Polymorphs and morphological changes during dissolution in aging of CaCO₃

1. Introduction

CO₂ concentrations have been increasing in the atmosphere over the past century and are one of the root causes of global warming. The sea is an important reservoir of CO₂ absorption, and CO₂ absorption causes ocean acidification [1]. The formation of inorganic calcium carbonate (ICC) [2] and the deposition of biological CaCO₃ (BCC) are methods of CO₂ capture and deposition in the ocean [3]-[5]. BCC is a component of the shell and outer skeleton of shellfish and corals. A decrease in the pH and acidification of the seawater lowers the concentration of carbonate ions, CO₃^2-, and the supersaturation ratio of CaCO₃. A decrease in the supersaturation ratio lowers the formation of ICC and BCC [1],[3]-[5].

Laboratory synthesis of ICC [6] and the biomimetic deposition of CaCO₃ have been widely studied. Calcium carbonate has three polymorphs, i.e., calcite, aragonite, and vaterite. The polymorphs and morphologies of ICC are influenced by the solution pH, solution temperature, supersaturation ratio, and the added additives. In the formation of BCC, apart from the aforementioned factors, acidic biomolecules are known to be involved in the polymorphs and morphologies at the crystal growth stage [7]-[10].

Thus far, few studies have been conducted regarding the change in ICC and the dissolution in BCC during aging [11]. It has been reported that the calcification rate of corals and seashells is decreasing due to ocean acidification [1][2]. The decrease in the net calcification is reported to cause the reduction in the number seashells and corals [3]-[5].

In this study, we devised an experimental setup to achieve accelerated aging when dissolving calcite crystals due to solution acidification in the laboratory. CaCO₃ synthesis was carried out in a Petri dish, and the formed CaCO₃ was aged for several days in the solution, causing it to be acidic during aging. We observed the pH values of the solutions and polymorphs, and the morphological changes during aging in the absence and presence of polyacrylic acid (PAA).

2. Experimental Setting

The experimental setup was designed to simulate the dissolution of ICC and BCC due to ocean acidification in the laboratory. First, the calcification of CaCO₃ was conducted in an aqueous 50 mL 10 mM CaCl₂ solution with magnetic stirring at 400 rpm. The formed CaCO₃ was aged in the solution. 100 mM tris (tris (hydroxy methyl) amino methane) was added to increase the pH of the solution for the synthesis of CaCO₃. The morphology was controlled using polyacrylic acid (PAA; molecular weight of 2,000 g/mol). The samples were referred to as PAA50 (50 μg/mL = 2.5 × 10^-5 M PAA added) and PAA100 (100 μg/mL = 5 × 10^-5 M PAA added), depending on the amount of PAA added. After calcification, the solution was kept in aging condition. Aging was performed at room temperature while stirring at 400 rpm. The concentration change in the calcium ions in the solution was checked using the ethylenediaminetetraacetic acid (EDTA) titration method. The aging time was evaluated by stirring the solution with a magnetic stirrer. At each stage of calcification and aging, the precipitates were dried and characterized. The morphologies of CaCO₃ were examined using scanning electron microscopy (SEM, Hitachi S-4800) and transmission electron microscopy (TEM, Tecnai20). The crystal structure of CaCO₃ was investigated using X-ray diffraction (XRD) with 0.154 nm Cu Kα radiation (D/max 2500, Rigaku).

3. Results and Discussion

3. 1 Precipitation and Dissolution in the Aging of CaCO₃

This study was conducted to evaluate the accelerated aging of ICC sediment and deposited BCC caused by ocean acidification. As shown in Figure 1 a, the pH of the solution suddenly decreased after one day and then decreased gradually, which was attributed to the absorbance of CO₂ from the atmosphere. The dissolved CO₂ was converted into bicarbonate (HCO_3^-) and carbonate (CO₃^2-) ions. During calcification, calcium ions and carbonates reacted to form CaCO₃, which was deposited on a plate. The addition of the acidic polymeric additive resulted in a slightly lower pH than that in the absence of PAA. Figure 1 b shows the amounts of EDTA titration in the presence and absence of PAA. The titrated EDTA amounts were converted to concentration of calcium ions in the solution, and Figure 1 c shows the concentration change of the measured calcium ions in the solution at different aging times. It is assumed that calcium, excluding the amount of titrated calcium ions, is used to produce calcium carbonate (See Figure 1 d). PAA is usually used as an acidic polymer as it is capable of playing the role of acidic biomolecules in biomimetic mineralization studies in the laboratory. In the absence of PAA, the amount of calcium ions in the solution reached the minimum value after aging for one day. In the presence of PAA, the concentration of calcium ions reached a minimum on the third day of aging, which was slightly higher than that in the absence of PAA. This implies that the formation of calcium carbonate was slowed by the addition of PAA. In addition, the deposition amount of calcium carbonate was less than that without PAA. This implies that the PAA addition reduced the deprotonation of bicarbonate, which is needed for the formation of carbonate ions. This phenomenon slowed the formation of CaCO₃ and finally reduced the amount.

