Evolution Mechanism of Calcium Carbonate in Solution

Transcription

1 CHINESE JOURNAL OF CHEMICAL PHYSICS VOLUME 23, NUMBER 6 DECEMBER 27, 2010 ARTICLE Evolution Mechanism of Calcium Carbonate in Solution Ya-ping Guo a,b,c, Hai-xiong Tang b, Yu Zhou b, De-chang Jia b, Cong-qin Ning c, Ya-jun Guo a a. Department of Chemistry, Shanghai Normal University, Shanghai , China b. Institute for Advance Ceramics, Harbin Institute of Technology, Harbin , China c. State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai , China (Dated: Received on August 7, 2010; Accepted on October 14, 2010) Calcium carbonate was synthesized in a CaCl 2 /NaCO 3 mixed solution by using ethylenediaminetetraacetic acid (EDTA) as an additive. The thermodynamics and kinetics analyses indicate that although the driving force of amorphous calcium carbonate (ACC) precipitation is always less than that of calcite and vaterite precipitation, the nucleation rate of ACC is greater than that of calcite and vaterite at the initial stage of the precipitation reaction. With the increasing incubation time, vaterite and calcite particles nucleate heterogeneously by using the as-formed particles as active sites. Scanning electron microscopy images indicate that the transformation mechanism of ACC and vaterite to calcite is the dissolution-recrystallisation reaction. The presence of EDTA not only improves the stabilities of ACC and vaterite, but also leads to forming enlongated, connected rhombohedral calcite crystals after incubation 7 days in solutions. The ACC and vaterite are stabler in air than in solutions at room temperature, although the dissolution-recrystallisation reaction occurs on the surface. Key words: Calcium carbonate, Dissolution-recrysatllisation reaction, Phase transition I. INTRODUCTION Recently, biological materials have attracted considerable research interests due to the unique structure and better performance. Among more than 60 kinds of inorganic minerals in various organisms, calcium carbonate is one of the most abundant biominerals [1]. CaCO 3 has three anhydrous crystalline polymorphs including calcite, aragonite, and vaterite, with rhombohedral, orthorhombic, and hexagonal structures, respectively [2, 3]. Biogenic single crystals of calcium carbonate are often produced by the transformation of amorphous calcium carbonate (ACC). ACC, a metastable phase, has been found to exist widely in biological organisms [4, 5]. This mineral is obtained difficultly under ambient conditions in an inorganic environment, and, if formed, it will be transformed into thermodynamically stable phases. Stabilization of ACC in biological systems is attributed to magnesium ions [6, 7] and organic molecules, including glutamate and hydroxyamino acids [8], betacyclodextrins [9], and ethanol [10]. An important advantage of ACC is that it can be easily molded into many different shapes through oriented nucleation on well-defined, chemically modified substrate surfaces. Author to whom correspondence should be addressed. ypguo@shnu.edu.cn Under certain conditions, the transient ACC phase will be transformed into calcite phase [4]. Based on the biomineralization principles, a crystallization strategy has been developed that the ACC bulks or films are used as predetermined micropatterned frameworks to define the location and crystallographic orientation of the aragonite or calcite nucleus [11 13]. Previous studies demonstrate that the transformation mechanisms of ACC into vaterite, aragonite, and calcite include the dissolution-recrystallisation reaction and solid-state transformation reaction [2, 14, 15]. Under high humidity, the transformation mechanism of ACC into calcite is the dissolution-recrystallisation reaction, whereas at high temperatures, it is the solidsolid transition. ACC in solutions is converted to aragonite, vaterite, and calcite mainly via the dissolutionprecipitation reaction [2, 15]. However, the transformation rate of ACC into crystalline phases is difficult to control because the transformation process is not only thermodynamically favored but also kinetically fast. In this work, we fabricate calcium carbonate using ethylenediaminetetraacetic acid (EDTA) as a stabilizing additive and study the transformation mechanism both in solution and in air. II. EXPERIMENTS With magnetic stirring, a 0.1 mol/l calcium chloride solution (200 ml) was added into a mixed solution (200 ml) including 0.1 mol/l sodium carbonate 731

