Calcifiers can Adjust Shell Building at the Nanoscale to Resist Ocean Acidification

Ocean acidification is considered detrimental to marine calcifiers based on laboratory studies showing that increased seawater acidity weakens their ability to build calcareous shells needed for growth and protection. In the natural environment, however, the effects of ocean acidification are subject to ecological and evolutionary processes that may allow calcifiers to buffer or reverse these short‐term negative effects through adaptive mechanisms. Using marine snails inhabiting a naturally CO₂‐enriched environment over multiple generations, it is discovered herein that they build more durable shells (i.e., mechanically more resilient) by adjusting the building blocks of their shells (i.e., calcium carbonate crystals), such as atomic rearrangement to reduce nanotwin thickness and increased incorporation of organic matter. However, these adaptive adjustments to future levels of ocean acidification (year 2100) are eroded at extreme CO₂ concentrations, leading to construction of more fragile shells. The discovery of adaptive mechanisms of shell building at the nanoscale provides a new perspective on why some calcifiers may thrive and others collapse in acidifying oceans, and highlights the inherent adaptability that some species possess in adjusting to human‐caused environmental change.     

Simple synthesis of ZrO2/SiOx core/shell nanofibers using ZrSi2 with gallium

ZrO₂/SiO_x core/shell nanofibers with diameter ~ 50 nm were synthesized by the thermal oxidation of ZrSi₂ substrates with gallium. The crystalline ZrO₂ cores were grown with amorphous SiO_x shells. It is proposed that the growth of crystalline ZrO₂ core was guided by the prior supersaturation of Zr species in the molten gallium film, whereas the amorphous SiO_x shell could be attributed to the deposition of SiO vapor on the surface of ZrO₂ core. In addition, the ZrO₂/SiO_x core/shell nanofibers show a wide visible photoluminescence (PL) emission at 480 nm, which should originate from the SiO_x shells.     

Temperature effects on freshwater snail shells: Pomacea canaliculata Lamarck as investigated by XRD,EDX, SEM and FTIR techniques

In this study, the XRD, EDX, SEM and FTIR analyses are used for the characterization of the thermal decomposition process of Pomacea canaliculata Lamarck (PCL) samples. All these shells are abundant in Thailand. The shell was ground into fine powders. A set of four samples each was then separately annealed for 2 h in air atmosphere at 300°C, 400 °C, 450 °C and 500 °C, respectively. The PCL shell mainly consists of aragonite and a fraction of calcite. If the PCL powder samples were annealed at a temperature higher than 450 °C, it resulted in an irreversible phase transformation from aragonite to calcite. The FTIR spectra analyses of PCL show that, after an annealing, the relative intensities of CO₃²⁻ absorption bands and the intensities of OH⁻ absorption band increased.     

Photoemission studies of polymorphic CaCO3 materials

The surface analysis of polymorphic calcium carbonate (CaCO₃) compounds, namely, calcite, aragonite, and vaterite were carried out by X-ray photoelectron spectroscopy (XPS). XPS results clearly demonstrate that vaterite is different from the other two and exhibit a low binding energy (BE) for all its constituent elements. It is attributed to the perpendicular orientation of CO₃²⁻ to ab plane in vaterite. Aragonite shows less calcium and more oxygen and indicates the surface is carbonate terminated. Intergrowth of calcite and aragonite and natural dolomite samples were also analysed and compared with the above CaCO₃ compounds. Ca:Mg=3 suggest that the dolomite surface is dominated by Ca.     

ACBC to Balcite: Bioinspired Synthesis of a Highly Substituted High‐Temperature Phase from an Amorphous Precursor

Energy‐efficient synthesis of materials locked in compositional and structural states far from equilibrium remains a challenging goal, yet biomineralizing organisms routinely assemble such materials with sophisticated designs and advanced functional properties, often using amorphous precursors. However, incorporation of organics limits the useful temperature range of these materials. Herein, the bioinspired synthesis of a highly supersaturated calcite (Ca_0.5Ba_0.5CO₃) called balcite is reported, at mild conditions and using an amorphous calcium–barium carbonate (ACBC) (Ca_{1? x} Ba _x CO₃·1.2H₂O) precursor. Balcite not only contains 50 times more barium than the solubility limit in calcite but also displays the rotational disorder on carbonate sites that is typical for high‐temperature calcite. It is significantly harder (30%) and less stiff than calcite, and retains these properties after heating to elevated temperatures. Analysis of balcite local order suggests that it may require the formation of the ACBC precursor and could therefore be an example of nonclassical nucleation. These findings demonstrate that amorphous precursor pathways are powerfully enabling and provide unprecedented access to materials far from equilibrium, including high‐temperature modifications by room‐temperature synthesis.     

