Crystalline Calcium Carbonate Research Articles

Amorphous Calcium Carbonate Enhances Fracture Healing in a Rat Fracture Model

Background: Delayed and failed fracture repair and bone healing remain significant public health issues. Dietary supplements serve as a safe, inexpensive, and non-surgical means to aid in different stages of fracture repair. Studies have shown that amorphous calcium carbonate (ACC) is absorbed 2 to 4.6 times more than crystalline calcium carbonate in humans. Objectives: In the present study, we assessed the efficacy of ACC on femoral fracture healing in a male Wistar rat model. Methods: Eighty male Wistar rats were randomly divided into five groups (n = six per group): sham, fracture + water, fracture + 0.5× (206 mg/kg) ACC, fracture + 1× ACC (412 mg/kg), and fracture + 1.5× (618 mg/kg) ACC, where ACC refers to the equivalent supplemental dose of ACC for humans. A 21-gauge needle was placed in the left femoral shaft, and we then waited for three weeks. After three weeks, the sham group of rats was left without fractures, while the remaining animals had their left mid-femur fractured with an impactor, followed by treatment with different doses of oral ACC for three weeks. Weight-bearing capacity, microcomputed tomography, and serum biomarkers were evaluated weekly. After three weeks, the rats were sacrificed, and their femur bones were isolated to conduct an evaluation of biomechanical strength and histological analysis. Results: Weight-bearing tests showed that treatment with ACC at all the tested doses led to a significant increase in weight-bearing capacity compared to the controls. In addition, microcomputed tomography and histological studies revealed that ACC treatment improved callus formation dose-dependently. Moreover, biomechanical strength was improved in a dose-dependent fashion in ACC-treated rats compared to the controls. In addition, supplementation with ACC significantly lowered bone formation and resorption marker levels two–three weeks post-fracture induction, indicating accelerated fracture recovery. Conclusions: Our preliminary data demonstrate that ACC supplementation improves fracture healing, with ACC-supplemented rats healing in a shorter time than control rats.

Influence of typical soil minerals on ureolytic bacteria‐induced carbonate precipitation

AbstractMicrobial‐induced carbonate precipitation (MICP) has been widely applied in soil remediation, stabilization and soil carbon storage. This study investigated the effects of different soil minerals on carbonate production during MICP. Aqueous sorption experiments, along with chemical and microstructural analyses, were conducted to monitor the progress and performance of MICP. Montmorillonite significantly enhanced carbonate production, while quartz, kaolinite and goethite adsorbed free Ca2+ from the solution, slowing the formation of calcium carbonate without affecting the overall carbonate yield. Calcite and vaterite were identified as the primary carbonate phases, with vaterite dominating (accounting for ~90% of the crystalline calcium carbonate). However, montmorillonite substantially increased the proportion of calcite (from 9% to 52%). The enhancement may be attributed to the lower Ca2+ adsorption capacity of montmorillonite, which helps maintain sufficient Ca2+ in solution and promote direct calcite precipitation. Additionally, interactions between montmorillonite and ureolytic bacteria may also protect bacteria from being co‐precipitated with carbonate minerals. Soil minerals were also found to accelerate the urease activity of ureolytic bacteria, with goethite having the strongest effect. Since carbonate was not a limiting factor in the experiment, goethite had no significant impacts on carbonate yield. This study sheds light on the potential application of MICP under different soil mineral conditions.

Oxygen isotope fractionation during amorphous to crystalline calcium carbonate transformation at varying relative humidity and temperature

