The effect of ultrasonication on calcium carbonate crystallizationin the presence of biopolymer

Semra Kirboga n, Mualla Oner, Emel AkyolYildiz Technical University, Department of Chemical Engineering, Davutpasa Campus Esenler, Istanbul 34210, Turkey

a r t i c l e i n f o

Keywords:B1. Calcium carbonateB1. Crystallization

a b s t r a c t

Synthesis of calcium carbonate (CaCO3) was carried out using sonication in aqueous solution medium.The effect of the probe immersion depth (PID) and the amplitude of sonicator on calcium carbonatecrystallization were studied in the absence and presence of biopolymer, carboxymethyl inulin (CMI).Calcium carbonate crystals synthesized with and without ultrasound were compared. X-ray diffraction(XRD) analysis showed that calcium carbonate obtained in the presence of biopolymer was a mixture ofcalcite and vaterite whereas there was only calcite polymorph in the absence of biopolymer. In thepresence of biopolymer, the relative fraction of vaterite increased with the application of sonicationprocess. The higher amplitude resulted in the higher relative vaterite fraction. The results showed thatthe probe immersion depth and the amplitude affected the morphology of calcium carbonate.

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1. Introduction

The synthesis of inorganic particle has been attracted a great dealof attention in many research areas. Calcium carbonate is the mostwidely inorganic minerals. It is used in many industrial applicationssuch as rubber, plastics, coating, cosmetic, pharmaceuticals, food,ceramic etc. It also appears in many mineralized tissues in biologicalsystems. Calcium carbonate has three crystalline polymorphs: calcite,aragonite and vaterite [1–6]. Calcite and vaterite can be producedfrom aqueous solution at room temperature whereas aragonite canbe obtained at high temperatures [7]. Polymorphs having differentcrystal structures have an important role in industrial applicationsdue to differences in their properties, so morphology and crystalstructure are essential parameters for industrial applications [1,2].

The synthesis condition of calcium carbonate can influence theproperties of calcium carbonate such as particle shape, particle sizeand morphology. The ultrasound (US) process is a promising andgreener method to synthesis of CaCO3. A narrow particle size ofcalcium carbonate can be obtained with application of US process[4,8,9]. In addition, the nucleation of crystals can be affected by thepresence of US waves. Particle size and shape can be changed byvarying the US process conditions [5,9]. Different polymorphs can besynthesized by applying US waves [5]. Sonawane et al. [4] hasinvestigated calcite using sonochemical carbonization method. Theyobtained only calcite polymorph in their studies. Kojima et al. [7] hasalso investigated the effect of frequency and amplitude on the

morphological of vaterite at high supersaturation without additives.Synthetic conditions such as temperature, pH and supersaturation inliquid reaction were found the most important parameter to controlthe morphology and polymorph of CaCO3.

In this work we have studied calcium carbonate crystallization byusing US in the presence of biopolymer with low amplitude at lowsupersaturation. The effect of the US process conditions on theformation of calcium carbonate crystals was investigated. Ultrasoundamplitude and PID were investigated in the absence and presence ofbiopolymer, carboxymethyl inulin. CMI is a biodegradable, environ-mentally friendly polysaccharide-based polycarboxylate [1–3,10]. CMIwas chosen as a biopolymer because the biodegradability and non-toxicity of CMI allows a wide application possibility in industries.Various polymorphs have been carefully obtained, yielding newinformation on the effect of ultrasound on the precipitation of CaCO3

from solution.

2. Experimental

Calcium chloride (CaCl2) and sodium carbonate (Na2CO3) (reagentgrade) were from Merck. CMI of 3022 molecular weight (MW) (CMI-20) was from thermPhos, Switzerland as Dequest DPB-116AB (whereAB¼20 for CMI-20). The number AB also indicates the degree ofsubstitution (DS). DS is defined as the average number of carboxylatemoieties per fructose unit (DS¼2.0).

