Surfactant assisted synthesis of precipitated calcium carbonate nanoparticles using dolomite: Effect of pH on morphology and particle size

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Original Research Paper

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Surfactant assisted synthesis of precipitated calcium carbonate nanoparticles using dolomite: Effect of pH on morphology and particle size

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M.R. Abeywardena a,b, R.K.W.H.M.K. Elkaduwe a,b, D.G.G.P. Karunarathne b, H.M.T.G.A. Pitawala c, R.M.G. Rajapakse d, A. Manipura b, M.M.M.G.P.G. Mantilaka a,e,⇑ a

Postgraduate Institute of Science, University of Peradeniya, Peradeniya 20400, Sri Lanka Department of Chemical and Process Engineering, University of Peradeniya, Peradeniya 20400, Sri Lanka c Department of Geology, Faculty of Science, University of Peradeniya, Peradeniya 20400, Sri Lanka d Department of Chemistry, Faculty of Science, University of Peradeniya, Peradeniya 20400, Sri Lanka e Sri Lanka Institute of Nanotechnology, Nanotechnology and Science Park, Mahenwatta, Pitipana, Homagama, Sri Lanka b

a r t i c l e

i n f o

Article history: Received 5 June 2019 Received in revised form 2 October 2019 Accepted 20 October 2019 Available online xxxx Keywords: Precipitated calcium carbonate Calcium-sucrate Nanoparticles Dolomite

a b s t r a c t Synthesis of nanomaterials from readily available minerals for industrial applications is a growing research area. Understanding the causes of their properties becomes handy in utilization. In this study, an effective sucrose solution based method was employed for the extraction of calcium from dolomite to synthesize precipitated calcium carbonate nanostructures with different morphologies and sizes. It was found that 30% (w/v) sucrose solution extracted approximately 91% of calcium from dolomite forming a calcium-sucrate complex. Carbonation was achieved by CO2 bubbling and aqueous sodium carbonate addition. Precipitation was performed under different pH values of 7.5, 10.5 and 12.5 in the absence of an anionic surfactant and in the template of sodium dodecyl sulfate (SDS)/calcium-sucrate at pH 12.5. It was found that CO2 bubbling slightly promotes smaller particles. The anionic surfactant enables particle size and agglomeration reduction while introducing some hydrophobicity. The smallest particles were achieved at a range of 40–55 nm in the presence of SDS/sucrose template and were of spherical morphology. By changing the pH, a tendency to form different polymorphs and shapes of calcium carbonate was observed. Ó 2019 Published by Elsevier B.V. on behalf of The Society of Powder Technology Japan. All rights reserved.

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1. Introduction

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Identification and development of advanced methodologies to control the morphology and size of particles of different materials has gained the interest of many researchers. Nevertheless, the study of their employment in various applications has been a blooming field of research [1–4]. Nanotechnology has opened pathways for this research area and has supported the synthesis of materials with novel properties. One such approach is on synthesis of precipitated calcium carbonate (PCC) nanoparticles from readily available naturally occurring earth resources such as dolomite, calcite, chalk, limestone and marble. As one of the most abundant minerals in nature, calcium carbonate (CaCO3) occupies

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⇑ Corresponding author at: Sri Lanka Institute of Nanotechnology, Nanotechnology & Science Park, Mahenwatta, Pitipana, Homagama, Sri Lanka. E-mail address: [email protected] (M.M.M.G.P.G. Mantilaka).

around 5% of the earth’s crust which has a great potential in utilizing for many applications [5-7]. Dolomite is a carbonate mineral which has chemical formula of CaMg(CO3)2 [8]. In dolomite, the composition of calcium, magnesium and other elements and compounds such as Sr, Na, Cu, Fe and SiO2 may vary depending on the region they exist [9]. Although there are numerous dolomite mines present worldwide, very least number of works on PCC nanomaterials using dolomite has been reported. Mantilaka et al., have successfully synthesized hollow bone-like [10] structure of CaCO3 while in another study, they have developed stable amorphous CaCO3 (ACC) starting from naturally occurring dolomite [11]. Somarathna et al., have synthesized high purity calcium carbonate micro- and nanostructures from dolomite using polyethylene glycol templates [12]. In all these studies, dolomite has been used as the calcium source. However, it is necessary to point out that plenty of knowledge gaps exists in this particular approach. It is contemporary to perform a deep study in order to identify more

https://doi.org/10.1016/j.apt.2019.10.018 0921-8831/Ó 2019 Published by Elsevier B.V. on behalf of The Society of Powder Technology Japan. All rights reserved.

