Abstract

Given widespread fluoride in the ground water, there is a need for effective defluoridation in several geographical areas. In this regard, we explored heavily doped cationic nano-composites of hydroxyapatite (HA) given its surface chemistry for adsorption of the specific anion. We synthesized and extensively characterized HA nano-rods (HA-NR), Al/Mg-HA nanocomposites and amorphous aluminum hydroxide, and optimized their efficient defluoridation. The kinetics and thermodynamics of adsorption were further evaluated to establish the mechanistic rationale and its spontaneity. We report the optimized ideal adsorbents for the near-total removal of fluoride that demonstrated 99.99% and 99.98% efficiency with adsorption capacities of 83.3 and 81.3mg/g respectively. The adsorbent composites were (Mg-HA)-Al(OH)3 and (HA-NR)-Al(OH)3 in 1:1 ratio. The optimal conditions for defluoridation were 25mg of adsorbent in 25ml (10mg/L) fluoride solution at room temperature agitated for 10h in the pH range of 4.88–7.20.

INTRODUCTION

Fluoride ion in the range of 0.5–1.5mg/L is required for the good health of the human body. However, when beyond that concentration, it results in dental and skeletal fluorosis (Zhang & Jia 2016; Nayak et al. 2017). Defluoridation in this scenario assumes prime importance as the only practical way out of this problem. In this regard, there has been a lot of promising work using natural wastes (Parmar et al. 2006), commercial activated carbons (Tembhurkar & Dongre 2006; Emmanuel et al. 2008), graphite (Karthikeyan & Elango 2008), and CNTs (Li et al. 2003). There were consequent studies using ion exchange resins (Samadi et al. 2014), clays like pumice (Malakootian et al. 2011), granular red mud (Tor et al. 2009), bauxite (Das et al. 2005) and even plaster of Paris (Gopal & Elango 2007). Minerals too have undergone a thorough investigation as potential adsorbents with promising results ranging from laterite (Sarkar et al. 2006), hydroxyapatite (HA) (Fan et al. 2003), carbonate-impregnated hydroxyapatite (Terasaka et al. 2016), and nano-crystalline HA (Chandrasekar et al. 2013).

The use of HA specifically attained greater importance given its biocompatibility as the alternative bone substitute in humans. HA nano-rods (HA-NR) as adsorbents yielded significant results that led to the exploration of their cation-doping in low amounts of (Al/Mg/La)-HA for evaluating their potential as adsorption membranes (Chen et al. 2018). Another promising recent report was the use of amorphous aluminum hydroxide where Zhang & Jia (2016) investigated the defluoridation phenomenon using amorphous Al(OH)3 with hydroxyl groups, acetate anions and chlorine anions' enriched surface. In light of these findings, the present work studies defluoridation using composites of heavily cation-doped HA (Al/Mg at 33%/50% respectively) with amorphous aluminum hydroxide. The proposed adsorbents were synthesized, characterized and optimized for their defluoridation capacity for the various parameters. In addition, the thermodynamics and kinetic models of adsorption were also explored.

MATERIALS AND METHODS

Synthesis

Hydroxyapatite and amorphous Al(OH)3 were prepared using the routes prescribed (Zhang & Jia 2016; Chen et al. 2018). The synthesis of HA-NR was undertaken via the hydrothermal method prescribed by Li et al. (2014). For the synthesis of heavily doped (33%) Al-HA, 200ml of 0.14 M Al(NO3)3.9H2O was prepared in DI water to which 200ml 0.5 M Ca(NO3)2.4H2O was added. For the synthesis of 50% doped Mg-HA, 200ml of 0.2 M Mg(NO3)2.6H2O in DI water was prepared to which 200ml of 0.5 M Ca(NO3)2.4H2O in DI water was added. These solutions were homogenously mixed following which, 1.67 M 100ml ortho-phosphoric acid was added at 1ml/min under vigorous stirring. The pH of the reaction mixture was maintained at 9–10 by adding dilute ammonia periodically. After precipitation, it was continuously stirred for 0.5h. The products obtained were filtered and successively washed with DI water repeatedly to neutral. The filtered products were then obtained by drying in a hot air oven at 105 °C for 3h followed by grinding (Chen et al. 2018).

