Vaterite Synthesis via GasâLiquid Route under Controlled pH

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Vaterite Synthesis via Gas−Liquid Route under Controlled pH Conditions Ion Udrea,† Constantin Capat,† Elena A. Olaru,†,* Raluca Isopescu,‡ Mihaela Mihai,‡ Carmencita D. Mateescu,§ and Corina Bradu†,⊥ †

Research Center for Environmental Protection and Waste Management, University of Bucharest, 90 Panduri Street, Bucharest, Romania, 050663 ‡ Department of Chemical Engineering, Polytechnic University of Bucharest, 1-5 Gh. Polizu Street, Bucharest, Romania, 011061 § National Institute of Materials Physics, 105 Atomistilor Street, Magurele, Romania ⊥ Faculty of Chemistry, University of Bucharest, 4-12 Regina Elisabeta Avenue, Bucharest, Romania, 070346 ABSTRACT: The purpose of this work was to obtain precipitated calcium carbonate (PCC) particles in polymorphic form of vaterite via gas−liquid route in controlled pH conditions. The effect of CO2 concentration (12.5−100%), feed gas (CO2−air) flow rate, pH, and conductivity of solution upon the PCC particles properties was studied. On the basis of the experimental data, the main factors leading to vaterite formation as major product were established. It was found that the buffer solution has a decisive role in determining polymorphic phase of PCC while CO2 concentration and feed gas flow rate have no significant influence. It was demonstrated that spherical vaterite particles of high purity can be produced under controlled reaction conditions. Also, some considerations on the mechanism of carbonation process were formulated.

1. INTRODUCTION Calcium carbonate is one of the most spread salts in nature, in the lithosphere and in the biosphere.1 In recent decades, both natural and precipitated calcium carbonate (PCC) had a wide use as a coating pigment, filler or extender in the production of paper, plastics, rubber, textiles, paints, adhesives, sorbents for SO2, detergents, and medicines.2−5 Technical applications of PCC are determined by its physicochemical and structural properties as pore size distribution, specific surface area, brightness, adsorption capacity of industrial oils, and chemical purity.6−8 It is well-known that there are three anhydrous crystalline polymorphic forms of PCC with different morphologies: spherical vaterite, acicular aragonite, and rhombic calcite.9−11 Calcite is the most stable thermodynamically form, while vaterite is the least stable and easily transforms into one of the other two polymorphic forms. In practice, the stability of these polymorphic forms depends mainly on temperature12,13 and additives.14−16 The relationship between precipitation conditions and morphology of CaCO3 is the object of many experimental studies but it still is disputed.17 It was proved that organic additives can stabilize a given polymorphic phase by changing the interfacial energy. This property was used to obtain pure aragonite. Some divalent cations also affect the nucleation, growth, and transformation of CaCO3 polymorphs.18 Spherical vaterite particles are formed by nanosize crystallites (25−35 nm) agglomeration that gives them a porous structure with a larger surface area and a greater hydrophilicity than the other more stable polymorphic phases.19,20 Owing to these characteristics, vaterite is recently used to obtain new materials with promising prospects in various fields. Thus, in the medical field, owing to its hydrophilicity, vaterite was used to obtain the © 2012 American Chemical Society

hydroxyl apatite, which is the starting material for artificial bones and tooth preparation,21,22 but it also finds applications as a component of orthopedic cements.23 Given their porous structure and hydrophilicity, spherical particles of vaterite can successfully replace silica in the coating pigments. These are used in the manufacture of inkjet paper because the resulting product presents high quality at low price.24,25 As filler in polymer composites, vaterite improves the crystallization temperature, the crystallinity degree, and the size of composite spherulites.26 According to Ostwald’s rule of phases,27,28 in aqueous media, the less-stable polymorph can nucleate first and then converts into the most stable polymorph later, sometimes via structures of intermediate stability. The initial phase that may be obtained is the amorphous calcium carbonate (ACC) and the subsequent transformation and crystallization of ACC can follow a downhill pathway in the free energy: ACC → vaterite → aragonite →calcite.29 If the solution supersaturation reaches the solubility constant value for vaterite, this metastable phase will be formed predominantly. Polymorphic vaterite → calcite transformation involves two processes that take place simultaneously: vaterite dissolution and calcite precipitation.30−32 There are two main methods for vaterite synthesis: mixing a calcium salt solution with carbonate salt solution24,25,33,34 or bubbling CO2 gas in ammoniacal calcium salt solution.17,35,36 Kralj et al.33,34 and Han et al.17 obtained vaterite by a liquid− liquid route from diluted solutions of calcium ion in the range Received: Revised: Accepted: Published: 8185

