Introduction

Glass industries produce a lot of wastes ranging from the products of the used raw materials to the damaged glass products. Glass in general formed from a mixture of materials such as silicate, soda ash, and CaCO3 with other additives for coloring or improvement the properties of the end product by melting at high temperature followed by cooling to the solidification without crystallization. On the other hand, the used fuel during glass production generates a lot of waste. The fuel waste is more pollutant than that produced from the melting process1.

The main environmental challenges for the glass industry are polluting emissions and energy consumptions. Most of glass production used natural gas as a source of energy. The production of the glass passes through different stages of heating such as the decomposition of the start materials, melting and finishing. In general, the amount of pollution emission depends on the type of glass produced, the raw materials, the types of furnace and the fuel used. The recent literatures2,3 refer that the main evolved gas are CO, CO2 and NO2.

Unfortunately, various works4,5,6,7 were aimed to minimize the waste during the melting process, while a little attention was paid to manage the produced fumes and gases from the fuel combustion. In most cases, when the fuel burned produces carbon oxides, water vapor and some amounts of other dangerous gases. According to the regulation of the governments, the gases fuel should be removed or minimize the environmental pollution. The technology of glass manufacture refers that the waste fuel gases could be absorbed in some adsorbent materials such as Ca(OH)28. After a period of work, the adsorbent becomes saturated and it should be replaced.

The exhausted Ca(OH)2 becomes a source of pollution, so that it is necessary to utilize the Ca(OH)2 which loaded with the fuel gases. In our previous works, several managements to minimize the pollution from the by-product of some chemical industries such as phosphogypsum produced from the phosphoric acid manufacture or cirtogypsum produced from citric acid or treatment the copper and zinc scrap were carried out9,10. As continuation of this strategy, the present work aims to convert the loaded Ca(OH)2 with gases to valuable materials. This study directed to produce high calcium content materials such as hydroxyapatite.

Hydoxyapatite (HAp), could be prepared from raw materials rich in Ca2+ and PO4–3 such as calcium carbonate, phosphate ores, phosphoric acid, ammonium dihydrogen phosphate and etc.11. Otherwise, HAp was prepared from some biogenic wastes such as egg shells, sea shells, animal bone and corals12,13,14,15.

HAp particles have several advantages such as Ca/P ratio equal 1.67 similar to that found in the human bones, thermal stability and high surface area. Owing to these properties it used in several applications such as biocompatibility, osteogenic ability and in removal of some pollutant elements from water. HAp used mainly in different biomedical applications such as implant coating, bone scaffold, bone filler and drug delivery. Moreover its structure can be grafted with special cations such as Ag and Zn to increase the bioactivity effects16,17,18,19.

Experimental

Materials

The solid waste, collected during one month of Ca(OH)2 which absorbed the evolved gases from combustion of the fuel, was supplied kindly from Guardian Glass Industry, 10th Ramadan city, Egypt. Caustic soda (99%, Merck), HCl (33% Adwic), H2SO4 (98% Adwic), ammonium dihydrogen phosphate (NH4)H2PO4, 99%, Merck), ammonia solution (26%Adwic) and distilled water were used without further purification.

Methodology

Treatment of the waste

To determine the best method for waste treatment, the waste was treated by the following methods;

  1. a.

    Washed with water for several times

  2. b.

    Treatment with alkaline solution (10%)

  3. c.

    Treatment with HCl (50%) and H2SO4 (10%)

  4. d.

    Effect of sulfuric acid concentration ((2, 4, 6, 8 and 10%)

All the samples after treatment were separated, washed and exposed to chemical analysis by XRF technique (Table 1).

Table 1 XRF analysis of industrial waste, treated waste with distilled water (sample 1), alkaline solution (sample 2), HCl (sample 3) and H2SO4 (sample 4).
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a. Water treatment

The waste (100 g) was washed with distilled water (500 ml) for three times by tap water, then filtered and dried at 100 °C (sample 1).

b. Alkaline treatment

The industrial waste was rinsed in a 10% caustic soda solution for one hour at room temperature, and the mixture was left for 24 h. Then the excess of caustic soda was removed by washing with tap water for several times until the pH of the filtrate become neutral. Then the treated sample was filtered and dried at 100 °C (sample 2).

c. Acidic treatment

The industrial waste was mixed separately in 50% of HCl (sample 3) and 10% of H2SO4 (sample 4) solutions for one hour at room temperature and the mixtures were left for 24 h. Then the excess acid solution was removed and the treated sample washed with tap water for several times until the pH of the filtrate become neutral. The treated samples were filtered, dried at 100 °C.

d. Effect of sulfuric acid concentration

The washed waste sample was treated with different concentrations of sulfuric acid (2, 4, 6, 8 and 10%) and calcined at 450 °C for 2 h to identify the best concentration. The produced phases were compared using XRD (Fig. 1).

