Plastics, when introduced into the environment as pollutants, not only scar the natural beauty of terrestrial and aquatic ecosystems, but they also pose threats to the inhabitants of these ecosystems. When degraded, plastics made from petrochemicals, which are the conventional plastics we all use, produce microplastics that become a part of the food chain when animals, such as fish, mistake them for food. These microplastics then accumulate in the higher trophic levels of the food chain, where we are, and can have a toxic impact on the health of many organisms. Even in their nondegraded form, plastics can strangle animals like sea turtles. While these problems result from all plastics, no type of plastic is a greater threat to life than single-use plastics, simply because of their abundance. It has become almost impossible to go a day without coming across single-use plastics such as bags, bottles, cups, cutlery, packaging, and straws. Now, we find ourselves trying to figure out ways of getting rid of single-use plastics and exploring other possible alternatives. But if plastics can be so harmful, why were they invented in the first place? What have scientists done to approach this predicament? What is yet to be done?

 

Friend or Foe?

About a century ago, trees were increasingly being cut down to make paper bags. These bags, however, could not last long because of their poor strength, so people constantly needed to replace their old bags with new ones – creating a steady demand for paper bags. Inevitably, as the demand for bags increased, more trees were cut down. As you can imagine, cutting down thousands of trees for the manufacturing of paper bags didn’t positively impact the environment, and Sten Gustaf Thulin – a Swedish engineer – felt strongly about that.

A BBC story in October 2019 showed that, in 1965, the Swedish engineer patented his invention – the plastic bag – to reduce the number of trees being cut down.  As shown in Figure 1, Thulin said “It is the object of the invention to produce at low [economic and environmental] cost by a simple method a strong bag with handles which, contrary to what is the case with, for example, conventional paper bags with handles, are safely connected with the bag portion” (BBC News World 2019).

Surely, Thulin thought that his invention would save trees and reduce the overall need for paper bags since plastic bags are stronger and last longer than paper ones. However, the threat this invention was meant to reduce, it actually amplified. People often misused plastic bags by throwing them away after little use. The plastic bag then went on to inspire the invention of plastic bottles and many other plastic products, most of which we now identify as single-use plastics. Owing to the high demand for single-use plastics, the amount of carbon dioxide (a greenhouse gas) created by plastic production greatly increased.

Carbon dioxide is produced throughout the life of plastics made from petrochemicals. For example, at “birth,” the transportation of raw materials produces carbon dioxide. The manufactured plastics then eventually decompose (aging) to produce more carbon dioxide. And, finally, if the plastic waste is burned (death), then even more carbon dioxide is produced. In her article “What Is the Carbon Footprint of a Plastic Bottle?” (2018), Marie-Luise Blue, an assistant professor at Harvard Medical School, suggested that manufacturing a pound of PET (polyethylene terephthalate, a petrochemical plastic used to make bottles) can produce up to three pounds of carbon dioxide. About a year later, in their article “Plastic’s Carbon Footprint,” Sangwon Suh and Jaijai Zheng – researchers at the University of California, Santa Barbara – found that the total carbon emission resulting from petrochemical plastics in 2015 was about 1.8 million metric tons. So, what can be done to prevent all this carbon from entering the atmosphere?

The most intuitive answer to the problems that result from plastic use is to simply stop using plastics. However, with their use as coffee cups, water bottles, food packaging, and grocery bags, plastics have become so highly integrated into our daily lives that giving them up has become too difficult; we need an alternative.

 

The First Barrier: Finding an Alternative

Fossil fuels are essentially a carbon sink – their formation removes carbon from the carbon cycle, as shown in figure 3. So, when petrochemical plastics are made, stored carbon becomes active in the carbon cycle again, which increases the amount of atmospheric carbon and the rate of global warming. To find an alternative source of raw materials to produce plastic, we are now restricted by the criterion that the source must not be a carbon sink; otherwise, we risk intensifying the severity of global warming. We should aim for a source that can at least absorb atmospheric carbon—perhaps, one that can grow from the ground.

While trees seem like a viable option, turning to them for raw material would defeat the purpose of an alternative material to petrochemical plastics by increasing deforestation. Max Roser and Hanna Ritchie, the respective founder and senior researcher of Oxford’s Our World in Data, found that global plastic production is increasing exponentially. Between the years 1965 (when Thulin patented his invention) and 2000, production went from 17 million to a staggering 213 million tons of plastic per year. Now, our plastic production exceeds 300 million tons annually. Even back in 2000, for the tree-produced alternative to meet the demand of plastics, we would have had to chop down even more of our forests, threatening their indigenous people and animals while doing so. Fortunately, trees’ being an unsustainable source for producing an alternative to plastic guided scientists towards another possible option: crops.

