Stability of alkalinity in Ocean Alkalinity Enhancement (OAE) approaches – consequences for durability of CO2 storage

• Published in: Biogeosciences
• Main Authors: Hartmann, Jens, Suitner, Niels, Lim, Carl, Schneider, Julieta, Marin-Samper, Laura, Aristegui, Javier, Renforth, Phil, Taucher, Jan, Riebesell, Ulf
• Format: Article in Journal/Newspaper
• Language: English
• Published: Copernicus Publications (EGU) 2023
• Subjects: Ocean acidification
• Online Access: https://oceanrep.geomar.de/id/eprint/57648/; https://oceanrep.geomar.de/id/eprint/57648/7/bg-20-781-2023.pdf; https://bg.copernicus.org/articles/20/781/2023/bg-20-781-2023.html; https://doi.org/10.5194/bg-20-781-2023

According to modelling studies, ocean alkalinity enhancement (OAE) is one of the proposed carbon dioxide removal (CDR) approaches with large potential, with the beneficial side effect of counteracting ocean acidification. The real-world application of OAE, however, remains unclear as most basic assumptions are untested. Before large-scale deployment can be considered, safe and sustainable procedures for the addition of alkalinity to seawater must be identified and governance established. One of the concerns is the stability of alkalinity when added to seawater. The surface ocean is already supersaturated with respect to calcite and aragonite, and an increase in total alkalinity (TA) together with a corresponding shift in carbonate chemistry towards higher carbonate ion concentrations would result in a further increase in supersaturation, and potentially to solid carbonate precipitation. Precipitation of carbonate minerals consumes alkalinity and increases dissolved CO2 in seawater, thereby reducing the efficiency of OAE for CO2 removal. In order to address the application of alkaline solution as well as fine particulate alkaline solids, a set of six experiments was performed using natural seawater with alkalinity of around 2400 µmol kgsw−1. The application of CO2-equilibrated alkaline solution bears the lowest risk of losing alkalinity due to carbonate phase formation if added total alkalinity (ΔTA) is less than 2400 µmol kgsw−1. The addition of reactive alkaline solids can cause a net loss of alkalinity if added ΔTA > 600 µmol kgsw−1 (e.g. for Mg(OH)2). Commercially available (ultrafine) Ca(OH)2 causes, in general, a net loss in TA for the tested amounts of TA addition, which has consequences for suggested use of slurries with alkaline solids supplied from ships. The rapid application of excessive amounts of Ca(OH)2, exceeding a threshold for alkalinity loss, resulted in a massive increase in TA (> 20 000 µmol kgsw−1) at the cost of lower efficiency and resultant high pH values > 9.5. Analysis of ...