Studies on Electrospun Alumina Nanofibers for the Removal of

Dec 25, 2012 - The maximum uptakes of chromium(VI) and fluoride by electrospun .... Treatment and resource recovery from inorganic fluoride-containing...
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Studies on Electrospun Alumina Nanofibers for the Removal of Chromium(VI) and Fluoride Toxic Ions from an Aqueous System A. Mahapatra, B. G. Mishra, and G. Hota* Department of Chemistry, NIT Rourkela, Orissa, India 769008 S Supporting Information *

ABSTRACT: In this study, we have synthesized alumina nanofibers by using an electrospinning method. The obtained alumina nanofibers were used as adsorbents for the removal of chromium(VI) and fluoride ions from an aqueous system. The morphology and size of the obtained nanofibers were characterized by scanning electron microscopy and transmission electron microscopy analytical techniques. The X-ray diffraction study of the nanofibers indicated the formation of pure and crystalline αalumina. The effects of the pH, adsorbent dose, contact time, and initial concentration of the adsorbate on the alumina nanofiber surface have been studied. The optimum pH values for maximum removal of chromium(VI) and fluoride on the alumina nanofiber surface were found to be pH 5 and 7, respectively. The experimental data obtained from the adsorption studies by alumina nanofibers were analyzed by the Langmuir and Freundlich equation using a linearized correlation coefficient at room temperature. The Freundlich isotherm was found to fit well with the equilibrium data for adsorption of chromium(VI) and fluoride ions. The adsorption kinetics could be modeled by a pseudo-second-order rate expression. The maximum uptakes of chromium(VI) and fluoride by electrospun alumina nanofibers were found to be 6.8 and 1.2 mg/g, respectively. among the consumers.19 According to WHO reports, a value of more than 1 mg/L in drinking water can cause dental and skeletal fluorosis.20,21 However, by crossing a certain level of fluoride intake, it causes many bone diseases, including mottling of teeth and lesions of the endocrine glands, thyroid, liver, and other organs.22 Hence, it is an important task to supply water with safe fluoride levels. Fluorine is the most electronegative and reactive among all of the elements in the periodic table. Because of its great reactivity, fluorine cannot be found in nature in its elemental state. It exists either as inorganic fluorides (including the free anion F−) or as organic fluoride compounds.23 Electrospinning is a simple and versatile method for fabricating continuous fibers with diameters ranging from micrometers to several nanometers.24 Fibers that have large specific surface areas and high adsorption rates have attracted great attention in heavy-metal removal from wastewater in recent years.25,26 A similar type of work has been done by Hota et al. for the removal of heavy-metal ions like CdII. Here the sorption of CdII was done by using a boehmite-impregnated electrospun membrane. The adsorption capacity was found to be 0.20 mg/g.27 Recently Zhu et al. have reported the preparation of a PVC ultrafine fibrous membrane using an electrospinning method, used for the adsorption of cationic dye.28 The adsorption of metal ions from aqueous solutions of adsorbents is usually controlled by the properties of the surface functional groups of the adsorbents.29 The defluoridation performances of five different types of aluminas such as aluminum oxide, hydroxide, and oxyhydroxide were inves-

