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Journal of Crystal Growth 289 (2006) 269–274

www.elsevier.com/locate/jcrysgro

Crystallization and transformation of vaterite at controlled pH

Yong Sheng Han, Gunawan Hadiko, Masayoshi Fuji, Minoru Takahashi�

Ceramics Research Laboratory, Nagoya Institute of Technology, 10-6-29 Asahigaoka, Tajimi 507-0071, Japan

Received 1 September 2005; received in revised form 28 October 2005; accepted 6 November 2005

Communicated by K. Nakajima

Abstract

Precipitated calcium carbonate particles were prepared by passing CO2/N2 mixed gases into CaCl2 solution. The nucleation,

crystallization and transformation of vaterite at controlled pH were investigated in this paper. The control of pH was conducted by the

addition of ammonia. The results showed that addition of ammonia induced a local high supersaturation and continuous nuclei

formation, which is in favor of the growth and agglomeration of vaterite nuclei while inhibiting the transformation of vaterite to calcite.

The nearly pure vaterite is ready to form at the control of pH.

r 2005 Elsevier B.V. All rights reserved.

PACS: 61.10Nz; 61.66.Fn; 64.70.Kb

Keywords: A1. Crystal morphology; A1. Nucleation; A1. X-ray diffraction; B1. Calcium compounds; B1. Oxides

1. Introduction

The precipitation of calcium carbonate has attractedmore and more attentions because of its applications in theareas of plastics, rubbers, papermaking, biomedical im-plant and drug delivery [1–6]. The application of calciumcarbonate particles is mainly determined by the poly-morphs of CaCO3. Calcium carbonate has three crystalpolymorphs, namely rhombic calcite, needle-like aragoniteand spherical vaterite. Calcite is the most stable phase atroom temperature under normal atmospheric conditions,while aragonite and vaterite are metastable polymorphsthat readily transform into the stable phase (calcite). Thepreference of polymorphs formation for calcium carbonateis mainly determined by the precipitation condition, suchas pH, temperature and supersaturation. pH is usuallyconsidered to be an important factor affecting thepolymorph of CaCO3. Although the preparation conditionof CaCO3 has been widely investigated, there are a fewworks reporting the influence of pH on the polymorphs of

e front matter r 2005 Elsevier B.V. All rights reserved.

rysgro.2005.11.011

ing author. Tel.: +81572 27 6811; fax: +81 572 27 6812.

esses: m-takahashi@nitech.ac.jp,

ua.edu.cn, han@crl.nitech.ac.jp (M. Takahashi).

CaCO3. Even in these few works the results seemunambiguous and contradictory. For example, Spanosand Koutsoukos [7] have concluded that pH has no effecton the transformation of vaterite to calcite, which indicatedthat pH has no effect on the formation of vaterite andcalcite. While Chen et al. [8] observed that the pH ofsolution is the most important factor affecting thepolymorphs of calcium carbonate, they produced nearlypure vaterite at pH below 8. Besides, Hostomsky and Jones[9] found that at pHX9.5, mainly vaterite was precipitatedin the form of agglomerates of spherical particles and Kraljand Brecevic [10] precipitated pure vaterite within pHbetween 9.3 and 9.9. These reported results are totallydifferent, which make readers confused. Hence, it isnecessary to investigate the influence of pH on thepolymorphs of CaCO3 systematically to elucidate therelationship between pH and polymorphs of CaCO3.In this paper, the calcium carbonate particles were

prepared by passing CO2/N2 mixed gas into CaCl2 solutionunder controlled pH condition. The initial pH of thesolution varied in the range 7.5–11.5. The pH was keptconstant during precipitation by the addition of ammonia.The main purpose of this paper is to investigate theinfluence of pH on the polymorphs of CaCO3.

ARTICLE IN PRESSY. Sheng Han et al. / Journal of Crystal Growth 289 (2006) 269–274270

