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Highly efficient and robust electrospun nanofibers for selective removal of acid dye
Accepted Manuscript Highly efficient and robust electrospun nanofibers for selective removal of acid dye
Umair Ahmed Qureshi, Zeeshan Khatri, Farooq Ahmed, Abdul Sameeu Ibupoto, Muzamil Khatri, Faraz Ahmed Mahar, Rafi Zaman Brohi, Ick Soo Kim PII: DOI: Reference:
S0167-7322(17)33053-2 doi: 10.1016/j.molliq.2017.08.129 MOLLIQ 7870
To appear in:
Journal of Molecular Liquids
Received date: Revised date: Accepted date:
10 July 2017 24 August 2017 28 August 2017
Please cite this article as: Umair Ahmed Qureshi, Zeeshan Khatri, Farooq Ahmed, Abdul Sameeu Ibupoto, Muzamil Khatri, Faraz Ahmed Mahar, Rafi Zaman Brohi, Ick Soo Kim , Highly efficient and robust electrospun nanofibers for selective removal of acid dye, Journal of Molecular Liquids (2017), doi: 10.1016/j.molliq.2017.08.129
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ACCEPTED MANUSCRIPT Highly efficient and robust electrospun nanofibers for selective removal of acid dye Umair Ahmed Qureshi a,b, Zeeshan Khatri†a,c , Farooq Ahmed a , Abdul Sameeu Ibupoto a, Muzamil Khatri c, Faraz Ahmed Mahar a, Rafi Zaman Brohi d, Ick Soo Kim c†† a
Nanomaterials Research Lab, Department of Textile Engineering, Mehran University of Engineering and Technology, Jamshoro 76062,
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Pakistan
Government Boys Degree College Qasimabad, Hyderabad, 71000, Pakistan.
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Nano Fusion Technology Research Lab, Division of Frontier Fibers, Institute for Fiber Engineering (IFES), Interdisciplinary Cluster for
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Department of Environmental Engineering, Mehran University of Engineering and Technology Jamshoro, 76062, Pakistan
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d
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Cutting Edge Research (ICCER), Shinshu University, 3-15-1, Tokida, Ueda, Nagano 386-8567, Japan.
E-mail: [email protected]
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†Corresponding Author: Zeeshan Khatri Dr.Eng.
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Address: Nanomaterials Research Lab, Department of Textile Engineering, Mehran University of Engineering and Technology, Jamshoro 76060, Pakistan.
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Tel: 0092 (0) 22 2772250
††Corresponding Author: Ick Soo Kim, Dr.Eng.
E-mail: [email protected]
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Address: Nano Fusion Technology Research Group, Shinshu University, 3-15-1, Tokida, Ueda City, Nagano 386-8567, Japan.
Tel.: +81 268 21 5439; Fax: +81 268 21 5482.
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Graphical abstract
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Abstract
The highly efficient and economic nylon-6 nanofibers without pre/post electrospinning treatment were
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prepared for the fast and selective removal of anionic dye. i.e., Acid Blue 117 (AB 117), in just 25 min
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of contact. This is the first time the application of pure nylon-6 nanofibers for dye removal is reported. Nylon-6 , in nanofibers form, possessed greater adsorption capability compared to its pellet form. The
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adsorption data fitted well to pseudo second order and Freundlich isotherm. Nylon-6 nanofibers showed excellent efficiency for AB117 in single and higher selectivity in binary component system. The nanofibers were also used as a membrane for removing AB117 from continuous streams by varying the number of layers. The maximum adsorption capacity in batch mode was found 58.8 mg/g and 0.321 mg/cm2 in continuous mode. The proposed nanofibers offer maximum permeation flux of 113.2 L/m2.h which is much higher than conventional nanofiltration membranes with 10 L/m2.h.bar. Keywords Filtration, electrospinning, nanofibers, selectivity, acid blue 117, adsorption 2
ACCEPTED MANUSCRIPT Introduction Dye pollution and its control is a critical global issue because they are refractory and lead increased biochemical oxygen demand. In addition to this, dyes in water inhibit photosynthetic activities by resisting the penetration of sunlight in water bodies
[1,2]
. Acid blue 117 (AB 117) is an anionic, azo
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classified dye having azo group (-N=N-) with aromatic groups in its chemical structure. Presence of
[3]
. Additionally, continuous exposure to such dyes causes dysfunction of
liver, kidney, brain and reproductive systems as well
[4]
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biodegradation respectively
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amine groups and other aromatic groups in the azo dyes make them highly toxic and more resistant to
. It is generally used for coloring wool, nylon,
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silk and rayon blended fabrics. The effluents generated from industries, particularly textile industry,
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contain approximately 15% of total dye that deteriorates not only environment but also reduces availability of drinkable water resources. Therefore, it is the utmost task to prevent discharge of
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industrial effluents to an aquatic environment from eco-toxicological and aesthetic point of view.
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Conventionally, various chemical and biological processes are employed to remove dyes from wastewater such as photochemical, coagulation, ozonation, membrane filtration and adsorption.
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Nevertheless, the use of these techniques is limited by high operation cost, generation of waste sludge
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and ineffectiveness in treating effluent with low dye concentrations [5]. Adsorption has proven the best in terms of ease of operation, low cost, simple design and its flexibility towards the type of adsorbent
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used in the process.
Many cheap and efficient powdered materials have been studied for removing anionic dyes such as, overripe Cucumis sativus peels [3], chitosan-metal complex [6], hydroxyl double salts [7] etc. but their efficient performance in presence of other anionic dyes and electrolytes of different nature was not addressed properly. Additionally, powdered adsorbents also require tiresome filtration steps along with sample loss and generation of secondary pollution. The industrial wastewater, especially textile industry, is not loaded with single dye but numbers of aesthetic dyes are being discharged at a time with 3
ACCEPTED MANUSCRIPT many folds greater concentrations of salts together with acids and bases. In addition to this, most of the research is focused on batch wise adsorption process which is virtually not efficient in terms of time and energy saving purpose. Therefore, the selective removal and recovery of dyes particularly acid dyes and their removal through continuous process without being affected due to highly acidic, basic pH or electrolyte rich environment is a challenging issue. Some studies manifested selective and efficient , polydopamine-polyethylene imine ultrathin
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[8]
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separation of anionic dyes such as, Fe3O4@NH2@PEI
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coating on polyvinyl alcohol-polyethylene imine (PDA/PEI@PVA/PEI) nanofibers [9] but, such studies are limited by the higher cost of starting materials and lengthy synthetic procedures. Therefore, search
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of economic, simple and efficient material is the emphasis of current research. Nowadays, nanofibers have received hallmark achievement both scientifically and technologically due to exceptionally higher
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surface area, easy handling and highly porous morphology. To achieve better removal efficiency for
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anionic dyes, variety of nanofibers and their composites have been proposed but either their prolonged preparations times or delayed equilibrium time (20 to 24 h), limit their applications for practical [10, 11]
.
Nylon-6 nanofibers have gained reasonable attention due to abundant
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implementation
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availability and higher mechanical strength. Although, many research papers are published based on surface modified nylon-6 nanofibers such as, graphene flakes loaded nylon 6 nanofibers through [12]
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supersonic kinetic spraying
for methylene blue (MB) removal and polyaniline coated nylon 6 [13]
; but, their applications are limited to only lower dye
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nanofibers for methyl orange (MO) removal
concentrations (5 and 10 mg/L respectively) and greater equilibrium time (400-600 min) for MO removal. The application of commercially available nylon-6 nanofibers alone for removal of few anionic dyes is reported, which is not sufficiently performed to resolve the issues pertaining interference, selectivity of dyes and lack information regarding appropriate adsorption mechanism [14,15]
. Its importance in terms of economic feasibility has also not been highlighted in previous studies
and thus remained a gap. 4
ACCEPTED MANUSCRIPT We, therefore, for the very first time are reporting application of pure nylon 6 nanofibers for the efficient removal of anionic dye (AB117) without time consuming pre/post electrospinning treatment. Our main objective is to use easily available unmodified nylon-6 nanofibers for the selective removal of anionic acid dye within shortest time both batch wise and continuous filtration system, under ambient conditions in order to save cost, time and energy. Further, the main concern related to its efficient
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performance in presence of interfering ions is also the main goal of current research. The detailed
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mechanism of dye-nylon-6 has also been proposed through advance instrumental approach. Experimental
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Material and methods
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Nylon-6 polymer (pellets 3mm) was purchased from Sigma Aldrich, Japan. Formic acid (98%) was obtained from Wako Pure Chemical Industries, Japan. Acid blue 117 (anionic dye), Reactive black 5,
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Reactive red 195, Acid orange 67 and Methyl orange were supplied by Archroma, Pakistan Ltd. and the
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corresponding dye structures are given in Table 1. HCL (37%) and NaOH were purchased from Sigma
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Electrospinning of nylon-6
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Aldrich, USA.
Nylon-6 nanofibers were obtained by electrospinning technique. The procedure was reproduced from
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our recent published work
[16]
. A 22 wt% nylon-6 solution was prepared using formic acid as solvent.
