MIT researchers and the possibility of removing carbon dioxide directly from ocean water

In Massachusetts, recently, the possibility of removing carbon dioxide directly from ocean water has emerged as another promising possibility for mitigating CO₂ emissions, one that could potentially someday even lead to overall net negative emissions. But, like air capture systems, the idea has not yet led to any widespread use, though there are a few companies attempting to enter this area.

Offering a potentially efficient and inexpensive treatment, a process was developed by a team of researchers at the Massachusetts Institute of Technology (MIT).

The ocean is the world’s main CO2 ‘sink’, soaking up 30-40% of all the gas produced by human activities. Existing methods for removing the emissions from water rely on expensive membranes, however, across which a voltage is applied to acidify a ‘feed stream’ by water splitting. This converts bicarbonates in the water to molecules of CO2, which can then be removed under vacuum. Chemicals driving electrode reactions at either end of the ‘stack’ also add to the expense and complexity.

The team developed a reversible process using membrane-free electrochemical cells. Reactive electrodes are used to release protons to the seawater fed to the cells, driving the release of the dissolved CO2 from the water.

The cyclic process first acidifies the water to convert dissolved inorganic bicarbonates to molecular CO2, which is collected as a gas under vacuum. The water is then 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. The roles of the two cells are periodically reversed when one set of electrodes is depleted of protons and the other has been regenerated during alkalisation.

The removal of CO2 and reinjection of alkaline water could slowly start to reverse local acidification of the water, which threatens coral reefs and shellfish, said Professor Kripa Varanasi. 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, the researchers said.