Electrospun Zein Nanofiber as a Green and Recyclable Adsorbent for

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Electrospun zein nanofiber as green and recyclable adsorbent for the removal of reactive black 5 from aqueous phase Umair Ahmed Qureshi, Zeeshan Khatri, Farooq Ahmed, Muzamil Khatri, and Ick-Soo Kim ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b00402 • Publication Date (Web): 07 Apr 2017 Downloaded from http://pubs.acs.org on April 8, 2017

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ACS Sustainable Chemistry & Engineering

Electrospun zein nanofiber as green and recyclable adsorbent for the removal of reactive black 5 from aqueous phase Umair Ahmed Qureshia,b, Zeeshan Khatria,c†, Farooq Ahmeda, Muzamil Khatria,c, Ick-Soo Kimc†† a

Department of Textile Engineering, Mehran University of Engineering and Technology, Jamshoro 76062, Sindh, Pakistan b

c

Government Boys Degree College Qasimabad, Hyderabad, 71000, Sindh, Pakistan.

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

†Corresponding Author: Zeeshan Khatri Dr.Eng.

E-mail: [email protected]

Address: Nanomaterials Research Lab, Department of Textile Engineering, Mehran University of Engineering and Technology, Jamshoro 76060, Pakistan. Tel: 0092 (0) 22 2772250

††Corresponding Author: Ick Soo Kim, Dr.Eng.

E-mail: [email protected]

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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ABSTRACT

Zein polymer is drawing intensive interest due to its nontoxicity, biodegradability, and unique structural properties. Zein based electrospun nanofibers are prepared from 80% ethanolic solution and are used as nanoadsorbent material for the confiscation of reactive black 5 (RB5) dye from aqueous solutions. The electrospun nanofibers possessed extraordinary high capacity for RB5 uptake compared to their powder and film analogues. Zein nanofibers possessed exceptionally efficient performance in removing RB5 in just 20 minutes of contact at room temperature and normal working pH. The mechanism of dye-zein interaction was chiefly controlled via hydrophobic, electrostatic forces and hydrogen bond interactions. Experimental data followed pseudo second order model and Langmuir adsorption isotherm was the most appropriate mechanism governing RB5 adsorption. The dye loaded zein nanofibers were directly re-electrospun in order to achieve a recyclable and green adsorbent; preventing the consumption of toxic and expensive reagents for elution of RB5.

KEYWORDS nanofibers; zein; adsorption; dye removal; reactive black 5 INTRODUCTION The dye loaded wastewater from textile, pharmaceutical, printing, leather, cosmetics and food industries is one of the serious public and scientific concerns [1,2]. The presence of dyes in water inhibits the penetration of sunlight and oxygen that deteriorates the aquatic life. Additionally, most of the reactive dyes and their metabolites are considered toxic, carcinogenic and are difficult to remove due to their complex chemical structure [3].Unfortunately, there are no official guidelines available to show the permissible limits of dye effluents coming out of industries [4]. Globally, a good number of research works are underway to develop powerful treatment methods in order to alleviate those toxic pollutants from natural and wastewaters. Different conventional treatments, like, coagulation-floccultion, advance oxidation, adsorption, ozonation, photo-chemical degradation


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and membrane technology, and further including biological methods are carried out to treat dye containing waste water [5]. However, the selection of a treatment technique depends on its efficiency as well as its cost, while, current research findings are preferring adsorption as the most appropriate, simple, robust and versatile technique, as it can eliminate different types of pollutants more efficiently. One of the key requirements of this technique is the exploration of the adsorbent that should be cheaper, abundant and must have greater adsorption efficiency. Activated carbon being easily available has some limitations, such as, longer equilibrium time, improper removal efficiencies and the variable nature of porosity to capture some complex dye molecules [6]. CI reactive black 5 (RB5) is commonly used dye in textile industry, and it possesses poor binding ability on textile surface; as a consequence it is released in industrial effluents. The adsorbents tested for RB5 removal include activated carbon, but due to its poor adsorption efficiency, prolonged equilibrium time and anionic nature of RB5, the surface of activated carbon requires some chemical treatments, such as quaternization with cationic surfactants [7]. Other adsorbents reported are chitosan [8], the surface modified chitosan [9,10], pumice and walnut activated carbon [11], polyaniline coated lignocellulosic composite [12] and etc. but such materials require either tedious preparation steps, toxic chemicals for synthesis or longer adsorption process times; hence search of a highly efficient, easily available, low cost, sustainable and environmentally benign adsorbent is still a challenging issue. Nanotechnology has benefits in many research areas encompassing water treatment, sensors and engineering. Cutting-edge developments in nanotechnology include design of particle shapes and morphology. Recently, the utilization of 1-dimensional nanoscale adsorbents, especially nanofibers, is of interest as adsorbents due to their superior advantages for wastewater applications. Thus, nanofiber adsorbents when compared to traditional adsorbents such as activated carbons offer advantages such as lower sample loss, easy separation without filtration, faster and higher adsorption efficiency. Many synthetic and natural polymers as well as inorganic precursors can be electrospun to make nanofibers with unique dimensions and properties. These have been proven efficient materials in removing the ACS Paragon Plus Environment