Figure 1
figure 1

XRD pattern of waste sample.

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Conversion the waste to hydroxyapatite

The conversion method was used elsewhere to convert the phosphogypsum to hydroxyapatite17,18,19. In this method, the treated waste with 6% H2SO4 (10 g) was suspended in amount of water (500 mL) under vigorous stirring at room temperature for 30 min. The required amount (5.0 g) of ammonium dihydrogen phosphate (NH4)H2PO4 solution was added dropwise with continue the stirring. The pH of the mixture was adjusted at 11 by using ammonia solution (18%) the chemical reaction was continued for 2 h. The solid part was separated by using centrifuge, dried at 100 °C and calcined at 700 °C for 2 h. The chemical reaction was preceded according to the following equation:

$${\text{5CaSO}}_{{4}} + {\text{3NH}}_{{4}} {\text{H}}_{{2}} {\text{PO}}_{{4}} + {\text{7NH}}_{{4}} {\text{OH}} \to {\text{Ca}}_{{5}} \left( {{\text{PO}}_{{4}} } \right)_{{3}} {\text{OH}} + {5}\left( {{\text{NH}}_{{4}} } \right)_{{2}} {\text{SO}}_{{4}} + {\text{6H}}_{{2}} {\text{O}}$$

Characterization

The chemical compositions of industrial waste, all treated samples by water, alkaline and acidic solutions were analyzed by using Axios advanced Sequential WD_XRF Spectrometer, PANalytical2005 to quantify their percentages. The produced phases and crystalline nature of the prepared materials were studied using X-ray diffraction (XRD), Bruker (D8 advance) diffractometer (Germany) with copper (Ka) radiation which works at (40 kV and 40 mA) with 0.02°/0.4 s. The characteristic groups were measured by using JASCO–FT/IR-3000E infrared spectrometer from 4000 to 400 cm−1. The surface morphology of HAp was investigated by SEM (JEOL JXA-840A, Electron Probe Micro-Analyzer, Japan) at 20 kV. The shape and size of HAp nanoparticles were performed using high resolution-transmission electron microscope (HR-TEM, JEM-1230, Japan) operated at 200 kV. Thermal stability (TG and DTG) of HAp was performed using thermogravimetric analyzer (Shimadu TGA-50 H) under N2 flow over rate 30 ml/min at 10°/min). Brunauer–Emmett–Teller surface area (SBET, m2/g) and pore size distributions were measured using nitrogen adsorption analysis at − 196 °C (BEL-Sorp-max, Microtrac Bel Crop, Japan).

Results and discussion

The waste sample was investigated using XRD (Fig. 1) where several compounds were occurred as CaCO3, CaSO4, Ca2(SiO4) and Na2SO4.To determine the content of each component and other minor compounds, XRF was carried out and the results were recorded in Table 1. The results show that the native waste contains CaO (23.13%) and SO3 (38.59%) with other oxides such as SiO2, Al2O3, Fe2O3 and etc. This means that Ca(OH)2 is partially converted to calcium sulfate and calcium carbonate by action of both sulfur oxide and carbon oxide gases which evolved from the fuel combustion. Thus, many trials were carried out to treat the industrial waste for eliminating all other impurities and to only one phase which used for synthesis of HAp.

For the effects of the treatment with water, alkaline and acids, it may be indicated that the washing with distilled water (sample 1) led to a drastic decrease from 21.63 to 0.09% in the percentage of sodium oxide. Moreover, an increase in the percentage of calcium from 23.13 to 41.52% and water content is obtained. This finding means that the sodium content dissolved in water so, the Ca content increases.

XRF results show that the treatment with 10% of caustic soda (sample 2) led to a considerable increase in the percentage of calcium from 41.52 to 47.79% while the SO3 decreases from 36.68 to 5.17%. The decrease in percentage of sulfate is due to the chemical reaction which occurred between sodium ions and sulphate ions.

The chemical treatment with HCl acid (sample 3) has negative effect where the calcium carbonate content converted to soluble calcium chlorides solution. In case of H2SO4 (sample 4), the carbonates converted into insoluble sulfates where theSO3% increased from 38.5 to 59.6% (Table 1). Hence, the acidic treatment with H2SO4 is the preferable usage to produce calcium sulfate as starting material for preparing HAp.

Because sulfuric acid exhibited good results in the waste treatment process as mention above, a series of experiments were carried out to determine the best H2SO4 concentration. Figure 2 represents the XRD patterns of industrial waste after treatment with different concentrations of H2SO4 (2, 4, 6, 8 and 10 v/v%) and calcined at 450 °C for 2 h. It was 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%.

