Electrospun polyacrylonitrile nanofibers functionalized with EDTA for adsorption of ionic dyes

Accepted Manuscript Electrospun polyacrylonitrile nanofibers functionalized with EDTA for adsorption of ionic dyes E.F.C. Chaúque, J.C. Ngila, Adedeji A. Adelodun, C.J. Greyling, L.N. Dlamini PII:

S1474-7065(16)30131-0

DOI:

10.1016/j.pce.2016.10.008

Reference:

JPCE 2524

To appear in:

Physics and Chemistry of the Earth

Received Date: 5 June 2016 Accepted Date: 3 October 2016

Please cite this article as: Chaúque, E.F.C., Ngila, J.C., Adelodun, A.A., Greyling, C.J., Dlamini, L., Electrospun polyacrylonitrile nanofibers functionalized with EDTA for adsorption of ionic dyes, Physics and Chemistry of the Earth (2016), doi: 10.1016/j.pce.2016.10.008. 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.

ACCEPTED MANUSCRIPT

Electrospun polyacrylonitrile nanofibers functionalized with EDTA for adsorption of ionic dyes E.F.C. Chaúquea, J.C. Ngilaa1, Adedeji A. Adeloduna, C.J. Greylingb, LN Dlaminia a

Department of Applied Chemistry, University of Johannesburg, Doornfontein 2028 Johannesburg,

South Africa. Department of Chemical Engineering and Technology Station in Clothing and Textiles, Cape

RI PT

b

Peninsula University of Technology, Belville Campus, P.O. Box 7535, Cape Town.

SC

ABSTRACT

The manipulation of nanofibers’ surface chemistry could enhance their potential application toward the removal of ionic dyes in wastewater. For this purpose, surface

M AN U

modification of electrospun polyacrylonitrile (PAN) nanofibers with

ethylenediaminetetraacetic acid (EDTA) and ethylenediamine (EDA) crosslinker was experimented. The functionalized EDTA-EDA-PAN nanofibers were characterized using Fourier transform infrared (FT-IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and Brunauer-Emmett-Teller

TE D

(BET) technique. The impregnation of EDA and EDTA chelating agents on the surface of PAN changed the distribution of nanofibers as proximity is increased (accompanied by reduced softness), but the nanofibrous structure of the pristine PAN nanofibers was not substantially altered. Adsorption equilibrium studies were performed with

EP

Freundlich, Langmuir and Temkin isotherm models with the former providing better correlation to the experimental data. The modified PAN nanofibers showed efficient sorption of methyl orange (MO) and reactive red (RR) from aqueous synthetic samples,

AC C

evinced by the maximum adsorption capacities (at 25 ºC) of 99.15 and 110.0 mg g-1, respectively. The fabricated nanofibers showed appreciable removal efficiency of the target dye sorptives from wastewater. However, the presence of high metal ions content affected the overall extraction of dyes from wastewater due to the depletion of the adsorbent´s active adsorptive sites. Key words: Polyacrylonitrile; Nanofibers; Adsorption; Electrospinning; EDTA; Azodyes.

1

Corresponding author: Tel: +27 11 5596196; Fax: +27 11 5596425; Email: [email protected]

1

ACCEPTED MANUSCRIPT 1. INTRODUCTION Along years, societal development via industrialization has been accompanied by environmental pollution, as industrial effluents usually contain toxic and hazardous substances. Hence, the need for mitigation approaches cannot be over emphasized as stringent regulations have been introduced worldwide for ecological restoration. Among

RI PT

the synthetic pollutants released to the aquatic environment, dyes and dyestuffs are of high concern owned to their persistence and recalcitrance (Savin and Butnaru, 2008). Over 100.000 dyes are commercially available with an estimate production up to

700.000 tons of dyestuffs per annum worldwide (Rafatullah et al., 2010). Among these

SC

pollutants are azo dyes, with known widespread use, due to their ability to covalently bond textile substrates (Kusic et al., 2013). In addition, these dyes have broad colour

M AN U

range, high wet fastness, low energy consumption and easy application. Large amount of water is often required during textile processing. However, inefficient dyeing process does result in the elution of coloured wastewater into aquatic systems, serving as final sinks. These effluents are characterized by strong coloration, high content of organic matter, pigments, metals, broad pH range (pH 3 ̴ 12), and recalcitrant substances (Savin and Butnaru, 2008). As a result, the ecosystem of receiving water bodies is subject to

TE D

devastating adverse effects which have been the topic of scientific scrutiny (Nguyen and Juang, 2013; Riu et al., 1997). Therefore, the recyclability of dye (from their effluents) in the drive toward optimizing their use represents an ecological and economical

EP

challenge for the textile industry.

The treatment of dye wastewater usually involves physicochemical and biological processes. Biological treatment of dye effluents makes use of microorganisms such as

AC C

bacteria, protozoa, fungi, and algal cells (Keskin et al., 2012). This method is environmentally and economically viable although technically challenging and ineffective (i.e. exceeds the standards for colour and nitrogen) (Szpyrkowicz et al., 2001). Furthermore, this process is also affected by low biodegradability of dyestuffs, presence of toxic substances and dark coloration (Pagga and Brown, 1986). On the other hand, physicochemical processes used for pre-treatment and removal of colour from dye wastewater includes flocculation, coagulation, membrane filtration, ozonolysis, hydrogen peroxide under UV radiation, Fenton´s reactions, photodegradation, and adsorption (Kusic et al., 2013; Nguyen and Juang 2013; Szpyrkowicz et al., 2001). Some of these methods are effective although economically restrictive. 2

ACCEPTED MANUSCRIPT The adsorption of dyes usually involves the use of activated carbon, organic waste, microalgae, fibers, among others (Keskin et al., 2012; Zheng et al., 2012; Szyguła et al., 2008). The surface chemistry of adsorbents is widely reported to play a pivotal role in the sorption of dyes and dyestuffs. This is owned to the complex physicochemical nature of these pollutants which are usually grouped as acidic, basic, reactive, direct,

RI PT

dispersed, sulphur-based, and metallic dyes (Graham et al., 2001; Al-Degs et al., 2000; Cooper, 1993). Powdery adsorbents have been widely used due to their high surface area per unit mass. However, the use of these materials is often accompanied by

challenges involving their removal after being spent (for regeneration or disposal)

SC

which usually result in secondary pollution. Hence, we opine that the use of nanofibers may present an effective alternative for the remediation of organic pollutants from wastewater because these nanomaterials also possess comparably high surface area per

M AN U

unit mass (Leary and Westwood, 2011). In addition, the surface chemistry of nanofibers can be modified for preferential purposes without necessarily compromising their nanofibrous structure. Therefore, the potentials of nanofibers as adsorbents is immense owing to the combined high surface area per unit mass, developed surface chemistry and easy separation of spent nanofibers from the treated effluents.