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variety of dyes from wastewater [13,14]. Additionally, nanofibers either structurally modified or unadorned show improved dye fixation ability due to an exceptionally greater surface area [15,16]. Other interesting areas for milestone applications of these new generation nanomaterials include sensor development, decontamination, energy storage, biomedical and catalysis [17]. Literature survey reveals that a limited work has been reported using electrospun nanofibers for RB5 removal such as, microalgae immobilized polysulphone nanofiber web [18]; however, the equilibrium time for RB5 removal was 14 days in addition to the difficult loading procedure of biomass on polymer nanofibers that impairs process efficiency both technically and economically. Dooto et al., [19] used chitosan modified polyamide nanofibers for RB5 removal, but the removal efficiency was not adequate to meet the quality standard and optimum higher temperature for RB5 removal is another energy intensive process. Therefore, search of highly efficient, sustainable and green adsorbent that should be capable to remove recalcitrant RB5 dye within a short time, under wide pH range and ambient temperature is still the issue to be resolved. Presently, biopolymers from renewable resources such as zein have drawn rigorous attention for economic and environmental reasons. Zein is the major storage protein in corn or maize, amphiphilic amorphous polymer and a product of the bio-ethanol industry. It is exclusively found in the corn endosperm, but due to a major proportion of nonpolar amino acids, it is insoluble in water. Zein is generally considered safe (GRAS) biopolymer. Due to its unique properties, such as nontoxicity, biodegradability and biocompatibility, zein nanofibers have drawn intensive interest in many areas such as encapsulation of essential oils, controlled drug delivery of anti-microbial drugs with enhanced cell adhesion and proliferation [20], as an efficient, edible and biodegradable antifungal coating material in apples to inhibit fungal proliferation [21] and a very promising wound dressing nanofiber mat when mixed with Ag [22]. When mixed with hydrophilic bacterial cellulose films, these nanofibers improved water resistance properties of bacterial cellulose [23]. Apart from those applications, zein polymer has also drawn keen interest in wastewater treatment such as, treating reactive dye polluted wastewater by changing surface morphology into hollow spherical form to confer greater contact area to capture dye ACS Paragon Plus Environment

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[24]. An effective dye removal (acid blue113 and acid yellow 110) has also been studied when zein was combined with TiO2 that provided dual benefits through adsorption and photocatalytic decolorization [25] but such experiments could not justify the reusability issue of used adsorbent materials. It has been also utilized with iron (III) for arsenic (V) removal [26]. But the performance of zein in the form of nanofibers for dye removal in minimum possible time and its recyclability is still required to be investigated in detail. Keeping above mentioned limitations of some conventional and nanofibrous adsorbents, the present work is aimed to prepare environmentally friendly, sustainable and recyclable zein nanofiber adsorbents for RB5 removal through electrospinning zein solution as it is believed to possess the greater surface area compared to its native form or film form. The purpose of present work is to overcome the limitations associated with tedious preparation steps, cost effective harmful chemical reagents for preparing efficient materials, longer adsorption times to achieve adequate removal for RB5 decontamination through the application of environmentally gentle and sustainable zein nanofibers. This is the first experimental investigation focusing on application of zein nanofibers as low cost, rapid acting and sustainable adsorbent for RB5 attenuation. EXPERIMETAL SECTION Materials

Zein from maize (melting point 266-283oC) was purchased from Sigma-Aldrich (USA); anionic dye, C.I reactive black 5 (vinylsulfone) was supplied by the Sumitomo Chemical Company, Ltd., Japan. Preparation of zein nanofibers and zein film Zein nanofibers were produced through the procedure reproduced from our previous study with slight modifications [27]. A solution of 25% (w/v) zein in 80% aqueous ethanol was prepared followed by stirring at 80oC for 2 h. The solution was then supplied through 5 mL plastic syringe attached to a capillary tip with an inner diameter of 0.6 mm. The syringes were placed at an angle of 10o from the horizontal plane. A copper wire was connected to the positive terminal (anode) dipped into the zein solution and a negative terminal (cathode) was attached to ground collector. A voltage of 20kV was applied and the tip-to-collector distance was set at 12 cm. The electrospun zein nanofibers were ACS Paragon Plus Environment

