Composition and Properties of Triple Superphosphate Obtained from Oyster Shells and Various Concentrations of Phosphoric Acid (2024)

Triple superphosphates[TSPs, Ca(H₂PO₄)₂·H₂O] were produced by exothermic reactionsof oyster shells and different concentrations of phosphoric acid (10,20, 30, 40, 50, 60, and 70% w/w) in a molar ratio of 1:2. The percentageyields, P₂O₅ and CaO contents, metal impurities,and thermal behaviors of all the as-prepared products are dependenton the concentrations of phosphoric acid added during the productionprocesses, which confirm to get the best optimum of 60% w/w phosphoricacid. All the as-prepared products were characterized by several characterizationmethods [X-ray fluorescence, thermal gravimetric/derivative thermalgravimetric analysis, powder X-ray diffraction, Fourier-transforminfrared spectroscopy, and scanning electron microscopy], verifyingthat all the obtained compounds are TSP that can be used as fertilizerswithout metal toxic contaminants. From the successful results, themethod for TSP production can be applied in the fertilizer industrybased on starting waste materials of oyster shells that can replacethe use of unsustainable phosphate or calcium minerals obtained fromnonliving things.

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2.1. Production Results

The preparationconditions and percentage yields of TSP products obtained from thereaction between oyster shell powders and different concentrationsof phosphoric acid are given in Table 1. When the concentration of phosphoric acid increased,the degree of the exothermic reaction increased as well. As a result,preparation methods with higher concentrations of phosphoric acidcaused the powder to rapidly dry. For example, for TSP70, it tookonly 3 h. With the low concentrations of phosphoric acid used, lowpercentage yields of the product were obtained because there wereinadequate amounts of phosphoric acid to react with the oyster shell.This resulted in the mixed phases of calcium carbonate and superphosphate,which will be discussed and confirmed by TGA, Fourier-transform IR(FTIR), and XRD data in the next parts. The synthesized powder thathas the highest percentage yield was TSP60 (94.62%), while TSP70 whichused 70% w/w phosphoric acid gave the percentage yield of only 71.77%.This effect may be related to the number of water molecules in thecrystal structure because the strong exothermic reaction occurredin the preparation process for TSP70, which will be explained in TGA,XRF, and XRD information. On the basis of production, the short preparationtime and highest percentage yield are the preferred features and,in this research, it was the reaction between 60% w/w phosphoric acidand oyster shell powder (CaCO₃) with the molar ratio ofP/Ca = 2/1.

2.2. X-ray Fluorescence

Chemical compositionsof all the as-prepared samples are presented in Table 2. The seven synthesized superphosphates (TSP10–TSP70)contain large proportions of phosphorus pentoxide (41.40–77.54%)and calcium oxide (18.20–35.70%) and minor oxide impurities(SO₃, K₂O, Fe₂O₃, Al₂O₃, and SiO₂). The quantities of theminor oxide impurities decreased from TSP10 to TSP60 samples, butthey increased in the TSP70 sample. This result indicates that concentratedphosphoric acid can leach the oxide impurities that were specificto some ranges and the best concentration for preparation was 60%w/w phosphoric acid, which was revealed by much less contained impurities.The observed phosphorus pentoxide contents in the range of 60.40–77.54%for the six obtained superphosphates (TSP20–TSP70) were higherthan that of TSP in the fertilizer industry (40–58%)^1,2 and other research works.^3,4 For the quantity ofcalcium oxide in TSP fertilizer, the theoretical value is 22.22% andthe previous works were reported as 20–22%, which are closeto the observed values (18.20–19.70%) of TSP30–TSP60samples.^29,35 The high calcium oxide contents of 35.70%for TSP10, 24.70% for TSP20, and 31.00% for TSP70 indicated that solidphases of calcium carbonate, dicalcium phosphate dihydrate, and MCPMhave been mixed, which were supported by the P/Ca molar ratio obtainedat below 1.0.^32,35,36 For the P/Ca molar ratio of TSP fertilizer, the theoretical valuesand previously reported works⁵⁻⁷ were found to be in the rangeof 1.5–2.5 and the TSP30–TSP60 samples were found tohave ratios in this range (1.48–1.67). From these obtainedXRF data, the much fewer the impurities, the higher the P₂O₅ content, and the P/Ca molar ratio in the range of 1.5–2.5was used as a basis for the selection of 60% w/w phosphoric acid asthe optimum concentration. The obtained results of XRF well supportthe production results. Additionally, TSPs obtained from oyster shellpowder in this work did not have the contents of toxic trace elements(Cr, Sr, Cd, etc.), unlike TSPs produced from phosphate rock and ores.^{1,2,5−7,26,37} Nevertheless, the accumulationof toxic elements in oyster shells may occur depending on the source.Therefore, the composition of the elements should be examined beforeuse.

2.3. Thermal Analysis

The TG/DTG thermogramsof all the as-prepared samples are shown in Figure ​Figure11. The TG traces (Figure ​Figure11a) illustrate percentage mass losses in therange of 30–800 °C, which are related to DTG peaks (Figure ​Figure11b), indicating thedecomposition mechanism of reactions. The TG/DTG curves observed weresimilar for the samples of TSP20 and TSP70, TSP30 and TSP50, and TSP40and TSP60, whereas the TG/DTG curves of TSP10 were significantly differentfrom other results. At a temperature below 100 °C, the mass lossesand DTG peaks observed in all the as-prepared samples were assignedto the dehydration of moistures. The TG curves of the TSP10 sampleappear in the range of 100–200, 480–520, and 650–750°C corresponding to DTG peaks at 140, 186, 518, and 706 °C,which were related to the first two steps of dehydration, deprotonationof hydrogen phosphate, and decarbonation reaction, respectively. Thetotal mass loss was about 23% which corresponded to the retained massof about 77%, which confirmed that this prepared sample was the mixedsolid phases of CaCO₃ and calcium phosphates [CaHPO₄·2H₂O and Ca(H₂PO₄)₂·H₂O]. The TG curves of the TSP20 and TSP70samples were similar in the range of 100–200 and 300–400°C corresponding to four DTG peaks at 140, 186, 217, and 395°C, which corresponded to a mass loss of about 17% (the retainedmass of about 83%). The TG curves of TSP30, TSP40, TSP50, and TSP60samples appeared similarly in the range of 100–200 and 300–400°C corresponding to four DTG peaks at 140, 186, 217, and 330°C. The total mass losses (the respective retained mass) werefound to be 21 (79%), 22 (78%), 24 (76%), and 24% (76%), respectively.The total mass losses of the TSP30 and TSP40 samples were close tothe theoretical value (21.43%, 3 mol H₂O)³⁷ of Ca(H₂PO₄)₂·H₂O while those of the TSP50 and TSP60 samples were more than3% because moistures were absorbed, which can be seen from their TGtraces at below 100 °C. Additionally, the total mass losses ofthe TSP20 and TSP70 samples were lower than the theoretical valuebecause impurities were mixed in the target compound [Ca(H₂PO₄)₂·H₂O], which must be discussedclearly. Basically, thermal transformation reactions of the targetcompound, TSP [Ca(H₂PO₄)₂·H₂O], that included two reactions, dehydration and deprotonationof hydrogen phosphate, produced the final decomposed product calciumpolyphosphate [Ca(PO₃)₂] at 600 °C accordingto reactions 2 and 3(5,35)

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Figure 1

TG(a) and DTG (b) curves of Ca(H₂PO₄)·H₂O produced from oyster shells and phosphoric acid with concentrationsof 10 (TSP10), 20 (TSP20), 30 (TSP30), 40 (TSP40), 50 (TSP50), 60(TSP60), and 70% (TSP70) w/w.

For the obtained thermal analysis results of the TSP20–TSP70samples, each reaction of 2 and 3 was separated into two steps because of the distinct environmentof the water molecule and dihydrogen phosphate anion in the structureof the as-prepared samples. The thermal transformation reactions areproposed as follows^29,35

(n = x + y = 1).

The number of thermal transformation stepsmay be less than twosteps (reactions 2 and 3) because of the overlapping reactions caused by close heating energyfor the eliminated crystal water molecules with similar environmentsin the structure. On the other hand, the number of thermal transformationsteps could also be more than two steps because of the splitting reactionscaused by different distinct crystal water molecules in the structureaffecting the energy used differently to break bonds. In general,the different thermal transformation behaviors for the compounds ofthe same formula can occur depending on different parameters suchas the methods of preparations, reagents, synthesis conditions, andso forth.^35,36,38 The thermalanalysis results obtained clearly indicated that the thermal behaviorsof TSPs were affected by the concentrations of phosphoric acid duringtheir preparation.

2.4. IR Spectroscopy

The traditional methodfor the identification of material compounds is using IR absorptionspectrum (FTIR) based on the identification of chemical functionalgroups contained within the molecules of the as-prepared compounds.The target compound synthesized has the general chemical formula ofCa(H₂PO₄)₂·H₂O thatis well known in the fertilizer field as TSP and in the academic fieldas MCPM. This target compound contains the block units of CaO, [H₂PO₄]⁻ ion, and H₂Omolecule within the structure. Therefore, all spectra of the preparedsamples will be identified based on the fundamental vibrational modesof these molecules.³⁷ The characteristicfrequency bands of Ca–O bonds are observed at around 877, 720,and 500 cm^–1. The fundamental vibrational modesof the [H₂PO₄]⁻ ion includethe bending of O–P–O, the P–O stretching, in-planeP–O–H, and out-of-plane P–O–H, which areobserved in the region of 600–450, 1100–900, 1250–1200,and 900–800 cm^–1, respectively.^29,35 The fundamental vibrational modes of H₂O molecules consistof three bands: bending, symmetry, and asymmetry stretching of H–O–Hbonds, which appear at 1630, 3100, and 3340 cm^–1, respectively.^8,29,35 Additionally, the bands at center that include 2980, 2875, 2513,and 794 cm^–1 are harmonic vibrations of these elongationmodes.^35,38 The FTIR results of all prepared samplesshow fundamental vibrations of the block units that are within theCa(H₂PO₄)₂·H₂O structure,resulting in their similar spectra (Figure ​Figure22).³⁵ The FTIRspectra of TSP30, TSP40, TSP50, and TSP60 samples are very closelysimilar and resemble that of Ca(H₂PO₄)₂·H₂O reported in the previous literature,³⁵ while the FTIR spectra of TSP10 and TSP20 samplesshow double peaks at 3400–3600 cm^–1, correspondingto the O–H bonds matching with a low concentration of HPO₄^2– in the sample. This indicates that theTSP10 and TSP20 samples are the mixed solid phases of CaHPO₄·2H₂O and Ca(H₂PO₄)₂·H₂O in the structure. Furthermore, the FTIR spectrumof the TSP70 sample is different from the other spectra at over 3000cm^–1, showing a broad band that can be assumed inthe same way.

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Figure 2

IR spectra of Ca(H₂PO₄)·H₂O produced from oyster shells and phosphoric acid with concentrationsof 10 (TSP10), 20 (TSP20), 30 (TSP30), 40 (TSP40), 50 (TSP50), 60(TSP60), and 70% (TSP70) w/w.

2.5. X-ray Diffraction

Figure ​Figure33 shows the X-ray diffractogramsof all the as-prepared samples. The XRD patterns of TSP30, TSP40,TSP50, and TSP60 were shown quite similar, but intense peaks are different.There were two sharp characteristic peaks at 2θ = 22.95 and24.18° corresponding to (021) and (120) reflections for the anorthiccrystal structure of Ca(H₂PO₄)₂·H₂O, respectively.³⁶ According tothe standard data PDF# 700090, the T-labeled diffraction peaks canbe indexed, which confirms that the as-prepared Ca(H₂PO₄)₂·H₂O crystal structure is inthe anorthic system with the space group P$\bar{i}$.^8,29 For the XRD patterns of TSP10, TSP20, and TSP70,the four observed strong intense peaks at 2θ = 11.95, 21.09,26.91, and 29.38° are related to (100), (200), (002), and (210)reflections, respectively, for the crystal structure of CaHPO₄·2H₂O, which are indexed by using the standarddata PDF#72-0713.³⁸ Furthermore, the sharppeaks at 2θ = 50.05° appeared in the patterns of TSP10and TSP20 samples, which are reflected for the crystal structure ofCaHPO₄·2H₂O.³⁸ The XRD results obtained indicate that mixed phases between CaHPO₄·2H₂O and Ca(H₂PO₄)₂·H₂O occurred in the samples of TSP10, TSP20,and TSP70.^36,38 They could be dependent on differentconcentrations of phosphoric acid from the preparation process. TheFTIR and XRD results obtained are well consistent.

2.6. Structure and Morphology of Superphosphates

Figure ​Figure44 shows theSEM micrographs of all the prepared samples. The SEM micrograph ofTSP10 (Figure ​Figure44a) showssome chip-like woods with various large and small pieces horizontally.The SEM micrograph of TSP20 (Figure ​Figure44b) shows some sheet-like woods with various sizes andshapes in different directions. The SEM micrographs of TSP30, TSP40,TSP50, TSP60, and TSP70 samples (Figure ​Figure44c–g) show a group of polyhedral sheet-likefragments with different particle sizes of about 2–10 μm.Morphologies of TSP30, TSP40, TSP50, TSP60, and TSP70 samples havea hom*ogeneous microstructure in which the particle size tends to decreasewith the increasing phosphoric acid concentration. It can be deducedthat the product morphologies were influenced by the concentrationof phosphoric acid.

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Figure 4

SEM image of Ca(H₂PO₄)·H₂O produced from oyster shells and phosphoric acid with concentrationsof (a) 10 (TSP10), (b) 20 (TSP20), (c) 30 (TSP30), (d) 40 (TSP40),(e) 50 (TSP50), (f) 60 (TSP60), and (g) 70% (TSP70) w/w.

4.1. Raw Material Preparations

The oystershells were collected from a shell dumping place in Sriracha district,Chonburi Province, Thailand. Oyster shells were cleaned using saturatedsodium hypochlorite solution until meat particles were removed. Thecleaned shells were dried in an oven at 110 °C for 1 h. Afterthat, the oyster shells were ground to obtained oyster shell powderswith a particle size of 100 meshes (approximately 140 μm) asa raw material.

Industrial-grade concentrated phosphoric acid(85% w/w) was used without further purification. This concentratedacid was diluted with deionized water to prepare seven concentrationsof phosphoric acids (10, 20, 30, 40, 50, 60, and 70% w/w). It shallbe noted that the dilutions were strongly exothermic, so the solutionswere left to cool down before further use.

4.2. Productionof TSPs [Ca(H₂PO₄)₂·H₂O]

The target compounds,TSPs [Ca(H₂PO₄)₂·H₂O] with different chemical compositions and properties, were synthesizedbased on the following replacement reaction

For example, 58.17 mL of 10% w/w phosphoricacid was added into a beaker containing 10 g of oyster shell powder(CaCO₃). The mixed suspension was exothermic and stirredat 100 rpm until carbon dioxide was not evolved (up to 30 min). Thepale yellow-white product obtained was dried at room temperature for12 h and designated as TSP10. For other products, the process wasrepeated with different concentrations of phosphoric acids (42.19mL of 20%, 26.31 mL of 30%, 18.53 mL of 40%, 13.97 mL of 50%, 11.01mL of 60%, and 8.95 mL of 70% w/w) and the products were labeled TSP20,TSP30, TSP40, TSP50, TSP60, and TSP70, respectively. The yield ofTSP considered according to reaction 8 was calculatedby

where m_obs and m_theor are the massof the obtained TSP powderin the preparation process from each phosphoric concentration andthe mass of the TSP product calculated by theory, respectively.

4.3. Characterization

Chemical compositionsof all the as-prepared samples were identified by an X-ray fluorescencespectrometer (Brand Model, City, Country). The structure and crystallinesize of the as-prepared samples were identified by an X-ray powderdiffractometer (Bruker AXS, Karlsruhe, Germany) with the Cu Kαspectral line (λ = 1.54056 Å) as the incident radiation.FTIR spectra were recorded by an FTIR spectrophotometer (PerkinElmerSpectrum GX, City, UK) from 4000 to 400 cm^–1 with16 scans at a resolution of 4 cm^–1 by mixing 1 mgof the samples with 20 mg of KBr powder. TG/DTA (PerkinElmer PyrisDiamond, City, Country) was implemented to give the TGA curve of thesample in nitrogen gas from room temperature to 900 °C at a heatingrate of 10 °C/min. Finally, surface morphologies of the superphosphatecompounds were observed by a scanning electron microscope (Zeiss LEOVP1450, Oberkochen, Germany) after gold sputtering.

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