In an article published on November 2, 2024, in the Journal of Hazardous Materials, Lisbet Sørensen from SINTEF Ocean AS, Trondheim, Norway, and co-authors compared the chemical profile of a wide range of plastic and elastomer (rubber) products upon subjecting them to different extraction and leaching conditions.
For the first set of experiments, the scientists selected 51 thermoplastic and elastomer plastic products, including food packaging, and cut them into small pieces. These were extracted with ethyl acetate-dichloromethane solvent supported by ultrasonication, methanol, or thermal desorption. In a second set of experiments, the influence of environmental parameters on the leaching of chemicals from plastic particles was investigated using four elastomer products: dishwashing gloves, balloons, laboratory gloves, and car tire rubber. The scientists cryo-milled the elastomer samples and subjected the resulting particles, < 1000 µm, in a concentration of 0.5 g/L to salinity (seawater, 20 °C, 1-25 days), different temperatures (5 vs 20°C, 7 days), and turbulence (static, low 1 rpm, vs. 235 rpm, 1-35 days). The scientists also compared chemical leaching from different particle concentrations; 0.5 g/L and 2 g/L, after 7 days at 20 °C under high turbulence. Extracted or leached chemicals were analyzed by non-targeted (pyrolysis) gas chromatography-mass spectrometry (py-GC-MS).
Sørensen and co-authors found the number of chemical features in extracts varied with the applied extraction method, with chemical features as approximations of different types of chemicals. The greatest number of features were extracted using ethyl acetate-dichloromethane, followed by methanol, while thermal desorption showed much fewer features. For instance, the car tire sample showed 4075 peaks in the ethyl acetate-dichloromethane extract, 2456 in methanol, and only 804 in the thermal desorption chromatogram. These were also the samples with the highest number of chemical features detected, indicating high chemical complexity. The authors pointed out that thermal desorption can only be applied to some polymers while solvent extraction can be applied to a wide range of materials.
By comparing the chemical features from the 51 samples with the mass spectral library NIST17, a total of 475 were tentatively identified in ethyl acetate-dichloromethane extracts, 385 in methanol, and 155 in the samples undergoing thermal desorption. Most were unique to the respective extraction method, with only 19 features overlapping between the three methods. These included polymer monomers (e.g., caprolactam, CAS 105-60-2), softeners (e.g., n-butylbenzenesulfonamide, CAS 3622-84-2), antioxidants (e.g., 2, 6-di-tert-butyl-4-methylphenol, CAS 128-37-0), and flame retardants (e.g., triphenyl phosphate, CAS 115-86-6).
Comparing the chemicals leaching (or, migrating) from the four selected products ground into microplastic into seawater with those in the corresponding extracts showed that the chemical complexity was comparable, but the composition was very different. While the impacts of salinity and temperature on leaching depended on the chemical, turbulence generally had a minimal impact. The scientists further found that an increase in particles subjected to leaching did not only change the concentration but also the composition of chemicals in the leachate. The authors concluded, “that multiple factors play a role in chemical leaching, where the physicochemical properties of individual chemicals result in different factors dominating.”
Based on their findings, Sørensen and co-authors provide recommendations for the experimental design of aquatic toxicity studies and risk assessment. They emphasize that harmonized protocols are needed to “increase the comparability of polymer material leachate studies and toxicity studies.” In particular, they call for microplastic leachate toxicity studies to be performed with and without the presence of particles.
Reference
Sørensen, L. et al. (2024). “Towards realism in hazard assessment of plastic and rubber leachates – Methodological considerations.” Journal of Hazardous Materials. DOI: 10.1016/j.jhazmat.2024.136383
