Efficient and low-cost removal of fluoride from water has attracted wide attention. Here, aluminum-modified hydroxyapatite (Al-HAP) was prepared by a homogeneous hydrothermal co-precipitation method. The physicochemical properties of the Al-HAP surface were characterized by SEM, XRD, FT-IR, BET, and zeta potential, and the adsorption performances were evaluated. It showed that Al-HAP has a larger specific surface area (121.97 m2/g, which is 2.3 times larger than that of HAP), more surface-active hydroxyls and positively charged at pH less than 7, which indicate that Al-HAP is beneficial to the adsorption of negatively charged fluoride. Al-HAP had a higher fluoride adsorption capacity (56.44 mg/g) than that of HAP (28.36 mg/g), and not sensitive to the interference of coexisting ions except CO32-. Based on the adsorption kinetics and adsorption isotherm experiments, the proposed two-stage kinetic model and Freundlich isotherm model can better describe the adsorption process. From the results of XPS and FT-IR, it indicated that the ion exchange between hydroxyl group on the surface and fluoride ions is the main driven force for the adsorption, and electrostatic adsorption is also helpful. The present study provides an improved HAP to effectively remove fluoride from water.

  • Aluminum-modified hydroxyapatite (Al-HAP) was prepared by a co-precipitation method.

  • The adsorption efficiency of Al-HAP was 30% higher than that of HAP.

  • Al-HAP belongs to mesoporous structure with larger specific surface area.

  • The equilibrium adsorption capacity of Al-HAP is 56.44 mg/g at 298 K.

  • The main mechanism is ion exchange.

Fluoride contamination of groundwater is one of the major global environmental issues. In many countries, groundwater is the sole source of drinking and cooking water (Jagtap et al. 2012; Carrard et al. 2019). In over 200 countries, including India and China, approximately 2.5 billion people consume drinking water with excessive fluoride content (Yadav et al. 2019; Podgorski & Berg 2022). The World Health Organization (WHO) stipulates a maximum fluoride limit of 1.5 mg/L in drinking water. Excessive fluoride intake can lead to dental and skeletal fluorosis (Amini et al. 2008), functional disorders (Chinnakoti et al. 2016), and in severe cases, even neurological diseases (Bhatnagar et al. 2011; He et al. 2020; Scheverin et al. 2022).

Fluoride removal from drinking water can be achieved through various methods such as chemical precipitation (Chang & Liu 2007; Budyanto et al. 2015), adsorption (Wu et al. 2016; Zhang et al. 2016), membrane separation (He et al. 2014), and ion exchange (Meenakshi & Viswanathan 2007; Jia et al. 2015). Among these, membrane and ion exchange methods are costly, chemical precipitation struggles to completely remove fluoride from wastewater, resulting in relatively high fluoride concentrations in the treated water (Maliyekkal et al. 2008; Mohapatra et al. 2009). Adsorption processes are relatively simple and cost-effective (Viswanathan & Meenakshi 2008). Various adsorbents have been reported in the literature, such as aluminum oxides, carbon-based materials, ion exchange resins, bone char, and zeolites (Chatterjee et al. 2018; Pang et al. 2020; Alhassan et al. 2021; Wang et al. 2022). Activated alumina has been used commercially for fluoride removal but exhibits low adsorption capacity. In recent years, efforts have been focused on researching and developing various synthetic, naturally occurring, and waste material-based novel fluoride adsorbents.

Hydroxyapatite (HAP), also known as hydroxy phosphatic lime or basic calcium phosphate, appears as a white, powder-like substance without visible impurities to the naked eye, and its molecular formula is . It is a major component of bones and teeth. Due to its strong adsorption and ion exchange capabilities and good stability in water, HAP can be widely used as an eco-friendly material in water treatment (He et al. 2016; Rathnayake et al. 2022). HAP has a certain capacity for removing fluoride from water, primarily through ion exchange and electrostatic interactions, with reactions primarily occurring on the crystal surface. However, this leads to low adsorption capacity, limiting HAP's practical applications (Muthu Prabhu & Meenakshi 2014). Hence, numerous researchers have enhanced HAP's adsorption capacity through doping and surface modification, such as with magnesium, aluminum, and zirconium-based compounds (Huang et al. 2020; Alturki et al. 2021; Alagarsamy et al. 2022; Yan et al. 2022). Yu developed a biocomposite material by modifying HAP with cellulose, showing high efficiency in removing fluoride from drinking water (Yu et al. 2013). Others have improved fluoride removal rates by hydrothermally modifying hydroxyapatite (Gogoi & Dutta 2016). Huang synthesized Al(OH)-nHAP nanosheets by modifying nHAP, which enhanced its defluorination capacity, and the maximum adsorption capacity could reach 171.88 mg/g (Huang et al. 2020). Wang biosynthesis of HAP via bimetallic hydroxide modification, the maximum adsorption of fluoride by La-Fe-HAP and La-Al-HAP was 17.04 and 24.41 mg/g, respectively (Wang et al. 2024). Adamu et al. synthesized bismuth-doped HAP nanorods with a maximum adsorption of 60.67 mg/g by co-precipitation via (Adamu et al. 2023).