Figure 2
figure 2

XRD patterns of treated industrial waste by different concentrations of H2SO4 (2, 4, 6, 8 and 10%).

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The obtained calcium sulfate from chemical reaction of solid waste with 6% sulfuric acid was used as calcium source to prepare hydroxyapatite (HAp). Figure 3 depicts the XRD patterns of the calcium hydroxyapatite which confirmed that HAp was obtained in weakly crystalline form. After calcination at 700 °C for 2 h, the calcium hydroxyapatite was formed as a monophasic material belonging to reference (JCPDS (76-0694)) with low crystallinity20,21,22.

Figure 3
figure 3

XRD patterns of HAp dried at 100 °C and calcined at 700 °C.

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Figure 4 illustrates the FTIR spectrum of HAp calcined at 700 °C where the characteristic functional groups of HAp are observed. The appearance of absorption bands at 3443–2851 cm−1 and at 1640 cm−1corresponds to O–H stretching and bending vibrational modes, respectively (Fig. 4a)23,24,25. The double bands at 606 and 561 cm−1 which attributed to the vibrational modes of phosphorous groups for HAp are appeared26. The band at 1022–1100 cm−1 which existed as a doublet or a shoulder is related to vibrational mode of P–O groups (Fig. 4b)27,28,29. While the weak band occurred at 1430 cm−1isascribed to the asymmetric stretching vibrations of CO32− (Fig. 4a). This finding indicates that partially carbonated hydroxyapatite can be formed during preparation process30,31.

Figure 4
figure 4

FTIR spectrum of HAp calcined at 700 °C.

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The morphological properties of HAp sample using TEM tool is presented in Fig. 5a. It is clear that hydroxyapatite formed in a nanostructure of rods-like shape (11–15 nm of thickness and 25–32 nm of length) which would candidate for many important applications in the science of bone tissue engineering. From the brightness and intensity of the corresponding selected area electron diffraction (SAED) pattern, the material is polycrystalline and the diffraction rings were assigned to the structure of pure hydroxyapatite (Fig. 5b). The SEM image of HAp sample is presented in Fig. 6a. 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 particles32. The elemental analysis of HAp was investigated by using (EDX) where the Peaks connected to Ca, P, and O confirmed that HAp formed with its necessary ions (Fig. 6b). The atomic ratio of Ca/P was 1.56 which is close to the standard stoichiometric ratio of the HAp (1.67).

Figure 5
figure 5

TEM image (a) and SAED pattern (b) of HAp calcined at 700 °C.

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Figure 6
figure 6

SEM micrograph (a) and EDX (b) of HAp calcined at 700 °C.

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The thermal gravimetric analysis for prepared HAp at 700 °C is illustrated in Fig. 7. The TGA and DTA profiles of HAp showed three regions. The first one is appeared at 155 °C with mass loss about 6% which corresponded to evaporation of moisture water and volatile matter. The second region at 290 °C with mass loss of 22% is due to the removal of residual ammonia. The last one at 790 °C with small mass loss of 4% is attributed to the removal of carbonates and water molecule as a result of partial conversion of HAp to tricalcium phosphate (TCP)33.

Figure 7
figure 7

TG and DTG of prepared HAp.

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In order to confirm the porous textures of HAp sample, N2 gas adsorption was performed. 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 according to NLDFT theory is illustrated in Fig. 8. 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 structure34,35.

Figure 8
figure 8

Pore size distribution analysis of HAp calcined at 700 °C by NLDFT.

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Conclusions

From the analysis of the experiments and characterization of the prepared products, it may conclude that:

  1. 1.

    The Ca(OH)2 waste loaded with carbon oxide and sulfur oxide which results from combustion of fuel gas could be converted to pure calcium sulphate by action of 6 v/v%H2SO4.

  2. 2.

    The obtained calcium sulphate could be converted completely to HAp nanoparticles (11–15 nm of thickness and 25–32 nm of length) by reaction with ammonium dihydrogen phosphate at alkaline medium and calcination at 700 °C for 2 h as confirmed by XRD, FTIR, SEM and TEM analyses.

  3. 3.

    The composition of prepared HAp was investigated by EDX technique where the essential elements of HAp (O, Ca and P) were occured and the Ca/P ratio was 1.56, indicating that the prepared HAp is closely to natural HAp that exists in the human bone structure.

  4. 4.

    The produced HAp exhibited high surface area (146 m2/g) and a mesoporous structure which can candidate for medical and water purification applications.

Therefore, this work can be considered as a case study for achieving environmental and economic aspects through converting industrial wastes of glass industry to valuable materials.