TE D

Since the surface of nanofibers can be modified with suitable functional groups (depending on the chemistry of the precursor polymer), the incorporation of chelating agents on the surface of PAN nanofibers should add value in targeting ionic dyes by simple manipulation of the pH of the media. Therefore, this study is focused in the

EP

modification of the surface chemistry of electrospun PAN nanofibers with EDTA-EDA chelating agents for the sorption of selected popular dyes (methyl orange (MO) and

AC C

reactive red 120 (RR)) from synthetic water and wastewater samples.

2. EXPERIMENTAL 2.1. Reagents

Reagents such as EDTA, pyridine, diethyl ether, acetic anhydride,

ethylenediamine (EDA), triethylamine, tetrahydrofuran, methanol, MO, RR, NaOH and HCl were supplied by Sigma Aldrich (Gauteng, South Africa). All chemicals were of analytical grade and used as received without further purification. The PAN fibers which are composed of 93% acrylonitrile (AN), 6% methyl acrylate (MA) and 1% itaconic acid (IA) were sourced from Bluestar Fibres Company Limited (United

3

ACCEPTED MANUSCRIPT Kingdom). Wastewater samples were collected from Johannesburg Water (Northern Wastewater Treatment Works), Gauteng, South Africa.

2.2 Chemical immobilization of EDTA-EDA chelating agents on PAN nanofibers The surface modification of self-fabricated (by electrospinning) PAN nanofibers

RI PT

was carried out and applied for the removal of ionic dyes. This was sequel to their use in the adsorption of trace metals described in our previous work (Chaúque et al., 2016). The lab-scale PAN nanofibers were prepared by electrospinning 12.5% (w/v) PAN

solutions in DMF at 0.6 mL h-1 flow rate under 15 kV voltage, and 17.5 cm spinneret–

SC

aluminum foil collector distance. The EDTAD was prepared by adding 10 g of EDTA powder to a mixture of 16 and 14 mL of pyridine and acetic anhydride, respectively.

M AN U

The final mixture was heated at 70 ºC for 24 h under the N2 atmosphere. The formed crystals were collected by filtration and washed with dry diethyl ether before drying to constant weight at 50 ºC (Geigy, 1969).

The electrospun PAN nanofibers were modified as follows: ca. 0.2 g of the fibers was refluxed with EDA in an oil bath at 95 ºC for 2 h duration. The aminated nanofibers (EDA-PAN nanofibers) were washed with de-ionized water and dried in the oven at 60

TE D

ºC to constant weight. This was followed by EDTA impregnation, using EDTAD in tetrahydrofuran-triethylamine solvent (40:7, v/v) in a ring opening reaction. After the reaction has reached completion, the nanomaterial was neutralized with 2 M HCl solution before copiously rinsing with de-ionized water and dried in the oven at 50 ºC.

AC C

further use.

EP

Finally, modified nanofibers (EDTA-EDA-PAN) were then stored in a desiccator prior

2.3 Instrumentation

The surface area and porosity of nanofibers was examined using the surface area

analyser (ASAP 2020, Micromeritics Instruments, USA). With the aid of TESCAN (Performance in Nanospace, Model – Vega 3LMH, no. VG 97 31276ZA; Vega TC3 software, USA), SEM micrographs of pristine and modified PAN nanofibers were collected at the following analytical conditions: 50/60 Hz, 230 V, and 1300 VA. The EMSCOPE SC-500 sputtering apparatus was used to coat nanofibers with a thin layer of gold prior to the surface imaging. An imageJ™ software was used to estimate the nanofibers’ average diameter with 45 individual segment measurements. The surface 4

ACCEPTED MANUSCRIPT functional groups of the nanofibers were accessed using the FT-IR spectrometer (PerkinElmer Spectrum 100, USA). Here, nanofiber samples mixed with KBr were pelletized prior to the analysis in the 4000-450 cm-1 range. The core-level C 1s, N 1s, O 1s spectra were measured using the AXIS SUPRA X-ray photoelectron spectroscopy (XPS) equipment at the University of South Africa (UNISA). This analytical tool is

RI PT

usually applied to assess the distribution of electrons around the core-level (given by the binding energy) in order to monitor the functional groups present on the surface of

adsorbents. All measurements were taken under the same experimental conditions as

follows: working pressure of 1.8x10-8 torr, 15 mA emission current, resolution 80, dwell

SC

time 100 and sweeps 2. In addition, charge neutralizer mode was applied due to non-

conducting nature of the samples. Changes in the concentration of dyes were estimated using the UV-Vis spectrophotometer (Agilent Technologies, Cary 60 UV-Vis, USA).

M AN U

Finally, the zeta (ξ) potential was determined following the procedure available in open literature (Deng and Bai, 2003) and described in our previous work (Chaúque et al., 2016).

2.4 Adsorption experiments

TE D

The adsorption of MO and RR onto EDTA-EDA-PAN nanofibers was carried out batch wise at 464 and 512 nm maximum absorbance, respectively. The adsorption capacity of the nanofibers was determined by introducing 25 mL of dye solutions into 40 mL glass bottles containing 0.05 g of sorbent. The stock dye solutions were prepared

EP

by dissolution of their powders in Millipore water (type I). The pH of dye solutions was adjusted using 0.1 M NaOH and HCl solutions. The mixtures of dyes and nanofibers were shaken at room temperature at 75 rpm using a bench shaker. After predefined time

AC C

intervals, the nanofibers were withdrawn from the solution, rinsed with de-ionized water and dried in the oven at 50 ºC before storing in a desiccator. Synthetic aqueous dye solutions, prepared without the addition of nanofibers were used as controls. The amount of dyes sorbed onto the EDTA-EDA-PAN nanofibers was calculated by using equation 1:
 =

   

(1)

where,
 is the amount of the dye extracted (mg g-1), C0 and Ce are the initial and equilibrium dye concentrations (mgL-1), respectively, V is the solution volume (L) and m the amount of EDTA-EDA-PAN nanofibers employed (g). 5

ACCEPTED MANUSCRIPT The effect of pH, initial concentration and contact time for the adsorption of MO and RR onto EDTA-EDA-PAN nanofibers was also examined. This was achieved by varying the initial pH from 3 to 9, and the concentration of the adsorbing dye solution

2.5 Reusability of EDTA-EDA-PAN nanofibers

RI PT

from 5 to 300 mgL-1 for a period up to 30 h.

An investigation into the sorption/desorption cycles of MO and RR on EDTA-EDA-

SC

PAN nanofibers was carried out in order to examine the reutilization of the modified nanofibers. This was done by introducing ca. 0.1 g of modified PAN nanofibers into 100 mL 10 mgL-1 model dye solutions in glass bottles. The desorption of dyes from

M AN U

dye-laden EDTA-EDA-PAN nanofibers was done by washing them with 2 M NaOH and HCl solutions, alternatively, followed by copious washing with methanol and deionized water. The sorption/desorption cycle was repeated five times. The dye removal efficiency was calculated using equation 2:

  

100 

(2)

TE D

    =

where, C0 and Ce (mgL-1) are the initial and equilibrium concentrations of the dye in the

EP

liquid phase.

3. RESULTS AND DISCUSSION

AC C

3.1 Morphology of nanofibers

The BET results showed reduction in both surface area and average pore diameter

after the chemical modification of PAN nanofibers. A decrease from 10.6 to 9.9 m²g-1 and 14.4 to 11.4 nm were recorded for PAN and EDTA-EDA-PAN, respectively. These observations corroborate a recent report where similar study was done (Makaremi et al., 2015). Such depletion in the specific surface area and pore size is attributed to the coating of the impregnants on the external surface and inner channels of the nanofiber mats. Results on the surface (BET) analysis suggest that no substantial alterations were induced to the structural properties of the pristine PAN nanofibers when treated with EDTA-EDA complexing agents.

6

ACCEPTED MANUSCRIPT Structural changes brought to the surface of the nanofibers via chemical modification were further investigated using SEM, where alterations in the surface morphology could be monitored. Fig. 1 shows SEM micrographs of PAN, EDA-PAN and EDTA-EDA-PAN nanofibers, from which average diameters of 289 ± 31, 294 ± 55 and 288 ± 68 nm were respectively observed. From these results, one could infer that

RI PT

chemical modification of PAN induced negligible increase in the average diameter of nanofibers (although with increase in varying sizes as suggested by the measure of

dispersion). This observation is in line with the report of Zhao and co-workers (2015), where they partially converted the surface nitrile groups into amidoxime groups for

SC

subsequent remediation of trace metals pollution. Despite that the incorporation of EDA and EDTA onto PAN surface changed the distribution of the nanofibers as proximity is increased (accompanied by reduced softness), the nanofibrous structure of the pristine

M AN U

PAN nanofibers was not altered (Saeed et al., 2008).

Results on porous and micrographic (BET and SEM) investigations (section 3.1) indicate non-substantial changes of structural properties of the pristine PAN nanofibers when treated with EDTA-EDA complexing agents. Therefore, we opine that any

TE D

noticeable differences (improvement or depreciation) in the removal efficiencies of pristine and modified PAN nanofibers against the target model dyes to be attributed to alterations in the fiber’s surface chemistry, rather than the textural properties.

EP

3.2 Surface chemistry

From the infrared spectrograms illustrated in Fig. 2, the absorption frequencies for

AC C

the electrospun PAN nanofibers are assigned as follows: 3437 cm-1 for OH stretching, 2939 and 2872 cm-1 are attributed to CH stretching in CH, CH2, and CH3 groups, 2243 cm-1 evinced C≡N stretching, 1734 cm-1 signifies C=O stretching, 1454 cm-1 for CH blending, 1384 cm-1 shows symmetric blending of CH3 in CCH3, and 1072 cm-1 signifies C–O stretching in acetate ester. Upon reacting EDA with PAN nanofibers, significant changes were observed in the spectrum of the resulting EDA-PAN nanofibers. They are described as follows: the weak and broad peak at 3437 cm-1, attributed to the OH stretching in PAN nanofibers was transformed to a strong and broad band with maximum frequency at 3389 cm-1 for the EDA-PAN nanofibers. This band is derived from the combined intensities of OH 7

ACCEPTED MANUSCRIPT and NH stretching and vibration bands on the surface of EDA-PAN nanofibers as similarly pointed out in a published work, where chemically binding of diethylenetriamine with PAN fibers was reported (Deng et al., 2003). The significant reduction in intensity of the peak at 2243 cm-1 for EDA-PAN nanofibers indicates partial conversion of the surface nitrile groups with ethylenediamine during the

RI PT

formation of the amidine groups (N–C=N) which absorbs at wavenumber 1639 cm-1 (Zhao et al., 2015; Kampalanonwat and Supaphol, 2010; Anirudhan and Ramachandran, 2008). In addition, the reduction in peak intensities at 1731 cm-1 (C=O stretching), 1454 cm-1 (CH blending), 1384 cm-1 (symmetric blending of CH3 in CCH3) and 1072 cm-1

SC

(C–O stretching) suggests the hydrolysis of the esters of itaconic and methacrylic acids monomers (Deng and Bai, 2003). These reductions were accompanied by the appearance of new peaks at 1674 and 1612 cm-1, assigned to the stretching vibrations of

M AN U

C=O groups in amides and primary amines (NH2), respectively, suggesting that both functional groups were introduced onto the surface of the EDA-PAN nanofibers. After the incorporation of EDTA on the surface of the nanofibers (through ring opening reaction between EDA-PAN nanofibers with EDTAD), new peaks were observed. The broad band with absorption peak at 3389 cm-1, for the EDA-PAN nanofibers was

TE D

substituted by three distinctive peaks at 3110, 3250 and 3395 cm-1 assigned to the O–H stretching vibrations in carboxylic acids, and N–H stretching vibrations in amines and amides, respectively. In addition, the peaks at 1674 and 1612 cm-1 respectively associated with the stretching vibrations of C=O in carboxylic and N–H in primary

EP

amines were enhanced on the surface of EDTA-EDA-PAN nanofibers. From the FT-IR analysis, the hypothetical reaction mechanism for the surface modification of PAN

AC C

nanofibers is illustrated in the Scheme 1:

The surface chemistry of the EDTA-EDA-PAN nanofibers was further investigated

using the XPS technique. The XPS wide range spectrum for the EDTA-EDA-PAN nanofibers exhibits C 1s, O 1s and N 1s peaks as depicted in Fig. 3 and the peak characteristics are shown in the Table 1.