This study employs a homogeneous hydrothermal co-precipitation method to prepare hydrophilic nano-hydroxyapatite materials. By using anhydrous aluminum chloride for in situ surface modification through co-precipitation, Al-HAP is synthesized to enhance the material's fluoride removal performance. The study optimizes the material preparation conditions and characterizes the materials using XRD, FT-IR, etc., analyzing the material's phase structure. The fluoride adsorption performance of Al-HAP under different conditions is explored. Adsorption experiments are conducted to verify the adsorption capacity of Al-HAP, and further, the adsorption mechanisms are investigated through adsorption kinetics and thermodynamics experiments.

Experimental reagents

All reagents were of analytical grade. Anhydrous calcium chloride, anhydrous aluminum chloride, sodium dihydrogen phosphate, ammonia solution, and anhydrous aluminum chloride were purchased from Greagent, China. Anhydrous ethanol was obtained from Macklin, sodium hydroxide was obtained from Aladdin, and concentrated hydrochloric acid was obtained from Yonghua Chemical Technology Co., Ltd, China.

Preparation and characterization methods

Preparation and modification of HAP

  • (1) Preparation of HAP: HAP was synthesized using a homogeneous hydrothermal co-precipitation method. Anhydrous calcium chloride and sodium dihydrogen phosphate were weighed in a calcium-to-phosphorus ratio of 1.67 and dissolved separately in 50 mL of ultrapure water. The solutions were combined and magnetically stirred at room temperature. The pH of the mixed solution was adjusted to 9.0 ± 0.1 by adding 25% ammonia solution dropwise, stirring for 20 min. The mixed reaction solution was then transferred to a hydrothermal reactor and maintained at 130 °C for 12 h. After the reaction, the product was washed with pure water and ethanol until neutral, dried in an oven at 60 °C for 24 h, and ground for later use.

  • (2) Preparation of Al-HAP: A certain amount of anhydrous aluminum chloride and HAP were weighed and dissolved in 50 mL of water. Al3+ was added in the amounts of 2.5 mmol/g HAP, 5 mmol/g HAP, 10 mmol/g HAP, and 15 mmol/g HAP. Under magnetic stirring, a 1 mol/L NaOH solution was slowly added to the above mixture to adjust the pH to 7.0 ± 0.1. After the addition was complete, the mixture was sealed and stirred at room temperature for an additional hour, aged for 12 h, and the reaction product was washed several times with pure water and ethanol until neutral. The product was then dried at 60 °C for 12 h, cooled, and ground to obtain the Al-HAP samples.

Characterization methods

Fourier transform infrared spectroscopy (FT-IR) was used to analyze the functional groups in the samples and characterize the surface functional groups of the materials. X-ray diffraction (XRD) was used to characterize the crystalline structure of the materials. The surface morphology and elemental composition of the materials were observed through scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The Zeta potential of the materials as a function of pH was measured using a Malvern particle size analyzer. X-ray photoelectron spectroscopy (XPS) was conducted to analyze the elemental composition within the materials.

Adsorption experiments

To investigate the adsorption performance and mechanism of Al-HAP toward fluoride, batch experiments were conducted. Fluoride standard solution and deionized water were used to prepare the fluoride stock solution, which was diluted to the desired concentration before use. The fluoride concentration in the solution before and after adsorption was measured using an ion chromatograph.

