Sunday, January 6, 2013

Microbial degradation of plastics

Title figure: Mark W. Slater (Natural Line Studio)

My first post covered general topics of the capabilities of micro-organisms with regard to the manufacture and destruction of plastics. Now for the beginning of the slightly more nitty gritty. I'll start off with what plastics are, followed by what makes them amendable (or not) to bio-degradation. Lastly we'll see if the technology is at a stage where it can be used on an industrial scale.
What are plastics:
Figure 1A. Ethylene 
large ball - carbon atoms
small balls - hydrogen atoms
Figure 1B: Polyethylene
hydrogen atoms not shown
Figure 2: low density polyethylene (LDPE)
hydrogen atoms not shown
Plastic forms such a daily part of our lives –  packaging, toys, electronics, furniture, clothing (fleece, waterproof clothing), shoes, building materials; the list is endless. And when you think of the different forms plastic takes, the diversity is mind boggling; thin and foldable, thick and rigid, filled with air,  thread like, crinkly, fun to stamp on (i.e. bubble wrap) etc. 

Plastics can be divided into two types. 1. Thermoplastics and 2. Thermoset plastics.

Thermoplastics are formed from alkenes. The simplest alkene is ethylene (CH2=CH2) (also called ethene) and is shown in figure 1A. During polymerization the double bond between the two carbons is broken, resulting in a single bond between the two carbons and the use of the two halves of the second single bond to form two new single bonds with two new ethylene molecules on either side of the original ethylene. Thus a chain of ethylene molecules -(CH-CH)n- as in figure 1B is generated. This is called polyethylene or PE. Other thermoplastics are polyvinyl chloride and polystyrene. Modification of the monomer or mixing two or more different monomers together in the chain, changes the properties of the end plasic such as its  temperature tolerance or flexibility. Modification of the chain also changes the end plastic. For example, by adding side chains to the central strain of PE, causes the chains to pack less tightly, which decrease density creating low density polyethylene (LDEP - figure 2, rrepresentative illustration) and allows it to be stretched into thin sheets such as in your supermarket plastic bag. 

The single carbon bond in thermoplastics is stable, resistant to degradation and hydrolytic cleavage which is the cleavage of chemical bonds by the use of water. Thermoplastics have been generally regarded as non-biodegradable. Unfortunately, they make up the majority of our plastic use. In 2004 thermoplastics made up 92% of the distributed plastic resins in the USA (ref 1).
Figure 3 Ester and Amide reactions

Figure 4 Urethane reaction
The second type of plastic are thermoset plastics. These plastics are made by joining an alcohol (R-OH) or amine (R-NH2) with a carbonate group (R-C(O)OH) in a process known as condensation as a water molecule is released and an ester (-C(O)OC(H2)-.or amide (-C(O)NC(H2)-) is formed (figure 3). Examples of thermoset plastics are polyesters and  polyurethanes. Urethane is little more complex as it involves the reaction of an isocynate group with an alcohol group to generate a urethane group (figure 4). You will notice that no water is produced and this is not a condensation reaction but the result is an amide-ester bond or urethane bond. The small black dots in the structures represent carbons and the R groups, the rest of the molecule. R1 and R2 in polyurethane would then need contain another isocyanate and alcohol reactive group attached to whatever structure that was inbetween such as an alky or aryl group (figure 5). In this way, both ends of the molecule could react to form a polyurethane chain. From the differences in the R groups, its apparent that the polyurethanes can encompass great diversity (ref 2). The ester, amide or urethane link is susceptible to enzymes produced by bacteria and fungi and is thus biodegradable or at least more biodegradable than the thermoplastics.
Figure 5. Polyurethane synthesis reaction with a di-isocyante (2 isocyanate groups) and a diol (2 -OH groups)

Now that we have some basic ideas about plastic, lets see how biodegradation works. We know that thermoset plastics, because of their chemical composition are biodegradeable. Does that mean that addition of a few choice micro-organisms will cause automatic meltdown? Not quite. Polyesters for example consist of two types: aliphatic and aromatic polyesters. Though there is some controversy, aliphatic polyesters are thought to be biodegradable but aromatic polyesters are not (ref 1). Aromatic polyesters contain six carbon rings and form plastics used to make carpet, clothing and soda bottles. Aliphatic polyesters are rather weak plastics and are used to make medical and scientific items such as tissue scaffolds and dissolving drug delivery systems. Polyurethane is a different type of plastic to polyester (though sometime refered to as the polyester polyurethane) and is used in many products including furniture, paints and construction materials and its biodegradation has been shown to be due to the microbial destruction of the ester bonds (ref 2). Additionally, when plastics are made, additives are blended that improve their enviromental stability such as their resistance to heat and light and these generally hinder biodegradation. In biodegradation research, ways in which plastic stability can be maintained while improving susceptibility to micro-organisms are investigated. Various ways have also been devised to improve the poor biodegradation of thermoplastics. Before micro-organisms can attack the long polymer chains, they need to be transformed into bite size pieces of 500 Da or less. As many plastics are resistant to conditions that allow the chains to be broken up, additives called pro-oxidants are mixed in with the polymers that make the plastics hydrophilic and catalyze the breakdown of the hydrocarbon chains during photolysis or thermolysis (see title figure above). Pro-oxidants consist of transient metal ions (e.g cobalt or manganese) added in the form of an organic ligand complex. The chemistry is beyond the scope of this blog (for now!) but see ref 3 if you want to dig in. In addition to pro-oxidants, other truly biodegradable material such as starch are added to the polymers and this enhances the biodegradation process.

