The history of synthetic long-chain polymer plastics is the story of the modern world, which would not exist without plastics. From preventing contamination of our food to making aircraft light enough to leave the ground, these remarkable molecules are intimately connected to conveniences we now take for granted.
Versatile, lightweight, colourful, and strong, plastics began to change society beginning in the 19th century with collodion or Parkesine. Among the earliest polymers to reach commercial application was celluloid, created in the 1860s by American John Wesley Hyatt. He combined cellulose with camphor to create a material that could be moulded into a variety of shapes. The invention stopped elephants’ trajectory towards extinction by replacing ivory billiard balls. Hyatt’s invention also saved tortoises, which were being decimated due to demand for their shells, used for jewellery and other decorative items.
Later, plastics made from fossil fuel hydrocarbons would influence the outcome of the Second World War, with acrylic polymers or poly(methyl methacrylate) like Plexiglass replacing glass airplane windows. Nylons also replaced silk in parachutes, while other polymers were used in body armour and helmets.
Ubiquitous plastics have greatly benefitted society but the very qualities that make plastic useful have an unfortunate consequence. Unlike the natural materials they replaced — wood, bone, ivory and shell — plastics are resistant to the process of decay that impacts all other biological material. Although plastics can and do break down, the rate of degradation can be extremely slow: some polymers can persist in the environment for decades or even centuries. In the years since their creation, plastics have accumulated not only in landfills but increasingly in ocean environments and landscapes. The most famous example is the Great Pacific garbage patch, also described as the Pacific trash vortex. Covering an area three times the size of France — which is about 550,000 square kilometres — the patch is estimated to contain more than 80,000 tonnes of plastic or 1.8 trillion pieces of plastic, the vast majority being tiny pieces less than half a centimetre in size.
David Levin, University of Manitoba, Department of Biosystems Engineering
While innovation continues into new uses for plastics, there has been far less research into dealing with it as a waste product. One of the scientists addressing this challenge is molecular biologist and biotechnologist David Levin of the University of Manitoba’s Department of Biosystems Engineering. Like many of his colleagues, he is motivated by the sheer scale of the problem.
“From 1950 to 2015, 4,900 megatonnes — about 4.9 billion kilograms — of plastic were discarded in landfills and the environment,” he says. “Only about nine percent has been recycled. And recycling doesn’t get rid of plastic. It simply reconstitutes it in another form, so it returns to the environment.”
Plastics in the environment eventually erode into micro and nanosized particles that are washed into freshwater and marine environments, including the Great Pacific garbage patch. Once they are small enough, the particles end up in the digestive tracts of zooplankton and other tiny organisms. They are then bioaccumulated by filter feeders such as shellfish and crustaceans and from there continue to the top of the food chain. A report, released this past October by the major consulting body Environment Agency Austria, revealed that humans are now ingesting plastic from a variety of sources, including fish, tap water and soft drinks.
“There is a huge pending ecological catastrophe we’re facing with these nano- and micro-sized particles,” says Levin.
Yet, he holds out hope that nature itself may yet inspire a solution to the problem. This past November, Levin and his team published “Microbial degradation of low-density polyethylene and synthesis of polyhydroxyalkanoate polymers” in the Canadian Journal of Microbiology. The paper reported on what he describes as the first direct evidence for simultaneous bioconversion of low-density polyethylene (LDPE) to biodegradable polyhydroxyalkanoates. In other words, it seems possible that bacteria might ultimately provide the solution to plastic pollution by converting non-degradable plastics into degradable ones.
Bacteria bring biodegradability
Levin and his fellow researchers chose to work with LDPE, the most abundant commercially produced plastic, which accounts for around 60 percent of all discarded plastics, mostly in the all too familiar form of bags. The research was jumpstarted by Zahra Montazer, a student who recently completed a PhD in Levin’s lab after originally contacting him while studying plastics degradation in a landfill in her native Iran. Montazer was trying to isolate bacteria that could degrade low-density polyethylene and a scholarship from the Ferdowsi University of Mashhad, held by her supervisor in Iran, Habibi Najafi, facilitated further studies in Canada.
