DOI: 10.1021/cg901240n
Composition of Calcium Carbonate Polymorphs Precipitated Using Ultrasound
2011, Vol. 11 39–44
Gareth J. Price,* Mary F. Mahon, James Shannon, and Crispin Cooper Department of Chemistry, University of Bath, Bath, BA2 7AY, U.K. Received October 7, 2009; Revised Manuscript Received October 25, 2010
ABSTRACT: The precipitation of calcium carbonate (CaCO3) from saturated solutions prepared using calcium chloride and sodium hydrogen carbonate under the influence of ultrasound (20 kHz, intensity ranging from 1.5-18.5 W cm-2) has been studied. For short reaction times or treatment at low intensity, precipitates containing up to 90% vaterite were recovered; longer (>30 min) times or higher intensities yielded up to 100% calcite. Changes in the crystal sizes were noted. The results are consistent with the literature model of CaCO3 crystallization by the initial formation of vaterite followed by a dissolution-recrystallization process. Although a complete explanation cannot be offered, the results help to explain some of the apparently conflicting information previously published on this topic.
*To whom correspondence should be addressed. E-mail: g.j.price@ bath.ac.uk.
to be beneficial in the synthesis of inorganic materials17,19 and the preparation of pharmaceutical agents.20,21 There have been some studies which indicate that different polymorphs can be formed when systems are irradiated with ultrasound. One of the first was the investigation22 of the crystallization of tripalmitin and trilaurin fats using in situ synchrotron X-ray scattering showed that, whereas without ultrasound application both polymorphic forms β and β0 crystallized on cooling from the melt, only the β form of both fats formed between 50 and 30 °C, when ultrasound was applied for 2 s. A marked decrease of induction time and an increased nucleation rate were also reported. In later work, Gracin et al.23 found that sonication favored the precipitation of the R form of p-aminobenzoic acid over the β polymorph as well as reducing the induction time for nucleation. It was suggested that ultrasound disturbs the building up of the acid dimer structures required to form the R form in the early phases of crystal formation. In their study of the sonocrystallization of glycine, Louhi-Kultanen and co-workers reported24 that both the operating temperature and intensity of the ultrasound affected the glycine polymorphism; the effect of ultrasound was greater at lower temperatures together with a more uniform distribution of smaller crystal sizes. As a further example, sonication has also been reported25 to favor formation of the metastable δ phase during the freeze-drying of mannitol from aqueous solution. Sonication has previously been applied to the precipitation of calcium carbonate. In an early report, Dalas26 reported that its crystal growth rate was in fact retarded by over 62% when performed in an ultrasound field and that there was no effect on the formation mechanism or the nature, morphology, or size of the crystals formed. In contrast, Isopescu et al.27 reported that preparing CaCO3 in a batch reactor by mixing aqueous solutions of K2CO3 and Ca(NO3)2 under 20 kHz ultrasound led to a smaller mean particle size with narrower distribution compared with stirring. They ascribed this to the ultrasound preventing nucleated crystals from agglomerating. He et al.28 reported similar observations but suggested that the reduced particle size was due to rapid nucleation caused by supersaturation of Ca2þ ions around collapsing cavitation bubbles. Nishida29 applied ultrasound from a 24 kHz horn system over a
r 2010 American Chemical Society
Published on Web 11/17/2010
Introduction Calcium carbonate (CaCO3) is among the most widely occurring natural minerals.1 It has a wide range of uses as a raw material, for example, as a component of pharmaceuticals and foodstuffs, in water treatment, and as a filler in ceramics, plastics, coatings, and paper. While the natural material is often used, for more demanding applications CaCO3 is purified by precipitation to form precipitated calcium carbonate (PCC). In addition to the amorphous phase, calcium carbonate occurs in three main crystal polymorphs: calcite, aragonite, and vaterite. Of these, calcite, which forms rhombohedral crystals, is the most thermodynamically stable under ambient conditions. Aragonite usually forms needle-like orthorhombic crystals and is favored at high temperatures and pressures. It is metastable, converting slowly to calcite. Vaterite is the thermodynamically least stable polymorph, and its hexagonal crystals are rarely seen in the naturally occurring mineral.2,3 