1. INTRODUCTION The removal of toxic heavy-metal ions from wastewater has received significant attention because of their high impact on the environment and public health.1 It is a major concern because of their nondegradability and threat to human life and environment. This toxic metal removal from aqueous solutions has been traditionally carried out by chemical precipitation.2 Many more preconcentration methods are well-known.3 Most of them are complicated and time-consuming. Among these methods, however, the adsorption method is simple and convenient.4−8 Adsorption is the accumulation of gas, vapor, or liquid molecules on an interface.9 The adsorption processes appear to be multidetermined and depend on both the characteristic of the adsorbent and the operating conditions. The primary need for a better adsorption process is an adsorbent with sufficient selectivity, high sorption capacity, and resistance to high temperatures and high radiation levels.10 A literature survey shows that some scientists have already studied different ways for the removal of toxic metals and hazardous waste management.11,12 Chromium is one of the regulated toxic metals in the environment and is found in various oxidation stages ranging from II− to VI+ but mainly in two states, as CrIII and CrVI. Between the two forms, CrVI is the most toxic, and it is carcinogenic and mutagenic to living organisms.13,14 It is highly toxic because of its oxidizing properties and tends to accumulate in living organisms, causing serious damage for bacteria, plants, and animals. CrVI has high water solubility and mobility,15,16 and because of its negative impact on the aquatic ecosystem, the hexavalent state is more concerned by the research.17 The maximum permissible level of CrVI in drinking water according to World Health Organization (WHO) is 50 μg/L.18 Similarly, fluoride is also considered to be both an essential element and a potent environmental pollutant at high concentrations, causing a number of disorders (fluorosis) © 2012 American Chemical Society

Received: Revised: Accepted: Published: 1554

June 16, 2012 December 14, 2012 December 25, 2012 December 25, 2012 dx.doi.org/10.1021/ie301586j | Ind. Eng. Chem. Res. 2013, 52, 1554−1561

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tigated recently by Gong et al. Those five aluminas were synthesized at different pH values and calcination temperatures, and the fluoride removal behavior was studied.30 Similarly, Kamble et al. have also reported the adsorption potential of commercially available alkoxide-origin alumina materials for defluoridation of drinking water using batch and continuous modes of operations.20 In the present work, we have prepared alumina nanofibers by using an electrospinning method. The adsorption of chromium(VI) and fluoride ions from an aqueous solution by using alumina nanofibers as adsorbent materials was investigated. Batch studies were conducted at room temperature, and the effects of the contact time, pH, adsorbent dose, and initial metal-ion concentration were investigated to optimize the conditions for maximum chromium(VI) and fluoride removal. The experimental data obtained were calculated and fitted using an adsorption isotherm kinetic model in order to understand the nature of adsorption.

diffractometer (PAN) using nickel-filtered Cu Kα (λ = 1.541 Å) radiation. The surface morphology of Al2O3 nanofibers was characterized using a JEOL JSM-5300 scanning electron microscope and was operated at 20 kV. The nanofibers were deposited on a carbon tape before mounting on a sample holder for scanning electron microscopy (SEM) analysis. The Al2O3 nanofibers were dispersed in ethanol, and then a drop of the above dispersion was taken on a carbon-coated copper grid (300 mesh) for transmission electron microscopy (TEM) imaging. A digital pH meter (Sartorius Mechatronics India Pvt. Ltd.), fitted with a glass electrode, was used for the pH adjustment. The residual concentration of CrVI following sorption experiments was determined by using a PerkinElmer atomic adsorption spectrophotometer (PM) fitted with a chromium lamp. The concentration of fluoride in the test solution was measured using an Orion ion-selective electrode and an Orion 720 A+ ion analyzer. A total ionic strength adjusting buffer (TISAB-III) solution was added to both samples and standards in the ratio 1:10.32 TISAB-III contains 300 g of sodium citrate·2H2O (fw = 294.10), 22 g of 1,2cyclohexanediamine-N,N,N′,N′-tetraacetic acid (CDTA) and 60 g of NaCl in a volume of 1000 mL (pH 5−5.5). CDTA forms stable complexes with polyvalent metal cations (e.g., AlIII and FeIII) in a water and/or an aqueous solution (e.g., Al3+ and Fe3+), which are more stable than metal−fluoride complexes in solution.33 2.4. Adsorption Experiments. The electrospun alumina nanofibers were used as adsorbent materials for the removal of chromium(VI) and fluoride from an aqueous system. We have conducted batch adsorption studies in order to determine the optimum conditions for the removal of chromium(VI) and fluoride from an aqueous system, which were performed using small (50 mL) neat and clean screw-capped glass and polypropylene bottles respectively. A stock solution (1000 mg/L) of chromium(VI) and fluoride ions was prepared by dissolving exact amounts of K2Cr2O7 (Merck) and sodium fluoride (Merck) in deionized water. Solutions with the desired concentrations (10−100 mg/L) of chromium(VI) and fluoride were prepared by successive dilutions of the stock solution. Batch adsorption experiments were performed by the addition of different quantities of alumina nanofibers (10−100 mg) into a 20 mL stock solution of chromium(VI) and fluoride of varying concentrations (10−100 mg/L). All of the adsorption experiments were carried out at room temperature (25 °C) with constant stirring by using an orbital shaker. The effects of the pH on the adsorption of chromium(VI) and fluoride ions by alumina nanofibers were studied by varying the pH of the solution over the range of 3−9 by the addition of a 0.1 M NH4OH and 0.1 M HNO3 aqueous solution. A separate set of experiments were conducted to test the time required to reach the equilibrium condition, by keeping the initial concentration of the test solution at 10 mg/L and the adsorbent dose at 0.01 g for a chromium(VI) solution and 0.1 g for a fluoride solution at optimum pH values of 5 and 7, respectively. Similarly, the effect of the adsorbent dosage on the adsorption of chromium(VI) and fluoride ions was performed by shaking 20 mL of a 10 mg/ L stock solution for 1 h with different adsorbent doses (0.01− 0.1 g) at room temperature. After equilibrium was reached, the adsorbent was separated from the aqueous solution by filtration using Whatman-42 filter paper and the residual concentrations of the metal ions in the aliquot were determined by AAS and an Orion ion-selective electrode. Adsorption data obtained in this