Synthesis and structure of the perovskite oxide carbonate Ba4ScCu2O7−x(CO3)y (x ≈ 0.3, y ≈ 0.9): a new example of ordering in the small cation sites

In this paper we report further work on the perovskite oxide carbonate, Ba₄ScCu₂O_7−x(CO₃)_y (x≈0.3, y≈0.9). The X-ray diffraction pattern for this phase can be indexed on a 2a_p×2a_p×c_p cell (where a_p, c_p are the primitive perovskite cell lengths). On heating this phase to higher temperature, partial loss of CO₃²⁻ occurs and the pattern now can be indexed on a √2a_p×√2a_p×2c_p cell, similar to that previously observed for the system Ba₄YCu_2+xO_6+y(CO₃)_z. Structural studies using powder neutron diffraction show that Ba₄ScCu₂O_7−x(CO₃)_y (x≈0.3, y≈0.9) exhibits a new example of ordering within the small cation sites, with an ordered arrangement of Sc, Cu and C (from the carbonate group). It appears, however, from the refined data that this ordering may not be complete, which can probably be explained by the presence of small regions of the low carbonate content phase. Discussion of the factors influencing the relative stabilities of the high and low carbonate content phases is also presented.     

Mineral carbonation for carbon sequestration in cement kiln dust from waste piles

Alkaline earth metals, such as calcium and magnesium oxides, readily react with carbon dioxide (CO₂) to produce stable carbonate minerals. Carbon sequestration through the formation of carbonate minerals is a potential means to reduce CO₂ emissions. Calcium-rich, industrial solid wastes and residues provide a potential source of highly reactive oxides, without the need for pre-processing. This paper presents the first study examining the feasibility of carbon sequestration in cement kiln dust (CKD), a byproduct generated during the manufacturing of cement. A series of column experiments were conducted on segments of intact core taken from landfilled CKD. Based on stoichiometry and measured consumption of CO₂ during the experiments, degrees of carbonation greater than 70% of the material's potential theoretical extent were achieved under ambient temperature and pressure conditions. The overall extent of carbonation/sequestration was greater in columns with lower water contents. The major sequestration product appears to be calcite; however, more detailed material characterization is need on pre- and post-carbonated samples to better elucidate carbonation pathways and products.     

Thermal decomposition of calcium carbonate polymorphs precipitated in the presence of ammonia and alkylamines

Precipitated calcium carbonate was obtained by CO₂ bubbling in CaCl₂ solution. Ammonia and alkylamines were used to enhance CO₂ absorption and control the polymorphic phases. Vaterite and calcite mixtures were obtained, mainly vaterite in ammonia and mono-methylamine and mostly calcite in tri-methylamine environment. The thermal decomposition of precipitated calcium carbonate leads to high purity CaO. During the thermal decomposition the vaterite to calcite transformation was noticed but the final spherical shape of vaterite was maintained. As the use of CaO as catalyst in biodiesel synthesis may recommend a spherical shape and high contact surface, a precipitated calcium carbonate, rich in vaterite phase, may be a good precursor for CaO catalyst preparation.     

Production and characterization of synthetic aragonite prepared from dolomite by eco-friendly leaching–carbonation process

Dolomite is an alternative material for producing precipitated calcium carbonate (PCC) particles, which have widespread industrial applications depending on their morphology and particle sizes. These properties are readily controlled by the production conditions such as reaction time, temperature, stirring speed, and CO₂ flow rate. In this paper, we investigate the influences of these experimental conditions on the production of synthetic aragonite crystals from dolomite using a leaching carbonation process. The proposed process is believed so be more eco-friendly than other methods suggested in the literature because the CO₂ released from the dolomite during the leaching stage is stored for use in the carbonation stage. The experimental results indicate that the morphology of the produced PCC is influenced not only by the reaction time and temperature, but also the stirring speed and CO₂ flow rate. The required reaction time decreases with an increase in the CO₂ flow rates. However, calcite forms along with the aragonite crystals at higher CO₂ flow rates. We successfully synthesized pure aragonite crystals in the reaction temperature range of 40–70 °C at a fixed CO₂ flow rate of 3.00 l/min, and at a stirring speed of 750 rpm. The d₉₀ values of the aragonite crystals at various temperatures ranged from 18.47 to 25.99 μm. We fit the experimental results by a single-term exponential model.Additionally, we obtained a Mg-rich solution and CO₂ gas as by-products, which are in high demand.     

