The Perpetual Plastic Puzzle

BY KERRY MILLER

GRAPHICS BY TERRA PLUMB

Picture it – it is Friday afternoon, the sun is shining, and you have finally put the finishing touches on your Jacobson’s Catalyst report for organic chem, so you decide to take a nice, peaceful stroll down Owen’s Beach. The birds are singing, the Sound smells fresh – everything is as it should be. But then, out of the blue, it happens. An assault on your senses. The peace is shattered. A plastic water bottle on the beach? A discarded Chaco tangled in the seaweed? A vape pen laying smashed under a rock? What is going on?

We have learned about the environmental harm that comes with littering since we were kids, and know that the solution lies in ‘reduce, reuse, recycle’, but with plastics, things aren’t all that simple. Most plastic made since the 1950s still exists today, polluting rivers and oceans and mucking up beaches and forests. Only about 10% of the plastic produced every year ends up getting recycled due to both chemical and economic difficulties (1). Luckily, scientists have been working hard the last few decades to come up with some solutions to this puzzle, and hopefully soon, we will be able to walk on the beach with no plastic in sight.

What is Plastic?

Have you ever picked up a plastic bottle and seen the recycle symbol on it with a number inside? Those numbers can tell you which type of plastic you have! There are 6 plastics that are most commonly used in consumer products (number 7 means “other”), and these six are most responsible for creating the bulk of the plastic waste in landfills and the ocean. But what is a plastic exactly?

Plastics are polymers. A polymer is a long chain of molecules with repeating units called monomers (Fig. 1). As seen in table 1, each of these common plastics have a different monomer, and that change is enough to give each plastic its own set of properties. But whatever their differences, for the most part they are pretty similar: they don’t dissolve in water, they are bendy, and all of them can be melted down and reshaped for recycling purposes. However, melting down the plastic degrades its purity, and each piece of plastic can only be recycled so many times before it is too unstable to be used. Because of this, scientists have been looking for alternate ways of degrading existing plastics since the late 70s (2). There are now hundreds of new ways of recycling plastics, all with their pros and cons, and the research into them being used on an industrial scale continues.

Fig. 1: Structure of a polymer.

Table 1: Although very similar, these common plastics vary slightly in some important qualities (3).

Part 1: Re-refining

Plastic is made from crude oil products, specifically naphtha (a mix of small carbon molecules). Depending on which hydrocarbon is used, a different plastic will form. In theory, another way of recycling plastic is to turn it back into these small hydrocarbons (polyethylene to ethene instead of ethene to polyethylene). There are a few ways this has been accomplished, the two most common are incinerating the plastic and converting the resulting gasses back into the starting reagents, or heating and pressurizing the plastic until the chains broke apart (4, 5). This takes an immense amount of energy and requires gas capture – a notoriously difficult process to perfect.

A plastic chain is very chemically stable and does not like to react with other chemicals, and a lot of heat is needed to break that stability. But if the plastic is simply burned (oxidizing the carbon-carbon bonds in the backbone), the resulting products will be CO2 and CO (along with solid carbon and water), which we do not want to be releasing. If these gasses are captured, they can be converted to methanol and methyl chloride, which can be converted back into ethylene, but not very easily (Fig. 2). Breaking the chains with heat and pressure is not much better – so much heat has to be used (Fig. 3). Both of these processes are so expensive, and creating new plastic is pretty cheap. Even if these processes were more efficient, there would be no economic incentive to use them. This area of research has not seen much progress, with more research devoted to inventing a plastic that could be recycled cyclically for far cheaper than any currently existing plastic (6).

Figure 2: Cyclic recycling of polyethylene using incineration. All those catalysts and heat becomes expensive! Due to this (and the complexity of the process leading to loss of material) this is pretty much a theoretical process and is not used industrially (5, 7, 8, 9)

Figure 3: Cyclic recycling of polyethylene using depolymerization. This process is more common than the incineration method, but because it is impossible to control where the polymer chain will break, much of the product is lost as random hydrocarbons that cannot reform plastic without additional steps and reagents. (4,5)

Part 2: Biological degradation

Hundreds of biological organisms have shown potential in their ability to degrade various types of plastic. Plastic degrading bacteria make up the bulk of this research, but a number of fungi, insects, worms, and amoeba have also been observed eating away at plastics (10). Each species uses a slightly different pathway, and each plastic reacts slightly differently in different environments, making these organisms highly specific in the plastic they consume. This specificity means that biological degradation of plastics is generally not thought to be feasible on an industrial scale, since most recycled plastics are mixed. However, if the plastic degrading enzymes in these organisms are isolated, there is a chance that the process could be recreated outside the organism on a large scale, making this sort of research highly valuable to the recycling industry.