2. EXPERIMENTAL SECTION 2.1. Materials. The polymer poly(vinylpyrrolidone) (PVP; Mn = 1300000) was obtained from Sigma-Aldrich (USA). Aluminum sulfate 16 hydrate (reagent grade, GR) was purchased from Merck India. Barium acetate [Ba(Ac)2, AR grade] was supplied by SD Fine Chem Ltd., India. Acetic acid (AA) was purchased from Rankem, India. The rest of the chemicals, such as potassium dichromate, sodium fluoride, HNO3, and NH4OH, were purchased from Merck, India. A Merck chromium standard was used for standardization of atomic absorption spectroscopy (AAS). All of the chemicals were used without further purification. Double-distilled water and 25 mL neat and cleaned glass bottles were used throughout the experiments. 2.2. Fabrication of an Alumina Nanofiber Adsorbent. We have previously reported the synthesis of alumina nanofibers by an electrospinning method.31 In a typical procedure, the PVP solution (10 wt %) was prepared by dissolving PVP polymer powder in absolute ethanol under constant and vigorous stirring. Aluminum acetate was used as an aluminum precursor and was prepared by mixing a saturated solution of Ba(Ac)2 and aluminum sulfate. The aluminum acetate sol was then mixed with the previously prepared PVP/ ethanol solution. In order to avoid the turbidity, 1−2 drops of AA was added to the above solution. The polymer-to-aluminum precursor’s weight ratio was maintained at 3:1. The resulting PVP/aluminum acetate solution was loaded into a 3 mL plastic syringe fitted with a metallic needle. The polymer solution was pushed to the needle tip using the syringe pump, and the flow rate was kept at 1.0 mL/h. The positive terminal of 14 kV power was applied to the needle tip by using a variable-highvoltage power supply (Glassman Japan, 0−30 kV), whereas the negative terminal was connected to the grounded collector, which was covered with aluminum foil and served as the counter electrode. All of the experiments were conducted at room temperature with a relatively low-humidity (45−50%) condition. After electrospinning, the as-spun PVP/aluminum acetate composite fibers were calcined in air for 2 h at higher temperatures (1000 °C) at a heating rate of 20 °C/min in order to obtain pure Al2O3 nanofibers. The obtained alumina nanofibers were used as adsorbents for the removal of chromium(VI) and fluoride from an aqueous system. 2.3. Adsorbent Characterization. The powder X-ray diffraction (XRD) patterns were recorded on a PAN analytical 1555

