Porous hydroxyapatite (HAp) granules have been successfully fabricated from a HAp powder precursor and polyvinyl alcohol (PVA) additive by a simple sintering process. The composition and microstructures of the HAp were characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM) equipped with an energy dispersive X-ray (EDX) spectrometer. The effects of sintering temperature and PVA/HAp mass ratios on color, water stability, morphology, and chemical composition of HAp are discussed. Optimum conditions for the fabrication of HAp granules were found to be a PVA/HAp mass ratio of 3/20 and a sintering temperature of 600°C for 4 h. Accordingly, the obtained HAp is white in color, is in the granular form with a size of about 2 × 10 mm, and has a specific surface area of 70.6 m2/g. The adsorption of Pb2+ onto the as-prepared HAp granules was carried out in aqueous solution by varying the pH, the adsorbent dose, the initial concentration of Pb2+, and the contact time. The results of adsorption stoichiometry of Pb2+ on the HAp granule adsorbent were fitted to the Langmuir adsorption isotherm model (
Nowadays, pollution of the water environment caused by heavy metals, mainly from industrial waste, becomes one of the most serious problems. Various methods of removing heavy metal ions have been widely studied, such as chemical precipitation, electrochemical deposition, membrane filtration, ion exchange, adsorption, biological treatment, and stabilization/solidification [
Synthetic HAp has a similar chemical composition to the inorganic matrix of the natural bone and has good biological activity [
Ca(NO3)2·4H2O 98%, (NH4)2HPO4 99%, NH3 25–28%, Pb(NO3)2 99%, polyvinylalcohol (PVA), KNO3 99%, KOH 99%, and HNO3 65% are pure chemicals and are ordered from Merck.
HAp powder is prepared by a wet chemical precipitation method from Ca(NO3)2·4H2O and (NH4)2HPO4 salts in water following reaction (
The porous HAp granules are fabricated by the sintering method from HAp powder and PVA additive (with different PVA/HAp mass ratios of 2/20, 3/20, 4/20, and 5/20 denoted H2, H3, H4, and H5, respectively) at calcining temperatures: 400, 600, and 700°C. The sintering time for each experiment is 4 h.
The phase component of HAp powder and granules is analyzed by X-ray diffraction (XRD) using a Siemens D5000 diffractometer, CuK
A mixture of 0.30000 g of HAp granules in 50.0 mL of 0.01 M KNO3 solution is shaken for 60 minutes at room temperature. The initial pH values (pH0) are adjusted in the range of 2.5–9.5 using 0.1 M KOH or 0.1 M HNO3 solution. After equilibration, the pH values are measured once again (pHf), and the value of pHPZC (point of zero charge) is determined from the ∆pH =
The experiments are carried out by mixing a quantity of HAp granules with 50 mL of Pb2+ ion solution. The effect of physicochemical parameters on the adsorption process is investigated: the initial concentration of Pb2+ solution varies between 5 and 100 mg/L, the contact time varies between 5 and 60 min, the initial pH of the solution is studied between 2.5 and 7.5, and the dose of HAp granules is in the range of 2–26 g/L. The experiments are carried out at room temperature and with continuous shaking using a mechanical shaker at 100 rpm. After filtration to remove the solid, the remaining concentration of Pb2+ is determined by using an atomic absorption spectrophotometer (AAS).
The adsorption capacity and the removal efficiency are calculated by using equations (
The obtained experimental data are analyzed using the Langmuir and Freundlich isotherm models [
The adsorption kinetics is described by the pseudo-first-order and pseudo-second-order kinetic models using equations (
The weight loss of HAp powder is measured in the temperature range of 30 to 1000°C by the thermogravimetric analysis method. As shown in Figure
TGA curves of (a) HAp powder and (b) PVA.
The XRD patterns of HAp samples prepared at different sintering temperatures are shown in Figure
XRD patterns of (a) HAp powder, after sintering at (b) 700°C, (c) 850°C, and (d) 1000°C.