Characterization

The aforementioned synthesized materials were then subjected to various characterization techniques to establish their physico-chemical properties. X-ray powder diffraction was undertaken using the Pananalytical, X-Pert-3 diffractometer at a scan speed of 2°/min with Cu-Kα radiation (λ = 1.54Å) in the 2θ range of 10° to 80°. The crystallite size (Xc) was determined using the Scherrer equation. Attenuated total reflection (ATR) spectroscopy was used to identify the functional groups using Agilent Technologies Cary 630 Fourier transform infrared (FT-IR) spectrometer by the powder technique recorded from 4,000–400 cm−1 in transmission mode. The defluoridation studies were undertaken using the Fluoride Ion Selective Electrode with the Thermo Electron Corporation Orion 720A+ Advanced ISE/pH/mV/ORP Model #S/N 088150.

Sample and standard solution preparation

A 200mg/L fluoride stock solution was prepared in DI water using NaF. It was kept in a sealed standard flask under cool and dark conditions. Various concentrations ranging from 1 to 200mg/L (1, 5, 10, 30, 50, 80, 100, 160, 200) were obtained by diluting the above standard fluoride solution with DI water, and 50ml of 0.1 M HCl and NaOH standard solutions were prepared to maintain the pH at around 7.20 (natural pH of the solution at 0 adsorption time). To measure the fluoride ion concentration and eliminate the unnecessary interference of complexing ions, while maintaining the pH in the range (5.3–5.5), total ionic adjustment buffer (TISAB) was added.

Adsorption experiments

The composites of the five different kinds of adsorbents synthesized were evaluated for defluoridation using varying combinations in multiple ratios. The adsorbents were taken in 250ml polypropylene flasks containing 25ml of varying concentrations of fluoride and agitated at 200rpm. All the experiments, including the optimized conditions experiments, were conducted in triplicate for the statistical significance of the data analysis.

Kinetics studies protocols

The dosage optimization for the best possible combination of adsorbents was done by agitating the 10mg/L fluoride solution with varying proportions of the adsorbents. Finally, for the ideal composites, the conditions of pH, temperature and dwell time were optimized for maximum defluoridation using the various formalisms provided in the kinetics protocol, in the Supplementary Data, available with the online version of this paper.

RESULTS AND DISCUSSION

Morphology

We note that the hydrothermal treatment at 150 °C results in the morphology of nano-rods for HA as depicted in the Supplementary Data, Figure S1. We observe the extremely high porosity within the hollow cavity of these rods that can be rationalized to exhibit more surface contact with the highly nucleophilic fluoride ion. Figure S2 in the Supplementary Data captures the scanning electron microscopy (SEM) images of the doped cationic hydroxyapatite where we notice the agglomeration. The quantitative elemental analysis for the formation of HA and its cationic doping was confirmed using the energy-dispersive X-ray spectroscopy (EDX) studies. The SEM images of the Al(OH)3 are provided in the Supplementary Data, Figure S3, which exhibit its high surface area. This rationalizes its potential for the interaction of fluoride with the hydroxyl groups on the adsorbent, and consequently greater adsorption capacity with increasing contact time. We also observe very high surface porosity in the surface morphology of these materials. (Figures S1–S3 are available with the online version of this paper.)

