- Email: [email protected]
Carbon bead-supported hollow carbon nanofibers synthesized via templating method for the removal of hexavalent chromium
Accepted Manuscript Title: Carbon Bead-supported Hollow Carbon Nanofibers Synthesized via Templating Method for the Removal of Hexavalent Chromium Author: Shraddha Mishra Nishith Verma PII: DOI: Reference:
S1226-086X(16)30003-X http://dx.doi.org/doi:10.1016/j.jiec.2016.02.025 JIEC 2847
To appear in: Received date: Revised date: Accepted date:
22-7-2015 28-2-2016 28-2-2016
Please cite this article as: S. Mishra, N. Verma, Carbon Bead-supported Hollow Carbon Nanofibers Synthesized via Templating Method for the Removal of Hexavalent Chromium, Journal of Industrial and Engineering Chemistry (2016), http://dx.doi.org/10.1016/j.jiec.2016.02.025 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Highlights Carbon bead-supported and TEA-grafted hollow CNFs prepared by templating. Significant adsorption capacity of the material for Cr(VI) ions in water.
Ac ce
pt
ed
M
an
us
cr
ip t
Negligible interference on adsorption from co-existing metal ions.
1
Page 1 of 32
Carbon Bead-supported Hollow Carbon Nanofibers Synthesized via Templating Method for the Removal of Hexavalent Chromium
a
ip t
Shraddha Mishra a and Nishith Vermaa, b*
Center for Environmental Science and Engineering, Indian Institute of Technology Kanpur,
Department of Chemical Engineering, Indian Institute of Technology Kanpur, Kanpur-208016,
us
b
cr
Kanpur - 208016, India
Ac ce
pt
ed
M
an
India
*Corresponding author. Tel.: +91 512 259 6767; fax: +91 512 259 0104 E-mail address: [email protected], [email protected] (N. Verma), [email protected] (S. Mishra). 2
Page 2 of 32
ABSTRACT Novel triethanol amine-grafted hollow carbon nanofibers (CNFs), supported on carbon beads, were synthesized via templating method for the removal of hexavalent chromium (Cr(VI))
ip t
ions from water. CNFs were grown on carbon beads by chemical vapor deposition using the in situ incorporated Cr nanoparticles in the beads, as the catalyst. The beads were ultra-sonicated to
cr
remove the nanoparticles from the tip of the CNFs, thereby creating the hollow CNFs. The
us
Cr(VI)-adsorption capacity of the material was determined to be ~51 mg/g at 125 ppm-initial concentration and 4.5-solution pH. Negligible interference from coexisting metal ions in water
an
was found with the Cr(VI)-adsorption.
Keywords: templating method; carbon nanofibers; carbon beads; adsorption; hexavelent
Ac ce
pt
ed
M
chromium.
3
Page 3 of 32
Introduction Hexavalent chromium (Cr(VI)) is one of the most toxic metal pollutants in water, released from the electroplating, metal finishing, leather tanning, photography, dye and
ip t
textile industries [1, 2]. A redox active element, Cr exists as Cr(III) and Cr(VI) species in water. Cr(VI) is more toxic than Cr(III) and can cause several diseases such as cancer, mucosal
cr
ulceration, kidney disorder and chronic dermatitis [1].
us
Adsorption is considered to be an energy efficient and a simple method [3]. Different types of materials such as carbon, polymers and micellar compounds have been used as
an
adsorbents for Cr in aqueous systems [3-11]. Adsorbents with large surface area, ordered/uniform pore structure and surface activity are preferred for environmental remediation
M
applications. At present, template method is considered to be a powerful tool for producing adsorbents with high surface area. It is a fast developing method for designing a pattern with
ed
uniform pore size distributions. The method commonly includes treating the precursor of a substrate, viz., silica, alumina and titania, with different inorganic or organic reagents used as the
Ac ce
extraction.
pt
template molecules, and then removing the molecules by heat or acid treatment, or soxhlet
Several silica-based templates have been developed for the removal of various metal ions such as Pb(II), Cd(II), Hg(II)), Ni(II), Zn(II) and Cu(II) from aqueous solutions [12-18]. Among the salient studies, Lee et al. prepared a mesoporous silica using dodecylamine as the template for the removal of Hg(II) ions. The surface hydrophilicity of the material was enhanced by surface functionalization using the mercaptopropyl and aminopropyl-containing chelating ligands [12]. Heidari et al. synthesized the amine-functionalized mesoporous and nanomesoporous silica for the simultaneous removal of the Ni(II), Cd(II) and Pb(II) ions from 4
Page 4 of 32
aqueous solution [13]. Aguado et al. also synthesized the amine-functionalized mesopurous silica, using two different methods, namely, grafting and co-condensation. The amine-grafted materials exhibited adequate mesoscopic order with high amino content, and adsorbed significant
ip t
amounts of the Cu(II), Cd(II), Pb(II), Ni(II) and Zn(II) ions [14]. A major drawback of such silica-based materials is that they are destroyed in alkaline medium, caused by the hydrolysis of
cr
Si-O-Si bonds in the materials.
us
Mesoporous alumina- and titania-based templates have also been synthesized for the adsorption of metal ions. Rengaraj et al. [19] and Kim et al. [20] have prepared mesoporous
an
alumina using stearic acid as the template for the removal of Ni(II) and arsenic(V) ions, respectively. Wu et al. synthesized mesoporous titania beads of uniform size, using calcium
M
alginate template for the removal of Cr(VI) [21]. These studies, however, did not consider the effects of co-existing metal ions in water on the adsorption of the solutes. Zhang et al.
ed
synthesized titania-based template using D311 resin as the template for the removal of the Pb(II), Cd(II) and Zn(II) ions in a multi component system [22]. Researchers have also used templating
pt
methods for the synthesis of porous carbon materials including carbon nanotubes (CNTs) and
Ac ce
carbon nanofibers (CNFs), however, for electrochemical applications [23, 24]. This is the first study on the development of hollow CNFs by templating method, for the efficient removal of Cr(VI) ions from aqueous solution. The CNFs have gained a great technological significance because of their excellent physico-chemical properties such as large Brunauer-Emmet-Teller (BET) surface area, amenable to functionalization, and stability in acidic and basic medium. The CNFs contain exposed edge planes on their hexagonal surface, which facilitate chemical bonding with the functional groups-containing compounds [25, 26]. The novelties of the present study are in the synthesis aspects. (1) The in-situ doped Cr nanoparticles 5
Page 5 of 32
in the beads catalyzed the growth of the CNFs as well as they served as the template. (2) The templates were removed by simple ultrasonication of the nanofibrous beads, thereby creating the hollow CNFs. A technical advantage of using such materials is that the synthesized carbon beads
ip t
can be efficiently used as an adsorbent in packed bed columns under flow condition without
cr
maldistribution or channeling of the liquid to be treated.
us
Experimental Chemicals
an
All chemicals used in this study were of analytical reagent grade. Phenol, formaldehyde, triethylamine, hexamethylene tetramine (HMTA), polyvinyl alcohol (PVA), chromium nitrate
M
(Cr nitrate), potassium dichromate and triethanol amine (TEA) were purchased from Merck, Germany. All samples were prepared in Milli-Q water.
ed
Synthesis of Cr-doped polymeric beads
pt
The experimental setup used for the preparation of beads is described elsewhere [27]. Briefly, a 2 L-three-necked round bottom glass reactor was used for the polymerization reaction.
Ac ce
The reactor was equipped with a reflux condenser, mechanical stirrer, and thermometer. The Crdoped phenolic beads were prepared using suspension polymerization with phenol (50 g) as the monomer, formaldehyde (63 mL) as the solvent, and triethylamine (1.5 mL) as the catalyst. The mixture was stirred to prepare a homogenous solution at room temperature (30 ± 5 °C) for 8 h. Next, 200 mL of water was mixed into the reaction mixture. After approximately 30 min, 3.5 g of HMTA, used as the cross-linking agent, was mixed into the solution and the solution was heated 3 °C per min until the solution-temperature reached 100 °C. Approximately 4 g-PVA as a 6
Page 6 of 32
suspension stabilizer was mixed into the solution. After approximately 30 min, the solution turned into a gel-like emulsion. At the incipience of the gel formation, approximately 3 g of the aqueous solution of Cr nitrate, used as the precursor for Cr, was mixed into the reaction mixture.
ip t
The Cr nitrate solution was prepared using 0.3% (w/w) sodium dodecyl sulfate (anionic surfactant) to achieve approximately uniform dispersion of Cr ions on the beads without
cr
agglomeration. After ~3 h, the beads were formed. Heating was stopped. Stirring was continued.
us
After cooling the reaction product to room temperature, the produced solid beads were separated from the residual liquid, using a 20 mesh-sieve. The beads were repeatedly washed with Milli-Q
an
water, methanol and acetone in that sequence. The washed beads were dried at room temperature. The yield of the synthesized spherical beads (~0.8 mm-average size) was ~36 g.
M
The prepared materials were termed as Cr-PhB for the reference purposes in this study.
ed
Growth of CNFs on Cr-PhB
Carbonization, activation, reduction and chemical vapor deposition (CVD) were
pt
sequentially performed on the prepared Cr-PhBs in a horizontal inconel-tubular reactor (30
Ac ce
mm-diameter and 1000 mm-length) installed in the horizontal electric furnace, having provisions for gas inlet and outlet [28]. The polymeric beads were carbonized at 5° per min to 900 °C for 1 h in a nitrogen (N2) atmosphere (150 standard cc per min (sccm)). Activation was performed at the same temperature for another 1 h, using a mixture of N2 (100 sccm) and steam (0.3 g per min). Approximately 40% weight-loss was measured in the produced carbon beads
post-carbonization and activation steps. The average particle diameter also decreased to ∼0.5 7
Page 7 of 32
mm. The porous carbon beads (Cr-PhB-A) were then subjected to reduction at 800 °C for 2 h in a hydrogen atmosphere (150 sccm). Prior to the reduction, temperature programmed reduction analysis was performed on the activated beads to determine the optimum reduction temperature
ip t
(800 °C) for Cr oxides. The catalytic CVD was performed on the H2-reduced beads at 600 °C for 45 min to grow the CNFs, using acetylene as the carbon-source and the in-situ doped Cr
cr
nanoparticles (NPs) as the CVD catalyst. Such beads were termed as CNFb. Fig. 1 shows the
us
photograph of Cr-PhB and the SEM images of the Cr-PhB-A and CNFb samples.
an
Synthesis of TEA-functionalized CNFb
The prepared CNFbs (0.5 g) were ultrasonicated in a 50 mL of 0.05 M-HNO3 aqueous
M
solution for 5 min to dislodge the Cr NPs from the tips of the CNFs. The sonicated carbon beadsupported hollow CNFs were termed as CNFb-S for the reference purposes. Next, the CNFb-S
ed
beads were grafted with TEA. Approximately 0.5 g of CNFb-S were first oxidized with 50 mLHNO3 (65% v/v) at 100 °C for 120 min to functionalize the surface of the beads with carboxylic
pt
functional groups (-COOH), and also, enhance the hydrophilicity of the material. The beads were
Ac ce
cooled to room temperature and then washed with Milli-Q water until the pH of the surface was 7. The oxidized CNFb-S (CNF-S-COOH) samples were dried in a vacuum oven at 70 °C for 2 h. Next, the dried beads were mixed with 20 mL-TEA in a 100 mL-beaker. The beaker was heated on a hotplate at 80 °C for 1 h, thereby allowing the covalent coupling between the CNF-SCOOH and TEA via the formation of an ester group. The synthesized CNFb-S-TEA beads were washed with Milli-Q water and dried in the vacuum oven at 50 °C for 2 h. Batch adsorption study
8
Page 8 of 32
A stock solution of 200 ppm-Cr(VI) in water was prepared using potassium dichromate salt. Test solutions (50 mL) of different concentrations (10-150 ppm) were prepared from the stock solution and transferred to conical flasks (100 mL volume) for the adsorption study.
ip t
Approximately 0.05 g of the prepared CNFb-S-TEA adsorbents were mixed into the test solutions and the samples were kept on a mechanical shaker (150 rpm) at 30 °C for 12 h.
cr
Separate adsorption tests were performed using the Cr-solution of 100 ppm-concentration, mixed
us
together with the 10 ppm-concentration solution, each of Cu, Ni, Cd, Zn, and As salts, to study the interference of impurity ions on the adsorption of Cr by the prepared adsorbents in this study.
an
The concentration of Cr ions in the solution was determined using a flame atomic absorption spectrometer (FAAS) (Varian AA-240, USA) at the wavelength of 357.9 nm, using
M
an air-acetylene flame. Prior to the analysis, the instrument was calibrated using the standard solutions. The amount of Cr adsorbed by the material was calculated from the species balance
ed
equation q = (Ci - Ce) V/W, where q is the adsorption capacity (mg/g); Ci and Ce are the initial and equilibrium concentrations (mg/L), respectively; V is the volume (L) of the solution and W is
pt
the weight (g) of the adsorbent.
Ac ce
Characterization of adsorbents
The surface characterization of the prepared materials was performed using various
analytical techniques. The surface morphology was studied using SEM (Supra 40 VP, Zeiss, Germany) and TEM (FEI Technai, 20 U Twin, USA). The metal compositions were determined using EDX (Oxford Inc., Germany). The area-mapping of the surface of the samples was captured at different locations to examine the distribution of different elements. FT-IR analysis was performed to determine the surface functional groups. The spectra were recorded in the 9
Page 9 of 32
frequency range of 4000-400 cm-1 using the Vertex 70 (Brucker, Germany) instrument. The elemental analysis was performed using an elemental analyzer (CE-440 EL, Exeter Analytical, Inc., USA). The Autosorb-1C instrument (Quantachrome, USA) was used for the surface area
ip t
and pore size distribution (PSD) analysis. The specific BET surface area was determined from the nitrogen adsorption/desorption isotherm at 77 K. The PSD was calculated using the Barrett-
cr
Joyner-Halenda (BJH) method and density functional theory (DFT) for mesopores (2-50 nm) and
us
micropores (< 2 nm), respectively. Prior to the analysis, the samples were degassed at 200 °C to remove the impurities and moisture. XPS analysis was performed using the kratos axis ultra x-
an
ray photoelectron spectrometer (PHI 5000 Versa Prob II, FEI Inc, USA) with a 165 mm-
M
hemispherical electron energy analyzer. Results and Discussion
ed
SEM analysis Micro-nanostructures of Cr-PhB-A, CNFb, CNFb-S, CNFb-S-TEA and Cr-adsorbed
pt
CNFb-S-TEA (CNFb-S-TEA-ads) samples are shown in Fig. 2, at low (10 kx) and high (100 kx)
Ac ce
magnifications. The SEM images of Cr-PhB-A showed a highly porous surface of the beads, and pores were clearly visible in the high magnification-image (Fig. 2a, b). A dense and an approximately uniform distribution of the CNFs was observed on the bead surface (Fig. 2c,d). The shiny tips of the CNFs were clearly visible in the high magnification-image, attributed to the presence of metal NPs at the tips. The growth of the CNFs occurred by the tip-growth mechanism and the Cr NPs moved out from within the pores to the external surface of the beads, along with the nanofibers during the CVD step. The Cr NPs were leached out from the tips of the CNFs during sonication and the surface texture of the CNFs became relatively less 10
Page 10 of 32
bright (Fig. 2e,f). The grafting with TEA changed the surface morphology of the beads (Fig. 2g,h). The surface became rough and the fibers were covered with the coating, indicating that the grafting was effective over the entire surface of CNFb. A white precipitate (the complexation
ip t
product of TEA and Cr) was visible on the surface of the CNFb-S-TEA-ads samples (Fig. 2i,j).
cr
EDX and elemental mapping
us
The EDX spectra confirmed the presence of the Cr NPs on the surface of CNFb (Fig. 3a). The sonication of CNFb caused the significant reduction in the amounts of Cr. Therefore, a
an
negligible amount (0.19%) of Cr was observed in CNFb-S-TEA (Fig. 3b). However, the amount significantly increased in the CNFb-S-TEA-ads samples (Fig. 3c).
M
The elemental mapping (shown underneath the EDX spectra in Fig. 3) was performed for the (pre-sonicated) CNFb samples, and the CNFb-S-TEA samples (before and after adsorption).
ed
The red and green dots represent C and Cr elements, respectively. Approximately uniform and dense distribution of Cr was observed in the respective mapping of the CNFb and CNFb-S-TEA-
Ac ce
TEM analysis
pt
ads samples, indicating the suitability of the prepared CNFb-S-TEA adsorbents for Cr.
