Experimental procedure

We compared the effectiveness of two different materials in the removal of fluoride from potable well water, the first fibrillar gamma alumina nanopowder (γ-Al₂O₃), and the second a commercial activated charcoal powder (CA). The first material, γ-Al₂O₃, was prepared by homogeneous precipitation. A basic aluminum sulfate (BAS) was synthesized from a mixture of aluminum sulfate and ammonium bisulfite solutions. The BAS was then neutralized in a solid/liquid reaction with ammonia solution to transform it to aluminum hydroxides. Then, hydroxides were oven dried at 110 °C to obtain pseudoboehmite which was in turn used as a γ-Al₂O₃ precursor since it transforms to this phase at 450 °C. (^{Sugita, Contreras, Juárez, Aguilera & Serrato, 2001}; ^{Zamorategui, Soto & Sugita, 2012a}). Purity of the second material, commercial charcoal (Karal), is described in Table 1. Specific surface area of both adsorbent materials was determined by single-point Brunauer, Emmett and Teller (BET) method (ASAP, 2010 Micromeritrics Instrument Corp., USA). Morphology and particle size was examined by Field Emission Scanning Electron Microscopy (FE-SEM; JEOL, JSM 7401F). Zeta potential (pZ), particle size of the γ-Al₂O₃, and dispersed activated carbon in water were measured by electroacoustic technique with a particle size analyzer (AcoustoSizer II, ESA; Colloidal Dynamics, USA) using a 10 mM solution of potassium chloride (KCl). Remaining fluoride ion in the final treated water was measured using a fluoride ion selective electrode.

Table 1 Parameters of the Langmuir equation (pH 5 and pH 7). 

Source: Author’s own elaboration.

Samples were prepared with concentrations of 5 ppm (mg/L), 10 ppm (mg/L), 25 ppm (mg/L), 50 ppm (mg/L), 100 ppm (mg/L), 125 ppm (mg/L), 150 ppm (mg/L) and 200 ppm (mg/L) of fluoride ion in deionized water. We used 0.3 grams of adsorbent material, and the pH (5 and 7) was adjusted with hydrochloric acid. Samples were left for five days at room temperature (28 °C). After five days, solution was filtered and final concentrations of fluoride ions were determined. It was considered that equilibrium is reached when the fluoride concentration obtained was the same or that there was a difference of 5% over the earlier recorded period.

Particle size and morphology

Figure 1a shows the FE-SEM image of the activated γ-Al₂O₃ powder prepared by homogeneous precipitation. The powder consists of nanofibers about 100 nm in length that tend to form amorphous agglomerates due to their high surface energy, therefore generating high porosity. In comparison with the morphology of the activated charcoal powder (Figure 1b), we can see that the powder exhibits a higher degree of agglomeration and consequently has a lot of small pores.

Source: Author's own elaboration.

Figure 1 FE-SEM images of a) activated γ-Al₂O₃ and b) activated charcoal (CA). 

Adsorption/desorption of N₂, BET analysis

Figure 2 shows the adsorption-desorption isotherms of the γ-Al₂O₃ and CA. The γ-Al₂O₃ isotherm shows a type IV according to the International Union of Pure and Applied Chemistry (IUPAC) definition, which is characteristic of a mesoporous material (^{Kim, Lee, Jun, Park & Potdar, 2007}; ^{Park & Jeong, 2008}). Irregular shape isotherms with the hysteresis loop type E indicate that the pores in the material have an inkwell-type shape. The γ-Al₂O₃ synthesized by homogeneous precipitation has a smaller specific surface area (332 m² • g^-1) than that determined for the CA (601 m² • g^-1) (^{Lavado-Meza, Sun-Kou & Recuay-Arana, 2012}). The smaller area presents an isotherm type I, according to the IUPAC classification, which is characteristic of processes where micropore filling occurs. The major consumption of N₂ in the adsorption-desorption isotherm of the CA occurred at a low relative pressure (< 0.2) and reached a plateau at high relative pressure. In agreement with the porosity observed by FE-SEM, and the above-mentioned pore distribution, the CA and γ-Al₂O₃ are micro and mesoporous materials respectively (Figure 2b).

