Recrystallization of Triple Superphosphate Produced from Oyster Shell Waste for Agronomic Performance and Environmental Issues

: Calcium dihydrogen phosphate monohydrate (Ca(H 2 PO 4 ) 2 · H 2 O) (a fertilizer) was successfully synthesized through a recrystallization process using prepared triple superphosphate (TSP) derived from oyster shell waste as the starting material. This bio-green, eco-friendly process to produce an important fertilizer can promote a sustainable society. The shell-waste-derived TSP was dissolved in distilled water and kept at 30, 50, and 80 ◦ C. Non-soluble powder and TSP solution were obtained. The TSP solution fractions were then dried, and the recrystallized products (RCP30, RCP50, and RCP80) were obtained and conﬁrmed as Ca(H 2 PO 4 ) 2 · H 2 O. Conversely, the non-soluble products (NSP30, NSP50, and NSP80) were observed as calcium hydrogen phosphate dihydrate (CaHPO 4 · 2H 2 O). The recrystallized yields of RCP30, RCP50, and RCP80 were found to be 51.0%, 49.6%, and 46.3%, whereas the soluble percentages were 98.72%, 99.16%, and 96.63%, respectively. RCP30 shows different morphological plate sizes, while RCP50 and RCP80 present the coagulate crystal plates. X-ray diffractograms conﬁrmed the formation of both the NSP and RCP. The infrared adsorption spectra conﬁrmed the vibrational characteristics of HPO 42 − , H 2 PO 4 − , and H 2 O existed in CaHPO 4 · 2H 2 O and Ca(H 2 PO 4 ) 2 · H 2 O. Three thermal dehydration steps of Ca(H 2 PO 4 ) 2 · H 2 O (physisorbed water, polycondensation, and re-polycondensation) were observed. Ca(H 2 PO 4 ) 2 and CaH 2 P 2 O 7 are the thermodecomposed products from the ﬁrst and second steps, whereas the ﬁnal product is CaP 2 O 6 .

In the agricultural industry, calcium phosphate, an essential fertilizer, is called triple superphosphate (TSP) [3]. TSP fertilizer in the form of Ca(H 2 PO 4 ) 2 ·H 2 O (or MCPM) provides a high concentration of phosphorus (P) (more than 40% of diphosphorus pentoxide, P 2 O 5 ). It was widely used in the 20th century and became the most common phosphorus source used in many countries until the mid-1970s for plant development [4]. Phosphorus is one of the macronutrients employed for plant growth that plays an important role in many physiological and biochemical plant activities [5]. One of the advantages of feeding plants phosphorus is creating deeper and more abundant roots, early maturation, decreasing grain mold, increasing chlorophyll content in leaves, and positively affecting photosynthesis, resulting in the improvement of crop quantity and quality [6]. However, the agronomic performance of the fertilizer for plants is limited by its insolubility in water, which causes a very slow phosphorus delivery rate. To increase the soluble phosphorus contents and purity of the TSP fertilizer, it can be recrystallized to obtain soluble Ca(H 2 PO 4 ) 2 ·H 2 O. TSP is firstly dissolved in water, and a filtration technique is then employed to remove the insoluble fraction from the solution. When the dissolved cation and anion species are recrystallized, pure water-soluble Ca(H 2 PO 4 ) 2 ·H 2 O can be recovered [7]. Owing to its high solubility, Ca(H 2 PO 4 ) 2 ·H 2 O can deliver phosphorus at a higher rate [7]. Readily soluble phosphorus has the potential to supply phosphorus to plants immediately after application. When the fertilizers (i.e., Ca(H 2 PO 4 ) 2 ·H 2 O) are used, the moisture that exists in the soil is absorbed by Ca(H 2 PO 4 ) 2 ·H 2 O (in both granular and powder forms). Ca(H 2 PO 4 ) 2 ·H 2 O is dissolved, leading to the release of phosphorus fertilizer into the soil, followed by the uptake of plants for growth and development.
There are many synthesis methods in the production of calcium phosphates such as hydrothermal [4,6], microwave-assisted [8], precipitation [9,10], sol-gel [11], and electrochemical [12] techniques. The method used in each case depends on the desired type of morphology, structure, and chemical composition. In general, calcium phosphates have been synthesized from phosphoric acid (H 3 PO 4 ) and commercial calcium (Ca 2+ ) sources [7]. However, using shells (one of the important wastes observed in many countries) as a raw Ca 2+ -source material can reduce the cost of calcium phosphate production [13]. Oyster, one of the bivalve mollusks, can be found in coastal areas where brine water mixes with fresh water. Thailand is one of the oyster-rich countries. Eastern and southern parts of Thailand are areas where oysters can be found, and the two highest oyster production provinces are Chonburi (Eastern of Thailand) and Surat Thani (Southern of Thailand). Crassostrea iredalei, Saccostrea cucullata, and Crassostrea belcheri are the three major oyster species widely cultivated [14]. In 2019, Thailand's coastal aquaculture produced about 17,903 tons of oysters [15]. The results by Chilakala et al. [15] indicated that the major phase of raw oyster was CaCO 3 (92.6 wt% calcite and 2.71 wt% aragonite), which is an important source of calcium (Ca 2+ ). The X-ray fluorescence (XRF) results reported by Chilakala et al. [15] also demonstrated that the chemical composition of an oyster was mostly calcium oxide (CaO) (53.66 wt%). Furthermore, oxides of silicon (SiO 2 , 0.45 wt%), aluminum (Al 2 O 3 , 0.12 wt%), iron (Fe 2 O 3 , 0.06 wt%), magnesium (MgO, 0.26 wt%), potassium (K 2 O, 0.06 wt%), sodium (Na 2 O, 0.55 wt%), titanium (TiO 2 < 0.01 wt%), manganese (MnO, 0.01 wt%), and phosphorus (P 2 O 5 , 0.16 wt%) were also observed [15].
Consequently, recycling waste materials (i.e., oyster shells) by applying a Ca 2+ source (in the form of CaCO 3 ) is beneficial for reducing waste and most useful for various industrial uses. The aim of this work is to recrystallize the soluble calcium phosphate (Ca(H 2 PO 4 ) 2 ·H 2 O) from the synthesized triple superphosphate (TSP) compound. Firstly, the oyster-shell waste was used as the precursor to prepare CaCO 3 . This oyster-derived CaCO 3 then reacted with H 3 PO 4 , resulting in the formation of TSP. After that, the obtained TSP compounds were dissolved in deionized water at three different temperatures. The non-soluble part was filtrated, whereas the solution part was dried, resulting in the recrys-tallization (formation) of Ca(H 2 PO 4 ) 2 ·H 2 O. This obtained Ca(H 2 PO 4 ) 2 ·H 2 O exhibits the desired properties, namely high crystallinity, high purity, and high solubility. To confirm the formation of the target Ca(H 2 PO 4 ) 2 ·H 2 O material, the physicochemical properties of the recrystallized products were identified and characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), thermogravimetric/differential thermal analysis (TG/DTA), and scanning electron microscopy (SEM). In addition, the recrystallized yield and the solubility of Ca(H 2 PO 4 ) 2 ·H 2 O were also calculated to demonstrate the influence of the operating condition on the properties of the synthesized Ca(H 2 PO 4 ) 2 ·H 2 O compound. By successfully producing Ca(H 2 PO 4 ) 2 ·H 2 O, we have provided an efficient method to produce this valuable compound and also demonstrated a way to recycle oyster shell waste, which will lead to a higher income and a reduction in oyster shell waste to reduce environmental issues.

Preparation
Oyster shell waste was used as the starting material (CaCO 3 as Ca 2+ source) to prepare the triple superphosphate (TSP). After that, the oyster-shell-derived TSP compound was then used as the precursor to recrystallize Ca(H 2 PO 4 ) 2 ·H 2 O (MCPM). Figure 1 shows the schematic diagram designed for MCPM preparation. CaCO3 then reacted with H3PO4, resulting in the formation of TSP. After that, the obtained TSP compounds were dissolved in deionized water at three different temperatures. The non-soluble part was filtrated, whereas the solution part was dried, resulting in the recrystallization (formation) of Ca(H2PO4)2·H2O. This obtained Ca(H2PO4)2·H2O exhibits the desired properties, namely high crystallinity, high purity, and high solubility. To confirm the formation of the target Ca(H2PO4)2·H2O material, the physicochemical properties of the recrystallized products were identified and characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), thermogravimetric/differential thermal analysis (TG/DTA), and scanning electron microscopy (SEM). In addition, the recrystallized yield and the solubility of Ca(H2PO4)2·H2O were also calculated to demonstrate the influence of the operating condition on the properties of the synthesized Ca(H2PO4)2·H2O compound. By successfully producing Ca(H2PO4)2·H2O, we have provided an efficient method to produce this valuable compound and also demonstrated a way to recycle oyster shell waste, which will lead to a higher income and a reduction in oyster shell waste to reduce environmental issues.

Preparation
Oyster shell waste was used as the starting material (CaCO3 as Ca 2+ source) to prepare the triple superphosphate (TSP). After that, the oyster-shell-derived TSP compound was then used as the precursor to recrystallize Ca(H2PO4)2·H2O (MCPM). Figure 1 shows the schematic diagram designed for MCPM preparation. The waste was collected from a local shellfish market in Chonburi, Thailand, and was cleaned with sodium hypochlorite (NaOCl, saturated solution) until the shell's meat was completely destroyed. After the cleaning process, the shells were kept at 110 °C in an oven for 60 min. The dried shells were milled and then sieved by a 100-mesh sieve. Using this process, approximately 140 μm oyster-shell-derived CaCO3 powders were achieved and The waste was collected from a local shellfish market in Chonburi, Thailand, and was cleaned with sodium hypochlorite (NaOCl, saturated solution) until the shell's meat was completely destroyed. After the cleaning process, the shells were kept at 110 • C in an oven for 60 min. The dried shells were milled and then sieved by a 100-mesh sieve. Using this process, approximately 140 µm oyster-shell-derived CaCO 3 powders were achieved and used as a raw material. A concentration of 50% w/w H 3 PO 4 prepared from 85% w/w industrial grade H 3 PO 4 was also used as a reagent for the TSP preparation. The process described by Seesanong et al. [16] was used to prepare the TSP. A volume of 13.97 mL of 50% w/w H 3 PO 4 was added to a beaker containing 10 g of shell-derived CaCO 3 powder. The mixed suspension was stirred at 100 rpm until carbon dioxide (CO 2 ) gas was not evolved (approximately 30 min). The pale yellow-white product was obtained and exposed to ambient conditions for 12 h to dry. Equation (1) shows the chemical reaction between oyster-shell-derived CaCO 3 and H 3 PO 4 , and TSP (Ca(H 2 PO 4 ) 2 ·H 2 O) was then synthesized from this reaction.
Afterward, 1 kg of synthesized TSP was then dissolved in 3 L of deionized water and kept at three different temperatures, namely 30, 50, and 80 • C for 2 h. According to the hydrolyzed reaction, Equation (2), TSP was dissolved, while phosphoric acid (H 3 PO 4 ) and calcium hydrogen phosphate (CaHPO 4 ) species were generated from this hydrolyzed reaction, which are the intermediates used to recrystallize and obtain MCPM (Ca(H 2 PO 4 ) 2 ·H 2 O).
However, a fraction of TSP did not dissolve in water. Therefore, the resulting solution was filtered by filter paper using a Buchner funnel under reduced pressure. After filtration, the non-soluble powders on the filter paper were dried at 30 • C for 1 h, and the non-soluble powders processed at 30, 50, and 80 • C were labeled NSP30, NSP50, and NSP80, respectively.
In the case of the soluble fraction, the obtained TSP solutions were kept and dried at 30 • C for 3 h. Using this drying process, the recrystallized Ca(H 2 PO 4 ) 2 ·H 2 O (MCPM) products were obtained. The recrystallized products prepared at 30, 50, and 80 • C were coded as RCP30, RCP50, and RCP80, respectively. The percent yields of the recrystallized products were calculated using Equation (3).
where m obs is the mass of the synthesized Ca(H 2 PO 4 ) 2 ·H 2 O powder from each recrystallized temperature and m theor is the mass of the theoretical Ca(H 2 PO 4 ) 2 ·H 2 O (mass before recrystallization process).

Characterization
Dried powder of the recrystallized product was then ground by the ball mill technique at 60 rpm for 15 min under an ambient temperature. The solubility performance of the recrystallized samples was determined by adding 1 g of each sample into 100 mL of deionized water with the stirring mode at 60 rpm for 1 h. The resulting mixture was then filtered by a Buchner funnel, dried at 60 • C for 6 h, and weighed to determine the solubility value of the recrystallized samples.
The non-soluble powders (NSP30, NSP50, and NSP80) and the recrystallized products (RCP30, RCP50, and RCP80) were characterized by X-ray powder diffractometer (XRD, D8 Advance, Bruker AXS, Karlsruhe, Germany) to identify the structure, purity, and crystallinity of the prepared samples. Two theta (2θ) angles ranged from 5 • to 60 • with the Cu-K α X-ray source. The selected scan speed was 1 s per step at a 0.01 • increment. The International Centre for Diffraction Data (ICDD) database was used to compare and confirm the crystal structure and purity of samples obtained from experimental XRD results. Surface morphologies of samples were observed by a scanning electron microscope (SEM, FEI Quanta 250, Hillsboro, OR, USA) with energy-dispersive X-ray (EDX) mode. This EDX technique was used to evaluate the elemental compositions of samples. The gold-coated technique was used for sample preparation [17]. Infrared adsorption spectra were recorded on the Fourier transform infrared spectrophotometer (FTIR, Spectrum GX, PerkinElmer, Waltham, MA, USA). The measurement range is between 4000 and 370 cm −1 . A resolution of 2 cm −1 was selected with 32 scans to decrease the noise signal [18]. To prepare the infrared adsorption sample, each non-soluble (NSP30, NSP50, and NSP80) and recrystallized (RCP30, RCP50, and RCP80) product was homogeneously mixed with Minerals 2022, 12, 254 5 of 12 the potassium bromide (KBr, spectroscopic grade) powder at a mass ratio of 1:10 [19]. The obtained mixture was compressed into a small pellet form using a manual hydraulic press and then placed inside a sample holder of the infrared spectrophotometer [20]. The thermal decomposition behavior of the recrystallized samples was analyzed with a thermogravimetric/differential thermal analysis (TG/DTA, Pyris Diamond, Perkin Elmer) instrument. The decomposition characteristics of the recrystallized samples were observed from the obtained TG/DTA profiles at a heating rate of 10 • C·min −1 from room temperature to 800 • C under inert (N 2 ) gas at a flow rate of 80 mL·min −1 . The sample powder was loaded into an alumina pan without a lid and pressing process using the calcined α-Al 2 O 3 as the reference material [21].

Yield and Soluble Percentage
The masses (g) of the recrystallized products, their corresponding yields (%), and the solubility values (%) were investigated, and the results are shown in Table 1. Using 10 g of shell-derived TSP, 5.10, 4.96, and 4.63 g of recrystallized Ca(H 2 PO 4 ) 2 ·H 2 O products were obtained for RCP30, RCP50, and RCP80, respectively. The obtained masses of the products indicated that the yields of the product obtained from the recrystallization process (Table 1) decreased as the temperature used to dissolve the TSP increased, and the product obtained from the dissolved temperature of 30 • C showed the highest recrystallized yield (51.0%). After that, the solubility of the recrystallized products was investigated. From the experimental results, the solubilities of RCP30 and RCP50 were 98.72% and 99.16%, respectively, whereas the solubility of RCP80 (96.63%) was significantly lower than the other samples. According to the previous research, it was reported that the solubility of efficient TSP fertilizer (Ca(H 2 PO 4 ) 2 ·H 2 O) is usually more than 85% [22]. This solubility performance is dependent on the fertilizers' sources (i.e., reactant grade used for Ca(H 2 PO 4 ) 2 ·H 2 O preparation) and the operating conditions (i.e., neutral or basic or acidic condition). Consequently, this observation indicated that all recrystallized Ca(H 2 PO 4 ) 2 ·H 2 O obtained in this research are excellent soluble fertilizers in terms of agronomic performance. Additionally, the solubility of the product recrystallized at a low dissolution temperature was higher than the product recrystallized at a higher dissolution temperature, and 50 • C is the suitable dissolution temperature to recrystallize the soluble Ca(H 2 PO 4 ) 2 ·H 2 O fertilizer.  [7] according to hydrolyzed reaction, as shown in Equation (2). However, the reactions did not complete because the acid (H 3 PO 4 ) was not removed from the system [7]. Therefore, for the solution system without the addition of other chemical reagents, such as a basic reagent, especially sodium hydroxide (NaOH), the equilibrium phenomenon will be obtained, and the H 3 PO 4 will remain between amorphous CaHPO 4 and crystalline Ca(H 2 PO 4 ) 2 ·H 2 O. Therefore, when the Ca(H 2 PO 4 ) 2 ·H 2 O product, recrystallized from Ca 2+ -rich oyster waste TSP, was employed as the fertilizer, the largest amount of phosphorus content with the lowest impurities will be valuable for the cultivation process.

Morphology and Chemical Composition (SEM-EDX)
SEM technique was applied to observe the morphological characteristics of the recrystallized samples. SEM images of RCP30, RCP50, and RCP80 products are shown in Figure 2. The micrograph of RCP30 (Figure 2a) shows many plates of crystal intersperse in different sizes, ranging from around 10-110 µm. Conversely, the morphologies of RCP50 (Figure 2b) and RCP80 (Figure 2c) products present the coagulate plate of crystals with sizes of around 1-90 µm. According to the SEM results, it can be seen that the crystal size of RCP80 (1-50 µm) is smaller than that of the crystal size of RCP50 (5-90 µm). Using the EDX analysis, coupled with the SEM technique, the elemental composition was investigated, and the EDX results, as demonstrated in Table 2, indicate the presence of calcium (Ca), phosphorus (P), and oxygen (O) elements, which are characteristic of the Ca(H 2 PO 4 ) 2 ·H 2 O recrystallized in the present work. Furthermore, the results of EDX show the presence of potassium (K) and aluminum (Al) elements (slight amount) as the adulterate ions.

Morphology and Chemical Composition (SEM-EDX)
SEM technique was applied to observe the morphological characteristics of the recrystallized samples. SEM images of RCP30, RCP50, and RCP80 products are shown in Figure 2. The micrograph of RCP30 (Figure 2a) shows many plates of crystal intersperse in different sizes, ranging from around 10-110 μm. Conversely, the morphologies of RCP50 ( Figure 2b) and RCP80 (Figure 2c) products present the coagulate plate of crystals with sizes of around 1-90 μm. According to the SEM results, it can be seen that the crystal size of RCP80 (1-50 μm) is smaller than that of the crystal size of RCP50 (5-90 μm). Using the EDX analysis, coupled with the SEM technique, the elemental composition was investigated, and the EDX results, as demonstrated in Table 2, indicate the presence of calcium (Ca), phosphorus (P), and oxygen (O) elements, which are characteristic of the Ca(H2PO4)2·H2O recrystallized in the present work. Furthermore, the results of EDX show the presence of potassium (K) and aluminum (Al) elements (slight amount) as the adulterate ions.

X-ray Diffractogram (XRD)
D8 Advance XRD was employed to determine the diffraction pattern of the samples obtained from both non-soluble powders and the recrystallized products. The X-ray diffractograms (or XRD patterns) of non-soluble powders (NSP30, NSP50, and NSP80) samples are displayed in Figure 3a. The diffractogram results indicated that all peaks present the diffraction characteristic of CaHPO4·2H2O based on the standard ICDD # 72-0713 [23]. From the previous work, the lattice parameters of CaHPO4·2H2O are a = 5.812, b = 15.180, and c = 6.239 Å with the monoclinic crystal system and space group of Ia, whereas the β angle equals 116.42° [24]. Its structural layers are comprised of PO4 tetrahedra and CaO8

X-ray Diffractogram (XRD)
D8 Advance XRD was employed to determine the diffraction pattern of the samples obtained from both non-soluble powders and the recrystallized products. The X-ray diffractograms (or XRD patterns) of non-soluble powders (NSP30, NSP50, and NSP80) samples are displayed in Figure 3a. The diffractogram results indicated that all peaks present the diffraction characteristic of CaHPO 4 ·2H 2 O based on the standard ICDD # 72-0713 [23]. From the previous work, the lattice parameters of CaHPO 4 ·2H 2 O are a = 5.812, b = 15.180, and c = 6.239 Å with the monoclinic crystal system and space group of Ia, whereas the β angle equals 116.42 • [24]. Its structural layers are comprised of PO 4  octahedra. Six O atoms of CaO8 are bound to PO4, and the other two O atoms of CaO8 belong to H2O [25]. The H is bound to the O atom with the longest P−O bond in the PO4 [26]. Layers within CaHPO4·2H2O are held together by H bonds and are parallel to the caxis [27]. The H in the two H2O forms strong bonds with O of PO4; weak H bonds exist between O of H2O and H of P−OH and also between the molecules of H2O [28]. The molecules of H2O in CaHPO4·2H2O are not identical, and the H−O−H bond angles of 105.4° and 106.6° were reported by Schofield et al. [25] and Curry and Jones [26]. Furthermore, one molecule of H2O had the H-bond angles of 167.3° and 165.8°, and another molecule had the angles of 175.7° and 173.3° [26].  Figure 3b shows the XRD patterns of recrystallized products (RCP30, RCP50, and RCP80). The peaks with the highest intensities (in 2θ) are observed around 7.8°, 15.4°, 23.1°, 24.4°, 30.7°, and 46.6°. The XRD pattern results indicate and confirm the formation of calcium dihydrogen phosphate monohydrate (Ca(H2PO4)2·H2O) according to the ICDD # 009-0347, which can be applied as a nutrient, yeast food, leavening agent, hardener, and buffer [3]. According to the previous work, it was reported that Ca(H2PO4)2·H2O crystallizes in a triclinic crystal system, and the lattice parameters are a = 6.25 Å, b = 11.89 Å, and c = 5.63 Å with and α = 96.67°, β = 114.20°, and γ = 92.95° [3]. The crystal structure of Ca(H2PO4)2·H2O was well described by Dickens and Bowen [29]. The chains of CaH2PO4 + form corrugated layers. The layers of H2PO4 − and H2O were observed in the layers between CaH2PO4 + layers. The positions of two H sets differ by approximately 0.28 Å. One H bond of the H2O-donor molecule is apparently bifurcated, whereas other H atoms form normal H bonds, and each site of H atoms was fully occupied [29]. Furthermore, the diffractogram of RCP80 also indicates some residues at 10.26°, which can be attributed to hydrogenated hydrated calcium chloride (CaClH2PO4·H2O) [3] according to the ICDD # 044-000746.

Vibrational Spectroscopy (FTIR)
Infrared adsorptions of non-soluble powders (NSP30, NSP50, and NSP80) are recorded by Spectrum GX FTIR spectrophotometer, and the resulting spectra are shown in Figure 4a. Infrared adsorption frequencies of non-soluble powders are in line with results reported by Tortet et al. [30] and Dosen and Giese [27]. The vibrational characteristics of hydrogen phosphate anion (HPO4 2− ) and water (H2O) molecules were used to explain the infrared spectra obtained in the present work. The vibrational characteristic modes, sorted from high to low wavenumber (cm   , which can be applied as a nutrient, yeast food, leavening agent, hardener, and buffer [3]. According to the previous work, it was reported that Ca(H 2 PO 4 ) 2 ·H 2 O crystallizes in a triclinic crystal system, and the lattice parameters are a = 6.25 Å, b = 11.89 Å, and c = 5.63 Å with and α = 96.67 • , β = 114.20 • , and γ = 92.95 • [3]. The crystal structure of Ca(H 2 PO 4 ) 2 ·H 2 O was well described by Dickens and Bowen [29].  [29]. Furthermore, the diffractogram of RCP80 also indicates some residues at 10.26 • , which can be attributed to hydrogenated hydrated calcium chloride (CaClH 2 PO 4 ·H 2 O) [3] according to the ICDD # 044-000746.

Vibrational Spectroscopy (FTIR)
Infrared adsorptions of non-soluble powders (NSP30, NSP50, and NSP80) are recorded by Spectrum GX FTIR spectrophotometer, and the resulting spectra are shown in Figure 4a. Infrared adsorption frequencies of non-soluble powders are in line with results reported by Tortet et al. [30] and Dosen and Giese [27]. The vibrational characteristics of hydrogen phosphate anion (HPO 4 2− ) and water (H 2 O) molecules were used to explain the infrared spectra obtained in the present work. The vibrational characteristic modes, sorted from high to low wavenumber (cm  Table 3 summarizes the vibrational modes and their corresponding wavenumbers obtained in this research. According to the obtained results, it was confirmed that CaHPO 4 ·2H 2 O is the compound found in the non-soluble powder. and O-P-O(H) bending of HPO4 2− , respectively. Table 3 summarizes the vibrational modes and their corresponding wavenumbers obtained in this research. According to the obtained results, it was confirmed that CaHPO4·2H2O is the compound found in the nonsoluble powder.  Table 3. Vibrational characteristics (modes) and vibrational positions (wavenumbers/cm −1 ) observed in the non-soluble CaHPO4·2H2O (NSP30, NSP50, NSP80) and the recrystallized Ca(H2PO4)2·H2O (RCP30, RSP50, RCP80) compounds.

Thermal Analysis (TG/DTA)
The thermal decomposition behaviors of RCP30, RCP50, and RCP80 samples recorded by Pyris Diamond TG/DTA are shown in Figure 5a-c, respectively. The existence of the endothermic peaks located under 200 °C is identified by the loss of physisorbed water (first dehydration, Equation (4)) on the surface of samples, resulting in the formation of calcium dihydrogen phosphate anhydrous (Ca(H2PO4)2). The endothermic phenomenon occurred at 234 °C (Figure 5a,b) and 258 °C (Figure 5c), which are related to the dehydroxylation characteristic (polycondensation (orthophosphate PO4 3− → pyrophosphate P2O7 4− ), second dehydration, Equation (5)) of Ca(H2PO4)2 to form calcium dihydrogen pyrophosphate anhydrous (CaH2P2O7). Another thermal behavior above 320 °C is the re-polycondensation (pyrophosphate P2O7 4− → metaphosphate P2O6 2− ) process (third dehydration, Equation (6)), which corresponds to the formation of calcium metaphosphate anhydrous (CaP2O6) [34,35]. Consequently, the thermal decomposition equations of the recrystallized Ca(H2PO4)2·H2O product are shown below: First dehydration of physisorbed water: Second dehydration of polycondensation: Third dehydration of re-polycondensation:  Some calcium phosphate compounds showed safety and biological compatibility in organism tissues [34], and CaP 2 O 6 , one of the phosphate-based compounds with a good biocompatibility property, has been used in biomedical fields [34]. Its mechanical values, i.e., high bending strength (~200 Mpa) and low elastic modulus (~45 GPa), are similar to the natural cortical bone [35]. CaP 2 O 6 is composed of a large porosity (~70%), indicating that CaP 2 O 6 with flexible and high-strength properties can be applied as fillers to repair the bone [34]. When it is implanted in bone tissues, the spaces between tissues are filled. The fillers would be embedded by the high-convoluted growth of natural bone into the spaces. Therefore, it can be concluded that the final thermal decomposition product (CaP 2 O 6 ) of the recrystallized Ca(H 2 PO 4 ) 2 ·H 2 O compound obtained in this work can be further applied as the biocompatible material for biomedical fields.

Conclusions
The recrystallization process was used to prepare a well-soluble Ca(H 2 PO 4 ) 2 ·H 2 O fertilizer by using synthesized triple superphosphate (TSP) as a precursor, whereas oystershell-derived CaCO 3 was used as the raw material for TSP preparation. Using this recrystallized technique, the waste was transformed into valuable material. Under three different dissolved temperatures, RCP30, RCP50, and RCP80 (Ca(H 2 PO 4 ) 2 ·H 2 O) were obtained. The non-soluble products, namely NSP30, NSP50, and NSP80, were also investigated and observed as CaHPO 4 ·2H 2 O. The RCP30 showed the highest recrystallized yield of 51.0%, whereas the highest soluble percentage of 99.16% was observed from RCP50. Applying the SEM technique, the plate of crystal intersperse in different sizes of RCP30 was observed, whereas the RCP50 and RCP80 show the coagulate crystal plate. The formation and purity of the soluble Ca(H 2 PO 4 ) 2 ·H 2 O and non-soluble CaHPO 4 ·2H 2 O compounds were confirmed by the X-ray diffractograms. Various vibrational modes of HPO 4 2− , H 2 PO 4 − , and H 2 O presented in the crystal structure of the synthesized products observed from the infrared adsorptions also confirm the characteristic of Ca(H 2 PO 4 ) 2 ·H 2 O and CaHPO 4 ·2H 2 O. The thermal decomposition of the recrystallized products was investigated. The thermal dehydration of physisorbed water, polycondensation (dehydroxylation), and re-polycondensation processes were observed, and CaP 2 O 6 was the final thermodecomposed product.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.