The solution was mixed using magnetic stirrer for 12 h at room temperature. The prepared solution was electrospun using electrospinning apparatus installed with high voltage supply (Har-100∗12, Matsusada Co., Tokyo, Japan). A 5 ml syringe was used for electrospinning such that a positive electrode connected with the metallic electrode in the syringe and negative electrode was attached with the rotating metallic collector. Distance between the tip and metallic rotating collector was 15 cm and a 15
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ACCEPTED MANUSCRIPT kV voltage was used to prepare nylon-6 nanofiber webs. The nanofiber webs were fabricated at 30.6 ± 2 °C under relative humidity of 40%.
Characterization
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The morphology of nanofibers was studied using scanning electron microscope (SEM, JEOL model
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JSM 6010 LA). Samples were sputtered coated with Pd-Pt prior to examination. Nanofiber diameter
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was calculated using image J software. Mechanical tests were conducted on universal testing machine
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(Tensilon RTC1250A; A&D Company Ltd, Japan) equipped with a 120 N load cell using stretching rate of 5 mm/min. Dog-bone-shaped specimens were cut with a length of 3 cm and width of 0.2 cm
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with the help of rotary cutter. The surface functional groups on nylon 6 nanofibers and their role in dye fixation was studied by Fourier Transform Infrared Spectroscopy (Shimadzu 8900-FT-IR spectrometer
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Tokyo, Japan). X-ray photoelectron spectroscopy (XPS) measurements were performed with AXIS
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Ultra by Shimadzu equipped with dual-anode X-ray source Al/Mg and the HSA hemispherical sector
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analyzer detector with the vacuum pressure maintained at 1.4 × 10−9 Torr. A Mg Kα X-ray source (1253.6 eV) was used for XPS measurements. Data analysis and curve fitting were performed using
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Origin Pro 9.0 with a Gaussian−Lorentzian product function.
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Dye adsorption experiment
Both batch and continuous methods were implemented to study dye removal by nylon-6 nanofibers. For batch experiments, 20 mg nylon-6 nanofibers were mixed with 5mL of AB117 dye solution (50 mg/L). The parameters studied to find the optimum conditions were adsorption time (0-25 min), pH (1-11), mass of nanofibers (10-40 mg), initial dye concentrations (25-400 mg/L). The influence of commonly occurring different interfering ions such as, Na+, Ca2+, Mg2+, Cl- and SO42- was also studied by preparing 100 mg/L solution of each ions separately from their precursor salts namely NaCl, 6
ACCEPTED MANUSCRIPT CaCl2.2H2O, MgCl2.6H2O and Na2SO4 respectively. Further, the selective response of nylon-6 nanofibers towards AB117 was investigated by conducting adsorption experiment with individual anionic dye solution, i.e. AB117, RB5, RR195, MO and AO 67 (50 mg/L each). The wavelength maximum of each dye is given in Table 1. The selectivity experiment was also performed by mixing 20 mg nylon-6 nanofibers in binary mixture of dyes containing AB117: RR 195 (1:1 w/w) and AB117 :
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MO (1:1 w/w). The dye removal efficiency (AE %) was calculated through UV-Vis spectrophotometer
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Cf AE% 1 Co
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using the equation (1) :
(1)
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Where Co and Cf (mg/L) are the initial and remaining concentrations of dye obtained after dye-nylon contact. The maximum adsorption capacity qe (mg/g) of nylon-6 was also calculated through the
V m
(2)
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q e C o C f
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equation (2):
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Where V is the volume (L) of dye solution and m is the mass of nylon-6 nanofibers (g). The feasibility of nylon-6 nanofibers for large industrial scale application was also assessed
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through continuous mode. Briefly, nylon-6 nanofibers were loaded on conventional filter holder with 13 mm diameter. The dye solution (50 mg/L) was loaded in plastic syringe and was installed on syringe pump usually used for electrospinning purpose. The dye solution was carefully passed through the filtration cell holder and the residual concentration after every interval was monitored. The adsorption capacity of membrane was determined through equation (3):
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Co Vs A
(3)
ACCEPTED MANUSCRIPT Where Co is the initial dye concentration, Vs is the volume (L) of permeate when the dye removal was less than 99%. A is the effective area (cm2). All experiments were performed in triplicate and the mean values are provided in current work. To further validate the linear fittings of kinetic and isotherm models apart from regression coefficient (R2), sum of squared errors (SSE) was also
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determined through the equation (4): SSE q cal q exp
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where qcal and qexp (mg/g) are the calculated and experimentally determined adsorption
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capacities of nylon-6 nanofibers. The smaller the SSE values, the better will be the corresponding
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fitting.
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Table 1. Physical properties of different anionic dyes λmax
Molecular weight
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Dye name
588
594.57
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Acid Blue 117
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(g/mol)
Methyl Orange
463
327.3
Acid Orange 67
477
604.5
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Structure
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1136
Reactive black 5
590
991.8
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Reactive red 195
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Results and discussion
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SEM Analysis
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Figure 1a presents the SEM micrographs of nylon-6 nanofibers. From the images, the nanofibers obtained were uniform and bead free. The nanofibers were found to exist in both thick and thin
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morphologies. The average nanofiber diameter was found to be 198.6 nm (Figure 1b).
Figure 1. (a) SEM micrograph of nylon-6 nanofibers and (b) Histogram of nanofiber diameter distribution. Mechanical Test: 9
ACCEPTED MANUSCRIPT Figure 2 shows typical stress-strain curve of nylon-6 nanofibers that exhibited a tensile strength of 5.6MPa at break of around 26% strain. This strength is much greater than other nanofibers [17,18] and
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hence, exhibit greater stiffness. The nanofiber specimen possessed toughness of 21.8 J/g.
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Figure 2. Typical stress-strain curve of nylon-6 nanofibers XPS Analysis
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XPS survey scan of the nylon-6 nanofibers is shown in Figure 3a. The survey spectrum indicated the
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presence of C (284 eV), N (398 eV) and O (530 eV) as anticipated. The main information regarding chemical composition was collected through deconvolution of C1s, N1s and O1s peaks. The deconvolution of peak at 284 eV (Figure 3b) indicated four chemical environments for carbon atom. The peaks at 284.7 eV, 287.1 eV, 287.4 eV and 290 eV suggest the presence of C in the forms of CH2CH2, CH2-NH, CH2-C=O and HN-C=O respectively. The high resolution XPS spectrum of N1s at 402 eV is probably due to N-C=O, amide bond (Figure 3c). The O1s region was deconvoluted into two
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ACCEPTED MANUSCRIPT chemical states, i.e. 531.eV due to O=C-N and 533.8 eV which may be ascribed to the terminal OH
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group [19] (Figure 3d).
Figure 3. (a) XPS full scan survey spectrum of nylon-6 nanofibers; (b) high resolution core level C1s spectrum; (c) N1s and (d) O1s spectrum of nylon-6 nanofibers Effect of time The optimum time required for decolorization of AB117 was investigated by analyzing the residual dye concentration after different intervals. Figure 4 shows that significant and rapid decrease of AB117 concentration could be achieved in 1 min, and the adsorption process slowed down and attained the 11
ACCEPTED MANUSCRIPT equilibrium within 25 min. While nylon-6 nanofibers showed extremely superior performance towards AB 117 decolorization; polymeric pellets on the other hand presented poor efficiency. Nylon-6 pellets are the aggregates of polymer chains with many foldings due to hydrogen bonds between chains; as a result most of the active sites remain masked. Nanofibers are formed by stretching polymer chains from electrified polymer jet due to increase in the ratio of electrostatic repulsion to surface tension. This
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electric induction tends to open the hidden active sites by inducing the positive charges on polymer
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chains that tend to extend as a result of electric repulsion. This process tends to increase number of active sites and hence surface area that consequently results in enhancement in removal efficiency for
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AB117. Due to the poor performance of nylon-6 pellets, they were not tested further.
Figure 4. Effect of contact time of nylon-6 nanofibers and polymer pellets on AB117 adsorption efficiency. 12
ACCEPTED MANUSCRIPT To further elucidate the characteristics of adsorption process, including the rate controlling steps and potential mechanism of adsorption; the adsorption kinetic data was evaluated using pseudo first-order and pseudo-second order [20,21]. The rate equations of models are given in equations (5) and (6): ln qe qt ln qe k1t
(5)
(6)
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t 1 t 2 qt k 2 qe qe
where qe (mg/L) and qt (mg/g) are the amounts of AB117 adsorbed on nanofibers at equilibrium and at
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time t (min), respectively. k1 (1/min) and k2 (g/mg.min) are the rates of pseudo first and second order
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models. Table 2 lists the parameters of kinetic models and error values. The smaller error values and higher regression co-efficient support the major contribution of pseudo-second order model in
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determining adsorption rate suggesting the rate controlling step is chemical adsorption through sharing
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of electrons between dye and nanofibers. Further confirmation was made through the calculated value of qe obtained from the pseudo-second order which was exactly the same as experimental qe value.
