Adsorbents based on carbon microfibers and carbon nanofibers for the removal of phenol and lead from water

Journal of Colloid and Interface Science 359 (2011) 228–239

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Adsorbents based on carbon microfibers and carbon nanofibers for the removal of phenol and lead from water Anindita Chakraborty a, Dinesh Deva b, Ashutosh Sharma a,b,⇑, Nishith Verma a,⇑ a b

Department of Chemical Engineering, Indian Institute of Technology Kanpur, Kanpur 208 016, India DST Unit on Nanosciences, Indian Institute of Technology Kanpur, Kanpur 208 016, India

a r t i c l e

i n f o

Article history: Received 13 December 2010 Accepted 17 March 2011 Available online 30 March 2011 Keywords: Activated carbon fiber (ACF) Carbon nanofiber (CNF) Phenol Lead Adsorption

a b s t r a c t This paper describes the production, characteristics, and efficacy of carbon microfibers and carbon nanofibers for the removal of phenol and Pb2+ from water by adsorption. The first adsorbent produced in the current investigation contained the ammonia (NH3) functionalized micron-sized activated carbon fibers (ACF). Alternatively, the second adsorbent consisted of a multiscale web of ACF/CNF, which was prepared by growing carbon nanofibers (CNFs) on activated ACFs via catalytic chemical vapor deposition (CVD) and sonication, which was conducted to remove catalytic particles from the CNF tips and open the pores of the CNFs. The two adsorbents prepared in the present study, ACF and ACF/CNF, were characterized by several analytical techniques, including SEM–EDX and FT-IR. Moreover, the chemical composition, BET surface area, and pore-size distribution of the materials were determined. The hierarchal web of carbon microfibers and nanofibers displayed a greater adsorption capacity for Pb2+ than ACF. Interestingly, the adsorption capacity of ammonia (NH3) functionalized ACFs for phenol was somewhat larger than that of the multiscale ACF/CNF web. Difference in the adsorption capacity of the adsorbents was attributed to differences in the size of the solutes and their reactivity towards ACF and ACF/CNF. The results indicated that ACF-based materials were efficient adsorbents for the removal of inorganic and organic solutes from wastewater. Ó 2011 Elsevier Inc. All rights reserved.

1. Introduction Industrial effluents are the primary source of toxic organic compounds and heavy metals. Contamination of groundwater by pollutants via percolation or run-off through soils and landfills is a major concern. Thus, the development of adsorbents based on carbon fibers for the removal of organic solutes and metal ions from wastewater was the primary focus of the present study. Phenol and lead are common pollutants present in wastewater and were used as examples of organic and inorganic adsorbates, respectively. The primary sources of wastewater contaminated with phenol are the industries producing dyes, pesticides, insecticides, explosives, plastics, leather, paint, and pharmaceutical products [1,2]. Industries responsible for the discharge of lead in wastewater include battery manufacturing, printing, painting, and dying [3,4]. Activated carbons (ACs) prepared from various precursors, such as bagasse ash, wood, agricultural wastes, and coconut shells, have been extensively used for the adsorptive removal of phenol and lead from wastewater [5–12]. Recently, activated carbon fibers ⇑ Corresponding authors. Address: Department of Chemical Engineering, Indian Institute of Technology Kanpur, Kanpur 208 016, India (N. Verma). Fax: +91 512 2590104. E-mail addresses: [email protected] (A. Sharma), [email protected] (N. Verma). 0021-9797/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2011.03.057

(ACFs), carbon nanotubes (CNTs), carbon nanoparticles, and carbon microspheres have been used as adsorbents for the aqueous phase removal of phenol and metallic ions, including Pb2+ [13–18]. In our recent studies, a phenolic resin precursor based on ACF was prepared from non-activated fabrics for the removal of gaseous phase pollutants [19–22]. More recently, we developed a hierarchal web of carbon microfibers and nanofibers, which were effective in the removal of NOx from air [23], and fluoride and arsenic (V) ions from water [24,25]. The multiscale carbon web was prepared by growing CNF on ACF by catalytic chemical vapor deposition (CVD) using Fe or Ni as a catalyst and benzene as a carbon source. The ACF/CNF composite consisted of a web of carbon microfibers and carbon nanofibers with hierarchal pore sizes and fiber diameters. Unlike CNF (or CNT) grown on metal oxides or silica, ACF does not have to be removed from the final product, and the web can be used directly in adsorptive applications. In the present study, the surface of ACF was functionalized with NH3, a basic reagent, to enhance the adsorption of phenol, which is a Lewis acid. The results indicated that surface functionalized ACF possessed a larger adsorption capacity for phenol than ACF/CNF composites. Alternatively, the hierarchal web of ACF/CNF displayed a larger adsorption capacity for Pb2+ than ACF. To further improve equilibrium loading, ACF/CNF was sonicated in an acidic medium to remove Ni particles from the tip of the CNFs. Compared to the

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results obtained under extreme sonication conditions, the texture of CNF was retained after sonication under mild conditions. As a result, a uniform adsorption of Pb2+ was achieved on the surface of the material.

2. Materials and methods 2.1. Adsorbents 2.1.1. ACF The ACF-based phenolic resin precursor was procured from Kynol Inc., Tokyo (Japan) and was soaked in de-ionized (DI) water for 6 h to remove impurities and to enhance the effectiveness of subsequent preparation steps (metal impregnation or surface functionalization). The spent water was analyzed, and ions, such as chlorides and nitrates, were detected in the solution. Treated ACF samples were dried in a vacuum oven at 200 °C for 12 h.

