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Morphological and crystallization process of CaCO3 in the presence of Aqua soft 330 (AS 330)
Powder Technology 225 (2012) 58–64
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Morphological and crystallization process of CaCO3 in the presence of Aqua soft 330 (AS 330) Shanmukha Prasad Gopi, P. Vijaya, V.K. Subramanian ⁎ Department of Chemistry, Annamalai University, Annamalai nagar-608 002, Tamilnadu, India
a r t i c l e
i n f o
Article history: Received 27 October 2011 Received in revised form 19 December 2011 Accepted 24 March 2012 Available online 30 March 2012 Keywords: Morphology Calcium carbonate Scale, Inorganic compounds
a b s t r a c t CaCO3 was crystallized from CaCl2 solution containing commercially available 1-hydroxyethylidene-1,1diphosphonic acid, Aqua soft having a chelating value of 330 (AS 330). The effect of the concentration of AS 330 and the temperature of crystallization on the morphology and crystalline structure of CaCO3 was investigated. The samples were characterized using scanning electron microscopy (SEM), powder X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR). The presence of AS 330 favored ternary calcite–vaterite–aragonite mixture at 60 and 80 °C and a binary calcite–vaterite mixture at 100 °C. Rhomboidal calcite, needle/rod like aragonite and flower like vaterite were obtained under different conditions. In contradiction to the usual process of the transformation of amorphous CaCO3 into different polymorphic forms, the data confirmed calcite is the precursor for the flower structured vaterite. The mechanism of formation of the flower structured vaterite is discussed in detail. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Calcium carbonate (CaCO3) is one of the most abundant biominerals found in different polymorphs [1]. Generally it exists in three anhydrous crystalline polymorphs: calcite, aragonite, and vaterite and three metastable forms: amorphous calcium carbonate (ACC), crystalline hexahydrate and crystalline monohydrate [2,3]. Out of the above different polymorphs, calcite is the most stable while aragonite and vaterite are metastable polymorphs which can be easily be transformed into the stable phase—calcite. The controlled synthesis of inorganic materials with specific size and morphology is an important aspect in the development of new materials in many fields such as advanced materials, catalysis, medicine, electronics, ceramics, pigments, cosmetics, etc. [4]. In the recent past, attention has been received on the controlled synthesis of CaCO3 particle due to its wide application in industrial fields such as paper, rubber, plastics, paint, etc. [5]. The strategy of using organic templates or modifiers with complex functionalization patterns to control the nucleation, growth, and alignment of inorganic crystals has been widely adapted for the biomimetic synthesis of inorganic materials with complex form [6]. Many studies have shown that a wide range of additives can influence the morphology and crystalline phase of CaCO3 [4–11]. These studies reveal that the crystalline phase and morphology CaCO3 mainly depend on the precipitation conditions such as pH, temperature, presence of additives, etc.
⁎ Corresponding author. Tel.: + 91 94862 82324. E-mail address: [email protected] (V.K. Subramanian). 0032-5910/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2012.03.032
Chemicals for water treatment have been in use for more than a century [12]. A large number of additive formulations are commercially available. Commonly used anti-scalants are chemicals derived from chelating agents [13,14]. These complexing agents not only form complexes with scale causing constituents and inhibit the supersaturation but also are able to weaken its adherence to surface, probably due to the lack of symmetry in the morphology which inhibits building up of scale. For this reason, usage of derivatives of phosphonic acids, acetic acid and other sequestrants as scale inhibitors is very common in water treatment. The formation of scale on the surface of a metal is a process of deposition of insoluble salts of calcium and magnesium layer over layer. CaCO3 is one of the major constituent of scales and hence careful morphological and phase control could help keep them in suspension and preclude the deposition of scale on the metal surface. Although various methods for morphology control have been reported, the advantage of structural and morphological asymmetry for the inhibition of scale has not been explored adequately. AS 330 is a commercially available water treatment chemical widely used in many industrial applications including aquaculture for the control of hardness in India. Considering its wide commercial applications and cost effectiveness, in the present work, crystallization of CaCO3 from CaCl2 was carried out in its presence and morphological and phase transformation studies were performed. The effect of concentration of AS 330 and temperature in the range 60–100 °C on the morphology and polymorphic composition of CaCO3 was investigated in detail. Flower like vaterite similar to those reported by G Yan et al. [9], and H. Tang et al. [15] and rhomboidal calcite and needle/rodlike aragonite were formed under different conditions. This study might be helpful to provide new understanding in the prevention of scale by controlling the morphology and crystalline phase.