2. Experimental sections

Calcium chloride (Wako Pure Chemicals, Japan) wasdiluted with distilled water to form a solution with theconcentration of 0.1mol/l. Ammonia (Wako Pure Chemi-cals, Japan) was added to the prepared solution to adjustthe pH to an expected initial value. Then the freshlyprepared solution reacted with a mixed gas (33.3 vol%CO2+66.6 vol% N2) that was introduced into the solutionat a flow rate of 0.3 l/min. The reaction temperature waskept at 20 1C by a water bath. During the carbonation, thepH of solution was kept constant by the addition ofammonia. The total volume of the working solution was500ml and the solution was continuously stirred at aconstant rate by means of Teflon-coated magnetic stirringbar. The completion of precipitation was determined bythat there was no more decrease in pH even after theaddition of ammonia was stopped. The prepared solidswere collected by filtering through membrane filters(0.2 mm) and dried at 105 1C for at least 24 h, and usedfor measurements. SEM (JEOL JSM-6100, Japan) wasused to observe the morphologies of the samples. XRD(RINT 1100, Rigaku, Japan) measurements were con-ducted using Cu Ka radiation (40 keV, 30mA) to identifythe composition of the samples. The scanning step is 0.02o

and 2y ranges from 201 to 601.

Fig. 1. SEM images of sample

3. Results and discussion

The overall process of the precipitation of calciumcarbonate in our experiments is

CaCl2 ðaqÞ þ CO2 ðgÞ þH2O ¼ CaCO3 # þ2Hþ

þ 2Cl�: (1)

This reaction starts with the dissolution of CO2 intosolution. After CO2 dissolves into water in the form ofH2CO3 and then converts to H+, HCO�3 and CO2�

3 ions,the Ca2+ reacts with CO2�

3 to form the temporary resultantof amorphous calcium carbonate that is an unstable formand quickly transforms to crystalline phase. Some studiesreported that vaterite is the firstly formed crystal phase incalcium carbonate formation [11]. Due to its instability,vaterite is ready to transform to the stable phase (calcite) atroom temperature. Hence, the product of precipitatedcalcium carbonate is usually the mixture of vaterite andcalcite at room temperature under normal atmosphericcondition.Fig. 1 shows the micrographs of CaCO3 samples

prepared at different pH. The pH of solution was keptconstant (with the precision of 0.1) during precipitation bythe addition of ammonia. At pH 11.1, spherical particles,as the major phase, were observed with a mixture of some

s prepared at different pH.

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20 30 40 50 60

C018C104

V224

V118V304

V300V114V112

V110V004

7.9

8.3

9

9.6

10.4

10.9

11.1

11.2

2θ

Fig. 2. XRD patterns of CaCO3 samples prepared at different pH.

7.5 8.0 8.5 9.0 9.5 10.0 10.5 11.0 11.5

0.75

0.80

0.85

0.90

0.95

1.00

Fra

ctio

n of

vat

erite

pH

Fig. 3. Fraction of vaterite changing with pH.

Y. Sheng Han et al. / Journal of Crystal Growth 289 (2006) 269–274 271

rhombic particles. Some of the spherical particles attach toeach other and form the irregular particles (as shown inFig. 1a). At pH 10.4, a few rhombic particles were alsoobserved and the major particles have ellipse shape. Someof the ellipse-like particles were agglomerated and formedbig congeries (as shown in Fig. 1b). When the pH decreasedto 8.9, regular spherical particles with different size wereprepared and the rhombic particles were also observed (asshown in Fig. 1c). At pH 7.9, the sample was comprised ofonly spherical particles with different sizes while therhombic particles were not observed.

Fig. 2 shows the X-ray diffraction (XRD) patterns ofCaCO3 samples prepared at different pH. The XRDpattern exhibits the characteristic reflection of calcite(C104) and vaterite (V110, V112, V114). With the decreaseof pH, the peak of calcite (C104) decreases gradually.When the pH decreased to 7.9, the peak of C104disappeared, which indicates that calcite was not formedat pH 7.9. This result has good accordance with the SEMobservation. To analyze the formation of vaterite quanti-tatively, we employ the Rao equation to express the relativefraction ðf vÞ of vaterite in the crystalline phase [12]

f v ¼ ðI110V þ I112V þ I114VÞ=ðI110V þ I112V þ I114V

þ I104CÞ: ð2Þ

The subscripts V and C here indicate vaterite and calcite,respectively. The calculation results of f v at different pHare shown in Fig. 3. It is easy to conclude that the fractionof vaterite decreases with pH. The change of f v with theincrease of pH may be attributed to the competition ofvaterite crystallization and transformation.

