OUTER PLANET 62 PETplanet Insider Vol. 25 No. 04/24 www.petpla.net OUTER PLANET Putting an end to bio-based plastic anxiety Bio-based plastics such as polylactic acid (PLA) came from a great idea: they were invented to help solve the plastic waste crisis by being constructed so as to break down in the natural environment. Unfortunately, they can make waste management more challenging because they look and feel the same as conventional, petroleum-based plastics. As a result, many products are added to the recycling stream by well-intentioned consumers, get shredded and melted down with the recyclable plastics and bring down the quality of the mixture, making it harder to manufacture functional products out of recycled plastic resin. The only solution, currently, is to try to separate the different plastics at recycling facilities. Yet even with the most high-end, automated sorting tools, some biobased plastics end up contaminating the sorted streams. Scientists at Lawrence Berkeley National Laboratory (Berkeley Lab) and the Joint BioEnergy Institute (JBEI) are collaborating with X – the moonshot incubator led by Alphabet, Google’s parent company (not to be confused with Elon Musk’s company) – to skip the separation step and make a better product. The team has invented a process to break down mixtures of petroleum- and bio-based plastics, using naturally-derived salt solutions paired with specialised microbes, in a single vat. The salts act as a catalyst to break the materials down from polymers into individual molecules (monomers), which the microbes ferment into a biodegradable polymer that can be made into fresh commodity products. The process is described in a One Earth paper published last October. The paper describes a series of laboratory-scale experiments with mixtures of PET and PLA, the most common bio-based plastic, using an amino-acid-based salt catalyst and a strain of Pseudomonas putida, engineered by scientists at Oak Ridge National Laboratory. This combination successfully broke down 95% of the PET/PLA mixture and converted the molecules into a type of biodegradable polyhydroxyalkanoate (PHA) polymer. The team’s approach could enable bio-based manufacturing of other valuable products, with the same bacteria. Biofuels or even medicines could be made from plastic waste. “We foresee the potential to replace sugars, traditional carbon sources for microbes, with processed hard-to-recycle mixed plastics that can be converted to valuable products through fermentation,” said Zilong Wang, a UC Berkeley postdoctoral researcher working at JBEI. The team is searching for an organic salt catalyst that is both highly effective at breaking polymers down and can be reused in multiple batches, to lower costs, and is also modeling the process in large scale, real-world recycling facilities. This chemical recycling process is currently proven only for mixtures of PET and biodegradable PLA but it is claimed to be beneficial for diverse plastic streams typically encountered in real recycling facilities and can be completely integrated with them. This would be very helpful in dealing with commercial products such as fleece jackets, which comprise PET-based polyesters alongside polyolefins or polyamides. The one-pot process can separate the polyester component from that mixture and convert it into a bioplastic. The monomers are soluble in water but polyolefins or polyamides are not, which means that they can be easily removed by filtration and then sent off for a traditional mechanical recycling process. Chemical recycling has been difficult to achieve at commercial scale, because the separation steps are so expensive, according to Ning Sun, staff scientist at ABPDU, lead author and principal investigator of this project. “By using a biocompatible catalyst in water, the microbes can directly convert the depolymerised plastics without extra separation steps,” he said. Co-authors Nawa R. Baral and Corinne Scown, experts in technoeconomic analysis in JBEI and Berkeley Lab’s Biosciences Area, also demonstrated that, once optimised with a reusable salt solution, the process could reduce the cost and carbon footprint of PHAs by 62% and 29%, respectively, compared with today’s commercial PHA production. www.lbl.gov A graphic showcasing the scientists’ streamlined one-pot process. (Credit: Bianca Susara/Berkeley Lab)
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