2.1.2. Surface functionalization of ACF with NH3 Vacuum dried ACFs were treated with NH3 at elevated temperature. Fig. 1a shows a schematic of the set up used for the vapor phase functionalization of ACFs. The same apparatus was also used to prepare ACF/CNF via calcination, reduction, and CVD, as described later in the text. ACF (approx. 3 g) was wrapped on an Inconel 625 (an alloy of nickel–chromium–molybdenum) perforated, tubular (I.D. = 2.5 cm, O.D. = 2.8 cm, L = 6 cm) reactor. One end of the tube was sealed, and the outer surface of the tube was perforated with 0.1-mm holes at center-to-center distances of 0.4 cm. The tube was connected to a nitrogen source and mounted in a vertically movable furnace. The temperature of the furnace was controlled with a thermostat and a PID controller. Nitrogen was used as a carrier gas and was bubbled at a constant flow rate of 100 cc per min (measured under standard conditions) through a bubbler containing 5 M NH3. The temperature of the bubbler was set to 40 °C. The vapor phase concentration of NH3 in nitrogen was equal to the saturation vapor pressure of the liquid and was approximately 15% (v/v).

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The temperature of the perforated tube-furnace was increased at the rate of 10 °C/min under a constant flow of nitrogen. The contact temperature for functionalization was varied between 400 °C and 750 °C, and the contact time was set to 1 h. Contact between the vapor phase and the solid phase resulted in the covalent attachment of nitrogenous functional groups on the surface of ACF, as later described in the paper while discussing the FT-IR data. After functionalization, the system was cooled to room temperature by purging with nitrogen. 2.1.3. Preparation of ACF/CNF The hierarchal web of ACF/CNF was prepared by growing CNF on ACF by the CVD of benzene in the presence of a Ni catalyst. The details of the synthetic method are described in the literature [24]. In short, the preparation of ACF/CNF was achieved by impregnating ACF with nickel nitrate, followed by calcination, reduction by hydrogen, and CVD. In the wet incipience method, a 0.4 M solution of nickel nitrate salts in acetone was used to impregnate ACF with nickel nitrate. All the other solutions, including those for the adsorption study, were prepared with Milli-Q water, which was obtained from an ultrapure water purification system (Millipore, Milford, CT). Vacuum dried ACF (approx. 3 g) was wrapped on a perforated inconel 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). The reactor was equipped with a solution inlet and outlet, and one end of the tube was sealed. The outer surface of the tube was perforated with 0.1-mm holes at center-to-center distances of 0.4 cm. With a peristaltic pump, the solution in the glass container was continuously re-circulated at a constant flow rate through the ACF-wrapped tubular reactor. Impregnation was carried out under a continuous flow for 6 h. Fig. 1b shows a schematic depiction of the experimental set up. After impregnation, the sample was dried for 6 h in air and an additional 6 h at 50 °C inside a hot air-oven to remove moisture. Thereafter, the sample was calcined for 1 h in the oven at 300 °C to convert Ni(NO3)26H2O to NiO. The reaction is as follows:

Fig. 1a. Experimental set up used for calcination, reduction, CVD, and functionalization.

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Fig. 1b. Experimental set up used for impregnation.

NiðNO3 Þ2  6H2 OðsÞ ! NiOðsÞ þ 2NO2 ðgÞ þ 3=2O2 ðgÞ þ 6H2 OðgÞ In a previous study, metals impregnated ACF was prepared for the catalytic oxidation of volatile organic compounds (VOC) [21]. The method of preparation included the impregnation of ACF with the nitrate solutions of a few metals, including Ni, followed by drying and calcination. The operating conditions were the same as used in the present study. Using the X-ray diffraction (XRD) and temperature-programmed reduction (TPR) analysis, it was demonstrated that nickel nitrate is converted to nickel oxide during calcinations. After calcination, NiO was reduced to the metallic state by treatment at 550 °C under a flow of hydrogen at a rate of 0.16 L/min (measured under standard conditions) for 1 h. The same experimental set up used for surface functionalization was employed for calcination and reduction; however, additional provisions were applied to deliver nitrogen and hydrogen gas. After reduction, nitrogen was bubbled through benzene and passed through the reactor, which was packed with metal-impregnated ACF (Ni-ACF). A Freon refrigeration unit (R-15) was used to control the temperature of the bubbler and the concentration of benzene vapor in the carrier gas. Benzene saturated nitrogen was passed through the reactor at 750 °C for one hour, which resulted in the decomposition of benzene over Ni active sites to carbon and the formation and growth of CNF. Upon completion, the system was cooled to room temperature by purging the system with nitrogen. The hierarchal web of ACF/CNF was sonicated in dilute nitric acid to remove Ni from the tips of the CNFs, which improved the accessibility of the fibers to the solutes. The concentration of acid and the sonication time were varied to optimize the performance of the adsorbent. Several ACF/CNF samples were not subjected to sonication to determine the effects of the synthetic conditions. The importance of the sonication conditions and the effects of sonication on the performance of the materials will be discussed later in the text. All samples were prepared in triplicate, and the coefficient of variation of the intrinsic properties of the materials, including the surface area and elemental composition, was less than 5%. 2.2. Adsorption (batch and flow) study Stock solutions containing 200 ppm (mg/L) of phenol were prepared from Milli-Q water and commercially available phenol. All standards with lower concentrations than the stock solution were prepared by diluting the stock solution. Similarly, a stock solution containing 100 ppm of lead nitrate was prepared, and dilutions were made as required. For the adsorption study, the concentration range of phenol and lead were 0–100 ppm and 0–50 ppm, respectively. In a typical batch adsorption experiment, different amounts of adsorbent are mixed to a fixed (pre-determined) volume of the aqueous solutions at constant initial concentration levels. Alternatively, fixed amounts of the adsorbent may be mixed with a fixed volume of the aqueous solutions, however, at different initial concentration levels. The obtained isotherm (the solid phase concen-

