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Busy Little Recyclers: Chemical process, microbial metabolism transform trash-bound plastics

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12:36pm, February 22, 2006
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A two-step approach that converts a common plastic into a biodegradable polymer could cut the number of packing peanuts and Styrofoam cups that end up in landfills, researchers suggest.

In 2003, U.S. manufacturers produced more than 2 million tons of polystyrene to make such items as food packaging, packing materials, and furniture. Nearly all of this plastic was discarded, according to the Environmental Protection Agency. There is little demand for recycled polystyrene because it's of lower quality than the polystyrene made directly from petroleum components, notes Kevin E. O'Connor of the University College Dublin.

Last year, O'Connor's group reported that the bacterium Pseudomonas putida CA-3 could metabolize pure styrene, the precursor of polystyrene, and convert it into polyhydroxyalkanoate (PHA), a biodegradable polymer that can be made into good-quality plastic. While other members of this microbe family turn sugars and other plant-based materials into PHA, converting styrene is rare, says O'Connor.

In the April 1 Environmental Science & Technology, O'Connor and his colleagues describe adding a chemical step to the microbial process, thereby changing polystyrene into PHA.

First, they heated the polystyrene to 520°C in a closed reactor, which broke down the polymer chains and produced styrene oil containing a few other chemical compounds.

The researchers then cooled the liquid and added it to P. putida CA-3 growing in the lab. O'Connor says that he was not sure whether the microbes would tolerate the impure styrene, but "they grew well. You can take your dirty oil and just feed it to the bacteria rather than having to clean it up."

As in their previous work, the researchers induced the bacteria to turn styrene into PHA by limiting the microbes' supply of nitrogen, which they need to make amino acids. Under such conditions, which don't support growth, some bacteria go dormant, says O'Connor, but others react by storing carbon as a polymer that will be useful if more-favorable conditions return.

After 48 hours of fermentation, the bacteria had produced 1.6 grams of medium–chain-length PHA from 16 g of styrene oil, the researchers report. This biodegradable polymer, with repeating units of 6 to 14 carbons, could be used to make paints or medical devices.

"We recognize that it's a nice concept, but it needs to be improved to make it more economical," O'Connor says. The researchers are now investigating ways to improve the 10 percent styrene-to-PHA conversion rate.

Stephen McCarthy of the University of Massachusetts at Lowell agrees that the approach is not practical at this stage but adds that it "could be of value in the future."

However, he notes that rather than making biodegradable materials from petrochemical plastics, it would be better to use only biodegradable plastics in the first place. They can be made from renewable resources such as farm crops. "That's going to help us to wean ourselves off of fossil fuels more than this particular approach," says McCarthy.

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