The high resolution C 1s spectrum of the nanofibers is shown in the Fig. 4. The peaks at the assigned binding energy (BE, eV) of 286.3 (CC=O, carbonyl), 287.6 (CC–N and N–C=N,

amine and amidine) and 289.6 (COC=O, carboxyl) are in agreement with reports in 8

ACCEPTED MANUSCRIPT the open literature (Singh et al., 2013; Zubavichus et al., 2004; McCafferty and Wightman, 1998). This observation demonstrates successful incorporation of EDTAEDA chelating agents on the surface of PAN nanofibers and confirms the findings earlier provided from the FT-IR analysis.

RI PT

The deconvoluted N 1s for the EDTA-EDA-PAN nanofibers (Fig. 5) shows 2 distinct peaks at 400 and 401.6 eV, which are assigned to amines (–NH2 and –NH–) and π* orbital delocalized over the amide (O=C–NH) groups, respectively (Barazzouk and Daneault, 2012; Liu et al., 2006). The peak at 400 eV indicates the chemical

SC

incorporation of EDA on the surface of PAN nanofibers while the peak at 401.6 eV

suggests chemical immobilization of EDTA on the surface of PAN nanofibers through the EDA crosslinker. These observations demonstrate successful impregnation of

from FT-IR assessment.

M AN U

EDTA-EDA chelating agents on the surface of PAN nanofibers and support the findings

3.3 Electrostatic properties of nanofibers

The determination of zeta (ξ) potentials of both pristine PAN and EDTA-EDA-PAN

TE D

nanofibers at various pH are shown in Fig. 6. For the neat PAN nanofibers, ξ potentials were found to be positive at pH values below 1.3. After the impregnation of EDTA (using the EDA as the cross-linker), various carboxylic and amine groups were introduced onto its surface. The pH at zero point of charge (pHzpc) shifted from 1.3 for

EP

PAN to 7.6 for EDTA-EDA-PAN nanofibers as a result of the protonation of the surface of nanofibers. Similar finding was previously reported by Deng and Bai (2003) when

AC C

introducing OH and NH groups to the surface of PAN fibers.

3.4 Adsorption studies

The collection of dyes to the surface of adsorbents is affected by parameters

such as pH, dosage and contact time. Hence the optimization of adsorption parameters is important mainly when up-scaling the removal of color from textile effluents. Therefore, the effect of pH, initial concentration and contact time on the sorption of MO and RR are described in the following session. 3.4.1 Effect of pH The degree of ionization of both the dye molecule and the functional groups present on the surface of adsorbents change according to the pH of the contacting solution 9

ACCEPTED MANUSCRIPT (Dastbaz and Keshtkar, 2014; Rafatullah et al., 2010). The amine and carboxylic groups on the surface of EDTA-EDA-PAN nanofibers tend to protonate or deprotonate in the acidic or basic media, respectively. Here, the pH for optimum adsorption of MO and RR was investigated in the range 3-9 at 25 ºC as shown in Fig. 7. From our observation, the optimum pH value of 4 was chosen for further adsorption experiments for both MO and

RI PT

RR dye molecules. When the pH of the media is varied from 3 to 9, the adsorption capacities for the

sorption of MO and RR on the pristine PAN nanofibers decrease while a non-substantial change is observed for the modified PAN nanofibers. From ξ potentials analysis (given

SC

in the session 3.3), it can be seen that the surface of pristine PAN nanofibers are

negatively charged while the EDTA-EDA-PAN nanofibers surface charge shifts from positive to negative within the pH range 3 to 9. The highest adsorption capacities for the

M AN U

collection of MO and RR on the unmodified PAN nanofibers occurred in acidic media and it can be explained by the electrostatic attraction forces between the negatively charged nanofiber´s surface and positively charged dye molecule (owned to protonation of amine groups in lower pH). As the pH of the media increases, the target dye molecules deprotonate becoming negatively charged (basic). As a result, the

TE D

electrostatic repulsion forces between the negatively charged pair (adsorbent and adsorbate) limit the collection of the sorptives on the neat PAN nanofibers. However, an opposite behavior is observed for the collection of MO and RR dye molecules on the EDTA-EDA-PAN nanofibers. In this case, increments in the pH of the media from 3 to

EP

9 did not reduce the sorption of MO and RR on the modified PAN nanofibers. This observation indicates that the sorption of MO and RR on the modified nanofibers cannot be explained exclusively by the ionic interaction forces between the oppositely charged

AC C

surfaces. In this case, the presence of hydrogen bonds formed between the surface amine and carboxylic groups of the fabricated nanofibers and the functional groups of the dye molecules must have played a pivotal role for the removal of the sorptives. This observation corroborates the findings given by the sorption of MO and RR on the surface of pristine PAN nanofibers (inset graph on Fig. 7). Under the same pH range investigated (i.e. pH 3 ̴ 9) the surface of pristine PAN nanofibers was found to be negatively charged (section 3.3) and yet presented adsorption capacities for the removal of the sorptives ca. 6.9 and 14.7 times lower than the ones observed for the modified PAN nanofibers for the collection of MO and RR, respectively. Hence, we opine that the high adsorption capacities observed for the sorption of MO and RR on EDTA-EDA10

ACCEPTED MANUSCRIPT PAN nanofibers is better explained by the combined synergistic effects of the electrostatic forces and non-electrostatic interactions (i.e. hydrogen bonding, hydrophobic interactions, and van der Waals forces) between the pair.

3.4.2 Effect of contact time

RI PT

The collection of dye molecules on the surface of adsorbents is a time dependent process. Hence the time necessary for quantitative sorption of MO and RR was

optimized in order to estimate the adsorbent´s utilization period. The adsorption

experiments were carried out for a contact time up to 30 h. Results shown in Fig. 8

SC

indicate that a period of 8 h was required for most of the sorption of dye molecules to reach equilibrium, while that of RR slowly drags up to 15 h. This observation demonstrates the effect of molecular weight on the adsorption of dye molecules on

M AN U

nanofibers. In this case, the time required for the collection of the sorptives to reach equilibrium increased following increments in the molecular weight of the target dye molecules (i.e. MO < RR). This fact is expected based on relatively lower mobility of heavier molecules. Therefore, an average of 24 h was used in the subsequent adsorption

TE D

experiments as the optimum adsorption time.

