Reusable carbon nanofibers for efficient removal of methylene blue from aqueous solution

Accepted Manuscript Title: Reusable carbon nanofibers for efficient removal of methylene blue from aqueous solution Authors: Abdul Sameeu Ibupoto, Umair Ahmed Qureshi, Farooq Ahmed, Zeeshan Khatri, Muzamil Khatri, Maryam Maqsood, Rafi Zaman Brohi, Ick Soo Kim PII: DOI: Reference:

S0263-8762(18)30330-7 https://doi.org/10.1016/j.cherd.2018.06.035 CHERD 3245

To appear in: Received date: Revised date: Accepted date:

21-12-2017 15-4-2018 26-6-2018

Please cite this article as: Ibupoto, Abdul Sameeu, Qureshi, Umair Ahmed, Ahmed, Farooq, Khatri, Zeeshan, Khatri, Muzamil, Maqsood, Maryam, Brohi, Rafi Zaman, Kim, Ick Soo, Reusable carbon nanofibers for efficient removal of methylene blue from aqueous solution.Chemical Engineering Research and Design https://doi.org/10.1016/j.cherd.2018.06.035 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Reusable carbon nanofibers for efficient removal of methylene blue from aqueous solution Abdul Sameeu Ibupoto a,b, Umair Ahmed Qureshi b, Farooq Ahmed b, Zeeshan Khatri b, Muzamil Khatric, Maryam Maqsood a, Rafi Zaman Brohi a, Ick Soo Kimc a

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Institute of Environmental Engineering & Management, Mehran University of Engineering and Technology, Jamshoro, Pakistan bCentre

of Excellence in Nanotechnology and Materials, Mehran University of Engineering and Technology, Jamshoro, 76060, Pakistan c

Nano Fusion Technology Research Lab, Division of Frontier Fibers, Institute for Fiber Engineering (IFES), Interdisciplinary Cluster for Cutting Edge Research (ICCER), Shinshu University, 3-15-1, Tokida, Ueda, Nagano 386-8567, Japan

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Email: [email protected]

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Graphical abstract

Highlights

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Carbon nanofibers were prepared from polyacrylonitrile through thermal treatment Carbon nanofibers efficiently removed Methylene Blue dye Carbon nanofibers have good adsorption ability and reusability Carbon nanofibers were characterized through SEM, EDX and FTIR The developed carbon nanofibers had negligible fragility and good strength

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Abstract: This work demonstrates the preparation of polyacrylonitrile (PAN) based activated carbon nanofibers (ACNFs) through electrospinning followed by thermal treatment. Resulted activated carbon nanofibers having diameters in the range of 240-280 nm were then examined for the adsorption capability for methylene blue dye from aqueous solution. Batch mode experiments 1

were carried out at room temperature to study the effect of amount of nanofiber, contact time and pH on dye adsorption. It was found that activated carbon nanofibers showed remarkable adsorption efficiency while completely decolorizing the dye solution within 60 min of contact. Results revealed that the adsorption data followed Langmuir isotherm giving maximum adsorption capacity of 72.46 mg/g and pseudo second order kinetic model. Furthermore, the

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reusability of activated carbon nanofibers had shown removal efficiency more than 80% up to 3 cycles. Morphology and structure of PAN nanofibers and ACNFs were characterized by scanning electron microscopy (SEM), Energy dispersive X-ray (EDX) and Fourier transform infrared spectroscopy (FTIR).

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Keywords: Carbon nanofibers; Cationic dyes; Methylene Blue; Water treatment.

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Introduction:

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Dyes in most of the industrial effluents pose adverse health impacts in human beings as they are capable of causing cancer and other mutagenic effects (Gad et al., 2009). Many industries like

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textiles, leather, paper, pharmaceuticals and food use dyes to color their products before draining

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them into water bodies resulting water pollution (L. Ai et al., 2011; Hameed et al., 2009). The presence of such coloring compounds in water bodies is serious concern as they block light

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permeation leading to eutrophication, as a consequence pose serious threat to aquatic life (Uddin et al., 2009). Methylene Blue (MB) also called tetramethylthionine chloride is a cationic dye, not

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much harmful but can be dangerous while continuous exposure to it causing vomiting and increased heart rate (Nasuha et al., 2010). Therefore, it is very much necessary to remove such contaminants from wastewater before discharging them in order to keep environment safe.