trations vs. the final aqueous phase equilibrium concentrations) is unique as long as the temperature is set constant. The latter procedure was adopted in the present study to obtain the equilibrium data (isotherm), i.e., a fixed amount of the adsorbent equilibrated with a fixed volume of aqueous solutions of phenol or lead, however, at different initial concentrations. A fixed amount (0.1 g) of the adsorbent was placed in 150 mL of aqueous solutions of phenol or lead at different pre-determined concentrations. Flasks containing different concentrations of the solutions were placed in a water bath shaker at a constant temperature, and the solutions were periodically collected and analyzed. A Varian UV–Vis spectrophotometer (model: Cary 100) was used to determine the concentration of phenol, and a METROHM ion chromatograph (model: IC 861) was used to determine the quantity of lead ions in the aqueous solutions. The measurements were conducted until the concentration of the solution remained constant, and the equilibrium concentration was attained. The equilibration times for phenol and lead were approximately 12 and 36 h, respectively. The pH of the phenol- and lead solutions was also measured pre- and post adsorption. The UV–Vis spectrophotometer and IC were calibrated prior to analysis. The same apparatus used to impregnate ACF was used in the dynamic (breakthrough) tests, as shown in Fig. 1b. However, the valve used to re-circulate the solution between the conical flask and the container was sealed. The solution was continuously delivered to the perforated tubular reactor at a constant flow rate, and spent solution was continuously withdrawn from the conical flask. DI water was initially pumped through the tube. At t = 0+, the inlet concentration was increased incrementally by switching to a solution with a pre-determined concentration of the adsorbate. The exit concentration increased asymptotically until it was equal to the inlet concentration. Samples were collected at specific time intervals, and the entire procedure was repeated at different solution flow rates and inlet concentrations.

3. Surface characterization ACF and ACF/CNF samples were characterized by several analytical techniques, including BET and pore size distribution (PSD) analysis, scanning electron microscopy (SEM), energy dispersive X-ray (EDX), CHN elemental analysis, and FTIR spectroscopy. The adsorption/desorption isotherms and pore volumes of the adsorbents were determined by nitrogen adsorption–desorption at 77 K with a Quantachrome Autosorb 1C system. Prior to the experiment, the samples were degassed at 200 °C for 8 h in the outgassing station of the instrument to remove adsorbed moisture or entrapped gases in the samples. The total surface area and pore volume of the materials were determined from the nitrogen adsorption/desorption multi-point isotherms. The well-know BET equation linearized as: 1/[W(P0/P)  1] vs. P/P0 was applied in the region of the P/P0 range between 0.05 and 0.35 in calculating specific surface area. The BET plot is usually linear in this range. Moreover, most of the micropores are filled in this range. The total pore volume was measured from the amount of N2 adsorbed at the relative pressure close to unity (0.9994). The surface morphology of the samples was examined with a Supra 40 VP Field Emission SEM from Zeiss. All SEM images were captured with an inlens detector operating at an accelerating voltage of 10 kV, a filament current of 2.37 A, and a working distance of 2–4 mm. SEM images and EDX mappings of the samples were taken at various locations throughout the samples. Variations in the chemical composition (predominantly carbon) were investigated by elemental (CHNO) analysis (Exeter Analytical Inc., model: CE 440). In addition, a Bruker Tensor 27 type Fourier transform infrared spectrometer was used to determine the identity of surface functional groups. A

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DLa TGS detector with a KBr window was employed, and the resolution was set to 4 cm1. The Raman analysis was carried out to estimate the degree of graphitization in the prepared adsorbent. For the analysis, the laser Raman spectra of samples were taken by the confocal Raman instrument (Model: Alpha, Make: 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 cm1 at room temperature in air.

4. Results and discussion 4.1. BET area analysis The BET area, total pore volume, and PSD of the materials at different stages of the preparation process were calculated from the isotherms and are summarized in Table 1. As shown in Table 1, the BET area and pore volume decreased after impregnation. The external surface of ACF was measured to be 30 m2/g. Considering that the external surface area of ACF was negligible in comparison to the BET area (1300 m2/g), the reduction is attributed to the incorporation of nickel nitrate within the pores of ACF. In the present study, the PSD analysis was carried

out using the instrument’s (Quantachrome) in-built data reduction software. The PSD was calculated from desorption isotherms by the method of Barrett, Joyner, and Halenda (BJH) for mesopores (2–40 nm), and the density functional theory (DFT), in particular non local density functional theory (DLDFT), for micropores (<2 nm), assuming slit type pores in ACF. Macropores consisted of pores with size >40 nm, as per the IUPAC norms. Thus, it is reasonable to assume that nano-size nitrate-crystals were incorporated from the impregnating solution into the pores of ACF. It may be pointed out that such BET area and pore-volume measurements and PSD analysis have also been carried out in a number of studies to corroborate the incorporation of crystals within the pores of ACF [26,27]. Calcination and H2-reduction opened up the pores and increased the BET area and pore volume of the material. In addition, the PSD of Ni-ACF (row 3 of Table 1) was significantly different from that of the parent material (ACF, row 1). As also observed from Table 1, ACFs are predominantly microporous containing insignificant macropore and mesopore volumes, whereas the micropore and mesopore volumes in Ni-ACF were approximately 60% and 30% of the total pore volume, respectively. Therefore, it may be inferred that Ni-ACF partially possessed mesoporous features. Moreover, the surface area and pore volume of the material