Since different polymorphs impart different properties,4 control over the crystal form is important to end users. In recent years, ultrasound has been used in the preparation of a wide range of solid materials,5,6 taking advantage of the various effects arising from cavitation.7 One of the earliest reports concerned the preparation of amorphous metals,8 and this has been followed by the preparation of metal,9,10 oxide,11,12 sulfide,13 and other nanomaterials.14 The main advantage of using sonochemistry is to greatly accelerate the reactions, although the unique conditions offered, for example, rapid cooling rates, allow some materials to be produced that cannot be produced by more conventional means. Some control over the size and shape of the particles is also possible5 in some cases by varying the sonication conditions. In addition to its use to prepare novel materials, power ultrasound (i.e., ultrasound with sufficient intensity to cause physicochemical change in contrast to low intensity “diagnostic” ultrasound) can also be used to influence crystallization from solution.15-17 Applying ultrasound (“sonication”) has been shown to influence ice formation in freezing water18 and
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range of intensities and showed that the rate of precipitation of CaCO3 from solution was faster under sonication and suggested this was due to the physical mixing of the solution rather than any intrinsic effects of cavitation. None of these papers reported the formation of different CaCO3 polymorphs. However, Wang et al. showed30 that the conditions of formation did impact the polymorph produced, the major influences being the degree of agitation and the concentration of the reactants. Uniform needle-like aragonite particles were formed without stirring or at low-power ultrasound; rhombohedral calcite formed when the mixing was carried out with stirring, while irregular particles of mixed vaterite and calcite were generated when high concentrations of Ca2þ were used. Zhou and co-workers31 also showed that aragonite could be prepared using high-power sonication and that the intensity of the ultrasound affected its morphology. They also suggested that dissolved CO2 played an important role in crystal nucleation and growth. In this paper, we attempt to reconcile these differing reports and describe the formation of CaCO3 crystals under varying experimental conditions. The types and amounts of the various polymorphs have been carefully evaluated, yielding new information on the effect of ultrasound on precipitation and crystallization of CaCO3 from solution. Experimental Section CaCO3 synthesis. Solutions of 0.040 mol dm-3 NaHCO3 and 0.020 mol dm-3 CaCl2 (both Aldrich, >99%) were prepared in deionized water (resistance >12 MΩ). The usual precipitation procedure32 involved mixing 25 cm3 of each solution in an open vessel with a jacket through which thermostatted water was circulated to maintain the temperature. Specific conditions pertaining to individual experiments are listed below. Where saturation with CO2 was required, the separate solutions were bubbled with CO2 gas at a slow rate for 10 min prior to mixing, and the reaction vessel was fitted with a lid to maintain a CO2 atmosphere. Sonication was carried out using a Sonic Systems L500 system fitted with a 1 cm diameter titanium horn operating at 20 kHz. The solution temperature was monitored by a thermocouple. Ultrasound intensities were measured calorimetrically in the usual manner.33 Except where noted below, reactions were conducted for 30 min, after which the precipitates were recovered by vacuum filtration through a 0.45 μm pore-size nylon membrane, washed twice with distilled water, once with acetone, and dried in air at 80 °C. The recovered yields were generally 45-60% of the theoretical maximum. Methods of Analysis. Samples were analyzed by powder X-ray diffraction (pXRD) on a Bruker AXS D8 Advance diffractometer using copper radiation. The approximate ratios of the different polymorphs were estimated using PowderCell software.34 Computed patterns for calcite aragonite and vaterite were generated from single crystal data, imported into Powdercell, where a simple Rietveld refinement was performed on the recorded diffraction pattern for each sample, to ascertain the mass percentage of each polymorph present. Diffraction peaks were modeled in Powdercell using a Gaussian function, because the peaks in the calculated polymorph patterns approximated most closely to this profile. Scanning electron microscopy (SEM) was carried out on a JEOL JSM6310 microscope. Samples were coated with a thin, conductive layer of gold.
