Potential of nano crystalline calcium hydroxyapatite for Tin(II) removal from aqueous solutions: Equilibria and kinetic processes

2.1 Preparation of nano crystalline hydroxyapatite sorbents

All chemicals used in this work were of analytical grade and the aqueous solutions were prepared using double distilled water. Nanocrystalline hydroxyapatite compounds were prepared by a solution-precipitation method (Mobasherpour et al., 2007) using (NH₄)₂HPO₄ (Merck No. 1205) and Ca(NO₃)₂·4H₂O (Analar No. 10305) as starting materials and ammonia solution as agents for pH adjustment. A suspension of Ca(NO₃)₂·4H₂O was vigorously stirred and its temperature was maintained at 25 °C. A solution of (NH₄)₂HPO₄ was slowly added dropwise to the Ca(NO₃)₂·4H₂O solution. In all experiments the pH of Ca(NO₃)₂·4H₂O solution by adding ammonia solution was 11. The precipitate HAP was removed from the solution by the centrifuge method at a rotation speed of 3000 rpm. The resulting powder was dried at 100 °C. The particles thus synthesized were characterized by the following methods. Transmission electron microscopy (TEM) was used to characterize the synthesized particles of HAp. For this purpose, particles were deposited onto Cu grids, which support a ‘‘holey'' carbon film. The particles were deposited onto the support grids by deposition from a dilute suspension in acetone or ethanol. The crystalline shapes and sizes were characterized by diffraction (amplitude) contrast and, for crystalline materials, by high resolution (phase contrast) imaging. The specific surface area was determined from N₂ adsorption isotherm by the BET method using a Micromeritics surface area analyzer model ASAP 2010. The concentrations of Ca²⁺ and ${\text{PO}}_4^{3-}$ in the synthesized particles were determined by inductively coupled plasma atomic emission spectrometer (ICP-AES) by first dissolving the particles in HCl solution. The Fourier transform infrared spectra (FT-IR) of nano hydroxyapatite were recorded using a Perkin–Elmer 2000 FTIR spectrometer calibrated with a deuterated triglycine sulfate (DTGS) detector covering the frequency range of 500–4000 cm⁻¹. The sample cell was purged with nitrogen gas throughout data collection to exclude carbon dioxide and water vapor. Ten milligrams of the dried samples was dispersed in 200 mg of spectroscopic grade KBr to record the spectra.

The crystal phase was identified by powder X-ray diffraction (XRD) using Siemens (30 kV and 25 mA) X-ray diffractometer with Cu K_α radiation ( $\lambda =1.5404\text{A}˚$ ) and XPERT software.

2.2 Sorption study

All sorption experiments were carried out without any pre equilibrium processes that were imposed during the performance of any experiments. In order to determine the sorption capacity of nano HAp for Sn²⁺ cations, as well as the influence of the initial concentration of Sn²⁺ ion, adsorbent dosage and temperature, sorption experiments were performed by the batch equilibration technique.

Aqueous solutions containing Sn²⁺ ions of concentration (80, 100,120 and 140 mg/L) were prepared from Sn²⁺ chloride (SnCl₂·2H₂O, Merck No. 7815). 0.01 g of nano HAp was introduced in a stirred tank reactor containing 500 mL of the prepared solution. The stirring speed of the agitator was 300 rpm. The temperature of the suspension was maintained at 25 ± 1  C. The initial pH of the solution was adjusted to the value 6.5 by adding NH₃ and HCl. Samples were taken after mixing the adsorbent and Sn²⁺ ion bearing solution at pre determined time intervals (5, 10, 20, 30, 60, and 120 min) for the measurement of residual metal ion concentration in the solution and to ensure equilibrium was reached. After any specified time the sorbents were separated from the solution by centrifuge and filtration through the filter paper (Whatman grade6). The exact concentration of metal ions was determined by AAS (GBC 932 Plus atomic absorption spectrophotometer). All experiments were carried out twice. The mass balance of tin is given by:

The percent removal (%) was calculated using the following equations:

3.1 Characteristics of adsorbent

TEM micrograph of the HAp powders after drying is seen in Fig. 1a. The microstructure of the HAp crystalline after drying is observed to be almost like needle shape, with size in the range of 20–30 nm. The crystal structure analysis of HAp particles was performed, using X-ray diffraction, and the obtained diffractograms are represented in Fig. 1b. The produced reflection patterns match the ICDD standards (JCPDS) for HAp. The patterns show only the peaks characteristic of the synthesized HAp with no obvious evidence on the presence of other additional phases. The broad patterns around (2 1 1) and (0 0 2) indicate that the crystallites are very tiny in nature with much atomic oscillations. The sample was indexed in the hexagonal system with space group P6₃/m and Calcium Phosphate Hydroxide ICSD name (Ref. Code.01-074-0565 in XPERT software). The unit cell parameters of nano Hydroxyapatite synthesized were a = b = 0.9424 nm and c = 0.6879 nm. The analysis of the HAp sample has confirmed a low-crystalline product, with the specific surface area of 94.9 m²/g. The Ca/P molar ratio was found to be 1.65 ± 0.05 instead of 1.67 that represents the characteristic stoichiometric ratio. This cation deficiency is common for the sample obtained by the wet method.

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Figure 1

TEM micrograph (a) and XRD pattern (b) of calcium nanocrystalline hydroxyapatite after drying at 100 °C.

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The structure of the powder was analyzed using FT-IR spectroscopy after drying at 100 °C, as shown in Fig. 2. In the FT-IR analysis, mainly the peaks for ${\text{PO}}_4^{3-}$ and OH^– groups in the hydroxyapatite can be identified in Fig. 2. Peaks at 560–610 and 1000–1100 cm⁻¹ must be due to ${\text{PO}}_4^{3-}$ . For the OH^– group the peak positions are 636 and 3572 cm⁻¹.

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Figure 2

FTIR Spectroscopy analysis of the nano hydroxyapatite powder after drying at 100 °C.

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3.2 Effect of initial Sn²⁺ concentration and adsorbent dosage

The sorption of Sn²⁺ ions was carried out at different initial Sn²⁺ ion concentrations ranging from 80 to 140 mg/L, at pH 6.5, at 300 rpm with 120 min of contact time using nano-HAp. A rapid kinetic reaction of Sn removal by sorbent occurred within the first 10 min (Fig. 3a). The aqueous Sn concentration at 10 min decreased to 42, 62, 80 and 98 mg/L by nano-HAp for 80, 100, 120 and 140 mg/L initial concentration respectively. Uptake of the Sn²⁺ also increased with increasing the initial metal concentration tending to saturation at higher metal concentrations. As shown in Fig. 3b, when the initial Sn²⁺ concentration increased from 80 to 140 mg/L, the uptake capacity of nano HAp increased from 1985 to 2400 mg/g. A higher initial concentration provides an important driving force to overcome all mass transfer resistances of the pollutant between the aqueous and solid phases, thus increasing the uptake (Aksu and Tezer, 2005).

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Figure 3

Time dependent concentration (a) and effect of initial concentration on removal of aqueous Sn²⁺ by nano hydroxyapatite sorbents (pH 6.5, adsorbent dosage = 0.01 g/L, 300 rpm agitating rate).

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The effect of nano-HAp dosage is presented in Fig. 4. It is evident that adsorption increases with the increase in the mass of sorbent and the uptake capacity of Sn²⁺ decreased from 2400 mg/g (34.28% removal) to 1233.33 mg/g (52.86% removal) with the increasing nano-HAp concentration from 0.01 to 0.03 g/L. This is because at the higher dosage of sorbent due to increased surface area, more adsorption sites are available causing higher removal of Sn²⁺.

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Figure 4

Effect of adsorbent dosage on removal of Sn²⁺ by nano hydroxyapatite (pH 6.5, initial metal concentration = 140 mg/L, 300 rpm agitating rate).

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3.3 Effect of temperature

To study the effect of this parameter on the uptake of Sn²⁺ ions by nano-HAp, we selected the following temperatures: 25, 45 and 65  C. Fig. 5, illustrates the relationship between temperature and the amount of Sn²⁺ ions adsorbed onto nano-HAp at equilibrium time (120 min). As seen in Fig. 5, the adsorption of Sn²⁺ on nano-HAp increased from 2400 mg/g (34.28% removal) to 2750 mg/g (39.28% removal) when temperature was increased from 25 to 65 °C at an initial concentration of 140 mg/L. The increase in the equilibrium sorption of Sn²⁺ with temperature indicates that Sn²⁺ ions removal by adsorption on nano-HAp favors a high temperature. This may be a result of increase in the mobility of the Sn²⁺ ion with temperature. An increasing number of molecules may also acquire sufficient energy to undergo an interaction with active sites at the surface. Furthermore, increasing temperature may produce a swelling effect within the internal structure of nano-HAp enabling metal ions to penetrate further (Dogan and Alkan, 2003).

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Figure 5

Uptake capacity of Sn²⁺ ions at different temperatures (pH 6.5, initial metal concentration = 140 mg/L, adsorbent dosage = 0.01 g/L, 300 rpm agitating rate).

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3.4 Determination of thermodynamic parameters

Thermodynamic parameters such as free energy change (ΔG^o (ΔH^o), and entropy change (ΔS^o) can be estimated using equilibrium constants changing with temperature. The free energy change of the sorption reaction is given by the following equation:

3.5 Adsorption isotherms

Analysis of the equilibrium data is important to develop an equation which accurately represents the results and which could be used for design purposes (Aksu, 2002). Several isotherm equations have been used for the equilibrium modeling of adsorption systems.

The sorption data have been subjected to different sorption isotherms, namely, Langmuir, Freundlich, and Dubinin–Kaganer–Radushkevich.

The equilibrium data for metal cations over the concentration range from 80 to 140 mg/L at 25  C have been correlated with the Langmuir isotherm (Langmuir, 1918):

The linearized Langmuir adsorption isotherms of Sn²⁺ ions are given in Fig. 6a. An adsorption isotherm is characterized by certain constants whose values express the surface properties and affinity of the sorbent and can also be used to find the sorption capacity of the sorbent.

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Figure 6

Linear fits of experimental data obtained using Langmuir (a), Freundlich (b) and D–K–R (c) sorption isotherms for the adsorption of Sn²⁺ onto nano Hydroxyapatite (pH 6.5, initial metal concentration = 80, 100, 120 and 140 mg/L, adsorbent dosage = 0.01 gr/L, 300 rpm agitating rate).

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The Freundlich sorption isotherm, one of the most widely used mathematical descriptions, usually fits the experimental data over a wide range of concentrations. This isotherm gives an expression encompassing the surface heterogeneity and the exponential distribution of active sites and their energies. The Freundlich adsorption isotherms were also applied to the removal of Sn²⁺ on nano-HAp (Fig. 6b).

The Dubinin–Kaganer–Radushkevich (D–K–R) equation has been used to describe the sorption of metal ions on clays. The D–K–R equation has the form:

The correlation factors R (0.997, 0.929 and 0.935 for Langmuir, Freundlich and D–K–R models, respectively) confirm good agreement between the two theoretical models and our experimental results. The maximum sorption capacity, Q₀, calculated from the Langmuir equation is 2500 mg/g, while Langmuir constant K is 0.07 L/mg. The values obtained for Sn²⁺ from the Freundlich model showed a maximum adsorption capacity (K_f) of 899.64 mg/g with an affinity value (n) equal to 4.58. The values of sorption constants, derived from the D–K–R model are: 5092.06 mg/g (42.89 mmol/g) for X_m, −2 × 10⁻⁹ mol²/J² for β and 15.81 kJ/mol for E.

The values indicate that the adsorption pattern for Sn²⁺ on nano-HAp followed thrid the Langmuir isotherm (R² > 0.997), the D–K–R isotherm (R² > 0.935), and the Freundlich isotherm (R² > 0.929) at all experiments. It is clear that the Langmuir isotherm is the best fit for the sorption of Sn²⁺ on nano-HAp. When the system is in a state of equilibrium, the distribution of Sn²⁺ between nano HAp and the Sn²⁺ solution is of fundamental importance in determining the maximum sorption capacity of nano HAp for the Sn²⁺ ion from the isotherm. The E values are 15.81 kJ/mol for Sn²⁺, on the nano HAp. It is the orders of an ion-exchange mechanism, in which the sorption energy lies within 8–16 kJ/mol.

