Electrospun Zein Nanofiber as a Green and ... - ACS Publications

Research Article pubs.acs.org/journal/ascecg

Electrospun Zein Nanofiber as a Green and Recyclable Adsorbent for the Removal of Reactive Black 5 from the Aqueous Phase Umair Ahmed Qureshi,†,‡ Zeeshan Khatri,*,†,§ Farooq Ahmed,† Muzamil Khatri,†,§ and Ick-Soo Kim*,§ †

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

ABSTRACT: Zein polymer is drawing intense research interest because of its nontoxicity, biodegradability, and unique structural properties. Zein-based electrospun nanofibers are prepared from an 80% ethanolic solution and are used as a nanoadsorbent material for the confiscation of reactive black 5 (RB5) dye from aqueous solutions. The electrospun nanofibers possessed an extraordinarily high capacity for RB5 uptake compared to their powder and film analogues. Zein nanofibers exhibited exceptionally efficient performance in removing RB5 after contact for only 20 min at room temperature and a normal working pH. The mechanism of dye−zein interaction was chiefly controlled via hydrophobic, electrostatic forces and hydrogen bond interactions. Experimental data followed a pseudo-second-order model, and the Langmuir adsorption isotherm was the most appropriate mechanism governing RB5 adsorption. The dye-loaded zein nanofibers were directly re-electrospun 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

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INTRODUCTION Dye-loaded wastewater from textile, pharmaceutical, printing, leather, cosmetics, and food industries is a serious public and scientific concern.1,2 The presence of dyes in water inhibits the penetration of sunlight and oxygen, which harms aquatic life. Additionally, most of the reactive dyes and their metabolites are considered toxic and carcinogenic and are difficult to remove because of 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 studies are underway to develop powerful treatment methods to alleviate those toxic pollutants from natural waters and wastewaters. Different conventional treatments, such as coagulation−flocculation, advance oxidation, adsorption, ozonation, photochemical degradation, and membrane technology, and additional biological methods are used to treat dye-containing wastewater.5 However, the selection of a treatment technique depends on its efficiency as well as its cost, while current research findings point to 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 and abundant and must have greater adsorption efficiency. Being easily acquired, activated carbon has some limitations, such as a longer equilibrium time, improper removal efficiencies, and the variable nature of porosity for capturing some complex dye molecules.6 © 2017 American Chemical Society

CI reactive black 5 (RB5) is a dye commonly used in the textile industry, and it possesses poor binding ability on a textile surface; as a consequence, it is released in industrial effluents. The adsorbents tested for RB5 removal include activated carbon, but because of its poor adsorption efficiency and prolonged equilibrium time and the 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 pumiceand walnut-activated carbon,11 polyaniline-coated lignocellulosic composite,12 etc., but such materials require either tedious preparation steps, toxic chemicals for synthesis, or longer adsorption process times; hence, the search for a highly efficient, easily available, inexpensive, sustainable, and environmentally benign adsorbent is ongoing. Nanotechnology provides benefits in many research areas encompassing water treatment, sensors, and engineering. Cutting-edge developments in nanotechnology include the design of particle shapes and morphology. Recently, the utilization of one-dimensional nanoscale adsorbents, especially nanofibers, as adsorbents is of interest because of their superior advantages for wastewater applications. Thus, nanofiber adsorbents when compared to traditional adsorbents such as activated carbons Received: February 9, 2017 Revised: March 27, 2017 Published: April 7, 2017 4340

DOI: 10.1021/acssuschemeng.7b00402 ACS Sustainable Chem. Eng. 2017, 5, 4340−4351

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ACS Sustainable Chemistry & Engineering

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 an inexpensive, rapid-acting, and sustainable adsorbent for RB5 attenuation.

