Chemical components of the waste sample before and after physical and chemical treatments as recorded by XRF are listed in Table 1. The results showed that the studied solid waste contains high weight percent of CaO, Na2O and SO3. This means that Ca(OH)2 is partially converted to calcium sulfate and CaCO3 by action of both sulfur oxide and carbon oxide gases. Thus, many trials were carried out to treat the industrial waste for eliminating all other impurities and to obtain a pure calcium sulfate which used for production of HAp. Different factors were being studied to optimize the conditions of treatment as follows.
Effect Of Water Treatment
Comparing to chemical compositions of industrial waste, XRF analysis of treated waste by water washing (sample1) indicated that the washing with distilled water led to a drastic decrease in the percentage of sodium oxide. Otherwise, an increase in the percentage of calcium and water content is obtained. Thus, there is an enhancement in calcium content on expense the dissolution of sodium salts in the waste sample as a result of water washing (Table 1).
Effect Of Alkaline Treatment
XRF results affirmed that the treatment with 10% of caustic soda (sample 2) led to a considerable increase in the percentage of calcium and water content of the sample. Whereas it is negatively affected the percentage of sulfate, as the weight percentage is decreased from 36.58–5.17%. This observation is due to the chemical reaction which occurred between alkaline solution and sulfate ions (Table 1).
Effect Acidic Treatment
Treatment with H2SO4 acid obviously is converted all unwanted impurities into soluble materials in addition to raising the concentration of sulfate ions. But treatment with HCl acid has negative influence on the amount of carbonates that react with acid and turn into soluble chlorides. In case of H2SO4, the carbonates convert into insoluble sulfates (Table 1). Hence, the acidic treatment with H2SO4 is the preferable usage to produce calcium sulfate as starting materials for preparing HAp.
Effect Of Sulfuric Acid Concentration
Because sulfuric acid exhibited good results in above section, series of experiments were carried out to explore the best H2SO4 concentration. (Fig. 1). represents the XRD patterns of industrial waste after treatment with different concentrations of H2SO4 (2, 4, 6, 8 and 10%) and calcined at 450oC for 2 hours. It is obvious that the pattern of the sample which treated with 2% contains traces of CaCO3 (JCPDS (88-1809)) bedside the main phase of CaSO4 (JCPDS (72–0916)) while all the patterns of the other treated samples contains only one pure phase of CaSO4. Depending on the phase purity and good crystallinity, the best acid concentration for treatment was 6%.
Characterization Of Hap
The obtained calcium sulfate from chemical treatment of solid waste with 6% sulfuric acid was calcined at 700oC for 2h to prepare hydroxylapatite (HAp) which characterized by the following techniques.
(Fig. 2) depicts the XRD patterns of the calcium hydroxyapatite which was obtained after drying at 100°C for 2 hours. XRD patterns confirmed that HAp was obtained in weakly crystalline form. After calcination at 700°C for 2 hours, the calcium hydroxyapatite was formed as a monophasic material belonging to reference (JCPDS (76–0694)) with high crystallinity.
(Fig. 3) illustrates the FTIR spectrum of HAp calcined at 700°C where the characteristic functional groups of HAp are observed. For example, the appearance of O–H stretching bands at 3443–2851 cm− 1 confirms the formation of HAp and the band at 1640 cm− 1 corresponds to O–H bending vibrational mode . The double bands at 606 and 561 cm− 1 which attributed to the ν4 vibrational modes of phosphorous groups are appeared . The band at 1022–1100 cm− 1 which existed as a doublet or a shoulder is related to ν3 vibrational mode of P-O groups [28, 29]. While the weak band occurred at 1430 cm− 1 is ascribed to the ν3 asymmetric stretching vibrations of CO32−. This finding indicates that partially carbonated hydroxyapatite can be formed during preparation process in agreement with results reported by Stanic et al. .
The TEM image of the synthesized sample (HAp) is presented in (Fig. 4a). It is clear from TEM that hydroxyapatite formed in a nanostructure of rods-like shape (11-15nm of thickness and 25-32nm of length) which would candidate for many important applications in the science of bone tissue engineering. It is also evident from the corresponding selected area electron diffraction (SAED) patterns of prepared hydroxyapatite that the brightness and intensity of the rings are strong, so the material is well crystalline (Fig. 4b). The SEM image of HAp sample is presented in (Fig. 4c). It could be observed that the surface morphology of the sample was appeared as an ellipsoidal shape. The SEM micrograph emphasized that the HAp nanoparticles formed with high agglomeration as result of nanometric dimensions of the particles .
The thermal gravimetric analysis for prepared HAp at 700oC is illustrated in (Fig. 5). The TGA profile of Hap showed that three regions are occurred according to DTG also. The first one is appeared at 155oC with mass loss about 6% which corresponded to evaporation of humidity water and volatile matter. The second region at 290 oC with mass loss of 22% is due to the removal of residual ammonia. The last one at 790 oC with small mass loss of 4% is attributed to the partial conversion of HAp to tri-calcium sulfate (TCP) . This finding confirmed the high thermal stability of the produced HAp sample.
Bet Surface Area Measurements
The measured BET surface area was found to be 146 m2/g with pore diameter (16.3 nm) and total pore volume (0.593 nm). The pore size distribution analysis is illustrated according to NLDFT theory at (Fig. 6). It is noted that the maximum pore size of the HAp sample is mainly centered at 9.6 nm, confirming that the prepared sample has mesoporous structure .
Determination Of Ca/p Molar Ratio
XRF data of HAp sample shows that the Ca /P molar ratio is 1.60 as presented in Table 2. The obtained value is closely to the Ca /P ratio (1.67) that found in human bone . This result affirmed the validity of utilizing of glass waste in production of HAp.