Please cite this article as: M. R. Abeywardena, R. K. W. H. M. K. Elkaduwe, D. G. G. P. Karunarathne et al., Surfactant assisted synthesis of precipitated calcium carbonate nanoparticles using dolomite: Effect of pH on morphology and particle size, Advanced Powder Technology, https://doi.org/10.1016/j. apt.2019.10.018

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effective and feasible techniques along with the development of novel materials. Moreover, the usage of dolomite to synthesis nano-PCC has an extra advantage over limestone [13], which is usually used rock in CaCO3 production. A proper extraction of calcium component yields a magnesium-rich byproduct which is useful in vast applications. Hence we see the potential of producing multiple products hand in hand, while using dolomite as the mineral raw material. Our work targets to identify an innovative and cost-effective process to extract and synthesize nano-PCC from dolomite and study morphological changes of products under different conditions. Considering the lesser solubility in the carbonates of calcium and magnesium as well as their similarities in chemistry, it has become a tougher task to separately extract the two metal ions in dolomite. In literature, several methods such as leaching, hydration, flotation, calcination, forming carbonates and formates, chlorine compounds and alkalis are suggested for separation of calcium and magnesium from dolomite [14]. Feasibility of most of such methods are challenged by their low efficiency, high-cost factor and high wastage. However, usage of sucrose solution has proven to be an extremely successful method to separate calcium from calcined dolomite. It is also a commercially available material with low-cost. CaO shows high solubility in a sucrose solution where MgO does not. Therefore, calcined dolomite, composed of CaO and MgO can be effectively used to isolate calcium as Ca-sucrate [15,16]. CaCO3 displays three anhydrous crystalline polymorphs including calcite, aragonite and vaterite [5,6,17,18] and amorphous [11] form. The most thermodynamically stable form of calcium carbonate under ambient conditions is calcite while the other two anhydrous crystalline polymorphs are meta-stable in nature. Calcite acquires different morphologies, most commonly, trigonal, rhombohedral [19], needle-like, mesoporous [20] and scalenohedral [17,21,22]. Some of the common morphological structures for aragonite are orthorhombic, needle-like [7,19] while vaterite is the least unstable form among them which shows a hexagonal morphology [5,18,23]. The main sources of particulate CaCO3 are ground calcium carbonate (GCC) mined from calcite deposits and PCC produced by means of a chemical reaction [6,18]. Due to its properties such as uniform shape and size, PCC is more effective than any other type of CaCO3 [6]. PCC is very important because of their biodegradability, biocompatibility, easy preparation and readily availability of cheap raw materials in large quantities for their preparation [10,24,25]. The application of CaCO3 particles depends on various factors. These can be identified as morphology, structure, size, specific surface area, brightness, oil adsorption and chemical purity [6]. PCC is increasingly used in the paper, plastics, rubber, tooth paste, paint, textile, pharmaceutical industries, cosmetics, food and beverages, nutritional supplements, adhesives & sealants and in some other industries [5,6,17,26]. When calcium carbonate particles are synthesized in nanometer scale, they demonstrate novel and improved characteristics compared to regular sized particles where these characteristics are highly advantageous in certain applications. These characteristics are gained due to the presence of large number of unsatisfied valences at the surface resulting in a ‘‘small size” effect and a ‘‘surface effect” that is not present in ordinary calcium carbonate [27,28]. Nanoparticles exhibit a high surface free energy and a tendency to aggregate each other [22]. In contrast they demonstrates different optical, electrical, magnetic, self-healing, gas barrier, thermal and mechanical properties than regular particles [22]. Classical crystallization mechanisms of CaCO3 particle synthesis are associated with a nucleation step followed by the crystal growth which is controlled by thermodynamic or kinetic approaches [29]. These crystallization mechanisms involve a prenucleation process in which ions form stable clusters of charged

