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Legacy Plastics Release Toxic Metal(oid)s During Environmental Degradation
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Scientists found that old plastic items on beaches contain toxic metals like lead and chromium that get released when the plastic breaks down from sunlight and wear. When these "legacy plastics" degrade, they create tiny cracks that allow dangerous metal particles to escape into the environment as separate pollutants. This matters because these toxic metals can end up in our food chain and water supply, potentially harming human health beyond the risks we already know about from microplastics alone.
Metallic additives are extensively incorporated into plastics materials to improve their functional properties. Although the use of hazardous substances is now largely restricted in the EU, numerous metal(oid)s, such as lead, cobalt and hexavalent chromium (Cr(VI)), were historically used in plastic formulation1,2. Yet, as these legacy plastics undergo environmental degradation, the fate of the metal(oid)s they contain remains poorly understood.Chromium speciation in plastics debris collected from beaches in Guadeloupe (France) was first investigated using micro- X-ray absorption spectroscopy (micro-XAS) combined with micro-X-ray fluorescence (micro-XRF). Chromium was found predominantly as Cr(III) within cobalt chromite (CoCr2O4), but also as toxic Cr(VI) in crocoite (PbCrO4) or potassium chromate (K2CrO4)3. Mineral phases occur as micro- and nanoscale particulate additives embedded within the polymer matrix.To assess the behavior of such chromium additives during plastic degradation, representative PP fragments, containing high chromium concentrations, were altered in environmentally relevant conditions, following Blancho et al.4. Micro-XAS analyses combined with micro- and nano-XRF imaging reveal that crocoite micro- and nanoparticles dominate chromium speciation in both macro- and microplastics. However, chromium contents and speciation are drastically modified in nanoplastics. A ~95% reduction in total chromium concentration is observed. Remaining chromium occurs as Cr(III) diffused within the polymer matrix.Micro and nano-tomography imaging, in addition to SEM observations, were conducted to track the processes leading to the fragmentation and the release of toxic additives such as crocoite. In the plastic, before UV-C exposure, particulate additives are embedded in the polymer matrix, with air pockets around them. These additives are separated by more than 100 nanometers. UV-C irradiation induces the development of fracture networks from the plastics surface to subsurface, which connect the additive pockets to each other.Altogether, this study combining laboratory and field experiments demonstrates a new model for the release of metallic additives during plastic alteration. UV-C exposure creates a network of fractures that compromises the polymer integrity by interconnecting the pockets of additives particles. Subsequent mechanical erosion promotes polymer fragmentation. This two-step process results in the liberation of nanoplastics that are free from metallic particulate additives, which are released on their own. This model is crucial to understand the mechanisms governing the environmental release of toxic metal(oid)s during plastic alteration. References1 Bridson et al., (2021), Journal of Hazardous Materials, 414, 125571.2 Turner and Filella, (2021), Environment International, 156, 106622.3 Catrouillet et al., (2021), Environ. Sci.: Processes Impacts, 23, 553–558.4 Blancho et al., (2021) Environ. Sci.: Nano, 8, 3211–3219.
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