offer advantages such as a smaller sample loss, easy separation without filtration, being faster, and a 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 proven to be efficient materials in removing the variety of dyes from wastewater.13,14 Additionally, nanofibers either structurally modified or unadorned show improved dye fixation ability because of an exceptionally larger surface area.15,16 Other interesting areas for milestone applications of these new-generation nanomaterials include sensor development, decontamination, energy storage, biomedicine, and catalysis.17 A literature survey reveals that limited work has been reported using electrospun nanofibers for RB5 removal, such as a microalga-immobilized polysulfone 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 chitosanmodified polyamide nanofibers for RB5 removal; however, the removal efficiency was not adequate to meet the quality standard, and the optimal higher temperature for RB5 removal is another energy intensive process. Therefore, the search for a highly efficient, sustainable, and green adsorbent that should be capable of removing recalcitrant RB5 dye within a short time over a wide pH range at ambient temperature is ongoing. At present, biopolymers from renewable resources such as zein have attracted a great deal of attention for economic and environmental reasons. Zein is the major storage protein in corn or maize, amphiphilic amorphous polymer, and a product of the bioethanol industry. It is exclusively found in the corn endosperm, but because of a major proportion of nonpolar amino acids, it is insoluble in water. Zein is generally considered a safe (GRAS) biopolymer. Because of 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 antimicrobial drugs with enhanced cell adhesion, and proliferation20 as an efficient, edible, and biodegradable antifungal coating material on apples to inhibit fungal proliferation21 and a very promising wound dressing nanofiber mat when mixed with Ag.22 When mixed with hydrophilic bacterial cellulose films, these nanofibers improved the water resistance properties of bacterial cellulose.23 Apart from those applications, zein polymer has also drawn keen interest in wastewater treatment such as the treatment of reactive dye-polluted wastewater by changing surface morphology into hollow spherical form to confer greater contact area to capture dye.24 Effective dye removal (acid blue 113 and acid yellow 110) has also been studied upon combination of zein with TiO2 that provided dual benefits through adsorption and photocatalytic decolorization,25 but such experiments could not answer the question of reusability of used adsorbent materials. It has also been utilized with iron(III) for arsenic(V) removal.26 However, the performance of zein in the form of nanofibers for dye removal in the least possible time and its recyclability still need to be investigated in detail. With limitations of some conventional and nanofibrous adsorbents mentioned above, we aimed to prepare environmentally friendly, sustainable, and recyclable zein nanofiber adsorbents for RB5 removal by electrospinning a zein solution as it is believed to have a surface area larger than that of its native form or film form. The purpose of this work is to overcome the limitations associated with tedious preparation steps, cost-effective harmful chemical reagents for preparing efficient materials, and longer

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EXPERIMENTAL SECTION

Materials. Zein from maize (melting point of 266−283 °C) was purchased from Sigma-Aldrich; anionic dye CI reactive black 5 (vinylsulfone) was supplied by the Sumitomo Chemical Company, Ltd. Preparation of Zein Nanofibers and Zein Film. Zein nanofibers were produced through the procedure described in our previous study with slight modifications.27 A solution of 25% (w/v) zein in 80% aqueous ethanol was prepared and then stirred at 80 °C for 2 h. The solution was then supplied through a 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 10° 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 a ground collector. A voltage of 20 kV was applied, and the tip−collector distance was set at 12 cm. The electrospun zein nanofibers were deposited continuously over a stationary metallic drum. After the electrospinning process had been completed, samples were subjected to drying at room temperature overnight for the removal of residual solvents prior to the adsorption experiment. The relative humidity within the laboratory atmosphere was 40%. The average thickness of zein nanofibers was found to be 49 μm. Zein films were fabricated through the well-known evaporationinduced method. Briefly, a 25% zein polymer solution in 80% aqueous ethanol was cast into the flat glass plate. The solution was left to dry in the presence of air at room temperature overnight. This process led to automatic peeling off the zein film from the surface of the glass plate. The average thickness of the film was found to be 178 μm. Characterization. The chemical structure of nanofiber sheets was analyzed through Fourier transform infrared (FTIR) spectroscopy (model IR Prestige-21, Shimadzu) in ATR mode. The surface morphology of zein nanofibers was obtained via scanning electron microscopy (SEM) (model S-3000N, Hitachi). All samples were sputtered with platinum under vacuum before being assessed. The average diameter of electrospun nanofibers was measured from SEM micrographs using image analysis software (Image pro Plus, version 5.1, Media Cybernetics, Inc.). X-ray photoelectron spectroscopy (XPS) measurements were performed with AXIS Ultra by Shimadzu equipped with dual-anode X-ray source Al/Mg and the HSA hemispherical sector 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 Origin Pro 9.0 with a Gaussian−Lorentzian product function. The BET surface area and pore size distribution were measured from N2 adsorption desorption at 77 K using a Quantachrome ASiQwin surface area analyzer. Ultraviolet−visible (UV−vis) spectroscopy was used to measure the absorbance of RB5 solutions with different concentrations at a λmax of 590 nm using a Lambda 35 UV−vis spectrophotometer (PerkinElmer). Batch Adsorption Experiments. An adsorption experiment was conducted by adding 5 mL of 50 mg/L RB5 to a Pyrex glass tube. The reason behind selecting the small 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 and room temperature by means of a Gallenkamp shaker until equilibrium was achieved. The following parameters influenced the removal efficiency of zein nanofibers: adsorption time (1−30 min), solution pH (1−12), nanofiber mass (10−60 mg), and initial RB5 concentration (20−200 mg/L). The pH of the dye solution was adjusted by adding 0.1 M HCl and 0.1 M NaOH. The dyed zein nanofibers were separated manually after the completion of the adsorption experiments. The residual concentration after the adsorption experiment was analyzed using an UV−vis spectrophotometer at a λmax of 590 nm. The equilibrium adsorption 4341