Adsorption capacity: Different doses of adsorbent (0.5, 1, 2, 3, 4, 5 g/L) were added to a 10 mg/L fluoride solution, the pH was adjusted to 5.0 ± 0.1, and the mixture was agitated in a constant temperature water bath shaker at 25 °C and 180 rpm for 24 h. After settling for half an hour, the supernatant was filtered, and the residual fluoride ion concentration was determined.

Effect of pH on adsorption: 20 mg of adsorbent was added to 20 mL of fluoride solution, and the pH was adjusted to values between 3 and 8 using HCl and NaOH. After reaching adsorption equilibrium, the supernatant was filtered through a 0.22 μm filter membrane, and the remaining fluoride ion content was measured.

Adsorption kinetics: To study the change in fluoride ion concentration over time, 500 mL of fluoride solution (10 mg/L) was placed in a 1-L open beaker, and the pH was adjusted to 5.0 ± 0.1. At 25 °C, the solution was stirred magnetically, and at regular intervals, a 5 mL solution was taken and filtered through a 0.22 μm filter membrane to measure the remaining fluoride ion concentration.

Adsorption thermodynamics: 20 mL of fluoride solution of different concentrations (5–200 mg/L) were placed in 50 mL conical flasks, 0.02 g of adsorbent was added, and the pH was adjusted to 5.0 ± 0.1. The flasks were then placed in a constant temperature water bath shaker and shaken for 24 h at 25, 35, and 45 °C. After settling for half an hour, the supernatant was filtered to determine the remaining fluoride ion concentration.

Effect of coexisting ions: 20 mL of fluoride solution (10 mg/L) containing different concentrations (10–100 mg/L) of anions was placed in 50 mL conical flasks, 0.02 g of adsorbent was added, and the pH was adjusted to 5.0 ± 0.1. The flasks were then placed in a constant temperature water bath shaker and shaken for 24 h at 25 °C. After settling for half an hour, the supernatant was filtered to determine the remaining fluoride ion concentration.

Characterization of adsorbent materials

SEM and EDS and FT-IR analysis

Scanning electron microscopy (SEM) was used to analyze the surface morphology and structure of the samples before and after modification, as shown in Figure 1(a) and 1(b). From Figure 1(a) and 1(b), it can be seen that the morphology changes from short rod-like to approximately spherical particles, with poorer dispersion and agglomeration of grains, likely due to the substitution of some Ca2+ by Al3+ and the encapsulation of HAP by Al (Kaygili et al. 2014). Figure 1(c) and 1(d) present the EDS spectra of the samples, showing a decrease in the Ca/P molar ratio, indicative of reduced Ca content and the presence of Al, along with an increased relative content of oxygen. This suggests partial substitution of Ca in the lattice by other ions. The Al/Ca molar ratio is 1.4, which is higher than the 0.0684 (Chen et al. 2018) and 0.3298 (He et al. 2017) reported in other studies, with the aluminum ions providing more adsorption sites, resulting in a larger adsorption capacity for Al-HAP. The infrared absorption spectra of HAP and Al-HAP are shown in Figure 1(e), HAP exhibits a weak broad peak of –OH stretching vibration at 3,443 cm−1 and a sharp bending vibration at 1,636 cm−1. Peaks corresponding to vibrations are present at 563, 603, 1,034, and 1,107 cm−1, with characteristic peaks of asymmetric stretching modes of the phosphate group in the 1,000–1,200 region (Singh et al. 2020). From the FT-IR spectrum of HAP, peak values at 1,034 and 1,107 cm−1 are characteristic peaks of symmetric stretching vibrations of the phosphate group, and peaks at 563 and 603 cm−1 are attributed to bending vibrations of the phosphate group (Wang et al. 2011). In the spectrum of Al-HAP, the absorption peaks at 3,443 and 1,646 cm−1 belong to hydroxyl stretching vibrations, where the peak at 1,646 cm−1, which is likely attributed to Al–OH–A, exhibits a blue shift and a significant increase in intensity, indicating that Al modification has increased the number of –OH groups in the material (Gogoi et al. 2015; Tomar et al. 2015). At the same time, the phosphate group peaks after Al modification show slight variations, possibly due to changes in the bending vibrations of the P···O bond caused by the Al···P bond (Bouiahya et al. 2019), indicating that the Al-HAP material modified by aluminum ions does not change the position and structure of functional groups but simply increases the –OH content, and its main structure remains that of hydroxyapatite. The XRD patterns of HAP and Al-HAP samples are shown in Figure 1(f). Clear diffraction peaks are observed in the XRD spectra, with the main diffraction peaks of HAP consistent with the standard card PDF#09-0432, indicating that the experimentally prepared HAP material has good crystallinity and high purity. After modification, the characteristic absorption peaks of Al-HAP in the spectrum are consistent with those of HAP (Chen et al. 2010), signifying that the crystal structure has not changed (Goldberg et al. 2019), only the intensity has slightly varied. The sharper characteristic peaks of Al-HAP may be due to the substitution of Al3+ for Ca2+ (Kaygili et al. 2014; Kim et al. 2019).
Figure 1

SEM spectra: (a) HAP; (b) Al-HAP, EDS spectra; (c) HAP; (d) Al-HAP; (e) XRD patterns of HAP and Al-HAP; and (f) infrared absorption spectra of HAP and Al-HAP.

Figure 1

SEM spectra: (a) HAP; (b) Al-HAP, EDS spectra; (c) HAP; (d) Al-HAP; (e) XRD patterns of HAP and Al-HAP; and (f) infrared absorption spectra of HAP and Al-HAP.

Close modal

BET and zeta potential analysis

The nitrogen adsorption–desorption isotherms and pore size distribution curves of HAP and Al-HAP are shown in Figure 2(a). It is evident from the data that the specific surface area of the material has significantly increased after modification, with Al-HAP having a specific surface area of 121.97 m2/g, which is 2.27 times that of pure HAP. Moreover, the average pore size of the modified material has increased from 5.43 to 7.77 nm, and the pore volume has expanded, making the material more porous with a mesoporous structure. The enriched specific surface area and pore volume increase the number of active sites on the surface of Al-HAP, enhancing the material's adsorption capacity, suggesting that Al-HAP is highly suitable for use as an adsorbent.
Figure 2

(a) Specific surface area and average pore size of HAP and Al-HAP and (b) zeta potential at different pH levels.

Figure 2

(a) Specific surface area and average pore size of HAP and Al-HAP and (b) zeta potential at different pH levels.

Close modal

The surface potential of HAP and Al-HAP was tested, with results shown in Figure 2(b). At pH 3.0, the surface potential of HAP is slightly above 0, and at acidic (pH = 5), neutral (pH = 7), and alkaline (pH = 9) conditions, the potential of HAP is negative, consistent with the literature (Fahami et al. 2016). The introduction of Al comprehensively improves the surface potential of Al-HAP, with an isoelectric point of 7.0. Its surface carries a positive charge within the pH range less than 7.0, which is advantageous for the adsorption of anions in water.

Adsorption performance of Al-HAP

Effect of Al-HAP dosage

Modification of HAP surface by addition of different amounts of Al3+. Al-HAP modified with Al3+ displays a significant improvement in adsorption capacity compared to pure HAP. When the amount of Al3+ added reaches 5 mmol/g HAP, the removal rate can reach 85%. As the Al3+ content continues to increase, the removal rate of Al-HAP only increases slightly. This may be due to excessive Al3+ occupying active sites on the HAP surface, leading to a decrease in its inherent adsorption effect. Therefore, in subsequent experiments, Al-HAP with an Al3+ loading of 5 mmol/g HAP was selected as the subject for further research.

Figure 3(a) shows the impact of different dosages of adsorbent on the adsorption efficacy. It is evident that, under a constant initial fluoride concentration, an increase in the dosage of the adsorbent significantly enhances the fluoride removal efficiency. Increasing the dosage from 0.5 to 1 g/L, the adsorption efficiency rapidly increases from 51.02 to 81.1%. The rate of removal growth is relatively steady as the dosage increases from 1 to 5 g/L. With the increase in dosage, more active adsorption sites are provided, but an excess of these active sites may mask each other, reducing their effective utilization rate, leading to a gradual decrease in the effective adsorption capacity per unit of adsorbent. As the dosage increases from 1 to 5 g/L, the growth in material adsorption efficiency is quite slow compared to the rate of dosage increase. To improve the efficiency of material use, subsequent experiments will fix the dosage at 1 g/L.
Figure 3

(a) Impact of different dosages on material adsorption efficacy, (b) adsorption efficiency of the two materials for F at different pH levels, and (c) impact of coexisting ions on adsorption capacity.

Figure 3

(a) Impact of different dosages on material adsorption efficacy, (b) adsorption efficiency of the two materials for F at different pH levels, and (c) impact of coexisting ions on adsorption capacity.

Close modal

Effect of pH

The impact of initial pH on fluoride removal by Al-HAP is depicted in Figure 3(b). Within the studied pH range, the fluoride removal efficiency of HAP first increases and then decreases, with the highest adsorption efficiency at pH = 5.0, reaching 55%, a 26% difference from the lowest point of adsorption efficiency. Al-HAP exhibits an adsorption efficiency nearly 30% higher than pure HAP, with significantly reduced sensitivity to pH, with the highest difference in adsorption efficiency being 16%.

The pH also affects the state of fluoride in the aqueous solution. With a pKa of hydrofluoric acid at 3.18, most F will convert to HF below this pH. Even at a pH lower than 5, some F will still exist in the solution as HF (Zhao et al. 2021), meaning that when pH < 5.0, the contribution of electrostatic attraction to adsorption is reduced, thereby decreasing the fluoride adsorption efficiency of both HAP and Al-HAP. At low pH, Al-HAP may also experience the dissolution of aluminum from the surface, leading to a further decrease in fluoride adsorption efficiency. When the pH of the solution is greater than 7.0 and progressively increases, the fluoride adsorption efficiency of Al-HAP gradually decreases. This is because an increase in negative charge on the Al-HAP surface results in electrostatic repulsion with F in the solution, and the abundant OH ions in the solution compete with F for adsorption sites on the material's surface, causing a decrease in fluoride adsorption efficiency.

Effect of coexisting ions

In natural waters and industrial wastewaters, fluoride ions coexist with many other ions, such as Cl, , NO3−, , etc. The presence of these anions can affect the adsorption of fluoride ions by Al-HAP. Adsorption experiments were conducted on fluoride solutions containing different concentrations of anions, with conditions controlled at 25 °C, a dosage of 1 g/L, and an initial F concentration of 10 mg/L. The impact of coexisting ions on the adsorption efficacy of Al-HAP is shown in Figure 3(c). It was observed that Cl, , NO3−, have minimal impact on fluoride removal efficiency. However, significantly affects the adsorption capacity of Al-HAP, with a gradual decrease in adsorption capacity as its concentration increases. The interference of carbonate with the adsorption of fluoride ions by the adsorbent is mainly due to its hydrolysis producing OH, leading to an increase in pH, which inhibits fluoride adsorption, which is consistent with results of previous studies (Singh & Chaudhari 2023).

Table 1

Pseudo-first-order and pseudo-second-order kinetic fitting parameters

T (k)Pseudo-first-order
Pseudo-second-order
qe,expqe,calK1R2qe,expqe,calK1R2
298 7.06 3.844 0.0036 0.935 7.06 7.0947 0.003 0.998 
T (k)Pseudo-first-order
Pseudo-second-order
qe,expqe,calK1R2qe,expqe,calK1R2
298 7.06 3.844 0.0036 0.935 7.06 7.0947 0.003 0.998 

Adsorption mechanism

Adsorption kinetics

Adsorption kinetics are evaluated using the rate of adsorption to assess the dynamic equilibrium state. Therefore, by studying the contact time required to reach equilibrium and fitting with pseudo-first-order, pseudo-second-order kinetics, and the intra-particle diffusion model, the adsorption kinetics mechanisms are investigated. As can be seen from Figure 4(a), the adsorption rate is fast at the beginning due to the availability of active sites on the adsorbent surface, reaching about 70% of the saturation adsorption amount in approximately 120 min. In the later stage, as fluoride ions occupy most of the active sites on the adsorbent, the adsorption rate approaches equilibrium and slows down. Table 1 show the fits for the pseudo-first-order and pseudo-second-order kinetic models, respectively. It can be observed that the regression coefficient (R2 = 0.935) for the pseudo-first-order kinetic equation is less than that for the pseudo-second-order kinetic equation (R2 = 0.998), implying that the pseudo-second-order kinetic model aligns more closely with the experimental results, indicating that the process of Al-HAP adsorbing fluoride ions is chemisorption. Figure 4(b) shows the intra-particle diffusion model and the relative parameters are listed in Table 2. The first stage corresponds to the rapid adsorption phase and is related to the particle diffusion process, with a higher Kd1 value. The final stage is the adsorption equilibrium phase, where the adsorption rate is slower, corresponding to the chemisorption process.
Table 2

Intra-particle diffusion model fitting parameters

T (K)Intra-particle diffusion
Kd1C1R2Kd2C2R2
298 0.61847 −0.23078 0.91761 0.04545 4.56935 0.97375 
T (K)Intra-particle diffusion
Kd1C1R2Kd2C2R2
298 0.61847 −0.23078 0.91761 0.04545 4.56935 0.97375 
Figure 4

(a) Al-HAP adsorption kinetics of F and (b) intra-particle diffusion model.