September 28, 2011 May 20, 2012 May 30, 2012 May 30, 2012 dx.doi.org/10.1021/ie202221m | Ind. Eng. Chem. Res. 2012, 51, 8185−8193

Industrial & Engineering Chemistry Research

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of 10−3−10−1 M. Han35,36 prepared calcium carbonate with more than 70% vaterite using a gas−liquid method, through a CO2 and N2 gas mixture with 33% CO2 at 3 L/min flow rate, and 66% CO2 at 0.9 L/min, respectively. All experiments were carried out at slightly alkaline pH obtained by ammonia addition, which determines local high supersaturation, continuous nucleation, growth, and agglomeration of the vaterite nuclei, inhibiting vaterite transformation into calcite. Vaterite− calcite mixtures were obtained, with up to 80% vaterite content in these conditions. In the present study, various CO2−air mixtures were bubbled into a CaCl2 solution with high Ca2+ concentration. Instead of adding ammonia during the carbonation process, NH3/NH4Cl buffer was added from the beginning to the liquid phase containing Ca2+ in order to maintain a pH range favorable to vaterite formation. The influences of feed gas flow rate and CO2 content upon precipitated calcium carbonate polymorphic phase were studied. The adequate working conditions for high purity vaterite synthesis were established.

experiment. The feed gas flow rate and CO2 concentration were varied in geometric progression with a ratio r ≈ 1.68, in a series of 25 experiments. The experiments were stopped at the moment when the carbonate solution pH decreased to 6.50. The Ca2+ concentration was monitored with a calcium ion selective electrode (ISE Ca800, WTW) throughout the entire process. The PCC was separated on a filter equipped with polytetrafluoroethylene (PTFE) membrane discs (0.2 μm), washed with deionized water and dried for 24 h at 105 °C. The morphology and particle size were examined by analysis of scanning electron microscopy (SEM) images using a SEM Quanta 3D FEG D9399 microscope (Figure 5) and Zeiss EVO 50XVP microscope (Figure ); the particle size distribution (PSD) was measured with Mastersizer 2000 analyzer using the laser diffraction method. The solid phase composition and the crystallite size of PCC samples were determined by X-ray diffraction with a Bruker D8 Advance diffractometer. The instrument provides high quality patterns due to an error of 2θ ≤ ±0.01°. The patterns were recorded with Cu Kα1 radiation with wavelengths 1.54060 Å in the angular range: 2θ = 20−100° and step width of 0.02 degree. The phase composition was estimated using the Rietveld method and Topas program which ensure, in the case of powder samples, a virtual separation of the overlapping peaks due to the large numbers of crystallites and allow an accurate determination of the structure. The Topas program allows also an evaluation of the crystallites size based on Scherrer formula knowing that vaterite spheres are formed by agglomeration of a large number of submicrometer crystallites. The polymorphic forms of PCC were also evaluated by IR spectroscopy using a Varian 3100 Excalibur spectrometer, equipped with a Harrick Praying Mantis diffuse reflectance accessory in the 4000−500 cm−1 range. Samples from all experiments were subjected to X-ray, FTIR, and PSD analysis. In the PSD analysis, three consecutive measurements were performed for each sample, and the error in successive samples was in the range of 0.8−1.7%.

2. EXPERIMENTAL SECTION The experiments were carried out at room temperature, in a 0.5 L glass bubble column as reactor (0.25 L working volume) equipped with a microporous glass gas distributor (22G3). The feed gas used in the experiments was a CO2−air mixture, with the CO2 composition varying in the 12.5−100% range. The feed gas flow rate (Q), measured with a Cole-Parmer flow meter (Figure 1), varied between 125 and 1000 mL·min−1.