Results and Discussion The particular polymorph formed during precipitation of CaCO3 depends on a number of experimentally controllable conditions including the degree of supersaturation, pH, temperature, and the presence of additives such as polymers or other metal ions. In order to determine the effects of ultrasound, all
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Figure 1. Library pXRD patterns of calcite, vaterite, and aragonite.36
Table 1. Experimental Conditions and Results Obtained for Precipitation of CaCO3 entry gasa 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
CO2 CO2 CO2
intensity time finalb- percent. polymorph (W cm-2) (min) temp calcite aragonite 0 0 9.0 13.0 18.5 9.0 13.0 18.5 9.0 18.5 1.5 3.5 6.0 13.0 13.0 13.0 13.0
30 30 30 30 30 30 30 30 30 30 30 30 30 10 20 45 60
19 70 53 69 76 61 70 78 19 23 35 46 54 58 60 72 76
100 84 51 75 92 94 98 85 33 80 10 21 38 18 8 97 98
7
formedcvaterite 8 49 25 8
6 15 67 20 90 79 62 82 92 3 2
a Unless shown, precipitations were conducted in an open vessel under air. b All experiments started at 19-22 °C. c Calculated from pXRD patterns. Uncertainty (3-5% (see text).
experiments were conducted in the absence of additives, at natural pH, and at a constant initial supersaturation (as defined by Neilson and Toft35) of ca. 400 (although this value will change through the course of some experiments due to changes in temperature). The highest purity reagents and water were used to minimize contamination by other inorganic ions. Figure 1 shows the pXRD patterns calculated from single crystal data for pure samples of each of the three polymorphs.32 Note that peaks at 2θ = 29.5°, 39.3°, and 47.5° are characteristic of calcite, those at 2θ = 25° and 44° are characteristic of vaterite, and those at 2θ = 26.2°, 37.9°, and 45.9° are characteristic of aragonite so that each of the polymorphs can be readily distinguished and quantified. Table 1 lists the proportions of each polymorph formed and the conditions used for each experiment. For comparison with the sonochemical experiments, initial precipitations of CaCO3 were carried out in the absence of ultrasound at 19 and 70 °C, these being the extremes of temperature reached during sonication. As expected (Table 1, entries 1 and 2), precipitation at the lower temperature produced almost pure calcite. The higher temperature sample contained small amounts of vaterite and aragonite, high temperatures being known to favor aragonite formation.37 The effect of sonication at two different intensities during precipitation is shown by Figure 2.
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Figure 3. Scanning electron micrographs (magnification 7000) of CaCO3 precipitated using ultrasound at 9 W cm-2 (a) in air (b) in CO2 gas and at 18.5 W cm-2 (c) in air (d) in CO2 gas.
Figure 2. Scanning electron micrographs (magnification 1200) of CaCO3 precipitated (a) with stirring at 70 °C (b) using ultrasound at 9 W cm-2 (c) using ultrasound at 18.5 W cm-2 (insets: magnification 7000).
