Microplastic extraction protocols can impact the polymer structure

Abstract

Although microplastics are ubiquitous in today’s natural environments, our understanding of the materials, quantities, and particle sizes involved remains limited. The recovery of microplastics from different types of environmental matrices requires standardized matrix digestion protocols that allow inter-laboratory comparisons and that have no effect on the polymers themselves. A number of commonly used digestion methods rely on oxidation with concentrated hydrogen peroxide solutions to remove organic matter from the matrix. However, this can alter the nature of polymers through hydrolysis and often does not lead to a complete matrix removal. We have therefore investigated the use of two altered matrix digestion protocols, an acidic (Fenton) protocol and a new alkaline (Basic Piranha) protocol, focusing mainly on the effect on biodegradable polymers (polylactide, polybutylene adipate terephthalate, polybutylene succinate) and polymers with known degradation pathways via hydrolysis (thermoplastic polyurethanes, polyamide). Comparing the initial surface textures, chemical compositions, and particle size distributions with those obtained after digestion revealed that the Fenton protocol left most of the polymers unchanged. The ferrous residue that remains following Fenton digestion had no effect on either the polymer composition or the particle size distribution, but could disturb further analytics (e.g. Raman microscopy due to auto-fluorescence). While increasing the chance of complete matrix removal, the more powerful Basic Piranha protocol is also more likely to affect the polymer properties: Polylactide polymers in particular showed signs of degradation under alkaline digestion (reduced polylactide content, holes in the polymer matrix), indicating the unsuitability of the Basic Piranha protocol in this specific case. Polyamide, however, remained stable during the Basic Piranha treatment, and the surface chemistry, the particle size as well as the molar mass distribution of the investigated thermoplastic polyurethanes were also not affected. Hence, this protocol offers a powerful alternative for microplastic analysis with focus on particle size in more complex environmental matrices (e.g. removal of cellulose in soil), while avoiding ferrous Fenton residue. Unexpectedly, also tire rubber, a frequent target analyte in microplastic monitoring, was found to be susceptible to artefact structures by both oxidation protocols. In summary, controls for the specific combination of polymer and sample preparation protocol are highly recommended to select the most fitting protocol. Here selected suitable combinations are reported.

Hide-and-seek: Threshold values and contribution towards better understanding of recovery rate in microplastic research

“…The Fenton reaction has been used widely and effectively to purify material removing biogenic organic matter. Its appropriateness and safety as to polymers degradation has been approved by previous studies [22 , 27] . Therefore, we assume, the last treatment steps in rather long protocol may result both in increased loss of the particles and number of deformed particles (see graphical abstract) due to cumulative effect.…”

SEM/EDX analysis of stomach contents of a sea slug snacking on a polluted seafloor reveal microplastics as a component of its diet

“…The complexity of the processes of cause/effect characterizing microplastic implies the use of different protocols each of them optimized for a specific target of study. Additionally, even if studying microplastic is nowadays of a central importance and several standardized protocols of extraction and analysis have been published 97 – 102 , the continuous search for the most effective or performing one is still ongoing 103 – 105 . Furthermore, even though considerable research effort focused on several target species, (mainly vertebrates from fish to mammals) 17 , 19 , 21 , few invertebrates were investigated so far, and, among them, filter-feeder molluscs (bivalves) were mainly studied 15 , 31 , 39 , 57 .…”

Effect of Polymer Properties on the Biodegradation of Polyurethane Microplastics

“…A Non-destructive Extraction Process Is the Premise for Reliable Investigations on Microplastic Biodegradation in Complex Environmental Matrices. Harsh conditions can alter the particle properties, 39,40 which is why we focused on the development and validation of an efficient and mild extraction method. The use of intentionally added microplastics <500 μm in compost allowed removal of larger compost components by sieving (<1 mm).…”