3. RESULTS AND DISCUSSION The process parameters for the all 25 experiments are presented in Table 1: initial Ca2+ concentration (M), reaction time (trc, min) and final pH (pHf) corresponding to complete precipitation, conductivity (κ, mS·cm−1) and vaterite concentration of the final product (V %). Increasing the CO2 concentration and/or flow rate of the feed gas mixture causes the reaction time to decrease from 245 min (experiment A) to 5 min (experiment Y). Figure 2 shows a representative example for Ca 2+ concentration evolution and liquid phase pH variation noticed for A, K, and U, experiments, when the CO2 concentration was 12.5% and flow rate varied between 125 and 1000 mL min−1. The minimum Ca2+ concentration value corresponds to a 7.50−7.80 pH value (pHf). The time required to reach this concentration is denoted by trc (final reaction time). The reaction period when precipitation occurs in pH range controlled by the buffer (tbuff), represents more than 60% from the total reaction time. It is well-known that for alkaline solutions, the pH domain in which the buffer effect is efficient is given by

Figure 1. Experimental model of laboratory installation used in the carbonation process.

The reactor was equipped with a pH electrode and temperature sensor (Ioline, Schott), a conductivity cell (LF431, Schott) connected to pH/ISE/conductivity measuring instrument (Prolab 4000, Schott). Process parameters (pH, temperature, and conductivity) were continuously monitored using an online data acquisition system. CaCl2·6H2O (min. 99.1% purity, Merck) was used as calcium salt, while the buffer system was prepared from ammonium chloride (99.8% purity, Merck) and 25% aqueous ammonia (Merck). A 0.6−0.7 M Ca2+ ions solution with an initial pH (pHi) of 9.50−9.70 (the molar ratios Ca2+/NH3 and NH3/ NH4+ were 0.5 and 3.0, respectively) was used for each

pH buff = (14 − pK B) ± 1 8186

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dx.doi.org/10.1021/ie202221m | Ind. Eng. Chem. Res. 2012, 51, 8185−8193


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Table 1. Experimental Operational and Chemical Parameters and the Final Results: Reaction Time, Ph Liquid Phase and Vaterite Concentration of Product

Ca2+, M κ, mS/cm pHi trc, mina V, %b pHfa

QCO2 = 125 mL·min−1


A 0.699 129 9.73 245 72.7 7.8

F 0.671 129 9.74 150 99.4 7.7


B 0.659 127 9.54 148 98.3 7.6

G 0.632 127 9.49 83 98.4 7.6


C 0.665 132 9.74 84 98.3 7.6

H 0.640 130 9.66 53 98.3 7.9


D 0.692 129 9.69 46 98.4 7.6

I 0.615 129 9.73 32 99.5 7.6


E 0.710 127 9.72 33 98.8 7.5

J 0.647 128 9.66 19 99.2 7.5

QCO2 = 350 mL·min−1 CCO2 % = 12.5



P 0.675 130 9.79 62 98.2 7.6

U 0.640 130 9.81 41 97.1 7.8

CCO2

K 0.618 130 9.66 85 100 7.8 % = 21.0

Q 0.675 129 9.67 37 99.1 7.7

V 0.666 130 9.63 24 97.3 7.6

CCO2

L 0.612 127 9.52 51 98.9 7.6 % = 35.0

R 0.596 128 9.7 21 98.7 7.7

W 0.575 130 9.71 15 98.6 7.6

CCO2

M 0.598 129 9.73 30 98.9 7.7 % = 60.0

S 0.620 130 9.74 14 99.3 7.5

X 0.665 128 9.7 9 100 7.7

CCO2

N 0.615 129 9.74 18 99.4 7.5 % = 100.0 O 0.647 128 9.66 11 100 7.5

T 0.688 130 9.68 8 99.8 7.5

Y 0.663 130 9.68 5 100 7.6

Reaction time (trc) and final pH (pHf) were determined from the time evolution curves of the Ca2+ concentration and pH, respectively, during reaction; these values correspond to the minimum concentration of Ca2+ (Figure 2). bVaterite concentration calculated from FTIR spectra. a

The carbonation process of calcium salt solutions in presence of ammonia can be described by the following steps:

In the present case, when a NH4Cl/NH3 buffer solution was involved, KB = 1.75 × 10−5 and pHBuff varies in the 8.25−10.25 range. The pH variation in all 25 experiments (from pHi 9.50− 9.70 to pHf 7.50−7.80) falls largely in this area, maintaining a high supersaturation that helps to trap kinetically the metastable vaterite form, preventing the transformation into calcite. This is supported by the fact that the sample A (the less supersaturated) transforms to calcite at the end of experiments. This assumption is also consistent with the results of Chen et al.37 who found that the pH value is essential in determining polymorphic form of PCC. Figure 3 shows the influence of CO2 concentration on the carbonation process and according to Table 1, it can be noticed that a high CO2 content in feed gas leads to almost pure vaterite.

absorption of CO2 in liquid phase: CO2 (g) → CO2 (aq)

(2)

dissociation of ammonium hydroxide: 2NH4OH(aq) ⇆ 2NH4 +(aq) + 2OH−(aq)

(3)

formation of bicarbonate ion: CO2 (aq) + OH−(aq) ⇆ HCO3−(aq)

(4)

formation of carbonate ion: HCO3−(aq) + OH−(aq) → CO32 −(aq) + H 2O(l) 8187

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predominant and the metastable vaterite precipitates. In our experiments, a 7.50−7.80 pHf corresponds to the almost complete calcium carbonate precipitation. This is due to the use of ammonia/ammonium chloride solution which helps buffer the reaction environment and thus increases the time available for vaterite nucleation from solution. Below these pH values, dissolution of vaterite and calcite crystallization may occur. This phenomenon is well revealed in experiment A, where for about 150 min, the pH was lower than pH = 7.50 (see Figure 6). In the same time supersaturation decreases35 and the equilibrium of the following reaction is shifted to the left: HCO3− ⇆ CO32 − + H+

Vaterite and calcite concentrations were determined by two methods: from XRD spectra, by Rietveld analysis using the Topas program, and from FTIR spectra, where the content of both polymorphs are proportional to the peak areas: the 745 cm−1 peak corresponds to vaterite and 713 cm−1 peak to calcite.36 Figure 4 shows the FTIR and XRD spectra for samples A and F and the polymorphic compositions calculated with both methods. The results reveal that the polymorphic compositions of products, calculated by the two methods, are very close. Toward the end of the experiment, when supersaturation is low and the pH is almost 6.5, the metastable vaterite particles are transformed in calcite by the dissolution−recrystallization mechanism.30−32 This fact explains the presence of calcite for experiments with higher overall reaction times (including aging period). Thus, for 130 min maturation time (experiment A) the calcite content in the sample was 27.3%. This result is unlike the results of other authors35 who reported that the total transformation of vaterite into calcite took place within minutes. The SEM images reveal that vaterite spherical particles were obtained in all experiments (Figure 5a,b) except for those performed at low CO2 concentration and small flow rates, when the particles habitus were mostly irregular (Figure 5c,d). To elucidate the evolution of particles morphology during the precipitation process, samples were taken at various reaction times. The intermediated moments of sampling were chosen based on analysis of the pH variation during experiment A. As figure 6 shows, the pH curve can be divided in three regions: the first region (I), lasting up to 150 min, in which pH slightly decreases from 9.73 to 9.00 that corresponds to the efficiency domain of the NH4OH/NH4Cl buffer solution; the second region (II) during the interval 150−275 min in which the pH decreases down to 6.64 with high slope; the third region (III) lasting until the end of the process (275−375 min), in which the pH is almost constant (6.64−6.43). On the basis of this analysis of the pH curve, the reaction times 75, 200, 275, and 375 min were considered appropriate for the before mentioned investigations. The samples corresponding to these reaction times (separate experiments) were denoted as: A75, A200, A275, and A375, the latter being identical to the experiment A. The polymorphic composition was investigated by X-ray diffraction (Figure 7) and FTIR spectroscopy (Figure 8). X-ray diffraction proved that the dominant phase within the analyzed powder was hexagonal vaterite with lattice constants; a = 4.13(1) and c = 8.49(2). Side phase is rhombohedral calcite with lattice numbers: a = 4.9896 and c = 17.061. The sample morphology was revealed by SEM (Figure 9) and the particles size distribution was measured by laser ray diffraction (Figure 10).

Figure 2. Evolution of Ca2+ concentration and liquid phase pH vs time at different gas flows with 12.5% CO2 (experiments A, K, and U).

