Calcium carbonate is one of the most abundant materials on our planet and has been quite early used in ground form to produce polymer composites. It has three anhydrous crystalline polymorphs which have typical morphologies: rhombohedral calcite, orthorhombic aragonite, and hexagonal vaterite.
Of them, Aragonite is the stable phase at high pressure and low temperature. The process leading to the formation of various sizes, shapes and surface properties of aragonite has been thoroughly studied for its importance in biological sciences and geography and for many industrial applications.Aragonite is an needle-like shape crystals having a very high aspect ratio and can be used as an industrial raw material such as a rubber, a plastic, a filler for paints and a pigment for papers (Li et al.
, 2013; Wada and Suzuki, 2003), ceramics, pharmaceuticals and can improve brightness and transparency due to complicated surface structure (Matsumoto et al., 2010). Biogenic aragonite has complex structures with specific characteristics that are difficult to find in general inorganic crystals (Kim et al.
, 2010). Such as aragonite containing protein polysaccharides has been studied to have a strength of over 3,000 times that of aragonite composed of inorganic materials. For many technological applications a precise control over the particle size, morphology, and specific surface area would be highly desirable.Several reports have shown that organic acids introduced to the crystallization processes of CaCO3 modify the shape of crystals, retard nucleation and growth rate, and control polymorphism (Reddy and Hoch 2000; Wada et al.,2001; Westin and Rasmuson 2003; Aschauer et al. ,2010; Kim et al.,2011; Karar et al., 2014).
They quantified the effects of different organic molecules on CaCO3 crystal growth by adding seed crystals to supersaturated solution.In their work, Wada et al. studied with the pH-drift method and the light scattering technique to study the influence of five natural carboxyl acids, namely malonic acid, maleic acid, succinic acid, tartaric acid and citric acid (Table 2), on CaCO3 crystallization (Wada et al., 2001). It was shown that CaCO3 growth seemed to be inhibited by the adsorption of the carboxyl acids on to the CaCO3 surface. The authors concluded that the inhibitory capacity of the organic molecules depended on the number of carboxyl groups in the molecule. However, they suggested that other factors, such as the conformation of the acids, could also play a role in their inhibition properties. Reddy et al.
used the constant composition technique to determine the CaCO3 growth rate in the presence of organic molecule. The authors showed that citric acid exhibited only moderate calcite crystal growth rate (Reddy et al., 2001). They suggested that structural conformation of organic molecule could be related to the crystal growth mechanisms, such as cyclic, rigid polycarboxyl acids like tetrahydrofurantetracarboxyl acid or cyclopentanetracarboxyl acid were much more efficient towards inhibition of CaCO3. The constant composition technique was also used by Malkaj et al.
to investigate the kinetic of CaCO3 growth in the presence of leucine, one of the amino acids. In this case, Leucine had effect on the morphology and the particle size of the CaCO3 was observed (Malkaietal.,2004). The same results have been previously studied with another amino acid, namely glutamic acid (Manoli and Dalas 2001).
They suggested that the active growth site s of CaCO3 surface might be blocked by adsorption of amino acids and the Ca-Complexes of the crystal surface with their terminalcarboxyl groups or through hydrogen bonding maybe effect on the growth of crystals. Finally, Amjad has studied the influence of benzenepolycarboxyl acids on CaCO3crystalsbyconstant-addition method. The results of these studies found that the different structural characteristic of benezenepolycarboxyl acids effect upon the rate of crystal growth may be due to the respective complex formation with Ca2+ with organic molecules. However, despite such a number of researches, previous studies have been mostly focused on the calcite rather than aragonite crystals among calcium carbonate minerals. Also, structural characteristics of organic acids containing carboxyl groups on aragonite are not fully understood yet.Therefore, In this study, effects of organic acids which have different structural characteristics, i.e., citric acid, malic acid, acetic acid, glutamic acid, aspartic acid and phthalic acid, on aragonite crystals were investigated using seeded constant-addition method.
In addition, the species of organic acids in supersaturated solution was explained together with PHREEQC speciation modeling results.2. Materials and methods2.1 MaterialsAll chemicals are commercially available and analytical grade used as received without further purification. Calcium chloride dihydrate (CaCl2·2H2O), Sodiumbicarbonate (NaHCO3), Sodiumcarbonate (Na2CO3) Sodiumchloride (NaCl), Magnesiumchloridehexahydrate (MgCl2·6H2O) and organic acids (citric acid, malic acid, acetic acid, glutamic acid, aspartic acid, phthalic acid) used in this study were purchased from Sigma-Aldrich Chemical Reagent Co., Ltd. Analytical grade hydrochloric acid (HCl) and sodium hydroxide (NaOH) were used to adjust the pH whenever necessary. Deionized water was used in all experiments.
