Microplastics as a raw material for artificial photosynthesis

A Korean-Dutch research group has developed a process to produce valuable chemicals from microplastics. They mimic photosynthesis.

Plastic weathers over time – and accumulates as microplastics in the environment.

A research group led by Chan Beum Park from the Korea Advanced Institute of Science and Technology has succeeded in combining two promising future technologies in one process. Their process mimics the photosynthesis of plants, utilizing microplastics in the process. As the team reports in Nature Synthesis, an electrode made of zirconium and iron oxide snatches electrons from the microplastic with the help of sunlight. These reach enzymes via an electric circuit, which uses the electrons to produce valuable chemicals. So far, however, the study has only shown that the principle works. It is still unclear whether the process is economically worthwhile, and whether it can be implemented on a large scale and actually contributes to reducing plastic waste.

Around 390 million tons of plastics are produced worldwide every year – and almost three-quarters of them are thrown away again shortly afterwardThe majority of plastic waste ends up in incinerators or accumulates in landfills and in nature. An even greater ecological threat is the smallest plastic particles with a size of fewer than five millimeters. This so-called microplastic includes, for example, cosmetic beads, clothing fibers, or tire abrasion.

To utilize such waste, Chan Beum Park and his colleagues Jinhyun Kim, Jinha Jang, Thomas Hilberath, and Frank Hollmann from the University of Technology in Delft are now using a principle that plants have used since time immemorial. They use PET as a raw material, which is what plastic bottles are made of, for example. Sunlight provides the energy to extract electrons from the plastic and transfer them to the anode. The electrons are conducted to a carbon fiber paper cathode, which absorbs the electrons and transmits them to enzymes.

Microplastics as an electron source

These use high-energy electrons to produce valuable chemicals. As the working group shows, the principle works with various enzymes that allow technically important reactions to take place. So you can specifically choose which substances you produce with the help of plastic waste. Meanwhile, plastic waste produces formic and acetic acid, which also have a number of industrial applications. “These results indicate that the photoanode can extract electrons from the microplastics and produce organic fuels from them,” they write, but at the same time point to weaknesses in their process.

The technology is not yet very effective. The researchers were only able to produce 24 micrograms of formic acid and 11 micrograms of acetate from a solution of one milligram of PET to one milliliter of sodium hydroxide. They, therefore, propose follow-up experiments in which the surface structure of the zirconium-containing electrode should be changed so that more high-energy charge carriers can pull electrons out of the plastic more effectively. They also suspect that the yield could also be improved in the enzyme reactions if the excited electrons could be more easily transferred to the biocatalysts.

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