Generally, HAp selectivity toward divalent metal cations is a result of the ion-exchange process with Ca²⁺ ions (Monteil Rivera and Fedoroff, 2002). Ionic radius of Sn²⁺ (0.71 Å) slightly differ from that of Ca²⁺ (0.99 Å), and it can substitute Ca²⁺ in the HAp crystal lattice. Fig. 7 presents the XRD patterns of the Sn²⁺-loaded sample. No structural changes of nano HAp were detected by the powder X-ray diffraction analysis of the solid residue with the maximum amount of uptake capacity of Sn²⁺, obtained after interaction of 0.01 g/L of nano HAp with 140 mg/L Sn²⁺ solution, at pH 6.5, at 300 rpm with 120 min of contact time. The sample was indexed in the hexagonal system with space group P6₃/m. The diffractogram’s evidence clarifies that all XRD peaks were shifted toward upper diffraction angles for Sn-HAp particles (maximum peak: from $2\theta ={31.94}^{°}\text{to}2\theta ={32.11}^{°}$ ). These shifts are indicative of the decrease in unit cell dimensions which is due to the replacement of Sn²⁺ (ionic radius 0.71 Å), which is smaller than Ca²⁺ (ionic radius 0.99 Å), into the crystal lattice of apatite molecules.

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Figure 7

XRD pattern of the solid residue with maximum amount of uptake capacity of Sn²⁺ (pH 6.5, initial metal concentration = 140 mg/L, adsorbent dosage = 0.01 g/L, 300 rpm agitating rate).

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The reaction mechanism corresponds to the equimolar exchange of tin and calcium yielding Ca_10−xSn_x(PO₄)₆(OH)₂, where x can vary from 0 to 10 depending on the reaction time and experimental conditions. Our results of synthesized nano-HAp agreed with those described elsewhere that the proposed mechanism for Sn²⁺ removal by HAp comprises two steps: firstly, rapid surface complexation of Sn²⁺ on the ≡POH sites of HAp which causes the decrease of the pH (from pH = 6.5 to pH = 6.0 for initial metal concentration = 140 mg/L, dosage = 0.01 g/L, 300 rpm agitating rate) and secondly, partial dissolution of calcium followed by the precipitation of an apatite with formula: Sn_xCa_10−x(PO₄)₆(OH)₂.

In which Sn²⁺ ions are first adsorbed on the nano HAp surface and substitution with Ca²⁺ ion occurs as described by the following equation:

3.6 Sorption kinetics

Sorption kinetic studies were carried out in order to understand the behavior of nano-HAp toward Sn²⁺ metal. The sorption kinetics includes two phases: a rapid metal sorption stage followed by a much slower stage before the equilibrium was established. It was found that the mass transfer was the key factor in the metal sorption (Chen and Wang, 2004). The sorption kinetics describes the metal sorption rate, which in turn governs the residence time of the sorption reaction and also the efficiency of the sorption process. Out of the several kinetic models available to examine the controlling mechanism of the sorption kinetic process and to test the experimental data, the Lagrangian equation or pseudo-first-order equation and pseudo-second-order equation have been used for the metal sorption kinetics of nano-HAp.

The linearized form of the pseudo-first-order equation:

The linearized form of the pseudo-second-order equation for the kinetics of absorption described by Ho and Chiang (2001) is as follows:

The second-order rate constant (k₂) and q_{e cal} were determined from the slope and intercept of the plot obtained by plotting t/q_t vs t.

Linear plots of log (q_e − q_t) versus t and t/q_t versus t are depicted in Fig. 8a and Fig. 8b at 25, 45 and 65  C, respectively. A comparison of the results with the correlation coefficients is shown in Table 3. The pseudo-second-order kinetic model obtained for Sn²⁺ sorption at various temperatures showed a better correlation of the result than the pseudo-first-order equation model. The correlation coefficients for the second order kinetic model obtained at 140 ppm concentrations at different temperatures were high. The values of k₂ at 25, 45 and 65  C were varied from 0.00023 to 0.000191 min⁻¹. The higher rate of metal sorption in the beginning (Fig. 3b) could be due to the presence of the active site in the nano-HAp surface, available for the sorption of metals. Once the sorptive sites are exhausted, the uptake rate may be controlled by the rate of intra particle diffusion. The activation energy Ea was determined using the Arrhenius equation (Aksu, 2002):

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Figure 8

Linear fit of experimental data obtained using pseudo-first order kinetic model (a) and pseudo-second order kinetic model (b) (pH 6.5, initial metal concentration = 140 mg/L, adsorbent dosage = 0.01gr/L, 300 rpm agitating rate).

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