FLUORIDE REMOVAL FROM WATER SOLUTION BY ADSORPTION ON ACTIVATED ALUMINA PREPARED FROM PSEUDO-BOEHMITE

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1 J. Environ. Eng. Manage., 18(5), (28) FLUORIDE REMOVAL FROM WATER SOLUTION BY ADSORPTION ON ACTIVATED ALUMINA PREPARED FROM PSEUDO-BOEHMITE Roberto Leyva-Ramos, 1, * Nahum A. Medellin-Castillo, 1 Araceli Jacobo-Azuara, 2 Jovita Mendoza- Barron, 1 Lilia E. Landin-Rodriguez, 1 Jose M. Martinez-Rosales 2 and Antonio Aragon-Piña 3 1 Centro de Investigacion y Estudios de Posgrado Universidad Autonoma de San Luis Potosi San Luis Potosi 7821, Mexico 2 Centro de Investigaciones en Quimica Inorganica Universidad de Guanajuato Guanajuato 365, Mexico 3 Instituto de Metalurgia Universidad Autonoma de San Luis Potosi San Luis Potosi 7821, Mexico Key Words: Activated alumina, adsorption, drinking water, fluoride ABSTRACT The adsorption of fluoride from a water solution on an activated alumina prepared from pseudo-boehmite was investigated in this work. The activated alumina was characterized to determine its physicochemical and textural properties. It was found that the fluoride adsorption capacity of the activated alumina was considerably dependent upon the solution ph and diminished considerably with increasing solution ph from 4 to 11. The effect of the ph was attributed to the electrostatic interactions between the surface of the activated alumina and the fluoride in solution. The activated alumina was dissolved at ph lower than 4. It was not possible to ascertain the effect of the temperature on the adsorption of fluoride because the adsorption capacity increased and then decreased while the temperature was augmented from 15 to 25 C and from 25 to 35 C, respectively. The adsorption equilibrium was not reversible at ph of 5 but was reversible at ph of 11. The solution ph was a very important factor in desorbing fluoride and more mass of fluoride was desorbed increasing the solution ph. The adsorption of fluoride on activated alumina was mainly due to both electrostatic attraction and chemisorption mechanisms, but not to ion exchange. INTRODUCTION The level of fluoride in drinking water is a very important physicochemical factor, which must be considered when assessing water quality for human consumption. It is known that its low concentration as well as its excess causes health problems to the human beings. The consumption of drinking water with a fluoride level greater than about 4 mg L -1 causes prevailing dental fluorosis in the population, and the chronic consumption of water containing high levels of fluoride between 4 and 15 mg L -1 provokes skeletal fluorosis that is associated with serious bone abnormalities [1]. On the other hand, if the concentration of fluoride is below.5 mg L -1, the incidence of dental caries increases considerably [1]. A guideline value of fluoride in drinking water of 1.5 mg L -1 was recommended by World Health Organization [2]. In several regions of the world it is well documented that the level of fluoride in drinking water exceeds this guideline value [3]. The concentration of fluoride in water for human consumption may be reduced below the permissible limit by the following methods: ion exchange on polymeric resins, adsorption, reverse osmosis, and electrodialysis. The most commonly employed method is the adsorption on activated alumina [3-6]. Activated alumina is an inorganic porous material, semi-crystalline and essentially comprised by aluminum oxide. It is a compound with an amphoteric character because the activated alumina behaves like a base in acidic solutions, and as an acid in basic solutions [7]. Currently activated alumina is considered the *Corresponding author [email protected]

2 32 J. Environ. Eng. Manage., 18(5), (28) most effective adsorbent for reducing fluoride concentrations in water for human consumption at levels below the permissible level [3,6,8]. Activated alumina can be prepared by different methods from aluminum salts and the main activated alumina phase used as an adsorbent is the gamma-alumina (γ-al 2 O 3 ). The capacity of the activated alumina to adsorb fluoride depends on its crystalline form and its preparation and activation processes and varies between 1 and 12 mg g -1, depending on the operating conditions [6,9]. The adsorption of fluoride on activated alumina has been extensively studied and several adsorption mechanisms have been proposed; however, the predominant mechanism has not been reported. Besides, it has been reported in various works that the activated alumina may be dissolved at an acid ph [1,11]. However, the chemical stability of the activated alumina in an acid ph has not been