3.4.3 Effect of initial concentration and temperature The effect of initial concentration on the adsorption of the model dyes onto the fabricated nanofibers was investigated using a concentration range 10 to 300 mgL-1 at

EP

three different temperatures (i.e. 25, 35 and 45 ºC). As expected, increase in the initial concentration of the sorptives resulted in the depletion of the adsorption sites on the surface of EDTA-EDA-PAN nanofibers. This is often exhibited by rapid (exponential)

AC C

adsorption rate (Fig. 9). Such rapid adsorption is often evinced by the formation of multi-layers of dye molecules on the surface of the sorbent. Here, it could be as a result of the combined electrostatic attraction forces and and non-electrostatic interactions such as hydrogen bonds, hydrophobic interactions, and van der Waals forces between the adsorbent/adsorbate pair. The sorption of MO and RR dye molecules was enhanced by increments in the temperature with prominence to the collection of RR. This indication can be explained by increased mobility and permeation of adsorbate molecules through the pores of the EDTA-EDA-PAN nanofibrous. This observation is in line with findings by Ghasemi and Asadpour (2007) when adsorbing methylene blue dye molecules onto activated carbon at different temperatures. 11

ACCEPTED MANUSCRIPT The aforementioned inference suggests that the EDTA-EDA-PAN nanofibers can be optimized for each, a pair or all the dye depending on their presence and concentrations in the matrix wastewater.

RI PT

3.5 Adsorption isotherm study Adsorption isotherms are usually applied to describe the liquid and solid phase

interactions between the adsorbent and adsorbate under certain conditions (i.e. ionic

strength, fixed mass and particle of the adsorbent) (Tran et al., 2016). In this study, the

SC

experimental adsorption data were fitted into non-linearized plots obtained from

Langmuir, Freundlich and Temkin models using mathematical software (Sigma plots). The Langmuir model (Langmuir, 1916) is based in the following major conditions: (i)

M AN U

the adsorbent´s surface have limited number of adsorbing sites with equal energy; (ii) the adsorption process is reversible; (iii) there is no interaction among the adsorbate molecules in the monolayer, (iv) the sorption of adsorbate molecules in one adsorbing site does not influence the adsorption of other molecules in neighboring sites, and (v) the intermolecular attraction forces between the adsorbate molecules in the monolayer

TE D

and the ones in the solution decreases drastically with distance (Foo and Hameed, 2010; Ng et al., 2003). The non-linearized form of the Langmuir model is expressed as


 =

 !"  #$!" 

(3)

EP

follows:

AC C

where, Ce and qe are the equilibrium concentration (mgL-1) and adsorption capacity (mg g-1) of dye molecules, respectively, given in the equation 1; qm is the maximum adsorption capacity of the saturated monolayer (mg g-1) and KL (Lmg-1) is the Langmuir constant related to the adsorption energy.

Unlike the Langmuir, the Freundlich isotherm advocates the formation of a multilayer of sorptives on heterogeneous adsorbent´s surface due to non-limitation by the Henry´s law (Freundlich, 1906). This model assumes that the collection of adsorbing molecules onto the surface of adsorbent is directly proportional to concentration of the adsorbing

12

ACCEPTED MANUSCRIPT solution and is not constant at a given concentration. The linear form of this model is expressed as follows: #/)


 = %& '

(4)

The Temkin isotherm model advocates reduction of the heat of adsorption due to the

RI PT

interaction between the adsorbent/adsorbate pair (Temkin and Pyzhev, 1940). This model is also based on the assumption uniform distribution of the binding energy (at

least up to a maximum binding energy). The linear expression of this model is given as follows:

SC


 = *+,/- 0 × ln4. × '

(5)

M AN U

.

where T is the absolute temperature in K and R is the universal gas constant (8.314109 kJmol-1 K-1). The constant bT is related to the heat of adsorption (kJmol1

), AT is the constant of equilibrium binding (Lg-1) corresponding to the maximum

binding energy.

TE D

The selected isotherm models are given in the Figs. 10a and b for MO and RR, respectively, and the respective extrapolated theoretical parameters are listed in the Table 2. It was observed that the Langmuir model shows substantial difference between the experimental (qe) and theoretical (qm) values for all sorptives. Judging by their

EP

corresponding error function (R2) given in Table 2, this model did not provide an appreciable correlation with the experimental data. In furtherance, the dimensionless

AC C

Langmuir parameter known as the separation factor (RL) was used to evaluate the adsorbate-adsorbent interactions. Based on the RL values (Table 2), the collection of the model dyes on the surface of the modified PAN nanofibers is favorable for RR and linear for MO (Samadi et al., 2015). Among the three models examined, the Temkin isotherm provided the poorest correlation to the experimental data. However, the high AT values (which is related to the binding strength at the adsorption interphase) for MO and RR suggests strong interaction (toward chemisorption) between MO and RR with the EDTA-EDA-PAN nanofibers. In addition, the strength of the binding forces between the adsorbent-adsorbate pair was further evinced by difficulties to quantitatively elute the sorptives from the dye-laden spent EDTA-EDA-PAN nanofibers 13

ACCEPTED MANUSCRIPT for regeneration (session 3.7). The order of elution of dye molecules from the spent dyeladen nanofibers follows the sequence MO > RR which is the opposite order of the available adsorbate´s functional groups used to bind to the nanofibers. Overall, the Freundlich model provided the best fit to the experimental data. Therefore, we infer that the sorptives (i.e. MO and RR) collect as a multilayer on a heterogeneous adsorbent´s

RI PT

surface, chemically modified with EDTA-EDA chelating agents with no uniform distribution of the adsorption heat (Adamson and Gast, 1997). Considering the values of the Freundlich constant n (i.e. 0 < n < 1) for MO and RR, we conclude that the

adsorption process follows a concave isotherm, otherwise known as the solvent-affinity

SC

type isotherm (Febrianto et al., 2009; Site, 2001).