The TEM images of the CNF and CNF-S-TEA samples are exhibited in Fig. 4. The black spot seen at the tip of the fiber indicated that the metal was adhered at the tip of the CNF. After the sonication, the tips of the CNFs were opened up, creating hollow tube like structures of uniform size. Further, the grafting with TEA utilized the expose edge of the CNFs, without
11
Page 11 of 32
removing the cavity at the tip of the CNFs. The average internal diameter of the fiber was
cr
ip t
measured to be ∼20 nm, using Image J software.
us
FTIR Spectra
an
Fig. 5 shows the FTIR spectra of the CNFb-S, CNFb-S-COOH, CNFb-S-TEA and CNFb-S-TEA-ads samples. A common peak is observed at 3417 cm-1 in all samples, attributed to
M
-OH stretch. Peaks observed at 1163 and 1623 in CNFb-S-COOH are attributed to >C-O and >C=O stretches of carboxylic functional group, respectively, caused by the oxidation of CNFb-S.
ed
The >C-N stretch observed in the spectrum of CNFb-S-TEA confirmed the attachment of TEA on CNFb-S-COOH. The stretching frequencies observed at 572 cm-1 (Cr-O stretch) and 431 cm-1
pt
(Cr-N stretch) in CNFb-S-TEA-ads (i.e., post Cr-adsorbed sample) indicated the formation of the
Ac ce
Cr-TEA complex (inset in Fig. 5) Elemental analysis
Elemental analysis was performed to determine the elemental compositions (N and O) of CNFb-S and CNFb-S-TEA. The N-contents were determined to be 2.01 and 3.46% in CNFb-S and CNFb-S-TEA, respectively, whereas the O-contents were 22.1 and 34.46% in CNFb-S and CNFb-S-TEA, respectively. The higher N- and O-contents in CNFb-S-TEA indicated the attachment of the ligand (TEA) to the CNFb-S-TEA surface. 12
Page 12 of 32
Surface area and PSD Table 1 summarizes the SBET, total pore volume (VT) and PSD of the materials, calculated
ip t
from the isotherms data. The Cr-PhB-A beads were highly microporous, with the microporosity
us
cr
content determined to be ∼87% of the total pore volume. On growing the CNFs, the BET surface
an
area of the beads decreased because the CNFs partially blocked the pore entrance. The BET surface area increased in CNFb-S because of the removal of the Cr NPs from the tip of the
M
CNFs, thereby exposing the internal surface area to the N2 adsorbate used for the measurements of BET surface area. Further, the grafting of CNFb-S with TEA blocked the micropores,
ed
resulting in the reduction of pore volume and BET surface area, however, with an increase in the
pt
mesoporous contents of CNFb-S-TEA.
Ac ce
X-ray photoelectron microscopy
XPS analysis was performed on CNFb-S-TEA-ads to confirm the adsorption of Cr on the surface of the material. Fig 6 shows the C1s, O1s, N1s and Cr2p spectra. The deconvolution of C1s spectrum identified three peaks at 284.34 eV (C-C) , 285.74 eV (C-N, C-OH and C=O) and 289.11 eV (O-C=O). Next, the deconvolution of the O1s spectrum confirmed the peak at 532.02 eV, attributed to the presence of C=O, O-C=O, and that at 533.25 eV, attributed to C-OH group. The deconvolution of N1s spectrum confirmed the peak at 399.41 eV, attributed to tertiary amine [29] and that at 401.08 eV, attributed to the quaternary nitrogen atoms [30]. This also confirmed 13
Page 13 of 32
that the N atom of the TEA was protonated at acidic pH. The speciation of the Cr on the surface of CNFb-S-TEA-ads was also characterized from the XPS spectra. The spectrum showed two bands at 576.89 eV (Cr(III)) and 586.51 eV (Cr(VI)), the first corresponding to the Cr2p3/2
ip t
orbital and the second corresponding to the Cr2p1/2 orbital [5], indicating the presence of both
cr
Cr(III) and Cr(VI) species on the adsorbent surface.
us
Adsorption capacity
Adsorption tests were performed at solution pH (~4.5 ± 0.3) over 10-150 ppm-
an
concentration range. The adsorbents were mixed in the Cr(VI) solutions and stirred at 30 °C. The Cr-equilibrium loadings increased with the aqueous phase concentrations of Cr and the
M
equilibrium loading on CNFb-S-TEA was higher than that on CNFb-S at all concentrations (Fig. 7).
ed
The adsorption capacity of CNFb-S-TEA was compared with the other adsorbents including polymer-, micellar compounds- and carbon-based materials discussed in the literature
pt
[1, 3-11] and the data are summarized in Table 2. The comparison was made considering
Ac ce
different pH and initial aqueous phase concentrations of Cr (mg/l) used in the studies. (1) The adsorption capacity of CNFb-S-TEA is considerably higher than most of the materials. (2) In a few cases where the (literature) materials exhibited high capacity, either the solution was considerably acidic (pH < 3), which is not suitable for practical remediation applications, or the aqueous phase concentration levels were high, which is not a realistic representation of industrial effluents. Effect of pH and adsorption mechanism
14
Page 14 of 32
The effect of pH on the adsorption of Cr(VI) was studied over the pH range of 1-8 at 100 ppm-initial Cr concentration. The adsorption capacity of CNFb-S-TEA decreased from ~61 to 23 mg/g with the increase of solution pH from 2 to 8, indicating that the adsorption of Cr(VI)
ip t
significantly depended on pH (Fig. 8). The Cr(VI) ions exist in various anionic forms such as HCrO4-, Cr2O72- and CrO42-, depending on the pH. HCrO4- is the predominant Cr(VI) species at
cr
acidic pH, whereas CrO42- predominantly exists at basic pH [31, 32]. At relatively lower pH
us
value, the surface functional groups of the adsorbents (CNFb-S-TEA) are protonated, which facilitate the adsorption of the negatively charged HCrO4- via electrostatic attraction. Increasing
an
pH causes the deprotonation of the surface, repelling the negatively charged CrO42- ions in the solution, and therefore, resulting in less adsorption. The adsorption capacity of the TEA-
M
functionalized material was higher than that of CNF-S at all pH, attributed to the relatively larger amounts of the functional groups (-OH and -N<) present on the surface of CNFb-S-TEA, and
ed
therefore, a higher extent of protonation. This is consistent with the data shown in Fig. 7. The CrO and Cr-N stretching vibrations observed in the FT-IR spectrum of CNFb-S-TEA-ads also
pt
corroborated the bonding of Cr with -OH and -N< groups (Fig. 5). Fig. 9 illustrates the proposed
Ac ce
mechanism for the adsorption of Cr ions on CNFb-S-TEA. The redox potential (Eo) of Cr(VI) is highly positive (1.33-1.38 V) in acidic solution. Therefore, part of HCrO4- ions is reduced to Cr(III) by the redox reaction between HCrO4- and electron donor groups [6, 21, 32]: 3CxOH + HCrO4- + 4H+ ↔ 3CxO- + Cr3+ + 4H2O
(1)
The produced Cr(III) ions are subsequently captured by sorption or ion-exchange on the weak acidic surface groups [6]. The XPS spectrum of Cr corroborated the existence of Cr(III) in the
15
Page 15 of 32
adsorbed samples (Fig. 6). The mechanism also explains a decrease in the adsorption capacity of the prepared adsorbent at a very low pH (~ 2) (Fig. 8).
ip t
Effect of co-existing ions Traces of metal ions such as Cu, Ni, Cd, Zn, and As are commonly present in the Cr-
cr
laden industrial effluents. Fig. 10 shows the test results of the interference tests performed on
us
CNFb-S-TEA. The results are presented for the worst scenario, when all metals were present as impurities at the maximum 10-ppm concentration level in the solution, along with Cr at 100
an
ppm-concentration. At acidic pH, Cr(VI) existed in the form of HCrO4- species, whereas the other metal ions (Cu+2, Ni+2, Cd+2, Zn+2) were positively charged. Therefore, the negatively
M
charged HCrO4- ions were predominantly adsorbed on the protonated functional groups of TEA by electrostatic attraction. Only arsenic ions existed as the negatively charged species in the form
ed
of H2AsO4-. Therefore, the adsorption capacity of CNFb-S-TEA for Cr(VI) decreased in the mixed solution, although by a small amount (10%), attributed to the competitive adsorption
Ac ce
Conclusions
pt
between the HCrO4- and H2AsO4- ions on the active sites of the material.