According to Juvekar and Sharma [13], the rate-controlling step in the carbonation process is the CO2

dissolution, as shown in the following:

CO2 ðaqÞ þOH� ðaqÞ ¼ HCO3� ðaqÞ: (3)

The rate of CO2 dissolution is given by the following [5]:

rA ¼ KCBðCA � CA;eqÞ, (4)

where rA is the rate of CO2 dissolution, CA and CA,eq

denote the CO2 concentration in solution at non-equili-brium and equilibrium, respectively, and CB the concentra-tion of OH� in solution, K is the rate constant. From Eq.(4), it can be concluded that CO2 dissolves quickly intosolution at high pH.In our experiments, at the high pH condition, the quick

dissolution of CO2 resulted in a high supersaturation,which lead to a rapid nucleation of CaCO3 precipitatesaccording to the Gibbs–Thomson formula of classicalnucleation theory [14,15]:

J ¼ A exp½�BðlnSÞ�2�; (5)

S ¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi½Ca2þ�½CO2�

3 �r2

Kso

s, (6)

where J and S are nucleation rate and supersaturationratio, r is the activity coefficient of divalent ions, Kso is thesolubility product of calcium carbonate, A is constant, B isrelative to the interfacial energy of polymorphs. It can beexpressed as follows [16]:

B ¼ 16pg3n2=3ðkTÞ3, (7)

where k and T are Boltzmann constant and absolutetemperature, respectively, g is interfacial free energy, n issolid density. Based on the ‘‘Ostwald step rule’’ [17], at highsupersaturation the difference of the interfacial energy (B)between polymorphs becomes relatively dominated and themetastable form may tend to precipitate, which indicatesthat the metastable vaterite prefers to form at the highsupersaturation. This is well consistent with our experi-mental results.It is important to point out that the nucleation rate of

calcite may increase when the pH of solution decreases tolow value (such as lower than 8) due to the decrease insupersaturation resulting from change of CO2�

3 to HCO�3 .

ARTICLE IN PRESSY. Sheng Han et al. / Journal of Crystal Growth 289 (2006) 269–274272

HCO�3 is the major ion in the pH of 6–7 while CO2�3 mainly

exists in the high pH, such as 10–11 [6]. Since HCO�3cannot directly contribute to supersaturation, the super-saturation may decrease with pH, which is in favor of thenucleation of stable polymorphs (calcite). Due to theautomatic decrease of pH during carbonation, it isimportant to control the pH of solution during precipita-tion for the purpose of obtaining pure vaterite.

After nucleation, the freshly formed nuclei grow upquickly at the high supersaturation condition according toKralj and Brecevic [10]

dr=dt ¼ kðS � 1Þ2, (8)

where dr/dt denotes growth rate of crystal and k and S�1are rate constant and supersaturation, respectively.

Since the crystal grows up quickly at the high pH, theprecipitation reaction proceeded quickly and finished in ashort time. Fig. 4 shows the reaction time cost of theexperiments with different pH. The completion of reactionwas determined by that there is no more pH decrease evenafter the addition of ammonia is stopped. At pH 11.1, theprecipitation was finished in 6min. Since each experimentwas kept passing CO2/N2 10min, there is no moreprecipitation in the last 4min for pH 11.1. At the last4min, the prepared samples just stayed in the solution andsuffered the transformation of vaterite to calcite. At thatending stage, the supersaturation of the solution isrelatively low due to the complete consumption of Ca2+

and the metastable vaterite particles are ready to dissolveand transform to calcite in the solution. This is why weobserved some rhombic particles at the high pH as shownin Fig. 1. After filtration, the remnant vaterite particles arestable in solid products because the transformation ofvaterite to calcite happens only in the solution with theassistance of water.

Here we should mention that the transformation ofvaterite to calcite during precipitation is very low in our

8.0 8.5 9.0 9.5 10.0 10.5 11.0 11.55

6

7

8

9

10

prec

ipita

tion

time,

min

pH

Fig. 4. Precipitation time changing with pH.

experiments because of the continuous addition ofammonia to keep pH constant. After the addition ofammonia, the fresh ammonia droplet forms a local highsupersaturation in the solution due to the high pH in theammonia droplet, which results in more nuclei formationin this droplet. We can observe the turbidity of ammoniadroplet in the solution. The turbid droplet quicklydisappears in the disturbed solution. With the formationof more and more nuclei, the growing nuclei tends toagglomerate and form metastable vaterite particles. Fig. 5shows the magnification micrographs of vaterite particles.It can be seen that the vaterite particles were composed of alot of nano-sized fine particles. The fine particles are thegrown up nuclei. The fine particles prefer to agglomeratetogether and form the spherical particles to meet the rule oflowest energy. The agglomerated particles are relativelystable compared with the nano-sized vaterite nuclei, whichis easy to be understood if the following equation wasconsidered [18,19]:

