Dual mineralization of dicalcium phosphate and hydroxyapatite formation on both sides of porous membrane: Alternating current induces pH-modulated crystal deposition

Dual mineralization on a porous membrane was carried out using an electrochemical approach. The porous membrane was interposed between a pair of glass cells, and calcium chloride (CaCl₂) and sodium hydrogenphosphate (Na₂HPO₄) solutions were separately injected into the cells. After inserting platinum electrodes into the cells, an alternating current with a sine waveform was applied for a given period of time. The resulting membrane was removed from the glass cells and rinsed with ultrapure water. The minerals formed on the membrane were analyzed by using spectroscopic methods such as scanning electron microscopy (SEM), infrared (IR) spectroscopy, and X-ray diffraction (XRD) and identified to be dicalcium phosphate and hydroxyapatite. It was noted that dicalcium phosphate formed on one side of the membrane, while hydroxyapatite was formed on the other side. Thus, dual mineralization could be successfully achieved on both sides of the porous membrane under an alternating current. This process of dual mineralization is very useful for the formation of versatile organic–inorganic hybrids and also for the regulation of the polymorphs on either side of a membrane.     

Growth of hierarchical calcium carbonate crystals templated by biomolecules of lotus root

In this paper, crystal growth of calcium carbonate (CaCO₃) in the presence of biomolecules of lotus root was investigated. Scanning electron microscopy, Fourier transform infrared spectroscopy and X‐ray powder diffractometry were used to characterize the products. The results indicate that calcite spherical particles were constructed from small rhombohedral subunits. Similar CaCO₃ crystals were also gained when crystal growth of CaCO₃ in aqueous solution containing extracts of lotus root was performed, suggesting that the soluble biomolecules of lotus root play a crucial role in directing the formation of hierarchical calcite spherical particles. The possible formation mechanism of the CaCO₃ crystals by using lotus root is also discussed, which can be interpreted by particle‐aggregation based non‐classical crystallization laws. The biomolecules of lotus root might induce and control the nucleation and growth of calcium carbonate crystals. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Influence of stirring speed on the crystallization of calcium carbonate

Control over crystal morphology of calcium carbonate (CaCO₃) was investigated by simply changing the stirring speeds in the process of CaCO₃ formation. Scanning electron microscopy (SEM) and powder X‐ray diffraction (XRD) measurements explore the morphology evolution of CaCO₃ at varying stirring speeds. As the stirring speeds increase, rhombohedral calcite, spherical vaterite, and monoclinic crystal with coexistence of calcite phase and vaterite phase were formed, suggesting a facile control over calcium carbonate crystallization in constructing crystals with desired morphology. Moreover, almost pure vaterite spherical particles of narrow particle size distribution were formed at optimum stirring speed. Finally, also elucidated in this work is the mechanism investigation into the construction of various crystal forms via this simple route. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth of aragonite calcium carbonate nanorods in the biomimetic anodic aluminum oxide template

In this study, a biomimetic template was prepared and applied for growing calcium carbonate (CaCO₃) nanorods whose shape and polymorphism were controlled. A biomimetic template was prepared by adsorbing catalytic dipeptides into the pores of an anodic aluminum oxide (AAO) membrane. Using this peptide-adsorbed template, mineralization and aggregation of CaCO₃ was carried out to form large nanorods in the pores. The nanorods were aragonite and had a structure similar to nanoneedle assembly. This aragonite nanorod formation was driven by both the AAO template and catalytic function of dipeptides. The AAO membrane pores promoted generation of aragonite polymorph and guided nanorod formation by guiding the nanorod growth. The catalytic dipeptides promoted the aggregation and further dehydration of calcium species to form large nanorods. Functions of the AAO template and catalytic dipeptides were verified through several control experiments. This biomimetic approach makes possible the production of functional inorganic materials with controlled shapes and crystalline structures.     

