Crystallization of CaCO3 in the presence of sulfate and additives: experimental and molecular dynamics simulation studies

The effects of sulfate and BHTPMP (Bis (hexamethylene) triaminepentakis (methylene phosphonic acid)) on the crystallization rate, phase composition and morphology of calcium carbonate have been studied. It was observed that sulfate reduces the nucleation rate and favors the formation of aragonite form in the calcium carbonate precipitate. Moreover, in the presence of sulfate the rhombohedral morphology of the calcite crystals is modified, and during the formation of calcite, the development of {104} faces are more significantly prohibited than {110} faces. In the presence of sulfate together with BHTPMP, the crystallization process is inhibited and the modified morphology and the dominant calcite form are observed in the solid. The results from molecular dynamics simulations show the more strong combination of sulfate with calcite surface, in particular the {104} face, in comparison with the aragonite surface. The strong interaction of BHTPMP with sulfate and the aragonite surface favors the formation of the dominant calcite phase in the precipitate.     

Effects of Carboxylic Acids on Calcite Formation in the Presence of Mg2+ Ions

The effects of seven carboxylic acids on calcite formation in the presence of Mg2+ ions, whose molar concentration ratio Mg2+/Ca2+ = 0.5 exclusively induced aragonite precipitation in the absence of carboxylic acids, were studied using a double diffusion technique. The presence of carboxylic acids, acrylic acid, maleic acid, tartaric acid, malonic acid, malic acid, succinic acid, and citric acid in the gel medium favored the formation of magnesian calcite relative to the amount of the additives. Induction time and the positions of the first precipitation were measured to analyze the behavior of crystallization based on the equivalency rule. The formation of magnesian calcite was also studied with the help of Avrami's equation (solid-state model for transformation). The results of applying this equation suggested that aragonite transformed into calcite through a solid-to-solid process. The formation of magnesian calcite was interpreted as the following process: aragonite nuclei, formed owing to Mg2+ ions at the initial stage of CaCO3 crystallization, transformed into calcite nuclei through a solid-to-solid process while their growth was inhibited by the adsorption of carboxylic acids. The magnesian calcite crystals grew on crystal seeds of calcite formed from aragonite nuclei. Copyright 1999 Academic Press.     

The Application of Ammonium Hydroxide and Sodium Hydroxide for Reagent Softening of Water

The calcium carbonate crystallization from aqueous solutions in the presence of alkali additives such as sodium hydroxide and ammonium hydroxide has been researched. It is found CaCO₃ crystallizes predominantly in the modification of aragonite in the presence of ammonium hydroxide. The calcium carbonate formation rate in an alkaline medium and the gaseous reaction products due to sorption of gas bubbles on crystal surfaces, affect the aragonite structure formation. It is shown use of ammonium hydroxide for water treatment can solve two urgent tasks such as water softening and exclusion sediment of deposits on the equipment surfaces by a calcium carbonate crystallization in the form of aragonite.     

Nylon 6/Calcium Carbonate Nanocomposites: Characterization and Properties

Two polymorphic modifications of calcium carbonate nanoparticles (calcite and aragonite) characterized by different shape and coated with fatty acids were used as reinforcement phases of Nylon 6. Nylon 6 based nanocomposites filled with 1% and 5% by weight of calcite and aragonite were prepared by melt mixing. Morphological analysis performed on the fractured surface of nanocomposites showed that the coating agent permits to obtain uniform and fine nanoparticles dispersion. DMTA analysis revealed that nanoparticles increase the glass transition temperature of Nylon 6 up to 12 °C in the case of calcite, while a less pronounced increase was recorded for aragonite. Finally, structural analyses (FT-IR and WAXS) underlined that calcite nanoparticles promote and stabilize the γ-crystalline form of Nylon 6, while in the case of aragonite nanofillers the α-crystalline form was still dominant.     

