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Earth Science

MIT Team Makes a Case For Direct Carbon Capture From Seawater, Not Air 131

Posted by BeauHD from the alternative-options dept.
The oceans soak up enormous quantities of carbon dioxide, and MIT researchers say they've developed a way of releasing and capturing it that uses far less energy than direct air capture -- with some other environmental benefits to boot. New Atlas reports: According to IEA figures from 2022, even the more efficient air capture technologies require about 6.6 gigajoules of energy, or 1.83 megawatt-hours per ton of carbon dioxide captured. Most of that energy isn't used to directly separate the CO2 from the air, it's in heat energy to keep the absorbers at operating temperatures, or electrical energy used to compress large amounts of air to the point where the capture operation can be done efficiently. But either way, the costs are out of control, with 2030 price estimates per ton ranging between US$300-$1,000. According to Statista, there's not a nation on Earth currently willing to tax carbon emitters even half of the lower estimate; first-placed Uruguay taxes it at US$137/ton. Direct air capture is not going to work as a business unless its costs come way down.

It turns out there's another option: seawater. As atmospheric carbon concentrations rise, carbon dioxide begins to dissolve into seawater. The ocean currently soaks up some 30-40% of all humanity's annual carbon emissions, and maintains a constant free exchange with the air. Suck the carbon out of the seawater, and it'll suck more out of the air to re-balance the concentrations. Best of all, the concentration of carbon dioxide in seawater is more than 100 times greater than in air. Previous research teams have managed to release CO2 from seawater and capture it, but their methods have required expensive membranes and a constant supply of chemicals to keep the reactions going. MIT's team, on the other hand, has announced the successful testing of a system that uses neither, and requires vastly less energy than air capture methods.

In the new system, seawater is passed through two chambers. The first uses reactive electrodes to release protons into the seawater, which acidifies the water, turning dissolved inorganic bicarbonates into carbon dioxide gas, which bubbles out and is collected using a vacuum. Then the water's pushed through to a second set of cells with a reversed voltage, calling those protons back in and turning the acidic water back to alkaline before releasing it back into the sea. Periodically, when the active electrode is depleted of protons, the polarity of the voltage is reversed, and the same reaction continues with water flowing in the opposite direction. In a new study published in the peer-reviewed journal Energy & Environmental Science, the team says its technique requires an energy input of 122 kJ/mol, equating by our math to 0.77 mWh per ton. And the team is confident it can do even better: "Though our base energy consumption of 122 kJ/mol-CO2 is a record-low," reads the study, "it may still be substantially decreased towards the thermodynamic limit of 32 kJ/mol-CO2."
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MIT Team Makes a Case For Direct Carbon Capture From Seawater, Not Air More

MIT Team Makes a Case For Direct Carbon Capture From Seawater, Not Air

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  • the team says its technique requires an energy input of 122 kJ/mol, equating by our math to 0.77 mWh per ton.

    By my calculation, that's $150-$300 USD per ton.

    • Re:Cost (Score:5, Interesting)

      by timeOday ( 582209 ) on Friday February 17, 2023 @10:27PM (#63302747)
      Burning a gallon of gas creates 20 lbs of CO2.

      So 100 gallons of gas creates a ton of CO2.

      So $1.50 cleanup per gallon of gas at the low end.

      https://climatekids.nasa.gov/r... [nasa.gov]

    • The actual paper doesn't appear to mention energy requirements in MWh equivalent values. However, they do give a direct cost estimate of $50-$100/tCO2.

  • Wonder how areas of lower CO2 might impact growth of algae and other sealife. Sort of the opposite of an algal bloom from fertilizer runoff.

    I assume the top layer of ocean is the important part. Where the sunlight, gases, and other necessary parts all come together in the water for photosynthesis (one of the ways nature removes carbon from the atmosphere already). Wouldn't lower CO2 in the top layer mean less photosynthesis across the same area?