3.6 Adsorption kinetics

M AN U

The modelled adsorption kinetics (i.e. pseudo-first and second order kinetic models) for the sorption of MO and RR onto the surface of functionalized nanofibers were obtained through the examination of data for the effect of contact time given in the section 3.5.2. The pseudo-first order kinetic model adsorption process is based in the assumption that the rate of adsorption process is directly related to the number of unoccupied adsorption

TE D

sites (Kampalanonwat and Supaphol, 2014) whose linear expression is given as follows: !

> log
 −
= = log
 − ?.9@9 

(6)

where, qt is the adsorption capacity of dye molecules at a time t (mg g-1) and k1 is the

EP

pseudo-first-order rate constant (min-1).

AC C

On the other hand, the pseudo-second-order kinetic model advocates that the adsorption process is given by the difference of squared product of total number of sorption sites occupied and unoccupied in the equilibrium (Kampalanonwat and Supaphol, 2014). The linear expression of the pseudo-second-order kinetic model is given as follows: =

A

=!

#

B B 

=

+



(7)

where, k2 is the pseudo-second-order rate constant (g mg-1min-1).

14

ACCEPTED MANUSCRIPT The plots of log(qe-qt) vs t (Fig. 11a) and t/qt vs t (Fig. 11b) were used to acquire the rate constants, k1 and k2 for the pseudo-first and pseudo-second order kinetic models, respectively. The k1 and k2 with the extrapolated qe values for MO and RR are shown in the Table 3.

RI PT

The pseudo-second-order kinetic model showed a better fit to the experimental data as evinced by the higher correlation factor (R2) and proximity of values for the calculated and experimental adsorption capacity (Table 3). This observation indicates that the rate of adsorption of dye molecules on the EDTA-EDA-PAN nanofibers is a function of the

SC

sorbent’s active sites as well as the concentration of the sorptives (Ho and McKay, 1998). Therefore, from the kinetic study we infer the suitability of this model to

nanofibers.

3.7 Adsorption thermodynamics

M AN U

describe the adsorption process of MO and RR model dyes onto EDTA-EDA-PAN

The determination of the adsorption strength/intensity is a key tool in up-scaling the application of adsorbents. This is usually done by investigating the thermodynamics of

TE D

the adsorption process. The sorption of adsorbate molecules on the surface of adsorbent can either be physical (physisorption) or chemical (chemisorption), depending on the strength of the binding force (Tran et al., 2016; Zhou and Zhou, 2014). Physisorption is characterized by weak interactions (i.e. van der Waals) while chemisorption involves

EP

strong binding forces (i.e. chemical bond). From the change in magnitude and polarity of thermodynamic parameters such as Gibbs energy (∆Gº), enthalpy (∆Hº) and entropy (∆Sº), physisorption can be distinguished from chemisorption, as well as the

AC C

determining the spontaneity of the adsorption process. The change in Gibb’s free energy is determined as follows: ΔG° = −+,G

(8)

It is related to the other two parameters (enthalpy and entropy) by the following equation: ΔG° = −ΔH° + ,ΔS°

(9)

The Gibbs free energy can also be determined from the Langmuir constant KL as follows (Liu, 2009): 15

ACCEPTED MANUSCRIPT JK° = −+,[

M" N

1  O# ]

(10)

where, γe is the activity coefficient of MO and RR dye molecules in the adsorption equilibrium. The Debye-Huckel limiting law expresses the γe as a function of the ionic strength and

RI PT

charge of adsorbates (i.e. MO and RR dye molecules) at the adsorption equilibrium. However, since the target dye molecules along with most organic compounds have

weak charges, in dilute solutions the γe ≈ 1 (Liu, 2009). Therefore, the Gibbs free energy can be approximately determined from the Langmuir constant (KL) expressed in Lmol-1

SC

units as follows: JK° = −+,GQ

(11)

M AN U

The KL values (Table 4) can be converted into Lmol-1 units using following equation: GQ O # = 1000 × RS   # × GQ O#

(12)

where, Mw is the molecular weight of MO and RR dye molecules.

The equation 11 was used to directly determine the ∆G° values at different absolute

TE D

temperatures (T). Then, from the linear plots of ∆G° versus T, the ∆H° and ∆S° are obtained as the intercept and slope, respectively. In current work, the collection of MO dye molecules onto the surface of EDTA-EDAPAN nanofibers released the heat of adsorption to its surroundings (i.e. ∆Hº = -33.6

EP

kJmol-1), thereby resulting in ∆Hº<0 whereas the sorption of RR behaved otherwise (i.e. ∆Hº = 58.3 kJmol-1). Thus, the adsorption process is termed exothermic for the

AC C

collection of MO and endothermic for the sorption of RR. Usually, exothermic adsorption processes involve one or both of physisorption and chemisorption whereas endothermic adsorption is mostly related to chemisorption (Tran et al., 2016). The ∆Gº describes the spontaneity of the adsorption process. The ∆Gº values for the collection of MO and RR dye molecules onto the fabricated PAN nanofibers are negative (i.e. ∆Gº<0) at all temperatures under study. This means that the sorption of RR and MO on the modified PAN surface is favorable and spontaneous. The high negative values observed (i.e. ∆Gº = ˗31.2 ± 0.3 and ˗31.9 ± 2.9 kJmol-1 for MO and RR, respectively), evinces the high spontaneity of the adsorption process. The ∆Sº for MO is positive (∆Sº = 8 Jmol-1) and negative for RR (∆Sº = ˗292 Jmol-1). The entropy entity is related to 16

ACCEPTED MANUSCRIPT degree of order (∆Sº<0) or disorder (∆Sº>0) of the RR and MO during their collection to the surface of the fabricated nanofibers. The more randomness in the sorption of MO (∆Sº>0) demonstrates its easy collection to the surface of the modified PAN nanofibers

3.8 Reusability of EDTA-EDA-PAN nanofibers

RI PT

whereas the collection of RR behaved otherwise.

The adsorption and desorption cycles of MO and RR onto the EDTA-EDA-PAN nanofibers was carried out to evaluate the reusability of the adsorbent. 2 M HCl and

NaOH solutions followed by methanol and de-ionized water were sequentially used to

SC

elute the model dyes from the dye-laden spent nanofibers in this regeneration study. As shown in Fig. 12, after 5 cycles of adsorption/desorption, a steady removal efficiency

M AN U

above 92% was achieved for MO and RR although the sorptives could not be quantitatively washed out from the dye-laden spent nanofibers after each recycle. From these observations, we infer that the modified nanofibers can be recycled 5 times without substantially loosing the adsorbing capacity.

