Defining Enhanced Weathering
Enhanced weathering is a carbon dioxide removal method in which natural chemical weathering of minerals is amplified. Chemical weathering is the breaking down of material through contact with the atmospheric or biologically produced chemicals, without displacement and with little or no movement.
How Does Natural Chemical Weathering Work?
When some special minerals (silicate and carbonate) come in contact with rainwater and carbon dioxide (CO₂), they dissolve in the rainwater, remove CO₂ from the atmosphere and create bicarbonate ions (equations (1) and (2)) ending up in the ocean with the rainwater. They are then formed back into carbonate minerals by calcifying organisms (often to make their shells) in time periods of 1000s of years. An increase in this kind of weathering will therefore result in a build up of bicarbonate ions and with it the alkalinity in the ocean (this counters the effects of ocean acidification) and removes carbon dioxide from the atmosphere storing it in solid carbonate minerals.
Mg₂SiO₄ + 4CO₂ + 4H₂O → 2Mg₂⁺ + 4HCO₃⁻ + H₄SiO₄
CaCO₃ + CO₂ + H₂O → Ca₂⁺ + 2HCO₃⁻
To sequester CO₂ with enhanced weathering, minerals like Olivine are mined, ground into sand and spread on land surface or coastal regions. The minerals then react with the CO₂ in the air and the water from rain over time until completely dissolved.
Over geological time periods - hundreds of thousands to millions of years - these natural processes act as a thermostat stabilising Earth’s climate: Volcanoes emit CO₂, which would heat our planet if natural chemical weathering would not sequester the emitted CO₂ and stabilise Earth’s temperature. Unfortunately this cycle happens in a time frame much larger than the human one and can thus not regulate our current excess of CO₂ which has accumulated since the industrial revolution.
CO₂ Sequestration Rate
The speed in which the minerals sequester CO₂ depends on the type of mineral; sequestration rate increases with finer grinding grades due to higher surface area; increases with higher CO₂ concentrations; and is dependent on the saturation of the dissolving mineral in the solution - if there is not enough rain water this could slow down the sequestration rate.
Due to the current excess of CO₂ in the atmosphere, Earth’s oceans absorb a larger amount than they would naturally. This results in ocean acidification - the ongoing decrease of the ocean’s pH - which has a range of harmful consequences for marine life, such as coral bleaching. Enhanced weathering counters ocean acidification as it increases the amount of bicarbonate (HCO₃⁻) which increases ocean alkalinity.