MIT team explores removing CO2 from ocean

The ocean is the world’s largest carbon dioxide ‘sink’, soaking up around 30 to 40 per cent of all of the gas produced by human activities.

Recently, the idea of removing CO2 directly from ocean water has emerged as a possibility for mitigating emissions. Whilst it has not yet led to widespread use, a team of researchers at MIT (Massachusetts Institute of Technology) believes they may have found the key to a ‘truly efficient and inexpensive removal mechanism’. Their findings are published in Energy and Environmental Science.

Existing methods for removing CO2 from seawater apply a voltage across a stack of membranes to acidify a feed stream by water splitting. This converts bicarbonates in the water to CO2 molecules, which can then be removed under vacuum.

T. Alan Hatton, co-author of the paper and Ralph Landau Professor of Chemical Engineering at MIT, noted that the membranes are expensive and chemicals are required to drive the overall electrode reactions at either end of the stack, adding further to the expense and complexity of the processes.

“We wanted to avoid the need for introducing chemicals to the anode and cathode half cells and to avoid the use of membranes if at all possible,” he said.

The team explained that they came up with a reversible process consisting of membrane-free electrochemical cells. Reactive electrodes are used to release protons to the seawater fed to the cells, driving the release of the dissolved carbon dioxide from the water.

The process is cyclic, first acidifying the water to convert dissolved inorganic bicarbonates to molecular carbon dioxide, which is collected as a gas under vacuum. Then, the water is fed to a second 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 sea.

Periodically, the roles of the two cells are reversed once one set of electrodes is depleted of protons (during acidification) and the other has been regenerated during alkalisation.

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According to MIT professor of mechanical engineering Kripa Varanasi, this removal of CO2 and reinjection of alkaline water could slowly start to reverse, at least locally, the acidification of the oceans that has been caused by CO2 buildup, which has threatened coral reefs and shellfish.

The reinjection of alkaline water could be done through dispersed outlets or far offshore to avoid a local spike of alkalinity that could disrupt ecosystems, Varanasi said.

Once the CO2 is removed from the water, it still needs to be disposed of, as with other carbon removal processes.

 “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,” said Hatton. “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.”

Initially, the team’s idea would be to couple the scalable system with existing or planned infrastructure that already processes seawater, such as desalination plants, as a ‘simple add-on’.

It could be implemented by ships that would act as ‘ocean scrubbers’, processing water as they travel, to help mitigate the significant contribution of ship traffic to overall emissions, Varanasi said.

The team believes the system could also be implemented at locations such as offshore drilling platforms, or at aquaculture farms, and could eventually lead to deployment of global free-standing carbon removal plants.

According to Hatton, the process could be more efficient than air-capture systems because the concentration of CO2 in seawater is more than 100 times greater than in air. In direct air capture systems it is first necessary to capture and concentrate the gas before recovering it. He explained that there is ‘no capture step, only release’, meaning the volumes of material that need to be handled are much smaller, potentially simplifying the process and reducing the footprint requirements.

One goal for the research is finding an alternative to the present step that requires a vacuum to remove the separated CO2 from the water. Another need is to identify operating strategies to prevent precipitation of minerals that can foul the electrodes in the alkalinisation cell, an inherent issue that reduces overall efficiency in all reported approaches.

Hatton noted that ‘significant progress’ has been made on these issues, but that it is still too early to report on them. The team expects that the system could be ready for a practical demonstration project within about two years.