Ocean Alkalinity Enhancement – Avoiding runaway CaCO3 precipitation during quick and hydrated lime dissolution

• Main Authors: Moras, Charly Andre, Bach, Lennart Thomas, Cyronak, Tyler, Joannes-Boyau, Renaud, Schulz, Kai Georg
• Format: Text
• Language: English
• Published: 2021
• Subjects: Ocean acidification
• Online Access: https://doi.org/10.5194/bg-2021-330; https://bg.copernicus.org/preprints/bg-2021-330/

Ocean Alkalinity Enhancement (OAE) has been proposed as a method to remove carbon dioxide (CO 2 ) from the atmosphere and to counteract ocean acidification. It involves the dissolution of alkaline minerals such as quick lime, CaO, and hydrated lime, Ca(OH) 2 . However, a critical knowledge gap exists regarding their dissolution in natural seawater. Particularly, how much can be dissolved before secondary precipitation of calcium carbonate (CaCO 3 ) occurs is yet to be established. Secondary precipitation should be avoided as it reduces the atmospheric CO 2 uptake potential of OAE. Here we show that both CaO and Ca(OH) 2 powders (> 63 µm of diameter) dissolved in seawater within a few hours. However, CaCO 3 precipitation, in the form of aragonite, occurred at a saturation (Ω Ar ) threshold of about 5. This limit is much lower than what would be expected for typical pseudo-homogeneous precipitation in the presence of colloids and organic materials. Secondary precipitation at unexpectedly low Ω Ar was the result of so-called heterogeneous precipitation onto mineral phases, most likely onto CaO and Ca(OH) 2 prior to full dissolution. Most importantly, this led to runaway CaCO 3 precipitation by which significantly more alkalinity (TA) was removed than initially added, until Ω Ar reached levels below 2. Such runaway precipitation would reduce the CO 2 uptake efficiency from about 0.8 moles of CO 2 per mole of TA down to only 0.1 mole of CO 2 per mole of TA. Runaway precipitation appears to be avoidable by dilution below the critical Ω Ar threshold of 5, ideally within hours of the addition to minimise initial CaCO 3 precipitation. Finally, model considerations suggest that for the same Ω Ar threshold, the amount of TA that can be added to seawater would be more than three times higher at 5 °C than at 30 °C, and that equilibration to atmospheric CO 2 levels during mineral dissolution would further increase it by a factor of ~6 and ~3 respectively.