Table 1 Surface area and PSD data. Sample

BET (m2/g)

Total pore volume (cc/g)

Micropore volume (cc/g)

Mesopore volume (cc/g)

Macropore volume (cc/g)

ACF NiNO3 impregnated ACF Ni-ACF ACF/CNF

1375 1041 1260 970

0.709 0.528 0.718 0.537

0.620 0.468 0.466 0.356

0.052 0.029 0.231 0.163

0.037 0.031 0.020 0.019

Fig. 2. SEM images of ACF/CNF at a magnification of (a) 5 K, (b) 50 K, (c) 150 K, and (d) 200 K.

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decreased after the growth of CNFs on Ni-ACF. A reduction in the BET area of nanoporous materials has been reported in previous studies [27]. 4.2. ACF/CNF morphology and surface elemental analysis The morphology of ACF and ACF/CNF before and after sonication was examined by SEM, and surface elemental analyses were conducted by EDX. SEM images and EDX maps were taken from various locations within the samples. Fig. 2a–d shows representative SEM images of CNFs grown on ACFs at different magnifications. The regions in Fig. 2a and b are marked, which were magnified into Fig. 2b and c, respectively. As shown in Fig. 2b, the distribution of CNF on ACF was uniform and dense. Moreover, shiny nickel particles on the tip of the nanofibers, as observed in Fig. 2c, were indicative of the growth of CNF by the tip-growth mechanism, by which the nickel particles remain attached to the tips of the fibers during the growth [28]. After the growth of CNF, the hierarchal web of micro/-nanofibers was sonicated to remove nickel particles from the tip of the nanofibers, which would allow the interior surface area within the pores of the fibers to become available for adsorption. Sonication was conducted with different concentrations of acid (0.01– 0.5 M) for various lengths of time (5–30 min). Fig. 3a–d displays representative SEM images (taken at different magnifications) of ACF/CNF-HS, which was sonicated under extreme conditions (high acid concentrations (0.1–0.5 M) and long durations (15–30 min)). The regions in Fig. 3a and b are marked, which were magnified into Fig. 3b and c, respectively. Under harsh conditions, the texture of

the nanofibers was adversely affected by sonication, and severe agglomeration of the fibers occurred as observed in Fig. 3b and c. In addition, the uniformity of the fiber diameter was also adversely affected. Namely, the diameter of the nanofibers increased significantly due to swelling in the sonication medium. The post-sonicated solution contained carbon fibers, which were removed from the web during the sonication process. Moreover, as discussed later in the text, ACF/CNF-HS did not provide reproducible results in the adsorption tests (equilibrium loading). Fig. 4a–d displays representative SEM images of the carbon web (ACF/CNF-LS) after sonication under mild conditions (low concentrations of acid (0.01–0.03 M) and short durations (5 min)). Under mild conditions, CNFs remained intact, and agglomeration was not observed. Moreover, the growth of CNF on ACF was not affected by mild sonication. The number of shiny Ni particles on the fibers was relatively smaller, suggesting that sonication under mild conditions was an effective procedure. Based on the results, ACF/ CNF adsorbents were sonicated for 5 min at an acid concentration of 0.01–0.03 M. Fig. 5a–d displays the representative SEM images of the substrate ACF and the ACF/CNF composites with and without sonication, after Pb-adsorption. Comparing the SEM images to those displayed in Figs. 2, 3, 4, a distinct change in the morphology of the web may be observed following the adsorption of Pb ions. EDX analyses were conducted to obtain elemental mappings of the surface of the samples. The model, INCA Energy System from Oxford Instruments, UK was used for the EDX elemental mapping. The mapping was carried out at 10 kV keeping the working distance of 10 mm. The high magnification SEM images were first recorded using the Inlens detector at a working distance of 3 mm

Fig. 3. SEM images of ACF/CNF-HS at a magnification of (a) 2 K, (b) 50 K, (c) 150 K, and (d) 200 K.

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233

Fig. 4. SEM images of ACF/CNF-LS at a magnification of (a) 5 K, (b) 50 K, (c) 100 K, and (d) 200 K.

