The gigaton-scale challenge of CO₂ removal by using natural silicates
The Intergovernmental Panel on Climate Change (IPCC) has repeatedly emphasized that limiting global warming to 1.5 ºC will require removing hundreds of gigatons (Gt) of CO₂ from the atmosphere by 2100. While technologies like Direct Air Capture (DAC) have gained traction, their energy demands (1.8–2.7 MWh per ton of CO₂) and costs remain prohibitive for large-scale deployment.
Mg-rich silicates, such as olivine (Mg₂SiO₄) and serpentine (Mg₃Si₂O₅(OH)₄), have vast geological reserves (>10⁵ gigatons) and high theoretical CO₂ sequestration capacity. However, their natural weathering processes are kinetically limited, requiring centuries to millennia to react with CO₂ to form stable carbonates (CO₃²⁻) or bicarbonates (HCO₃⁻). The extremely slow reaction rates under ambient conditions renders them ineffective for practical, large-scale CO₂ removal within the urgent timescales needed to address climate change.
Using natural silicates for carbon dioxide removal (CDR) on the multi-gigaton scale requires finding ways to accelerate the weathering process.
Various CDR efforts involve trying to enhance weathering by grinding silicates to ultrafine powders and spreading them in soils or aquatic environments. Although these approaches are simple, they are energy-intensive (>400 kWh per ton) and pose health risks due to airborne particulate matter.
Compared with Mg-rich silicates, Mg(OH)₂ is much more reactive with CO₂ in ambient air. It has been shown that wetted, large particles of MgO, which form Mg(OH)₂ spontaneously, react with CO₂ in ambient air at rates corresponding to 18% carbonation in 1 year (ref.). MgO particles are also used for Direct Ocean Capture (DOC). For example, Planetary Technologies developed a floating platform which mixes MgO particles and seawater to form Mg(OH)₂ solution. The Mg(OH)₂ solution is safely added to seawater to sequester dissolved CO₂ in seawater.
Therefore, the main challenge of using Mg-rich silicates for CDR on a gigaton scale is to release the alkalinity (that is, MgO) that is effectively trapped in the silicate lattice using as little energy and resources as possible.
The breakthrough science: liberating trapped alkalinity in natural silicates
The Stanford University team introduces a novel thermochemical process to transform inert magnesium-rich silicates into highly reactive materials to accelerate the weathering process for gigaton-scale CO₂ removal. This innovation could revolutionize CDR by leveraging Earth’s vast mineral reserves while cutting energy use in half compared to leading DAC systems. Their breakthrough study was published in Nature.
The team used a calcium oxide (CaO) source (CaCO₃ or CaSO₄) to react quantitatively with inert Mg-rich silicates to form CDR material, that is Ca-enriched silicates (Ca₂SiO₄) and reactive MgO. The method is depicted in the diagram below.
The synthesis of CRD material involves two-step thermal process:
Step 1: calcination:
Calcium carbonate (CaCO₃) or calcium sulfate (CaSO₄) is heated to ~900 ºC to produce CaO, releasing CO₂ that is captured for sequestration.
CaCO₃ → CaO + CO₂
Step 2: ion exchange
At 1,200 ºC, CaO reacts with Mg-silicates in a 2:1 molar ratio, displacing Mg²⁺ ions and forming calcium silicate (Ca₂SiO₄) and magnesium oxide (MgO).
2CaO + Mg₂SiO₄ → Ca₂SiO₄ + 2MgO
This reaction, termed “Ca²⁺/Mg²⁺ exchange,” converts structurally locked Mg into free MgO—a material far more reactive toward CO₂. Crucially, the process also generates Ca₂SiO₄, a calcium silicate that rapidly carbonates under ambient conditions.
To overcome slow solid-state ion diffusion, the team introduced molten salts like Na₂SO₄ as fluxes. These additives lower reaction temperatures and accelerate kinetics, enabling complete conversion of silicates into CDR material (Ca₂SiO₄ + MgO) in just 1 hour at 1,200 ºC. Without fluxes, reactions remained incomplete even after 4 hours.
The team validated their process at multi-kilogram scales, producing 2 kg of CDR material (Ca₂SiO₄ + MgO) in a single batch using industrial-grade furnaces. This scalability suggests compatibility with existing cement or steel infrastructure, where high-temperature kilns are already commonplace.
Carbonation performance
The synthesized CDR material (Ca₂SiO₄ + MgO) is then deployed to remove CO₂ from air or CO₂-enriched environments (for example, soil) to form CaCO₃ and MgCO₃ (terrestrial storage) or soluble Ca(HCO₃)₂ and Mg(HCO₃)₂ (aquatic storage), as depicted in the diagram below.
For every ton of CDR material produced:
- 4 tons of CO₂ are removed as carbonates.
- 8 tons of CO₂ are removed as bicarbonates if deployed in aquatic environments.
In ambient air, wet Ca₂SiO₄ converts to CaCO₃ and silicic acid within weeks, while MgO partially carbonates to hydromagnesite (Mg₅(CO₃)₄(OH)₂). Full carbonation of Ca₂SiO₄ is achieved in 50 days, with MgO lagging due to its lower alkalinity.
By contrast, raw olivine shows no carbonation after six months in air.
Energy and cost analysis: A game-changer?
The study estimates an energy demand of 0.6–1.3 MWh per ton of CO₂ removed, depending on whether bicarbonates (aquatic storage) or carbonates (terrestrial storage) are the end products. This is roughly 50% lower than state-of-the-art DAC systems. Key factors include:
The calcination dominates energy use. Decomposing CaCO₃ accounts for ~75% of energy input. However, recycling 40% of waste heat from kilns reduces energy needs. Using oxyfuel combustion (burning methane in pure O₂) or renewable electricity could mitigate emissions. Optimizing reactions for lower temperatures or using alternative fluxes could further reduce energy use.
If CaSO₄ replaces CaCO₃, sulfuric acid (H₂SO₄) is produced—a valuable chemical for fertilizers or industrial processes.
Open questions
While promising, several hurdles remain:
- High-temperature requirements: Operating kilns at 1,200 ºC demands significant energy, though this is comparable to cement production. Transitioning to renewables is critical.
- Iron byproducts: Reactions with Fe-containing silicates (e.g., olivine with Fe²⁺) yield Ca₂Fe₂O₅, which has unclear environmental impacts if deployed at scale.
- Material deployment: Spreading CDR materials in soils or oceans requires careful monitoring. Excessive Mg(HCO₃)₂ could alter local pH or ecosystems.
- Economic viability: While energy costs are lower, mining, grinding, and transporting silicates add logistical expenses.
Conclusion
The Stanford study represents a paradigm shift in carbon management. By transforming inert silicates into reactive CDR materials, this approach taps into a virtually unlimited CO₂ sequestration resource while sidestepping the energy bottlenecks of current technologies. If scaled, it could remove tens of gigatons annually—making it a cornerstone of climate mitigation strategies.
However, success hinges on industrial collaboration, policy support, and continued innovation. As the authors note, “The chemistry described here could unlock Mg-rich silicates as a vast resource for safe and permanent CDR.” In a race against time, this breakthrough offers a much-needed tool for turning the tide on climate change.
For further reading, refer to the original article: Chen, Y. & Kanan, M.W. Thermal Ca²⁺/Mg²⁺ exchange reactions to synthesize CO₂ removal materials. Nature 638, 972–979 (2025).
Patent
WO2024112368A3 Methods for transforming silicate-containing materials into materials for co2 removal and other applications
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