reported and the effect of dissolution of the activated alumina upon its adsorption capacity has not been investigated. The main aim of the present work is to study the capacity of an activated alumina prepared from a pseudo-boehmite phase to adsorb fluoride from an aqueous solution, as well as the effects of the solution ph and temperature on the adsorption capacity. Moreover, the adsorption mechanism of fluoride is elucidated and the acid stability of the activated alumina is studied to better understand the adsorption of fluoride on the activated alumina. MATERIALS AND METHODS 1. Activated Alumina The activated alumina used in this work was prepared from a pseudo-boehmite phase using the procedure reported elsewhere [12]. The average particle diameter of the activated alumina was 2.5 mm. 2. Characterization of the Activated Alumina The textural properties (surface area, pore volume, and average pore diameter) of the activated alumina were determined by the N 2 -BET method using a surface area and porosimetry analyzer, Micromeritics, model ASAP 21. The density of the solid was assessed by the helium displacement method with a pycnometer, Micromeritics, AccuPyc 13. The external surface of the activated alumina was examined with a Scanning Electron Microscope (Philips, model XL 3) equipped with an EDAX-DX- 4 disperse energy microanalysis system. X-Ray diffraction analysis was used to identify the crystalline species present in the activated alumina employing an X-ray diffractometer (Rigaku, model DMAX 2). 3. Determination of Acidic and Basic Sites The acidic and basic sites of the activated alumina were determined by the acid-base titration method proposed by Boehm [13]. The acid sites were neutralized with a.1 N NaOH solution and the basic sites with a.1 N HCL solution. The acidic and basic sites were determined by adding 1 g of the activated alumina to a volumetric flask, which contained 5 ml of the neutralizing solution. The flask was partially submerged in a constant temperature water bath set at 25 C and it was left there for 5 d. During this period, the flask was shaken manually twice a day. Afterwards, a sample of 1 ml was taken and titrated with.1 N NaOH or HCl solutions, depending on the case. The titration was carried out by triplicate employing a potentiometer, Orion, model EA Determination of the Point of Zero Charge The point of zero charge (PZC) of the activated alumina was determined by the mass titration procedure developed by Noh and Schwarz [14]. Different masses of the activated alumina ranging from.5 to 3 g and 1 ml of a.1 N NaCl solution were added to 25 ml plastic bottles. The bottle was capped and was stirred continuously with a Teflon-coated stirring bar for 48 h. After this time, the final ph of the solution was measured. The final ph was graphed against the mass of the activated alumina and the final ph approached an asymptotic value increasing the mass of the activated alumina. This asymptotic value of the final ph was the PZC. 5. Method for Determining the Acid Stability The acid stability of the activated alumina was determined by a procedure similar to that recommended by Wingenfelder et al. [15]. An activated alumina mass of.5 g and 4 ml of an acid solution with an initial ph varying from.8 to 6 were added to a plastic bottle. The acid solutions were prepared from a concentrated HNO 3 solution. The activated alumina and acid solution were left in contact for 7 d and the final ph of the acid solution was measured. Moreover, the concentration of aluminum dissolved in the acid solution was determined by atomic absorption spectroscopy. The absorbance of a sample was measured with a double beam atomic absorption spectrophotometer (Varian, model SpectrAA-2), at a wavelength of 39.3 nm and the concentration of aluminum in the sample was estimated using a calibration curve. 6. Determination of Fluoride Concentration in Water Solution The concentration of fluoride in an aqueous solution was determined using a potentiometer (Orion, model SA 72), equipped with a fluoride ion selective electrode. The concentration of fluoride in a water sample was determined by a calibration curve, which