S.P. Gopi et al. / Powder Technology 225 (2012) 58–64
2. Experimental details 2.1. Reagents and materials Analytical grade CaCl2, disodium salt of EDTA and Na2CO3 were obtained from Sigma-Aldrich chemical company. 1-hydroxyethylidene1,1-diphosphonic acid (AS 330) with a chelating capacity of 330 was obtained from Satyajith Chemicals P Ltd, Mumbai. The reagents were used as such. Double distilled water was used for the preparation of aqueous solutions. Analytical grade hydrochloric acid and sodium hydroxide were used to adjust the pH whenever necessary. 2.2. Synthesis A procedure similar to Guowei Yan et al. [9] was used for the synthesis of CaCO3. In a typical synthesis, 50 ml of CaCl2 (500 ppm) was taken in a round bottom flask and 50 ml AS 330 (100 ppm) was added. The pH of the solution was adjusted to7. The solution was heated in a rotamantle to 60 °C and 50 ml Na2CO3 was introduced from a burette 15 min after attaining the temperature and kept for 12 h at the same temperature for digestion. A water condenser was used to avoid the loss of water. The precipitate was filtered using a Whatman No. 40 filter paper and dried at 40 °C for 6 h and then kept in desiccators. The experiment was repeated at 80 and 100 °C and also using different volumes of AS 330. 2.3. Characterization Microscopic morphological images were obtained using JEOL JSM 5610 LV scanning electron microscope (SEM) at an applied voltage of 20 kV. The samples were coated with Au prior to examination. FT-IR spectra were taken in the range 500 to 4000 cm− 1 using an Avatar330 FTIR spectroscopy after KBr pelletization. The X-ray diffraction (XRD) patterns were recorded on a Bruker D8 Advanced XRD diffractometer with Cu Ka radiation at λ = 1.5406 Å. The pH measurements were made using Elico pH meter model LI-120. The relative contents for three CaCO3 crystal forms were calculated using the following equations Yc ¼ 1=1 þ 3:9½Ia=Ic þ 2:9½Iv=Ic Ya ¼ 3:9½Ia=IcYc and Yv ¼ 2:9½Iv=IcYc where Yc, Ya, Yv are the mass fractions of calcite, aragonite and vaterite, respectively; Ic, Ia, Iv are the peak areas of characteristic adsorptions for calcite (2θ=29.369), aragonite (2θ=26.213) and vaterite (2θ=32.74), respectively. 3. Results and discussion 3.1. Interpretation of XRD data The powder XRD patterns of the samples are presented in Fig. 1. The details of sequestrant used, temperature of precipitation, Ia, Iv and Ic values and ratio of different polymorphs obtained are given in Table 1. 3.1.1. Effect of temperature The XRD patterns of samples prepared at 60, 80 and 100 °C are presented in Fig. 1a, b and c respectively. The d spacing values corresponding to the 104 plane of calcite are 3.037, 3.007 and 3.025 respectively. This confirmed the presence of calcite in all the samples. The XRD spectrum of sample 1 confirmed the presence of calcite (JCPDS 17-0528), vaterite (JCPDS 02-0261) and aragonite (JCPDS 76-0606). Sample
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prepared at 80 °C also showed the presence of a ternary mixture containing calcite (JCPDS 87-1803), vaterite (JCPDS 74-1867) and aragonite (JCPDS 05-0453). The XRD patterns of samples 1 and 2 showed peaks at 2θ = 26.15 and 26.42 respectively, confirming the presence of aragonite in these samples. However, the XRD pattern confirmed the sample prepared at 100 °C was a binary mixture of calcite and vaterite (JCPDS No. 04-0636 and 74-1867). There were no peaks corresponding to aragonite. The above data confirmed the presence of aragonite in all the samples except 3. The samples synthesized at 60 and 80 °C showed only marginal changes in their calcite to vaterite ratio (Table 1). The data suggests that, at 100 °C there was a remarkable change in the crystallization process and formation of aragonite was less favored over vaterite and calcite. The absence of aragonite in the sample and an increase in calcite and vaterite were observed. Samples 1 and 2 had almost the same composition of calcite (about 12%) and vaterite (about 33%) and aragonite (about 55%). At 100 °C there was greater tendency towards the formation of vaterite and calcite with a relative increase in the composition of calcite (from 12 to 27%) and vaterite (from 32.7 to 72%) (Table 1).
3.1.2. Effect of concentration of AS330 The change in percentage composition of calcite, vaterite and aragonite when 25, 75 and 100 ml AS 330 were used at 100 °C is presented in Table 1 (samples 4–6). The corresponding XRD spectra are shown in Fig. 1d–f. The table and XRD data showed the presence of all the three polymorphs in all these samples. The percentage of calcite was found to be more in sample 6. This was in good agreement with the SEM image (Fig. 2f) which had more rhomboidal structures. The vaterite fraction was maximum when 25–50 ml AS 330 was used at 100 °C. The above observations suggest that 25–50 ml AS 330 can act as versatile sequestrants at 100 °C. From the results it was also concluded that the increase in concentration of AS 330 favors the formation of calcite and aragonite.
3.2. Interpretation of SEM images We investigated the effect of temperature on the morphology of CaCO3 by varying the temperature of precipitation at fixed concentrations of CaCO3, AS 330 and Na2CO3. Fig. 2(a–c) shows the SEM images of the CaCO3 obtained at 60, 80 and 100 °C respectively. Fig. 2a and b shows the presence of rhomboidal structures with 8–10 μm length, needle like structures with 5–10 μm along with flower like structures. However, at 100 °C the needle like structures were absent and the presence of flower and rhomboidal like structures increased in number (Fig. 2c). The flower like particles showed large central protuberance with a diameter of about 2 μm and having six petals arranged around the orbicular centre. A similar structure has been reported in the precipitation of CaCO3 from ethanol/water system containing Pluronic 127 [9] and polyelectrolyte poly-(sodium 4-styrene-sulfate) (PSSS) [15]. SEM images Fig. 2(d, e and f) depict the CaCO3 samples precipitated from the CaCl2 in the presence of 25 ml, 75 ml and 100 ml of AS 330 respectively at 100 °C. The sample prepared with 25 ml AS 330 (Fig. 2e) showed uniform distribution of all the three morphologies. At higher concentrations of AS 330 (Fig. 2e and f) showed more rhomboidal structure along with some needle structures. This confirmed the presence of more calcite and aragonite under these conditions. When the concentration was increased from 25 ml to 50 ml there was drastic reduction in the needle like aragonite structure. However, further increase in AS 330 showed all the above mentioned three morphologies with increased calcite percentage (Fig. 2d, e). This confirmed the formation of calcite, vaterite and aragonite at higher concentrations of AS 330. This could be due to the favorable microscopic interfacial dynamics for the formation of different polymorphs under different conditions.
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3.3. Interpretation of FTIR data FTIR spectrum of samples prepared at 60, 80 and 100 °C is shown in Fig. 3a–c. The samples prepared at different concentrations 25 ml, 75 ml and 100 ml of AS 330 are presented in Fig. 3d–f. Simultaneous occurrence of peaks at 876 and 712 cm− 1 confirmed the presence of calcite in all samples [16]. Appearance of peaks at 1084 and 745 cm− 1 confirmed the presence of vaterite in these samples. The FTIR peak
intensities corresponding to calcite and vaterite in samples 2 were similar to sample 1. This re-ascertain the ratio of these polymorphs in samples 1 and 2 were the same as observed from the XRD data (Table 1). The above results reveal that the CaCO3 prepared from AS 330 at 60 and 80 °C and those prepared at different concentrations of 25 ml, 75 ml and 100 ml of AS 330 were a ternary calcite, aragonite and vaterite mixture. The peaks due to vibrations of water molecules were observed at 3300 and 1645 cm − 1. The values assigned for
Fig. 1. Powder XRD of CaCO3 samples precipitated under different conditions (a, b and c) in the presence of AS 330 at 60 °C, 80 °C and 100 °C, (d, e and f) in the presence of 25 ml, 75 ml and 100 ml AS 330 at 100 °C.
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Fig. 2. SEM images of CaCO3 samples precipitated under different conditions (a, b and c) in the presence of AS 330 at 60 °C, 80 °C and 100 °C, (d, e and f) in the presence of 25 ml, 75 ml and 100 ml AS 330 at 100 °C.
vaterite were 875, 745 and 1084 cm − 1. The FTIR of the sample prepared at 100 °C in the presence of AS 330 showed the absence of aragonite. These results reconfirm the XRD data from Fig. 1a–f. It has been reported earlier that the more the vaterite appears in the scale, the more efficient the scale inhibitor is [17]. When 50 ml AS 330
Table 1 Details of the volume of sequestrants used, precipitation temperature and Ya, Yc and Yv calculations. Sample no.
Sequestrant used
Temp. °C
Vol of sequestrant
Yc
Ya
Yv
1 2 3 4 5 6
AS AS AS AS AS AS
60 80 100 100 100 100
50 ml 50 ml 50 ml 25 ml 75 ml 100 ml
0.125 0.125 0.277 0.178 0.298 0.589
0.547 0.547 0 0.064 0.112 0.201
0.326 0.327 0.722 0.758 0.590 0.210
330 330 330 330 330 330
was used, the formation of vaterite was more favored at higher temperature which confirmed its greater scale inhibition capacity. Secondly, under the above conditions the prevention of transformation of vaterite to calcite functioned effectively probably due to the dissolution– recrystallization mechanism [12]. Report suggests that the rich variety of CaCO3 structures in nature may be due to the amorphous character of ACC which enables it to easily mold into many different shapes [3]. Phase transformation from vaterite or aragonite to calcite [18,19] and multistep phase transformation form ACC to calcite and ACC which in turn leads to calcite and aragonite [3] are reported. However, the evidence from the SEM images (Fig. 4d) shows that, in contradiction to the usual observation of the formation of different polymorphic forms from ACC, vaterite was formed from calcite precursor. The rhomboidal calcite particles have bulbous deformations at the centre which forms the centre part of the flowerlike structure (Fig. 4a–d). This conclusion is reasonable as similar deformations [4] are observed in the SEM images of CaCO3 particles synthesized in the presence of 0.5 g/l PAA at 80 °C and at pH 10. Reports are also available on the deformation of
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Fig. 3. FTIR spectrum of CaCO3 samples precipitated under different conditions (a, b and c) in the presence of As 330 at 60 °C, 80 °C and 100 °C, (d, e and f) in the presence of 25 ml, 75 ml and 100 ml AS 330 at 100 °C.
