The plastic problem – accumulation before alternatives

The plastic problem – accumulation before alternatives

By Karlee Bamford, Treasurer for the GCI

Plastics undoubtedly play a central role in our daily lives and played a pivotal role in the development of consumer societies across the globe for over a century. Concurrent with newfound materials and newfound possibilities, unprecedented environmental problems have emerged as a result of our reliance on plastics. The accumulation of plastics in allocated disposal sites (e.g. landfills) and in otherwise uninhabited spaces (e.g. beaches, open ocean) present threats to human health, water security, and food supply. These challenges now impact communities globally, irrespective of their actual contribution to the generation of plastic waste, and affect individuals of all economic backgrounds.

Figure 1. Examples of waste plastic accumulation in landfills and the environment. Images source: Pixabay.

Given the scale and significance of these challenges, is there anything that chemists can do to resolve this panhuman problem? A recent blog post from the Green Chemistry Initiative (https://greenchemuoft.wordpress.com/category/author/molly-sung/) highlighted the advances that have been made in synthetic and materials chemistry towards plant-derived and biodegradable plastics as alternatives to traditional petroleum-derived plastics. While this is undoubtedly a crucial area of research as humanity has become permanently dependent on plastics, the design of next generation plastics that are inherently sustainable will not mitigate the overwhelming impacts of existing plastic waste. Arguably, attenuating the problem of plastic waste is more important than finding alternatives to traditional plastics. Indeed, the decomposition time for products made from the top four families of commodity plastics (PP, PE, PVC, PET), produced on a 224.6 million tonne-scale alone in 2017,1 is estimated at 1 to 600 years in marine environments2 and considerably longer in landfills due to lack of moisture.4

Figure 2. Examples of the top five most-produced commodity polymers and their production scale in 2017.1,3

Traditional plastic-recycling methods are not equipped to resolve the issue of waste plastic accumulation either. Recycling can be broken down into three distinct varieties: primary, secondary, and tertiary.5 Primary recycling, which is equivalent to repurposing or reusing, is used limitedly for products such as plastic bottles, typically made of PET, which be directly reused following the necessary sterilization. Secondary recycling involves mechanical processing of plastics into new materials and frequently results in reduction of the plastics overall quality or durability due to the thermal or chemical processes involved. Primary and secondary recycling account for the majority of recycling efforts, however, as a consequence of poor consumer compliance (e.g. <10 % in the US and 30-40 % in the EU)6 and the deteriorating value of plastics with repeated secondary recycling, all plastics eventually become waste. The last and most underutilized form of recycling is tertiary recycling, the degradation or depolymerization of plastics into useful chemicals or materials. In the last year alone, numerous high profile editorial and review articles have appeared in Science7,8,9 and Nature6,10 emphasizing the incredible potential of chemical (tertiary) recycling as means of reducing plastic waste and as a new, sustainable chemical feedstock for the polymer (plastics) industry.

The challenge of chemical recycling is immediately evident: plastics have been expertly designed to be highly durable and chemically resistant, and thus, plastics cannot be easily transformed chemically. Ideally, polymers used in plastics could be depolymerized to monomer for subsequent repolymerization. For condensation polymers, such as polyethylene terephthalate (PET), the reverse of the polymerization reaction is the addition of a small molecule to the polymer to reform monomer. While completely reversible on paper or in theory, such depolymerization strategies have had limited success for PET.

Reacting the polymeric PET material with protic reagents (e.g. amines, alcohols) followed by hydrolysis to give monomers that can be repolymerized, if of sufficient purity (Figure 3), requires high temperature (250-300 °C) and high pressure (0.1-4 MPa) conditions unless additives, such as strong acids and bases or metal salts, are used.11 The action of many additives is not well understood, thus precluding rational improvement of the system. Hydrolysis of PET itself, especially at neutral pH, is the most challenging approach to PET chemical recycling as water is a relatively poor nucleophile. Hence stronger nucleophiles, such as ethylene glycol, are preferred.

Figure 3. Depolymerization of PET by glycolysis.

