Offering a potentially efficient and inexpensive treatment, the 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.
“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,” said MIT Professor T Alan Hatton.
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.
“We’re not going to be able to treat the entire planet’s emissions,” said Varanasi. But the reinjection could be done in places such as fish farms, which tend to acidify the water. The captured CO2 could be buried underground, or converted into compounds such as ethanol.
Initially, the systems could be installed at existing or planned infrastructure that already processes seawater, such as 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 Varanasi.
He added: “With desalination plants, you’re already pumping all the water, so why not co-locate there? A bunch of capital costs associated with the way you move the water… could already be taken care of.”
The system could also be implemented by ships that would process water as they travel, helping to mitigate the significant contribution of ship traffic to overall emissions. There are already international mandates to lower shipping’s emissions, and “this could help shipping companies offset some of their emissions, and turn ships into ocean scrubbers,” Varanasi said.
It could also be integrated at locations such as offshore drilling platforms, or at aquaculture farms. Eventually, it could lead to a deployment of globally-distributed free-standing carbon removal plants.
The process could be more efficient than air-capture systems, Hatton said, because the concentration of CO2 in seawater is more than 100-times greater than it is in air. Direct air-capture systems first need to capture and concentrate the gas before recovering it.
“The oceans are large carbon sinks, however, so the capture step has already kind of been done for you,” he said. “There’s no capture step, only release.” That means the volumes of material that need to be handled are much smaller, potentially simplifying the whole process and reducing the footprint.
The researchers aim to find an alternative to the requirement for a vacuum to remove separated CO2 from the water, and to identify operating strategies to prevent precipitation of minerals that can foul the electrodes.
“The carbon dioxide problem is the defining problem of our life, of our existence,” said Varanasi. “So, clearly, we need all the help we can get.”
The work was reported in the journal Energy and Environmental Science.
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