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Removal of fluoride from aqueous environment by modified Amberlite resin
Journal of Hazardous Materials 171 (2009) 815–819
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Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Removal of fluoride from aqueous environment by modified Amberlite resin Imam Bakhsh Solangi, Shahabuddin Memon ∗ , M.I. Bhanger National Center of Excellence in Analytical Chemistry, University of Sindh, Jamshoro 76080, Pakistan
a r t i c l e
i n f o
Article history: Received 11 February 2009 Received in revised form 11 April 2009 Accepted 15 June 2009 Available online 21 June 2009 Keywords: Fluoride remediation Thar Desert Adsorption isotherms Solid phase extraction Ion chromatography Ion exchange resin
a b s t r a c t Fluoride in drinking water above permissible level is responsible for human being affected by skeletal fluorosis. In this study, Amberlite XAD-4TM has been modified by introducing amino group onto the aromatic ring for its application in fluoride remediation. Characterization of the modified resin was made by, FT-IR and elemental analysis (CHNS) techniques. The pH 9 was optimum value for quantitative sorption of fluoride in both batch and column experiments. The desorption of fluoride was achieved by using 10% HCl. The batch and column sorption studies of fluoride with modified resin were carried out to evaluate sorption isotherms too. Thus equation isotherms such as Langmuir, Freundlich, and Dubinin–Radushkevich (D–R) were successfully used to model the experimental data. The sorption capacity of modified Amberlite XAD-4 resin was found as 5.04 × 10−3 mol g−1 . From the D–R isotherm parameters, it has been evaluated that the uptake of fluoride by modified resin occurs through ion exchange adsorption mechanism. The study will contribute toward the removal of fluoride from the aqueous environment as well as in the field of analytical and environmental chemistry. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Fluoride in drinking water has significant advantages as well as detrimental effects to human health depending on its concentration. Fluoride has major role for the proper growth of different organs in human beings. Generally, in order to strengthen teeth and to minimize cavities fluoride is added in the drinking water in low concentration [1]. However, maintaining fluoride concentration below the permissible limit (i.e. 1.5 mg L−1 ) in the dietary intake surely minimizes the skeletal and dental problems [2]. Moreover, when the concentration of fluoride is above this limit, it causes dental and skeletal fluorosis and lesions of the endocrine glands, thyroid and liver, etc. All these serious health effects are observed in different areas all over the world, such as China, India, Mexico, Africa and in some areas of Thar Desert of Pakistan [3]. The analytical investigation of groundwater quality regarding fluoride concentration of Thar Desert of Pakistan has been recently reported in the range of 0.01–12.0 mg L−1 [3]. For the removal of excessive amount of fluoride from drinking water, several methods, i.e. adsorption [4], ion exchange [5–10], precipitation [11], Donnan dialysis [12], electrodialysis [13], reverse osmosis [14], nanofiltration [15] and ultrafiltration [16] have been investigated.
∗ Corresponding author. Tel.: +92 22 2772065; fax: +92 22 2771560. E-mail addresses: [email protected], [email protected] (S. Memon). 0304-3894/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2009.06.072
Various researchers [4,17] have emphasized the advantages of solid phase extraction (SPE) over other techniques and in particular over liquid–liquid extraction. Chelating resins have been frequently used in SPE’s as they provide good stability, high sorption capacity for ions and good flexibility in working conditions. Iminodiacetate resins [18] such as chelex 100 and Amberlite IRC-718 are widely used for this purpose [19] but with insignificance in selectivity [20,21]. Other chelating resins that are employed for such purpose are, 4-vinyl pyridine-divinylbenzene/acrylonitrile-divinylbenzene copolymers, Amberlite IRA-400, Biorad AGMP-1, silica based C18 support, Amberlyst A-26, Dowex-2 and Merrifield chloromethylated resins. Of these, Amberlite-XAD resins have superior physical properties like durability and chemical stability toward hard environments [22,23]. Besides this, for the removal of fluoride different adsorbent material. For example, Biswas and coworkers [24] used synthetic iron(III)–aluminum(III) mixed oxide for adsorption of fluoride. Viswanathan et al. [25,26] have reported chitosan beads for the removal of fluoride from aqueous solution. Sarkar et al. [27] have designed and operated fixed bed laterite column for the removal of fluoride from water. Ramanaiah et al. [28] used the waste fungus (Pleurotus ostreatus 1804) for the adsorption of fluoride from aqueous solution. Cengeloglu and co-workers utilized granular red mud [6] for the removal of fluoride from water. Recently, the hydrated cement, perfluorinated surfactants, geomaterials and acid treated spent bleaching earth had been employed for the adsorption of fluoride [29–32]. However, it has been revealed from the literature that instead of going toward more complicated non-durable and expensive
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Scheme 1. Structural representation of modified Amberlite XAD-4 resin.