particles in equilibrium with their ions. These clusters can develop or colloid to produce CaCO3 nuclei [30]. Introduction of certain additives into the crystal growing medium changes the surface charge of the growing nuclei [29]. Newly produced CaCO3 particles can be stabilized in nano-scale by modifying the surface potential of the particles. In a colloidal system, the surface potential is related to the magnitude of the zeta potential. Particles are considered as stable when the zeta potential values are more positive than +30 mV or more negative than
30 mV [31]. It was determined that the surface potential for CaCO3 is influenced by various parameters such as additives, aging, pH, surface modifiers and ions [32,33]. In literature, it has been reported that the zeta potential values for CaCO3 are sometimes positive, sometimes negative and sometimes possesses a variable value [33,34]. It was reported normally to be about
10 mV and vary with the presence of poten+
2
tial determining ions such as Ca2+, CO2
3 , HCO3 , H and OH in the case of calcite. It indicates that the newly produced CaCO3 clusters are naturally unstable [34]. The point of zero charge (PZC) of CaCO3 exists around a pH value of 9–10 although the zeta potential of colloids is pH dependent [33]. Previous studies claim that the zeta potential value for CaCO3 particles was more than +30 mV in Ca (OH)2 solution and it is certainly a natural stabilizer for the newly formed CaCO3 particles [35]. Different methods have used to synthesis CaCO3 in nanometer scale such as hydrothermal [36], solvothermal [37], reactive precipitation [38], microemulsion [39] and templates [10,40]. Furthermore the introduction of surfactants are observed to control the self-assembly, growth and crystallization which internally influences the properties of particles including size, crystal structure, morphology, and surface properties [22,41,42]. Jiang et al., have studied the formation of CaCO3 particles by purging carbon dioxide (CO2) through calcium hydroxide slurry which was added with different long chain fatty acids such as lauric acid, palmitic acid and stearic acid added [43]. Guo et al., have been successful in preparing CaCO3 with different polymorphs and morphologies using three different amino acids including L-valine, L-serine, and Larginine as organic matrices through a facile gas diffusion method based on a biomimetic strategy [44]. As most of the templates are high cost organic polymers that are not feasible for industrial scale production, low cost and effective template should be considered. In this work, an innovative, decent and industrially viable method has been recognized to isolate calcium from dolomite and to synthesize calcium carbonate nanoparticles with different shapes and sizes. Literature reveals that the sucrose is commercially available, nontoxic and low-cost material that can be used to extract calcium from dolomite effectively. Herein, we have determined that apart from being an effective extractor of calcium, sucrate serves in the process of controlling particle size and morphology. Furthermore it induces the ability to tune the said aspects by changing the pH. The addition of anionic surfactants alongside sucrose has also found to improve the quality of the products. The structural and morphological properties of the fabricated CaCO3 particles were investigated using XRD, SEM, TGA and FTIR analysis.

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2. Experimental

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2.1. Materials

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Freshly prepared distilled water was used for the preparation of all the solutions. Locally purchased sucrose was employed for the extraction of calcium from calcined dolomite and for the synthesis process. Sodium carbonate (Na2CO3), calcium chloride (CaCl2), hydrochloric acid (HCl), sodium hydroxide (NaOH), sodium lauryl/dodecyl sulfates (SDS), absolute alcohol of analytical grade were

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purchased from Aldrich. Dolomite was obtained from the quarry in Digana, Sri Lanka.

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2.2. Characterization of raw materials

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First, dolomite was ground and sieved through 400 µm mesh. Then it was digested by adding concentrated HCl, followed by addition of distilled water. The mixture was stirred, and filtered to separate the supernatant. The residue was subjected to drying and then was weighed to determine the undesirable insoluble material present. The supernatant obtained was subjected to atomic absorption spectroscopic (AAS) analysis to determine the calcium, magnesium and iron content.

