Low temperature and limited water activity reveal a pathway to magnesite via amorphous magnesium carbonate

Decreasing Hygroscopicity Slows Forsterite Carbonation under Low-Water Conditions

A fundamental understanding of processes that slow divalent metal silicate carbonation is important for developing effective strategies to durably store carbon dioxide and mitigate atmospheric CO2. This study presents a detailed investigation of a passivation effect unique to low-water conditions during the carbonation of forsterite (Mg2SiO4) and highlights the importance of hygroscopicity in influencing metal silicate carbonation. Integrated in situ and ex situ experimental results showed that the decrease in the carbonation rate of forsterite observed after ∼10 h in humid supercritical CO2 (50 °C, 90 bar) correlates with a reduction in water film thickness, and in particular, weakly hydrogen bonded adsorbed water that facilitates ion transport. We attribute the decrease in thickness to a drop in the concentrations of hygroscopic Mg2+, MgHCO3+, and HCO3- ions within the film as the predominate forsterite carbonation product evolves from amorphous magnesium carbonate (AMC) to magnesite (MgCO3). When more soluble AMC is present, hygroscopic ion concentrations are higher, drawing more water from the supercritical phase to the forsterite surface. Carbonation rates are faster because thicker water films can better mobilize ions to growing carbonates. In contrast, when less soluble magnesite predominates, hygroscopic ion concentrations are lower, water films are thinner, and carbonate rates are slower.

Cobalt substitution slows forsterite carbonation in low-water supercritical carbon dioxide

Cobalt recovery from low-grade mafic and ultramafic ores could be economically viable if combined with CO2 storage under low-water conditions, but the impact of Co on metal silicate carbonation and the fate of Co during the carbonation reaction must be understood. In this study, in situ infrared spectroscopy was used to investigate the carbonation of Co-doped forsterite ((Mg,Co)2SiO4) in thin water films in humidified supercritical CO2 at 50 °C and 90 bar. Rates of carbonation of Co-doped forsterite to Co-rich magnesite ((Mg,Co)CO3) increased with water film thickness but were at least 10 times smaller than previously measured for pure forsterite at similar conditions. We suggest that the smaller rates are due to thermodynamic drivers that cause water films on Co-doped forsterite to be much less oversaturated with respect to Co-doped magnesite, compared to the pure minerals.

Facile Synthesis of Low-Dimensional and Mild-Alkaline Magnesium Carbonate Hydrate for Safe Multiple Protection of Paper Relics

Paper-based cultural relics inevitably face a variety of diseases such as acidification, yellowing, and strength loss during long-term preservation, where weakly alkaline inorganic materials play an important role in their deacidification treatments. In this work, by simply adjusting the supersaturation of crystal growing solution without the use of any organic additives, one-dimensional (1D) and two-dimensional (2D) weakly alkaline materials-magnesium carbonate hydrates (MCHs)-were controllably synthesized. It is worth noting that the coatings of 1D/2D MCHs not only cause little change in chromatic aberration and water wettability, but also ensure their safety for alkali-sensitive pigments. Meanwhile, the deacidification, anti-aging, strength-enhancing, and flame-retardant effects of these materials have been tested on ancient book papers, all of which achieved good protective effects. In contrast, 1D MCH materials brought about significant enhancement in both mechanical strengths and flame-retardant effects, and the related effects were investigated. Based on this facile micromorphology control strategy, more low-dimensional nanomaterials are expected to be synthesized by design for the protection of paper-based relics, which will expand our knowledge on functional deacidification and protection mechanisms.