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ACS Sustainable Chemistry & Engineering capacity of zein nanofibers was determined using the following equation:

qe =

Co − Ce V m

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 with an enhanced surface area and contact sites for RB5 interaction as a result of 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 the micropore volume was 0.013 cm3/g. However, zein in powder form, although it makes more contact with the dye solution, had some drawbacks in terms of handling because of 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, because they have the smallest surface area (0.01204 m2/g), may have a limited number of active sites that consequently lead to the low permeability of dye molecules that penetrate the film surface. Characterization of Material. The SEM images of zein nanofibers are shown in Figure 2b. It is obvious that zein exhibited a smooth and bead free morphology. The average diameter of zein nanofibers was found to be 325 nm as shown in Figure 2c. Other properties such as tensile strength, flexural rigidity, and bending modulus were also measured. Zein nanofibers showed a tensile strength of 1.4 MPa that is reasonably effective in sustaining the water pressure. The flexural rigidity was found to be 0.2 mg/cm and the bending modulus 20.5 MPa. These results suggest that zein nanofibers can withstand moderate stress. Figure 3 shows the typical FTIR spectrum of electrospun zein nanofibers. Neat zein nanofibers displayed a 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, 1538, 1305, and 1229 cm−1 were indicative

(1)

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

SSE =

∑ (qcal − qexp)2 i=1

(2)

where qcal and qexp are the calculated and experimental adsorption capacities of zein nanofibers, respectively.

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RESULTS AND DISCUSSION The chemical structure of RB5 is shown in Figure 1. Initially, a comparison experiment was performed to demonstrate the importance of using zein in its nanofiber form. The results depicted in Figure 2a illustrate the performance of zein in three different physical forms, i.e., powder, film, and nanofiber

Figure 1. Chemical structure of the RB5 dye.

Figure 2. (a) Adsorption efficiencies of different physical forms of zein. (b) SEM image of zein nanofibers. (c) Distribution of nanofiber diameters. 4342


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in C (284 eV), N (399 eV), and O (530 eV). The highresolution XPS spectra of C 1s (Figure 4b) were obtained with three fitted peaks; the component located at 284.05 eV corresponds to C−C or C−H, and the components at 285 and 287.3 eV correspond to C−N and N−CO, respectively. The N 1s peak was further fitted into three more components (Figure 4c); the peaks at 399.9, 400.43, and 401 eV were related to N−C, N−CO, and N−H, respectively, particularly positively charged nitrogen.30 The oxygen spectrum, as shown in Figure 4d, exhibited two components from OC at 530.56 eV and OC−N at 531.32 eV.31 Optimization of Adsorption Parameters. Effect of Time. To achieve maximal removal of RB5 from the 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 (Figure 5a). It was observed that the adsorption process was rapid in the first 5 min and thereafter proceeded at a relatively slower rate that almost led to complete decolorization within 20 min (Figure 5b). This time was found to be sufficient to attain equilibrium conditions. Therefore, an adsorption contact time of 20 min was selected for subsequent work. This remarkable performance of zein nanofibers within the shortest possible equilibrium time may become important for industrial applications as it saves processing time compared to other high-efficiency adsorbents.7,8 The rapid uptake may be