High-magnesian calcite mesocrystals: a coordination chemistry approach

While biogenic calcites frequently contain appreciable levels of magnesium, the pathways leading to such high concentrations remain unclear. The production of high-magnesian calcites in vitro is highly challenging, because Mg-free aragonite, rather than calcite, is the favored product in the presence of strongly hydrated Mg(2+) ions. While nature may overcome this problem by forming a Mg-rich amorphous precursor, which directly transforms to calcite without dissolution, high Mg(2+)/Ca(2+) ratios are required synthetically to precipitate high-magnesian calcite from solution. Indeed, it is difficult to synthesize amorphous calcium carbonate (ACC) containing high levels of Mg, and the Mg is typically not preserved in the calcite product as the transformation occurs via a dissolution-reprecipitation route. We here present a novel synthetic method, which employs a strategy based on biogenic systems, to generate high-magnesian calcite. Mg-containing ACC is produced in a nonaqueous environment by reacting a mixture of Ca and Mg coordination complexes with CO(2). Control over the Mg incorporation is simply obtained by the ratio of the starting materials. Subsequent crystallization at reduced water activities in an organic solvent/water mixture precludes dissolution and reprecipitation and yields high-magnesian calcite mesocrystals with Mg contents as high as 53 mol %. This is in direct contrast with the polycrystalline materials generally observed when magnesian calcite is formed synthetically. Our findings give insight into the possible mechanisms of formation of biogenic high-magnesian calcites and indicate that precise control over the water activity may be a key element.     

Effects of calcium carbonate polymorph on the structure and properties of soy protein-based nanocomposites

Novel protein-based nanocomposites were well prepared by in vivo synthesis and co-precipitation of soy protein isolate (SPI) with calcium carbonate (CaCO3) in an aqueous solution. The resultant CaCO3 in the nanocomposites was identified as calcite- and aragonite-type, respectively. The morphology and structure of the CaCO3/SPI composites were investigated by means of wide-angle X-ray diffraction, Fourier transform infrared spectra, scanning electron microscopy, and high-resolution transmission electron microscopy. The results revealed that the polymorph and the size of CaCO3 in the nanocomposites were dependent on its content, pH, and the conformation of soy protein. At the content of more than 5%, CaCO3 was changed into calcite crystal with the preference of growing along (104) plane. However, at lower content of less than 5%, CaCO3 preferred to form aragonite in the composite as a result of the modulation by soy protein. The aragonite nanocrystals were arrayed in the direction of (111) plane and self-assembled along beta-sheet planes of soy protein polypeptides. The mechanical properties, thermal stability, and water resistance of the CaCO3/SPI nanocomposites were significantly improved as a result of the nanosized effects. Interestingly, the aragonite/SPI nanocomposite exhibited higher tensile strength (about 50 MPa) than that of calcite/SPI, owing to a good compatibility and strong interaction between aragonite and soy protein polypeptides. This work provided a simple pathway to develop the soy protein-based bio-hybrid materials with high mechanical strength and valuable information on their structure-properties relationship.     

Investigation of scale inhibition mechanisms based on the effect of scale inhibitor on calcium carbonate crystal forms

To probe the scale inhibition mechanisms,calcium carbonate scale occurring before and after the ad- dition of scale inhibitors was collected.The results from scale SEM confirm that,without scale inhibitor, calcium carbonate scale shows rhombohedron and hexagon,which are the characteristic feathers of calcite.After addition of inhibitors,morphology of scale is changed,and the more efficient the scale inhibitor is,the more greatly the morphology is modified.To elucidate the scale constitute,they were further analyzed by FT-IR,XRD.Besides calcite,vaterite and aragonite occur in calcium carbonate scale after addition of inhibitors,and the higher scale inhibition efficiency is,the more vaterite presents in scale.It can be concluded that the alteration of morphology is ascribed to the change of crystal form. There are three stages in the crystallizing process including occurrence and disappearing of unstable phase,occurrence and disappearing of metastable phase,development of stable phase.Without scale inhibitors,metastable phases usually transform into stable phase,thus the main constitute of formed scale is calcite.When scale inhibitors are added,both formation and transformation of metastable phases are inhibited,which results in the occurrence of aragonite and vaterite.From the fact that more vaterite presents in scale with a more efficient scale inhibitor added,we can see that the function of scale inhibitor is realized mainly by controlling the crystallizing process at the second stage.     

