Seawater Enables High-Quality Carbon Removal

Abstract

We present the mass balances associated with carbon dioxide (CO₂) removal (CDR) using seawater as both the source of reactants, and as the reaction medium via electrolysis following the “Equatic™” (formerly known as “SeaChange”) process (La Plante et al. in ACS EST Eng. https://doi.org/10.1021/acsestengg.3c00004, 2023). This process, extensively detailed in La Plante et al. (Chem Eng. https://doi.org/10.1021/acssuschemeng.0c08561, 2021), involves the application of an electric overpotential that splits water to form H⁺ and OH^– ions, producing acidity and alkalinity, i.e., in addition to gaseous co-products, at the anode and cathode, respectively. The alkalinity that results, i.e., via the “continuous electrolytic pH pump” results in the instantaneous precipitation of calcium carbonate (CaCO₃), hydrated magnesium carbonates (e.g., nesquehonite: MgCO₃·3H₂O, hydromagnesite: Mg₅(CO₃)₄(OH)₂·4H₂O, etc.), and/or magnesium hydroxide (Mg(OH)₂) depending on the CO₃^2– ion-activity in solution. This results in the trap**, and hence durable and permanent (at least ~ 10,000–100,000 years) immobilization of CO₂ that was originally dissolved in water, and that is additionally drawn down from the atmosphere within: (a) mineral carbonates, and/or (b) as solvated bicarbonate (HCO₃^–) and carbonate (CO₃^2–) ions (i.e., due to the absorption of atmospheric CO₂ into seawater having enhanced alkalinity). Taken together, these actions result in the net removal of ≈ 4.6 kg of CO₂ per m³ of seawater catholyte processed. Overall, this analysis provides direct quantifications of the ability of the Equatic™ process to serve as a means for technological CDR to mitigate the worst effects of accelerating climate change.