or less. The mode of the measurement was thereafter switched to the EDX detection for the elemental mapping. The detector’s working distance was increased to optimize counts and dead time, however, at reduced magnification. Fig. 6a–c shows the EDX maps of ACF/CNF, ACF/CNF-HS, and ACF/CNF-LS, which correspond to the SEM images displayed in Figs. 3–5. All of the maps were obtained at a magnification of 200 K. As shown in the figures, sonicated samples presented less Ni particles than pre-sonicated samples. In addition, fewer Ni particles were observed in the map of ACF/ CNF-HS than that of ACF/CNF-LS because more particles were removed from the fibers under extreme sonication conditions. Namely, the elemental analyses revealed that the Ni content of ACF/CNF, ACF/CNF-HS, and ACF/CNF-LS were approximately 13%, 2%, and 7% (w/w), respectively. Fig. 7 shows the elemental mappings of ACF, ACF/CNF, ACF/CNFHS, and ACF/CNF-LS after Pb2+ adsorption, corresponding to the SEM images displayed in Fig. 5. Red and green1 dots represent Pb2+ and Ni particles, respectively. As observed in the maps, all four adsorbents, ACF, ACF/CNF, ACF/CNF-HS, and ACF/CNF-LS adsorbed Pb2+. ACF/CNF-LS adsorbed the most Pb2+, followed by ACF/CNF and ACF. As described later in the text, the adsorption tests corroborated the EDX results. However, the adsorption data of ACF/CNFHS were highly variable, and differences of ±75% were observed. As previously stated, nanofiber agglomerations were observed along the surface of ACF/CNF-HS.

1 For interpretation of color in Figs. 2–9, the reader is referred to the web version of this article.

4.3. C–H–N–O elemental analysis Table 2 summarizes the results of the elemental analyses. As shown in the table, the O/C ratio (amount of oxygen relative to carbon) significantly increased after sonication. The relatively larger amount of oxygen in sonicated ACF/CNF samples was attributed to the incorporation of oxygen from nitric acid used as the sonication medium. The sonication in nitric acid causes the oxidation of the ACF/CNF surface. The oxidation in turn results in the incorporation of carbonyl and carboxylic surface groups which contain oxygen atoms [29]. 4.4. FTIR analysis As previously mentioned, the ACFs were functionalized with NH3 to enhance the adsorption of phenol. Fig. 8 shows the FTIR spectra of unmodified ACF and ACF treated with NH3 at different temperatures. The peak observed at 1580–1600 cm1 was attributed to the presence of C@O groups in ACF, and the presence of free OH was confirmed by the peak at 3600–3700 cm1. NH3 treated samples exhibited C–N stretching bands at 1020–1100 cm1, which indicated that nitrogen was incorporated into the material by covalent bonding. The intensity of the C–N peak increased with increase in the treatment temperature, and the largest C–N peak was observed in the sample treated at 700 °C. The FTIR results are in accordance with the results obtained from the adsorption studies. Namely, maximum adsorption was observed when ACF was treated at 700 °C. The incorporation of nitrogenous groups increases the alkalinity of activated carbon, which improves the adsorption of acidic substances, such as phenol [30]. The absence of OH group in NH3-treated ACF indicated that water molecules

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Fig. 5. SEM images of (a) ACF, (b) CNF, (c) CNF-HS, and (d) CNF-LS after lead adsorption.

were expelled from ACF at elevated temperatures and that nitrogenous species were incorporated into the material. 4.5. Raman analysis Fig. 9 describes the Raman spectrum of CNF grown on ACF. Table 3 lists the corresponding data for the Raman intensities obtained from the spectrum. As observed from the spectrum, the first peak at 1342.8 cm1 corresponds to D-Band and is attributed to the disordered phase of carbon, whereas the second peak at 1587.3 cm1 corresponds to G-band and is attributed to the ordered (graphitic) phase of the material. The ratio Id/Ig reflects the extent of disorder in the carbon-material. The smaller is the ratio, the more highly ordered is the graphitic structure. For the present material, the ratio is calculated as 2.71, indicating the presence of disordered graphite components in ACF/CNF. Furthermore, ACF has a turbostratic carbon structure and the crystalline size of CNF is larger than that of ACF [25]. 4.6. Adsorption isotherms

Fig. 6. Elemental mapping of (a) ACF/CNF, (b) ACF/CNF-HS, and (c) ACF/CNF-LS, corresponding to the SEM images displayed in Figs. 2b, 3b, and 4b, respectively.

The concentration of the solute in the solid phase or the solute loading, q was determined theoretically by calculating the species balance: q = V  (C0  C)/w, where C0 and C are the concentration of aqueous phase solutes before and after equilibrium was attained, respectively, V is the volume of solution in contact with the adsorbents, and w is the weight of the adsorbent. The equilibrium data for both lead ions and phenol were fitted by the Freundlich isotherm equation q = K  C1/n, where C (mg/L) and q (mg/g)

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235

Fig. 7. Elemental mappings of lead adsorbed samples (a) ACF, (b) ACF/CNF, (c) ACF/CNF-HS, and (d) ACF/CNF-LS.

Table 2 C–H–N–O elemental analysis data. Sample

%C

%H

%N

%O

O/C

ACF/CNF-HS ACF/CNF-LS ACF/CNF

68.05 70.16 81.26

0.48 1.04 1.25

2.48 2.15 2.00

6.82 1.81 0.77

0.100 0.025 0.009

Transmittance (%)

(a)

O-H

(b) (c) (d)

C-N str

Fig. 9. Raman spectrum of ACF/CNF composites.