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Therefore, pseudo-second order appears to be the best in describing adsorption kinetics.
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Table 2. Kinetic fittings and parameters for the adsorption of AB117 on nylon-6 nanofibers. Pseudo-first order
qe, exp (mg/g) ±SDa
qe, cal (mg/g)
k1 (min-1)
R2
SSE
12±0.85
1.51
0.0844
0.967
0.105
Pseudo-second order qe, exp (mg/g) ±SD
qe, cal (mg/g)
k2 (g/mg.min)
R2
SSE
12±0.85
12.05
0.265
0.999
0.013
a=Standard deviation 13
ACCEPTED MANUSCRIPT Effect of solution pH and mass of nanofibers The removal of AB117 as a function of pH was studied in the range of pH values from 1 to 11 including the pH that was unaltered (unaltered pH of actual solution ≈5.5). Figure 5 shows that pH has substantial influence on dye removal efficiency of nylon-6 nanofibers. The removal efficiency declines
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as the pH changes from acidic to basic. This is obvious as the acidic pH tends to protonate the surface
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of nylon-6 nanofibers, making highly positive surface that attracts negatively charged AB117
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molecules possibly making electric double layer. The increase in pH causes charge imbalance by decreasing positive charge density and increasing negative charges on surface that lead majority of
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active sites with negative charges. This increase in negative charge suppresses the removal efficiency of
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nylon-6 nanofibers. Similar behavior was also noted by Raghunath et al. and Elwakeel et al. for anionic dye removal by proline based polymer nanocomposite
and modified glycidyl methacrylate resin for
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reactive black 5 removal, respectively [23].
[22]
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From the above results, adsorption of AB117 may be assumed to be partly electrostatic in
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nature as the adsorption efficiency was still 62±1.9% at extremely basic pH (pH 11) which suggest that the mechanism of adsorption of AB117 on nylon-6 nanofibers was not purely electrostatic; but some
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other secondary interactions such as, hydrogen bonding and hydrophobic interactions could also be crucial. From the results, the best pH chosen for subsequent experiment was unaltered pH of actual
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solution (≈5.5).
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Figure 5. Effect of solution pH and mass of nanofibers on AB117 adsorption
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The amount of nanofiber was varied between 10 to 40 mg. The mixtures containing 50 mg/L dye solution and nanofiber samples of different masses were agitated at room temperature for 25 min.
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Figure 5 shows the relationship between nanofiber mass and dye removal efficiency. By increasing the
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mass of nanofiber from 10 to 40 mg, the adsorption efficiency increased from 70 ± 0.25% to 95 ± 2.5%. Beyond 20 mg (AE% 90± 2), the adsorption efficiency was relatively constant with slow rate of removal. The increase in AE% with mass of nanofibers is due to increase in active sites and exposed area for efficient dye removal [24].
Effect of initial dye concentration
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ACCEPTED MANUSCRIPT The effect of initial dye concentration on its removal by nylon-6 nanofibers is shown in Figure 6a. By increasing initial dye concentration from 25 to 400 mg/L, the adsorption efficiency reduced from 97 ±2 % to 58 ±1.9 %. This trend can be explained with the fact that the nanofiber possessed a limited amount of active sites, causing exhaustion on nanofiber surface beyond certain concentration. According to
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Figure 6a, the adsorption capacity increases linearly with increasing initial dye concentration
Figure 6. (a) Nylon-6 adsorption efficiency and capacity curves as a function of initial AB117 concentration, (b) Linear Langmuir adsorption isotherm and (c) linear Freundlich adsorption isotherm.
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ACCEPTED MANUSCRIPT The maximum adsorption capacity at different concentrations at constant temperature can be evaluated by application of adsorption isotherms. The equilibrium adsorption data obtained in our study were analyzed using Langmuir and Freundlich
[25, 26]
isotherm models which are expressed by equations (7)
and (8). Ce C 1 e qe qm K L q m
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1 log C e log K f n
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log q e
(7)
(8)
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Where, Ce is the equilibrium concentration of AB117 in the bulk solution, qe (mg/g) is the equilibrium
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adsorption capacity of nylon-6 nanofiber, KL is (L/ mg) is the Langmuir equilibrium constant, qm is the maximum adsorption capacity when adsorption achieves monolayer coverage. n is the adsorption index
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the value of which must lay greater than 1 for the adsorption system to be favorable. It also determines 1/n
) is the Freundlich constant
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the degree of heterogeneity of adsorbent surface. Kf (mg/ (g (L/mg)
related to the adsorption capacity. Figure 6 b and 6 c illustrate the linear plots of Langmuir and
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Freundlich isotherms. The experimental data fitted well with Freundlich isotherm with a higher
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correlation coefficient (R2=0.983). The data also fitted well with Langmuir isotherm (R2=0.962) but, it is usually applicable under the conditions of lower adsorbate concentrations. When the dye
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concentration increases, more molecules will be available per unit area of nanofiber. Due to this, multilayer formation of dyes on nanofiber surface would occur; as a result adsorption capacity increased with rise in initial dye concentration. The actual experimental qm was found to be 58.8 mg/g. The preferential fitting of Freundlich isotherm also suggests that the multilayer adsorption of AB117 takes place on energetically heterogeneous sites which is actually true as the surface of nylon-6 nanofibers is highly dense with CH2-NH, NH-C=O and CH2-CH2 groups that are reasonably chemically and energetically heterogeneous. These results are in good agreement with the work reported by Metin 17
ACCEPTED MANUSCRIPT et al. for the removal of acid black194 (AB194) using chitosan/zeolite biocomposites acid red 138 (AR138) using blast furnace slag [29]
[28]
[27]
, removal of
and removal of acid blue 113 by activated red mud
. The experimental adsorption capacity of AB 117 at 25oC shown in Table 3 was comparable to
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the adsorption capacities of some other efficient adsorbents reported for other types of acid blue dyes.
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Nylon-6 nanofibers require just 25 min contact for sufficient removal compared to other materials,
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serving efficient material for saving processing time. Additionally, the morphology of nanofibers was
ES1) suggesting chemical stability of nanofibers .
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not affected during the adsorption experiment (SEM images can be seen in supplementary material
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Table 3. Adsorption capacities and other operational parameters for acid blue dyes reported in
Acid Blue 113
Activated carbon from rubber tire
2
Temperature Amount (K) (g/L) 298 4
Adsorption Reference Capacity 58.8 This Work
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298
6
83.3
[29]
40
298
10
9.2
[30]
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3
Time (min) 25
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Activated red mud
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Acid Blue 117 Acid Blue 113
Adsorbent Operating pH Nylon 6 5.5 NF
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Dye
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literature.
Acid Alkali Blue 25 treated saw dust
2
60
300
2
24.3
[31]
Acid Cationized Blue 25 starch
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50
298
1
317.7
[32]
Acid Chitin gels Blue 74
5.7
1200
-
5
40
[33]
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Interference of cations and anions Dye wastewater normally contains high concentrations of electrolytes. For practical applications and monitoring the reliable sustainability of nylon-6 nanofibers in removing AB117 successfully in
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presence of common cations (Na +, Ca2+ and Mg2+) and anions (Cl- and SO42-), we selected NaCl,
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CaCl2.2H2O, MgCl2.6H2O and Na2SO4 as precursor. Experiments were conducted under optimized
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conditions by taking 5 ml of 50 mg/L AB117 containing 100 mg/L of cations and anions separately.
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Figure 7 shows that the presence of either cations or anions did not pose negative impact on adsorption efficiency of nylon-6 nanofibers, suggesting their consistent behavior in electrolyte rich dye
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wastewater. The addition of electrolytes could strengthen the electric double layers between the interface of nanofibers and bulk solution or would reduce the solubility of dye [34]. It is usually believed
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that electrolyte addition promotes hydrophobic interactions
[35]
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for complete dye removal in presence of these ions could also be increased hydrophobic interactions.
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Figure 7. Effect of different cations and anions on adsorption efficiency of nylon-6 nanofibers for
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Dye selectivity study:
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AB117
The selective removal of dyes by nylon-6 nanofibers was examined by selecting five different anionic
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dyes including AB117, RB5, MO, AO 67 and RR 195 in separate solutions. In a typical experiment, 20mg nylon-6 nanofibers were dispersed in 5mL of individual dye solution at pH 5.5 with their concentrations of 50 mg/L. Figure 8 a represents adsorption performance of nylon-6 nanofibers for different anionic dyes. The removal performance of nylon-6 towards AB 117 was found highly specific as it was able to decolorize only AB117 effectively. Other dyes such as MO and RB5 were not satisfactorily removed. The equilibrium concentrations of MO, AO 67, RR195, RB5 and AB117 were 30.25, 11.6, 11.4, 14.2 and 2 mg/L respectively. This indicated that nylon-6 nanofibers selectively 20
ACCEPTED MANUSCRIPT adsorbed AB117 dye. In order to ascertain the selectivity of nylon-6 towards AB117, binary mixture of dyes containing AB117 and RR195, AB117 and MO of equal concentrations were prepared and adsorption experiments were conducted under optimum conditions. The results in Figure 8b show highly selective and efficient removal performance of nylon-6 towards AB117 that removed appreciable amount of AB117 dye from binary mixture and left the other dye in solution. The removal
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efficiency for AB 117 was 83 ±1.8% while for RR 195 was 70.8 ± 0.6%, similarly the removal efficiency for the same dye was 85 ± 0.75% in presence of MO (45.5 ± 0.9% AE%) In a study reported
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by [36], the standard dye affinity increases proportionally with the size of anionic dye; as the mode of
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interaction changes from ion-ion to van der Waal forces and small enthalpy changes contribute to greater affinity. Therefore, it may be proposed that AB117 was specifically adsorbed on nylon-6
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nanofibers due to above mentioned facts. Although its size was small than RR195 but due to the
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presence of one SO3- group in AB117, its selectivity supersedes RR195. From this behavior, it is also
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responsible in AB117 removal.