Montazer’s PhD project complemented Levin’s work on production of polyhydroxyalkanoates (PHAs), a family of biodegradable and biocompatible polymers synthesized by a variety of bacteria, including Cupriavidus necator (C. necator) and Pseudomonas putida (P. putida). C. necator produces polyhydroxybutyrate, a short-chain PHA consisting of subunits with only four carbon atoms, while P. putida produces medium-chain PHAs, which consist of a mixture of subunits with six, eight, 10, 12, or 14 carbon atoms. PHA polymers produced by C. necator are already being used in some commercial applications, such as medical stitches that naturally degrade as the wound heals. One of the interesting observations made in the paper is that the PHA polymers synthesized by C. necator and P. putida after consuming polyethylene were very different from the polymers these bacteria synthesize when grown on other substrates, like sugars or fatty acids.
Dozens of C. necator and P. putida variations have already been isolated around the world. Some strains of P. putida, a common soil bacterium, break down hydrocarbon petroleum products while others promote plant growth or produce antibiotics. Levin tested a well-characterized strain of P. putida as well as two other Pseudomonas species (P. chloroaphis and P. montiellii). Montazer isolated an additional P. putida strain sourced from the landfill in Iran. Of all the bacteria tested, the C. necator and P. putida strains were best at degrading LDPE.
Montazer began by pre-adapting bacteria, growing them on paraffin oil, which has linear hydrocarbon molecules similar to the structure of LDPE. Gradually, the paraffin oil was reduced until the P. putida grew solely on LDPE powder. Of the eight bacteria tested, some were more effective than others, with the best ones reducing the mass of the LDPE by 30 percent, “which is quite remarkable,” says Levin.
As impressive as that rate may be in the lab, it will take a lot more to make a dent in the large amounts of LDPE currently spilling into the environment. Levin is already considering the chemical barriers that may be keeping C. necator and P. putida from degrading the LDPE even more. He hypothesizes that the branched nature of the polymer may be playing a role.
“We think the bacteria are creating enzymes that break down those extensions from the circles and chew it back into a kind of core,” he says. “But once you get it chewed back to the core, the enzymes can’t penetrate that core to break the chains inside to completely release those hydrocarbons for complete mineralization.”;
Levin is looking forward to the next stage of research into biodegradable polymer production. The key questions that need to be answered: what enzymes are the bacteria using to degrade the LPDE; what is the mechanism of degradation; and why is only 30 percent of the LDPE consumed? Levin believes that the enzymes degrading the LDPE are the same ones the bacteria use to degrade hydrocarbons like paraffin oil. Because the genomes of C. necator and P. putida are sequenced, Levin and his team will be able to identify the proteins secreted by the bacteria during both the pre-culture phase on paraffin oil as well as when the bacteria are going on 100 percent LDPE using proteomics. Once the enzymes are known, the team can use purified enzymes to study the mechanisms by which they bind to and oxidize LDPE. This may provide some clues as to why the LDPE is not degraded completely.
If the hypothesis that hydrocarbon degrading enzymes are also degrading LDPE, then it may be possible that these bacteria also degrade other synthetic plastics, such as polystyrene. Polystyrene foam is a very commonly used packing material. It consists of a linear hydrocarbon backbone with aromatic (ring structures) hydrocarbon side-chains. As both C. necator and P. putida can use linear and aromatic hydrocarbons for growth, they may also be able to degrade polystyrene — inquiries that Levin and his team will be investigating further.
Polymers with promise
While Levin and his team work with bacterial strains that could break down existing plastics, other scientists are imagining new polymers that could be designed with an end-of-life plan already baked into their structure. In other words, what if a polymer could be engineered to self-destruct on command?
Elizabeth Gillies, Western University, Departments of Chemistry and Chemical and Biochemical Engineering
Elizabeth Gillies, a professor in the Departments of Chemistry and Chemical and Biochemical Engineering at Western University in London, Ont. works on a group of materials known as self-immolative polymers, which she has been researching for more than a decade.
