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 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 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.

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