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the XRD curve for the calcined alumina fibers that are prepared by electrospinning of a PVP/aluminum acetate solution. Diffraction peaks with corresponding 2θ values indicate the formation of a highly crystalline α-Al2O3 phase for the sample sintered at 1000 °C for 2 h. No other peaks associated with the presence of impurities are observed in the XRD patterns; suggesting the formation of pure alumina. All of the observed diffraction peaks are indexed to the α-Al2O3 structure and match well with the literature value (JCPDS card no. 42-1468). Figure 2 indicates the SEM micrographs of alumina nanofibers obtained after sintering PVP/aluminum acetate composite fibers at 1000 °C. These figures indicate the formation of pure ultrafine cylindrical Al2O3 fibers with smooth surface morphology. The diameters of the alumina fibers (Figure 2a) are found to be in the range of 200−600 nm. The SEM elemental detection X-ray analysis (SEM-EDX) clearly suggests the presence of aluminum and oxygen elements (Figure 2b). This result indicates the formation of pure alumina fibers. Figure 2c shows the SEM image of alumina nanofibers after adsorption experiments. It is observed from the figure that the fiberlike structure of alumina fibers was fully retained after adsorption studies without affecting the fiber morphology. The SEM-EDX of the alumina fibers after adsorption (Figure 2d), suggests the presence of Al, Cr, and O. This result might be due to the sorption of Cr ion onto the surface of alumina fibers. In order to obtain more information about the formation, morphology, and dimensions of individual Al2O3 ultrafine fibers, we have carried out TEM imaging. Figure 3a shows a TEM micrograph of Al2O3 ultrafine fibers. It was observed from TEM imaging that the diameters of the nanofibers were not uniform; instead, a range of fibers were formed (100−500 nm), which is also consistent with the fiber diameters as observed by the magnified SEM micrographs. Figure 3b shows the selected-

experimental study were evaluated with the Freundlich and Langmuir isotherm models. The percentage of chromium(VI) and fluoride ions adsorbed were determined from the difference between initial Ci and final Cf concentrations of both ions in aqueous solution, before and after contact. % removal =

C i − Cf × 100 Ci

(1)

3. RESULTS AND DISCUSSION 3.1. Characterization of Alumina Nanofibers. The formation, crystalline phase, and purity of electrospun alumina nanofibers were identified with XRD analysis. Figure 1 shows

Figure 1. XRD patterns of alumina nanofibers: (a) PVP/Alac/as-spun nanofiber and (b) Al2O3 nanofiber obtained after sintering at 1000 °C.

Figure 2. SEM micrographs of alumina nanofibers (a) before and (c) after adsorption. EDX spectra (b) before and (d) after adsorption. 1556

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of chromium(VI) on the alumina nanofiber surface occurs at pH 5, whereas in the case of fluoride, pH 7 shows maximum removal. The percentage removal efficiency increased from 38% (pH 3) and reached 72% (pH 5). This result can be interpreted as follows. Depending on the solution pH values and the concentration of CrVI ions in solution, CrVI species may be present in solution, as dichromate (Cr2O72−), hydrochromate (HCrO4−), or chromate (CrO42−). In an acidic environment (lower pH), CrVI exists as either HCrO4− or Cr2O72−, and in an alkaline environment, it predominately exists as CrO42− ions.34 Highly acidic conditions (pH 3) are not favorable because of competition between protons and anionic metallic species (HCrO4−), which explains the weak adsorption. With an increase in the pH, the degree of protonation of anionic metallic species gradually reduces and the surface of the alumina sorbent becomes positive and as a result the uptake of CrVI ions increases. Hence, the adsorption of CrVI onto the alumina surface increases at pH 5. Furthermore, upon an increase in the pH, there is competition between OH− and chromate ions (CrO42−), with the former being the dominant species at higher pH values. The net positive surface potential of the sorbent decreases, resulting in a weakening of the electrostatic forces between sorbent and sorbate, which ultimately leads to a reduced sorption capacity.35−37 Because maximum removal of CrVI by alumina nanofibers was observed at pH 5 and no further significant change in the adsorption capacity was noticed between pH 5 and 8, we have chosen pH 5 for all adsorption experiments of CrVI ions. In the case of the fluoride ion, the maximum removal percentage (50%) was observed at pH 7, as shown in Figure 4. Fluoride adsorption may be due to the combined effect of both chemical and electrostatic interactions between the aluminum oxide surface and fluoride ion and also the availability of active sites on the adsorbent surfaces.38,39 At acidic pH, protonation creates a positively charged alumina surface, which is attributed to a greater increase in the attractive force between the positively charged surface and negatively charged fluoride ions, while at higher pH 9, the adsorption percentage decreases. Here the efficiency decreases because the basic solution acquires a negative charge in the alkaline pH and there is repulsion between the negatively charged alumina surface and fluoride ions. Here the negative hydroxyl ions also compete with fluoride ions, leading to a decrease in its uptake, whereas at highly acidic pH 3,

