Sequestration of CO2 within stable mineral carbonates (e.g., CaCO3) represents an attractive emission reduction strategy because it offers a leakage-free alternative to geological storage of CO2 in an environmentally benign form. However, the pH of aqueous streams equilibrated with gaseous streams containing CO2 (pH < 4) are typically lower than that which is required for carbonate precipitation (pH > 8). Traditionally, alkalinity is provided by a stoichiometric reagent (e.g., NaOH) which renders these processes environmentally hazardous and economically unfeasible. This work investigates the use of regenerable ion-exchange materials to induce alkalinity in CO2-saturated aqueous solutions such that the pH shift required for mineralization occurs without the need for stoichiometric reagents. Na+-H+ exchange isotherms (at [H+] = 10−8–10−1 M) and rates were measured for 13X and 4A zeolites and TP-207 and TP-260 organic exchange resins in batch equilibrium and fixed-bed exchange experiments, respectively. At solutions equilibrated with CO2 at 1.0 atm (pH = 3.9), H+ exchange capacities for the materials were similar (1.7–2.4 mmol H+/g material) and resulted in pH increases from 3.9 to greater than 8.0. Multi-component mixtures using Ca2+ and Mg2+ cations (at 10−3–10−1 M) in CO2-saturated water were used to probe competitive ion exchange. The presence of divalent cations in solution inhibited H+ exchange, reducing capacities to as low as 0.2 mmol H+/g for both resins and zeolites. Dynamic H+ exchange capacities in fixed-bed ion exchange columns were similar to equilibrium values for resins (∼1.5 mmol/g) and zeolites (∼0.8 mmol/g) using inlet solutions that were equilibrated with gaseous streams of CO2 at 1.0 atm. However, exchange kinetics were limited by intraparticle diffusion as indicated by the increased rate parameters with increasing inlet flow rates (20–160 cm3 min−1). Experimental calcite precipitation from mixing the alkaline CO32−-rich water solution obtained from the ion-exchange column with a simulated liquid waste stream solution achieved thermodynamic maximum yields. The results from these studies indicate that ion exchange processes can be used as an alternative to the addition of stoichiometric bases to induce alkalinity for the precipitation of CaCO3, thereby opening a pathway toward sustainable and economic mineralization processes.

The sequestration of CO 2 within stable mineral carbonates (e.g., CaCO 3 ) represents an attractive emissionsreduction strategy because it offers an energy efficient, environmentally benign, and leakage-free alternative to geological storage. However, the pH levels of aqueous streams equilibrated with CO 2containing gas streams (pH ∼ 4) are lower than the pH required for carbonate precipitation (pH > 8). Thus, the use of regenerable ion exchange materials is proposed to induce alkalinity in CO 2containing aqueous streams to achieve the pH required for mineralization without the addition of expensive stoichiometric reagents such as caustic soda (e.g., NaOH). Herein, geochemical and process-modeling software was used to identify the optimum thermodynamic conditions and to quantify the energy intensity and CO 2 reduction potential of a process that sequesters CO 2 (dissolved in wastewater) as solid calcium carbonate (CaCO 3 ). CaCO 3 yields were maximized when the initial calcium to CO 2 ratio in the aqueous phase was 1:1. The energy intensity of the process (0.22−2.10 MW•h/t of CO 2 removed) was dependent on the concentration of CO 2 in the gas phase (i.e., 5−50 vol %) and the produced water composition, with the nanofiltration and reverse osmosis steps used to recover magnesium and sodium ions requiring the most energy (0.07−0.80 MW•h/t of CO 2 removed). Energy consumption was minimized under conditions where CaCO 3 yields were maximized for all produced water compositions and CO 2 concentrations. The ratio of net CO 2 to gross CO 2 removal for the process ranged from 0.05 to 0.90, indicating a net CO 2 reduction across all conditions studied. The results from these studies indicate that ion exchange processes can be used as alternatives to the addition of stoichiometric bases to provide alkalinity for the precipitation of CaCO 3 at the CO 2 concentrations studied, thereby opening a pathway toward sustainable and economic mineralization processes.

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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.

Surface modification offers a straightforward means to alter and enhance the properties and performance of materials, such as nanofiltration membranes for water softening. Herein, we demonstrate how a membrane's surface charge can be altered by grafting different electrostatically varying copolymers onto commercial membrane surfaces using perfluorophenylazide (PFPA) photochemistry for enhanced ion separation performance. The native membrane's performancei.e., in terms of divalent cation separationwith copolymer coatings containing a positively charged quaternary ammonium (−N(Me) 3 + ), a negatively charged sulfonate (−SO 3 − ), and an essentially neutral zwitterion (sulfobetaine, −N(Me) 2 R 2 + , and −SO 3 − ), respectively, indicates that: (a) the sulfonated polymer induces robust Coulombic exclusion of divalent anions as compared to the negatively charged native membrane surface on account of its higher negative charge; (b) the positively charged ammonium coating induces exclusion of cations more effectively than the native membrane; and significantly, (c) the zwitterion polymer coating, which reduces the surface roughness and improves wettability, in spite of its near-neutral charge enhances exclusion of both divalent cations and anions on account of aperture sieving by the compact zwitterion polymer that arises from its ability to limit the size of ions that transport through the polymer along with dielectric exclusion. The outcomes thereby inform new pathways to achieve size-and charge-based exclusion of ionic, molecular, and other species contained in liquid streams.

Alkaline industrial wastes (e.g., slags: ordered crystalline solids, and fly ashes: disordered solids) represent abundant reservoirs of elements such as silicon and calcium. Rapid elemental extractions from these wastes, however, have often relied on the use of “stoichiometric additives” (i.e., acids or bases). Herein, we demonstrate that acoustic stimulation enhances the release of network-forming Si species from crystalline blast furnace slags and amorphous fly ashes at reaction temperatures less than 65 °C. These additive-free enhancements are induced by cavitation processes which reduce the apparent activation energy of solute dissolution (E a, kJ/mol) by up to 40% as compared to unstimulated conditions. Because of the reduction in the apparent activation energy, acoustic stimulation features an energy intensity that is up to 80% lower in promoting dissolution, as compared to other additive-free methods such as enhancing the solute’s surface area, introducing heat, or convectively mixing the solvent. Based on atomic topology analysis, we show that the reduction in apparent dissolution activation energy upon acoustic stimulation scales with the number of weak topological constraints per atom in the atomic network of the dissolving solute, independent of their ordered or disordered nature. This suggests that sonication breaks the weakest constraints in the solute’s atomic network, which, in turn, facilitates dissolution. The results suggest the ability of acoustic stimulation to enhance waste utilization and circularity, by enabling efficient resource extraction from industrial wastes.

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