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Move over direct air capture. Pulling CO2 from the ocean could be more efficient.

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Move over direct air capture. Pulling CO2 from the ocean could be more efficient.

Engineers have devised an elegantly simple new system to pull greenhouse gas from seawater. No membranes. No chemicals. And a lot less energy required.
February 23, 2023

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Oceans soak up about a third of all carbon dioxide emissions released into the atmosphere, making it the world’s biggest carbon reservoir. And now, researchers have come up with a way to remove carbon dioxide from the oceans in an effort to reduce the overall levels of the harmful greenhouse gas in our environment.

The approach is easy to deploy, and does not require expensive membranes or chemicals, write the MIT engineers in a paper published in the journal Energy & Environmental Science. It needs less energy than other technologies to capture carbon, such as directly capturing the gas from air. Plus, the researchers’ preliminary analysis suggests that this ocean capture system could be economically feasible.

Oceans have become more acidic in recent decades as they have soaked up carbon dioxide. This acidification destroys coral reefs and harms shellfish and other marine life. Removing carbon dioxide from the oceans would not only mitigate that, it would also help to reduce atmospheric concentrations of the gas. That’s because decarbonated oceans would continue to absorb more carbon emissions from the air.

Besides, say the researchers, the concentration of the gas in seawater is over 100 times higher than in air. So removing carbon dioxide from oceans should be less expensive and energy-intensive than direct air capture, which is slowly gaining traction globally.

Existing methods for removing carbon dioxide from seawater require membranes or chemicals, which make the process complex and expensive. T. Alan Hatton and Kripa Varanasi of MIT and their colleagues wanted to avoid using these materials.

The new ocean carbon removal system could be used by ships and offshore platforms. Credit: MIT

Their system is made of two electrochemical cells that work in tandem. In the first one, a bismuth electrode releases protons into the seawater. This converts the carbonate and bicarbonate in the water to carbon dioxide.

A vacuum chamber removes carbon dioxide from the water as it passes through into the second cell. In the second cell, a reverse voltage soaks up the protons, turning the acidic water back to alkaline, so it can be released back into the ocean.

These carbon dioxide removal modules could be installed on cargo ships plying the oceans, or on platforms located near offshore wind farms or floating solar islands, the researchers say. The system could also be integrated with processes such as desalination that already have installed facilities to handle seawater. The captured carbon dioxide could be injected directly into geologic structures under the seabed for long-term storage, or could be collected on land to make fuels or chemicals.

 

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The researchers estimate the cost of the system to be between $50–100 per ton of carbon dioxide. Direct air capture today costs between $250–600 per ton. However, the MIT team’s calculations do not account for additional costs for intake filers, pumps and gas separation. They are still improving the system, and expect that it could be ready for a practical demonstration project in about two years.

Source: Seoni Kim et al. Asymmetric chloride-mediated electrochemical process for CO2 removal from oceanwater. Energy and Environmental Science, 2023.

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