2 732 Chin. J. Chem. Phys., Vol. 23, No. 6 Ya-ping Guo et al. (a) FIG. 1 (a) SEM image and (b) FTIR spectrum of CaCO 3 extracted immediately from the CaCl 2/EDTA/NaCO 3 solution after the milky color becomes visible. FIG. 2 XRD patterns of (a) calcium carbonate obtained in the CaCl 2/NaCO 3/EDTA solution, and followed by different incubation time: (b) 1 day, (c) 2 days, (d) 3 days, (e) 7 days. and 6 mmol/l EDTA for 6 min. In order to examine the metastable phase of CaCO 3, some samples were extracted as soon as white precipitates are formed. After the solution was stirred continuously for 30 min, CaCO 3 particles were obtained. These samples were divided into two parts: (i) the samples were incubated continuously in the mother solution at room temperature for 1, 2, 3, and 7 days, respectively; (ii) the samples were filtered off, washed with deionized water, and dried in an oven at 60 C for 48 h. To examine the stability of CaCO 3 in a solid state, the second part of samples was placed in air for 6 months at room temperature. The morphologies of samples were investigated by scanning electron microscopy (SEM, S-4800, Hitachi). The crystalline phases of samples were examined with X-ray powder diffraction (XRD, D/max-II B) using Cu Kα radiation. Fourier Transform Infrared spectra (FTIR, VECTOR22, BRUKER) were collected at a room temperature using the KBr pellet technique working in the range of wavenumbers cm 1 at resolution of 2 cm 1 (number of scans about 60). FIG. 3 FTIR spectra of (a) calcium carbonate obtained in the CaCl 2/NaCO 3/EDTA solution, and followed by different incubation time: (b) 1 day, (c) 2 days, (d) 3 days, (e) 7 days. III. RESULTS The CaCO 3 nanoparticles are extracted immediately from the CaCl 2 /EDTA/NaCO 3 mixed solution as soon as white precipitates are formed. Figure 1(a) shows that the particle size is nm. Figure 1(b) indicates the characteristic absorption bands of ACC such as the symmetric stretch in non-centrosymmetric structures at 1074 cm 1 (ν 1 ), the carbonate out-of-plane bending absorption at 866 cm 1 (ν 2 ), and the split peak at 1475 and 1412 cm 1 (ν 3 ) [5, 16]. After reaction for 30 min, the characteristic diffraction peaks belonging to vaterite and calcite are observed in Fig.2(a). Each phase of CaCO 3 has distinct IR spectral characteristics. Figure 3(a) shows the characteristic peaks of CaCO 3, corresponding to CO 3 2 at

3 Chin. J. Chem. Phys., Vol. 23, No. 6 Evolution Mechanism of Calcium Carbonate (a) (b) (c) (d) (e) (f) 733 FIG. 4 SEM micrographs of (a) and (b) calcium carbonate obtained in the CaCl2 /NaCO3 /EDTA solution, and those followed by different incubation time: (c) 1 day, (d) 2 days, (e) 3 days, (f) 7 days. The arrows indicate the hollow microspheres. FIG. 5 (a) XRD pattern and (b) FTIR spectrum of CaCO3 particles placed in air for 6 months. (ν3 ), 876 (ν2 ), 746, and 713 cm 1 (ν4 ), indicating the presence of both calcite and vaterite [17, 18]. ACC is preserved partly after reaction for 30 min, as confirmed by the presence of the characteristic band of ACC at 1074 cm 1 (Fig.3(a)). The strength of the characteristic peaks attributed to vaterite and ACC decreases with the incubation time in the mother solution, while that attributed to calcite increase (Fig.2 and Fig.3). The results demonstrate that the vaterite and ACC are converted to the calcite with the increasing incubation time. Figure 4 shows the morphological evolution of CaCO3 particles with the incubation time. After the suspension is stirred for 30 min, spherical particles with the diameter of 2 5 µm and rhombohedral calcite are formed (Fig.4(a)). With the increasing incubation time, the spherical particles are dissolved gradually into the solution, while the rhombohedral calcite crystals begin to nucleate and grow (Fig.4 (c) (e)). After incubation for 7 days, only the enlongated rhombohedral crystals are observed, as shown in Fig.4(f). In order to examine further the stability and transformation mechanism of the CaCO3 particles, the samples have been placed in air at room temperature for 6 months. The XRD pattern and FTIR spectrum show that ACC and vaterite remained partly (Fig.5). However, the CaCO3 particles become irregular, and some particles are connected together (Fig.6(a)). As compared with Fig.4(a), lots of distinct nanoparticles are formed on the surfaces (Fig.6(b)), indicating that ACC and vaterite are unstable in air. IV. DISCUSSION A. Formation mechanism of ACC at initial stage At ordinary temperatures and pressures, the thermodynamics stability of CaCO3 decreases in the following order: calcite>aragonite>vaterite. However, ACC precipitates firstly in the CaCl2 /EDTA/NaCO3 mixed soc 2010 Chinese Physical Society

