Had to read most of the way through to get to what I was most interested in:

  The process could be more efficient than air-capture systems, Hatton says, because the concentration of carbon dioxide in seawater is more than 100 times greater than it is in air. In direct air-capture systems it is first necessary to capture and concentrate the gas before recovering it. 

Sounds like a promising area to explore. I wonder how long it takes water to absorb CO2 in the first place. I picture eventually engineering some kind of big ponds that pull CO2 out of the air and leave limestone sediment behind

Seawater <> CO2 reactions are quite complex, because various compounds form from dissolved gaseous CO2 in the water and the molecules interact in ways that are difficult to quantify (I am not a chemist). My layperson understanding is that carbonic acid ("normal" dissolved CO2) can break down into bicarbonate ions, which themselves can further break down into single carbonate ions. Then depending on what other ions are present in the water e.g. calcium you get various new compounds forming from the bicarbonate and carbonate ions and the metals. So you have a multi-step equilibration process that kind of recursively effects itself and also depends on local pressures, density, presence of other ions, temperature and so on.

In reference to the "big ponds" - yes, people are doing this (myself included) but our process relies on using microalgae as the agent of decarbonisation, and our R&D is going into more efficient ways to produce and harvest the microalgal cells. Photosynthesis is still a pretty good way to capture CO2, and microalgae grow the fastest.

Is or could photosynthesis be more efficient in water because of the higher amount of CO2? Do plants pull it out of the water directly?

I've also wondered about whatever mineralization happens in making sea shells - I understand that's another natural process that fixes CO2, is that something that could be replicated, artificially or through growing a high concentration of little shellfish?

> Is or could photosynthesis be more efficient in water because of the higher amount of CO2?

Could be - the limiting factor after CO2 access is sunlight. Microalgae have the tendency to self-shade which limits growth. Again, photosynthesis is also a very complex process and there are many pathways to increasing efficiency. Lots of work is being done on improving photon activation in microalgae, meaning fewer photons are required to trigger individual photosynthesis reactions.

> Do plants pull it out of the water directly?

yes

> I've also wondered about whatever mineralization happens in making sea shells - I understand that's another natural process that fixes CO2, is that something that could be replicated, artificially or through growing a high concentration of little shellfish?

Again possible however I believe shells are limited by the availability of calcium ions, finding an appropriate organism etc. I am sure it is possible to genetically engineer some kind of ultra fast growth oyster hybrid. The question is can it be done in reasonable time, cost of deployment etc etc

Cool, thanks for answering!

What do you do with the resulting algae? What's a good way to sequester that captured carbon? I guess on industrial scales we should really just pump an algae sludge right back into the holes we pumped them out of.

Pump it into old oil wells

You can sink it in the ocean - beneath around 500m it just gets compressed and sinks to the bottom. So that would be "permanent" sequestration. The other option is to make it into "biochar" which is basically anaerobically combusting it to leave carbon residues, that you can then bury in the ground. This is scientifically accepted as also being permanently sequestered. This has the benefit of acting as a soil amendment also, increasing water retention and facilitating microorganism development.

> concentration of carbon dioxide in seawater is more than 100 times greater than it is in air

This is straight up wrong. Unless they mean alkalinity, which is carbonate, but not dissolved CO2.

I keep both reef and freshwater aquariums. I have to inject CO2 into my freshwater aquariums to keep the plants happy, and the maximum I can go without killing off the fish is ~30ppm. Given that the PPM of CO2 in air is 4, are they trying to say saltwater has 400PPM of CO2? That would mean ocean water is 0.04% CO2. A wave would have so much foam because of how much carbonation that'd be in the water.

They're doing some funny math.

> PPM of CO2 in air is 4

This is wrong, CO2 is currently 420PPM in Earth's atmosphere.

Wow, how did I manage that. My numbers are all off by 100x.

So that means they're saying the ocean is 4% CO2?

Ocean is also about 1000x as dense as air, so it only needs 40ppm of CO2 to be 100x more concentrated as air.

The molecules in water are more dense than in air. They are talking about density not ratio (PPM)

"Concentration" could either mean ppm or p/m^2. When they go on to say "the volumes of material that need to be handled are much smaller" they're making it quite explicit that they mean the later.

> Sounds like a promising area to explore.

Indeed, and it has been. Google's Project Foghorn's process was (2014) was based on the same concept.

> Sounds like a promising area to explore.

It's so completely obvious in hindsight. It's amazing how many great ideas share that property.

This isn't a new idea. It's possible it was also obvious in hindsight a few decades ago when it was first proposed.

It's a new idea to me, and is completely obvious to me in hindsight.

I mean comparing to direct air capture is A comparison. I wouldn't call direct air capture efficient by any stretch.

So they have the carbon as bicarbonate, and they are reforming carbon dioxide?

I'm no chemist, but ... is there something solid that it could be converted to instead? Like baking soda, rather than a difficult to store gas?

