Adsorptive Removal of Fluoride by Micro-nanohierarchal Web of

Apr 28, 2009 - Adsorptive Removal of Fluoride by Micro-nanohierarchal Web of Activated Carbon Fibers. Amit Kumar ... Fax: +91-512-2590104. This articl...
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Ind. Eng. Chem. Res. 2009, 48, 9697–9707

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Adsorptive Removal of Fluoride by Micro-nanohierarchal Web of Activated Carbon Fibers Amit Kumar Gupta, Dinesh Deva, Ashutosh Sharma, and Nishith Verma* Department of Chemical Engineering and DST unit on Nanosciences, Indian Institute of Technology Kanpur, Kanpur, 208016, India

This study describes the development of aluminum impregnated hierarchal web of carbon fibers for the removal of dissolved fluoride in water. Micrometer-sized activated carbon fibers (ACF) were used as a substrate to grow carbon nanofibers (CNF) by chemical vapor deposition (CVD) using Ni as a catalyst. The hierarchal web of micro-nanocarbon fibers (ACF/CNF) thus prepared was impregnated with aluminum (in its metallic state) and then tested for the adsorption of fluoride ions over the concentration range of 1-50 ppm in water under both batch and flow conditions. The adsorbent web showed significant adsorption of fluoride ions. In addition, the total fluoride uptake was observed to be larger on aluminum impregnated ACF/CNF web than on its parent material, the ACF alone impregnated with Al. Various analytical techniques including the scanning electron microscopy (SEM), Raman spectroscopy, and the elemental analyzer were employed to characterize the Al-impregnated ACF/CNF. The SEM images showed a complex 3D network of nanofibers uniformly deposited with Al. This study is a step in developing a general platform suitable for producing potable water that also specifically addresses the problem of fluoride removal. 1. Introduction According to World Health Organization norms, the upper limit of fluoride concentration in drinking water is 1.5 mg/l.1 Excessive presence of fluoride (F) in potable water continues to be a serious public health concern in many parts of the world, including India. Adsorption has shown considerable potential in defluoridation of wastewater. The viability of such technique is greatly dependent on the development of suitable adsorptive materials. In this respect, literature is replete with studies on defluoridation, albeit using mostly alumina and oxides of a few metals as adsorbents.2-10 In most of these studies, performance of the adsorbents has been evaluated under batch conditions, with temperature, pH, and dose of adsorbents being the primary variables.2-7 Few column studies have also been carried out with a view to determining breakthrough curves and throughput volume during dynamic (flow) conditions in packed beds.8-10 Traditional adsorbent like granular activated carbons (GAC) has also been tested for defluoridation, although without much success.11,12 The application of modified GAC in removal of dissolved solutes has drawn interest.13-17 Although the modified GAC showed considerable potential in removing organic pollutants such as phenolic compounds, and to some extent, arsenic and manganese in wastewater, only partial success has been reported on defluoridation. In one such study, equilibrium and kinetic studies were carried out on fluoride adsorption from water by zirconium ion impregnated coconut-fiber-derived carbon (ZICFC).18 In another study on defluoridation using aluminum (Al)-impregnated activated carbon, the adsorption of fluoride was shown to be strongly dependent on pH of the solution and as a consequence the method required pre- and post-treatment of the aqueous solution.19 On the basis of literature survey, it is fair to say that despite several studies carried out on the adsorptive removal of fluoride ions, the maximum removal efficiency reported is between 60-90%. Removal in excess of 90% has also been reported, however, in acidic medium (pH < 4), thus requiring post* To whom correspondence should be addressed. E-mail: nishith@ iitk.ac.in. Tel.: +91-512-2597704. Fax: +91-512-2590104.

treatment for preparing potable water at suitable pH. In addition to these limitations, there are several operational difficulties such as plugging, fouling, and excessive pressure-drop, encountered during defluoridation using packed beds of granular adsorbent particles, leading to periodic disruption of operation. In this study, we have described the development of a micronanohierarchal web (MiNaHiWe) consisting of activated carbon fibers (ACF) and carbon nanofibers (CNF), impregnated with Al for the removal of fluoride ions over the concentration range of 1-50 ppm in wastewater. We chose impregnation by aluminum for this study because of the known strong affinity and selectivity toward fluoride ions of the adsorbents such as activated alumina, zeolites, calcium aluminum silicate, and clay, all of which contain aluminum. The preparation steps primarily include impregnation of the fibers with the aqueous solution of aluminum nitrate, calcinations at high temperature to convert nitrate to oxide, and, subsequently, the reduction of oxide of aluminum to its metallic state in a hydrogen atmosphere. In this work, we have focused on impregnation of the carbon web with Al for fluoride removal. However, preparation of the web with other appropriate methods of activation and impregnation has the potential to develop MiNaHiWe into a multifunctional platform for catalytic and adsorptive purification of other gas and liquid mixtures. The advantages of using the hierarchal mixture of micro- and nanofibers are as follows: (1) The active surface area is increased due to creation of nanopores. (2) As shown in the manuscript, metal impregnated web (Al-CNF) performs far superior to ACF alone or ACF impregnated with metal or CNF alone, and thus a multiscale structure is found to be the best. (3) CNF has to be grown on a support like metal oxides and has to be subsequently removed and/or transferred to another porous substrate for use. The synthesis of hierarchical structure obviates the need for these multiple steps and directly provides a robust platform for adsorption applications such as making a filter. In our recent study we have successfully demonstrated the application of CNF grown this way in removing NO by adsorption and catalytic reaction.20 Inspired by our study on the application of MiNaHiWe of carbon in the gaseous phase reaction, we have carried