Crystalline calcium carbonate isotope compositions have been widely used to reconstruct past environments. However, if their isotopic compositions are modified because of crystallization from an amorphous precursor, their reliability as paleo-geochemical proxies can be compromised. This study explored the changes in the oxygen isotope compositions during the transformation of amorphous calcium carbonate (ACC) into crystalline carbonate under different conditions of relative humidity (RH of 33–95 %), temperature (T of 5 °C and 20 °C) and in the presence/absence of atmospheric CO2. The data showed that at low RH and T (e.g., RH ≤ 45 % and 5 °C) when a complete ACC-crystalline carbonate transformation did not take place then the original ACC δ18O values (δ18OCaCO3 = −15.9 ± 1.0 ‰, VPDB) were preserved throughout the experimental runtime (up to 144 days). In contrast, in fully crystallized CaCO3 (e.g., at RH ≥ 60 %) the δ18OCaCO3 values increased rapidly over the first few days, followed by a slower and gradual increase. By the end of the experiments (i.e., after 103–144 days) the crystalline δ18OCaCO3 values ranged from −10.4 ‰ to −8.1 ‰ in the presence of atmospheric CO2 and from −12.6 ‰ to −9.5 ‰ in the CO2-free experiments. These changes in oxygen isotope compositions of the CaCO3 reaction products (calcite and/or vaterite) were mainly driven by exchange with H2O from the hydrated ACC i.e. the synthesis fluid. In CO2-present experiments, oxygen isotope fractionation factors between the CaCO3 reaction products and the synthesis fluid (18αc–w) approached or exceeded oxygen isotope equilibrium values. This could be explained by a decrease in the initially high pH of the aqueous fluid released from ACC dissolution during CO2 hydration/hydroxylation, which would have increased the oxygen isotope exchange kinetics between H2O and dissolved inorganic carbon (DIC). In some experiments, the hydration/hydroxylation of 18O-enriched CO2, due to isotopic salt-effects, might have also resulted in 18O-enriched calcium carbonates and calculated fractionation factors that exceeded equilibrium values. In the CO2-free experiments, isotopic equilibrium between the crystalline phase and the synthesis fluid was not reached. This oxygen isotope disequilibrium suggests that without the pH lowering effect of the hydroxylation/hydration of CO2, the CO32− released during ACC/calcite dissolution-reprecipitation may have not isotopically equilibrated with the high pH synthesis fluid due to the long equilibration times required to reach isotope equilibrium at high pH values, leading to the self-buffering of δ18OCaCO3 values. The results suggest that the oxygen isotopic compositions of natural carbonates formed from ACC transformation in air and at low water/solid ratio (e.g., biominerals or carbonates formed in caves) are complex and cannot be used as simple proxies if the reaction kinetics (RH/CO2/T) and H2O sources are not known and quantified.

Enhanced subsurface chloride transport resistance of cement pastes via optimizing CO2 curing time

The chloride erosion of CO2 cured cementitious materials could be a concern for concrete structure application. This study aims to clarify the chloride transport and binding behavior by elucidating the alteration of free and bound chloride content from the outer towards the core (5 layers with 2 mm intervals). The carbonation depth and chloride binding ratio at the carbonation zone were also determined. The results showed that dense carbonated surface significantly reduced the penetration of chloride, especially in the carbonation zone, resulting in a decrease in total chloride content (48 %–83 %) and a slight increase in the chloride binding ratio (up to 22 %). It is found that the benefits of minimizing surface porosity to impede chloride penetration significantly outweigh the drawbacks associated with the consumption of AFm phase and C-S-H gel. However, as prolonged the CO2 curing time to 7 days, the surface areas with the highest carbonation degree exhibited more chloride content than the inward partially carbonated zone. This is because excessive carbonation at early age could increase the surface pore structures, resulting in the promotion of chloride transport. It was found that the content of chloride and CaCO3 are strongly correlated with CO2 curing time, and CO2 at first 24 h curing can achieve greater benefits in reducing chloride penetration. Moreover, amorphous calcium carbonate gradually transformed into crystalline calcium carbonate during the process of chloride attack, conducive to the further improvement of the compactness of the sample.

Liquid-liquid phase transition as a basis for novel materials for skin repair and regeneration.