Fig. 8. FTIR spectra of various adsorbents used for the removal of phenol (a) ACF, (b) ACF + NH3 at 400 °C, (c) ACF + NH3 at 500 °C, (d) ACF + NH3 at 600 °C, and (e) ACF + NH3 at 700 °C.

surface energies by multilayer adsorption. The equilibrium data were also fitted by the Langmuir isotherm, q = Q  k  C/ (1 + k  C), where Q (mg/g) is the maximum solute loading (mg/ g) and k (L/mg) is the Langmuir coefficient. However, the data could not be explained with the respective Langmuir isotherms for the solutes, suggesting that the surface coverage with the solutes was more than a monolayer. To this end, the values for Freundlich constants, K (measure of the adsorption capacity) and the power of isotherm n (measure of adsorption intensity) are reported in the figures. In general, a value of n less than unity indicates favorable adsorption.

are the concentration in the solution and amount of solute adsorbed at equilibrium, with the non-linear least-squares adjustment. The Freundlich isotherm describes the heterogeneous

4.6.1. Lead adsorption Fig. 10 shows the equilibrium isotherms for the adsorption of lead on ACF, ACF/CNF, and acid-sonicated ACF/CNF. The following

(e) C=O

4000

3500

3000

2500

2000

1500

1000

Wavenumber (cm-1 )

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Table 3 The Raman data corresponding to the Raman spectrum of CNF/ACF composites shown in Fig. 9. Peak

Area

Center

Width

Height

Structure

1 2

Id = 10468.9 Ig = 3859.0

1342.8 1587.3

50.6 58.7

165.0 52.3

Disordered Graphite

trend was observed in the adsorption capacity of the adsorbents: sonicated ACF/CNF > ACF/CNF > ACF. Specifically, the maximum adsorption capacity for the hierarchal carbon web was approximately four times greater than that of ACF. Moreover, upon sonication, the adsorption capacity of ACF increased by approximately 30% because the acid-sonication resulted in the removal of Ni from the hierarchal web, and oxidation by HNO3 increased the cationexchange capacity of carbon [15]. An increase in the oxygen content was confirmed by C–H–N–O analysis, as previously reported in Table 2.

4.6.2. Adsorption of phenol Fig. 11 represents the equilibrium isotherm for the adsorption of phenol on ACF, ACF/CNF, and the NH3-treated ACF. Contrary to the results obtained with Pb2+, the adsorption capacity of ACF was larger than that of the hierarchal web of ACF/CNF. Namely, the maximum adsorption capacity of ACF for phenol was 200 mg/g. In comparison, ACF/CNF possessed an adsorption capacity of 150 mg/g. Functionalization of ACF with NH3 increased the adsorption capacity to 275 mg/g, as shown in the figure. The introduction of nitrogenous functional groups to the surface of the adsorbent increased the basicity of the material, which enhanced the adsorption of phenol. An increase in the basicity of a substance due to the incorporation of nitrogenous surface groups has been observed in a previous study [30]. The results of the adsorption tests also revealed that maximum phenol adsorption was observed on ACF treated with NH3 at 700 °C, which is in accordance with the FTIR results. Namely, the IR spectra of NH3-treated ACF indicated that basic functional groups were incorporated into the adsorbent.

Fig. 10. Lead adsorption isotherms at 38 °C (solid lines show Freundlich isotherm, q = K  Cn, where K = 11, 7, 4, and n = 0.30, 0.33, and 0.15, for ACF/CNF-LS, ACF/CNF, and ACF, respectively).

Fig. 11. Phenol adsorption isotherms at 38 °C (solid lines show Freundlich isotherm, q = K  Cn, where K = 143, 130, 17, and n = 0.16, 0.11 and 0.53, for ammonia treated ACF, ACF and ACF/CNF, respectively).

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The batch equilibrium adsorption experiments were also carried out at different temperatures (38, 50, and 60 °C). The adsorbents, ammonia functionalized ACF and CNF, exhibiting the maximum adsorption capacity for phenol and lead, respectively, were selected for the test. The adsorption results showed that there was practically no change observed in the equilibrium concentrations with increasing temperatures. For brevity, the adsorption isotherms for two solutes at different temperatures are not produced herein. 4.7. Breakthrough experiments Breakthrough or column experiments were performed on ACF/ CNF samples and NH3-treated ACF to determine the adsorption of Pb2+ and phenol under dynamic conditions. Two important parameters are commonly used to describe the effectiveness of adsorbents: (1) the throughput volume (the volume of feed solution passed through the bed until saturation is achieved), and (2) the total uptake of the adsorbate. The former was calculated as QDt, where Q is the flowrate of the solution laden with the solutes and Dt is the time until saturation. The latter was determined from the following equation:

 Z q ¼ Q C in T 

T

 C exit dt ;

ð1Þ

0

where q is the uptake of the solute by the adsorbent, T is the total time until saturation of the adsorbents, Q is the flowrate of the solution laden with the solutes, Cin is the concentration of the solute in the solution at the inlet to the column, and Cexit is the concentration of the solute at the exit the column. In principle, the specific uptake (mg/g) of the solute should be equal to the equilibrium concentration of the adsorbate in the batch tests. The purpose of the breakthrough or column experiment was to identify channeling effects that can alter the distribution of liquid in the packed bed adsorber and can result in the underutilization of adsorbents, which would reduce the uptake of the solutes. Figs. 12a and 12b describe the breakthrough characteristics of Pb2+ ions and phenol, respectively, at different inlet concentrations. The hatched area shown in the uppermost curve of Figs. 12a and 12b corresponds to the total solute uptake at the inlet Pb2+ concentration of 50 ppm (or mg/L) and the inlet phenol concentration of 20 ppm. According to Eq. (1), the specific uptakes were approximately equal to the equilibrium concentration of Pb2+ (Fig. 10) and phenol (Fig. 11), which suggested that the adsorbent was completely utilized and channeling effects in the perforated tubular adsorber were negligible. Table 4 summarizes the breakthrough results for lead and phenol under different operating conditions of inlet solute concen-

Fig. 12a. Breakthrough curve for the adsorption of lead onto ACF/CNF at different inlet concentrations (W = 0.5 g, Q = 20 mL/min, temperature = 38 °C).