3 Leyva-Ramos et al.: Flouride Removal by Treated Pseudo-boehmite 33 was prepared with standard solutions of fluoride concentrations ranging from.1 to 1 mg L -1. The experimental error in determining the fluoride concentration was estimated to be less than 1%. 7. Experimental Batch Adsorber Adsorption equilibrium data were obtained using an experimental batch adsorber consisting of a 5 ml plastic bottle. An aqueous solution of fluoride and a Nylon mesh basket holding the activated alumina were placed inside the batch adsorber. The adsorber was partially submerged in a constant temperature water bath placed over a magnetic stirrer. The adsorber solution was stirred continuously with a Teflon-coated stirring bar. 8. Method for Obtaining the Adsorption Equilibrium Data The procedure for obtaining the adsorption equilibrium data is as follows. A water solution of a known initial concentration of fluoride was prepared, the ph of this solution was adjusted to a given value, and a sample was taken to corroborate its initial concentration. A 48 ml portion of this fluoride solution was added to a batch adsorber and a Nylon basket containing a certain mass of the activated alumina was placed inside the adsorber solution. The activated alumina doses were.5 or 1. g and the initial concentrations of fluoride varied from 1 to 2 mg L -1. The solution ph was measured periodically with a phmeter and kept constant by adding.1,.5 and.1 N HNO 3 or NaOH solutions as required. The total volume of the HNO 3 and NaOH solutions added to the adsorber was recorded to be considered in the mass balance. The solution and activated alumina were left in contact until equilibrium was attained. Preliminary experiments showed that 3 d were enough to reach equilibrium. Samples were taken at various times (1, 2 and 3 d) to follow the progress of adsorption, and the fluoride concentration of each sample was determined as described previously. Equilibrium was reached when the concentrations of two consecutive samples did not change over time. The mass of fluoride adsorbed at equilibrium was calculated by a fluoride mass balance. The experimental error in the mass of fluoride adsorbed was assessed to be less than 3%. 9. Method for Obtaining the Desorption Equilibrium Data The reversibility of fluoride adsorption on the activated alumina was investigated by carrying out desorption experiments, which consisted of performing an adsorption experiment as already described. Once equilibrium was reached, the Nylon basket containing the activated alumina saturated with fluoride was removed from the fluoride solution, washed with deionized water and placed inside a batch adsorber containing 48 ml of a desorbing solution without fluoride. The objective of the washing step was to remove the fluoride solution caught between the activated alumina particles and was achieved by submerging the Nylon basket with the activated alumina into deionized water for 5 s. The saturated activated alumina and the desorbing solution were left in contact for 7 d until they attained new equilibrium; it was assumed that new equilibrium was reached when the fluoride concentration of two consecutive samples did not vary more than 2%. The initial ph of the desorbing solution was 7 or 12 and the ph was kept constant during desorption, by adding.1 and.1 N HNO 3 or NaOH solutions. The mass of fluoride that remained adsorbed on the activated alumina was calculated by a mass balance. 1. Equivalents of Fluoride and Hydroxyl The equivalents of F - and OH - exchanged during the adsorption of fluoride were determined by the following procedure. A 5 ml portion of a water solution without fluoride and a Nylon basket containing 1 g of the activated alumina were placed inside the batch adsorber. The solution ph was between 3.9 and The solution ph was measured periodically with a ph-meter and kept constant by adding.1,.5 and.1 N HNO 3 or NaOH solutions as required. Once the solution ph did not change over time, the solution ph was registered as the initial ph, and a 1 ml portion of a solution of known initial concentration of fluoride and at the same ph of the adsorber solution was added to the adsorber. Afterwards, the solution ph was measured periodically but was not adjusted. The equilibrium was attained when the solution ph did not change over time. The final ph of the solution was registered and the final concentration of fluoride in the solution at equilibrium was determined as described previously. RESULTS AND DISCUSSION 1. Characterization of the Activated Alumina An image of the surface of an activated alumina particle is illustrated in Fig. 1. It was observed that the surface of the particle was fractured, rough and porous. The particles presented a compacted morphology and some particles had a spheroidal shape. The elemental chemical composition of the alumina surface was determined by means of the X-ray microanalysis probe (Energy Dispersive Spectroscopy) coupled to the microscope and the spectrum is depicted in Fig. 2. The elemental chemical analysis revealed that the activated alumina was mainly composed of Al and O. This is due to that the activated alumina was essen-