Functional characterization

The HA spectrum (Supplementary Data, Figure S4, available online) contains the two peaks of the hydroxyl group owing to its weak stretching at 3,440 cm−1 and the weak bending vibration at 1,626 cm−1 in addition to the phosphate group at 601 cm−1 characteristic of peaks in pure HA (Lala et al. 2016). Further, the spectrum of the cation-doped HA exhibits the characteristic peaks of HA with the weak stretching and bending vibrations at 3,441 and 1,629 cm−1 respectively in addition to the phosphate vibration that occurs at 564 cm−1. However, it is clear that both the vibrational peaks of the OH emerged as a blue shift with increasing intensity, as reported by earlier studies (Lala et al. 2016), while the peaks corresponding to the phosphate remain unchanged in comparison with the pure HA. Figure S4 in the Supplementary Data also confirms the formation of the synthesized aluminum hydroxide. We note the key peak at 3,215 cm−1 that can be attributed to the stretching vibration of the hydroxyl groups, while the peaks at 2,695 and 2,770 cm−1 correspond to the C-H stretching mode. The band at 1,139 cm−1 is attributable to the bending vibrations of the C-O group in the ethylene glycol. The peak at 741 cm−1 can be reasoned to the Al-O stretching vibration (Persson et al. 1998; Strathmann & Myneni 2004; Peterangelo et al. 2007; Ibrahim & Abu-Ayana 2008; Lala et al. 2016).

X-ray diffraction (XRD)

The X-ray powder diffraction patterns of synthesized powders are captured in the online Supplementary Data, Figure S5. The key planes are found to be comparable to hydroxyapatite (standard reference, JCPDS: 98-028-9993) with the matching 100% and most intense peaks at the hkl planes corresponding to (201), (211) and (213). We notice between the two spectra corresponding to the two doped HA composites, that the key difference lies with respect to (400) and (203). These could be demarcated as the characteristic peaks accordingly for the two cations present in the crystal lattice. The XRD spectrum in Figure S6, Supplementary Data (available online), confirms the formation of amorphous Al(OH)3 with several wide diffraction peaks comparable to peaks obtained by Fukushi et al. (2006). The mean distribution of the theoretical crystallite size of the powders as calculated from the Scherrer equation using the XRD peaks returned the sizes for HA, Al-HA, Mg-HA at 57.6, 46.8, and 71.35nm respectively.

Adsorbents and their adsorption capacities

The kinetics of the adsorption of fluoride on 25mg of the following adsorbents and their combinations were investigated at 30 °C and 7.20pH. The amount of fluoride adsorbed as determined by corresponding (qe) values using Equations (1) and (2) in the kinetics section, online Supplementary Data, are recorded in Table 1. We clearly observe that amorphous (Mg-HA)-Al(OH)3 and (HA-NR)-Al(OH)3 are the combinations exhibiting the best qe of 9.990 and 9.987mg/g respectively. As expected, the hydroxyapatite that was taken as the control exhibited the least qe of 6mg/g.

Table 1

The percentage removal efficiency of defluoridation as achieved by the various composites

Types of adsorbents Removal (%) qe (mg/g) 
Hydroxyapatitea 60.0 6.00 
Hydroxyapatite nano-rods 90.0 9.00 
Al-HA 76.2 7.62 
Mg-HA 95.2 9.52 
Al(OH)3a 63.9 6.39 
Al-HA + Mg-HA + Al(OH)3a 88.9 8.10 
Al(OH)3a + Al-HA 81.0 8.89 
Al(OH)3a + Mg-HA 99.9 9.99 
Al(OH)3a + HA-NR 99.8 9.98 
Types of adsorbents Removal (%) qe (mg/g) 
Hydroxyapatitea 60.0 6.00 
Hydroxyapatite nano-rods 90.0 9.00 
Al-HA 76.2 7.62 
Mg-HA 95.2 9.52 
Al(OH)3a 63.9 6.39 
Al-HA + Mg-HA + Al(OH)3a 88.9 8.10 
Al(OH)3a + Al-HA 81.0 8.89 
Al(OH)3a + Mg-HA 99.9 9.99 
Al(OH)3a + HA-NR 99.8 9.98 

aAmorphous.

Therefore, the consequent studies were undertaken for the two adsorbents that returned the best adsorption capacity: amorphous (HA-NR)-Al(OH)3 and (Mg-HA)-Al(OH)3.