In 1997, Tillman U. Gerngross, a professor of bioengineering at Dartmouth College, and Steven C. Slater, a finance professor at the University of Bradford, took part in a joint project between Dow Chemical Company and the agricultural firm Cargill. The scientists there aimed to make the production of bioplastics from crops a reality. And, after three years, the project was deemed “successful”; scientists were able to convert sugar from corn into a bioplastic called PLA (polylactide). This was a great achievement: if these crops could replace single-use plastics, it would mean greatly reducing the amount of carbon pumped into the carbon cycle through petrochemical plastics. However, using corn itself would mean farmers have to grow enough corn to feed humans and to meet companies’ demand. To address that issue, scientists targeted the corn stover (the stem and leaves of the crop) for plastic production. And, later on, PHA (polyhydroxy-alkanoate, another type of bioplastic) was produced in a more efficient process, whereby, instead of making the plastic outside of the plant through a chemical reaction, the crops were genetically modified so as to produce the plastic within bacteria inside the plant. Everything seemed promising.

Gerngross and Slater then found that several barriers arise if we are to produce PHA at a commercial level. Large amounts of land are required to grow enough of the corn stover to meet the demand, and the process of producing PHA is very energy demanding. Producing the same mass of PHA as PE (polyethene, a fossil-based plastic) would require 300% more energy. Given that the majority of the world’s energy is produced from fossil fuels, PHA would actually be a much worse alternative to petrochemical plastics for two reasons. The first is that PHA-based plastic production emits more carbon dioxide than petrochemical plastic production. The second is that farm land is commonly obtained through a deforestation method called slash and burn – which, while commonly used, has many drawbacks.

So, how did scientists overcome the energy barrier?

 

The Second barrier: Energy

There were two evident solutions to the energy requirements that make bioplastics unsustainable: increase the energy efficiency of the production process, or replace the fossil-fuel-based power supply with renewable power. Renewable energy can provide a clean source of energy that produces a negligible amount of carbon dioxide compared to fossil-fuel-powered sources. However, the price of renewable energy is still high relative to that of power produced by fossil fuels. Nevertheless, something can be done to decrease the energy consumption in producing bioplastic.

In October 2019, a research group from Department of Agricultural Engineering from the University of Cairo, led by Mohamed Samer, demonstrated that varying the chemical composition of bioplastics varies the amount of energy consumed in the production process. For example, in a bioplastic composed of starch, glycerin, vinegar, and water, changing the ratio of each component changes the energy needed to produce the end product. The experiment carried out by these researchers showed that the energy consumed can vary by a factor of two. The consumed energy for eight chemical compositions proved to range from about 1.63 kilojoules to about 3.65 kilojoules per gram of bioplastic produced. Even though potato peels were used here, as opposed to corn stover in the earlier production of PHA, these variations in chemical composition suggest how to reduce energy consumption. However, another major issue faces bioplastics produced from crops: the large amount of agricultural land needed for the farming.

 

The Third Barrier: Resources and Land

Given the industrial demand for plastics, if bioplastics were to replace petrochemical plastics, then there would also be a demand for the crop that produced the bioplastic. This demand would cause farmers to shift towards planting corn, since consumers are guaranteed. Then, competition between corn crops and other crops for fresh water and agricultural land would increase. Consequently, fewer food crops would be harvested. Simultaneously, monocropping for the production of commercial crops would result into soil degradation – potentially turning a once fertile land into a desert. New land would be needed for crops as the cultivated soil degrades. Ultimately, this process would kick-start a vicious cycle that turns fertile land into desert. In fact, as shown by the United Nations’ Department of Economic and Social Affairs, the United Nations’ 2030 Agenda for Sustainable Development (SDGs) aims to achieve the opposite effect of that cycle through Goal 2, titled “Zero Hunger” (involves promoting sustainable agriculture), and Goal 15, titled “Life on Land” (involves combating desertification, and halting reverse land degradation). As Gerngross and Staler also found, even if the energy consumed to produce bioplastics from corn stover decreased and the crop yield increased, fulfilling the great demand for plastics would still require large tracts of land.

Scientists also started exploring plants that do not need arable land, and would not, therefore, compete with food crops. Fortunately, they stumbled upon a plant that wasn’t considered about two decades ago when bioplastics were still an emerging technology: algae. And perhaps also its relative, cyanobacteria.

Algae and cyanobacteria are plants that grow just like trees, but instead of receiving nutrients from soil, these plants absorb nutrients from water. Through genetic modification, microalgae and cyanobacteria can be changed to produce bioplastic within their cells, which can then be extracted and synthesized. In the article “Green Bioplastics as Part of a Circular Bioeconomy” in a recent issue of Trends in Plant Science (2019), researchers suggested that the advantage algae-based bioplastics have over crop-based bioplastics is that they can grow on saline and nonarable land. So, in effect, cyanobacterial-based and microalgal-based bioplastics eliminate the fresh water and fertile land competition that exist with crop-based bioplastics. Owing to their sustainability, cyanobacterial-based and microalgal-based bioplastics have another advantage over crop-based bioplastics: the bioplastic produced can serve as both a biodegradable bioplastic (which is carbon neutral in theory), and a nonbiodegradable bioplastic (which can serve as a carbon sink). Since cyanobacterial-based and microalgal-based bioplastics absorb atmospheric carbon, their nonbiodegradable products should essentially store carbon the same way trees do. These nonbiodegradable products can be used as infrastructure materials. The problem, however, is very similar to that of solar panels when they were first being developed: the bioplastic produced from cyanobacteria and microalgae is too expensive to be feasible at a large economic scale. This problem partly stems from inefficiency in bioplastic production processes.