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relevant that the dye removal is not governed by size exclusion mechanism but surface adsorption is
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Figure 8. (a) Adsorption efficiency of nylon-6 nanofibers for different dyes; (b) Selective
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adsorptive removal of AB117 in different binary mixtures. Dye-nanofiber adsorption mechanism IR spectra of neat nylon 6 nanofibers and AB117 treated nylon-6 nanofibers are shown in Figure 9. The band at 3429 cm-1 can be related to N-H stretching vibrations. The peaks at 2930 cm-1 and 2848 cm-1 may be associated with –CH asymmetric and symmetric stretching vibrations, respectively. The bands at 1640 cm-1 and 1542 cm-1 may be assigned to amide I (C=O stretching vibrations) and amide II (N-H bending vibration) 23
[37]
. The band at 694 cm-1 is attributed to bending of O-C-N
[13]
. The FTIR
ACCEPTED MANUSCRIPT analysis after dye (AB117) treatment of nylon-6 nanofibers showed a considerable increase in intensities as well as sharpening of N-H stretching, suggesting addition of amine groups from AB117 molecules and involvement of some hydrogen bond interactions between AB117 and nylon-6 nanofibers. The bands due to amide I and amide II became weak and less intense possibly due to participation of N-H and C=O sites for dye capture. The characteristic bands of –SO3 due to . In addition to this, a band at 930
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[38]
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asymmetric stretching vibrations are observed at 1022-1030 cm-1
[39]
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cm-1 was emerged due to dye capture that was attributable to S-O stretching from SO2 group in AB117 . From the above facts, it can be considered that the nature of dye nanofiber interactions was
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combination of multiple interactions.
Figure 9. FTIR spectra of nylon-6 nanofibers before and after dye treatment. Further assistance regarding mode of interaction and probable binding sites was gained through XPS technique. The C1s high resolution spectrum of nylon-6 after AB117 treatment is shown in Figure 10 a. The main binding energies due to CH2-CH2 (284.9 eV), CH2-C=O (287.2 eV) and O=C-N (289.9 24
ACCEPTED MANUSCRIPT eV) were slightly shifted from their original positions that suggest participation of amide groups for AB117 capture. The positive shift of CH2-CH2 is more likely due to hydrophobic interactions of alkyl chain with alkyl groups of dye. Moreover, the peak due to CH2-NH diminished after dye treatment suggesting main point of dye interaction. To the best of our prediction the amide group in nylon-6 affects adjoining –CH2- group by generating polarity in N C bond that creates charges; hence,
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possibly endowing Lewis acid and base sites for dye attachment. The dye was also subjected to XPS
states are provided in supplementary material (ES2-ES6)
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analysis and the relevant figures and binding energies of atoms along with their different chemical
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The deconvolution of N1s peak at 402 eV indicated the arousal of extra peak at 399 eV which
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through extensive survey of reference data could be due to N-C or N-CO-N bond [40] (Figure 10 b). The overall mechanism of dye-nylon-6 nanofibers is a complex system, but for simplicity we may assume
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that a new bond between carbonyl carbon of nylon-6 and N of dye molecule have been generated
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through attack of nucleophilic nitrogen from dye to the more electrophilic carbonyl carbon of nylon 6.
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When the O1s region of dye treated nylon 6 was fitted, three different components were achieved, i.e. the peaks at 530.19 eV, 531.5 eV and 533 eV as shown in Figure 10 c. The interpretation
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of this region unveiled some interesting facts; the peak that emerged at 530.19 eV may be attributed to [41]
, this could be possible as the dye was sodium salt hence Na+ may
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metal-oxygen (M-O) bond
possible have some interfacial interactions with electronegative oxygen of nylon-6. Additionally, the peak ratio of O=C-N/O-H (0.81) changed to 0.2 that suggest that O=C-N group has predominantly taken main participation in dye fixation. The above results and the results from FTIR are in good concurrence with each other. From the results achieved through these both techniques, it may be suggested that dye-nylon-6 interactions can be categorized to partly hydrophobic, electrostatic and hydrogen bond interactions. The conclusive interactions may be illustrated in the Scheme-1.
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Figure 10 (a). XPS high resolution core level C1s; (b) N1s and (c) O1s.
26
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O S O Na
Dye
O δ+
*
δ-
N
* N
Proposed dye nylon-6 adsorption mechanism.
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Scheme 1.
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Dye
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H H
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Membrane filtration study
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Since the adsorption results accomplished through the batch mode showed excellent adsorption performance in terms of time, efficiency and capacity, it was realized to use nylon-6 nanofiber
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membrane as filtration medium to evaluate its adsorption performance for AB117 removal. For
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achieving satisfactory dye removal from aqueous solution for over sufficient volume, we made different foldings corresponding to layers 5, 10 and 20 of nylon-6 membrane in conventional filter
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holder. The thickness of membranes with 5, 10 and 20 foldings were 193 ± 20, 280 ± 15and 490 ± 30
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μm, respectively. The dye solution (50 mg/L) was passed through the membrane filter at a flow of 1mL/min. Figure 11 shows the influence of number of layers on breakthrough efficiency of nanofiber
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membrane. The 50% breakthrough with 5 layers was achieved earlier (9 mL) whereas for the membrane with 10 and 20 layers, the breakthrough was achieved through passing 17 mL and 22 mL respectively. The probable reason for the entire removal of AB117 in early filtration process could be pressure driven convection mass transport through the interstitial spaces between nanofiber membranes and rapid adsorption kinetics due to abundant available active sites
[42]
. The adsorption capacity for
AB117 in this design was 0.169, 0.283 and 0.321 mg/cm2 when passed through 5, 10 and 20 layers respectively with a high permeation flux 113.2 L/m2.h. This flux is much greater than conventional 27
ACCEPTED MANUSCRIPT nanofiltration membranes (10 L/m2.h.bar)
[43]
.The initial experimental survey suggests that membrane
stacking could afford greater potential to filter greater volume of dye contaminated water. Since the membranes are highly permeable therefore, they offer negligible drop in flux and can be conveniently applied in continuous adsorption by stacking multiple membranes of the 20 foldings to increase the adsorption capacity and render delayed breakthrough and service time.
[44].
The resulting nanofiber
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membrane has many desirable properties such as high removal efficiency, permeation flux, stability,
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20 Layers 10 Layers 5 Layers
50
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Outlet dye concentration, mg/L
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manufacturing scalability and cost-effectiveness.
5-10
11-15
16-20
21-25
26-30
31-35
Permeation Volume, mL
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1-4
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Figure 11. Breakthrough curves obtained after passing dye solution through nylon-6 nanofiber membranes with different layers. Conclusion
Highly robust and selective nylon-6 nanofibers were fabricated that offered complete removal of AB117 within 25 min; that is one of the promising characteristics of this nanoadsorbent. The optimum pH was found natural working pH for processing adsorption experiment. The nanofibers with 20 mg were sufficient for achieving 90± 2% removal. The adsorption efficiency reduced with an increase in 28
ACCEPTED MANUSCRIPT initial dye concentration with maximum experimental adsorption capacity of 58.8 mg/g. The presence of commonly found ions did not affect the removal performance of nylon-6 nanofibers that is worth applicable in industrial applications. Moreover, the dye was found highly selective and specific in both individual as well as binary mixtures of other anionic dyes. Apart of batch studies, nylon-6 nanofibers also proved worth applicable in treating AB117 in continuous system with maximum adsorption
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capacity 0.321 mg/cm2 by increasing the number of layers that offered permeation flux of 113.2
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L/m2.h; which is much greater than conventional nanofiltration membranes 10 L/m2.h.bar. The dye nylon-6 binding mechanism is the combination of both physical and chemical interactions. The nylon-6
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nanofibers are simple, economic and offer minimum removal time compared to other expensive and
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synthetic materials. Acknowledgement
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The work was supported by Mehran University of Engineering and Technology Jamshoro Pakistan and
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Shinshu University Japan.
