Abstract

In the current work, alumina modified natural zeolite (Z-Al) was used for fluoride adsorption in aqueous solution. Effects of process parameters such as pH, temperature, initial concentration and contact time were investigated. Box–Behnken design was found effective in defining the operating conditions for fluoride sorption onto Z-Al. Confirmatory experiments were conducted to examine the reliability of the regression equation. The predicted (2.261 mg g−1) and experimental (2.289 mg g−1) capacities were found to be similar, demonstrating the accuracy of the model. The fluoride adsorption onto Z-Al was well described by the Freundlich model. Kinetic studies revealed that the adsorption followed a pseudo-second-order reaction. Thermodynamic parameters depicted that the fluoride adsorption on the alumina modified zeolite was a spontaneous and exothermic process. The co-existing ions affected the defluoridation performance significantly. Regeneration of exhausted Z-Al was achieved with H2SO4.

INTRODUCTION

Fluoride is considered as a beneficial constituent at concentrations in water ranging between 0.4 and 1.0 mg L−1 while excessive intake of fluoride can cause skeletal or dental fluorosis. The World Health Organization (WHO 2004) and European Union have established fluoride concentration as 1.5 mg L−1 in drinking water. The optimum range in the United States for fluoride in water is 0.7‒1.2 mg L−1, while in Turkey, the tolerance limit of fluoride in drinking water was regulated as 1.5 mg L−1. However, groundwater has very high fluoride concentrations (up to 30 mg L−1) in some countries, so fluorosis in humans has been a worldwide problem (Brindha & Elango 2011; Tomar et al. 2014). Therefore, different methods such as adsorption, ion exchange, membrane processes and precipitation–coagulation have been evaluated for defluoridation of water. Among them, adsorption is the preferred technique due to its simplicity and high performance and the availability of a broad range of adsorbents (Singh et al. 2017).

One of the fundamental questions for the application of the adsorption technique is the selection of a suitable adsorbent. So far, various adsorbents such as activated alumina, fly ash, alum sludge, hydroxyapatite, calcite, double layer oxides, zeolites, clays and soils have been used for fluoride removal (Mohapatra et al. 2009; Bhatnagar et al. 2011; He et al. 2016; Tang & Zhang 2016). Of these, natural zeolites are good candidates for the defluoridation of aqueous solutions (Díaz 2017). As zeolites have negative surface charges, modification is generally required in order to provide higher adsorption capacity for anions. Studies have shown that the treatment of zeolites with Al3+ ions can result in higher defluoridation performance (Onyango et al. 2004, 2006; Samatya et al. 2007; Vázquez-Guerrero et al. 2016). However, as far as we know, no detailed study including optimization, kinetics of reaction and regeneration has been reported on fluoride removal by alumina modified zeolite. In the current work, alumina modified Turkey clinoptilolite was evaluated in the removal of fluoride. The optimization of experimental parameters such as pH, temperature and initial fluoride concentration was examined using experimental design methodology. Kinetic behavior of the system was also investigated. The kinetic data of adsorption onto adsorbents were applied to the models, such as the pseudo-first-order, pseudo-second-order, intra-particular diffusion and Elovich models.

MATERIALS AND METHODS

Chemicals and reagents

Clinoptilolite from Gördes-Manisa was supplied from the Rota Mining Company. The sample was composed of 88%–95% clinoptilolite, 2%–5% montmorillonite, 3%–5% feldspar, 0%–3% muscovite and 0%–2% cristobalite mineral. AlCl3 (anhydrous) was used for zeolite modification. NaF, HCl (37%), HNO3 (65%), NaOH, SPADNS reagent (sodium 2-(parasulfophenylazo)-1,8-dihydroxy-3,6-naphthalene di-sulfonate), and ZrOCl2·8H2O were used in the sorption experiments and fluoride analysis.

