Crystallization Of Amorphous Calcium Carbonate Research Articles

Producing amorphous calcium carbonate using waste Ca-rich solution generated through the bio-leaching of slag

This study presents a new method for treating waste Ca-rich solutions (WCS), from which amorphous calcium carbonate (ACC) nanoparticles, an industrial product in high demand, are produced. The distinctive feature of this method is the use of tannic acid (TA) as a surfactant, employing a CO2 bubbling method under ambient conditions. By modulating the concentration of TA, the morphology and crystallization of the generated CaCO3 nanoparticles can be effectively controlled. Experimental results reveal that with increasing TA dosage, the crystallinity of CaCO3 decreases until an amorphous phase is obtained. The crystallization of ACC in solution was completely inhibited when TA was added at a concentration of more than 1%. Therefore, the proposed method provides a new strategy for the efficient preparation of ACC from WCS.

Pressure-induced crystallization and densification of amorphized calcium carbonate hexahydrate controlled by interfacial water

Amorphous calcium carbonate (ACC) is widely known as a metastable precursor in the formation of crystalline calcium carbonate biominerals. However, the exact role of water during the crystallization of ACC remains elusive. Here, a novel ACC with high specific surface area and nanopores is synthesized by solvent-induced dehydration and amorphization of crystalline calcium carbonate hexahydrate (ikaite), denoted as I-ACC. Comparing I-ACC and typical spherical ACC (S-ACC) nanoparticles, it reveals that the crystallization pathways of ACC under heating or pressure are not dictated by the total amount of water in ACC as reported, but rather the interfacial water that is released from ACC bulk and adsorbed on the surface of the particles. We show that the crystallization pathways of I-ACC to calcite single crystal with high specific surface area or vaterite can be easily controlled by tuning the release of water during heating. In addition, densely packed pure vaterite can be obtained via pressured-induced transformation of I-ACC at room temperature, which is otherwise difficult to form using S-ACC. These insights contribute to the understanding of the biological control of mineral formation via amorphous precursors and offer new opportunities to bioprocess inspired fabrication of strong bulk material at room temperature.

Nonclassical Crystallization of Amorphous Calcium Carbonate in the Presence of Phosphate Ions

Phosphate ions are commonly found in amorphous biominerals and play critical roles in controlling the stability and crystallization pathways of amorphous calcium carbonate (ACC). However, the role of phosphate ions during the transformation process of ACC remains elusive. Here, we show that while the onset of ACC transformation in solution could not be effectively delayed by addition of phosphate ions up to 5% (molar fraction in the initial carbonate solution), interestingly, the transformation process can be significantly prolonged. This is ascribed to the formation of a more stable phosphate-enriched amorphous phase during transformation and the inhibition of vaterite dissolution and calcite crystallization. Phosphate ions can also promote the nonclassical crystallization of calcite, where calcite crystals grow via oriented attachment of nanoparticles. In addition, nanoporous calcite single crystals can be obtained through crystallization of ACC after being embedded in the crystals. Our results are of significance to understand the crystallization mechanisms of amorphous materials and biomineralization processes.

Forming amorphous calcium carbonate within hydrogels by enzyme-induced mineralization in the presence of N-(phosphonomethyl)glycine

Amorphous inorganic materials have a great potential in material science. Amorphous calcium carbonate (ACC) is a widely useable system, however, its stabilization often turns out to be difficult and the synthesis is mostly limited to precipitation in solution as nanoparticles. Stable ACC in bulk phases would create new composite materials. Previous work described the enzyme-induced mineralization of hydrogels with crystalline calcium carbonate by entrapping urease into a hydrogel and treating this with an aqueous mineralization solution containing urea und calcium chloride. Here, this method was modified using a variety of crystallization inhibitors attached to the hydrogel matrix or added to the surrounding mineralization solution. It was found that only N-(phosphonomethyl)glycine (PMGly) in solution completely inhibits the crystallization of ACC in the hydrogel matrix. The stability of the homogeneously precipitated ACC could be accounted to the combination of stabilizing effects of the additive and stabilization through confinement. The crystallization could be accelerated at higher temperatures up to 60 °C. Here, a combination of Mg ions and PMGly was required to stabilize ACC in the hydrogel. Variation of these two compounds can be used to control a number of different calcium carbonate morphologies within the hydrogel. While the ACC nanoparticles within the hydrogel are stable over weeks even in water, a calcite layer grows on the surface of the hydrogel, which might be used as self-hardening mechanism of a surface.

