(Bio-)Degradation of Plastic in the Environment

This chapter covers the conditions and mechanisms of plastic degradation in the environment. Physical factors such as UV light and heat, and the presence or absence of oxygen and water influence these processes. Mechanical forces lead to the formation of physical fragments resulting in micro-and nanoparticles. Depolymerization of the polymers that make up the plastic can occur by purely chemical reactions, for example, hydrolysis and oxidation, or can be mediated by enzymes released by capable microbes or a combination of all these factors. Products of polymer chain scission that reach a size small enough to be taken up by microbes can be further metabolized, used as a source of energy and carbon, and finally be converted to carbon dioxide, methane, and water. Apart from the chemical structure of the polymer, which must allow the access of suitable enzymes to cleave specific bonds, the rate of biodegradation highly depends on environmental conditions, with temperature, water, and the abundance of able microbes being the main drivers. The chapter gives an overview on what is known about the biodegradation of synthetic conventional polymers and biodegradable polymers. It also covers the natural nonplastic polymers used in cellulosic fibers and touches on the "cellulose enigma." In addition, the trend toward using (bio-)degradability as an engineered property of plastics is critically summarized. Finally, considerations are given to different classes of environmental (bio-)degradation rates and the implications for what is acceptable as a lifetime of a plastic item released to the environment.