Volcanic rocks can quickly turn pollution into stone

Greenhouse gases are a major contributor to climate change. But a new technique can turn one of these gases into solid rock before it floats up into the atmosphere. If widely adopted, such a move might just help slow global warming.

As part of a test program, researchers in Iceland injected gaseous carbon dioxide (CO₂) into basalt (Bah-SALT). It’s the type of rock that lava turns into when it cools quickly. About two years later, the scientists checked back. More than 95 percent of the gas had turned to stone, they found.

Solidifying the gas effectively locked it away. Now the gas is no longer a threat to Earth’s climate. Such a relatively fast process could help counteract the world’s greenhouse-gas emissions, the researchers conclude.

“It’s working,” says Jeurg Matter. “It’s feasible and it’s fast enough to be a permanent solution for storing CO₂ emissions,” he adds. As a geochemist, Matter studies the chemical composition of solid material on Earth — and how it changes. He works at the University of Southampton in England.

Matter’s group published its new results in the June 10 Science.

What makes basalt special

People have tried different approaches for storing CO₂. Many rely on pumping the gas into rocky vaults underground. But sometimes the gas can leak back to the surface. That’s unlikely to happen with basalt, however. As much as one-fourth of basalt is made up of elements that react with CO_2.. This allows them to form solid minerals, such as limestone.

This process is called mineralization. It occurs naturally as basalt is exposed to harsh weather. Researchers had thought this process took hundreds to thousands of years. That would make it far too slow to be useful for combating the immediate threat of climate change.

For their new experiment, Matter and his colleagues mixed groundwater with 230 tons of CO₂ . The gas had been emitted by a geothermal-energy power plant. Combining that gas with the water created a mixture that was a lot like seltzer water. The researchers injected this bubbly liquid into basaltic rock some 400 to 800 meters (about 1,300 to 2,600 feet) below ground. When the team sampled that rock about two years later, almost all of the CO₂ had mineralized.

Turning CO₂ emissions into minerals is not cheap. It costs $17 per ton. That’s roughly twice as much as existing storage methods. But it doesn’t require long-term monitoring to prevent leaks, Matter notes. A second benefit: This technique requires only water and basalt. And in theory, he adds, “We have enough basalt globally to take care of all anthropogenic CO₂ emissions.”

A second group’s work seems to back up the new findings. Peter McGrail is a geochemist at the Energy Department’s Pacific Northwest National Laboratory in Richland, Wash. He and his colleagues did similar tests using pure CO₂, no groundwater added. They haven’t yet published their results. But McGrail says their findings resemble those that Matter’s group has just reported.