Effect of Strontium Ions on Crystallization of Amorphous Calcium Carbonate

AbstractIn biomineralization, amorphous precursors enable formation of complicated crystal shapes and incorporation of organic and inorganic impurities. In some cases the concentrations of the inorganic impurity atoms greatly exceed the thermodynamical (equilibrium) solubility limits. To shed additional light on this, crystallization experiments with Sr‐rich amorphous calcium carbonate (ACC) are carried out. The Sr‐concentration in ACC, Sr/(Ca+Sr) ≈0.3, is more than 60 times larger than its solubility limit in calcite. Crystallization of ACC is accomplished using two different pathways: i) in a high humidity environment at room temperature, and ii) upon heating at 300 °C. In both cases, X‐ray diffraction measurements revealed the formation of particular crystal phases with an ability to take up substantial amounts of Sr. Crystals formed via the humidity route comprised spatially separated regions of almost pure rhombohedral calcite and orthorhombic aragonite/strontianite solid solution with a Sr/(Ca+Sr) ratio equal nearly 0.5. Crystallization of ACC upon heating resulted in the appearance of a new calcite‐like phase with reduced space symmetry, , instead of for regular calcite. Symmetry reduction in this Sr‐rich ‐phase is due to the rotational disorder of flat carbonate groups about the c‐axis, which is responsible for the enhanced lattice capacity of Sr incorporation.

Effect of pH on formation of single-phase vaterite

This paper reports on the effect of pH on polymorphism of calcium carbonate (vaterite) formed in the CaCl2-Na2CO3-H2O reaction system. The initial pH of CaCl2 aqueous solution was adjusted to the initial pH 2.0–11.3 using HCl and NH3 aqueous solution. The initial pH was 6.5 without pH adjustment. When the reaction was carried out, amorphous calcium carbonate (ACC) was formed at the early stage of the reaction and crystallized to calcite and vatrite after 3 min of stirring. Formation ratio of vaterite at this time was 60％. Single-phase vaterite was formed at initial pH 2.0–2.5. Further, focusing on the pH during ACC crystallization. When HCO3− ions were 80% or more during ACC crystallization, single-phase vaterite was formed.

Alv Protein Plays Opposite Roles in the Transition of Amorphous Calcium Carbonate to Calcite and Aragonite during Shell Formation

Amorphous calcium carbonate (ACC) is an important precursor in biominerals such as shells, coral, foraminiferans, and urchin spine. However, the mechanism underlying the transition from ACC to stable biosynthetic crystals is still poorly understood. Herein, we identified a matrix protein referred to as Alv in Pinctada fucata, which has dramatically opposite functions during the different transition processes from ACC to stable crystals—calcite and aragonite in shell formation. The functions of Alv were studied by RNA interference, binding of recombinant Alv (rAlv) to chitin, calcite and aragonite assay, ACC transition, in vitro crystallization, calcium carbonate precipitation, and near-UV CD spectra. We found that rAlv could promote nucleation during ACC crystallization, stimulate the transition from ACC to calcite, but suppress transition from ACC to aragonite. It is concluded that Alv is involved in the transition of ACC, and plays a crucial role in the formation of shells. As far as we know, Alv is one...

The Crystallization of Amorphous Calcium Carbonate is Kinetically Governed by Ion Impurities and Water.

