Investigation of the Different Carbonate Phases and Their Formation Kinetics during Mg(OH)2 Slurry Carbonation

Reaction Layer Formation on MgO in the Presence of Humidity

Mineralization by MgO is an attractive potential strategy for direct air capture (DAC) of CO2 due to its tendency to form carbonate phases upon exposure to water and CO2. Hydration of MgO during this process is typically assumed to not be rate limiting, even at ambient temperatures. However, surface passivation by hydrated phases likely reduces the CO2 capture capacity. Here, we examine the initial hydration reactions that occur on MgO(100) surfaces to determine whether they could potentially impact CO2 uptake. We first used atomic force microscopy (AFM) to explore changes in reaction layers in water (pH = 6 and 12) and MgO-saturated solution (pH = 11) and found the reaction layers on MgO are heterogeneous and nonuniform. To determine how relative humidity (R.H.) affects reactivity, we reacted samples at room temperature in nominally dry N2 (∼11-12% R.H.) for up to 12 h, in humid (>95% R.H.) N2 for 5, 10, and 15 min, and in air at 33 and 75% R.H. for 8 days. X-ray reflectivity and electron microscopy analysis of the samples reveal that hydrated phases form rapidly upon exposure to humid air, but the growth of the hydrated reaction layer slows after its initial formation. Reaction layer thickness is strongly correlated with R.H., with denser reaction layers forming in 75% R.H. compared with 33% R.H. or nominally dry N2. The reaction layers are likely amorphous or poorly crystalline based on grazing incidence X-ray diffraction measurements. After exposure to 75% R.H. in air for 8 days, the reaction layer increases in density as compared to the sample reacted in humid N2 for 5-15 min. This may represent an initial step toward the crystallization of the reaction layer. Overall, high R.H. favors the formation of a hydrated, disordered layer on MgO. Based on our results, DAC in a location with a higher R.H. will be favorable, but growth may slow significantly from initial rates even on short timescales, presumably due to surface passivation.

Morphology and stability of mineralized carbon influenced by magnesium ions

Ex situ mineralization of CO₂ is a promising technology that employs Ca- and Mg-rich industrial wastes but it simultaneously produces end products. Although Mg is a major mineralization source, it can adversely impact carbonate precipitation and crystal stability during co-precipitation in combination with Ca²⁺. In this study, the effects of Mg²⁺ ions on the mineralization process and its products were investigated using precipitates formed at different aqueous concentrations of Mg²⁺. The final phases of the precipitates were quantitatively evaluated at the end of each process. The alterations undergone by the calcite crystals, which constituted the dominant carbonate phase in each experiment, were analyzed using a sophisticated crystallographic approach. Aragonite was detected at high Mg²⁺ concentrations (Mg²⁺/Ca²⁺ ratio of 2.00), although brucite was the sole phase of the Mg crystal. The increase in Mg²⁺ ion concentration induced the formation of an amorphous solid. The results revealed that a drastic transformation of the calcite lattice occurred when the ratio of Mg²⁺/Ca²⁺ exceeded 1.00, agreeing with the shifts observed in the calcite structure upon comparing the precipitates formed at the Mg²⁺/Ca²⁺ ratios of 1.00 and 2.00, wherein microstrain and crystallite sizes changed from 0.040 and 55.33 nm to 0.1533 and 12.35 nm, respectively. At a Mg²⁺/Ca²⁺ ratio of 2.00, 6.51% of the Ca²⁺ ions in the calcite structure were substituted by Mg²⁺, increasing the surface energy of the crystal and the solubility of the carbonate. Therefore, Mg²⁺ is a potential hindrance that can impede the precipitation of carbonates and increase instability at certain concentrations.