4 34 J. Environ. Eng. Manage., 18(5), (28) Fig. 1. Photomicrograph of the surface of an activated alumina particle. Magnification factor = 11. Fig. 2. Energy dispersive spectrum of the activated alumina surface. Relative intensity γ-al2o3 γ-al2o3 γ-al2o3 γ-al2o θ Fig. 3. XRD pattern of the activated alumina. tially aluminum oxide (Al 2 O 3 ). The X-ray diffraction pattern of the activated alumina is shown in Fig. 3 and revealed that the activated alumina presented a low degree of crystallization. The maximum diffraction peaks observed in the X-ray pattern were characteristic of the γ-al 2 O 3, indicating that the activated alumina was mainly composed of γ-al 2 O 3. The physicochemical and textural properties of the activated alumina depend on the raw material used, as well as the preparation and activation methods. The textural and physicochemical properties of the activated alumina are presented in Table 1. The surface area and the pore volume of the activated alumina have been determined in several works [1,16,17] and the surface area and pore volume of the activated alumina ranged from 5 to 5 m 2 g -1 and from.2 to.76 cm 3 g -1, respectively. The values shown in Table 1 are within these ranges. The average pore diameter of the activated alumina was 11 nm indicating that the activated alumina was a mesoporous solid [18]. The solid density and the particle density are listed in Table 1 and they are slightly different from the values reported by Martinez-Rosales [12], which were 3. and 1.25 g cm 3, respectively. The concentrations of acid sites and basic sites in the activated alumina were.35 and.58 meq g -1, respectively. Hao and Huang [1] reported that the concentration of acid sites in a commercial activated alumina (ALCOA F-1) was.27 meq g -1. This value is slightly smaller than the one reported in this work. No value of the concentration of basic sites was found in the literature reviewed for this work. The PZC of the activated alumina was 7.8, which indicated that the surface of the activated alumina was slightly basic. This result corroborated that the concentration of basic sites was slightly higher than that of the acid sites. Choi and Chen [19] reported that the PZC of several activated aluminas ranged from 6.2 to 8.9. Ku and Chiou [11] found a PZC value of 8. for a commercial activated alumina. Therefore, the PZC of the activated alumina employed in this work was within the range of values reported in the literature. The acid stability results for the activated alumina were graphed as the concentration of dissolved aluminum in the acid solution versus the final ph of the solution (See Fig. 4). The concentration of dissolved aluminum in the acid solution was increased considerably while reducing the solution ph below 4.3 but was almost negligible when the solution ph was in the range from 4.3 to 7. Hence, the activated alumina was not chemically stable in acid solutions at ph lower than 4.3 because the aluminum from the alumina framework would be dissolved by the acid solution. 2. Adsorption Isotherm of Fluoride The Freundlich and Langmuir isotherm models were fitted to the experimental adsorption equilibrium data of fluoride on the activated alumina. These models are represented by the following equations: qmkc q = (1) 1 KC 1/ n q = kc (2)

5 Leyva-Ramos et al.: Flouride Removal by Treated Pseudo-boehmite 35 Table 1. Physicochemical and textural properties of the activated alumina Surface area (m 2 g -1 ) Pore volume (cm 3 g -1 ) Average pore diameter (nm) Solid density (g cm -3 ) Particle density (g cm -3 ) Acid sites (meq g -1 ) Basic sites (meq g -1 ) PZC Concentration of Aluminium (mg L -1 ) Fig. 4. Acid stability of the activated alumina at different ph values and T = 25 C. The constants for these isotherms were estimated by a least-squares method based in the Rosenbrock- Newton optimization algorithm and the values of the constants are shown in Table 2. The average absolute percentage deviation, %D, was calculated for each isotherm by applying the following equation: N 1 q %D = N exp q q i= 1 exp ph pred 1% (3) The average percentage deviations varied between 6 and 16% for the Langmuir isotherm and between 2 and 9% for the Freundlich isotherm. Both isotherms matched all the experimental data well because the average percentage deviations were less than 16%. The isotherm best fitting the experimental data was assumed to be that having the lowest average absolute percentage deviation. Of the 6 isotherm cases shown in Table 2, the Freundlich best fitted 4 cases and the Langmuir isotherm 2 cases. The fluoride can be adsorbed on the different active sites of the activated alumina and by different mechanisms. Hence, the