Thermal Ca2+/Mg2+ exchange reactions to synthesize CO2 removal materials

Abstract

Most current strategies for carbon management require CO₂ removal (CDR) from the atmosphere on the multi-hundred gigatonne (Gt) scale by 2100 (refs. ^1,2,3,4,5). Mg-rich silicate minerals can remove >10⁵ Gt CO₂ and sequester it as stable and innocuous carbonate minerals or dissolved bicarbonate ions^3,6,7. However, the reaction rates of these minerals under ambient conditions are far too slow for practical use. Here we show that CaCO₃ and CaSO₄ react quantitatively with diverse Mg-rich silicates (for example, olivine, serpentine and augite) under thermochemical conditions to form Ca₂SiO₄ and MgO. On exposure to ambient air under wet conditions, Ca₂SiO₄ is converted to CaCO₃ and silicic acid, and MgO is partially converted into a Mg carbonate within weeks, whereas the input Mg silicate shows no reactivity over 6 months. Alternatively, Ca₂SiO₄ and MgO can be completely carbonated to CaCO₃ and Mg(HCO₃)₂ under 1 atm CO₂ at ambient temperature within hours. Using CaCO₃ as the Ca source, this chemistry enables a CDR process in which the output Ca₂SiO₄/MgO material is used to remove CO₂ from air or soil and the CO₂ process emissions are sequestered. Analysis of the energy requirements indicates that this process could require less than 1 MWh per tonne CO₂ removed, approximately half the energy of CO₂ capture with leading direct air capture technologies. The chemistry described here could unlock Mg-rich silicates as a vast resource for safe and permanent CDR.