Figure 3. FTIR spectrum of zein nanofibers.

of amide I (CO stretching vibration), amide II (N−H bending), and amide III (axial deformation vibrations of C−N stretching).28 There were no peaks related to the presence of either β sheets or β turns (1662, 1614, and 1631 cm−1, respectively) that indicated the predominant presence of α helices in zein.29 Figure 4a shows the survey spectrum obtained through XPS analysis of zein nanofibers. The surface was found to be rich

Figure 4. (a) XPS full survey scan of zein nanofibers. (b) C 1s, (c) N 1s, and (d) O 1s core levels of zein nanofibers. 4343


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Figure 5. (a) RB5 uptake kinetics by zein nanofibers. (b) Decrease in UV−vis absorption of RB5 with time. (c) Pseudo-second-order fitting of RB5 adsorption. (d) Intraparticle diffusion model of RB5 adsorption. (e) Optical view of RB5 samples achieved after contact with zein nanofibers at different time intervals.

qt = k it 0.5 + C

due to the larger surface area and numerous binding sites of zein nanofibers. Over time, the concentration gradients gradually decreased because of the accretion of dye molecules onto available sites of nanofibers that resulted in a decreased adsorption rate. To determine the adsorption rate and investigate the ratecontrolling step, the time-dependent RB5 adsorption data were 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-order, pseudo-second-order, and intraparticle diffusion models are given in eqs 3−5, respectively. log(qe − qt ) = log qe − t 1 t = + qt qe k 2qe 2

k1 t 2.303

(5)

where qt is the RB5 concentration (milligrams per gram) on nanofibers at the time t, qe is the solution phase concentration (milligrams per liter) of RB5 at equilibrium, and k1, k2, and ki are the rate constants associated with pseudo-first-order, pseudosecond-order, and intraparticle diffusion models, respectively. Analysis and validation of experimental data from different models suggested that the adsorption of RB5 can be better explained by a pseudo-second-order model (Figure 5c). Furthermore, the calculated qe,cal values for the pseudo-second-order model show good concurrence with experimental qe,exp values compared to those of the pseudo-first-order model, implying poor fitting of the pseudo-first-order model to the experimental results (Table 1). The 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

(3)

(4) 4344


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ACS Sustainable Chemistry & Engineering Table 1. Kinetic Parameters for the Adsorption of RB5 by Zein Nanofibers pseudo-first-order

a

pseudo-second-order

qe,exp ± SDa

k1 (min−1)

qe,cal (mg/g)

R2

SSE

6.05 ± 0.01

0.076

2.44

0.98

0.17

qe,exp ± SDa

k2

(g mg−1 min−1)

qe,cal (mg/g)

R2

6.05 ± 0.01

0.084

6.36

0.99

intraparticle diffusion SSE

ki (mg g−1 min−0.5)

C

R2

SSE

0.13

0.563

3.17

0.95

0.204

Standard deviation.

pH could also cause significant damage to protein as well as hydrolysis of RB5 that resulted in a decline in adsorption efficiency toward RB5.36 This result is in agreement with previously published results.37,38 In the textile industry, the pH of the effluent is usually alkaline; therefore, with a change in the pH to either acidic or near neutral, this adsorbent can be recommended for treating dye effluents. Because the adsorption performance of zein nanofibers was excellent in the working pH of dye solutions, a normal working pH was selected for further optimization. Effect of Nanofiber Mass. The adsorption efficiency depends heavily on the adsorbent mass; therefore, the reliance of the removal efficiency of RB5 on the mass of zein nanofibers was investigated at laboratory temperature and the unaltered pH of the dye solution in the presence of amounts of nanofibers varying from 20 to 60 mg (Figure 7). The efficiency of dye

diffusion, (2) pore diffusion, and (3) adsorption at the site on the adsorption surface. This information can be extensively acquired through the Weber and Morris intraparticle model. According to this model, the plot of qt versus t0.5 should be linear and pass through the origin for the intraparticle diffusion to be the rate-limiting step. The linear plot of the intraparticle diffusion model showed that the plot of RB5 adsorption on zein nanofibers was not linear but could be divided into two linear regimes (Figure 5d). Depending upon different structural and morphological properties of the 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 because of 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. Effect of pH. The role of pH in adsorption of RB5 on zein nanofibers was investigated by preparing 50 mg/L RB5 solutions with different pH values ranging from 2 to 12. Figure 6 shows that pH has a considerable influence on surface