4 734 Chin. J. Chem. Phys., Vol. 23, No. 6 (a) 5 Ya-ping Guo et al. µm FIG. 7 Free energy change G of CaCO3 precipitated in the CaCl2 /EDTA/NaCO3 solution as a function of Ca2+ concentrations. (a) ACC, (b) vaterite, and (c) calcite. (b) 1 µm FIG. 6 SEM micrographs of CaCO3 particles placed in air for 6 months. (a) low magnification, (b) high magnification. where γ is the interfacial tension, v the molecular volume, kb the Boltzmann constant, and A the kinetic factor. The CaCO3 molecular volume is defined by the crystal structure, which can be calculated: v= lution (Fig.1). Why is the first phase ACC, rather than calcite and vaterite? In order to answer the above question, the driving force and nucleation rate of CaCO3 in the CaCl2 /EDTA/NaCO3 solution have been analyzed based on the classical crystallization theory. The driving force G for crystallizations of CaCO3 is expressed as Gibbs free energy of transference from supersaturated to equilibrium solution. RT IP ln n Ksp RT = ln S n G = where IP is the activity product of precipitating salt, Ksp the thermodynamic solubility product, n the number of ions in the molecule, R the gas constant, T the absolute temperature, and S the supersaturation. According to Eq.(1), the G values for calcite, vaterite, and ACC in the CaCl2 /EDTA/NaCO3 solution are estimated for a range of reaction time, as shown in Fig.7. As soon as the CaCl2 solution was added into the Na2 CO3 /EDTA solution, the formation of calcite, vaterite and ACC is all thermodynamically feasible. If more than one reaction is thermodynamically possible, the resulting reaction is not the one that is thermodynamically most likely, but the one that has the fastest rate. The kinetics analysis is focused on the CaCO3 nucleation rate J, which can be estimated based on the classical model of homogeneous nucleation [19]: 16πγ 3 v 2 (2) J = A exp 3kB 3 T 3 (ln S)2 (3) where M is the molecular weight, ρ the density, and N the Avogadro constant. A in Eq.(2) is proportional to the probability P that the appropriate ion units of CaCO3 meet to compose a nucleus in the solution, i.e., A=A0 P, in which A0 is a constant. Hence, J can be also expressed by JR, which does not affect the comparison of nucleation rates of ACC, vaterite and calcite. JR = (1) M Nρ J A0 = P exp 16πγ 3 v 2 3kB 3 T 3 (ln S)2 (4) P depends on the concentrations of the ion units in the solutions, which can be calculated according to the method of Boistelle and Lopez-Valero [20]: P = 2[Ca2+ ][CO3 2 ] ([Ca2+ ] + [CO3 2 ])2 (5) where [Ca2+ ] and [CO3 2 ] are the concentrations of Ca2+ and CO3 2, respectively. According to Eq.(4), the analysis results of JR of CaCO3 in the CaCl2 /EDTA/NaCO3 solution are presented in Fig.8. Although the driving forces of calcite and vaterite are always larger than those of ACC (Fig.7), the nucleation rate of ACC is greater than that of calcite and vaterite at the initial stage (Fig.8). Hence, the first phase which precipitates in the CaCl2 /EDTA/NaCO3 solution is mainly ACC, rather than calcite and vaterite. c 2010 Chinese Physical Society

5 Chin. J. Chem. Phys., Vol. 23, No. 6 Evolution Mechanism of Calcium Carbonate 735 FIG. 8 (a) lgj R of CaCO 3 precipitated in the CaCl 2/EDTA/NaCO 3 solution as a function of Ca 2+ concentrations. (b) The enlarged curve. B. Evolution of CaCO 3 in aqueous solutions With the increasing reaction time, the concentrations of Ca 2+ reduce, and thus both the driving forces and the nucleation rates of CaCO 3 decrease (Fig.7 and Fig.8). The driving force of calcite is always larger than that of both ACC and vaterite, but the phase with the largest nucleation rate changes with the decrease of Ca 2+ concentrations. The phase with the largest nucleation rate is ACC when [Ca 2+ ] 47 mmol/l, it is vaterite when 41 mmol/l<[ca 2+ ]<47 mmol/l, and it is calcite when [Ca 2+ ] 41 mmol/l. Owing to the presence of ACC, the heterogeneous nucleation of vaterite and calcite may occur. The rate of heterogeneous nucleation can be estimated based on the classical model of heterogeneous nucleation [19]: [ J = A exp 16πγ3 v 2 ] ϕ 3k 3 (6) B T 3 (ln S) 2 where ϕ is the contact angle function. Because of the partial affinity of CaCO 3 (0<ϕ<1), the rate of heterogeneous nucleation for vaterite and calcite is greater than that of homogeneous nucleation (Eq.