3.9 Application of EDTA-EDA-PAN nanofibers

TE D

The EDTA-EDA-PAN nanofibers were applied for the removal of MO and RR from wastewater samples. This was done by adding known amounts of the model dyes and metals to the wastewater in order to simulate dye effluents rich in organic matter

EP

and metals. The adsorption efficiency of the fabricated nanofibers was examined using individual and the mixture of MO and RR model dyes in the wastewater spiked with metals at neutral pH. Briefly, 1 mL 100 mgL-1 multi elements standard solution

AC C

(composed of Al, Ba, Cd, Ca, Cr, Co, Cu, Fe, Pb, Mg, Mn, Ni, Na, Ti and Zn) and 2 mL 500 mgL-1 model dyes (i.e. MO and RR) solutions were added to 100 mL volumetric flasks and filled to the mark with wastewater making up a concentration of 1 and 10 mgL-1 of metals and dyes, respectively. Results in Fig. 13 show moderate removal efficiency (~ 50%) of the target sorptives when individually present in wastewater. However, when applying the fabricated nanofibers in a mixture of the model dyes in wastewater, a significant reduction in the removal efficiency of RR was observed. In this case, the collection of dyes on the nanofibers reduced in the following order MO > RR. This observation is in line with the findings presented on the effect of contact time for the collection of the sorptives (session 3.4.2) where RR required longer time to 17

ACCEPTED MANUSCRIPT reach the adsorption equilibrium compared to the MO. Therefore, MO having higher mobility, quickly diffused and permeated through the pores of the modified PAN nanofibers occupying more adsorbent´s active sites. Overall, the organic matter and metal ions in the wastewater at ca. 1 mgL-1 reduced the removal of dyes molecules (Fig. 13). This observation is in part explained

RI PT

by the depletion of the adsorbent´s active sites as a result of the complexation of metal ions by the surface carboxylic and amine groups on the surface of the fabricated nanofibers as demonstrated in our previous report (Chaúque et al., 2016).

SC

4. CONCLUSIONS

The EDTA-EDA chelating agents were chemically impregnated on the surface of electrospun PAN nanofibers and applied for the removal of model dyes from synthetic

M AN U

wastewater samples. The fabricated nanofibers showed high affinity for MO and RR as derived from their high adsorption capacities. The sorption of dye molecules follows a pseudo-second order reaction which suggests that the rate of adsorption is a function of the sorbent’s active sites as well as the concentration of the sorptives. The non-linear isotherm model showed a better fit of the experimental data to the Freundlich model.

TE D

This observation implies that the MO and RR are collected as a multilayer on a heterogeneous nanofiber´s surface. The electrostatic attraction forces between the sorptives and the functionalized nanofiber´s surface could not exclusively account for the removal mechanism as the strong intermolecular forces (i.e. hydrogen bond) as

EP

indicated by the thermodynamic parameters played a pivotal role. The EDTA-EDAPAN nanofibers could be reutilized without appreciable drop in performance for the removal of both sorptive (MO and RR) for five cycles. The fabricated nanofibers can

AC C

remove dye molecules in the wastewater with low content of metal ions. Oppositely, the presence of multi-elemental metal ions ca. 1 mgL-1 (each metal ion concentration) in wastewater was found to reduce the removal of MO dye molecules. This observation suggests competition of the metal ions and dye molecules to the adsorbent´s active adsorption sites. Overall, the fabricated nanofibers have demonstrated potential application for the extraction of dyes from wastewater with low content of metal ions.

ACKNOWLEDGEMENTS The study was supported by the Water Research Commission (WRC) (Contract #K5/2386) (EFCC, CJG, JCN). We also acknowledge useful and insightful comments 18

ACCEPTED MANUSCRIPT of the unknown reviewers and the partial funding from the Centre for Nanomaterials Science Research, in the Department of Applied Chemistry at UJ.

REFERENCES Adamson A.W., Gast A.P., 1997. Physical Chemistry of Surfaces, sixth ed., Wiley-

RI PT

Interscience, New York.

Al-Degs Y., Khraisheh M.A.M., Allen S.J., Ahmad M.N., 2000. Effect of carbon

surface chemistry in the removal of reactive dyes from textile effluent. Wat Res

SC

34, 927–35.

Anirudhan T.S., Ramachandran M., 2008. Synthesis and characterization of

M AN U

amidoximated polyacrylonitrile/organobentonite composite for Cu(II), Zn(II), and Cd(II) Adsorption from aqueous solutions and industry wastewaters. Ind Eng Chem Res 47, 6175–6184.

Barazzouk S., Daneault C., 2012. Amino acid and peptide immobilization on oxidized nanocellulose: spectroscopic characterization, Nanomaterials 2, 187-205.

TE D

Chaúque E.F.C., Dlamini L.N., Adelodun A.A., Greyling C.J., Ngila J.C., 2016. Modification of electrospun polyacrylonitrile nanofibers with EDTA for the removal of Cd and Cr ions from water effluents, Appl. Surf. Sci. 369, 19–28.

EP

Cooper P., 1993. Removing colour from dyehouse waste waters—a critical review of technology available. J Soc Dyers Colour 109, 97–100.

AC C

Dastbaz A., Keshtkar A.R., 2014. Adsorption of Th4+, U6+, Cd2+, and Ni2+ from aqueous solution by a novel modified polyacrylonitrile composite nanofiber adsorbent prepared by electrospinning. Appl Surf Sci 293, 336–344.

Deng S., Bai R., Chen J.P., 2003. Aminated polyacrylonitrile fibers for lead and copper removal. Langmuir 19, 5058-5064. Deng S., Bai R.B., 2003. Aminated polyacrylonitrile fibers for humic acid adsorption: behaviors and mechanisms. Environ Sci Technol 37, 5799–5805.

19

ACCEPTED MANUSCRIPT Febrianto J., Kosasih A.N., Sunarso J., Ju Y.H., Indraswati N., Ismadji S., 2009. Equilibrium and kinetic studies in adsorption of heavy metals using biosorbent: A summary of recent studies – Review. J Hazard Mater 162, 616–645. Foo K.Y., Hameed B.H., 2010. Insights into the modeling of adsorption isotherm

RI PT

systems, Chem. Eng. J. 156, 2–10. Freundlich H.M.F., 1906. Over the adsorption in solution, J. Phys. Chem. 57, 385–471. Ghasemi J., Asadpour S., 2007. Thermodynamics’ study of the adsorption process of

Thermodynamics 39, 967–971.