Figure 1:

pH values and the precipitated CaCO3 in the solution at different aging times. a) pH, b) EDTA titration volume, c) Ca2+ concentration, d) the precipitated CaCO3

The dissolution of CaCO₃ rapidly progressed up to five days but slowed down afterwards. For the solutions with PAA, the amount of CaCO₃ was the maximum after only three days of aging. However, the maximum amounts of the synthesized CaCO₃ were lesser than that of the solution without PAA. The dissolution of the synthesized CaCO₃ was initiated after only three days and then the deposited CaCO₃ amounts decreased. Compared to the condition without PAA, the dissolution of CaCO₃ was slower in the presence of PAA. This implies that the added PAA functioned as a buffering component in the solution.

For the case of 100 μg/mL PAA addition, the amount of CaCO₃ decreased rapidly up to 5 days but remained constant until 12 days. The minimum amount of CaCO₃ was ~15 mg and the pH of the solution was in the range of 6.5 to 7 after aging for 12 days. This was due to the dissolved CO₂. Without the addition of the basic component, the concentrations of the carbonate and bicarbonate ions remained relatively unchanged, and chemical equilibrium of the system was achieved. Therefore, there was no longer any noticeable difference in the precipitated amount of CaCO₃.

3. 2 Morphological Change in the Aging of CaCO₃ Formed in the Absence of PAA

Figure 2 shows the morphological changes of CaCO₃ at different aging times in the solution without PAA. As shown in the figure, rhombohedron calcite was dominant at room temperature and atmospheric pressure [12]. The rhombohedron polymorph of calcite was unchanged during aging. However, the cubes began to erode after two days, as observed in Figure 2 b.

Figure 2:

SEM images of CaCO3 in the absence of PAA at different aging times. a) 1 day, b) 2 days, c) 5 days, d) 7 days, e) 9 days, and f) 12 days

When aged for five days, some cubes were eroded inside the cube, and the hole inside enlarged progressively (Figure 2 c). Some appeared to degrade into smaller pieces. When aging had progressed for seven and nine days, erosion had occurred such that the facet was smoothened, and many fragments were found around the flattened crystals, as observed in Figure 2 d and 2 e. This implies that the cleavage of the crystals had majorly progressed in the specific direction [8][9]. On the 12^th day, the facets of the crystals had disappeared significantly, and the shape of the sheet appeared as observed in Figure 2 f. It can be inferred that this sheet was formed by the overlapping of several eroded crystal plates.

The TEM image in Figure 3 a shows that the crystals cracked when aged for five days, and there was some debris around the mother crystal. On the seventh day, the facets had eroded, and some sheet forms appeared (Figure 3 b). Thin, irregularly shaped lumps overlapped, the crystal face disappeared, and some holes were observed inside the lump when aged for 12 days, as shown in Figure 3 c.

Figure 3:

TEM images of CaCO3 in the absence of PAA at different aging times. a) 5 days, b) 7 days, and c) 12 days

Figure 4 a shows the XRD patterns in the absence of PAA at different aging times. After one day, the calcite pattern was dominant. As the diffraction peaks appeared at the same position of two theta degrees, this implies that the polymorph was not altered by aging. The intensities at each peak are shown to be changed at different aging times. The (104) plane corresponded with the peak 2θ = 29.4 in the XRD diffractogram and was dominant in the calcite crystal. The relative peak intensity of the (104) plane to other planes became stronger gradually with aging time. In particular, the intensity ratios significantly increased after 12 days of aging, as observed in Figure 4 b. This implies that the (104) plane of the calcite was worn out and removed by aging. It can be explained that these calcite crystals tended to crack mainly on the (104) plane. This result is in accordance with the SEM observations of the dissolved calcites. Lardge and Wang suggested that Ca²⁺ and CO₃^2- ionic bonds form across the (104) plane, and this bond can be cleavaged through this crystal plane by external stress [13][14].

