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https://doi.org/10.5194/egusphere-2024-108
https://doi.org/10.5194/egusphere-2024-108
22 Jan 2024
 | 22 Jan 2024

A comprehensive assessment of electrochemical ocean alkalinity enhancement in seawater: kinetics, efficiency, and precipitation thresholds

Mallory Ringham, Nathan Hirtle, Cody Shaw, Xi Lu, Julian Herndon, Brendan Carter, and Matthew Eisaman

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 CO2 as dissolved bicarbonate (HCO3-). The SEA MATE (Safe Elevation of Alkalinity for the Mitigation of Acidification Through Electrochemistry) process uses electrochemistry to convert some of the salt (NaCl) in seawater or brine into aqueous acid (HCl), which is removed from the system, and base (NaOH), which is returned to the ocean with the remaining seawater. 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 pCO2. The shift in the pCO­2 results in enhanced CO2 uptake or reduced CO2 loss by the seawater due to gas exchange. The net result of this process is the increase of surface-ocean DIC, where it is durably stored as mostly bicarbonate and some carbonate. In this study, we systematically test the efficiency of CO2 uptake in seawater treated with NaOH at beaker (1 L), aquaria (15 L), and tank (6000 L) scales to establish operational boundaries for safety and efficiency in scaling up to field experiments. Preliminary results show CO2 equilibration occurred on 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/ 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 Ωaragonite exceeded 30.0. This precipitation was dominated by Mg(OH)2 over hours to 1 day before shifting to CaCO3, aragonite precipitation. These data, combined with models of the dilution and advection of alkaline plumes, will allow for 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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Mallory Ringham, Nathan Hirtle, Cody Shaw, Xi Lu, Julian Herndon, Brendan Carter, and Matthew Eisaman

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2024-108', Anonymous Referee #1, 29 Feb 2024
    • AC1: 'Reply on RC1', Mallory Ringham, 26 Mar 2024
  • RC2: 'Comment on egusphere-2024-108', Anonymous Referee #2, 04 Mar 2024
    • AC2: 'Reply on RC2', Mallory Ringham, 26 Mar 2024

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2024-108', Anonymous Referee #1, 29 Feb 2024
    • AC1: 'Reply on RC1', Mallory Ringham, 26 Mar 2024
  • RC2: 'Comment on egusphere-2024-108', Anonymous Referee #2, 04 Mar 2024
    • AC2: 'Reply on RC2', Mallory Ringham, 26 Mar 2024
Mallory Ringham, Nathan Hirtle, Cody Shaw, Xi Lu, Julian Herndon, Brendan Carter, and Matthew Eisaman
Mallory Ringham, Nathan Hirtle, Cody Shaw, Xi Lu, Julian Herndon, Brendan Carter, and Matthew Eisaman

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Latest update: 22 Jun 2024
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Short summary
Ocean alkalinity enhancement leverages the large surface area and carbon storage capacity of the oceans to store atmospheric CO2 as dissolved bicarbonate. We monitored CO2 uptake in seawater treated with NaOH to establish operational boundaries for carbon removal experiments. Preliminary results show CO2 equilibration occurred on order of weeks to months, was consistent with values expected from equilibration calculations, and was limited by mineral precipitation at high pH and CaCO3 saturation.