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2.3. Preparation of calcium-sucrate solution

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Ground dolomite was calcined at 1000 °C to convert it into calcined dolomite. Then, 5% (w/v) of calcined dolomite was added into 30% (w/v) sucrose solution to obtain calcium-sucrate solution (Table S.4, Fig. S.2 in the supplementary document). Resulting solution was subjected to AAS analysis in order to obtain the concentration of calcium and magnesium. The observed pH of the solution was 13.

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2.4. Preparation of CaCl2 solution

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According to AAS results, the concentrations of calcium and magnesium were found to be 0.41 mol dm
3 and 0.005 mol dm
3 respectively. Hence a solution of 0.41 mol dm
3 CaCl2 controlled solution was prepared at pH 6.5.

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2.5. Synthesis of products

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Solutions were prepared as in the Table 1 before carrying out the precipitation through the introduction of specified additives. Sample (a)–(g) were prepared using calcium-sucrate solution while sample (h) and (i) were prepared using 0.41 mol dm
3 CaCl2 solution. Fig. 1 illustrates a schematic diagram of the two precipitation methods that described. Precipitation was done by (1) adding freshly prepared 0.41 mol dm
3 sodium carbonate solution dropwise (Fig. 1a), (2) bubbling CO2 gas through the solution of 100 ml of calcium precursor (Fig. 1b). The process was performed while stirring using magnetic stirrer until a white thick formation was visible. Concentration of 1 mol dm
3 sodium hydroxide and 1 mol dm
3 HCl solutions were used to control the pH of the solutions. The constant stirring was continued for 1 h. The resulting suspension was washed for three times with 50 ml of distilled water and once with 50 ml of alcohol under a centrifugation process. Resulting products were dried at 60 °C under vacuum drying

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Fig. 1. Schematic diagram of experimental set-up of, (a) addition of aqueous sodium carbonate solution and (b) CO2 bubbling.

for 6 h. Entire synthesis procedure was performed under ambient temperature.

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2.6. Characterization

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Elemental quantities of the solutions were determined by the Atomic Absorption Spectroscopic analysis (AA-70000, SHIMATZU). The crystalline phases of obtained samples were characterized by X-ray powder diffraction (Siemens D5000 Pow-der X-ray Diffractometer, with the Cu Ka radiation of wavelength k = 0.1540562 nm, at the scan rate of 1° min
1) with the 2h angle ranging from 20° to 60°. The morphologies and the particle size of samples were visualized by scanning electron microscope (SEM, ZEISS EVO LS15) measurements with accelerating voltage of 10 kV. The Fourier Transform Infrared (NICOLET iS50 ATR/FT-IR, Thermo SCIENTIFIC) spectra of the final products were recorded. Contact angle was measured to study the hydrophobicity of the products. Decomposition of the products was monitored by TGA (PerkinElmer TGA 4000) under O2 atmosphere.

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3. Results and discussion

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Almost half of the composition of dolomite is consisted of magnesium carbonate as shown in Table S.1 in the supplementary document. Accordingly the calcium carbonate and magnesium carbonate contents in dolomite were calculated to be as 48.86% and 43.20% respectively. The undesirable content which was insoluble in acid was found to be 7.14% (w/w) as in Table S.2. By considering the mass reduction during the calcination process of dolomite, the theoretical content of calcium oxide and magnesium oxide that can be gained is determined as 48.88% and 36.93%

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Table 1 Sample preparation conditions with respective particle properties. Sample No

(a) (b) (c) (d) (e) (f) (g) (h) (i) *

Sucrose %(w/v)*

30 30 30 30 30 30 30 – –

SDS %(w/v)*

– – – – – 0.5 0.5 0.5 0.5

pH

12.5 12.5 10.5 7.5 7.5 12.5 12.5 12.5 12.5

Precipitation method CO2 bubbling

Na2CO3 addition

U – U U – U – U –

– U – – U – U – U

Particle morphology

Particle size

Rod-like Catkin-like agglomeration Rod-like Highly agglomerated Spheres Highly agglomerated Spheres Spherical Spherical Spherical + Rhombohedral Spherical + Rhombohedral

80–100  250–400 nm 180–200  250–400 nm 250–300  550–650 nm 35–50 nm 40–50 nm 40–55 nm 50–65 nm 80–120 nm 100–130 nm

Weight Percentage to the initial volume.