Synthesis of alumina loaded zeolite

Modification of zeolite by alumina was achieved by following the procedure as described in our previous study (Bilgin Simsek et al. 2013). Briefly, zeolite (clinoptilolite) – particle size between 0.15‒0.50 mm – was carefully washed with distilled water and dried at 200 °C. Then, a defined amount of zeolite was mixed with 2.0 M NaCl solution at room temperature for 24 h and coded as Z-Na. The Z-Na sample was mixed with 1 M AlCl3 solution and then 5 M NaOH was added to raise the pH of the solution to pH 7.0. The resulting mixture was stirred for 1 h and aged at 65 °C for 2 days. The dried sample was washed with distilled water until reaching neutral pH and then placed into the oven to dry at 65 °C for 12 h. The resultant sample was coded as Z-Al.

Adsorption experiments

In the preliminary adsorption tests, the fluoride adsorption performances of the Z-Na and Z-Al samples were compared. The experiments were conducted with different adsorbent dosages (1.2, 2.4 and 3.6 g/L) and pH values (pH 5.0 and 7.0).

In order to obtain adsorption isotherms, a series of batch sorption studies were conducted. Defined amounts of Z-Al were placed in 50 mL glass flasks to contact with fluoride solution (Ci = 10 mg L−1). To investigate the effects of pH on fluoride adsorption, the isotherm studies were carried out at different pH values (3.0, 5.0 and 7.0). The effect of temperature (25, 40 and 55 °C) was examined at the optimum pH of 3.0. During experiments, solution pHs were checked and adjusted with H2SO4 or NaOH twice in a day. Once the solutions reached equilibrium, they were filtered and equilibrium fluoride concentrations were analyzed with a spectrophotometer (Thermo Aquamate UV-VIS Spectrophotometer) at a wavelength of 570 nm by using the SPADNS method (APHA 2005). Each experiment was triplicated under identical conditions and average results were reported. The samples were analyzed in duplicate.

For the kinetics investigation, the Z-Al sample was mixed with 10 mg L−1 fluoride solution (at pH 3.0), and the studies were conducted at 25, 40 and 55 °C. At regular time intervals, aliquots (5 mL) were collected and fluoride concentrations were analyzed. The removal of fluoride in the presence of co-existing ions such as chloride (Cl), nitrate , sulfate , carbonate , and phosphate was investigated with the concentration of 50 mg L−1 and with an initial fluoride concentration of 10 mg L−1 at room temperature (25 °C).

Experimental design modelling studies

The response surface method (RSM) is a set of statistical methods for evaluating and designing experiments, and finding optimum conditions of process parameters to predict responses (Tripathi et al. 2009). Box–Behnken design (BBD) is a type of RSM model including points at the center and at the midpoints of edges.

In the current work, the BBD technique was applied to investigate the effect of process parameters. Three parameters, pH (x1), temperature (x2), and initial fluoride concentration (x3), were designated in the quadratic equation, while the sorption capacity of Z-Al (mg g−1) was defined as the dependent variable (Yi). STATISTICA (Ver. 8.0, StatSoft Inc., USA) software was utilized for developing the second-order equation.

Firstly, the distributions of residuals were investigated by the Anderson–Darling normality test (Anderson 1987). When the p-value is equal to or smaller than 0.05 and A2 is higher than the critical value (0.787), the test rejects the hypothesis of normality.

Next, an analysis of variance (ANOVA) was examined to understand the BBD model approximation with 95% confidence limits (α = 0.05). The statistical significance of variables was investigated by F- and p-values and Student's t-test.