Figure 7. Effect of the mass of zein nanofibers.

removal increased abruptly with an increase in the mass of zein nanofibers due to the increase in the surface area of nanofibers and greater availability of binding sites. The maximal level of adsorption removal was achieved when the nanofiber mass was 40 mg. The maximal removal percentage was ∼97.19 ± 1.8%. At the same time, the adsorption capacity of zein nanofibers gradually decreased with an increase in their amount, which can be due to unsaturation of active sites.39 From these results, it may be estimated that for the treatment of 1000 L of industrial wastewater, only 8 kg of this economically viable adsorbent is required. Therefore, this adsorbent may serve as an efficient alternate for the treatment of dye-loaded waste streams in textile industries. Effect of Initial Dye Concentration. Color is noticeable at a dye concentration as low as 1 mg/L. In textile industries, production of the large amount of colored 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 the removal

Figure 6. Effect of pH on RB5 uptake by zein nanofibers.

properties of zein and the degree of ionization of dye molecules. The adsorption efficiency of zein nanofibers decreased as the pH increased from 2 to 12. An acidic pH facilitated complete removal of RB5 because an acidic pH tended to protonate -NH and -COOH groups in zein, promoting electrostatic interactions between dye-SO3− and +-zein nanofibers.24 Zein exhibits zero ζ 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 the net negative charge on zein nanofibers. As a result, electrostatic repulsion between dye molecular ions and zein nanofibers would have occurred within the pH range of 8−12. In addition to the increase in the net negative charge, a basic 4345


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Figure 8. (a) Effect of initial RB5 concentration. (b) Nonlinear adsorption isotherm.

of pores that are smaller than the dimensions of the adsorbate because zein nanofibers possessed an 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 were correlated with those of Langmuir and Freundlich42,43 to model the adsorption mechanism. Langmuir and Freundlich equations can be illustrated as

efficiency of zein nanofibers to examine their capacities and performance in higher concentration ranges. Figure 8a shows that the level of adsorption of RB5 decreased slowly from 25 to 75 mg/L and then further decreased when the 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 in the possession of adsorbed RB5 molecules, consequently leading to the reduction in adsorption efficiency. Figure 8b shows a typical shape of a type I adsorption isotherm according to BET classification.41 Such an isotherm is characteristic of nonporous adsorbents, microporous adsorbents, or adsorbents having a limited number

Ce C 1 = + e qe bqmax qmax

log qe = log K f +

Table 2. Isotherm Parameters for the Adsorption of RB5 on Zein Nanofibers isotherms Ce qe

=

1

qmax (18.18 mg/g)

bqmax

+

Ce qmax

b (0.1443 L/g) R2 (0.969) SSE (0.33) 1/n (1.831)

Freundlich

log qe = log K f +

1 n

log Ce

1 log Ce n

(7)

where qmax is the maximal adsorption capacity (milligrams per gram), Ce is the equilibrium solution phase concentration, b is related to the adsorption free energy and specifies the adsorbent−dye affinity, Kf is the adsorption capacity, and 1/n from the 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 the adsorption mechanism would preferably be physical in nature.44 The isotherm parameters of adsorption of RB5 on zein nanofibers are listed in Table 2. The Langmuir model presented the best fitting (favored by R2 as well as lower SSE values) that

parameters

Langmuir

(6)

Kf (0.158 mg/g) R2 (0.639) SSE (0.79)