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Worldwide researchers and scientist are paying attention to the quick, easy and economical decolorization techniques and methods. Membrane filtration and adsorption techniques are proving the best in terms of efficacy and ease (Qureshi et al., 2017) among conventional techniques such as ion exchange, coagulation and flocculation, photochemical degradation, ozonation and other biological methods.

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Furthermore, a cheap and affordable sorbent material makes the adsorption technique more effective and economically viable. Many researchers have reported different adsorbents for the removal of methylene blue (MB) dye from aqueous solutions, for example; Granular activated carbon, multiwalled carbon nanotubes, clay, chitosan and by-products of agricultural wastes such as rise hull, cotton plant waste, garlic peels etc. Recently in 2017 Esra et al. have

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used iron loaded activated carbon for the removal of methylene blue (MB) dye while maximum adsorption capacity was 303 and 357.1 mg g−1 for activated carbon and iron loaded activated carbon respectively (Altıntıg et al., 2017). Another study was carried out by Sheng-Tao Yang et al. in which methylene blue was absorbed by Graphene oxide having adsorption capacity of 715 mg g-1(Yang et al., 2011) but such materials limit their applications either due to intensive cost of materials or prolonged preparation time. Some materials also tend to yield lower adsorption

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capacities and longer equilibrium times are needed to reach maximum adsorption of dyes.

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Nanotechnology has played a vital role in the fields of electronics, medical,

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environmental friendly technologies such as wastewater treatments, air purification techniques

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and other so many fields while minimizing health effects and making environment green. Various nanosized structures are used for the removal of methylene blue such as multiwall carbon nanotubes, Zeolite-iron oxide magnetic nano composites due to high surface area to

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volume ratio, high pore size distribution (Dwivedi et al., 2016). The main concern of such

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studies reported yet is either longer adsorption equilibrium time or tedious preparation processes along with very expensive adsorbent materials. Therefore, sustainability, low cost, ease, high

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efficiency and easy handling and separation of sorbent material is the first and foremost need of the present time. Moreover, conventional method either physical or chemical for developing

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carbon is to use higher furnace temperatures and optimization to obtain tunable porosities. Such methods are more energy intensive and tend to produce carbons with lower mass production. The present solution for mass production issue is to recycle the spent adsorbent and reuse it

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efficiently that is rarely achieved in previously used material or to modify the method of carbon production. Therefore, the main purpose of present study is to develop a low cost, highly efficient and reusable carbon nanofibers based adsorbent for the substantial removal of Methylene blue dye. The main advantage of such material is its reusability and easy separation from aqueous solution 3

which makes it the most prominent, economical and sustainable towards the environment. Though the polymer polyacrylonitrile (PAN) used as a precursor of carbon nanofibers is little expensive, but the reusability of carbon nanofibers overcomes the problem of high cost, thus making it the promising sorbent material for the removal of methylene blue (MB) dye.

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Experimental Material and methods

Polyacrylonitrile (PAN) (Mw = 150,000 g/mol) and N, N-Dimethylformamide (DMF) was purchased from Sigma Aldrich (USA); Cationic Dye Methylene Blue (MB) was supplied by Archroma Pakistan, Sodium hydroxide (NaOH) and hydrochloric acid (HCL) was purchased

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Preparation of activated carbon nanofibers (ACNFs)

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from Sigma Aldrich, USA.

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Electrospinning

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Polyacrylonitrile (PAN) solution was dissolved in N, N-Dimethylformamide (DMF) and stirred at room temperature for 6 h to form a 10 wt. % solution of PAN. The prepared solution was then

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filled into the 3 ml syringe attached to a capillary tip with an inner diameter of 0.6 mm. The syringes were installed in the home-built electrospinning setup where a copper wire (anode) was

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dipped into the solution while the negative terminal (cathode) was attached to a grounded rotary collector drum. The voltage of 8 kV was applied and the tip to collector distance was set to 20

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cm. Finally, a non-woven membrane of continuous electrospun PAN nanofibers having an average thickness of 90 µm ±5 was collected onto aluminum foil wrapped onto the metal rotary

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collector drum.