Influence of pH

The influence of pH on the adsorption ranging from 1.65 to 9.20 was evaluated as captured in Figure 1(a). The tabulated data, Table ST1 in the Supplementary Data (available online), indicate that the ideal window of pH (4.5–6.5) exhibited the most significant qe values, comparable to the values also obtained at the natural pH of 7.20. However, we clearly note the considerable decrease in adsorption at the extreme pH conditions. Therefore, we confirm that the optimum pH for the fluoride adsorption is 7.20 as we prefer this economic condition to the acidic range of pH 4.5–6.5. Subsequently, all the adsorption experiments were undertaken maintaining the pH constant at 7.20 using 0.1 M NaOH every 2h, to restore the neutrality from the basicity observed due to the displacement of the OH ions by the fluoride.

Figure 1

The influence of the different conditions on adsorption of fluoride on both the adsorbents: (a) the influence of pH, (b) the effect of the dwell time, and (c) the influence of contact time.

Figure 1

The influence of the different conditions on adsorption of fluoride on both the adsorbents: (a) the influence of pH, (b) the effect of the dwell time, and (c) the influence of contact time.

Optimizing the dwell time

The time required to achieve the maximum adsorption was obtained by plotting the adsorption capacity at constant pH of 7.20 as a function of dwell time in the range of 2–24h as compiled in Table ST2 in the online Supplementary Data. Figure 1(b) provides the comparison between the amount of fluoride adsorbed on both the adsorbents. After 10h of agitation, we record their saturation limit in terms of fluoride adsorption capacity. This demonstrates that 10h of contact time is the ideal time required for the maximum adsorption.

Influence of contact time on adsorption

Figure 1(c) plots the fluoride adsorption as a function of contact time for the two adsorbents to determine the kinetics of adsorption as tabulated (Supplementary Data, Tables ST3 and ST4, available online). We observe the evident initial rapid adsorption of fluoride in both cases. For 10mg/L, almost 99.9% and 99.8% removal of fluoride was observed within the first 20 minutes of contact time, and equilibrium was achieved for the adsorption process from the 11th minute onwards for the respective adsorbents. The dwell time for the increase in concentration from 10 to 20mg/L has been accordingly recorded.

Kinetic modelling

Reaction-based models

In order to explain the adsorption of fluoride, we pursued the commonly adopted pseudo-first-order and pseudo-second-order models to rationalize the observations. The mathematical formalisms of the two models are provided in the kinetics section in the online Supplementary Data (Equations (3) and (4)). On comparison of the correlation equations based on the coefficient r2 for both types of adsorbents (Figure 2 and Supplementary Data, Table ST5, available online), we note that the pseudo-second-order adsorption model has higher values than that of the pseudo-first-order adsorption model. Further, interestingly, only for the pseudo-second-order model, the calculated qe are almost equal to the experimentally obtained qe while they are significantly lower for the pseudo-first-order adsorption model. Thus, the adsorption kinetics for fluoride can be attributed to the pseudo-second-order model.

Figure 2

Kinetic models for the defluoridation by (Mg-HA)/Al(OH)3 and HA nano-rods/Al(OH)3 plotting the corresponding (a,d) pseudo-first-order and (b,e) pseudo-second-order kinetics and the (c,f) inter-particle diffusion plot.

Figure 2

Kinetic models for the defluoridation by (Mg-HA)/Al(OH)3 and HA nano-rods/Al(OH)3 plotting the corresponding (a,d) pseudo-first-order and (b,e) pseudo-second-order kinetics and the (c,f) inter-particle diffusion plot.

Diffusion-based model

Further, to understand the diffusion mechanism of the fluoride adsorption, the kinetic results were validated by employing the Weber and Morris intra-particle diffusion model as provided in the online Supplementary Data (Equation (5)). The obtained values of kip for the adsorbents are listed in the Supplementary Data, Table ST5. Based on this model, we observe that for the initial fluoride concentration at 10mg/L, the correlation coefficient r2 of the intra-particle diffusion model is 0.9041 and 0.86103 for the (Mg-HA)-Al(OH)3 and (HA-NR)-Al(OH)3 adsorbents respectively. The large significance of correlation thus confirms the mechanism of intra-particle diffusion as the dominant adsorption process.