Figure 4

(a) Al-HAP adsorption kinetics of F and (b) intra-particle diffusion model.

Close modal

Adsorption isotherms

To better investigate the performance of Al-HAP material in adsorbing fluoride ions in aqueous solutions, adsorption experiments of different F concentrations were conducted at 25, 35, and 45 °C. The experimental data were fitted with the Langmuir and Freundlich isotherm models, with the adsorption isotherms shown in Figure 5 and fitting parameters summarized in Table 3.
Table 3

Langmuir and freundlich model fitting parameter

T (k)Langmuir
Freundlich
qm,expqm,calKL (L/mg)R21/nKFR2
298 56.44 65.02 0.069 0.993 0.378 9.036 0.959 
308 59.79 84.01 0.067 0.995 0.395 9.091 0.972 
319 64.73 92.61 0.069 0.994 0.401 9.633 0.974 
T (k)Langmuir
Freundlich
qm,expqm,calKL (L/mg)R21/nKFR2
298 56.44 65.02 0.069 0.993 0.378 9.036 0.959 
308 59.79 84.01 0.067 0.995 0.395 9.091 0.972 
319 64.73 92.61 0.069 0.994 0.401 9.633 0.974 
Figure 5

(a) HAP adsorption equilibrium curve, (b) Al-HAP adsorption isotherm curves, (c) Al-HAP lnKd and Ce fitting relationship at different temperatures.

Figure 5

(a) HAP adsorption equilibrium curve, (b) Al-HAP adsorption isotherm curves, (c) Al-HAP lnKd and Ce fitting relationship at different temperatures.

Close modal

Figure 5(a) shows the equilibrium adsorption curve of HAP at 298 K. The equilibrium adsorption capacity of HAP at an initial concentration of 200 mg/L was 28.36 mg/g. Figure 5(b) presents the adsorption isotherm curves of Al-HAP at different temperatures. The pollutant chosen for testing the material's adsorption performance at three different temperatures was fluoride, with initial concentrations of 5, 10, 20, 50, 75, 100, 150, and 200 mg/L, and a dosage of 1 g/L. The pH was adjusted to 5.0 using HCl and NaOH, and the samples were reacted in a water bath shaker at 180 rpm and 25 °C for 24 h. After the reaction, the samples were allowed to settle for half an hour, and the supernatant was filtered through a 0.45 μm filter membrane for fluoride concentration determination by ion chromatography. The adsorption amount of Al-HAP for F in solution increased with the initial F concentration. When the initial concentration exceeded 100 mg/L, the adsorption amount tended to plateau, which can be attributed to a limited number of adsorption sites available for the pollutant when the adsorbent dosage is fixed. As the initial concentration of the pollutant increased, the adsorption sites became saturated, leading to a slower increase in adsorption capacity. It can also be seen from the figure that with changes in solution temperature, the adsorption capacity varied, indicating different chemical interactions between the Al-HAP adsorbent and pollutant at different temperatures. Higher temperatures may produce more active sites or change the diffusion rate within the Al-HAP adsorbent's micropores, suggesting that Al-HAP adsorption of F is an endothermic reaction. At 25, 35, and 45 °C, and an initial concentration of 200 mg/g, the equilibrium adsorption capacities of Al-HAP were 56.44, 59.79, and 64.73 mg/g, respectively. Figure 5(b) and Table 3 show that the Langmuir model better describes the adsorption of F by Al-HAP, indicating that the adsorbent surface has uniformly distributed active sites and that the adsorption of F is monolayer.