Most bacteria generally use the same process to degrade plastics, which is actually very similar to how humans break down fats! Because breaking the main chain into smaller carbon molecules is generally the most difficult part of plastic degradation, identifying the enzymes used for this process is usually the purpose of studying these organisms (10).

One of the more promising studies is that of the bacterium Ideonella Sakaiensis, identified in soil samples near landfills in Japan in 2016 (2). This bacterium uses a metabolic pathway very similar to many other species of bacteria, but given its location, seems to have evolved to be far more efficient at plastic metabolism than other related species. Since plastic is not a naturally occurring substance, it is likely that these bacteria adapted to the high abundance of plastic in their soil until they were able to live and grow on it as a sole energy source, while other bacteria are used to using smaller, more easily degraded carbon molecules. Ideonella Sakaiensis is specific to PET, one of the most common single use plastics. Due to its structure (containing more oxygen than other plastics), PET has seen more luck with biodegradability than most other plastics. The two enzymes isolated from I. Sakaiensis are both hydrolases (Fig. 4). This combination of enzymes (PETase and MHETase) is the only process identified for any plastic that yields nearly full degradation outside of the bacteria, and is being investigated as a method for mass degradation of PET.

Figure 4: How I. Sakaiensis is able to degrade PET. When it is hungry (all the time), the bacterium secretes 2 enzymes: PETase and MHETase. After some cool hydrolysis and oxidation reactions, two small and tasty molecules are created (TPA and EG) which are then eaten by the bacterium. After processing these for energy, the bacterium excretes them as carbon dioxide and other small organic molecules (bacteria poop). Hooray! No more plastic! (2, 10)

Part 3: Repurposing

Because, like everything else, recycling is a business, economic feasibility is just as important to the recycling process as the science that makes it happen. That is why more and more, we see plastic repurposing a lot more often than biological degradation. One of the more exciting propositions helps fix two environmental issues at once: plastic pollution and sand harvesting. Although it seems like it is everywhere, sand is not a renewable resource, and the harvesting of sand can sometimes destroy the environment it came from – lots of things live in sand! Most sand harvested is used in the concrete industry, but only sand from river beds and the ocean can be used (11). These sand particles are jagged and are essential for holding the concrete together. Unfortunately, this sand is also where many sand-dwelling creatures live! To help reduce on the amount of sand needed as aggregate, waste plastic and microplastic particles have been proposed as an alternative. However, if the strength of the concrete is compromised by the plastic, it can’t be used. Since plastic is generally valued for its flexibility and low melting point, this is an issue.

In 2017, researchers at MIT came up with a clever idea to fix this issue (12). The rigidity of a plastic can be increased by increasing crystallinity and cross linking in the plastic. Plastics are generally not highly crystalline due to the extremely high molecular mass of a single strand. Crystallinity can be increased by shortening the strand, which is a bit of an issue (carbon-carbon bonds are strong!). Degree of crosslinking is basically the amount of bonds between strands. Luckily, both crystallinity and crosslinking can be increased with one process: subject the plastic to gamma radiation. High levels of radiation can form free radicals in the plastic chains, which leads to excessive crosslinking and shorter strands.

When tested against normal strength concrete, this gamma radiated plastic-concrete was still not as strong or heat resistant, but fared much better than concrete with untreated plastic. Although there is still a long way to go with this research, there is clearly some promise. This study only worked with one type of plastic (PET again), and it would be interesting to see how results would vary with PE family plastics. One idea would be to try crosslinking with polysilane (basically a one-dimensional strand of sand). Since it is unknown if the binding of the sand to the cement is mechanical (the growth of interlocking crystals) or chemical (covalent bonding to the sand), perhaps introducing the chemical that makes up sand would yield better results. Sounds like a fun project to me!

Conclusions

From worms eating plastic bags to melting water bottles to plastic sand to reverse plastic, the number of ways plastic can be gotten rid of is rising every day. While none of them are perfect yet, the research continues to grow and the answer seems closer every day. Just last August, while the rest of us were hunkering down getting ready for the start of another Zoom semester, a group of pioneering chemists unveiled the newest addition to the growing list of ‘infinitely recyclable’ plastics: PBTL. With high rigidity, thermal resistance, and 100% recyclability, PBTL shows great promise in becoming a suitable plastic alternative in the near future (6). Theoretically, these totally recyclable polymers like PBTL would drastically reduce the need to continuously produce and throw away plastics. So do not fret! Even in the middle of a global pandemic with the earth warming, animals dying, and plastic threatening to suffocate us all, science will come to the rescue! So, next time you find yourself confronted by that lone Chaco in the seaweed, or that sad water bottle in the sand, don’t pout, throw it out! Who knows? Maybe that crushed vape pen sitting in a landfill will help to evolve an incredible vapepen-eating bacterium that will save the world one day. At the very least, your post-Jacobsen lab report walk will be a tiny bit more pretty.