materials for the fluoride ion remediation; we can use modified cost effective and durable Amberlite XAD-4 modified resin which is reported for the first time. This can be prepared easily in a very large quantity from cheap raw material and could be regenerated successfully. Therefore, herein we report the modification of Amberlite XAD-4 resin and its application for the removal of fluoride ion from the groundwater of the Thar Desert area of Pakistan. 2. Experimental
2.3. Sorption procedures 2.3.1. Static method for the sorption of fluoride Batch-wise adsorption study was carried out for fluoride at room temperature, i.e. 25 ± 1◦ C. The sample solution (20 mL) containing fluoride ion (5 mg L−1 ) was taken in a 50 mL Erlenmeyer flask. The modified resin (0.1 g) was added and the mixture was equilibrated for a fixed period of time (60 min). The resin was filtered and the adsorbed fluoride was analyzed by IC. The adsorption of fluoride was calculated as follows: Ci − Cf × 100 Ci
2.1. Apparatus and reagents
% adsorption =
The Ion-Chromatography (IC) (Metrohm, Switzerland) instrument used was 861 Advance Compact with 833 IC liquid handling unit equipped with self-regenerating suppressor, which consists a double gradient peristaltic pump along with conductivity detector. The IC anion column (4.0 mm × 250 mm) METROSEP A SUPP 4-250 (6.1006.430) was used to quantitate the anions in the aqueous media. The pH measurements were made with pH meter (781pH/Ion meter, Metrohm, Herisau Switzerland) with glass electrode and internal reference electrode. Janke & Kunikel automatic shaker model KS 501 D, Singapor, was used for the batch experiments at ambient temperature (ca. 25 ◦ C). IR spectra were recorded on a Thermo Nicollet 5700 FTIR spectrometer (WI 53711, USA) as KBr pellets. Elemental analyses were performed using a CHNS instrument model Flash EA 1112 elemental analyzer (20090 Rndano, Milan-Italy). All chemicals used were of analytical or equivalent grade. Stock standard solutions of fluoride (1000 mg L−1 ) has been prepared. The pH (1–10) of the solution was adjusted by mixing appropriate amount of 0.1 M (HCl/NaOH). Amberlite XAD-4TM (surface area of 825 m2 g−1 , pore diameter 14.4 nm and bead size 20.50 mesh) was procured from Fluka, Germany. The surface area, pore diameter and mesh size were quoted by the supplier.
where Ci (mol L−1 ) is the initial concentration of solution before the adsorption and Cf (mol L−1 ) is the final concentration after the adsorption of the fluoride ion. 2.3.2. Dynamic method for sorption of fluoride The modified resin (500 mg) was poured into a Pyrex glass column (Ø 5 mm × 50 mm). A small amount of glass wool was placed at the bottom and top of resin in order to prevent any loss of the resin beads during the sample loading. A solution of fluoride was passed through the column at optimized flow rate, i.e. 1 mL min−1 . The concentration of fluoride in the column effluents was determined by IC. 3. Results and discussion 3.1. Modification and application of Amberlite resin The Amberlite XAD-4TM resin has been modified successively in to amino groups containing resin by the method described in Section 2.1. The FT-IR spectrum of Amberlite XAD-4TM is previously interpreted by Flores et al. [33]. The nitro and amino derivatives of Amberlite XAD-4 were confirmed by FT-IR results as in Fig. 1. At
2.2. Synthesis Amberlite XAD-4TM has been modified (Scheme 1) by the treatment of beads of resin (10.0 g) with 20 mL of concentrated HNO3 and 50 mL of concentrated H2 SO4 for 30 min at 50 ◦ C with continuous stirring. The reaction mixture was poured into ice-cold water and filtered. The nitrated resin was repeatedly washed with distilled water until free from acid. The reduction of nitro groups introduced was made thereafter by treating with SnCl2 ·2H2 O (30 g, 0.13296 mol) in 30 mL of concentrated HCl and 40 mL of ethanol. This mixture was refluxed for 10 h. The solid beads were washed with distilled water followed by 2.0 M NaOH in order to release SnCl2 from amino resin (R–NH2 ). Finally, the amino resin was washed with excess of distilled water. The modification of the resin was confirmed by the examination of its FT-IR spectroscopy and elemental analysis. IR (KBr) 3442 and 3381 cm−1 (–NH2 ). Found: C, 67.31; H, 6.98; N 4.78%. Calculated for C18 H20 NCl·2H2 O: C, 67.17; H, 7.51; N 4.35%.
(1)
Fig. 1. FT-IR spectrum: (a) XAD-4 nitro derivative (b) XAD-4 amino derivative.
I.B. Solangi et al. / Journal of Hazardous Materials 171 (2009) 815–819
817
modified resin was explored at fixed concentration of fluoride ion. The results obtained are shown in Fig. 2. The % adsorption increases by increasing the pH of solution and attain a maximum value at pH 9.0 for fluoride ions. 3.5. Sorption mechanism
Fig. 2. Effect of dosage of modified resin on adsorption of fluoride (F−1 = 5 mg L−1 , shaking time = 60 min; V = 20 mL; T = 25 ◦ C).