Mass of fluoride adsorbed (mg g -1 ) ph = 3 ph = 4 ph = 5 ph = 7 ph = Concentration of fluoride at equilibrium (mg L -1 ) Fig. 5. Effect of solution ph on the adsorption isotherm of fluoride on the activated alumina at T = 25 C. The lines represent the Freundlich isotherm. Freundlich isotherm was chosen over the Langmuir isotherm because the Freundlich isotherm implied that the fluoride was adsorbed on an energetically heterogeneous surface. The Langmuir isotherm constant q m represents the maximum adsorption capacity. As noted in Table 2, the value of q m at ph = 7 is greater than the value of q m at ph = 11 indicating that the maximum adsorption capacity increases with augmenting the solution ph. As explained in the next section, this tendency is opposite to the actual dependence of the adsorption capacity regarding the solution ph. This result further supports the selection of the Freundlich isotherm. 3. Effect of the Solution ph on the Fluoride Adsorption This effect was studied by determining the adsorption isotherm of fluoride on the activated alumina at ph values of 3, 4, 5, 7 and 11, and the results are depicted in Fig. 5. As shown in this figure, the adsorption capacity presented a maximum at ph = 4, and de- Table 2. Freundlich and Langmuir isotherm constants for the adsorption of fluoride on the activated alumina T Freundlich Langmuir ( C) ph k q (mg 1-1/n L 1/n g -1 n %D m K ) (mg g -1 ) (L mg -1 %D )

6 36 J. Environ. Eng. Manage., 18(5), (28) creased considerably when the ph was either increased from 4 to 11 or diminished from 4 to 3. The fluoride was adsorbed on the active sites of the activated alumina that are formed in the aluminum atoms of the activated alumina framework [11]. The adsorption capacity was reduced with the diminishing solution ph from 4 to 3 because of the loss of active sites of the activated alumina due to dissolution of the aluminum present in the activated alumina. Several researchers [11,19] have found that the adsorption capacity of the activated alumina typically increased and diminished when the solution ph was lessened from 8 to 3 and presented a maximum capacity at an optimal ph. The optimal ph for fluoride removal ranged from 3 to 7 depending on the characteristics of the activated alumina [5,1]. Nevertheless, in all these works it was not argued that this reduction in the adsorption capacity at ph less than 4 had been caused by the dissolution of the activated alumina in acid solutions. In Fig. 5, it can be also observed that the shape of the adsorption isotherm at ph = 3 was very different from that presented by the isotherms at ph greater than 3. The adsorption isotherm at ph = 3 was concave upward, whereas the adsorption isotherms at ph greater than 3 were concave downward. This different behavior can possibly be attributed to the decrease of active sites of the activated alumina surface caused by the dissolution of the aluminum from the activated alumina framework. At a concentration of fluoride at equilibrium of 1.5 mg L -1, the masses of fluoride adsorbed were 2.68, 1.95, 1.25, and.38 mg g -1 at ph values of 4, 5, 7 and 11, respectively. Comparing these values it can be noted that the adsorption capacity of the alumina was reduced 1.4, 2.2 and 7.1 times when the solution ph was increased from 4 to 5, 4 to 7 and 4 to 11, respectively. Therefore, the capacity of the activated alumina for adsorbing fluoride was considerably dependent upon the solution ph. The effect of the solution ph on the adsorption capacity can be ascribed to the electrostatic or coulombic interactions between the activated alumina surface and the fluoride ions in the aqueous solution. The activated alumina surface was charged positively at ph less than the PZC of 7.8 and hence the fluoride ion was attracted to the alumina surface. On the contrary the surface was negatively charged at ph greater than 7.8 then the fluoride ion was repelled from the surface. The electrostatic attractions favored the adsorption of fluoride at ph lower than 7.8. Furthermore, the positive charge of the surface increased gradually when the ph of the solution was decreased below the PZC. This explains why the mass of fluoride adsorbed increased while reducing the ph from 7 to Effect of the Temperature on the Fluoride Adsorption Isotherm The effect of temperature on the fluoride adsorp- Mass of fluoride adsorbed (mg g -1 ) T T=15 = 15 蚓 C T T=25 = 25 蚓 C T T=35 = 35 蚓 C Concentration of fluoride at equilibrium (mg L -1 ) Fig. 6. Effect of temperature on the adsorption isotherm of fluoride on the activated alumina at ph = 5. The lines represent the Freundlich isotherm. tion capacity of the activated alumina is displayed