RESULTS AND DISCUSSION

Selection of sorbent for removal fluoride from aqueous phase

Preliminary adsorption studies were conducted with Z-Na and Z-Al adsorbents and the results are shown in Figure 1(a). Both Na- and Al-loaded zeolites showed better adsorption performance at pH 5.0. The sorption performance of Z-Al increased from 1.383 ± 0.09 mg g−1 to 1.814 ± 0.08 mg g−1 as the adsorbent dosage was increased from 1.2 to 3.4 g/L. Similar phenomena were observed with the Z-Na sample. Moreover, the alumina modified Z-Al adsorbent showed a better fluoride removal capacity (qZ-Al = 1.814 ± 0.08 mg g−1, pH 5) than the Z-Na (qZ-Na = 0.767 ± 0.12 mg g−1, pH 5). Therefore, the remaining fluoride sorption experiments were conducted with the Z-Al sample.

Figure 1

(a) Fluoride adsorption performances of Z-Na and Z-Al samples; (b) effect of pH on fluoride adsorption; (c) kinetic studies at different temperatures.

Figure 1

(a) Fluoride adsorption performances of Z-Na and Z-Al samples; (b) effect of pH on fluoride adsorption; (c) kinetic studies at different temperatures.

Effect of solution pH

Solution pH is a key parameter in a sorption process as it affects the active functional groups of the surface and the chemical speciation of the adsorbate. The point of zero charge (PZC) is a pH value at which the net surface charge is zero while the isoelectric point (IEP) is a pH value at which the zeta potential is zero. In our previous study, the pHPZC and pHIEP values of a Z-Al sample were found as 4.62 and 4.58, respectively (Bilgin Simsek et al. 2013). When the solution pH is below the PZC and IEP values, the surface is positively charged and shows higher affinity to anionic compounds. In the acidic environment, anionic fluoride is adsorbed on the positively charged surface of alumina loaded zeolite owing to electrostatic forces (Li et al. 2011).

The adsorption performance at different pH values (3.0, 5.0 and 7.0) is shown in Figure 1(b). When the equilibrium concentration was 0.29 ppm with 97% removal of fluoride, the removal capacity was calculated as 0.99 ± 0.11 mg g−1 at pH 3.0. However, increase of pH (>3.0) decreased fluoride removal while a progressive decrease was obtained above pH 5.0 indicating exchange reactions between surface hydroxyl groups and the fluoride ions are less favorable. As the concentration of OH and AlO increases, AlOH2+ sites decrease. Therefore, increase in the pH induces electrostatic repulsion between the negatively charged AlO surface and the fluoride. On the other hand, the decrease in adsorption performance with increasing pH could be attributed to the competition between OH and F ions on the surface. The fluoride adsorption capacities at pH 5.0 and pH 7.0 were calculated as 0.77 ± 0.43 mg g−1 (93% removal) and 0.45 ± 0.14 mg g−1 (64% removal), respectively. Reaction mechanisms during F adsorption on the surface can be depicted as (Valdivieso et al. 2006):  
formula
(1)
 
formula
(2)

In the literature, maximum fluoride adsorption capacities were generally found between pH 3.0 and 4.0 (Valdivieso et al. 2006; Wajima et al. 2009; Li et al. 2011).

Adsorption isotherms

The equilibrium distribution between the adsorbate molecules and the solution is essential since it describes a comprehensive understanding of the adsorption system. The equilibrium adsorption isotherms (Langmuir, Freundlich, Dubinin–Radushkevich (D-R); see Supplementary material, available with the online version of this paper) were applied by nonlinear regression analysis using the STATISTICA (Ver. 8.0, StatSoft Inc., USA) software package. The amount of adsorbed fluoride as a function of equilibrium concentrations is plotted in Figure S1. The regression coefficients (R2) were found as 0.937, 0.960 and 0.948 for the Langmuir, Freundlich and D-R isotherms, while chi-square values (χ2) were found as 6.56, 1.46 and 1.92, respectively (Table S1). According to the higher R2 and lower χ2 values, the Freundlich and D-R models were found to be more suitable than the Langmuir. The Freundlich isotherm parameter 1/n was determined as 0.653, which is a sign of chemical adsorption. The mean adsorption energy (E) value of the D-R model was found as 9.449 kJ mol−1 indicating a chemisorption process.

Adsorption kinetics

The adsorption kinetics of fluoride onto the Z-Al sample were conducted at different temperatures and the results are shown in Figure 1(c). After 8 h, 90.5%, 84% and 76.6% of fluoride was removed at 25, 40 and 55 °C, respectively. It was obvious that sorption rates were quite high in the early minutes and decreased throughout the adsorption. In the reactions involving chemical forces, the sorption rate decreases with time due to the increased surface coverage (Aharoni & Tompkins 1970). After 25 h, the fluoride removal was calculated as 98.9% at 25 °C while that for 55 °C was found as 76.7%.

In order to analyze the adsorption mechanism, the pseudo-first-order, pseudo-second-order, and Elovich models (Supplementary material, available with the online version of this paper) were applied by linear and nonlinear methods. The model parameters calculated by using linear and nonlinear methods are shown in Tables S2 and S3, respectively. The pseudo-second-order and Elovich models showed the best fit for both linear and nonlinear regression with higher R2 values (Figures S2 and S3). The adsorption performance of the linear second-order model was calculated as 1.004 mg g−1 (Table 1), which was closer to the experimental value (0.99 mg g−1). The higher R2 values for the pseudo-first-order and Elovich models by the nonlinear method when compared to that of obtained by the linear method indicates that nonlinear regression is a better method to estimate the best-fitting model. The difference in R2 values obtained by linear and nonlinear regression could be due to error alterations in the linearized form (Kumar & Sivanesan 2006). The second-order adsorption capacity calculated from the nonlinear method was found as 0.962 mg g−1, which was closer to the experimental value when compared with the pseudo-first-order model (0.919 mg g−1) (Table S3). The equilibrium factors of the Elovich equation (RE) were found as 0.147 (25 °C) and 0.148 (40 °C) indicating that fluoride adsorption was carried out at a mild speed at these temperatures (Table S2). The first-order model showed a low degree of correlation between the theoretical and experimental data (Table S2).

Table 1

Pseudo-second-order model parameters by linear and nonlinear methods for the sorption of F onto Z-Al

ParameterUnitLinear method
Nonlinear method
25 °C40 °C55 °C25 °C40 °C55 °C
qe mg g−1 1.004 ± 0.03 0.911 ± 0.06 0.7561 ± 0.09 0.962 ± 0.04 1.028 ± 0.07 0.661 ± 0.02 
k2 g mg−1 min−1 0.0143 ± 0.011 0.0220 ± 0.06 0.0639 ± 0.03 0.019 ± 0.005 0.009 ± 0.003 0.549 ± 0.184 
H mg g−1 min−1 0.0144 0.0183 0.0365 0.0175 0.009 0.239 
R2 – 0.99 0.99 0.99 0.966 0.978 0.977 
ParameterUnitLinear method
Nonlinear method
25 °C40 °C55 °C25 °C40 °C55 °C
qe mg g−1 1.004 ± 0.03 0.911 ± 0.06 0.7561 ± 0.09 0.962 ± 0.04 1.028 ± 0.07 0.661 ± 0.02 
k2 g mg−1 min−1 0.0143 ± 0.011 0.0220 ± 0.06 0.0639 ± 0.03 0.019 ± 0.005 0.009 ± 0.003 0.549 ± 0.184 
H mg g−1 min−1 0.0144 0.0183 0.0365 0.0175 0.009 0.239 
R2 – 0.99 0.99 0.99 0.966 0.978 0.977 

Moreover, the linear intra-particle diffusion model was also applied to the experimental data (Table S2). The kid-1 values were found to be greater than kid-2 indicating fast diffusion of fluoride ions to the zeolite at the beginning of the sorption. Moreover, the lines of the diffusion model (Figure S2) did not pass through the origin, indicating that particle diffusion was not the rate-controlling step. Similar results were obtained by Ghorai & Pant (2005) and Srivastav et al. (2013).

Adsorption thermodynamics

The Gibbs free energy change (ΔG°) was found to be negative (−18.95, −18.63, −18.31 kJ mol−1), reflecting the spontaneous behavior of the adsorption. Standard enthalpy change (ΔH°) was determined as −25.32 kJ mol−1 indicating that the adsorption followed an exothermic path. The negative ΔS° value (−0.0214 kJ mol−1 K−1) showed that randomness decreased at the adsorbent‒solution interface during adsorption. In order to calculate the activation energy by the Arrhenius equation , the second-order model constant (k2) was used. The activation energy was found as 40.285 kJ mol−1 K−1, which demonstrates a chemical reaction to be involved in the fluoride adsorption.

Effect of co-existing ions

As the groundwater contains a variety of anions such as phosphate, sulfate, nitrate and chloride, which compete with fluoride ions, the effect of different ions on defluoridation efficiency was investigated. During the experiments, the solution pH was not controlled and the pH values were measured to be 7.4, 6.7, 7.0, 9.7 and 7.9 in the presence of Cl, , , and , respectively. Results are depicted in Figure 2(a). There was no significant influence of chloride and nitrate on fluoride removal. It can be seen that phosphate ions have higher interference in the adsorption of fluoride by the Z-Al adsorbent. At the end of 8 h, the fluoride removal efficiency decreased from 94.6% to 88.0%, 86.4% and 82.0% in the presence of , and , respectively. The influence of on adsorption could be due to the significant increase in pH of the solution (Kumar et al. 2009). Moreover, in order to examine the water matrix effect, fluoride at a 10 mg/L concentration level was subjected to a kinetic test in tap water and the results are shown in Figure 2(b). After 6 hours of treatment in distilled water, the equilibrium F concentration was found as 1.30 mg/L while it was found as 2.54 mg/L for tap water media. The reduction in the removal efficiency is mainly attributed to co-existing anions such as carbonate and sulfate in tap water.

Figure 2

(a) Effect of co-existing anions on fluoride adsorption; (b) adsorption kinetics in different water matrices; (c) desorption efficiencies for different agents; (d) adsorption capacity change for each cycle.

Figure 2

(a) Effect of co-existing anions on fluoride adsorption; (b) adsorption kinetics in different water matrices; (c) desorption efficiencies for different agents; (d) adsorption capacity change for each cycle.

Desorption studies

In order to investigate the reusability of Z-Al zeolites, the exhausted sample was subjected to desorption tests. In the literature, NaOH has been generally used as the desorbing agent in fluoride adsorption/desorption tests (Wu et al. 2007; Biswas et al. 2010; Paudyal et al. 2011). In the current work, although different concentrations of NaOH (0.5‒5.0 M) were applied to release fluoride from the exhausted zeolite, no fluoride was detected in the desorption media. Therefore, further experiments were performed at acidic pHs by using HCl (2.5 and 5.0 M) and H2SO4 (0.1, 2.5 and 5.0 M). Among various desorption media studied (Figure 2(c)), H2SO4 showed the highest desorption efficiency (85.8% for 5.0 M H2SO4) when compared with HCl (58.3% for 5.0 M HCl). Moreover, the desorption percentages were increased from 10.1% (0.1 M H2SO4) to 76.4% (2.5 M H2SO4) with increasing acidity. According to the obtained results, 2.5 M H2SO4 was chosen as the agent (76.4% desorption) for the regeneration cycle experiments. Figure 2(d) shows the change in the capacity before and after desorption, which decreased slightly after the fifth cycle. At the end of the second and third cycles of desorption, the desorption percentage was determined as 70.3% and 65.9%, respectively.

Experimental design modelling studies

The predicted and observed capacities were found to be in good agreement. Figure 3(a) shows the normal probability plot of the residuals, which distributed around the straight line. As a result of an Anderson–Darling normality check, the A2 value was found as 0.153, which was below the critical value (A2critical value = 0.787).

Figure 3

(a) Normal probability plot and (b) Pareto chart of Z-Al sample.