the end of each reaction the main transmittance were observed, as in case of nitro derivative two bands at 1528 and 1350 cm−1 for NO2 and –C–N, respectively confirm its formation. Whereas, on the reduction of nitro derivative into amino derivative the disappearance of 1528 cm−1 and appearance of new bands of –NH2 at 3442 and 3381 cm−1 confirm the conversion. The modified resin was used in the fluoride sorption experiments carried out to optimize its efficiency and reusability. The samples of different areas of Thar Desert have been collected and analyzed. It has been found that the fluoride ion concentration in the samples vary from 0.1 to 12 mg L−1 [3]. Moreover, most of the areas have fluoride ion concentration in between this range. Therefore it has been decided that the experiments should be carried out on the fluoride ion solutions with 5 mg L−1 concentration. 3.2. Effect of adsorbent dosage Adsorbent dosage is a significant parameter because this concludes the capacity of an adsorbent for given initial concentration of the adsorbate at the operating conditions. The influence of immobilized resin on the adsorption of fluoride is represented in Fig. 2. The adsorption of fluoride increases by increasing the dosage of immobilized resin and the adsorption was almost constant at higher dosage than 100 mg. This was because of the availability of more and more adsorbent surface for the fluoride to be adsorbed. 3.3. Optimum shaking time The effect of shaking time on the adsorption of fluoride onto the modified resin was studied. The time intervals studied were from 0 to 60 min, at pH 9, using 20 mL of 5 mg L−1 fluoride ion solution and 100 mg of sorbent. It has been noticed that fluoride sorption increases by increasing shaking time; the equilibrium was established within 40 min and showing a slight increase in percent sorption up to 60 min. Therefore, further experimental work was carried out at 60 min shaking time in order to avoid sorption error. 3.4. pH effect on adsorption of fluoride ions To recognize the effect of pH, sorption studies were carried out at different pHs (1–10). The influence of pH plays a key role on separation sciences, as in this study the adsorption of fluoride ions onto
The mechanism of fluoride removal by modified Amberlite XAD4 resin is shown in Scheme 2. However, resin may be considered as hard acid as they possess H+ ion and hence would prefer to bind with hard base, most preferably with high electronegative and low polarisable fluoride ion [34]. The fluoride saturated resin was regenerated with mineral acid. All the regeneration experiments were carried out at room temperature. Desorption efficiency of the resin was studied using 10% HCl as eluent. It has been observed that 98.5% desorption can successfully be made. 3.6. Adsorption isotherms Investigation of data by isotherm is main parameter required for the design of a adsorbent and it is important for calculating the adsorption capacity of the adsorbent system. Several models have been used for this purpose. The mathematical expressions of Freundlich, Langmuir and D–R are used in this study. The details of the isotherms are also available elsewhere in literature [35,36]. Freundlich log(Cads = log A + (1/n) log Ce ), Langmuir [(Ce /Cads ) = (1/Qb) + (Ce /Q)] and Dubinin–Radushkevich (D–R) (ln Cads = ln Xm − ˇε2 ) and ε = RT ln [1 + (1/Ce )] equations were applied, where, Cads is the amount of fluoride ion adsorbed per unit mass of the adsorbent and Ce is the amount of fluoride ion in liquid phase at equilibrium. A, n, Q, b, Xm and ˇ are Freundlich, Langmuir and D–R constants, respectively. These equations are applied to correlate the amount of the fluoride adsorbed per unit amount of the adsorbent and the initial concentration of adsorbate is in the range of 2.1 × 10−4 to 4.3 × 10−3 mol dm−3 using 100 mg adsorbent per 20 mL of adsorbate and 60 min shaking time at 25 ◦ C. The Langmuir and Freundlich models are widely used isotherms. The analytical equation of Langmuir model was plotted Ce /Cads versus Ce gives a straight line, indicating that the Langmuir adsorption is also followed by the adsorption data very well. The essential characteristic of Langmuir isotherm can be expressed in terms of a dimensionless constant, separation factor RL , which describes the type of isotherm [36] and is calculated by RL = 1 + (1/bCi ) where, Ci is the initial concentration of fluoride ions. The values of RL calculated were 0.338–0.0249, representing highly favorable adsorption of fluoride ions onto modified resin. The Freundlich adsorption isotherm is based on heterogeneous surface was applied for the fluoride removal by adsorption [35]. The constants of Freundlich adsorption model were calculated by plotting graph log Cads versus log Ce . From the intercept and slope, the values A and 1/n have been evaluated and compiled in Table 1. The Dubinin–Radushkevich (D–R) isotherm is more general to model the physical/chemical adsorption phenomena [35]. This isotherm is more common than Langmuir isotherm model. It assumes heterogeneity of energies over the surface. The values of E calculated from the constants (ˇ) of D–R isotherm model using
Scheme 2. Mechanism of fluoride sorption on modified Amberlite XAD-4 resin and it’s desorption.
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Table 1 Freundlich, Langmuir and D–R characteristic constants for fluoride ions on modified resin. Langmuir −1
Q (mol g
Freundlich )
−3
5.04 × 10
−3
b (mol dm 8972.39
)
2
−1
RL
R
A (mol g
0.338–0.025
0.99
0.4763
equation (E = 1/ −2ˇ) is 10.425 kJ mol−1 . The linear form of the D–R isotherm is plotted ln Cads versus ε2 . The constants of this model were calculated and are given in Table 1. The adsorption intensity of the Freundlich constant (n) for fluoride ion was found to be 2.08. The value of n lies between 1 and 10 indicating favorable adsorption [36]. The value of adsorption capacity is different for all mentioned adsorption isotherms. This difference in adsorption capacity can be evaluated in terms of the assumptions taken into consideration while deriving these adsorption models. It is known that the magnitude of E is very common for evaluating the type of adsorption and if this value lies in between 8 and 16 kJ mol−1 , the adsorption can be explained by ion exchange [37]. Hence, in this case the value of E is 10.425 kJ mol−1 and it is very likely that the fluoride ions are adsorbed onto modified Amberlite XAD-4 resin predominantly by ion exchange mechanism. To understand the adsorption phenomenon, it has been concluded from the experimental results carried out at different pH (Fig. 3) that modified resin contains aromatic amine binding sites and hence possesses capability of capturing anions. It can be speculated that modified resin based on the electron accepting nature of the amino-containing phenyl groups and the electron-donating nature of anions, the ion exchange mechanism could be preferentially considered (Scheme 2). For instance, anions such as fluoride ion may attach itself to aromatic amines which can accept electron, forming coordination bond between them. It is then readily be understood that the equilibrium is quite dependent on the concentration of chloride ion present in modified Amberlite XAD-4 resin. At higher pH, the fluoride ions compete with chloride ions at the exchange sites of the modified resin, thereby releasing the chloride.