Crystallographic characterization of naturally occurring aragonite and calcite phase: Rietveld refinement

This research has comprehended the crystallographic characterization of two naturally occurring calcium carbonates phases e.g. aragonite and calcite in Pila globosa (P. globosa) and eggshells respectively. The tools employed to confirm the phases of aragonite and calcite were X-ray diffractoion (XRD) and Fourier Transform Infrared (FT-IR) spectroscopy. Several important crystallographic parameters like crystallite size, lattice parameters, dislocation density, crystallinity index, microstrain, volume of the unit cell, relative intensity of a certain plane, preference growth, and specific surface area were taken into account while assessing the desired aragonite and calcite phases. A number of well-known models e.g. Straight-line model of the Scherrer model, the Monshi-Scherrer model, the Williamson-Hall model, the Sahadat-Scherrer model, and the three-peak model aided the estimation of crystallite size. In all the cases, the observed crystallite size of calcite was larger than that of aragonite. The percentage of calcite and aragonite in eggshell and P. globosa were assessed by Rietveld refinement method. Observed results revealed that 97.4% calcite and 2.6% aragonite phases are present in eggshell while in P. globosa these percentages exist inversely, i.e. 93.2% aragonite and 6.8% calcite phases.     

Investigation of Calcium Carbonates Enhanced Poly(ε-caprolactone) Materials for Biomedical Applications

In this study poly(ε-caprolactone) – calcium-carbonate composites were obtained by melt-mixing. Two crystal-modifications of calcium-carbonate were used, namely calcite and aragonite. Compressive and tensile tests were executed on samples with various compositions to analyze the effect of filler content and particle geometry. Both minerals improved the compressive modulus and strength significantly, however the influence of calcite was superior. The tensile modulus was also highly increased. The elongation at break remained high even at 50 wt% aragonite filling, but decreased with two orders in the case of calcite. Biocompatibility tests were also carried out with human osteoblast cells and the results were promising. The relative cell number increased due to calcium-carbonate. Both filler material is able to enhance the mechanical and biological properties of poly(ε-caprolactone) significantly. Aragonite samples remained more ductile compared to calcite ones, but the calcite filled scaffolds are stiffer, stronger and slightly more biocompatible than aragonite filled materials.     

Solid‐State Transformation of Amorphous Calcium Carbonate to Aragonite Captured by CryoTEM

Early‐stage reaction mechanisms for aragonite‐promoting systems are relatively unknown compared to the more thermodynamically stable calcium carbonate polymorph, calcite. Using cryoTEM and SEM, the early reaction stages taking place during aragonite formation were identified in a highly supersaturated solution using an alcohol–water solvent, and an overall particle attachment growth mechanism was described for the system. In vitro evidence is provided for the solid‐state transformation of amorphous calcium carbonate to aragonite, demonstrating the co‐existence of both amorphous and crystalline material within the same aragonite needle. This supports non‐classical formation of aragonite within both a synthetic and biological context.     

Morphology and Kinetics of Growth of CaCO<Subscript>3</Subscript> Precipitates Formed in Saline Water at 30°C

The crystallization kinetics and morphology of CaCO₃ crystals precipitated from the high salinity oilfield water were studied. The crystallization kinetics measurements show that nucleation and nuclei growth obey the first order reaction kinetics. The induction period of precipitation is extended in the high salinity solutions. Morphological studies show that impurity ions remain mostly in the solution phase instead of filling the CaCO₃ crystal lattice. The morphology of CaCO₃ precipitates can be changed from a smooth surface (calcite) to rough spheres (vaterite), and spindle rod bundles, or spherical, ellipsoid, flowers, plates and other shapes (aragonite).     

The influence of electrogalvanic device on scaling

The use of an electrogalvanic device for scale neutralisation is descibed in this paper. Physico-chemical analyses were performed before and after the treatment. The results were compared with those obtained by using magnetic water treatment device. By measuring some individual parameters and the implementation of chemical analysis, the satisfactory functioning of the electrogalvanic device was demonstrated. The quality of drinking water did not change much after the water treatment method. The results of determination of calcium carbonate saturating index showed that the raw drinking water is in carbonate equlibrium as well as both treated water samples. The calcite/aragonite ratio was studied by means of microscopy and X-ray powder diffraction. Inspection of crystals formed during the experiments with microscopy indicated that aragonite crystal structure of the precipitates prevailed over the calcite stucture. The diffractograms showed that the share of aragonite increased after using the electrogalvanic device compared with raw drinking water samples where the share of calcite was higher.      

