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 PlanteE.C. La Plante, E.C. ACS Sustain. Chem. Eng.2021910731089, 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.

Direct Electrochemical Seawater Splitting for Green Hydrogen and Artificial Reefs

“…Besides traditional chemical methods, Rau et al also proposed electrochemical technologies to capture CO 2 previously. − In their proof of concept, CaCO 3 was utilized to react with the OH – anions generated from water electrolysis to produce Ca­(HCO 3 ) 2 and Ca­(OH) 2 in the cathode part, which would capture CO 2 from the atmosphere to reform CaCO 3 . La Plante et al proposed a single-step carbon sequestration and storage approach that relies on combining dissolved CO 2 in water bodies with Ca and Mg species in an aqueous medium using electrolytic flow reactors and described the mass balances associated with CO 2 removal using seawater as both the source of reactants and the reaction medium by geochemical simulations. , Oloye et al also demonstrated direct air capture of CO 2 using electrochemical approaches, where the efficiency could be increased by the addition of monoethanolamine because CO 2 solubility increased in water . Here, instead of using capture agents, the atmosphere was injected directly into the electrolyzer, where OH – generation by ORR and CO 2 capture occurred simultaneously.…”

Assessing technical aspects of ocean alkalinity enhancement approaches

“…31-34), where aqueous brine (approximately 26 wt %) and NaOH (approximately 28 wt %) are converted into less concentrated brine (approximately 24 wt %), more concentrated NaOH (approximately 30 wt %), hydrogen gas (H 2 ), and chlorine gas (Cl 2 ) (Kumar et al, 2021). The development of efficient and durable oxygen-selective electrodes is critical to making seawater electrolysis more feasible (La Plante et al, 2023).…”

“…The precipitation of CaCO 3(s) reduces alkalinity (resulting either in lower efficiency of CO 2 removal per unit of added alkalinity in the case of OAE or a release of CO 2 in cases where the precipitation occurs in the absence of an alkalinity addition), making this second version relatively inefficient from a CO 2 removal perspective, but this may be pursued if other considerations such as ease of verification outweigh this inefficiency. A third version of this approach relies primarily on the precipitation of Mg(OH) 2 , in addition to the precipitation of some CaCO 3, and prevents release of CO 2 in the process of CaCO 3 precipitation by generating alkalinity at a sufficiently high rate to keep the pH at a constant target value (La Plante et al, 2021Plante et al, , 2023.…”

Reaction Layer Formation on MgO in the Presence of Humidity

“…Presently, the majority of DAC plants use liquid amines as absorbents for the CO 2 , but mineralization is also a potentially promising technique . Mineralization strategies include saline mineralization, , the conversion of silicate minerals, and the weathering of silicate or oxide minerals. − However, there are challenges associated with mineralization strategies, for example, passivation layers inhibiting CO 2 uptake can form on silicate minerals due to the polymerization of silicic acid during dissolution and the formation of silica-rich leached layers. ,− DAC with alkaline metal oxide minerals, such as CaO and MgO, may be cheaper and more feasible compared to other mineral systems if a mineral looping process is used. In this approach, metal carbonates are calcined to form metal oxides that are typically amorphous and highly reactive.…”

“…1 Presently, the majority of DAC plants use liquid amines as absorbents for the CO 2 , 2 but mineralization is also a potentially promising technique. 3 Mineralization strategies include saline mineralization, 4,5 the conversion of silicate minerals, 6 and the weathering of silicate or oxide minerals. 7−11 However, there are challenges associated with mineralization strategies, for example, passivation layers inhibiting CO 2 uptake can form on silicate minerals due to the polymerization of silicic acid during dissolution and the formation of silica-rich leached layers.…”