Electrochemical Splitting of Calcium Carbonate to Increase Solution Alkalinity: Implications for Mitigation of Carbon Dioxide and Ocean Acidity

Abstract

Electrochemical splitting of calcium carbonate (e.g., as contained in limestone or other minerals) is explored as a means of forming dissolve hydroxides for absorbing, neutralizing, and storing carbon dioxide, and for restoring, preserving, or enhancing ocean calcification. While essentially insoluble in water, CaCO₃ can be dissolved in the presence of the highly acidic anolyte of a water electrolysis cell. The resulting charged constituents, Ca²⁺ and CO₃²⁻, migrate to the cathode and anode, respectively, forming Ca(OH)₂ on the one hand and H₂CO₃ (or H₂O and CO₂) on the other. By maintaining a pH between 6 and 9, subsequent hydroxide reactions with CO₂ primarily produce dissolved calcium bicarbonate, Ca(HCO₃)_2aq. Thus, for each mole of CaCO₃ split, there can be a net capture of up to 1 mol of CO₂. Ca(HCO₃)_2aq is thus the carbon sequestrant that can be diluted and stored in the ocean, in natural or artificial surface water reservoirs, or underground. The theoretical work requirement for the reaction is 266 kJ_e per net mole CO₂ consumed. Even with inefficiencies, a realized net energy expenditure lower than the preceding quantity appears possible considering energy recovery via oxidation of the H₂ produced. The net process cost is estimated to be <$100/tonne CO₂ mitigated. An experimental demonstration of the concept is presented, and further implementation issues are discussed.