Every year, several hundred million tons of plastic are produced worldwide, of which more than 10 million tons end up in the oceans, costing the lives of countless marine animals. Less conspicuous but just as dangerous are so-called microplastics – plastic particles that are less than 5 mm in length. They result, for example, from the abrasion of synthetic textile fibers or tires, or the decay of larger plastic parts due to weathering processes. These small particles also pose a danger to humans, as we unknowingly ingest these particles through our food, drinking water, and the air.
Methods already exist to filter out microplastics from water but only for particles larger than 5-20 µm in size. A team of researchers consisting of Dr. Francisca Contreras Leiva and Professor Ulrich Schwaneberg from the Chair of Biotechnology and Professor Thomas Wintgens and Professor Volker Linnemann from the Institute of Environmental Engineering is now working on a method to filter out even smaller particles from water.
The researchers took advantage of the properties of peptides, molecules consisting of amino acids: different peptides have different binding specificity for different materials, i.e. some peptides bind to the filter material, others to plastic.
The researchers initially identified the peptides that bind well to the filter and to plastic. They then combined these two types of peptides, resulting in a biadhesive peptide that binds to the filter on one side and to the microplastic particles on the other. Finally, the biadhesive peptides were placed on the filter. In subsequent experiments, the research team's hypothesis was confirmed: More microparticles could indeed be filtered out of the water with the peptide-coated filter compared to filters that were not coated with peptides.
The research team still faced a number of challenges, however:
At first, the particles could not be detected because of their size, but the researchers quickly found a way to make the tiny particles visible. They added a fluorescent dye to the peptides, which could also bind to the plastic particles.
Another challenge was measuring the filtered particles. To quantify how many microplastic particles could be filtered out of the water with their newly developed method, the researchers used so-called spiked water. This is water that has been artificially enriched with microparticles so that the concentration of particles in the water can be precisely determined before and after filtration.
This is precisely the problem that the research team would like to address in the future and find a solution for better determining and quantifying the filtered microplastic particles.
Finally, questions remain open as to whether other materials can be used as filters and whether the process can or must be modified so that, for example, even more particles can be filtered out by repeated filtration.