Inorganic materials are of increasing interest not only for bone repair but also for other applications in regenerative medicine. In this study, the combined effects of energy-providing, regeneratively active inorganic polyphosphate (polyP) and also morphogenetically active pearl powder on wound healing were investigated. Aragonite, the mineralic constituent of pearl nacre and thermodynamically unstable form of crystalline calcium carbonate, was found to be converted into a soluble state in the presence of a Ca2+-containing wound exudate, particularly upon addition of sodium polyP (Na-polyP), driven by the transfer of Ca2+ ions from aragonite to polyP, leading to liquid-liquid phase separation to form an aqueous Ca-polyP coacervate. This process is further enhanced in the presence of Ca-polyP nanoparticles (Ca-polyP-NP). Kinetic studies revealed that the coacervation of polyP and nacre aragonite in wound exudate is a very rapid process that results in the formation of a stronger gel with a porous structure compared to polyP alone. Coacervate formation, enabled by phase transition of crystalline aragonite in the presence of Na-polyP/Ca-polyP-NP and wound exudate, could also be demonstrated in a hydroxyethyl cellulose-based hydrogel used for wound treatment. Furthermore, it is shown that Na-polyP/Ca-polyP-NP together with nacre aragonite strongly enhances the proliferation of mesenchymal stem cells and promotes microtube formation in the in vitro angiogenesis assay with HUVEC endothelial cells. The latter effect was confirmed by gene expression studies, applying real-time polymerase chain reaction, using the biomarker genes VEGF (vascular endothelial growth factor) and hypoxia-inducible factor-1 α (HIF-1α). Division of Escherichia coli is suppressed when suspended in a matrix containing Na-polyP/Ca-polyP-NP and aragonite. The potential medical relevance of these findings is supported by an animal study on genetically engineered diabetic mice (db/db), which demonstrated a marked increase in granulation tissue and microvessel formation in regenerating experimental wounds treated with Ca-polyP-NP compared to controls. Co-administration of aragonite significantly accelerated the wound healing-promoting effect of polyP in db/db mice. Based on these results, we propose that the ability of polyP to form a mixed coacervate with aragonite, in addition to its energy (ATP)-generating function, can decisively contribute to the regenerative activity of this polymer in wound repair.

Impact of dissolved CO2 on calcification in two large, benthic foraminiferal species.

Rising atmospheric CO2 shifts the marine inorganic carbonate system and decreases seawater pH, a process often abbreviated to 'ocean acidification'. Since acidification decreases the saturation state for crystalline calcium carbonate (e.g., calcite and aragonite), rising dissolved CO2 levels will either increase the energy demand for calcification or reduce the total amount of CaCO3 precipitated. Here we report growth of two large benthic photosymbiont-bearing foraminifera, Heterostegina depressa and Amphistegina lessonii, cultured at four different ocean acidification scenarios (400, 700, 1000 and 2200 ppm atmospheric pCO2). Using the alkalinity anomaly technique, we calculated the amount of calcium carbonate precipitated during the incubation and found that both species produced the most carbonate at intermediate CO2 levels. The chamber addition rates for each of the conditions were also determined and matched the changes in alkalinity. These results were complemented by micro-CT scanning of selected specimens to visualize the effect of CO2 on growth. The increased chamber addition rates at elevated CO2 concentrations suggest that both foraminifera species can take advantage of the increased availability of the inorganic carbon, despite a lower saturation state. This adds to the growing number of reports showing the variable response of foraminifera to elevated CO2 concentrations, which is likely a consequence of differences in calcification mechanisms.

Investigation on mechanical properties of biomimetic mineralized mortar incorporated with boric acid modifier

Microbially induced carbonate precipitation (MICP) has been intensively studied as a potential method to improve engineering properties of soil. However, MICP has limitations due to the introduction of microorganisms and the difficulty of controlling the calcium carbonate crystal precipitation model. This study proposed biomimetic mineralized mortar based on the biomimetic-chemically induced carbonate precipitation (BCICP) method with boric acid as a crystal modifier. Similar to the bacteria in MICP, boric acid can promote the nucleation of calcium carbonate and the adsorption of calcium carbonate on the surface of sand particles. Besides, the boric acid concentration can effectively change the precipitation mode of calcium carbonate. Unconfined compression strength (UCS) test and microscopic characterization were conducted to investigate the mechanism of boric acid modifying calcium carbonate crystal growth and precipitation morphology. The results show that with the increase of boric acid concentration, the reinforcement efficiency of calcium carbonate increased. With the increase of boric acid concentration, the crystalline calcium carbonate changed from a cube-shaped calcite with a smooth surface to a spherical calcite with a rough surface and the coating pattern of calcium carbonate distribution on the surface of sand particles was weakened, while the bonding-filling pattern was enhanced. Besides, the cementing fluid concentration can also affect the reinforcement efficiency of calcium carbonate and with the participation of boric acid, biomimetic mineralized mortar can use the high concentration cementing fluid to achieve effective strength enhancement. This study confirms that boric acid could effectively modify the form and distribution of calcium carbonate between sand particles, and enhance the cementation between calcium carbonate and sand particles.