10.1021/ie801688k CCC: $40.75  2009 American Chemical Society Published on Web 04/28/2009

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out the present study on the Al-impregnated CNF for the removal of fluorides in the aqueous phase. We have also characterized the adsorbents for their surface characteristics using the several state-of-the-art analytical techniques including scanning electron microscopy (SEM), energy dispersive X-ray (EDX), and Raman spectroscopy. The method described in this study for the synthesis of Al-CNF is new and environmentally benign, without requiring any pre- or post-treatments. The design of the perforated tubular reactor used in this study for carrying out adsorption tests is also new. The liquid-solid contact arrangement made in the tubular reactor not only ensures uniform flow of the solution through the wrapped cloth, but also permits conduct of the tests under both batch and continuous flow conditions. In addition, the possibility of channeling or nonwetting of the adsorbent surfaces is minimal. We have used a similar type of arrangement (perforated tubular contactor) in our earlier studies on ACF for the gas-phase reactions.21-24 The experimental results in the present study demonstrate larger uptake of the solute (fluoride ions) and throughput volume of the fluoride laden water by the prepared adsorbents (Al-CNF) in comparison to those by alumina, GAC, modified GAC, and other commercial adsorbents, as reported in open literature. For comparison, Al-ACF was also prepared in this study. This study assumes significance from the perspective of developing suitable defluoridation technique for wastewater and the application of nanotechnology in the context of environmental pollution control. 2. Experimental Details 2.1. Preparation of Al-ACF Samples. The starting substrate used was the phenolic-based activated carbon fibers (Kynol Inc., Japan). The specific surface area of the prepared ACF samples was calculated by N2 adsorption. The adsorption isotherms were measured by N2 adsorption at 77 K using BET surface area analyzer (model: Autosorb 1C, Quantachrome, FL). The analysis showed typically 1200-1400 m2/g of BET area. To begin with, the as-received samples of fibers were leached by dipping them in DI water for around 6 h to remove any undesirable ions from the surface and to render the subsequent steps (impregnation, etc.) effective. The ion analysis of the spent water showed chlorides, nitrates, and phosphate ions in the range of 1-5 ppm. All chemicals used in the study were of high purity grade (Merk). The aluminum nitrate solution of 0.4 M concentration was used for impregnation. The salt concentration in excess of 0.4 M resulted in precipitation of aluminum nitrate crystals over ACF surface. The X-ray diffraction (XRD) test confirmed the existence of crystals at 0.5 M or larger concentration. The samples impregnated with relatively larger nitrate concentration solution also showed insignificant adsorption of fluoride ions due to the blocking of pores of ACF by the precipitated crystals. In our earlier studies on the application of metal impregnated ACF in the adsorptive or catalytic removal of the gaseous species, we observed the similar adverse effect because of the precipitation of the crystals at relatively larger concentration levels.21 Figure 1 is the schematic of the experimental setup used in the study. Impregnation was carried out in an inconel made tubular reactor (i.d. ) 2.5 cm, o.d. ) 2.8 cm, L ) 6 cm) mounted inside an inconel shell (i.d. ) 4.0 cm, L ) 10 cm) with provision made for the water inlet and outlet. One end of the tube was closed. The outer surface of the tube was perforated with holes of diameter 0.1 mm at center-to-center distances of 0.4 cm. ACF was wrapped over the perforated section of the

Figure 1. Schematic of the experimental setup for the preparation of impregnated activated carbon fiber and adsorption studies.

