Here's how carbon dioxide is pulled out of seawater
Massachusetts, US: Researchers from all around the world have been working for years to find effective techniques to remove carbon dioxide from the atmosphere as it continues to accumulate in the planet's atmosphere. The ocean, on the other hand, serves as the planet's top "sink" for atmospheric carbon dioxide, absorbing between 30 and 40 per cent of all the gas generated by human activity.
Another interesting option for reducing CO2 emissions that could eventually result in net negative emissions is the prospect of drawing carbon dioxide directly out of ocean water. This possibility has recently come to light. Although there are a few businesses trying to break into this market, the notion has not yet resulted in any broad adoption, similar to air capture systems.
Now, a team of researchers at MIT says they may have found the key to a truly efficient and inexpensive removal mechanism. The findings were reported this week in the journal Energy and Environmental Science, in a paper by MIT professors T. Alan Hatton and Kripa Varanasi, postdoc Seoni Kim, and graduate students Michael Nitzsche, Simon Rufer, and Jack Lake.
The existing methods for removing carbon dioxide 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 molecules of CO2, which can then be removed under a vacuum. Hatton, who is the Ralph Landau Professor of Chemical Engineering, notes 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 says.
The team 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: It first acidifies the water to convert dissolved inorganic bicarbonates to molecular carbon dioxide, which is collected as a gas under a 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 into 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 alkalization.
This removal of carbon dioxide and reinjection of alkaline water could slowly start to reverse, at least locally, the acidification of the oceans that have been caused by carbon dioxide buildup, which in turn has threatened coral reefs and shellfish, says Varanasi, a professor of mechanical engineering. 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, they say.
"We're not going to be able to treat the entire planet's emissions," Varanasi says. But the reinjection might be done in some cases in places such as fish farms, which tend to acidify the water, so this could be a way of helping to counter that effect.
Once the carbon dioxide is removed from the water, it still needs to be disposed of, as with other carbon removal processes. For example, it can be buried in deep geologic formations under the sea floor, or it can be chemically converted into a compound like ethanol, which can be used as a transportation fuel, or into other speciality chemicals. "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," says 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, at least, the idea would be to couple such systems with 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," Varanasi says. There, 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.
"With desalination plants, you're already pumping all the water, so why not co-locate there?" Varanasi says. "A bunch of capital costs associated with the way you move the water, and the permitting, all that could already be taken care of."
The system could also be implemented by ships that would process water as they travel, in order to help mitigate the significant contribution of ship traffic to overall emissions. There are already international mandates to lower shipping emissions, and "this could help shipping companies offset some of their emissions, and turn ships into ocean scrubbers," Varanasi says.