Enhanced weathering

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.^[1] 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 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.^[2][3] Biological activity in soils has been shown to promote the dissolution of silicate minerals (see discussion in,^[4] 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,^[5] although others^[6] 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^[7] 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,^[8] although the higher pH of seawater may substantially decrease the rate of dissolution,^[9] 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.^[10] 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.^[11] 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.

Mineral carbonation

The enhanced dissolution and carbonation of silicates ('mineral carbonation') was first proposed by Seifritz,^[12] and developed initially by Lackner et al.^[13] and further by the Albany Research Center.^[14] 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, e.g. through direct air capture or biomass-CCS.

See also

• Olivine#Uses

References

1. Berner, Robert A. Berner; Kothavala, Zavareth (2001). "GEOCARB III: A revised model of atmospheric CO₂ over Phanerozoic time". American Journal of Science 301 (2): 182–204. doi:10.2475/ajs.301.2.182.
2. Schuiling, R. D.; Krijgsman, P. (2006). "Enhanced Weathering: An Effective and Cheap Tool to Sequester CO₂". Climatic Change 74: 349–54. doi:10.1007/s10584-005-3485-y.
3. Manning, D. A. C. (2008). "Biological enhancement of soil carbonate precipitation: Passive removal of atmospheric CO₂". Mineralogical Magazine 72 (2): 639–49. doi:10.1180/minmag.2008.072.2.639.
4. Manning, David A. C.; Renforth, Phil (2013). "Passive Sequestration of Atmospheric CO₂ through Coupled Plant-Mineral Reactions in Urban soils". Environmental Science & Technology 47 (1): 135–41. Bibcode:2013EnST...47..135M. doi:10.1021/es301250j. PMID 22616942.
5. Köhler, Peter; Hartmann, Jens; Wolf-Gladrow, Dieter A.; Schellnhuber, Hans-Joachim (2010). "Geoengineering potential of artificially enhanced silicate weathering of olivine". Proceedings of the National Academy of Sciences 107 (47): 20228–33. Bibcode:2010EGUGA..12.6986K. doi:10.1073/pnas.1000545107. JSTOR 25756680. PMC 2996662. PMID 21059941.
6. Schuiling, Roelof D.; Wilson, Siobhan A.; Power, lan M. (2011). "Enhanced silicate weathering is not limited by silicic acid saturation". Proceedings of the National Academy of Sciences 108 (12): E41. Bibcode:2011PNAS..108E..41S. doi:10.1073/pnas.1019024108. PMC 3064366. PMID 21368192.
7. Renforth, P. (2012). "The potential of enhanced weathering in the UK". International Journal of Greenhouse Gas Control 10: 229–43. doi:10.1016/j.ijggc.2012.06.011.
8. Schuiling, R.D.; de Boer, P.L. (2010). "Coastal spreading of olivine to control atmospheric CO₂ concentrations: A critical analysis of viability. Comment: Nature and laboratory models are different". International Journal of Greenhouse Gas Control 4 (5): 855–6. doi:10.1016/j.ijggc.2010.04.012.
9. Hangx, Suzanne J.T.; Spiers, Christopher J. (2009). "Coastal spreading of olivine to control atmospheric CO₂ concentrations: A critical analysis of viability". International Journal of Greenhouse Gas Control 3 (6): 757–67. doi:10.1016/j.ijggc.2009.07.001.
10. Harvey, L. D. D. (2008). "Mitigating the atmospheric CO₂ increase and ocean acidification by adding limestone powder to upwelling regions". Journal of Geophysical Research 113. Bibcode:2008JGRC..113.4028H. doi:10.1029/2007JC004373.
11. Kheshgi, Haroon S. (1995). "Sequestering atmospheric carbon dioxide by increasing ocean alkalinity". Energy 20 (9): 915–22. doi:10.1016/0360-5442(95)00035-F.
12. Seifritz, W. (1990). "CO₂ disposal by means of silicates". Nature 345 (6275): 486. Bibcode:1990Natur.345..486S. doi:10.1038/345486b0.
13. Lackner, Klaus S.; Wendt, Christopher H.; Butt, Darryl P.; Joyce, Edward L.; Sharp, David H. (1995). "Carbon dioxide disposal in carbonate minerals". Energy 20 (11): 1153. doi:10.1016/0360-5442(95)00071-N.
14. O’Connor, W. K.; Dahlin, D. C.; Rush, G. E.; Gedermann, S. J.; Penner, L. R.; Nilsen, D. N. (March 15, 2005). Aqueous mineral carbonation, Final Report (PDF). National Energy Technology Laboratory.