Bacteria plus polystyrene equals a novel way to dispose of packaging waste and deliver new eco-friendly material, writes Dick Ahlstrom
A UCD scientist has turned a common soil bacterium into an environmental hero, discovering a method for converting polystyrene into a biodegradable plastic. The work could also help clean up one of the world's most polluting chemical processes.
Polystyrene is the packaging material we love to hate. An extremely useful and versatile material, we use it in packaging to protect computers and stereo equipment, in lightweight cups that keep our fingers from getting burned by hot coffee and around the corners of heavy white goods in transit.
It is also an environmentalist's nightmare, says UCD lecturer, Dr Kevin O'Connor. More than 14 million metric tonnes of polystyrene are produced annually worldwide but 70 per cent of it ends up in landfill within 12 months.
Ultimately 99 per cent of all the polystyrene produced will end up claiming space in landfills where unfortunately it also stubbornly resists degradation. It takes literally thousands of years to decompose naturally.
Recycling might seem a tempting option, and polystyrene can be recycled, O'Connor says, but unfortunately no more than one per cent of this material gets sent back for reuse. No one will take it. "Polystyrene really isn't recyclable because nobody wants it."
Polystyrene's main ingredient, styrene, is another environmental problem, Dr O'Connor adds. It disturbs the central nervous system and is dispersed widely in our natural environment.
"The US EPA (Environmental Protection Agency) suggests it is the fifth most polluting process in the world," he says. "It is a nasty compound."
Dr O'Connor, of UCD's school of biomolecular and biomedical science and the centre for synthesis and chemical biology, has sought an answer to this problem for some years. His PhD supervisor had isolated a bacterium that seemed happy to digest polystyrene, and this took him on a quest to tackle both left over waste styrene and its twin nemesis, polystyrene.
This was not simply going to be a matter of finding hungry bacteria. Variations in polystyrene production meant each waste stream was an unknown quantity. Also, there was no obvious method on hand to help the bacteria break down extremely stable polystyrene.
Dr O'Connor started scanning the literature and came across his later collaborator, Prof Walter Kaminsky of the University of Hamburg. Prof Kaminsky is an expert in the chemical degradation of plastics. He uses a technique called pyrolysis where the plastic is heated in a vacuum to temperatures as high as 520 degrees.
The lack of oxygen prevents the plastic from igniting and the high heat breaks the plastic, in this case polystyrene, down into a liquid. "There are no emissions from this," says Dr O'Connor. "It leaves an oily liquid we refer to as styrene oil."
The oil is 83 per cent styrene and Prof Kaminsky's original idea was to use it as an energy source, a synthetic fuel oil, given you get 10 times more energy back out as fuel than you put in to process it. Unfortunately, cheap oil supplies made the process unattractive.
However, Dr O'Connor held the other puzzle pieces in the form of a bacterium, Pseudomonas putida CA-3. He knew this tiny soil bacterium well having studied ways of encouraging it to break down styrene. Not only that, the bacterium also obligingly takes carbon from the styrene and converts it into another biodegradable plastic called polyhydroxalkanoate or PHA.
"It has picked up a few genes during its evolution and these mutations allow it to degrade styrene to PHA," Dr O'Connor says. The PHA forms naturally as tiny white granules inside the bacteria.
He researched this process and found ways to enhance PHA production. "We stimulate the cell by tricking the bacterium to take more carbon and store it as PHA," he says. This is done with low nitrogen levels which cause the bacterium to store the plastic rather than use it as an energy source for growth.
The two-part system involving initial pyrolysis and later bacterial digestion of the styrene oil works very well, Dr O'Connor says. The bacteria are grown up in a fermentation tank and then styrene is added gradually. "They completely destroy the styrene oil," he says.
He also devised a method for getting the PHA out of the bacteria afterwards. Polluting solvents were one possibility but instead he uses mild detergents that break up the bacterial cell walls to release the PHA.
There is a ready market for PHA which can be used in things ranging from plastic bottles to surgical parts, Dr O'Connor says. The two scientists also continue to refine their process.
Dr O'Connor is devising ways to get more PHA out of the process by stimulating the bacteria. Meanwhile, Prof Kaminsky is looking at re-distilling the crude styrene oil to produce a cleaner feedstuff for the bacteria, while using the pyrolysis residue as a fuel source to drive the entire process.