Figure 3

(a) Normal probability plot and (b) Pareto chart of Z-Al sample.

The ANOVA results (Table 2) indicated that both the linear and quadratic effects of concentration were significant according to the p-values, while the quadratic effect of pH and the interaction of temperature and concentration were found to be insignificant due to their higher p-values (>0.05).

Table 2

ANOVA of fluoride removal

FactorSum of squaresdfMean squareF-valuep-value
 2.540748 2.540748 1463.687 0.000003 
 0.005208 0.005208 3.001 0.158275 
 0.097394 0.097394 56.107 0.001699 
 0.301592 0.301592 173.743 0.000191 
 1.171677 1.171677 674.986 0.000013 
 0.041759 0.041759 24.057 0.008016 
 0.038667 0.038667 22.275 0.009174 
 0.195626 0.195626 112.697 0.000446 
 0.004540 0.004540 2.615 0.181140 
Lack of fit 0.066260 0.022087 12.724 0.016325 
Pure error 0.006943 0.001736 – – 
Total SS 4.457142 16 – – – 
 0.98358 – – – – 
 0.96246 – – – – 
FactorSum of squaresdfMean squareF-valuep-value
 2.540748 2.540748 1463.687 0.000003 
 0.005208 0.005208 3.001 0.158275 
 0.097394 0.097394 56.107 0.001699 
 0.301592 0.301592 173.743 0.000191 
 1.171677 1.171677 674.986 0.000013 
 0.041759 0.041759 24.057 0.008016 
 0.038667 0.038667 22.275 0.009174 
 0.195626 0.195626 112.697 0.000446 
 0.004540 0.004540 2.615 0.181140 
Lack of fit 0.066260 0.022087 12.724 0.016325 
Pure error 0.006943 0.001736 – – 
Total SS 4.457142 16 – – – 
 0.98358 – – – – 
 0.96246 – – – – 

The significance of the independent process variables and their interactions were examined with a Pareto chart (Figure 3(b)). Linear terms of pH and fluoride concentration of solution were found to be more effective than their quadratic terms. Moreover, the interaction of pH and concentration significantly affected the sorption performance of Z-Al. Negative coefficients of pH , temperature , quadratic temperature and the combined effects showed an unfavorable effect on fluoride removal. On the other hand, the positive coefficients of concentration and its quadratic form indicated a positive effect on removal performance. Moreover, the linear term of concentration had a favorable effect (t = 25.98048) on fluoride removal.

After neglecting the insignificant effects , based on the regression coefficients, the second-order equation for the Z-Al sample can be written as:  
formula
(3)

Effect of process variables

As seen in Figure 4(a), the dark red color distribution indicates better removal, which was obtained at low pHs. Initial fluoride level was also found significant for fluoride adsorption. The removal performance increased when the initial concentration was close to 15 ppm. The adsorption capacity increased from 1.131 mg g−1 (Ci: 5 mg L−1) to 1.797 mg g−1 (Ci: 15 mg L−1) at pH 5.0. Decrease in the fluoride concentration (5 ppm) and the temperature (40 °C) resulted in a decrease in the adsorption capacity (0.879 mg g−1).

Figure 4

3D surface graphs of (a) pH and temperature (Ci: 15 mg L−1), (b) concentration and pH (T: 25 °C), (c) temperature and concentration (pH 3.0). Please refer to the online version of this paper to see this figure in color: http://dx.doi.org/10.2166/ws.2017.081.

Figure 4

3D surface graphs of (a) pH and temperature (Ci: 15 mg L−1), (b) concentration and pH (T: 25 °C), (c) temperature and concentration (pH 3.0). Please refer to the online version of this paper to see this figure in color: http://dx.doi.org/10.2166/ws.2017.081.