)
D–R 1/n
R
Xm (mol g−1 )
E (kJ mol−1 )
R2
0.484
0.97
0.0245
10.425
0.98
2
Table 2 Interference of Br− , NO2 − , NO3 − , HCO3 − and SO4 − on the sorption of fluoride onto modified resin at pH 9. Interfering ions
F/Br− F/NO2 − F/NO3 − F/SO4 − F/HCO3 −
Fluoride
Percent sorption/fluoride:An− 1:1
1:2
1:3
1:4
1:5
92.7
87.640 90.629 92.809 90.010 93.919
80.320 89.901 90.908 89.808 96.659
74.119 88.672 89.931 89.153 96.705
64.784 84.551 88.649 89.024 98.209
54.458 84.096 83.870 87.684 99.092
The fluoride ions are completely released under circumstances of extreme conditions, i.e. at low pH. In such a case therefore, modified resin may be used as regenerable preconcentration agent. Moreover, this phenomenon may reflect the ‘hard and soft acids and bases’ concept introduced by Pearson [34]. As this environment exists and because, the fluoride is harder as compared to chloride which replaces it at the normal conditions. The proposed mechanism of the modified Amberlite XAD-4 resin with fluoride ion is shown in Scheme 2. 3.7. Dynamic sorption studies 3.7.1. Effect of flow rate The effects of flow rate play an important role for dynamic study on the adsorption phenomena. The flow rate was adjusted 1–5 mL min−1 at optimized pH (i.e. pH 9) for maximum adsorption (Fig. 4). It was observed that at a flow rate of 3 mL min−1 and even greater than this, there was a decrease in % adsorption. This decrease in adsorption with increasing flow rate is due to the decrease in equilibration time between two phases. 3.8. Interference of other ions The studies regarding interferences of other selected anions (Br− , NO2 − , NO3 − , HCO3 − and SO4 − ) on the sorption ability of modified Amberlite XAD-4 resin (Table 2) show that there is no significant effect of the other anions on the sorption ability of resin for fluoride ion. From these observations we conclude that it may be due to the nature of fluoride demonstrated as a hard anion [34].
Fig. 3. Adsorption of fluoride ions onto modified () and pure Amberlite XAD-4 resin ().
Fig. 4. % Adsorption of fluoride at different flow rate.
3.9. Analytical application of modified resin The analytical applicability of modified Amberlite XAD-4 resin was tested by real samples of Thar Desert. The samples were collected from the groundwater of Mithi sub-district of Thar Desert. A 20 mL aliquot of sample was taken and then agitated with 0.1 g of modified resin for 60 min fluoride was analyzed before and after sorption using IC, the % sorption was calculated. The fluoride content reduced to 0.82 from 1.55 mg L−1 by treating with modified Amberlite resin. The other competing anions were also analyzed and it was observed that chloride was reduced up to 49.57 from 54.44 mg L−1 and nitrate up to 53.97 from 81.27 mg L−1 along with fluoride ions. The other parameters like pH and electrical conductivity reduces up to 6.24 and 9.77 mS cm−1 from 8.18 and 11.18 mS cm−1 , respectively.
I.B. Solangi et al. / Journal of Hazardous Materials 171 (2009) 815–819
4. Conclusions The present study reveals that the fluoride ion can be efficiently adsorbed using modified Amberlite XAD-4 resin. It has been observed that the modified resin is efficient for the removal of fluoride ion from aqueous solution at various pHs particularly at 9 pH. It has also been found that the resin is effective even in the presence of other anions such as Br− , NO2 − , NO3 − , HCO3 − and SO4 − ions. It is suitable adsorbent and can be applied for the removal of fluoride ion from the drinking water of Thar Desert and can be regenerated several times with mineral acid, i.e. HCl (10%). The study will find its applicability in various fields of material science. Acknowledgements We thank the Higher Education Commission (Grant number 20-713/R&D/06) Islamabad and National Centre of Excellence in Analytical Chemistry, University of Sindh, Jamshoro for the financial support of this work. References [1] A.R. Judy, “Fluoride” in Encyclopedia of Water Science, Marcel Dekker, New York, 2003. [2] WHO, WHO Guidelines for Drinking-Water Quality, 3rd ed., WHO, Geneva, 2004. [3] T. Rafique, S. Naseem, M.I. Bhanger, T.H. Usmani, Fluoride ion concentration in the ground water of Mithi sub-district, the Thar Desert, Pakistan, Environ. Geol. 56 (2008) 317–326. [4] A. Tor, N. Danaoglu, G. Arslan, Y. Cengeloglu, Removal of fluoride from water by using granular red mud: batch and column studies, J. Hazard. Mater. 164 (2009) 271–278. [5] K.M. Popat, P.S. Anand, B.D. Dasare, Selective removal of fluoride ions from water by the aluminium form of the aminomethylphosphonic acid-type ion exchanger, React. Polym. 23 (1994) 23–32. [6] S. Meenakshi, N. Viswanathan, Identification of selective ion-exchange resin for fluoride sorption, J. Colloid Interface Sci. 308 (2007) 438–450. [7] M.J. Haron, W.M. Yunus, Removal of fluoride ion from aqueous solution by a cerium–poly(hydroxamic acid), J. Environ. Sci. Health A 36 (2001) 727–734. [8] C.S. Sundaram, N. Viswanathan, S. Meenakshi, Defluoridation chemistry of synthetic hydroxyapatite at nano scale: equilibrium and kinetic studies, J. Hazard. Mater. 155 (2008) 206–215. [9] C.S. Sundaram, N. Viswanathan, S. Meenakshi, Uptake of fluoride by nanohydroxyapatite/chitosan, a bioinorganic composite, Bioresour. Technol. 99 (2008) 8226–8230. [10] N.I. Chubar, V.F. Samanidou, V.S. Kouts, G.G. Gallios, V.A. Kanibolotsky, V.V. Strelko, I.Z. Zhuravlev, Adsorption of fluoride, chloride, bromide, and bromate ions on a novel ion exchanger, J. Colloid Interface Sci. 291 (2005) 67–74. [11] M.G. Sujana, R.S. Thakur, S.N. Das, S.B. Rao, Defluorination of waste waters, Asian J. Chem. 4 (1997) 561–570. [12] M. Hichour, F. Persin, J. Sandeaux, C. Gavach, Fluoride removal from waters by Donnan dialysis, Sep. Purif. Technol. 18 (2000) 1–11. [13] N. Kabay, O. Arar, S. Samatya, U. Yuksel, M. Yuksel, Separation of fluoride from aqueous solution by electrodialysis: effect of process parameters and other ionic species, J. Hazard. Mater. 153 (2008) 107–113.