Table 3. Comparison of Adsorption Capacities and Operational Parameters of Previously Reported Adsorbents with Zein Nanofibers adsorbent zein nanofibers cetylpyridinium chloride-modified activated carbon chitosan DD (90%) cross-linked chitosan-functionalized 3-amino-1,2,4-triazole, 5-thiol polyaniline-modified lignocellulosic material Chlamydomonas reinhardtii immobilized polysulfone nanofibrous web chitosan polyamide nanofibers polyaniline nanofibers Amberlite IRA-458 Amberlite IRA-958 Eichhornia crassipes/chitosan composite glycidyl methacrylate resin modified with tetraethylenepentamine

adsorption capacity (mg/g)

temperature (K)

2−6 − 4 3

18.1 0.1 mmol/g (99.1 mg/g) 1441.8 616.9

298 293 295 298

2 6

357.1 18.7

318 293

0.5 0.2

12 18

1 2−6 − − 2.5−3 3

456.9 434.7 1295.9 1723.9 0.606 0.603 mmol/g (597.6 mg/g)

328 318 318 308 298 318

0.2 1 10 10 10 1

19 45 46 46 47 48

time (min)

pH

20 3600 10080 (168 h) 180 60 2.01 × 104 (14 days) 240 1440 180 180 60 60 4346

amount (g/L) 8 10 1 2

ref this study 7 8 9


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Figure 9. (a) FTIR spectrum presenting zein nanofibers before and after RB5 adsorption. High-resolution XPS spectra of (b) C 1s, (c) N 1s, and (d) O 1s of RB5-loaded zein nanofibers.

(0.1443 L/g), suggesting the higher affinity of RB5 with peptide bonds due to different modes of interaction generated because of different atoms (C, N, H, and O) in the peptide linkage. In a nut shell, zein protein may be thought to contain diversified functional groups, but a major part of those groups include amide (due to peptide linkage); as a result, the major part of the polymer contains identical sites through the entire chain. The adsorption capacity of the current adsorbent was compared with those of previously reported materials under

supported the chemical nature of adsorption. In our case, 1/n (1.831; n = 0.54) was found to be >1, which implied adsorption equilibrium of RB5 on a zein nanofiber was more Langmuir-like with a majority of energetically homogeneous sites. As the zein was abundant in peptide bonds (R-CO-NH-), it may be supposed that such sites predominantly prevailed throughout the entire polymer chain, which may have resulted in the overall uniform distribution of active sites. Langmuir constant b, which is the free energy of adsorption, was found to be greater 4347


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ACS Sustainable Chemistry & Engineering optimized conditions. The importance of the adsorption process can be evaluated by the cost of the adsorbent, its removal efficiency, the shortest possible time to achieve adequate removal, and ambient operational parameters. Table 3 shows that zein nanofibers possessed moderate adsorption capacity, but other operational parameters such as pH (2−6), time (20 min), temperature (298 K), and mass of nanofibers (8 g/L) were within the framework of a 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 prolonged contact time between the dye solution and the material of interest or acidic pH and higher temperatures for processing dyed wastewater. Some of the adsorbents reported may also require multisynthesis steps that could affect the cost and efficiency of the process. On the basis of these facts, the use of zein nanofibers, being a natural biopolymer, may be considered to serve as green, efficient, and inexpensive process in industrial operations for water treatment. Recyclability. Keeping in mind the issue of disposal of the used adsorbent and minimizing the environmental pollution due to waste disposal and prevention of elution of toxic reagents of dye from zein nanofibers, we attempted to recycle the zein nanofibers after the first attempt of the adsorption experiment. For this purpose, the used zein waste adsorbent was redissolved in 80% aqueous ethanol along with the addition of pure zein polymer to compensate for the weight loss of the sample and fortify the used materials with new active sites. The resulting solution was again electrospun under the conditions mentioned in the preceding section. The experimental observations showed that the freshly prepared dye-loaded zein nanofiber was capable of removing 72 ± 2% of RB5 (50 mg/L). Binding Mechanism of the 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 (Figure 9a) obtained after the adsorption of the dye on zein nanofibers showed substantial changes in absorption peaks with less intense amide I−III bands. The two most prominent peaks emerged at 1126 and 1048 cm−1, which were assigned to the asymmetric and symmetric stretchings of SO3− present in RB5, respectively.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. An XPS study was performed to demonstrate the mode of interaction between dye and zein nanofibers. Two samples, i.e., zein nanofibers and dyeloaded zein nanofibers, were examined, and changes associated with their peak positions (binding energies), percentages, and full widths at half-maximum (FWHM) are listed in Table 4. Figure 9b shows a high-resolution spectrum attributed to C 1s after dye adsorption. Binding energy (BE) values of C 1s after dye adsorption have shifted to 283.5, 284.4, and 286.5 eV. In addition to this, the peak that emerged at 282.9 eV was associated with CC, which was preferably related to aromatic rings of RB5. The change in C−C binding energy and the decrease in its percentage (from 48.06 to 37%) revealed the involvement of some hydrophobic interactions such as van der Waals forces between alkyl groups present in both RB5 and zein nanofibers. The decrease in percentages of C−N and