Confirmation of BBD model

In order to analyze the availability of the BBD model, confirmatory experiments were carried out in two replicates at the optimum points (pH: 3.0, T: 50 °C, Ci: 15 mg L−1) proposed by the model. The predicted fluoride uptake capacity was found as 2.348 mg g−1 while the experimental capacities of parallel samples were determined as 2.261 and 2.289 mg F g−1, respectively. In order to examine the suitability of experimental data for the predicted value, the chi-square value was calculated as 0.00638, which is very close to zero, demonstrating the accuracy of the applied model for fluoride adsorption onto the Z-Al sample under experimental conditions.

CONCLUSIONS

It has been successfully shown that Z-Al adsorbent can be potentially used to decrease fluoride level in aqueous media to healthy limits of 0.50‒1.50 ppm. The maximum fluoride adsorption removal was observed at pH 3.0. The experimental data were fitted well to the Freundlich isotherm model and pseudo-second-order kinetic model. The activation energy was calculated as 40.285 kJ mol−1 K−1 indicating that the fluoride adsorption onto Z-Al is related to chemisorption on heterogeneous surfaces. The common co-existing anions had a slight effect on the F adsorption onto Z-Al. In desorption studies, the adsorption capacities did not change significantly after the fifth cycle, demonstrating that the synthesized adsorbent has the potential of practical application for fluoride removal from drinking water.

ACKNOWLEDGEMENTS

This study was supported by Yildiz Technical University, Scientific Research Projects Coordinating Department under Project No. 2014-07-01-YL04.