(2) and Eq.(6)). As the vaterite and calcite particles precipitate heterogeneously on the surface of ACC, the nanoparticles aggregate to form microspheres. It is noted that the phase with the largest nucleation rate is vaterite when 41<[Ca 2+ ]<47 mmol/l. Therefore, the main phases of the core of microspheres are ACC and vaterite. With the increase of the reaction time and the decrease of the Ca 2+ concentrations, the main phase of the precipitated CaCO 3 is calcite, which served as the shell of the microspheres. The phenomena of heterogeneous nucleation can be confirmed by the SEM micrographs in Fig.4 (a) (d). As we know, the crystal, which has a higher solubility, is dissolved into the solution more easily. Since the solubility product of ACC ( ) and vaterite ( ) are higher than that of calcite ( ) [21], ACC and vaterite are dissolved into solution more easily than calcite. After incubation of 1 2 days, many hollow microspheres appear due to the dissolution of ACC and vaterite (Fig.4 (c) and (d)), which suggests that the core of the spherical particles is ACC and vaterite, and the shell is calcite. Based on the kinetics analysis and SEM images in Fig.4 (c) and (d), the conclusion can be made that the formation of vaterite and calcite is a typical heterogeneous nucleation process. Figures 2 and 3 show that the strength of the characteristic peaks attributed to vaterite and ACC decreases with the incubation time, while that attributed to calcite increases. After incubation of 7 days, no ACC and vaterite are observed in Fig.4(f). How are ACC and vaterite converted to calcite? In order to address this question we studied the morphological evolution of CaCO 3 particles, as shown in Fig.4. At high supersaturation in solutions, Ca 2+ react with CO 2 3 to form the ACC in a diameter nm (Fig.1(a)) [22]. After the suspension is stirred for 30 min, the microspheres with a diameter of 2 5 µm and the rhombohedral calcite are formed (Fig.4(a)). With the increasing incubation time, the spherical particles are dissolved gradually into the solution, while the rhombohedral calcite crystals begin to nucleate and grow (Fig.4 (c) (e)). After incubation of 7 days, only the enlongated rhombohedral crystals are obtained, as shown in Fig.4(f). The morphologic changes of CaCO 3 particles demonstrate that the transformation mechanism of vaterite and ACC into rhombohedral calcite is the dissolution-recrystallisation reaction. The rhombohedral calcite crystals grow at the expense of the dissolving ACC and vaterite particles (Fig.4 (c) (f)). The inset in Fig.4(d) shows that the surfaces of vaterite and ACC particles are used as active sites for the heterogeneous nucleation of calcite. With increasing the incubation time, the vaterite and ACC particles begin to dissolve, resulting in the formation of some hollows with the diameter of approximately 5 µm in calcite crystals (Fig.4 (e) and (f)). The diameter of hollows is consistent with that of the microspheres. The growth mechanism can be suggested that as the vaterite and ACC microspheres are dissolved into solutions, the

6 736 Chin. J. Chem. Phys., Vol. 23, No. 6 Ya-ping Guo et al. rhombohedral calcite crystals nucleate and grow by using the microspheres as active sites. The hollows are formed in the calcite crystals, after the microspheres are dissolved completely into solutions. The calcite crystals grow with the incubation time (Fig.4 (d) (f)), which indicates that Ostwald ripening is the underlying mechanism operative in this transformation process. Ostwald ripening, that is, the growth of large crystals at the expense of small ones, results from the difference in the chemical potential or solubility of the crystals [23]. The solubility of calcium carbonate crystals increases with decreasing the radius, so the smaller crystals dissolve into the solution, and redeposit upon larger ones, leading to an overall increase in average size of calcite crystals (Fig.4 (d) (f)). Based on the above analyses, the evolution procedures of calcium carbonate in the mother solution may be suggested: (i) ACC precipitates immediately after the addition of the CaCl 2 solution into the EDTA/NaCO 