Bottling Liquid-Like Minerals for Advanced Materials Synthesis.

Materials synthesis via liquid-like mineral precursors has been studied since their discovery almost 25 years ago, because their properties offer several advantages, for example, the ability to infiltrate small pores, the production of non-equilibrium crystal morphologies or mimicking textures from biominerals, resulting in a vast range of possible applications. However, the potential of liquid-like precursors has never been fully tapped, and they have received limited attention in the materials chemistry community, largely due to the lack of efficient and scalable synthesis protocols. Herein, the "scalable controlled synthesis and utilization of liquid-like precursors for technological applications" (SCULPT) method is presented, allowing the isolation of the precursor phase on a gram scale, and its advantage in the synthesis of crystalline calcium carbonate materials and respective applications is demonstrated. The effects of different organic and inorganic additives, such as magnesium ions and concrete superplasticizers, on the stability of the precursor are investigated and allow optimizing the process for specific demands. The presented method is easily scalable and therefore allows synthesizing and utilizing the precursor on large scales. Thus, it can be employed for mineral formation during restoration and conservation applications but can also open up pathways toward calcium carbonate-based, CO2 -neutral cements.

Transformation of amorphous calcium carbonate in the presence of magnesium, phosphate, and mineral surfaces

Amorphous calcium carbonate (ACC) is known as a precursor to crystalline calcium carbonate polymorphs (CaCO3) in various natural and synthetic systems. While ACC has been shown to form by heterogeneous nucleation in biological systems, it is less obvious whether it plays any role in inorganic settings where carbonate minerals grow on pre-existing surfaces. Tight assemblies of calcium carbonate (CaCO3) phases and clay minerals occur in rock-forming quantities in marlstones, are abundant in Earth’s critical zone, in the sediments of aqueous environments, and in the atmosphere as aerosol particles. Previous studies suggested that the co-occurrence of clay and carbonate minerals (especially smectite and calcite) results from the heterogeneous nucleation of CaCO3 on the surface of smectite. In order to understand the role of ACC in the heterogeneous nucleation of carbonate minerals and the effects of common silicate mineral surfaces on the stability of ACC, we synthesized ACC both in the presence and absence of smectite, kaolinite, and quartz, monitored the pH of the solution during the experiments, and studied the precipitated phases with various transmission electron microscopy techniques. Since Mg2+ and (PO43−) ions are known to prolong the lifetime of ACC and are important components in natural aqueous environments, we tested their effects on ACC transformation in both homogeneous and heterogeneous systems.In all studied systems ACC was the first condensed carbonate phase. The presence of smectite in the solution reduced the lifetime of ACC and led to the rapid formation of calcite. We suggest that the main factor driving the transition was the enhanced growth of ACC particles on the surfaces of smectite flakes. On the other hand, the presence of kaolinite and quartz slightly increased ACC stability; ACC typically rimmed the silicate particles and transformed slower than separated, individual ACC particles. When Mg2+ was present in the solution, ACC stability significantly increased and led to the formation of aragonite instead of calcite in both homogeneous and heterogeneous systems. The presence of (PO4)3− ions caused only a minor delay in ACC transformation and the crystalline product was calcite, irrespective of the presence of silicate minerals. These observations suggest that smectite minerals play important roles in the formation of carbonate minerals by providing a suitable surface for the rapid growth and coalescence of ACC particles, leading to larger particle sizes which, in turn, result in the conversion of ACC into calcite.