Crystal growth of aragonite and calcite in presence of citric acid, DTPA, EDTA and pyromellitic acid

The influence of four calcium complexing substances, i.e., citric acid (CIT), diethylenetriaminepentaacetic acid (DTPA), ethylenediaminetetraacetic acid (EDTA) and pyromellitic acid (PMA), on the crystal growth rate of the calcium carbonate polymorphs aragonite and calcite has been studied. Using a seeded constant supersaturation method supersaturation was maintained at 4 by keeping a constant pH of 8.5 through addition of sodium carbonate and calcium chloride solutions. The unique composition of each solution was calculated using chemical speciation. The growth rate was interpreted in terms of an overall growth rate. For both calcite and aragonite, the crystal growth rate is significantly reduced in the presence of the calcium complexing substances. The growth retarding effect depends on both the concentration and the polymorph. The relative crystal growth rate was correlated to the total complexing agent concentration using a Langmuir adsorption approach. Aragonite appeared fully covered for lower total concentrations than calcite. Furthermore, CIT very efficiently blocked aragonite growth contrary to what was observed for calcite. This is thought to be related to certain distinct features of the dominant aragonite crystal faces compared to the dominant calcite faces.     

Calcium Carbonate Polyamorphism and Its Role in Biomineralization: How Many Amorphous Calcium Carbonates Are There?

Although the polymorphism of calcium carbonate is well known, and its polymorphs—calcite, aragonite, and vaterite—have been highly studied in the context of biomineralization, polyamorphism is a much more recently discovered phenomenon, and the existence of more than one amorphous phase of calcium carbonate in biominerals has only very recently been understood. Here we summarize what is known about polyamorphism in calcium carbonate as well as what is understood about the role of amorphous calcium carbonate in biominerals. We show that consideration of the amorphous forms of calcium carbonate within the physical notion of polyamorphism leads to new insights when it comes to the mechanisms by which polymorphic structures can evolve in the first place. This not only has implications for our understanding of biomineralization, but also of the means by which crystallization may be controlled in medical, pharmaceutical, and industrial contexts.     

Continuous Preparation of Calcite,Aragonite and Vaterite,and of Magnesium‐Substituted Amorphous Calcium Carbonate (Mg‐ACC)

The three water‐free calcium carbonate polymorphs calcite, aragonite and vaterite were prepared from aqueous solutions without additives using standard laboratory equipment in a continuous process. Variation parameters were the way of mixing, the solution concentrations, and the reactor residence time. The samples were crystallographically and chemically pure, but a thorough elemental analysis revealed the presence of small amounts of sodium carbonate which was not detectable by X‐ray powder diffraction. The continuous process avoids the inherent variability of batch syntheses. By adapting the crystallization parameters, magnesium‐substituted amorphous calcium carbonate (molar ratio of Mg:Ca of 1:2.68) was prepared in this continuous process.     

类珍珠质结构的再生丝素蛋白/文石复合材料的仿生制备

详细研究了不同浓度的聚丙烯酸(分子量为2000, PAA-2k)和镁离子对碳酸钙在再生丝素蛋白(RSF)膜表面结晶的影响. 发现单独采用PAA-2k时, 碳酸钙主要以方解石形式在RSF膜表面沉积成膜; 若加入一定量的镁离子参与共同调控, 碳酸钙则有可能在RSF膜表面形成以文石为主的连续薄膜, 进而得到了具有类珍珠质结构的层状RSF/文石复合材料. 我们认为, 吸附在RSF膜表面的PAA对碳酸钙成核诱导作用及其溶液中PAA对碳酸钙结晶抑制作用共同导致RSF膜表面碳酸钙薄膜的形成.     

