New technology converts problematic polystyrene waste into useful material
New research shows that microbes can help transform discarded polystyrene—including the Styrofoam used extensively for convenience foods—into PHA, a useful thermoplastic.
Although polystyrene foam—commonly known as Styrofoam—has many excellent qualities that make it ideal for disposable coffee cups and convenience food packaging, it has always been difficult to recycle. As a result, most polystyrene waste ends up in landfills where it breaks down very slowly. But a new technology described in research posted today to ES&T’s Research ASAP website (DOI: 10.1021/es0517668) shows how microbes can help transform the waste into a useful biodegradable plastic.
The technology is the joint effort of a group of microbiologists who teamed up with experts in pyrolysis, a way to transform materials by heating them in the absence of oxygen. The scientists carried out the pyrolysis of polystyrene in a fluidized-bed reactor to produce an oil composed of 83% styrene. Next, they used a special strain of Pseudomonas putida, a common soil bacterium, that can feed on styrene. The authors already knew that these bacteria could synthesize the biodegradable plastics known as PHA (polyhydroxyalkanoates), which have numerous uses in medicine and other areas.
However, the authors were surprised to learn that the bacteria thrived on the dirty pyrolysis oil. “We fed that dark black liquid to our bacteria. We thought, maybe it’s a bit too toxic for them, but they grew quite well and they accumulated plastic,” says Kevin O’Connor of University College Dublin, the paper’s corresponding author.
Currently, polystyrene constitutes only [iii] 0.6% of the solid waste generated in the U.S., according to data from the U.S. EPA. But that amounts to approximately 14 million tons each year. Overall, plastics constitute roughly 11% of U.S. solid waste. The strategy developed to recycle polystyrene into PHA might also work for other plastics or even batches of mixed plastics, O’Connor says.
Bioenergy expert Bruce Logan of Pennsylvania State University finds the strategy interesting, but he warns that pyrolysis is an energy-demanding process and can generate hazardous wastes, which would make the process less “green”.
However, paper author and pyrolysis expert Walter Kaminsky, with the Institute for Technical and Macromolecular Chemistry (Germany), says that one way to fuel the system would be by redistilling the crude pyrolysis oil. This cleaner styrene oil can be consumed by the bacteria, while the remaining material can be burned to produce energy for the process. “We have investigated the energy balance of different kinds of plastic,” says Kaminsky. According to numbers provided by Kaminsky, the energy needed to pyrolyze polystyrene is about 2700 kilojoules per kilogram (kJ/kg); the residual 10% of the oil (100g) has a heating value by combustion of 50,000 kJ/kg (5000 kJ/100g). This calculation shows that the energy is more than sufficient for the process’s energy requirements. The fluidized-bed pyrolysis process is totally closed; hence, no pollutants can escape. “The burning of the oil residue after distillation . . . produces only CO2 and water,” he adds.
Currently, O’Connor’s team is concentrating on increasing the yield of the PHA plastic, but they eventually plan to evaluate the costs. “You could imagine a municipal recycling scheme where people are actually putting their polystyrene into a collection system, and so that polystyrene is at low cost,” he says.
Environmental biotechnologist Bruce Rittmann of Arizona State University cautions that it remains to be determined whether the process can be done economically. However, he says, “If your alternative is throwing it in the landfill, now you are capturing a resource out of it. There is always a big benefit to [turning] a waste into something valuable.” —BARBARA BOOTH
