Removing carbon dioxide from seawater

As carbon dioxide continues to build up in our planet’s atmosphere, scientists are investigating ways to eliminate this gas more efficiently from the air. However, the world’s leading carbon sink is the ocean, which captures 30 to 40 percent of all the CO2 produced by human activities. Thus, many researchers have recently taken into consideration the possibility of removing CO2 directly from seawater in order to mitigate emissions.

The existing methods for doing this usually apply a high voltage across a stack of membranes to acidify a feed stream by water splitting, a process that converts bicarbonates in the water to CO2 molecules. However, such membranes are expensive, and other chemicals are also required to drive the overall electrode reactions at both sides of the stack, making the procedure even more expensive and complex. Recently though, a team of researchers led by the Massachusetts Institute of Technology (MIT) may have found the key to a highly efficient and inexpensive removal mechanism.

“We wanted to avoid the need for introducing chemicals to the anode and cathode half cells and to avoid the use of membranes at all if possible,” said study senior author Alan Hatton, a professor of Chemical Engineering at MIT.

The experts devised a reversible process consisting of membrane-free electrochemical cells, in which reactive electrodes are used to release protons to the seawater fed to the cells, triggering the release of the dissolved CO2 from the water. This process is cyclical: it first acidifies the water to convert dissolved inorganic bicarbonates to molecular CO2 that is collected as a gas under vacuum, while afterwards, the water is fed to another 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 ocean. Once one set of electrodes is depleted of protons during acidification and the other is regenerated during alkalization, the roles of the two cells is reversed.

Since CO2 buildup has increased ocean acidification, threatening vast populations of corals and shellfish, the removal of this gas and reinjection of alkaline water – which could be done through dispersed outlets or far offshore to avoid dangerous local spikes in alkalinity – could slowly start to reverse this process. 

However, once the CO2 is removed from seawater, it will still need to be disposed of. This could be done by burying it in deep geologic foundations beneath the seafloor or by chemically converting it into a compound such as ethanol that can be later used as a transportation fuel or integrated 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. You’ll run out of markets for all the products you produce, so not matter what, a significant amount of the captured CO2 will need to be buried underground,” Hatton explained.

Since desalination plants are already processing ocean water, these new carbon removal systems could efficiently be integrated into such already existing plants. Other locations could include ships that would process water as they travel, offshore drilling platforms, or aquaculture farms. Ultimately, it could lead to the global deployment of free-standing carbon removal plants.

“The carbon dioxide problem is the defining problem of our life, of our existence. So clearly, we need all the help we can get,” concluded co-author Kripa Varanasi, a mechanical engineer at MIT.

The study is published in the journal Energy & Environmental Science.

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By Andrei Ionescu, Earth.com Staff Writer

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