the corners of the rhomboidal calcite get rounded off [16,20,21] to form spherical vaterite. Apart from the usual dissolution–recrystallization mechanism, from the data it could be concluded that the efficiency of AS 330 as a good scale inhibitor could be due to the morphological dissimilarities that might prevent the three dimensional growth of scale beyond a certain limit. The results from the SEM images of samples indicate that a sunflower like morphology is present in all these samples. The XRD data confirmed the presence of only calcite and vaterite in all samples. Aragonite was absent in the sample prepared at 100 °C. Rhomboidal
morphology is the most well established structure for calcite and is present in all these samples. Hence the flower structure can be assigned only to vaterite. The absence of other morphological compositions and earlier reports [9,15] confirm this morphology to vaterite. 3.4. Mechanism of formation of flower like vaterite On the basis of the above experimental results a four step mechanism of formation of this different sunflower structure of vaterite is presented in Fig. 5. A spherical fraction from the centre of the rhomboidal calcite
S.P. Gopi et al. / Powder Technology 225 (2012) 58–64
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Fig. 4. SEM images of CaCO3 samples precipitated in the presence of AS 330 (a) at 100 °C, 75 ml AS 330 (b) at 60 °C, 50 ml As 330 (c) at 100 °C, 100 ml AS 330 and (d) showing deformity at the centre of the cubical structure.
forms the core part of the sunflower (Fig. 5, Step 1). Generally, crystal growth is governed by kinetic factors and the observed crystal morphology reflects the relative rates of growth of the crystal to different directions by the inhibitor [22]. It is expected that this provides the site for further spherical fractions to join (Fig. 5, Step 2). However due to steric factors this could not go beyond a certain number of layers. At this stage the dispersion of the second and further layers of spheres start (Fig. 5, Step 3) to form petal structure of the flower keeping the central sphere intact. In the last step the spreading of layers ceases leading to the sunflower structure (Fig. 4, Step 4). The above observations were further confirmed by the uniformity in the diameter of the bulbous part in the sunflower structure and the spherically deformed portion of the rhomboidal calcite, Fig. 4. The above mechanism of vaterite formation from the more thermodynamically stable calcite in the presence of AS
330 along with the morphological structural differences could be another major reason for its effective anti-scalant nature despite an increase in calcite composition. 4. Conclusions Crystallization of CaCO3 from CaCl2 using Na2CO3 in the presence of AS330 generated calcite ranging 12–27% between 60 and 100 °C. In the presence of AS 330 formation of aragonite was inhibited at 100 °C. Sunflower flower like morphology for calcium carbonate was observed when AS 330 was used. The interpretation of SEM, XRD and FTIR data confirmed this structure can be attributed to vaterite. Calcite was found to be the precursor for the formation of this nature of vaterite. A ternary calcite–aragonite–vaterite mixture was the most favored
Fig. 5. Suggested mechanism of formation of the flower structure.
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composition. However, at 100 °C and in the presence of 50 ml AS 330, a binary calcite–vaterite mixture is produced. The observed results might be helpful in selecting the anti scaling sequestrant in systems operating between 60 and 100 °C. Acknowledgements The authors thank the University Grants Commission, New Delhi, for the financial assistance provided through Major Research Project Scheme No. F-37-40/2009 (SR) and the UGC Networking Resource Centre, University of Hyderabad, Hyderabad for providing facility for carrying out instrumental analysis. References [1] S. Mann, Journal of Materials Chemistry 5 (1995) 935. [2] X. Yang, Xiaodeng Yang, Guiying Xu, Yijian Chen, Teng Liu, Hongzhi Mao, Weiping Sui, Mingqi Ao, Fang He, Powder Technology 204 (2010) 228. [3] H. Tang, J. Yu, X. Zhao, Materials Research Bulletin 44 (2009) 831. [4] J. Yu, M. Lei, Bei Cheng, X. Zhao, Journal of Solid State Chemistry 177 (2004) 681. [5] J. Yu, M. Lei, B. Cheng, Materials Chemistry and Physics 88 (2004) 1.