Fig. 12b. Breakthrough curve for the adsorption of phenol onto ammonia treated ACF at different inlet concentrations (W = 0.2 g, Q = 20 mL/min, temperature = 38 °C).

Table 4 Breakthrough data for lead and phenol (Temp. = 38 °C). S. No.

Cin (ppm)

W (g)

Q (mL/min)

Lead 1 2 3 4 5

Uptake of lead/phenol (mg/g)

50 40 30 30 30

0.5 0.5 0.5 0.5 0.5

20 20 30 20 15

25 20 18 18 18

Phenol 7 8 9 10 11

5 10 20 20 20

0.2 0.2 0.2 0.2 0.2

20 20 20 30 40

180 220 250 247 249

tration (Cin), amount of adsorbent (W) used in the column, and the flowrate (Q) of the test solution. The specific uptake (q) of the solute (Pb2+/phenol) was calculated from Eq. (1) to show that the specific uptake data calculated from the column experiments were approximately equal to the data obtained from the respective isotherms (shown in Figs. 10 and 11). The salient result of the present study is that the phenoladsorption capacity of ACFs (or ammonia functionalized ACFs) is larger than that of ACF/CNF composites. On the other hand, the lead-adsorption capacity of ACF/CNF is larger than that of ACFs or ammonia functionalized ACFs. The different adsorption capacities of ACF and ACF/CNF for phenol and lead ions are due to the combined effects of the size of the adsorbate molecules vis a vis the pore structure of the adsorbents, the surface chemical functional groups, and the electrostatic force induced by the surface charge on the adsorbent. The ionic radius of Pb ions is 0.119 nm [31]. Therefore, Pb2+ can easily penetrate into the micropores of ACF or micro- and mesopores of ACF/CNF. The CNF of the ACF/CNF composites possesses graphitic planes with dense p-electron clouds. As a result, the negatively charged cloud facilitates the adsorption of cationic lead ions. Therefore, the adsorption capacity of ACF/CNF composites for Pb2+ is larger than that of ACF. As pointed out earlier in the texts, the elemental analysis confirmed increase in the O/C contents and decrease in nitrogen (N) contents of ACF/CNF, which suggests the incorporation of the acidic oxygenated groups into the webs. The sonication of ACF/CNF webs using HNO3 further augments the adsorption capacity of the composites. Therefore, sonication not only opens-up the pores by removing Ni-particles from the tips, but also introduces the acidic carboxylic surface groups responsible for increasing the adsorption capacity for Pb2+. Such increase in the adsorption capacity of the HNO3 modi-

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A. Chakraborty et al. / Journal of Colloid and Interface Science 359 (2011) 228–239 Table 5 A comparison of the adsorption capacity of ACF and ACF/CNF with the literature data. Reference Lead Present study [4] [7] [15] [36] [37] Phenol Present study [9] [10] [11] [12] [16] [18] [38]

Adsorbent

Equilibrium concentration (ppm)

Loading (mg/g)

Sonicated ACF/CNF Commercial carbon Granular palm shell activated carbon CNT Gel type weak acid resin (110-H) Polymer-supported nanosized hydrous manganese dioxide

0–50 50 50 5 40 100

0–40 36 10 1.5 510 400

NH3 treated ACF Mesoporous carbon CMK-3 Carbon cryogel microshperes Activated carbon fiber Date-pit activated carbon (DP-AC) Activated carbons prepared from wood particleboard wastes Coal-reject-derived adsorbents INDION polymeric resins

0–30 30 30 30 30 0.2 50 720

0–290 90 140 50 45 250 50 160

fied carbonaceous materials CNF and CNT for Pb2+ and Cd2+ ions have also been confirmed in other investigations [15,29]. With regard to the adsorption of phenol by ACF and ACF/CNF, the phenol molecules can hydrogen bond to form long chains. The typical dimensions of such long chains of hydrogen bonded phenol are reported as 0.67 nm  0.15 nm  0.80 nm [16]. Due to the long chain, the adsorption of phenol molecules into the nanopores of CNF by ‘‘sliding’’ between the graphite platelets is inhibited [32]. The rapid uptake of phenol from water by porous carbonaceous materials, including ACF, having relative larger micropore volume or wider pore size is also observed in other studies [9,18]. The interaction of phenol containing p-electron clouds with the graphitic planes of CNF is likely to be electrostatic (repulsive) driven, as the latter also contains dense p-electron clouds. The adsorption of phenol on CNF may occur by physisorption due to the van der Waals forces. Such forces are also responsible for the adsorption of phenol on the parent material ACF. However, in the case of CNF, the combined effect of electrostatic interaction and size of the adsorbate-molecule restricts the adsorption of phenol on CNF, which is lesser than that on ACF. Re-visiting the discussion of the adsorption of phenol, it was mentioned that the introduction of nitrogenous functional groups into the surface of ACF by functionalization with ammonia resulted in the enhanced adsorption of phenol by ACF. The increase in the adsorption of phenol because of the presence of basic groups such as cyclic amides and N–H in mesoporous carbon has also been observed elsewhere [33]. The solution-pH may also affect adsorption via the surface charge induced on the adsorbents. Depending on the pH of the solution, the surface charge can be positive or negative, thereby possibly affecting the adsorption of a solute by the electrostatic interactions. In the present study, the pH of water was measured between 6.6 and 6.8. The pH-variation of the solution during the adsorption tests was found to be negligible (<±0.5). It has been reported in several studies that the surface charge of carbonaceous materials, including carbon nanotubes (CNTs), activated carbons (ACs) and activated carbon fibers (ACFs) is negative at pH > 6 [17,34,35]. It may, therefore, be inferred that the adsorption on CNF of the positively charged Pb2+ ions and the phenol with its pi-electron clouds was possibly facilitated and hindered, respectively, due to the pH effects. The effects are consistent with the adsorption data of Pb2+ and phenol on ACF/CNF composite, as discussed in the preceding paragraphs. It is also important to mention that the pH of the test water used in the present study was measured between 6.6 and 6.8. In most cases, pH of ground or potable water is in the vicinity of 7