Thermal treatment

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The prepared PAN nanofibers were then placed in the air dryer at 280 °C with a heating rate of 3 °C/min and 1 hour hold time at peak temperature to carry out stabilization process (Rahaman et al., 2007; Zhou et al., 2009). The brownish colored stabilized PAN nanofiber sheet was carbonized and activated by a new method called plate sandwich method. This new method was adapted to prevent sample fragility and formation of ashes during treatment at higher temperatures. In this method stabilized nanofiber sheet was placed in between two rectangular 4

stainless steel plates tighten by nuts and bolts on all four sides. Plates were then put into a concentric steel container with lid, inside space of the container was filled with the sand up to the brim, such arrangement assures the total absence of air to the carbonizing material. The container was then placed into a muffle furnace at 600 °C with a heating rate of 5 °C and 1 hour hold time at peak temperature. Activated carbon nanofibers (ACNF) were finally obtained and

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characterized. Characterization

The produced materials were characterized by scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) (S-3000N by Hitachi, Japan) for surface morphological structure and elemental analysis, respectively. Image analysis software (ImageJ proR Plus, Version 5.1, Media

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Cybernetics, Inc.) was used to check the average diameter of nanofibers from SEM micrographs.

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FTIR (IR Prestige-21 by Schimadzu, Japan) analysis was carried out using ATR mode to check

and graphing.

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MB dye stock solution and its analysis

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the chemical structural changes. Origin® 9.0 Pro from OriginLab® was used for data analysis

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Standard stock solution of methylene blue dye was prepared by dissolving certain amount of dye into the distilled water to get 1000 ppm of MB dye solution, further dilution was carried out to

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get different concentrations of dye solution. The amount of MB dye into the solution before and after the adsorption process was quantified using a UV-vis spectrophotometer UV 1800

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(Shimadzu Japan) at a maximum wavelength 668 nm.

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Adsorption Experiments

A stock solution of MB with a concentration of 1000 mg/L was prepared and further different concentrations were obtained from a series of dilution. 0.1M NaOH and 0.1M HCL was used to

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adjust pH of the solutions. All the experiments were carried out in 10 ml solution for the removal of dye from water. Batch experiments were conducted by taking 7 mg of ACNF into the 10 ml solution of MB with initial concentration of 25 mg/L. The parameters affecting adsorption were optimized by varying one factor at a time keeping others same. The influence of time was determined by shaking the dye solution and nanofiber mixture at 100 rpm. The different time intervals selected were 15, 30, 45, 60 and 70 min. The influence of pH was monitored by varying 5

pH within 3-10. The optimum dosage was selected by measuring nanofiber mass from 2-10 mg. The influence of initial concentration was determined by selecting MB concentration from 25250 mg/L. The reusability was performed by shaking the dye loaded nanofiber in 0.01 M HCL

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solution (5 mL) for 15 min. The residual dye concentration was analyzed by UV-Vis spectrophotometer through calibration of different initial concentrations of MB. The percent adsorption was calculated by using the following equation; % Adsorption 

C

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Cf Ci

 100

Eq.1

where Ci is an initial dye concentration (mg/L), Cf is the final dye concentration (mg/L),

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V is the volume of the solution (L), m is the mass of nanofibers (g). Batch kinetic studies were

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carried out and the kinetic experimental procedures were similar to that of equilibrium tests. The

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concentration of a dye in aqueous samples was measured at different time intervals. The

C o  Ct V m

Eq. 2

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qt 

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adsorption amount at time t, qt (mg/g), was calculated using equation given below;

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where Co is an initial concentration (mg/L), Ct is the concentration at any time t (mg/L), V is the volume of a solution (L), m is the mass of nanofiber (g).

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Results & discussion

Effect of thermal treatment on PAN nanofiber

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The synthesis of ACNFs from polyacrylonitrile (PAN) nanofibers begins with the heating of PAN nanofibers in an air atmosphere at 280 °C for one hour, the process is called stabilization.

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During the stabilization process, PAN nanofibers underwent physical stress and apparently contracted. Chemically, it went through the cyclization resulting formation of ring structure often called ladder structure in which C=N double bonds are formed. Furthermore, dehydrogenation happens when the stabilized PAN nanofibers are further carbonized keeping nanofiber morphology intact (Gu et al., 2005; Kozhitov et al., 2016). It is usually believed that carbon

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nanofibers having C=C aromatic rings are formed, leaving the hydrogen atom at the edges after carbonization as shown in Fig.2.

