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 favored 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.
Industrial use
Traditional cement manufacture releases large amounts of carbon dioxide, but newly developed cement types from Novacem can absorb CO2 from ambient air during hardening. A similar technique was pioneered by TecEco, which has been producing "EcoCement" since 2002.
In Estonia, oil shale ash, generated by power stations could be used as sorbents for CO2 mineral sequestration. The amount of CO2 captured averaged 60–65% of the carbonaceous CO2 and 10–11% of the total CO2 emissions.
Chemical scrubbers
Various carbon dioxide scrubbing processes have been proposed to remove CO2 from the air, usually using a variant of the Kraft process. Carbon dioxide scrubbing variants exist based on potassium carbonate, which can be used to create liquid fuels, or on sodium hydroxide. These notably include artificial trees proposed by Klaus Lackner to remove carbon dioxide from the atmosphere using chemical scrubbers.
Basalt storage
Carbon dioxide sequestration in basalt involves the injecting of CO2 into deep-sea formations. The CO2 first mixes with seawater and then reacts with the basalt, both of which are alkaline-rich elements. This reaction results in the release of Ca2+ and Mg2+ ions forming stable carbonate minerals.
Underwater basalt offers a good alternative to other forms of oceanic carbon storage because it has a number of trapping measures to ensure added protection against leakage. These measures include “geothermal, sediment, gravitational and hydrate formation.” Because CO2 hydrate is denser than CO2 in seawater, the risk of leakage is minimal. Injecting the CO2 at depths greater than 2,700 meters (8,858 ft) ensures that the CO2 has a greater density than seawater, causing it to sink.
One possible injection site is Juan de Fuca plate. Researchers at the Lamont-Doherty Earth Observatory found that this plate at the western coast of the United States has a possible storage capacity of 208 gigatons. This could cover the entire current U.S. carbon emissions for over 100 years.
Acid neutralisation
Adding crushed limestone or volcanic rock] to oceans enhances the solubility pump, which naturally removes CO2 from the atmosphere. Various other scientists have explored this technique, and suggested a variety of different bases that added to the ocean, increase CO2 absorption.
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