Synthesis of iron-substituted hydroxyapatite nanomaterials by co-precipitation method for defluoridation

The point of zero charge (pzc) is the point at which initial and final pH becomes equal (Mahmood et al 2011). Moreover, it is determined by plotting the change in pH (the difference in initial and final) versus initial pH (pH_i). The point of zero charge of the synthesized pure and iron-doped hydroxyapatite were found to be 7.6 and 8.2, respectively (figure 5(a)). This indicates that the surface charge of the materials is positive below the corresponding values of pzc and negatively charged above the values. The high pzc value of iron-doped hydroxyapatite indicates that the surface of Fe-HA has extra positive charge. The solution pH is one factor affecting the removal of fluoride since a change in pH of the solution could also changes the surface properties and dissolution behavior of materials (Wang et al 2017, Fernando et al 2019, Fernando et al 2021). At pH less than 3, both pure and iron-doped hydroxyapatite were not stable, so that pH influence was optimized by varying it from 3–11 using 0.1 N HCl and 0.1 N NaOH solutions. The fluoride removal tendency of the pure hydroxyapatite was found to increase with increase in pH and attained maximum value of 78.46% at pH = 6 and then sharply decreased in the alkaline media to 45.31% at pH = 11 (figure 5(b)). The decrease in fluoride removal in the acidic solution may be due to apatite dissolution, according to the reaction given in different reports (Dorozhkin 2012, Nayak et al 2017, Wimalasiri et al 2021). However, in the case of alkaline pH, the electrostatic repulsion between fluoride and OH⁻ ions decreases the removal of percentage (Tang et al 2018, Fernando et al 2021). In the case of iron-doped hydroxyapatite, the adsorption efficiency was high in acidic media and then dropped to a lower value in the basic media. This indicated that the surface charge of iron-doped hydroxyapatite is positive in acidic media and becomes negative in the alkaline condition. This result agrees with previous reports on tri-valet metal ion-doped hydroxyapatites (Nie et al 2012, Chen et al 2018). During the pH change from 3 to 11, the percentage removal was observed to change from 89.8%–71.5% for iron-doped hydroxyapatite. The result also showed that iron-doped hydroxyapatite was less affected by pH change when compared with the pure hydroxyapatite.

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The variation of fluoride removal capacity of HA and Fe-HA was determined separately by varying the time of interaction from 10–240 min using 10 mg l⁻¹ of F^–, pH 6.5 and 0.5 g L⁻¹ of adsorbents. The result is shown in figure 6(a). Similarly, the non-linear fitting plots for pure and iron-doped hydroxyapatites are given in figures 6(b) and (c) respectively.

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The rate of adsorption processes of pure and iron-doped hydroxyapatites (figure 6(a)) varied very fast in the first 60 min and then slowly increased up to 180 min which was considered as the equilibrium time. In order to further understand the rate and mechanism of adsorption, the experimental data were non-linearly fitted with pseudo-first order, pseudo-second order and Elovich models based on equations (9), (10) and (11), respectively (Nayak et al 2017).

$$\begin{eqnarray}&&ln\left({q}_{e}-{q}_{t}\right)=ln{q}_{e}-{k}_{1}t\end{eqnarray} \tag{ 9 }$$

$$\begin{eqnarray}&&\displaystyle \frac{t}{{q}_{t}}=\,\displaystyle \frac{1}{{k}_{2}{{q}_{e}}^{2}}+\,\displaystyle \frac{t}{{q}_{e}}\end{eqnarray} \tag{ 10 }$$

$$\begin{eqnarray}&&{q}_{t}=\displaystyle \frac{1}{\beta \,}\,\mathrm{ln}(\alpha \beta )+\displaystyle \frac{1}{\beta }\,\mathrm{ln}\,t\end{eqnarray} \tag{ 11 }$$

Where, ${q}_{t}$ and ${q}_{e}$ are the amounts (mg g⁻¹) of fluoride ion adsorbed at time t and equilibrium respectively ${k}_{1}$(1/min) ${\rm{is}}$ the pseudo-first order rate constant, ${k}_{2}$ (g/(mg.min)) the pseudo-second order rate constant, α (mg/(g.min)) the initial sorption rate constant and β (mg g⁻¹) is the extent of surface coverage during chemisorption.

