Comparative of the removal of Pb²⁺, Cd²⁺ and Ni²⁺ by nano crystallite hydroxyapatite from aqueous solutions: Adsorption isotherm study

2.1 Preparation of nano crystallite 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 via solution-precipitation method (Mobasherpour et al., 2007) using Ca(NO₃)₂·4H₂O (Analar No. 10305) and (NH₄)₂HPO₄ (Merck No. 1205) as starting materials and ammonia solution as agents for pH adjustment. A suspension of Ca(NO₃)₂·4H₂O was vigorously stirred at constant temperature 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 was kept 11 using ammonia solution. The precipitin 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 synthesized were characterized by the following methods. Transmission electron microscopy (TEM) was used to characterize the HAp particles. For this purpose, particles were deposited onto Cu grids, which support a ‘‘holey’’ carbon film. The particles were deposited onto the support grids 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 crystal phase was identified by powder X-ray diffraction (XRD) using Siemens (30 kV and 25 mA) X-ray diffractometer with Cu Kα radiation (λ = 1.5404 Å) and XPERT software.

2.2 Reagents

Inorganic chemicals were supplied by Merck analytical-grade reagents and deionized water was used. The metal ions studied were Pb²⁺, Cd²⁺ and Ni²⁺. We prepared a synthetic stock solution of cadmium and nickel using their sulfate salts, 3CdSO₄·8H₂O (Merck Art No. 2026) and NiSO₄·7H₂O (Merck Art No. 6725), respectively, in deionized water. The stock solution of lead was prepared using Pb(NO₃)₂ (Merck Art No. 7397) salts.

2.3 Batch sorption study

All sorption experiments were carried without imposing any pre equilibration processes during the performance of any experiments. Batch adsorption experiments were conducted using 0.1 g, 0.25 g and 2 g of adsorbent with 500 ml of solutions containing Pb²⁺, Cd²⁺ and Ni²⁺ ions of desired concentrations, respectively, at constant temperature (20 ± 1 °C) in 1000 ml glass bottles. The bottles were stirred for 120 min. The agitator stirring speed was 300 rpm. After 120 min, the sorbents were separated from the solution by centrifuge and filtration through the filter paper (Whatman grade 6).

The initial pH of the solution was adjusted to the value between 5 and 6 by adding NH₃ and HCl. The exact concentrations of metal ions were determined by AAS (GBC 932 Plus atomic absorption spectrophotometer). All experiments were carried out twice.

The mass balance of heavy metal ion is given by:

The distribution ratio (K_d) were calculated using the equations:

3.1 Characteristics of adsorbent

TEM micrograph of the HAp powders after drying at 100 °C is shown in Fig. 1(a). The microstructure of the HAp crystalline after drying was almost needle shape, with a 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. 1(b). The reflection patterns matched the ICDD standards (JCPDS) for HAp phase. The patterns only showed the peaks’ characteristic of HAp with no obvious evidences on the presence of other additional phases. The broad peaks around (2 1 1) and (0 0 2) planes indicated that the crystallites were very tiny in nature with much atomic oscillations. The analysis of the HAp sample has confirmed a low-crystalline product, with the specific surface area 94.9 m²/g.

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Figure 1

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

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3.2 Adsorption of metals on nano crystalline hydroxyapatite

The adsorption of Pb²⁺, Cd²⁺ and Ni²⁺ onto nano crystalline hydroxyapatite as a function of their concentrations was studied at 20 °C by varying the metal concentration from 100 to 400 mg/L while keeping all other parameters constant. The results are shown in Figs. 2 and 3. Amount of heavy metal ion removed (q) for Pb²⁺, Cd²⁺ and Ni²⁺ increases with increasing metal concentration in aqueous solutions.

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Figure 2

Uptake capacity of metal ions by nano hydroxyapatite sample as a function of initial concentration: V = 500 ml, pH 5–6, time = 120 min.

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Figure 3

Variation of metal ions on nano hydroxyapatite as a function of initial concentration: V = 500 ml, pH 5–6, time = 120 min.

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As shown in Fig. 2, when the initial metal cations concentration increased from 100 to 400 mg/L, the uptake capacity of nano HAp increased from 430 to 700 mg/g, 134 to 142 mg/g and 20 to 36.25 mg/g for Pb²⁺, Cd²⁺ and Ni²⁺, respectively. A higher initial concentration provided an important driving force to overcome all mass transfer resistances of the pollutant between the aqueous and solid phases thus increased the uptake (Aksu and Tezer, 2005).

Fig. 3 illustrates K_d as a function of metal ions concentration. The K_d values increase with the decreasing concentration of metal ions. In other words, the K_d values increase as dilution of metal ions in solution proceeds. These results indicate that energetically less favorable sites become involved with increasing metal concentration in the aqueous solution.

3.3 Adsorption isotherms models

Analysis of the equilibrium data is important to develop an equation which accurately represents the results and can be used for the 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 (DKR). An adsorption isotherm is characterized by certain constants which values express the surface properties and affinity of the sorbent and can also be used to find the sorption capacity of sorbent.

The equilibrium data for metal ions over the concentration range from 100 to 400 mg/L at 20 °C have been correlated with the Langmuir isotherm (Langmuir, 1918):

The Langmuir model parameters and the statistical fits of the sorption data to this equation are given in Fig. 4 and Table 1. The Langmuir model effectively described the sorption data with all R² values >0.985. According to the Q₀ parameter, sorption on nano hydroxyapatite is produced following the sequence Pb²⁺ > Cd²⁺ > Ni²⁺. The preference of sorption exhibited by the nano hydroxyapatite for Pb over Cd and Ni may be attributed to Pb’s smaller hydrated radius (Pb²⁺ = 0.401 nm, Cd²⁺ = 0.426 nm, Ni²⁺ = 0.404 nm) and hydration energy (Pb²⁺ = −1481 kJ/mol, Cd²⁺ = −1807 kJ/mol, Ni²⁺ = −2106 kJ/mol).

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Figure 4

Langmuir plots for metal ions adsorption onto nano hydroxyapatite.

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The groups present on the nano hydroxyapatite are OH⁻ and ${\text{PO}}_4^{3-}$ group, which are hard Lewis bases. Pb²⁺ is a borderline hard Lewis acid while Cd²⁺ and Ni²⁺ is a soft Lewis acid. This could be one of the reasons for greater affinity of Pb as compared to Cd and Ni. The other reason for affinity could be the higher electronegativity of Pb than Cd and Ni for electrostatic and inner sphere surface complexation reactions.

According to LeGerose and LeGerose (1984), cations with ionic radii smaller than Ca²⁺ (0.099 nm) have less of a chance to be incorporated into a hydroxyapatite structure compared with cations with larger ionic radii. Therefore, precipitation of Ni²⁺ (0.072 nm) with Ca²⁺ would be less likely compared with the precipitation of larger cations Pb²⁺ (0.118 nm) and Cd²⁺ (0.097 nm).

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 gave 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 Pb²⁺, Cd²⁺ and Ni²⁺ on nano-HAp (Fig. 5).

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Figure 5

Freundlich plots for metal ions adsorption onto nano hydroxyapatite.

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

The plot of ln C_ads against ɛ² for metal ion sorption on nano hydroxyapatite is shown in Fig. 6. The DKR parameters are calculated from the slope of the line in Fig. 6 and listed in Table 3. As shown in Table 3, the E value are 15.811 for Pb²⁺, 40.825 for Cd²⁺, and 12.909 kJ/mol for Ni²⁺ on the nano HAp. The E values are 15.811 and 12.909 kJ for Pb²⁺ and Ni²⁺, on the nano HAp, respectively. They are the orders of an ion-exchange mechanism, in which the sorption energy lies within 8–16 kJ/mol. on the other hand, the E values is 40.825 kJ for Cd²⁺ cations on the nano HAp. It is the orders of a stronger chemical adsorption than ion exchange. The sorption capacity X_m in the DKR equation is found to be 1003.726 for Pb²⁺, 150.028 for Cd²⁺, and 57.812 mg/g for Ni²⁺.

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Figure 6

DRK plots of metal ions on nano hydroxyapatite at constant temperature.

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Results show that, it is clear that the Langmuir isotherm has best fitted for the sorption of heavy metal cations on nano-HAp. When the system is in a state of equilibrium, the distribution of cations between the nano HAp and the cations solution is of fundamental importance in determining the maximum sorption capacity of nano HAp for the metal ion from the isotherm.

The values of the adsorption capacities for the adsorption of Pb²⁺, Cd²⁺ and Ni²⁺ cations on different adsorbents used in the literature with adsorbent of the present study are summarized in Table 4. Although direct comparison of the nano HAp with other adsorbent materials is difficult, owing to the differences in experimental conditions, it was found that the adsorption capacity of nano HAp was higher than adsorbents presented in Table 4.