As the world continues its slow shift to renewable energy, it would be great to limit the carbon dioxide produced from the fossil fuels we'll burn in the meantime. Some researchers are working on capturing that CO 2 from smokestacks using as little energy as possible. Others are working on places to put it.

Deep, briny aquifers are an obvious choice. The concern there is the risk of leakage. Once we put the CO 2 deep into the Earth, we want it to stay there. Eventually, the CO 2 dissolved in those brines can precipitate as carbonate minerals (which won't be going anywhere), but that takes a pretty long time.

Brines aren't the only option for locking away captured CO 2 , though. There are also volcanic rocks that will readily react with CO 2 , potentially speeding things along. In 2012, a pilot project got rolling in Iceland to inject CO 2 into basalt—something the island nation has in abundance. An impressive outcome from this pilot is reported in a new paper published in Science.

A team led by University of Southampton researcher Juerg Matter was involved in the CarbFix project, located next to a geothermal power plant outside Reykjavik. This plant basically taps a source of steam above Iceland’s shallow magma chambers, but some volcanic CO 2 and sulfur gas come along with it. The goal is to capture that gas and stick it back underground. That’s done with an injection well drilled down into basalt bedrock, a material that was born as erupted lavas.

Carbon dioxide is often compressed to a supercritical fluid for these injections, but this project has chosen to dissolve it in water instead. That requires a lot of water—about 25 tons of water for every ton of CO 2 —but the solution isn’t buoyant so there’s little risk of the CO 2 escaping upwards. The researchers also added a few chemical tags so they could follow the fluid after it was injected between 400 and 800 meters below the surface. Those tags included some CO 2 high in carbon-14, and a couple gases like sulfur hexafluoride that would simply go along for the ride.

Monitoring wells at various depths and distances away were frequently tested for signs of these chemical tags after the injections. The nonreactive sulfur hexafluoride showed up on schedule, diluted as the injected water spread out and mixed with groundwater. The carbon in those samples, however, told a different story.

Very quickly, the amount of carbon (and carbon-14) fell to less than 5 percent of what would be expected if all the CO 2 they injected was still there. This suggests it must have precipitated out along the way. In fact, a pump in one of the wells stopped working because it became encased in precipitated calcium carbonate. The carbon-14 in that calcium carbonate showed that it did indeed come from the injected CO 2 .

The surprise here is how quickly it all happened. The monitoring reported here covered less than two years. Laboratory experiments have shown that it ought to take decades for CO 2 injected into basalt to mineralize. The researchers write, “The results of this study demonstrate that nearly complete in situ CO 2 mineralization in basaltic rocks can occur in less than 2 years. Once stored within carbonate minerals, the leakage risk is eliminated and any monitoring program of the storage site can be significantly reduced, thus enhancing storage security and potentially public acceptance.”

Following on this early success, Reykjavik Energy, which operates the geothermal power plant, has ramped up injection over the past couple years. According to a Columbia University press release, they’ll soon be injecting a quarter of the CO 2 released by the plant. Since they don’t have to purify the gas they capture, the cost of the project is comparatively minor—about $30 per ton of CO 2 .

So is this a breakthrough demonstration of carbon storage that can be emulated around the world? Not necessarily. It’s not entirely clear what it is about the CarbFix site that allowed such rapid mineralization. It could be some combination of characteristics of the geology and groundwater chemistry, although the researchers think their approach of dissolving the CO 2 in water before injection played a role.

Dr. Charlotte Sullivan studies CO 2 storage in basalt at the Pacific Northwest National Laboratory. She told Ars that slower rates of mineralization are expected in other locations that have been studied. Sullivan also described the trade-offs of the large volume of water used in the CarbFix project, with the need to manage fluid pressure underground. Simply injecting supercritical CO 2 (to depths where it stays compressed) is not without its advantages.

Even if CarbFix is an anomaly, getting these results is more encouraging than having the pilot program falter. But there are some big challenges to scaling this: if the CarbFix site was storing CO 2 emissions from a single 300 megawatt coal plant, the injection rate would have to increase by a factor of 80. Putting a dent in our greenhouse gas emissions with this technology is going to take some effort.

Science, 2016. DOI: 10.1126/science.aad8132 (About DOIs).