One practical problem in the chemical recycling of any plastic is its insolubility. Phase transfer catalysts –  species capable of transferring from one phase to another – have been used to address the insolubility of PET12 and have permitted the direct hydrolysis of PET at operating temperatures as low as 80 °C, as in the work of Karayannidis and coworkers (Figure 4). The phases in these systems are the insoluble PET polymer (the organic phase) and the basic solution (the aqueous phase) surrounding it.13

Figure 4. Phase transfer catalyzed hydrolysis of PET (catalyst shown in blue).

Addition polymers, such as polypropylene (PP) or polyethylene (PE), cannot be depolymerized to monomer form using the above strategies as their polymerization does not involve the loss of small molecules. Until very recently, the best end-of-life purpose for the majority of plastics has been energy recovery through incineration. The work of Huang and coworkers on the chemical degradation of PE plastics is a break-through for the field of plastic recycling. While previous studies have reported that thermolysis of PE yields poorly defined mixtures of hydrocarbons, these authors have found a remarkable, highly targeted method for converting PE to a narrow distribution of fuels (3 to 30 carbons in length) using a dehydrogenative metathesis strategy (Figure 5).14 The homogeneous iridium catalysts employed were previously reported in the literature for alkane dehydrogenation (step 1) and hydrogenation (step 3), but no such polymer substrates had apparently been attempted for main-chain dehydrogenation. Similarly, the authors used a previously-established rhenium oxide/aluminium oxide catalyst for olefin metathesis (step 2).

Figure 5. The transition-metal catalyzed degradation of PE to liquid fuels reported by Huang and Guan (catalysts shown in blue).14

The chemical recycling of PET by phase transfer catalysis and of PE by dehydrogenative-metathesis have very little in common with one another on a technical level. What unites these two strategies is the desire to transform the problematic, highly abundant and inexpensive resource that is waste plastic into useful commodities. Perhaps more importantly, these two examples both take revolutionary approaches to old problems through inspiration from fundamental research and parallels found in small molecule catalysis. Rethinking the plastic problem into a challenge for catalysis, rather than solely a call for clever materials design, is critical if we wish to reduce the threats that waste plastics pose to our health and our environment.

References:

  1. Tavazzi, L., et al., The Excellence of the Plastics Supply Chain in Relaunching Manufacturing in Italy and Europe, The European House, Ambrosetti, 2013 (as cited in Bühler‐Vidal, J. O. The Business of Polyethylene. In Handbook of Industrial Polyethylene and Technology; Spalding, M. A.; Chatterjee, A. M., Eds.; John Wiley & Sons: Hoboken, NJ, 2017; p. 1305).
  2. Mote Marine Laboratory Biodegradation Timeline; 1993. Available from: https://www.mass.gov/files/documents/2016/08/pq/pocket-guide-2003.pdf ; accessed July 10, 2018.
  3. Image sources: Image sources: (Plastic recycling symbols) http://naturalsociety.com/recycling-symbols-numbers-plastic-bottles-meaning/ ; (PP) https://www.screwfix.com/p/stranded-polypropylene-rope-blue-6mm-x-30m/98570 ; (LLDPE) https://www.polymersolutions.com/blog/differences-between-ldpe-and-hdpe/ ; (HDPE) https://chemglass.com/bottles-high-density-polyethylene-hdpe-wide-mouths ; (PVC) https://omnexus.specialchem.com/selection-guide/polyvinyl-chloride-pvc-plastic ; (PET) https://ecosumo.wordpress.com/2009/06/04/what-does-the-recycle-symbol-mean-part-2/
  4. Andrady, A. L. Journal of Macromolecular Science, Part C: Polymer Reviews, 1994, 34(1), 25-76.
  5. Hopewell, J.; Dvorak, R.; Kosior, E. Trans. R. Soc. B, 2009, 364, 2115–2126.
  6. Rahimi, A.; García, J. M. Nature Reviews Chemistry, 2017, 1, 0046.
  7. MacArthur, E. Science, 2017, 358 (6365), 843.
  8. García, J. M.; Robertson, M. L. Science, 2017, 358(6365), 870-872.
  9. Sardon, H.; Dove, A. P. Science, 2018, 360(6387), 380-381.
  10. The Future of Plastic. Nature Communications, 2018, 9, 2157.
  11. Venkatachalam, S.; Nayak, S. G.; Labde, J. V.; Gharal, P. R.; Rao, K.; Kelkar, A. K. Degradation and Recyclability of Poly (Ethylene Terephthalate). In Polyester; Saleh, H. E. M., Ed.; InTech: London, 2004; p. 78.
  12. Glatzer, H. J.; Doraiswamy, L. K. Eng. Sci. 2000, 55(21), 5149-5160.
  13. Kosmidis, V. A.; Achilias, D. S.; Karayannidis, G. P. Mater. Eng. 2001, 286(10), 640-647.
  14. Jia, X.; Qin, C.; Friedberger, T.; Guan, Z.; Huang, Z. Science Advances 2016, 2(6), e1501591.