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SEM and EDX analysis The polyacrylonitrile (PAN) nanofibers and activated carbon nanofibers (ACNF) were characterized by SEM in order to observe morphological changes due to heat treatment. Fig 4a. and Fig 4b. show the SEM micrographs of as-spun PAN nanofibers and ACNFs respectively. Image analysis software was used to check the average diameter of nanofibers from SEM micrographs. The nanofibers derived from PAN were found bead free and smooth in

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morphology. The average diameter of PAN nanofibers was found to be 315 nm .After the

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carbonization process , the mat retained its shape and the fiber diameter was dramatically shrank to 250 nm; such morphological changes with few surface defects could be due to micro

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combustion and pyrolysis of PAN. Fig.4c shows the histogram of ACNFs , it can be seen that the

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diameter of the nanofibers was mostly in between 250-300 nm, confirming the uniformity of produced ACNFs. Furthermore, energy dispersive X-ray (EDX) technique was used to study the

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elemental analysis and can be seen in Fig.4d that the produced material contained major part of

FTIR analysis

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carbon and can prove itself as an adsorbent material for the removal of contaminants from water.

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The PAN nanofibers and activated carbon nanofibers (ACNFs) were characterized for chemical

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structural changes. FTIR analysis was carried out using ATR mode. Fig.5 shows FTIR spectra of as-spun PAN nanofibers and ACNFs; characteristic peaks in PAN nanofibers are very much clear, vibrations at 2242 cm-1 correspond to a major (C≡N) nitrile group (Dalton et al., 1999;

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Wangxi et al., 2003), the vibrations at 2922 cm-1, 1450 cm-1 are associated with different aliphatic CH groups (CH, CH2 bonds) while the band at 1665 shows the presence of amide groups. While for ACNFs, FTIR spectral data was much similar to the graphene oxide. Results showed that the major peak from PAN nanofibers 2244 cm-1 of C≡N was diminished due to the cyclization (Dalton et al., 1999) and the emergence of two major peaks at 1580 cm-1 and 1260 cm-1 results from C=C stretching vibrations in aromatic rings due to the dehydrogenation. The 7

band at 1260 cm-1 indicates formation of heteroaromatic rings with double bonds or may be attributed to C-H groups (Arshad, 2011; Dalton et al., 1999). Influence of time and pH The removal of MB dye solution was carried out at different times (15 min-70 min) to determine

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the optimum time taken by ACNFs to remove 25 mg/L of dye solution. Fig 6a shows that the instant removal was achieved within 15 min (85%) due to availability of abundant vacant sites that became saturated with dye adsorption with the further passage of time. A complete removal was noticed within 60 min hence, this time was considered as equilibrium time for optimization of further parameters.

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Furthermore, the influence of pH of dye solution was probed to reach the maximum removal efficiency. Fig 6c shows that the alkaline pH was very much favorable in effectively

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removing MB from aqueous phase. This behavior may be attributed to the presence of negative

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surface charge on ACNF at alkaline pH that rendered electrostatic interactions between

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positively charged MB molecules and ACNF. However, the lower removal efficiency of MB dye in acidic pH may be due to protonation of surface functional groups in ACNF that may have

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rendered the surface with positive charges thereby retarding the adsorption of MB molecules. Such trend can be considered favorable for treating Textile industry wastewater as it usually has

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proposed adsorbent.

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alkaline pH, hence it is more appropriate to treat that water without pH adjustments using the

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Influence of dye concentration and mass of nanofibers The effect of an initial dye concentration on adsorption of MB dye was determined by preparing different concentrations of dye from 25-250 mg/L. Adsorption efficiency and performance of

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ACNFs were determined and it was observed that the removal efficiency decreased with increase in initial MB concentration as shown in Fig.7. This noticeable behavior may be because of limited active sites present on ACNFs resulting reduction in efficiency of adsorption. Influence of an adsorbent dosage on the removal efficiency of MB was studied and is presented in Fig. 7.Different amounts of nanofibers ranging from 2-10 mg were agitated for 60 8

min using an initial concentration of 25 mg/L of MB. It was observed that the removal efficiency of ACNF increased with further increase in their masses. A complete removal was achieved using 10 mg ACNF only that is economic and can save secondary pollution due to sludge generation. This trend has been noted in many well-known research studies and may be correlated to the increment in the surface area and available active sites onto the activated carbon

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nanofiber in a result of an increased adsorbent dosage (Gupta et al., 2016; Uddin et al., 2009).

