By Alex Waldie, Graduate Student, Faculty of Science
The oxidation and eventual decomposition of plastics in the environment has been gaining attention as the public becomes increasingly aware of microplastics. These small plastic particles are becoming ubiquitous in the environment and their effects on human and environmental health are now beginning to be identified. Understanding how these plastic particles degrade overtime could provide new ways to accelerate their decomposition and the recycling of bulk plastic into small molecules that could be repurposed. This could reduce the amount of plastic that enters landfills and help create value from plastic waste.
While researchers generally understand the environmental processes for the mechanical degradation, oxidation, and decomposition of plastics, a mechanistic understanding of the chemical and enzymatic processes responsible is less well defined. This is especially true for plastics like polyethylene, which only contain carbon-carbon bonds to connect the monomers and are therefore sometimes called non-hydrolysable. This makes it very difficult for an enzyme to degrade since there are no functional groups like esters to exploit. Of the 5,600 million metric tons (Mmt) of global plastic waste resin produced by the end of 2018 over 4,300 Mmt (76%) is non-hydrolysable including over 2,300 Mmt of polyethylene, which generally enters the environment as single use plastic such as plastic bags, films, and containers.1
Previous studies have shown that oxidized regions of polymers like polyethylene, caused by manufacturing defects or environmental processes like UV exposure, are the first to be broken off and metabolized by microorganisms. Chemical methods of oxidizing polyethylene have been developed which use inorganic catalysts to introduce alcohol groups that may be further oxidized for the purposes of degradation or used for advanced polymer modifications. However, these chemical methods tend to use chlorinated solvents, high temperatures, expensive reagents, and introduce unwanted modifications. Instead, the relatively mild aqueous conditions that most enzymes operate at offers an opportunity to reduce these unwanted modifications while at the same time provide insight into the initial steps of biodegradation.
Before screening the activity of oxidative enzymes, previously characterized chemical catalysts for the oxidation of polyethylene are being synthesized. The purpose of this is to refine the analytical procedure used to determine the extent of oxidation of polyethylene as well as to provide references for future work with oxidative enzymes. These samples may also be used by other research groups to better understand how environmental oxidation changes the spectroscopic signature of polyethylene. This could aid in determining a microplastic particle’s history and the environments it has been exposed to.
The catalysts used to create these reference standards have been previously characterized by the Hartwig Group2,3 and utilize oxygen donors to introduce specific oxygen functional groups, like alcohols, to polyethylene. These catalysts use a central metal atom to coordinate the oxidizer so that it may more efficiently attack the polyethylene. The oxidized polyethylene can then be characterized by instrumental methods such as proton nuclear magnetic resonance (1H-NMR) and Fourier-transform infrared spectroscopy (FTIR). Refining this procedure is important so that we have a consistent approach to characterizing the oxidative effects of untested enzymes and methods. If effective enzymes are identified using these processes, the mechanism of oxidation will be characterized and attempts will be made to increase its efficiency. Hopefully, the insight into the enzyme oxidative process, gained from these experiments, will aid in addressing the plastic waste crisis.
References
- Geyer, R. Chapter 2: Production, Use, and Fate of Synthetic Polymers. In Plastic Waste and Recycling; Letcher, T. M. Elsevier: Amsterdam, 2020; 13-30.
- Hartwig, J. F.; Li, Q.; Lee, S.; Bunescu, A. Catalytic Hydroxylation of Polyethylenes. ACS Cent. Sci. 2017, 3, 895-903. DOI: 10.1021/acscentsci.7b00255
- Hartwig, J. F.; Messersmith, P. B.; Sanchez, D.; Uliana, A.; Malollari, K. G.; Chen, Liye, C. Selective Catalytic Oxidation of C-H Bonds in Polyethylenes Produce Functional Materials with Enhanced Adhesion. Chem. 2021, 7, 137-145. DOI: 10.1016/j.chempr.2020.11.020