    I applaud the outside the box thinking, but I hesitate to su

    • It literally won't make a dent. The sea has been fine without us pumping billions of tons of CO2 into the air and 40-60% of that getting absorbed in the ocean.

      It will be perfectly fine to fallback a little bit.

      • Re: (Score:3)

        by Temkin ( 112574 )

        It literally won't make a dent. The sea has been fine without us pumping billions of tons of CO2 into the air and 40-60% of that getting absorbed in the ocean.

        The sea is mostly a nutrient desert. Toss a little iron in, and the plankton & algae go nuts fixing carbon, and the rest of the sea life chain is down steam from there. This is how all that carbon got sequestered as coal & oil in the first place. The 1991 Mt. Pinatubo eruption offers good clues here, but its somewhat obfuscated by the co-morbid atmospheric SO2 injection.

    • Re:CO2 is used in the ocean. (Score:5, Informative)

      by The Evil Atheist ( 2484676 ) on Friday February 17, 2023 @10:34PM (#63302771)
      I would add, in fact, we've pumped too much CO2 into the oceans that it's becoming more acidic, killing off coral reefs.

      • This is the first thing that came to mind for me - CO2 acidifies the ocean as well (surprised that isn't mentioned in the brief), so pulling CO2 out of the water hopefully will result in a more favourable PH.

        But that's a LOT of carbon to remove.

        • But isn't the constant releasing of co2 into the ocean the problem? And the ocean naturally processes this co2? So the issue in not removing it after we dump it but not dumping it in the first place?

          • Yes....

            -Humans are not going to stop releasing CO2 into the atmosphere.
            -The ocean will continue to absorb CO2 from the atmosphere.

            Scientists have discovered that they may be able to reclaim CO2 from the ocean more efficiently than from the atmosphere.

            This is a treatment, not a prevention, but it may help. It is not perfect, but it may be good.

            I say "may" because this is experimental. It remains to be seen if it can, or will, be implemented at all -much less on an effective scale.

    • Wonder how areas of lower CO2 might impact growth of algae and other sealife.

      It will make no difference. Carbon is not the bottleneck. For life in the ocean, iron and phosphorous are the bottlenecks.

    • Wonder how areas of lower CO2 might impact growth of algae and other sealife.

      The ocean currently is overacidified (technically, insufficiently alkaline, since the point of comparison is neutral Ph) because there's too much CO2 in it.

  • mWh? (Score:4, Informative)

    by Pinky's Brain ( 1158667 ) on Friday February 17, 2023 @10:22PM (#63302735)

    Not mWh clearly, that needs to be MWh. Still, if that is the dominant cost it's still quite cheap.

  • We did it slashdot (Score:3)

    by NFN_NLN ( 633283 ) on Friday February 17, 2023 @10:22PM (#63302737)

    > Best of all, the concentration of carbon dioxide in seawater is more than 100 times greater than in air.

    1. Co2 creates global warming
    2. global warming melts polar ice caps which means more sea water
    3. ??? --> more sea water absorbs more Co2
    4. less Co2 means global warming

    We finally solved the equation!

  • Called it! (Score:3)

    by The Evil Atheist ( 2484676 ) on Friday February 17, 2023 @10:30PM (#63302753)
    I literally made the case for this in the other stories about capturing from air.

    Still won't really make up for pumping billions of tons of it into the atmosphere in the first place.

    • Re: (Score:2)

      by jwhyche ( 6192 )

      This exactly. We really don't need to be concentrating on our efforts on removing CO2 from the atmosphere. The environment can do that just fine. We need to concentrate on reducing our emissions and let nature do what it does best.

  • Yes, this action is valuable in itself, but it'll go down smoother with something one can SELL.
    Is there something else we can extract at the same time, something like the 17 rare earth elements, precious metals, lithium, or uranium that can be found in sea water?

    Perhaps one or more of these will just naturally collect on the emitter/receiver plates?

