Despite the ubiquitous uses of plastics, from an environmental perspective they are a menace. Rebecca Hosking's "plastic bag" campaign is well known, when driven by the horror of her first-hand experience as a wildlife photographer seeing birds and sea-creatures tangled-up in plastic that had crossed the world's oceans, she persuaded her home town of Modbury in Devon to ban plastic bags in the shops there. Most plastics are extremely resistant to biological degradation and are expected hang around for centuries, causing much environmental calamity. They are also a significant component of landfill. What then, if a simple, cheap and eco-friendly process could be devised with which to not only decompose plastic waste, but to turn it into useful products? There may indeed be such a solution according to the promise of preliminary results from labs around the world, in the form of pseudomonas putida - a bacterium found in soil, known for its ability to destroy naphthalene as a soil-contaminant.
A modified version of the pseudomonas bacterium has been shown able to decompose styrene, which is recovered by pyrolysis (thermal decomposition in the absence of oxygen) of styrofoam (polystyrene), and to convert it into polyhydroxyalkanoates (PHA), which are themselves useful plastics, e.g. in medical procedures including skin-grafts, but are biodegradable. Dr Kevin O'Connor at University College Dublin thinks that the bacteria can save the world from being suffocated by toxic plastic waste. The bacteria seem to show an affinity for aromatic molecules, and so feed on polystyrene, polyethyeneterephthalate (PET), which in low-grade form is used to make plastic drinks bottles. Thus, rather than all of this ending up in landfill, it can be used as a feedstock for production of PHA in digesters in which pseudomonas putida grow, using the waste plastic as an energy source.
Around 126 million pounds (sixty million tonnes or so) of styrene waste is released into the environment in the United States each year, contaminating ground, water and air. Styrene is itself carcinogenic (causes cancer). O'Connor believes that within five years, each pound of styrene will be convertible by pseudomonas into half a pound of useful PHA. To place this in context, 126 million pounds of styrene waste could yield 63 million pounds of PHA which is about the same amount that Americans buy each year in terms of plastic goods. When exposed in soil, air or water for several weeks, the plastic simply degrades like a banana-peel. It may be possible to genetically modify the bacteria so that it eats other kinds of toxic waste and converts that to different kinds of useful, biodegradable plastic.
The new, useful plastic can be recovered from the bacterial cells simply by treating the mixture with a mild detergent, which breaks down the cell walls and releases the PHA as tiny granules - nonetheless, separating the plastic from dead cell debris does pose a challenge. The pound to half a pound conversion figure, while being an optimistic projection in five years time, is quite a leap from the mere pound to a tenth of a pound that is obtained using the current technology. If the process can be made cheaper and the price of the PHA pound for pound reduced to a similar order as that for conventional oil-based plastics, then business is likely to be more interested, and that is likely key to the success of this technology. I am enthused by this, as a good example of working with nature to find a solution to an environmental problem, which does not add a greater burden, e.g. using detergents to break-up oil slicks, but that a living organism can be grown to do the job rather than placing a further reliance on oil-based chemicals, which have both caused the problem in the first place and are likely to soon run into short supply.
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