Many organisms use amorphous calcium carbonate (ACC) and control its stability by various additives and water; however, the underlying mechanisms are yet unclear. Here, the effect of water and inorganic additives commonly found in biology on the dynamics of the structure of ACC during crystallization and on the energetics of this process is studied. Total X‐ray scattering and pair distribution function analysis show that the short‐ and medium‐range order of all studied ACC samples are similar; however, the use of in situ methodologies allow the observation of small structural modifications that are otherwise easily overlooked. Isothermal calorimetric coupled with microgravimetric measurements show that the presence of Mg2+ and of PO4 3− in ACC retards the crystallization whereas increased water content accelerates the transformation. The enthalpy of ACC with respect to calcite appears, however, independent of the additive concentration but decreases with water content. Surprisingly, the enthalpic contribution of water is compensated for by an equal and opposite entropic term leading to a net independence of ACC thermodynamic stability on its hydration level. Together, these results point toward a kinetic stabilization effect of inorganic additives and water, and may contribute to the understanding of the biological control of mineral stability.

Control of Polymorph Selection in Amorphous Calcium Carbonate Crystallization by Poly(Aspartic Acid): Two Different Mechanisms.

Poly(aspartic acid) (pAsp) is known to stabilize amorphous calcium carbonate (ACC) and affect its crystallization pathways. However, little is known about the mechanisms behind these phenomena. Here it is shown that ACC is stabilized by pAsp molecules in the solution rather than by the amount of pAsp incorporated into the ACC bulk, and that the effect of pAsp on the polymorph selection is entirely different at low and high concentration of pAsp. At low concentrations, pAsp is more effective in inhibiting the nucleation and growth of vaterite than calcite. At high concentrations, when calcite formation is prevented, the crystallization of vaterite proceeds via a pseudomorphic transformation of ACC nanospheres, where vaterite nucleates on the surface of ACC nanospheres and grows by a local transformation of the bulk ACC phase. These results shed some light on the function of pAsp during an ACC-mediated biomineralization process and provide an explanation for the presence of metastable vaterite at conditions where calcite is thermodynamically favored.

Opposite Particle Size Effect on Amorphous Calcium Carbonate Crystallization in Water and during Heating in Air

Calcium carbonate is a common constituent of many natural materials, such as shells and skeletons of marine animals. While it is well-documented that additives (organic and inorganic) modulate the crystallization of amorphous calcium carbonate (ACC), the effects of the intrinsic physicochemical characteristics of ACC, such as particle size, shape, and water content on the transformation to crystalline polymorphs, are still poorly understood. Here, we investigate the effect of particle size by preparing ACC nanoparticles with an average size ranging from ∼66 to ∼196 nm using a high-resolution titration setup. Our results show that the particle size determined the polymorph selection in solution; an increasing proportion of vaterite to calcite was observed with decreasing particle size. The polymorph selection was ascribed to a higher apparent solubility of ACC with decreasing particle size, a parameter from which we could determine the surface energy of ACC to be ∼0.33 J/m2. Upon heating, particle size sho...

Phase‐controlled crystallization of amorphous calcium carbonate in ethanol‐water binary solvents

The evolution of amorphous calcium carbonate (ACC) into crystals in ethanol/water binary solvents under ambient temperature was investigated, and it was found to depend on the volume ratio of ethanol to water (R). Calcite remained dominant when the amount of water was high (R = 1/3). A slight change in the amount of ethanol (R = 3/1) could lead to a dramatic change in the polymorph from calcite to aragonite. However, when poly (allylamine hydrochloride) (PAH) was added at R = 3/1, almost pure vaterite could be obtained, which has a specific morphological variation (from hollow microspheres to cloud‐like). This study provides an alternative polymorphic route for the CaCO3 mineral by using the evolution of ACC in different solvent environments, which provides some useful clues for understanding the importance of kinetic control of the morphologies and polymorphs of a wide range of inorganic materials. In addition, this simple mild phase‐controlled synthetic method could be scaled up as a green chemistry route for the industrial production of different polymorphs of CaCO3.

