CO₂ emissions as a source of biodegradable plastic

Biotechnologists use the greenhouse gas carbon dioxide (CO₂) to produce bioplastics. The resulting polyester is compatible with living tissue and biodegradable, writes a group led by Sang Yup Lee and Hyunjoo Lee from the Korea Advanced Institute of Science and Technology in Daejeon in the "Proceedings" of the US National Academy of Sciences ("PNAS"). .

If a biotechnological plant is operated with electricity from renewable energies, the production of the plastic is climate-neutral. "Converting man-made CO₂ into value-added products using renewable energy has received much attention to achieve a sustainable carbon cycle," the researchers write.

It was already known that the bacterium Cupriavidus necator can produce the polyester polyhydroxybutyric acid (PHB). However, the quantities achieved so far have been quite small. The research team has now realized that the biotechnological process can be improved if the electrochemical conversion of CO₂ and the production of PHB take place in two separate vessels.

The CO₂ is bound in formic acid at a gas diffusion electrode. However, this creates electrical currents as well as oxygen and nitrogen compounds that are harmful to the bacterial cells. By moving C. necator's fermentation to a different vessel, the scientists created better growth conditions for the bacterium. To do this, however, they had to develop an electrolyte liquid that enables both the electrochemical binding of the CO₂ and the production of bioplastics with high yields in the bacterial cells.

Because they succeeded, the team was able to set up a cycle: Formic acid is constantly being produced in the CO₂ electrolyser, which flows into the fermenter with the electrolyte, whereby substances harmful to the bacteria are filtered out. In the fermenter, the bacteria use the formic acid as a food source to produce PHB, which accumulates in their cells. The biotechnologists achieved a yield of PHB that corresponded to 83 percent of the bacteria's dry mass.

The C. necator bacteria removed from the fermenter are replaced with freshly cultivated ones. After the bacterial cells have been filtered out and returned to the fermenter, the electrolyte flows back into the CO₂ electrolyser.

"This work proposes an extraordinary strategy to reduce carbon emissions and produce eco-friendly bioplastics," the study authors write. The process operated stably over a period of 18 days, maintaining constant concentrations of formic acid and PHB. The researchers are therefore confident that their process can be scaled up from the gram range to enable commercial PHB production.

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