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deposited continuously over a stationary metallic drum. After completing the electrospinning process, samples were subjected to drying at room temperature overnight for removing residual solvents prior adsorption experiment. The relative humidity within the laboratory atmosphere was 40%. The average thickness of zein nanofibers was found to be 49µm. Zein films were fabricated through the well-known evaporation induced method. Briefly, 25% zein polymer solution in 80% aqueous ethanol was casted into flat glass plate. The solution was left to dry in the presence of air at room temperature overnight. This process led automatic peeling off the zein film from the surface of glass plate. The average thickness of the film was found to be 178 µm. Characterization The chemical structure of nanofiber sheets was analyzed through FTIR spectroscopy (IR Prestige-21 by Schimadzu, Japan) using ATR mode. The surface morphology of zein nanofibers was obtained under SEM (S-3000N by Hitachi, Japan). All samples were sputtered with platinum under the vacuum before assessment. The average diameter of electrospun nanofibers was measured from SEM micrographs using image analysis software (Image pro® Plus, Version 5.1, Media Cybernetics, Inc.). XPS measurements were performed with AXIS Ultra by Schimadzu equipped with the dual anode X-ray source Al/Mg and the HSA hemispherical sector analyzer detector with vacuum pressure maintained at 1.4 × 10-9torr. Mg

Kα

X-ray source (1253.6 eV) was used for XPS measurements. Data analysis and

curve fitting was performed using Origin Pro 9.0 with Gaussian-Lorentzian product function. BET surface area and pore size distribution were measured from N2 adsorption desorption at 77K using Quantachrome ASiQwin surface area analyzer. Ultraviolet-visible (UV-vis) spectroscopy was used to measure absorbance of RB5 solutions with different concentrations at λmax590 nm using a Lambda 35 UV-vis spectrophotometer (Perkin Elmer USA). Batch adsorption experiments Adsorption experiment was conducted by taking 5 mL of 50 mg/L RB5 into pyrex glass tubes. The reason behind selecting the low volume was prevention of waste disposal. Zein nanofibers (40±0.2 mg) were added to the reaction mixture. The mixture was agitated at 200 rpm at room temperature by means ACS Paragon Plus Environment

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of Gallenkamp shaker until equilibrium was achieved. The parameters that influenced removal efficiency of zein nanofibers were: adsorption time (1-30 min), solution pH (1-12), nanofiber mass (1060 mg) and initial RB5 concentration (20-200 mg/L). The pH of dye solution was adjusted through adding 0.1M HCl and 0.1M NaOH. The dyed zein nanofibers were separated manually after completing adsorption experiments. The residual concentration after adsorption experiment was analyzed using UVvis spectrophotometer at λmax 590 nm. The equilibrium adsorption capacity of zein nanofibers was determined by using the following equation:

qe =

(Co − Ce )V

(1)

m

where Co and Ce are initial and equilibrium concentrations of RB5 respectively; qe is the amount of RB5 adsorbed per unit mass ‘m’ of nanofiber (mg/g) at equilibrium. V is the volume of dye solution in L. Error analysis was also established to compare the validity of kinetic and isotherm models using following relation: N

2

SSE = ∑ (qcal − qexp )

(2)

i =1

where q cal and q exp are the calculated and experimental adsorption capacities of zein nanofibers.

RESULTS AND DISCUSSION The chemical structure of RB5 is given in Fig. 1. Initially, a comparison experiment was attempted to demonstrate the importance of using zein in its nanofiber form. The results in Fig. 2 a illustrate performance of zein under three different physical forms, i.e. powder, film and nanofiber form for RB5 removal. It is evident from the results that zein in nanofiber form showed superior performance. The transformation of zein in nanofiber form has rendered enhanced surface area and contact sites for RB5 interaction as a result efficient removal was achieved. The measured BET surface area of zein nanofibers was 556.2 m2/g ,the average pore size distribution was 1.1 nm and micro pore volume 0.013 cc/g . However, zein in powder form although having more contact with the dye solution had some ACS Paragon Plus Environment

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drawbacks in handling due to its sticky nature. The zein in powder form made aggregations and attached to the walls of the glass tube during shaking and hence could not efficiently remove RB5. However, the zein films due to having lowest surface area (0.01204 m2/g), may have limited number of active sites that consequently led low permeability of dye molecules to be penetrated inside the film surface.

Figure 1. Chemical structure of RB5 dye Material characterization The SEM images of zein nanofibers are shown in Fig. 2b. It is obvious that zein exhibited smooth and bead free morphology. The average diameter of zein nanofibers was found to be 325 nm as shown in Fig 2c.