Figure 3. Influence of CO2 concentration in feed gas on the carbonation process.

formation of CaCO3: Ca 2 +(aq) + CO32 −(aq) → CaCO3(s)

(6)

These steps can be briefly expressed by the global reaction: Ca 2 +(aq) + 2NH4OH(aq) + CO2 (g) → CaCO3(s) + 2NH4 +(aq) + H 2O(l)

(7)

According to Juvecar,38 step 4 controls the process. At pH values over 9, the OH− concentration is sufficiently high to form CO32− (step 5) that, in the presence of high Ca2+ ions (initial concentration, 29 g/L), can lead to high supersaturations and consequently to high nucleation rates for CaCO3:31,35 J = a exp[(− 16πγ 3υ2 /3kT )(ln S)−2 ]

(9)

(8)

where J is nucleation rate, S is supersaturation ratio, a is a constant, γ is interfacial free energy, υ is solid density, k is Boltzmann constant, and T represents absolute temperature. According to Ostwald’s step rule,27 at high supersaturation, the difference between interfacial energy of polymorphs is 8188



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Figure 4. XRD (a) and FTIR (b) spectra for samples A and F, and the corresponding calculated polymorphic composition.

The results presented in Figures 6, 7, and 10 suggest that in the range of the buffer effect, nucleation and precipitation (domain I, Figure 6) occur accompanied by an increase in the size of crystallites (dc) and particles (dp) of vaterite (Table 2). Simultaneously, pH and supersaturation decrease. During the time interval corresponding to the second region (II), the decrease in supersaturation and in the pH results in the decrease of vaterite precipitation rate and increase in rate of dissolution−recrystallization process leading to calcite particles formation. If the carbonation reaction is stopped at pHf = 7.5−7.8, the obtained PCC contains 99% vaterite (Figures 6 and 7). However, if carbonation is allowed to continue, the aging period occurs (zone III), when the pH becomes slightly acidic (6.5) and the supersaturation vanishes. As a consequence, vaterite is transformed into calcite and, HCO3− is generated, leading to a slight increase of Ca2+ ion formed by the reaction: CaCO3 + CO2 + H 2O ⇆ Ca(HCO3)2

The analysis of the PCC at different time during experiment A reveals that (1) the habitus of the particles is continuously transformed during the reaction from spherical (A75) to irregular (A200, A275, A375), as SEM images show (Figure 9) and (2) the peak at 887 cm−1 emphasized by the FTIR spectra (Figure 8) in region II might be assigned to the out of plane bending vibration (ν2) of free carbonate anion.40,41 The irregular shape of calcium carbonate particles in experiment A (Figure 9) is attributed to the phase transformation from vaterite to calcite by dissolution and recrystallization and also to the agglomeration process. The phenomenon takes place at pH around 8. This value marks the lower limit of pH favorable for vaterite formation, and, at the same time, the upper limit for calcite recrystallization. Under these conditions, the rate of vaterite dissolution is higher than the rate of calcite recrystallization. This is also suggested by the shape of the solution conductivity curve in Figure 6. The continuous increase of the liquid phase conductivity seems surprising since the ions concentration decreases by the

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Figure 7. RX patterns of PCC samples from experiment A corresponding to 75, 200, 275, and 375 min.

The variation of particle size distribution (PSD) (Figure 10) in time (75, 200, 275, and 375 min, respectively) reveals a dominant growth mechanism during the first stages of carbonation process. This assumption is sustained by similar shapes of particle size distribution, shifted toward larger sizes of samples A75 and A200. Agglomeration accompanies the growth process19,32 which creates a slight asymmetry of PSD and is more important in the last stages when larger particles are formed (sample A275 and A375). These remarks are consistent with SEM images (Figure 9) and particles characteristics revealed by X-ray patterns (Table 2).

Figure 5. SEM images of vaterite obtained in various conditions: (a) experiment E, (b) experiment I, (c) experiment A, and (d) experiment F.

precipitation of calcium carbonate. Ukrainczyk et al.42 have considered that the increase of conductivity vs time curve after the end of the PCC formation was a consequence of PCC dissolution in the excess of the carbonic acid remaining in the system. In our case, the phenomena could be explained by reaction 6 which shows that ammonium ions are continuously produced, in a double amount as compared to the consumed calcium ions. On the other hand, as pH value decreases down to 8, the vaterite starts to dissolve and to dissociate in solution, representing an extra factor in the conductivity increase.

4. CONCLUSIONS Calcium carbonate particles were obtained by carbonation of calcium chloride solutions using NH3/NH4Cl as buffer. It was found that at any CO2 concentration values and feed gas flow

Figure 6. Evolution of Ca2+ concentration, pH, and conductivity in the liquid phase, during experiment A (dc = average crystallite diameter, dp = average particle diameter). 8190



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Figure 10. PSD analyses (laser beam diffraction) of PCC samples from experiment A, corresponding to 75, 200, 275, 375 min.

Table 2. Particles Characteristics for Experiment A (Q = 125 mL·min−1 and 12.5% CO2) polymorphic phase

sample region A75 A200 A275 A375

I II II, III III

crystallite size dc, (nm)

particle size dp, (μm)

41 49 50 51

0.5 0.5 1.5 1.8

vaterite, % calcite, % 100.00 99.0 98.5 74.0

0.0 1.0 1.5 26.0

habitus spherical irregular irregular irregular

rates used in this work, the vaterite was obtained as major product. Its purity level was confirmed by X-ray diffraction and FTIR spectroscopy, emphasizing that in controlled conditions no calcite was formed. Introducing ammonium chloride into ammonia calcium solutions resulted in keeping a pH value favorable to vaterite formation. Its transformation into calcite was favored by long carbonation times and low pH (∼6.5) values, as a result of dissolution−recrystallization processes which are accentuated as the reaction time (aging) increases. Maintaining a high supersaturation in the buffered pH range helps to trap kinetically the metastable vaterite form, preventing the transformation in calcite. Increasing CO2 concentration or gas flow supply led to lower reaction times from several hours to minutes but did not significantly influence the polymorphic form of PCC and particles size distribution. At low concentrations of CO2 (12.5%; 21.0%) or low feed gas flow rates (125−210 mL·min−1) vaterite samples show an irregular habitus while at higher concentration of CO2 (60%− 100%) or high flow rates of feed gas mixture (regardless of CO2 concentration) characteristic spherical vaterite particles are formed. The irregular shape of calcium carbonate particles obtained in the longest experiment (experiment A, 375 min) was attributed to vaterite−calcite transformation mediated by solution and also to the agglomeration process. The study stressed that the vaterite formation under controlled pH conditions (7.50−9.50) can be applied for a wide CO2 concentration range leading to vaterite with purity over 98%. This could enable application of vaterite synthesis in mitigation emissions of greenhouse gases by CO2 sequestration in inorganic compounds of industrial interest.

Figure 8. FTIR spectra of PCC samples from experiment A, corresponding to 75, 200, 275, and 375 min.

Figure 9. SEM images of PCC samples from experiment A, corresponding to 75, 200, 275, 375 min.