Accelerated and natural carbonation of a municipal solid waste incineration (MSWI) fly ash mixture: Basic strategies for higher carbon dioxide sequestration and reliable mass quantification

The carbonation of alkaline wastes is an interesting research field that may offer opportunities for CO2 reduction. However, the literature is mainly devoted to studying different waste sequestration capabilities, with lame attention to the reliability of the data about CO2 reduction, or to the possibilities to increase the amount of absorbed CO2.In this work, for the first time, the limitation of some methods used in literature to quantify the amount of sequestered CO2 is presented, and the advantages of using suitable XRD strategies to evaluate the crystalline calcium carbonate phases are demonstrated. In addition, a zero-waste approach, aiming to stabilize the waste by coupling the use of by-products and the possibility to obtain CO2 sequestration, was considered. In particular, for the first time, the paper investigates the differences in natural and accelerated carbonation (NC and AC) mechanisms, occurring when municipal solid waste incineration (MSWI) fly ash is stabilized by using the bottom ash with the same origin, and other by-products. The stabilization mechanism was attributed to pozzolanic reactions with the formation of calcium silicate hydrates or calcium aluminate hydrate phases that can react with CO2 to produce calcium carbonate phases. The work shows that during the AC, crystalline calcium carbonate was quickly formed by the reaction of Ca(OH)2 and CaClOH with CO2. On the contrary, in NC, carbonation occurred due to reactions also with the amorphous Ca. The sequestration capability of this technology, involving the mixing of waste and by-products, is up to 165 gCO2/Kg MSWI FA, which is higher than the literature data.

Amorphous Calcium Carbonate Cluster Nanospheres in Water-Deficient Organic Solvents.

As the key intermediate phase of crystalline calcium carbonate biominerals, amorphous calcium carbonate (ACC) remains mysterious in its structures because of its long-range disorder and instability. We herein report the synthesis of ACC nanospheres in a water-deficient organic solvent system. The obtained ACC nanospheres are very stable under dry conditions. Cryo-TEM reveals that each nanospheres consists of smaller nanosized clusters. We further demonstrate that these clusters can precipitate on other substrates to form an ultrathin ACC coating, which should be an ACC cluster monolayer. The results demonstrate that the presence of small ACC clusters as the subunits of larger aggregates is inherent to ACC synthesized in water-alcohol system but not induced by polymer additives.

Amorphous Calcium Carbonate Cluster Nanospheres in Water‐Deficient Organic Solvents

AbstractAs the key intermediate phase of crystalline calcium carbonate biominerals, amorphous calcium carbonate (ACC) remains mysterious in its structures because of its long‐range disorder and instability. We herein report the synthesis of ACC nanospheres in a water‐deficient organic solvent system. The obtained ACC nanospheres are very stable under dry conditions. Cryo‐TEM reveals that each nanospheres consists of smaller nanosized clusters. We further demonstrate that these clusters can precipitate on other substrates to form an ultrathin ACC coating, which should be an ACC cluster monolayer. The results demonstrate that the presence of small ACC clusters as the subunits of larger aggregates is inherent to ACC synthesized in water‐alcohol system but not induced by polymer additives.

Bio-inspired films of crystalline calcium carbonate: 2D patterning by surface-driven liquid–liquid phase separation and hybrid amorphous-to-crystal transformation

Bio-inspired films of crystalline calcium carbonate: 2D patterning by surface-driven liquid–liquid phase separation and hybrid amorphous-to-crystal transformation

Biocalcifying Potential of Ureolytic Bacteria Isolated from Soil for Biocementation and Material Crack Repair.

Microbially induced calcium carbonate precipitation (MICP) has been highlighted for its application in civil engineering, and in the environmental and geotechnical fields. Ureolytic activity is one of the most promising bacterial mechanisms in terms of inducing calcium carbonate formation. In this study, four bacterial isolates with high-yield urease production capabilities were obtained from two-step screening using a high-buffered urea medium. The highest urease activity and calcium carbonate formation was observed in Lysinibacillus fusiformis 5.1 with 4.40 × 103 unit/L of urease and 24.15 mg/mL of calcium carbonate, followed by Lysinibacillus xylanilyticus 4.3 with 3.93 × 103 unit/L of urease and 22.85 mg/mL of calcium carbonate. The microstructure of the precipitated crystalline calcium carbonate was observed using scanning electron microscopy. X-ray diffraction analysis confirmed that the main polymorph of the calcium carbonate particle obtained from both isolates was calcite. Examination of the material-crack filling in mortar specimens showed that calcite layers had formed along the crack edges and inside after 10 days, and gradually filled the cracks up to the upper surface. These results showed that these two isolates presented robust characteristics of potential MICP-inducing bacteria for civil engineering and material engineering applications.