Carbonation, Cementation, and Stabilization of Ultramafic Mine Tailings

Tailings dam failures can cause devastation to the environment, loss of human life, and require expensive remediation. A promising approach for de-risking brucite-bearing ultramafic tailings is in situ cementation via carbon dioxide (CO₂) mineralization, which also sequesters this greenhouse gas within carbonate minerals. In cylindrical test experiments, brucite [Mg(OH)₂] carbonation was accelerated by coupling organic and inorganic carbon cycling. Waste organics generated CO₂ concentrations similar to that of flue gas (up to 19%). The abundance of brucite (2-10 wt %) had the greatest influence on tailings cementation as evidenced by the increase in total inorganic carbon (TIC; +0.17-0.84%). Brucite consumption ranged from 64-84% of its initial abundance and was mainly influenced by water availability. Higher moisture contents (e.g., 80% saturation) and finer grain sizes (e.g., clay-silt) that allowed for a better distribution of water resulted in greater brucite carbonation. Furthermore, pore clogging and surface passivation by Mg-carbonates may have slowed brucite carbonation over the 10 weeks. Unconfined compressive strengths ranged from 0.4-6.9 MPa and would be sufficient in most scenarios to adequately stabilize tailings. Our study demonstrates the potential for stabilizing brucite-bearing mine tailings through in situ cementation while sequestering CO₂.

CO2 utilization in built environment via the PCO2 swing carbonation of alkaline solid wastes with different mineralogy

Carbon mineralization to solid carbonates is one of the reaction pathways that can not only utilize captured CO2 but also potentially store it in the long term. In this study, the dissolution and carbonation behaviors of alkaline solid wastes (i.e., waste concrete) was investigated. Concrete is one of the main contributors to a large carbon emission in the built environment. Thus, the upcycling of waste concrete via CO2 utilization has multifaceted environmental benefits including CO2 emission reduction, waste management and reduced mining. Unlike natural silicate minerals such as olivine and serpentine, alkaline solid wastes including waste concrete are highly reactive, and thus, their dissolution and carbonation behaviors vary significantly. Here, both conventional acid (e.g., hydrochloric acid) and less studied carbonic acid (i.e., CO2 saturated water) solvent systems were explored to extract Ca from concrete. Non-stoichiometric dissolution behaviors between Ca and Si were confirmed under far-from-equilibrium conditions (0.1 wt% slurry density), and the re-precipitation of the extracted Si was observed at near-equilibrium conditions (5 wt% slurry density), when the Ca extraction was performed at a controlled pH of 3. These experiments, with a wide range of slurry densities, provided valuable insight into Si re-precipitation phenomena and its effect on the mass transfer limitation during concrete dissolution. Next, the use of the partial pressure of CO2 for the pH swing carbon mineralization process was investigated for concrete, and the results were compared to those of Mg-bearing silicate minerals. In the PCO2 swing process, the extraction of Ca was significantly limited by the precipitation of the carbonate phase (i.e., calcite), since CO2 bubbling could not provide a low enough pH condition for concrete-water-CO2 systems. Thus, this study showed that the two-step carbon mineralization via PCO2 swing, that has been developed for Mg-bearing silicate minerals, may not be viable for highly reactive Ca-bearing silicate materials (e.g., concrete). The precipitated calcium carbonate (PCC) derived from waste concrete via a pH swing process showed very promising results with a high CO2 utilization potential as an upcycled construction material.

Alkaline thermal treatment of seaweed for high-purity hydrogen production with carbon capture and storage potential

Current thermochemical methods to generate H₂ include gasification and steam reforming of coal and natural gas, in which anthropogenic CO₂ emission is inevitable. If biomass is used as a source of H₂, the process can be considered carbon-neutral. Seaweeds are among the less studied types of biomass with great potential because they do not require freshwater. Unfortunately, reaction pathways to thermochemically convert salty and wet biomass into H₂ are limited. In this study, a catalytic alkaline thermal treatment of brown seaweed is investigated to produce high purity H₂ with substantially suppressed CO₂ formation making the overall biomass conversion not only carbon-neutral but also potentially carbon-negative. High-purity 69.69 mmol-H₂/(dry-ash-free)g-brown seaweed is produced with a conversion as high as 71%. The hydroxide is involved in both H₂ production and in situ CO₂ capture, while the Ni/ZrO₂ catalyst enhanced the secondary H₂ formation via steam methane reforming and water-gas shift reactions.

While biomass may serve as a renewable source of carbon-neutral hydrogen, it is challenging both to utilize as-found bio-resources and to suppress CO₂ formation. Here, authors convert wet, salty seaweed using alkaline thermal treatment to produce high-purity hydrogen and suppress carbon emission.