All glassware (beakers and small piece of PFA substrates) were washed and dipped in 10% HCl bath for 1 day, rinsed thoroughly with deionized water, and then rinsed with deionized water and finally dried in oven 60? overnight 2.2 Organic acidsTable 2 shows characteristics of organic acids used in this study. It composed of carboxyl groups, but they have different structures with the amount of carboxyl or functional groups. In this study, the organic acids applied to the experiment were classified to three groups.
Group 1 is classified according to the number of carboxyl groups composed with the organic acid structure. this group 1 was aimed to find the physicochemical effect on mineral growth by quantitative classification of carboxyl groups in organic acids. Group 2 consists of organic acids including amino groups. glutamic acid and aspartic acid contain two carboxyl groups and one amino group.
The purpose of this group 2 was to investigate the physicochemical effects of amino group during the growth of aragonite mineral with carboxyl groups and to confirm whether the amount of incorporated organic acids are proportional to the smaller molecular size of organic molecules. Phthalic acid, which constitutes of group 3, is an aromatic compound with two carboxyl group. it was applied to find the possibility of incorporating aromatic group with complex chelate structure. 2.3 Aragonite seed preparationSynthesis of aragonite seeds was carried for seeded constant-addition methods. CaCl2·2H2O 7 mM, NaCl 0.1 M, and 0.5 M of MgCl2·6H2O were first dissolved in 3.
5 L of deionized water with vigorous stirring by a magnetic stirrer at 25?. Then, NaHCO3 7 mM was added to the solution and the mixture was stirred for 1-6 hr to find proper size of aragonite seeds for the aragonite mineral coprecipitation experiment with organic acids. Mixtures were filtered with 0.45 ?m pore size of the membrane filter and dried in oven at 60? overnight. seeds characterization was conducted by XRD, SEM and Particle Size Distribution analysis.
2.4 Coprecipitation methods Experimental conditions for this experiment are shown Table 1. 0.1-1.0 mM of individual organic acids were added to a reaction vessel containing 700 mL of an initial growth solution having the following molar concentration CaCl2·2H2O 0.007 M ,NaHCO3 0.
007M ,NaCl 0.1 M, 0.5M of MgCl2·6H2O and 0.1 g of aragonite seeds. This initial solution was slightly oversaturated with respect to aragonite.
A constant solution composition in the reaction vessel was maintained by addition of 0.3 M CaCl2 + 0.1 M NaCl with individual 0.1-1.0 mM of organic acid and 0.3M Na2CO3 + 0.1M NaCl solutions from separate syringes at constant rate of 1.
0 mL / min for 10 hr. Air was also injected constantly to the solution to keep equilibrium with CO2(g) in atmosphere. The reaction solution was stirred by teflon coated stirring magnet with 200 rpm throughout the entire synthesis process and maintained at 25 ?. At the end of experiments, precipitates in the solution were filtered with 0.45 ? (cellulose nitrate) membrane filter, rinsed several times with pH 8.3 DI-Water, and dried for overnight in an oven at 60 ?.2.
5 CharacterizationAll samples were ground before characteristic analysis. Mineral phase and crystallinity of the synthesized samples were found by X-ray Diffractometer (XRD, Rigaku SmartLab). Crystal morphology of the samples was analyzed by Field Emission Scanning Electron Microscopy (FE-SEM, FEI Quanta 250 FEG).
High-Resolution Transmission Electron Microscopy (HR-TEM, FEI Tcnai 20) was conducted to obtain significantly high-resolution image of crystal of the crystals and to observe lattice patterns of the crystals by Selected Area Electron Diffraction (SAED) patterns. The average particle size of aragonite seeds was measured by Particle size distribution analyzer (PSD, Otsuka Electronics ELSZ-1000). High-Performance Liquid Chromatography (HPLC, Agilent Technologies 1200s) was performed to measure the concentration of organic additives in coprecipitated aragonite. XRD was conducted using Cu-K? at 40 kV and 100 mA in 2 theta steps of 0.01° with 0.5 °/min scan rate.
SEM analysis was conducted at 15 kV using platinum coated samples. TEM analysis was conducted using Ni-Si grid at acceleration voltage of 200 kV. 1.0 N HCl was used for dissolution the samples before analysis of using HPLC. 2.6 Theoretical species calculationsThe species calculations of organic acids in the experimental conditions were carried out by using the geochemical computer program PHREEQC and the database WATEQ4f.dat containing the data for the organic species.