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Issues of Sustainability in Laboratories Outside the Field of Chemistry: Pipette Tips

Issues of Sustainability in Laboratories Outside the Field of Chemistry: Pipette Tips

By David Djenic, Member-at-Large for the GCI

As a biochemistry student in the Green Chemistry Initiative, I’m interested in looking at how to implement the principles of green chemistry in molecular biology and biochemistry labs. While molecular biology labs focus more on studying biological systems and molecules rather than synthesizing new molecules, like in synthetic chemistry, there are still problems when it comes to performing environmentally sustainable research.

Pipette tips and pipette tip racks are major contributors to non-chemical waste in biomedical labs because of the volume of tips thrown out and the lack of recycling programs to deal with tips and racks. Pipette tip racks are commonly used because they reduce the risk of contaminating pipette tips. Pipette tip racks are made of #5 plastic (polypropylene), the same material as yogurt cups, medicine bottles and David_blog 1microwavable containers, making them lightweight and very safe to use [1].
However, #5 plastics are rarely accepted by curbside recycling programs and are placed in landfills and incinerators instead [2]. The plastic from the empty polypropylene racks take hundreds, if not thousands, of years to degrade [3].

Biomedical companies have worked in the past 10 years to reduce the amount of waste from pipette tip racks. For example, Anachem, a pipette and pipette tip manufacturing company in the UK, has collaborated with a plastic recycling company to collect racks from qualifying laboratories, ground them down, melt them, and remould into new products [3]. A similar program is run at the Environment, Health and Safety (EHS) division of the National Cancer Institute at Frederick (NCI-Frederick), where, from 2003 to 2006, approximately 8,400 pounds of pipette tip boxes were recycled, saving approximately $7,400 in medical waste contract money [4].

David_blog 2

Pipette tip box waste to be recycled through the EHS program [4].

There aren’t many statistics on the waste produced by the pipette tips themselves. But whenever I’m in a biochemistry lab course, the orange bins where used tips are thrown are filled to the brim with pipette tips, microcentrifuge tubes, Falcon tubes, etc. It is more difficult to reduce and recycle tips rather than tip racks because they are heavily contaminated after use. GreenLabs at the University of Chicago offers some interesting suggestions on reducing pipette waste, such as using pipette tip refills, buying pipette tips made from sustainable material, and generally reducing pipette tip use when possible. However, more research on pipette tip waste is needed to quantifiably analyze the impact of tips and come up with solutions to reduce potential waste.

I think undergraduate biomedical teaching and research labs do apply basic green chemistry principles, even if they are not explicitly brought up. Many of the reactions are done in very small, precise quantities and waste is generally disposed of in the proper place. However, there does not seem to be much exposure, if at all when it comes to green chemistry issues; biochemistry and biomedical students aren’t aware of the environmental impact they generate in labs. Introducing green chemistry education in biomedical laboratories at U of T, especially when it comes to the issue of pipette tips and racks, would help U of T reduce its environmental impact even more.