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c 40 35 Frequency of nanofibers

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30 25 20 15 10 5 0 200

300

400

Nanofiber diameter, nm

Figure 2. (a) The adsorption efficiencies of different physical forms of zein; (b) the SEM image of zein nanofibers; (c) the distribution of nanofiber diameters. Other properties such as tensile strength, flexural rigidity and bending modulus were also measured. Zein nanofibers showed a tensile strength of 1.4 MPa which is reasonably good to sustain the water pressure. The flexural rigidity was found 0.2mg/cm and bending modulus 20.5 MPa. These results suggest that zein nanofibers are able to withstand moderate stress. Fig. 3 shows the typical FTIR spectrum of electrospun zein nanofibers. Neat zein nanofibers displayed the broad absorption peak at 3301 cm-1 that is associated with amide A (–NH2 stretching vibrations). In addition to this, the characteristic peaks at 1649 cm-1, 1538 cm-1,1305 cm-1 and 1229 cm-1 were indicative of amide I (C=O stretching vibration), amide II (N-H bending) and amide III (axial deformation vibrations of C-N stretching) respectively [28]. There were no peaks related to the presence of either β sheets or β turns (1662, 1614 and 1631 respectively), that indicated the predominant presence of α helices in zein [29].


9

4000

1229

1305

1443

1538

3500

2951 2854

3301

Absorbance

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1649


3000

2500

2000

1500

1000

Wavenumber (cm-1)

Figure 3.The FTIR spectrum of zein nanofibers Fig.4a shows survey spectrum obtained through XPS analysis of zein nanofibers. The surface was found rich in C (284eV), N (399eV) and O (530eV). The high resolution XPS spectra of C1s (Fig.4b) was obtained with three fitted peaks; the component located at 284.05 eV corresponds to C-C or C-H; the components at 285 eV and 287.3 eV correspond to C-N and N-C=O respectively. The N1s peak was further fitted into three more components (Fig.4c); the peaks at 399.9 eV, 400.43 eV and 401 eV were related to N-C, N-C=O and N-H particularly positively charged nitrogen [30] respectively. The oxygen spectrum, as shown in Fig.4d, exhibited two components assigned from O=C at 530.56 eV and O=C-N at 531.32 eV [31].


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Figure 4. (a) XPS full survey scan of zein nanofibers, (b) C1s, (c) N1s, (d) O1s core levels of zein nanofibers. Optimization of adsorption parameters The effect of time To achieve maximum removal of RB5 from aqueous phase, the effect of contact time on adsorption of RB5 was studied to determine the time taken by zein nanofibers to remove 50 mg/L RB5 (Fig. 5a). It was observed that the adsorption process was rapid in the first 5 min and thereafter proceeded at the relatively slower rate that almost led the entire decolorization within 20 min (Fig. 5b). This time was ACS Paragon Plus Environment

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found sufficient to attain equilibrium conditions. Therefore, adsorption contact of 20 min was selected for subsequent work. This remarkable performance of zein nanofibers within the shortest possible equilibrium time may become meaningful in industrial applications as it saves processing time compared to other well efficient adsorbents [7, 8]. The rapid uptake may be due to the greater surface area and numerous binding sites of zein nanofibers. As the time continued, the concentration gradients gradually reduced due to accretion of dye molecules onto available sites of nanofibers that resulted decrease in adsorption rate. To determine the adsorption rate and investigating rate controlling step, the time-dependent RB5 adsorption data was fitted to different kinetic models such as pseudo first order, pseudo second order and intraparticle diffusion models [32-34]. The linear forms of pseudo first, pseudo second and intraparticle diffusion models are given in equations 2, 3 and 4 respectively. log(q e − q t ) = log q e −

k1 t 2.303

(3)

t 1 t = + 2 qt k 2 qe qe

(4)

q t = k i t 0.5 + C

(5)

Where qt is the RB5 concentration (mg/g) on nanofibers at the time t, qe is the solution phase concentration (mg/L) of RB5 at equilibrium. k1, k2 and ki are the rate constants associated with pseudo first, pseudo second and intra particle diffusion models, respectively. Analysis and validation of experimental data to different models suggested that the adsorption of RB5 can be better explained by pseudo second order model Fig. 5c. Furthermore, the calculated qe, cal values for the pseudo second order model show good concurrence with experimental qe,exp values compared to the pseudo first order implying poor fitting of the pseudo first order model to experimental results (Table 1).


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Table 1. Kinetic parameters for the adsorption of RB5 by zein nanofibers.

Pseudo first order qe, exp± Sa 6.05±0.01

a

k1

(min-1)

0.076

qe,cal

R2

Pseudo second order SSE

(mg/g)

2.44

qe, exp± S

0.98

0.17

a

6.05±0.01

k2

qe,cal

(g/mg/min)

(mg/g)

0.084

6.36

R2

Intraparticle diffusion SSE

ki

C

R2

SSE

0.563

3.17

0.95

0.204

(mg/g/min0.5)

0.99

0.13

Standard deviation.

