Preparation and defluorination mechanism of a novel copolymerized hydroxyapatite–aluminium chloride material

Abstract

As an attempt to avoid the low defluoridation capacity of conventional adsorbents, this study offers a novel copolymerized hydroxyapatite–aluminum (HAP–PAC) adsorbent and evaluates its performance in fluoride removal of drinking water, and a possible fluoride removal mechanism is proposed based on the characterization results obtained by FTIR, XRD, BET, SEM and EDS. The results indicate that the copolymerization material prepared by co-crystallization with aluminum chloride is featured by the chemical combination between hydroxyapatite (HAP) and poly-aluminium chloride (PAC). A certain amount of Ca²⁺ and OH⁻ in the HAP crystal lattice are partially replaced by Al³⁺ and Cl⁻ doping from PAC, and the structure of the copolymerization material shows a uniform trend. In addition, mainly by chemical adsorption, the highest defluoridation capacity (DC) of the copolymerization material can reach up to 18.12 mg g⁻¹, an increase of 14.02 mg g⁻¹ as compared with conventional hydroxyapatites. When aluminum chloride is replaced by bauxite in the study, the highest DC is 13.72 mg g⁻¹ due to the co-occurrence of chemical and physical combinations and relatively lower homogeneous structure caused by the co-existence of HAP and PAC while electrostatic attraction and chemical adsorption are both involved. As for the material prepared by the physical mixing method, in which the combination between HAP and PAC is mainly dominated by physical effects, its maximum DC is 13.08 mg g⁻¹, with physical adsorption as its main form of defluoridation.

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1 Introduction

Fluorine is an essential micronutrient for calcium and phosphorus metabolism in human body.^1,2 Its intake mainly comes from drinking water. An appropriate amount of fluorine can increase the hardness of bones and teeth, promote the metabolism of enzymatic system and help to transfer nerve excitement.³ However, when the level of fluoride is more than 1.5 mg L⁻¹, it can lead to various diseases such as skeletal and dental fluorosis.^4,5 Therefore, it is significant to explore ways to reduce excessive fluoride in drinking water.

Adsorption is widely applied nowadays to remove excessive fluorine in drinking water due to its simple operation on instruments and low running cost;^6,7 the kernel for this method is the choice of an appropriate adsorbent. Popular adsorbents include activated alumina,^8,9 bone charcoal,¹⁰ biological macromolecule fluoride agents,¹¹ kaolin,¹² and hydroxyapatite.^13,14 Of all the adsorbents, activated alumina, widely employed several years ago, is seldom used now because of its low defluoridation capacity (DC), which consequently leads to a lot of regenerations and secondary pollution by adding chemical reagents, and the need to adjust the values of pH of the water system, which increases the complexity of its normal usage. To improve the adsorption efficacy of activated alumina, researchers have tried to modify the surface of alumina. For instance, La³⁺-modified activated alumina (La-AA) was studied by Jiemin Cheng et al.¹⁵ The comparison of adsorption characteristics of La-AA and original alumina in the removal of fluoride concluded that the maximum DC values of AA and La-AA were 2.74 and 6.70 mg g⁻¹ at pH = 7.0, respectively. A novel adsorbent of sulfate-doped Fe₃O₄/Al₂O₃ nanoparticles with magnetic separability was prepared, characterized and applied by Liyuan Chai et al.¹⁶ Test results showed that the adsorption by this adsorbent for fluoride by a two-site Langmuir model was 70.4 mg g⁻¹ at pH = 7.0 with initial fluoride concentrations of 160 mg L⁻¹. Bansiwal et al.¹⁷ examined copper oxide coated alumina (COCA) by impregnating alumina with copper sulphate solution followed by calcination at 450 °C in the presence of air. The analysis indicated that the DC of COCA obtained through the Langmuir model was 7.22 mg g⁻¹, which was advanced by 5.0 mg g⁻¹ as compared with unmodified AA. One disadvantage of these adsorbents is that certain amounts of aluminum dissolves in the water during the defluoridation process, which poses a potential threat to human health in the long-term drinking.

