Seawater cement: a CO2 sink?

By Kelly

In last week’s edition of Science, a special section was devoted to the science of waste management. There was an article that caught this oceanographer’s  attention that exemplified a rather unique solution. This solution takes advantage of some fundamental principles of carbonate chemistry to sequester that most pervasive of waster products: carbon dioxide.  In California, an entrepreneur and biomineralization expert has decided to tackle elevating CO2 levels in a rather novel way. He is not proposing to plant trees, or to pump CO2 into the ground, rather he is advocating cement: seawater cement. Cement that paves the way quite literally to a cleaner future.

Brent Constantz states the idea stems from his study of biomineralization; a process that calcareous organisms have used for hundreds of millions of years to form hard shells. By combining simple seawater with CO2 Constantz hopes to remove vast amounts of CO2. But the idea is not without problems, indeed this is Constantz second attempt at marketing the idea. If we think back to the fundamental principles of carbonate chemistry then the idea is genius.Under the right conditions CO2 in seawater will dissociate to form bicarbonate and then carbonate ions. The carbonate ions when combined with calcium will precipitate as calcium carbonate…simple right? Not only can the CO2 trapped in seawater can be sequestered in cement but you are offsetting the emissions from cement production, which in itself has been estimated responsible for 5% of global emissions; a tonne of concrete produces (quite literally) a tonne of CO2. Constantz is looking at the big picture.  We produce ~2.5 billion tonnes of cement ever year, aswell as ~12.5 billion tonnes of concrete and close to three times that of aggregate. He advocates that using this material to trap carbon is “a properly scaled solution to the problem”.

However the problem lies in maintaining the ‘right’ conditions for the chemistry to occur. The initial plan was heavily criticized as to elevate the pH to ensure a constant supply of carbonate ions, massive amounts o f sodium hydroxide needed to be added, and the process became cost prohibitive, and the carbon footprint less desirable. Not to be discouraged Constantz went back to his home institute Stanford and came up with another solution. Again, looking to how nature solves the problem of calcification, Constantz is now adding protein to the (cement) mix. Carbonic anhydrase (CA) is the carbon concentrating enzyme in our bodies that catalyses the removal of carbon dioxide from our blood. If he can use these principles to catalyze and concentrate the CO2 to be bound in his cement, he hope to circumvent the need for such large amounts of sodium hydroxide, and instead use ash byproducts from power production to raise the pH.

While I applaud the efforts, and sincerely hope that Constantz finds success, I can’t help thinking of the other industry that has struggled under similar constraints…algal biofuels. For the record algae, does not rely solely on sunlight and CO2 to photosynthesize. Without the right addition of nutrients, think nitrate, yes fertilizer, the algae won’t be able to grow to full potential, and this ‘green’ solution can end up with a rather large carbon footprint. But that discussion is for another day…Today I’d like to think that Constantz will get it right.

If you can access Science, then then all of the details can be found here.

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