A CNF-based template was successfully synthesized on a carbon bead substrate for the effective removal of Cr(VI) ions from water by adsorption. The grafting of the CNFs with TEA increased the Cr-adsorption capacity of the material to ~51 mg/g at pH 4.5, which was significantly larger than that of the adsorbents discussed in the literature. The presence of the coexisting metal ions in the model Cr-laden wastewater had negligible effects on the adsorption capacity of the prepared CNFb-S-TEA beads for Cr(VI) ions, confirming the high selectivity of 16
Page 16 of 32
the material for Cr(VI). Various surface characterization techniques, used to characterize the CNFb-S-TEA, confirmed the success of different steps used in the synthesis of the material. This approach used nano-sized templates for the removal of Cr(VI) ions, with large adsorption
ip t
capacity and high selectivity. The CNF-based template described in this study is novel and simple to synthesize, and the method can be potentially extended to develop the similar (CNF-
cr
based) templates for the removal of various other metal ions from wastewater.
us
Acknowledgements
an
The authors acknowledge support from DST (New Delhi, India) in the form of a research
M
grant.
ed
References
Ac ce
pt
[1] S. Deng, R. Bai, Water Res. 38 (2004) 2424-2432. [2] J. Yang, M. Yu, W. Chen, J. Ind. Eng. Chem. 21 (2015) 414-422. [3] R. Kumar, M. Ehsan, M.A. Barakat, J. Ind. Eng. Chem. 20 (2014) 4202-4206. [4] G. Bayramoglu, M.Y. Arica, J. Hazard. Mater. 187 (2011) 213-221. [5] P. Yuan, M. Fan, D. Yang, H. He, D. Liu, A. Yuan, J. Zhu, T. Chen, J. Hazard. Mater. 166 (2009) 821-829. [6] J. Hu, C. Chen, X. Zhu, X. Wang, J. Hazard. Mater. 162 (2009) 1542-1550. [7] N.M. Mubarak, R.K. Thines, N.R. Sajuni, E.C. Abdullah, J.N. Sahu, P. Ganesan, N.S. Jayakumar, Korean J. Chem. Eng. 31 (2014) 1582-1591. [8] Z. Sadaoui, S. Hemidouche, O. Allalou, Desalination 249 (2009) 768-773. [9] M. Jain, V.K. Garg, K. Kadirvelu, J. Environ. Manage. 91 (2010) 949-957. [10] V. Neagu, S. Mikhalovsky, J. Hazard. Mater. 183 (2010) 533-540. [11] H. Deveci, Y. Kar, J. Ind. Eng. Chem. 19 (2013) 190-196. [12] B. Lee, Y. Kim, H. Lee, J. Yi, Microporous and Mesoporous Mater. 50 (2001) 77-90. [13] A. Heidari, H. Younesi, Z. Mehraban, Chem. Eng. J. 153 (2009) 70-79. [14] J. Aguado, J.M. Arsuaga, A. Arencibia, M. Lindo, V. Gascón, J. Hazard. Mater. 163 (2009) 213-221. [15] L.-C. Lin, M. Thirumavalavan, Y.-T. Wang, J.-F. Lee, J. Chem. Eng. Data 55 (2010) 36673673. [16] S. Hao, Y. Zhong, F. Pepe, W. Zhu, Chem. Eng. J. 189 (2012) 160-167. 17
Page 17 of 32
Ac ce
pt
ed
M
an
us
cr
ip t
[17] L. Mercier, T.J. Pinnavaia, Environ. Sci. Technol. 32 (1998) 2749-2754. [18] J. Brown, R. Richer, L. Mercier, Microporous and Mesoporous Mater. 37 (2000) 41-48. [19] S. Rengaraj, J.-W. Yeon, Y. Kim, W.-H. Kim, Ind. Eng. Chem. Res. 46 (2007) 2834-2842. [20] Y. Kim, C. Kim, I. Choi, S. Rengaraj, J. Yi, Environ. Sci. Technol. 38 (2004) 924-931. [21] N. Wu, H. Wei, L. Zhang, Environ. Sci. Technol. 46 (2011) 419-425. [22] D. Zhang, C.-l. Zhang, P. Zhou, J. Hazard. Mater. 186 (2011) 971-977. [23] X.Y. Chen, H. Song, Z.J. Zhang, Y.Y. He, Electrochim. Acta 117 (2014) 55-61. [24] J.K. Kasi, A.K. Kasi, W. Wongwiriyapan, N. Afzulpurkar, P. Dulyaseree, M. Hasan, A. Tuantranont, Adv. Mat. Res. 557 (2012) 544-549. [25] J. Kang, D.H. Shin, K.N. Yun, F.A. Masud, C.J. Lee, M.J. Kim, Carbon 79 (2014) 149-155. [26] A. Chakraborty, D. Deva, A. Sharma, N. Verma, J. Colloid Interface Sci. 359 (2011) 228239. [27] A. Sharma, N. Verma, A. Sharma, D. Deva, N. Sankararamakrishnan, Chem. Eng. Sci. 65 (2010) 3591-3601. [28] M. Bikshapathi, S. Singh, B. Bhaduri, G.N. Mathur, A. Sharma, N. Verma, Colloids Surf., A: Physicochemical and Engineering Aspects 399 (2012) 46-55. [29] L. Pei, C.A. Lucy, J. Chromatograph. A 1365 (2014) 226-233. [30] A.-Y. Wang, B. Chen, L. Fang, J.-J. Yu, L.-M. Wang, Electrochim. Acta 108 (2013) 698706. [31] N. Ballav, H. Choi, S. Mishra, A. Maity, J. Ind. Eng. Chem. 20 (2014) 4085-4093. [32] C. Jung, J. Heo, J. Han, N. Her, S.-J. Lee, J. Oh, J. Ryu, Y. Yoon, Sep. Purif. Technol. 106 (2013) 63-71.
18
Page 18 of 32
Figure captions Fig. 1. Photograph and SEM images of beads. Fig. 2. SEM images of (a, b) Cr-PhB-A, (c, d) CNFb, (e, f) CNFb-S, (g, h) CNFb-S-TEA and (i,
ip t
j) CNFb-S-TEA-ads, at low (10 kx) and high (100 kx) magnifications. Fig. 3. EDX spectra and Elemental mapping of (a) CNFb, (b) CNFb-S-TEA and (c) CNFb-S-
cr
TEA-ads.
us
Fig. 4. TEM images of (a) CNF and (b) CNF-S-TEA.
Fig. 5. FTIR spectra of CNFb-S, CNFb-S-COOH, CNFb-S-TEA and CNFb-S-TEA-ads.
an
Fig. 6. Deconvolution of XPS spectra: C1s, O1s, N1s and Cr2p peaks of CNFb-S-TEA-ads. Fig. 7. Comparative adsorption data of the CNFb-S and CNFb-S-TEA adsorbents for Cr(VI).
M
Fig. 8. Effect of pH on the adsorption of Cr(VI) by the CNFb-S and CNFb-S-TEA adsorbents. Fig. 9. Effect of co-existing 10% (w/w) Cu/Ni/Zn/Cd/As metal ions on the adsorption of Cr(VI)
ed
by the CNFb-S-TEA adsorbent.
Ac ce
pt
Fig. 10. Proposed mechanism for the adsorption of Cr(VI) on CNFb-S-TEA.
19
Page 19 of 32
Table 1
SBET (m2/g)
PSD (%)
VT (cc/g)
Vmicro
Vmeso
(cc/g)
(cc/g)
Vmacro
(cc/g)
cr
Materials
ip t
SBET, VT and PSD of the prepared materials.
1419
0.868
87.33
9.33
3.34
CNFb
892
0.629
65.02
20.03
14.94
CNFb-S
1150
0.738
65.71
24.79
9.48
CNFb-S-TEA
506
0.336
64.58
27.68
7.74
Ac ce
pt
ed
M
an
us
Cr-PhB-A
20
Page 20 of 32
Table 2 Comparative study of different adsorbents for the removal of Cr(VI).
Concent ration (ppm)
Loading (mg/g)
pH
51.09
~4.5
This study
5.60
5.0
[1]
Reference
CNFb-S-TEA
125
2.
Aminated polyacrylonitrile fibers
50
3.
Carbon/AlOOH composite
100
32.60
2.0
[3]
4.
Cr(VI)-imprinted poly(4-vinylpyridine-cohydroxyethyl methacrylate)
200
3.31
4.0
[4]
5.
Montmorillonite-supported magnetite NPs
50
15.30
2.5
[5]
6.
Oxidized multiwalled CNTs
750
2.60
2.9
[6]
7.
Functionalized CNTs
61
2.48
5.0
[7]
8.
Micellar compounds
70
9.53
2.0
[8]
9.
Carbonaceous waste biomass
250
53.40
2.0
[9]
10. Crosslinked poly(4-vinylpyridines)
1000
142.85
4.5
[10]
75
3.53
1.5
[11]
us
an
M
ed
pt
11. Bio-char
cr
1.
Ac ce
Adsorbent
ip t
Sr. No
21
Page 21 of 32
Ac
ce
pt
ed
M
an
us
cr
i
fig. 1.tif
Page 22 of 32
Ac ce p
te
d
M
an
us
cr
ip t
fig. 2.tif
Page 23 of 32
Ac
ce
pt
ed
M
an
us
cr
i
fig. 3.tif
Page 24 of 32
Ac
ce
pt
ed
M
an
us
cr
i
fig. 4.tif
Page 25 of 32
Ac
ce
pt
ed
M
an
us
cr
i
fig. 5.tif
Page 26 of 32
Ac
ce
pt
ed
M
an
us
cr
i
fig. 6.tif
Page 27 of 32
Ac
ce
pt
ed
M
an
us
cr
i
fig. 7.tif
Page 28 of 32
Ac
ce
pt
ed
M
an
us
cr
i
fig. 8.tif
Page 29 of 32
Ac
ce
pt
ed
M
an
us
cr
i
fig. 9.tif
Page 30 of 32
Ac
ce
pt
ed
M
an
us
cr
i
fig. 10.tif
Page 31 of 32
Ac
ce
pt
ed
M
an
us
cr
i
*Graphical Abstract
Page 32 of 32
S1226-086X(16)30003-X http://dx.doi.org/doi:10.1016/j.jiec.2016.02.025 JIEC 2847
To appear in: Received date: Revised date: Accepted date:
22-7-2015 28-2-2016 28-2-2016
Please cite this article as: S. Mishra, N. Verma, Carbon Bead-supported Hollow Carbon Nanofibers Synthesized via Templating Method for the Removal of Hexavalent Chromium, Journal of Industrial and Engineering Chemistry (2016), http://dx.doi.org/10.1016/j.jiec.2016.02.025 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Highlights Carbon bead-supported and TEA-grafted hollow CNFs prepared by templating. Significant adsorption capacity of the material for Cr(VI) ions in water.