Figure 4:

a) X-ray diffraction patterns of CaCO3 formed in the absence of PAA during aging. b) Peak intensity ratio to the (104) plane

3. 3 Morphological Change in Aging of CaCO₃ Formed in the Presence of PAA

Only the calcite polymorph was found from the solution in the presence of PAA. As shown in Figure 5, cone structures were dominant. The cone structures developed in two days. They were configured with stacks of nanofibers. In previous studies, researchers have reported mesoscale assembly processes for the development of the cone structures of CaCO₃ and BaSO₄[15][16]. In such studies, nanospheres were formed and assembled into nanofibers, and finally, cone structures were formed [12],[17]-[19]. The nanostructures are believed to be formed first, and then they are assembled into a conical structure. An acidic polymer PAA reacts with the specific face of the growing CaCO₃, and this action controls the direction of growth and the nanostructured fibers of CaCO₃.

Figure 5:

SEM images of CaCO3 in the presence of PAA at different aging times. a) 1 day, b) and c) 2 days, d) 5 days, e) 9 days, and f) 12 days

During the dissolution process of CaCO₃ grown in the presence of PAA, the decomposition of the cone structure and the destruction of nanofibers were not observed macroscopically, despite aging for 9 and 12 days, as can be observed in the SEM images of Figures 5 e and 5 f.

For aging for one day, nanofibers with clean surfaces appeared, as shown in the TEM image (Figure 6 a). Figure 6 b displays a disrupted nanofiber and a couple of fragmented nanoparticles scattered around the fibers after aging for nine days. The nanofibers fragmented into nanoparticles, indicating that the specific cleavage did not develop well. Such crack development behavior differed from that in the absence of PAA. An acidic biomolecule involved in biomineralization is known to control the polymorphs and crystal growth of biominerals [7]-[10]. It has been reported that the involved acidic biomolecule influences the enhancement of the mechanical properties of biominerals [9]-[10],[20]. The added biomolecule causes the biomineralized structures to follow a distorted fracture path when a crack develops, which causes retardation of fracture propagation. In this study, the acidic polymer PAA seems to control not only the crystal growth, but also the dissolution rate and fracture behavior of the biominerals.

Figure 6:

TEM images of CaCO3 at different aging times, prepared with 50 μL/mL of PAA. a) 1 day, b) 9 days

Figure 7 a shows the XRD patterns of CaCO₃ grown in the presence of PAA. The typical XRD pattern of calcite was maintained irrespective of the aging time. The relative intensity of each peak to the (104) plane was significantly higher than that of calcite grown in the absence of PAA. As shown in Figure 7 b, the ratio increased with aging and exhibited a significant increase after nine days. This trend was similar to that in the absence of PAA.

Figure 7:

a) X-ray diffraction patterns of CaCO3 grown in the presence of PAA during aging. b) Peak intensity ratio to the (104) plane

4. Conclusions

The dissolution of CaCO₃ crystals at different aging times were observed in the presence and absence of acidic polymer PAA. In the absence of PAA, the dissolution of CaCO₃ was rapid and definite during aging. The cracks seemed to propagate in the (104) plane leaving large sheet-like debris. In the presence of PAA, cone-structured assembled nanofibers developed. With aging, the nanofibers broke into nanoparticles and showed a complicated crack path. It is considered that the addition of PAA in the CaCO₃ dissolution experiment imparted the resistance to the crack.

This study shows that the aging study of calcium carbonate by ocean acidification can be reproduced in the laboratory. We designed this experimental setup to evaluate the dissolution of ICC and examine the role of acidic biomolecules in biomineralized structures when aging is conducted for a long-time duration.

Author Contributions

Conceptualization, H. C. Lee and S. Lee; Methodology, H. C. Lee and S. Lee; Software, S. Lee; Validation, H. C. Lee and S. Lee; Formal Analysis, H. C. Lee and S. Lee; Investigation, H. C. Lee and S. Lee; Resources, H. C. Lee and S. Lee; Data Curation, S. Lee; Writing—Original Draft Preparation, H. C. Lee and S. Lee; Writing—Review & Editing, H. C. Lee and S. Lee; Visualization, H. C. Lee and S. Lee; Supervision, S. Lee; Project Administration, S. Lee; Funding Acquisition, S. Lee.

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