Can Chemists Solve the Plastic Problem?

 

    An article published in Science News talks about how chemists are working on trying to solve one of the biggest problems to our environment: plastics. In 2018, 27 million tons of plastic made it into landfills, while only 3 million tons were recycled. This large disparity is a result of recycling not producing useful products that manufactures can use. In the U.S., there are two major types of plastics that are recycled: high-density polyethylene (HDPE), which can be found in milk jugs and detergent containers, and polyethylene terephthalate (PET), which is found in soda bottles and will be the main plastic talked about in this post. The article in Science News specifically talks about four ways different ways chemists are trying to solve this recycling issue, but I will only highlight two in this post: breaking products down into different plastics and breaking plastics down to a molecular level.

     A major issue when trying to recycle plastic products is that each individual plastic needs to be separated, because when certain plastics are mixed together, they yield unwanted results. Normally a simple assembly line where people or machines are picking out plastics works sufficiently enough to separate out the different types of plastics, especially when talking about milk jugs and soda bottles, but some plastic products are complicated, like deodorant containers, where the cap, crank, and bottle are all different types of plastics.

    George Huber of the University of Wisconsin-Madison and colleagues set out to solve this issue by coming up with a process that uses liquid solvents to dissolve individual plastic components, which allows for a separation of the individual plastics in a complex product. The process Huber and colleagues produced was successful in getting 95% of each plastic out of the product. The process involved stirring the plastic in a toluene solvent, which dissolved the polyethylene layer and then putting it into DMSO, which stripped off ethylene vinyl alcohol (EVOH), leaving only PET. The use of antisolvent chemicals was then used to separate the polyethylene and EVOH from the solvents. Huber plans to look at different solvents and more plastics to eventually make this method a way of effectively sorting even the most complicated plastic products.

    Another issue is that recycled plastics obtain the characteristics from the original product, making it harder for manufactures to produce something useful out of them and recycling breaks chemical bonds allowing products to only be recycled a certain number of times.

     To solve this issue companies started working on a new type of recycling called chemical recycling. Chemical recycling involves breaking down the plastic at a molecular level, but this recycling method is costly, energy inefficient, and varies for each plastic type. One of the leading companies in this research is a company in France called Carbios. They are studying PET first because it is the easiest to take apart and they found that cutinase, which is an enzyme produced by microorganisms, cutinase breaks down PET when it has been mutated. The mutated cutinase broke down 90% of PET in 10 hours, whereas the non-mutated cutinase only broke down 50%. Cutinase breaks down PET into ethylene glycol and terephthalic acid, which can be made into recycled materials more easily than just using PET.

    Both of these methods can still be improved on, as well as the methods not mentioned in this post. Chemists are actively trying to solve the global plastic problem, but many of these methods may take years to fully develop, so other actions should be taken to reduce the use of plastics.

Sources:

https://www.sciencenews.org/article/chemistry-recycling-plastic-landfills-trash-materials

Tournier, V., Topham, C.M., Gilles, A. et al. An engineered PET depolymerase to break down and recycle plastic bottles. Nature. 2020, 580, 216-219. DOI: 10.1038/s41586-020-2149-4.

Walker, T.W., Frelka, N., et al. Recycling of multilayer plastic packaging materials by solvent-targeted recovery and precipitation. Sci. Adv. 2020, 6. DOI: 10.1126/sciadv.aba7599.

https://www.worldwildlife.org/stories/what-do-sea-turtles-eat-unfortunately-plastic-bags

https://www.chemistryworld.com/news/plastic-eating-bacteria-show-way-to-recycle-plastic-bottles-sustainably/9556.article