10 Dec 2021
10 Dec 2021
Status: a revised version of this preprint was accepted for the journal BG.

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

Charly Andre Moras1, Lennart Thomas Bach2, Tyler Cyronak3, Renaud Joannes-Boyau1, and Kai Georg Schulz1 Charly Andre Moras et al.
  • 1Faculty of Science and Engineering, Southern Cross University, Lismore, NSW, Australia
  • 2Institute for Marine and Antarctic Studies, Ecology & Biodiversity, University of Tasmania, Hobart, TAS, Australia
  • 3Department of Marine and Environmental Sciences, Nova Southeastern University, Fort Lauderdale, FL, USA

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.

Charly Andre Moras et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on bg-2021-330', A. Mucci, 25 Dec 2021
  • RC2: 'Comment on bg-2021-330', Anonymous Referee #2, 30 Dec 2021
  • CC1: 'Comment on bg-2021-330', Eyal Wurgaft, 31 Dec 2021

Charly Andre Moras et al.

Charly Andre Moras et al.


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Short summary
This research presents the first laboratory results of quick and hydrated lime dissolution in natural seawater. These two minerals are of great interest for Ocean Alkalinity Enhancement, a strategy aiming to decrease atmospheric CO2 concentrations. Following the dissolution of these minerals, we identified several hurdles and presented ways to avoid them or completely negate them. Finally, we proceeded to various simulations in today’s oceans to implement the strategy at its highest potential.