Solubility product of amorphous magnesium carbonate

Investigation of the structure and dynamics of amorphous calcium carbonate by NMR: stabilization by poly-aspartate and comparison to monohydrocalcite

Dense amorphous phases are key intermediates in biomineralization pathways. Structural information is required to understand these pathways, but is, as per the amorphous nature, difficult to obtain. We report an investigation of amorphous calcium carbonate (ACC) with magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy. Mimicking the use of acidic proteins, we stabilize ACC against crystallization with poly-aspartate (PAsp). Spectra are in agreement with incorporation of PAsp into ACC nanoparticles and show that it forms an α-helix. The pH of the solution, from which PAsp-stabilized ACC is synthesized, affects the 13C chemical shift of carbonate in a way that is identical for additive-free ACC. Generally, we observe that the magnetic properties of the 1H and 13C nuclei in the rigid environment of ACC are similar (though not identical) to those in monohydrocalcite (MHC). This allows us to establish, based on 1H-13C correlation spectra, relaxation properties, and spin dynamics simulations, that the structural water molecules in ACC undergo 180° flips on a millisecond time scale.

Structural order of Mg-stabilized amorphous calcium carbonate and its associated phase transformation

Biominerals formed by marine organisms exhibit intricate structures and support a remarkable range of functionalities. Recent advances in our understanding of biomineralization highlight the pivotal role of magnesium-stabilized amorphous calcium carbonate (Mg-ACC) as a transient precursor in the formation of calcareous biominerals. This feature article reviews recent in vitro studies of Mg-ACC, illustrating the concepts of particle attachment, secondary nucleation, domain segregation, and mesocrystal formation. Some conceptual issues associated with the ongoing debate between classical nucleation theory and non-classical nucleation theory are discussed. We suggest that solid-state NMR measurements of the van Vleck second moment provide a stringent test for any proposed atomic model of Mg-ACC. The coordination environment of Mg2+ ions and the significance of bicarbonate ions in Mg-ACC are discussed. The diffusion of Mg2+ ions within the calcitic lattice of high-Mg calcite offers a mechanistic insight into the 'dolomite problem'. The fusion of neighboring high-Mg calcite spherulites highlights their potential role as versatile building blocks for calcareous biomineral skeletal frameworks. Altogether, this article aims to provide a comprehensive understanding of the structural complexity and transformation pathways of Mg-ACC.

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.

Soil microbial community structure and environmental effects of serpentine weathering under different vegetative covers in the serpentine mining area of Donghai County, China

The use of serpentine biological weathering to capture atmospheric CO₂ has attracted much attention. In the long-term mining activities in a serpentine mining area, a large amount of serpentine powder diffused into the surrounding forest and farmland soil. The study of the serpentine weathering in soils of different vegetative covers and the composition characteristics of soil carbonate has important implications for understanding the serpentine weathering and carbon sequestration under natural conditions. The microbial diversity on exposed rock serpentine surfaces and soil under different vegetative covers in the serpentine mining area in Donghai County, China was investigated by high-throughput sequencing technology, and the characteristics of serpentine weathering and soil carbonate in related area were also explored by XRF, XRD, SEM-EDS, and chemical analysis methods. The results showed that the richness and uniformity of the bacteria species community increased significantly with the increasing complexity of plant groups covering the rock surface, but the species richness and uniformity of fungi showed an overall declining trend. Furthermore, high‑magnesium calcite (HMC) is ubiquitous on the exposed rock surface and the soil under different vegetative covers in this area. Based on these results, combined with the verification test results of HMC fixed heavy metal ions, the model of serpentine weathering in serpentine mining soil to synthesize carbonate and fix heavy metal ions was developed. That is, with the increase in the degree of rock weathering and the colonization of plants, the soil and plants seem to shape jointly a relatively stable microbial community structure adapted to the environment of the serpentine mining area, which promotes the serpentine weathering coupled with the formation of HMC and immobilization of metal ions in the serpentine soil. This study provides a theoretical basis for the serpentine bio-weathering in the mine area to capture atmospheric CO₂.

New Insights into Fabrication of Al-Based Foam with Homogeneous Small Pore-Structure Using MgCO3/Zn Composite Powder as a Foaming Agent

Due to its excellent mechanical properties and ultra-lightweight, Al-based foam with homogeneous small pore-structures has wide applicational prospects in many industrial fields. However, during the foaming process of molten Al, it is difficult to manipulate the pore structures of the Al-based foam by means of the ALPORAS© production route due to the violent gas-releasing performance of TiH2 as a traditional foaming agent. Herein, we developed the melt-foaming route, that is, using MgCO3/Zn composite powder as a foaming agent instead of TiH2, the Al-based foam with homogeneous small pore-structures (average diameter was about 1 mm) was prepared successfully. Meanwhile, the decomposition model of the MgCO3/Zn composite powder was proposed and further verified experimentally. The decomposition kinetics of the MgCO3/Zn composite powder was also analyzed. Our findings not only shed light on the practical manufacturing of Al-based foam with homogeneous small pore-structures, but provide an insightful improvement for melt-foaming approaches.