Turning Into Stone

Power plants around the world release 40 billion metric tons of carbon dioxide into the atmosphere each year—on track to meet or exceed the amount that will trigger the most dire climate change predictions of scientists. But what if those gases could be captured before they entered the air and rendered harmless? That’s what a team of geoscientists that includes Professor Martin Stute has accomplished. The group’s study, conducted in Iceland, took carbon dioxide emissions from a power plant and—for the first time—transformed them into stone.

The key to the process is basalt, a naturally occurring volcanic rock that interacts with the gases to turn them into inert crystals. While scientists have known about the process for a long time, it has in the past appeared to take too long to be a practical solution to greenhouse gases. “People thought this was happening on geological timescales—perhaps thousands of years,” says Stute, a professor of environmental science who began teaching at Barnard in 1993. As scientists began experimenting with the process, they revised those estimates to decades, and then to 10 years. Stute’s team accomplished it in two—an incredibly swift timetable no one thought possible. The findings were published in Science in June. “This opens another door for getting rid of carbon dioxide or storing carbon dioxide in the subsurface that really wasn’t seen as a serious alternative in the past,” Stute says in Scientific American, one of dozens of publications that reported on the study.

The so-called CarbFix project grew out of a talk that preeminent Columbia University earth scientist Wally Broecker gave at the University of Iceland around 2008. After the speech, the president of Iceland, who was in the audience, approached Broecker with the audacious dream of making Iceland the first country to eliminate all emissions of CO₂ into the atmosphere. Knowing of the research on turning CO₂ into rock, Broecker tapped Stute to help design a similar system to test on Iceland’s main utility, which uses a geothermal power plant, fueled by heat under the earth’s surface, outside of Reykjavik. Other institutions on the team include the University of Iceland, the National Center for Scientific Research in France, the University of Copenhagen, the University of Southampton, and Columbia’s Lamont-Doherty Earth Observatory, where Stute is an adjunct senior research scientist.

Several Barnard students have also worked on the project. “Their work is critical to helping us understand what the results mean,” Stute says. Bailey Griswold ’12 traveled to Iceland in 2011 to analyze the sampling process, while Claudia Mack ’15 and Rory Vinokor ’15 studied samples in Stute’s laboratory at Columbia’s Lamont-Doherty Earth Observatory and injected them with tracers to track the movement of CO₂.“I learned the importance of being able to convey scientific lessons to non-scientists,” says Mack, who now works with an environmental consulting firm in San Francisco. “I constantly think about the scale of CarbFix and its importance in addressing a huge environmental problem as I begin to work toward a career in environmental engineering.” Students are eager to work on the project because they are concerned about climate change and “they want to know what to do about it,” Stute says.

Stute also offers students hands-on experience with real-world problems in a workshop course on environmental sustainability. In a recent class, students prepared a report for the Hudson River Foundation that made detailed recommendations on how climate change would affect the parkland along the river and what the foundation could do to mitigate the effects. Another class explored arsenic contamination of New Jersey drinking water by creating a series of videos to educate the public. The videos demonstrated how people can test their water for arsenic and install treatment systems.

Stute’s background is the study not of stone but water. His past research analyzed the age and composition of groundwater to reconstruct past climate conditions. More recently, he looked at hydraulic fracturing, better known as fracking. His knowledge of water flow turned out to be key to the process of sequestering the carbon gas. While geothermal energy—which is used at Iceland’s power plant—emits much less carbon than coal or oil plants, it is not emission-free; trapped carbon dioxide and hydrogen sulfide (the source of the telltale “rotten egg” smell) are released from the ground as gases.

Stute helped design a system to capture those gases and transmit them into a well drilled 450 meters into the fine-grained rock called basalt. There, the gases were bubbled into the water, dissolving to make a fizzy carbonic acid. That water then flows slowly through the tiny cracks in the porous rock. “It flows 60 meters in two years, so it’s a very, very slow movement,” says Stute.

While it is flowing, the mild carbonic acid eats away at the rock, dissolving minerals including iron, magnesium, and calcium. Those minerals then react with the carbon dioxide to create carbonate salts that slowly precipitate out of the water, trapping the carbon emissions in a solid form. “You are basically dissolving the rock, which reacts with the CO₂ to form another kind of rock,” says Stute. He isn’t sure yet why the reaction was so much faster than predicted. It’s possible that the surface area of the rock over which the reaction is taking place is greater, causing more minerals to come into contact with the carbon dioxide gas.

What is clear is that this process has the potential to be adopted more widely to sequester carbon gas from power plants across the world. Basalt forms some 10 percent of the earth’s land masses, including major deposits in the Pacific Northwest of the United States, as well as 90 percent of the ocean floor. “There is no shortage in terms of the capacity to dissolve CO₂,” Stute says. Especially for power plants near the ocean, the gas could be transported through a pipeline and injected directly into the ocean floor, where it could turn into stone without ever touching the open ocean. “The only danger is if the pipeline itself breaks,” says Stute, but even in that case, the gas would be rapidly diluted in the ocean.

This approach to carbon sequestration would be much safer than current methods, which rely on injecting carbon dioxide at high pressure into caverns deep within the earth, or dissolving it into groundwater. While those repositories are sealed, the gas still could move upward. “If it finds a crack or an old well, there is the chance it will come to the surface,” warns Stute. By contrast, once the gas is mineralized into stone, it is completely safe and nonreactive.

The biggest challenge to implementing the system is cost. The actual process for transforming gas into stone is cheap, something on the order of $30 per metric ton, according to Stute. Unlike geothermal plants where the CO₂ is relatively pure, however, CO₂ in coal plants is mixed with ash and toxic chemicals, which must be removed before the gas can be sequestered—driving costs up to as much as $100 to $150 per metric ton. Then there is the cost of transporting the CO₂ to a source of basalt or mining basalt to bring to the power plants.

Despite those challenges, the process is not likely to be more expensive than other forms of sequestration. Some scientists—including Klaus Lackner, formerly of Columbia and now at Arizona State University—are working on a method to extract CO₂ directly from the air, cutting transportation costs to zero.

Stute’s work holds great promise, but he cautions that mineralization of CO₂ isn’t in itself a solution to climate change. “We can’t keep producing CO₂ and sequestering it—it’s too vast a quantity to deal with.” However, the process could provide a vital stopgap to buy time while the world continues to convert to more renewable sources of energy. The Iceland study shows for the first time that the process could work rapidly to turn CO₂ into harmless rock, without any leakage or side effects. Iceland’s utility company, which currently captures 25 percent of emissions, is considering expanding the project to capture more. To prove the viability for conventional power plants, however, a much bigger project would be needed, says Stute, whose team is working to secure funding for a project in Washington State that would pump CO₂ into offshore basalt.

Scientists often spend their career studying an esoteric question that may be of interest to a small group of people. But Stute and his team are contributing to our understanding of climate change on a large scale. It’s the kind of opportunity that happens once in a lifetime, if you’re lucky—and Stute is pleased to do his part to make a significant difference to the world.