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respectively. Hence the maximum amount of calcium that can be extracted into a solution is 0.45 mol dm
3. In our process of sucrose based calcium extraction the AAS analysis shows (Table S.3) that the concentration of calcium separated from calcined dolomite were 0.41 mol dm
3. The amount of magnesium extracted into the sucrose matrix was at a 0.005 mol dm
3. Therefore, it shows that 30% sucrose solution has been effective in extracting approximately 91% calcium with very least amount of magnesium. Moreover the ability to gain a magnesium ion solution with a simple purification process and it can be used to synthesis magnesium base products is an added advantage of using dolomite. Carbonation of the calcium-sucrate/surfactant mixture to yield CaCO3 has done in two ways. Introduction of CO2 gas through the reaction mixture is undergo following mechanism to produce PCC.

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CO2 (g) CO2 (aq)

ðaÞ

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CO2 (aq) + OH
(aq) ! HCO3
(aq)

ðbÞ

298 300

HCO3
(aq) + OH
(aq) ! H2 O + CO3 2
(aq)

ðcÞ

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Ca2þ (aq) + CO3 2
(aq) ! CaCO3 (s) [45,46]

ðdÞ

Reactions (c) and (d) are spontaneous while rate of formation of CaCO3 is controlled by the diffusion of CO2 gas in liquid [46]. Reduction of the pH was observed due to the consumption of hydroxyl ion upon bubbling of CO2 through the reaction mixture. Addition of aqueous NaOH solution was required to maintain the pH at a constant value. Direct addition of carbonate ion to the reaction mixture follows the step (d) determines the nucleation and particle growing process. But in case of addition of CO2
to the 3 ca-sucrate solution more washing is needed to remove the Na+ and extra added CO2
3 to maintain the purity than the CO2 bubbling method.

3.1. Morphological effect of sucrate template in different pH

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Studies show that sucrose can act as a di basic acid at higher pH values [47]. However still no clear evidence of the second dissociation step below pH 14 [48]. Hydroxyl groups present in sucrose can undergo ionization at higher pH to form its anionic form of sucrate. As a result, ‘‘–O
” groups that are able to form electrostatic bonding with different cationic species such as Ca2+ and Ca(OH)+ are produced. Therefore, the dissolution mechanism of calcium hydroxide in sucrate is expected to undergo the reactions (i)–(iv).

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Sucrose + OH
! Sucrate
+ H2 O

ðiÞ

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CaO + H2 O Ca(OH)2

ðiiÞ

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Ca(OH)2 Ca(OH)þ + OH


ðiiiÞ

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Ca(OH)þ + Sucrate
! Ca-Sucrate

ðivÞ

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Negative behavior on the oxygen atom due to the existence of lone pairs in the hydroxyl groups may influence the highly unstable nanoparticle surface which leads to a change in the surface potential. Moreover, the higher OH
concentration at higher pH causes the hydroxyl ions to adsorb on to the particle surface further changing surface potential of the particles that assist to stabilize them in such small scale. Concentration of Ca2+ ions may be regulated as per reaction (v) and it also can be considered to effect the nucleation and growth of the particles. Fig. 2 illustrates the bonding between PCC particles and sucrose in solution.

Ca-Sucrate Ca(OH)þ + Sucrate
Ca2þ + Sucrate
+ OH


ðvÞ

SEM images shown in Fig. 3 were obtained under different pH correspond to samples (a), (b), (c), (d) and (e) as stated in Table 1 respectively. It can be seen that the products in Fig. 3a and b which are synthesized only in the presence of sucrose leads to produce one dimensional aggregates of tiny nanoparticles. This Catkin-like morphology was achieved in both instances at pH 12.5. Moreover as it

Fig. 2. Schematic diagram of interaction between sucrose molecules and PCC particles.

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Fig. 3. SEM images of PCC products obtained under conditions: (a) Ca-sucrate/pH 12.5/CO2 bubbling, (b) Ca-sucrate/pH12.5/Na2CO3 (aq), (c) Ca-sucrate/pH 10.5/CO2 bubbling, (d) Ca-sucrate/pH 7.5/CO2 bubbling (aq), and (e) Ca-sucrate/pH 7.5/Na2CO3 (aq).

