Ocean Alkalinity Enhancement – Avoiding runaway CaCO 3 precipitation during quick and hydrated lime dissolution

Abstract

Abstract. Ocean Alkalinity Enhancement (OAE) has been proposed as a method to remove carbon dioxide (CO2) 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 (CaCO3) occurs is yet to be established. Secondary precipitation should be avoided as it reduces the atmospheric CO2 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, CaCO3 precipitation, in the form of aragonite, occurred at a saturation (ΩAr) 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 ΩAr 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 CaCO3 precipitation by which significantly more alkalinity (TA) was removed than initially added, until ΩAr reached levels below 2. Such runaway precipitation would reduce the CO2 uptake efficiency from about 0.8 moles of CO2 per mole of TA down to only 0.1 mole of CO2 per mole of TA. Runaway precipitation appears to be avoidable by dilution below the critical ΩAr threshold of 5, ideally within hours of the addition to minimise initial CaCO3 precipitation. Finally, model considerations suggest that for the same ΩAr 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 CO2 levels during mineral dissolution would further increase it by a factor of ~6 and ~3 respectively.