Controllable mineralization of calcium carbonate on regenerated cellulose fibers

Gel–forming fibers (GF fibers) can serve as nucleation sites to prepare calcium carbonate (CaCO₃) because they can adsorb large amounts of Ca²⁺ due to their porous structure. In this paper, mineralization behavior of CaCO₃ on GF fibers in ethanol–water mixed solvents without any additives has been investigated. The results showed that some crystals covered the fibers, while others were embedded in fibers. Twin–sphere based vaterite, zonary and rodlike calcite with large aspect ratio could be prepared successfully. The effect of ethanol content inside GF fibers, concentration of Ca²⁺ and CO₃^2‐, mineralization time, miscibility between alcohol and water, and temperature were studied. Lastly, a possible mineralization mode was suggested. This work could provide a new method to prepare inorganic/polymer hybrid materials. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Precipitation of sparingly soluble salts in packed sandbeds in the presence of miscible and immiscible organic substances

Sparingly soluble salts precipitation, e.g. calcium carbonate or calcium sulfate, results in pore clogging in rock formations and in the concomitant reduction of the local permeability of oil wells during the oil extraction processes. On the other hand, in situ controlled salt precipitation is desirable in various applications e.g. waterproofing of concrete constructions suffering from leakages, etc. In the present study, calcium carbonate (CaCO₃) precipitation in sandbeds was investigated, in the presence of organic solvents simulating the conditions prevalent in oil‐well zones. CaCO₃ precipitation was investigated from supersaturated solutions prepared by in‐situ mixing of NaHCO₃ and CaCl₂.2H₂O solutions before the inlet of sandbeds. The solution resulting from the mixing of the two solutions was supersaturated with respect to all calcium carbonate polymorphs. Three types of experiments were performed depending on the supersaturated solutions: a) aqueous solutions b) aqueous supersaturated solutions in contact with sandbeds pre‐saturated with n‐dodecane c) aqueous solutions containing monoethylene glycol (MEG). Results showed that oil–water interfaces enhanced the heterogeneity of the supersaturated solutions and accelerated crystal growth of calcium carbonate at the inlet of the sandbed, resulting in early pore clogging and limitation of local permeability. Maximum sandbed consolidation was obtained with the solutions containing MEG.     

Influence of calcium binding proteins on the precipitation of calcium carbonate: A kinetic and morphologic study

In our experiments, the thermodynamic effect of calcium binding proteins (CBP) on the growth of calcium carbonate (CaCO₃) was studied in vitro. The CaCO₃ crystals obtained in systems with and without CBP were characterized by scanning electron microscope (SEM), Fourier Transform Infrared spectrograph (FT‐IR) and powder X‐ray diffractometer (XRD). The kinetic process was studied by monitoring the conductivity and pH value, which revealed the obvious inducement effect of CBP on the CaCO₃ crystals growth, and the possible formation mechanism of CaCO₃ in CBP solution was discussed. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Study on crystallization of calcium carbonate from calcium sulfate and ammonium carbonate in the presence of magnesium ions

Batch reactive crystallization of calcium carbonate (CaCO₃) from ammonium carbonate ((NH₄)₂CO₃) and calcium sulfate (CaSO₄) was investigated in the presence of magnesium (Mg²⁺) ions. It was observed that Mg²⁺ ions partly inhibited the conversion of CaSO₄ into CaCO₃. When the content of Mg²⁺ was less than 2%, the reduction in conversion rate of CaSO₄ was less than 2%, and the effect of Mg²⁺ ions could be ignored. Effect of impurity on crystallization kinetics of CaCO₃, including the growth rate and nucleation rate, was investigated. The results revealed that when Mg²⁺ ions content was less than 1%, Mg²⁺ could promote the growth of CaCO₃ and inhibit the nucleation process, which was favorable for the filtration of CaCO₃.When the content of Mg²⁺ ions was greater than 1%, Mg²⁺ inhibited the growth of CaCO₃, which resulted in explosion nucleation and led to a large number of particles in the solution, which was unfavorable for the filtration of CaCO₃. Based on the Bransom model, the particle size distribution equations of CaCO₃ were established. X‐ray diffraction patterns and scanning electron microscopy images exhibited the existence of spherical vaterite of CaCO₃ due to the reaction of CaSO₄ with (NH₄)₂CO₃ under the effect of Mg²⁺ ions, which was inconsistent with the results reported in the literatures.     