Adsorption behavior of any solute is a combination of different complex processes. However, this process is considered to proceed through three main stages: (1) film or external diffusion, (2) pore diffusion and (3) adsorption at the site on the adsorption surface. This information can be extensively achieved through the Weber and Morris intraparticle model. According to this model, the plot of qt vs. t0.5 should be linear and pass through the origin in order for the intraparticle diffusion to be the rate-

limiting step. The linear plot of the intra particle diffusion model showed that the plot of RB5 adsorption on zein nanofibers was not linear but could be divided into two linear regimes (Fig. 5d). Depending upon different structural and morphological properties of adsorbent and size of adsorbate molecules, many research groups have suggested two types of events taking place following this prototype of the plot [35]: (1) film diffusion and (2) pore diffusion. In our case, the first part of the plot was linear due to boundary layer diffusion. The second part was the final equilibrium stage. From the plot obtained, one can assume that RB5 was rapidly adsorbed on the surface of nanofibers followed by penetration into the mesh pores or microscale interstitial spaces between nanofibers where they were finally attached leading to equilibrium.


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Figure 5. (a) RB5 uptake kinetics by zein nanofibers, (b) decrease in UV-Vis absorption of RB5 with time, (c) the pseuo second order fitting of RB5 adsorption, (d) the intraparticle diffusion model of RB5 adsorption and (e) the optical view of RB5 samples achieved after contact with zein nanofibers at different time intervals. ACS Paragon Plus Environment

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The effect of pH The role of pH in adsorption of RB5 on zein nanofibers was investigated by preparing 50mg/L RB5 solutions with different pH values ranging 2-12. Fig. 6 shows that pH has considerable influence on surface properties of zein and degree of ionization of dye molecules. The adsorption efficiency of zein nanofibers decreased as pH increased from 2 to 12. The acidic pH facilitated in complete removal of RB5 since acidic pH tended to protonate –NH and – COOH groups in zein promoting electrostatic interactions between dye-SO3- and +-zein nanofibers [24]. Zein exhibits zero zeta potential at pH 6 [24]. However, the basic pH restricted the uptake of RB5 on zein nanofibers as the basic pH would have deprotonated both amino and carboxyl groups thereby increasing net negative charge on zein nanofibers. As a result, electrostatic repulsion between dye molecular ions and zein nanofibers would have repelled each other within 8-12 pH. In addition to increase in net negative charge, basic pH could also cause significant damage to protein as well as hydrolysis of RB5 that resulted in decline in adsorption efficiency towards RB5 [36]. This result is in agreement with previously published results [37, 38]. In textile industry, the pH of effluent is usually alkaline; therefore, adjusting pH to either acidic or near neutral, this adsorbent can be recommended for treating dye effluents. Since, the adsorption performance of zein nanofibers was excellent in working pH of dye solutions; therefore, normal working pH was selected for further optimization.


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100

Adsorption Efficiency (%)

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80

60

40

20

0 2

4

6

8

10

12

pH

Figure 6. The effect of pH on RB5 uptake by zein nanofibers. The effect of the mass of nanofiber The adsorption efficiency depends heavily upon mass of adsorbent; therefore, the reliance of removal efficiency of RB5 on the mass of zein nanofibers was investigated at laboratory temperature and unaltered pH of dye solution in the presence of nanofiber masses varying from 20-60mg (Fig. 7). The dye removal increased abruptly with the rise in the mass of zein nanofibers due to increase in the surface area of nanofibers and greater availability of binding sites. The maximum adsorptive removal was achieved when the nanofiber mass was 40mg. The maximum percentage removal was about 97.19 ±1.8%. At the same time, adsorption capacity of zein nanofibers gradually decreased with the increase in their amount, which can be due to unsaturation of active sites [39]. From present results, it may be estimated that for treating 1000L of the industrial waste water; only 8kg of this economically viable adsorbent is required. So, this adsorbent may serve as an efficient alternate for the treatment of dye loaded waste streams in textile industries.


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100 24

Adsorption Efficiency (%) Adsorption Capacity (mg/g)

20 18

90

16 14 12 10

80

8 6

Adsorption Capacity (mg/g)

22

Adsorption Efficiency (%)

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4 10

20

30

40

50

60

Mass of Nanofibers , mg

Figure 7. The effect of the mass of zein nanofibers The effect of initial dye concentration Colour is noticeable at a dye concentration as low as 1 mg/L. In textile industries, production of the large amount of coloured wastewater originates from dyeing and pigmentation applications that range from 10 to >100 mg/L [40]. Therefore, it is imperative to study the effect of different initial concentrations of dyes on removal efficiency of zein nanofibers to examine their capacities and performance in higher concentration ranges. Fig. 8a shows that adsorption of RB5 decreased slowly from 25 to 75 mg/L then further decreased when initial concentration was further increased to 200 mg/L. This behavior may be attributed to the finite number of identical sites on zein nanofibers that were under possession of adsorbed RB5 molecules consequently leading to the reduction in adsorption efficiency. Fig. 8b shows a typical shape of type-I adsorption isotherm according to BET classification [41].