3 solution; (ii) with the increasing incubation time, the vaterite and few calcite particles nucleate heterogeneously by using as-obtained CaCO 3 particles as active sites; (iii) the vaterite and ACC phases are converted to calcite phase by the dissolutionrecrystallization reaction; (iv) the calcite phase grows into enlongated, connected rhombohedral crystals. C. Effect of EDTA on the formation of CaCO 3 EDTA is one of important complexing agents, which can react with Ca 2+ to form Ca-EDTA complex. In this work, EDTA used as the organic additive may affect the morphology, crystal phase, nucleation, and growth of CaCO 3 particles. Firstly, EDTA decreases the activation energy for the nucleation formation of the ACC and vaterite, and thus promotes them to crystallize preferentially. Secondly, the Ca-EDTA complex can decrease the surface free energy of ACC and vaterite, so the metastable phases can be stabilized in solution for a longer time than those without EDTA. Thirdly, the EDTA with negative charges may attach to the surfaces of CaCO 3 particles, resulting in the changes of interfacial ion distributions. The interfacial ion distributions of Ca 2+ and CO 2 3 which deviate from the stoichiometry for CaCO 3 can promote heterogeneous nucleation and growth of calcite on the surfaces of metastable phases [24]. In addition, EDTA will affect the morphology of calcite particles. Figure 4(f) shows that the calcite particles grow into enlongated, connected rhombohedral crystals after incubation of 7 days due to the presence of EDTA. Without it, the calcite crystals exhibit multidimensional morphology via the heterogeneous nucleation process [1]. D. Evolution of CaCO 3 in air The stability and transformation mechanism of the CaCO 3 particles are investigated by placing the samples in air. After 6 months, ACC and vaterite are remained partly, as confirmed by XRD pattern and FTIR spectrum (Fig.5). However, the microspheres and rhombohedral calcite particles become irregular, and some particles are connected together (Fig.6(a)). As compared with Fig.4(a), lots of distinct nanoparticles are formed on the surfaces (Fig.6(b)), indicating that the ACC and vaterite is unstable in air. The changes of calcium carbonate in morphology, shape, and size suggest that the dissolution-recrystallisation reaction occurs in air due to the adsorbed water on the surface of ACC and vaterite. The characteristic peak at 1637 cm 1 corresponding to adsorbed water can be observed in Fig.5(b). Since the solubility of ACC and vaterite is higher than that of calcite, the saturated solution is unstable so that calcite crystals will spontaneously nucleate and grow from it. Xu et al. suggest that the increase of the humidity will induce and speed up the transformation of ACC by supplying a matrix in which it can dissolve and recrystallize [14]. At room temperature, ACC and vaterite are transformed into calcite via the dissolution-recrystallisation reaction both in solutions and in air, but their transformation rates are obviously different. In solutions, the ACC and vaterite have been transformed completely into calcite after only 7 days; while, in air, the ACC and vaterite are still evident even after 6 months. Thus, the aqueous solution can be used as an important intermediate to induce the transformation of ACC and vaterite into calcite. V. CONCLUSION The formation and transformation of CaCO 3 in the CaCl 2 /EDTA/NaCO 3 solution have been analyzed based on classical crystallization theories of thermodynamics and kinetics. The analysis results indicate that the first phase precipitated in the solution is ACC. With the increasing incubation time, the vaterite and calcite particles nucleate heterogeneously by using the asobtained particles as active sites. SEM images indicate that the transformation mechanism of ACC and vaterite to calcite is the dissolution-recrystallisation reaction. In addition, EDTA plays an important role in improving the stabilities of ACC and vaterite, and leading to forming enlongated, connected rhombohedral calcite crystals after incubation 7 days in solutions. ACC and vaterite can be stable in air for more than 6 months, although the dissolution-recrystallisation reaction occurs on the surfaces. VI. ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (No and No ), the National Postdoctor Science Founda-

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