced through sonication and naturally aged MP shows the addition of polar groups to the surface of the particles in the OH, C=O, and COOH region, making these particles suitable reference materials for secondary MP.

TUM-ParticleTyper: A detection and quantification tool for automated analysis of (Microplastic) particles and fibers

“…Reference materials were produced by ultrasonication of solid polymers and filtrated onto gold coated polycarbonate filters [24]. The filters carrying the reference materials were then PLOS ONE used to optimize the camera settings of the Raman microscope (alpha300R Raman Microscope, WITec GmbH, Germany) and the scanning electron microscope (Sigma 300 VP, Carl Zeiss AG, Germany).…”

Particle Size and Pre-Treatment Effects on Polystyrene Microplastic Settlement in Water: Implications for Environmental Behavior and Ecotoxicological Tests

“…ED is the ratio between calculated suspended solid volume Vcss and theoretical suspended solid volume Vtss and indicates the state of the dispersion (fully dispersed vs. agglomerated/attached to surfaces etc.). Vtss was calculated using suspended solid mass and a PS density of 1.05 g/cm 3 . Vcss was calculated using the particle counter data and the lower channel boundary (see SI, section D) and volume formula for spherical shape.…”

“…This has to be seen critically since usual contact times of adsorption experiments are in the magnitude of 48-72 hours [28,29] with even less efficient dispersion. Theoretical position of an exemplary cryomilled polystyrene (PS) particle with a density of 1.05 g/cm 3 during settling in the course of a measurement with PAMAS particle counter. The particle starts to sink at the water level.…”

“…MP exposure experiments conducted with only a few details on applied mass, particle size distribution and concentration are hard to compare [1]. Prerequisites have been formulated for particle characterization used in toxicology studies regarding surface area or size distribution but also homogeneity of dispersion [2,3]. Only fully dispersed, stable dispersions with most particles existing separately allow for correct extrapolations of mass/number concentration effects.…”

“…Artificial weathering can be achieved by UV photodegradation or chemical oxidation using hydrogen peroxide or ozone. Van der Esch used an alkaline oxidation with potassium hydroxide to simultaneously fragment and oxidize polystyrene (PS), polyethylene terephthalate (PET) and polyactic acid (PLA) polymers to obtain a stable dispersion [3]. The advantage of surface activation lies in the mode of stabilization which can closely resemble environmental weathering processes and thus create more realistic scenarios.…”