Magnesium Ions Direct the Solid‐State Transformation of Amorphous Calcium Carbonate Thin Films to Aragonite, Magnesium‐Calcite, or Dolomite

AbstractAmorphous calcium carbonate (ACC) is a common precursor to crystalline calcium carbonate, and is of particular importance in biomineralization, where its crystallization in privileged environments ensures a pseudomorphic transformation. While organisms regulate this process using organic molecules and magnesium ions to selectively form calcite or aragonite, it has proven highly challenging to replicate this polymorph selectivity synthetically. Here, it is demonstrated that remarkable control can be achieved over the chemical composition and structure of crystalline calcium carbonate by using heat to drive a pseudomorphic transformation of ACC thin films. The crystal polymorph can be tuned from low magnesium‐calcite to pure aragonite, high magnesium‐calcite, and ultimately dolomite according to the magnesium content of the ACC, and mosaics of large single crystals are generated at elevated temperatures rather than the spherulitic structures formed at room temperature. This methodology also enables an in situ investigation of the ACC crystallization mechanism using transmission electron microscopy. Finally, the approach can be combined with templating methods to generate arrays of large aragonite single crystals with preselected morphologies. These results demonstrate that exceptional control can be achieved through the solid‐state transformation of Mg‐ACC, which has relevance to both synthetic and biological systems.

Removal of Aqueous Cu2+ by Amorphous Calcium Carbonate: Efficiency and Mechanism

Crystalline calcium carbonate (CaCO3, such as calcite) could scavenge aqueous metals via adsorption and coprecipitation. As a precursor to crystalline CaCO3, amorphous calcium carbonate (ACC) is poorly understood on metals removal. Herein, we synthesized silica-stabilized ACC and investigated its Cu2+ removal efficiency and mechanism. The results showed that the Cu2+ removal efficiency by ACC is controlled by the initial solution pH, initial Cu2+ concentration, contacting time, and ACC dosage. The maximum Cu2+ removal capacity was 543.4 mg/g at an ACC dosage of 1 g/L, an initial pH of 5.0, an initial Cu2+ concentration of 1000 mg/L, and an equilibrium time of 20 h. X-ray powder diffraction (XRD) and scanning electron microscope with an energy dispersive spectrometer (SEM-EDS) revealed that Cu2+ precipitated as paratacamite (Cu2(OH)3Cl, space group: R3¯) at an ACC dosage of 1 g/L, whereas botallackite (Cu2(OH)3Cl, space group: P21/m) was the Cu-bearing product for crystalline calcite using the same dosage as ACC. However, Cu2+ preferred to incorporate into calcite, which is transformed from ACC at high ACC loading (such as 4 g/L). Our results demonstrated that the crystallinity and dosage of CaCO3 could control the Cu2+ removal mechanism.

An exceptionally stable and widespread hydrated amorphous calcium carbonate precipitated by the dog vomit slime mold Fuligo septica (Myxogastria)

Biogenic amorphous calcium carbonate (ACC) is typically metastable and can rapidly transform through aging, dehydration, and/or heating to crystalline calcium carbonate. Gaining insight into its structure and properties is typically hampered by its tendency to crystallize over short time periods once isolated from the host organism, and also by the small quantities that are usually available for study. Here we describe an exceptionally stable hydrated ACC (HACC) precipitated by the cosmopolitan slime mold Fuligo septica (L.) F.H. Wigg. (1780). A single slime mold can precipitate up to a gram of HACC over the course of one night. Powder x-ray diffraction (XRD) patterns, transmission electron microscopy images, infrared absorption spectra, together with the lack of optical birefringence are consistent with an amorphous material. XRD simulations, supported by thermogravimetric and evolved gas analysis data, are consistent with an intimate association of organic matter with ~ 1-nm-sized ACC units that have monohydrocalcite- and calcite-like nano-structural properties. It is postulated that this association imparts the extreme stability of the slime mold HACC by inhibiting loss of H2O and subsequent crystallization. The composition, structure, and thermal behavior of the HACC precipitated by F. septica collected over 8000 km apart and in markedly different environments, suggests a common structure, as well as similar biochemical and biomineralization mechanisms.

