Nanoplastic Particles Are Looking For A Connection: Bayreuth Researchers Analyze The Degradation Of Polyethylene In The Environment

Almost a third of the world's plastic waste consists of polyethylene, a cheap and easy-to-process plastic. An interdisciplinary team from the University of Bayreuth has researched the progressive degradation of polyethylene in the environment for the first time. This process leads to fragmentation into smaller and smaller particles. Nevertheless, isolated nanoplastic particles are hardly found in the environment. The reason: These decomposition products do not like to be left alone. They quickly attach to larger colloidal systems that occur naturally in the environment. The researchers present their results in the journal Science of the Total Environment.

Polyethylene is a plastic that occurs in various molecular structures. Low-density polyethylene (LDPE) is often used for packaging of everyday consumer goods, for example in the food sector, and is one of the most common polymers worldwide due to increasing demand. Until now, there have only been estimates of how this common 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. For this purpose, the scientists have developed a novel, technically sophisticated experimental setup. This makes it possible to independently simulate in the laboratory two well-known processes of plastic degradation that are linked to one another in the environment: photooxidation, in which the long polyethylene chains are gradually broken down into smaller, potentially water-soluble molecules under the influence of light, and increasing fragmentation due to mechanical stress. On this basis, it was possible to gain detailed insights into the complex physical and chemical processes involved in the degradation of LDPE.

The last stage of LDPE degradation is of particular interest for studies dealing with the possible effects of the environmental pollution caused by polyethylene. As the researchers have found, 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, which consist of organic or inorganic molecules and are part of the material cycle in the environment. Examples of such colloidal systems are clay minerals, humic acids, polysaccharides or biological particles from bacteria and fungi. “This process of aggregation prevents individual, Nanoparticles formed by polyethylene degradation are freely available in the environment and interact with animals and plants. But that doesn't mean the all clear. Larger aggregates that participate in the material cycle in the environment and contain nanoplastics are often ingested by living organisms. This is how nanoplastics can eventually get into the food chain,” says Teresa Menzel, one of the three first authors of the new study and doctoral student in the field of polymer materials.

To identify the degradation products that form when polyethylene breaks down, the researchers used a method that has not been used very often in microplastic research: cross-polarization with multiCP sequences in solid-state NMR spectroscopy. "This method even enables us to quantify the degradation products that are formed as a result of photooxidation," says co-author Anika Mauel, a doctoral student in the field of inorganic chemistry.

The 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. In this respect, too, the degradation of LDPE poses a potential threat to natural ecosystems. These relationships must be examined more closely in the future,” adds co-author Nora Meides, PhD student in the field of 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 the coordinated use of the latest research technologies on the Bayreuth campus. These include, in particular, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), X-ray diffractometry (XRD), NMR spectroscopy, Fourier transform infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC).