Simple Synthesis of Calcium Carbonate Nanoparticles and Calcium Carbonate-Silver Nanocomposites with Three Various Micro-Wave, Ultrasonic Waves and Hydrothermal Methods

INTRODUCTION
Calcium carbonate (CaCO3) is the most abundant mineral in nature, which makes it a cheap and inorganic substance. Synthetic forms of CaCO3 in oil, paints, paper, plastics, coatings, environmentally friendly materials, calcium-enriched foods, Portland cement and steelmaking, as agricultural inputs for soil amendment and food additives in soy milk and products Dairy has many uses. In addition to various industrial applications as an alloying material, calcium is also an essential biological element found in bones, teeth, and shells. Calcium carbonate nanoparticles are synthesized through precipitation of calcium nitrate and saturated sodium carbonate solution. Catalysts activated in the presence of light are known as photocatalysts, that is one of application of advanced oxidation processes (AOP) and has been widely used in many fields, such as settling the energy problem, destruction the synthetic dyes, eliminating greenhouse effect and so on. The use of nano-scale photocatalysts increases the photocatalytic efficiency but nano size substances can cause irreparable environmental hazard. Due to of the nanoparticles size, separating this materials upon completion of the photocatalytic reaction via ordinary filtration procedures are impractical. Unique features emerge when the particle size is less than critical size; this size depends on the nature of the material. Various photocatalytic are prepared and investigated for potential application for the decay of biological wastes. Photocatalytic materials reduce organic compounds by advance oxidation process. Hospitals treat microbial contaminated wastewater by either way such as autoclaving and irradiation methods for remove or inactivate prior to discharge. All these systems have their own advantages and limitations. In the last decade, photocatalysis, a new technology which is based on advance oxidation processes and is capable of producing various reactive kinds such as oxygen radicals and hydroxyl radicals have attracted the attention for the degradation of bio-organic wastes [1-10].
This research purpose is presentation an efficient photocatalysts for degradation dyes under ultra violet or sun light radiation. In this work, nanocomposite was used to degrade methyl orange dye. Photocatalytic reactions are carried out in a heterogeneous system with solvent; Finding a low-cost photocatalyst material is a very important factor in selecting precursors for industrial acceptance.

MATERIALS AND METHODS
Materials
XRD patterns were recorded by a Philips, X-ray diffractometer using Ni-filtered CuKα radiation. Size and morphology of nanoparticles were investigated via scanning electron microscopy (model MIRA TESCAN). The golden thin film was used as a conductive coating on the surface of the samples (Prevent the accumulation of electrical charge and make better contrast(. Magnetic attributes were studied using a vibrating sample magnetometer (VSM) at room temperature in an applied magnetic field sweeping between ±10000 Oe. 
About 100 kg of samples in three types with different grades of RS-A (Rostami Sayeh Seng- Kar Front A (Abbasabad Mahallat)), RS-B (Rostami Sayeh Seng- Kar Front B (Abbas Abad Mahallat)) and MR (Mehrgan Mine) - Hamedan) was prepared from the mine and sent to Arak University of Technology. The size of most of the larger stones brought was 15 cm. With the help of jaw and roller crusher, all three samples were converted into millimeter dimensions.

Preparation of calcium carbonate from calcium oxide
In the first step calcium carbonate was clacinated at 700 0C for preparation of calcium oxide. Then 20 grams of prepared calcium oxide was added little by little in distilled water and stirred well. At this stage, calcium oxide is dissolved in water and impurities remain. This step is considered a purification step. A small amount of calcium oxide can be dissolved in distilled water and produce calcium hydroxide. Carbonation is a chemical reaction in which calcium hydroxide reacts with carbon dioxide according to the following formula and forms insoluble calcium carbonate. Impurities and large amounts of calcium oxide in the previous step were not dissolved in distilled water and settled. These solids were separated from the resulting solution. To form calcium carbonate precipitate, we added CO2 to the desired solution and gave the desired carbon dioxide gas to the desired solution, solid carbon dioxide (dry ice): We added dry ice to the solution and used a magnet to stir the solution.

Preparation of calcium carbonate from recrystallization
2 g of rock sample was dissolved with 3 ml of acid (HCl). Solving the sample with acid and doing the opposite reaction, some impurity is formed, and due to the fact that calcium chloride is dissolved in the acid, the impurities settle and are separated from the above solution with the help of filter paper.
Sodium hydroxide was used to neutralize the solution, it was added little by little to the solution containing the sample and checked until the solution turned from acidic to neutral. 25 ml of the prepared sodium carbonate (or ammonium carbonate) 0.1M solution was added to these two solutions separately while stirring. Then it was placed in the microwave for 10 minutes (On:20s, Off:20s) . Then it was placed in a centrifuge and the deposited sediment was separated and placed in the oven.
The previous steps were repeated and ammonium carbonate (or sodium carbonate) was added to both solutions and placed inside the hydrothermal (200 0C, 12 h). Then it was placed in a centrifuge and the deposited sediment was separated and placed in the oven.
On the third reaction the solutions was placed in the ultrasonic device (1200W, 60 min). Then it was placed in a centrifuge and the deposited sediment was separated and placed in the oven.
In this method, all steps are similar to the second method, with the difference that instead of sodium carbonate, ammonium carbonate is used for sedimentation. As results confirm ammonium carbonate show higher purity compare to other precipitating agents. 
By using all three methods (microwave, hydrothermal and ultrasonic) and by adding ammonium carbonate solution (ammonium carbonate in distilled water: 0.1 M) the following reaction occurs and finally calcium carbonate precipitate is formed which is in the form of nanoparticles and has no impurities is.