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(Supporting information is provided)
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ACCEPTED MANUSCRIPT Highlights
Nylon-6 nanofibers without pre/post electrospinning treatment were prepared
Nylon-6 nanofibers selectively and efficiently removed acid blue 117
The maximum adsorption capacity was 58.8 mg/g
Nanofibers were characterized by FTIR, SEM and XPS
Nanofibers were also successfully used as filtration medium
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Umair Ahmed Qureshi, Zeeshan Khatri, Farooq Ahmed, Abdul Sameeu Ibupoto, Muzamil Khatri, Faraz Ahmed Mahar, Rafi Zaman Brohi, Ick Soo Kim PII: DOI: Reference:
S0167-7322(17)33053-2 doi: 10.1016/j.molliq.2017.08.129 MOLLIQ 7870
To appear in:
Journal of Molecular Liquids
Received date: Revised date: Accepted date:
10 July 2017 24 August 2017 28 August 2017
Please cite this article as: Umair Ahmed Qureshi, Zeeshan Khatri, Farooq Ahmed, Abdul Sameeu Ibupoto, Muzamil Khatri, Faraz Ahmed Mahar, Rafi Zaman Brohi, Ick Soo Kim , Highly efficient and robust electrospun nanofibers for selective removal of acid dye, Journal of Molecular Liquids (2017), doi: 10.1016/j.molliq.2017.08.129
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ACCEPTED MANUSCRIPT Highly efficient and robust electrospun nanofibers for selective removal of acid dye Umair Ahmed Qureshi a,b, Zeeshan Khatri†a,c , Farooq Ahmed a , Abdul Sameeu Ibupoto a, Muzamil Khatri c, Faraz Ahmed Mahar a, Rafi Zaman Brohi d, Ick Soo Kim c†† a
Nanomaterials Research Lab, Department of Textile Engineering, Mehran University of Engineering and Technology, Jamshoro 76062,
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Pakistan
Government Boys Degree College Qasimabad, Hyderabad, 71000, Pakistan.
c
Nano Fusion Technology Research Lab, Division of Frontier Fibers, Institute for Fiber Engineering (IFES), Interdisciplinary Cluster for
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b
Department of Environmental Engineering, Mehran University of Engineering and Technology Jamshoro, 76062, Pakistan
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d
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Cutting Edge Research (ICCER), Shinshu University, 3-15-1, Tokida, Ueda, Nagano 386-8567, Japan.
E-mail: [email protected]
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†Corresponding Author: Zeeshan Khatri Dr.Eng.
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Address: Nanomaterials Research Lab, Department of Textile Engineering, Mehran University of Engineering and Technology, Jamshoro 76060, Pakistan.
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Tel: 0092 (0) 22 2772250
††Corresponding Author: Ick Soo Kim, Dr.Eng.
E-mail: [email protected]
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Address: Nano Fusion Technology Research Group, Shinshu University, 3-15-1, Tokida, Ueda City, Nagano 386-8567, Japan.
Tel.: +81 268 21 5439; Fax: +81 268 21 5482.
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Graphical abstract
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Abstract
The highly efficient and economic nylon-6 nanofibers without pre/post electrospinning treatment were
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prepared for the fast and selective removal of anionic dye. i.e., Acid Blue 117 (AB 117), in just 25 min
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of contact. This is the first time the application of pure nylon-6 nanofibers for dye removal is reported. Nylon-6 , in nanofibers form, possessed greater adsorption capability compared to its pellet form. The
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adsorption data fitted well to pseudo second order and Freundlich isotherm. Nylon-6 nanofibers showed excellent efficiency for AB117 in single and higher selectivity in binary component system. The nanofibers were also used as a membrane for removing AB117 from continuous streams by varying the number of layers. The maximum adsorption capacity in batch mode was found 58.8 mg/g and 0.321 mg/cm2 in continuous mode. The proposed nanofibers offer maximum permeation flux of 113.2 L/m2.h which is much higher than conventional nanofiltration membranes with 10 L/m2.h.bar. Keywords Filtration, electrospinning, nanofibers, selectivity, acid blue 117, adsorption 2
ACCEPTED MANUSCRIPT Introduction Dye pollution and its control is a critical global issue because they are refractory and lead increased biochemical oxygen demand. In addition to this, dyes in water inhibit photosynthetic activities by resisting the penetration of sunlight in water bodies
[1,2]
. Acid blue 117 (AB 117) is an anionic, azo
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classified dye having azo group (-N=N-) with aromatic groups in its chemical structure. Presence of
[3]
. Additionally, continuous exposure to such dyes causes dysfunction of
liver, kidney, brain and reproductive systems as well
[4]
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biodegradation respectively
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amine groups and other aromatic groups in the azo dyes make them highly toxic and more resistant to
. It is generally used for coloring wool, nylon,
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silk and rayon blended fabrics. The effluents generated from industries, particularly textile industry,
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contain approximately 15% of total dye that deteriorates not only environment but also reduces availability of drinkable water resources. Therefore, it is the utmost task to prevent discharge of
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industrial effluents to an aquatic environment from eco-toxicological and aesthetic point of view.
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Conventionally, various chemical and biological processes are employed to remove dyes from wastewater such as photochemical, coagulation, ozonation, membrane filtration and adsorption.
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Nevertheless, the use of these techniques is limited by high operation cost, generation of waste sludge
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and ineffectiveness in treating effluent with low dye concentrations [5]. Adsorption has proven the best in terms of ease of operation, low cost, simple design and its flexibility towards the type of adsorbent
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used in the process.
Many cheap and efficient powdered materials have been studied for removing anionic dyes such as, overripe Cucumis sativus peels [3], chitosan-metal complex [6], hydroxyl double salts [7] etc. but their efficient performance in presence of other anionic dyes and electrolytes of different nature was not addressed properly. Additionally, powdered adsorbents also require tiresome filtration steps along with sample loss and generation of secondary pollution. The industrial wastewater, especially textile industry, is not loaded with single dye but numbers of aesthetic dyes are being discharged at a time with 3
ACCEPTED MANUSCRIPT many folds greater concentrations of salts together with acids and bases. In addition to this, most of the research is focused on batch wise adsorption process which is virtually not efficient in terms of time and energy saving purpose. Therefore, the selective removal and recovery of dyes particularly acid dyes and their removal through continuous process without being affected due to highly acidic, basic pH or electrolyte rich environment is a challenging issue. Some studies manifested selective and efficient , polydopamine-polyethylene imine ultrathin
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[8]
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separation of anionic dyes such as, Fe3O4@NH2@PEI
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coating on polyvinyl alcohol-polyethylene imine (PDA/PEI@PVA/PEI) nanofibers [9] but, such studies are limited by the higher cost of starting materials and lengthy synthetic procedures. Therefore, search
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of economic, simple and efficient material is the emphasis of current research. Nowadays, nanofibers have received hallmark achievement both scientifically and technologically due to exceptionally higher
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surface area, easy handling and highly porous morphology. To achieve better removal efficiency for
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anionic dyes, variety of nanofibers and their composites have been proposed but either their prolonged preparations times or delayed equilibrium time (20 to 24 h), limit their applications for practical [10, 11]
.
Nylon-6 nanofibers have gained reasonable attention due to abundant
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implementation
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availability and higher mechanical strength. Although, many research papers are published based on surface modified nylon-6 nanofibers such as, graphene flakes loaded nylon 6 nanofibers through [12]
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supersonic kinetic spraying
for methylene blue (MB) removal and polyaniline coated nylon 6 [13]
; but, their applications are limited to only lower dye
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nanofibers for methyl orange (MO) removal
concentrations (5 and 10 mg/L respectively) and greater equilibrium time (400-600 min) for MO removal. The application of commercially available nylon-6 nanofibers alone for removal of few anionic dyes is reported, which is not sufficiently performed to resolve the issues pertaining interference, selectivity of dyes and lack information regarding appropriate adsorption mechanism [14,15]
. Its importance in terms of economic feasibility has also not been highlighted in previous studies
and thus remained a gap. 4
ACCEPTED MANUSCRIPT We, therefore, for the very first time are reporting application of pure nylon 6 nanofibers for the efficient removal of anionic dye (AB117) without time consuming pre/post electrospinning treatment. Our main objective is to use easily available unmodified nylon-6 nanofibers for the selective removal of anionic acid dye within shortest time both batch wise and continuous filtration system, under ambient conditions in order to save cost, time and energy. Further, the main concern related to its efficient
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performance in presence of interfering ions is also the main goal of current research. The detailed
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mechanism of dye-nylon-6 has also been proposed through advance instrumental approach. Experimental
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Material and methods
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Nylon-6 polymer (pellets 3mm) was purchased from Sigma Aldrich, Japan. Formic acid (98%) was obtained from Wako Pure Chemical Industries, Japan. Acid blue 117 (anionic dye), Reactive black 5,
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Reactive red 195, Acid orange 67 and Methyl orange were supplied by Archroma, Pakistan Ltd. and the
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corresponding dye structures are given in Table 1. HCL (37%) and NaOH were purchased from Sigma
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Electrospinning of nylon-6
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Aldrich, USA.
Nylon-6 nanofibers were obtained by electrospinning technique. The procedure was reproduced from
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our recent published work
[16]
. A 22 wt% nylon-6 solution was prepared using formic acid as solvent.
The solution was mixed using magnetic stirrer for 12 h at room temperature. The prepared solution was electrospun using electrospinning apparatus installed with high voltage supply (Har-100∗12, Matsusada Co., Tokyo, Japan). A 5 ml syringe was used for electrospinning such that a positive electrode connected with the metallic electrode in the syringe and negative electrode was attached with the rotating metallic collector. Distance between the tip and metallic rotating collector was 15 cm and a 15
5
ACCEPTED MANUSCRIPT kV voltage was used to prepare nylon-6 nanofiber webs. The nanofiber webs were fabricated at 30.6 ± 2 °C under relative humidity of 40%.
Characterization
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The morphology of nanofibers was studied using scanning electron microscope (SEM, JEOL model
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JSM 6010 LA). Samples were sputtered coated with Pd-Pt prior to examination. Nanofiber diameter
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was calculated using image J software. Mechanical tests were conducted on universal testing machine
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(Tensilon RTC1250A; A&D Company Ltd, Japan) equipped with a 120 N load cell using stretching rate of 5 mm/min. Dog-bone-shaped specimens were cut with a length of 3 cm and width of 0.2 cm
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with the help of rotary cutter. The surface functional groups on nylon 6 nanofibers and their role in dye fixation was studied by Fourier Transform Infrared Spectroscopy (Shimadzu 8900-FT-IR spectrometer
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Tokyo, Japan). X-ray photoelectron spectroscopy (XPS) measurements were performed with AXIS
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Ultra by Shimadzu equipped with dual-anode X-ray source Al/Mg and the HSA hemispherical sector
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analyzer detector with the vacuum pressure maintained at 1.4 × 10−9 Torr. A Mg Kα X-ray source (1253.6 eV) was used for XPS measurements. Data analysis and curve fitting were performed using
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Origin Pro 9.0 with a Gaussian−Lorentzian product function.
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Dye adsorption experiment
Both batch and continuous methods were implemented to study dye removal by nylon-6 nanofibers. For batch experiments, 20 mg nylon-6 nanofibers were mixed with 5mL of AB117 dye solution (50 mg/L). The parameters studied to find the optimum conditions were adsorption time (0-25 min), pH (1-11), mass of nanofibers (10-40 mg), initial dye concentrations (25-400 mg/L). The influence of commonly occurring different interfering ions such as, Na+, Ca2+, Mg2+, Cl- and SO42- was also studied by preparing 100 mg/L solution of each ions separately from their precursor salts namely NaCl, 6
ACCEPTED MANUSCRIPT CaCl2.2H2O, MgCl2.6H2O and Na2SO4 respectively. Further, the selective response of nylon-6 nanofibers towards AB117 was investigated by conducting adsorption experiment with individual anionic dye solution, i.e. AB117, RB5, RR195, MO and AO 67 (50 mg/L each). The wavelength maximum of each dye is given in Table 1. The selectivity experiment was also performed by mixing 20 mg nylon-6 nanofibers in binary mixture of dyes containing AB117: RR 195 (1:1 w/w) and AB117 :
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MO (1:1 w/w). The dye removal efficiency (AE %) was calculated through UV-Vis spectrophotometer
100
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Cf AE% 1 Co
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using the equation (1) :
(1)
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Where Co and Cf (mg/L) are the initial and remaining concentrations of dye obtained after dye-nylon contact. The maximum adsorption capacity qe (mg/g) of nylon-6 was also calculated through the
V m
(2)
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q e C o C f
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equation (2):
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Where V is the volume (L) of dye solution and m is the mass of nylon-6 nanofibers (g). The feasibility of nylon-6 nanofibers for large industrial scale application was also assessed
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through continuous mode. Briefly, nylon-6 nanofibers were loaded on conventional filter holder with 13 mm diameter. The dye solution (50 mg/L) was loaded in plastic syringe and was installed on syringe pump usually used for electrospinning purpose. The dye solution was carefully passed through the filtration cell holder and the residual concentration after every interval was monitored. The adsorption capacity of membrane was determined through equation (3):
qc 7
Co Vs A
(3)
ACCEPTED MANUSCRIPT Where Co is the initial dye concentration, Vs is the volume (L) of permeate when the dye removal was less than 99%. A is the effective area (cm2). All experiments were performed in triplicate and the mean values are provided in current work. To further validate the linear fittings of kinetic and isotherm models apart from regression coefficient (R2), sum of squared errors (SSE) was also
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determined through the equation (4): SSE q cal q exp
(4)
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i 1
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N
where qcal and qexp (mg/g) are the calculated and experimentally determined adsorption
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capacities of nylon-6 nanofibers. The smaller the SSE values, the better will be the corresponding
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fitting.
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Table 1. Physical properties of different anionic dyes λmax
Molecular weight
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Dye name
588
594.57
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Acid Blue 117
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(g/mol)
Methyl Orange
463
327.3
Acid Orange 67
477
604.5
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Structure
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1136
Reactive black 5
590
991.8
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Reactive red 195
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Results and discussion
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SEM Analysis
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Figure 1a presents the SEM micrographs of nylon-6 nanofibers. From the images, the nanofibers obtained were uniform and bead free. The nanofibers were found to exist in both thick and thin
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morphologies. The average nanofiber diameter was found to be 198.6 nm (Figure 1b).
Figure 1. (a) SEM micrograph of nylon-6 nanofibers and (b) Histogram of nanofiber diameter distribution. Mechanical Test: 9
ACCEPTED MANUSCRIPT Figure 2 shows typical stress-strain curve of nylon-6 nanofibers that exhibited a tensile strength of 5.6MPa at break of around 26% strain. This strength is much greater than other nanofibers [17,18] and
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hence, exhibit greater stiffness. The nanofiber specimen possessed toughness of 21.8 J/g.
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Figure 2. Typical stress-strain curve of nylon-6 nanofibers XPS Analysis
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XPS survey scan of the nylon-6 nanofibers is shown in Figure 3a. The survey spectrum indicated the
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presence of C (284 eV), N (398 eV) and O (530 eV) as anticipated. The main information regarding chemical composition was collected through deconvolution of C1s, N1s and O1s peaks. The deconvolution of peak at 284 eV (Figure 3b) indicated four chemical environments for carbon atom. The peaks at 284.7 eV, 287.1 eV, 287.4 eV and 290 eV suggest the presence of C in the forms of CH2CH2, CH2-NH, CH2-C=O and HN-C=O respectively. The high resolution XPS spectrum of N1s at 402 eV is probably due to N-C=O, amide bond (Figure 3c). The O1s region was deconvoluted into two
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group [19] (Figure 3d).
Figure 3. (a) XPS full scan survey spectrum of nylon-6 nanofibers; (b) high resolution core level C1s spectrum; (c) N1s and (d) O1s spectrum of nylon-6 nanofibers Effect of time The optimum time required for decolorization of AB117 was investigated by analyzing the residual dye concentration after different intervals. Figure 4 shows that significant and rapid decrease of AB117 concentration could be achieved in 1 min, and the adsorption process slowed down and attained the 11
ACCEPTED MANUSCRIPT equilibrium within 25 min. While nylon-6 nanofibers showed extremely superior performance towards AB 117 decolorization; polymeric pellets on the other hand presented poor efficiency. Nylon-6 pellets are the aggregates of polymer chains with many foldings due to hydrogen bonds between chains; as a result most of the active sites remain masked. Nanofibers are formed by stretching polymer chains from electrified polymer jet due to increase in the ratio of electrostatic repulsion to surface tension. This
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electric induction tends to open the hidden active sites by inducing the positive charges on polymer
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chains that tend to extend as a result of electric repulsion. This process tends to increase number of active sites and hence surface area that consequently results in enhancement in removal efficiency for
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AB117. Due to the poor performance of nylon-6 pellets, they were not tested further.
Figure 4. Effect of contact time of nylon-6 nanofibers and polymer pellets on AB117 adsorption efficiency. 12
ACCEPTED MANUSCRIPT To further elucidate the characteristics of adsorption process, including the rate controlling steps and potential mechanism of adsorption; the adsorption kinetic data was evaluated using pseudo first-order and pseudo-second order [20,21]. The rate equations of models are given in equations (5) and (6): ln qe qt ln qe k1t
(5)
(6)
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t 1 t 2 qt k 2 qe qe
where qe (mg/L) and qt (mg/g) are the amounts of AB117 adsorbed on nanofibers at equilibrium and at
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time t (min), respectively. k1 (1/min) and k2 (g/mg.min) are the rates of pseudo first and second order
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models. Table 2 lists the parameters of kinetic models and error values. The smaller error values and higher regression co-efficient support the major contribution of pseudo-second order model in
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determining adsorption rate suggesting the rate controlling step is chemical adsorption through sharing
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of electrons between dye and nanofibers. Further confirmation was made through the calculated value of qe obtained from the pseudo-second order which was exactly the same as experimental qe value.
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Therefore, pseudo-second order appears to be the best in describing adsorption kinetics.