Figure 2. Schematic of experimental setup for drying/calcination/reduction of impregnated ACF.

tube. With the aid of a peristaltic pump, the solution in a glass container was continuously recirculated to the inlet of the tube, as shown in the schematic. Valve A was kept closed and B was kept open. It was observed that this method (impregnation under flow condition) resulted in uniform dispersion of the solutes in ACF as compared to other traditional methods like impregnation by continuously stirring ACF in salt solution or leaving ACF in the solution under batch condition without stirring. Additionally, the total time of impregnation was also considerably reduced (less than 6 h) in the case of continuous recycling, as against 12-14 h taken under the stagnant condition. After impregnation, the tube-shell assembly was taken out without removing the wrapped ACF for further processing of the sample. First, the sample was dried for 3 h at 50 °C in a small nitrogen flow to remove surface moisture. Drying and the subsequent treatment were carried out in a vertical quartz tube as schematically described in Figure 2. After drying, the sample was calcined in an oxygen flow at 300 °C for 2 h to convert nitrate into oxide. Following calcination, the sample was cooled to room temperature. To convert oxide into the corresponding metallic state, reduction was carried out in the hydrogen flow of 0.16 splm and at 350 °C for 1 h. The reduced sample (Al-ACF) is ready for testing. The stock solution of 100 mg/L fluoride was prepared by dissolving 221 mg of anhydrous NaF in 1 L of distilled water for the subsequent experiments. 2.2. Preparation of CNF and Al-CNF. The method of growing CNF on ACF is described in our recent study and is not reproduced here for the sake of brevity.20 In brief, ACF was impregnated with the nickel nitrate hexahydrate solution prepared in acetone, subsequently calcined at high temperature and then reduced in a hydrogen atmosphere to convert the salt of Ni to its metallic state, the procedure of impregnation to

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reduction being identically the same as that followed in preparing Al-ACF. CNF was subsequently grown on the prepared Ni-ACF in a CVD reactor at the reaction temperature of 1023 K using benzene as a source of carbon. The hierarchal web of ACF/CNF was then sonicated in dilute nitric acid to remove Ni from the tips or the surfaces of the grown CNF. Some samples of CNF without sonication were also prepared for the comparative study. The prepared CNF was reimpregnated with Al(NO3)3 and subsequently processed to prepare Al-CNF. 2.3. Batch (Equilibrium) and Column (Flow) Studies. A fixed amount of the adsorbent (either ACF, Al-ACF, CNF, or Al-CNF) was put in 150 mL of aqueous solution of sodium fluoride. The temperature of the solution was maintained constant. The experimental arrangement was the same as in the previous case (Figure 1). The solution was periodically collected and analyzed for fluoride ions using METROHM ion chromatography (model: IC 861). The measurement was carried out until the concentration of the solution remained unchanged and adsorption equilibrium concentration was attained. The procedure was repeated for varying amounts of the sample, temperature, and initial fluoride concentration. It may be pointed out that similar to the previous case (impregnation of ACF with Al), total time to reach equilibrium was considerably reduced by recirculation of the solution. However, the total amounts of fluorides adsorbed, that is, equilibrium concentrations, were found to be approximately the same (within a difference of (5%) in two cases (with and without flow). The arrangement for the experimental setup used for conducting dynamic (breakthrough) tests was the same as that for impregnation and equilibrium studies, except that the valve A was kept open and B was kept closed. The solution was continuously delivered at a constant flow rate with the aid of a peristaltic pump to the perforated tubular reactor wrapped with the Al-ACF/Al-CNF sample. To begin with, DI water was pumped through the tube. At t ) 0+, a step change in the inlet concentration was induced by switching the inlet to the tube to the fluoride solution of a certain concentration. Samples for the concentration measurement were collected at the exit of the tube over a certain time interval, until the exit concentration level reached approximately that at the inlet to the tube. The entire procedure was repeated for different amounts of the samples, pump speed (solution flow rate), and inlet fluoride concentration. 3. Batch (Equilibrium) Study The concentration of solute in the solid phase was theoretically determined by the mass balance: q )V(C0 - C)/w, where C0 and C are the aqueous phase solute concentrations before and after equilibrium is attained. V is the volume of the solution in contact with the adsorbent of weight w. Figure 3 presents the equilibrium concentrations of the ions in the solid phase as a function of the aqueous phase concentration for five types of samples: ACF, Al-ACF, CNF, and Al-CNF with and without sonication (acid treatment). The best fits to these data are essentially the equilibrium isotherms of fluoride ions versus different samples at 35 °C over the aqueous phase concentrations between 0.1 and 50 mg/L. For each sample, concentrations in the solid phase are observed to monotonically increase with increasing solution concentrations and are best explained by the Freundlich isotherm in the aforesaid concentration range, q ) KCn, where q is the amount of adsorbed fluoride ion in mg per g of ACF, K is the Freundlich constant, and n is the power of isotherm. The values of the constant n less than unity indicate a favorable adsorption for the systems reported here.