[6] J. Yu, X. Zhao, B. Cheng, Q. Zhang, Journal of Solid State Chemistry 178 (2005) 861. [7] Y.S. Han, G. Hadiko, M. Fuji, M. Takahashi, Journal of Crystal Growth 276 (2005) 541. [8] W. Wang, G. Wang, Y. Liu, C. Zheng, Y. Zhan, Journal of Materials Chemistry 11 (2001) 1752. [9] Guowei Yan, Lina Wang, Jianhua Huang, Powder Technology 192 (2009) 58–64. [10] K. Naka, Y. Tanaka, Y. Chujo, Y. Ito, Chemical Communications (1999) 1931. [11] E. Altay, T. Shahwan, M. Tanoğlu, Powder Technology 178 (2007) 194. [12] R. Ketrane, B. Saidani, O. Gil, L. Leleyter, F. Baraud, Desalination 249 (2009) 1397. [13] S. Ghizellaoui, J. Ledion, S. Ghizellaoui, A. Chibani, E. De, Desalination 166 (2004) 315. [14] D. Lisitsin, Q. Yang, D. Hasson, R. Semiat, Desalination 183 (2005) 289. [15] H. Tang, J. Yu, X. Zhao, D.H.L. Ng, Journal of Alloys and Compounds 463 (2008) 343. [16] M. Dinamani, P. Vishnu Kamath, R. Seshadri, Materials Research Bulletin 37 (2002) 661. [17] Z. GuiCai, G.E. JiJiang, S. Ming Qin, P. BinLin, M. Tao, S. Zhao Zheng, Science in China. Series B, Chemistry 50 (2007) 114–120. [18] T. Ogino, T. Suzuki, K. Sawada, Journal of Crystal Growth 100 (1990) 159. [19] C. JimEnez-Lopez, E. Caballero, F.J. Huertas, C.S. Romanek, Geochimica et Cosmochimica Acta 65 (2001) 3219. [20] A.N. Kulak, P. Iddon, Y. Li, S.P. Armes, H. Colfen, O. Paris, R.M. Wilson, F.C. Meldrum, Journal of the American Chemical Society 129 (12) (2007) 3729 9. [21] Dong-Ki Keum, Kensuke Naka, Yoshiki Chujo, Journal of Crystal Growth 259 (2003) 411. [22] R. Lacmann, A. Herden, Chr. Mayer, Chemical Engineering and Technology 22 (1999) 279.
Contents lists available at SciVerse ScienceDirect
Powder Technology journal homepage: www.elsevier.com/locate/powtec
Morphological and crystallization process of CaCO3 in the presence of Aqua soft 330 (AS 330) Shanmukha Prasad Gopi, P. Vijaya, V.K. Subramanian ⁎ Department of Chemistry, Annamalai University, Annamalai nagar-608 002, Tamilnadu, India
a r t i c l e
i n f o
Article history: Received 27 October 2011 Received in revised form 19 December 2011 Accepted 24 March 2012 Available online 30 March 2012 Keywords: Morphology Calcium carbonate Scale, Inorganic compounds
a b s t r a c t CaCO3 was crystallized from CaCl2 solution containing commercially available 1-hydroxyethylidene-1,1diphosphonic acid, Aqua soft having a chelating value of 330 (AS 330). The effect of the concentration of AS 330 and the temperature of crystallization on the morphology and crystalline structure of CaCO3 was investigated. The samples were characterized using scanning electron microscopy (SEM), powder X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR). The presence of AS 330 favored ternary calcite–vaterite–aragonite mixture at 60 and 80 °C and a binary calcite–vaterite mixture at 100 °C. Rhomboidal calcite, needle/rod like aragonite and flower like vaterite were obtained under different conditions. In contradiction to the usual process of the transformation of amorphous CaCO3 into different polymorphic forms, the data confirmed calcite is the precursor for the flower structured vaterite. The mechanism of formation of the flower structured vaterite is discussed in detail. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Calcium carbonate (CaCO3) is one of the most abundant biominerals found in different polymorphs [1]. Generally it exists in three anhydrous crystalline polymorphs: calcite, aragonite, and vaterite and three metastable forms: amorphous calcium carbonate (ACC), crystalline hexahydrate and crystalline monohydrate [2,3]. Out of the above different polymorphs, calcite is the most stable while aragonite and vaterite are metastable polymorphs which can be easily be transformed into the stable phase—calcite. The controlled synthesis of inorganic materials with specific size and morphology is an important aspect in the development of new materials in many fields such as advanced materials, catalysis, medicine, electronics, ceramics, pigments, cosmetics, etc. [4]. In the recent past, attention has been received on the controlled synthesis of CaCO3 particle due to its wide application in industrial fields such as paper, rubber, plastics, paint, etc. [5]. The strategy of using organic templates or modifiers with complex functionalization patterns to control the nucleation, growth, and alignment of inorganic crystals has been widely adapted for the biomimetic synthesis of inorganic materials with complex form [6]. Many studies have shown that a wide range of additives can influence the morphology and crystalline phase of CaCO3 [4–11]. These studies reveal that the crystalline phase and morphology CaCO3 mainly depend on the precipitation conditions such as pH, temperature, presence of additives, etc.
⁎ Corresponding author. Tel.: + 91 94862 82324. E-mail address: [email protected] (V.K. Subramanian). 0032-5910/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2012.03.032
Chemicals for water treatment have been in use for more than a century [12]. A large number of additive formulations are commercially available. Commonly used anti-scalants are chemicals derived from chelating agents [13,14]. These complexing agents not only form complexes with scale causing constituents and inhibit the supersaturation but also are able to weaken its adherence to surface, probably due to the lack of symmetry in the morphology which inhibits building up of scale. For this reason, usage of derivatives of phosphonic acids, acetic acid and other sequestrants as scale inhibitors is very common in water treatment. The formation of scale on the surface of a metal is a process of deposition of insoluble salts of calcium and magnesium layer over layer. CaCO3 is one of the major constituent of scales and hence careful morphological and phase control could help keep them in suspension and preclude the deposition of scale on the metal surface. Although various methods for morphology control have been reported, the advantage of structural and morphological asymmetry for the inhibition of scale has not been explored adequately. AS 330 is a commercially available water treatment chemical widely used in many industrial applications including aquaculture for the control of hardness in India. Considering its wide commercial applications and cost effectiveness, in the present work, crystallization of CaCO3 from CaCl2 was carried out in its presence and morphological and phase transformation studies were performed. The effect of concentration of AS 330 and temperature in the range 60–100 °C on the morphology and polymorphic composition of CaCO3 was investigated in detail. Flower like vaterite similar to those reported by G Yan et al. [9], and H. Tang et al. [15] and rhomboidal calcite and needle/rodlike aragonite were formed under different conditions. This study might be helpful to provide new understanding in the prevention of scale by controlling the morphology and crystalline phase.