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Table 2. Kinetic fittings and parameters for the adsorption of AB117 on nylon-6 nanofibers. Pseudo-first order
qe, exp (mg/g) ±SDa
qe, cal (mg/g)
k1 (min-1)
R2
SSE
12±0.85
1.51
0.0844
0.967
0.105
Pseudo-second order qe, exp (mg/g) ±SD
qe, cal (mg/g)
k2 (g/mg.min)
R2
SSE
12±0.85
12.05
0.265
0.999
0.013
a=Standard deviation 13
ACCEPTED MANUSCRIPT Effect of solution pH and mass of nanofibers The removal of AB117 as a function of pH was studied in the range of pH values from 1 to 11 including the pH that was unaltered (unaltered pH of actual solution ≈5.5). Figure 5 shows that pH has substantial influence on dye removal efficiency of nylon-6 nanofibers. The removal efficiency declines
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as the pH changes from acidic to basic. This is obvious as the acidic pH tends to protonate the surface
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of nylon-6 nanofibers, making highly positive surface that attracts negatively charged AB117
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molecules possibly making electric double layer. The increase in pH causes charge imbalance by decreasing positive charge density and increasing negative charges on surface that lead majority of
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active sites with negative charges. This increase in negative charge suppresses the removal efficiency of
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nylon-6 nanofibers. Similar behavior was also noted by Raghunath et al. and Elwakeel et al. for anionic dye removal by proline based polymer nanocomposite
and modified glycidyl methacrylate resin for
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reactive black 5 removal, respectively [23].
[22]
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From the above results, adsorption of AB117 may be assumed to be partly electrostatic in
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nature as the adsorption efficiency was still 62±1.9% at extremely basic pH (pH 11) which suggest that the mechanism of adsorption of AB117 on nylon-6 nanofibers was not purely electrostatic; but some
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other secondary interactions such as, hydrogen bonding and hydrophobic interactions could also be crucial. From the results, the best pH chosen for subsequent experiment was unaltered pH of actual
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solution (≈5.5).
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Figure 5. Effect of solution pH and mass of nanofibers on AB117 adsorption
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The amount of nanofiber was varied between 10 to 40 mg. The mixtures containing 50 mg/L dye solution and nanofiber samples of different masses were agitated at room temperature for 25 min.
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Figure 5 shows the relationship between nanofiber mass and dye removal efficiency. By increasing the
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mass of nanofiber from 10 to 40 mg, the adsorption efficiency increased from 70 ± 0.25% to 95 ± 2.5%. Beyond 20 mg (AE% 90± 2), the adsorption efficiency was relatively constant with slow rate of removal. The increase in AE% with mass of nanofibers is due to increase in active sites and exposed area for efficient dye removal [24].
Effect of initial dye concentration
15
ACCEPTED MANUSCRIPT The effect of initial dye concentration on its removal by nylon-6 nanofibers is shown in Figure 6a. By increasing initial dye concentration from 25 to 400 mg/L, the adsorption efficiency reduced from 97 ±2 % to 58 ±1.9 %. This trend can be explained with the fact that the nanofiber possessed a limited amount of active sites, causing exhaustion on nanofiber surface beyond certain concentration. According to
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Figure 6a, the adsorption capacity increases linearly with increasing initial dye concentration
Figure 6. (a) Nylon-6 adsorption efficiency and capacity curves as a function of initial AB117 concentration, (b) Linear Langmuir adsorption isotherm and (c) linear Freundlich adsorption isotherm.
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ACCEPTED MANUSCRIPT The maximum adsorption capacity at different concentrations at constant temperature can be evaluated by application of adsorption isotherms. The equilibrium adsorption data obtained in our study were analyzed using Langmuir and Freundlich
[25, 26]
isotherm models which are expressed by equations (7)
and (8). Ce C 1 e qe qm K L q m
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1 log C e log K f n
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log q e
(7)
(8)
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Where, Ce is the equilibrium concentration of AB117 in the bulk solution, qe (mg/g) is the equilibrium
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adsorption capacity of nylon-6 nanofiber, KL is (L/ mg) is the Langmuir equilibrium constant, qm is the maximum adsorption capacity when adsorption achieves monolayer coverage. n is the adsorption index
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the value of which must lay greater than 1 for the adsorption system to be favorable. It also determines 1/n
) is the Freundlich constant
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the degree of heterogeneity of adsorbent surface. Kf (mg/ (g (L/mg)
related to the adsorption capacity. Figure 6 b and 6 c illustrate the linear plots of Langmuir and
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Freundlich isotherms. The experimental data fitted well with Freundlich isotherm with a higher
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correlation coefficient (R2=0.983). The data also fitted well with Langmuir isotherm (R2=0.962) but, it is usually applicable under the conditions of lower adsorbate concentrations. When the dye
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concentration increases, more molecules will be available per unit area of nanofiber. Due to this, multilayer formation of dyes on nanofiber surface would occur; as a result adsorption capacity increased with rise in initial dye concentration. The actual experimental qm was found to be 58.8 mg/g. The preferential fitting of Freundlich isotherm also suggests that the multilayer adsorption of AB117 takes place on energetically heterogeneous sites which is actually true as the surface of nylon-6 nanofibers is highly dense with CH2-NH, NH-C=O and CH2-CH2 groups that are reasonably chemically and energetically heterogeneous. These results are in good agreement with the work reported by Metin 17
ACCEPTED MANUSCRIPT et al. for the removal of acid black194 (AB194) using chitosan/zeolite biocomposites acid red 138 (AR138) using blast furnace slag [29]
[28]
[27]
, removal of
and removal of acid blue 113 by activated red mud
. The experimental adsorption capacity of AB 117 at 25oC shown in Table 3 was comparable to
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the adsorption capacities of some other efficient adsorbents reported for other types of acid blue dyes.
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Nylon-6 nanofibers require just 25 min contact for sufficient removal compared to other materials,
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serving efficient material for saving processing time. Additionally, the morphology of nanofibers was
ES1) suggesting chemical stability of nanofibers .
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not affected during the adsorption experiment (SEM images can be seen in supplementary material
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Table 3. Adsorption capacities and other operational parameters for acid blue dyes reported in
Acid Blue 113
Activated carbon from rubber tire
2
Temperature Amount (K) (g/L) 298 4
Adsorption Reference Capacity 58.8 This Work
60
298
6
83.3
[29]
40
298
10
9.2
[30]
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3
Time (min) 25
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Activated red mud
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Acid Blue 117 Acid Blue 113
Adsorbent Operating pH Nylon 6 5.5 NF
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Dye
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literature.
Acid Alkali Blue 25 treated saw dust
2
60
300
2
24.3
[31]
Acid Cationized Blue 25 starch
6
50
298
1
317.7
[32]
Acid Chitin gels Blue 74
5.7
1200
-
5
40
[33]
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Interference of cations and anions Dye wastewater normally contains high concentrations of electrolytes. For practical applications and monitoring the reliable sustainability of nylon-6 nanofibers in removing AB117 successfully in
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presence of common cations (Na +, Ca2+ and Mg2+) and anions (Cl- and SO42-), we selected NaCl,
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CaCl2.2H2O, MgCl2.6H2O and Na2SO4 as precursor. Experiments were conducted under optimized
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conditions by taking 5 ml of 50 mg/L AB117 containing 100 mg/L of cations and anions separately.
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Figure 7 shows that the presence of either cations or anions did not pose negative impact on adsorption efficiency of nylon-6 nanofibers, suggesting their consistent behavior in electrolyte rich dye
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wastewater. The addition of electrolytes could strengthen the electric double layers between the interface of nanofibers and bulk solution or would reduce the solubility of dye [34]. It is usually believed
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that electrolyte addition promotes hydrophobic interactions
[35]
. Therefore, one of the possible reasons
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for complete dye removal in presence of these ions could also be increased hydrophobic interactions.
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Figure 7. Effect of different cations and anions on adsorption efficiency of nylon-6 nanofibers for
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Dye selectivity study:
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AB117
The selective removal of dyes by nylon-6 nanofibers was examined by selecting five different anionic
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dyes including AB117, RB5, MO, AO 67 and RR 195 in separate solutions. In a typical experiment, 20mg nylon-6 nanofibers were dispersed in 5mL of individual dye solution at pH 5.5 with their concentrations of 50 mg/L. Figure 8 a represents adsorption performance of nylon-6 nanofibers for different anionic dyes. The removal performance of nylon-6 towards AB 117 was found highly specific as it was able to decolorize only AB117 effectively. Other dyes such as MO and RB5 were not satisfactorily removed. The equilibrium concentrations of MO, AO 67, RR195, RB5 and AB117 were 30.25, 11.6, 11.4, 14.2 and 2 mg/L respectively. This indicated that nylon-6 nanofibers selectively 20
ACCEPTED MANUSCRIPT adsorbed AB117 dye. In order to ascertain the selectivity of nylon-6 towards AB117, binary mixture of dyes containing AB117 and RR195, AB117 and MO of equal concentrations were prepared and adsorption experiments were conducted under optimum conditions. The results in Figure 8b show highly selective and efficient removal performance of nylon-6 towards AB117 that removed appreciable amount of AB117 dye from binary mixture and left the other dye in solution. The removal
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efficiency for AB 117 was 83 ±1.8% while for RR 195 was 70.8 ± 0.6%, similarly the removal efficiency for the same dye was 85 ± 0.75% in presence of MO (45.5 ± 0.9% AE%) In a study reported
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by [36], the standard dye affinity increases proportionally with the size of anionic dye; as the mode of
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interaction changes from ion-ion to van der Waal forces and small enthalpy changes contribute to greater affinity. Therefore, it may be proposed that AB117 was specifically adsorbed on nylon-6
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nanofibers due to above mentioned facts. Although its size was small than RR195 but due to the
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presence of one SO3- group in AB117, its selectivity supersedes RR195. From this behavior, it is also
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responsible in AB117 removal.