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involves different mechanisms depending on the precipitation method it is observed that the employed methodology also has a slight effect on the particle size and the dimensional aggregates. Particles of 80–100 nm of width and 250–400 nm of length were achieved under CO2 bubbling method while the aqueous carbonate ion addition was successful in yielding particles with 180–200 nm of width and 250–400 nm of length. This variation may be due to the influence of electrolyte (natrium ion) [49] on the surface potential and the possibility of the ion to have an interaction with sucrose molecules [50,51] may cause this change. SEM images (Fig. 3c–e) are of the synthesized PCC samples (c), (d) and (e) of Table 1 respectively. At the pH 10.5 (Sample c), it is seen to promote a blend of tiny spherical particles and rod-like formations with 250–300 nm of width and 550–650 nm of length. Images suggest that they are formed of needle like particles. At pH 7.5 (Sample d and e) tiny PCC nanoparticle formed into highly agglomerated large spheres where the tiny particles produced from CO2 bubbling method was

of 35–50 nm. The sodium carbonate addition resulted in tiny nanoparticles of 40–50 nm. But relatively lower yield was seen in CO2 bubbling method [45].

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3.2. Influence of SDS on the morphology

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SEM images (Fig. 4a and b) show the morphology of in-situ prepared calcium carbonate in the presence of SDS surfactant using the calcium-sucrate template. It is found that both the precipitation methods yield tiny zero dimension nanoparticles. The particles obtained under CO2 bubbling method promotes spherical particles with a size of about 40–55 nm and carbonate ion addition method introduced particles about 50–65 nm. In comparison with the products shown in (Fig. 3a and b), it is seen that SDS has inhibited the growth of particles by stabilizing them in nanometer scale. Synthesized nanoparticles have shown serious agglomeration since they have high surface area to volume ratio. SEM images (Fig. 4c and d)

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Fig. 4. SEM images of PCC products obtained under conditions: (a) Ca-sucrate/SDS/pH 12.5/CO2 bubbling, (b) Ca-sucrate/SDS/pH12.5/Na2CO3 (aq), (c) CaCl2 (aq)/SDS/pH 12.5/ CO2 bubbling, and (d) CaCl2 (aq)/SDS/pH 12.5/Na2CO3 (aq).

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were obtained for the samples prepared using calcium chloride in the presence of SDS and in the absence of sucrose. SDS has overlaid surface of nanoparticles and they have grown separately into spherical nanoparticles. Particle sizes were relatively higher compared to the samples synthesized in the presence of sucrose. Images illustrate a blend of spherical particles along with some rhombohedral particles in a size range of 80–120 nm and 100– 130 nm respectively. It is evident that the anionic surfactant, SDS has played an important role in the controlling of particle size and morphology. In comparison of carbonation methods CO2 bubbling process is found to promote lesser particles sizes with minimized agglomeration than Na2CO3 addition. Moreover the purity of the samples is better compared to Na2CO3 addition as cations like Na+ and excess amounts of CO2
3 anions are not present in the solution. Hence the bubbling method reduces the requirement of washing of the product which is an advantage when in industrial utilization. Additionally unlike aqueous sodium carbonate addition, the bubbling can be done the calcium sucrate solution the volume of the vessels required becomes smaller which reduces the capital cost in building an industrial plant. To minimize effects of pH, it was maintained at 12.5 in all the four synthesis procedures. Fig. 5 illustrates the schematic diagram of the stabilization mechanism of nanoPCC in presence of the anionic surfactant. SDS has a molecular formula with an amphiphilic nature as shown in Fig. S.1. Since the CaCO3 particles are hydrophilic [52], the surfactant molecules can adsorb to CaCO3 particles to form a monolayer with the hydrophilic anionic head group bonded to the particles’ surface. The long tails of the surfactant line up perpendicular to the surface introducing hydrophobic character to the particle surface as shown in Fig. 6 and the contact angle is measured to be

120.8° & 119.5°. FTIR and TGA analysis are also used to identify the surface adsorbed surfactants.