S.P. Gopi et al. / Powder Technology 225 (2012) 58–64
2. Experimental details 2.1. Reagents and materials Analytical grade CaCl2, disodium salt of EDTA and Na2CO3 were obtained from Sigma-Aldrich chemical company. 1-hydroxyethylidene1,1-diphosphonic acid (AS 330) with a chelating capacity of 330 was obtained from Satyajith Chemicals P Ltd, Mumbai. The reagents were used as such. Double distilled water was used for the preparation of aqueous solutions. Analytical grade hydrochloric acid and sodium hydroxide were used to adjust the pH whenever necessary. 2.2. Synthesis A procedure similar to Guowei Yan et al. [9] was used for the synthesis of CaCO3. In a typical synthesis, 50 ml of CaCl2 (500 ppm) was taken in a round bottom flask and 50 ml AS 330 (100 ppm) was added. The pH of the solution was adjusted to7. The solution was heated in a rotamantle to 60 °C and 50 ml Na2CO3 was introduced from a burette 15 min after attaining the temperature and kept for 12 h at the same temperature for digestion. A water condenser was used to avoid the loss of water. The precipitate was filtered using a Whatman No. 40 filter paper and dried at 40 °C for 6 h and then kept in desiccators. The experiment was repeated at 80 and 100 °C and also using different volumes of AS 330. 2.3. Characterization Microscopic morphological images were obtained using JEOL JSM 5610 LV scanning electron microscope (SEM) at an applied voltage of 20 kV. The samples were coated with Au prior to examination. FT-IR spectra were taken in the range 500 to 4000 cm− 1 using an Avatar330 FTIR spectroscopy after KBr pelletization. The X-ray diffraction (XRD) patterns were recorded on a Bruker D8 Advanced XRD diffractometer with Cu Ka radiation at λ = 1.5406 Å. The pH measurements were made using Elico pH meter model LI-120. The relative contents for three CaCO3 crystal forms were calculated using the following equations Yc ¼ 1=1 þ 3:9½Ia=Ic þ 2:9½Iv=Ic Ya ¼ 3:9½Ia=IcYc and Yv ¼ 2:9½Iv=IcYc where Yc, Ya, Yv are the mass fractions of calcite, aragonite and vaterite, respectively; Ic, Ia, Iv are the peak areas of characteristic adsorptions for calcite (2θ=29.369), aragonite (2θ=26.213) and vaterite (2θ=32.74), respectively. 3. Results and discussion 3.1. Interpretation of XRD data The powder XRD patterns of the samples are presented in Fig. 1. The details of sequestrant used, temperature of precipitation, Ia, Iv and Ic values and ratio of different polymorphs obtained are given in Table 1. 3.1.1. Effect of temperature The XRD patterns of samples prepared at 60, 80 and 100 °C are presented in Fig. 1a, b and c respectively. The d spacing values corresponding to the 104 plane of calcite are 3.037, 3.007 and 3.025 respectively. This confirmed the presence of calcite in all the samples. The XRD spectrum of sample 1 confirmed the presence of calcite (JCPDS 17-0528), vaterite (JCPDS 02-0261) and aragonite (JCPDS 76-0606). Sample
59
prepared at 80 °C also showed the presence of a ternary mixture containing calcite (JCPDS 87-1803), vaterite (JCPDS 74-1867) and aragonite (JCPDS 05-0453). The XRD patterns of samples 1 and 2 showed peaks at 2θ = 26.15 and 26.42 respectively, confirming the presence of aragonite in these samples. However, the XRD pattern confirmed the sample prepared at 100 °C was a binary mixture of calcite and vaterite (JCPDS No. 04-0636 and 74-1867). There were no peaks corresponding to aragonite. The above data confirmed the presence of aragonite in all the samples except 3. The samples synthesized at 60 and 80 °C showed only marginal changes in their calcite to vaterite ratio (Table 1). The data suggests that, at 100 °C there was a remarkable change in the crystallization process and formation of aragonite was less favored over vaterite and calcite. The absence of aragonite in the sample and an increase in calcite and vaterite were observed. Samples 1 and 2 had almost the same composition of calcite (about 12%) and vaterite (about 33%) and aragonite (about 55%). At 100 °C there was greater tendency towards the formation of vaterite and calcite with a relative increase in the composition of calcite (from 12 to 27%) and vaterite (from 32.7 to 72%) (Table 1).
