New research from MIT uses effective and inexpensive electrochemistry to remove CO2 from seawater, combatting ambient CO2 levels and acidification of oceans.

Known as the primary driver of climate change, atmospheric concentrations of CO2 continue to increase, wreaking havoc on the environment. With levels being at the highest ever recorded, it is widely recognised both the production and mitigation of existing CO2 desperately need to be reduced.

The ocean acts as one of the largest sinks of CO2, holding between 30 and 40% of anthropogenic CO2 emissions. As CO2 is absorbed it combines with water molecules to form the weak acid, carbonic acid (H2CO3). This dissociates in water, releasing acidifying hydrogen ions (H+) and bicarbonate ions (HCO3-).

Whilst this provides the benefit of removing CO2 from air and in turn reducing the damage atmospherically, unfortunately it provides its own set of issues under the sea. With the primary effect being acidification of the ocean, rising CO2 levels have seen destruction of coral reefs as well as harm to shellfish and other marine life.

Current technologies exist to attempt to remove CO2 directly from air, but suffer from issues such as complex and expensive processes. More recently, the possibility of removing CO2 from seawater has emerged as a promising alternative. This route offers advantages over air capture as the concentration in seawater is over 100 times that in the air. However, similarly to air capture technologies, this idea has not led to any widespread use despite considerable research in this area.

Now, a team of researchers at Massachusetts Institute of Technology (MIT) have found a breakthrough in this area which could be the key to inexpensive and efficient removal of CO2 from seawater.

Published in the journal Energy and Environmental Science, the technology uses electrochemistry to remove the CO2 before neutralising the resultant seawater and returning it back to the sea. In turn, the returned seawater can re absorb further CO2, decreasing atmospheric pressures as well as combatting the effects of acidification.

“The carbon dioxide problem is the defining problem of our life, of our existence”

Kripa Varanasi, paper co-author

The process uses an electrochemical cell with reactive electrodes. The seawater is pumped into one side containing a bismuth electrode where a reaction takes place producing H+ ions. As the acidity increases, the bicarbonates are converted to CO2 gas which can be tapped off. The remaining acidified water is pumped into the second cell where the reaction is reversed, removing the acidifying H+ ions to alkalise the water. This can then be pumped back into the ocean.

“This is what we feed back to the ocean, so we’ve actually done both the CO2 removal and the alkalisation […] of the ocean water,” said one of the study’s co-authors, Alan Hatton.

As with any carbon removal process, the captured gas needs to be properly disposed of. Current routes involve using it as a feedstock for alternative chemical production such as ethanol, although the market for these processes is limited with the amount of CO2 captured. Deep geological storage is a more likely fate where the gas is buried underground.

The researchers talk of initial application of this technology in existing infrastructure that processes seawater, for example in 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,” said one of the authors, Kripa Varanasi.

Further implementation could come in the form of ships that would process the water while travelling.

“This could help shipping companies offset some of their emissions, and turn ships into ocean scrubbers,” Varanasi said.

Eventually the research team hopes to see the technology in free-standing carbon removal plants around the globe. “The carbon dioxide problem is the defining problem of our life, of our existence,” Varanasi continued. “So clearly, we need all the help we can get.”