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3.3. XRD analysis of the products

419

XRD patterns of PCC products show (Fig. 7a–d) related to products a-d as in the Fig. 4 respectively. XRD peaks are located at 2h values of 23.01, 29.41, 36.01, 39.41, 43.11, 47.41, 48.51, 56.51 and 57.31 with corresponding basal planes of (1 1 2), (1 0 4), (1 1 0), (1 1 3), (2 0 2), (0 1 8), (1 1 6), (2 1 1) and (1 2 2) can be assigned to calcite crystalline form of CaCO3 (JCPDS card no. 83-1762). It can be seen that synthesized products has high purity and crystallinity as there are no any peaks related to other materials. Fig. 8 represents the XRD patterns of the products of calcium carbonated precipitated at different pH values of 12.5, 10.5 and 7.5 from Ca-sucrate solution. XRD pattern (a) and (b) correspond to the products obtained at pH 12.5 via CO2 bubbling method and carbonate ion addition method respectively. The pattern (c), (d) and (e) relate to the samples (d), (e) and (c) of Table 1 respectively. Though XRD patterns of Fig. 8a–d show the same crystalline form of calcite while pattern Fig. 8e shows the mixture of two crystalline phases of calcite and aragonite. Literature clearly support that the pH range 10–11 lead to the formation of aragonite under certain conditions. Moreover the presence of some impurities such as magnesium ions acts favorable in the formation of aragonite. The existing slight amounts of magnesium in the calcium-sucrate solution are shown in Table S.3. Furthermore the presence of organic macromolecules have been found to facilitate aragonite phase [7,53–55]. Ultimately with the above mentioned factors, formation of the aragonite phase can be justified. However, the

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Fig. 5. Schematic diagram of formation and stabilazation of PCC nano particles presence of anionic surfactant SDS.

Fig. 6. Contact angle image of the PCC obtained with Ca-sucrate/SDS/pH 12.5/CO2 bubbling.

Fig. 7. XRD patterns of PCC products obtained under conditions: (a) Ca-sucrate/ SDS/pH 12.5/CO2 bubbling, (b) Ca-sucrate/SDS/pH 12.5/Na2CO3 (aq), (c) CaCl2 (aq)/ SDS/pH 12.5/CO2 bubbling, and (d) CaCl2 (aq)/SDS/pH 12.5/Na2CO3 (aq).

Fig. 8. XRD patterns of PCC products obtained under conditions: (a) Ca-sucrate/pH 12.5/CO2 bubbling, (b) Ca-sucrate/pH12.5/Na2CO3 (aq), (c) Ca-sucrate/pH 7.5/Na2CO3 (aq), (d) Ca- sucrate/pH 7.5/CO2 bubbling, and (e) Ca-sucrate/pH 10.5/CO2 bubbling.

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Fig. 9. FTIR/ATR spectrum of (a)–(i) are of PCC samples (a)–(i) as in the Table 1. Inset: FTIR/ATR spectrum for sample (c).

Fig. 10. (a) TGA/(b) DTG curve for the (a0 ) PCC/Sucrose/SDS at pH 12.5 with CO2 bubbling and (b0 ) PCC/Sucrose at pH 12.5 with addition of Na2CO3(aq).

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amounts of two phases were not quantified since more analysis is needed.

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3.4. FTIR analysis of the products

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FTIR spectra (Fig. 9) of CaCO3 polymorphs display characteristic absorption bands corresponding to the CAO bond vibrations, that are symmetric stretching (t1), out-of-plane bending (t2), a doubly degenerate asymmetric stretching (t3) which is a strong broad band, and a doubly degenerate in-plane bending (t4) that is a weak

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absorption band. Three of these bands are observed for calcite and four are observed for vaterite in their infrared spectra while all six bands are infrared active for aragonite because of the nondegeneracy of the t3 and t4 bands due to the reduction in symmetry [23,56]. FTIR spectra of all the products exhibit characteristic infrared bands at 712 cm
1, 872 cm
1 and 1430 cm
1 that can be attributed to the (t4 mode), (t2 mode) and (t3 mode) of carbonate anion, respectively confirming the formation of calcite [40,44]. The presence of a broad t1 band around 1073 cm
1 which is not infrared active in calcite but consistent with the presence of amorphous