Langmuir adsorption isotherm

A key aspect of kinetic modelling that studies the energy of adsorption and the adsorption process is the Langmuir adsorption isotherm, represented by Equation (6) in the online Supplementary Data. Figure S7(a) and S7(b) in the online Supplementary Data demonstrate the experimental consonance to the Langmuir adsorption isotherm for each of the adsorbents. The corresponding values of unknowns qm and b for each of the adsorbents are shown in Table 2, which confirm the mechanism to be via a monolayer and homogeneous in nature with the maximum adsorption capacities for (Mg-HA)-Al(OH)3 and (HA-NR)-Al(OH)3 as 83.3 and 81.3mg/g respectively at 30 °C.

Table 2

The comparison of the various adsorption parameters determined for the Langmuir and Freundlich adsorption models

LAI (35 °C) qm(mg/g) b(L/mol) RL r2 
(Mg-HA)-AlOH3 50 0.0183 0.9077 0.9916 
(HA-NR)-AlOH3 50.01 0.0181 0.9071 0.9925 
LAI (30 °C) qm(mg/g) b(L/mol) RL r2 
(Mg-HA)-AlOH3 83.3 0.0129 0.9080 0.9951 
(HA-NR)-AlOH3 81.3 0.0132 0.9080 0.9953 
LAI (15 °C) qm(mg/g) b(L/mol) RL r2 
(Mg-HA)-AlOH3 82.9 0.029 0.9080 0.9915 
(HA-NR)-AlOH3 81.1 0.030 0.9070 0.9917 
FAI (1/n) KF(mg/g)(L/mg)(1/n) r2 
(Mg-HA)-AlOH3 0.585 1.514 0.9527 
(HA-NR)-AlOH3 0.579 1.523 0.9518 
LAI (35 °C) qm(mg/g) b(L/mol) RL r2 
(Mg-HA)-AlOH3 50 0.0183 0.9077 0.9916 
(HA-NR)-AlOH3 50.01 0.0181 0.9071 0.9925 
LAI (30 °C) qm(mg/g) b(L/mol) RL r2 
(Mg-HA)-AlOH3 83.3 0.0129 0.9080 0.9951 
(HA-NR)-AlOH3 81.3 0.0132 0.9080 0.9953 
LAI (15 °C) qm(mg/g) b(L/mol) RL r2 
(Mg-HA)-AlOH3 82.9 0.029 0.9080 0.9915 
(HA-NR)-AlOH3 81.1 0.030 0.9070 0.9917 
FAI (1/n) KF(mg/g)(L/mg)(1/n) r2 
(Mg-HA)-AlOH3 0.585 1.514 0.9527 
(HA-NR)-AlOH3 0.579 1.523 0.9518 

LAI, Langmuir adsorption isotherm; FAI, Freundlich adsorption isotherm.

To ascertain the feasibility of the adsorption isotherm, the adsorption intensity RL was invoked. From Table 2, the values of RL obtained for (Mg-HA)-Al(OH)3 and (HA-NR)-Al(OH)3 are 0.9080 and 0.9080 respectively in the range 0–1. This confirms that the Langmuir adsorption model explains the adsorption most comprehensively.

Freundlich adsorption isotherm

Figure S7(c) and S7(d) in the Supplementary Data plot the result obtained in principle with Equation (8), corresponding to the Freundlich adsorption isotherm. The corresponding values of KF and 1/n are provided in Table 2 where the values of 1/n for (Mg-HA)-Al(OH)3 and (HA-NR)-Al(OH)3 are 0.585 and 0.579 respectively in the range 0–1, thus confirming the favourable conditions for fluoride adsorption. However, given that these r2 values of the corresponding adsorbents are lower in the case of Freundlich adsorption in comparison with the RL values provided above, we confirm the mechanism to pursue the Langmuir model.

The adsorbents were then evaluated for the function of temperature whose qm and b parameters are tabulated in Table 2 for both the substrates with their corresponding isotherms and van't Hoff's plots for the fluoride removal captured in the Supplementary Data, Figures S8 and S9 respectively (available online).

