From waste to wealth

We are amid a global plastic waste crisis, with just 9% of the 400 million tonnes of plastic we have ever produced being recycled. The remainder is burnt, sent to landfill, or worse ends up polluting the planet’s natural environments. As well as being an environmental catastrophe, this is also an economic and wider sustainability problem. Plastics are derived from high value, finite petrochemical feedstocks (ie oil and gas) and the current linear life cycle - make, use, discard or convert into lower value recycled materials - is estimated to equate to a loss of $10 billion to the global economy each year. It is therefore imperative for both environmental and economic sustainability that we develop new technologies to valorise plastic waste as a feedstock to produce high-value, second generation products.

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Research by Joanna Sadler and Stephen Wallace (School of Biological Sciences) addresses this by developing sustainable, bio-based methods of converting post-consumer plastic waste into industrially important chemicals. They target chemicals which would otherwise be prepared directly from finite fossil fuel resources. Most recently, they developed a technology to convert a waste plastic bottle (made of the plastic polyethylene terephthalate, PET) into the vanilla flavour molecule vanillin using an engineered microbe, a strain of the common laboratory host Escherichia coli. Vanillin is currently produced at >20 000 tonnes pa scale, with over 80% of this demand being met via chemical synthesis from petrochemical feedstocks.

Their novel PET-to-vanillin process involved two steps, carried out sequentially in a single reaction vessel. First, they used a thermostable enzyme to degrade the PET into its constituent monomers, ethylene glycol and terephthalate. Next, they added E. coli which had been specially engineered as whole cell biocatalysts able to convert terephthalate into the target molecule vanillin. This involved co-expressing four non-native enzymes in the E. coli cells to catalyse sequential steps in the pathway. Only when terephthalate and these enzymes were present were we able to detect vanillin in the reaction mixture.

Plastic can be viewed as a resource, rather than simply a problematic waste product

 Dr Joanna Sadler, School of Biological Sciences

The team next set out to maximise the amount of vanillin being formed through optimising the process conditions, and were rapidly able to increase vanillin titres over 150-fold simply through adjusting parameters such as temperature, buffer additives and pH. They are now looking to apply modern synthetic biology workflows to this pathway to optimise the genetic design of the microbes and further increase yields and process efficiency. This work shows that plastic can be viewed as a resource, rather than simply a problematic waste product. It is an alternative feedstock for production of high-value chemicals and the next challenge is to develop the technologies to realise the exciting potential of valorising plastic waste to transition towards a sustainable, circular economy.

For more information on this work, please see Sadler & Wallace, Green Chemistry, 2021.