The experiments were conducted in a 0.5 dm3 water-jacketedreactor providing a constant-temperature at 2570.1 1C. The totalmolar concentration of calciumwas 100 mmol with calcium/carbonatemolar ratio of 1. Calcium carbonate was always precipitated by mixing

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n Corresponding author. Tel.: þ90 212 383 4783; fax: þ90 212 383 4725.E-mail address: skirboga@yildiz.edu.tr (S. Kirboga).

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equal volumes (100 cm3) of CaCl2 and Na2CO3 solutions. Reactionsolution was prepared by adding calcium solution to the reactorfirst. Sodium carbonate solution and inhibitor solution werequickly poured into the reactor. The experiments were carriedout under either US waves or magnetic stirring at 400 rpm. Theabove reaction solution was subjected to sonication (Sonics VibraCell, 20 kHz and 13 mm with threaded end and replaceable tipprobe) at room temperature. The amplitude value was ranged 25%to 50% whereas PID was varied from 1 cm to 2 cm below the liquidsurface. The obtained CaCO3 particles were filtered through a0.2 mm cellulose nitrate membrane filter, dried at 100 1C for 24 h.

3. Results and discussions

Samples were coded to include the information about the synthesisconditions. The first number in the code of samples indicates thesonicator amplitude (%). The second number coming after dash showsthe biopolymer concentration (g/L), the last number shows the probeimmersion depth (cm). While S0-0/0 denotes sample synthesizedwithout US waves in the absence of biopolymer without the probeimmersion depth, S25-0.5/1 denotes sample synthesized with 25% ofUS waves in the presence of 0.5 g/L biopolymer with 1 cm of probeimmersion depth (Table 1).

X-ray diffraction, Fourier transform infrared (FT-IR) and scanningelectron microscopy (SEM) were used in order to characterize thesynthesized CaCO3 crystals. X-ray diffraction analysis was carried outby means of Panalytical X'pert Pro PW 3040/60 powder diffractometeroperating with Cu Kα radiation in operating at 40mA and 45 kV. The

2θ range was from 51 to 901 at scan rate of 0.0261 step�1. The ratios ofthe different polymorphs were determined by semi-quantitativeanalysis of the XRD results using the Rietveld method with HighScorePlus software (Table 1). The samples were analyzed using FT-IRspectral analysis (Bruker Alpha-P) in the 4000–400 cm�1 region at aresolution of 4 cm�1. The morphology of crystals was analyzed byscanning electron microscopy (JEOL JSM 6335F and JSM 6510LV). Thespecific surface area (SSA) of the CaCO3 crystals was determined bynitrogen sorption isotherms according to the multiple-point BET(Brunauer, Emmett and Teller) method using a continuous flowapparatus (COSTECH Kelvin Sorptometer 1042). Calcium carbonatesamples were first outgassed at 80 1C. The nitrogen adsorptionisotherm has been performed by additions of gaseous nitrogen tothe tube containing the sample at 77 K.

The addition of additive is a general method which is used tocontrol the morphology of crystals [11–13]. Fig. 1 shows XRDdiagram of obtained crystals in the absence and presence of CMI.The XRD pattern (Fig. 1) indicates two crystalline polymorphs,calcite and vaterite. We observed the characteristic peaks of calciteat 2θ of 29.41, 35.91 and 39.51 which are corresponding to (1 0 4),(1 1 0) and (1 1 3) crystallographic planes of calcite [1–3]. Thecharacteristic peaks of calcite were observed not only in thepresence (Fig. 1b) but also in the absence of the biopolymer(Fig. 1a). The introduction of CMI biopolymer caused the formationof a new absorption peak presented vaterite form. The diffractionpeaks at 24.921, 26.991 and 32.781, which were obtained only inthe presence of CMI, proved the existence of vaterite [1,2]. Thediffraction peaks of the crystals can be indexed as the (1 1 0),(1 1 2) and (1 1 4) reflection of vaterite at 24.921, 26.991 and32.781, respectively [1]. The addition of CMI caused the changesin calcium carbonate polymorphs, probably due to its strongspecific interaction with calcium carbonate [1–3].