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Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Removal of fluoride from aqueous environment by modified Amberlite resin Imam Bakhsh Solangi, Shahabuddin Memon ∗ , M.I. Bhanger National Center of Excellence in Analytical Chemistry, University of Sindh, Jamshoro 76080, Pakistan
a r t i c l e
i n f o
Article history: Received 11 February 2009 Received in revised form 11 April 2009 Accepted 15 June 2009 Available online 21 June 2009 Keywords: Fluoride remediation Thar Desert Adsorption isotherms Solid phase extraction Ion chromatography Ion exchange resin
a b s t r a c t Fluoride in drinking water above permissible level is responsible for human being affected by skeletal fluorosis. In this study, Amberlite XAD-4TM has been modified by introducing amino group onto the aromatic ring for its application in fluoride remediation. Characterization of the modified resin was made by, FT-IR and elemental analysis (CHNS) techniques. The pH 9 was optimum value for quantitative sorption of fluoride in both batch and column experiments. The desorption of fluoride was achieved by using 10% HCl. The batch and column sorption studies of fluoride with modified resin were carried out to evaluate sorption isotherms too. Thus equation isotherms such as Langmuir, Freundlich, and Dubinin–Radushkevich (D–R) were successfully used to model the experimental data. The sorption capacity of modified Amberlite XAD-4 resin was found as 5.04 × 10−3 mol g−1 . From the D–R isotherm parameters, it has been evaluated that the uptake of fluoride by modified resin occurs through ion exchange adsorption mechanism. The study will contribute toward the removal of fluoride from the aqueous environment as well as in the field of analytical and environmental chemistry. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Fluoride in drinking water has significant advantages as well as detrimental effects to human health depending on its concentration. Fluoride has major role for the proper growth of different organs in human beings. Generally, in order to strengthen teeth and to minimize cavities fluoride is added in the drinking water in low concentration [1]. However, maintaining fluoride concentration below the permissible limit (i.e. 1.5 mg L−1 ) in the dietary intake surely minimizes the skeletal and dental problems [2]. Moreover, when the concentration of fluoride is above this limit, it causes dental and skeletal fluorosis and lesions of the endocrine glands, thyroid and liver, etc. All these serious health effects are observed in different areas all over the world, such as China, India, Mexico, Africa and in some areas of Thar Desert of Pakistan [3]. The analytical investigation of groundwater quality regarding fluoride concentration of Thar Desert of Pakistan has been recently reported in the range of 0.01–12.0 mg L−1 [3]. For the removal of excessive amount of fluoride from drinking water, several methods, i.e. adsorption [4], ion exchange [5–10], precipitation [11], Donnan dialysis [12], electrodialysis [13], reverse osmosis [14], nanofiltration [15] and ultrafiltration [16] have been investigated.
∗ Corresponding author. Tel.: +92 22 2772065; fax: +92 22 2771560. E-mail addresses: [email protected], [email protected] (S. Memon). 0304-3894/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2009.06.072
Various researchers [4,17] have emphasized the advantages of solid phase extraction (SPE) over other techniques and in particular over liquid–liquid extraction. Chelating resins have been frequently used in SPE’s as they provide good stability, high sorption capacity for ions and good flexibility in working conditions. Iminodiacetate resins [18] such as chelex 100 and Amberlite IRC-718 are widely used for this purpose [19] but with insignificance in selectivity [20,21]. Other chelating resins that are employed for such purpose are, 4-vinyl pyridine-divinylbenzene/acrylonitrile-divinylbenzene copolymers, Amberlite IRA-400, Biorad AGMP-1, silica based C18 support, Amberlyst A-26, Dowex-2 and Merrifield chloromethylated resins. Of these, Amberlite-XAD resins have superior physical properties like durability and chemical stability toward hard environments [22,23]. Besides this, for the removal of fluoride different adsorbent material. For example, Biswas and coworkers [24] used synthetic iron(III)–aluminum(III) mixed oxide for adsorption of fluoride. Viswanathan et al. [25,26] have reported chitosan beads for the removal of fluoride from aqueous solution. Sarkar et al. [27] have designed and operated fixed bed laterite column for the removal of fluoride from water. Ramanaiah et al. [28] used the waste fungus (Pleurotus ostreatus 1804) for the adsorption of fluoride from aqueous solution. Cengeloglu and co-workers utilized granular red mud [6] for the removal of fluoride from water. Recently, the hydrated cement, perfluorinated surfactants, geomaterials and acid treated spent bleaching earth had been employed for the adsorption of fluoride [29–32]. However, it has been revealed from the literature that instead of going toward more complicated non-durable and expensive
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Scheme 1. Structural representation of modified Amberlite XAD-4 resin.