Effects of limestone powder on CaCO3 precipitation in CO2 cured cement pastes

The concept of using limestone powder as supplementary materials to accelerate cement hydration has been explored widely. There have also been interests in using CO₂ curing to enhance the early strength of concretes. This study investigated the effects of incorporating limestone powder on CO₂ curing of cement-based materials. The results showed that using limestone powder to partially replace cement could significantly increase the CO₂ curing degree of the cement pastes. QXRD analysis showed that calcite was the major reaction product, accompanied by amorphous calcium carbonate. The mass ratio of poorly crystallized calcium carbonate to highly crystallized calcium carbonate (DC/HC ratio) formed was affected by both the applied CO₂ pressure and the use of limestone powder. At low pressure, incorporating limestone powder led to an increase in the DC/HC ratio. However, a reversed trend was observed in the case of high CO₂ pressure.     

Growth of calcium carbonate crystal imitating stalagmite growth in nature

Calcite crystals were prepared by dropping a saturated calcium carbonate aqueous solution on a substrate. The calcite crystals were grown with a growth rate of approximately 0.7 μm/day under the condition of aqueous temperature of 2 °C, aqueous concentration of 0.006 mol/l, and substrate temperature of 40 °C. When the calcite substrate was used, calcite crystals were grown epitaxially. Na₂CO₃ and CaCl₂ aqueous solutions were dropped and reacted on the calcite crystal substrate with a CaCO₃ concentration of 0.06 mol/l, which is 10 times the saturated concentration at an aqueous temperature of 40 °C. Rhombohedral calcite was also deposited epitaxially with a growth rate of about 6.3 μm/day under this condition.     

In vitro growth of calcium carbonate crystals on bivalve shells: Application of two methods of synthesis

The classic ammonium carbonate vapor diffusion method (VDM) and the coprecipitation method (CM) in its modified form were applied for in vitro growth of calcium carbonate crystals on glass substrate and on calcitic and aragonitic shell layers of blue mussel Mytilus edulis (L.) and Iceland scallop Chlamys islandica (M.). The experiments were carried out using large volumes of growth medium (250 ml and 1000 ml). Crystallization using the VDM is relatively slow, but faster with the CM. The formation of calcium carbonate polymorphs is strongly influenced by the mineralogical phase in the uppermost layer of the shell substrate bathed in the experimental solution, even if magnesium ions are added to solution with the CM. The morphology of calcium carbonate crystals clearly differs between methods, and is influenced by the type of substrate. The effect of biomacromolecules released from the shell substrate on morphology and organization of calcium carbonate crystals is clearly observed with both methods of crystallization.     

Chemical Interaction of Calcium Oxide and Calcium Hydroxide with CO<Subscript>2</Subscript> during Mechanical Activation

The reactions of CaO and Ca(OH)₂ with CO₂ during mechanical activation are studied by IR spectroscopy, x-ray diffraction, and thermal analysis. The results indicate that the earlier described extensive sorption of CO₂ by Ca-containing silicates during grinding is not related to the formation of CaO or Ca(OH)₂. Mechanical activation in CO₂ converts calcium oxide to amorphous CaCO₃ and calcium hydroxide to calcite, whereas Ca-containing silicates under such conditions homogeneously dissolve CO₂ to form a material similar to analogous carbonate-containing silicate glasses.     

Fabrication of porous low crystalline calcite block by carbonation of calcium hydroxide compact

Calcium carbonate (CaCO₃) has been widely used as a bone substitute material because of its excellent tissue response and good resorbability. In this experimental study, we propose a new method obtaining porous CaCO₃ monolith for an artificial bone substitute. In the method, calcium hydroxide compacts were exposed to carbon dioxide saturated with water vapor at room temperature. Carbonation completed within 3 days and calcite was the only product. The mechanical strength of CaCO₃ monolith increased with carbonation period and molding pressure. Development of mechanical strength proceeded through two steps; the first rapid increase by bonding with calcite layer formed at the surface of calcium hydroxide particles and the latter increase by the full conversion of calcium hydroxide to calcite. The latter process was thought to be controlled by the diffusion of CO₂ through micropores in the surface calcite layer. Porosity of calcite blocks thus prepared had 36.8–48.1% depending on molding pressure between 1 MPa and 5 MPa. We concluded that the present method may be useful for the preparation of bone substitutes or the preparation of source material for bone substitutes since this method succeeded in fabricating a low-crystalline, and thus a highly reactive, porous calcite block.     