What are the bacteria and fungi that are used to chew up plastics once it is in a compatible form? Of course its variable! Without getting too specific about genus and species, lets look at two examples - the thermoplastic polyethylene and the thermoset plastic polyurethane.

Biodegradation of thermoplastic polyethylene
PE takes up to 1000 years to degrade in the environment and makes up 64% of the plastic mass produce each year globally (ref. 4). To put that in perspective 500,000,000 (500 billion) to 1,000,000,000,000 (1 trillion) plastic bags are used annually, world wide. And thats not counting the PE thats used to make all the other products. It threatens wildlife, domestic life, marine life and gets into the food chain as its broken down by the action of heat and sunlight. Only a fraction is recycled (ref. 4). In fact, the EPA says that only 8% of all plastic is recycled! As mentioned above, because of the lack of atoms that are subject to electrophilic or nucleophilic attack such as oxgen and nitrogen, PE is regarded as essentially resistant to microbial attack. Pro-oxidants incorporated into the PE during manufacture and resident mainly in pockets of the PE that are less crystalline and more amorphous, are then used post-consumer to break up the chains of PE using heat or light. The broken up chains are susceptible to microbial digestion (see title figure at top of blog). For example, the bacteria Rhodococcous rhodocrous, is part of a genus able to metabolise a diverse array of substances due to their robust physiology and large genome that incorporates an array of catabolic genes These are genes that make products able to break down (catabolise) many different substances and this genus is used for other environmental purposes as well as PE degredation (ref 5). A comprehensive table of the different bacterium tried is given in reference 3 and 4 and the authors admit that while consumption occurs in the pockets of PE broken up by pro-oxidant action, further, more complete degradation does not occur. Some success was met mixing pro-oxidant heat treated PE into compost where up to 70% conversion of PE to carbon dioxide was achieved, perhaps because diverse populations of bacteria are present able to produce a more diverse set of enzymes and thus support each other. However from the review article (ref 4) published this year, it can be seen that while some bio-degradation is possible, we are a long way from having a commercially viable process for bio-degradation of PE. For the small dent it can make in improving our current and future environment - keep reaching for your canvas bags and avoid extra packaging wherever possible.

Biodegradation of Thermoset plastic, polyurethane.
So much for PE! What about polyurethane? In 2004, 2,722,000 tons of polyurethane was produced in the United States which encompasses 25% of the market. To put that more visually, the average African male elephant weighs 12,000lbs. There are 2000lbs in a US ton. So, in 2004 about 227 African Elephants could have been modeled from of polyurethane. And thats only a 2004. Production has assuredly increased since then. The mind boggles.

As described above, polyurethane is made from isocynate precursors interacting with polyol (precursors containing at least one -OH group). Because of the diversity of the precursors available, polyurethane (PU) can make many different types of products. The bond between the two precursors is called an amide ester or urethane (figure 4). When the first polyurethanes were made it was noticed that they were degraded by fungi and efforts were made to make them less susceptible by mixing in additives (ref 2). It was also noted the degredation by enzymes was dependent on the chemical structure of the PU and that more amorphous regions were degraded more rapidly.  As the amount of plastics in landfills became a problem, the biodegradability was looked on as an advantage, as long as it did not compromise the function of the plastic. There are a number of fungi and bacteria that are efficient at degrading various PUs. However, until very recently, biostimulation (micro-organism growth stimulus by addition of growth promoting material such as yeast extract) and bioaugmentation (enrichment of particular micro-organism in an environment by external addition of those micro-organisms) were necessary for optimal degradation. The most recent research (ref. 6) has identified a species of fungus isolated  from the Amazonian rainforest called Pestalotiopsis
, which, remarkably
can live on PU alone. This suggests that these type of fungi are a source of efficient PU biodegraders.

So, Abbracadabra?
From these two examples we can summarize that PE is some way from being biodegradable by current techniques and though PU is much further along, the technology is still at the research stage. Is bioremediation of soils of plastics of any sort a commercial venture yet? It turns out the my two examples give a rough idea of the state of affair - lots of encouraging research but no large scale applications yet. We live in hope.

When plastic goes, what do you get instead?
Ok, popping the plastic landfill balloon needs a sharper needle, but say we did have a large scale technology available - what would happen to the plastic once the microbes get their proverbial teeth into it? Well, boys and girls, thats the story for next time! Await the delights, January 30th! or there abouts.

NB: I think there's been some difficulty posting comments which I have addressed. If you would like to post a comment, use the little arrow to select a pull down tab. From the resulting pull down menu, you can select an ID or you can comment as annonymous, which is the last option. You can also e-mail me at


Lifestyle magic - the soda wash.
Wash  your hair with baking soda. 2 tablespoons of soda to 2-3 cups of water. Use as much or little as you need per wash. I've been doing this for the last 6 weeks and I'm really pleased. I have fine, fly away hair (alas, NOT a crowning glory) that needs frequent washing. My hair is as clean and fluffy as with any shampoo BUT - less comes out during brushing and my scalp does not itch as it often did with shampoos. Neither do I need conditioner. If you do, an altnative is dilute apple cider vinegar with honey added optionally.

No comments:

Post a Comment