Electrolytic seawater mineralization and how it ensures (net) carbon dioxide removal

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^TM” (formerly known as “SeaChange”) process. The process involves the application of an 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), magnesium carbonates (MgCO₃), and/or magnesium hydroxide (Mg(OH)₂) depending on the CO₃^2– activity in solution. This results in the trapping, 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 up to ≈4.6 kg of CO₂ per m³ of seawater processed. Geochemical simulations quantify the extents of net CO₂ removal including the dependencies on the process configuration. It is furthermore indicated that the efficiency of realkalinization of the acidic anolyte using alkaline solids depends on their H⁺ neutralization capacity and dissolution reactivity. We also assess changes in seawater chemistry resulting from Mg(OH)₂ dissolution with emphasis on the change in its alkalinity and saturation state. Overall, this analysis provides direct quantifications of the ability of the Equatic^TM process to serve as a means for technological CDR to mitigate the worst effects of accelerating climate change.