Enhanced extraction of heavy metals from gypsum-based hazardous waste by nanoscale sulfuric acid film at ambient conditions

Low-cost, low-energy extraction of heavy metal(loid)s (HMs) from hazardous gypsum cake is the goal of the metallurgical industry to mitigate environmental risks and carbon emissions. However, current extracting routes of hydrometallurgy often suffer from great energy inputs and substantial chemical inputs. Here, we report a novel solid-like approach with low energy consumption and chemical input to extract HMs by thin films under ambient conditions. Through constructing a nanoscale sulfuric acid film (NSF) of ∼50 nm thickness on the surface of arsenic-bearing gypsum (ABG), 99.6% of arsenic can be removed, surpassing the 50.3% removal in bulk solution. In-situ X-ray diffraction, infrared spectral, and ab initio molecular dynamics (AIMD) simulations demonstrate that NSF plays a dual role in promoting the phase transformation from gypsum to anhydrite and in changing the ionic species to prevent re-doping in anhydrite, which is not occurred in bulk solutions. The potential of the NSF is further validated in extracting other heavy metal(loid)s (e.g., Cu, Zn, and Cr) from synthetic and actual gypsum cake. With energy consumption and costs at 1/200 and 1/10 of traditional hydrometallurgy separately, this method offers an efficient and economical pathway for extracting HMs from heavy metal-bearing waste and recycling industrial solid waste.

CO2 Mineralization by MgO Nanocubes in Nanometric Water Films

Water films formed by the adhesion and condensation of air moisture on minerals can trigger the formation of secondary minerals of great importance to nature and technology. Magnesium carbonate growth on Mg-bearing minerals is not only of great interest for CO2 capture under enhanced weathering scenarios but is also a prime system for advancing key ideas on mineral formation under nanoconfinement. To help advance ideas on water film-mediated CO2 capture, we tracked the growth of amorphous magnesium carbonate (AMC) on MgO nanocubes exposed to moist CO2 gas. AMC was identified by its characteristic vibrational spectral signature and by its lack of long-range structure by X-ray diffraction. We find that AMC (MgCO3·2.3-2.5H2O) grew in sub-monolayer (ML) to 4 ML thick water films, with formation rates and yields scaling with humidity. AMC growth was however slowed down as AMC nanocoatings blocked water films access to the reactive MgO core. Films could however be partially dissolved by exposure to thicker water films, driving AMC growth for several more hours until nanocoatings blocked the reactions again. These findings shed new light on a potentially important bottleneck for the efficient mineralization of CO2 using MgO-bearing products. Notably, this study shows how variations in the air humidity affect CO2 capture by controlling water film coverages on reactive minerals. This process is also of great interest in the study of mineral growth in nanometrically thick water films.

Capillary suction under unsaturated condition drives strong specific affinity of ions at the surface of clay mineral

Mineral-solution interface is of great importance in many soil and geochemical processes as well as industrial applications. Most relevant studies were based on saturated condition and given the corresponding theory, model, and mechanism. However, soils are usually in the non-saturation with different capillary suction. Our study presents substantially different scenery for ions interacting with mineral surface under unsaturated condition using molecular dynamics method. Under partially hydrated state, both cations (Ca²⁺) and anions (Cl^-) can be adsorbed as outer-sphere complexes at the montmorillonite surface, and the number significantly increased with the increase of unsaturated degree. Ions preferred to interact with clay mineral instead of water molecules under unsaturated state, and the mobility of both cations and anions substantially decreased with the increase of capillary suction as reflected by the diffusion coefficient analysis. Potential of mean force calculations further clearly revealed that the adsorption strength of both Ca²⁺ and Cl^- increased with capillary suction. Such an increase was more obvious for Cl^- compared to Ca²⁺, despite the adsorption strength of Cl^- was much weaker than Ca²⁺ at a certain capillary suction. Therefore, it is the capillary suction under unsaturated condition that drives the strong specific affinity of ions at the surface of clay mineral, which was tightly related to the steric effect of confined water film, the destruction of EDL structure, and the cation-anion pair interaction. This suggests that our common understanding of mineral-solution interaction should be largely improved.

