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
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.
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.
"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.
