Adsorbents made from food waste for direct air capture
Removing CO2 from the ambient air (direct air capture, DAC) is one option for generating negative emissions. Researchers at ETH Zurich have developed adsorbents for this purpose, which they can produce from waste generated during cheese and tofu production.
Why we need direct air capture
In early June 2026 the German Federal Environment Agency reported that global CO2 concentrations in the atmosphere have risen steadily since a level of 280 ppm in the pre-industrial era, up to 422.79 ppm in 2024. Although CO2 is not the only greenhouse gas, it accounts for the largest share, contributing 66 per cent to the greenhouse effect. To meet the two-degree limit on atmospheric temperature rise agreed at the Paris Climate Conference in 2015, the total concentration of all long-living greenhouse gases must be stabilised at 450 ppm CO2 equivalents by the end of the century; yet the figure for 2023 was already 534 ppm CO2 equivalents.
Consequently, simply preventing greenhouse gas emissions is far from enough – greenhouse gases resulting from past emissions must be removed from the atmosphere.
What are the requirements for DAC technologies?
422 ppm of CO2 in the air is a high level in climate terms, but very little from the perspective of separation technology: at an air temperature of 20°C and standard pressure (1013 hPa), this amounts to approximately 0.78 g of CO2 per cubic metre of air. For a filtration system, this means that a large volume of air must be passed through the filter in order to capture a relatively small amount of CO2.
A DAC system should therefore be made of inexpensive materials that have a high CO2 sorption capacity and high air permeability; the latter is necessary to ensure that the energy required to transport the air through the filter is not too high.
Typical materials that selectively bind CO2 are amine-functionalised (plastic) particles. To remove the CO2 adsorbed onto them, they must be heated to temperatures of 100 °C and above. This process is energy-intensive and, if this energy cannot be supplied from renewable sources, it generates new CO2 emissions.
How can protein residues be converted for capturing CO2?
Milk processing and tofu production generate large quantities of protein-containing solutions, only a small proportion of which is used in food production. The rest is discarded as waste. From these waste solutions, the scientists isolate proteins and use them to form long, thread-like chains known as amyloid fibrils. These are treated with potassium hydroxide solutions. The potassium hydroxide deposits on the surface of the fibrils and acts as a CO2 scavenger, reacting to form hydrogen carbonate. The KOH-treated, thread-like chains are shaped into spheres of approximately 5–10 mm diameter.
“The resulting material is like a sponge that can absorb large quantities of CO2 via the potassium hydroxide,” Prof Raffaele Mezzenga explains. He is group head of the Laboratory of Food and Soft Materials at ETH Zurich.
The fibril beads have a high adsorption capacity for this greenhouse gas: for adsorption from ambient air, the capacity is up to 2.2 mmol/g, which corresponds to just under 10 per cent of the beads’ mass.
Easy regeneration and high stability
The protein beads demonstrate their particular advantage during regeneration: whilst with synthetic adsorbents desorption requires elevated temperatures of 100 °C or higher, or a vacuum, regeneration from the protein beads can be carried out at room temperature. Alternating spraying it with a mild acid and a mild base breaks the chemical bonds, allowing the CO2 to be isolated. The study reports over 30 loading and regeneration cycles, during which the protein beads showed virtually no loss of efficiency.
When the protein beads reach the end of their lifetime as CO2 adsorbents, they could be used further as fertilizer or base material for the production of biofuels.
So far, the beads have been produced and tested on a laboratory scale. However, Raffaele Mezzenga’s team is convinced that the technology can be scaled up, thereby making a more cost-effective and sustainable DAC process available in the future.
The original publication of the study can be found here.
Featured image: Whey is a by-product of cheese production. Among other things, it can be used as a raw material for the production of CO₂ scrubbers. Photo: dyhuettner / Pixabay