Using an electrochemical technique to study the effective variables on morphology and deposition of CaCO3 and BaSO4 at the metal surface

In this work using an electrochemical technique, deposition and crystal growth of calcium carbonate and barium sulphate at a stainless steel electrode is investigated through a rotating disk electrode involving oxygen reduction under diffusion condition. The influence of some parameters such as pressure, temperature, surfactant, cosalt and pH on morphology and deposition of CaCO₃ and BaSO₄ at surface of the stainless steel are studied. The results of the temperature tests reveal that the surface deposition is reduced by increase of the temperature and decrease of pH. The pressure also proves to have a significant influence on the morphology and the structure of calcium carbonate and barium sulphate deposition at the metal surface. With establishing a flow condition at high pressure, nucleation and deposition of calcium carbonate and barium sulphate at the metal surface generate the nano size of CaCO₃ and BaSO₄ crystals and leads to reduction of the coverage of the surface. In the presence of surfactant, it is shown that deposition of the calcium carbonate decreases the surface coverage so that after the point of the critical molar concentration of surfactant, a reduction of deposition of the calcium carbonate and barium sulphate at the surface can be clearly observed. Finally, influence of monovalent cosalts such as NaCl and KCl are investigated so that it does not present any certain trend in the deposition; however the morphology of the deposited crystal considerably changes.     

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.     

Effects of pH and temperature on CaCO3 crystallization in aqueous solution with water soluble matrix of pearls

Water soluble matrix (WSM) was extracted from pearls originated from Hyriopsis cumingii in Zhuji, Zhejiang province, China. WSM was regarded as an additive in mineralization experiments in order to study the effect of WSM on CaCO₃ crystallization. The experiments were carried out at different pH and temperatures by gas diffusion method and solution titration method, respectively. Scanning electron microscopy (SEM) and Raman spectroscopy (Raman) were used as powerful techniques to analyze the co-effect of pH value, temperature and WSM on crystal growth of CaCO₃. The results showed that WSM could induce aragonite at different pH values of mineralization solution, and the pH value had remarkable influence on morphology of calcite rather than aragonite due to distinct supersaturation and ionic strength related to various pH values. At different solution temperatures, WSM had little effect on crystal growth of calcium carbonate while the solution temperature had notable effect on polymorph and morphology of CaCO₃ crystals. This work can provide some basic information for the polymorph and morphology control of calcium carbonate.     

Preparation of calcium carbonate crystals in the presence of trisodium citrate

The dumbbell‐like calcium carbonate (CaCO₃) crystals were synthesized in the presence of trisodium citrate. Different morphologies were obtained by changing the reaction temperature and the trisodium citrate concentration. The obtained samples were characterized by means of X‐ray diffraction (XRD) and scanning electron microscopy (SEM). The results showed that the morphology of CaCO₃ crystals was markedly affected by the reaction temperature and citrate anion concentration. The possible growth mechanism of CaCO₃ crystals was proposed. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Conductometric study of calcium carbonate prenucleation stage: underlining the role of CaCO3o ion pairs

Calcium carbonate crystallization process, especially the prenucleation stage, has increasingly been the subject of several works. In the present work, a simple method based on electrical conductivity modeling applied to the FCP (Fast Controlled Precipitation) method data is used to highlight the role of CaCO₃^o ion pairs on calcium carbonate prenucleation stage. A good agreement was obtained between the resistivity vs pH curves estimated by the McCleskey model equation and obtained experimentally in a FCP test. Results showed that the nucleation process begins with the formation of CaCO₃^o ion pairs as pre‐nuclei as soon as the calcite‐equilibrium pH is reached. Additionally CaCO₃^o content increases with pH to form aggregates, which depend on the saturation state of the solution. Basing on our thermodynamic data, these aggregates do not form amorphous calcium carbonate ACC as an intermediate phase. They lead to the formation of stable calcium carbonate nuclei which will further evolve to crystallize. Furthermore we demonstrate that in addition to their inhibitory effect on the Ca²⁺ and CO₃²⁻ association to form ion pairs, the two scale inhibitors sodium triphosphate (STP) and sodium polyacrylate (RPI) reduce ion pairs aggregation rate.     