in Fig. 6. The adsorption capacity did not vary with the temperature for fluoride concentrations at equilibrium less than 1 mg L -1. However, the adsorption capacity of the activated alumina presented a maximum at 25 C and decreased in the average 1.5 and 1.3 times when the temperature was diminished from 25 to 15 C and increased from 25 to 35 C, respectively. This unusual behavior cannot be ascribed to the experimental error because the decrease in the adsorption capacity is greater than the magnitude of the experimental error in measuring the mass of fluoride adsorbed. Several authors have reported that the adsorption equilibrium in liquid-solid systems can be affected by the temperature in the following three ways [8]: (1) The adsorption equilibrium is favored by diminishing the temperature, (2) The adsorption equilibrium is favored by increasing the temperature, and (3) The adsorption equilibrium is independent on the temperature. One possible explanation to this unusual behavior is that the predominant adsorption mechanism of fluoride might have changed with the temperature. 5. Reversibility of the Adsorption of Fluoride on Activated Alumina The reversibility of the fluoride adsorption was studied by carrying out adsorption-desorption experiments. First, the fluoride was adsorbed on the activated alumina then the fluoride was desorbed from the alumina. Fluoride was desorbed from the alumina by placing the alumina saturated with fluoride in a water solution without fluoride. In this fashion, the fluoride would desorb back to the solution and reach new equilibrium. If the desorption equilibrium data were on the adsorption isotherm, the adsorption would be reversible. The adsorption data were obtained at ph = 5 and T = 25 C and desorption data at ph of 5 or 11 and T = 25 C. The experimental data of adsorption-

7 Leyva-Ramos et al.: Flouride Removal by Treated Pseudo-boehmite 37 desorption of fluoride on the activated alumina, as well as the adsorption isotherms of fluoride at ph of 5 and 11, are shown in Fig. 7. It can be observed in Fig. 7 that the desorption equilibrium data at ph = 5 were well above the adsorption isotherm at this ph. Thus, the fluoride adsorption was not reversible at ph = 5. On the other hand, it was observed that the desorption equilibrium data at ph = 11 were practically coinciding with the adsorption isotherm at ph = 11. This revealed that the fluoride adsorbed at ph = 5 was considerably desorbed with from the activated alumina at ph = 11, and confirmed that the adsorption of fluoride was reversible at ph = 11. The amount of fluoride desorbed from the activated alumina was augmented as the ph of the desorbing solution increased as well. This result can be explained with recalling that the concentration of OH - was raised augmenting the solution ph and the OH - ions were adsorbed by displacing F - ions adsorbed on the activated alumina. 6. Mechanism of Fluoride Adsorption on Activated Alumina Several authors have investigated the mechanism of fluoride adsorption on the activated alumina. The active sites of the activated alumina are the following surface groups: Al-OH 2, Al-OH and Al-O -, where represents the surface of the activated alumina. The first and last groups are formed when the Al-OH group is protonated and deprotonated according to the following reactions [1,11]: Al - OH H Al - OH2 Al - OH Al - O The fluoride ion exchange mechanism may occur on the surface of the activated alumina according to the following reaction: Al - OH F H Al - F OH The contribution of the ion exchange mechanism to the adsorption of fluoride on activated alumina was assessed by calculating the equivalents of F - and OH - exchanged during the adsorption of fluoride with the following equations: Equivalents of F - exchanged from the solution to the alumina = V (C F -C Ff ) (4) Equivalents of OH - exchanged from the alumina to the solution = V (C OHf -C OH ) (5) The equivalents of F - and OH - exchanged are shown in Table 3 and the equivalents of OH - exchanged from the alumina to the solution are negative at initial ph values near 11. This indicated that the OH - ions were exchanged from the solution to the Mass of fluoride adsorbed (mg g -1 ) Adsorption at ph = 5 Adsorption at ph = 5-Desorption at ph = 5 Adsorption at ph = 5-Desorption at ph = 11 Adsorption Isotherm at ph = 5 Adsorption Isotherm at ph = Concentration of fluoride at equilibrium (mg L -1 ) Fig. 7. Adsorption and desorption isotherms of fluoride on the activated alumina at different ph values and T = 25 C. The lines represent the Freundlich isotherm. alumina. The following ratio was evaluated from the equivalents of ions exchanged: - - F