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Figure 8. (a) The effect of initial RB5 concentrations and (b) nonlinear adsorption isotherm.

Such type of isotherm is characteristic of nonporous, microporous or adsorbents having a limited number of pores having size less than the dimensions of adsorbate since it is evident from the fact that zein nanofibers possessed the average pore size of 1.1 nm. It also demonstrates a high degree of adsorption at lower concentrations followed by stabilization and formation of the saturation plateau as the adsorption sites were progressively occupied at higher concentrations. Experimentally derived adsorption process data was correlated with Langmuir and Freundlich [42,43] in order to model the adsorption mechanism. Langmuir and Freundlich equations can be illustrated as: Ce 1 C = + e qe b × qmax qmax

(6)

1 log qe = log K f + log Ce n

(7)

Where qmax is the maximum adsorption capacity (mg/g), Ce is the equilibrium solution phase concentration, b is related to adsorption free energy and specifies the adsorbent-dye affinity. Kf is adsorption capacity and the value 1/n from Freundlich isotherm gives information about the relative distribution of active sites; the smaller the value of 1/n the greater the availability of heterogeneous active sites and adsorption mechanism would preferably be physical in nature [44].


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The isotherm parameters of RB5 adsorption on zein nanofibers are listed in the Table 2. The Langmuir model presented the best fitting (favored by R2 as well as lower SSE values) that supported chemical nature of adsorption. In our case, the value 1/n (1.831, n=0.54) was found greater than 1 that implied adsorption equilibrium of RB5 on zein nanofiber was more Langmuir like with the majority of energetically homogeneous sites. As the zein was abundant in peptide bonds (R-CO-NH-); therefore, it may be supposed that such sites were predominantly prevailed throughout the entire polymer chain that may have resulted in the overall uniform distribution of active sites. The Langmuir constant b, which is the free energy of adsorption was found greater (0.1443 L/g) suggesting the higher affinity of RB5 with peptide bonds due to different modes of interactions generated owing to different atoms (C, N, H and O) in peptide linkage. In a nut shell, it may be considered that zein protein contains diversified functional groups but major part of those groups include amide (due to peptide linkage) as a result the major part of polymer contains identical sites through the entire chain. Table 2. Isotherm parameters for the adsorption of RB5 on zein nanofibers. Isotherms

Parameters

Langmuir

qmax (18.18 mg/g)

Ce 1 C = + e qe b × qmax qmax

b (0.1443 L/g) R2 (0.969) SSE (0.33)

Freundlich

1/n

(1.831)

1 log qe = log K f + log Ce n

Kf

(0.158 mg/g)

R2 (0.639) SSE (0.79)


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Page 20 of 36

The adsorption capacity of current adsorbent was compared with previously reported materials under optimized conditions. The importance of the adsorption process can be evaluated by the cost of adsorbent, its removal efficiency, the shortest possible time to achieve adequate removal and ambient operational parameters. The Table 3 shows that zein nanofibers possessed moderate adsorption capacity, but the other operational parameters such as pH (2-6), time (20min), temperature (298 K) and mass of nanofibers (8g/L) were lying within the framework of sustainable and green process that should be implemented in current industrial cleanup processes. Other adsorbents having higher adsorption capacities may be suggested but some of those materials required either the prolonged contact time between dye solution and material of interest, or acidic pH and higher temperatures for processing dyed wastewater. Some of the adsorbents reported may also require multi synthesis steps that could affect the cost and efficiency of the process. On the basis of above facts, it may be considered that using zein nanofibers, being a natural biopolymer, may serve as green, efficient and cost saving process in industrial operations for water treatment. Table 3. Comparison of adsorption capacities and operational parameters of previously reported adsorbents with zein nanofibers Adsorbent

Time

pH

(min)

Adsorption capacity (mg/g)

Zein nanofibers

20

2-6

18.1

Temperature Amount (K)

(g/L)

298

8

Reference

Present study

Cetylpyridinium

3600

-

chloride modified

0.1

mmol/g 293

10

[7]

(99.1 mg/g)

activated carbon Chitosan DD * 90%

10,080

4

1441.8

295

1

[8]

3

616.9

298

2

[9]

(168 h) Crosslinked chitosan

180


20

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functionalized 3amino-1,2,4 triazole,5thiol Polyaniline modified

60

2

357.1

318

0.5

[12]

14 days

6

18.7

293

0.2

[18]

240

1

456.9

328

0.2

[19]

Polyaniline nanofibers

1440

2-6

434.7

318

1

[45]

Amberlite IRA-458

180

-

1295.9

318

10

[46]

IRA-958

180

-

1723.9

308

10

[46]