Molecular-scale mechanisms of CO2 mineralization in nanoscale interfacial water films

The calamitous impacts of unabated carbon emission from fossil-fuel-burning energy infrastructure call for accelerated development of large-scale CO₂ capture, utilization and storage technologies that are underpinned by a fundamental understanding of the chemical processes at a molecular level. In the subsurface, rocks rich in divalent metals can react with CO₂, permanently sequestering it in the form of stable metal carbonate minerals, with the CO₂-H₂O composition of the post-injection pore fluid acting as a primary control variable. In this Review, we discuss mechanistic reaction pathways for aqueous-mediated carbonation with carbon mineralization occurring in nanoscale adsorbed water films. In the extreme of pores filled with a CO₂-dominant fluid, carbonation reactions are confined to angstrom to nanometre-thick water films coating mineral surfaces, which enable metal cation release, transport, nucleation and crystallization of metal carbonate minerals. Although seemingly counterintuitive, laboratory studies have demonstrated facile carbonation rates in these low-water environments, for which a better mechanistic understanding has come to light in recent years. The overarching objective of this Review is to delineate the unique underlying molecular-scale reaction mechanisms that govern CO₂ mineralization in these reactive and dynamic quasi-2D interfaces. We highlight the importance of understanding unique properties in thin water films, such as how water dielectric properties, and consequently ion solvation and hydration behaviour, can change under nanoconfinement. We conclude by identifying important frontiers for future work and opportunities to exploit these fundamental chemical insights for decarbonization technologies in the twenty-first century.

A Database of Solution Additives Promoting Mg2+ Dehydration and the Onset of MgCO3 Nucleation

Formed via aqueous carbonation of Mg²⁺ ions, the crystallization of magnesite (MgCO₃) is a promising route to carbon capture and reuse, albeit limited by the slow precipitation of MgCO₃. Although magnesite is naturally abundant, forming at low temperature conditions, its industrial production is an energy-intensive process due to the temperatures required to prevent the formation of hydrated phases. The principal difficulty in aqueous conditions arises from the very strong Mg²⁺···H₂O interaction, with high barriers to Mg²⁺ dehydration. Using atomistic simulations, we have investigated the influence of 30 additive anions (X ^n-, n = 1-3), ranging from simple halides to more complex molecules, on the first two steps of MgCO₃ aggregation from solution, as follows: Mg²⁺ dehydration and subsequent prenucleative Mg²⁺···CO₃ ^2- pairing. We have computed the thermodynamic stabilities of solvent shared ion pairs (Mg²⁺···H₂O···X ^n-) and contact ion pairs (Mg²⁺···X ^n-) to reveal the propensity of solution additives to inhibit or promote Mg²⁺···CO₃ ^2- formation. We have determined the stabilization of undercoordinated hydrated Mg²⁺ states with a vacant coordination site to which CO₃ ^2- can bind, subsequently initiating MgCO₃ nucleation or Mg²⁺ incorporation into the crystal lattice. Extensive molecular dynamics simulations of electrolyte solutions containing Na₂CO₃ with different sources of Mg²⁺ (i.e., MgCl₂, MgSO₄, and Mg(CH₃COO)₂) further show that the degree of dehydration of Mg²⁺ and the structure of prenucleation MgCO₃ clusters change depending on the counterion identity. Through a fundamental understanding of the role of solution additives in the mechanism of Mg²⁺ dehydration, our results help to rationalize previously reported experimental observation of the effect of solvation environments on the growth of magnesite. This understanding may contribute to identifying the solution composition and conditions that could promote the low-temperature CO₂ conversion into MgCO₃ at industrially relevant scales.

Low-Temperature Synthesis of Disordered Dolomite and High-Magnesium Calcite in Ethanol-Water Solutions: The Solvation Effect and Implications

