CO2 removal from seawater by Electrochemical reaction

[taricha]

I saw this article, and thought it was interesting but didn't have near enough info to make it clear what was going on

https://phys.org/news/2023-04-big-sponge-co2-tech-oceans.html

This write-up gives a bit more detail on the basic idea for the CO2-stripping with electrochemical cells (but isn't about the specific project in the first article)
https://www.chemistryworld.com/news...xide-extract-it-from-seawater/4017113.article

And the underlying paper for the chemistryworld article works out the mechanics in plenty of detail....
Asymmetric chloride-mediated electrochemical process for CO2 removal from oceanwater (Kim et al)

(process overview)


The basics are (I think) that they use electricity in one cell to acidify ocean water to low enough pH that all the carbonate/bicarbonate becomes CO2 in solution, then they strip out the CO2 gas (in the paper, they strip CO2 out by vacuum).
Then they take this CO2-free acidified seawater and put it to the other electrochemical cell where the charges go the opposite way and they raise the pH back. Then they have CO2-free good "alkaline" seawater that they dump back in the ocean. But it doesn't look to be "alkaline" in the sense of having much alkalinity the way we use that term in the hobby - there's no carbonate/bicarbonate going back into the seawater it seems?


The floating plant that's being operated in the first link (phys.org) article is doing this process on a boat that moves around so it isn't stuck doing this to too much water in one spot.
The detailed Kim paper doesn't suggest what to do with the CO2 that you strip out by vacuum, but the boat is dumping it back in - in the form of solid CaCO3. So they are either adding a whole bunch of Calcium on the boat, or they are stripping a lot of Ca from seawater to turn all the CO2/carbonate/bicarbonate from the treated seawater into CaCO3?

It's cool that it's possible (and crazy that this might be way cheaper than other CO2 capture) but I'm not seeing how the chemistry of the treated water they are putting back in the ocean looks much like seawater?

 

[biom]

The corals will do the same (carbon sequestration in form of CaCO3 ) big scale and free of charge . We just need to give them a chance.

 

[MyFirstCar]

The corals will do the same (carbon sequestration in form of CaCO3 ) big scale and free of charge . We just need to give them a chance.

Interestingly, corals probably release CO2, rather than sequester it. The ph change from the carbonate formation can release carbon.

 

[MyFirstCar]

Interestingly, corals probably release CO2, rather than sequester it. The ph change from the carbonate formation can release carbon.

Adoptez un corail

Adoptez un corail et agissez pour la biodiversité marine

www.coralguardian.org

 

[Randy Holmes-Farley]

Adoptez un corail

Adoptez un corail et agissez pour la biodiversité marine

www.coralguardian.org

FWIW, I've only thought about this for a few minutes, but I think their analysis is overly simplistic and flawed. It's like a new chemistry student run amok writing reactions without any regard to how much they actually happen.

Yes, the reactions (1-3) they write are correct, to the extent they happen at all, but they are only part of the story, and are not happening quantitatively. I could write those same reactions in a way that does not appear to add CO2:

Ca2+ + HCO3− ⇔ CaCO3 + H+ (2)

OK, equation 2 is fine.

Equation 3 is NOT fine. One cannot just take the H+ and quantitatively combine it with more bicarbonate to form CO2. That is a VERY minor process at pH 8. The main process happening to that H+ at pH 8 is:

H+ + CO3-- --> 2HCO3- (4)

So while 3

Ca2+ + 2 HCO3−⇔ CaCO3 + CO2 + H2O (3)

does happen to a small extent, the large majority of the quantitative changes to seawater composition whent he pH remains in the 8 range are better summarized by my equation 5:

Ca++ + CO3-- --> CaCO3 (5)

 

[taricha]

The corals will do the same (carbon sequestration in form of CaCO3 ) big scale and free of charge .

Thanks. The analogy helps me imagine what the no-carbonate/bicarbonate seawater they are putting back is actually like.

Corals use the carbonate/bicarbonate and Ca from water making CaCO3.
If we imagine this process pushed to 100% using a bunch of electricity, then all the alkalinity and some of the Ca would be pulled out and make a bunch of CaCO3.
So what they end up with may be close to an extreme version of seawater after a bunch of coral calcification.

 

[biom]

FWIW, I've only thought about this for a few minutes, but I think their analysis is overly simplistic and flawed. It's like a new chemistry student run amok writing reactions without any regard to how much they actually happen.

Yes, the reactions (1-3) they write are correct, to the extent they happen at all, but they are only part of the story, and are not happening quantitatively. I could write those same reactions in a way that does not appear to add CO2:

Ca2+ + HCO3− ⇔ CaCO3 + H+ (2)

OK, equation 2 is fine.

Equation 3 is NOT fine. One cannot just take the H+ and quantitatively combine it with more bicarbonate to form CO2. That is a VERY minor process at pH 8. The main process happening to that H+ at pH 8 is:

H+ + CO3-- --> 2HCO3- (4)

So while 3

Ca2+ + 2 HCO3−⇔ CaCO3 + CO2 + H2O (3)

does happen to a small extent, the large majority of the quantitative changes to seawater composition whent he pH remains in the 8 range are better summarized by my equation 5:

Ca++ + CO3-- --> CaCO3 (5)

It is not entirely their fault actually. They were citing a scientific paper which is old and surprisingly incorrect as you noticed.

 

[biom]

Thanks. The analogy helps me imagine what the no-carbonate/bicarbonate seawater they are putting back is actually like.

Corals use the carbonate/bicarbonate and Ca from water making CaCO3.
If we imagine this process pushed to 100% using a bunch of electricity, then all the alkalinity and some of the Ca would be pulled out and make a bunch of CaCO3.
So what they end up with may be close to an extreme version of seawater after a bunch of coral calcification.

It looks they have built an expensive replica of a coral
Joke aside I don't believe this approach is promising I really hope they will do a proper environment impact assessment before make it large scale. Cant imagine what is happening to the plankton in those chambers with altering Ph from 6 to 12 and chlorine... I know I am biased but always prefer Nature based Solutions (NbS) than engineered ones.