Thermodynamic study

The extent of the feasibility of the process was determined by thermodynamics. The Gibbs free energy (ΔGo) was evaluated for (Mg-HA)-Al(OH)3 and (HA-NR)-Al(OH)3 to be −1.801 × 103 and −1.844 × 103 kJ/mol respectively. The corresponding details are enumerated in Table 3. The negative Gibbs free energies thus confirm the favourable spontaneity of the mechanism of uptake/adsorption of the fluoride onto the adsorbents.

Table 3

The evaluated thermodynamic parameters of the two adsorbents

Adsorbents kd
 
ΔG° (kJ/mol)
 
(kJ/mol)  (kJ/mol) 
35 °C 15 °C 35 °C 15 °C 
(Mg-HA)-AlOH3 1.018 1.60 −4.097 −3.831 −10.38 −35.09 
(HA-NR)-AlOH3 1.017 1.65 −3.098 −3.882 −10.94 −33.34 
Adsorbents kd
 
ΔG° (kJ/mol)
 
(kJ/mol)  (kJ/mol) 
35 °C 15 °C 35 °C 15 °C 
(Mg-HA)-AlOH3 1.018 1.60 −4.097 −3.831 −10.38 −35.09 
(HA-NR)-AlOH3 1.017 1.65 −3.098 −3.882 −10.94 −33.34 

Post-defluoridation SEM analysis

SEM images of the adsorbent (Mg-HA)-Al(OH)3 taken post-adsorption are presented in Figure 3(a) and 3(b) for different scales of resolution. The adsorption of fluoride is confirmed by the EDX study of the elemental composition of the adsorbent that demonstrates the uptake of fluoride present in the adsorbent as captured in Figure 3(c).

Figure 3

SEM images of the composite after deflouridation: (a) (Mg-HA)-Al(OH)3 at 10 µm, (b) (Mg-HA)-Al(OH)3 at 2 µm, and (c) the corresponding EDX spectrum with emphasis on the fluoride uptake peak.

Figure 3

SEM images of the composite after deflouridation: (a) (Mg-HA)-Al(OH)3 at 10 µm, (b) (Mg-HA)-Al(OH)3 at 2 µm, and (c) the corresponding EDX spectrum with emphasis on the fluoride uptake peak.

CONCLUSION

The synthesized composites of amorphous aluminum hydroxide with nano-rods of hydroxyapatite (HA) and heavily cation-doped Al/Mg-HA were characterized. The optimization for maximal defluoridation was obtained using the aforementioned composites with 99.99% and 99.98% efficiency, and adsorption capacities of 83.3 and 81.3mg/g respectively. The ideal combination of adsorbents was demonstrated to be 25mg of (Mg-HA)-Al(OH)3 and (HA-NR)-Al(OH)3 in 25ml fluoride-ion concentration at room temperature, agitated for 10h in the pH range 4.88–7.20. Kinetic studies revealed a pseudo-second-order model of adsorption with the dominant Weber and Morris intra-particle diffusion mechanism fitted to the Langmuir adsorption isotherm. The thermodynamic studies too ascertained the spontaneity of the adsorption process confirmed by the EDX studies on the adsorbent. A comparison with the emerging adsorbents reviewed (Yadav et al. 2018) complements the potential of this material for defluoridation. Moreover, where there were worries about the leaching of Al during the defluoridation, Al impregnation within the lattice of our material mitigates that potential toxicity. The current work leaves a positive scope for research into the translation of these materials to effective defluoridation technologies.

ACKNOWLEDGEMENTS

We are indebted to the founding Chancellor Bhagawan Sri Sathya Sai Baba for his societal vision in transforming lives that inspired this work. We are thankful to the Central Research Instruments Facility (CRIF), SSSIHL for the support and resources in the relevant studies. We thank Dr Rajni Bhandari, Assoc. Prof., SSSIHL, for lending the fluoride meter in a timely manner.

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Supplementary data