Chemical and Morphological Analysis of Calcium Oxide (CaO) Powder from Sea Urchin (Diadema setosum) Shell

Calcium carbonate (CaCO3) has been taken from sea urchin (Diadema setosum) shell and calcinated at several temperatures to obtain calcium oxide (CaO). X-Ray Fluorescence Spectroscopy (XRF) revealed that the powder from this shell had a high Ca level, which increased at a higher temperature. Fourier Transform Infrared Spectroscopy (FTIR) spectra agreed to an increase in Ca level that showed the presence of CaO at 900 °C and 1100 °C; otherwise, CaCO3 disappeared. Using Scanning Electron Microscopy (SEM), the powder morphology was more homogeneous at 900 °C and 1100 °C than at lower temperatures. This morphology was encouraged by decreasing particle size, indicating compound decomposition in the powder and Ca leaved. This decomposition was confirmed by an Energy Dispersive X-ray Spectroscopy (EDS) analysis that showed increased Ca content with higher mass and atomic level at a higher temperature.


Introduction
Sea urchin (Diadema setosum) shell is the source of Ca in the form of calcium carbonate (CaCO3), which highly potential for synthesizing CHAp [1], like other natural shells such as common cockle shell [2], cockle shell [3], and oyster shell [4] with high of Ca level. Calcium (Ca) contribute to several functions in the human body [5]. Ca can be used for biomedical purposes, even several environmental technologies [6,7]. In other applications, CaCO3 is decomposed into calcium oxide (CaO), which shows potential as biomaterials for bone and teeth implants [8]. Transforming CaCO3 into CaO needed a calcination process with calcination temperature that depended on the nature of the compounds before the shell was calcinated [9]. In the previous study, the abalone mussel shell had a Ca level of 33.58% before calcination at 1000 o C, and the Ca level increased up to 56.32% [10]. In another study, green mussel shell has been reported Ca level 30.43% at calcination temperature 600 o C, then increase until 49.58% at 950 o C which higher and potential as a precursor for hydroxyapatite synthesis, at this calcination temperature indicated breaking of chemical bond of CaCO3 into CaO is faster, confirmed by infrared spectra analysis [11]. Calcination temperature can affect the characteristics and phase transition of CaO [9]. Previous research has studied the effect of calcination temperature at 850 o C for aquatic materials, including oyster shells, mussel shells, and cuttlebone, and obtained optimum material conditions [12]. Another study reported that crab shell was calcinated at a temperature of 700 o C and obtained appropriate material for synthesizing hydroxyapatite [13]. In this study, sea urchin shell was used as raw materials. We analyzed the effects of calcination temperature on the powder's chemical and morphological characteristics derived from the shell.

Materials and Methods
Synthesis of CaO Powder. Sea urchin shells were taken from Bali. The shells were washed with distilled water and boiled for 30 minutes, then sun-dried for 8 hours. The shells were soaked in acetone for 24 hours and then sun-dried for 6 hours. As dried, to obtain CaCO3 powder, the shells were milled for 30 seconds in 3 times repetition. The powder was calcined in a furnace for 6 hours with temperature variations of 600 o C, 800 o C, 900 o C, and 1100 o C to obtain pure CaO.
Characterization of CaO Powder. The chemical composition of powders was characterized by X-Rays Fluorescence (XRF [RIGAKU-NEX + QuanTEZ]), including atomic level. The functional group of powders were characterized by FTIR (Thermo Nicolet iS10, Japan); this FTIR characterization used a range of wavenumber 650-4000 cm -1 . The morphology of powders was observed using Scanning Electron Microscopy (SEM, Joel JSM-6510LA-1400, Japan) with an accelerating voltage of 15 kV and magnification of 1000 times. Powders' particle size distribution was calculated using ImageJ by measuring 60 random particles. To determine the composition of calcium and other compounds in powders, used Energy Dispersive X-ray Spectroscopy (EDS) that included while performing SEM.