Ac ce
pt
ed
M
an
us
cr
ip t
Negligible interference on adsorption from co-existing metal ions.
1
Page 1 of 32
Carbon Bead-supported Hollow Carbon Nanofibers Synthesized via Templating Method for the Removal of Hexavalent Chromium
a
ip t
Shraddha Mishra a and Nishith Vermaa, b*
Center for Environmental Science and Engineering, Indian Institute of Technology Kanpur,
Department of Chemical Engineering, Indian Institute of Technology Kanpur, Kanpur-208016,
us
b
cr
Kanpur - 208016, India
Ac ce
pt
ed
M
an
India
*Corresponding author. Tel.: +91 512 259 6767; fax: +91 512 259 0104 E-mail address: [email protected], [email protected] (N. Verma), [email protected] (S. Mishra). 2
Page 2 of 32
ABSTRACT Novel triethanol amine-grafted hollow carbon nanofibers (CNFs), supported on carbon beads, were synthesized via templating method for the removal of hexavalent chromium (Cr(VI))
ip t
ions from water. CNFs were grown on carbon beads by chemical vapor deposition using the in situ incorporated Cr nanoparticles in the beads, as the catalyst. The beads were ultra-sonicated to
cr
remove the nanoparticles from the tip of the CNFs, thereby creating the hollow CNFs. The
us
Cr(VI)-adsorption capacity of the material was determined to be ~51 mg/g at 125 ppm-initial concentration and 4.5-solution pH. Negligible interference from coexisting metal ions in water
an
was found with the Cr(VI)-adsorption.
Keywords: templating method; carbon nanofibers; carbon beads; adsorption; hexavelent
Ac ce
pt
ed
M
chromium.
3
Page 3 of 32
Introduction Hexavalent chromium (Cr(VI)) is one of the most toxic metal pollutants in water, released from the electroplating, metal finishing, leather tanning, photography, dye and
ip t
textile industries [1, 2]. A redox active element, Cr exists as Cr(III) and Cr(VI) species in water. Cr(VI) is more toxic than Cr(III) and can cause several diseases such as cancer, mucosal
cr
ulceration, kidney disorder and chronic dermatitis [1].
us
Adsorption is considered to be an energy efficient and a simple method [3]. Different types of materials such as carbon, polymers and micellar compounds have been used as
an
adsorbents for Cr in aqueous systems [3-11]. Adsorbents with large surface area, ordered/uniform pore structure and surface activity are preferred for environmental remediation
M
applications. At present, template method is considered to be a powerful tool for producing adsorbents with high surface area. It is a fast developing method for designing a pattern with
ed
uniform pore size distributions. The method commonly includes treating the precursor of a substrate, viz., silica, alumina and titania, with different inorganic or organic reagents used as the
Ac ce
extraction.
pt
template molecules, and then removing the molecules by heat or acid treatment, or soxhlet
Several silica-based templates have been developed for the removal of various metal ions such as Pb(II), Cd(II), Hg(II)), Ni(II), Zn(II) and Cu(II) from aqueous solutions [12-18]. Among the salient studies, Lee et al. prepared a mesoporous silica using dodecylamine as the template for the removal of Hg(II) ions. The surface hydrophilicity of the material was enhanced by surface functionalization using the mercaptopropyl and aminopropyl-containing chelating ligands [12]. Heidari et al. synthesized the amine-functionalized mesoporous and nanomesoporous silica for the simultaneous removal of the Ni(II), Cd(II) and Pb(II) ions from 4
Page 4 of 32
aqueous solution [13]. Aguado et al. also synthesized the amine-functionalized mesopurous silica, using two different methods, namely, grafting and co-condensation. The amine-grafted materials exhibited adequate mesoscopic order with high amino content, and adsorbed significant
ip t
amounts of the Cu(II), Cd(II), Pb(II), Ni(II) and Zn(II) ions [14]. A major drawback of such silica-based materials is that they are destroyed in alkaline medium, caused by the hydrolysis of
cr
Si-O-Si bonds in the materials.
us
Mesoporous alumina- and titania-based templates have also been synthesized for the adsorption of metal ions. Rengaraj et al. [19] and Kim et al. [20] have prepared mesoporous
an
alumina using stearic acid as the template for the removal of Ni(II) and arsenic(V) ions, respectively. Wu et al. synthesized mesoporous titania beads of uniform size, using calcium
M
alginate template for the removal of Cr(VI) [21]. These studies, however, did not consider the effects of co-existing metal ions in water on the adsorption of the solutes. Zhang et al.
ed
synthesized titania-based template using D311 resin as the template for the removal of the Pb(II), Cd(II) and Zn(II) ions in a multi component system [22]. Researchers have also used templating
pt
methods for the synthesis of porous carbon materials including carbon nanotubes (CNTs) and
Ac ce
carbon nanofibers (CNFs), however, for electrochemical applications [23, 24]. This is the first study on the development of hollow CNFs by templating method, for the efficient removal of Cr(VI) ions from aqueous solution. The CNFs have gained a great technological significance because of their excellent physico-chemical properties such as large Brunauer-Emmet-Teller (BET) surface area, amenable to functionalization, and stability in acidic and basic medium. The CNFs contain exposed edge planes on their hexagonal surface, which facilitate chemical bonding with the functional groups-containing compounds [25, 26]. The novelties of the present study are in the synthesis aspects. (1) The in-situ doped Cr nanoparticles 5
Page 5 of 32
in the beads catalyzed the growth of the CNFs as well as they served as the template. (2) The templates were removed by simple ultrasonication of the nanofibrous beads, thereby creating the hollow CNFs. A technical advantage of using such materials is that the synthesized carbon beads
ip t
can be efficiently used as an adsorbent in packed bed columns under flow condition without
cr
maldistribution or channeling of the liquid to be treated.
us
Experimental Chemicals
an
All chemicals used in this study were of analytical reagent grade. Phenol, formaldehyde, triethylamine, hexamethylene tetramine (HMTA), polyvinyl alcohol (PVA), chromium nitrate
M
(Cr nitrate), potassium dichromate and triethanol amine (TEA) were purchased from Merck, Germany. All samples were prepared in Milli-Q water.
ed
Synthesis of Cr-doped polymeric beads
pt
The experimental setup used for the preparation of beads is described elsewhere [27]. Briefly, a 2 L-three-necked round bottom glass reactor was used for the polymerization reaction.
Ac ce
The reactor was equipped with a reflux condenser, mechanical stirrer, and thermometer. The Crdoped phenolic beads were prepared using suspension polymerization with phenol (50 g) as the monomer, formaldehyde (63 mL) as the solvent, and triethylamine (1.5 mL) as the catalyst. The mixture was stirred to prepare a homogenous solution at room temperature (30 ± 5 °C) for 8 h. Next, 200 mL of water was mixed into the reaction mixture. After approximately 30 min, 3.5 g of HMTA, used as the cross-linking agent, was mixed into the solution and the solution was heated 3 °C per min until the solution-temperature reached 100 °C. Approximately 4 g-PVA as a 6
Page 6 of 32
suspension stabilizer was mixed into the solution. After approximately 30 min, the solution turned into a gel-like emulsion. At the incipience of the gel formation, approximately 3 g of the aqueous solution of Cr nitrate, used as the precursor for Cr, was mixed into the reaction mixture.
ip t
The Cr nitrate solution was prepared using 0.3% (w/w) sodium dodecyl sulfate (anionic surfactant) to achieve approximately uniform dispersion of Cr ions on the beads without
cr
agglomeration. After ~3 h, the beads were formed. Heating was stopped. Stirring was continued.
us
After cooling the reaction product to room temperature, the produced solid beads were separated from the residual liquid, using a 20 mesh-sieve. The beads were repeatedly washed with Milli-Q
an
water, methanol and acetone in that sequence. The washed beads were dried at room temperature. The yield of the synthesized spherical beads (~0.8 mm-average size) was ~36 g.