How dolomite [CaMg(CO₃)₂] forms is still underdetermined, despite over a century of efforts. Challenges to synthesizing dolomite at low temperatures have hindered our understanding of sedimentary dolomite formation. Unlike calcium, magnesium's high affinity toward water results in kinetic barriers from hydration shells that prevent anhydrous Ca-Mg carbonate growth. Previous synthesis studies show that adding low-dielectric-constant materials, such as dioxane, dissolved sulfide, and dissolved silica, can catalyze the formation of disordered dolomite. Also, polar hydrophilic amino acids and polysaccharides, which are very common in biomineralizing organisms, could have a positive role in stimulating Mg-rich carbonate precipitation. Here, we show that disordered dolomite and high-magnesium calcite can be precipitated at room temperature by partially replacing water with ethanol (which has a lower dielectric constant) and bypassing the hydration barrier. Increasing the ethanol volume percentage of ethanol results in higher Mg incorporation into the calcite structure. When the ethanol volume percentage increases to 75 vol %, disordered dolomite (>60 mol % MgCO₃) can rapidly precipitate from a solution with [Mg²⁺] and [Ca²⁺] mimicking seawater. Thus, our results suggest that the hydration barrier is the critical kinetic inhibitor to primary dolomite precipitation. Ethanol synthesis experiments may provide insights into other materials that share similar properties to promote high-Mg calcite precipitation in sedimentary and biomineral environments.

Reactive force fields for aqueous and interfacial magnesium carbonate formation

We develop Mg/C/O/H ReaxFF parameter sets for two environments: an aqueous force field for magnesium ions in solution and an interfacial force field for minerals and mineral-water interfaces. Since magnesium is highly ionic, we choose to fix the magnesium charge and model its interaction with C/O/H through Coulomb, Lennard-Jones, and Buckingham potentials. We parameterize the forcefields against several crystal structures, including brucite, magnesite, magnesia, magnesium hydride, and magnesium carbide, as well as Mg²⁺ water binding energies for the aqueous forcefield. Then, we test the forcefield for other magnesium-containing crystals, solvent separated and contact ion-pairs and single-molecule/multilayer water adsorption energies on mineral surfaces. We also apply the forcefield to the forsterite-water and brucite-water interface that contains a bicarbonate ion. We observe that a long-range proton transfer mechanism deprotonates the bicarbonate ion to carbonate at the interface. Free energy calculations show that carbonate can attach to the magnesium surface with an energy barrier of about 0.22 eV, consistent with the free energy required for aqueous Mg-CO₃ ion pairing. Also, the diffusion constant of the hydroxide ions in the water layers formed on the forsterite surface are shown to be anisotropic and heterogeneous. These findings can help explain the experimentally observed fast nucleation and growth of magnesite at low temperature at the mineral-water-CO₂ interface in water-poor conditions.

Thin Water Films Enable Low-Temperature Magnesite Growth Under Conditions Relevant to Geologic Carbon Sequestration

Injecting supercritical CO₂ (scCO₂) into basalt formations for long-term storage is a promising strategy for mitigating CO₂ emissions. Mineral carbonation can result in permanent entrapment of CO₂; however, carbonation kinetics in thin H₂O films in humidified scCO₂ is not well understood. We investigated forsterite (Mg₂SiO₄) carbonation to magnesite (MgCO₃) via amorphous magnesium carbonate (AMC; MgCO₃·xH₂O, 0.5 < x < 1), with the goal to establish the fundamental controls on magnesite growth rates at low H₂O activity and temperature. Experiments were conducted at 25, 40, and 50 °C in 90 bar CO₂ with a H₂O film thickness on forsterite that averaged 1.78 ± 0.05 monolayers. In situ infrared spectroscopy was used to monitor forsterite dissolution and the growth of AMC, magnesite, and amorphous SiO₂ as a function of time. Geochemical kinetic modeling showed that magnesite was supersaturated by 2 to 3 orders of magnitude and grew according to a zero-order rate law. The results indicate that the main drivers for magnesite growth are sustained high supersaturation coupled with low H₂O activity, a combination of thermodynamic conditions not attainable in bulk aqueous solution. This improved understanding of reaction kinetics can inform subsurface reactive transport models for better predictions of CO₂ fate and transport.

New insights into the role of solution additive anions in Mg2+ dehydration: implications for mineral carbonation

Computer simulations of the Mg²⁺ dehydration mechanism show that solution additives can stabilise undercoordinated Mg²⁺ hydration configurations, opening up coordination sites on the central Mg²⁺ ion, promoting Mg-carbonates nucleation and growth.