Growth of calcium carbonate crystal on various substrates from a saturated calcium carbonate solution utilizing difference in solubility

Crystalline calcium carbonate was deposited on various substrates (substrate temperature of 80 °C) from a saturated calcium hydrogen carbonate solution (solution temperature of 10 °C). A single aragonite phase deposited on the Al substrate, whereas a mixed phase of aragonite and calcite grew on the soda lime glass, borosilicate glass, silica glass, and polycarbonate substrates. With increasing deposition time, the crystal size and the proportion of calcite on the soda lime silicate glass substrate increased. Furthermore, calcite crystals were preferentially deposited on an Al substrate with a thin film of Au as a buffer layer (Au/Al substrate).

Structural analyses of amorphous calcium carbonate before and after removing strontium ions from an aqueous solution

Amorphous calcium carbonate (ACC) is a precursor of crystalline calcium carbonate; hence, its structural information at the atomic level is very important for controlling the morphology of crystalline calcium carbonate. In this study, we attempted to elucidate the process of Sr extraction from aqueous solution by using ACC for the purpose of removal of radioactive Sr from the contaminated water leaked after the Fukushima Daiichi nuclear accident. The pair distribution functions, g(r) obtained by X-ray and neutron diffraction (ND) measurements show that ACC has a structure partially similar to that of monohydrocalcite, suggesting that ACC is transferred to the crystalline calcium carbonate starting from its crystal nucleus. Rietveld analysis of the ND data showed that the ACC that removed Sr was crystallized to calcite. However, the Sr–O coordination analyzed using extended X-ray absorption fine structure (EXAFS) implies that the local environment of O around Sr is similar to that in crystalline calcium carbonate aragonite.

Nested Formation of Calcium Carbonate Polymorphs in a Bacterial Surface Membrane with a Graded Nanoconfinement: An Evolutionary Strategy to Ensure Bacterial Survival.

It is the intention of this study to elucidate the nested formation of calcium carbonate polymorphs or polyamorphs in the different nanosized compartments. With these observations, it can be concluded how the bacteria can survive in a harsh environment with high calcium carbonate supersaturation. The mechanisms of calcium carbonate precipitation at the surface membrane and at the underlying cell wall membrane of the thermophilic soil bacterium Geobacillus stearothermophilus DSM 13240 have been revealed by high-resolution transmission electron microscopy and atomic force microscopy. In this Gram-positive bacterium, nanopores in the surface layer (S-layer) and in the supporting cell wall polymers are nucleation sites for metastable calcium carbonate polymorphs and polyamorphs. In order to observe the different metastable forms, various reaction times and a low reaction temperature (4 °C) have been chosen. Calcium carbonate polymorphs nucleate in the confinement of nanosized pores (⌀ 3–5 nm) of the S-layer. The hydrous crystalline calcium carbonate (ikaite) is formed initially with [110] as the favored growth direction. It transforms into the anhydrous metastable vaterite by a solid-state transition. In a following reaction step, calcite is precipitated, caused by dissolution of vaterite in the aqueous solution. In the larger pores of the cell wall (⌀ 20–50 nm), hydrated amorphous calcium carbonate is grown, which transforms into metastable monohydrocalcite, aragonite, or calcite. Due to the sequence of reaction steps via various metastable phases, the bacteria gain time for chipping the partially mineralized S-layer, and forming a fresh S-layer (characteristic growth time about 20 min). Thus, the bacteria can survive in solutions with high calcium carbonate supersaturation under the conditions of forced biomineralization.