Tuning Reactive Crystallization Pathways for Integrated CO2 Capture, Conversion, and Storage via Mineralization

ConspectusAchieving carbon neutrality requires realizing scalable advances in energy- and material-efficient pathways to capture, convert, store, and remove anthropogenic CO2 emission in air and flue gas while cogenerating multiple high-value products. To this end, earth-abundant Ca- and Mg-bearing alkaline resources can be harnessed to cogenerate Ca- and Mg-hydroxide, silica, H2, O2, and a leachate bearing high-value metals in an electrochemical approach with the in situ generation of a pH gradient, which is a significant departure from existing pH-swing-based approaches. To accelerate CO2 capture and mineralization, CO2 in dilute sources is captured using solvents to produce CO2-loaded solvents. CO2-loaded solvents are reacted Ca- and Mg-bearing hydroxides to produce Ca- and Mg-carbonates while regenerating the solvents. These carbonates can be used as a temporary or permanent store of CO2 emissions. When carbonates are used as a temporary store of CO2 emissions, electrochemical sorbent regeneration pathways can be harnessed to produce high-purity CO2 while regenerating Ca- and Mg-hydroxide and coproducing H2 and O2. Figure 1 is a schematic representation of this integrated approach.Tuning the molecular-scale and nanoscale interactions underlying these reactive crystallization mechanisms for carbon transformations is crucial for achieving kinetic, chemical, and morphological controls over these pathways. To this end, the feasibility of (i) crystallizing Ca- and Mg-hydroxide during the electrochemical desilication of earth-abundant alkaline industrial residues, (ii) accelerating the conversion of Ca- and Mg-carbonates for temporary or permanent carbon storage by harnessing regenerable solvents, and (iii) regenerating Ca- and Mg-hydroxide while coproducing high-purity CO2, O2, and H2 electrochemically is established.Evidence of the fractionation of heterogeneous slag to coproduce silica, Ca- and Mg-hydroxide, and a leachate bearing metals during electrochemical desilication provides the basis for further tuning the physicochemical parameters to improve the energy and material efficiency of these pathways. To address the slow kinetics of CO2 capture and mineralization starting from ultradilute emissions, reactive capture pathways that harness solvents such as Na-glycinate are shown to be effective. The extents of carbon mineralization of Ca(OH)2 and Mg(OH)2 are 97% and 78% using CO2-loaded Na-glycinate upon reacting for 3 h at 90 °C. During the regeneration of Ca- and Mg-hydroxide and high-purity CO2 from carbonate sources, charge efficiencies of as high as 95% were observed for the dissolution of MgCO3 and CaCO3 while stirring at 100 rpm. Higher yields of Mg(OH)2 are observed compared to that for Ca(OH)2 during sorbent regeneration due to the lower solubility of Mg(OH)2. These findings provide the scientific basis for further tuning these reactive crystallization pathways for closing material and carbon cycles to advance a sustainable climate, energy, and environmental future.