Figure 3. (a) TEM micrograph and (b) electron diffraction pattern of alumina nanofibers.

area electron diffraction pattern of alumina nanofibers. The corresponding diffraction rings and bright spot on the electron diffraction pattern indicate the formation of highly crystalline alumina fibers, which is also consistent with XRD results. 3.2. Effect of the pH on the Adsorption of Chromium(VI) and Fluoride Ions. The removal of chromium(VI) and fluoride ions from aqueous solution by alumina nanofiber adsorbents was studied by varying the pH of the solution over the range of 3−9, and the results obtained are shown in Figure 4. It is observed from the figure that the maximum adsorption

Figure 4. Percentage removal of chromium(VI) and fluoride ions by alumina nanofibers with a change in the pH.

Figure 5. (a) Percentage removal of chromium(VI) and fluoride ions as a function of time (min) by alumina nanofibers and (b) loading capacity with changes in time. 1557

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Figure 6. Pseudo-second-order plot for chromium(VI) and fluoride ion removal by alumina nanofibers.

hydrofluoric acid forms and hence adsorption of fluoride decreases.20,40 3.3. Effect of the Contact Time on the Adsorption of Chromium(VI) and Fluoride Ions. The time of contact of the adsorbate and adsorbent plays a significant role for the removal of pollutant species from aqueous solution during batch experimental studies. In order to determine the optimum time for the complete removal of chromium(VI) and fluoride ions from the aqueous solution, we have carried out a number of adsorption experiments at different contact times from 0 min to 3 h, keeping the initial chromium(VI) and fluoride ion concentration of 10 mg/L at pH 5 and 7 and the adsorbent doses of 0.01 and 0.1 g, respectively. Figure 5 shows the effect of the contact time on the adsorption of chromium(VI) and fluoride ions by alumina nanofibers. The result indicates that the adsorption rate reaches equilibrium after 1 h in the case of both ions. The percentage removal of chromium(VI) ions was found to be 70%, whereas for fluoride ions, it is 50% in 60 min of contact time. There was no significant change in the equilibrium concentration from 1 to 3 h of contact time. Therefore, it is clear that the adsorption of chromium(VI) and fluoride ions by electrospun alumina nanofibers is rapid and after 1 h complete adsorption equilibrium is obtained. In Figure 5b, the loading capacity is plotted with changes in time. This result indicates that the sorption capacity of electrospun alumina nanofibers for the chromium(VI) ion is 6.8 mg/g and for that for the fluoride ion is 1.2 mg/g at equilibrium. 3.4. Adsorption Kinetics. The study of adsorption kinetics is important because it provides valuable insight into the reaction pathways and the mechanism of adsorption. The adsorption mechanism depends on the physical and chemical characteristics of the adsorbent and also on the mass-transfer process.41 In this present work, we have studied the kinetics of the removal of chromium(VI) and fluoride ions for understanding the adsorption behavior of the alumina nanofiber. The experimental data obtained are applied to pseudo-first-order42 and pseudo-second-order models43 to clarify the sorption kinetics of chromium(VI) and fluoride ions onto the alumina nanofiber surface. The pseudo-second-order model can be expressed as follows: dQ = k 2(qe − qt)2 dt