M
The prepared materials were termed as Cr-PhB for the reference purposes in this study.
ed
Growth of CNFs on Cr-PhB
Carbonization, activation, reduction and chemical vapor deposition (CVD) were
pt
sequentially performed on the prepared Cr-PhBs in a horizontal inconel-tubular reactor (30
Ac ce
mm-diameter and 1000 mm-length) installed in the horizontal electric furnace, having provisions for gas inlet and outlet [28]. The polymeric beads were carbonized at 5° per min to 900 °C for 1 h in a nitrogen (N2) atmosphere (150 standard cc per min (sccm)). Activation was performed at the same temperature for another 1 h, using a mixture of N2 (100 sccm) and steam (0.3 g per min). Approximately 40% weight-loss was measured in the produced carbon beads
post-carbonization and activation steps. The average particle diameter also decreased to ∼0.5 7
Page 7 of 32
mm. The porous carbon beads (Cr-PhB-A) were then subjected to reduction at 800 °C for 2 h in a hydrogen atmosphere (150 sccm). Prior to the reduction, temperature programmed reduction analysis was performed on the activated beads to determine the optimum reduction temperature
ip t
(800 °C) for Cr oxides. The catalytic CVD was performed on the H2-reduced beads at 600 °C for 45 min to grow the CNFs, using acetylene as the carbon-source and the in-situ doped Cr
cr
nanoparticles (NPs) as the CVD catalyst. Such beads were termed as CNFb. Fig. 1 shows the
us
photograph of Cr-PhB and the SEM images of the Cr-PhB-A and CNFb samples.
an
Synthesis of TEA-functionalized CNFb
The prepared CNFbs (0.5 g) were ultrasonicated in a 50 mL of 0.05 M-HNO3 aqueous
M
solution for 5 min to dislodge the Cr NPs from the tips of the CNFs. The sonicated carbon beadsupported hollow CNFs were termed as CNFb-S for the reference purposes. Next, the CNFb-S
ed
beads were grafted with TEA. Approximately 0.5 g of CNFb-S were first oxidized with 50 mLHNO3 (65% v/v) at 100 °C for 120 min to functionalize the surface of the beads with carboxylic
pt
functional groups (-COOH), and also, enhance the hydrophilicity of the material. The beads were
Ac ce
cooled to room temperature and then washed with Milli-Q water until the pH of the surface was 7. The oxidized CNFb-S (CNF-S-COOH) samples were dried in a vacuum oven at 70 °C for 2 h. Next, the dried beads were mixed with 20 mL-TEA in a 100 mL-beaker. The beaker was heated on a hotplate at 80 °C for 1 h, thereby allowing the covalent coupling between the CNF-SCOOH and TEA via the formation of an ester group. The synthesized CNFb-S-TEA beads were washed with Milli-Q water and dried in the vacuum oven at 50 °C for 2 h. Batch adsorption study
8
Page 8 of 32
A stock solution of 200 ppm-Cr(VI) in water was prepared using potassium dichromate salt. Test solutions (50 mL) of different concentrations (10-150 ppm) were prepared from the stock solution and transferred to conical flasks (100 mL volume) for the adsorption study.
ip t
Approximately 0.05 g of the prepared CNFb-S-TEA adsorbents were mixed into the test solutions and the samples were kept on a mechanical shaker (150 rpm) at 30 °C for 12 h.
cr
Separate adsorption tests were performed using the Cr-solution of 100 ppm-concentration, mixed
us
together with the 10 ppm-concentration solution, each of Cu, Ni, Cd, Zn, and As salts, to study the interference of impurity ions on the adsorption of Cr by the prepared adsorbents in this study.
an
The concentration of Cr ions in the solution was determined using a flame atomic absorption spectrometer (FAAS) (Varian AA-240, USA) at the wavelength of 357.9 nm, using
M
an air-acetylene flame. Prior to the analysis, the instrument was calibrated using the standard solutions. The amount of Cr adsorbed by the material was calculated from the species balance
ed
equation q = (Ci - Ce) V/W, where q is the adsorption capacity (mg/g); Ci and Ce are the initial and equilibrium concentrations (mg/L), respectively; V is the volume (L) of the solution and W is
pt
the weight (g) of the adsorbent.
Ac ce
Characterization of adsorbents
The surface characterization of the prepared materials was performed using various
analytical techniques. The surface morphology was studied using SEM (Supra 40 VP, Zeiss, Germany) and TEM (FEI Technai, 20 U Twin, USA). The metal compositions were determined using EDX (Oxford Inc., Germany). The area-mapping of the surface of the samples was captured at different locations to examine the distribution of different elements. FT-IR analysis was performed to determine the surface functional groups. The spectra were recorded in the 9
Page 9 of 32
frequency range of 4000-400 cm-1 using the Vertex 70 (Brucker, Germany) instrument. The elemental analysis was performed using an elemental analyzer (CE-440 EL, Exeter Analytical, Inc., USA). The Autosorb-1C instrument (Quantachrome, USA) was used for the surface area
ip t
and pore size distribution (PSD) analysis. The specific BET surface area was determined from the nitrogen adsorption/desorption isotherm at 77 K. The PSD was calculated using the Barrett-
cr
Joyner-Halenda (BJH) method and density functional theory (DFT) for mesopores (2-50 nm) and
us
micropores (< 2 nm), respectively. Prior to the analysis, the samples were degassed at 200 °C to remove the impurities and moisture. XPS analysis was performed using the kratos axis ultra x-
an
ray photoelectron spectrometer (PHI 5000 Versa Prob II, FEI Inc, USA) with a 165 mm-
M
hemispherical electron energy analyzer. Results and Discussion
ed
SEM analysis Micro-nanostructures of Cr-PhB-A, CNFb, CNFb-S, CNFb-S-TEA and Cr-adsorbed
pt
CNFb-S-TEA (CNFb-S-TEA-ads) samples are shown in Fig. 2, at low (10 kx) and high (100 kx)
Ac ce
magnifications. The SEM images of Cr-PhB-A showed a highly porous surface of the beads, and pores were clearly visible in the high magnification-image (Fig. 2a, b). A dense and an approximately uniform distribution of the CNFs was observed on the bead surface (Fig. 2c,d). The shiny tips of the CNFs were clearly visible in the high magnification-image, attributed to the presence of metal NPs at the tips. The growth of the CNFs occurred by the tip-growth mechanism and the Cr NPs moved out from within the pores to the external surface of the beads, along with the nanofibers during the CVD step. The Cr NPs were leached out from the tips of the CNFs during sonication and the surface texture of the CNFs became relatively less 10
Page 10 of 32
bright (Fig. 2e,f). The grafting with TEA changed the surface morphology of the beads (Fig. 2g,h). The surface became rough and the fibers were covered with the coating, indicating that the grafting was effective over the entire surface of CNFb. A white precipitate (the complexation
ip t
product of TEA and Cr) was visible on the surface of the CNFb-S-TEA-ads samples (Fig. 2i,j).
cr
EDX and elemental mapping
us
The EDX spectra confirmed the presence of the Cr NPs on the surface of CNFb (Fig. 3a). The sonication of CNFb caused the significant reduction in the amounts of Cr. Therefore, a
an
negligible amount (0.19%) of Cr was observed in CNFb-S-TEA (Fig. 3b). However, the amount significantly increased in the CNFb-S-TEA-ads samples (Fig. 3c).
M
The elemental mapping (shown underneath the EDX spectra in Fig. 3) was performed for the (pre-sonicated) CNFb samples, and the CNFb-S-TEA samples (before and after adsorption).
ed
The red and green dots represent C and Cr elements, respectively. Approximately uniform and dense distribution of Cr was observed in the respective mapping of the CNFb and CNFb-S-TEA-
Ac ce
TEM analysis
pt
ads samples, indicating the suitability of the prepared CNFb-S-TEA adsorbents for Cr.
