MIT Breakthrough: How To Efficiently Remove Carbon Dioxide From the Ocean

Removing Carbon Dioxide From Ocean

Researchers have found an effective new method for removing carbon dioxide from the ocean. It could be implemented by ships that would process seawater as they travel, or at offshore drilling platforms or aquaculture fish farms. Credit: Courtesy of the researchers

A new method for removing the greenhouse gas from the ocean could be far more efficient than existing systems for removing it from the air.

As carbon dioxide continues to build up in the Earth’s atmosphere, research teams around the world have spent years seeking ways to remove the gas efficiently from the air. Meanwhile, the world’s number one “sink” for carbon dioxide from the atmosphere is the ocean, which soaks up some 30 to 40 percent of all of the gas produced by human activities.

Recently, the possibility of removing carbon dioxide directly from ocean water has emerged as another promising possibility for mitigating CO2 emissions, one that could potentially someday even lead to overall net negative emissions. But, like air capture systems, the idea has not yet led to any widespread use, though there are a few companies attempting to enter this area.

Now, a team of researchers at MIT says they may have found the key to a truly efficient and inexpensive removal mechanism. The findings were reported recently in the journal Energy and Environmental Science, in a paper by MIT professors T. Alan Hatton and Kripa Varanasi, postdoc Seoni Kim, and graduate students Michael Nitzsche, Simon Rufer, and Jack Lake.

The existing methods for removing carbon dioxide from seawater apply a voltage across a stack of membranes to acidify a feed stream by water splitting. This converts bicarbonates in the water to molecules of CO2, which can then be removed under vacuum. Hatton, who is the Ralph Landau Professor of Chemical Engineering, notes that the membranes are expensive, and chemicals are required to drive the overall electrode reactions at either end of the stack, adding further to the expense and complexity of the processes. “We wanted to avoid the need for introducing chemicals to the anode and cathode half cells and to avoid the use of membranes if at all possible,” he says.

Removing Carbon Dioxide From Ocean Desalination Plant

Initially, the system can use existing or planned infrastructure that already processes seawater, such as desalination plants, but the system is scalable. This rendering shows how the new method could also be used by ships and offshore platforms. Credit: Courtesy of the researchers

The team came up with a reversible process consisting of membrane-free electrochemical cells. Reactive electrodes are used to release protons to the seawater fed to the cells, driving the release of the dissolved carbon dioxide from the water. The process is cyclic: It first acidifies the water to convert dissolved inorganic bicarbonates to molecular carbon dioxide, which is collected as a gas under vacuum. Then, the water is fed to a second set of cells with a reversed voltage, to recover the protons and turn the acidic water back to alkaline before releasing it back to the sea. Periodically, the roles of the two cells are reversed once one set of electrodes is depleted of protons (during acidification) and the other has been regenerated during alkalization.

This removal of carbon dioxide and reinjection of alkaline water could slowly start to reverse, at least locally, the acidification of the oceans that has been caused by carbon dioxide buildup, which in turn has threatened coral reefs and shellfish, says Varanasi, a professor of mechanical engineering. The reinjection of alkaline water could be done through dispersed outlets or far offshore to avoid a local spike of alkalinity that could disrupt ecosystems, they say.

“We’re not going to be able to treat the entire planet’s emissions,” Varanasi says. But the reinjection might be done in some cases in places such as fish farms, which tend to acidify the water, so this could be a way of helping to counter that effect.

Once the carbon dioxide is removed from the water, it still needs to be disposed of, as with other carbon removal processes. For example, it can be buried in deep geologic formations under the sea floor, or it can be chemically converted into a compound like ethanol, which can be used as a transportation fuel, or into other specialty chemicals. “You can certainly consider using the captured CO2 as a feedstock for chemicals or materials production, but you’re not going to be able to use all of it as a feedstock,” says Hatton. “You’ll run out of markets for all the products you produce, so no matter what, a significant amount of the captured CO2 will need to be buried underground.”

Initially at least, the idea would be to couple such systems with existing or planned infrastructure that already processes seawater, such as desalination plants. “This system is scalable so that we could integrate it potentially into existing processes that are already processing ocean water or in contact with ocean water,” Varanasi says. There, the carbon dioxide removal could be a simple add-on to existing processes, which already return vast amounts of water to the sea, and it would not require consumables like chemical additives or membranes.

“With desalination plants, you’re already pumping all the water, so why not co-locate there?” Varanasi says. “A bunch of capital costs associated with the way you move the water, and the permitting, all that could already be taken care of.”

The system could also be implemented by ships that would process water as they travel, in order to help mitigate the significant contribution of ship traffic to overall emissions. There are already international mandates to lower shipping’s emissions, and “this could help shipping companies offset some of their emissions, and turn ships into ocean scrubbers,” Varanasi says.

The system could also be implemented at locations such as offshore drilling platforms, or at aquaculture farms. Eventually, it could lead to a deployment of free-standing carbon removal plants distributed globally.

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. “The oceans are large carbon sinks, however, so the capture step has already kind of been done for you,” he says. “There’s no capture step, only release.” That means the volumes of material that need to be handled are much smaller, potentially simplifying the whole process and reducing the footprint requirements.

The research is continuing, with one goal being to find an alternative to the present step that requires a vacuum to remove the separated carbon dioxide from the water. Another need is to identify operating strategies to prevent precipitation of minerals that can foul the electrodes in the alkalinization cell, an inherent issue that reduces the overall efficiency in all reported approaches. Hatton notes that significant progress has been made on these issues, but that it is still too early to report on them. The team expects that the system could be ready for a practical demonstration project within about two years.

“The carbon dioxide problem is the defining problem of our life, of our existence,” Varanasi says. “So clearly, we need all the help we can get.”

Reference: “Asymmetric chloride-mediated electrochemical process for CO2 removal from oceanwater” by Seoni Kim, Michael Nitzsche, Simon B Rufer, Jack R. Lake, Kripa Kiran Varanasi and T. Alan Hatton, 13 February 2023, Energy & Environmental Science.
DOI: 10.1039/D2EE03804H

The work was supported by ARPA-E.

9 Comments on "MIT Breakthrough: How To Efficiently Remove Carbon Dioxide From the Ocean"

  1. Boils down to a nasty cyclic problem regarding CO2 release. Maybe pump it back into the oceans??? Making ethanol to capture CO2 and burning it is a cyclic process. Burning any carbon compound creates CO2 and other “nasty” stuff. The research does not seem to address the major problem.

  2. Typically non-organic chemistry academics and mechanical engineers tend to have little knowledge in biological fields. This technique would require the intake of massive amounts of seawater. It is loaded with living organisms, including the phytoplankton that generates much of the oxygen on the planet. It is clear that the effects upon such organisms, of their proposed process including the subjecting the water to a vacuum, wasn’t considered in these experiments.

    Many of the environmental problems we are now trying to find solutions to are the unintended consequences of other environmental problems we thought we were effectively addressing.

  3. It is not uncommon for researchers to propose injecting the CO2 underground without considering the sad history of that effort during the last two decades. In many cases the effort wasn’t done unless the CO2 was sold to the fossil fuel industry to force more fossil fuels out of nearly depleted reservoirs.

    Those who propose converting the CO2 into fuels, such as ethanol, may be lacking in understanding chemical thermodynamics. Such conversions requires adding energy to create the fuel so when it is burned it will then release the artificially added energy. Upon burning the fuel will likely emit CO2 as a waste product into the atmosphere. The entire process is simply a way of shifting the emissions further to our offspring’s future.

  4. This seems to be a good technology maybe for saving the oceans specifically things like the Great barrier reef which I do believe acidifying the environment is destroying that reef. But like someone mentioned before you are going to be killing organisms that directly take CO2 out of the atmosphere. And that taking the CO2 out of the atmosphere is what would reduce global warming so I’m a bit confused perhaps thinking that this is a way to capture a bunch of the funds for reducing CO2 in the atmosphere but it’s actually not doing that at all.

  5. Rachel Findley | March 1, 2023 at 1:07 am | Reply

    How does the process work thermodynamically?
    The excess carbon dioxide came from burning fossil fuels.
    First, they acidify the seawater, and the carbon dioxide bubbles out. How does that work? Doesn’t carbon dioxide acidify seawater? So how does further acidifying seawater make it less able to hold carbon dioxide?
    Then they use the carbon dioxide to make ethanol, and store the ethanol. Carbon dioxide is a product of burning ethanol. Does it take energy to turn the carbon dioxide back into ethanol? How much?
    Then they de-acidify the seawater, and return it to the ocean. How much energy does de-acidifying the seawater take? Or does it release energy?
    Then they burn the ethanol. What??? Doesn’t that put the carbon dioxide right back into the atmosphere, ready to dissolve into the ocean again?
    Seems like a futile cycle.

  6. Now, if only CO2 was an actual problem.

  7. Sounds like a ‘feel good’ article with no direct communication as to how this will be viable. CO2 is a doozy to remove, and the ‘byproducts’ of CO2 and alkaline water is only the beginning of the issues faced. A good direction but a clumsy article on the details side(CO2 in water, so acidify water, why?)as I wished to understand the mechanisms. Next time use other products that CO2 can make. You picked Ethanol. A bit self defeating, dont you think?

  8. Earlier the Better | March 7, 2023 at 12:28 pm | Reply

    When are we going to see a scaled up version of it? How Soon? We are also reading that there is a lot of Hydrogen and Helium just like thermal energy underneath the Soil. Deserts are such a waste. Why not go there ASAP and pump that H2 and He from there. It will not matter if there is going to be an earthquake because of it in the Deserts or the Earth Collapses there !

    • Earlier the Better | March 7, 2023 at 12:37 pm | Reply

      May be asking AI about these issues than asking other people will be of some help? We cannot let decades and centuries to pass by without any interesting answer.

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