Equivalents of F exchanged from the solution to the alumina = - - OH Equivalents of OH exchanged from the alumina to the solution (6) This ratio was positive when the F - ions were exchanged in the opposite direction to the OH - ions, whereas it was negative when both ions were exchanged in the same direction. It is very important to point out that in these experiments the activated alumina was first allowed to equilibrate in a water solution without fluoride. In this way the buffering effect of the activated alumina did not affect the equivalents of OH - exchanged. The F - /OH - ratio was greater than 2, and 28 when the initial ph was close to 4 and 6, respectively. This implied that for each OH - ion exchanged from the alumina to the solution there were more than 2, and 28 F - ions exchanged from the solution to the alumina at the initial ph values near 4 and 6, respectively. Besides, the OH - ions were exchanged from the alumina to the solution since the ph of the solution increased slightly during fluoride adsorption. In the ph range from 5 to 7.6, the fluoride ion was considerably adsorbed on the alumina but few OH - ions were exchanged from the alumina to the solution. Hence, the contribution of the ion exchange mechanism to the fluoride adsorption on the activated alumina was insignificant for ph below 7.6 and can be disregarded for all practical purposes. At initial ph values close to 11, it was observed that the solution ph was diminished during fluoride adsorption. This revealed that the concentrations of OH - and F - ions decreased since both ions were adsorbed or exchanged from the solution to the alumina. For this reason, the F - /OH - ratio was negative (See Table 3). The F - /OH - ratio varied between -.5 and -.41 at the initial ph of 11 (See Table 3). This means

8 38 J. Environ. Eng. Manage., 18(5), (28) Table 3. Ions exchanged during the adsorption of fluoride on the activated alumina Experiment No. Concentration of fluoride Equivalents of ions exchanged (mg L -1 Solution ph ) (meq) Initial Final Initial Final F - OH - Ratio F - /OH - 2-F ,5 1-F , 2-F ,4 1-F F F F F F < -.1 that for each F - ion adsorbed on the alumina there were between 2 and 2 OH - ions adsorbed on the alumina. At the initial ph of 11 no ion exchange occurred between the OH - ions on the alumina and the F - ions from the solution but rather the OH - ions in solution were competing with the F - ions in the solution for the activated alumina sites. Moreover, at this ph the activated alumina was much more selective toward OH - ions than F - ions. As pointed out earlier, the PZC of the activated alumina was 7.8 indicating that at ph lower than 7.8 the predominant surface groups were Al-OH 2, whereas at ph greater than 7.8 the predominant groups were Al-O -. At ph below the PZC the fluoride in solution was attracted to the activated alumina surface by the electrostatic forces because the surface was positively charged. The reversibility studies demonstrated that the adsorption of fluoride was not reversible and only a part of the adsorbed fluoride was desorbed. The above may be explained by considering that the fluoride was adsorbed by a reversible mechanism and an irreversible one. The reversible mechanism may be the electrostatic attraction between the basic sites of the alumina and the fluoride ion in solution. This mechanism can be schematically represented by the following reaction: Al - OH2 F Al - OH2F The irreversible mechanism may be the chemisorption of fluoride on the basic sites that occurred by the following reaction: Al - OH2 F Al - F H2O This mechanism was proposed by Hao and Huang [1]. The contribution of each of these mechanisms to the adsorption of fluoride on the activated alumina was dependent upon the ph since the adsorption of fluoride was reversible at ph of 11 but not at ph 5. CONCLUSIONS The character of the activated alumina surface was basic (PZC = 7.8) because the concentration of basic sites was slightly greater than that of the acid sites. The capacity of the activated alumina for adsorbing fluoride was considerably dependent on the ph of the solution and exhibited a maximum at ph of 4. The adsorption capacity diminished when the ph was increased from 4 to 11 and this behavior was attributed to the electrostatic interactions between the fluoride ions in solution and the surface charge of the activated alumina. The aluminum of the activated alumina framework was dissolved at ph below 4.3 causing a decrease in the adsorption capacity of the activated alumina. The effect of the temperature on the adsorption capacity of the activated alumina was very unusual. A possible explanation was that the adsorption