Dehydration and crystallization of amorphous calcium carbonate in solution and in air

The mechanisms by which amorphous intermediates transform into crystalline materials are poorly understood. Currently, attracting enormous interest is the crystallization of amorphous calcium carbonate, a key intermediary in synthetic, biological and environmental systems. Here we attempt to unify many contrasting and apparently contradictory studies by investigating this process in detail. We show that amorphous calcium carbonate can dehydrate before crystallizing, both in solution and in air, while thermal analyses and solid-state nuclear magnetic resonance measurements reveal that its water is present in distinct environments. Loss of the final water fraction—comprising less than 15% of the total—then triggers crystallization. The high activation energy of this step suggests that it occurs by partial dissolution/recrystallization, mediated by surface water, and the majority of the particle then crystallizes by a solid-state transformation. Such mechanisms are likely to be widespread in solid-state reactions and their characterization will facilitate greater control over these processes.

The Effect of Aspartic Acid and Glycine on Amorphous Calcium Carbonate (ACC) Structure, Stability and Crystallization

The effect of organic molecules on CaCO3 crystallization, in particular on the formation of the initial amorphous calcium carbonate (ACC) phase, is poorly understood despite this knowledge being crucial for designing biomimetic compounds with specific function, strength and stability. We monitored ACC crystallization in the presence of varying concentrations of aspartic acid (ASP) and glycine (GLY). We observed an increase in ACC lifetime with increasing amino acid concentrations and showed that the amino acid molecules sorbed onto the ACC particles. However, little if any difference in composition and atomic structure or the so formed ACC was observed. Similarly, the crystallization pathway of ACC via vaterite and calcite although delayed, was only slightly affected by the added amino acids. The only exemption was at the highest tested ASP concentration where ACC formation was inhibited, The calcite crystals that formed in the presence of ASP had rounded edges and rough surfaces, features that are not observed for the pure, inorganic calcite or calcite formed in the presence of GLY. Overall, the results suggest that the amino acids affected ACC lifetime through the inhibition of crystal nucleation and growth, more so in the presence of ASP than GLY.

Onset of Orientational Order in Amorphous Calcium Carbonate (ACC) upon Dehydration

Crystallization of amorphous calcium carbonate (ACC) into one of the anhydrous crystalline polymorphs (vaterite, calcite, and aragonite) is a topic of extensive current research in biomineralization. We performed molecular dynamics simulations of ACC to characterize the dehydration-induced variation in the distance-dependent orientational order of Ca2+ cations around CO32- anions and to examine how such changes in the local structure facilitates nucleation in ACC. Our observations provide molecular scale basis for the formation of vaterite by internal reorganization of ACC nano-particles, which is in-line with the previous experimental demonstration that vaterite is the precursor for the crystallization of calcite or aragonite.

Ethanol assisted synthesis of pure and stable amorphous calcium carbonate nanoparticles

Stable monodispersed amorphous calcium carbonate (ACC) nanoparticles can be synthesized in ethanol media by a facile method, and crystallization of ACC is kinetically controlled, resulting in the formation of three polymorphs in a mixed solvent of ethanol-water at different pH values.

The Effect of Additives on Amorphous Calcium Carbonate (ACC): Janus Behavior in Solution and the Solid State

AbstractAmorphous calcium carbonate (ACC) is an important intermediate in the formation of crystalline CaCO3 biominerals, where its crystallization is controlled using soluble additives. However, although this transformation often occurs in the solid state, experiments mainly focus on the effect of additives on ACC crystallization in solution. This paper addresses this issue and compares the crystallization, in solution and in the solid state, of ACC precipitated in the presence of a range of additives. Surprisingly, these results show that some additives exhibit a Janus behavior, where they retard crystallization in solution, yet accelerate it in the solid state. This is observed for all of the larger molecules examined, while the small molecules retard crystallization in both solution and the solid state.

Mechanistic Insights into the Crystallization of Amorphous Calcium Carbonate (ACC)