respectively. The integrated form of the above equation can be expressed as follows: t 1 1 = + t 2 qt qe k 2qe (3) The values of qe and k2 can be determined by the slope and intercept of the straight line of the plot t/qt versus t, respectively (Figure 6). The results obtained demonstrate a highly significant linear relationship for the removal of chromium(VI) and fluoride ions onto the alumina nanofiber surface in Table 1. The values of Table 1. Values of Pseudo-Second-Order Kinetic Parameters for the Removal of Chromium(VI) and Fluoride Ions by Alumina Nanofibers pseudo-second-order reaction chromium(VI) fluoride

K2, min−1

R2

Qe,calc, mg/g

7.246 0.032

0.995 0.988

1 1.239

the correlation coefficient (R2 = 0.995 and 0.9884) clearly indicate that the experimental data are in good agreement with the pseudo-second-order rate law based on the adsorption capacity. 3.5. Effect of the Initial Concentration on the Adsorption of Chromium(VI) and Fluoride Ions. The effect of the initial concentration on the adsorption of chromium(VI) and fluoride ions by alumina nanofibers was studied by varying the chromium(VI) and fluoride concentrations from 10 to 100 mg/L for 1 h of contact time. Figure 7 shows the effect of the initial concentration on the adsorption of chromium(VI) and fluoride ions by alumina nanofibers. We observed from this figure that the percentage removal of chromium(VI) increases with an increase in the initial concentration; the reason may be due to the availabilty of more chromium(VI) ions in the solution for sorption onto the available binding sites on the surface of the alumina nanofiber adsorbent. The percentage removal of chromium(VI) increases from 70% to 93% with an increase in the initial concentration of the solution from 10 to 50 mg/L. Upon further increases in the concentration of the initial solution (up to 100 mg/L), there is no further appreciable change in the percentage removal. This is due to the achievement of equilibrium and saturation of the binding sites of the adsorbent surface.44,45 It is also observed from Figure 7 that the percentage removal of the fluoride ion decreases with an increase in the initial fluoride ion

(2)

where k2 [g/(mg min)] is the rate constant of the pseudosecond-order equation and qe and qt are the amounts of solute adsorbed on the adsorbent at equilibrium and at time t, 1558

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where Ce and qe are the concentrations of adsorbate (mg/L) and amount adsorbed (mg/g) at equilibrium. qm (mg/g) and b (L/mg) are the Langmuir constants and are related to the adsorption capacity and energy of adsorption, respectively. The equilibrium data obtained were also examined by the Freundlich adsorption isotherm. This model is based on the assumption that adsorption of adsorbate molecules occurs on a heterogeneous adsorbent surface. This equation is employed to describe the heterogeneous system and is characterized by the heterogeneity factor n. The equation is represented as follows: ln qe = ln KF + (1/n) ln Ce