Hydroxyapatite (HAP), as a new type of fluoride removal material, is superior to activated alumina in terms of its high fluoride removal capacity and availability. However, its unfixed property can induce turbidity, water filtration difficulty and the reactivation process of HAP is also extremely complex. To solve these problems, Weihua Xu et al.¹⁸ studied a novel type of high hardness granular filter material made by hydroxyapatite with a modification with attapulgite. However, this material has lower DC because of its smaller contact area with water systems. Then, Li Feng et al.¹⁹ reported a method for heat regeneration of the hydroxyapatite/attapulgite composite beads for defluoridation of drinking water and its total DC can reach up to the level of the powdered material of HAP. Another adsorbent of alginate bioencapsulating nano-hydroxyapatite (n-HApAlg), synthesized by Pandi K et al.,²⁰ has a higher DC (3.87 mg g⁻¹) than the original n-Hap (1.296 mg g⁻¹) and calcium alginate (CaAlg) composites (0.68 mg g⁻¹). Yulun Nie et al.²¹ stated that aluminum modified calcium hydroxyapatite (Al-HAP) nanoparticles produced by the co-precipitation method possessed a higher DC of 32.57 mg g⁻¹ with an initial fluoride concentration of 10 mg L⁻¹. The adsorption capacity of nano-materials can be increased because of its huge specific surface area. However, its difficulty in interception cannot eliminate the possibility of entering the human body. In addition, its fluoride removal capacity is greatly influenced by the concentration of nano-materials in fluorine-containing water: the higher the concentration is, the larger the capacity will be.¹⁹

Poly aluminium chloride (PAC), as an inorganic polymer flocculation, is commonly applied in wastewater treatment owing to advantages such as strong coagulability, big floc, less dosage, and high water purification efficiency.

This study proposes a new sorbent of HAP–PAC copolymerization material based on the analysis of fluoride removal of HAP/active aluminum and the flocculation effects of PAC. Because of its high DC, it is capable of removing fluoride intensively and producing drinkable water without any health concerns.

2 Materials and methods

2.1 Chemicals and materials

NaF and all other reagents used in this study were of analytical grade and they were purchased from Shanghai Chemical Corporation, China. Poly aluminium chloride (PAC) was obtained from Shandong Zhongke Tianze Water Purification Materials Co., Ltd and its effective substance content (Al₂O₃) was 28%. The main ingredients of bauxite were Al₂O₃ (53.87%), SiO₂ (18.13%), and Fe₂O₃ (9.53%).

2.2 Characterization

Examination with a scanning electron microscope (SEM) (FEI Quanta250 model) fitted with an energy dispersive X-ray analyzer (EDS) allowed qualitative detection and localization of elements within the samples. X-ray powder diffraction (XRD) patterns were obtained using a Bruker D8 Advance X-ray diffractometer using Cu Kα radiation, a nickel filter, and a Lynx Eye detector. FTIR spectra of the copolymerization materials were obtained on Nicolet 380 FTIR Spectrometer to confirm the presence of functional groups.

2.3 Preparation of filtering material

2.3.1 Preparation of HAP–PAC copolymerization material.
2.3.1.1 Method no.1 crystallization of aluminum chloride (AlCl₃·6H₂O) for use as the raw material. First, a certain amount of AlCl₃·6H₂O was weighed and placed into a muffle furnace with the temperature set at 290 °C. After heating for 30 min, it was stirred with the addition of certain amount of distilled water, during which some sticky resin material, namely, semi-finished PAC, was produced. After this process, some of this material was taken as a sample and its content of Al₂O₃ was titrated according to the National Standard Method (HG/T 2677-2009). Then, after the instilment of phosphoric acid, an appropriate amount of the semi-finished PAC was added to the synthesis reaction of HAP,²² a process that accompanied the preparation of the semi-finished PAC. After 60 min maturation at the temperature of 90 °C, the sample was taken out and cooled down to room temperature naturally. Then, it was filtered, washed, dried for 12 hours at 105 °C and then ground. Finally, the material prepared under these optimized conditions was labeled as H1.
2.3.1.2 Method no.2 bauxite as the raw material. First, bauxite was smashed and sieved by a 100 mesh sieve. Afterwards, a certain amount of bauxite was weighed and placed into a muffle furnace. After being heated for 60 min with the temperature set at 650 °C, the calcined bauxite and certain amount of 20% HCl solution were charged into a three-necked flask and mixed. The ratio of the bauxite mass and the HCl volume was 1 : 1.5. The next step was acid treatment, in which Allihn condenser reflux was adopted to conduct backflow, at the temperature of the mixture between 85 °C and 95 °C for duration of 2 hours. After this stage, an appropriate amount of NaAlO₂ was slowly added to conduct polymerization that lasted for 4 hours at a temperature of 90 ± 2 °C (the mass ratio of the bauxite and NaAlO₂ was 5 : 1). After cooling down to room temperature naturally, the processed material was filtered to obtain the liquid PAC. Then, a sample was taken and its Al₂O₃ content was titrated based on the National Standard Method (HG/T 2677-2009). An appropriate amount of the newly-produced liquid PAC was added to the synthesized HAP when the dripping of phosphoric acid finished during the synthesis of HAP,²² a process concurrent with the preparation of the liquid PAC. After the maturation of this mixture for 60 min at 90 °C, some of the mixture was sampled and cooled down to room temperature naturally. Then, it was filtered, washed, dried for 12 h at 105 °C and then ground. Finally, the material prepared under these optimal conditions was labeled as H2.

2.3.2 Method no.3 HAP and PAC physical mixing. The prepared HAP²² and PAC (commercially available) were mixed according to a certain mass ratio and the sample produced under these ideal conditions was labeled as H3.

2.4 Batch adsorption experiments

A 5 mg L⁻¹ fluoride solution was prepared by dissolving 0.1105 g of NaF in tap water for a volume of 10 L to be used as the fluoride-containing water sample.

The prepared water sample was transferred into a coagulation blender container until the volume reached 0.6 L. Then, 0.09 g of filtering material was added into the container followed by stirring at the speed of 200 rpm for 30 min. After maturating for 30 min, 25 mL of supernatant was pipetted into a 50 mL volumetric flask and 10 mL of TISAB solution was added. During the whole process, the volume was maintained. Finally, the electromotive force E of the solution was tested by a fluoride ion-selective-electrode, pF-1 (made in China). A standard curve could be drawn by setting the electromotive force E of the standard solution as the Y-axis vs. the logarithm of solution concentration as the X-axis. The equation obtained was y = −57.071x + 235.23 with a correlation index R² = 0.9999. The concentration (mg L⁻¹) of fluoride ion in the solution could be calculated based on this curve.

3 Results and discussion

3.1 The effect of Al/Ca molar ratio on fluoride removal

The Al/Ca molar ratio is applicable to the characterization of the molar ratio between Al measured by Al₂O₃ in PAC and Ca in HAP. Filtering materials were prepared by physical mixing of a set molar ratio of Al/Ca as 0, 0.022, 0.044, 0.088, 0.132, 0.176, 0.22, 0.264, and 0.308. The effects of different Al/Ca molar ratios on the adsorption capacity are evaluated by the defluoridation capacity (DC) and the results are shown in Fig. 1.

Fig. 1 The effects of different Al/Ca molar ratios on defluoridation capacity on physical mixing.

As Fig. 1 shows, the DC of pure HAP is 4.10 mg g⁻¹, and its capacity shows no distinct variation as a small quantity of PAC is added to make the ratio turn into 0.022. It then rises quickly from 0.022 to 0.132 and possesses the maximum DC of 13.08 mg g⁻¹ when the Al/Ca molar ratio is 0.132. However, the DC decreases with further increase in the Al/Ca molar ratio. The possible explanation for this phenomenon lies in the fact that a certain type of synergistic effect between HAP and PAC may be induced with the increase of Al/Ca molar ratio, thus improving the capacity of defluoridation. Moreover, the flocculation precipitation caused by the flocs from PAC and turbidity from fluor-hydroxyapatite accelerates the adsorption reaction between adsorbent and fluoride in water, which in turn mitigates the potential dangers of turbidity value decreases in water and aluminum dissolved from PAC. It can be observed that the filtering material with the best proportion exhibits the highest efficiency for the removal of fluoride. As the ratio increases beyond the optimal one, further additions of PAC means more impurities in the filtering material, which lowers its adsorption ability, and thus weakening its DC.

Fig. 2 shows the impact on DC as filtering materials prepared by chemical synthesis under different Al/Ca molar ratio. The preparation methods are divided into the crystalline aluminum chloride (AlCl₃·6H₂O) method and the bauxite method due to the difference in the choice of the raw materials.

Fig. 2 The influence of different Al/Ca molar ratio on defluoridation capacity on chemical synthesis.

It can be observed from Fig. 2 that the overall trend of the two curves displays some similarity, that is, the fluoride removal capacity will increase firstly and then decrease with increasing Al/Ca molar ratio, and the DC of the bauxite method is relatively lower. An account for this is that the two filtering materials form a stable structure when copolymerization occurs between special amounts of HAP and PAC in the preparation process and leads to their relatively higher DC. However, if Al/Ca molar ratio surpasses its optimal value, then it will finally induce physical mixture, this procedure will lead to the decrease of DC. As compared with the AlCl₃·6H₂O method, the effective content of the prepared product is relatively lower in view of the high content of impurities in raw bauxite, which triggers its poor copolymerization with HAP. Consequently, the adsorption capacity of the filtering material generated from the bauxite method is not as effective as that created from the AlCl₃·6H₂O method. The optimal Al/Ca molar ratio of filtering materials produced by the AlCl₃·6H₂O method and the bauxite method demonstrates an identical value of 0.22, and the corresponding DC are 18.12 mg g⁻¹ and 13.72 mg g⁻¹, respectively. Both methods increase the DC by 14.02 mg g⁻¹ and 9.62 mg g⁻¹ over conventional hydroxyapatite.

3.2 Characterization of adsorbents

3.2.1 FTIR analysis. The functional group structures of H1, H2, H3 and pure HAP, PAC are characterized via FTIR and the infrared spectra are shown in Fig. 3.

Fig. 3 FTIR spectra of filtering materials (in Fig. 3: H1 is the copolymerization material prepared by AlCl₃·6H₂O; H2 is the filtering material prepared by bauxite; H3 is the filtering material prepared by physical mixing method).

Fig. 3 indicates that HAP shows a strong P–O stretching vibration peak at 1040 cm⁻¹ and a P–O bending vibration double peak at 605 cm⁻¹ and 570 cm⁻¹. In addition, HAP also presents a relatively weaker –OH stretching vibration peak at 3450 cm⁻¹ and a relatively weaker –OH bending vibration peak at 1640 cm⁻¹. The characteristic peaks of PAC are listed below. A relatively stronger broad peak at 3410 cm⁻¹ and a relatively stronger sharp peak at 1640 cm⁻¹ are ascribed to the stretching vibration and bending vibration of –OH, respectively. The characteristic peak at 598 cm⁻¹ is assigned to the bending vibration of Al–Cl.

In comparison with the infrared spectra of pure HAP and PAC, both the spectra of H1 and H2 at near 3410 cm⁻¹ and 1640 cm⁻¹ show relatively stronger –OH stretching vibration and bending vibration peaks, which are similar to those of PAC. However, it is worth noting that a red shift occurs at 3410 cm⁻¹, a blue shift occurs at 1640 cm⁻¹, and their wavenumbers display slight differences. This result clearly suggests that the material structure is different from that of pure HAP and PAC. While a relatively higher P–O stretching vibration peak, which is similar to HAP in this respect, appears at near 1040 cm⁻¹, the band exhibits obvious shifts to higher wavenumbers when the intensity of the absorption peak decreases significantly and the wavenumber changes slightly. This may be due to the bending vibration of the Al–OH–Al groups. In addition, the single peaks at 604 cm⁻¹and 565 cm⁻¹, respectively, may be attributed to the bending vibration of the O=P⋯Cl groups. A part of the hydroxyl groups in HAP may be replaced by Cl in PAC. On the basis of these aforementioned results, it can be said that the chemical action plays a dominant role in the synthesis of H1 and H2 and that the degree of the action and the results display slight differences.

The spectra of H3 at 3425 cm⁻¹ and 1640 cm⁻¹ have the relative stronger stretching vibration and bending vibration peaks of the –OH group, which bears certain similarities to PAC at 3410 cm⁻¹ and 1640 cm⁻¹. The change of the wavenumber at 3425 cm⁻¹ may be the result of the overlap of peaks between PAC and HAP. There is also a relatively higher P–O stretching vibration peak, which is also shown in HAP at 1040 cm⁻¹, and doublets at 604 cm⁻¹ and 565 cm⁻¹ just like HAP. The change of wavenumbers may be attributed to the overlap of peaks at 598 cm⁻¹. On the grounds of the aforementioned results, it can be concluded that physical action plays an important role in the formation procedure of H3 but does not change the respective structures of HAP and PAC.

3.2.2 XRD analysis. The crystal structures of pure HAP and HAP–PAC copolymerization materials are characterized by X-ray diffraction and the results are presented in Fig. 4.

Fig. 4 XRD spectra of powder HAP (a) and filtering materials (b).

The XRD patterns of Fig. 4(a) show that the crystalline peaks occur at 2θ = 25.9°, 32°, 33°, 35.5° and 40°, thus confirming the formation of a hydroxyapatite structure. Based on existing data, the characteristic peaks of PAC are 7.5°, 8.3°, 10.2°, 10.8°, 12.7°, and 13.5°.²³ In comparison with Fig. 4(a) and (b), it can be found that filtering materials H1 and H2 have different characteristic diffraction peaks with pure HAP or PAC. Evidence obtained from the XRD study suggests that these synthesized filtering materials have changed the respective molecular structures of HAP and PAC. However, the formation procedure of filtering material H3 does not alter the respective structures of HAP and PAC because multiple HAP and PAC characteristic peaks are found in H3. This means that the mixing between these two materials mainly belongs to a physical measure, which conforms to the information as obtained from FTIR.

As can be observed in Fig. 4(b), the crystallization degree of H3 is the lowest with relatively more impurity peaks, whereas the crystallization degrees of H1 and H2 are relatively higher. The peak shape is extremely sharp, the peak width higher and narrower, and the background thinner. These results imply that the chemical reaction is able to enlarge the grain crystals of H1 and H2, and stimulate the decrease of crystal defects and intercrystalline disordered structures, which further demonstrates that the structures of H1 and H2 are more evenly distributed than that of H3.

3.2.3 BET analysis. BET results are presented in Table 1. It can be observed that H1 owns the largest specific surface area and the optimal averaging chemical mixture, which is consistent with its highest defluoridation capacity. The specific surface area of H2 and H3 shows little difference with each other due to the fact that both of their physical mixture modes are non-uniform. There is no doubt that their defluoridation capacity shows no great difference. Nonetheless, FTIR and XRD analysis indicate that H2 also contains certain chemical reactions. Therefore, H2 possesses a relatively higher defluoridation capacity than that of H3. In conclusion, the BET results show high degrees of consistency with the results obtained from static adsorption experiments.

Table 1 The results of BET analysis

Adsorbent	as,BET (m^2 g^−1)	Average pore diameter (nm)	Total pore volume (cm^3 g^−1)
H1	43.807	5.206	0.057
H2	22.783	4.305	0.025
H3	28.429	12.844	0.091

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3.2.4 SEM with EDS analysis. The analysis of the morphology and structure in the thin surface layers were tested by SEM with EDS. The SEM micrographs of these three filtering materials are presented in Fig. 5. By comparing these three images, it is clear that the structure of H1 is relatively uniform and there are no obvious HAP and PAC particles, which testifies that HAP and PAC in H1 can chemically form HAP–PAC as a uniform copolymer with a regular structure and higher copolymerization degrees. But there are obvious particles with varying sizes and irregular shapes in H3, thus demonstrating that what occurs in H3 is mainly a physical process when HAP and PAC are mixed, and that the respective structures of HAP and PAC undergo no changes during the preparation. Therefore, H3 shows a general non-uniform structure. The shape and particle size of H2 are in their intermediate state and there are multiple non-uniform particles. The size of the particles is also smaller than that of H3, which leads to the conclusion that there is a certain chemical reaction occurring during the preparation of H2, though it is not sufficient enough. Moreover, this procedure is accompanied by a certain physical mixture and there are multiple ingredients in H2.

Fig. 5 SEM images of H1 (a), H2 (b), and H3 (c).

EDS area-scan diagrams and EDS line-scan diagrams of H1, H2, and H3 are displayed in Fig. 6(a)–(c). The EDS line-scan diagram indicates that all surfaces of H1, H2, and H3 contain elements Ca, Al, Cl, P, and O. Surface scanning energy spectrum in Fig. 6(a) and (b) suggest that the morphological structures of all elements except oxygen show identical trends and they are uniformly distributed on the surfaces of H1/H2. This proves that the elements in H1 and H2 are specified by homogeneous distribution and are in a stable combination. In view of the alternation of FTIR functional groups and the preparation process, it can be speculated that copolymerization occurs between HAP and PAC and that the possible combination form for this procedure is that a certain amount of Ca²⁺ and OH⁻ in the HAP crystal structure is partially doped and replaced by Al³⁺ and Cl⁻ in PAC. The elements Ca, P, Al and Cl in H3 display identical morphology and structure. However, the morphology and structure between these two groups is apparently different, which indicates Ca and Al in the filtering material are unevenly distributed. On the basis of the preparation process, the conclusion can be drawn that HAP and PAC are physically mixed with each other during the synthetic process and that there is no occurrence of a copolymerization reaction. The information reflected is consistent with FTIR and XRD.

Fig. 6 (a) EDS spectrum for copolymerization material H1. (b) EDS spectrum for filtering material H2. (c) EDS spectrum for filtering material H3.

3.3 Adsorption kinetics and the effects of pH

The kinetics of adsorption is an important parameter for designing adsorption systems and is required for selecting optimum operating conditions for full-scale batch process. The different parameters of adsorption kinetics onto these three filter materials are illustrated in Fig. 7. In our study, the kinetic data of H1, H2 and H3 fit well with pseudo-second-order equations, the adsorption process of them generally belongs to chemical adsorption.²⁴ From the shape of the kinetic curve, it is evident that fluoride adsorption onto HAP–PAC copolymerization material is a two-step process, i.e. initial rapid adsorption during the first 25 min and a slower rate of adsorption until the equilibrium is reached. The pseudo-second-order equation is shown below.
where Q_t is the amount of fluoride on the surface of filter materials at any time (mg g⁻¹), k₂ is the pseudo-second-order rate constant (g mg⁻¹ min⁻¹), Q_e is the amount of fluoride adsorbed at equilibrium (mg g⁻¹) and t is the reaction time (min).

Fig. 7 Defluoridation capacity onto H1 (a), H2 (b) and H3 (c) at various time, and the inserted small figure indicates pseudo-second-order adsorption rate of F⁻.

The pH of the medium is one important variable, which significantly affect the extent of adsorption of fluoride. Fig. 8 shows the effect of the initial solution pH on fluoride adsorption onto H1, H2 and H3 at the given conditions. Clearly, the maximum defluoridation capacity (DC) is recorded at pH = 5.69 and shows a gradual decreasing trend with increases in the solution pH. The reduction of fluoride removal in the alkaline pH range should be attributed to competition of hydroxyl ions with fluoride for adsorption sites because of the similarity in charge and ionic radii for fluoride and the hydroxyl ion.

Fig. 8 Defluoridation capacity onto H1 (a), H2 (b) and H3 (c) at various initial pH values.

3.4 Adsorption isotherm

Analysis of equilibrium data is important for developing an equation that can be used to compare different adsorbents under different operational conditions and to design and optimize an operating procedure. The adsorption isotherms of these three filtering materials are shown in Fig. 9.

Fig. 9 Adsorption isotherms of H1 (a), H2 (b) and H3 (c) at different temperatures.

Data from the adsorption isotherms are modeled using the Langmuir and Freundlich isotherm models with the resulting isotherm constants presented in Table 2.

Table 2 Langmuir and Freundlich isotherm constants for F⁻ adsorption by filtering materials

Adsorbent	Adsorption model	Relevant parameter	308 K	318 K	328 K
H1	Langmuir	Qm (mg g^−1)	19.194	24.876	32.030
		a	0.181	0.168	0.144
		R^2	0.9997	0.9884	0.9772
	Freundlich	n	2.045	1.873	1.814
		KF	3.973	4.575	4.974
		R^2	0.9781	0.9869	0.9772
H2	Langmuir	Qm (mg g^−1)	17.544	23.866	29.412
		a	0.318	0.25	0.191
		R^2	0.9396	0.974	0.9914
	Freundlich	n	2.599	2.211	2
		KF	5.562	6.109	6.142
		R^2	0.8972	0.9647	0.9704
H3	Langmuir	Qm (mg g^−1)	12.180	20.202	24.450
		a	0.469	0.246	0.163
		R^2	0.9183	0.9526	0.9233
	Freundlich	n	3.293	2.149	1.985
		KF	5.033	5.018	4.711
		R^2	0.9152	0.9572	0.9242

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Langmuir model

Freundlich model

where C_e is the equilibrium concentration (mg L⁻¹), Q_e is the amount adsorbed at equilibrium (mg g⁻¹), Q_m is adsorption capacity for Langmuir isotherms and ‘a’ is an energy term, which varies as a function of surface coverage strictly due to variations in the heat of adsorption. ‘n’ indicates the degree of favorability of adsorption and K_F is the isotherm constant for the Freundlich model.

As observed in Fig. 9 and Table 2, fluoride adsorption onto H1 is well described by the Langmuir model. It indicates that homogeneous distribution of active sites on the adsorbents surface and the adsorption of fluoride occur in a monolayer adsorption manner. The Langmuir model of H2 is not as good as that of H1 on account of the participation of physical adsorption in H2. H3 is well described by both models, it indicates that H3 is dominated by physical adsorption, and it only belongs to the Langmuir model when the pores are sufficiently small. Furthermore, as the temperature increases from 35 °C to 55 °C, a positive effect is observed on the adsorption of fluoride (Table 2). This phenomenon is because of the increased tendency of fluoride attached to filtering materials, which may also indicate that the adsorption of fluoride onto filtering materials is endothermic in nature. To study the feasibility of the process, the thermodynamic parameters were obtained from the following equations.

ΔG^θ = −RT ln K^θ

The value of K^θ can be obtained from the intercept of a plot of ln(Q_e/C_e) vs. C_e, in which C_e is the equilibrium concentration (mg L⁻¹), and Q_e is the amount adsorbed at equilibrium (mg g⁻¹). The values of ΔH^θ and ΔS^θ can be obtained from the slope and intercept of a plot of ln K vs. 1/T and the results are presented in Table 3. The adsorption of fluoride onto H1, H2 and H3 are endothermic in nature.

Table 3 Thermodynamic parameters for F⁻ removal by filtering materials

Adsorbent	Temperature	K	ΔG (kJ mol^−1)	ΔH (kJ mol^−1)	ΔS (J mol^−1 K^−1)
H1	308 K	2.017	−1.796	27.889	81.796
	318 K	2.181	−2.061
	328 K	2.308	−2.281
H2	308 K	1.300	−0.672	14.246	51.490
	318 K	1.451	−0.954
	328 K	1.505	−1.047
H3	308 K	1.102	−0.248	9.571	33.345
	318 K	1.239	−0.548
	328 K	1.243	−0.557

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3.5 Mechanism of fluoride removal by the sorbents

The defluoridation process of HAP–PAC copolymerization material is extremely complicated. In view of multiple evaluations, comparisons among infrared spectra, analyses on surface morphology of filtering materials, and related research on fluoride removal, it can be reasoned that the formula of the copolymerization material is [Al_xCa_10−x(PO₄)₆Cl_y(OH)_z]^{(x+2−y−z)+}, and from the result of the influence of pH value, we can know that the M–OH on the surface of filtering materials belong to active sites. It can also be concluded that the quantitative substitution of the M–OH groups by F⁻ plays a key role in F⁻ adsorption.²¹ The more adsorption sites are formed on the HAP–PAC copolymerization material, which possesses abundant surface hydroxyl groups, the more efficient F⁻ removal occurs. However, anion exchange has happened during the modification process, when hydroxide ion is dissociated from Ca–OH, elevating the pH value. Under this circumstance, the aluminum ion can combine with the hydroxyl ion forming the tetrahydroxoaluminate ion, and the ion-exchanging reaction can occur between HAP and the tetrahydroxoaluminate ion to generate more hydroxide ions.²⁵ Based on this speculation, the reaction equation is described as follows.

Al³⁺ + 4OH⁻ → [Al(OH)₄]⁻ (1)

Ca₁₀(PO₄)₆(OH)₂ + [Al(OH)₄]⁻ → Ca₁₀(PO₄)₆OH·Al(OH)₄ + OH⁻ (2)

During the defluoridation procedure, electrostatic adsorption can be encouraged as positively charged filtering material contacts with F⁻. Moreover, the large number of –OH functional groups in the crystal lattices are capable of conducting replacement reactions or ion exchange reactions with F⁻ in water. In addition, [Al(OH)₄]⁻ and Ca₁₀(PO₄)₆OH·Al(OH)₄ are produced during the copolymerization process for this type of material, which makes it possible for the occurrence of chemical adsorption between them and fluoride ions. Moreover, the HAP–PAC copolymerization material is also accompanied by the generation of a hydrolytic condensation product [Al₁₃O₄(OH)₂₄]⁷⁺ and gel [Al(OH)₃].²⁶ The ligand hydroxyl ions embedded in these products can exchange with fluorine ions.

Based on the comprehensive analysis of these three types of filtering materials, it can be theorized that the defluorination process of H1 is dominated by chemical adsorption and partly accompanied by physical adsorption, of which the main reaction equations are expressed as below ((7)–(12)). Both electrostatic adsorption and partial chemical adsorption will occur during the defluorination process of H2, its reaction equations being shown as below ((3)–(12)). Physical adsorption is the main reaction form on the surface of H3 when contacting with F⁻ (moreover, unbroken ingredients of HAP may also conduct relatively weaker electrostatic adsorption and ion exchange process), its chief reaction equations are conveyed as below ((3)–(6)).

