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Direct air capture

Method of carbon capture from carbon dioxide in air

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Through Carbon dioxide, Alkali and Solvent

DAC is in contrast to carbon capture and storage (CCS), which captures CO2 from point sources, such as a cement factory or a bioenergy plant. After the capture, DAC generates a concentrated stream of CO2 for sequestration or utilization. Carbon dioxide removal is achieved when ambient air makes contact with chemical media, typically an aqueous alkaline solvent or sorbents. These chemical media are subsequently stripped of CO2 through the application of energy (namely heat), resulting in a CO2 stream that can undergo dehydration and compression, while simultaneously regenerating the chemical media for reuse.

DAC has yet to be integrated into emissions trading because, at over US$1000, the cost per ton of carbon dioxide is many times the carbon price. The current high cost of DAC is driven by the scale of deployment and energy factors. In 2025 DAC costs exceeded $1000 per tonne CO2. Some claim that for plant scales of 1 Mtpa (million tonnes per annum) and above, DAC cost could be reduced below $200 per tonne of atmospheric CO2 removed, but others dispute this. Future innovations may reduce the energy intensity of this process.

DAC was suggested in 1999 and is still in development. Several commercial plants are planned or in operation in Europe and the US. Large-scale DAC deployment may be accelerated when connected with economical applications or policy incentives.

In contrast to carbon capture and storage (CCS) which captures emissions from a large point source such as a factory, DAC reduces the carbon dioxide concentration in the atmosphere as a whole. Flying across oceans and getting into space are expected to emit a significant amount of greenhouse gases in the mid-21st century. DAC can be used to capture carbon dioxide before or after its use in jet fuel. Although biofuels are an alternative to efuels, as of 2026 the most sustainable balance between them is unclear.

Methods of capture

There are the three stages of CO2 capture in DAC: the contacting stage, the capture stage, and the separation stage. In the contacting stage, the DAC system transports atmospheric air containing CO2 to the equipment using large-scale fans. Subsequently, in the CO2 capture stage, CO2 rapidly and effectively binds with liquid solvents in chemical reactors or solid sorbents in filters, which must possess binding energies equivalent to that of CO2. Later in the CO2 separation stage, external energy sources facilitate the separation of CO2 from the solvents or sorbents, yielding pure CO2 and regenerated solvents or sorbents. Following the completion of these three stages, the separated pure CO2 is either utilized or stored, while the recovered solvents or sorbents are recycled for reuse in the CO2 capture process.

Generally, solid sorbents DAC (S-DAC) uses low temperature process DAC, while liquid (amine or metallic hydroxides) sorbents DAC (L-DAC) uses low or high temperature process. S-DAC and L-DAC feature different properties in terms of kinetics and heat transfers. Currently, L-DAC and S-DAC represent two mature technologies for industrial deployment. Additionally, several emerging DAC technologies, including electro-swing adsorption (ESA), moisture-swing adsorption (MSA), and membrane-based DAC (m-DAC), are in different stages of development, testing, or limited practical application.

More recently, Ireland-based company Carbon Collect Limited has developed the MechanicalTree™ which simply stands in the wind to capture CO2. The company claims this 'passive capture' of CO2 significantly reduces the energy cost of Direct Air Capture, and that its geometry lends itself to scaling for gigaton CO2 capture.

Most commercial techniques use a liquid solvent—usually amine-based or caustic—to absorb CO2 from a gas. For example, a common caustic solvent: sodium hydroxide reacts with CO2 and precipitates a stable sodium carbonate. This carbonate is heated to produce a highly pure gaseous CO2 stream. Sodium hydroxide can be recycled from sodium carbonate in a process of causticizing. Alternatively, the CO2 binds to solid sorbent in the process of chemisorption. Through heat and vacuum, the CO2 is then desorbed from the solid.

Among the specific chemical processes that are being explored, three stand out: causticization with alkali and alkali-earth hydroxides, carbonation, and organic−inorganic hybrid sorbents consisting of amines supported in porous adsorbents.

Other explored methods

The idea of using many small dispersed DAC scrubbers—analogous to live plants—to create environmentally significant reduction in CO2 levels, has earned the technology a name of artificial trees in popular media.

Moisture swing sorbent

In a cyclical process designed in 2012 by professor Klaus Lackner, the director of the Center for Negative Carbon Emissions (CNCE), dilute CO2 can be efficiently separated using an anionic exchange polymer resin called Marathon MSA, which absorbs air CO2 when dry, and releases it when exposed to moisture. A large part of the energy for the process is supplied by the latent heat of phase change of water. The technology requires further research to determine its cost-effectiveness.

Metal-organic frameworks

Other substances which can be used are metal–organic frameworks (MOFs).

Metal–organic frameworks are crystalline porous materials composed of metal ion clusters linked by organic molecules, resulting in highly ordered three-dimensional structures with exceptionally large internal surface areas. Compared to conventional solid sorbents such as activated carbons and zeolites, MOFs offer greater tunability: their pore size, geometry, and surface chemistry can be systematically varied by changing the metal centers or organic linkers, enabling materials to be designed with specific CO₂ binding properties. For DAC applications, amine-functionalized MOFs have attracted particular interest, as the amine groups can selectively bind CO₂ through chemisorption even at the extremely low atmospheric concentration of approximately 400 ppm. A key challenge for MOFs in DAC is moisture stability: since atmospheric air always contains water vapor at concentrations far exceeding CO₂, water molecules compete for the same binding sites and can cause structural degradation of the framework over repeated adsorption–desorption cycles. Engineering MOFs that are both selective for CO₂ and resistant to humidity under real-world operating conditions remains an active area of research.

Identifying effective MOF sorbents for DAC is challenging due to the vast number of possible framework combinations. In 2023, researchers from Meta AI's Fundamental AI Research (FAIR) team and Georgia Institute of Technology released the Open Direct Air Capture (OpenDAC) dataset to address this problem computationally. The dataset contains results from approximately 400 million CPU hours of quantum chemistry calculations screening MOF candidates for CO₂ adsorption properties, making it one of the largest open computational datasets in the field. Alongside the dataset, machine learning interatomic potential models were released to enable rapid screening of candidate materials at a fraction of the cost of traditional density functional theory simulations, with the goal of accelerating the discovery of viable DAC sorbents.

Membranes

Membrane-based separation (m-DAC) employs semi-permeable membranes. This method requires little water and has a smaller footprint. Typically polymeric membranes, either glassy or rubbery, are used for direct air capture. Glassy membranes typically exhibit high selectivity with respect to carbon dioxide; however, they also have low permeabilities. Membrane capture of carbon dioxide is still in development and needs further research before it can be implemented on a larger scale.

Electro-Swing Adsorption

Electro-swing adsorption (ESA) has also been proposed.

Rock flour

Rock flour, soil ground into nanoparticles by glacier ice, has potential both as a soil conditioner and for carbon capture. Glacier melting deposits one billion tons of rock flour annually, and one ton of Greenlandic rock flour can capture 250 kilograms (550 lb) of carbon.

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