Eichhornia

60

2.5-3

0.606

298

10

[47]

60

3

0.603mmol/g 318

1

[48]

lignocellulosic material Chlamydomona reinhardtii immobilized polysulphone

2.01× 104 min

nanofibrous web Chitosan polyamide nanofibers

crassipes/chitosan composite Glycidyl methacrylate resin modified with tetraethelenepentamine

(597.6 mg/g)

Recyclability Keeping in view the disposal issue of used adsorbent, minimizing the environmental pollution due to waste disposal and prevention of toxic reagents for elution of dye from zein nanofibers; it was realized to recycle the zein nanofibers after the first attempt of adsorption experiment. For this purpose, used ACS Paragon Plus Environment

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Page 22 of 36

zein waste adsorbent was redissolved in 80% aqueous ethanol along with the addition of pure zein polymer to compensate the weight loss of the sample and fortifying the used materials with new active sites. The resulting solution was again electrospun under the conditions already mentioned in the preceding section. The experiment observations showed that the freshly prepared dye loaded zein nanofiber was capable to remove 72±2% of RB5 (50mg/L).

The binding mechanism of dye The potential role of active groups hosting RB5 in zein nanofibers was explicitly explained by two well-known techniques, i.e FTIR and XPS technique. The IR absorption peaks (Fig. 9a) obtained after dye adsorption on zein nanofibers showed substantial changes in absorption peaks with less intense amide I,II and III bands. The two most prominent peaks were emerged at 1126 and 1048 cm-1 that were assigned to the asymmetric and symmetric stretchings of SO3- present in RB5 [49]. The presence of these additional peaks supported the fact that RB5 was immobilized on zein. The change in intensity and peak positions with respect to -CH2 stretching suggested the existence of some hydrophobic interactions of alkyl groups in the zein polymer chain with RB5. Moreover, the changes in amide peaks and disappearance of a peak at 1305 cm-1 suggested the involvement of peptide bond interactions with RB5. XPS study was carried out to demonstrate the mode of interaction between dye and zein nanofibers. Two samples i.e., zein nanofibers and the dye loaded zein nanofibers were examined and changes associated with their peak positions (binding energies), percentages and full width half maximum (FWHM) are listed below in Table 4.


22

Page 23 of 36


Zein nanofibers Zein nanofibers- RB5 dye

a Zein Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1637

1533 1442 1246 1126

3300 2962

2866 1048

4000

3500

3000

2500

2000

1500

1000

Wavenumber (cm-1)

Figure 9. (a) The FTIR spectrum presenting zein nanofibers before and after RB5 adsorption, (b) XPS high resolution of C1s, (c) N1s and (d) O1s of RB5 loaded zein nanofibers. ACS Paragon Plus Environment

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Page 24 of 36

Fig. 9b shows a high resolution spectrum attributed to C1s after dye adsorption. Binding energy (BE) values of C1s after dye adsorption have shifted to 283.5, 284.4 and 286.5 eV. In addition to this, the peak emerged at 282.9eV was associated with C=C, which was preferably related to aromatic rings of RB5. The change in C-C binding energy and its decrease in percentage (from 48.06% to 37%) revealed involvement of some hydrophobic interactions such as vander Waal forces between alkyl groups present in both RB5 and zein nanofibers. The decrease in percentages of C-N and O=C-N in zein nanofibers along with their FWHMs suggested the changes associated with chemical bonds and states of C due to the attack of RB5 molecules through their negative centers such as SO3- , NH2 or OH. The peptide bonds gain stability by delocalization of lone pairs of nitrogen in amides. The C in zein serves as an electrophilic center due to the presence of oxygen hence, nitrogen shares its lone pair to this positive center (shown in the scheme 1) that results in the stable resonance structure containing highly electronegative oxygen as an anion making nitrogen a positive center.

Scheme 1. Resonance structures in amide


24

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Table 4. Assignment of peaks in the XPS spectra of zein nanofibers and RB5 loaded zein nanofibers together with binding energies, peak full width at half maximum and chemical bond. percentages. Before Adsorption

After Adsorption

C1s (BE)

FWHM

% composition

C1s (BE)

FWHM

% composition

C-C (284.05)

1.06

47.8

C=C (283.8)

0.788

28

C-N (284.9)

1.53

37

C-C (284.4)

0.905

37

O=C-N (287.3)

1.3

15

C-N (285.4)

1.35

23

O=C-N (287.5)

1.223

11

N1s

FWHM

% composition

N1s

FWHM

% composition

N-C (399.8)

0.835

29

N-C (399.9)

0.704

22.2

N-C=O (400.43)

0.803

42

N-C=O (400.3)

0.75

38.2

N-H or +NH (401)

0.87

27.8

N-H (400.6)

1.06

39.4

O1s

FWHM

% composition

O1s

FWHM

% composition

530.5 (O=C)

1.005

38.7

530.8 (O=C)

1.12

37.8

531.3 (O=C-N)

1.46

61.3

531.6(O=C-N)

1.52

62

Hence, negative centers of RB5 may have preferred to attack N in zein nanofibers that may have resulted decrease in the number and weakening of bonds between N and C or N-C=O thereby tended to reduce FWHM. This type of mode can be considered as electrostatic. The N1s high resolution spectra are given in Fig. 9c. The percentage decrease in N-C and N-C=O suggested that N also played a fundamental role in binding RB5. The type of interaction could be considered as electrostatic between N atom and negative centers from RB5.The decrease in FWHM values may be due to similar reason as mentioned for C-N and C-N=O. The increase in FWHM of N-H suggested that N has changed its chemical state and increased number of bonds could be achieved in addition to covalent bonds with C and H. This may preferably be due to the formation of hydrogen bonds between OH or NH2 groups of ACS Paragon Plus Environment

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Page 26 of 36

RB5 and N-H of zein. This supported the fact that hydrogen bonding was also engaged in binding RB5 with zein nanofibers. The mode of interaction with O (Fig. 9d) in zein was understood by analyzing high resolution O1s. The increase in percentage of O-C after dye adsorption suggested the addition of chemically similar O-C groups from RB5. The increase in FWHM values of O-C and O=C-N may be due to making of hydrogen bond or restoration of carbonyl double bond after combining of RB5. From the XPS and FTIR results, it may be assumed that binding of RB5 is governed by three important modes of interaction. i.e, hydrophobic interactions probably due to pendant alkyl groups in RB5 and surface alkyl groups on zein, ionic interactions possibly due to either positively charged N or carbonyl carbon of zein and negative centers of RB5 and hydrogen bonding between hydrogen bond donors in RB5 (NH2 or OH) and hydrogen bond acceptors in zein (O or N) or vice versa. The possible adsorption interactions between RB5 and zein nanofibers are illustrated in Fig. 10

H N O

H H

H

N

O

O

H H R

C

N R O

O

R

R

S

O

O

N H

R' S

O

O

Hydrogen bonding Electrostatic Interactions Hydrophobic Interactions

Figure 10. The proposed mechanism of RB5 adsorption on zein nanofibers.


26

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Conclusion Zein nanofibers are acknowledged as versatile and green nanoadsorbent materials due to their high surface area, the greater number of exposed active sites and environmental friendly nature. Their binding with RB5 was highly efficient and rapid compared to their powder and film analogues. The interactions governing adsorption of RB5 were mainly hydrophobic, ionic and hydrogen bonding. The zein nanofibers exhibited 97±1.8% RB5 removal within the shortest time period of 20 min, normal working pH, lower dye concentration (20-100mg/L), lower nanofiber mass (40 mg). Moreover, the recycled zein nanofiber showed 72 ±2% of RB5 removal. Their enhanced surface area, easy separation and ambient operational conditions save experimental time and filtration steps.

ACKNOWLEDGMENT This work was supported by Mehran University of Engineering and Technology Jamshoro Pakistan and Shinshu University, Japan

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[47] El-Zawahry, M. M.; Abdelghaffar, F.; Abdelghaffar, R. A.; Hassabo, A. G. Equilibrium and kinetic models on the adsorption of Reactive Black 5from aqueous solution using Eichhornia crassipes/chitosan composite, Carbohydr. Polym. 2016, 136, 507–515. [48] Elwakeel, K. Z.; Rekaby, M. Efficient removal of Reactive Black 5 from aqueous media using glycidyl methacrylate resin modified with tetraethelenepentamine, J. Hazard. Mater.2011, 188 , 10–18. [49] Neoh, C.H.; Lam, C. Y.; Lim, C. K.; Yahya, A.; Bay, H. H.; Ibrahim,

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Page 34 of 36

For Table of Contents Use Only


34

Syringe pump

Page ACS 35Sustainable of 36 Chemistry & Engineering Power supply 20 KV

1 2 Zein polymer Zein 3 solu.on 4 5 ACS Paragon Plus Environment Dye removal efficiency 6 7

Electrospun zein nanofiber as green and recyclable ACS Sustainable Chemistry & Engineering Page 36 of 36 adsorbent for the removal of reac@ve black 5 from aqueous phase 1 Authors: Umair Ahmed Qureshi, Zeeshan Khatri, 2 Farooq Ahmed, Muzamil Khatri, Ick-Soo Kim 3 4 Synopsis 5 ACS Paragon Plus Environment Electrospun zein nanofibers from corn lie within the 6 framework of sustainable and green technology for 7 dye removal within twenty minutes

Electrospun Zein Nanofiber as a Green and Recyclable Adsorbent for

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