materials for the fluoride ion remediation; we can use modified cost effective and durable Amberlite XAD-4 modified resin which is reported for the first time. This can be prepared easily in a very large quantity from cheap raw material and could be regenerated successfully. Therefore, herein we report the modification of Amberlite XAD-4 resin and its application for the removal of fluoride ion from the groundwater of the Thar Desert area of Pakistan. 2. Experimental
2.3. Sorption procedures 2.3.1. Static method for the sorption of fluoride Batch-wise adsorption study was carried out for fluoride at room temperature, i.e. 25 ± 1◦ C. The sample solution (20 mL) containing fluoride ion (5 mg L−1 ) was taken in a 50 mL Erlenmeyer flask. The modified resin (0.1 g) was added and the mixture was equilibrated for a fixed period of time (60 min). The resin was filtered and the adsorbed fluoride was analyzed by IC. The adsorption of fluoride was calculated as follows: Ci − Cf × 100 Ci
2.1. Apparatus and reagents
% adsorption =
The Ion-Chromatography (IC) (Metrohm, Switzerland) instrument used was 861 Advance Compact with 833 IC liquid handling unit equipped with self-regenerating suppressor, which consists a double gradient peristaltic pump along with conductivity detector. The IC anion column (4.0 mm × 250 mm) METROSEP A SUPP 4-250 (6.1006.430) was used to quantitate the anions in the aqueous media. The pH measurements were made with pH meter (781pH/Ion meter, Metrohm, Herisau Switzerland) with glass electrode and internal reference electrode. Janke & Kunikel automatic shaker model KS 501 D, Singapor, was used for the batch experiments at ambient temperature (ca. 25 ◦ C). IR spectra were recorded on a Thermo Nicollet 5700 FTIR spectrometer (WI 53711, USA) as KBr pellets. Elemental analyses were performed using a CHNS instrument model Flash EA 1112 elemental analyzer (20090 Rndano, Milan-Italy). All chemicals used were of analytical or equivalent grade. Stock standard solutions of fluoride (1000 mg L−1 ) has been prepared. The pH (1–10) of the solution was adjusted by mixing appropriate amount of 0.1 M (HCl/NaOH). Amberlite XAD-4TM (surface area of 825 m2 g−1 , pore diameter 14.4 nm and bead size 20.50 mesh) was procured from Fluka, Germany. The surface area, pore diameter and mesh size were quoted by the supplier.
where Ci (mol L−1 ) is the initial concentration of solution before the adsorption and Cf (mol L−1 ) is the final concentration after the adsorption of the fluoride ion. 2.3.2. Dynamic method for sorption of fluoride The modified resin (500 mg) was poured into a Pyrex glass column (Ø 5 mm × 50 mm). A small amount of glass wool was placed at the bottom and top of resin in order to prevent any loss of the resin beads during the sample loading. A solution of fluoride was passed through the column at optimized flow rate, i.e. 1 mL min−1 . The concentration of fluoride in the column effluents was determined by IC. 3. Results and discussion 3.1. Modification and application of Amberlite resin The Amberlite XAD-4TM resin has been modified successively in to amino groups containing resin by the method described in Section 2.1. The FT-IR spectrum of Amberlite XAD-4TM is previously interpreted by Flores et al. [33]. The nitro and amino derivatives of Amberlite XAD-4 were confirmed by FT-IR results as in Fig. 1. At
2.2. Synthesis Amberlite XAD-4TM has been modified (Scheme 1) by the treatment of beads of resin (10.0 g) with 20 mL of concentrated HNO3 and 50 mL of concentrated H2 SO4 for 30 min at 50 ◦ C with continuous stirring. The reaction mixture was poured into ice-cold water and filtered. The nitrated resin was repeatedly washed with distilled water until free from acid. The reduction of nitro groups introduced was made thereafter by treating with SnCl2 ·2H2 O (30 g, 0.13296 mol) in 30 mL of concentrated HCl and 40 mL of ethanol. This mixture was refluxed for 10 h. The solid beads were washed with distilled water followed by 2.0 M NaOH in order to release SnCl2 from amino resin (R–NH2 ). Finally, the amino resin was washed with excess of distilled water. The modification of the resin was confirmed by the examination of its FT-IR spectroscopy and elemental analysis. IR (KBr) 3442 and 3381 cm−1 (–NH2 ). Found: C, 67.31; H, 6.98; N 4.78%. Calculated for C18 H20 NCl·2H2 O: C, 67.17; H, 7.51; N 4.35%.
(1)
Fig. 1. FT-IR spectrum: (a) XAD-4 nitro derivative (b) XAD-4 amino derivative.
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817
modified resin was explored at fixed concentration of fluoride ion. The results obtained are shown in Fig. 2. The % adsorption increases by increasing the pH of solution and attain a maximum value at pH 9.0 for fluoride ions. 3.5. Sorption mechanism
Fig. 2. Effect of dosage of modified resin on adsorption of fluoride (F−1 = 5 mg L−1 , shaking time = 60 min; V = 20 mL; T = 25 ◦ C).
the end of each reaction the main transmittance were observed, as in case of nitro derivative two bands at 1528 and 1350 cm−1 for NO2 and –C–N, respectively confirm its formation. Whereas, on the reduction of nitro derivative into amino derivative the disappearance of 1528 cm−1 and appearance of new bands of –NH2 at 3442 and 3381 cm−1 confirm the conversion. The modified resin was used in the fluoride sorption experiments carried out to optimize its efficiency and reusability. The samples of different areas of Thar Desert have been collected and analyzed. It has been found that the fluoride ion concentration in the samples vary from 0.1 to 12 mg L−1 [3]. Moreover, most of the areas have fluoride ion concentration in between this range. Therefore it has been decided that the experiments should be carried out on the fluoride ion solutions with 5 mg L−1 concentration. 3.2. Effect of adsorbent dosage Adsorbent dosage is a significant parameter because this concludes the capacity of an adsorbent for given initial concentration of the adsorbate at the operating conditions. The influence of immobilized resin on the adsorption of fluoride is represented in Fig. 2. The adsorption of fluoride increases by increasing the dosage of immobilized resin and the adsorption was almost constant at higher dosage than 100 mg. This was because of the availability of more and more adsorbent surface for the fluoride to be adsorbed. 