Removing CO2 from seawater could be more efficient than carbon capture

Scientists say removing carbon dioxide from seawater could be more efficient and less costly than air capture systems, showing how me might reduce emissions in the future.  

The ocean is the world’s biggest carbon sink, soaking up 30-40% of all greenhouse gases, which offers a key opportunity for mitigating CO2 emissions.  

Now researchers from the Massachusetts Institute of Technology (MIT) have discovered a new method which can capture carbon dioxide and could reverse the acidification of the oceans.  

According to their investigations, the process could be more efficient than air-capture systems, as carbon dioxide concentration are 100 times greater in seawater compared to air.  

Professor of Chemical Engineering, T. Alan Hatton, said the volumes of material needed are much smaller, simplifying the process and reducing the amount of energy needed: ‘The oceans are large carbon sinks, however, so the capture step has already kind of been done for you. There’s no capture step, only release.’  

Existing methods involve applying a voltage across a stack of membranes and converting bicarbonates in the water to CO2 molecules which can be removed under a vacuum.  

However, this can be an expensive process, so the team devised a reversible membrane-free procedure with electrochemical cells driving the release of dissolved carbon dioxide from the water. 

The cyclic process first acidifies the water to convert dissolved bicarbonates into molecular carbon dioxide which can be collected as a gas. The water is then fed through a second set of cells with a reversed voltage which turns the acidic water back to alkaline before its released back into the sea.  

Scientists believe the method could help to reduce ocean acidification, due to carbon dioxide build-up, which threatens coral reefs and shellfish.  

The alkaline water could also be targeted in certain areas such as fish farms which suffer from acidic water to counteract the effect.  

Carbon dioxide removed from the water could then be buried in geologic formations under the sea floor or could be converted into compounds like ethanol other chemicals.  

Hatton explained: ‘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 no matter what, a significant amount of the captured CO2 will need to be buried underground.’  

Researchers say the method could be used initially within existing sea water processing infrastructure, like desalination plants, or be used by ships to process water as they travel.  

‘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 Professor of Mechanical Engineering, Kripa Varanasi. ‘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.’  

Photo by Matt Hardy