Thermal Ca^2+/Mg^2+ Exchange Reactions to Transform Abundant Silicates Into Alkaline Materials for Carbon Dioxide Removal

The removal of CO2 from the atmosphere (CDR) on the multi-hundred gigaton (Gton) scale is essential for nearly all strategies to achieve net-zero greenhouse gas emissions and limit global warming to 2°C by 2100. CDR must capture CO2 from air and safely sequester it. Mg-rich silicate minerals have the capacity to remove ~10^5 Gton CO2 and sequester it in the form of indefinitely stable and innocuous Mg carbonates, but their carbonation rates in air are far too slow for practical use. Here we show that simple thermochemical Ca^2+/Mg^2+ exchange reactions convert CaO and diverse Mg silicates (olivine, serpentine, augite) into Ca2SiO4 and MgO. While the input mineral shows no carbonation in ambient air over 6 months, Ca2SiO4 completely carbonates to form CaCO3 and SiO2 within weeks. Although MgO carbonates at a slower rate in air, it can be fully carbonated under 1 atm CO2 at ambient temperature within hours. By combining it with CaCO3 calcination to generate CaO, this chemistry enables a closed cycle for CDR wherein the Ca2SiO4 generated by Ca^2+/Mg^2+ exchange is used to capture CO2 from air and MgO is used to recapture the CO2 released by calcination. Analysis of the thermal requirements indicates that this cycle could be performed with an energy demand per ton CO2 removed that is comparable to the energy demand for just capturing CO2 using direct air capture (DAC) technologies. We also demonstrate analogous transformations using CaSO4 as the CaO source. The chemistry described here could unlock the use of Mg-rich silicates as a vast resource for safe, permanent, and verifiable CDR.