Adsorption isotherms

The adsorption isotherm studies useful in understanding the behavior of molecular distribution between liquid and solid phase when adsorption equilibrium stage reaches. Monolayer

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adsorption of molecules is assumed in Langmuir isotherm where adsorption occurs onto a

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surface having limited number of homogeneous adsorption sites. The correlation coefficients, R2

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was judged to check the fitting of isotherm equations.

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The adsorption data of MB on ACNFs were analyzed with the following linear form of

Eq. 3

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 1  Ce 1 Ce    qe Q max b  Q max 

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Langmuir isotherm:

Where Ce is the equilibrium concentration of the MB (mg/L), qe is the amount of adsorbed MB

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(mg/g), and Qmax and b are Langmuir constants associated to capacity of adsorption and affinity of adsorption, respectively. Fig.8a shows that the MB dye adsorption was well fitted to Langmuir

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isotherm, where b is 0.159 L/g and Qmax is 72.46 mg/g. Following logarithmic equation of Freundlich model was used to study the behavior of

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adsorption of MB dye on ACNFs. 1

𝐿𝑜𝑔 𝑞𝑒 = 𝐿𝑜𝑔 𝑘𝑓 + (𝑛) log 𝐶𝑒

Eq. 04

qe = Adsorbed amount of adsorbate at equilibrium (mg/g) Ce = equilibrium concentration of adsorbate

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kf = adsorption capacity 1/n = heterogeneity factor kf and n are the two Freundlich constants. The value of kf defines the coefficient of distribution and signifies the amount of MB dye adsorbed onto the ACNFs at equilibrium concentration,

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where n shows the favorability of adsorption process. 1/n is the slope which ranges from 0 to 1 and is the measure of surface heterogeneity factor or the intensity of adsorption, surface will be more heterogeneous if the value is nearer to zero. if the value of 1/n is less than one, it shows the adsorption process is more likely to be Langmuir isotherm while value greater than one indicates mutual adsorption behavior.

In case of adsorption of MB dye onto ACNFs graph of log qe vs. log Ce shown in Fig.8b.

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suggests that the adsorption process is not Freundlich isotherm giving the correlation coefficient

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R2 value 0.25, adsorption capacity Kf was calculated and found 64.7 mg/g whereas, the value of

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n is 9.34 which shows that surface of the adsorbent is homogeneous (Haghseresht et al., 1998).

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Adsorption Kinetics

In order to determine kinetic mechanism of MB adsorption and to evaluate rate of adsorption,

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different kinetic models namely pseudo first-order and pseudo second order were applied to

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experimental results. The pseudo first order and pseudo second order rate equations are depicted as:

ln qe  qt   ln qe  k1t

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Eq. 5

1 t 1  t   qt k 2 q 2 e  qt 

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Eq.6

Where qe (mg/g) is the amount of MB adsorbed at equilibrium , qt (mg/g) is the amount of MB

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adsorbed at time t (min), k1 (min-1) and k2 (g.mg -1.min-1) are the rates of pseudo first and pseudo second order equations. In case of pseudo first order, the plot of log (qe-qt) vs. t was found linear with the correlation coefficient value R2=0.916 (Fig 9b) but the experimental value of qe did not coincide with the calculated one suggesting that the adsorption of MB onto ACNFs cannot be pseudo first order type.

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On the other hand, applying pseudo second order gave linear plot of t/qt vs. t as shown in Fig.9a. The higher correlation coefficient (R2=0.997) and good agreement between the calculated and experimental qe values together are supporting this model effectively suggesting the process

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of adsorption of MB on ACNFs is pseudo second order in nature with chemical interactions occurring between MB and ACNF functional groups.

Reusability:

The reusability of any adsorbent makes it prominent adsorbent material for the adsorption and

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makes it more economical too, in this regard reusability of ACNFs for the MB dye adsorption

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was studied by carrying adsorption and desorption experiments. ACNFs membranes obtained

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after adsorption were dried at room temperature and were shaken in 5 ml of 0.01 M HCl for 15 min in order to achieve MB desorption. Adsorption experiment was carried out under optimum

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conditions subsequently; it was observed that the ACNFs gave more than 80% removal of MB dye up to 3 cycles and then efficiency declined to 50% and 40% after fourth and fifth cycles

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respectively as shown in Fig.10

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The adsorption capacities and process time of other adsorbent materials for MB dye

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removal were compared to proposed ACNFs and are shown in Table 2. It is obvious that PAN based carbon nanofibers exhibited good adsorption capacity and required reasonable time for

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MB removal compared to previously reported materials. Other materials reported are either expensive or required prolonged chemical or thermal treatments that limit their applicability in

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practical implications. On the basis of those facts, currently used materials are both efficient, practically and economically feasible.

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The proposed binding mechanism between ACNFs and methylene blue dye could be evaluated from the FTIR spectra of before and after adsorption as shown in Fig.11a . It was observed that there was no any change in peaks except some shifting in C=C aromatic rings from benzenoid structure from 1580 cm-1 to 1592 cm-1, such change in chemical structure suggests that there could be π-π stacking between the aromatic rings of ACNFs and methylene blue dye as shown in

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Fig.11b.

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Conclusion

The current study suggests that the adsorption of MB on PAN based ACNFs is quite effective

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and economical as its preparation process is quite simple compared to other methods. This

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adsorbent material possesses the high adsorption capacity (Qmax 72.46 mg/g) within shortest

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equilibrium time compared to other adsorbents previously used for the removal of similar dye. Furthermore, isotherm studies revealed that the adsorption was monolayer adsorption following

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Langmuir isotherm model, while kinetic modeling was well supported by pseudo second order kinetic model. According to the results obtained from studies and FTIR results, the proposed

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adsorption mechanism was better defined through π - π stacking between aromatic rings of ACNFs and methylene blue dye. Another benefit the current adsorbent offers is the reusability

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up to three cycles without affecting adsorption efficiency (>80%) and material stability that prevents preparation of fresh sample requirement as well as cost. Furthermore, the adsorbent is

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easy to synthesize and offers thermal stability. These all features make these nanofibers highly desirable adsorbent material.

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Acknowledgement The work was supported by Mehran University of Engineering and Technology Jamshoro and Shinshu University Japan.

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Hameed, B., & Ahmad, A. (2009). Batch adsorption of methylene blue from aqueous solution by garlic peel, an agricultural waste biomass. J Hazard Mater, 164(2), 870-875. Kozhitov, L. V., V‫׳‬et, N. K., Kostikova, A. V., Zaporotskova, I. V., & Kozlov, V. V. (2016). The simulation of carbon material structure based on polyacrylonitrile obtained under IR heating. Modern Electronic Materials, 2(1), 13-17. doi:https://doi.org/10.1016/j.moem.2016.08.003 Kumar, K. V., Ramamurthi, V., & Sivanesan, S. (2005). Modeling the mechanism involved during the sorption of methylene blue onto fly ash. J Colloid Interface Sci, 284(1), 14-21. Nasuha, N., Hameed, B. H., & Din, A. T. M. (2010). Rejected tea as a potential low-cost adsorbent for the removal of methylene blue. J Hazard Mater, 175(1-3), 126-132. doi:10.1016/j.jhazmat.2009.09.138 Qureshi, U. A., Khatri, Z., Ahmed, F., Khatri, M., & Kim, I.-S. (2017). Electrospun Zein Nanofiber as a Green and Recyclable Adsorbent for the Removal of Reactive Black 5 from the Aqueous Phase. ACS Sustainable Chemistry & Engineering, 5(5), 4340-4351. doi:10.1021/acssuschemeng.7b00402 Rahaman, M. S. A., Ismail, A. F., & Mustafa, A. (2007). A review of heat treatment on polyacrylonitrile fiber. Polym Degrad Stab, 92(8), 1421-1432. doi:https://doi.org/10.1016/j.polymdegradstab.2007.03.023 Uddin, M. T., Islam, M. A., Mahmud, S., & Rukanuzzaman, M. (2009). Adsorptive removal of methylene blue by tea waste. J Hazard Mater, 164(1), 53-60. doi:10.1016/j.jhazmat.2008.07.131 Vadivelan, V., & Kumar, K. V. (2005). Equilibrium, kinetics, mechanism, and process design for the sorption of methylene blue onto rice husk. J Colloid Interface Sci, 286(1), 90-100. Wangxi, Z., Jie, L., & Gang, W. (2003). Evolution of structure and properties of PAN precursors during their conversion to carbon fibers. Carbon, 41(14), 2805-2812. doi:https://doi.org/10.1016/S0008-6223(03)00391-9 Yang, S.-T., Chen, S., Chang, Y., Cao, A., Liu, Y., & Wang, H. (2011). Removal of methylene blue from aqueous solution by graphene oxide. J Colloid Interface Sci, 359(1), 24-29. doi:10.1016/j.jcis.2011.02.064 Zheng, J., Cheng, C., Fang, W.-J., Chen, C., Yan, R.-W., Huai, H.-X., & Wang, C.-C. (2014). Surfactant-free synthesis of a Fe 3 O 4@ ZIF-8 core–shell heterostructure for adsorption of methylene blue. CrystEngComm, 16(19), 3960-3964. Zhou, Z., Lai, C., Zhang, L., Qian, Y., Hou, H., Reneker, D. H., & Fong, H. (2009). Development of carbon nanofibers from aligned electrospun polyacrylonitrile nanofiber bundles and characterization of their microstructural, electrical, and mechanical properties. Polymer, 50(13), 2999-3006. doi:https://doi.org/10.1016/j.polymer.2009.04.058

