CO2 storage by petrifying?

Greenhouse gas dissolved in water turns into carbonate rock in basalt with surprising rapidity

Porous basalt rocks act as a catalyst for the mineralization of CO2 to carbonate. © Kevin Krajick / Lamont-Doherty Earth Observatory
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Petrifying instead of liberating: The greenhouse gas carbon dioxide (CO2) could in future be stored more safely in the underground - by dissolving it in water and pumping it into porous basalt rock. As a pilot test on Iceland shows, the CO2 then reacts surprisingly fast with trace elements and becomes solid carbonate rock. Thus mineralized, the CO2 is thus permanently withdrawn from circulation, as the researchers report in the journal "Science".

In view of the hesitant progress in global climate protection, more and more researchers are pleading for the retention and storage of the greenhouse gas CO2 through technical solutions. For example, in Carbon Capture and Storage (CCS), CO2 is separated from exhaust fumes or air and then pumped into disused gas or oil reservoirs or other underground cavities.

Mineralization instead of gas storage?

How secure such underground gas storage are, however, is unclear. While some studies found evidence of leaks, cracks, and even earthquakes, other pilot tests did not pose a high safety risk.

Now, however, Juerg Matter of the University of Southampton and his colleagues have found an alternative to storing CO2 as a gas: turning it into rock. It has long been known that CO2 dissolved in water can become carbonate in the presence of silicate rock - as evidenced by deposits of thick carbonate layers in the subsurface. This mineralization is promoted by liberated from the rock trace elements such as calcium, magnesium and iron.

Volcanic rock as a trial field

The catch was the time: According to common assumption, the mineralization of CO2 to carbonates takes hundreds to thousands of years - and thus far too long to be useful as a geotechnical climate protection method. However, a pilot project started in 2012 at the Reykjavik Energy Hellisheidi geothermal power plant in Iceland now shows that the conversion of CO2 gas into rock can be much faster. display

Injecting the CO2 into the ground during a pilot test at the Hellisheidi geothermal plant in Iceland. Ur Sigurdur Gislason

For the experiment, the researchers pumped 250 tons of CO2 gas, 400 to 800 meters deep, into the basalt rock of the subsurface. Previously, they had labeled the CO2 with radioactive C-14 carbon and added water hygroscopic sulfide as a tracer. Over the months that followed, scientists sampled water from eight nearby wells and determined how much of the pumped gas was still circulating there.

White veins in the basalt

The result: After only several months, the indicators for the CO2 in the water samples dropped significantly. The composition showed that the greenhouse gas must have been mineralized underground. "Our results show that between 95 and 98 percent of the injected carbon dioxide has been mineralized in less than two years - that's amazingly fast, " says Matter.

The white carbonate veins in this basalt core prove that the dissolved CO2 has been mineralized. Annette K. Mortensen

Further evidence for the conversion of CO2 to carbonate was provided by drilling cores from the subsurface rock: the porous basalt from the depths was crossed by numerous white cores of carbonate rock. The scientists suspect that the CO2-rich water in the fine canals of basalt is virtually trapped and thus accumulates more quickly with the trace elements needed for mineralization.

Safe alternative to gas storage

According to the researchers, the pilot test demonstrates that CO2 gas can be converted into carbonates damit and therefore rocks schneller faster than expected. "This means that we can pump large amounts of CO2 into the ground and store it there safely for a very long time, " says co-author Martin Stute of Columbia University. In contrast to the storage of CO2 as gas, this method poses no risk for leaks and gas leaks.

"Our newly developed method could therefore be used for sustainable and environmentally friendly storage of CO2 emissions, " says Matter. This could be used anywhere where there is basalt in the ground and that is the case in many volcanic and formerly volcanic areas of the earth. "Basalt is one of the most common rocks on earth, " says Matter.

Useful especially at the coast

However, in order to get the mineralization going, the CO2 gas must be dissolved in water it takes about 25 tons of water for every tonne of CO2, as the researchers explain. Therefore, this form of CO2 storage would be particularly suitable along the coast, where there is plenty of seawater. The cheapest - even financially - it would be anywhere where, as Iceland already geothermal plants are.

"We have to do something about rising CO2 emissions, " says Matter. "And with this method, we have the ultimate permanent storage - we just turn it back to stone." He and his colleagues have already begun the next stage of their experiments at the Icelandic Hellisheidi power plant: Since 2014, 5, 000 tonnes of CO2 have been separated there each year and stored in pumped the basalt. (Science, 2016; doi: 10.1126 / science.aad8132)

(University of Southampton / The Earth Institute at Columbia University, 10.06.2016 - NPO)