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.     

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)     

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.     

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.     

Formation of various polymorphs of calcium carbonate on porous membrane by electrochemical approach

Calcium carbonate (CaCO₃) formation was observed without surface modification of the organic template and in the absence of chemical additives such as macromolecules and divalent cations. Our innovative electrochemical approach that involves the use of an alternating current facilitated the crystallization of CaCO₃ polymorphs on a porous polymer membrane. A solution of calcium chloride (CaCl₂) and sodium carbonate (Na₂CO₃) was filled in a glass cell, and the porous membrane was interposed in the cell. A sine waveform of 10 Hz was applied to the platinum electrodes using a high-speed bipolar power supply. An alternating current was generated for 60 min. The crystal morphology and crystal structure of the resulting hybrid membrane were studied. In this electrochemical approach, versatile polymorphs of vaterite, aragonite, and calcite were formed on the membrane, thereby showing that the alternating current induced the formation of various polymorphs of CaCO₃ on the porous membrane even in the absence of any additives.     

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.     

PSSS‐controlled synthesis of CaCO3 superstructures

Complex CaCO₃ superstructure can be easily synthesized by using poly (sodium 4‐styrenesulfonate) (PSSS) as a structure directing agent to direct the controlled precipitation of calcium carbonate from aqueous solution. The products were characterized by scanning electron microscopy (SEM), and powder X‐ray diffraction (XRD) analysis. The results revealed that the morphology of the products changed significantly with the increasing of the concentration of PSSS in solution, from rhombohedral particles to plate‐packed aggregates to spheres with smooth surface, to sponge‐like spheres and finally to complex spherical superstructure consisted of plate‐like sub‐units. We hypothesize that the observed sequential changes in morphology of CaCO₃ particles with added PSSS concentration may be due to the influence of PSSS on nucleation, growth and aggregation of CaCO₃ crystals. The formation mechanisms of CaCO₃ crystals with different morphologies were discussed. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

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.     

Speciation of Fe(II) and Fe(III) effect on CaCO3 crystallization

The present work was carried out to investigate separately the effect of Fe²⁺ and Fe³⁺ on the precipitation kinetics and the microstructure of CaCO₃. For this an experimental procedure was proposed. Precipitation tests were made by using the dissolved‐CO₂ degassing method. Both air and nitrogen were employed to strip the CO₂ from a Ca(HCO₃)₂ solution initially rich in this gas. At anoxic medium, it was shown that iron (II) prolongs the nucleation step and decelerates the crystalline growth rate. X‐ray diffraction analysis shows that its presence inhibits calcite and promotes aragonite variety. By using air, the reaction medium is rich in oxygen and iron (II) is rapidly oxidized. Seeing the higher solution pH (> 6.5), iron hydroxide forms before the onset of CaCO₃ precipitation and plays a role of seed permitting to initiate CaCO₃ nucleation. So, contrary to the observed effect of iron (II), the presence of iron (III) accelerates the precipitation rate of CaCO₃. As for iron (II), iron (III) inhibits calcite formation but favored the vaterite variety instead of the aragonite one.     

Preparation and structure of calcium peroxide‐templated porous calcium carbonate crystals

Crystalline calcium carbonate with randomly dispersed porous structure was prepared through co‐ crystallization with calcium peroxide and the following template elimination by a post heating treatment and washing with water. The artificial CaCO₃ possess abundant macro‐mesopores structures and high surface area. This approach may open a new general route for the preparation of crystals with high porosity and structure specialty. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

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)     

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)     

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)     

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)     

Crystallization habit of calcium carbonate in presence of sodium dodecyl sulfate and/or polypyrrolidone

The synthesis of calcium carbonate (CaCO₃) crystals from aqueous solutions containing sodium dodecyl sulfate (SDS), poly(N-vinyl-1-pyrrolidone) (PVP) or SDS/PVP complexes has been performed through a slow titration method. It was found that aragonite and calcite coexisted in the prepared crystals. The formation of aragonite in the precipitation systems without magnesium ions indicates that at ambient temperature ca. 26.0°C, initially formed amorphous CaCO₃ could also transfer into aragonite in the sedimentary phase, which indicates the controlling factor of organic additives in the nucleation and growth process of CaCO₃ crystals. The appearance of hexagonal crystals in the suspensible phase confirmed the hexagonal crystallization cell of vaterite, and revealed the colloidal-dispersion function of the SDS/PVP complex in the crystallization process of CaCO₃.     

The effect of polyacrylamide on the crystallization of calcium carbonate: Synthesis of aragonite single-crystal nanorods and hollow vatarite hexagons

CaCO₃ nanorods were synthesized via a facile solution route by polymer-controlled crystallization in the presence of polyacrylamide (PAM). The morphology, size and crystal structure were characterized by means of scanning electron microscope (SEM), transmission electron microscope (TEM), and X-ray diffraction (XRD). The results suggest that the as-synthesized product was CaCO₃ (aragonite) nanorods with diameter ca. 50 nm and length ca. 1 μm. Selected area electron diffraction (SAED) pattern shows the single-crystal nature of CaCO₃ nanorods. The reaction time, temperature, pH and reactant concentration were systemically investigated. With the increase in the reaction time, hollow vaterite hexagonal disks can be obtained.     

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.     

Temperature fluctuations in a LiNbO3 melt during crystal growth

The synthesis of calcium carbonate (CaCO₃) crystals from aqueous solutions containing sodium dodecyl sulfate (SDS), poly(N-vinyl-1-pyrrolidone) (PVP), or SDS/PVP complexes has been performed through a slow titration method. It was found that aragonite and calcite co-existed in the prepared crystals. The formation of aragonite in the precipitation systems without magnesium ions indicates that at ambient temperature ca. 26.0°C, initially formed amorphous CaCO₃ could also transfer into aragonite in the sedimentary phase, which indicates the controlling factor of organic additives in the nucleation and growth process of CaCO₃ crystals. The appearance of hexagonal crystals in the suspensible phase confirmed the hexagonal crystallization cell of vaterite, and revealed the colloidal-dispersion function of the SDS/PVP complex in the crystallization process of CaCO₃.     

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.     

Crystallization of potassium sulphate from glaserite

The crystallization of K₂SO₄ from aqueous solutions of glaserite (a double salt of sodium and potassium sulphate) was investigated in an MSMPR cooling apparatus at 35 °C. The precipitation of K₂SO₄ at an average purity of 93 wt.% was obtained and the crystallization kinetics were derived from the experimental data. The nucleation and growth rates here obtained were, respectively, remarkably higher and slightly lower than those relevant to pure K₂SO₄: this behaviour may be explained by a faster nucleation and a slower growth of crystals containing also Na₂SO₄. The effect of small (0.05%) calcium additions to the feed was also tested, which caused an increase of supersaturation and a depression of the growth kinetics, whereas the nucleation one was not appreciably modified.