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.

By focusing the power of sound, acoustic stimulation (i.e., often referred to as sonication) enables numerous “green chemistry” pathways to enhance chemical reaction rates, for instance, of mineral dissolution in aqueous environments. However, a clear understanding of the atomistic mechanism(s) by which acoustic stimulation promotes mineral dissolution remains unclear. Herein, by combining nanoscale observations of dissolving surface topographies using vertical scanning interferometry, quantifications of mineral dissolution rates via analysis of solution compositions using inductively coupled plasma optical emission spectrometry, and classical molecular dynamics simulations, we reveal how acoustic stimulation induces dissolution enhancement. Across a wide range of minerals (Mohs hardness ranging from 3 to 7, surface energy ranging from 0.3 to 7.3 J/m2, and stacking fault energy ranging from 0.8 to 10.0 J/m2), we show that acoustic fields enhance mineral dissolution rates (reactivity) by inducing atomic dislocations and/or atomic bond rupture. The relative contributions of these mechanisms depend on the mineral’s underlying mechanical properties. Based on this new understanding, we create a unifying model that comprehensively describes how cavitation and acoustic stimulation processes affect mineral dissolution rates.

Synthetic hydrotalcites were produced by a co‐precipitation method. The hydrotalcites are represented by the general formula [MII(1‐x)MIII(x)(OH)2][An−]x/n·zH2O, where MII is a divalent cation (eg, Mg2+or Ca2+), MIII is a trivalent cation (eg, Al3+) and An− is the interlayer anion. Herein, MII = Mg, and MIII = Al such that [Mg/Al] = [2, 3] (atomic units) and An−, represents intercalant species including: OH−, SO42− and CO32− anions. The thermochemical data of each compound including their solubility constants (Kso), density and molar volume were quantified at T = 25 ± 0.5°C, and P = 1 bar. The solubilities of the synthetic hydrotalcites, irrespective of their divalent‐trivalent cation partitioning ratio, scaled as CO32− < SO42− < OH−; in order of decreasing solubility. The type of anion, very slightly, affected the solubility with less than ±1 log unit of variation for [Mg/Al] = 2, and ±2 log units of variation for [Mg/Al] = 3. The solubilities of these phases were strongly correlated with that of gibbsite (Al(OH)3); such that activity of the [AlO2−] species was solubility determining with increasing pH. The tabulated thermodynamic data were used to construct solid‐solution models for phases encompassing both cation distribution ratios and to calculate stable phase equilibria relevant to alkali‐activated slag (AAS) systems for diverse activator compositions.

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.

An updated calcium silicate hydrate (C–S–H) model incorporating aluminium-containing end-members was used for thermodynamic modelling of blended cements using blast-furnace slag and Portland cement (BFS:PC) with ratios of 1:1, 3:1 and 9:1, using GEMSelektor. Selective dissolution and magic angle spinning nuclear magnetic resonance (MAS NMR) studies were performed to determine the degree of hydration (DoH) of the anhydrous material as an input parameter for the modelling work. Both techniques showed similar results for determining the DoH of the BFS within each sample. Characterisation of the hardened cement pastes over 360 days, using X-ray diffraction analysis and MAS NMR, demonstrated that the use of the updated C–S–H model can highlight the effect of different blend ratios and curing ages on the phase assemblages in these cements. Validation using this modelling approach was performed on 20 year old specimens from the literature to highlight its applicability for modelling later-age blended cements.