Ocean alkalinity enhancement – avoiding runaway CaCO 3 precipitation during quick and hydrated lime dissolution

• Published in: Biogeosciences
• Main Authors: C. A. Moras, L. T. Bach, T. Cyronak, R. Joannes-Boyau, K. G. Schulz
• Format: Article in Journal/Newspaper
• Language: English
• Published: Copernicus Publications 2022
• Subjects: Ecology; QH540-549.5; Life; QH501-531; Geology; QE1-996.5; Ocean acidification
• Online Access: https://doi.org/10.5194/bg-19-3537-2022; https://doaj.org/article/514c756e88604fff9ee2867ca8e6a052

Ocean alkalinity enhancement (OAE) is a method that can remove carbon dioxide (CO 2 ) from the atmosphere and counteract ocean acidification through the dissolution of alkaline minerals. Currently, critical knowledge gaps exist regarding the dissolution of different minerals suitable for OAE in natural seawater. Of particular importance is to understand how much alkaline mineral can be dissolved before secondary precipitation of calcium carbonate (CaCO 3 ) occurs, since secondary CaCO 3 precipitation reduces the atmospheric CO 2 uptake potential of OAE. Using two types of mineral proposed for OAE, quick lime (CaO) and hydrated lime (Ca(OH) 2 ), we show that both ( < 63 µ m of diameter) dissolved in seawater within a few hours. No CaCO 3 precipitation occurred at a saturation state ( Ω A ) of ∼5 , but CaCO 3 precipitation in the form of aragonite occurred above an Ω A value of 7. This limit is lower than expected for typical pseudo-homogeneous precipitation, i.e. in the presence of colloids and organic matter. Secondary precipitation at low Ω A ( ∼ 7) was the result of heterogeneous precipitation onto mineral surfaces, most likely onto the added CaO and Ca(OH) 2 particles. Most importantly, runaway CaCO 3 precipitation was observed, a condition where significantly more total alkalinity (TA) was removed than initially added. Such runaway precipitation could reduce the OAE CO 2 uptake efficiency from ∼ 0.8 mol of CO 2 per mole of added TA down to 0.1 mol of CO 2 per mole of TA. Runaway precipitation appears to be avoidable by dilution below the critical Ω A threshold of 5, ideally within hours of the mineral additions to minimise initial CaCO 3 precipitation. Finally, OAE simulations suggest that for the same Ω A threshold, the amount of TA that can be added to seawater would be more than 3 times higher at 5 ∘ C than at 30 ∘ C. The maximum TA addition could also be increased by equilibrating the seawater to atmospheric CO 2 levels (i.e. to a p CO 2 of ∼ 416 µ atm) during addition. This would allow for more TA to ...