REFERENCES

REFERENCES
Aharoni
,
C.
&
Tompkins
,
F. C.
1970
Kinetics of Adsorption and Desorption and the Elovich Equation. Advances in Catalysis and Related Subjects
,
Vol. 21
.
Academic Press
,
New York, USA
, pp.
1
49
.
Anderson
,
R. L.
1987
Practical Statistics for Analytical Chemists
.
Van Nostrand Reinhold
,
New York
,
USA
.
APHA, American Public Health Association
2005
Standard Methods for Examination of Water and Wastewater
, 21st edn.
American Public Health Association
,
Washington, DC, USA
.
Bhatnagar
,
A.
,
Kumar
,
E.
&
Sillanpää
,
M.
2011
Fluoride removal from water by adsorption – a review
.
Chemical Engineering Journal
171
,
811
840
.
Bilgin Simsek
,
E.
,
Özdemir
,
E.
&
Beker
,
U.
2013
Zeolite supported mono- and bimetallic oxides: promising adsorbents for removal of As(V) in aqueous solutions
.
Chemical Engineering Journal
220
,
402
411
.
Brindha
,
K.
&
Elango
,
L.
2011
Fluoride in groundwater: causes, implications and mitigation measures
. In:
Fluoride Properties, Applications and Environmental Management
(
Monroy
,
S. D.
, ed.),
Nova Science
,
New York, USA
, pp.
111
136
.
Ghorai
,
S.
&
Pant
,
K. K.
2005
Equilibrium, kinetics and breakthrough studies for adsorption of fluoride on activated alumina
.
Separation & Purification Technology
42
,
265
271
.
He
,
J.
,
Zhang
,
K.
,
Wu
,
S.
,
Cai
,
X.
,
Chen
,
K.
,
Li
,
Y.
,
Sun
,
B.
,
Jia
,
Y.
,
Meng
,
F.
,
Jin
,
Z.
,
Kong
,
L.
&
Liu
,
J.
2016
Performance of novel hydroxyapatite nanowires in treatment of fluoride contaminated water
.
Journal of Hazardous Materials
303
,
119
130
.
Kumar
,
E.
,
Bhatnagar
,
A.
,
Ji
,
M.
,
Jung
,
W.
,
Lee
,
S.-H.
,
Kim
,
S.-J.
,
Lee
,
G.
,
Song
,
H.
,
Choi
,
J.-Y.
,
Yang
,
J.-S.
&
Jeon
,
B.-H.
2009
Defluoridation from aqueous solutions by granular ferric hydroxide (GFH)
.
Water Research
43
,
490
498
.
Li
,
W.
,
Cao
,
C.-Y.
,
Wu
,
L.-Y.
,
Ge
,
M.-F.
&
Song
,
W.-G.
2011
Superb fluoride and arsenic removal performance of highly ordered mesoporous aluminas
.
Journal of Hazardous Materials
198
,
143
150
.
Mohapatra
,
M.
,
Anand
,
S.
,
Mishra
,
B. K.
,
Giles
,
D. E.
&
Singh
,
P.
2009
Review of fluoride removal from drinking water
.
Journal of Environmental Management
91
,
67
77
.
Onyango
,
M. S.
,
Kojima
,
Y.
,
Aoyi
,
O.
,
Bernardo
,
E. C.
&
Matsuda
,
H.
2004
Adsorption equilibrium modeling and solution chemistry dependence of fluoride removal from water by trivalent-cation-exchanged zeolite F-9
.
Journal of Colloid and Interface Science
279
,
341
350
.
Onyango
,
M. S.
,
Kojima
,
Y.
,
Kumar
,
A.
,
Kuchar
,
D.
,
Kubota
,
M.
&
Matsuda
,
H.
2006
Uptake of fluoride by Al3+ pretreated low-silica synthetic zeolites: adsorption equilibrium and rate studies
.
Separation Science and Technology
41
,
683
704
.
Paudyal
,
H.
,
Pangeni
,
B.
,
Inoue
,
K.
,
Kawakita
,
H.
,
Ohto
,
K.
,
Harada
,
H.
&
Alam
,
S.
2011
Adsorptive removal of fluoride from aqueous solution using orange waste loaded with multi-valent metal ions
.
Journal of Hazardous Materials
192
,
676
682
.
Samatya
,
S.
,
Yüksel
,
Ü.
,
Yüksel
,
M.
&
Kabay
,
N.
2007
Removal of fluoride from water by metal ions (Al3+, La3+ and ZrO2+) loaded natural zeolite
.
Separation Science and Technology
42
,
2033
2047
.
Srivastav
,
A. L.
,
Singh
,
P. K.
,
Srivastava
,
V.
&
Sharma
,
Y.
2013
Application of a new adsorbent for fluoride removal from aqueous solutions
.
Journal of Hazardous Materials
263
,
342
352
.
Tomar
,
V.
,
Prasad
,
S.
&
Kumar
,
D.
2014
Adsorptive removal of fluoride from aqueous media using Citrus limonum (lemon) leaf
.
Microchemical Journal
112
,
97
103
.
Tripathi
,
P.
,
Srivastava
,
V. C.
&
Kumar
,
A.
2009
Optimization of an azo dye batch adsorption parameters using Box–Behnken design
.
Desalination
249
,
1273
1279
.
Valdivieso
,
A. L.
,
Reyes Bahena
,
J. L.
,
Song
,
S.
&
Herrera Urbina
,
R.
2006
Temperature effect on the zeta potential and fluoride adsorption at the α-Al2O3/aqueous solution interface
.
Journal of Colloid and Interface Science
298
,
1
5
.
Vázquez-Guerrero
,
A.
,
Alfaro-Cuevas-Villanueva
,
R.
,
Rutiaga-Quiñones
,
J. G.
&
Cortés-Martínez
,
R.
2016
Fluoride removal by aluminum-modified pine sawdust: effect of competitive ions
.
Ecological Engineering
94
,
365
379
.
Wajima
,
T.
,
Umeta
,
Y.
,
Narita
,
S.
&
Sugawara
,
K.
2009
Adsorption behavior of fluoride ions using a titanium hydroxide-derived adsorbent
.
Desalination
249
,
323
330
.
World Health Organization
2004
Fluoride in Drinking Water. WHO Guidelines for Drinking Water Quality
.
World Health Organization
,
Geneva, Switzerland
.

Supplementary data