3.3. Optimum shaking time The effect of shaking time on the adsorption of fluoride onto the modified resin was studied. The time intervals studied were from 0 to 60 min, at pH 9, using 20 mL of 5 mg L−1 fluoride ion solution and 100 mg of sorbent. It has been noticed that fluoride sorption increases by increasing shaking time; the equilibrium was established within 40 min and showing a slight increase in percent sorption up to 60 min. Therefore, further experimental work was carried out at 60 min shaking time in order to avoid sorption error. 3.4. pH effect on adsorption of fluoride ions To recognize the effect of pH, sorption studies were carried out at different pHs (1–10). The influence of pH plays a key role on separation sciences, as in this study the adsorption of fluoride ions onto
The mechanism of fluoride removal by modified Amberlite XAD4 resin is shown in Scheme 2. However, resin may be considered as hard acid as they possess H+ ion and hence would prefer to bind with hard base, most preferably with high electronegative and low polarisable fluoride ion [34]. The fluoride saturated resin was regenerated with mineral acid. All the regeneration experiments were carried out at room temperature. Desorption efficiency of the resin was studied using 10% HCl as eluent. It has been observed that 98.5% desorption can successfully be made. 3.6. Adsorption isotherms Investigation of data by isotherm is main parameter required for the design of a adsorbent and it is important for calculating the adsorption capacity of the adsorbent system. Several models have been used for this purpose. The mathematical expressions of Freundlich, Langmuir and D–R are used in this study. The details of the isotherms are also available elsewhere in literature [35,36]. Freundlich log(Cads = log A + (1/n) log Ce ), Langmuir [(Ce /Cads ) = (1/Qb) + (Ce /Q)] and Dubinin–Radushkevich (D–R) (ln Cads = ln Xm − ˇε2 ) and ε = RT ln [1 + (1/Ce )] equations were applied, where, Cads is the amount of fluoride ion adsorbed per unit mass of the adsorbent and Ce is the amount of fluoride ion in liquid phase at equilibrium. A, n, Q, b, Xm and ˇ are Freundlich, Langmuir and D–R constants, respectively. These equations are applied to correlate the amount of the fluoride adsorbed per unit amount of the adsorbent and the initial concentration of adsorbate is in the range of 2.1 × 10−4 to 4.3 × 10−3 mol dm−3 using 100 mg adsorbent per 20 mL of adsorbate and 60 min shaking time at 25 ◦ C. The Langmuir and Freundlich models are widely used isotherms. The analytical equation of Langmuir model was plotted Ce /Cads versus Ce gives a straight line, indicating that the Langmuir adsorption is also followed by the adsorption data very well. The essential characteristic of Langmuir isotherm can be expressed in terms of a dimensionless constant, separation factor RL , which describes the type of isotherm [36] and is calculated by RL = 1 + (1/bCi ) where, Ci is the initial concentration of fluoride ions. The values of RL calculated were 0.338–0.0249, representing highly favorable adsorption of fluoride ions onto modified resin. The Freundlich adsorption isotherm is based on heterogeneous surface was applied for the fluoride removal by adsorption [35]. The constants of Freundlich adsorption model were calculated by plotting graph log Cads versus log Ce . From the intercept and slope, the values A and 1/n have been evaluated and compiled in Table 1. The Dubinin–Radushkevich (D–R) isotherm is more general to model the physical/chemical adsorption phenomena [35]. This isotherm is more common than Langmuir isotherm model. It assumes heterogeneity of energies over the surface. The values of E calculated from the constants (ˇ) of D–R isotherm model using
Scheme 2. Mechanism of fluoride sorption on modified Amberlite XAD-4 resin and it’s desorption.
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Table 1 Freundlich, Langmuir and D–R characteristic constants for fluoride ions on modified resin. Langmuir −1
Q (mol g
Freundlich )
−3
5.04 × 10
−3
b (mol dm 8972.39
)
2
−1
RL
R
A (mol g
0.338–0.025
0.99
0.4763
equation (E = 1/ −2ˇ) is 10.425 kJ mol−1 . The linear form of the D–R isotherm is plotted ln Cads versus ε2 . The constants of this model were calculated and are given in Table 1. The adsorption intensity of the Freundlich constant (n) for fluoride ion was found to be 2.08. The value of n lies between 1 and 10 indicating favorable adsorption [36]. The value of adsorption capacity is different for all mentioned adsorption isotherms. This difference in adsorption capacity can be evaluated in terms of the assumptions taken into consideration while deriving these adsorption models. It is known that the magnitude of E is very common for evaluating the type of adsorption and if this value lies in between 8 and 16 kJ mol−1 , the adsorption can be explained by ion exchange [37]. Hence, in this case the value of E is 10.425 kJ mol−1 and it is very likely that the fluoride ions are adsorbed onto modified Amberlite XAD-4 resin predominantly by ion exchange mechanism. To understand the adsorption phenomenon, it has been concluded from the experimental results carried out at different pH (Fig. 3) that modified resin contains aromatic amine binding sites and hence possesses capability of capturing anions. It can be speculated that modified resin based on the electron accepting nature of the amino-containing phenyl groups and the electron-donating nature of anions, the ion exchange mechanism could be preferentially considered (Scheme 2). For instance, anions such as fluoride ion may attach itself to aromatic amines which can accept electron, forming coordination bond between them. It is then readily be understood that the equilibrium is quite dependent on the concentration of chloride ion present in modified Amberlite XAD-4 resin. At higher pH, the fluoride ions compete with chloride ions at the exchange sites of the modified resin, thereby releasing the chloride.