Typically, the melting temperature of PVA is determined indirectly at about 200°C. When the PVA is heated under vacuum at 200°C, it decomposes into water and brown powder. Continuing with the heat of PVA at 400°C, it breaks down into volatile hydrocarbon molecules and nonvolatile products. The result of the thermogravimetric analysis is shown in Figure
To study the effect of the amount of PVA additive on the color, the durability in water, and the physicochemical characteristics of the obtained material, the HAp granules with different PVA/HAp mass ratios of 2/20 (H2), 3/20 (H3), 4/20 (H4), and 5/20 (H5) are fabricated. After that, the samples are sintered at 600°C for 4 h.
The result shows that, at 600°C and 4 h sintering, samples H2 and H3 are white, while H4 and H5 are still grey. This result can be explained by the fact that, with a higher mass ratio of PVA/HAp (4/20, 5/20), the sintering time of 4 h is not sufficient to completely decompose the PVA in the samples. However, increasing of the sintering time is not a benefit of the energy. Therefore, H4 and H5 granule samples will not be fabricated. The physicochemical characteristics of obtained H2 and H3 granules including the durability in water, specific surface area, and morphology are presented in Table
The color, durability in water, and specific surface area of HAp granules sintered at 600°C for 4 h.
Sample | Color | % mass of HAp disintegration in water |
| |
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4 h | 8 h | |||
H2 | White | 15.4 | 26.5 | 67.6 |
H3 | White | 10 | 20 | 70.6 |
H4 | Grey | |||
H5 | Grey |
SEM images of (a) H2 and (b) H3 granules sintered at 600°C for 4 h.
Both H2 and H3 samples have a relatively uniform surface, while the H3 sample shows lower disintegration in water and a larger specific surface area than H2, so its adsorption capacity is higher. This is because the increase of the PVA content leading to the binding in the material structure is better and the porous enhancements lead to an increase in the specific surface area. Therefore, the H3 granules are chosen for further studies.
As the sintering temperature increases, the mass percentage of HAp disintegration decreases. This means that the durability in water of granules increases after 4 h and 8 h shaking (Table
The effect of sintering temperature on the color, durability in water, and specific surface area of H3 granules.
Temperature (°C) | ||||||||||
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400 | 600 | 700 | ||||||||
Color | % mass | Color | % mass |
|
Color | % mass |
| |||
Black | 4 h | 8 h | White | 4 h | 8 h | 70.6 | White | 4 h | 8 h | 73.0 |
26 | 43 | 10 | 20 | 8 | 16.8 |
Sample H3 sintered at 400°C for 4 h is black because this temperature is not high enough to completely decompose the PVA in the sample. Accordingly, the disintegration in water is high, and the granules cannot be fabricated under this condition. H3 samples sintered at 600°C and 700°C for 4 h are white in color and are used for further study of the physicochemical characteristics.
The XRD results show that H3 granules sintered at 600°C and 700°C are a single phase of HAp (Figure
XRD patterns of H3 granules sintered at (a) 600°C and (b) 700°C.
EDX spectra of H3 granules sintered at (a) 600°C and (b) 700°C.
Component of elements in H3 granules sintered at 600°C and 700°C.
|
Content (%) | Element | ||
---|---|---|---|---|
P | Ca | O | ||
600 | weight | 18.98 | 40.31 | 40.71 |
atom | 14.71 | 24.21 | 61.08 | |
|
||||
700 | weight | 18.75 | 40.71 | 40.54 |
atom | 14.56 | 24.48 | 60.96 |
SEM images of H3 granules sintered at (a) 600°C and (b) 700°C.
The BET results show that the specific surface area increases slightly as the temperature increases from 600°C to 700°C (Table
Digital photo of H3 granules prepared at 600°C for 4 h (PVA/HAp = 3/20).
