Electrolytic Seawater Mineralization and the Mass Balances That Demonstrate Carbon Dioxide 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. This process, extensively detailed in La Plante E.C.; ACS Sustain. Chem. Eng.2021, 9, ( (3), ), 1073-1089, 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 coproducts, 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 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 ∼4.6 kg of CO₂ per m³ of seawater catholyte 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 acid neutralization capacity and dissolution reactivity. We also assess changes in seawater chemistry resulting from Mg(OH)₂ dissolution with emphasis on the change in seawater alkalinity and saturation state. 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.

Figure 1

(a) The concentration and speciation of CO₂ in seawater (solid curves) and freshwater (dashed curves) in equilibrium with an ambient atmosphere containing 420 ppm of CO₂ (0.042 vol % CO₂). The total dissolved CO₂ is the sum of the concentrations of HCO₃^–, CO₃^2–, and H₂CO₃*. H₂CO₃* represents the sum of CO_2(aq) and true carbonic acid (H₂CO₃). The speciation of CO₂ is calculated using equilibrium constants that vary with temperature and salinity. (b) The detail of (a) for 8 ≤ pH ≤ 9, showing the far greater solubility of CO₂ in seawater than freshwater. (c) The different aqueous species of DIC, including complexes with dissolved cations in seawater, and their relative amounts. The concentrations for each species are given in mol/kg (molal basis).

Figure 2

A schematic of the Equatic process showing major inlet and outlet feeds of the primary steps for CO₂ removal associated with the formation of: carbonate solids and (aqueous) dissolved CO₂ (Cases 1, 2a) and carbonate solids only (Case 2b). The major energy inputs include electrolysis, water processing and pumping, and rock grinding.

Figure 3

(a) The evolution of total dissolved CO₂ (ΣCO₂) and the pH of seawater with increasing Ca²⁺ and Mg²⁺ precipitation as CaCO₃, Mg–CO₃ hydrates, and/or Mg(OH)₂. (b) The equilibration with air of the catholyte effluent for pH values ranging from 9.5 to 13, where the catholyte is depleted of divalent cations and CO₂. The figure shows different extents of pH decrease (red–blue curves) with progressive CO₂ absorption as pCO₂ (gray curves) approaches −3.38 (i.e., atmospheric concentrations). For pCO₂ evolution, increasing darkness of the gray curves corresponds to increasing initial pH of the catholyte effluent.

Figure 4

(a) The total dissolved inorganic carbon (ΣCO₂) in the anolyte following dissolution (alkalinization) of Ca- or Mg-rich solids (e.g., Ca₂SiO₄ or Mg₂SiO₄). The dashed gray line indicates typical oceanic pH. (b) The distribution of inorganic carbon species as a function of the extent of realkalinization, showing the persistence of H₂CO₃* at low(er) pH and HCO₃^– and CO₃^2– at high(er) pH. M–HCO₃^– and M–CO₃^2– represent aqueous HCO₃^– and CO₃^2– complexes formed with Na⁺, Ca²⁺, and Mg²⁺ cations in solution.

Figure 5

Acid neutralization capacity (ANC: i.e., the effluent realkalinization capacity) of diverse alkaline solids (Table 4). While exact abundances are nontrivial to assess, these materials are available at levels ranging from 10s-to-100s of millions (e.g., slags) to 1000s of billions of tonnes (e.g., olivine).

Figure 6

(a) The dissolution rate of Ca and Mg silicates at 25 °C as a function of pH, following Schott et al. (2009). (b) The dependence of the specific surface area on the diameter, assuming monosized spheres. (c) The mass-normalized dissolution rate of forsterite as a function of the specific surface area for select anolyte pH values, calculated from (a) and (b).

Figure 7

Effect of Mg(OH)₂ addition/dissolution on the (a) total dissolved carbon and pH and (b) saturation indices of seawater with respect to aragonite and brucite. The molar effectiveness of Mg(OH)₂ addition for CO₂ removal for each 0.0002 mol of addition of Mg(OH)₂ is also shown. (c) and (d) show plots similar to (a) and (b) for a case where hydromagnesite precipitation occurs when supersaturation is reached. The CO₂ removal factor reduces to 0.8 mol of CO₂ per mol of hydromagnesite (1 mol CO₂ per mol nesquehonite). Similar plots as (a) and (b) for a case where aragonite precipitation occurs in excess of the original saturation index of seawater are shown in (e) and (f).

Figure 8

Changes in the mineral saturation indices of diverse minerals with (a) increasing Mg(OH)₂ dissolution or (b) increasing pH, see also Table 2.

Figure 9

Changes in (a) pH, (b) solid phase assemblage, and total dissolved CO₂ in the catholyte during reaction with CO₂ to achieve equilibrium pCO₂ equivalent to atmospheric conditions at 25 °C. These simulations show that the catholyte solids discharged include hydromagnesite and aragonite, in general agreement with our experiments.