Unlocking the Ocean's Potential: How to Pull Carbon Dioxide Out of Seawater

Researchers from all around the globe have been working for years to find effective techniques to remove carbon dioxide from the atmosphere as it continues to accumulate in the planet's atmosphere. The ocean, on the other hand, serves as the planet's top "sink" for atmospheric carbon dioxide, absorbing between 30% and 40% of all the gas generated by human activity.

 Another interesting option for reducing CO2 emissions that might eventually result in net negative emissions is the prospect of drawing carbon dioxide straight out of ocean water. This possibility has just come to light. Although there are a few businesses trying to break into this market, the notion has not yet resulted to any broad adoption, similar to air capture systems.

MIT researchers claim to have now discovered the secret to a really effective and affordable removal process. The researchers, including MIT professors T. Alan Hatton and Kripa Varanasi, postdoctoral fellow Seoni Kim, and graduate students Michael Nitzsche, Simon Rufer, and Jack Lake, published their findings this week in the journal Energy & Environmental Science.

In order to acidify a feed stream via water splitting, the current techniques for extracting carbon dioxide from seawater use a voltage across a stack of membranes. By doing this, bicarbonates in the water are changed into CO2 molecules, which may subsequently be extracted using a vacuum. Hatton, the Ralph Landau Professor of Chemical Engineering, points out that the processes are costly and complicated since membranes are necessary to power the total electrode reactions at either end of the stack. If at all feasible, he explains, "we sought to prevent the necessity to introduce chemicals to the anode and cathode half cells and to avoid the usage of membranes."

The group developed a reversible procedure using electrochemical cells without a membrane. To drive the release of the dissolved carbon dioxide from the water, reactive electrodes are utilized to release protons into the saltwater that is supplied to the cells. The procedure is cyclical: first, the water is made acidic to dissolve the inorganic bicarbonates and turn them into molecular carbon dioxide, which is then collected as a gas under vacuum. To recover the protons and convert the acidic water back to alkaline before returning it to the sea, the water is then injected into a second set of cells with a reversed voltage. Once one set of electrodes has been depleted of protons (during acidification) and the other has been restored (during alkalization), the responsibilities of the two cells are periodically switched.

According to Varanasi, a professor of mechanical engineering, this removal of carbon dioxide and reinjection of alkaline water could gradually start to reverse, at least locally, the acidification of the oceans that has been brought on by carbon dioxide buildup and has in turn threatened coral reefs and shellfish. To prevent a local alkalinity rise that can disturb ecosystems, they suggest reinjecting alkaline water through dispersed outlets or further offshore. Varanasi asserts that "we won't be able to treat the pollutants from the entire earth." Nevertheless, the reinjection may be carried out in some instances in locations like fish farms, which have a tendency to make the water more acidic; therefore, this could be a strategy to assist mitigate that impact.

 Like with previous carbon removal procedures, once the carbon dioxide has been taken out of the water, it still has to be disposed of. For instance, it may be chemically transformed into substances like ethanol, which may be used as a transportation fuel, or into other specialized compounds, or it may be buried in deep geologic formations beneath the ocean floor. You won't be able to use all of the CO2 that has been captured as a feedstock for the manufacturing of chemicals or materials, but you may surely think about it. In order to avoid running out of markets for all the things you make, a sizable portion of the CO2 that is gathered must be buried below.

At least initially, the concept would be to integrate such systems with infrastructure that is already in place or is being developed and processes seawater, such as desalination facilities. This technique is scalable, so Varanasi claims that it might be integrated into current procedures that are currently treating ocean water or coming into touch with it. It would not be necessary to use consumables like chemical additives or membranes there, since the carbon dioxide removal could be a straightforward addition to the processes that are already used to return enormous volumes of water to the ocean.

In order to lessen the sizeable contribution of ship traffic to total emissions, the system might also be used by ships that would treat water as they sailed. International regulations to reduce shipping's emissions already exist, and Varanasi claims that "this might enable shipping corporations offset part of their pollution, and transform ships into ocean scrubbers."

The technique may potentially be used in places like aquaculture farms or offshore drilling rigs. It may eventually result in the deployment of independent carbon removal facilities all over the world.

According to Hatton, the procedure could be more effective than air-capture devices since saltwater has a carbon dioxide content that is more than 100 times higher than that of air. Prior to recovering the gas in direct air-capture systems, the gas must first be captured and concentrated. Yet, he adds, "the capture stage has already kind of been done for you since the seas are big carbon sinks." "There is simply a release step, no capture process." This implies that the amounts of material that must be handled are substantially less, possibly streamlining the entire process and lowering the demands for footprint.

The objective of ongoing research is to develop a replacement for the current procedure, which involves removing the separated carbon dioxide from the water using a vacuum. A further requirement is the identification of operational tactics to stop the precipitation of minerals that might smear the electrodes in the alkalinization cell, a fundamental problem that lowers the general effectiveness of all described methods. Hatton observes that although these concerns have made great progress, it is still too early to report on them. After around two years, the team anticipates that the system will be prepared for a real-world demonstration project.