Chemical Processes
Carbon, in the form of CO2 can be removed from the atmosphere by chemical processes, and stored in stable carbonate mineral forms. This process is known as 'carbon sequestration by mineral carbonation' or mineral sequestration. The process involves reacting carbon dioxide with abundantly available metal oxides–either magnesium oxide (MgO) or calcium oxide (CaO)–to form stable carbonates. These reactions are exothermic and occur naturally (e.g., the weathering of rock over geologic time periods).
- CaO + CO2 → CaCO3
- MgO + CO2 → MgCO3
Calcium and magnesium are found in nature typically as calcium and magnesium silicates (such as forsterite and serpentinite) and not as binary oxides. For forsterite and serpentine the reactions are:
- Mg2SiO4 + 2CO2 = 2MgCO3 + SiO2
- Mg3Si2O5(OH)4+ 3CO2 = 3MgCO3 + 2SiO2 + 2H2O
The following table lists principal metal oxides of Earth's crust. Theoretically up to 22% of this mineral mass is able to form carbonates.
| Earthen Oxide | Percent of Crust | Carbonate | Enthalpy change (kJ/mol) |
|---|---|---|---|
| SiO2 | 59.71 | ||
| Al2O3 | 15.41 | ||
| CaO | 4.90 | CaCO3 | -179 |
| MgO | 4.36 | MgCO3 | -117 |
| Na2O | 3.55 | Na2CO3 | |
| FeO | 3.52 | FeCO3 | |
| K2O | 2.80 | K2CO3 | |
| Fe2O3 | 2.63 | FeCO3 | |
| 21.76 | All Carbonates |
These reactions are slightly more favorable at low temperatures. This process occurs naturally over geologic time frames and is responsible for much of the Earth's surface limestone. The reaction rate can be made faster, for example by reacting at higher temperatures and/or pressures, or by pre-treatment, although this method requires additional energy.
CO2 naturally reacts with peridotite rock in surface exposures of ophiolites, notably in Oman. It has been suggested that this process can be enhanced to carry out natural mineralisation of CO2.
Read more about this topic: Carbon Sequestration
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