Rice husk has been transformed into highly efficient material for extracting carbon dioxide directly from the air. Scientists from the Korea Institute of Science and Technology and the Allnex USA company have developed a sorbent that almost completely restores its properties when heated to just 75 °C and loses less than 2.5% efficiency after 20 operational cycles. The development could make direct CO₂ capture technologies significantly cheaper.
Direct extraction of carbon dioxide from the atmosphere is considered one of the most challenging tasks in modern climate engineering. While CO₂ concentrations in power plant flue gases reach 10 -15%, its concentration in the air is only about 0.04%. To extract such a small amount of carbon dioxide, the material must literally “catch” individual molecules among nitrogen, oxygen, and water vapor.
Today, one of the most promising materials for this purpose are solid silica-based sorbents with amines deposited on their surface representing organic compounds that chemically bind carbon dioxide. However, the production of silica itself requires significant energy input and is accompanied by additional CO₂ emissions, making such materials expensive and not particularly environmentally friendly.
To address this problem, the researchers proposed using ordinary rice husks instead of traditional raw materials – one of the most common agricultural wastes in the world. Millions of tons are generated annually, and after processing – which includes acid cleaning and calcination – it is converted into almost pure amorphous silicon dioxide. It was from this product that the researchers obtained a new porous material, the structure of which became a key factor in its high efficiency.
The pore volume of the new material reached 2.31 cubic centimeters per gram, and their average diameter was about 18 nanometers, approximately 3 to 5 times larger than that of many existing industrial sorbents. This three-dimensional structure resembles a sponge with a network of interconnected channels through which carbon dioxide molecules can freely penetrate into the material.
The scientists then filled the pores with polyethyleneimine, a polymer containing a large number of amine groups that directly bind CO₂. Typically, at high loading of such a polymer, the pores begin to clog and sorbent efficiency drops. However, thanks to the large channels of the new silica foam, the researchers managed to load up to 55% of the active substance without significantly compromising material permeability.
Tests showed that the sorbent captures carbon dioxide in two stages: first, amine groups located on the surface react quickly, then CO₂ molecules gradually penetrate deeper into the pores and bind to internal active sites. At optimal loading, the material absorbed 2.92 millimoles of CO₂ per gram in dry air.
One of the main advantages of the new development was the low regeneration temperature. While traditional liquid amine systems require heating to approximately 120–150 °C, the new material releases about 85% of the accumulated carbon dioxide in just half an hour at 60 °C, and at 75 °C it almost completely restores its absorption capacity. This allows the use of low-grade heat, for example from industrial facilities or solar collectors, significantly reducing the energy costs of the system.
The sorbent’s performance was tested under real atmospheric conditions. Ordinary outdoor air containing about 460 parts per million of CO₂ at 59% relative humidity was passed through a column containing the material. The sorbent operated effectively for nearly five hours and absorbed 2.43 millimoles of carbon dioxide per gram. Additional analysis showed that during repeated use, no irreversible by-products – which typically gradually degrade such sorbents – are generated in the material.
According to the researchers’ estimates, the cost of the new material is about $0.47–0.50 per gram, excluding energy and labor costs. Using rice husks instead of traditional raw materials also reduces the environmental footprint of production. For comparison: the industrial production of sodium silicate from quartz sand releases about 1.5 kilograms of CO₂ into the atmosphere for every kilogram of product.