Many organisms use amorphous calcium carbonate (ACC) during crystalline calcium carbonate biomineralization, as a means to control particle shape/size and phase stability. Here, we present an in situ small- and wide-angle X-ray scattering (SAXS/WAXS) study of the mechanisms and kinetics of ACC crystallization at rapid time scales (seconds). Combined with offline solid and solution characterization, we show that ACC crystallizes to vaterite via a three-stage process. First, hydrated and disordered ACC forms, then rapidly transforms to more ordered and dehydrated ACC; in conjunction with this, vaterite forms via a spherulitic growth mechanism. Second, when the supersaturation of the solution with respect to vaterite decreases sufficiently, the mechanism changes to ACC dissolution and vaterite crystal growth. The third stage is controlled by Ostwald ripening of the vaterite particles. Combining this information with previous studies, allowed us to develop a mechanistic understanding of the abiotic crystallization process from ACC to vaterite and all the way to calcite. We propose this is the underlying abiotic mechanism for calcium carbonate biomineralization from ACC. This process is then augmented or altered by organisms (e.g., using organic compounds) to form intricate biominerals. This study also highlights the applicability of in situ time-resolved SAXS/WAXS to study rapid crystallization reactions.

In Situ Study of the Precipitation and Crystallization of Amorphous Calcium Carbonate (ACC)

The precipitation and crystallization of amorphous calcium carbonate (ACC) from metastable calcium carbonate solutions in the concentration range 2–10 mM have been investigated in situ in solution in the absence of additives, and in the presence of Mg2+ and poly(acrylic acid). We demonstrate that measurement of the intensity of transmitted light provides an effective method for monitoring the early stages of precipitation of calcium carbonate in solution, where the nature of the scattering particles was confirmed with transmission electron microscopy. The recorded changes in light transmission with time and concentration can be related to the precipitation and aggregation of ACC, its subsequent crystallization to calcite (or vaterite), and the gravitational sedimentation of the growing crystals. Addition of Mg2+ or poly(acrylic acid) is shown to retard the process, delaying the precipitation of ACC and increasing the lifetime of this phase. These results show that measurement of turbidity provides an effe...

The role of pH and Mg on the stability and crystallization of amorphous calcium carbonate

The effects of pH and Mg on the crystallization of amorphous calcium carbonate (ACC) to vaterite and/or calcite were studied using a combination of in situ time resolved synchrotron-based techniques and electron microscopy. The experiments showed that Mg increased the stability of ACC and favoured the formation of calcite over vaterite. A neutral (∼7) starting pH during mixing promoted the transformation of ACC into calcite via a dissolution/reprecipitation mechanism. Conversely, when ACC formed in a solution that started with a high initial pH (∼11.5), the transformation to calcite occurred via metastable vaterite, which formed via a spherulitic growth mechanism. In a second stage this vaterite transformed to calcite via a surface-controlled dissolution and recrystallization mechanism. These crystallization pathways can be explained as a consequence of the pH-dependent composition, local structure, stability and dissolution rates of ACC.

Transformation and crystallization energetics of synthetic and biogenic amorphous calcium carbonate

Amorphous calcium carbonate (ACC) is a metastable phase often observed during low temperature inorganic synthesis and biomineralization. ACC transforms with aging or heating into a less hydrated form, and with time crystallizes to calcite or aragonite. The energetics of transformation and crystallization of synthetic and biogenic (extracted from California purple sea urchin larval spicules, Strongylocentrotus purpuratus) ACC were studied using isothermal acid solution calorimetry and differential scanning calorimetry. Transformation and crystallization of ACC can follow an energetically downhill sequence: more metastable hydrated ACC → less metastable hydrated ACC ⇒ anhydrous ACC ∼ biogenic anhydrous ACC ⇒ vaterite → aragonite → calcite. In a given reaction sequence, not all these phases need to occur. The transformations involve a series of ordering, dehydration, and crystallization processes, each lowering the enthalpy (and free energy) of the system, with crystallization of the dehydrated amorphous material lowering the enthalpy the most. ACC is much more metastable with respect to calcite than the crystalline polymorphs vaterite or aragonite. The anhydrous ACC is less metastable than the hydrated, implying that the structural reorganization during dehydration is exothermic and irreversible. Dehydrated synthetic and anhydrous biogenic ACC are similar in enthalpy. The transformation sequence observed in biomineralization could be mainly energetically driven; the first phase deposited is hydrated ACC, which then converts to anhydrous ACC, and finally crystallizes to calcite. The initial formation of ACC may be a first step in the precipitation of calcite under a wide variety of conditions, including geological CO(2) sequestration.