Silicate rocks are rich in cations such as calcium and magnesium. When these cations dissolve, they increase alkalinity and through a series of chemical reactions, converts CO₂ into dissolved inorganic carbon, predominantly, bicarbonate ions (HCO₃ -) (Eq. 1). One mole of Ca₂+ results in 2 moles of CO₂ removed.
1) CaSiO₃ + 2CO₂ + 3H₂O -> H₄SiO₄ + 2HCO₃⁻ + Ca²⁺
The intention of ERW is to promote this mineral dissolution, whereby the weathering products are then transported to the oceans where they increase ocean alkalinity and in turn, drawdown atmospheric CO₂. (see further reading for ocean alkalinity - CDR relationship).
Depending on the conditions in the soil and drainage waters, precipitation of solid carbonates can occur releasing one mole of CO₂ in the process (Eq.2). While this is still net removal, CDR is maximised when precipitation is limited and instead, the weathering products are delivered to the ocean via continental runoff.
2) Ca²⁺ + 2HCO₃⁻ -> CaCO₃ + CO₂ +H₂O
In general, ERW is most efficient in areas with high rainfall, warmer temperatures and slightly acidic soils. Even in ideal conditions, there will still be some CO₂ leakage due to the precipitation of carbonates and pH-dependent equilibration while the alkalinity makes its way to the ocean.
This process doesn’t come without its risks. The weathering of silicate rocks is often associated with the release of heavy metals, particularly nickel and chromium. These heavy metals can contaminate the soils, surface waters, and crops. There are currently no environmental regulations regarding heavy metals in the context of ERW however projects can monitor the amount of heavy metals going into the system, or their bioavailability in the soil to keep them to safe levels.
