Turning Food Waste Into Carbon Captors

In order to stabilise global warming at less than 1.5°C in the long term, there is a need not only for a drastic reduction in greenhouse gas emissions but also for technologies to remove and store hundreds of billions of tonnes of carbon dioxide (CO₂) from the atmosphere. This is also the underlying basis of the scenarios set out in the latest Assessment Report from the Intergovernmental Panel on Climate Change (IPCC).

For years, research groups and start-ups have therefore been working on ways to remove CO₂ directly from the air - a process known as "direct air capture". The company Climeworks, which was founded as an ETH spin-off in 2009, is one of the world's first commercial providers of DAC. To this day, however, the direct removal of CO₂ from the air remains an energy-intensive and expensive process.

Porous protein beads bind carbon dioxide

In a study recently published in the journal external page PNAS , researchers present a promising new approach to DAC. A group led by materials scientist Raffaele Mezzenga, a professor at the Department of Health Sciences and Technology of ETH Zurich, uses whey and by-products from tofu production for CO₂ absorption.

Dairy and tofu production generate large quantities of protein-containing solutions, only a small part of which is reprocessed in food production - the remainder goes to waste. From this waste, the researchers isolate proteins that they use to form long, threadlike chains known as amyloid fibrils. They then load these fibrils with potassium hydroxide and process them into beads with a diameter of between half and one centimetre. "The resulting material is like a sponge that can absorb large quantities of CO₂ via the potassium hydroxide," Mezzenga explains.

When the porous beads are exposed to ambient air, the potassium hydroxide reacts with CO₂ to form hydrogen carbonate, a salt of carbonic acid. This process removes the CO₂ from the air. "In our tests with ambient air, we were able to extract 97 milligrams of CO₂ with one gram of material," explains Zhou Dong, a postdoc in Mezzenga's group and lead author of the study. This is a very high rate, he says, and 10 to 50 percent greater than the capacity of conventional DAC methods. Dong assumes that, with one kilogram of protein beads, it would theoretically be possible to bind and isolate 100 grams of CO₂ per process cycle.

Technique for a circular economy

Conventional DAC methods generally use heat and negative pressure to release the carbon dioxide from the absorption material again. This is necessary in order to then store the CO₂ or convert it into other materials, thereby removing it from the atmosphere on a long-term basis. However, this process requires a great deal of energy, which is why DAC generally only makes sense nowadays - in terms of both energy and economics - where large amounts of renewable energy are available.

This is another area in which the researchers in Mezzenga's team are taking a different approach: in order to release the carbon dioxide from the protein beads again, the beads are alternately sprayed with a mild acid and base for around 10 minutes at room temperature. This breaks the chemical bonds so that the CO₂ can be isolated.

The acid, base and beads can then be reused. "The synthetic materials that are used to capture CO₂ today decompose quickly," says Dong. "By contrast, our protein beads remain stable for a long time." In the lab, the researchers tested 30 cycles of CO₂ adsorption and release without observing significant losses of efficiency.

Mezzenga assumes that the material would nevertheless need to be replaced after a few thousand cycles due to a decrease in adsorption capacity. However, the protein beads could then be used as fertiliser in agriculture or converted into biofuel, the researcher explains. The beads are made up entirely of organic material, he says, and are readily degradable - meaning that the system could therefore become part of a circular economy.

"The materials we use for this process are non-toxic and are food-grade," Mezzenga points out. In a life cycle analysis, the researchers show that their method generates less environmental pollution across the entire life cycle than other DAC methods.

Expected to be cheaper than other capture methods

Further tests are needed to reveal whether the technology is scalable for practical use and the high CO₂-absorption capacity will remain intact on a larger scale. For the recently published study, the researchers tested the method in a controlled laboratory environment with a few grams of protein beads, binding and isolating around 50 grams of CO₂.

Mezzenga is optimistic. He has been working with amyloid fibrils for nearly 20 years and is well acquainted with the material. In the past, he has used it to develop biodegradable alternatives to plastics as well as techniques for water purification. "We're confident that the technology is scalable," he says. According to Mezzenga, the spray system used to separate the CO₂ from the protein beads is geared towards existing techniques that are already used in industry. Postdoc Zhou Dong will now further examine the question of scalability.

Although the researchers are yet to make an exact calculation of the costs per captured tonne of CO₂, Mezzenga expects them to be significantly lower than with conventional DAC. "Our technology is cheaper and more sustainable because it requires little energy and is based on a widely available waste product," he says. "That could be a game changer for the future of removing CO₂ from the air."