3.1.2. Effect of concentration of AS330 The change in percentage composition of calcite, vaterite and aragonite when 25, 75 and 100 ml AS 330 were used at 100 °C is presented in Table 1 (samples 4–6). The corresponding XRD spectra are shown in Fig. 1d–f. The table and XRD data showed the presence of all the three polymorphs in all these samples. The percentage of calcite was found to be more in sample 6. This was in good agreement with the SEM image (Fig. 2f) which had more rhomboidal structures. The vaterite fraction was maximum when 25–50 ml AS 330 was used at 100 °C. The above observations suggest that 25–50 ml AS 330 can act as versatile sequestrants at 100 °C. From the results it was also concluded that the increase in concentration of AS 330 favors the formation of calcite and aragonite.
3.2. Interpretation of SEM images We investigated the effect of temperature on the morphology of CaCO3 by varying the temperature of precipitation at fixed concentrations of CaCO3, AS 330 and Na2CO3. Fig. 2(a–c) shows the SEM images of the CaCO3 obtained at 60, 80 and 100 °C respectively. Fig. 2a and b shows the presence of rhomboidal structures with 8–10 μm length, needle like structures with 5–10 μm along with flower like structures. However, at 100 °C the needle like structures were absent and the presence of flower and rhomboidal like structures increased in number (Fig. 2c). The flower like particles showed large central protuberance with a diameter of about 2 μm and having six petals arranged around the orbicular centre. A similar structure has been reported in the precipitation of CaCO3 from ethanol/water system containing Pluronic 127 [9] and polyelectrolyte poly-(sodium 4-styrene-sulfate) (PSSS) [15]. SEM images Fig. 2(d, e and f) depict the CaCO3 samples precipitated from the CaCl2 in the presence of 25 ml, 75 ml and 100 ml of AS 330 respectively at 100 °C. The sample prepared with 25 ml AS 330 (Fig. 2e) showed uniform distribution of all the three morphologies. At higher concentrations of AS 330 (Fig. 2e and f) showed more rhomboidal structure along with some needle structures. This confirmed the presence of more calcite and aragonite under these conditions. When the concentration was increased from 25 ml to 50 ml there was drastic reduction in the needle like aragonite structure. However, further increase in AS 330 showed all the above mentioned three morphologies with increased calcite percentage (Fig. 2d, e). This confirmed the formation of calcite, vaterite and aragonite at higher concentrations of AS 330. This could be due to the favorable microscopic interfacial dynamics for the formation of different polymorphs under different conditions.
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3.3. Interpretation of FTIR data FTIR spectrum of samples prepared at 60, 80 and 100 °C is shown in Fig. 3a–c. The samples prepared at different concentrations 25 ml, 75 ml and 100 ml of AS 330 are presented in Fig. 3d–f. Simultaneous occurrence of peaks at 876 and 712 cm− 1 confirmed the presence of calcite in all samples [16]. Appearance of peaks at 1084 and 745 cm− 1 confirmed the presence of vaterite in these samples. The FTIR peak
intensities corresponding to calcite and vaterite in samples 2 were similar to sample 1. This re-ascertain the ratio of these polymorphs in samples 1 and 2 were the same as observed from the XRD data (Table 1). The above results reveal that the CaCO3 prepared from AS 330 at 60 and 80 °C and those prepared at different concentrations of 25 ml, 75 ml and 100 ml of AS 330 were a ternary calcite, aragonite and vaterite mixture. The peaks due to vibrations of water molecules were observed at 3300 and 1645 cm − 1. The values assigned for
Fig. 1. Powder XRD of CaCO3 samples precipitated under different conditions (a, b and c) in the presence of AS 330 at 60 °C, 80 °C and 100 °C, (d, e and f) in the presence of 25 ml, 75 ml and 100 ml AS 330 at 100 °C.
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Fig. 2. SEM images of CaCO3 samples precipitated under different conditions (a, b and c) in the presence of AS 330 at 60 °C, 80 °C and 100 °C, (d, e and f) in the presence of 25 ml, 75 ml and 100 ml AS 330 at 100 °C.
vaterite were 875, 745 and 1084 cm − 1. The FTIR of the sample prepared at 100 °C in the presence of AS 330 showed the absence of aragonite. These results reconfirm the XRD data from Fig. 1a–f. It has been reported earlier that the more the vaterite appears in the scale, the more efficient the scale inhibitor is [17]. When 50 ml AS 330
Table 1 Details of the volume of sequestrants used, precipitation temperature and Ya, Yc and Yv calculations. Sample no.