    Or since we're doing the work of pumping it around and through a channel, does that make the mining operation cheaper to start with and thus make element

    • Re:Can this pay for itself? (Score:5, Interesting)

      by steveha ( 103154 ) on Friday February 17, 2023 @11:11PM (#63302809) Homepage

      Is there something else we can extract at the same time

      You suggested lithium. With the increasing popularity of electric cars, and with products like Tesla Megapack for grid storage, there is now huge demand for battery-grade lithium.

      My understanding is that it's possible but not cost-effective to extract lithium from seawater... but if you set up a desalinization plant, the more-concentrated brine you get when you remove some of the water as fresh water might be cost-effective for lithium recovery.

      I'm wondering how many things you can do at once and whether some of them help the others. Is it easier, or harder, to get carbon dioxide from desalinization brine than pure seawater?

      Also, Tony Seba and his think tank are predicting that we could transition to relying on "renewable" power only if we overbuild production by 3 to 5 times the minimum. That way, when it's a winter day (less sunlight) and the wind dies down, you aren't getting 100% production but you at least have more production of what you can get (and the plan also calls for enough backup battery power for 2 to 4 days). Well, the corollary of that is that most of the time, when sun and wind are normal, your overbuilt production makes excess energy that you can afford to sell cheaply.

      So I've started reading stories like this and wondered if they would be good candidates for the excess power from wind/solar on normal days. Collect lithium and carbon dioxide while the energy is cheap, and plan for some down time in December.

      For a real win, turn the carbon dioxide all the way into pure jet fuel, because long-haul jet airplanes aren't converting to battery power anytime soon. If you have carbon, and you can get hydrogen from the water (just need more electricity), you can make hydrocarbons. I don't know whether it would be economically feasible, but large amounts of cheap power might help with that.

      • Re: (Score:2)

        by HiThere ( 15173 )

        Well, the jet fuel returns the CO2 to the atmosphere, but I guess it prevents more fossil fuel being burned. The real win, though, is to turn it into something durable. Perhaps we could enter "the diamond age" with it. Graphite (and it's developed forms) are an easier alternative, though. (But that assumes enough energy to remove the oxygen. Plastics would be a lot easier.) Perhaps the easiest/best alternative is to use it for a synthetic photosynthesis process to produce various organic chemicals. W

  • What about sea life? (Score:3)

    by stevenm86 ( 780116 ) on Friday February 17, 2023 @10:33PM (#63302765)
    Sea water is a complex soup. What kind of impact will this process have on microscopic sea life, and/or any micronutrients that may be present in the water, as it passes through this process?

  • It seems to me that the recapturing of protons won't work perfectly, and some will wash out to sea. Simularly, anyone with a boat knows sea water has a lot of gunk in it, and some of it will be attracted to the electrodes, or just naturally stick to them, further degrading performance.

    The New Atas article talks about this briefly, but doesn't say anything about the material the electrodes are made of, and what it costs to replace them:

    > And mineral precipitates are fouling the electrodes on the alkaliniz

    • Re:Electrodes are consumables (Score:5, Informative)

      by mrwireless ( 1056688 ) on Friday February 17, 2023 @11:03PM (#63302801)

      Reading some more, they do acknowledge this:

      "The bismuth electrode is inexpensive and can robustly modulate the pH of chloridecontaining salt water regardless of the type and concentration of cations. On the other hand, although silver has the advantage of large capacity and low energy penalty, electrode dissolutionwill need to be overcome by modifying the silver particles or replacing silver with another material; this is to be the subject of future studies."

      Seems like a pretty relevant caveat for New Atlas to skip over.

      "Methods to overcome the fouling of electrodes during the regeneration step owing to local high surface pH conditions that promote the formation of precipitates such as Mg(OH)2 must be developed, and several engineering and electrochemical approaches are currently being considered to alleviate the fouling issues; one such method was discussed in which the degasified water was mixed with fresh ocean water before being fed to the regeneration cell to reduce the overall pH increase in the flow channel."