Adsorption thermodynamics

By calculating the thermodynamic fitting parameters (Gibbs free energy ΔG0, enthalpy change ΔH0, and entropy change ΔS0), one can determine the influence of temperature on adsorption and the changes in energy, which plays an important role in evaluating the performance of the adsorption reaction and rationally predicting the reaction mechanism. The calculation formulas are as follows:
formula
(1)
formula
(2)
formula
(3)
where is the distribution coefficient (L/g), is the equilibrium constant (L/g), T is the absolute temperature (K), and R is the ideal gas constant 8.314 J·mol−1·K−1. According to Equation (1), is calculated, and the versus Ce linear fitting relationship graph is plotted, as shown in Figure 5(c). ΔG0 is obtained by Equation (2) from . ΔH0 and ΔS0 values are calculated by Equation (3), with the thermodynamic parameters obtained summarized in Supplementary material, Table S1. The adsorption enthalpy change ΔH0 = 10.901 KJ/mol is greater than 0, indicating that the adsorption process is spontaneous and endothermic, and the reaction proceeds in the forward direction with increasing temperature. The value of ΔS0 is also greater than 0, suggesting that the overall adsorption reaction increases in disorder, representing an entropy-increasing process. The ΔG0 values at the three temperatures are less than 0, indicating that the adsorption process for fluoride ions is spontaneous at these temperatures and that the reaction is more favorable with increasing temperature.

XPS analysis

To probe the adsorption mechanism of Al-HAP on F, XPS and FT-IR characterizations of the material before and after adsorption were conducted. Figure 6(a) displays the XPS survey spectra of Al-HAP before and after adsorption. From Figure 6(b), it can be observed that the spectrum after adsorption features an additional F1s photoelectron peak, indicating that F has indeed been adsorbed onto the material's surface. Further deconvolution of the F1s peak also confirms the presence of fluoride, with the Ca–F bond accounting for 64.89%, suggesting that more F is binding with HAP and undergoing ion exchange with OH. The O1s high-resolution spectra before and after adsorption confirm that the adsorption mechanism involves an exchange between hydroxyl groups on the Al-HAP surface and fluoride ions. The hydroxyl oxygen content before adsorption constituted 40.74% of the total O1s spectrum, which declined to 34.38% after adsorption, emphasizing its critical role during fluoride ion adsorption. After adsorption, the binding energy of Ca2p3/2 increased from 347.32 to 347.55 eV, indicating strong interactions between F and Ca post-adsorption. Figure 6(f) presents the FT-IR spectra of Al-HAP before and after adsorption, revealing a notable decrease in the intensity of the OH peak after adsorption, further corroborating the significance of surface hydroxyl groups during the fluoride ion adsorption process. Thus, it can be concluded that the primary mechanism of fluoride ion adsorption by Al-HAP is the ion exchange between surface hydroxyl groups and fluoride ions.
Figure 6

(a) XPS survey spectra before and after Al-HAP adsorption, (b) F1 s high-resolution spectrum, (c) O1 s high-resolution spectrum before adsorption, (d) O1 s high-resolution spectrum after adsorption, (e) Ca2p high-resolution spectrum before and after adsorption, and (f) FT-IR spectra before and after adsorption.

Figure 6

(a) XPS survey spectra before and after Al-HAP adsorption, (b) F1 s high-resolution spectrum, (c) O1 s high-resolution spectrum before adsorption, (d) O1 s high-resolution spectrum after adsorption, (e) Ca2p high-resolution spectrum before and after adsorption, and (f) FT-IR spectra before and after adsorption.

Close modal

Al-HAP was successfully prepared via a method of hydrothermal homogeneous co-precipitation of anhydrous calcium chloride and sodium dihydrogen phosphate followed by chemical solution deposition of anhydrous aluminum chloride. SEM result showed that the morphology of Al-HAP was nearly spherical particles. With additional hydroxyl functional groups and larger specific surface area, the available adsorption sites of Al-HAP increased, expanding its capacity toward fluoride ion. The maximum adsorption capacity achieved 56.44 mg/g at pH 5.0, which is almost double that of HAP. The isotherm followed the Langmuir isotherm adsorption model and the dynamics followed the pseudo-second-order kinetic model, indicating it preferred monolayer adsorption pattern. The fluoride removal mechanism of Al-HAP is dominant by electrostatic adsorption and hydroxyl ion exchange. In conclusion, the Al-HAP prepared in this study shows stable and efficient removal toward fluoride ions from water and has great potential for practical application.

The authors are grateful for the financial support from National Natural Science Foundation of China (52070137) and Suzhou Social Development Science and Technology Innovation Project (SS202107).

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.

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