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(19) Brecevic, L.; Noethig-Laslo, V.; Kralj, D.; Popovic, S. Effect of Divalent Cations on the Formation and Structure of Calcium Carbonate Polymorphs. J. Chem. Soc. Faraday Trans. 1996, 92, 1017. (20) Nehrke, G.; Van Cappellen, P. Framboidal Vaterite Aggregates and their Transformation into Calcite: A Morphological Study. J. Cryst. Growth. 2006, 287, 528. (21) Kasuga, T.; Maeda, H.; Kato, K.; Nogami, M.; Hatabe, K.; Ueda, M. Preparation of Poly (lactic acid) Composites Containing Calcium Carbonate (Vaterite). Biomaterials 2003, 24, 3247. (22) Maeda, H.; Maquet, V.; Chen, Q. Z.; Kasuga, T.; Sawad, H.; Boccaccini, A. R. Bioactive Coatings by Vaterite Deposition on Polymer Substrates of Different Composition and Morphology. Mater. Sci. Eng., C 2007, 27, 741. (23) Tas, A. C. Use of Vaterite and Calcite in Forming Calcium Phosphate Cement Scaffolds. In Developments in Porous, Biological and Geopolymer Ceramics: Ceramics and Engineering Science Proceedings; Brito, M., Case, E., Kriven, W. M., Salem, J., Zhu, D., Eds.; John Wiley & Sons, Inc.: Hoboken, NJ, 2008; vol. 28, pp: 135−150. (24) Mori, Y.; Enomae, T.; Isogai, A. Application of Vaterite-Type Calcium Carbonate Prepared by Ultrasound for Ink Jet Paper. J. Imaging Sci. Technol. 2010, 54, 1. (25) Mori, Y.; Enomae, T.; Isogai, A. Preparation of Pure Vaterite by Simple Mechanical Mixing of Two Aqueous Salt Solutions. Mater. Sci. Eng., C 2009, 29, 1409. (26) Lyu, S. G.; Park, S.; Sur, G. S. The Synthesis of Vaterite and Physical Properties of PP/CaCO3 Composites. Korean J. Chem. Eng. 1999, 16, 538. (27) Ostwald, W. Lehrbuch fur Allgemeine Chemie; Engelman: Leipzig, Germany, 1902. (28) Noorduin, W. L; Vlieg, E.; Kellogg, R. M.; Kaptein, B. From Ostwald Ripening to Single Chirality. Angew. Chem., Int. Ed. 2009, 48, 9600. (29) Radha, A. V.; Forbesa, T. Z.; Killianb, C. E.; Gilbert, P. U. P. A.; Navrotskya, A. Transformation and Crystallization Energetics of Synthetic and Biogenic Amorphous Calcium Carbonate. Proc. Natl. Acad. Sci. U.S.A. 2010, 107, 16438. (30) Brecevic, L.; Skrtic, D.; Garside, J. Transformation of Calcium Oxalate Hydrates. J. Cryst. Growth. 1986, 74, 399. (31) Kralj, D.; Brecevic, L.; Kontrec, J. Vaterite Growth and Dissolution in Aqueous Solution III. Kinetics of Transformation. J. Cryst. Growth. 1997, 177, 248. (32) Brecevic, L.; Kralj, D. On Calcium Carbonates: from Fundamental Research to Application. Croat. Chem. Acta. 2007, 80, 467. (33) Kralj, D.; Brecevic, L.; Nielsen, A. E. Vaterite Growth and Dissolution in Aqueous Solution I. Kinetics of Crystal Growth. J. Cryst. Growth. 1990, 104, 793. (34) Kralj, D.; Brecevic, L.; Nielsen, A. E. Vaterite Growth and Dissolution in Aqueous Solution II. Kinetics of Dissolution. J. Cryst. Growth. 1994, 143, 269. (35) Han, Y. S.; Hadiko, G.; Fuji, M.; Takahashi, M. Crystallization and Transformation of Vaterite at Controlled pH. J. Cryst. Growth. 2006, 289, 269. (36) Han, Y. S.; Hadiko, G.; Fuji, M.; Takahashi, M. Effect of Flow Rate and CO2 Content on the Phase and Morphology of CaCO3 Prepared by Bubbling Method. J. Cryst. Growth. 2005, 276, 541. (37) Chen, P. C.; Tai, C. Y.; Lee, K. C. Morphology and Growth Rate of Calcium Carbonate Crystals in a Gas−Liquid−Solid Reactive Crystallizer. Chem. Eng. Sci. 1997, 52, 4171. (38) Juvekar, V. A.; Sharma, M. M. Absorption of CO2 in a Suspension of Lime. Chem. Eng. Sci. 1973, 28, 825. (39) Vagenas, N. V.; Gatsouli, A.; Kontoyannis, C. G. Quantitative Analysis of Synthetic Calcium Carbonate Polymorphs Using FT-IR Spectroscopy. Talanta 2003, 59, 831. (40) Su, C. M.; Suarez, D. L. In Situ Infrared Speciation of Adsorbed Carbonate on Aluminum and Iron Oxides. Clays Clay Miner. 1997, 45, 814. (41) Satao, M.; Matsuda, S. Structure of Vaterite and Infrared Spectra. Z. Kristallogr. 1969, 129, 405.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for financial support from R&D Programme “Partnerships in Priority Science &Technology Areas/2nd National Plan for Research, Development & Innovation, 2007-2013 (Grant: PNII 22-116/2008).

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REFERENCES

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Article

(42) Ukrainczyk, M.; Kontrec, J.; Babic-Ivancic, V.; Brecevic, L.; Kralj, D. Experimental Design Approach to Calcium Carbonate Precipitation in a Semicontinuous Process. Powder Technol. 2007, 171, 192.

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