Enhanced weathering refers to geoengineering approaches that use the dissolution of natural or artificially created minerals to remove carbon dioxide from the atmosphere. Since the carbon dioxide is usually first removed from ocean water, these approaches would attack the problem by first reducing ocean acidification.
Weathering and ocean alkalinity
Weathering is the natural process in which rocks are broken down and dissolved on the land surface. When silicate or carbonate minerals dissolve in rainwater, carbon dioxide is drawn into the solution from the atmosphere through the reactions below (Eq.1&2) to form bicarbonate ions:
Eq.1 Forsterite: Mg2SiO4 + 4CO2 + 4H2O → 2Mg2+ + 4HCO3− + H4SiO4
Eq.2 Calcite : CaCO3 + CO2 + H2O → Ca2+ + 2HCO3−
Rainwater and bicarbonate ions eventually end up in the ocean, where they are formed into carbonate minerals by calcifying organisms (Eq.3), which then sinks out of the surface ocean. Most of the carbonate is redissolved in the deep ocean as it sinks.
Eq.3 Ca2+ + 2HCO3− → CaCO3 + CO2 + H2O
Over geological time periods these processes are thought to stabilise the Earth's climate. For silicate weathering the theoretical net effect of dissolution and precipitation is 1 mol of CO2 sequestered for every mol of Ca2+ or Mg2+ weathered out of the mineral. Given that some of the dissolved cations react with existing alkalinity in the solution to form CO32− ions, the ratio is not exactly 1:1 in natural systems but is a function of temperature and CO2 partial pressure. The net CO2 sequestration of carbonate weathering (Eq.2) and carbonate precipitation (Eq.3) is zero.
Weathering and biological carbonate precipitation are thought to be only loosely coupled on short time periods (<1000 years). Therefore, an increase in both carbonate and silicate weathering with respect to carbonate precipitation will result in a build up of alkalinity in the ocean, n.
Enhanced weathering research considers how these natural processes may be enhanced to sequester CO2 from the atmosphere to be stored in solid carbonate minerals or ocean alkalinity.
Terrestrial enhanced weathering
'Enhanced Weathering' was initially used to refer specifically to the spreading of crushed silicate minerals on the land surface. Biological activity in soils has been shown to promote the dissolution of silicate minerals (see discussion in, but there is still uncertainty surrounding how quickly this may happen. As weathering rate is a function of saturation of the dissolving mineral in solution (decreasing to zero in fully saturated solutions), some have suggested that the quantity of rainfall may limit terrestrial enhanced weathering, although others suggest that secondary mineral formation or biological uptake may suppress saturation and promote weathering.
The amount of energy that is required for comminution depends on rate at which the minerals dissolve (less comminution is required for rapid mineral dissolution). Recent work has suggested a large range in potential cost of enhanced weathering largely down to the uncertainty surrounding mineral dissolution rates.
Oceanic enhanced weathering
To overcome the limitations of solution saturation and to utilise natural comminution of sand particles from wave energy, silicate minerals may be applied to coastal environments, although the higher pH of seawater may substantially decrease the rate of dissolution, and it is unclear how much comminution is possible from wave action.
Alternatively, the direct application of carbonate minerals to the up-welling regions of the ocean has been investigated. Carbonate minerals are supersaturated in the surface ocean but are undersaturated in the deep ocean. In areas of up welling this undersaturated water is brought to the surface. While this technology will likely be cheap, the maximum annual CO2 sequestration potential is limited.
Transforming the carbonate minerals into oxides and spreading this material in the open ocean ('Ocean Liming') has been proposed as an alternative technology. Here the carbonate mineral (CaCO3) is transformed into lime (CaO) through calcination. The energy requirements for this technology are substantial, and additional CO2 would be created.
The enhanced dissolution and carbonation of silicates ('mineral carbonation') was first proposed by Seifritz, and developed initially by Lackner et al. and further by the Albany Research Center. This early research investigated the carbonation of extracted and crushed silicates at elevated temperatures (~180°C) and partial pressures of CO2 (~15 MPa) inside controlled reactors ('Ex-situ mineral carbonation'). Some research explores the potential of 'In-situ mineral carbonation' in which the CO2 is injected into silicate rock formations to promote carbonate formation underground (see: CarbFix)
Mineral carbonation research has largely focused on the sequestration of CO2 from flue gas. It could be used for geoengineering if the source of CO2 was derived from the atmosphere, eg. though direct air capture or biomass-CCS.
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