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 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, 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 and carbonate precipitation is zero. Weathering and biological carbonate precipitation are thought to be only loosely coupled on short time periods. Therefore, an increase in both carbonate and silicate weathering with respect to carbonate precipitation will result in a buildup of alkalinity in the ocean. 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, 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. 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 use 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 has been proposed as an alternative technology. Here the carbonate mineral is transformed into lime through calcination. The energy requirements for this technology are substantial.
Mineral carbonation
The enhanced dissolution and carbonation of silicates 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 and partial pressures of CO2 inside controlled reactors. 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 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, e.g. through direct air capture or biomass-CCS.