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Figure 3. Comparative performance of Al-CNF (with and without sonication), Al-ACF, CNF, and ACF for fluoride removal (solid lines show Freundlich isotherms, q ) KCn; where K ) 1.30, 0.85, 0.59, 0.17, and 0.06 and n ) 0.61, 0.69, 0.75, 0.59, and 0.41 for Al-CNF (with and without sonication), Al-ACF, CNF, and ACF, respectively).

From the data shown in Figure 3, we conclude that the equilibrium loading of fluoride is larger on Al-CNF than on Al-ACF. The equilibrium loading of fluoride is also larger (approximately 10 times) on Al-CNF than CNF. In addition, sonication appears to have enhanced the equilibrium capacity of the prepared nanofibers, possibly due to the dislodging of Ni catalysts from the tips and other surfaces of the grown CNF, thereby creating additional sites for the incorporation of Al in the subsequent impregnation step. Considering the positive effect of sonication, all the subsequent data presented in this paper for Al-CNF refer to the samples prepared with sonication, unless specified otherwise. The equilibrium experiments were also carried out at varying temperatures (35, 50, 70 °C). However, there was practically no change or a marginal decrease observed in equilibrium concentration with increase in the bed temperature from 35 to 70 °C. The heat of adsorption for fluoride ions on Al-CNF was calculated and found to be approximately -3.0 kJ/mol, suggesting that adsorption of fluoride on Al-CNF is (slightly) exothermic. This value compares to 7.39 kJ/mol for that of fluoride on carbon slurry25 and -20 kJ/mol on plaster of Paris as reported in the literature.26 To this end, it is important to compare the loading (mg/g) of fluoride ions on Al-CNF obtained in this work with those reported in the literature for fluoride ions on different adsorbents such as activated carbon granules, activated alumina, and natural clays. From Figure 3, we note that the loading of fluoride ions on Al-CNF is 0.25-17 mg/g corresponding to the aqueous phase fluoride concentrations between 0.06-50 ppm. These values compare to 0.58 mg/g on activated alumina corresponding to 1 ppm of fluoride in water,6 1-5 mg/g on the zirconium impregnated coconut fiber carbon corresponding to 20-100 ppm of fluoride in water,18 and 2 mg/g on clays4 and 2.7 mg/g on activated titanium rich bauxite,10 both corresponding to 10 ppm of fluoride ions in the solution. In another study, fluoride ions loading of 16.5 mg/g was obtained on magnetic chitosan particles corresponding to the aqueous phase concentration of 100 ppm,27 which is nearly the same as that obtained in this work on Al-CNF. We have also noted that the reported literature values for the maximum loading of fluoride ions on a

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carbon or alumina. We now turn our attention to the study on the adsorption of fluoride ions by Al-CNF under dynamic conditions. 4. Breakthrough Characteristics

Figure 4. Breakthrough data for varying inlet fluoride concentration (WAl-CNF ) 3 g, Q ) 0.01 lpm, T ) 303 K).

few inorganic sorbents is ∼1 mg/g or less corresponding to the aqueous phase concentration of 1 mg/L or less. From the data presented in this section of the paper, we conclude that surface loading (equilibrium concentration) of fluoride ions on Al-CNF in this study is larger than those reported in literature, either on

In the typical experiments performed to study breakthrough characteristics using the perforated tubular reactor wrapped with fresh (unsaturated) carbon fibers, the aqueous solution of fluoride at a predetermined concentration was fed at a constant flow rate to the reactor. One can make a reasonably good estimate of the effective pore-diffusivity by fitting the experimental data with the theoretical breakthrough curve. Such technique is commonly used in estimating pore-diffusivity for the porous solids whose pore-size distribution may not be accurately determined experimentally. From the many breakthrough data collected in this work, we present here only representative results. Figure 4 describes solute concentrations, Cexit measured at the exit of the packed column of Al-CNF. The data are presented relative to the inlet concentration, Cin. As observed the exit concentration gradually increases, initially at a slow rate, until the outlet concentration asymptotically reaches the inlet concentration. At this time, the packed bed is saturated with fluoride ions and lacks capacity for further adsorption. As also observed, breakthrough occurs earlier for the relatively larger inlet concentration. It is also pointed out that breakage of nanofibers and release of nanometal particles were not observed during the experiment. As regards the importance of interparticle mass transfer coefficient, we have earlier pointed out that the diameter of carbon fibers is small (ca. 1-10 µm). As a result, the fluid to

Figure 5. SEM images of ACF (a) as-received, (b and c) post Al-reduction, and (d) EDX spectra of Al-ACF.

