Keeping global warming below 1.5°C over the long term will require more than major cuts in greenhouse gas emissions. According to climate scenarios outlined in the latest Assessment Report from the Intergovernmental Panel on Climate Change (IPCC), the world will also need technologies capable of removing and storing hundreds of billions of tons of carbon dioxide (CO2) already in the atmosphere.

One approach receiving increasing attention is “direct air capture” (DAC), a technology that removes CO2 directly from the air. Researchers and startups have spent years trying to improve DAC systems. One of the earliest commercial companies in the field is Climeworks, an ETH Zurich spin-off founded in 2009. Despite progress, direct air capture remains costly and requires significant amounts of energy.

Food Waste Turned Into Carbon Capture Material
Researchers at ETH Zurich have now developed a new DAC approach that could make the process more efficient and sustainable. In a study published in the journal PNAS, a team led by materials scientist Raffaele Mezzenga used waste products from dairy and tofu manufacturing to capture CO2.

Large amounts of protein-rich liquid are generated during the production of dairy products and tofu. Only a small portion is reused in food manufacturing, while much of the rest is discarded. The researchers extracted proteins from this waste stream and assembled them into long, thread-like structures called amyloid fibrils. These fibrils were then combined with potassium hydroxide and formed into porous beads measuring about half a centimeter to one centimeter in diameter.

“The resulting material is like a sponge that can absorb large quantities of CO2 via the potassium hydroxide,” Mezzenga explains.

When exposed to air, the potassium hydroxide inside the beads reacts with CO2 and converts it into hydrogen carbonate, a salt of carbonic acid. This reaction removes carbon dioxide from the atmosphere.

“In our tests with ambient air, we were able to extract 97 milligrams of CO2 with one gram of material,” explains Zhou Dong, a postdoctoral researcher in Mezzenga’s group and lead author of the study.

According to Dong, this performance is exceptionally strong, exceeding the capacity of conventional DAC technologies by 10 to 50 percent. Based on the team’s calculations, one kilogram of the protein beads could theoretically capture and isolate 100 grams of CO2 during a single operating cycle.

Lower Energy Carbon Removal
Most existing direct air capture systems rely on heat and negative pressure to release the captured carbon dioxide from the absorbent material. The recovered CO2 can then be stored or converted into other products, allowing it to be removed from the atmosphere over the long term. However, generating the necessary heat requires substantial energy, making DAC economically and environmentally viable mainly in places with abundant renewable energy.

The ETH Zurich team developed a different solution. To release the captured carbon dioxide, the protein beads are sprayed alternately with a mild acid and a mild base for approximately 10 minutes at room temperature. This process breaks the chemical bonds holding the CO2, allowing it to be collected.

The acid, base, and protein beads can all be reused afterward.