Synthesis of vaterite and aragonite crystals using biomolecules of tomato and capsicum

In this paper, biomimetic synthesis of calcium carbonate (CaCO₃) in the presence of biomolecules of two vegetables-tomato and capsicum is investigated. Scanning electron microscopy and X-ray powder diffractometry were used to characterize the CaCO₃ obtained. The biomolecules in the extracts of two vegetables are determined by UV-vis or FTIR. The results indicate that a mixture of calcite and vaterite spheres constructed from small particles is produced with the extract of tomato, while aragonite rods or ellipsoids are formed in the presence of extract of capsicum. The possible formation mechanism of the CaCO₃ crystals with tomato biomolecules can be interpreted by particle-aggregation based non-classical crystallization laws. The proteins and/or other biomolecules in tomato and capsicum may control the formation of vaterite and aragonite crystals by adsorbing onto facets of them.     

Nucleation activity of nanosized CaCO3 on crystallization of isotactic polypropylene, in dependence on crystal modification, particle shape, and coating

A detailed analysis of the effect of calcium carbonate nanoparticles on crystallization of isotactic polypropylene (iPP) is reported in this contribution. CaCO₃ nanoparticles with different crystal modifications (calcite and aragonite) and particle shape were added in small percentages to iPP. The nanoparticles were coated with two types of compatibilizer (either polypropylene-g-maleic anhydride copolymer, or fatty acids) to improve dispersion and adhesion with the polymer matrix.It was found that the type of coating agent used largely affects the nucleating ability of calcium carbonate towards formation of polypropylene crystals. CaCO₃ nanoparticles coated with maleated polypropylene can successfully promote nucleation of iPP crystals, whereas the addition of nanosized calcium carbonate coated with fatty acids delays crystallization of iPP, the effect being mainly ascribed to the physical state of the coating in the investigated temperature range for crystallization of iPP, as well as to possible dissolution by fatty acids of heterogeneities originally present in the polypropylene matrix.     

The Effect of Additives on the Early Stages of Growth of Calcite Single Crystals

As crystallization processes are often rapid, it can be difficult to monitor their growth mechanisms. In this study, we made use of the fact that crystallization proceeds more slowly in small volumes than in bulk solution to investigate the effects of the soluble additives Mg²⁺ and poly(styrene sulfonate) (PSS) on the early stages of growth of calcite crystals. Using a “Crystal Hotel” microfluidic device to provide well‐defined, nanoliter volumes, we observed that calcite crystals form via an amorphous precursor phase. Surprisingly, the first calcite crystals formed are perfect rhombohedra, and the soluble additives have no influence on the morphology until the crystals reach sizes of 0.1–0.5 μm for Mg²⁺ and 1–2 μm for PSS. The crystals then continue to grow to develop morphologies characteristic of these additives. These results can be rationalized by considering additive binding to kink sites, which is consistent with crystal growth by a classical mechanism.     

Removal of phosphate from solution by adsorption and precipitation of calcium phosphate onto monohydrocalcite

The sorption behavior and mechanism of phosphate on monohydrocalcite (CaCO₃?H₂O: MHC) were examined using batch sorption experiments as a function of phosphate concentrations, ionic strengths, temperatures, and reaction times. The mode of PO₄ sorption is divisible into three processes depending on the phosphate loading. At low phosphate concentrations, phosphate is removed by coprecipitation of phosphate during the transformation of MHC to calcite. The sorption mode at the low-to-moderate phosphate concentrations is most likely an adsorption process because the sorption isotherm at the conditions can be fitted reasonably with the Langmuir equation. The rapid sorption kinetics at the conditions is also consistent with the adsorption reaction. The adsorption of phosphate on MHC depends strongly on ionic strength, but slightly on temperature. The maximum adsorption capacities of MHC obtained from the regression of the experimental data to the Langmuir equation are higher than those reported for stable calcium carbonate (calcite or aragonite) in any conditions. At high phosphate concentrations, the amount of sorption deviates from the Langmuir isotherm, which can fit the low-to-moderate phosphate concentrations. Speciation–saturation analyses of the reacted solutions at the conditions indicated that the solution compositions which deviate from the Langmuir equation are supersaturated with respect to a certain calcium phosphate. The obtained calcium phosphate is most likely amorphous calcium phosphate (Ca₃(PO₄)₂?xH₂O). The formation of the calcium phosphate depends strongly on ionic strength, temperature, and reaction times. The solubility of MHC is higher than calcite and aragonite because of its metastability. Therefore, the higher solubility of MHC facilitates the formation of the calcium phosphates more than with calcite and aragonite.