mechanism of fluoride changed with temperature. The adsorption equilibrium of fluoride on the activated alumina was reversible at ph of 11 but was irreversible at ph of 5. The mass of fluoride desorbed was considerably increased when desorption was carried out at ph of 11. It was concluded that the solution ph played a very important role in desorbing the fluoride adsorbed on the activated alumina. The adsorption of fluoride on the activated alumina occurred by electrostatic interactions as well as chemisorption between the fluoride in solution and the basic sites of the activated alumina surface, but not by ion exchange. C C F C Ff C OH C OHf k NOMENCLATURE Concentration of fluoride at equilibrium, mg L -1 Initial concentration of fluoride in solution, meq L -1 Final concentration of fluoride in solution at equilibrium, meq L -1 Initial concentration of OH - in solution, meq L -1 Final concentration of OH - in solution at equilibrium, mg L -1 Constant of the Freundlich isotherm, mg 1 1/n L 1/n g 1

9 Leyva-Ramos et al.: Flouride Removal by Treated Pseudo-boehmite 39 K Constant of the Langmuir isotherm, L mg -1 n Constant of the Freundlich isotherm N Number of experimental data q Uptake of fluoride adsorbed on activated alumina, mg g -1 q exp Experimental uptake of fluoride adsorbed on activated alumina, mg g -1 q m Maximum uptake of fluoride adsorbed on activated alumina, mg g -1 q pred Uptake of fluoride adsorbed on activated alumina predicted with the adsorption isotherm, mg g -1 V Volume of the solution, L REFERENCES 1. Latham, M.C., Human Nutrition in the Developing World. Food and Agriculture Organization of the United, Rome, Italy (1997). 2. World Health Organization (WHO), Guidelines for Drinking-Water Quality. 3rd Ed., WHO, Geneva, Switzerland (24). 3. Fawel, J., K. Bailey, J. Chilton, E. Dahi, L. Fewtrell and Y. Magara, Fluoride in Drinking- Water. IWA Publishing, London, UK (26). 4. Chang, C.F., P.H. Lin and W. Höll, Aluminumtype superparamagnetic adsorbents: Synthesis and application on fluoride removal. Colloid. Surface. A, 28(1-3), (26). 5. Ghorai, S. and K.K. Pant, Equilibrium, kinetics and breakthrough studies for adsorption of fluoride on activated alumina. Sep. Purif. Technol., 42(3), (25). 6. Onyango, M.S. and H. Matsuda, Fluoride removal from water using adsorption technique. In: A. Tressaud (Ed.). Fluorine and the Environment. Elsevier, B.V., Amsterdam, The Netherlands, UK, pp (26). 7. Morel, J.P., N. Marmier, C. Hurel and N. Morel- Desrosiers, Effect of temperature on the acid-base properties of the alumina surface: Microcalorimetry and acid-base titration experiments. J. Colloid Interf. Sci., 298(2), (26). 8. Leyva-Ramos, R., Importancia y aplicaciones de la adsorcion en fase liquida. In J.C. Moreno- Pijaran (Ed.). Solidos Porosos. Preparacion, Caracterizacion y Aplicaciones. Ediciones Uniandes, Bogota, Colombia, pp (27). 9. Leyva-Ramos, R. and A. Juarez-Martinez, Adsorción de Fluoruros en varios. tipos Comerciales de Alumina Activada. Avan. Ing. Quím., 3, (1991) (in Spanish). 1. Hao, O.J. and C.P. Huang, Adsorption characteristics of fluoride onto hydrous alumina. J. Environ. Eng.-ASCE, 112(6), (1986). 11. Ku, Y. and H.W. Chiou, The adsorption of fluoride ion from aqueous solution by activated alumina. Water Air Soil Poll., 133(1-4), (22). 12. Martinez-Rosales, J.M., Control de textura de Alúminas Activadas vía Sustitución de Líquido Intermicelar, Tesis de Maestría, Universidad Autónoma Metropolitana, Noviembre (1994) (in Spanish). 13. Boehm, H.P., Some aspects of the surface chemistry of carbon blacks and other carbons. Carbon, 32(5), (1994). 14. Noh, J. and J. Schwarz, Estimation of the point of zero charge of simple oxides by mass titration. J. Colloid Interf. Sci., 13(1), (1989). 15. Wingenfelder, U., C. Hansen, G. Furrer and R. Schulin, Removal of heavy metals from mine waters by natural zeolites. Environ. Sci. Technol., 39(12), (25). 16. Suzuki, M., Adsorption Engineering. Kodansha LTD, Elsevier Science Pub. Co., Amsterdam, The Netherlands (199). 17. Ghorai, S. and K.K. Pant, Investigations on the column performance of fluoride adsorption by activated alumina in a fixed-bed. Chem. Eng. J., 98(1-2), (24). 18. Do, D.D., Adsorption Analyses: Equilibria and Kinetics. Imperial College Press, Singapore (1998). 19. Choi, W.W. and K.Y. Chen, The removal of fluoride from waters by adsorption. J. Am. Water Works Ass., 71(1), (1979) Discussions of this paper may appear in the discussion section of a future issue. All discussions should be submitted to the Editor-in-Chief within six months of publication. Manuscript Received: March 3, 28 Revision Received: July 1, 28 and Accepted: July 18, 28