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 stable and innocuous carbonate minerals or dissolved bicarbonate ions, but their reaction rates under ambient conditions are far too slow for practical and scalable CDR. Here we show that CaO reacts quantitatively with diverse Mg silicates (olivine, serpentine, augite) under thermochemical conditions to form Ca2SiO4 and MgO. Upon exposure to ambient air under wet conditions, Ca2SiO4 is quantitatively converted to CaCO3 and SiO2, and MgO is partially converted into a Mg carbonate within weeks, while the input Mg silicate shows no reactivity over 6 months. The mixture of Ca2SiO4, and MgO can also be completely carbonated to CaCO3 and Mg(HCO3)2 under 1 atm CO2 at ambient temperature within hours. By combining it with CaCO3 calcination to generate CaO, this chemistry enables a new process for CDR wherein the output Ca2SiO4/MgO material is used to remove CO2 from air or soil to form stable (bi)carbonates and the CO2 process emissions are sequestered. Analysis of the energy requirements indicates that this process could provide CDR at less than 1 MWh per ton CO2 removed, approximately half the energy required just to capture CO2 with leading direct air capture 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.