In order to investigate the influence of the ultrasound, pre-cipitations were carried out over two ranges of ultrasoundamplitudes (25% and 50%). With the increasing of the sonicatoramplitude, the intensity of peak at �29.41 decreased. It is clearthat the proportion of vaterite formed was higher when higheramplitude was used. On the other hand, the intensity of char-acteristic peak of vaterite at �24.91 increased (Fig. 1). These resultsindicated that the amount of vaterite was proportional with thesonicator amplitude. Similar results were obtained at all PID's(Table 1). The formation of vaterite, which is an unstable phase,was effectively induced by ultrasonic waves. The control of theformation of polymorph by chancing the amplitude was observedby other research groups. It was reported that aragonite could be

Table 1Synthesis conditions and the relative fraction of CaCO3 crystals obtained at 25 1C.

Samplecode

[CMI](g/L)

Amplitude(%)

PID(cm)

Relative calcitefraction (%)

Relative vateritefraction (%)

S0-0/0 0 – – 100 0S25-0/1 0 25 1 100 0S50-0/1 0 50 1 100 0S25-0/2 0 25 2 100 0S50-0/2 0 50 2 100 0S0-0.5/0 0.5 – – 46.97 53.03S25-0.5/1 0.5 25 1 36.42 63.58S50-0.5/1 0.5 50 1 13.90 86.10S25-0.5/2 0.5 25 2 43.13 56.87S50-0.5/2 0.5 50 2 18.33 81.67

10 20 30 40 50 60 70 80 90

C

(113)(110)

(104) C: Calcite

C

CCC

CC

C

C

S25-0/1

S0-0/0

Position (2 Tetha)20 30 40 50 60 70 80 90

(220)(202)

(018)C

(114)(112)

(113)(110)

(110)

(104)

C VCCCC V

V V

CC: CalciteV: Vaterite

S50-0.5/1

S25-0.5/1

Position (2 Tetha)

S0-0.5/0

Fig. 1. XRD analysis of obtained calcium carbonate crystals (a) in the absence of biopolymer, and (b) in the presence of biopolymer at 25 1C.

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synthesized by using high-power ultrasound irradiation. Aragonitewith different morphologies was obtained with 70% of the fullamplitude, and vaterite could also be obtained by using 75% or 80%of the full amplitude [14]. Calcite and vaterite polymorphs wereobtained with the ultrasound experiments conducted by Wagterveldet al. [15]. Stoica-Guzun et al. [6] synthesized calcite and vateritewith different crystal forms in the presence of ultrasonic irradia-tion. Price et al. [5] synthesized vaterite and calcite at differentultrasound intensities. It was indicated that the ultrasound agita-tion method favored to the synthesis of CaCO3 crystals with purepolymorph, uniform morphology and size [16]. The effect ofamplitude and frequency of ultrasonic irridation on the morpho-logical of vaterite has been investigated by Kojima et al. [7]. Ourresults were compatible with the results obtained by Kojimaet al. [7]. The presence of vaterite and calcite was also confirmedby FT-IR analysis. FT-IR analysis showed characteristic vibrationalbands at �1420, �874 and �712 cm�1 for calcite (Fig. 2). Thecharacteristic vibrational band for vaterite at �1070 and �745 cm�1

was also observed in the presence of biopolymer (Fig. 2). Theinfrared peak at �1590 cm�1 indicates the existence of CMI-20.In our previous studies, the mixture of vaterite and calcite wasobtained in the presence of CMI in a batch system with magneticstirring [1,2,17]. We have previously reported the effect of the feedrate of the reactant, the initial concentration of [Ca2þ], and theconcentration of the biopolymer on CaCO3 crystallization. Thecontrol of the feeding rate of reactant was provided by a peristalticpump. The maximum fraction of vaterite was obtained up to23.05% at 100 mmol of initial [Ca2þ] concentration in the presenceof 0.5 g/L CMI with magnetic stirring. However, the fraction ofvaterite changed from 53.03% to 86.10% by applying US waves(Table 1). Vaterite is an essential polymorph of calcium carbonateand is widely used in various industries, but rarely occurs innature. The values of SSA, solubility and dispersion of vaterite arehigher than other two CaCO3 polymorphs, calcite and aragonite[16,18]. The vaterite formwould perform better in various applicationssuch as filler in the plastic and paper industries due to the lowerspecific gravity, which would give more volume for same weight. Useof polymorphs as filler in the paper industry would give a betterquality paper due to its higher luminescence and better refractiveindex [1,18]. The inward absorption of inks can be promoted withvaterite. Vaterite-coated paper could provide high print quality for inkjet printing. Bleeding and feathering problem can be solved withvaterite [19]. Vaterite microsphere biocomposites can be used toaugment bonematrix formation [16]. Although there are many studieson the synthesis of CaCO3 crystals with different polymorphs andmorphologies, there is a great interest in the study of additives such