The variation of ΔpH versus pH0 (initial pH) of HAp granules is shown in Figure
The variation of ΔpH versus
Figure
The variation of Pb2+ adsorption capacity and removal efficiency according to contact time (mHAp granules = 6 g/L; C0 = 30 mg/L; pH0 5.5;
The removal efficiency of Pb2+ strongly depends on the pH of the solution because the surface properties of the adsorbent are modified by the pH. From pHPZC = 7.01, the experiments were investigated at a pH of about 7.01. However, by avoiding the precipitation of Pb(OH)2 in an alkaline media (pH > 7.5), the effect of pH is conducted with a pH ≤ 7.5. The variation of the adsorption capacity and the removal efficiency of Pb2+ with the pH values is illustrated in Figure
The effect of the pH value on the adsorption capacity and the removal efficiency of Pb2+ (
The effect of the mass of HAp granules on the adsorption capacity and the removal efficiency of Pb2+ is presented in Figure
The effect of HAp granules mass on the adsorption capacity and the removal efficiency of Pb2 + (
The initial concentration of Pb2+ has a significant effect on the adsorption capacity and the removal efficiency. When the concentration of Pb2+ is increased, the adsorption capacity increases while the removal efficiency decreases (Figure
The effect of initial Pb2+ concentration on the adsorption capacity and the removal efficiency of Pb2+ (
The appropriate conditions for the removal of Pb2+ are 6 g/L HAp granule, a contact time of 40 min, a pH0 value of 5.5, and a reaction temperature of 30°C. By varying the initial concentration of Pb2+ and determining the remaining Pb2+ concentration at equilibrium (
The values of ln
|
|
ln |
|
ln |
|
---|---|---|---|---|---|
30 | 0.32 | −1.13 | 4.95 | 1.60 | 0.06 |
40 | 0.97 | −0.03 | 6.50 | 1.90 | 0.15 |
50 | 2.34 | 0.85 | 7.94 | 2.07 | 0.30 |
60 | 2.7 | 0.99 | 9.49 | 2.25 | 0.28 |
80 | 7.35 | 2.00 | 12.10 | 2.49 | 0.57 |
100 | 17.35 | 2.85 | 13.77 | 2.62 | 1.26 |
Adsorption isotherm curves at 30°C follow (a) Langmuir and (b) Freundlich models.
Experimental constants
Langmuir | Freundlich | ||||
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|
|
|
|
|
14.75 | 0.79 | 0.99264 | 3.77 | 6.70 | 0.97397 |
From the results obtained, both adsorption isotherm models can describe experimental data of the adsorption of Pb2+ by HAp granules. However, the
By varying the reaction time, the graphs of pseudo-first-order (Figure
The description of the experimental data by (a) pseudo-first-order and (b) pseudo-second-order kinetic equations.
From Figure
The values of
Pseudo-first-order kinetic equation | Pseudo-second-order kinetic equation | Experimental | ||||
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|
|
|
|
|
|
6.32 | 0.12676 | 0.92245 | 5.43 | 0.03680 | 0.99648 | 4.97 |
HAp granules were fabricated from a HAp powder precursor and polyvinyl alcohol (PVA) additive by the sintering method with an average size of 2 × 10 mm and used as an effective adsorbent to remove Pb2+ in aqueous solution. The adsorption process depends on the pH, the adsorbent mass, the initial Pb2+ concentration, and contact time. Under the conditions studied, the adsorption of Pb2+ on HAp granules occurs rapidly and reaches equilibrium after 40 min. The adsorption process of Pb2+ follows the pseudo-second-order adsorption kinetic equation and the Langmuir adsorption isotherm model. The obtained results will open a direction of potential application in the adsorption column with the HAp granule adsorbent for the treatment of Pb2+ in polluted water.
The data used to support the findings of this study are included within the article.
The authors declare that there are no conflicts of interest regarding the publication of this paper.
The authors would like to thank the financial support of project of Ministry of Education and Training (code: B2017-15ĐT) for completing this work.