understanding the generation of micro and nano plastics

Gustave BERTIER

Plastics are ubiquitous in the environment. They accumulate there in diverse forms, mainly resulting from a combination of chemical modification of the plastic through photo-oxidation and fragmentation or abrasion caused by applied mechanical stress. Among these forms are included solid fragments called microplastics and nanoplastics, as well as soluble and volatile compounds. In order to better determine their impacts, more and more studies are being conducted to quantify and characterize these products from a physical and chemical point of view. In this study, we propose to develop and use innovative methods to study the generation of the various degradation products of two plastics at the molecular, microscopic, and macroscopic scales in order to quantify the kinetics of generation of these different products in a more relevant way and to suggest a scenario that explains all of the observed phenomena. This study was conducted on industrial plastic pellets made of low-density polyethylene (LDPE) and polybutylene adipate terephthalate (PBAT) exposed to UV light, then mechanically stirred in water to induce fragmentation of the photo-oxidized surface layer. This allows the degradation process in the environment to be reproduced in a controlled manner. LDPE was chosen as one of the most widely produced and consumed conventional plastics, while PBAT, which is considered biodegradable, is used to replace it in certain applications. Initially, all degradation products were quantified using an original method: total carbon balance, which focuses on an element present in all organic polymers and retained during the degradation process. This was achieved by analyzing the carbon masses in the degradation products through total organic carbon analysis in the aqueous phases and elemental analysis for solid products, over increasingly longer aging times. Thanks to this assessment based on the fate of carbon in different forms, it was possible to evaluate the quantities of by-products released by polyethylene in the form of volatile compounds (volatile alkanes, CO2, CO, etc.), which represent up to 12% of the products formed. In comparison, PBAT degrades very differently from LDPE under the conditions tested: surprisingly, cross-linking is predominant, inhibiting the formation and release of microplastics. In a second step, in order to complete our understanding of the mechanisms, we studied, using Fourier transform infrared spectroscopy (FTIR), the evolution of photo-oxidation markers such as the carbonyl index on LDPE granules and obtained, adjusted, and interpreted the oxidation profiles over time. At the same time, the cracking and fragmentation phenomena were studied under an electron microscope for both plastics. Finally, the molecular-sized compounds generated by plastics were isolated and studied, and chemical reaction pathways were also proposed for obtaining these compounds. Thus, for the first time, LDPE degradation products were characterized at different scales, from macroscopic to molecular. These results enable us to propose a comprehensive scenario for the degradation behavior of both plastics and to deduce potential consequences for plastic waste in the environment.