New Technique Turns Ordinary Rocks Into Carbon-Capturing Machines

Of all the planet-warming greenhouse gases human activity releases into the atmosphere, carbon dioxide is the most significant emission. As such, experts have suggested that, in addition to drastically lowering our fossil fuel use, we should actively remove carbon dioxide (CO2) from the atmosphere. What’s known as carbon capture technology, however, is usually expensive and/or energy-intensive, and necessitates carbon storage solutions.

Now, researchers at Stanford University have proposed a surprisingly practical strategy: make rocks do it for us.

They’re not kidding. Stanford chemists Matthew Kanan and Yuxuan Chen have developed a process that uses heat to transform minerals into materials that absorb CO2—permanently. As detailed in a study published Wednesday in the journal Nature, the process is practical and low-cost. Additionally, Kanan and Chen’s very helpful rocks could satisfy the needs of a common agricultural practice, hitting two birds with one stone.

“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,” Kanan, the senior author of the study, said in a Stanford statement. “Our work solves this problem in a way that we think is uniquely scalable.”

For decades, scientists have studied ways to accelerate some rocks’ natural absorption of CO2, a process called weathering that can take hundreds if not thousands of years. Kanan and Chen seem to have cracked the code by converting common slow-weathering minerals called silicates into fast-weathering minerals.

“We envisioned a new chemistry to activate the inert [not chemically reactive] silicate minerals through a simple ion-exchange reaction,” Chen explained. Ions are atoms or groups of atoms with an electrical charge. “We didn’t expect that it would work as well as it does.”

Kanan and Chen were inspired by cement production, where a kiln, or furnace, converts limestone (a sedimentary rock) into a reactive chemical compound called calcium oxide, which is then mixed with sand. The chemists replicated this process, but swapped sand out for a material called a magnesium silicate. Magnesium silicate contains two minerals that, with heat, exchanged ions and turned into magnesium oxide and calcium silicate: minerals that weather quickly.

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

To test their results, Kanan and Chen exposed wet calcium silicate and magnesium oxide to air. They turned into carbonate minerals—the result of weathering—within weeks to months.

“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.” This application could also be practical for farmers, who add calcium carbonate to soil when it’s too acidic: a solution called liming.

“Adding our product would eliminate the need for liming, since both mineral components are alkaline [basic, as opposed to acidic],” Kanan 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.”

Approximately one ton of magnesium oxide and calcium silicate could absorb one ton of CO2 from the atmosphere—and that estimate accounts for the CO2 emitted by the kilns themselves, which still require less than half the energy used in other carbon capture technologies.

Scaling this solution to an impactful level, however, would require millions of tons of magnesium oxide and calcium silicate, annually. Nevertheless, Chen points out that if estimates of natural reserves of magnesium silicates such as olivine or serpentine are accurate, they would be enough to remove all the human-emitted atmospheric CO2, and then some. Additionally, the silicates could be recovered from mine tailings (mining leftovers).

“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.”