Maybe something like this?

https://www.scientificamerican.com/article/desalination-brea...

Wow, I didn't know salinity doubled in the Arabian sea in only 10 years.

The first thing that came to mind when I saw the abstract was that existing bipolar membrane electrodialysis processes already provide a convenient way of performing the pH swing process they are developing, but with membranes that are already produced on km^2/year scale. Companies like Neosepta or Veolia (formerly Suez formerly GE Water formerly Ionics) produce bipolar membranes for this task, and it's a rapidly growing area of interest.

A bipolar membrane (BPM) consists of a polymer membrane full of positively charged groups (the anion-exchange resin) intimately bound to a polymer membrane full of negatively charged groups (the cation-exchange resin). The interface (reminiscent of a p-n junction) is known as a bipolar junction, and acts as an electrode under a sufficiently high potential gradient. They are made out of cheap materials which have been used in ion-exchange resins and membranes since the 60s, but the bipolar membrane process is niche and hasn't been anywhere as highly developed as other electrodialysis membranes. And electrodialysis is fairly niche, and hasn't been nearly as highly developed as membranes for gas separation, desalination, or removal of particulates (ultra- and micro-filtration).

It turned out that electrodialysis is less efficient for seawater desalination than reverse osmosis (the potential drop through the product water becomes really severe if you're trying to produce drinking water from seawater), so electrodialysis was half-abandoned in comparison to RO. Oddly, Japanese companies developed a lot of ED technology to its current state, including ion-selective cation exchange membranes, for producing table salt, since Japan doesn't have the climate necessary for normal salt evaporation. The ion-selective cation resins were developed for removing Mg from seawater for table salt, but are now popular for researchers trying to do lithium separations.

Anyway, while I agree with the authors that BPMs have unresolved challenges (related to efficiency, mechanical stability, and the fact that current membranes are required to be loaded with transition metal catalyst to get a decent water splitting rate at a low overpotential), I don't know that I'm convinced that their approach is better just because they call BPMs "expensive" four times. If we wanted to adjust the pH of a lot of water, we would need, as a guess, roughly the same amount of electrode catalyst surface area, or the same amount of bipolar junction surface area. However, the bipolar junction is made out of commodity polymer resins heat laminated together, while the electrodes in this study are made out of silver and bismuth. If the bipolar membrane is loaded with a metal catalyst, the most common one is iron. I don't see the BPMs being the more costly solution for very long.

For full disclosure, I recently started doing some work on BPMs, but I think the problems associated with it are solvable, especially for applications like this (as opposed to much more challenging conditions like CO2 electrolyzers).

> in places such as fish farms, which tend to acidify the water, so this could be a way of helping to counter that effect.

so basically overeating fish is to be offset by miracle technology. brilliant.

Similar to efforts to reduce methane from cows instead of actually facing the fact that industrialized meat production is, y'know, really wasteful and bad for the environment (and us, given how bad eating too much red meat is)

This article makes no mention of the power required for a process like this. A few days ago there was a post describing the older seawater method which stated that to remove as much carbon from the ocean as we are putting in would require ~70% of current global electricity production.

Unless the improvements made here are really significant, I don't see how this actually solves anything until we have moved to truly clean energy production.

The linked paper says this uses 122 kJ/mol. I translated that into more familiar units, and it came out to 770 kWh per metric ton CO2.

If you were dumb enough to power it with coal then you'd have net emissions, but put it someplace sunny, power it with solar at 2 cents/kWh and you're paying just $15.40 in energy cost per ton of CO2 absorption. One gallon of gasoline produces 20 pounds of CO2, and there are 2204 pounds in a metric ton, so you could pay for this by adding a surcharge of just 7 cents/gallon.

Of course that's just energy cost, there's also capital cost, and I don't have an estimate for that. But it's not obviously unworkable. Reducing emissions is usually better but I could see this being pretty helpful for cleaning up things that are hard to decarbonize, and once we hit net zero we'll need tech like this already scaling to bring CO2 back down.

Here is some more context on related technologies:

https://www.american.edu/sis/centers/carbon-removal/fact-she...

The MIT article presents an expensive strawman alternative, but ignores simpler, cheaper technologies that already exist.

In particular, since CO2 acidifies the ocean, dumping alkaline chemicals into sea water converts the CO2 to inert solids, which precipitate out. At least, that's the theory. Caveats in link.

Or, adding limestone converts CO2 and CO3(-2) into 2 HCO3(-1).

There is a whole industry springing up around this idea, it's known as "MCDR" (marine CO2 removal). Some startups:

https://www.heimdalccu.com/

https://www.vesta.earth/

https://www.ebbcarbon.com/

https://www.planetarytech.com/

https://capturacorp.com/

https://www.runningtide.com/

https://www.climatefoundation.org/xprize.html

One clam at a time