Facile preparation of diversified patterns of calcium carbonate in the presence of DTAB

Nano- and micro-sized calcium carbonate (CaCO₃) with various morphologies including multi-petal-flower-shaped, multi-step-cube-shaped, coral-shaped, dendrite-shaped and multi-antenna-shaped was successfully prepared using dodecyltrimethylammonium bromide (DTAB) micellar vevulsant. The effects of temperature, pH and the concentration of DTAB micellar solution on the morphology and crystalline form of CaCO₃ were systematically investigated. The prepared CaCO₃ was characterized by Scanning Electron Microscope (SEM), Transmission Electron Microscopy (TEM), X-ray Diffraction (XRD) and Fourier Transform Infrared Spectroscopy (FTIR). The concentration of DTAB micelle, pH and reaction temperature are found to play crucial roles in the morphology, size and crystalline form of the final products. On the base of the characterizations, a possible self-assembled mechanism was proposed. The novel multi-petal-flower-shaped and multi-antenna-shaped CaCO₃ may have some unique properties and potential applications in the future.     

The influence of xanthan on the crystallization of calcium carbonate

Calcium carbonate (CaCO₃) was crystallized in xanthan (XC) aqueous solutions. The CaCO₃ particles were characterized by scanning electron microscopy (SEM), powder X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy and thermogravimetry analysis (TGA) methods. The concentrations of XC, Ca²⁺ and CO₃^2? ions and the ratios [Ca²⁺]/[CO₃^2?] and [Mg²⁺]/[Ca²⁺] show evident influence on the aggregation and growth of CaCO₃ particles. The presence of Mg²⁺ ions influences not only the morphology, but also the polymorph of CaCO₃.     

Microwave-assisted synthesis of spheroidal vaterite CaCO3 in ethylene glycol–water mixed solvents without surfactants

Spheroidal vaterite CaCO₃ composed of irregular nanoparticals have been synthesized by a fast microwave-assisted method. The structures are fabricated by the reaction of Ca(CH₃COO)₂ with (NH₄)₂CO₃ at 90 °C in ethylene glycol–water mixed solvents without any surfactants. The diameters of the spheroidal vaterite CaCO₃ range from 1 to 2 μm, and the average size of the nanoparticals is about 70 nm. Bundle-shaped aragonite and rhombohedral calcite are also obtained by adjusting the experimental parameters. Our experiments show that the ratio of ethylene glycol to water, microwave power, reaction time, and two sources of ammonium ions and acetate anions are key parameters for the fabrication of spheroidal vaterite CaCO₃. A possible growth mechanism for the spheroidal structures has been proposed, which suggests that the spheroidal vaterite CaCO₃ is formed by an aggregation mechanism.     

Polymorphic composition and morphology of calcium carbonate as a function of ultrasonic irradiation

This paper reports on the precipitation of CaCO₃ polymorphs, having various crystal morphologies under different conditions. In particular, systems that were subject to ultrasonic irradiation were compared to the corresponding reference systems in the absence of such a treatment. The application of ultrasonic irradiation predominantly resulted in a change of particle size distribution and polymorphic composition of the precipitate, in comparison to the reference systems. Thus, it was found that the supersaturation and temperature influenced the size distribution, in both the reference and sonicated systems. A mixture of calcite, vaterite and aragonite was obtained in all reference systems, at 25 °C. At this temperature, the sonication caused the vaterite content to increase, while aragonite was not detected. In reference and sonicated systems at 80 °C, only aragonite precipitated. The results also indicate that the principle parameter responsible for the morphology of vaterite was the initial supersaturation: at higher supersaturation spherical vaterite particles precipitated, while at lower supersaturation hexagonal platelets were obtained. The morphological investigations also indicated different mechanisms of vaterite formation in the systems in which precipitation was initiated at higher supersaturation: spherulitic growth of vaterite was observed in sonicated systems, while the aggregation of primary particles was predominant in the reference systems. At lower supersaturation, the effect of c(Ca²⁺)/c(CO₃²⁻) on the morphology of hexagonal platelets of vaterite was observed as well. By varying the c(Ca²⁺)/c(CO₃²⁻), significant changes of the polymorphic composition were observed only in the sonicated systems, at 25 °C.     