The TEM images of the CNF and CNF-S-TEA samples are exhibited in Fig. 4. The black spot seen at the tip of the fiber indicated that the metal was adhered at the tip of the CNF. After the sonication, the tips of the CNFs were opened up, creating hollow tube like structures of uniform size. Further, the grafting with TEA utilized the expose edge of the CNFs, without
11
Page 11 of 32
removing the cavity at the tip of the CNFs. The average internal diameter of the fiber was
cr
ip t
measured to be ∼20 nm, using Image J software.
us
FTIR Spectra
an
Fig. 5 shows the FTIR spectra of the CNFb-S, CNFb-S-COOH, CNFb-S-TEA and CNFb-S-TEA-ads samples. A common peak is observed at 3417 cm-1 in all samples, attributed to
M
-OH stretch. Peaks observed at 1163 and 1623 in CNFb-S-COOH are attributed to >C-O and >C=O stretches of carboxylic functional group, respectively, caused by the oxidation of CNFb-S.
ed
The >C-N stretch observed in the spectrum of CNFb-S-TEA confirmed the attachment of TEA on CNFb-S-COOH. The stretching frequencies observed at 572 cm-1 (Cr-O stretch) and 431 cm-1
pt
(Cr-N stretch) in CNFb-S-TEA-ads (i.e., post Cr-adsorbed sample) indicated the formation of the
Ac ce
Cr-TEA complex (inset in Fig. 5) Elemental analysis
Elemental analysis was performed to determine the elemental compositions (N and O) of CNFb-S and CNFb-S-TEA. The N-contents were determined to be 2.01 and 3.46% in CNFb-S and CNFb-S-TEA, respectively, whereas the O-contents were 22.1 and 34.46% in CNFb-S and CNFb-S-TEA, respectively. The higher N- and O-contents in CNFb-S-TEA indicated the attachment of the ligand (TEA) to the CNFb-S-TEA surface. 12
Page 12 of 32
Surface area and PSD Table 1 summarizes the SBET, total pore volume (VT) and PSD of the materials, calculated
ip t
from the isotherms data. The Cr-PhB-A beads were highly microporous, with the microporosity
us
cr
content determined to be ∼87% of the total pore volume. On growing the CNFs, the BET surface
an
area of the beads decreased because the CNFs partially blocked the pore entrance. The BET surface area increased in CNFb-S because of the removal of the Cr NPs from the tip of the
M
CNFs, thereby exposing the internal surface area to the N2 adsorbate used for the measurements of BET surface area. Further, the grafting of CNFb-S with TEA blocked the micropores,
ed
resulting in the reduction of pore volume and BET surface area, however, with an increase in the
pt
mesoporous contents of CNFb-S-TEA.
Ac ce
X-ray photoelectron microscopy
XPS analysis was performed on CNFb-S-TEA-ads to confirm the adsorption of Cr on the surface of the material. Fig 6 shows the C1s, O1s, N1s and Cr2p spectra. The deconvolution of C1s spectrum identified three peaks at 284.34 eV (C-C) , 285.74 eV (C-N, C-OH and C=O) and 289.11 eV (O-C=O). Next, the deconvolution of the O1s spectrum confirmed the peak at 532.02 eV, attributed to the presence of C=O, O-C=O, and that at 533.25 eV, attributed to C-OH group. The deconvolution of N1s spectrum confirmed the peak at 399.41 eV, attributed to tertiary amine [29] and that at 401.08 eV, attributed to the quaternary nitrogen atoms [30]. This also confirmed 13
Page 13 of 32
that the N atom of the TEA was protonated at acidic pH. The speciation of the Cr on the surface of CNFb-S-TEA-ads was also characterized from the XPS spectra. The spectrum showed two bands at 576.89 eV (Cr(III)) and 586.51 eV (Cr(VI)), the first corresponding to the Cr2p3/2
ip t
orbital and the second corresponding to the Cr2p1/2 orbital [5], indicating the presence of both
cr
Cr(III) and Cr(VI) species on the adsorbent surface.
us
Adsorption capacity
Adsorption tests were performed at solution pH (~4.5 ± 0.3) over 10-150 ppm-
an
concentration range. The adsorbents were mixed in the Cr(VI) solutions and stirred at 30 °C. The Cr-equilibrium loadings increased with the aqueous phase concentrations of Cr and the
M
equilibrium loading on CNFb-S-TEA was higher than that on CNFb-S at all concentrations (Fig. 7).
ed
The adsorption capacity of CNFb-S-TEA was compared with the other adsorbents including polymer-, micellar compounds- and carbon-based materials discussed in the literature
pt
[1, 3-11] and the data are summarized in Table 2. The comparison was made considering
Ac ce
different pH and initial aqueous phase concentrations of Cr (mg/l) used in the studies. (1) The adsorption capacity of CNFb-S-TEA is considerably higher than most of the materials. (2) In a few cases where the (literature) materials exhibited high capacity, either the solution was considerably acidic (pH < 3), which is not suitable for practical remediation applications, or the aqueous phase concentration levels were high, which is not a realistic representation of industrial effluents. Effect of pH and adsorption mechanism
14
Page 14 of 32
The effect of pH on the adsorption of Cr(VI) was studied over the pH range of 1-8 at 100 ppm-initial Cr concentration. The adsorption capacity of CNFb-S-TEA decreased from ~61 to 23 mg/g with the increase of solution pH from 2 to 8, indicating that the adsorption of Cr(VI)
ip t
significantly depended on pH (Fig. 8). The Cr(VI) ions exist in various anionic forms such as HCrO4-, Cr2O72- and CrO42-, depending on the pH. HCrO4- is the predominant Cr(VI) species at
cr
acidic pH, whereas CrO42- predominantly exists at basic pH [31, 32]. At relatively lower pH
us
value, the surface functional groups of the adsorbents (CNFb-S-TEA) are protonated, which facilitate the adsorption of the negatively charged HCrO4- via electrostatic attraction. Increasing
an
pH causes the deprotonation of the surface, repelling the negatively charged CrO42- ions in the solution, and therefore, resulting in less adsorption. The adsorption capacity of the TEA-
M
functionalized material was higher than that of CNF-S at all pH, attributed to the relatively larger amounts of the functional groups (-OH and -N<) present on the surface of CNFb-S-TEA, and
ed
therefore, a higher extent of protonation. This is consistent with the data shown in Fig. 7. The CrO and Cr-N stretching vibrations observed in the FT-IR spectrum of CNFb-S-TEA-ads also
pt
corroborated the bonding of Cr with -OH and -N< groups (Fig. 5). Fig. 9 illustrates the proposed
Ac ce
mechanism for the adsorption of Cr ions on CNFb-S-TEA. The redox potential (Eo) of Cr(VI) is highly positive (1.33-1.38 V) in acidic solution. Therefore, part of HCrO4- ions is reduced to Cr(III) by the redox reaction between HCrO4- and electron donor groups [6, 21, 32]: 3CxOH + HCrO4- + 4H+ ↔ 3CxO- + Cr3+ + 4H2O
(1)
The produced Cr(III) ions are subsequently captured by sorption or ion-exchange on the weak acidic surface groups [6]. The XPS spectrum of Cr corroborated the existence of Cr(III) in the
15
Page 15 of 32
adsorbed samples (Fig. 6). The mechanism also explains a decrease in the adsorption capacity of the prepared adsorbent at a very low pH (~ 2) (Fig. 8).
ip t
Effect of co-existing ions Traces of metal ions such as Cu, Ni, Cd, Zn, and As are commonly present in the Cr-
cr
laden industrial effluents. Fig. 10 shows the test results of the interference tests performed on
us
CNFb-S-TEA. The results are presented for the worst scenario, when all metals were present as impurities at the maximum 10-ppm concentration level in the solution, along with Cr at 100
an
ppm-concentration. At acidic pH, Cr(VI) existed in the form of HCrO4- species, whereas the other metal ions (Cu+2, Ni+2, Cd+2, Zn+2) were positively charged. Therefore, the negatively
M
charged HCrO4- ions were predominantly adsorbed on the protonated functional groups of TEA by electrostatic attraction. Only arsenic ions existed as the negatively charged species in the form
ed
of H2AsO4-. Therefore, the adsorption capacity of CNFb-S-TEA for Cr(VI) decreased in the mixed solution, although by a small amount (10%), attributed to the competitive adsorption
Ac ce
Conclusions
pt
between the HCrO4- and H2AsO4- ions on the active sites of the material.
A CNF-based template was successfully synthesized on a carbon bead substrate for the effective removal of Cr(VI) ions from water by adsorption. The grafting of the CNFs with TEA increased the Cr-adsorption capacity of the material to ~51 mg/g at pH 4.5, which was significantly larger than that of the adsorbents discussed in the literature. The presence of the coexisting metal ions in the model Cr-laden wastewater had negligible effects on the adsorption capacity of the prepared CNFb-S-TEA beads for Cr(VI) ions, confirming the high selectivity of 16
Page 16 of 32
the material for Cr(VI). Various surface characterization techniques, used to characterize the CNFb-S-TEA, confirmed the success of different steps used in the synthesis of the material. This approach used nano-sized templates for the removal of Cr(VI) ions, with large adsorption
ip t
capacity and high selectivity. The CNF-based template described in this study is novel and simple to synthesize, and the method can be potentially extended to develop the similar (CNF-
cr
based) templates for the removal of various other metal ions from wastewater.
us
Acknowledgements
an
The authors acknowledge support from DST (New Delhi, India) in the form of a research
M
grant.