Table 4. Assignments of Peaks in the XPS Spectra of Zein Nanofibers and RB5-Loaded Zein Nanofibers Together with Binding Energies, Peak Full Widths at Half-Maximum, and Chemical Bond Percentages before adsorption C 1s (BE)

after adsorption

% FWHM composition

C−C (284.05) C−N (284.9) OC−N (287.3)

1.06 1.53 1.3

C 1s (BE)

47.8 37 15

CC (283.8) C−C (284.4) C−N (285.4)

0.788 0.905 1.35

28 37 23

OC−N (287.5)

1.223

11

before adsorption N 1s N−C (399.8) N−CO (400.43) N−H or +N−H (401) before O 1s OC (530.5) OC−N (531.3)

after adsorption

% FWHM composition 0.835 0.803

29 42

0.87

27.8

% FWHM composition

N 1s N−C (399.9) N−CO (400.3) N−H (400.6)

adsorption

0.704 0.75

22.2 38.2

1.06

39.4

after adsorption


% FWHM composition

38.7 61.3

O 1s OC (530.8) OC−N (531.6)


37.8 62

OC−N in zein nanofibers along with their FWHM 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 stability of the peptide bonds increases by delocalization of lone pairs of nitrogen in amides. The C in zein serves as an electrophilic center because of the presence of oxygen; hence, nitrogen shares its lone pair with this positive center (shown in Scheme 1), which results in a Scheme 1. Resonance Structures in Amide

stable resonance structure containing a highly electronegative oxygen as an anion, making nitrogen a positive center. Hence, negative centers of RB5 may have preferred to attack N in zein nanofibers that may have resulted in a decrease in the number and weakening of bonds between N and C or N−CO and thereby tended to reduce the FWHM. This type of mode can be considered as electrostatic. The N 1s highresolution spectra are shown in Figure 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 the N atom and negative centers from RB5. The reason for the decrease in FWHM may be similar to that mentioned for C−N and C−NO. The increase in the FWHM of N−H suggested that the chemical state of N has changed and an increased number of bonds could be achieved in addition to covalent bonds with C and H. 4348


Research Article


Figure 10. Proposed mechanism of adsorption of RB5 on zein nanofibers.

This may preferably be due to the formation of hydrogen bonds between OH or NH2 groups of RB5 and the N−H group of zein. This supported the fact that hydrogen bonding was also involved in the binding of RB5 to zein nanofibers. The mode of interaction with O (Figure 9d) in zein was understood by analyzing the high-resolution O 1s spectrum. The increase in the percentage of O−C after dye adsorption suggested the addition of chemically similar O−C groups from RB5. The increase in FWHM of O−C and OC−N may be due to the formation of a hydrogen bond or restoration of a carbonyl double bond after combination with RB5. From the XPS and FTIR results, one may assume that the 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 the 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 Figure 10.

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AUTHOR INFORMATION

Corresponding Authors

*Nanomaterials Research Lab, Department of Textile Engineering, Mehran University of Engineering and Technology, Jamshoro 76060, Pakistan. E-mail: zeeshan.khatri@faculty. muet.edu.pk. Telephone: 0092 (0) 22 2772250. *Nano Fusion Technology Research Group, Shinshu University, 3-15-1, Tokida, Ueda City, Nagano 386-8567, Japan. E-mail: [email protected]. Telephone: +81 268 21 5439. Fax: +81 268 21 5482. ORCID

Zeeshan Khatri: 0000-0001-8779-3805 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Mehran University of Engineering and Technology and Shinshu University.

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CONCLUSION

REFERENCES

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