Precipitated CaCO (PCC) can be synthesized or derived from natural resources or using chemicals. One of the carbonate-based natural resources is limestone. Indonesia has abundant carbonate reserves , including Madura island. Limestone in Madura Island only has low value . PCC has a full application in many industries; as a filler in paper, rubber, and plastic material, coatings [3,4]. The calcite phase PCC is widely used in the industry because of its superior appearance and bright colors .
The synthesis of PCC from limestone has also been carried out by researchers [6–11]. Limestone that has been widely used for the synthesis of CaCO is calcite [7,8] and dolomite [6,9,12,13]. No researcher has tried the synthesis of limestone-based PCC with an ankerite phase. The ankerite phase is polymorph from dolomite and kutnohorite . Limestone with an ankerite phase has the potential to be used as primary material for making PCC to be more valuable.
One way for the synthesis of PCC is bubbling method. Many researchers have used it [15–18]. The bubbling method is to flow CO gas into spherical bubbles to the solution. This method is low cost and easy procedure . The primary objective of the study is synthesizing of PCC used ankerite from Ambunten, Madura island (Indonesia).
2. Material and Methods
Calcium oxide (CaO) prepared from Ambunten's calcined limestone with Ca 95.37 wt% and ankerite phase  (Fig. 1a). CaO and aquades were dissolved and stirred to form 1 M Ca(OH) solution. Ca(OH) solution was precipitated for 24 hours. Then a clear solution of Ca(OH) is separated from the precipitate and the top layer.
PCC synthesis uses the bubble method by flowing CO gas into a clear Ca(OH) solution CO gas flow rate is 8 L/min and terminated when the pH of the solution has reached 7. The solution was filtered and dried in the oven at 105 C for 3 hours.
The crystalline phases of synthesized PCC powder were analyzed using X-Ray Diffraction test X-pert MPD with Cu-Kα radiation (λ = 1,5406 Å) and scanned from 10 to 60 . The crystal size of PCC was analyzed using Scherrer equation (Eq. 1):
with D is average crystallite size (nm), λ is X-Ray wavelength (nm), B is line broadening (radian), and θ is Bragg angle ( ).
The functional groups of PCC were tested using Fourier Transform Infra Red Thermo Scientific Nicolet iS10 and taken in the 500 – 4000 cm . The morphology was observed by Scanning Electron Microscope FEI Inspect S50 X'pertPRO PANalytical.
3. Results and Discussion
The flow of CO gas bubbles into the Ca(OH) solution is called the carbonation process so that the CO gas flow rate is one parameter in the formation of CaCO . After flowing CO gas, the newly formed CO reacts with Ca to form CaCO . The formation of CaCO formed initially is amorphous and transforms rapidly to form rhombic calcite . The carbonation process is ended if the pH of the solution has not changed. Carbonation reaction following previous research [15,16].
Crystal phase of precipitated CaCO (PCC)
The limestone diffraction pattern of Ambunten is shown in Fig. 1 (a) which shows all phases of ankerite and has been reported . Precipitated CaCO powder diffraction patterns produced by the bubble method are presented in Fig. 1 (b). Qualitative analysis using SEARCH and MATCH exhibits 100 % calcite phase (JCPDS no. 96-900-9668). Calcite is seen in 2 theta 23.02, 29.33, 31.36, 35.92, 39.43, 43.15, 47.05, 47.41, 48.43, 56.50, 56.60, 57.34, 57.47 and 57 . Characteristic of calcite at 2 theta values 29.33, corresponding to hkl 1 0 4, with trigonal (hexagonal axes) crystal systems and the lattice parameter of a = b = 4.9910 Å and c = 17.0680. The same results have also been reported , [16,19,20], . PCC powder crystal size is calculated using equation (1) with the mean of 43,85 nm. The crystal size distribution is shown in Fig. 2. Most of the crystal sizes are in the range 43 – 53 nm.
Analysis by Fourier transform Infra Red (FTIR)
Fig. 3 shows the FTIR spectra of synthesized PCC using the bubbling method. Six bands are visible, 420, 712, 873,1393, 1793, 2040 and 2360. Characteristic of calcite are seen at 712 and 873 [3,17,19], . Calcite has two absorption bands characteristics in the out of bending plane v 875 cm and doubly degenerate planar bending v 713 cm .
Observation by scanning electron microscope (SEM)
Morphology of the synthesized PCC powder using bubbling method shown in Fig. 4. Morphology of calcite is rhombohedral cubic. It is the same as previously reported [5,16], , . Calcite is the most stable polymorph of CaCO compared to vaterite and aragonite [5,25] at ambient conditions (temperature and pressure). In this study, calcite obtained formed at room temperature with the CO gas flow rate of 8 L / min.
In summary, the synthesis of precipitated CaCO (PCC) from ankerite using bubbling method has been successful. The PCC formed has a 100% calcite phase with morphology rhombohedral cubic. The average size of PCC powder crystals is 43.85 nm. The formation of PCC is also detected from the presence of the CaCO group from FTIR test results at 875 and 713 cm .