Mechanical properties and structure of Strombus gigas,Tridacna gigas,and Haliotis rufescens sea shells: A comparative study

Sea shells are composed of calcium carbonate crystals interleaved with layers of viscoelastic proteins, having dense, tailored structures that yield excellent mechanical properties. Shells such as conch (Strombus gigas), giant clam (Tridacna gigas), and red abalone (Haliotis rufescens) have hierarchical architectures that differ depending on growth requirements and shell formation of the particular mollusk. Mechanical tests have been carried out on these shells for a comparison of strength with respect to the microstructural architecture and sample orientation. The mechanical response is found to vary significantly from specimen to specimen and requires the application of Weibull statistics in order to be quantitatively evaluated. The complex micro-laminate structure of these biocomposite materials is characterized and related to their mechanical properties. The red abalone has the highest compressive (233–540 MPa) and flexure strengths of the three shells. The giant clam has the lowest strength (87–123 MPa) and the conch has an intermediate value (166–218 MPa) in compression. The high compressive strength observed in the abalone is attributed to an optimization of microstructural architecture in the form of 2-D laminates, enhancing the fracture toughness of this shell material and enabling higher stresses to develop before fracture.     

Growth of calcium carbonate on non-covalently modified carbon nanotubes

The crystallization of calcium carbonate was investigated on pristine and non-covalently modified carbon nanotubes (CNTs) using the vapor diffusion technique in a calcium chloride solution. Non-covalent modification was accomplished by treating the carbon nanostructures with the amphiphilic copolymer poly(isoprene-b-acrylic acid). Calcium carbonate crystals grown on the surface and in the interstitial channels of CNT buckypapers were observed in both cases. Scanning electron microscopy analysis of the untreated CNTs showed the characteristic rhombohedral morphology of calcite crystals, while in the case of modified material spherical and ellipsoidal crystals, consisted of nanocrystallites, were observed. X-ray diffraction analysis showed the presence of calcite crystals in both cases.     

Synthesis of hollow calcium carbonate particles by the bubble templating method

Hollow calcium carbonate (CaCO₃) is a potential component in many industrial fields such as plastics, rubbers, papermaking, and drug delivery. This paper described a novel approach to synthesize hollow CaCO₃ particles by using bubble as template via passing CO₂ bubbles into calcium chloride (CaCl₂) solution in the presence of ammonia (NH₃) at 27 °C. The CO₂ bubble is not only the reactive material, but also the template of hollow particles. The newly-formed primary particles attach to bubbles and form a solid shell. After filtering and drying the hollow CaCO₃ particles were obtained. Physical characteristics of the precipitate were evaluated using scanning electron microscopy (SEM) and X-ray diffraction (XRD).     

Combined effect of chill storage and modified atmosphere packaging on mussels (Mytilus galloprovincialis) preservation

Changes in biochemical indices, microbial growth, headspace and sensory quality of mussels which had been packaged in two modified atmospheres [Modified atmosphere packaging (MAP) 1: 60% CO₂/20% N₂/20% O₂ and MAP 2: 60% CO₂/40% N₂] and under vacuum (VP) were studied for 14 days. The results showed better quality retention and greater shelf life of mussels packaged under MAP 1 as compared to MAP 2 and VP samples. Increase in total volatile basic nitrogen followed the order: MAP 1 < MAP 2, VP < air (control) samples while increase in trimethylamine nitrogen followed the order: MAP 1 < air < MAP 2 < VP. The 2‐thiobarbituric acid (TBA) values of MAP 1 and air samples were significantly higher (p < 0.05) than the TBA values of VP and MAP 2 samples. MAP 1 showed a greater (p < 0.05) inhibition effect on total viable count of mussel samples than all other packaging conditions. Based primarily on odour scores, the MAP 1 samples remained acceptable up to ca. 10–11 days, the MAP 2 and VP up to ca. 7–8 days while the air‐packaged samples up to ca. 5–6 days of storage. Copyright © 2007 John Wiley & Sons, Ltd.     

Material properties of brachiopod shell ultrastructure by nanoindentation

Mineral-producing organisms exert exquisite control on all aspects of biomineral production. Among shell-bearing organisms, a wide range of mineral fabrics are developed reflecting diverse modes of life that require different material properties. Our knowledge of how biomineral structures relate to material properties is still limited because it requires the determination of these properties on a detailed scale. Nanoindentation, mostly applied in engineering and materials science, is used here to assess, at the microstructural level, material properties of two calcite brachiopods living in the same environment but with different modes of life and shell ultrastructure. Values of hardness (H) and the Young modulus of elasticity (E) are determined by nanoindentation. In brachiopod shells, calcite semi-nacre provides a harder and stiffer structure (H approximately 3-6 GPa; E=60-110/120 GPa) than calcite fibres (H=0-3 GPa; E=20-60/80 GPa). Thus, brachiopods with calcite semi-nacre can cement to a substrate and remain immobile during their adult life cycle. This correlation between mode of life and material properties, as a consequence of ultrastructure, begins to explain why organisms produce a wide range of structures using the same chemical components, such as calcium carbonate.