The crystals prepared under conventional, stirred conditions (Figure 2a) are well formed and the rhombohedral calcite and needle-like aragonite crystals are clearly visible. At higher magnification, the needle-like aragonite structures, 10-15 μm in length, and the spherical structures usually associated with vaterite can be observed. Similar comments can be made concerning those prepared under ultrasound (Figure 2b,c) except that the particle size is, in general, much smaller. The sample prepared at the higher intensity has a fairly uniform particle size between 2-5 μm while that from the lower intensity sonication contains mostly small crystals but some that are up to 6-7 μm in diameter. For all three samples, the yields of CaCO3 were approximately the same so that an effect on the overall kinetics of crystal growth would not explain the observed effects. As with previous work,16-18 we attribute the change in particle size to a combination of two effects. First, sonication produces a larger number of nucleation sites so that more, smaller crystals form. Second, large crystals and particles are broken up as a result of interparticle collisions in the strongly turbulent motion produced by the collapsing cavitation bubbles in the ultrasound field.5,38,39 The latter effect often produces particles with irregular or rounded edges, and these, as well as small fragments, were noted when suspensions of calcite crystals in water were sonicated under conditions similar to those used here. The presence of some well formed crystals seen at higher magnifications suggests that the collision mechanism cannot completely account for all the observed effects. It is also possible that the ultrasound caused the structures to break up as they were forming and so
preventing aggregation which is particularly prevalent in the growth of vaterite.40 X-ray diffraction (XRD) analysis (Table 1, entries 3, 4, 5; Figure S1, Supporting Information) shows that using ultrasound makes a significant difference to the product formed, with substantially higher proportions of the less stable vaterite being formed under sonication. It is notable that the proportion of calcite formed was higher when higher intensities were used, although this also resulted in larger temperature rises which may have a strong influence on the results; this point will be returned to below. In order to investigate the role of CO2, identified as important by Zhou et al., a series of precipitations were conducted while bubbling the reaction mixture with CO2 gas. Saturating the solutions prior to mixing and continuing gas sparging during the reaction yielded no detectable formation of CaCO3 under any of the conditions studied. Sonochemical processes are often minimized where CO2 is used since it is highly soluble and large gas bubbles form easily in solution. It is also a polyatomic gas with a relatively low polytropic ratio so that the temperatures and pressures reached inside cavitation bubbles are lower than when using, for example, argon.7 Saturating the solutions with CO2 prior to mixing but not maintaining the flow during precipitation at the same ultrasound intensities as above did produce CaCO3. Table 1 (entries 6, 7, 8; see Figure S2, Supporting Information) illustrates that, under these gaseous conditions, a much higher proportion of calcite was formed and that the minor component, formed only at the highest intensity, was aragonite, in contrast to that formed under air. In general, the yields were rather lower than the comparable reactions in air, and the crystals formed under CO2 were larger and better formed (see Figure 3) with fewer small fragments. Cavitation is less harsh in highly gassy environments such as a CO2 saturated solution, and this results in less breakage of growing crystals through collisions. In order to further investigate the influence of the ultrasound, precipitations were carried out over a wider range of ultrasound intensities (Table 1, entries 3, 4, 5, 11, 12, 13). As shown in Figure 4, each sample produced a mixture of calcite and vaterite, although there was an interesting variation in the proportions produced, illustrated in Figure 5. As higher intensities were used, the composition gradually changed from vaterite to calcite in a
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Figure 7. Variation of composition with treatment time for CaCO3 precipitated using ultrasound at 13.0 W cm-2. Figure 4. pXRD patterns from CaCO3 precipitated using ultrasound at varying intensities (Values shown in W cm-2).
Figure 5. Variation of composition with ultrasound intensity for sonochemically precipitated CaCO3 0 calcite; 2 vaterite.
Figure 6. Yield of CaCO3 precipitated using ultrasound at varying intensities.
virtually linear manner. Also interesting was the observation that the yield of CaCO3 increased with rising intensity until the maximum was reached as demonstrated by Figure 6. These results are consistent with the observations of Nishida29 in that ultrasound accelerates the rate of precipitation but further suggests that there is an optimum value beyond which no additional benefit is achieved. However, it should also be noted that as higher intensities are used, the final temperature of the solutions also increases so that this may again play a part in the nature of the precipitates obtained.