Physical adsorption process:

[Al_xCa_10−x(PO₄)₆Cl_y(OH)_z]^{(x+2−y−z)+} + nF⁻ → [Al_xCa_10−x(PO₄)₆Cl_y(OH)_z]^{(x+2−y−z)+}…nF⁻ (3)

[Al_xCa_10−x(PO₄)₆Cl_y(OH)_z]^{(x+2−y−z)+} + (x + 2 − y − z)F⁻ → [Al_xCa_10−x(PO₄)₆Cl_y(OH)_z]…F_x+2−y−z (4)

Ca₁₀(PO₄)₆(OH)₂ + nF⁻ → Ca₁₀(PO₄)₆(OH)₂…nF⁻ (5)

[Al₂(OH)_nCl_6−n]_m + nF⁻ → [Al₂(OH)_nCl_6−n]_m…nF⁻ (6)

Chemical adsorption process:

[Al_xCa_10−x(PO₄)₆Cl_y(OH)_z]^{(x+2−y−z)+} + mF⁻ → [Al_xCa_10−x(PO₄)₆Cl_y(OH)_z−mF_m]^{(x+2−y−z)+} + mOH⁻ (7)

[Al_xCa_10−x(PO₄)₆Cl_y(OH)_z]^{(x+2−y−z)+} + zF⁻ → [Al_xCa_10−x(PO₄)₆Cl_yF_z]^{(x+2−y−z)+} + zOH⁻ (8)

Ca₁₀(PO₄)₆OH·Al(OH)₄ + xF⁻ → Ca₁₀(PO₄)₆(OH)_1−yF_y·Al(OH)_4−x+yF_x−y + xOH⁻, (0 ≤ y ≤ 1, 0 ≤ x ≤ 5) (9)

[Al(OH)₄]⁻ + xF⁻ → [Al(OH)_4−xF_x]⁻ + xOH⁻, (0 ≤ x ≤ 4) (10)

[Al₁₃O₄(OH)₂₄]⁷⁺ + xF⁻ → Al₁₃O₄(OH)_24−xF⁷⁺_x + xOH⁻, (0 ≤ x ≤ 24) (11)

Al(OH)₃ + xF⁻ → Al(OH)_3−xF_x + xOH⁻, (0 ≤ x ≤ 3) (12)

The HAP–PAC copolymerization material can trap the particles in water during the procedure of defluorination. Then, these particles, treated as adsorbates, will precipitate together with the materials. Through this inner mechanism, the goal of fluoride removal in water can be thoroughly achieved. The general procedure of fluoride removal is illustrated in Fig. 10.

Fig. 10 Mechanism of fluoride removal by HAP–PAC copolymerization material.

4 Conclusion

(1) A novel adsorbent composed of copolymerized HAP–PAC material features a strong defluorination capacity, thus enormously improving the defluoridation capacity of HAP. At the same time, by virtue of the strong flocculation effects of PAC, this material can eliminate the concern that powder materials may enter the water supply network because of the difficulty in entrapment or sedimentation, therefore attaining the goal of thoroughly removing fluoride in water. Moreover, the quality of the processed water can meet the standards of drinking water, casting away any safety concerns. This obviously provides a new method and an original idea for the preparation of high-efficiency fluoride removal materials.

(2) The copolymerized HAP–PAC material prepared by means of aluminum chloride crystallization is characterized by the chemical combination between hydroxyapatite and poly aluminum chloride, and a certain amount of Ca²⁺ and OH⁻ in HAP crystal lattice is partially doped and replaced by Al³⁺ and Cl⁻ in PAC. The structure of the copolymerization material possesses a highly homogeneous structure. Moreover, its highest DC can reach up to 18.12 mg g⁻¹ when the Al/Ca molar ratio is 0.22, an increase of 14.02 mg g⁻¹ when compared with pure hydroxyapatite, and the fluoride removal process is mainly dominated by chemical adsorption process (ion exchange).

(3) As for the filtering material prepared by bauxite, a small part of Ca²⁺ and OH⁻ in HAP crystal lattice are doped and replaced by Al³⁺ and Cl⁻ in PAC, and both chemical combination and physical combination will occur during this procedure while the degree of uniformity is relatively weaker on account of the co-existence of HAP, PAC, and copolymerization material. The maximum DC is 13.72 mg g⁻¹ at the optimal Al/Ca molar ratio 0.22. Electrostatic attraction and ion exchange are the two main processes for defluorination.

(4) In consideration of the filtering material prepared by physical mixing, the combination between HAP and PAC is mainly physical effects, with its highest DC reaching 13.08 mg g⁻¹ at the best Al/Ca molar ratio 0.132. Physical adsorption is the governing form during this procedure of defluorination.

Acknowledgements

This study was supported by Jiangsu Province Science and Technology Key Research Program (Grant No. BE2015628), the 111 Project (B12030), the Fundamental Research Funds for the Central Universities (Grant No. 2014XT05), and the Priority Academic Program Development of Jiangsu Higher Education Institutions.

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