Geochemists Figure Out How to Turn Carbon Dioxide Emissions Into Stone

Researchers in Iceland have turned CO2 into stone in just two short years—mere nanoseconds on a geological scale.

Many scientists speculated it would take decades—if not centuries or millennia—for gaseous carbon dioxide to solidify. But an international project known as CarbFix, a project based at the Hellishedi geothermal power plant outside of Reykjavik, Iceland, accelerated that timetable by injecting CO2-laden water into the basaltic rock beneath the power plant. There, the mixture of minerals in the rock reacted with the dissolved gas to form carbonate minerals, a solid and stable form of rock that will lock the carbon away for millennia. The results were announced this week in the journal Science.

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The Hellishedi geothermal power plant.

Capturing and storing carbon dioxide emissions, a major contributor to global warming, has long plagued scientists. Previous efforts have been plagued by concerns that the gas will leak or worst yet escape explosively. Converting the carbon dioxide to a solid solves this problem and eliminates the need to monitor the storage site.

In geothermal power plants like Hellishedi, volcanically heated water produces steam. The superheated steam—along with other gases carbon dioxide and hydrogen sulfide—rise to the surface and spin turbines, which produce electricity.

Though geothermal power plants aren’t the source of most carbon emissions, they offer a convenient way to test the technology. For one, they already have the infrastructure to inject massive quantities of water in the ground, they just had to reconfigure the system to put the CO2 back into the water. The natural landscape also helps—over 90% of Iceland sits on basalt rock, the crucial ingredient required to mineralize carbon.

When CO2 and water mix with basalt, a chemical reaction occurs, transforming the carbon into a chalky, off-white substance reminiscent of limestone. At Hellishedi, the reaction happened so quickly that the scientists’ equipment occasionally broke down because it was coated in mineral carbon.

Mineralization naturally occurs over thousands of years deep underground, but it appears to happen much more rapidly closer to the surface level. “Basically we’re using a natural process and engineering it for climate-change mitigation,” Juerg Matter, associate professor of geoengineering at the University of Southampton and lead author of the study, told the New York Times in 2015.

To implement this technique on a broader scale, scientists will need to test the effects of much larger CO2 injections. Geothermal power plants like Hellishedi emit only 5% of the carbon dioxide that a equivalent coal power plant emits.

Still, researchers are hopeful that the experiment will scale, particularly since the necessary raw materials, water and basalt rock, are readily available. Nearly all of the ocean floor and 10% of Earth’s landmass the are lined with basalt.

Like other carbon storage techniques, cost remains the biggest hurdle for carbon mineralization. In the United States alone, power plants produce billions of tons of CO2 annually. Economics aside, the science of CarbFix is promising. After all, few things are more permanent than stone.