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Cited by 49Marine and coastal ecosystemsCO2 Sequestration and Geologic InteractionsClimate Change and Geoengineering

An assessment of ocean alkalinity enhancement using aqueous hydroxides: kinetics, efficiency, and precipitation thresholds

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Ringham, Mallory C., Hirtle, Nathan, Shaw, Cody, Lu, Xi, Herndon, Julian, Carter, Brendan R., Eisaman, Matthew D.·2024·Biogeosciences
DOI 10.5194/bg-21-3551-2024

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Abstract

Ocean alkalinity enhancement (OAE) is a promising approach to marine carbon dioxide removal (mCDR) that leverages the large surface area and carbon storage capacity of the oceans to sequester atmospheric CO 2 as dissolved bicarbonate (HCO3-). One OAE method involves the conversion of salt in seawater into aqueous alkalinity (NaOH), which is returned to the ocean. The resulting increase in seawater pH and alkalinity causes a shift in dissolved inorganic carbon (DIC) speciation toward carbonate and a decrease in the surface ocean p CO 2 . The shift in the p CO 2 results in enhanced uptake of atmospheric CO 2 by the seawater due to gas exchange. In this study, we systematically test the efficiency of CO 2 uptake in seawater treated with NaOH at aquarium (15 L) and tank (6000 L) scales to establish operational boundaries for safety and efficiency in advance of scaling up to field experiments. CO 2 equilibration occurred on the order of weeks to months, depending on circulation, air forcing, and air bubbling conditions within the test tanks. An increase of ∼0.7–0.9 mol DIC per mol added alkalinity (in the form of NaOH) was observed through analysis of seawater bottle samples and pH sensor data, consistent with the value expected given the values of the carbonate system equilibrium calculations for the range of salinities and temperatures tested. Mineral precipitation occurred when the bulk seawater pH exceeded 10.0 and Ω aragonite exceeded 30.0. This precipitation was dominated by Mg(OH) 2 over hours to 1 d before shifting to CaCO 3,aragonite precipitation. These data, combined with models of the dilution and advection of alkaline plumes, will allow the estimation of the amount of carbon dioxide removal expected from OAE pilot studies. Future experiments should better approximate field conditions including sediment interactions, biological activity, ocean circulation, air–sea gas exchange rates, and mixing zone dynamics.

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  • Once reaching a pH of ~10.5, Mg(OH)2 precipitates concomitantly with alkalinity increase.

    “Once reaching a pH of ~10.5, Mg(OH)2 precipitates concomitantly with alkalinity increase. 30-34,43”

    From A framework for understanding efficiency losses of Ocean Alkalinity Enhancement (2026) · cites this paper as [43]

    Section 3.3 reports Mg(OH)₂ precipitation immediately after NaOH addition, including in experiments with bulk pH of 10.0–10.3. It does not establish ~10.5 as the precipitation threshold, and it reports that total alkalinity decreased during sustained precipitation rather than increasing with it. Page numbers were not supplied for these passages. (LLM, conf=0.86)

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Cited by

Sorted by citations

Authors
Ringham, M. C., Hirtle, N., Shaw, C., Lu, X., Herndon, J., Carter, B. R., & Eisaman, M. D.
Affiliations
  • Stony Brook University (US)
  • National Oceanic and Atmospheric Administration (US)
  • Yale University (US)
ORCID
Venue
Biogeosciences
Publisher
Copernicus GmbH
Published
2024-08-09
Volume
21
Issue
15
Pages
3551-3570
ISSN
  • 1726-4170 (ISSN-L)
  • 1726-4189
DOI
10.5194/bg-21-3551-2024
OpenAlex
W4401459392
Cited by
49
References
7
Citations by year
2026: 29 · 2025: 15 · 2024: 5
FWCI
9.63
Type
journal article
Language
en
Open access
Licence
Locations
  • Biogeosciences (publishedVersion, cc-by)
  • Informationszentrum Lebenswissenschaften (ZB MED) (submittedVersion, cc-by)
  • NOAA Institutional Repository (submittedVersion, cc-by)
  • DOAJ (DOAJ: Directory of Open Access Journals) (submittedVersion, cc-by-sa)
  • OSTI OAI (U.S. Department of Energy Office of Scientific and Technical Information) (submittedVersion)
Funders
  • Grantham Foundation for the Protection of the Environment (SEA MATE (Safe Elevation of Alkalinity for the Mitigation of Acidification Through Electrochemistry))
  • Brookhaven National Laboratory (DE-SC0012704)
  • Cooperative Institute for Climate, Ocean, and Ecosystem Studies, University of Washington (NA20OAR4320271)
Topics
  • Physical Sciences › Earth and Planetary Sciences › Oceanography › Marine and coastal ecosystems
  • Physical Sciences › Environmental Science › Environmental Engineering › CO2 Sequestration and Geologic Interactions
  • Physical Sciences › Environmental Science › Global and Planetary Change › Climate Change and Geoengineering
Keywords
ocean alkalinity enhancement, marine carbon dioxide removal, sodium hydroxide, seawater alkalinity, seawater pH, aragonite precipitation, magnesium hydroxide precipitation, calcium carbonate precipitation, CO2 uptake, air-sea gas exchange

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