Sequestrant used
Temp. °C
Vol of sequestrant
Yc
Ya
Yv
1 2 3 4 5 6
AS AS AS AS AS AS
60 80 100 100 100 100
50 ml 50 ml 50 ml 25 ml 75 ml 100 ml
0.125 0.125 0.277 0.178 0.298 0.589
0.547 0.547 0 0.064 0.112 0.201
0.326 0.327 0.722 0.758 0.590 0.210
330 330 330 330 330 330
was used, the formation of vaterite was more favored at higher temperature which confirmed its greater scale inhibition capacity. Secondly, under the above conditions the prevention of transformation of vaterite to calcite functioned effectively probably due to the dissolution– recrystallization mechanism [12]. Report suggests that the rich variety of CaCO3 structures in nature may be due to the amorphous character of ACC which enables it to easily mold into many different shapes [3]. Phase transformation from vaterite or aragonite to calcite [18,19] and multistep phase transformation form ACC to calcite and ACC which in turn leads to calcite and aragonite [3] are reported. However, the evidence from the SEM images (Fig. 4d) shows that, in contradiction to the usual observation of the formation of different polymorphic forms from ACC, vaterite was formed from calcite precursor. The rhomboidal calcite particles have bulbous deformations at the centre which forms the centre part of the flowerlike structure (Fig. 4a–d). This conclusion is reasonable as similar deformations [4] are observed in the SEM images of CaCO3 particles synthesized in the presence of 0.5 g/l PAA at 80 °C and at pH 10. Reports are also available on the deformation of
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Fig. 3. FTIR spectrum of CaCO3 samples precipitated under different conditions (a, b and c) in the presence of As 330 at 60 °C, 80 °C and 100 °C, (d, e and f) in the presence of 25 ml, 75 ml and 100 ml AS 330 at 100 °C.
the corners of the rhomboidal calcite get rounded off [16,20,21] to form spherical vaterite. Apart from the usual dissolution–recrystallization mechanism, from the data it could be concluded that the efficiency of AS 330 as a good scale inhibitor could be due to the morphological dissimilarities that might prevent the three dimensional growth of scale beyond a certain limit. The results from the SEM images of samples indicate that a sunflower like morphology is present in all these samples. The XRD data confirmed the presence of only calcite and vaterite in all samples. Aragonite was absent in the sample prepared at 100 °C. Rhomboidal
morphology is the most well established structure for calcite and is present in all these samples. Hence the flower structure can be assigned only to vaterite. The absence of other morphological compositions and earlier reports [9,15] confirm this morphology to vaterite. 3.4. Mechanism of formation of flower like vaterite On the basis of the above experimental results a four step mechanism of formation of this different sunflower structure of vaterite is presented in Fig. 5. A spherical fraction from the centre of the rhomboidal calcite
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Fig. 4. SEM images of CaCO3 samples precipitated in the presence of AS 330 (a) at 100 °C, 75 ml AS 330 (b) at 60 °C, 50 ml As 330 (c) at 100 °C, 100 ml AS 330 and (d) showing deformity at the centre of the cubical structure.
forms the core part of the sunflower (Fig. 5, Step 1). Generally, crystal growth is governed by kinetic factors and the observed crystal morphology reflects the relative rates of growth of the crystal to different directions by the inhibitor [22]. It is expected that this provides the site for further spherical fractions to join (Fig. 5, Step 2). However due to steric factors this could not go beyond a certain number of layers. At this stage the dispersion of the second and further layers of spheres start (Fig. 5, Step 3) to form petal structure of the flower keeping the central sphere intact. In the last step the spreading of layers ceases leading to the sunflower structure (Fig. 4, Step 4). The above observations were further confirmed by the uniformity in the diameter of the bulbous part in the sunflower structure and the spherically deformed portion of the rhomboidal calcite, Fig. 4. The above mechanism of vaterite formation from the more thermodynamically stable calcite in the presence of AS
330 along with the morphological structural differences could be another major reason for its effective anti-scalant nature despite an increase in calcite composition. 4. Conclusions Crystallization of CaCO3 from CaCl2 using Na2CO3 in the presence of AS330 generated calcite ranging 12–27% between 60 and 100 °C. In the presence of AS 330 formation of aragonite was inhibited at 100 °C. Sunflower flower like morphology for calcium carbonate was observed when AS 330 was used. The interpretation of SEM, XRD and FTIR data confirmed this structure can be attributed to vaterite. Calcite was found to be the precursor for the formation of this nature of vaterite. A ternary calcite–aragonite–vaterite mixture was the most favored
Fig. 5. Suggested mechanism of formation of the flower structure.
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composition. However, at 100 °C and in the presence of 50 ml AS 330, a binary calcite–vaterite mixture is produced. The observed results might be helpful in selecting the anti scaling sequestrant in systems operating between 60 and 100 °C. Acknowledgements The authors thank the University Grants Commission, New Delhi, for the financial assistance provided through Major Research Project Scheme No. F-37-40/2009 (SR) and the UGC Networking Resource Centre, University of Hyderabad, Hyderabad for providing facility for carrying out instrumental analysis. References [1] S. Mann, Journal of Materials Chemistry 5 (1995) 935. [2] X. Yang, Xiaodeng Yang, Guiying Xu, Yijian Chen, Teng Liu, Hongzhi Mao, Weiping Sui, Mingqi Ao, Fang He, Powder Technology 204 (2010) 228. [3] H. Tang, J. Yu, X. Zhao, Materials Research Bulletin 44 (2009) 831. [4] J. Yu, M. Lei, Bei Cheng, X. Zhao, Journal of Solid State Chemistry 177 (2004) 681. [5] J. Yu, M. Lei, B. Cheng, Materials Chemistry and Physics 88 (2004) 1.
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