02 Jun 2022
02 Jun 2022
Status: this preprint is currently under review for the journal BG.

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

Jens Hartmann1,, Niels Suitner1,, Carl Lim2, Julieta Schneider3, Laura Marín-Samper4, Javier Arístegui4, Phil Renforth5, Jan Taucher3, and Ulf Riebesell3 Jens Hartmann et al.
  • 1Institute for Geology, Universität Hamburg, Bundesstrasse 55, D-20146 Hamburg, Germany
  • 2Faculty of Physics/Electrical Engineering, Universität Bremen, Otto-Hahn-Allee 1, 28359 Bremen, Germany
  • 3Geomar Helmholtz Centre for Ocean Research Kiel, Kiel, Germany
  • 4Instituto de Oceanografía y Cambio Global, Universidad de Las Palmas de Gran Canaria, Las Palmas, Spain
  • 5School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh, EH14 4AS, UK
  • These authors contributed equally to this work.

Abstract. According to modelling studies, ocean alkalinity enhancement (OAE) is one of the proposed carbon dioxide removal (CDR) approaches with large potential, and 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. Seawater is already supersaturated with respect to calcium carbonate minerals, and an increase in total alkalinity together with a corresponding shift in carbonate chemistry towards higher carbonate ion concentrations would result in 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 2,400 μmol/kgw. The application of CO2-equilibrated alkaline solution bears the lowest risk of losing alkalinity due to carbonate formation if added total alkalinity (ΔTA) is less than 2,400 μmol/kgw. The addition of reactive alkaline solids can cause a net loss of alkalinity if ΔTA > 600 μmol/kgw (e.g., for Mg(OH)2). Commercially available 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 supplied from ships. The application of excessive amounts of Ca(OH)2, exceeding a threshold for alkalinity loss, resulted in a massive increase in TA (> 20,000 μmol/kgw) at the cost of lower efficiency and resultant high pH values > 9.5.

Our results indicate that using an alkaline solution instead of reactive alkaline particles can avoid carbonate formation, unless alkalinity addition shifts the system beyond critical supersaturation levels. To avoid the loss of alkalinity and dissolved inorganic carbon (DIC) from seawater, the application of reactor techniques can be considered. These techniques produce an equilibrated solution from alkaline solids and CO2 prior to application. Differing behaviours of tested materials suggest that standardized engineered materials for OAE need to be developed to achieve safe and sustainable OAE with solids, if reactors technologies should be avoided.

Jens Hartmann 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-2022-126', olivier sulpis, 26 Jun 2022
  • RC2: 'Comment on bg-2022-126', Scott C. Doney, 09 Jul 2022
    • RC3: 'Reply on RC2', Scott C. Doney, 10 Jul 2022
      • AC4: 'Reply on RC3', Jens Hartmann, 27 Sep 2022
    • AC2: 'Reply on RC1 and RC2', Jens Hartmann, 25 Sep 2022
    • AC3: 'Reply on RC2', Jens Hartmann, 27 Sep 2022

Jens Hartmann et al.

Jens Hartmann et al.


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Short summary
CO2 can be stored in the ocean via increasing alkalinity of ocean water. Alkalinity is created via dissolution of alkaline materials, like limestone or soda. Presented research studies boundaries for increasing alkalinity in seawater. The best way to increase alkalinity was found using an equilibrated solution, as for example produced from reactors. Adding particles for dissolution into seawater on the other hand produces the risk of losing alkalinity and degassing of CO2 to the atmosphere.