where qe is the amount of adsorbate adsorbed per specific amount of adsorbent (mg/g), C e is the equilibrium concentration (mg/L), and Kf and n are Freundlich equilibrium constants. The constants Kf and n are incorporating factors affecting the adsorption process like the adsorption capacity and intensity of adsorption. Figure 8 gives the linear plot of log qe against log Ce. The values for the Freundlich constant and correlation coefficients (R2) are calculated using the Freundlich plot (Figure 8) and are shown in Table 2. The value of the Freundlich coefficient n fulfilled the condition 0 < n < 10; this suggests that the adsorption of chromium(VI) and fluoride ions on the surface of alumina fibers is favorable. Again from the correlation coefficient (R2 = 0.998 and 0.997), it is observed that chromium(VI) and fluoride ion adsorption is better fitted for the Freundlich adsorption isotherm. However, the equilibrium data are not fitted to the Langmuir isotherm model, as shown in Table 2 (plot not shown), indicating that the adsorption does not follow monolayer adsorption. Therefore, the uptake of chromium(VI) and fluoride ions preferably follows multilayer and heterogeneous adsorption processes. 3.7. Effect of the Adsorbent Dosage on the Adsorption of Chromium(VI) and Fluoride Ions. In order to study the effect of the adsorbent dosage for the removal of chromium(VI) and fluoride ions, we have carried out adsorption experiments by varying the amount of alumina nanofibers using 10 mg/L of a chromium(VI) and fluoride stock solution and maintaining the contact time of 1 h and the pH of the solutions at 5 and 7, respectively. The results obtained are shown in Figure 9. It is observed from the figure that, with an increase in the adsorbent dosage from 0.5 to 5 g/L, there was no appreciable change in the sorption of chromium(VI) ions. The percentage removal was found to be unchanged or maintained a constant value even if more alumina nanofiber was added. From this

Figure 7. Percentage adsorption graph of chromium(VI) and fluoride ions with changes in the concentration of the metal ion by alumina nanofibers.

concentration. In this case, the percentage removal of the fluoride ion was found to be 50% at 10 mg/L and 42% at 100 mg/L. This is probably due to the fact that, for a fixed adsorbent dosage, the total available adsorption sites are limited, thereby adsorbing almost the same amount of fluoride.46 3.6. Adsorption Isotherm. In order to describe the adsorption behavior of chromium(VI) and fluoride onto the alumina nanofiber surface, we have studied adsorption isotherms. The isotherm studies are conducted by varying the initial concentration of chromium(VI) and fluoride from 10 to 100 mg/L and maintaining the adsorbent dosages of 0.5 and 5 g/L, respectively. The equilibrium adsorption data obtained were analyzed using the Langmuir and Freundlich isotherm models.47−49 The Langmuir adsorption isotherm model is based on the assumption of monolayer coverage of the adsorbent surface, having a finite number of identical sites with equivalent energy. Therefore, the maximum adsorption corresponds to the formation of a saturated monolayer of adsorbate molecules [chromium(VI) and fluoride ions] on the adsorbent surface. The Langmuir equation is represented as follows: ce 1 1 = + ce qe qmb qm

(5)

(4)

Figure 8. Freundlich isotherm for the adsorption of chromium(VI) and fluoride ions by alumina nanofibers. 1559

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Table 2. Values of the Langmuir and Freundlich Constants for the Adsorption of Chromium(VI) and Fluoride Ions by Alumina Nanofibers Langmuir isotherm model chromium(VI) fluoride

Freundlich isotherm model

qm(mg/g)

b(L/mg)

R2

Kf(mg/g)(L/mg)1/n

n

R2

14.36 34.01

6.563 161.17

0.462 0.803

0.649 0.544

0.319 1.111

0.866 0.9925

of 1 h, and the adsorption kinetics was governed by the pseudosecond-order rate law. The isotherm studies indicate that the adsorption of chromium(VI) and fluoride ions onto the alumina nanofiber surface was successfully fit to the Freundlich isotherm model over all concentration ranges studied, which shows a heterogeneous adsorption process.

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ASSOCIATED CONTENT

S Supporting Information *

Comparative study on the sorption capacity of electrospun alumina nanofibers and conventional adsorbents. This material is available free of charge via the Internet at http://pubs.acs.org.

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Figure 9. Percentage removal graph of chromium(VI) and fluoride ions with a change in the dosage (gm/L) by electrospun alumina nanofibers.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] or [email protected]. Fax: 91661-2462651. Notes

The authors declare no competing financial interest.

result, we concluded that a dosage of 0.5 g/L of alumina nanofibers is good enough for the maximum removal of chromium(VI) ion present in a 10 mg/L stock solution, and the maximum percentage removal was found to be about 70% for chromium(VI) ions. In the case of fluoride ions, with an increase in the adsorbent dosage, the percentage removal increases up to 50% maximum. The enhancement of the adsorption with an increase of the dosage is due to the availability of active binding sites and to the presence of a greater surface area for adsorption of fluoride. The results showed that alumina nanofibers were efficient for 50% removal of fluoride ions at an adsorbent dosage of 5 g/L of a 10 mg/L stock solution, and hence this dosage was selected for further studies.50

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ACKNOWLEDGMENTS The authors acknowledge the DST Government of India for research funding.