 

The Fourth Barrier: Algae-based Bioplastic Production

There are two known methods to cultivate microalgae and cyanobacteria: open pond cultivation (shown in figure 4), and flat panel photobioreactors cultivation (shown in figure 5). The ultimate goal for each method is to produce large volumes of uncontaminated algae in a short period of time. Algae, like most plants, need sunlight, oxygen, carbon dioxide, and water; increasing the quantities of these three factors will increase the growth of algae. Since oceans and seas can provide great amounts of water, the limiting growth factors to the salt-tolerant algae become oxygen, carbon dioxide, and sunlight intensity. In their recent article “Bioplastic Production from Microalgae” (2020), Senem Onen Cinar and other researchers identified limitations to the two methods of cultivation. Summarized in figure 6, the advantages and disadvantages for each method mostly consist of a compromise between high volume production and the purity of the grown algae. For instance, open pond cultivation has a higher production rate, but the risk of contamination is also high. Cinar and her research team concluded that there is still much to be done before cyanobacterial-based and microalgal-based bioplastics can be produced at a large scale:

… there is still need for more development of bioplastic production processes from microalgae to overcome the economic feasibility problems in industrial-scale implementations, which prevents wider usage of bioplastic products from microalgae in the market. Here, a biorefinery concept where bioplastic is produced from by-products of high value chemical production from microalgae is promising.

As with solar panels, a combination of increasing efficiency and decreasing the cost of raw materials can result into a net decrease in the cost of microalgal-based and cyanobacterial-based bioplastics. However, here, the method of increasing efficiency greatly differs from that of solar panels. In the article “Algae-based biorefinery—How to make sense?”, Jayati Trivedi, a senior scientist at the Indian Institute of Petroleum, and her research team highlight that biorefining, the fermentation of the by-products produced from the process of bioplastic production to produce even more bioplastic, can be an effective way to reduce cost. However, more research needs to be done to increase the yield of microalgae- and cyanobacteria-produced bioplastics to ensure the economic feasibility of these potential petrochemical plastic alternatives. Currently, companies such as Cereplast are funding projects that aim to make the production of algae-based bioplastics economically feasible.

The Last Barrier?

When Thulin invented the plastic bag, it was certainly not his intent to invent the material that would cause 1.8 million tons of carbon emissions annually, and it was certainly not his intent to scar the beauty of nature while threatening the populations it sustains. Research revealed the negative impacts of plastic, and, luckily, the scientific community was quick to explore alternatives in its attempt to phase out petrochemical plastics. Eventually, scientists discovered that it is possible to harvest bioplastics from plants. Even so, methods of bioplastic production from conventional crops were unsustainable. Through algae and cyanobacteria, scientists then managed to produce microalgal-based bioplastic. While the production as of May 2020 was not at an industrial scale, the efforts to make mass production more feasible continue. However, even if these companies achieve their goal, as with Thulin’s bag, it is not certain if algae-based bioplastic will be the alternative Earth needs.

 

 

References

BBC, “Sten Gustaf Thulin, the inventor of the plastic bag who wanted to help the planet,” BBC News World, October 2019.

Blue M.-L, “What Is the Carbon Footprint of a Plastic Bottle?” Sciencing, June 2018.

Cinar S. O., Chong Z. K., Kucuker M. A., Wieczorek N., Cengiz U., & Kuchta K., “Bioplastic Production from Microalgae: A Review” International Journal of Environmental Research and Public Health, pages 1-21, May 2020.

Gerngross and Steven C. Slater, “How Green are Green Plastics?” Scientific American, Vol. 283, pages 37-41, August 2000.

Karan H., Funk C., Oey M., Grabert M., & Hankamer B., “Green Bioplastics as Part of a Circular Bioeconomy” Trends in Plant Science, Vol 24, pages 237-249, March 2019.

Roser M. and Ritchie H., “Global plastics production,” Our World in Data, September 2018.

Samer M., Khalefa Z., Abdelall T., Moawya W., Farouk A., Abdelaziz S., & Mohamed M., “Bioplastics production from agricultural crop residues” Department of Agricultural Engineering, Faculty of Agriculture, Cairo University, Vol. 21, pages 190-194, October 2019.

Sangwon S., Jaijai Z., “Plastic’s carbon footprint: Researchers conduct first global assessment of the life cycle greenhouse gas emissions from plastics,” ScienceDaily, April 2019.

Trivedi J., Aila M., Bangwal D., Kaul S., & Garg M., “Algae based biorefinery—How to make sense?” Renewable and Sustainable Energy Reviews, Vol. 47, pages 295-305, March 2015.

United Nations, “The 17 Goals”, Department of Economic and Social Affairs, September 2015.

 

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