Farm fertilizer could suck carbon dioxide from the atmosphere

If humanity wants to avoid a climate catastrophe, sucking up the carbon dioxide (CO2) it has already spewed into the atmosphere may be its last hope. One approach is to use naturally abundant minerals as CO2 sponges, but the process is slow. Now, a study reported in Nature suggests a way to accelerate it: by converting those minerals to compounds that bind CO2 faster and are similar to others already widely used in farming.

“This is a really great advance,” says Yogesh Surendranath, a chemist at the Massachusetts Institute of Technology. Sasha Wilson, a mineralogist at the University of Alberta, agrees. “It solves some of the long-standing problems related to carbon dioxide mineralization,” he says.

Since the start of the Industrial Revolution in the 1760s, humans have lofted some 2650 billion tons of CO2 into the atmosphere, raising the concentration of the heat-trapping gas by 50%. Researchers have proposed numerous strategies to soak up the excess, such as planting forests or capturing the gas with chemical filters and then burying it deep underground, an approach known as direct air capture (DAC). But trees release CO2 again when they burn or die and decay. DAC requires building expensive CO2 capture plants and consumes some 2 megawatt hours of energy for every ton of CO2 wrung from the air.

Another approach is carbon mineralization: spreading vast amounts of crushed alkaline rocks—usually abundant magnesium silicates, such as olivine and serpentine—on soils worldwide. The pulverized rock binds CO2, permanently locking it away in mineral form. Nature already performs carbon mineralization on a grand scale, in a process known as weathering.

But natural weathering takes millennia. “Nature already knows how to do this if you are willing to wait forever,” says Alissa Park, a chemical engineer at the University of California, Los Angeles. Crushing the rocks to increase the surface area exposed to the air—so called enhanced weathering—helps, but not enough. Even when the minerals are crushed to a powder, “they still don’t react fast enough,” says Matthew Kanan, a chemist at Stanford University.

The answer may be to change minerals. Unlike magnesium silicates, calcium silicates react quickly with CO2. If implemented on a global scale, say, by adding these crushed minerals to agricultural soils, the process could draw down billions of tons of atmospheric CO2 per year, Kanan and his postdoctoral assistant Yuxuan Chen estimate in the new research.

The catch is that unlike magnesium-based minerals, calcium silicates aren’t naturally abundant. So, the researchers had to find an efficient way to make them. They combined calcium oxide (CaO) with magnesium silicates and heated the mix to about 1200°C for 4 hours. A mineral square dance ensued, in which the magnesium swapped places with calcium. The process resulted in a final pairing of magnesium oxide and calcium silicates, both of which rapidly bind CO2.

When Kanan and Chen tested the reactivity of their materials in the lab, exposing them to water and air, they found that the magnesium oxide and calcium silicates bound all the CO2 they could possibly hold within months, thousands of times faster than natural weathering.

Whereas olivine and other magnesium silicates are cheap and readily available, getting the mountains of CaO needed for global scale CO2 removal would be trickier. Kanan and Chen propose to make it by heating limestone, or calcium carbonate, which transforms it into CaO—the same process used to make Portland cement. The process spews CO2, but the researchers envision capturing and sequestering it. Their accounting shows that even when they add in the energy costs of making CaO and mixing it with magnesium silicates, the total energy needed to capture CO2 is only about half that for DAC. The cost of CO2 removal would likely be about $100 per ton, a widely held CO2 removal target.

Even better, Kanan argues, farmers might be willing to pay for the CO2 sequestering materials. Farmers around the world already add about 1 billion tons of calcium carbonate and other alkaline materials to soils to lessen acidity and help their crops grow. Substituting calcium silicate and magnesium oxide would perform the same acid-neutralizing function and suck CO2 out of the air as a bonus. Before that’s done at scale, Kanan says researchers need to run field trials to ensure any impurities in the rocks, such as traces of nickel and other metals, don’t harm soil ecosystems. Kanan says he and his colleagues have already initiated such trials in Louisiana and New Jersey.