Enhanced weathering

Enhanced weathering, also termed ocean alkalinity enhancement when proposed for

Enhanced weathering is a chemical approach to remove carbon dioxide involving land- or ocean-based techniques. One example of a land-based enhanced weathering technique is in-situ carbonation of silicates. Ultramafic rock, for example, has the potential to store from hundreds to thousands of years' worth of CO₂ emissions, according to estimates.^[2][3] Ocean-based techniques involve alkalinity enhancement, such as grinding, dispersing, and dissolving olivine, limestone, silicates, or calcium hydroxide to address ocean acidification and CO₂ sequestration.^[4]

Although existing mine tailings^[5] or alkaline industrial silicate minerals (such as steel slags, construction & demolition waste, or ash from biomass incineration) may be used at first,^[6] mining more basalt might eventually be required to limit climate change.^[7]

History

Enhanced weathering has been proposed for both terrestrial and ocean-based carbon sequestration. Ocean methods are being tested by the non-profit organization Project Vesta to see if they are environmentally and economically viable.^[8][9]

In July 2020, a group of scientists assessed that the geo-engineering technique of enhanced rock weathering, i.e., spreading finely crushed basalt on fields – has potential use for carbon dioxide removal by nations, identifying costs, opportunities, and engineering challenges.^[10][11]

Natural mineral weathering and ocean acidification

Weathering is the natural process of rocks and minerals dissolving to the action of water, ice, acids, salts, plants, animals, and temperature changes.^[12] It is mechanical (breaking up rock—also called physical weathering or disaggregation) and chemical (changing the chemical compounds in the rocks).^[12] Biological weathering is a form of weathering (mechanical or chemical) by plants, fungi, or other living organisms.^[12]

Chemical weathering can happen by different mechanisms, depending mainly on the nature of the minerals involved. This includes

Carbonate and silicate minerals are examples of minerals affected by carbonation weathering. When silicate or carbonate minerals are exposed to rainwater or groundwater, they slowly dissolve due to carbonation weathering: that is the water (H₂O) and carbon dioxide (CO₂) present in the atmosphere form carbonic acid (H₂CO₃) by the reaction:^[12][14]

H₂O + CO₂ → H₂CO₃

This carbonic acid then attacks the mineral to form carbonate ions in solution with the unreacted water. As a result of these two chemical reactions (carbonation and dissolution), minerals, water, and carbon dioxide combine, which alters the chemical composition of minerals and removes CO₂ from the atmosphere. Of course, these are reversible reactions, so if the carbonate encounters H ions from acids, such as in soils, they will react to form water and release CO₂ back to the atmosphere. Applying limestone (a calcium carbonate) to acid soils neutralizes the H ions but releases CO₂ from the limestone^{[clarification needed]}.

In particular, forsterite (a silicate mineral) is dissolved through the reaction:

Mg₂SiO₄(s) + 4H₂CO₃(aq) → 2Mg²⁺(aq) + 4HCO₃⁻(aq) + H₄SiO₄(aq)

where "(s)" indicates a substance in a solid state and "(aq)" indicates a substance in an aqueous solution.

Calcite (a carbonate mineral) is instead dissolved through the reaction:

CaCO₃(s) + H₂CO₃(aq) → Ca²⁺(aq) + 2HCO₃⁻(aq)

Although some of the dissolved bicarbonate may react with soil acids during the passage through the soil profile to groundwater, water with dissolved bicarbonate ions (HCO₃⁻) eventually ends up in the ocean,^[14] where the bicarbonate ions are biomineralized to carbonate minerals for shells and skeletons through the reaction:

Ca²⁺ + 2HCO₃⁻ → CaCO₃ + CO₂ + H₂O

The carbonate minerals then eventually sink from the ocean surface to the ocean floor.^[14] Most of the carbonate is redissolved in the deep ocean as it sinks.

Over geological time periods these processes are thought to stabilize the Earth's climate.^[15] The ratio of carbon dioxide in the atmosphere as a gas (CO₂) to the quantity of carbon dioxide converted into carbonate is regulated by a chemical equilibrium: in case of a change of this equilibrium state, it takes theoretically (if no other alteration is happening during this time) thousands of years to establish a new equilibrium state.^[14]

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 reaction and carbonate precipitation reaction is zero.^{[clarification needed]}

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.^{[clarification needed]}

Terrestrial enhanced weathering

Enhanced weathering was initially used to refer specifically to the spreading of crushed silicate minerals on the land surface.^[16][17] Biological activity in soils has been shown to promote the dissolution of silicate minerals,^[18] but there is still uncertainty surrounding how quickly this may happen. Because weathering rate is a function of saturation of the dissolving mineral in solution (decreasing to zero in fully saturated solutions), some have suggested that lack of rainfall may limit terrestrial enhanced weathering,^[19] although others^[20] 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 the rate at which the minerals dissolve (less comminution is required for rapid mineral dissolution). A 2012 study suggested a large range in potential cost of enhanced weathering largely due to the uncertainty surrounding mineral dissolution rates.^[21]

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,^[22] although the higher pH of seawater may substantially decrease the rate of dissolution,^[23] and it is unclear how much comminution is possible from wave action.

Alternatively, the direct application of carbonate minerals to the upwelling regions of the ocean has been investigated.^[24] Carbonate minerals are supersaturated in the surface ocean but are undersaturated in the deep ocean. In areas of upwelling, 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.^[25] Here the carbonate mineral (CaCO₃) is transformed into lime (CaO) through calcination. The energy requirements for this technology are substantial.

Mineral carbonation

The enhanced dissolution and carbonation of silicates (

Mineral carbonation research has largely focused on the sequestration of CO₂ from

Soil

Where abundant electric surplus electricity is available, the electrolytic dissolution of silicate minerals has been proposed[30] and experimentally shown. The process resembles the weathering of some minerals. In addition, hydrogen produced would be a carbon-negative.^[31]

Cost

In a 2020 techno-economical analysis, the cost of utilizing this method on cropland was estimated at US$80–180 per tonne of CO₂. This is comparable with other methods of removing carbon dioxide from the atmosphere currently available (BECCS (US$100–200 per tonne of CO₂)-

An Irish company named Silicate has run trials in Ireland and in 2023 is running trials in the USA near Chicago. Using concrete crushed down to dust it is scattered on farmland on the ratio 500 tonnes to 50 hectares, aiming to capture 100 tonnes of CO2 per annum from that area. Claiming it improves soil quality and crop productivity, the company sells carbon removal credits to fund the costs. The initial pilot funding comes from prize money awarded to the startup by the THRIVE/Shell Climate-Smart Agriculture Challenge.[36]^[37]

See also

• Olivine#Uses

References