From Science and
From Science and Research Using Bugs to Recycle Plastic Researchers team up to convert PET Waste (iStockphoto) polystyrene and PET into a biodegradable plastic Article contributed by Dr. Kevin O’Connor, School of Biomolecular and Biomedical Science Centre for Synthesis and Chemical Biology University College Dublin, Ireland Worldwide, more than 14 million tonnes of polystyrene are produced annually and it is so durable that it takes thousands of years to decompose. Also millions of tonnes of PET are produced annually predominantly for use in food packaging. With 70% of polystyrene ending up in landfill within a year of manufacture and 99% of all polystyrene ultimately ending up in dumps, the long term problem created by this versatile plastic is causing major anxiety among local and national governments throughout the world. While PET is recycled at a much higher rate than polystyrene only 25% of the PET reaching the market place is collected and recycled. End of Life options The burying of plastic in landfill is viewed by many as a waste of a valuable resource. One alternative is the mechanical recycling with Germany boasting the highest rate in Europe (33%, [1]) even if a certain ‘downcyling’ has to be accepted. Other countries have a lower level of mechanical recycling and a higher rate of energy recovery from plastics (solid fuel). While the direct burning of plastic for energy may satisfy the energy needs of incinerators burning municipal waste the conversion of this plastic to a more versatile liquid fuel and other value added products is in the long term more valuable to society. Upcycling petrochemical plastic to Biodegradable plastic At University College Dublin we have combined chemistry and biology to develop a two step chemo-biotechnological process for the conversion of PS and PET respectively to polyhydroxyalkanoate (PHA). The process combines the thermal treatment of the plastic (pyrolysis, as described below) followed by microbial transformation of the thermally treated product to the biological polymer (PHA). The PHA produced contains repeating monomer units which are predominantly 10 carbons in length (R)-3-hydroxydecanoic acid. PHA composed of monomers which are 6 carbons or longer is referred to as medium chain length PHA (mclPHA). This polymer is a partially crystalline elastomer with a thermal degradation temperature close to 300°C and differs from the copolymer PHBcoPHV (polyhydroxybutyrate co polyhydroxyvalerate) which is a harder less flexible PHA with thermal degradation temperatures close to 200°C. 28 bioplastics MAGAZINE [06/08] Vol. 3
From Science and Research Pyrolysis The thermal treatment of plastic in the absence of air will not result in the burning of plastic but in the breaking of the bonds that hold the monomers together. This results in the conversion of petrochemical plastics and indeed other polymers into gases, liquids and solids. The pyrolysis of petrochemical plastics to generate fuels has been promoted since the 1970’s when the first major oil crisis occurred. Professor Walter Kaminsky from the University of Hamburg, Germany developed one of the modern pyrolysis processes for petrochemical polymers using quartz sand. The latter is heated so that it becomes molten and transfers heat to the plastic. The temperature of the pyrolysis reaction varies according to the polymer to be transformed e.g. polystyrene is pyrolysed at 520°C while PET is heated to 450°C. Polystyrene pyrolysis oil is composed of styrene (90 % by weight) and low levels of other aromatic compounds. Terephthalic acid is the predominant product of PET pyrolysis with gases and liquids as minor products. The reflux of 6% of the styrene oil product towards the burners that drive the pyrolysis reaction results in an energy output (3000 KJ/Kg) exceeding the initial energy input (2700 KJ/Kg) resulting in an energy positive process. Microbial production of polyhydroxyalkanoates from pyrolysis products While the products of pyrolysis are predominantly destined for the fuel market they are also excellent carbon substrates that microorganism and in particular bacteria can convert into polyhydroxyalkanoate (PHA) [2, 3, 4]. This styrene oil, when supplied as the sole source of carbon and energy allows for the growth of bacteria from the genus Pseudomonas, which are common soil bacteria. We have isolated a number of Pseudomonas species from soil that are capable of growth and PHA accumulation from pyrolysis products arising PS, PET and mixed plastic. The percentage of the bacterial cell that is PHA is generally between 20 and 30% when the bacteria are grown in shaken flasks. However we have shown the transfer to a fermentor (bioreactor) increases the PHA level 1.5 fold. Furthermore the manipulation of the fermentation conditions (substrate feeding and nutrient concentration) can significantly raise biomass and PHA levels [5]. However the biomass needs to be increased further to make the technology commercially viable. Experiments in the laboratory are focused on this task. Biorefinery: a single site for the conversion of renewable feedstock and waste to value added products In an oil refinery the primary products arising from crude oil are fuels. However, the minor fractions are an excellent resource for the synthesis of polymers and fine chemicals. Modern and future pyrolysis plants should operate a similar model to the oil refinery. In an extension to that idea the modern biorefinery, which is currently predominantly focused on renewable raw material conversion, should encompass the conversion of waste materials (arising from outside and within the biorefinery) to PHA in a petri-dish (Photo: Terence Union, University College) Polystyrene (iStockphoto) www.ucd.ie/biocatal bioplastics MAGAZINE [06/08] Vol. 3 29
