Chem321:Polyhydroxybutyrate

Polyhydroxybutyrate (PHB): Waste-stream Plastic

Sustainable development has sometimes been called the “responsible” way to encourage economic growth. The use of polymers to make ubiquitous everyday objects is a characteristic of modern, industrial nations. The abundance of polypropylene is evident by volume in landfills, which has been increasing six-fold over the last 40 years(1). Indeed, the properties of plastic make it ideal for a multitude of purposes. Health code regulations, increasing sanitation, and the disposable nature of fast-paced living in post-industrialized nations makes disposable plastics necessary for everyday life. Rising levels of affluence in a society demand lightweight, cheap, and disposable products. Currently, this need is met by petroleum-based products, such as polypropylene (PP).

The problem with using oil based products is partially linked to the rising price of oil, thus making this formerly cheap material more expensive. The second problem with petroleum-based products is that this use of technology is unsustainable because oil will undeniably eventually run out. Unless the human demand for plastic ceases, a new method must be developed by examining the whole life of the product, from cradle to grave. Perhaps the first solution to this problem may seem to be in developing a way to recycle the product to get the same raw materials needed to make PP. An abundance of PP exist in our landfills, but can the final product be safely converted back into the raw materials without producing hazardous waste products? The push for sustainable technology has led to research into other ways of making plastic for industrial uses without fossil fuels. One particularly “green” technology is the production of polyhydroxybutyrate.

Conventional Polypropylene

Polypropylene (PP), sometimes called polypropene, is a thermoplastic polymer currently used in many industrial processes throughout the life cycle of modern products(2). First polymerized to a crystalline isotactic polymer by Giulio Natta and his coworkers in March 1954, polypropylene has been a commercial product since 1957 onwards(3). Responsible for about 11% of municipal waste in 2000, the ubiquitous and disposable nature of this product makes it commercially appealing(4). Disposable and degradable imply two rather different ideas. Even though PP is thrown into a landfill, UV light generally does not reach plastic enough to degrade it. When thermal and chemical methods are used to recycle PP, chlorinated plastic films must first be removed in Japan, who recognized the need to regulate the recycling of the abundance of plastic waste from food containers present on the island and legislation was passed in April 2007 to regulate the industry(5). Hazardous materials that can be released from thermal and chemical processes used to isolate the polypropylene from comonomer ethylene of chlorinated polypropylene include hydrogen chloride and dioxins(6). Obviously, the continued release of these toxins will be collectively negative for human life. Processes have been developed to remove non-recyclable plastic from recyclable, but requires the input of energy to recover plastic and are generally not economically profitable.

In the short term, something must be done with all of the polyproylene plastic, but a more long term sustainable way to recycle products of this type must come at the design stages. PHB might offer a material similar enough to PP to maintain commercial appeal with consumers. The direct result of a law in Japan, which requires the recycling of all Containers and Packaging, has been more efficient and less toxic methods for recycling existing products, but does this solve or perpetuate the issue? According to a report in April 2006, the Berkeley Plastic Task force, processing used plastics often costs more than using virgin plastic(7). Rather than the economic incentive being to release toxic and hazardous materials by burning and recycling this product, a better, more efficient product can be substituted for true sustainability. Laws exist in the United States that prohibit the open burning of plastic, and is generally socially unacceptable, but open burning in rural, remote, or less developed areas remains the largest concern in terms of polluting the environment(8).

Polyhydroxybutyrate as an Alternative to Polypropylene

PHB is a form of PHA (polyhydroxyalkonoate) which is a polymer polyester. Originally isolated and characterized by French microbiologists Maurice Lemoigne, PHB offers the potential to sustainably produce plastics(9). As with any technology, this discovery can be used in a number of ways. When microbes experience macro-nutrient deficiency the state of physiological stress results in PHB production. Properties of PHB include water insolubility, oxygen permeability, ultra-violet light resistance, solubility in chloroform and other chlorinated hydrocarbons; although PHB has poor resistance to acids and bases(10). If PHB production is to be used in a sustainable way, this technology must utilize multiple waste streams produced by other industrial processes; for example using crude glycerol (CG) and sewage sludge.

