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 “EquaticTM” (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 (Mg–CO3), and/or magnesium hydroxide (Mg(OH)2) depending on the CO32– 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 (CO32–) 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 EquaticTM process to serve as a means for technological CDR to mitigate the worst effects of accelerating climate change.