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Figure 6. SEM images of ACF (a) impregnated with nickel nitrate, (b and c) post Ni- reduction, and (d) EDX spectra of Ni-ACF.

particle film mass transfer coefficient is significantly large in the packed bed of fibers even at small liquid flow-rate. Theoretical calculations indeed showed a typically large value of the liquid film mass transfer coefficient (of the order of 0.005 m/s) under moderate or small particle Reynolds numbers in the packed bed of Al-CNF. This is in contrast to the case of adsorption in packed beds of larger particles, such as alumina pellets or charcoal beads. In these cases, the mass transfer coefficient may be small at small liquid flow-rates, since the size of pellets or beads range from a few mm to cm. As a result, the mass transfer rate may be slow because it is controlled by interparticle diffusion. Figure 4 also describes theoretical curves obtained corresponding to the experimental conditions. The theoretical analysis is essentially based on three governing conservation equations: (a) species balance of the adsorbing component in the packed bed, (b) species balance of the component inside the pores of the fiber, and (c) surface adsorption and desorption rate within the pores. The analysis takes into account both inter- and intraphase diffusion resistances. In the present analysis, the governing equations (see appendix) are cast in the similar way as in our previous studies on the application of ACF in control of contaminants in gaseous phase.21-24 In the present case, the numerical value of pore-diffusivities for fluoride ions was adjusted at 5.5 × 10-10 m2/s to fit the data. One can calculate molecular diffusivity of these ions in dilute solution by Nearst-Haskell equation.28 On comparison it may be shown that pore diffusivity of fluoride ions in the pores of carbon fibers is 1 order of magnitude smaller than molecular diffusivity (∼1.0 × 10-9 m2/s). From the perspective of mass transfer rate, it is important to note that the characteristic time of intraparticle

diffusion in a pellet or fiber of diameter, df is proportional to the square of the particle or fiber size, that is, df2/Dpore, where Dpore is the effective pore diffusivity. Since CNF has small fiber size, characteristics time of pore-diffusion in fibers is much smaller than that in a pellet or bead. Thus, in the packed beds of the former, separation is seldom controlled by intraparticle diffusion. 5. Surface Characterization 5.1. SEM/EDX Analysis. The morphology of Al-ACF and Al-CNF with and without sonication was examined by SEM and EDX analysis using the Supra 40 VP Field Emission SEM procured from Zeiss. All images were captured with inlens detector at accelerating voltage of 10 kV and filament current 2.37 A at a working distance of 2-4 mm. The SEM images and EDX spectra of prepared samples were taken at a number of locations on the samples. A representative image and the corresponding EDX spectra of samples at different stages of the preparation are presented in Figures 5-8. As observed from the SEM image of the as-received ACF, shown in Figure 5a, the surface of the carbon fibers is smooth, with the diameter in a narrow range of 10-20 µm. On impregnation with the salts of Al followed by reduction, the surface of the fiber was uniformly dispersed with fine Al particles, as observed in the SEM images taken at different magnifications (see Figures 5b and 5c). Agglomeration of Al particles at a few locations may also be observed. The corresponding EDX spectra also confirmed the uniform dispersion of Al metal ions over the concentration range

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Figure 7. SEM images of CNF grown (a and b) as-grown, (c) EDX spectra on the tip of CNF, (d) EDX spectra on surface of CNF.

between 1.5 and 2.5% (wt/wt). One of such spectra is shown in Figure 5d. The SEM image in Figure 6a shows the homogeneous dispersion of nickel nitrate on the surface of ACF following the impregnation and before the calcinations step. Figure 6 panels b and c describe the SEM images of the ACF surface at different magnifications after the reduction step. Fine nanosized particles may be observed to be uniformly dispersed on the surface of the fiber. No sign of any deformation and loss of integrity or strength were found in the samples. The dispersed particles on the surface of ACF were confirmed as nickel particles using EDX spectra (see Figure 6d), with the concentrations obtained in the range, 3.0-3.5% (w/w). CNF were grown uniformly and densely on ACF as shown in Figure 7a,b corresponding to 400K× and 100K× magnifications, respectively. The diameter of most of the CNF is observed to be within the range of 30-40 nm and the length up to several micrometers. The bright, spherical Ni particles of nanosize may be observed at the tip of the CNF, with the size nearly the same as the diameter of CNF (see Figure 7a). However, in some cases, the diameter of grown CNF need not be the same as that of the Ni particles, since the Ni metal particles may undergo agglomeration, sintering, and fragmentation during the growth of CNF.29 The EDX spectra shown in Figure 7 c confirmed the