      I hope they figure it out.

  • Apparently seaweed has been removing carbon dioxide from the atmosphere for at least 500 million years, and you hipsters are using up all the world's seaweed with your trendy lunch-time sushi consumption. Think of the planet for once.

    • HAHA! This is actually a good point. It might be much cheaper to seed an area with nitrogen and phosphorus and iron, and periodically rip half the seaweed out and ... well, bury it, probably.

      There's a comment above about planting trees. I'll bet seaweed works better for carbon capture.

      • Yeah, but the ocean is not lacking for seaweed, or rather, algae. The problem is even they can't keep up by how fast we're pumping CO2 into the atmosphere, to get absorbed 40-60% into the ocean. Un-tethered CO2 in the ocean is raising acidity.

  • If the co2 is already in solution, and the end goal is to sequester it, like in minerals, then wouldn't pulling it out as a gas be, at best a lateral move, given that minerals are generally more easily formed by aqueous chemistry?

    • Re: (Score:2)

      by HiThere ( 15173 )

      This sounds quite plausible. I'm not *sure* it's true, but it's certainly plausible. Certainly iron rusts more easily in sea water than in dry air. I guess it may depend on how you intend to sequester it. (Many of the suggestions I've heard seem very temporary.)

  • 0.77 * 32 / 122 = 0.2 KWh/ton
    0.2 * 1000 KWh/MWh $0.10 = $20/ton
    1.6T * $0.02 = $32T at theoretical maximum efficiency

    humans have pumped about 1.6 trillion tons of carbon dioxide into the atmosphere

    Given their theoretical cheapest figure of 32 kJ/mol-CO2

    World/Gross domestic product
    96.51 trillion USD (2021)

    the team says its technique requires an energy input of 122 kJ/mol, equating by our math to 0.77 mWh per ton. And the team is confident it can do even better: "Though our base energy consumption of 122 kJ/mo

  • Why not add Coca Cola syrup to sea water and make it a soda instead? When we could use fossil fuels and have unlimited coke.

  • If they really wanted to sink carbon there are no shortages of smokestacks with way higher concentrations of CO2 to pick from.

    • I thought about that too, but then I realized they'd probably be scrubbing it out with water mist anyway, so this process is the same thing but using the atmosphere as a filter. What could possibly go wrong, etc etc, but I see why they want to do it this way.

  • Still too expensive (Score:4, Interesting)

    by WindBourne ( 631190 ) on Saturday February 18, 2023 @04:56AM (#63303075) Journal
    I posted about this before. Diablo power plant uses seawater for cooling. The initial exposure is well over 200C, which part of this is then flash distilled for fresh water. However, at the time of heating the water, the CO2 and other gases, are released. Capture it at that point.

    Note that the utility gets to have fresh water, pull out purified CO2, and pull out other elements from the concentrated brine. Oddly, 2 of the easiest to pull out is uranium and thorium. Turns out they will stick to treated paper.
  • This process collects CO2 as gas. How is it sequestered? Separating CO2 from air is going to be expensive. Collecting it from sea water seems more efficient. Still we have to sequester it.

    Wondering if it is possible to just add some sort of chemical to sea water, precipitate the dissolved CO2 into some nonsoluble inert carbonate particles that will simply sink to the sea bottom.

  • Some form of carbon scrubbing will be the main answer. Not hectoring, blaming, forming new eco-religions, etc.

  • 71 percent of the world is ocean which is already performing carbon capture naturally. What makes anyone think that this idea is going to make any significant difference?

    • The ocean capture hits a homeostasis based on temperature and other dissolved elements. Any carbon taken out will be replaced by the atmosphere at accelerated rates.

  • This pipe dream would require an infinite quantity of pipe.

  • Maybe some incompetent has, again, screwed up SI prefixes?
    https://www.nist.gov/pml/owm/m... [nist.gov]

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