As carbon dioxide continues to accumulate in Earth’s atmosphere, research teams around the world have spent years looking for ways to efficiently remove the gas from the air. The world’s largest “sink” of carbon dioxide from the atmosphere is now the ocean, which absorbs about 30 to 40 percent of the gas produced by human activities.
Recently, the ability to remove carbon dioxide directly from seawater has emerged as another promising way to mitigate CO2 Emissions that could one day even lead to overall negative net emissions. But like air capture systems, the idea has yet to see widespread adoption, although some companies are trying to break into this space.
Now a team of researchers from MIT says they may have found the key to a truly efficient and inexpensive removal mechanism. The results were published in the journal this week Energy and Environmental Sciencesin an article by MIT professors T. Alan Hatton and Kripa Varanasi, postdoc Seoni Kim, and graduate students Michael Nitzsche, Simon Rufer, and Jack Lake.
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 into CO molecules2, which can then be removed under vacuum. Hatton, Ralph Landau professor of chemical engineering, notes that the membranes are expensive and that chemicals are required to power the entire electrode reactions at both ends of the stack, further adding to the cost and complexity of the processes. “We wanted to avoid introducing chemicals into the anode and cathode half-cells and, if possible, do without membranes,” he says.
The team developed a reversible process consisting of membrane-free electrochemical cells. Reactive electrodes are used to donate protons to the seawater fed to the cells, driving the release of the dissolved carbon dioxide from the water. The process is cyclic: first, the water is acidified to convert dissolved inorganic bicarbonates into molecular carbon dioxide, which is collected as a gas under vacuum. Then the water is fed to a second set of reverse voltage cells to recover the protons and make the acidic water alkaline again before releasing it back into the sea. Periodically, the roles of the two cells are reversed as one set of protons is depleted (during acidification) and the other set is regenerated during alkalization.
That removal of carbon dioxide and re-injection of alkaline water could slowly begin to reverse, at least locally, ocean acidification caused by carbon dioxide build-up, which in turn has threatened coral reefs and shellfish, says Varanasi, a professor of mechanical engineering. Alkaline water reinjection could be through distributed vents or far offshore to avoid a local alkalinity spike that could disrupt ecosystems, they say.
“We won’t be able to deal with the entire planet’s emissions,” says Varanasi. But reinjection could in some cases be done in places like fish farms that tend to acidify the water, so this could be a way to counteract that effect.
Once the carbon dioxide has been removed from the water, it must be disposed of like other carbon removal processes. For example, it can be buried in deep geological formations under the sea floor, or chemically converted into a compound like ethanol, which can be used as a fuel for transportation, or other specialty chemicals. “You may well consider using the captured CO2 as feedstock for chemical or material production, but you won’t be able to use all of it as feedstock,” says Hatton. “You will run out of markets for any product you make, so no matter what, a significant amount of the CO captured2 must be buried underground.”
The idea would be, at least initially, to couple such systems with existing or planned infrastructure that already treats seawater, such as desalination plants. “This system is scalable, so we could potentially integrate it into existing processes that already process seawater or come into contact with seawater,” says Varanasi. There, carbon removal could be a simple add-on to existing processes that already return large amounts of water to the sea, and it would not require consumables such as chemical additives or membranes.
“With desalination plants, you’re already pumping all the water, so why not settle there together?” Varanasi says. “A lot of capital costs related to how you move the water and permitting, all of that could already be done.”
The system could also be implemented by ships, which would treat water while underway to help reduce shipping’s significant contribution to overall emissions. There are already international mandates to reduce emissions from shipping, and “this could help shipping companies offset some of their emissions and make ships ocean cleaners,” says Varanasi.
The system could also be implemented at sites such as offshore drilling platforms or aquaculture farms. Eventually, this could lead to the deployment of free-standing carbon removal plants distributed around the world.
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 higher than in the air. With direct air capture systems, it is first necessary to capture and concentrate the gas before it is recovered. “However, the oceans are major sinks of carbon, so the capture step has already been taken care of for you,” he says. “There is no capture step, only release.” This means that the amounts of material to be handled are much smaller, potentially simplifying the whole process and reducing the footprint.
Research continues, with one goal being to find an alternative to the current step, which requires a vacuum to remove the captured carbon dioxide from the water. Another need is to identify operational strategies to prevent the precipitation of minerals that can foul the electrodes in the alkalinization cell, an inherent problem that reduces the overall efficiency of all reported approaches. Hatton notes that significant progress has been made on these issues, but it is too early to report. The team expects 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, our existence,” says Varanasi. “So we clearly need all the help we can get.”
The work was supported by ARPA-E.