presence of Ni particles with the concentration ca. 20-23% (w/ w) at the tips. There were some Ni particles dispersed on the surface of ACF/CNF which could not catalyze the formation and subsequent growth of CNF (see Figure 7b). The corresponding EDX spectra (see Figure 7d) revealed the concentration ca. 4.0-20% (w/w) in such cases. As also observed from the SEM images, the nanofibers are not straight but have a crooked morphology with three-dimensional network structure. The SEM images shown in Figure 8a,b describe the morphology of CNF after sonication which essentially removed most of the Ni catalysts from the grown CNF. The density distribution and average diameter of CNF remained almost unchanged after sonication. However, small fractions of CNF were found to be removed during sonication. The EDX spectra shown in Figure 8c confirmed the removal of most of the Ni particles, with the concentration of the Ni particles remaining on the surface in the small range of 0.8-1.5% (w/w). Figure 8 panels d and e show the SEM images of sonicated Al-CNF. As observed, there is a distinct change in the morphology of CNF following loading with Al. The fibers were observed to be interwoven, possibly due to the agglomeration. The EDX spectra shown in Figure 8f revealed the presence of 5-7% (w/w) of Al along with 0.4-1.2% (w/w) of Ni.

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Figure 8. SEM images of CNF grown on ACF: (a and b) after acid treatment/sonication; (c) EDX spectra of CNF after acid treatment/sonication; (d and e) post Al-reduction, and (f) EDX spectra of Al-CNF.

5.2. Raman Analysis. The motivation of using Raman spectra was to characterize the ACF and CNF at various stages of preparation for ascertaining whether the material is amorphous or graphitic. The laser Raman spectra of all the samples were taken using the confocal Raman instrument (Alpha model; Witec, Germany). The data were collected using the Ar-ion laser of wavelength 514 nm as the excitation source and CCD as detector in the range of 800-2000 cm-1 at room temperature in air. Figure 9 describes the Raman intensities of ACF, Ni-ACF and CNF grown on ACF. Table 1 presents the corresponding data extracted from the Raman graphs.

As observed, there are two peaks for carbon in the Raman spectra. The Raman peak in the range of 1250-1450 cm-1 correspondes to D-band and is attributed due to the disordered components of carbon, whereas the peak in the range of 1550-1600 cm-1 corresponds to G-band and is attributed to the ordered component (graphite). The ratio ID/IG reflects the extent of disorder in carbons. For carbon nanotubes (CNT) grown on stainless steel (SS), the ratio is usually less than 1, whereas for CNF grown on low carbon mild steel (MS), the ratio is greater than 1.30 In another study, the ratio for CNF grown by the pyrolysis technique is reported to be 0.847.31 In

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Ind. Eng. Chem. Res., Vol. 48, No. 21, 2009 Table 3. Total Uptake of Fluoride during Adsorption on Al-CNF under Flow (Continuous Condition (Fiber Diameter ) 4.6 × 10-7 m) sample Cinlet W Q VR × 103 km × 103 Q∆t uptake of no. (mg/L) (g) (lpm) (m/s) (m/s) (L) fluoride (mg) 1 2 3 4 5 6 7 8 9

5 15 30 5 5 5 15 15 15

3 3 3 3 2 1 2 2 2

0.01 0.01 0.01 0.03 0.03 0.03 0.03 0.02 0.01

0.05 0.05 0.05 0.15 0.15 0.15 0.15 0.10 0.05

5.0 6.0 7.5 14.5 11.6 9.7 11.6 7.6 7.5

4.1 3.0 2.4 4.3 3.5 1.8 3.1 3.0 3.0

10.4 21.1 27.7 11.4 7.7 3.7 14.5 14.4 14.5

nificant increase in the carbon contents after CVD is obvious due to the deposition of amorphous carbon on Ni-ACF. Subsequent decrease in the carbon contents after acid treatment was due to the removal of excess soot-carbon in the sonication step. 6. Total Uptake of Fluorides and Throughput Volume

Figure 9. Raman spectrum of (a) ACF, (b) Ni-ACF, and (c) CNF grown on ACF. Table 1. Laser Raman Spectra Parameters peak shift (cm-1)

area

no.

sample

D-band

G-band

D-band

G-band

ID/IG

1 2 3

ACF Ni-ACF CNF

1331 1333 1328

1591 1596 1591

626583 24826 117779

245667 8833 40635

2.55 2.81 2.89

Table 2. CHN Analysis of Carbon Samples at Various Stages of Preparation no.

sample

%C

% residue % H % N (mostly Ni and O)

1 2 3 4 5

ACF Ni-impregnated Ni-reduced CNF (preacid treatment) CNF (postacid treatment)