Results and Discussions
The Ca level of the powders was characterized using XRF. As shown in Table 1, calcinated powder at 900 o C and 1100 o C had the highest Ca level; with the same value. Ca level significantly increased when calcination temperature increased, but at 1100 o C, the increase became slow. Fig. 1(a)-(d) shows that the powders had an increasing intensity of Ca-Kα while calcination temperature increased, which confirmed the increase in Ca level. As shown in Fig. 1(a) and (b), the intensity of the Ca-Kα peak slightly increased. As in Fig. 1(c) and (d), the increase in Ca-Kα intensity peak was significant compared with lower temperature. Sr impurity remained at a superficial level in all temperatures, not more than 1,2%.    [14,15]. The transmittance of C-O and C=O bonds increased while calcination temperature increased, which meant the CaCO3 level decreased. The C=O bond disappeared, then another C=O bond appeared at 2049.96 cm -1 at 900 o C, and the transmittance became lower at 1100 o C. OH-stretching functional group appeared for the first time at 3639.98 cm -1 at calcination temperature 800 o C, and transmittance became lower at higher calcination temperature, which confirmed the increase of CaO level according to the OH group. CaO bond functional group was present at 874.56 cm -1 at 900 o C, and the transmittance became lower at 1100 o C, which meant the CaO level increased. The increase in CaO was not significant; this could be seen in Fig. 2

(c) and (d).
This result concluded that CaCO3 was decomposed into CaO triggered by the calcination process, with 900 o C as the minimum temperature for the decomposition process [10]. CaCO3 received heat, causing atoms to move faster. This condition led breaking process of the CaCO3 chemical bond into CaO and CO2; at higher calcination temperature chemical bond is broken more quickly [11,16,17].  Fig. 3(a1,a2)-(d1,d2) shows the powders' morphology and particle size distribution characterized using SEM. It is shown that calcinated powder at higher temperatures caused smaller particle sizes. As shown in Fig. 3(a1,a2), the particle distribution was heterogeneous at 600 o C, and the particle size was large, about 8.10 μm. Fig. 3(b1,b2) shows that particles had been reduced into smaller particles size of 7.28 μm but remained with irregular and ununiform shapes at 800 o C. As seen in Fig. 3(c1,c2)-(d1,d2), the particles quickly became homogeneous and orderly distributed at higher calcination temperatures, at 900 o C and 1100 o C, with smaller particle sizes up to 4.78 μm. But at those Engineering Chemistry Vol. 3 temperatures, the particles had interconnected and agglomerated shapes in a solid structure, indicating optimum calcination temperatures. This particle size reduction defined particle compounds' decomposition at the atomic level, indicating that the calcination process released CO2 and created cavities [10]. It could be seen at 1100 o C; cavities created are more than 900 o C. In those temperatures, the distribution and size of particles were almost the same or slight differed compared with lower temperatures.   4 show the EDS spectra of the powders. It is shown in Fig. 4(a)-(d) that at higher calcination temperatures, CaKα and CaKβ had a higher peak, which meant the Ca level was increasing. The mass and atom count percentage of Ca obtained during characterization was performed in Table 2, which shows an increasing value which confirms the decomposition process of another compound and leaves Ca caused decreasing in particle size in SEM analysis. While the temperature was raised, decomposition occurred faster; this heat helped optimize decomposition [18]. At 900 o C and 1100 o C, the increase of Ca was low, indicating a slower decomposition process.

Conclusions
In this experiment, CaO was synthesized from sea urchin shells through calcination. The XRF analysis showed the powder had the highest Ca level, up to 96% at 900 o C and 1100 o C. FTIR results revealed changes in functional groups indicated phase transition from CaCO3 into CaO, CaO was present for the first time at 900 o C, and Ca level became higher at 1100 o C. The infrared spectra showed that 900 o C was possibly being minimum calcination temperature for CaO appearance through the decomposition process. SEM results showed particle distribution became more homogeneous and orderly distributed in higher calcination temperatures with smaller particle sizes, indicating the decomposition of powder leaving Ca content. This SEM analysis was confirmed by the EDS result that showed the percentage of Ca increased while calcination temperature increased. From all those analysis results, increasing Ca was fast until 900 o C and became slow when it reached 1100 o C. This high Ca content encourages the possibility for biomaterial application, certainly in the synthesis of hydroxyapatite (HAp) or carbonated hydroxyapatite (CHAp).