PHB is practical for medical applications, combined with a tensile strength of 40 MPa, which is comparable to polypropylene(11). The major US industry of pharmaceutical devices could use this technology to prevent going back into a person's body to remove medical devices, which can be designed to breakdown overtime within the patient's body. Targeted delivery of medication and imaging also can be made using PHB, as demonstrated by Jin Lee et al(12). Traditional polypropylene plastics used in medical devices require another surgery to remove the product once the purpose of the product has been served. Since PHB breaks down over time into nontoxic molecules, even if the patient's immune system responded, the negative consequences would likely be outweighed by targeted medication delivery. This use of PHB by the medical and pharmaceutical industry might drive the price down, making PHB eventually available for everyday applications.

One property that makes PHB more sustainable that polypropylene is PHB's ability to sink in water, which assists in anaerobic bio-degradation in sediments of water sources, which is possible because PHB is nontoxic(13). PHB will become an important technology in the future because the characteristics of this material are sufficient to replace many of the ways that petroleum-based plastics are currently used in society in aspects such as medicine, packaging and food-service industry(14). PHB is a bio-degradable plastic that can be decomposed by specific micro-organisms.

Microbial Production of PHB from Waste Water

Currently, waste water treatment plants spend about 60% of operating costs on sewage sludge treatment and disposal. PHB offers a potentially profitable way for this waste stream to become a feedstock as well as prevent environmental damage. Most waste is expensively disposed of by either incineration, composting, landfill, or ocean dumping. Sewage sludge can pollute the environment and affect human health(15). Pathogens and contaminants are an inevitable product of biological waste water treatment and the responsibility of the company to prevent water contamination can be driven by both moral and economic incentive. Since people are not going to stop producing waste of this type at any time soon, a solution needs to be worked out to prevent pollution and protect human's from themselves. No longer do we live in a world where we assume waste can indefinitely dissipate. The consequences of not dealing with sewage sludge are detrimental to human health and well-being. An example of a company that has been researching this area is Micromidas Inc(16).

Genetic Modification of Plants to Manufacture PHB from CG

Another industry that creates a waste stream that might be utilized is in the developing industry of bio-fuels. Although recent debates about using arable crop land to grow fuel have changed industrial practices, crude glycerol (CG) is a by-product of the bio-diesel production that offers the raw materials for PHB production. Tobacco can be genetically modified to express PhaB and consequently can produce more PHB. This process is helped by combining codon-optimization with highly active engineered enzymes in microbes, such as Ralstonia eutrophaix. Estimates have stated that this could generate up to $96,000 a year. An example of a company that utilizes this technology is Metabolix. The US company Metabolix received the Presidential Green Chemistry Award, in the small business category, for their development and commercialization of PHAs in general, while PHB is a specific type of PHA(17).

Initially considered to be not economically viable, the applications for PHB include areas diverse and widespread through many US industries. Several companies have already invested in research for production of PHB. Existing industries could easily and profitably retrofit factories that treat sewage and waste while concurrently producing PHB. The main economic incentive comes from money saved on treating and disposing of waste, although the PHB is a viable product of the process.

Polypropylene is currently used as a raw material and for packaging in a multitude of industries, ranging from pharmaceuticals, automobiles, household items, rugs, packaging, electronics and so on. Generally speaking, sustainable development has the potential to flourish in a recessed economic situation because the use of waste streams as feed stocks for industrial processes saves and the potential to make profits increase. PHB is an immediately profitable in monetary terms that would make an investment for any sewage treatment facility logical because of the money that PHB production can save by not producing a waste-stream that still needs neutralization.

PHB has the commercial prospect of replacing polypropylene, a derivative of fossil fuels by deriving feedstock from waste-stream treatment and using the concept of concurrent production. Conventional polypropylene is a derivative of oil products and one of the most commonly used polymers for consumer goods, but polyhydroxybutyrate is produced by micro-organisms from waste-stream products. The bio-degradable nature of PHB make an ideal replacement of polypropylene, which does not break down and runs the risk of polluting delicate ecosystems, such as in oceans.