Slow and Sustained Release of Carbonate Ions from Amino Acids for Controlled Hydrothermal Growth of Alkaline-Earth Carbonate Single Crystals

Alkaline-earth metal carbonate materials have attracted wide interest because of their high value in many applications. Various sources of carbonate ions (CO₃ ^2-), such as CO₂ gas, alkaline-metal carbonate salts, and urea, have been reported for the synthesis of metal carbonate crystals, yet a slow and sustained CO₃ ^2- release approach for controlled crystal growth is much desired. In this paper, we demonstrate a new chemical approach toward slow and sustained CO₃ ^2- release for hydrothermal growth of large alkaline-earth metal carbonate single crystals. Such an approach is enabled by the multiple hydrolysis of a small basic amino acid (arginine, Arg). Namely, the amino groups of Arg hydrolyze to form OH^- ions, making the solution basic, and the hydrolysis of the guanidyl group of Arg is hydrothermally triggered to produce urea and ammonia, followed by the hydrolysis of urea to produce CO₂ and ammonia and then the release of CO₃ ^2- because of the reaction between CO₂ and the OH^- ions hydrolyzed from ammonia. Such a CO₃ ^2- release behavior enables the slow and controlled growth of various carbonate single crystals over a wide range of pH values. The growth of uniform rhombohedron MgCO₃ single crystals with variable morphologies and crystal sizes is studied in detail. The influences of reaction temperature, solution pH, precursor type, and concentration on the morphology and size of the resulting MgCO₃ crystals are elucidated. The crystal evolution mechanism is also proposed and discussed with various supportive data.

Multiphase carbon mineralization for the reactive separation of CO2 and directed synthesis of H2

There is a need to capture, convert and store CO₂ by atom-efficient and energy-efficient pathways that use as few process configurations as possible. This need has motivated studies into multiphase reaction chemistries and this Review describes two such approaches in the context of carbon mineralization. The first approach uses aqueous alkaline solutions containing amine nucleophiles that capture CO₂ and eventually convert it into calcium and magnesium carbonates, thereby regenerating the nucleophiles. Gas-liquid-solid and liquid-solid configurations of these reactions are explored. The second approach combines silicates such as CaSiO₃ or Mg₂SiO₄ with CO and H₂O from the water-gas shift reaction to give H₂ and calcium or magnesium carbonates. Coupling carbonate formation to the water-gas shift reaction shifts the latter equilibrium to afford more H₂ as part of a single-step catalytic approach to carbon mineralization. These pathways exploit the vast abundance of alkaline resources, including naturally occurring silicates and alkaline industrial residues. However, simple stoichiometries belie the complex, multiphase nature of the reactions, predictive control of which presents a scientific opportunity and challenge. This Review describes this multiphase chemistry and the knowledge gaps that need to be addressed to achieve 'step-change' advancements in the reactive separation of CO₂ by carbon mineralization.

Accelerating Mineral Carbonation Using Carbonic Anhydrase

Carbonic anhydrase (CA) enzymes have gained considerable attention for their potential use in carbon dioxide (CO2) capture technologies because they are able to catalyze rapidly the interconversion of aqueous CO2 and bicarbonate. However, there are challenges for widespread implementation including the need to develop mineralization process routes for permanent carbon storage. Mineral carbonation of highly reactive feedstocks may be limited by the supply rate of CO2. This rate limitation can be directly addressed by incorporating enzyme-catalyzed CO2 hydration. This study examined the effects of bovine carbonic anhydrase (BCA) and CO2-rich gas streams on the carbonation rate of brucite [Mg(OH)2], a highly reactive mineral. Alkaline brucite slurries were amended with BCA and supplied with 10% CO2 gas while aqueous chemistry and solids were monitored throughout the experiments (hours to days). In comparison to controls, brucite carbonation using BCA was accelerated by up to 240%. Nesquehonite [MgCO3·3H2O] precipitation limited the accumulation of hydrated CO2 species, apparently preventing BCA from catalyzing the dehydration reaction. Geochemical models reproduce observed reaction progress in all experiments, revealing a linear correlation between CO2 uptake and carbonation rate. Data demonstrates that carbonation in BCA-amended reactors remained limited by CO2 supply, implying further acceleration is possible.