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relevant that the dye removal is not governed by size exclusion mechanism but surface adsorption is
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Figure 8. (a) Adsorption efficiency of nylon-6 nanofibers for different dyes; (b) Selective
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adsorptive removal of AB117 in different binary mixtures. Dye-nanofiber adsorption mechanism IR spectra of neat nylon 6 nanofibers and AB117 treated nylon-6 nanofibers are shown in Figure 9. The band at 3429 cm-1 can be related to N-H stretching vibrations. The peaks at 2930 cm-1 and 2848 cm-1 may be associated with –CH asymmetric and symmetric stretching vibrations, respectively. The bands at 1640 cm-1 and 1542 cm-1 may be assigned to amide I (C=O stretching vibrations) and amide II (N-H bending vibration) 23
[37]
. The band at 694 cm-1 is attributed to bending of O-C-N
[13]
. The FTIR
ACCEPTED MANUSCRIPT analysis after dye (AB117) treatment of nylon-6 nanofibers showed a considerable increase in intensities as well as sharpening of N-H stretching, suggesting addition of amine groups from AB117 molecules and involvement of some hydrogen bond interactions between AB117 and nylon-6 nanofibers. The bands due to amide I and amide II became weak and less intense possibly due to participation of N-H and C=O sites for dye capture. The characteristic bands of –SO3 due to . In addition to this, a band at 930
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[38]
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asymmetric stretching vibrations are observed at 1022-1030 cm-1
[39]
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cm-1 was emerged due to dye capture that was attributable to S-O stretching from SO2 group in AB117 . From the above facts, it can be considered that the nature of dye nanofiber interactions was
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combination of multiple interactions.
Figure 9. FTIR spectra of nylon-6 nanofibers before and after dye treatment. Further assistance regarding mode of interaction and probable binding sites was gained through XPS technique. The C1s high resolution spectrum of nylon-6 after AB117 treatment is shown in Figure 10 a. The main binding energies due to CH2-CH2 (284.9 eV), CH2-C=O (287.2 eV) and O=C-N (289.9 24
ACCEPTED MANUSCRIPT eV) were slightly shifted from their original positions that suggest participation of amide groups for AB117 capture. The positive shift of CH2-CH2 is more likely due to hydrophobic interactions of alkyl chain with alkyl groups of dye. Moreover, the peak due to CH2-NH diminished after dye treatment suggesting main point of dye interaction. To the best of our prediction the amide group in nylon-6 affects adjoining –CH2- group by generating polarity in N C bond that creates charges; hence,
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possibly endowing Lewis acid and base sites for dye attachment. The dye was also subjected to XPS
states are provided in supplementary material (ES2-ES6)
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analysis and the relevant figures and binding energies of atoms along with their different chemical
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The deconvolution of N1s peak at 402 eV indicated the arousal of extra peak at 399 eV which
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through extensive survey of reference data could be due to N-C or N-CO-N bond [40] (Figure 10 b). The overall mechanism of dye-nylon-6 nanofibers is a complex system, but for simplicity we may assume
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that a new bond between carbonyl carbon of nylon-6 and N of dye molecule have been generated
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through attack of nucleophilic nitrogen from dye to the more electrophilic carbonyl carbon of nylon 6.
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When the O1s region of dye treated nylon 6 was fitted, three different components were achieved, i.e. the peaks at 530.19 eV, 531.5 eV and 533 eV as shown in Figure 10 c. The interpretation
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of this region unveiled some interesting facts; the peak that emerged at 530.19 eV may be attributed to [41]
, this could be possible as the dye was sodium salt hence Na+ may
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metal-oxygen (M-O) bond
possible have some interfacial interactions with electronegative oxygen of nylon-6. Additionally, the peak ratio of O=C-N/O-H (0.81) changed to 0.2 that suggest that O=C-N group has predominantly taken main participation in dye fixation. The above results and the results from FTIR are in good concurrence with each other. From the results achieved through these both techniques, it may be suggested that dye-nylon-6 interactions can be categorized to partly hydrophobic, electrostatic and hydrogen bond interactions. The conclusive interactions may be illustrated in the Scheme-1.
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Figure 10 (a). XPS high resolution core level C1s; (b) N1s and (c) O1s.
26
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O S O Na
Dye
O δ+
*
δ-
N
* N
Proposed dye nylon-6 adsorption mechanism.
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Scheme 1.
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Dye
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H H
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Membrane filtration study
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Since the adsorption results accomplished through the batch mode showed excellent adsorption performance in terms of time, efficiency and capacity, it was realized to use nylon-6 nanofiber
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membrane as filtration medium to evaluate its adsorption performance for AB117 removal. For
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achieving satisfactory dye removal from aqueous solution for over sufficient volume, we made different foldings corresponding to layers 5, 10 and 20 of nylon-6 membrane in conventional filter
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holder. The thickness of membranes with 5, 10 and 20 foldings were 193 ± 20, 280 ± 15and 490 ± 30
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μm, respectively. The dye solution (50 mg/L) was passed through the membrane filter at a flow of 1mL/min. Figure 11 shows the influence of number of layers on breakthrough efficiency of nanofiber
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membrane. The 50% breakthrough with 5 layers was achieved earlier (9 mL) whereas for the membrane with 10 and 20 layers, the breakthrough was achieved through passing 17 mL and 22 mL respectively. The probable reason for the entire removal of AB117 in early filtration process could be pressure driven convection mass transport through the interstitial spaces between nanofiber membranes and rapid adsorption kinetics due to abundant available active sites
[42]
. The adsorption capacity for
AB117 in this design was 0.169, 0.283 and 0.321 mg/cm2 when passed through 5, 10 and 20 layers respectively with a high permeation flux 113.2 L/m2.h. This flux is much greater than conventional 27
ACCEPTED MANUSCRIPT nanofiltration membranes (10 L/m2.h.bar)
[43]
.The initial experimental survey suggests that membrane
stacking could afford greater potential to filter greater volume of dye contaminated water. Since the membranes are highly permeable therefore, they offer negligible drop in flux and can be conveniently applied in continuous adsorption by stacking multiple membranes of the 20 foldings to increase the adsorption capacity and render delayed breakthrough and service time.
[44].
The resulting nanofiber
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membrane has many desirable properties such as high removal efficiency, permeation flux, stability,
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20 Layers 10 Layers 5 Layers
50
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20
10
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Outlet dye concentration, mg/L
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manufacturing scalability and cost-effectiveness.
5-10
11-15
16-20
21-25
26-30
31-35
Permeation Volume, mL
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Figure 11. Breakthrough curves obtained after passing dye solution through nylon-6 nanofiber membranes with different layers. Conclusion
Highly robust and selective nylon-6 nanofibers were fabricated that offered complete removal of AB117 within 25 min; that is one of the promising characteristics of this nanoadsorbent. The optimum pH was found natural working pH for processing adsorption experiment. The nanofibers with 20 mg were sufficient for achieving 90± 2% removal. The adsorption efficiency reduced with an increase in 28
ACCEPTED MANUSCRIPT initial dye concentration with maximum experimental adsorption capacity of 58.8 mg/g. The presence of commonly found ions did not affect the removal performance of nylon-6 nanofibers that is worth applicable in industrial applications. Moreover, the dye was found highly selective and specific in both individual as well as binary mixtures of other anionic dyes. Apart of batch studies, nylon-6 nanofibers also proved worth applicable in treating AB117 in continuous system with maximum adsorption
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capacity 0.321 mg/cm2 by increasing the number of layers that offered permeation flux of 113.2
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L/m2.h; which is much greater than conventional nanofiltration membranes 10 L/m2.h.bar. The dye nylon-6 binding mechanism is the combination of both physical and chemical interactions. The nylon-6
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synthetic materials. Acknowledgement
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The work was supported by Mehran University of Engineering and Technology Jamshoro Pakistan and
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Shinshu University Japan.
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(Supporting information is provided)
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ACCEPTED MANUSCRIPT Highlights
Nylon-6 nanofibers without pre/post electrospinning treatment were prepared
Nylon-6 nanofibers selectively and efficiently removed acid blue 117
The maximum adsorption capacity was 58.8 mg/g
Nanofibers were characterized by FTIR, SEM and XPS
Nanofibers were also successfully used as filtration medium
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