To Combat Climate Change, Researchers Want to Pull Carbon Dioxide From the Ocean and Turn It Into Rock

A new method for combatting climate change feels like a bit of modern-day alchemy: scientists have figured out how to take carbon dioxide out of the ocean and turn it into harmless rock.

For every tonne of carbon dioxide we pump into the air, roughly a quarter of it gets absorbed by the ocean like a giant, watery sponge. All of this excess carbon dioxide is acidifying the water and threatening organisms, such as those with calcium carbonate shells, that are sensitive to the change.

To avert this fate, carbon emissions need to drop—fast. But many scientists also believe that active carbon capture—deliberately pulling carbon dioxide out of the environment—will be a necessary step to help curb, and potentially even reverse, the rise in emissions responsible for countless environmental impacts. However, capturing enough carbon to make a difference is a massive task, one that has so far proved challenging and expensive.

“You’re talking about removing some 10 to 20 gigatonnes of [carbon dioxide] per year, starting from 2050, probably for the next century,” says Gaurav Sant, a civil and environmental engineering professor and director of the Institute for Carbon Management at the University of California, Los Angeles.

To date, most efforts to capture carbon have focused on direct air capture—trying to pull the gas out of the atmosphere. But to make carbon capture more efficient, Sant’s research team is turning to the ocean for help.

Oceans and other large bodies of water can hold more than 150 times more carbon dioxide than the air. Sant and his colleagues’ idea is that if you can remove carbon from the ocean, the water will absorb more from the atmosphere to maintain a state of equilibrium. Now, they’re proposing an innovative way of getting carbon out of the ocean—by turning it into rock.

Seawater contains a lot of calcium and magnesium. When the calcium or magnesium ions combine with carbon dioxide, they form calcite or magnesite. The chemical reaction is similar to how many marine organisms build their shells. But by introducing a third ingredient, electricity, Sant and his team can make that reaction happen quickly, efficiently and, perhaps eventually, on a large scale. Putting this all together, the scientists have proposed a new technology that will run seawater through an electrically charged mesh, using electrolysis to trigger the chemical reactions needed to form carbonate rocks.

So far, the team has built a 1.5-by-1.5-meter prototype that they can flood with simulated seawater. They are collecting data on the amount of carbon dioxide that can be removed over various periods of time, analyzing the process efficiency and the amount of energy required. Aside from simply demonstrating the concept, they are using the model to determine what operational variables might impact the process.

“This is the formative step towards building larger systems and proving the process at a larger scale,” says Sant.

The process is a bit like a water treatment plant, but instead of taking in water and sifting out impurities, the proposed plant would use electricity to force carbon, calcium, and magnesium to react and become solids. The “purified” water would then be returned to the ocean.

“You are actually returning water that is slightly more alkaline than what you put in,” says Alan Hatton, a chemical engineer at the Massachusetts Institute of Technology who has worked on several unrelated carbon capture technologies. This more alkaline water could help mitigate the effects of ocean acidification in the immediate vicinity, he adds.

As well as pulling carbon out of seawater, the chemical reaction has a useful byproduct: hydrogen gas. By producing and selling the hydrogen, a plant could help offset its costs. Sant says that even if a proposed ocean carbon capture plant is powered by natural gas instead of renewable energy, the whole process could still be carbon negative because of this hydrogen gas byproduct.

While ocean carbon capture is a newer technology, a few other groups are also experimenting with it. Some of their projects, such as one by Halifax, Nova Scotia–based startup Planetary Hydrogen, are showing promise.

Like Sant’s team, Planetary Hydrogen is extracting carbon from seawater, trapping it in a solid, and indirectly making hydrogen gas. Rather than using electrolysis, however, they’re doing it with hydroxide. Hydroxide is an alkaline material that speeds up what is otherwise a natural process—rocks reacting with carbon dioxide and water to form alkaline forms of carbon—which would typically take place over geological timescales, says Greg Rau, the company’s lead researcher. While neither team is past the early stages of development, the two proposals seem to have a few benefits over trying to capture carbon out of the air.

Carbon dioxide is much less concentrated in the atmosphere than in the ocean, so direct air capture efforts typically need to be quite large to have a significant impact. Neither Hatton nor Sant believes ocean capture plants will require such real estate. And, according to Sant, his process will require half the energy cost of direct air capture and it won’t need a storage reservoir for the carbon dioxide.

There are some drawbacks to Sant’s proposal, though, that could make it difficult for the technology to progress. The biggest seems to be the amount of solids the process would create once it’s operating at a scale meaningful enough to affect climate change.

Removing 10 gigatonnes of carbon dioxide from the ocean, for instance, would yield 20 gigatonnes of carbonates—at a minimum, says Sant. He does have an idea for what to do with all these solids, though.

For the better half of a decade, Sant’s research has focused on streamlining a process of combining carbon dioxide from factory flue gas streams with calcium hydroxide to form concrete. “Because [my carbon dioxide sequestration method] effectively produces carbon neutral limestone, now you’ve got the ability to produce carbon neutral cement, and use the limestone solids for construction,” says Sant.

A lot of the solids produced by an ocean capture plant could be used that way, but there will still be tonnes left that would likely go back into the ocean, which could upset local marine ecosystems.

Hatton says it’s worth comparing the proposed plant’s potential impacts to the effects of a desalination plant on the surrounding ocean environment. While the main issue with desalination is the build-up of brine, the carbonate deposits from Sant’s plant could create other problems such as smothering plant life and significantly altering seafloor habitats. Just operating the plant, Hatton says, could also have physical effects on the behavior of the water near the facility, such as disturbing flow patterns.

Leaving the surrounding environment as undisturbed as possible is a top priority for Sant, although he recognizes that as this kind of technology becomes more prevalent there exists the potential for some unintended, as of yet unknown, consequences.

Once the team is able to demonstrate the technology can work on a large scale and is economically viable, they hope to eventually see hundreds if not thousands of plants built around the world. Ultimately, Sant hopes their work will open people’s minds to what carbon capture is capable of.

This article is from Hakai Magazine, an online publication about science and society in coastal ecosystems. Read more stories like this at hakaimagazine.com.

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