The presence of vaterite in ovoid structures approximately 1 μm in diameter along with a few rhomboidal calcite crystals was seen in the samples produced at low intensity (Figure S3, Supporting Information). There was relatively little variation in the particle size as the intensity changes, although some larger calcite crystals occur at the highest intensity. The next series of experiments (Table 1, entries 4, 14-17) investigated any time dependence of morphology that occurred during ultrasonic irradiation conducted at constant intensity. Again, there was some variation in the final temperature reached, but this was relatively small, particularly for sonication longer than 20 min. On the basis of the X-ray diffraction (XRD) results (Figure S4, Supporting Information), the predominant polymorph formed during initial treatment is vaterite. However, as is clear from Figure 7, as sonication continues, this is converted to the more stable calcite. A very small amount of aragonite was detected in the later samples, but this was within the measurement uncertainty of the method and so may not be significant. An inevitable consequence of using an ultrasonic horn is a significant rise in temperature of the bulk solution. In order to investigate this, two final experiments were carried out while circulating chilled water around the reaction vessel. This maintained the temperature rise to approx 5-10 °C rather than the up to 50 °C rise when sonicating at the highest intensities. It is noticeable from the results in Table 1 (pXRD analysis Figure S5, Supporting Information) that, in the lower temperature samples, a smaller proportion of calcite is formed. At 23 °C and 9 W cm-2, the precipitate contained 33% calcite compared with 51% when the solutions were allowed to heat up to 53 °C. This suggests that temperature has a major influence on the polymorph formed. Vaterite is formed at lower temperatures and low sound intensity; aragonite only forms at the highest temperature and intensity used (Table 1, entry 5). However, the variation in temperature does not explain all the observations. Comparing entries 9 and 10 in Table 1 produced using different intensities at very similar temperatures, it is apparent that ultrasound does have a true effect on the proportions of each polymorph formed. A higher proportion of calcite (80% compared with 33%) was formed at the higher intensity. Figure 8 compares products produced at the same intensity but different temperatures. It reveals the presence of some calcite together with a relatively high proportion of vaterite spheres, around 1 μm in diameter. The mean particle size was rather smaller for the samples produced at lower temperatures and there were more small fragments of broken crystals. This is consistent with cavitational collapse being more intense and hence producing stronger jets and more energetic collisions at
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Figure 8. Scanning electron micrographs (magnification 7000) of CaCO3 precipitated using ultrasound at 18.5 W cm-2 (a) temperature 19-23 °C, (b) temperature 19-76 °C.
lower temperature. As the temperature increases, more water vapor enters the bubble and cushions its collapse.7 Further Discussion. It is clear from the results that the precise distribution of polymorph formed depends critically on the exact conditions employed for the precipitation. Thus, it is perhaps not surprising that there is disagreement in the published studies of sonochemical precipitation of CaCO3. The work reported here does not investigate all of the possible variables but does reveal some of the effects of sonication and explains some of the apparent contradictions reported in the literature. The retardation effect reported by Dalas26 was not observed in this work and its origin is unclear. However, their report of no effect on the morphology or particle size can be explained due to the low ultrasound intensities (70 °C). While they suggested that the high cavitational temperatures and pressures were responsible for the formation of aragonite, if this were the case, aragonite would be expected to form in the solutions where CO2 was not used since the temperatures reached inside bubbles would be higher. It seems more likely that the combination of the higher solubility of aragonite and the high solution temperature favored aragonite formation. Cavitation on CO2 saturated solutions is relatively “soft” due to the relatively higher solubility compared with other gases so that the reaction products resemble those from solutions that are not sonicated and aragonite is formed. Conclusions Calcium carbonate has been precipitated under various conditions involving ultrasound. The results are consistent with previously published mechanisms of CaCO3 crystallization involving the initial formation of vaterite and subsequent transformation to calcite by a solution-mediated dissolutionrecrystallization process. Higher proportions of calcite were formed during prolonged exposure to high temperatures or higher intensity ultrasound. The results can explain some of the apparent contradictions in the literature concerning the effect of ultrasound on crystallization, many of the documented differences being reconciled by differences in the reaction conditions. While variation of the ultrasound intensity or treatment time produces different ratios of vaterite and calcite, the most important factor determining this ratio remains the starting concentrations and supersaturation of the solution. The precise effects of ultrasound during the initial stages of crystallization remain unclear, and further experiments are needed to completely deconvolute all of the effects. Supporting Information Available: pXRD patterns and SEMs from all experiments not included in the manuscript. This material is available free of charge via the Internet at http://pubs.acs.org.
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