(usually between 6 and 7.5). Considering that the pH-variation of the solution during the adsorption tests for either phenol or lead ions was found to be negligible (<±0.5), the results suggest that the post treatment of water for pH may not be required using the adsorbents prepared in this study. The removal of Pb2+ and phenol by the proposed adsorbents was compared to that of the other adsorbents reported in the literature, and the results are presented in Table 5. As shown in the table, the loading of lead ions is significantly larger on the sonicated ACF/CNF developed in this study than on CNT [15]. The loading may be considered to be larger or comparable to that on the activated carbon granules [4,7]. However, the adsorption by the noncarbonaceous adsorbents such as polymeric resins and metal oxide are observed to be significantly larger than that by the carbon based adsorbents [36,37]. With regard to the adsorption of phenol, the adsorption capacity of NH3 treated ACF is significantly larger than the other carbonaceous as well as non-carbonaceous adsorbents, including those derived from low-cost materials reported in literature [9,10–12,16,18,38]. In general, variation in the solute loading on different adsorbents is due to different types of material, surface functional groups, textural property, and operating conditions, including pH. It is also important to note that the equilibrium solute loading determined under flow conditions truly reflect the performance of the adsorbents, because mass transfer and hydrodynamic resistances are prevalent in such conditions. Considering that these resistances are usually significant in the granular materials, and the mal-distribution of flow due to channeling is common in packed beds of granular or powdered materials, the ACF based adsorbents are potentially practical candidates as packed bed fabric filters for removing contaminant solutes from wastewater.

5. Conclusions The hierarchal web of carbon micro/nano fibers developed in the present study was an efficient adsorbent for the removal of Pb2+. Alternatively, micron-sized ACF exhibited a superior adsorption capacity for phenol. Differences in the performance and selectivity of the adsorbents were attributed to the structure of the adsorbents and the size of the adsorbates. The surface functionalization of ACF by NH3 enhanced the adsorption of phenol due to the incorporation of nitrogenous groups onto the surface of the adsorbent. Alternatively, the sonication of ACF/CNF under mild conditions enhanced the adsorption of Pb2+ while retaining the texture of the nanofibers. Starting from the same material (micron-sized

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carbon fibers), different adsorbents were developed for the removal of different types of contaminants (i.e., inorganic heavy metal ions and organic phenolic compounds). Thus, ACF can be used in a wide range of applications. Acknowledgments The authors would like to thank the Department of Science and Technology (DST), New Delhi, for research grants and the support of the Center on Nanosciences at IIT Kanpur. References [1] G.J. Hathaway, N.H. Proctor, J.P. Hughes, M.L. Fischman, Proctor and Hughes’ Chemical Hazards of the Workplace, third ed., Van Nostrand Reinhold, New York, NY, 1991. [2] H. Herbert, P. Fang, O. Chan, Water Res. 31 (1997) 2229. [3] A. Groffman, S. Peterson, D. Brookins, Water Environ. Technol. 5 (1992) 54. [4] S. Halim, A. Shehata, M. Shahat, Water Res. 37 (7) (2002) 1678. [5] I.I. Salame, T.J. Bandosz, J. Colloid Interface Sci. 264 (2003) 307. [6] S. Mukherjee, S. Kumar, A. Misra, M. Fan, Chem. Eng. J. 129 (2007) 133. [7] G. Issabayeva, M. Aroua, N. Sulaiman, Bioresour. Technol. 97 (2006) 2350. [8] M. Streat, J. Patrick, M. Perez, Water Res. 29 (2) (1995) 467. [9] E. Haquen, V. Talapaneni, J. Jegal, Korean Chem. Soc. 31 (6) (2010) 232. [10] M.H. El-Naas, S. Al-Zuhair, M. Abu Alhaija, Chem. Eng. J. 162 (2010) 997. [11] P. Girodsa, A. Dufoura, V. Fierrob, Y. Rogaumea, C. Rogaumea, A. Zoulaliana, A. Celzardc, J. Hazard. Mater. 166 (2009) 491. [12] F. Haghseresht, G.Q. Lu, Energy Fuels 12 (6) (1998). [13] R.-S. Juang, F.-H. Wu, R.-L. Tseng, J. Chem. Eng. Data 41 (1996) 487. [14] C. López, G. Camargo, L. Giraldo, J. Piraján, Ecl. Quím. 32 (3) (2007) 61. [15] Y.-H. Li, S. Wang, J. Wei, X. Zhang, C. Xu, Z. Luan, D. Wu, B. Wei, Chem. Phys. Lett. 357 (2002) 263.

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