94.91 68.50 73.16 84.20 76.88

1.24 1.72 1.64 1.32 0.99

0.85 3.77 1.59 1.07 2.05

7.18 36.99 30.07 18.19 26.16

addition, the smaller is the ratio, the more highly ordered is the graphitic structure. In the present analysis, the ID/IG ratio of ACF was determined to be large, indicating the presence of disordered graphite components in the parent substrate material, that is, ACF. This disorder is observed to further increase during nickel impregnation. As observed, CNF grown on ACF also retain the short-range graphitic characteristics with an ID/IG ratio ) 2.89. 5.3. Elemental Analysis. Elemental analyzer was used to ascertain change in the elemental compositions of ACF following impregnation with Ni for growing CNF. The composition variation, predominantly in carbon, was investigated by elemental (CHN) analysis (Exeter Analytical Inc. made analyzer, model: CE 440). The data are presented in Table 2. As observed, the amount (percentage by weight) of carbon decreased in the Ni-impregnated sample and that of nitrogen increased as compared to ACF, due to the incorporation of nickel nitrate in ACF. Following calcinations and reduction steps, the carbon contents expectedly increased, although marginally. The sig-

The data discussed in this section pertain to the tests carried out on Al-CNF packed in a tubular reactor, with the flow Q and inlet concentration of fluoride solution set at prescribed values (see Figure 1). Valve A was kept open and B was kept closed. It is pointed out that breakthrough characteristics is a true reflection of the performance of an adsorbent under dynamic condition. In principle, steady-state conditions never exist during adsorption, except at the final stage of the operation when the adsorbent is nearly saturated with adsorbate, denoting asymptotic attainment of equilibrium. At this instance, the aqueous phase concentration at the exit of the adsorber equals that at the inlet to the adsorber. In such a process, two important parameters that may be used to describe the relative effectiveness of adsorbents are the throughput volume (total volume of feed solution passed through the bed until the bed is saturated) and the total uptake of the solute (fluoride in the present case) during that period. The former is calculated as Q∆t, where ∆t is the time spent until saturation of the bed, whereas the latter is determined from the breakthrough curve (i.e., hatched area marked in Figure 4 for the uppermost curve) by calculating the integral along the curve between the two time limits, the incipience of adsorption test and the instance when the bed is saturated. Mathematically the uptake of the solute (mg) is determined as follows: uptake (mg) ) Q(CinT -

∫

T

0

Cexitdt)

(1)

where T is the total time of adsorption until the bed is saturated with the solute in the influent stream. Table 3 summarizes the numerical values of two parameters, throughput volume and total uptake of fluoride ions, experimentally obtained under various conditions of concentration, flow-rate, and the amount of Al-CNF. Here, only representative data are presented for brevity. There are two salient observations that can be made. One, the fluid-to-particle (film) mass transfer coefficient is nearly independent of the fluid or solution flow-rate suggesting insignificant influence of the interparticle diffusion resistance on the removal of solutes from the fluid phase, under the experimental conditions used in the study. The numerical value (∼5 × 10-3 m/s) of km obtained in this study compares to 5.6 × 10-5 m/s reported by Pant and Ghorai8 for the packed bed of alumina particles (average size ) 2.5 mm), with the superficial

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Table 4. Comparison of Reported Values of Throughput Volume of the Liquid and Total Uptake of Fluoride in a Tubular Packed Bed Adsorber no.

parameters

this study

Pant and Ghorai8

Maliyekkal et al.9

1 2 3 4 5 6 7 8 9 10 11

adsorbent fiber or particle size (m) column I.D. (m) solution flow rate (lpm) superficial liquid velocity (m/s) km (m/s) amount of adsorbent (g) inlet fluoride concentration (mg/L) throughput volume (L) uptake (mg) specific uptake (mg/g)

Al-CNF 1 × 10-7 0.0198 0.01 5.0 × 10-3 0.005 3 5 4.1 10.4 3.47

activated alumina 3.5 × 10-3 0.0508 0.03 2.47 × 10-4 5.6 × 10-5 200 5 65 150 0.75

Mg-coated alumina 0.8 × 10-3 0.028 0.0225 6.09 × 10-4 310 (estimated) 3.56 100 267 0.86

