Stability of alkalinity in ocean alkalinity enhancement (OAE) approaches – consequences for durability of CO 2 storage

• Published in: Biogeosciences
• Main Authors: J. Hartmann, N. Suitner, C. Lim, J. Schneider, L. Marín-Samper, J. Arístegui, P. Renforth, J. Taucher, U. Riebesell
• Format: Article in Journal/Newspaper
• Language: English
• Published: Copernicus Publications 2023
• Subjects: Ecology; QH540-549.5; Life; QH501-531; Geology; QE1-996.5; Ocean acidification
• Online Access: https://doi.org/10.5194/bg-20-781-2023; https://doaj.org/article/7759dcf8e7324595b96a208b63be5d6e

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 CO 2 in seawater, thereby reducing the efficiency of OAE for CO 2 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 µ mol kgsw −1 . The application of CO 2 -equilibrated alkaline solution bears the lowest risk of losing alkalinity due to carbonate phase formation if added total alkalinity ( Δ TA) is less than 2400 µ mol kgsw −1 . The addition of reactive alkaline solids can cause a net loss of alkalinity if added Δ TA > 600 µ mol 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 µ mol kgsw −1 ) at the cost of lower efficiency and resultant high pH values ...