Biodegradable plastics get Naked

February 13, 2018

World tunes in: BBC's Naked Scientists interview Murdoch bioplastic researchers

Research into developing biodegradable plastics from wastewater at Murdoch University has hit the international spotlight with an appearance on the BBC’s The Naked Scientist, one of the world’s most popular science podcasts.

School of Engineering and Information Technology researchers Dr Damian Laird and Dr Leonie Hughes have been investigating an environmentally friendly solution for the use of oxalate, one of the major waste products of the alumina industry.

The Murdoch team used mixed microbial cultures, similar to what you find in a run-of-the-mill wastewater treatment plant, to convert the carbon in the wastewater into a potential product – a bioplastic known as a polyhydroxy alkanoate.

These plastics have properties similar to those manufactured with starting materials derived from crude oil.

The process works because most bacteria have a mechanism to store excess carbon when another essential nutrient, such as nitrogen or phosphorus or oxygen, is limited.

“Essentially it’s the bacterial equivalent of a human eating too many chips and not enough veggies – the excess gets stored as fat,” explained Dr Hughes.

“When a suitable nitrogen or phosphorous source becomes available again, the bacteria convert that stored fat back into biomass and the bacteria start growing again. What we do is manipulate this natural phenomenon by giving the bacterial culture a feed that has lots of accessible carbon, in the form of small organic molecules like acetic acid (vinegar) or lactic acid, but very little nitrogen or phosphorus.”

The carbon compound is converted into a polyester-like storage compound rather than being turned into bacterial biomass or respired as CO2 and Dr Hughes and Dr Laird can harvest the plastic.

“This plastic should be completely biodegradable when we’re finished using it as the compound itself has been derived from a naturally-occurring process and naturally-occurring bacteria and fungi already have the biochemical tools necessary to turn the plastic back into its original state – effectively they can eat it,” Dr Hughes explained.

“In a genetically modified pure culture, the stored plastic can represent more than 50 per cent of the bacteria’s biomass; given the right carbon source and optimal conditions.

“Unfortunately, this can be relatively expensive to achieve due to the costs associated with avoiding contamination from other species in the pure culture and the feedstock can be a bit pricey.”

Dr Laird said that industrial wastewater could be a much lower priced feedstock for biopolymer production.

“When an industry releases their wastewater into the sewage system or environment most of the focus is on making sure that toxic compounds like heavy metals, or nutrients that lead to eutrophication of local waterways (such as nitrate and phosphate), are removed from the water.

“Most people don’t realise quite how much carbon is contained in a lot of industrial wastewater – particularly from processes associated with the food and beverage industry, but also from areas such as the aluminium industry where organic matter in the soil is incorporated into the industrial plant. “This can then affect the outcome of the entire conversion of bauxite to aluminium. That carbon can be converted into a useful product.”

The Murdoch researchers are using industrial waste streams as the source of carbon compounds and mixed bacterial cultures to do the conversion.

“The use of a mixed culture will probably mean a smaller amount of plastic produced overall but there is less need to keep the culture in conditions that minimise the chance of contamination by other species,” Dr Laird said.

“We are really thrilled that our work has been featured by the Naked Scientists as we believe that this approach is a really exciting way re-use what has typically been considered a waste and convert it into a product that can be utilised in an environmentally-friendly manner.

“Just like upcycling clothes where you take a couple of old shirts and combine bits of each into a new product we are, essentially, upcycling waste carbon compounds into a biocompatible, biodegradable biopolymer.”

The team have recently shown that oxalate, a potential problem compound in alumina production, can be converted to a bioplastic using this approach. The research was published in the Journal of Environmental Chemical Engineering.

The Naked Scientist story can be heard here until 6 March 2018.

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