Rock solid technique for capturing CO2

Following the current trend of carbon emission mitigation, researchers from Pacific Northwest National Laboratory are endeavoring to subvert gaseous carbon dioxide by turning it into stone that cannot enter the atmosphere and warm our planet more.

"As global temperatures increase, so does the urgency to find ways to store carbon," said Pacific Northwest National Laboratory Lab Fellow Kevin Rosso, who co-authored a scientific article on converting CO2 to carbon minerals. "By taking a critical look at our current understanding of carbon mineralization processes, we can find the essential-to-solve gaps for the next decade of work."

Instead of emitting CO2 into the air, one option is to pump it into the ground. Theoretically, putting CO2 deep underground sequesters the carbon away. While a primary concern remains of the gas leaking back upwards, if it can be pumped into rocks rich in metals like magnesium and iron, the CO2 can be transformed into stable and common carbonate minerals.

"Mitigating human emissions requires fundamentally understanding how to store carbon," said PNNL chemist Quin Miller, co-lead author of the published work. "There is a pressing need to integrate simulations, theory, and experiments to explore mineral carbonation problems."

Basalt CO2 capture project

A recent scientific review article by PNNL researchers published in "Nature Reviews Chemistry" discusses how carbon dioxide converts from a gas to a solid in ultrathin films of water on underground rock surfaces. These solid minerals, known as carbonates, are both stable and, more importantly, common.

PNNL's Basalt Pilot Project at the Wallula field site in Washington is dedicated to studying CO2 storage in carbonates.

In 2013, PNNL researchers began a field demonstration of carbon storage by injecting approximately 1,000 metric tons of CO2 into a natural basalt formation at Wallula.

After two years of post-injection monitoring, the scientists obtained cores from within the injection zone and subjected them to detailed physical and chemical analysis.

Nodules found in small cavities throughout the cores were identified as the carbonate mineral ankerite, which includes calcium, iron, magnesium, and manganese. Upon further inspection, it was revealed that the nodules were chemically distinct from natural carbonates present in the basalt and in clear correlation with the isotopic signature of the injected CO2.

These findings provided field validation of rapid mineralized rates observed from years of laboratory testing with basalt.

As part of the recent work, the researchers said that although these basalt subsurface environments are generally dominated by water, the conversion of gaseous carbon dioxide to solid carbonate can also occur when injected CO2 displaces that water, creating extremely thin films of residual water in contact with rocks. However, these highly confined systems behave differently than CO2 in contact with a pool of water.

In thin films, the ratio of water and CO2 controls the reaction. Small amounts of metal leach out from the rocks, reacting both in the film and on the rock surface. This leads to the creation of new carbonate materials.

Previous work led by Miller, summarized in the review, showed that magnesium behaves similarly to calcium in thin water films. Thus, the nature of the water films plays a central role in how the system reacts.

Understanding how and when these carbonates form requires a combination of laboratory experiments and theoretical modeling studies.

Understanding the science

Fundamentally, the PNNL team outlined key questions that need answering to make this form of carbon storage practical. Essentially, researchers must understand how minerals react under different conditions, particularly in conditions that mimic real storage sites, including in ultrathin water films.

Mineralization has the potential to keep carbon safely stored underground. Knowing how CO2 will react with different minerals can help ensure that what gets pumped underneath the surface stays there.

The basic science insights from mineralization work can lead to practical CO2 storage systems. For PNNL, the Basalt Pilot Project represents an important study site that bridges small-scale basic science and large-scale research applications.

"This work combines a focus on fundamental geochemical insights with a goal of solving crucial problems," said Miller. "Without prioritizing decarbonization technologies, the world will continue warming to a degree humanity cannot afford."