Adsorption behavior of Cd²⁺ ions using hydroxyapatite (HAp) powder

2.1 Materials

HAp was synthesized by chemical precipitation using Ca(NO₃)₂ and (NH₄)₂HPO₄. The obtained HAp is a single-phase, cylinder-shaped, average crystal with size <100 nm [13].

Ca(NO₃)₂·4H₂O (M=236.15 g/mol, ≥99.0%), (NH₄)₂HPO₄ (M=132.05 g/mol, ≥99.0%), and 65% HNO₃ (D=1.37 g/ml, M=36.46 g/mol). NaOH (99.2%, M=40 g/mol) and Cd(NO₃)₂ (M=236.42 g/mol, ≥99.0%) are purified chemicals from Merck (Darmstadt, Germany).

2.2 Adsorption experiments

Cd²⁺ ion adsorption experiments were conducted using a 50 ml Cd(NO₃)₂ solution at various initial concentrations from 56 to 281 mg/l. HAp powder (0.1 g) was dispersed into the Cd(NO₃)₂ solution. The initial pH values of the solution were adjusted in the range from 2 to 10 by using 65% HNO₃ or 5% NaOH solution, and controlled with a pH meter (827 pH Lab). The mixtures were stirred at a rate of 750 rpm by using a magnetic stirrer (VMS-C7 Advanced) during different times (5, 10, 15, 20, 30, 45, 60, 90, and 120 min). Thereafter, the suspensions were separated by using a glass fiber filter with diameter of 90 mm (GA-55). The Cd²⁺ concentration in the solutions after treatment was determined using an atomic absorption spectrometer (AAS Perkin-Elmer 3110).

The Cd²⁺ adsorption capacity and efficiency were calculated according to the following equations:

where q (mg/g) is the Cd²⁺ adsorption capacity and H (%) is the HAp adsorption efficiency. C_o (mg/l) is the initial Cd²⁺ concentration in the solution, C_e (mg/l) is the equilibrium Cd²⁺ concentration in the solution after treatment, V (l) is the solution volume, and m (g) is the mass of HAp.

The dissolution of HAp at different pH values was determined through the concentration of calcium (Ca²⁺) in the solution when HAp is dispersed in water. The Ca²⁺ concentration leached was determined by using AAS.

The phase component of adsorbent before and after Cd²⁺ treatment were analyzed using X-ray diffraction (XRD) (Siemens D5000 diffractometer, CuK_α radiation, λ=1.54056 Å, with a step angle of 0.030°, scanning rate of 0.04285°/s, and 2θ in the range of 20–70°).

The crystallite diameter of HAp before and after treatment was calculated from (002) reflection in XRD patterns, using the Scherrer equation [14]:

where D (nm) is the crystallite size, λ is the wavelength of the X-ray radiation (CuKα), θ (rad) is the diffraction angle, and B is the full width at half-maximum (rad) of the peak along (002) direction.

3.1 Effect of contact time

The variation of the cadmium adsorption capacity and efficiency according to the contact time determined by using 0.1 g HAp powder with 281 mg/l initial Cd²⁺ concentration, pH 5.9, at 20°C, is presented in Figure 1. When the contact time increased from 5 to 60 min, the adsorption capacity also increased. The adsorption capacity increased rapidly within the first 20 min, then increased slowly and reached a plateau after 60 min. The adsorption efficiency followed the same trend in changed contact time. After 60 min, the efficiency was 86±0.5%, corresponding to an adsorption capacity of 122±0.5 mg/g. Therefore, for further experiments, a contact time of 60 min was chosen because a steady-state behavior was reached.

Figure 1:

Variation of Cd²⁺ adsorption capacity and efficiency of 0.1 g HAp according to the contact time.

3.2 Adsorption kinetics

The adsorption kinetics were investigated with 50 ml of a 281 mg/l solution of Cd²⁺ in the presence of 0.1 g HAp powder at a stirring rate of 750 rpm at 20°C. The amount of Cd²⁺ ions adsorbed at equilibrium time (q_e) was calculated using Eq. (4) below:

where C_o and C_e are the initial and equilibrium concentrations (mg/l) of Cd²⁺ ions in solution, V is the solution volume (l), and m is the absorbent mass (g) used during the experiments. The obtained experimental data after different times from 5 to 120 min were analyzed using three kinetic models: Lagergren’s pseudo-first-order law, McKay and Ho’s pseudo-second-order law, and the intra-particle diffusion model. The equations of the three models are given in Eqs. (5), (6), and (7), respectively:

where q_t (mg/g) is the adsorption capacity at time t, q_e (mg/g) is the adsorption capacity at equilibrium, and k₁ (min⁻¹) is the pseudo-first-order adsorption rate constant;

where k₂ (g/min·mg) is the pseudo-second-order rate constant for adsorption; and

where C is the intercept that provides the ideal boundary layer thickness and k_p is the intra-particle diffusion rate constant (mg/g·min^1/2 g).

Figure 2A presents the kinetic data plotted using Lagergren’s pseudo-first-order equation [Eq. (5)], while Figure 2B presents the kinetic data plotted using McKay and Ho’s pseudo-second-order equation [Eq. (6)]. A linear relationship with a high correlation coefficient (R²=0.9997) between t/q_t and t was obtained, which indicates the applicability of the pseudo-second-order model to describe the Cd²⁺ adsorption process. The calculated parameters of this model are given in Table 1.

Figure 2:

Adsorption data modeled using three kinetic models: (A) Lagergren’s pseudo-first order law, (B) McKay and Ho’s pseudo-second-order law, and (C) the intra-particle diffusion model.

3.3 Effect of Cd²⁺ concentration

The influence of the initial Cd²⁺ concentration on the adsorption capacity of HAp powder was examined with initial Cd²⁺ concentrations ranging from 56 to 281 mg/l. Figure 3 presents the adsorption capacity and efficiency of 0.1 g HAp adsorbent dispersed in 50 ml Cd²⁺ solution with different concentrations at 20°C and stirred at a rate of 750 rpm for 60 min. The results showed that when the initial Cd²⁺ concentration increased from 56 to 281 mg/l, the adsorption capacity increased strongly from 28±0.1 to 122±0.5 mg/g. The increase can be explained as follows: higher initial concentrations are able to overcome mass-transfer-related resistances existing between the aqueous and solid adsorbent phases by effectively creating a driving force [12].

Figure 3:

Variation of the Cd²⁺ adsorption capacity and efficiency of 0.1 g HAp according to the Cd²⁺ initial concentration.

3.4 Adsorption isotherm

In order to determine the adsorption capacity of HAp toward Cd²⁺ ions, sorption studies were carried out as follows: 0.1 g HAp adsorbent was dispersed into 50 ml Cd²⁺ solution with different initial concentrations from 56 to 281 mg/l, at 20°C; the mixtures were agitated at 750 rpm for a contact time of 60 min. The Langmuir and Freundlich adsorption isotherms were used to describe the Cd²⁺ removal process by HAp powder. The Freundlich isotherm equation used for modeling the equilibrium data is presented in its linear form below:

where k_F and n are Freundlich parameters and were determined by plotting log q_e versus log C_e.

The linear form of the Langmuir equation can be expressed as follows:

where q_m (mg/g) is the monolayer adsorption capacity; b (l/g) is the Langmuir constant that is related to the free energy of adsorption; and C_e (mg/l) and q_e (mg/g) are the equilibrium concentrations of adsorbate in solution and on the surface of HAp, respectively.

The parameters C_e, log C_e, Q, log Q, and C_e/Q for the two isotherms are determined and shown in Table 2. Figure 4 shows the linear fits of the experimental data using these adsorption isotherms. The results indicated that the adsorption became increasingly favored as the Cd²⁺ initial concentration increased. The analysis of results on the basis of the R² value for two isotherm equations showed that the Langmuir adsorption isotherm plot displayed good linear fit (R²=0.9962). From the slope of the fit, the calculated maximum monolayer adsorption capacity was about 119 mg/g.

Figure 4:

(A) Freundlich and (B) Langmuir adsorption isotherms for Cd²⁺ sorption by HAp powder.

3.5 Effect of solution pH

Subsequently, to investigate the effect of the initial pH of the solution on the removal of Cd²⁺ ions using HAp powder, the pH value at which the surface charge of HAp is zero (pH_pzc) was estimated. HAp (0.1 g) was dispersed into water with various initial pH values (pH_o) from 2 to 10, and the pH was adjusted using 65% HNO₃ or 5% NaOH solution. These mixtures were stirred at a rate of 750 rpm for 30 min at 20°C; thereafter, the pH was measured (pH₁). As the results of pH_o and pH₁, the ∆pH values were calculated and presented in Table 3. From the relationship between ∆pH and pH_o (Figure 5), the pH_pzc value corresponding to a ∆pH of 0 was determined, and its value was found to be about 7.2 in this study.

Figure 5:

Relationship between ∆pH and initial pH value of water containing 0.1 g HAp.

The concentration of Ca²⁺ leached from HAp powder into the solution with different initial pH values was determined using AAS (see Figure 6). The results showed that when the solution pH increased, the dissolution of HAp decreased. When the pH value was <4, HAp was partly dissolved, following Eq. (10). The results were confirmed by determining the Ca²⁺ concentration in the solution (Figure 6). In the pH range of 4–7, the pH₁ value increased. In contrast, final pH decrease occurred in the range of higher initial pH (7–10). The results can be explained according to the study by Wu et al. [15], as follows:

Figure 6:

Concentration of Ca²⁺ leached from HAp into the water according to solution pH.

In the lower range of initial pH value from 4 to 7, protons in the solution were consumed by protonation of the surface ≡P-O⁻ and ≡Ca-OH, resulting in final pH value increase. At higher initial pH (7–10), the final pH decreased due to OH⁻ consumption via deprotonation of surface ≡Ca-OH₂⁺ and ≡P-OH sites. Thus, neutral ≡Ca-OH and negatively charged ≡P-O⁻ species predominate in alkaline solutions, causing the HAp surface to become negatively charged in solutions with a high pH value [15], [16].

For Cd²⁺ adsorption, the suitable pH range is above the pH_pzc value, as the HAp surface is negatively charged in this range. In water, cadmium exists in different forms, e.g. Cd²⁺, Cd(OH)⁺, Cd(OH)2o, Cd(OH)_2(s), etc. [17], which are affected by the cadmium concentration and solution pH. Cd²⁺ ions are ionic species only in solutions with pH<6 [18]. At pH>8, cadmium forms dominant species as Cd(OH)₂ precipitates; in pH<8, Cd²⁺ and Cd(OH)⁺ are formed [19], [20]. Thus, the pH range chosen to investigate the Cd²⁺ adsorption capacity was from 2 to 8. Table 4 shows the effect of the initial pH on the adsorption capacity and efficiency of HAp. It was observed that at low solution pH (~2), the efficiency of Cd²⁺ removal was low. This can be explained on the basis of proton-competitive sorption reactions. At lower solution pH, H⁺ ions compete with Cd²⁺ ions for the surface binding sites of HAp, leading to the reduction of Cd²⁺ adsorption. When the solution pH increases, the competing effect of H⁺ ions decreases, whereas the efficiency of the Cd²⁺ removal process increases. In the pH range of 4–8, the efficiency does not change. Thus, pH 6 (pH_o) is the optimum pH value for the Cd²⁺ removal process.

3.6 Effect of adsorbent mass

The effect of HAp amount ranging from 0.05 to 0.15 g on the adsorption capacity and efficiency is presented in Table 5 and Figure 7. The data in Table 4 show that the amount of Cd²⁺ removed from the solution increased rapidly with increasing HAp amount from 0.05 to 0.1 g. However, when the amount of HAp continued to increase from 0.1 to 0.15 g, the efficiency increased slowly from 86% to 98%, whereas the adsorption capacity strongly decreased. Therefore, the optimum amount of HAp is 0.1 g.

Figure 7:

Effect of HAp mass on its adsorption capacity and efficiency.

3.7 Characterization of HAp adsorbent and Cd²⁺ exchanged samples

From the presented results, one can conclude that HAp powder has a maximum Cd²⁺ adsorption efficiency of about 86±0.5% at pH 6, for a contact time of 60 min, with 0.1 g amount of HAp. The phase composition of the obtained powder after treatment was investigated using X-ray analysis (Figure 8). In comparison with HAp, no changes were detected after treatment. From the XRD patterns, the crystal diameter was determined by using the Scherrer equation, and its value was 24.6 and 23.8 nm with HAp before and after treatment, respectively. The crystal diameter decreased slightly and can be explained by the ion exchange reaction between Ca²⁺ in HAp and Cd²⁺ adsorption on the surface of HAp, as the ionic radius of Cd²⁺ (0.97 Å) slightly differs from that of Ca²⁺ (0.99 Å).

Figure 8:

XRD patterns of HAp (A) before and (B) after Cd²⁺ treatment.

3.8 Cd²⁺ uptake mechanism

The Cd²⁺ uptake mechanism can be suggested as follows: the dissolution of HAp in aqueous solution containing Cd²⁺ ions follows Eq. (13).

The adsorption of Cd²⁺ on the surface of HAp and the ion exchange reaction between the Cd²⁺ adsorbed and the Ca²⁺ of HAp take place.

Moreover, the reaction to induce precipitation of Ca₁₀(PO₄)₆(OH)₂ or Ca_10−xCd_x(PO₄)₆(OH)₂ can occur as follows: