Water Air Soil Pollut (2012) 223:1073–1078 DOI 10.1007/s11270-011-0925-3

Fluoride Removal from Aqueous Solutions by Boehmite J. Jiménez-Becerril & Marcos Solache-Ríos & I. García-Sosa

Received: 9 May 2011 / Accepted: 5 August 2011 / Published online: 19 August 2011 # Springer Science+Business Media B.V. 2011

Abstract Boehmite was used for the removal of fluoride ions from aqueous solutions in a batch system. The pH, contact time, and fluoride concentration in the removal of fluoride ions by boehmite were evaluated. The removal of fluoride ions by boehmite was the highest between the pH values of 4.5 and 7.5. The kinetic fluoride sorption from aqueous solutions by boehmite was best described by the pseudo-secondorder model, and equilibrium was reached in about 24 h. The Freundlich model described the isotherm sorption process; the results indicate that the sorption mechanism is chemisorption on a heterogeneous material. Keywords Fluoride . Sorption . Boehmite 1 Introduction Fluoride is present in ground water in many areas of the world in concentrations ranging from 0.1 to 10 mg/l. Fluoride concentration levels between 0.5 and 1 mg/l are beneficial to health, providing substantial protection against dental caries, but higher concentrations of fluoride (1.5–2 mg/l) may lead to dental fluorosis. At 3–6 mg/l, skeletal fluorosis may be observed, and crippling skeletal fluorosis can develop with fluoride J. Jiménez-Becerril : M. Solache-Ríos (*) : I. García-Sosa Depto. de Química, Instituto Nacional de Investigaciones Nucleares, Apdo, Postal 18–1027, 11801 México, DF, Mexico e-mail: [email protected]

concentrations in drinking water that are higher than 10 mg/l (World Health Organization 2008). There are a variety of processes available for the treatment of fluoride-contaminated water (defluoridation): activated alumina technology, the reverse osmosis process, and the Nalgonda technique (addition of aluminum salts). Some studies on defluoridation techniques have shown that activated alumina and activated carbon are more efficient than other adsorbents (Munavalli and Patki 2009). Adsorption processes have been considered as an alternative technique for the removal of fluoride from drinking water and, as it is an anionic species, it is possible to remove it simultaneously with arsenic ions using alumina (Mishra et al. 2010) or other low-cost materials such that the adsorption of fluoride is controlled by the adsorbent structure and chemical properties (Fan et al. 2003). Alumina and its derivative compounds are common adsorption materials used in defluoridation; the parameters studied using these adsorbents are, primarily, pH, dose of adsorbent, contact time, initial concentration, co-ions, temperature of the adsorption process, and regeneration conditions (Tripathya et al. 2006; Karthikeyana and Elango 2009). On the other hand, boehmite is an aluminum oxyhydroxide (AlOOH) that is the most important precursor of alumina compounds that contain in their crystalline structure hydroxyl groups on the outer surface, which are considered adsorption sites for water molecules (Caiut et al. 2009). Boehmite has been synthesized and probed as an adsorption material for cobalt

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(Granados-Correa and Jiménez-Becerril 2009) or arsenious ions (Ogata et al. 2006), and its adsorption property was related to the surface hydroxyl groups. In the particular case of activated alumina prepared from pseudo-boehmite, the fluoride removal was attributed to electrostatic attraction and chemisorption mechanisms but not to ion exchange (Leyva-Ramos et al. 2008). The aim of this paper was to test a boehmite synthesized by a sol–gel method for the removal of fluoride from a water solution.

2 Materials and Methods 2.1 Adsorbent Preparation The boehmite was synthesized via sol–gel method at 298 K by the dropwise addition of an isopropanol and water mixture to 100 ml of a 0.5 mol l−1 aluminum isopropoxide/isopropanol solution under vigorous stirring over 24 h. The mixture was heated in a reflux system at 328 K for 5 h; then the solid was filtered and washed with deionized water, and the material was dried at 353 K (Vázquez et al. 1997). 2.2 Adsorbent Characterization The solid sample was characterized by X-ray diffraction using a Siemens D-5000 diffractometer coupled to a copper anode X-ray tube. The Ka radiation was selected with a diffracted beam monochromator. The compound was identified by comparing it with the JCPDS cards in the conventional way. 2.3 Fluoride Ions Measurements The concentration of fluoride ions in the solutions was determined using a selective electrode for fluoride ions (Radiometer ISEC301F Combined Fluoride Electrode). TISAB II (Total Ionic Strength Adjustment Buffer) was added to all fluoride standards and samples to control the ionic strength. The calibration curve was obtained using NaF standard solutions with a fluoride concentration ranging from 1 to 5 mg/l. 2.4 Sorption Kinetic The kinetic removal of fluoride ions by boehmite was performed as follows: 100 mg of the adsorbent and 10-ml

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aliquots of a 10 mg/l solution of fluoride ions were placed in centrifuge tubes and shaken occasionally for different times (5, 15, and 30 min, 1, 3, 5, 24, and 48 h); later, the samples were centrifuged and decanted. The fluoride ions concentrations in the solutions were determined as described above. The pH of each solution was measured before and after treatments. 2.5 Sorption Isotherms Samples (100 mg) of boehmite were placed in contact with 10 ml of different concentrations of fluoride solutions (5, 10, 15, 20, 25, 30, 40, and 50 mg/l) for 24 h. Later, the samples were centrifuged and decanted, fluoride concentrations were determined in the liquid phases as described above, and the pH was measured in each solution. 2.6 Effect of pH To check the effect of pH on fluoride ions uptake by boehmite, experiments were carried out with 10 ml of a 10 mg l −1 solution of fluoride ions and 100 mg of the adsorbent. The initial pH values were adjusted by adding 0.1 M HCl or NaOH to solutions; pH was measured with a pH meter (pH STAT Controller MeterLab PHM 290).

3 Results and Discussion 3.1 Characterization of Boehmite The X-ray diffraction patterns (Fig. 1a) show the boehmite phase (JPCDS 21–1307) with a small amount of gibbsite phase (JCPDS 33–0018). Very broad diffraction lines are observed and, therefore, the crystalline sizes of the AlOOH particles are very small. After the material was in contact with fluoride solution at pH 2 (Fig. 1b) and pH 8 (Fig. 1c), the gibbsite phase (Al(OH)3) disappeared. In a detailed investigation, well-crystallized boehmite (020) peak was found after the samples were in contact with the fluoride solutions; this behavior corresponds to a different rate of Al–OH interaction, as Al–OH interactions increase as the boehmite crystalline size decreases (Tipakontitikul et al. 2008). It has been shown that γ-Al2O3 surfaces undergo a transformation to bayerite (α-Al(OH)3), a polymorph

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(020)

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3.2.1 Pseudo-second-order Model

(051) (200)

(120) (031)

(002)

relative intensity

c)

b)

dqt ¼ k ð qe  qt Þ 2 dt

(002)

a)

(110)

The pseudo-second-order model, proposed by Ho and McKay (2002) and Ho (2006), is based on the assumption that the rate limiting step may be chemisorption involving valence forces through the sharing or exchange of electrons between adsorbent and adsorbate. This model can be represented in the following form:

and the linear form is: 10

20

30

40 2 theta

50

60

70

Fig. 1 X-ray diffraction patterns of a original synthesized boehmite, and after fluoride adsorption at b pH 2 and c pH 8

of gibbsite (γ-Al(OH)3), when hydrated for prolonged periods of time (Laiti et al. 1998), and α-Al2O3 produces a mixture of bayerite and gibbsite on its surface if it is exposed to water vapor for 1 h (Eng et al. 2000). Many studies have not specified whether the sorbent used was hydrated prior to sorption experiments. Using either dehydrated or partially hydrated sorbents may lead to morphological changes during sorption, thus making it difficult to elucidate a mechanism of sorption and to compare results. 3.2 Sorption Kinetic The sorption kinetic, as expressed in terms of the rate of the uptake of solute, which governs the residence time, is one of the important considerations for economical water treatment applications (Wang et al. 2006). Figure 2 shows the relationship between contact time and the sorption capacities of sorbents. According to the figure, the equilibrium was reached in about 24 h. The initial pH was 6.7 and the equilibrium pH of the solutions was between 7.6 and 8.0. Several kinetic models were applied to the experimental data of the sorption of fluoride ions by boehmite. The feature constants of sorption were obtained using a pseudo-first-order model, a secondorder model, and a pseudo-second-order model. The sorption behavior of fluoride was analyzed using both linear and nonlinear regression.

t 1 1 ¼ 2þ t qt kqe qe where qt and qe are the amounts adsorbed at time t and at equilibrium (mg/g), respectively; k is the pseudosecond-order rate constant for the sorption process (g/mg h). Thus, a plot of t/qt vs. t should give a linear relationship with a slope of 1/qe and an intercept 1=kq2e . The calculated k parameter was 1.58 g/mg h with r2 = 0.9951. It was found that the adsorption system was best described by the pseudo-second-order model; this behavior indicates that the sorption mechanism is chemisorption. The results could not be adjusted to the pseudo-firstorder model (Lagergren) and the second-order (Elovich). In the case of the nonlinear method, a trial-anderror method was used to determine the pseudosecond-order rate parameters by an optimization routine with the experimental data using the Origin 6.1 software, but the R2 value was inadequate. 3.3 Sorption Isotherms The maximum sorption capacity of sorbents is obtained from the sorption isotherms to optimize the use of this

Fig. 2 Sorption kinetic of fluoride ions by boehmite

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Fig. 3 Sorption isotherm of fluoride ions by boehmite

type of material. The experimental results are shown in Fig. 3; the initial pH was 6.8 and the equilibrium pH values were between 7.3 and 8.0. The results were analyzed by the nonlinear Langmuir, Freundlich, and Langmuir–Freundlich sorption models by using STATISTICA software. 3.3.1 Langmuir Model This model considers that the maximum sorption corresponds to a monolayer saturated with adsorbate molecules on the sorbent surface (Otero et al. 2003). The Langmuir isotherm is represented by the following equation: qe ¼

qo bCe 1 þ bCe

where qo is the amount of fluoride adsorbed per unit weight of adsorbent in forming a complete monolayer on the surface (mg/g), qe is the amount of fluoride adsorbed (mg/g), Ce is the concentration of the fluoride in the solution at equilibrium (mg/l), and b is the constant related to the energy or net enthalpy of the sorption. The parameters obtained by applying this model to the experimental results were as follows: qo = 2.057 mg/g and b=0.2806 with r=0.9212; the correlation obtained was not the highest using this model.

concentration of fluoride ions in solution (mg/l), Kf is the equilibrium constant indicative of adsorption capacity, and n is the adsorption equilibrium constant whose reciprocal is indicative of the heterogeneity of surface sorbent. The Kf, 1/n and qe parameters found applying this model were Kf =0.574 and 1/n=0.403 with r=0.965. According to the characteristics of the sorbents, it is better to apply this model to the results than the Langmuir model because the materials are heterogeneous. The value of 1/n is less than unity, which implies a heterogeneous surface structure with minimum interaction between adsorbed atoms (Abou-Mesalam 2004). 3.4 Effect of pH The pH is an important parameter in sorption procedures due to the ionization of surface functional groups and the composition of solutions. The impact of pH on the sorption of fluoride to alumina is significant in that it not only controls the maximum uptake capacity of the sorbent but also controls the solubility of various fluoride complexes that may be formed during the sorption process. Figure 4 shows the variation of fluoride sorption capacities at various pH values for boehmite. The removal of fluoride ions by boehmite was the highest between the pH values of 4.5 and 7.5, and then the sorption decreased very sharply at pH values higher than 7.5. When the pH of the solution increases to 7, the number of positively charged sorbent sites decreased and the number of –OH groups increased. Therefore, the positive sites of the sorbent and the presence of –OH groups are likely responsible for the low sorption at pH values higher than 7.5. Although the mechanistic details of fluoride sorption are not clear, the sorption of fluoride anions to alumina sorbents is often reported as a result of

3.3.2 Freundlich Model

qe ¼ Kf Ce1=n where qe is the amount of fluoride adsorbed per unit weight of adsorbent (mg/g), Ce is the equilibrium

0.8

mg/g NaF

The Freundlich model has been applied to adsorbents with heterogeneous surfaces and considers multilayer sorption (Torres-Pérez et al. 2008), which is given as follows:

1

0.6 0.4 0.2 0 0

2

4

6

8

pH

Fig. 4 Effect of pH on fluoride adsorption by boehmite

10

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electrostatic interactions with positive, and even neutral, surface charge (Stewart 2009). The decrease in sorption at lower pH values has been attributed to the dissolution of fluoride complexes at the surface, and the acid form (HF) increases, whereas, at higher pH values, competition between hydroxyl groups and fluoride anions account for this decrease. Because removal efficiencies drop significantly outside of the optimal pH range, exposing used activated alumina to acidic or basic conditions is often used as a regeneration method (Stewart 2009). Gao et al. (2009a, b) and Sundaram et al. (2008) reported that the maximum fluoride removal by hydroxyapatite was reached at pHinitial =2, attributing the higher fluoride removal in the acid medium to the attraction forces between the positively charged sorbent surface and the negatively charged fluoride ions and to the repulsion between the negatively charged surface and negatively charged fluoride in the alkaline medium.

4 Conclusions Fluoride ions can be removed from water by boehmite. The removal of fluoride ions was the highest between the pH values of 4.5 and 7.5. Pseudo-second-order and Freundlich models were successfully applied to the experimental data, which may indicate that the sorption mechanism of fluoride ions on boehmite is chemisorption on a heterogeneous material. Acknowledgement We acknowledge financial support from CONACYT, project 131174-Q.

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