velocity of the aqueous solution of fluoride ions at 0.003 m/s. Clearly, the small fiber size has caused significant improvement in the mass transfer coefficient. Two, the effect of fluoride concentration in the influent has larger effect on the uptake of the solute than on the throughput volume. Comparing the data for these two quantities in rows one and two in Table 3, total amount of fluoride removed is approximately doubled with increase in the inlet concentration from 5 to 15 ppm, whereas the throughput volume decreased by approximately 25%. The results are consistent with the breakthrough profiles shown in Figure 4. Although total adsorption time increases by not more than 25% with decrease in the influent concentration, total amount of solute removed varies significantly as the curve tends to be more concave down at low concentration than at high concentration, resulting in relatively larger area above the breakthrough curve. Similar concentration effects may be observed from the data reported in rows one and three of Table 3. With increase in the concentration from 5 to 30 ppm, the uptake amount is approximately tripled, whereas the throughput volume is halved. From the foregoing discussion, we conclude that the throughput volume and solute uptake are two parameters that must be used in screening adsorbents and comparing the relative performance of adsorbents under varying experimental conditions. We also point out that in this regard, most of the literature data on the removal of fluoride by adsorbent pertain to equilibrium conditions (under stirred condition in a batch reactor). The ions-loading in such condition may not truly reflect the removal efficiency of an adsorbent. In fact the adsorptive performance of an adsorbent must be ascertained under dynamic conditions, taking into account the influence of various diffusion resistances and finite adsorption and desorption rates. We close this section of the discussion by comparing our results to the limited published data on the throughput volume and fluoride uptake in a packed column of activated alumina beads.8,9 It is pointed out that we numerically determined the value of fluoride uptake from the breakthrough curves reported in the aforesaid studies using eq 1. Therefore, comparison may be made within the experimental and numerical errors only. As observed from Table 4, the throughput volume of the fluoride solution and uptake are 4.1 L and 10.4 mg corresponding to 3 g of Al-CNF used in this study, in comparison to 65 L and 150 mg, respectively corresponding to 200 g of activated alumina used by Pant and Ghorai.8 In another study the two parameters are obtained to be 100 L and 267 mg on Mn oxide coated alumina beads.9 In the latter study, the total amount of the adsorbent was 310 g, whereas the fluoride concentration in the influent stream was 3.56 mg/L. Thus, the comparative data presented in Table 4 is the obvious indicator of the superior

performance of Al-CNF in comparison to alumina when adsorbents are used under dynamic (flow) conditions. It is also important to note that (1) there are two types of sites on the surface of the synthesized material, Al-CNF. On the vacant sites unoccupied by metals, the adsorption (physisorption) of fluoride is reversible, whereas on the sites occupied by metal, the adsorption (chemisorption) of fluoride is irreversible,32 (2) although it is sensible to compare the performance of adsorbents using mg F/g adsorbent as metric, the comparison may also be made using mg F/m2 adsorbent. In the present study, the BET surface area of parent material, ACF was determined to be 1200-1400 m2/g, which is significantly larger than that of the other commercial adsorbents such as activated alumina (250 m2/g). The BET area of Al-CNF (due to N2 physisorption) was determined to be ∼850 m2/g, whereas the active metal sites area was measured by H2reduction (due to chemisorption) and determined to be ∼0.26 m2/g. At 5 mg/L of aqueous phase fluoride concentration, the solid-phase fluoride concentration on Al-CNF was determined to be ∼0.004 mg/m2, using the BET area as a basis, and ∼13.3 mg/m2, using the metal active surface area as a basis. The former compares favorably to ∼0.003 mg/m2 obtained for activated alumina. We also determined the amount (∼7.8 mg Al/g of Al-CNF) of aluminum impregnated on Al-CNF, using atomic absorption spectroscopy (Varian AA-240). From this measurement, fluoride concentration on Al-CNF may also be calculated as ∼0.44 mg F/mg Al at the aqueous phase fluoride concentration of 5 mg/L. 7. Variation in the Solution pH As shown below, pretreatment of the feedwater is not required while using the Al-CNF for treating the wastewater with pH between 5.0 and 8.0, which is also the typical pH range of the fluoride-rich potable water. However, under extreme conditions of very high or very low pH, pretreatment may be required. We have not performed any experiments on the extreme pH ranges to assess their effects (if any) on the web performance and stability. Figure 10 describes variation in pH values during fluoride removal at different initial fluoride concentration and solution temperature. As observed, initial pH value is relatively lower at larger fluoride concentration. The variation is in the range between 6.0 and 6.75 corresponding to the initial fluoride concentrations of 5 to 50 ppm. However, pH level thereafter remains approximately the same. Following the attainment of equilibrium, approximately maximum 4% increase is observed. For example, pH level in the solution of 5 ppm of initial fluoride concentration increases from 6.5 to approximately 6.75, whereas that of 50 ppm-solution increases from 6 to 6.5 only. From the results it may be inferred that the post-treatment (after defluo-

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ment of Science and Technology (DST), New Delhi in the form of research grants and through its Center on Nanosciences. The authors also acknowledge the supply of ACF from Kynol Inc. Japan. Appendix A. Governing Equations. 1. Species Balance of the Component in the Perforated Bed.

( )

DR ∂ ∂CG ∂CG ∂CG + VR R + ∂t ∂R R ∂R ∂R

( 1 -ε ε )k a(C m

G

- CFs) ) 0

2. Mass Balance of the Component Inside the Pores of the Fiber. R

∂Cp ∂Cs 1 ∂ + 2 (r2Nr) + a* )0 ∂t ∂r ∂t r

3. Adsorption/Desorption within the Pores. ∂Cs ) kaCG - kdCsn ∂t

Figure 10. Variation of pH levels at different initial fluoride concentrations.

ridation) of water containing fluoride at low concentration levels (