Polypropylene Versus Polyhydroxybutyrate Debate

The economic incentive for finding a profitable and less detrimental process to deal with waste is often considered an external cost, not included in the operation of the businesses. As pollution becomes less socially acceptable and as businesses aim to tighten up their processes to save money, businesses must find innovative ways to control waste produced. A major way that this can happen, with proper capital to support this emerging use of green chemistry, is if existing businesses could invest in infrastructure that would save them money while simultaneously producing a less environmentally damaging product. PHB is just one example of many sustainable technologies that are going to use the concept of concurrent production to make and save money with the potential to cause no harm to the environment.

References

1. “Disposable Planet?” BBC News <http://news.bbc.co.uk/hi/english/static/in_depth/world/2002/disposable_planet/waste/weeks_waste/plastics.stm> (accessed 27 June 2011)
2. Wikipedia contributors, "Polypropylene," Wikipedia, The Free Encyclopedia, http://en.wikipedia.org/w/index.php?title=Polypropylene&oldid=437227937 (accessed July 2, 2011).
3. Wikipedia contributors, "Polypropylene," Wikipedia, The Free Encyclopedia, http://en.wikipedia.org/w/index.php?title=Polypropylene&oldid=437227937 (accessed July 2, 2011).
4. “Disposable Planet?” BBC News <http://news.bbc.co.uk/hi/english/static/in_depth/world/2002/disposable_planet/waste/weeks_waste/plastics.stm> (accessed 27 June 2011)
5. Mallampati Srinivasa Reddy, Tetsuji Okuda, Wataru Nishijima, Mitsumasa Okada,“Recovery of polypropylene and polyethylene from plastic wastes without contamination of chlorinated plastic films by the combination process of wet gravity separation and ozonation,.” Waste Management vol 31 issue 8, Aug 2011, pg 1848-1851 (accessed 24 June 2011.)
6. Mallampati Srinivasa Reddy, Tetsuji Okuda, Wataru Nishijima, Mitsumasa Okada,“Recovery of polypropylene and polyethylene from plastic wastes without contamination of chlorinated plastic films by the combination process of wet gravity separation and ozonation,.” Waste Management vol 31 issue 8, Aug 2011, pg 1848-1851 (accessed 24 June 2011.)
7. http://www.ecologycenter.org/ptf/report1996/PTF_1996.pdf
8. “Disposable Planet?” BBC News <http://news.bbc.co.uk/hi/english/static/in_depth/world/2002/disposable_planet/waste/weeks_waste/plastics.stm> (accessed 27 June 2011)
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12. Jin Lee, Sung-Geun Jung, Cheon-Seok Park, Hae-Yeong Kim, Carl A. Batt, Young-Rok Kim“Tumor-specific hybrid polyhydroxybutyrate nanoparticle:Surface modification of nanoparticle by enzymatically synthesized functional block copolymer” Bioorganic & Medicinal Chemistry Letter, vol 21 issue 10, 15 May 2011, pg 2941-2944.
13. Wikipedia contributors, "Polyhydroxybutyrate," Wikipedia, The Free Encyclopedia, http://en.wikipedia.org/w/index.php?title=Polyhydroxybutyrate&oldid=430815386 (accessed July 2, 2011).
14. Ken'ichiro Matsumoto, Hirokazu Kobayashi, Koji Ikeda, Tasuku Komanoya, Atsushi Fukuoka, Seiichi Taguchi, “Chemo-microbial conversion of cellulose into polyhydroxybutyrate through ruthenium-catalysed hydrolysis of hydrolysis of cellulose into glucose” Bioresource Technology vol. 102 issue 3 3 February 2011 pg 3564-3567
15. Zhenggui Liu, Yuanpeng Wang, Ning He, Jiale Huang, Kang Zhu, Wenyao Shao, Haitao Wang, Weilong Yuan, Qingbiao Li,“Optimization of polyhydroxybutyrate (PHB) production by excess activated sludge and microbial community analysis,” Journal of Hazardous Materials, vol. 185 issue 1, 15 January 2011, pg 8-16.
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17. Zachary T. Dobroth, Shengjun Hu, Erik R. Coats, Armando G. McDonald,“Polyhydroxybutyrate synthesis on bio diesel waste water using mixed microbial consortia” Bioresource Technology vol. 102 issue 3 February 2011 pg 3352-3359.
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