Carbon dioxide sequestration through mineralization from seawater: The interplay of alkalinity, pH, and dissolved inorganic carbon

Marine carbon dioxide removal technologies rely on the equilibration of seawater with atmospheric CO₂, where a pH increase can facilitate CO₂ removal either through increased absorption capacity of CO₂, or precipitation of minerals. Here, we focus on explicitly defining the governing factors that influence seawater alkalinity with respect to CaCO₃ mineralization and atmospheric CO₂ uptake. We utilize an electrochemical setup and different water types, such as near-shore Mediterranean seawater and Red Sea Salt water. The setup allows us to separate mineralization from alkalinity enhancement via an electrochemical membrane cell, with a second compartment that allows evaluation of the interplay of ion concentration, precipitation, pH, DIC concentration, and total alkalinity under a controlled evironment. The effect of sparging additional CO₂ before, and after the precipitation of CaCO₃ was evaluated while utilizing both a mild pH swing (from, ∼8.3 to ∼9.5), and a larger pH swing (from, ∼8.3 to ∼10.2). A thermodynamic model is presented defining the energy consumptionrequired to increase the pH, and the influence of Mg thereon is explicated. Furthermore, we detail theoretical aquatic chemistry simulations via PHREEQC to rationalize the observed intricate dynamics of the carbonate system in seawater. We end with a discussion of the implications of total alkalinity, pH, salt complexation, and carbon uptake capacity relevant to of all ocean alkalinity enhancement and marine-based CO₂ removal systems, concluding that the carbon uptake capacity of water returned to the ocean must be verified in such technologies.