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EP

SC RI PT

TE

D

M

A

N

U

Figure 1. Chemical structure and UV- visible spectrum of methylene blue dye.

A

CC

Figure 2. Chemical structural changes in PAN nanofibers under thermal treatment.

15

SC RI PT U N A M D TE EP CC

A

Figure 3. (a) SEM image of as-spun PAN nanofibers. (b) SEM images of ACNFs. (c) Histogram of ACNFs. (d) EDX patterns of ACNFs.

16

SC RI PT U N A M

A

CC

EP

TE

D

Figure 4. FTIR spectra of PAN NF and ACNFs

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SC RI PT U N A M D

A

CC

EP

TE

Figure 5. (a) Adsorption efficiency of ACNFs for MB dye as a function of time (min); (b) UV-Vis spectra of MB dye after adsorption at different time intervals; (c) Effect of pH on adsorption of MB dye onto ACNFs.

18

SC RI PT U

CC

EP

TE

D

M

A

N

Figure 6. Effect of a mass of nanofiber and initial concentration on the adsorption efficiency.

A

Figure 7. (a) The Langmuir isotherm model for MB dye removal on ACNFs; (b) The Freundlich isotherm model for MB dye removal on ACNFs.

19

SC RI PT

CC

EP

TE

D

M

A

N

U

Figure 8.( a & b ) pseudo second order kinetic model and pseudo first order kinetic model, respectively for adsorption of MB dye onto ACNFs.

A

Figure 9. Different reusability cycles of ACNFs for MB dye adsorption.

20

SC RI PT

A

CC

EP

TE

D

M

A

N

U

Figure 10. (a) FTIR spectra of before and after adsorption of MB dye onto ACNFs, (b) Binding mechanism (π-π stacking) of MB dye onto ACNFs

21

Table 1. Kinetic parameters for the adsorption of MB on ACNF.

qe,exp

(mg−1)

(mg g−1)

Pseudo 2nd order

Pseudo 1st order k1

qe,cal (mg g−1)

R2

(min−1) 25

35.7

0.048

k2

[g

(mg

qe,cal (mg g−1)

R2

min−1] 20.65

0.91

0.004

SC RI PT

Initial conc.

38.5

0.99

Table 2. Comparison of previously reported adsorption capacities and equilibrium time of different adsorbents.

β -CD-SNHS*

99.22

A

178

M

Keratin nanofibers

clay

60 min

This Study

6 hr.

24 hr. >600 min 60 min

48.06 mg/g

2 hr

TE

graphene nanosheet/magnetite 43.82 (Fe3O4) composite 40.58 Rice husk

EP

Reference

58.2

D

M-MWCNT**

Eq. Time

U

Adsorption capacity (mg/g) Activated carbon nanofiber (PAN 72.46 based) Magnetite activated carbon (Acorn 357.1 Shell)

N

Adsorbent

20 min 2.5 hr.

40 and 35

50 min and 150 min

Carbonized citrus fruit peel

25.51

8 hr.

CC

Date stones and palm-trees waste

A

Fe3O4@ZIF-8 heterostructure Fly ash

core–shell 20.20

15 hr.

5.718

30 min

* β -cyclodextrin Silica nano hollow sphere ** Magnetite multiwall carbon nanotube

22

(Altıntıg et al., 2017) (Aluigi 2014) (Ebadi 2015) (Gürses 2006) (L. Ai 2011)

et al., et al., et al., et al.,

(Lunhong Ai et al., 2011) (Vadivelan et al., 2005) (Belala et al., 2011) (Dutta et al., 2011) (Zheng et al., 2014) (Kumar et al., 2005)