Based on the comparison of Adj. R², rate constants (k₁, k₂ and ${\rm{\alpha }}$) and the closeness of the experimental value to the calculated values of the equilibrium adsorption capacity (qe) given in table 2; the kinetics result fits more with pseudo-second order model. This indicated that the adsorption process is chemisorption through ion exchange or electrostatic interaction (Nayak et al 2017, Wang et al 2017, Chen et al 2018), is also confirmed by the high values of Adj.R² and the constants in the Elovich model.

Table 2. Calculated and experimental values of different kinetic parameters.

Kinetic models	Parameters	HA	Fe-HA
	${q}_{e},$ exp. (mg g−1)	15.62	17.81
SFO	${q}_{e}$ (mg g−1)	15.15	17.42
	${k}_{1\,}$(1/min)	0.086	0.094
	R2	0.988	0.993
SSO	${q}_{e}$ (mg g−1)	15.91	18.16
	${k}_{2}$ (g/(mg.min))	0.012	0.013
	R2	0.999	0.999
Elovich	${\rm{\alpha }}$ (mg/(g.min))	0.396	0.445
	β (mg g−1)	0.854	0.883
	R2	0.998	0.998

The effect of the amount of fluoride ion in the solution was optimized using fluoride ion concentration from 5–30 mg l⁻¹ at pH 6.5, contact time of 3 h and adsorbent dose of 0.5 g L⁻¹. It was perceived that initially, the removal percentage of the adsorbents (HA and Fe-HA) rose with increasing initial fluoride amount until 10 mg l⁻¹ and then it slightly decreased. However, the adsorption capacity increased even beyond 10 mg l⁻¹. The high fluoride ion uptake of the materials at the lower concentration indicates that there is limited amount of energetic sites of the materials (Nayak et al 2017). In order to understanding the adsorption mechanisms well, the obtained results were fitted non-linearly to Langmuir, Freundlich, and Temkin isotherm models. Langmuir isotherm of monolayer sorption onto homogenous surface (Girish 2017), is expressed by equation (12):

$$\begin{eqnarray}&&\displaystyle \frac{1}{{q}_{e}}=\,\displaystyle \frac{1}{b{q}_{o}{C}_{e}}+\displaystyle \frac{1}{{q}_{o}}\end{eqnarray} \tag{ 12 }$$

Where ${q}_{e}\,$the amount of fluoride ion adsorbed at equilibrium (mg g⁻¹), and ${C}_{e}$ is the equilibrium fluoride ion concentration (mg l⁻¹), b (l mg⁻¹) is a constant related to binding energy, and ${q}_{o}$ is monolayer adsorption capacity in (mg g⁻¹). Another important parameter (r) is needed to describe the feasibility of adsorption: $r=\,\displaystyle \frac{1}{1+b{C}_{o}};$ where ${C}_{o}$ is the initial concentration of F⁻ ion (mg l⁻¹) and r is the Langmuir isotherm constant.

The Freundlich isotherm model of multilayer sorption on heterogeneous surfaces (Girish 2017), is given by the equation:

$$\begin{eqnarray}&&log{q}_{e}=log{K}_{f}+\,\displaystyle \frac{1}{n}\,log{C}_{e}\end{eqnarray} \tag{ 13 }$$

Where Kf ((mg g⁻¹) (l mg⁻¹)^1/n) a constant is related with the adsorption capacity and n is an empirical parameter indicating the extent of adsorption.

Similarly, the Temkin isotherm that considers the induced heterogeneity due to the indirect adsorbate/adsorbent interaction (Ayawei et al 2017, Özsin et al 2019b), is expressed using:

$$\begin{eqnarray}&&{q}_{e}=\,{B}_{T}ln{K}_{T\,}+\,{B}_{T}ln{C}_{e}\end{eqnarray} \tag{ 14 }$$

Where, ${K}_{T\,}$ (l g⁻¹) is the Temkin isotherm binding constant, and ${B}_{T}$ (J/mg) is the constant related to the heat of sorption.

The plots of the isotherm models and the isotherm constants are presented in figure 7 and table 3 respectively. The data indicates that Langmuir model describes better the experimental result, it confirms that adsorption is more of a monolayer on homogeneous surface (Chen et al 2018). The value of r in any of the tested concentration for both pure and iron-doped hydroxyapatite is the range of 0 < r < 1, which indicates that the adsorption is favourable.

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Table 3. Calculated Values of isotherm constants of the different models.

Adsorbent	Langmuir			Freundlich			Temkin
	${q}_{o}$	b	R2	n	${K}_{f}$	R2	${B}_{T}$	${K}_{T}$	R2
HA	40.46	0.046	0.987	1.18	3.96	0.976	95.3	1.02	0.971
Fe-HA	83.86	0.280	0.988	1.67	9.16	0.977	305.54	3.36	0.970

The defluoridation capacity of Fe-HA and HA were 83.86 and 40.46 mg g⁻¹, respectively. The high adsorption capacity of Fe-HA is associated with the strong interaction of trivalent metal ion doped hydroxyapatite with fluoride ion (Chen et al 2018), high fluoride ion removal efficiency of iron-containing material (Sujana and Anand 2010, Corral-Capulin et al 2019, Ogata et al 2020, Scheverin et al 2021), the less size, high surface area, high pore volume and less crystalline nature of it as observed in BET, FE-SEM and XRD.

The effect of adsorbent amount was studied by taking 10 mg l⁻¹ of fluoride concentration, pH 6.5, contact time 3 h, agitation speed 150 rpm and varying the adsorbent doses from 1.0–10.0 g l⁻¹. Figure 8 showed that the adsorption capacity of pure and iron-doped hydroxyapatite decreased sharply with increasing the adsorbents amount up to 4 g L⁻¹, and after that, it became near to constant. This is because, at the lowest amount of adsorbent, the number of active sites for the adsorbent is insufficient to remove the fluoride ions. However, when the adsorbent amount increases, the number of active sites also increases, causing increment in the removal tendency. The decreasing tendency of defluoridation capacity by further increasing of adsorbent amount is due to adsorbent-adsorbent interaction (Nayak et al 2017), or it may be associated with the repulsion between the fluoride ions in solution and those that are attached on the surface of the materials (Roy and Das 2016). It was observed that the fluoride residual concentration was within the WHO recommended limit (1.5 mg l⁻¹) for Fe-HA in all the amounts of adsorbent used.

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The temperature effect on adsorption of fluoride ion was analysed in the range of 298–333 K using 10 mg l⁻¹ of F^– and 4 g l⁻¹ doses. The result indicates that adsorption capacity of pure and iron-doped hydroxyapatite was observed to increase with increase in temperature. This is because as the temperature increases, the free volume of the pores of the adsorbent increases as a consequence to the movement of the solute (Nayak et al 2017). In order to determine the viability and spontaneous nature of the adsorption process, thermodynamic parameters were determined using equations (15)–(17) (Fernando et al 2019).

$$\begin{eqnarray}&&{\rm{\Delta }}G^\circ ={\rm{\Delta }}H^\circ -T{\rm{\Delta }}S^\circ \end{eqnarray} \tag{ 15 }$$

$$\begin{eqnarray}&&{\rm{\Delta }}G^\circ =-RTlnKd\end{eqnarray} \tag{ 16 }$$

$$\begin{eqnarray}&&lnKd=-\displaystyle \frac{{\rm{\Delta }}H^\circ }{RT}+\,\displaystyle \frac{{\rm{\Delta }}S^\circ }{R}\end{eqnarray} \tag{ 17 }$$

Where, R is universal gas constant, T is absolute temperature, $Kd=\displaystyle \frac{{C}_{a}}{{C}_{e}}$ is the distribution coefficient of F⁻, ${C}_{a}$ is the F⁻ concentration on the adsorbent at the equilibrium and ${C}_{e}$ is the concentration of F⁻ in the solution at the equilibrium.

The plots of $lnKd$ versus 1/T and thermodynamics parameters for the pure and iron-doped hydroxyapatite are depicted in figure 9 and table 4 respectively. The negative values of Gibbs free energy change (ΔG°) for both materials in all the temperatures indicates that the adsorption process is spontaneous. Similarly, the positive values of enthalpy change (ΔH°) showed that the adsorption process is endothermic since energy is needed to break the high interaction between the fluoride ion and water molecule in the solution. In addition, the positive entropy change (ΔS°) for pure and iron-doped hydroxyapatite indicates that the adsorption process is endothermic and random (Chen et al 2018, Fernando et al 2019). Furthermore, the lower values of ΔH° and ΔS° for iron-doped hydroxyapatite indicates that fluoride ion was more readily adsorbed on the surface of the iron-doped hydroxyapatite than that of the pure hydroxyapatite.

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The effect of ionic strength was evaluated by taking 0–5 mM of sodium salts of chloride, nitrate, sulphate and carbonate ions with 20 mg l⁻¹ of fluoride and 4 g l⁻¹ of the adsorbents. The result (figure10 ) ilustrated that the ionic strengt of the ions such as chloride, sulphate and nitrate have a negligiable effect on the adsorption of fluoride ion on both pure and iron-doped hydroxyapatite. However, the precence of carbonate ion at different ionc strengths has an effect on the fluoride removal. The reduction in fluoride removal of the adsorbents in the presence of carbonate ion indicates that the competation of this ion is dependant mainly on the size and charge (Mondal et al 2016). Moreover, this effect is more pronounced in pure hydroxyapatite than in iron-doped hydroxyapatite, which implies that iron-modified hydroxyapatite has a more active site for fluoride interaction than pure hydroxyapatite.

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Since fluoride ion was poorly adsorbed in alkaline conditions for the two adsorbents, 1.0 M NaOH solutions was used as a regenerator. The regeneration analysis was performed using 10 mg l⁻¹ F⁻¹⁻, 4 g l⁻¹ adsorbents, 3 h contact time, and an agitation speed of 150 rpm at room temperature (25 ± 2 °C). Five consecutive adsorption - desorption cycles were performed to investigate the regeneration and recyclability of the pure and iron-doped hydroxyapatite. The results (figure 11) indicates that the removal percentage of the materials in the successive cycle was changed insignificantly assuring that the materials are suitable for regeneration and reuse.

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Adsorption mechanism prediction is highly significant for the study of materials characteristics (He et al 2017). To understand fluoride uptake through the pure and iron-doped hydroxyapatites, FT-IR analysis before and after adsorption, the adsorption tendency of fluoride ion at different pH range, adsorption models and thermodynamics data were considered. The probable dominant mechanisms of adsorption could be ion exchange and electrostatic interaction/complexation on the surfaces of the adsorbent materials, as shown in figure 12. FT-IR spectra of HA after fluoride adsorption (figure 2(b)) showed a decrease in the intensity of the O-H stretching at 3693 cm⁻¹ which is due to partial replacement of OH⁻ ions by fluoride ions (He et al 2017, Chen et al 2018, Nagaraj et al 2018). Similarly, the slight shift of the O-H stretching from 3427 cm⁻¹ to 3416 cm⁻¹ may indicate the formation of H.....F bond. A new band at 695 cm⁻¹ (less intense) may be associated with forming the H....F bond or it may be the electrostatic interaction between calcium ion and fluoride ion. For iron doped hydroxyapatite (figure 2(d)), the O-H peak was shifted from 3433 cm⁻¹ to 3331 cm⁻¹ with decreasing intensity and increasing broadness. It may be due to the formation of F-HA through the linkage of OH₂ ⁺...F⁻ (Nayak et al 2017). In addition, a new intense and sharp peak at 557 cm⁻¹ may be associated with the linkage of negative fluoride ions with the positive metal ions through complex formation. In this case, the carbonate ions also participated in the fluoride adsorption, as it is observed that the two carbonate peaks shifted from 826 cm⁻¹ to 869 cm⁻¹ (with decreasing intensity) and from 1391 cm⁻¹ to 1352 cm⁻¹ (with decreasing in intensity and broadness). Compared to the pure and hydroxyapatite, the high adsorption capacity of iron-doped hydroxyapatite indicates that the interaction of fluoride ion with Fe-HA is a complex process.

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The adsorption capacity of the pure and iron-doped hydroxyapatites was compared with the previously reported hydroxyapatite-based adsorbents (table 5). Iron-doped hydroxyapatite exhibited a high adsorption capacity at low temperature, neutral pH and moderate contact time than most of the adsorbents used previously except Al-HA nanowire (He et al 2017), which uses a high amount of initial fluoride ion (200 mg l⁻¹) and high temperature.