 

References

[1] http://www.davidsuzuki.org/publications/downloads/2010/plasticsbynumber.pdf

[2] http://earth911.com/home/recycling-mysteries-5-plastics/

[3] http://www.labnews.co.uk/features/consumables-dont-cost-the-earth-01-07-2005/

[4] G. A. Ragan, J. Chem. Health Saf. 2007, 14, (6) 17-20.  http://www.sciencedirect.com/science/article/pii/S1871553206001344

Recycling Perovskite Solar Cells

Recycling Perovskite Solar Cells

By Judy Tsao, Member-at-Large for the GCI

Solar energy is arguably the most abundant and environmentally friendly source of energy that we have access to. In fact, crystalline silicon solar cells have been employed in parts of the world at a comparable cost to the price of electricity derived from fossil fuels.1 The large-scale employment of solar cells, however, remains challenging as the efficiency of existing solar cells still needs to be improved significantly.

An important recent breakthrough the field of solar cells is the use of perovskite solar cells (PSC), which includes a perovskite-structured compound as the light-harvesting layer in the device (Figure 1). Perovskite is a name given to describe the specific 3-D arrangement of atoms in such materials. Even though the first PSC was reported only in 2009, its power conversion efficiency (PCE) has already been reported to exceed 20%, a milestone in the development of any new solar cells which typically takes decades of optimization to achieve.2

judy-blog-1

Figure 1. Thin-film perovskite solar cell manufactured by vapour deposition (photo credit: Boshu Zhang, Wong Choon, Lim Glenn & Mingzhen Liu)

PSC has several advantages compared with traditional solar cells, including low weight, flexibility, and low cost.3 There are, however, several challenges that must be overcome before PSC can be brought to the market. The most common PSC to date includes CH3NH3PbI3 and related materials, which contain soluble lead (II) salts that are toxic and strictly regulated.

Interestingly, there has been a consensus in the literature that the lead content in the perovskite layer is not actually the main issue in the environmental impact of PSC production.4 Part of the reason for this conclusion is simply that the thickness of perovskite layer required would amount to less than 1000 mg of lead in one square meter of material. This value is only modest compared to lead pollution from other human sources such as lead paints or lead batteries.5

The main environmental concerns regarding PSCs appear to lie in the use of gold and high temperature processes during the manufacturing of the devices.6 It has thus been suggested that, in order to reduce the environmental impact of PSCs, recycling of raw materials is very important. In a recent study by Kadro et al., 7 a facile protocol for the recycling of perovskite solar cell was developed. The entire procedure takes place at room temperature and takes less than 10 minutes (Figure 2).

judy-blog-2

Figure 2. Schematic process for recycling PSC components [7].

As it turns out, components of a fully assembled PSC can be extracted by sequentially placing the device in different solvents. Step 1 of the procedure uses chlorobenzene to remove the gold layer, while step two uses ethanol to dissolve CH3NH3I. This then leaves PbI2 to be the only component remaining on the device, which can be removed by just a few drops of N,N-dimethylformamide. It is also worth noting that the recycled materials can be fabricated into a complete PSC again without significant drop in performance.

Even though the discovery of PSC has only been made less than a decade ago, its potential in applications in photovoltaics has been underlined by numerous studies. It is especially gratifying to see that the environmental impacts of such devices are already under active research before PSCs are introduced to the market. While these studies have demonstrated that PSCs have low environmental impacts when properly recycled, there are other challenges still facing researchers in this field. In particular, the short lifetime of such devices needs to be improved to match that of traditional silicon-based solar cells. Nevertheless, the facile method of recycling PSCs without compromising the performance will certainly make them even more competitive than traditional solar cells.

References:

  1. Branker, K. et al. Renewable Sustainable Energy Rev. 2011, 15, 4470.
  2. Yang, W. S. et al. Science, 2015, 348, 1234.
  3. Snaith, H. J. Phys. Chem. Lett. 2013, 4, 3623.
  4. Serrano-Lujan, L. et al. Energy Mater. 2015, 5, 150119.
  5. Dabini, D. Phys., 2015, 6, 3546.
  6. Espinosa, N. et al. Adv. Energy Mater. 2015, 5, 1.
  7. Kadro, J. M. et al. Energy, Environ. Sci. 2016, 9, 3172.