Source: Author's own elaboration.

Figure 2 Graphs a) N₂ adsorption-desorption isotherm and b) Pore diameter of the γ-Al₂O₃ and CA. 

Zeta potential and particle size

Figure 3a shows the effect of pH variation on zeta potential. Profile of zeta potential vs. pH corresponds to changes in particle surface charge and takes the characteristic shape and isoelectric point typical for γ-Al₂O₃. Its isoelectric point (IEP) was found to be at pH 8.5, which is consistent with reported data (^{Bowen, Carry, Luxembourg & Hofmann, 2005}). As can be seen in Figure 3a, potential in the γ-Al₂O₃ suspension is high, either below or above the IEP, i.e., for negatively and positively charged particles and these changes in potential are related to the flocculation phenomena and the neutralized surface charge (^{Zamorategui, Surgita, Zárraga, Tanaka & Uematsu, 2012b}). In contrast with the CA, surface charge of the γ-Al₂O₃ is always positive at low zeta potential. This result can have an effect on the removal of the fluoride ion (F-). Furthermore, the particle size of γ-Al₂O₃ is smaller than that of the CA (Figure 3b) which is related to their zeta potential.

Source: Author's own elaboration.

Figure 3 Graphs a) Zeta potential and b) particle size of the γ-Al₂O₃ and CA. 

Langmuir isotherm

Removal of an ion is mainly attributed to the inter -action of surface groups and the ion through various mechanisms, such as: donor-acceptor reactions of electrons and/or formation of metal complexes such as COOH-M (^{Elmouwahidi, Zapata-Benabithe, Carrasco-Marín & Moreno-Castilla, 2012}; ^{Mor, Ravindra & Bishnoi, 2007}). Activated charcoal and γ-Al₂O₃ are well-known as adsorbents of substances interacting by van der Wall's forces.

Adsorption isotherms are important to describe how the adsorbate interacts with the adsorbent, and it is important to establish the most appropriate correlation for the equilibrium curves. Thus, several isotherms (namely, Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich) have been studied. Adsorption isotherms for γ-Al₂O₃ and activated charcoal are shown in Figure 4a and these are in agreement with the Langmuir equation which is represented as:

Source: Author's own elaboration.

Figure 4 Graphs a) Langmuir isotherms and b) Percent removal curves of the γ-Al₂O₃ and CA. 

Where qe is the amount of fluoride ion adsorbed per gram of adsorbent, Q_max is the maximum amount of fluoride ion per unit weight of absorbent to form a complete monolayer on the surface (mg • g^-1), k is the Langmuir constant, and C_e is the concentration of fluoride ion remaining in the solution (^{Ghorai & Pant, 2005}). Table 1 summarizes the parameters of the Langmuir equation at pH 5 and pH 7.

In order to determine the optimum pH for the maximum removal of fluoride ions, equilibrium adsorption of fluoride ions was carried out at pH 5 and pH 7. It is clear that the adsorption by mesoporous γ-Al₂O₃ and microporous CA is dependent on the initial pH of the solution, and the maximum uptake is obtained at pH 5, approximately 35 mg of F-/g and 8 mg of F-/g, respectively. Results show that γ-Al₂O₃ possesses a much higher adsorption capacity than the CA at both pH 5 and pH 7.

Removal curves of the γ-Al₂O₃ and CA at pH 5 and pH 7 are shown in Figure 4b. In this study, solutions of 50 ppm of fluoride ion were used. As can be seen, maximum rate of fluoride removal was during the first minute and the γ-Al₂O₃ is the better adsorbent (95.5%) in comparison with the CA (26%) at pH 5 and at pH 7 (89.8% and 59.4%).

Results indicate that the adsorption of fluoride ions occurs on the outermost surface of the adsorbent material, and this is not affected by the difference in porosities. Adsorption process is controlled by the diffusion of the fluoride ion in the liquid immediately adjacent the outer surface adsorbent material. Subsequently, fluoride ion must diffuse into the particle of adsorbent, which is the reason why the amount of fluoride ion adsorbed is less for the activated charcoal than for the activated alumina, since the phenome-non is influenced by the size of the pores through which the fluoride ion is transported.