The materials Gillies studies are based on four or five polymer backbones that can be triggered in some way in order to depolymerize rapidly. The idea is to make a polymer stable by introducing an end-cap designed to be responsive to a particular stimulus, such as light, acid, or oxidizing agents. When that stimulus comes, it cleaves off the end-cap and the polymer depolymerizes back to monomers or another small molecule byproduct, depending upon the chemical backbone. Examples of backbones that can be given this property include polyglyoxylate, which Gillies is working on for biomedical applications, and polyphthalaldehyde. Unfortunately, one of the byproducts of polyphthalaldehyde depolymerization is toxic, making it unsuitable for health applications. It can be used, however, on transient plastics and composites that can be broken down on command.
Gillies says that, due to higher cost in comparison to commodity plastics, self-immolative polymers are currently intended mainly for medical purposes or niche applications. They could, for example, be used to create smart packaging probes or labels that react with a colour change in case of temperature fluctuations or bacterial contamination. There are also potential military applications, where a plastic delivery system rapidly disappears in the field, leaving no trace of its existence, Gillies says. Such military applications, she adds, would require a very specific trigger, such as a particular wavelength of light. And while they won’t be replacing plastic water bottles any time soon, they could be used to displace some of the non-degradable materials in consumer electronics, which currently create huge waste problems due to a lack of recycling as well as planned obsolescence by manufacturers.
Another potential application is found in agriculture. Gillies and a colleague, funded by a Natural Sciences and Engineering Research Council (NSERC) grant, have experimented with self-immolating polymers for precise fertilizer, herbicide or pesticide release. Using polyglyoxylate polymer containers, fertilizers were released into the environment in response to chemical cues emitted by plant roots.
Even better, the product of polyglyoxylate degradation — ethyl glyoxylate hydrate — happens to be a metabolic intermediate in the glycolic acid cycle, meaning that it can be metabolized in the ground and environment. Although she and her research team have only undertaken proof-of-concept in a greenhouse so far, Gillies is “quite confident that our polymer is environmentally innocuous.”
Research by Gilles and Levin offer hope that the urgent problem of plastic pollution will one day be overcome. However, Gillies warns that it won’t just be up to scientists to save the day. Governments and ordinary citizens — who normally balk at the thought of digging deeper into their wallets to fight a problem that seems out of sight and out of mind will have to get behind solutions. “It comes down to this whole problem of who pays for pollution,” says Gillies. “There is no responsibility on the manufacturer to deal with the end life of their products, so ultimately that kind of change has to come from governments. Right now, it’s future generations who will ultimately pay.”
Wrangling over recycling
With industry statistics acknowledging some 80 percent of plastics wind up in landfills after they are used in consumer products, it is safe to conclude that this ubiquitous pollution represents a failure to manage one of the world’s major waste streams. From this perspective, it should be easy to argue that reigning in this stream would yield more progress — sooner, and with less expense — than changing the chemical structure of the waste itself.
That point was the centrepiece of a panel discussion held late last year in Ottawa, where representatives from industry and government tackled the niceties of trying to establish a circular economy for plastics, a recycling regime that would see this material captured for re-use before it wound up in the environment. The harsh reality is that discarded plastic comes from such a widely scattered array of sources that the real challenge is coming up a single, consistent target for individuals and organizations.
Across Canada, collection standards can vary widely with regulations set at the municipal or provincial level, often leaving participants confused about what goes where. The caricature of this problem is represented by spouses bickering over the contents of their household’s various recycling bins; industrial users of plastic may have more technical debates, but the result can be no less frustrating and leads to lower participation rates in recycling programs everywhere.
That also frustrates potential recyclers such as GreenMantra Technologies, a firm based in Brantford, Ont. that has developed a proprietary catalyst to transform waste plastic into polyethylene and polypropylene waxes and additives that can be incorporated into existing products, such as asphalt shingles.
“You need to have demand for recycled plastics for the entire circular economy to survive, and we need to focus most of our time to creating that demand,” argued Ryan L’Abbe, the company’s vice-president of operations.
Meanwhile, the companies that manufacture plastic continue to struggle with how to adapt their output to optimize this kind of recycling opportunity. Ken Faulkner, director of government relations for NOVA Chemicals, explained that stand-up pouches have become popular for all kinds of uses, but their strength and light weight stems from the fact they consist of multiple plastic layers, which makes them all the more difficult to break down.
“It sounds really simple but it’s not,” he insisted. “It takes a lot of effort, it takes a lot of money.”