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REFERENCES

(1) Lia, Y. H.; Ding, J.; Luan, Z.; Dia, Z.; Zhua, Y.; Xua, C.; Wu, D.; Wei, B. Competitive adsorption of Pb2+, Cu2+ and Cd2+ ions from aqueous solutions by multiwalled carbon nanotubes. Carbon 2003, 41, 2787. (2) Demirbas, A. Heavy metal adsorption onto agro-based waste materials: A review. J. Hazard. Mater. 2008, 157, 220. (3) Amin, Md. N.; Okada, H.; Itoh, S.; Suzuki, T.; Kaneco, S.; Ohta, K. Determination of chromium in river waters by electrothermal atomic absorption spectrometry with preconcentration on a tantalum wire. Fresenius’ J. Anal. Chem. 2001, 371, 1130. (4) Boddu, V. M.; Abburi, K.; Talbott, J. L.; Smith, E. D. Removal of Hexavalent Chromium from Wastewater Using a New Composite Chitosan Biosorbent. Environ. Sci. Technol. 2003, 37, 4449. (5) Fang, J.; Gu, Z.; Gang, D.; Liu, C.; Ilton, E. S.; Deng, B. Cr(VI) Removal from Aqueous Solution by Activated Carbon Coated with Quaternized Poly(4-vinylpyridine). Environ. Sci. Technol. 2007, 41, 4748. (6) Fathima, N. N.; Aravindhan, R.; Rao, J. R.; Nair, B. U. Solid Waste Removes Toxic Liquid Waste: Adsorption of Chromium(VI) by Iron Complexed Protein Waste. Environ. Sci. Technol. 2005, 39, 2804. (7) Hu, J.; Chen, G.; Lo, I. M. C. Removal and recovery of Cr(VI) from wastewater by maghemite nanoparticles. Water Res. 2005, 39, 4528. (8) Noroozifar, M.; Motlagh, M. K.; Gorgij, M. N.; Naderpour, H. R. Adsorption behavior of Cr(VI) on modified natural zeolite by a new bolaform N,N,N,N,N,N-hexamethyl-1,9-nonanediammonium dibromide reagent. J. Hazard. Mater. 2008, 155, 566. (9) Uysal, M.; Ar, I. Removal of Cr(VI) from industrial wastewaters by adsorption: Part I: Determination of optimum conditions. J. Hazard. Mater. 2007, 149, 482.

4. CONCLUSIONS Alumina nanofiber was synthesized by electrospinning of PVP and aluminum acetate salt precursors followed by heat treatment. XRD, SEM, EDX, and TEM characterization techniques confirmed the formation of pure nanosized alumina fibers. Batch experiments were performed for the removal of chromium(VI) and fluoride ions from aqueous solution, by using alumina nanofibers as adsorbent materials. Adsorption characterization has been examined at different pH values, initial adsorbate concentrations, contact times, and adsorbent dosages. It was observed that the efficient removal of chromium(VI) and fluoride ions was achieved at pH 5 and 7, respectively. The maximum removal was found to be 70% for Cr (VI) and 50% for fluoride ions. Furthermore, in the case of chromium(VI), with an increase in the initial concentrations, the percentage of adsorption increases because of an increase in the number of adsorption sites. The maximum uptakes of chromium(VI) and fluoride ions onto the alumina nanofiber surface were found to be 6.8 and 1.2 mg/g, respectively. Adsorption equilibrium was attained within a short contact time 1560

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dx.doi.org/10.1021/ie301586j | Ind. Eng. Chem. Res. 2013, 52, 1554−1561