as biopolymers in stabilizing a given phase and controlling thecrystallization of CaCO3. Spherical vaterite has been obtained inthe presence of divalent cations [20], bis(2-ethylhexyl)sodiumsulfate (AOT) [21], poly(styrenesulfonate) [22], poly(vinyl alcohol)[23]. To synthesize a single phase of vaterite by a convenientmethod is difficult. The particle size of spherical vaterite has animportant role for industrial applications such as pigments, fillers,and dentifrice. The studies on the control of spherical vaterite hasbeen restricted, especially studies of US waves. The effects ofultrasounds upon calcium carbonate crystallization are not fullyunderstood. Using US waves could give us an opportunity toproduce vaterite at room temperature.

When the effect of PID on the relative fraction of vaterite wascompared, at the same amplitude, an increase in PID caused adecrease in vaterite fraction (Table 1). The flow pattern of the liquidis related with the distance from the probe. Therefore, crystallizationcan be affected by the change in the flow rate. There is an optimumPID for irradiated medium [9]. We observed that the relative fractionof vaterite increased by decreasing the probe immersion depth.

The specific surface area was influenced by the applied USwaves and the presence of biopolymer. Fig. 3 shows that BETsurface area increased gradually with the increased the sonicatoramplitude in the presence of CMI. While the value of SSA rangedfrom 0.30 to 0.78 m2/g in the absence of CMI, the value of SSAobtained in the presence of CMI with using US waves increasedfrom 23.49 to 29.44 m2/g (Fig. 3). The presence of biopolymer hasaffected the SSA of calcium carbonate crystals synthesized not onlywith magnetic stirring [1,17] but also using US waves. In contrast,for the samples prepared with magnetic stirring at the samebiopolymer and initial [Ca2þ] concentration, SSA value reachedto maximum 23.05 m2/g [17]. The efficiency of biopolymer on SSAof CaCO3 increased with applying US waves. In addition, it is foundthat the SSA was proportional to the sonicator amplitude (Fig. 3).

The synthesized calcium carbonate was observed using SEM(Fig. 4). In the absence of CMI, the crystal shape of the calciteformed was of the typical rhombohedral form. Spherical vateriteand prolate spheroid crystal shape calcite were observed in thepresence of CMI. The prolate spheroid crystal shape of calcitewas also obtained in the presence of EDTA [24]. CMI dramaticallychanged the morphology of the calcite crystals, probably due toits strong specific interaction with calcium carbonate. Thisstrong interaction was enough to disturb the formation of typicalcalcite rhombohedra [1]. The dimensions of a minimum of 50crystals in each sample were measured from the SEM photomi-crographs. The average crystal size of the calcium carbonate wasgiven in Table 2.

1600 1400 1200 1000 800 600

S0-0/0

% T

Wavenumber (cm-1)

S50-0.5/1

S25-0.5/1

Fig. 2. FT-IR spectra of calcium carbonate crystals obtained at 25 1C.

5

10

15

20

25

30

35

0 5 15 25 35 4510 20 30 40 50

Spec

ific

Surf

ace

Are

a (m

2 /g)

Amplitude (%)

PID (1 cm)PID (2 cm)

Fig. 3. The change of the SSA in the obtained calcium carbonate crystals in thepresence of CMI versus the sonicator amplitude.