Structural and spectral studies on gypsum crystals under simulated conditions of phosphoric acid production with and without organic and inorganic additives

In the dihydrate process to produce phosphoric acid, phosphate ore [Ca₁₀F₂(PO₄)₆] is leached with sulfuric and weak phosphoric acids to produce phosphoric acid and gypsum as a by‐product. Crystallization of gypsum occurs as the leaching is taking place. The effect of organic and inorganic additives on the structure and spectrum of gypsum crystals under simulated conditions of phosphoric acid production is studied using x‐ray diffraction and infrared spectroscopy. Structure and spectrum of formed gypsum crystals in the absence of additives are slightly different from the standard gypsum crystals (card No. 6‐0046), which reflect the effect of preparation medium on the crystal structure of gypsum crystals. Presence of additives such as cetyl trimethyl ammonium bromide and 1,2‐dihydroxybenzene‐3,5‐disulfonic acid, Al³⁺ and Mg²⁺ increase the crystallinity of gypsum, while presence of additives such as citric acid and sodium dodecyl sulfate decrease the crystallinity of gypsum. Presence of Al³⁺ and Mg²⁺ as additives lead to the formation of calcium sulfate hemihydrate beside calcium sulfate dihydrate. Presence of sodium dodecyl sulfate as an additive inhibits the crystallization of gypsum and leads to the formation of anhydrite and calcium sulfate hemihydrate.     

Investigation of calcium carbonate precipitated in the presence of alkanols

The influence of myristyl alcohol (CH₃(CH₂)₁₃OH), cetyl alcohol (CH₃(CH₂)₁₅OH) and behenyl alcohol (CH₃(CH₂)₂₁OH) on the structure, morphology, size and surface properties of calcium carbonate (CaCO₃) has been investigated. Changes in the nature of the washing solvent, in the C_nOH/Ca²⁺ and CO₃²⁻/Ca²⁺ molar ratios and in temperature have been also evaluated. The sole polymorph produced was rhombohedral calcite. At room temperature, while microspheres composed of submicrocubes were produced at a high molar ratio CO₃²⁻/Ca²⁺ and low CH₃(CH₂)₁₅OH concentration, a stoichiometric molar ratio CO₃²⁻/Ca²⁺ and high CH₃(CH₂)₁₅OH concentration induced the formation of microcubes and microboxes. In the presence of this alkanol (12 % molar) a significant enhancement of the water contact angle (ca. 40 °) resulted in a sample obtained with a stoichiometric CO₃²⁻/Ca²⁺ ratio. These results emphasize the key role played by the three non‐ionic surfactants in the formation of materials with variable crystal shape and wettability and thus technological interest for a range of applications.     

Hierarchical strontium carbonate submicron spheres self‐assembled under hydrothermal conditions

Uniform hierarchical strontium carbonate (SrCO₃) submicron spheres bearing detectable cavities on the surfaces and porous structures within the body were efficiently obtained, by a facile hydrothermal treatment (190 °C, 12 h) of the room temperature precipitate derived from Na₂CO₃ and SrCl₂ solution. MgCl₂ and ethylene diamine tetraacetic acid (EDTA) disodium salt were used as the additives. The as‐obtained submicron spheres were self‐assembled by crystalline SrCO₃nanoparticles under hydrothermal conditions. The present hydrothermally synthesized hierarchical SrCO₃ submicron spheres would enlarge the potentials of SrCO₃ micro‐/nanostructures in the hierarchical architectures and porous materials family for further applications in the fields of catalysis, composites, adsorbents, and devices, etc. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)