Researchers analyze degradation of polyethylene in the environment – ScienceDaily – Advice Eating

Polyethylene, a plastic that is as cheap as it is easy to process, accounts for almost a third of the world’s plastic waste. An interdisciplinary team from the University of Bayreuth has investigated the progressive degradation of polyethylene in the environment for the first time. Although the degradation process leads to fragmentation into smaller and smaller particles, isolated nanoplastic particles are rarely found in the environment. The reason for this is that such decomposition products do not like to be left alone, but quickly attach themselves to larger colloidal systems that occur naturally in the environment. The researchers have now presented their results in the journal science of the whole environment.

Polyethylene is a plastic that occurs in various molecular structures. Low-density polyethylene (LDPE) is widely used to package everyday consumer goods such as food and is one of the most widely used polymers in the world due to increasing demand. So far, there are only estimates of how this widely used plastic degrades after it enters the environment as waste. A research team from the Collaborative Research Center “Microplastics” at the University of Bayreuth has now systematically investigated this question for the first time. To do this, the scientists developed a novel, technically sophisticated test setup. This allows two well-known and environmentally-related processes of plastic degradation to be simulated independently of one another in the laboratory: 1.) Photooxidation, in which the long polyethylene chains gradually break down into smaller, more water-soluble molecules under the influence of light, and 2.) increasing fragmentation due to mechanical stress. On this basis, detailed insights into the complex physical and chemical processes of LDPE degradation could be gained.

The final stage of LDPE degradation is of particular interest for studies addressing the potential impact of polyethylene on the environment. The researchers found that this degradation does not end with the decomposition of the packaging material released into the environment into many micro- and nanoplastic particles that have a high degree of crystallinity. This is because these tiny particles have a strong tendency to aggregate: They quickly attach themselves to larger colloidal systems made of organic or inorganic molecules and are part of the environmental material cycle. Examples of such colloidal systems include clay minerals, humic acids, polysaccharides, and biological particles from bacteria and fungi. “This aggregation process prevents individual nanoparticles, which result from polyethylene degradation, from being freely available in the environment and from interacting with animals and plants. However, this is not the all-clear. Larger aggregates that participate in the material cycle in the environment and contain nanoplastics are often ingested by living organisms and can thus eventually enter the food chain,” says Teresa Menzel, one of the three first authors of the new study and a doctoral student in the field of polymer materials.

In order to identify the degradation products formed during the degradation of polyethylene, the researchers used a method that is not very common in microplastics research: multi-cross polarization in solid-state NMR spectroscopy. “With this method, we can even quantify the degradation products of photooxidation,” says co-author Anika Mauel, PhD student in inorganic chemistry.

Bayreuth researchers have also discovered that the degradation and decomposition of polyethylene also leads to the formation of peroxides. “Peroxides have long been suspected of being cytotoxic, i.e. having a toxic effect on living cells. The degradation of LDPE also poses a potential threat to natural ecosystems. These relationships need to be further investigated in the future,” adds co-author Nora Meides, a PhD student in macromolecular chemistry.

The detailed analysis of the chemical and physical processes involved in the degradation of polyethylene would not have been possible without the interdisciplinary networking and coordinated use of state-of-the-art research technologies on the University of Bayreuth campus. These include, in particular, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), NMR spectroscopy, Fourier transform infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC).

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