Common Rocks And A Cement-Making Technique Could Transform How We Capture Carbon Dioxide

Chemists at Stanford University have developed a low-cost way to remove carbon dioxide (CO₂) from the atmosphere using an unlikely source: rocks.

The process involves heating common minerals, so they transform into materials that spontaneously pull carbon from the atmosphere and permanently contain it. Even more impressively, these reactive materials can be made in normal kilns, like the ones used to make cement.

There are currently various avenues being explored to remove CO₂ from the atmosphere using engineered systems, but these have often focused on either improving and scaling technologies for direct air capture – which use panels of large fans to drive ambient air through chemicals or other processes to remove CO₂.

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These technologies, although improving, are still limited when it comes to their high energy costs. Instead, researchers have started turning to alternative ways to capture CO₂ by turning to the planet's vast reserves of silicate minerals.

“The Earth has an inexhaustible supply of minerals that are capable of removing CO₂ from the atmosphere, but they just don’t react fast enough on their own to counteract human greenhouse gas emissions,” Matthew Kanan, a professor of chemistry at the Stanford School of Humanities and Science, said in statement.

“Our work solves this problem in a way that we think is uniquely scalable.”

Silicates are a common group of rock-forming minerals that comprise the basis of the Earth’s crust and mantle. When they react with water and atmospheric CO₂, silicates form stable bicarbonate ions and solid carbonate minerals through a process known as weathering.

In nature, this can take hundreds to thousands of years, but Kanan and Yuxuan Chen, a postdoctoral scholar at Stanford University and lead author of the study, have found a way to speed the reaction up, developing a new process for converting slow-weathering silicates into considerably more reactive minerals that capture and store atmospheric carbon more quickly.

“We envisioned a new chemistry to activate the inert silicate minerals through a simple ion-exchange reaction,” Chen, who developed the technique during their PhD, added. “We didn't expect that it would work as well as it does.”

In the efforts to overcome global warming, researchers have emphasized that we will need to both radically decrease our reliance on fossil fuels and permanently remove billions of tons of carbon from the atmosphere. This new development may contribute to this aim.

“Our process would require less than half the energy used by leading direct air capture technologies, and we think we can be very competitive from a cost point of view,” Kanan explained.

Old ideas make for new technologies

The new approach was inspired by an old one: the method for making cement. This begins heating up limestone in a kiln, converting it to calcium oxide, which is then mixed with sand to become a key component of cement.

During their laboratory research, Kanan and Chen adopted a similar process but replaced the sand with another mineral containing magnesium and silicate ions. When heated, the minerals exchange ions and become magnesium oxide and calcium silicate, two alkaline minerals that quickly react to acidic CO₂ in the air.

“The process acts as a multiplier,” Kanan explained. “You take one reactive mineral, calcium oxide, and a magnesium silicate that is more or less inert, and you generate two reactive minerals.”

In a lab test where the calcium silicate and magnesium oxide were exposed to water and pure CO₂, the two materials transformed into carbonate minerals with carbon trapped inside within just two hours.

Of course, lab conditions aren't the same as outdoors. Conducting a more realistic test, the team exposed wet samples of the minerals directly to air, which has a significantly lower concentration of CO₂ in it. The process under these conditions took weeks to months to occur, but it nevertheless thousands of times faster than it would in natural weathering.

Applications

“You can imagine spreading magnesium oxide and calcium silicate over large land areas to remove CO₂ from ambient air,” Kanan said. “One exciting application that we’re testing now is adding them to agricultural soil. As they weather, the minerals transform into bicarbonates that can move through the soil and end up permanently stored in the ocean.”

It is possible this approach could also benefit farmers, who often use a process called “liming” to increase soil pH by adding calcium carbonate to it.

“Adding our product would eliminate the need for liming, since both mineral components are alkaline,” he explained. “In addition, as calcium silicate weathers, it releases silicon to the soil in a form that the plants can take up, which can improve crop yields and resilience. Ideally, farmers would pay for these minerals because they’re beneficial to farm productivity and the health of the soil – and as a bonus, there's the carbon removal.”

Plans for the future

At the moment, Kanan’s lab can create around 15 kilograms (roughly 33 pounds) of the material a week. However, to make a significant impact on global carbon levels, production would need to scale up to millions of tons annually.

The team believes existing kilns used to make concrete could also produce the mix by using silicate minerals left over from mining.

“Each year, more than 400 million tons of mine tailings with suitable silicates are generated worldwide, providing a potentially large source of raw material,” Chen said. “It’s estimated that there are more than 100,000 gigatons of olivine and serpentine reserves on Earth, enough to permanently remove far more CO₂ than humans have ever emitted.”

Kanan is now looking for ways to develop kilns that do not run on fossil fuels to at least help in this effort.

“Society has already figured out how to produce billions of tons of cement per year, and cement kilns run for decades,” Kanan said. “If we use those learnings and designs, there is a clear path for how to go from lab discovery to carbon removal on a meaningful scale.”

The study is published in Nature.