Popular Science: Small is nice? And sometimes toxic.
Plastics production has been rising rapidly in recent decades. This has given rise to questions: what happens to plastics and how is our environment affected? Regrettably, despite recycling, plastics often remain in the nature, from the ocean to the forest. Try to look under your feet during a trip to nature. What can you find there? In addition, over time, these plastics break down into smaller parts, which – if smaller than 5 mm – are now called microplastics. As these substances are not inert, they can trigger various reactions in the environment. However, the specific dangers are often largely unknown.
Research studies have focused mainly on microplastics occurrence in the aquatic environment, as the separation of microplastics there is significantly easier than in soil. However, according to the results to date, there may be up to 10 times more microplastics in the terrestrial ecosystems than in the oceans. In addition, they have an adverse effect on both soil fauna (e.g., increased earthworm mortality) and the fundamental soil properties (bulk density, water holding capacity or sorption of persistent organic pollutants such as PAHs, DDT, PCBs).
Scent fences located along agricultural landscape and roads are being increasingly seen. Particularly, they are intended to protect agricultural crops or forest nurseries against deer or to prevent car accidents with animals. These scent fences are made of polyurethane foams, but these break down over time and their small particles fall into the surrounding environment. The soil around the roads is also heavily contaminated with PAHs from traffic. Therefore, Tereza Černá, together with her team, focused on the effect of microplastics from scent fences on the sorption of PAHs. Two localities that were contaminated with PAHs in the past due to industrial wood impregnation were selected for research: Soběslav and Srní. Two types of polyurethane foam were used for the experiment: biodegradable and conventional. The experiment, during which the microplastics were exposed to soil containing PAHs, lasted a total of 28 days.
Microplastics made of biodegradable polyurethane foam exhibited a strong tendency to accumulate PAH after only 7 days and their concentrations increased significantly over time; at the end of the experiment, the concentration of PAH in the microplastics was even 70 times higher than in the soil. By contrast, the sorption of PAHs on microplastics made of conventional polyurethane foam was significantly lower (maximum 3.6 times higher concentration than in soil). Accelerated ageing of the polyurethane foam, which resulted in enlargement and modification of the surface, did not have a significant effect on the sorption in any of the cases. Depending on the different sorption of the materials used in the experiment, the flexibility of the polyurethane polymeric network proved to be a possible main driving factor for the sorption.
The results of the experiment revealed the pathways of organic pollutants in the soil environment contaminated with microplastics. The individual types of polyurethane foams exhibited considerably different sorption of PAHs. An important question is whether PAHs absorbed in microplastics are no longer available to living organisms, or, conversely, their higher concentration in microplastics can lead to their increased intake and possible further effects of these dangerous substances on living organisms. Researchers will try to answer these and more questions in ongoing research.
Černá, T., et al. 2021. Polycyclic aromatic hydrocarbon accumulation in aged and unaged polyurethane microplastics in contaminated soil. Science of the Total Environment 770, 145254; https://doi.org/10.1016/j.scitotenv.2021.145254