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CaCO3. The combination bands of t1 + t4 and 2t2 + t4 appears at 1795 cm
1 and 2510 cm
1 respectively, and is called overtone [21,57]. The intensity ratio of the peaks (It2/It4) is indicated the existence of both the crystalline and amorphous phases while a high value for (It2/It4) denotes a higher content of ACC while for the pure calcite the ratio is 3 [58]. The (It2/It4) ratio for samples a, b, c, d, e, f, g, h and i are 3.76, 3.92, 5.74, 3.56, 3.11, 3.70, 3.83, 4.15 and 4.33, respectively indicate the presence of amorphous phase. Peaks arised at 2930 cm
1 and 2854 cm
1 are due to the stretching motion of C-H bonds of the surface adsorbed organic molecules. The formation of aragonite in the sample (c) can be seen in the degeneration of the peak at 712 cm
1 while generating two different peaks at 700 cm
1 and 713 cm
1 [23]. No other absorption peaks from impurities were detected indicating that the products obtained had higher purity.

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3.5. Thermal analysis of the products

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The thermal properties of the PCC nanoparticles synthesized in this study were investigated by the TG analysis. From the nucleation to further growth of the particles, moisture to other organic molecules can be adsoped on to the surface. This will be governed by the factors such as surface charge, existence of unsaturated Ca2+ ions on the surface etc. in the nanoparticle. The anionic organic groups can be adsorbed on to the unsaturated Ca2+ ions on the surface of the particle electrostatically in the formation of CaCO3-Casucrate and CaCO3-Ca-dodecyl sulfate. Usually upon heating of the particles, surface adsoped moisture starts to remove until 300 °C while the removal of other organic molecules prevail at a range of about 300–450 °C [59,60]. Fig. 10a and b shows the TGA/DTG analysis curves for the samples (f) and (b) in Table 1 respectively. There is a two-step successive mass-loss in each TGA profile of both samples. Mass-losses of Fig. 10a and b are found to be 4.79% and 4.56%, respectively in the temperature range 110–450 °C. For the Fig. 10a, this weight reduction is associated with the desorption of physically bound water molecules and combustion of surface attached sucrate molecules where in Fig. 10b, it is found to be of water molecule removal and combustion of sucrose/ SDS. Furthermore, PCC products show TGA mass-losses of 41.23% and 41.29% for the samples (f) and (b), respectively in the range between 600 and 810 °C, which is attributed to the decomposition of CaCO3 to CaO.

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4. Conclusions

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In this study nano-precipitated calcium carbonate was synthesised from dolomite. Sucrose has been successful in calcium extraction from dolomite. It is found that the carbonation method has an effect on paticle size. CO2 bubbling method promotes lower particle sizes compared to aqueous carbonate ion addition. Calcium-sucrate template alone worked with formation of calcite nano rods at high pH of 12.5. Formation of rod-like mixture of calcite and aragonite has been observed at pH 10.5 and nanospheres with high degree of agglomeration were obtained at pH 7.5. This confirms that pH also has a significant effect on polymorphsm, particle size and shape. Due to the grafting of anionic surfactant SDS over the produced nanoparticles the agglomeration has been reduced. The rod-like formations achieved in the absence of SDS has been converted into nanospheres of size of 40–55 nm in its presence.

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Declaration of Competing Interest

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The authors declared that there is no conflict of interest.

9

Acknowledgments

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The authors thank National Scientific Foundation of Sri Lanka (Grant No. TG/2016/Tech-D/02) for their financial support. All technical support from various laboratories at Faculty of Engineering and Faculty of Science, University of Peradeniya, Sri Lanka, Sri Lanka Institute of Nanotechnology (SLINTEC) and National Institute of Fundamental Studies (NIFS), Sri Lanka are acknowledged.

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Appendix A. Supplementary material

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Supplementary data to this article can be found online at https://doi.org/10.1016/j.apt.2019.10.018.

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