lnCðrÞ=Ce ¼ 2Mg=ðRTrrÞ (9)

where C(r) is the solubility of the particle with radius r, C e

is the usual solubility, T is absolute temperature, M is themolecular weight, g is the interfacial free energy, and r isthe crystal density. When the radius (r) of particles is small,the solubility of the particles is high. Since the transforma-tion of vaterite to calcite is controlled by the solubility ofvaterite, following the crystal growth of calcite [6], it can beexpected that the fine vaterite particles are ready totransform to calcite while the big agglomerations arerelatively stable. Besides, from Eq. (9), it can be concludedthat high density of crystal leads to a low solubility of thiscrystal, which further proves that the continuous additionof ammonia, resulting in more nuclei formation, leads to alow solubility of nuclei and most nuclei could be preservedand grown.To verify the stability of particles at different precipita-

tion stages, we have conducted the experiments with

Fig. 5. SEM image of the magnified vaterite particles.

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Fig. 6. SEM images of CaCO3 samples prepared at different bubbling times (a—bubble 2min, b—bubble 6min).

Y. Sheng Han et al. / Journal of Crystal Growth 289 (2006) 269–274 273

different reaction times. The results are shown in Fig. 6.Fig. 6a is the CaCO3 sample prepared at pH 11.1 bypassing CO2/N2 into CaCl2 solution for 2min, andstopping gas and keeping the precipitates in the solutionfor 3min, then filtering and drying the precipitates. It wasfound that almost all the particles are rhombic calcite,which indicates that the particles formed at the early stageof precipitation easily transform to calcite due to the smallsize of particles. Fig. 6b is the sample prepared at pH 11.1by passing CO2/N2 into CaCl2 solution for 5min, thenquickly filtering and drying the precipitates. It was foundthat the rhombic particles were not observed in the SEMimage and its XRD patterns showed that only vaterite wasformed in this sample, which indicated that the transfor-mation of vaterite to calcite during precipitation isfractional at the controlled pH.

One more interesting result is the morphology ofparticles change with pH. At high pH (here high pH refersto pH greater than 10), irregular congeries is formed (asshown in Fig. 1a). At low pH (here low pH refers to pH lessthan 9), spherical particles with different sizes are formed(as shown in Fig. 1c and d). The formation of irregularcongeries is the result of conglomeration of metastablevaterite particles. At the high pH, more nuclei are formedwith the addition of ammonia, and the single nuclei is notstable. Some growing nuclei agglomerated together andformed the metastable vaterite particles. Since the densityof mestable particles is still high at high pH, some vateriteparticles continue to agglomerate and form the irregularcongeries, while at the low pH, the dissolution of CO2 islimited and the supersaturation of solution is relativelylow. Even the nuclei were formed in the ammonia droplets,and some of them dissolve into solution quickly. Hence, thedensity of nuclei in the solution is relatively low. Theexisting nuclei grow up by precipitation of calciumcarbonate on their surface and the nuclei grow up toregular spherical particles to meet the rule of lowest energy.Since the nuclei were continuously formed in the addingammonia droplets during precipitation, some nucleiformed in the later stage of this process have short timeto grow up, resulting in smaller size. Hence, the samples

prepared at low pH were composed of spherical particleswith different sizes.

4. Conclusions

Calcium carbonate particles were prepared by passingthe mixed gas (CO2/N2) into CaCl2 solution at controlledpH. The influence of pH on polymorph of calciumcarbonate is discussed. The results show that the sampleprepared at low pH is mainly composed of vaterite, whichis attributed to the continuous addition of ammonia duringprecipitation. The addition of ammonia resulted in increaseof supersaturation and more nuclei formation, whichimproves the growth of vaterite nuclei and inhibitstransformation of vaterite to calcite. While at high pH,some rhombic calcite particles are formed in the samples,which was ascribed to the quick completion of theprecipitation reaction at high pH, resulting in a relativelylong time for the prepared vaterite to transform to calcite.Besides, the change of morphology of CaCO3 with pH wasalso discussed in this paper.

Acknowledgement

This study has been supported by a grant from theNITECH 21st Century COE Program ‘‘World CeramicsCenter for Environmental Harmony’’, and by the Ministryof Education, Science, Sports and Culture, Grant-in-Aidfor Scientific Research (B), 15310052, 2003, Japan. A partof this research has been financed by Japan Science andTechnology Agency (JST).

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