How the ocean can help remove excess CO2 from the atmosphere
Play the animation below for a basic overview.
The chemistry behind this
Squeezing the sponge
To get carbon out of seawater we need to make it more acidic (to about the acidity of apple juice). This causes the carbon stored securely in seawater (as carbonate and bicarbonate) to turn into CO2 that can 'bubble out' of the water and be captured.
Seawater is primarily water (H2O) and salt (sodium chloride - NaCl). The change in the acidity of the seawater can be generated by passing an electric current through the seawater so that the chemical elements that make up that seawater are arranged either side of a membrane to make a dilute acid and base. Click on the animation below for a more complete explanation.
To allow the water to recapture CO2 from the atmosphere, we need to nutralise the acid that was added. This means that when CO2 is absorbed from the air, 99% of it can again get converted into other forms (carbonate or bicarbonate) for long term storage in the seawater. Absorbtion of CO2 from the atmosphere returns the water chemistry to exactly what it was like before it got taken into the pilot plant.
Making the sponge more absorbent
The second approach can work in a very similar way. However, instead of using the acid to allow us to take CO2 from the water, we can use the same electrochemical approach take out the acid, or add a base. This leaves the water with a higher pH (less acidic), so it can store more CO2 away (as carbonate and bicarbonate), enabling it to absorb more carbon from the atmosphere.
Electrochemistry verses use of purchased chemicals
We leased a large electrochemical unit to generate the acid and base from seawater during our big trial in 2025. For shorter tests it will often make more sense to buy in the acid and the base rather than make it within the seawater. We would use exactly the same chemicals (hydrochloric acid and sodium hydroxide) as we produce within the seawater, and they would work in exactly the same way. Commercially, these chemicals are also produced electrochemically from salt water.