ed
References
Ac ce
pt
[1] S. Deng, R. Bai, Water Res. 38 (2004) 2424-2432. [2] J. Yang, M. Yu, W. Chen, J. Ind. Eng. Chem. 21 (2015) 414-422. [3] R. Kumar, M. Ehsan, M.A. Barakat, J. Ind. Eng. Chem. 20 (2014) 4202-4206. [4] G. Bayramoglu, M.Y. Arica, J. Hazard. Mater. 187 (2011) 213-221. [5] P. Yuan, M. Fan, D. Yang, H. He, D. Liu, A. Yuan, J. Zhu, T. Chen, J. Hazard. Mater. 166 (2009) 821-829. [6] J. Hu, C. Chen, X. Zhu, X. Wang, J. Hazard. Mater. 162 (2009) 1542-1550. [7] N.M. Mubarak, R.K. Thines, N.R. Sajuni, E.C. Abdullah, J.N. Sahu, P. Ganesan, N.S. Jayakumar, Korean J. Chem. Eng. 31 (2014) 1582-1591. [8] Z. Sadaoui, S. Hemidouche, O. Allalou, Desalination 249 (2009) 768-773. [9] M. Jain, V.K. Garg, K. Kadirvelu, J. Environ. Manage. 91 (2010) 949-957. [10] V. Neagu, S. Mikhalovsky, J. Hazard. Mater. 183 (2010) 533-540. [11] H. Deveci, Y. Kar, J. Ind. Eng. Chem. 19 (2013) 190-196. [12] B. Lee, Y. Kim, H. Lee, J. Yi, Microporous and Mesoporous Mater. 50 (2001) 77-90. [13] A. Heidari, H. Younesi, Z. Mehraban, Chem. Eng. J. 153 (2009) 70-79. [14] J. Aguado, J.M. Arsuaga, A. Arencibia, M. Lindo, V. Gascón, J. Hazard. Mater. 163 (2009) 213-221. [15] L.-C. Lin, M. Thirumavalavan, Y.-T. Wang, J.-F. Lee, J. Chem. Eng. Data 55 (2010) 36673673. [16] S. Hao, Y. Zhong, F. Pepe, W. Zhu, Chem. Eng. J. 189 (2012) 160-167. 17
Page 17 of 32
Ac ce
pt
ed
M
an
us
cr
ip t
[17] L. Mercier, T.J. Pinnavaia, Environ. Sci. Technol. 32 (1998) 2749-2754. [18] J. Brown, R. Richer, L. Mercier, Microporous and Mesoporous Mater. 37 (2000) 41-48. [19] S. Rengaraj, J.-W. Yeon, Y. Kim, W.-H. Kim, Ind. Eng. Chem. Res. 46 (2007) 2834-2842. [20] Y. Kim, C. Kim, I. Choi, S. Rengaraj, J. Yi, Environ. Sci. Technol. 38 (2004) 924-931. [21] N. Wu, H. Wei, L. Zhang, Environ. Sci. Technol. 46 (2011) 419-425. [22] D. Zhang, C.-l. Zhang, P. Zhou, J. Hazard. Mater. 186 (2011) 971-977. [23] X.Y. Chen, H. Song, Z.J. Zhang, Y.Y. He, Electrochim. Acta 117 (2014) 55-61. [24] J.K. Kasi, A.K. Kasi, W. Wongwiriyapan, N. Afzulpurkar, P. Dulyaseree, M. Hasan, A. Tuantranont, Adv. Mat. Res. 557 (2012) 544-549. [25] J. Kang, D.H. Shin, K.N. Yun, F.A. Masud, C.J. Lee, M.J. Kim, Carbon 79 (2014) 149-155. [26] A. Chakraborty, D. Deva, A. Sharma, N. Verma, J. Colloid Interface Sci. 359 (2011) 228239. [27] A. Sharma, N. Verma, A. Sharma, D. Deva, N. Sankararamakrishnan, Chem. Eng. Sci. 65 (2010) 3591-3601. [28] M. Bikshapathi, S. Singh, B. Bhaduri, G.N. Mathur, A. Sharma, N. Verma, Colloids Surf., A: Physicochemical and Engineering Aspects 399 (2012) 46-55. [29] L. Pei, C.A. Lucy, J. Chromatograph. A 1365 (2014) 226-233. [30] A.-Y. Wang, B. Chen, L. Fang, J.-J. Yu, L.-M. Wang, Electrochim. Acta 108 (2013) 698706. [31] N. Ballav, H. Choi, S. Mishra, A. Maity, J. Ind. Eng. Chem. 20 (2014) 4085-4093. [32] C. Jung, J. Heo, J. Han, N. Her, S.-J. Lee, J. Oh, J. Ryu, Y. Yoon, Sep. Purif. Technol. 106 (2013) 63-71.
18
Page 18 of 32
Figure captions Fig. 1. Photograph and SEM images of beads. Fig. 2. SEM images of (a, b) Cr-PhB-A, (c, d) CNFb, (e, f) CNFb-S, (g, h) CNFb-S-TEA and (i,
ip t
j) CNFb-S-TEA-ads, at low (10 kx) and high (100 kx) magnifications. Fig. 3. EDX spectra and Elemental mapping of (a) CNFb, (b) CNFb-S-TEA and (c) CNFb-S-
cr
TEA-ads.
us
Fig. 4. TEM images of (a) CNF and (b) CNF-S-TEA.
Fig. 5. FTIR spectra of CNFb-S, CNFb-S-COOH, CNFb-S-TEA and CNFb-S-TEA-ads.
an
Fig. 6. Deconvolution of XPS spectra: C1s, O1s, N1s and Cr2p peaks of CNFb-S-TEA-ads. Fig. 7. Comparative adsorption data of the CNFb-S and CNFb-S-TEA adsorbents for Cr(VI).
M
Fig. 8. Effect of pH on the adsorption of Cr(VI) by the CNFb-S and CNFb-S-TEA adsorbents. Fig. 9. Effect of co-existing 10% (w/w) Cu/Ni/Zn/Cd/As metal ions on the adsorption of Cr(VI)
ed
by the CNFb-S-TEA adsorbent.
Ac ce
pt
Fig. 10. Proposed mechanism for the adsorption of Cr(VI) on CNFb-S-TEA.
19
Page 19 of 32
Table 1
SBET (m2/g)
PSD (%)
VT (cc/g)
Vmicro
Vmeso
(cc/g)
(cc/g)
Vmacro
(cc/g)
cr
Materials
ip t
SBET, VT and PSD of the prepared materials.
1419
0.868
87.33
9.33
3.34
CNFb
892
0.629
65.02
20.03
14.94
CNFb-S
1150
0.738
65.71
24.79
9.48
CNFb-S-TEA
506
0.336
64.58
27.68
7.74
Ac ce
pt
ed
M
an
us
Cr-PhB-A
20
Page 20 of 32
Table 2 Comparative study of different adsorbents for the removal of Cr(VI).
Concent ration (ppm)
Loading (mg/g)
pH
51.09
~4.5
This study
5.60
5.0
[1]
Reference
CNFb-S-TEA
125
2.
Aminated polyacrylonitrile fibers
50
3.
Carbon/AlOOH composite
100
32.60
2.0
[3]
4.
Cr(VI)-imprinted poly(4-vinylpyridine-cohydroxyethyl methacrylate)
200
3.31
4.0
[4]
5.
Montmorillonite-supported magnetite NPs
50
15.30
2.5
[5]
6.
Oxidized multiwalled CNTs
750
2.60
2.9
[6]
7.
Functionalized CNTs
61
2.48
5.0
[7]
8.
Micellar compounds
70
9.53
2.0
[8]
9.
Carbonaceous waste biomass
250
53.40
2.0
[9]
10. Crosslinked poly(4-vinylpyridines)
1000
142.85
4.5
[10]
75
3.53
1.5
[11]
us
an
M
ed
pt
11. Bio-char
cr
1.
Ac ce
Adsorbent
ip t
Sr. No
21
Page 21 of 32
Ac
ce
pt
ed
M
an
us
cr
i
fig. 1.tif
Page 22 of 32
Ac ce p
te
d
M
an
us
cr
ip t
fig. 2.tif
Page 23 of 32
Ac
ce
pt
ed
M
an
us
cr
i
fig. 3.tif
Page 24 of 32
Ac
ce
pt
ed
M
an
us
cr
i
fig. 4.tif
Page 25 of 32
Ac
ce
pt
ed
M
an
us
cr
i
fig. 5.tif
Page 26 of 32
Ac
ce
pt
ed
M
an
us
cr
i
fig. 6.tif
Page 27 of 32
Ac
ce
pt
ed
M
an
us
cr
i
fig. 7.tif
Page 28 of 32
Ac
ce
pt
ed
M
an
us
cr
i
fig. 8.tif
Page 29 of 32
Ac
ce
pt
ed
M
an
us
cr
i
fig. 9.tif
Page 30 of 32
Ac
ce
pt
ed
M
an
us
cr
i
fig. 10.tif
Page 31 of 32
Ac
ce
pt
ed
M
an
us
cr
i
*Graphical Abstract
Page 32 of 32