)
D–R 1/n
R
Xm (mol g−1 )
E (kJ mol−1 )
R2
0.484
0.97
0.0245
10.425
0.98
2
Table 2 Interference of Br− , NO2 − , NO3 − , HCO3 − and SO4 − on the sorption of fluoride onto modified resin at pH 9. Interfering ions
F/Br− F/NO2 − F/NO3 − F/SO4 − F/HCO3 −
Fluoride
Percent sorption/fluoride:An− 1:1
1:2
1:3
1:4
1:5
92.7
87.640 90.629 92.809 90.010 93.919
80.320 89.901 90.908 89.808 96.659
74.119 88.672 89.931 89.153 96.705
64.784 84.551 88.649 89.024 98.209
54.458 84.096 83.870 87.684 99.092
The fluoride ions are completely released under circumstances of extreme conditions, i.e. at low pH. In such a case therefore, modified resin may be used as regenerable preconcentration agent. Moreover, this phenomenon may reflect the ‘hard and soft acids and bases’ concept introduced by Pearson [34]. As this environment exists and because, the fluoride is harder as compared to chloride which replaces it at the normal conditions. The proposed mechanism of the modified Amberlite XAD-4 resin with fluoride ion is shown in Scheme 2. 3.7. Dynamic sorption studies 3.7.1. Effect of flow rate The effects of flow rate play an important role for dynamic study on the adsorption phenomena. The flow rate was adjusted 1–5 mL min−1 at optimized pH (i.e. pH 9) for maximum adsorption (Fig. 4). It was observed that at a flow rate of 3 mL min−1 and even greater than this, there was a decrease in % adsorption. This decrease in adsorption with increasing flow rate is due to the decrease in equilibration time between two phases. 3.8. Interference of other ions The studies regarding interferences of other selected anions (Br− , NO2 − , NO3 − , HCO3 − and SO4 − ) on the sorption ability of modified Amberlite XAD-4 resin (Table 2) show that there is no significant effect of the other anions on the sorption ability of resin for fluoride ion. From these observations we conclude that it may be due to the nature of fluoride demonstrated as a hard anion [34].
Fig. 3. Adsorption of fluoride ions onto modified () and pure Amberlite XAD-4 resin ().
Fig. 4. % Adsorption of fluoride at different flow rate.
3.9. Analytical application of modified resin The analytical applicability of modified Amberlite XAD-4 resin was tested by real samples of Thar Desert. The samples were collected from the groundwater of Mithi sub-district of Thar Desert. A 20 mL aliquot of sample was taken and then agitated with 0.1 g of modified resin for 60 min fluoride was analyzed before and after sorption using IC, the % sorption was calculated. The fluoride content reduced to 0.82 from 1.55 mg L−1 by treating with modified Amberlite resin. The other competing anions were also analyzed and it was observed that chloride was reduced up to 49.57 from 54.44 mg L−1 and nitrate up to 53.97 from 81.27 mg L−1 along with fluoride ions. The other parameters like pH and electrical conductivity reduces up to 6.24 and 9.77 mS cm−1 from 8.18 and 11.18 mS cm−1 , respectively.
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4. Conclusions The present study reveals that the fluoride ion can be efficiently adsorbed using modified Amberlite XAD-4 resin. It has been observed that the modified resin is efficient for the removal of fluoride ion from aqueous solution at various pHs particularly at 9 pH. It has also been found that the resin is effective even in the presence of other anions such as Br− , NO2 − , NO3 − , HCO3 − and SO4 − ions. It is suitable adsorbent and can be applied for the removal of fluoride ion from the drinking water of Thar Desert and can be regenerated several times with mineral acid, i.e. HCl (10%). The study will find its applicability in various fields of material science. Acknowledgements We thank the Higher Education Commission (Grant number 20-713/R&D/06) Islamabad and National Centre of Excellence in Analytical Chemistry, University of Sindh, Jamshoro for the financial support of this work. References [1] A.R. Judy, “Fluoride” in Encyclopedia of Water Science, Marcel Dekker, New York, 2003. [2] WHO, WHO Guidelines for Drinking-Water Quality, 3rd ed., WHO, Geneva, 2004. [3] T. Rafique, S. Naseem, M.I. Bhanger, T.H. Usmani, Fluoride ion concentration in the ground water of Mithi sub-district, the Thar Desert, Pakistan, Environ. Geol. 56 (2008) 317–326. [4] A. Tor, N. Danaoglu, G. Arslan, Y. Cengeloglu, Removal of fluoride from water by using granular red mud: batch and column studies, J. Hazard. Mater. 164 (2009) 271–278. [5] K.M. Popat, P.S. Anand, B.D. Dasare, Selective removal of fluoride ions from water by the aluminium form of the aminomethylphosphonic acid-type ion exchanger, React. Polym. 23 (1994) 23–32. [6] S. Meenakshi, N. Viswanathan, Identification of selective ion-exchange resin for fluoride sorption, J. Colloid Interface Sci. 308 (2007) 438–450. [7] M.J. Haron, W.M. Yunus, Removal of fluoride ion from aqueous solution by a cerium–poly(hydroxamic acid), J. Environ. Sci. Health A 36 (2001) 727–734. [8] C.S. Sundaram, N. Viswanathan, S. Meenakshi, Defluoridation chemistry of synthetic hydroxyapatite at nano scale: equilibrium and kinetic studies, J. Hazard. Mater. 155 (2008) 206–215. [9] C.S. Sundaram, N. Viswanathan, S. Meenakshi, Uptake of fluoride by nanohydroxyapatite/chitosan, a bioinorganic composite, Bioresour. Technol. 99 (2008) 8226–8230. [10] N.I. Chubar, V.F. Samanidou, V.S. Kouts, G.G. Gallios, V.A. Kanibolotsky, V.V. Strelko, I.Z. Zhuravlev, Adsorption of fluoride, chloride, bromide, and bromate ions on a novel ion exchanger, J. Colloid Interface Sci. 291 (2005) 67–74. [11] M.G. Sujana, R.S. Thakur, S.N. Das, S.B. Rao, Defluorination of waste waters, Asian J. Chem. 4 (1997) 561–570. [12] M. Hichour, F. Persin, J. Sandeaux, C. Gavach, Fluoride removal from waters by Donnan dialysis, Sep. Purif. Technol. 18 (2000) 1–11. [13] N. Kabay, O. Arar, S. Samatya, U. Yuksel, M. Yuksel, Separation of fluoride from aqueous solution by electrodialysis: effect of process parameters and other ionic species, J. Hazard. Mater. 153 (2008) 107–113.
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