The need for zero carbon is paramount and is the focus of the various United Nations COP summits to reduce greenhouse gas emissions. The link between carbon dioxide levels and the greenhouse effect is covered in the 2023 syllabus under R1.3.3. Much research focuses on replacing fossil fuels with alternative energy sources but an alternative approach, which means that fossil fuels could be phased out more slowly, is to capture the carbon dioxide produced and so prevent it from entering the atmosphere.  

When carbon dioxide dissolves in water it is in dynamic equilibrium with carbonic acid, hydrogen carbonate ions and carbonate ions. In acidic aqueous solution, carbon dioxide gas is present as CO₂(aq), but in alkaline solution it reacts to form carbonate salts. This chemical reaction is reversible and is pH dependent as to whether it contains mainly carbon dioxide or mainly carbonate ions.

CO₂(g) ⇄ CO₂(aq) + H₂O(l) ⇆ H₂CO₃(aq)

H₂CO₃(aq) ⇄ HCO₃⁻(aq) + H⁺(aq)

HCO₃⁻(aq)  ⇄ CO₃²⁻(aq) + H⁺(aq)

The pH of aqueous solutions can be controlled by using compounds that can alter the pH by reacting with light. These are known as photoacids, examples include protonated merocyanines.  When photoacids are irradiated they form an excited state releasing a proton in the process which decreases the pH making the solutions more acidic. In the dark they return to their original state making the solution more alkaline. Effectively they undergo dissociation initiated by light to give a strongly acidic solution but undergo reassociation by heat (thermal reassociation) to give an alkaline solution.

The reversible excitation of protonated merocyanines

Theoretically, a mixture of carbon dioxide and air from fossil fuel combustion emissions could be passed through an alkaline liquid containing photoacids in the dark. The gaseous carbon dioxide would dissolve to form carbonates. Once the carbonate salts have accumulated to a significant degree the liquid would then be irradiated with light. This would make it acidic, and the equilibrium would be reversed. The CO₂(g) then bubbles out of the liquid and can be collected and stored in gas tanks. Once most of the CO₂(g) has been removed the light would be switched back off and the cycle restarts.

Up until recently, there have been two problems using photoacids to capture carbon dioxide by altering the pH of aqueous solutions. These need to be overcome for this to work in practice. Photoacids are not very resistant to hydrolysis and they have low solubility in water.

Now however, researchers in Zurich led by Maria R. Lukatskaya. claim to have overcome these two problems by using a mixture of aprotic dimethyl sulfoxide, (CH₃)₂SO, with water, rather than just water, as the solvent. When compared to pure water the pH modulation magnitude increases by 60% and is much more stable. The recycling process can continue for 350 hours compared to only 24 hours in water. This research obviously needs further development to capture carbon dioxide on an industrial scale but if that can be achieved the benefits are enormous as no heating or cooling is required.  Once the plant is built and the chemicals manufactured and transported etc., all that is needed in terms of energy is free sunlight!