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We have previously reported that a mixture of calcite andvaterite has been obtained in the presence of biopolymer [1,17].At the same biopolymer and [Ca2þ] concentration, the mean crystal

size of calcite reduced from 9.97 to 2.71 mm. Moreover, the meancrystal size of vaterite ranged 7.50 to 3.91 mm at the experimentscarried out using magnetic stirrer. As can be seen from Table 2, USwaves resulted in a decrease in the average crystal size of calciumcarbonate crystals. For samples synthesized by ultrasonic irradia-tion, the average particle size of vaterite decreased markedlydropping to about 0.91 mm. Furthermore, the crystal size tends todecrease with the sonicator amplitude. US waves hardly influ-enced the particle size of the calcium carbonate synthesized. It isclear that US waves is a more effective method than magneticstirring process.

The mean crystal size of calcium carbonates for all polymorphsdecreased by applying US waves. It can be attributed to masstransfer in the solution accelerated and the driving force increased.Increasing the US intensity increases the crystallization rate.Ultrasonication is a method to obtain small crystals with lowerequipment costs and the ability to operate under ambientconditions. There have been many studies supporting thedecrease in the crystal size with US irradiation [4,7,9]. US couldbe an effective method to control the crystal size of the calciumcarbonate.

Fig. 4. SEM photograph of obtained calcium carbonate crystals at 25 1C (a) S0-0/0, (b) S25-0/1, (c) S50-0/1, (d) S25-0.5/1 and (e) S50-0.5/1.

Table 2The average crystal size of the calcium carbonate obtained at 25 oC.

Sample code Rhombohedral (C) Calcite (C) Vaterite (V)

S0-0/0 9.49�8.21 mm – –

S25-0/1 3.62�3.23 mm – –

S50-0/1 3.52�3.18 mm – –

S25-0/2 3.88�3.56 mm – –

S50-0/2 3.58�3.28 mm – –

S0-0.5/0 – 8.70 mm 3.91 mmS25-0.5/1 – 1.69 mm 1.02 mmS50-0.5/1 – 1.54 mm 0.91 mmS25-0.5/2 – 1.83 mm 1.06 mmS50-0.5/2 – 1.62 mm 0.95 mm

C: Calcite, V: Vaterite.

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4. Conclusion

This study investigated the synthesis of calcium carbonate byapplying US waves. Variations in the sonicator amplitude and PID,and the presence of CMI were studied to control the morphologyand polymorph of calcium carbonate. The sonicator amplitude wasfound to be the most effective parameter to control the morphol-ogy of calcium carbonate. While the thermodynamically stablecalcite was produced in the absence of biopolymer, the polymorphof the synthesized calcium carbonate crystals changed from purecalcite to the mixture of vaterite and calcite in the presence of CMI.The inhibitory effect of biopolymer resulted in a mixture ofvaterite and calcite, due to partially inhibition of the transforma-tion of vaterite to calcite. Biopolymer influenced not only crystalmorphology but also polymorph. Furthermore, crystal morphologyand polymorph of calcium carbonate were very impressed withapplying US. The relative fraction of vaterite increased with theincrease in the sonicator amplitude whereas it decreased with theincreasing of the probe immersion depth. Maximum vaterite wasobtained at high amplitude with minimum probe immersiondepth. Ultrasound method favored the synthesis of vaterite inour experimental system. With addition of CMI, the crystalstructure of calcium carbonate changed from rhombohedral toprolate spheroid shape for calcite. In addition, spherical vateriteformed in the presence of CMI. Smaller particles can be achievedwith US at room temperature. The present results show thatcontrol of calcium carbonate is possible using US waves. US is agreen method to control the morphology, polymorph and crystalsize of minerals. A better understanding of the interactionsbetween US waves and inorganic phase may be expected to leadto new strategies for novel material with desirable shape andproperties.

Acknowledgements

We appreciate the support of YTU D.O.P. (Project No.: 2011-07-01-DOP03). Semra Kirboga gratefully acknowledges TUBITAK (TheScientific and Technological Research Council of Turkey) for ascholarship.

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