For preparation of nanocomposite 1 g of calcium carbonate nanoparticles that were achieved by microwave method was dispersed in 200 ml of distilled water, after that 1 g of silver nitrate was dissolved to the solution, after 5 h of mixing, 1 g of lactose as a capping agent was added to the solution, finally by heating at 90 0C, ammonia (10M) as a reducing agent was added, Final precipitated was washed and dried at oven (60 0C) for 24 h. 

RESULTS AND DISCUSSION
Fig. 1 shows phase characterization of carbonate calcium precursor. Elemental characterizations of three different precursors (MR, RS-A, RS-B) are shown in Fig. 2. Both analysis show high purity of the precursor, for obtaining higher purity this precursor was dissolved and was precipitated under different conditions.
Fig. 3 illustrate the X-ray diffraction patterns of pre-cursor nanocrystals obtained from mine, all the diffraction peaks at 2θ values of 23°, 29°, 36°, 39° , 43°, 47°, 49° and 57° are indexed to lattice planes of cubic CaCO3 spinel structure (JCPDS NO. 05-0586). It shows the results obtained from X-ray diffraction pattern that approve high percent purity (about 99 percent) of the precursor. 
Fig. 4 illustrates XRD pattern of calcium carbonate nanoparticles obtained by sono-chemical method. As we expected by ultrasound irradiation, nanoparticles are synthesized and because of power of collapsing of bubbles crytallinty was decreased. 
Fig. 5 shows XRD pattern of calcium carbonate nanoparticles synthesized under micro-wave irradiation. All the diffraction peaks are indexed to pure calcite structure (JCPDS NO. 72-1652). Results confirm microwave irradiation improve growth and crystallinty. Fig. 6 depicts XRD pattern of calcium carbonate nanoparticles synthesized with hydrothermal procedure.  By hyrothermal method, both calcite phase (JCPDS NO. 72-1652) and aragonite phase (JCPDS NO. 41-1475) were synthesized under high pressure condition. 
Fig. 7 depicts FT-IR absorption of calcium carbonate nanoparticles synthesized under micro-wave irradiation. The absorption peak around 700 cm-1 is ascribed to the Ca–O bonds, the absorption peaks about 3420 cm-1 is ascribed to O–H bonds on the surface of nanoparticles. The bonds for each absorptions peak are shown in the figure. A sharp peak near 900 cm−1 is assigned to C–O vibration. The absorption about 1600 cm-1 is assigned to C=O vibration and the absorption peaks at 2900 cm-1 is assigned to C–H vibration. FT-IR absorption of calcium carbonate nanoparticles synthesized with hydrothermal procedure is shown in Fig. 8. Results show the same characterization absorption peaks in the same regions. 
Morphological characteristics of the samples, the as-prepared pure calcium carbonate nanoparticles prepared using different methods were investigated by scanning electron microscopy. SEM images of CaCO3 nanoparticles are shown in Fig. 9. According to images, the average particles size about 80 nm was estimated. It seems that the use of microwave irradiation is more suitable for the preparation of pure nanoparticles. Hot spots that were prepared by microwave (or ultrasonic) methods prevent agglomeration and leads to smaller nanoparticles. SEM images of CaCO3-Ag nanocomposite are illustrated in Fig. 10 that approve average particle size are less than 100 nm. Cubic nanostructures with some agglomeration were synthesized, results approve by hydrothermal reaction (high pressure and temperature) preferential growth was observed; Images confirm cubic nanostructures has suitable surface to volume ratio for catalytic applications. 
The photocatalytic activity of the CaCO3–Ag nanocomposite was evaluated. Fig. 11 illustrates the effect of radiation time on methyl orange photo-degradation in presence of CaCO3–Ag nanocomposite. According to the energy gap of (Eg = 2.82 eV), visible light can cause the formation of electron-hole pairs in the matter. The conduction band position is lower (less negative) than the oxygen redox potential, therefore the electrons in the conduction band cannot to produce superoxide anions but via hydrogen peroxide produced hydroxyl radicals. Also, the valence band position is lower than the water oxidation potential and the formation of radical hydroxyl. Therefore, has a high oxidation capability and is able to oxidize water and create active radical species. 
The relative location of the conduction bands and valence bands of materials, create ideal conditions for the transfer of electrons Therefore, the amount of hydrogen peroxide in the photocatalyst atmosphere increases. Hydrogen peroxide reacts with the electron and leads to the formation of radical hydroxyl. With increasing radical production, photocatalytic degradation increases. 
Mechanisms of degradation could be on the following mentioned, first of all oxidative stress induction; in which the active radicals, cause the oxidation process in the cells that leads destroy proteins, reduce the activity of special enzyme, and finally interact with DNA. Maybe metal ions are infiltrate into the cell and directly interact with the –NH , –SH and –COOH group of nucleic acid and protein. Inactivation of bacteria’s via decreasing the important cellular metabolism such as energy, amino acid, carbohydrate, protein, and nucleotide without oxidative stress infusion [11-13]. As shown in the Figs. 12 with the creation of composites, the antibacterial properties have been improved due to the simultaneous presence of active radicals and silver.

CONCLUSION
CaCO3 and CaCO3-Ag nanocomposites were synthesized by various procedures, hydrothermal, sono-chemical and micro-wave irradiation. Photocatalytic performance was appraised for degradation of methyl orange dye under visible light. The photocatalytic degradation of the CaCO3-Ag nanocomposite was examined by the de-coloration reaction of methyl orange in an aqueous solution. This work introduces a novel smart photocatalyst for purification of toxic aromatic dyes from water.

CONFLICT OF INTEREST
The authors declare that there is no conflict of interests regarding the publication of this manuscript.