SC

methylene blue on activated carbon at different ionic strengths. J. Chem.

M AN U

Geigy, 1969. J.R.A.-G., Fr. Patent 1,548,888. (1968); C.A. 71:81380.

Graham N., Chen X.G., Jayaseelan S., 2001. The potential application of activated carbon from sewage sludge to organic dyes removal. Water Sci Technol 43, 245–52.

Ho Y.S., McKay G., 1998. The kinetics of sorption of basic dyes from aqueous solution

822-827.

TE D

by sphagnum moss peat. The Canadian Journal of Chemical Engineering, 76 (4),

Kampalanonwat P., Supaphol P., 2010. Preparation and adsorption behavior ofaminated electrospun polyacrylonitrile nanofiber mats for heavy metal ion removal. Appl

EP

Mat Interf 2, 3619–3627.

Kampalanonwat, P., Supaphol, P. 2014. The study of competitive adsorption of heavy

AC C

metal ions from aqueous solution by aminated polyacrylonitrile nanofiber mats.

Energy Procedia, 56, 142-151.

Keskin N.O.S., Celebioglu A., Uyar T., Tekinay T., 2015. Microalgae Immobilized by Nanofibrous Web for Removal of Reactive Dyes from Wastewater, Ind. Eng. Chem. Res. 54, 5802−5809. Kusic H., Koprivanac N., Bozic A.L., 2013. Environmental aspects on the photodegradation of reactive triazine dyes in aqueous media, J. Photochem. Photobiol. A 252, 131– 144.

20

ACCEPTED MANUSCRIPT Langmuir I., 1916. The constitution and fundamental properties of solids and liquids, J. Am. Chem. Soc. 38, 2221–2295. Leary R., Westwood A., 2011. Carbonaceous nanomaterials for the enhancement ofTiO2 Photocatalysis – Review, Carbon 49, 741–772.

RI PT

Liu X., Jang C.H., Zheng F., Jurgensen A., Denlinger J.D., Dickson K.A., Raines R.T., Abbott N.L., Himpsel F.J., 2006. Characterization of protein immobilization at silver surfaces by near edge X-ray absorption fine structure spectroscopy. Langmuir 22, 7719-7725.

Eng. Data 54 (7), 1981–1985.

SC

Liu Y., 2009. Is the free energy change of adsorption correctly calculated? J. Chem.

M AN U

Makaremi M., De Silva R.T., Pasbakhsh P., 2015. Electrospun nanofibrous membranes of polyacrylonitrile/halloysite with superior water filtration ability. J Phys Chem C 119, 7949−7958.

McCafferty E., Wightman J.P., 1998. Determination of the Concentration of Surface Hydroxyl Groups on Metal Oxide Films by a Quantitative XPS Method. Surf

TE D

Interface Anal 26, 549-564.

Ng J.C.Y., Cheung W.H., McKay G., 2003. Equilibrium studies for the sorption of lead from effluents using chitosan, Chemosphere 52, 1021–1030.

EP

Nguyen T.A., Juang R.S., 2013. Treatment of waters and wastewaters containing sulfur dyes: A review. Chemical Engineering Journal 219, 109–117.

AC C

Pagga U., Brown D., 1986. The degradation of dyestuffs: part II behaviour of dyestuffs in aerobic biodegradation tests, Chemosphere 15, 479-491.

Rafatullah M., Sulaiman O., Hashim R., Ahmad A., 2010. Adsorption of methylene blue on low-cost adsorbents: A review. Journal of Hazardous Materials, 177,

70–80). Riu J., Barcelo D., Rafols C., 1997. Determination of sulphonated azo dyes in water and wastewater. Trends in Analytical Chemistry 16 (7) 405-419.

21

ACCEPTED MANUSCRIPT Saeed K., Haider S., Ohb T.J., Park S.Y., 2008. Preparation of amidoxime-modified polyacrylonitrile (PAN-oxime) nanofibers and their applications to metal ions adsorption. J Memb Sci 322, 400–405. Samadi N., Hasanzadeh R., Rasad M., 2015. Adsorption isotherms, kinetic, and desorption studies on removal of toxic metal ions from aqueous solutions by

RI PT

polymeric adsorbent, J. Appl. Polym. Sci. 132, 41642-41654.

Savin I.I., Butnaru R., 2008. Wastewater characteristics in textile finishing mills. Environmental Engineering and Management Journal 7 859–64.

SC

Singh J., Gusain A., Saxena V., Chauhan A.K., Veerender P., Koiry S.P., Jha P., Jain

A., Aswal D.K., Gupta S.K., 2013. XPS, UV−Vis, FTIR, and EXAFS Studies to

M AN U

Investigate the Binding Mechanism of N719 Dye onto Oxalic Acid Treated TiO2 and Its Implication on Photovoltaic Properties. J Phys Chem C 117, 21096−21104.

Site A.D., 2001. Factors affecting sorption of organic compounds in natural sorbent/water systems and sorption coefficients for selected pollutants – A

TE D

review. J Phys Chem Ref Data 30, 187–439.

Szpyrkowicz L., Juzzolino C., Kaul S.N., 2001. A Comparative study on oxidation of disperse dyes by electrochemical process, ozone, hypochlorite and fenton

EP

reagent. Wat Res 35 (9), 2129–2136. Szyguła A., Guibal E., Ruiz M., Sastre A.M., 2008. The removal of sulphonated azodyes by coagulation with chitosan, Colloids and Surfaces A: Physicochem Eng

AC C

Aspects 330, 219–226.

Temkin M.I., Pyzhev V., 1940. Kinetics of ammonia synthesis on promoted iron catalyst, Acta Phys. Chim. USSR 12, 327–356.

Tran H.N., You S.J., Chao H.P., 2016. Thermodynamic parameters of cadmium adsorption onto orange peel calculated from various methods: A comparison study. J. Environ. Chem. Eng. 4, 2671–2682.

22

ACCEPTED MANUSCRIPT Zhao H., Liu X., Yu M., Wang Z., Zhang B., Ma H., Wang M., Li J., 2015. A study on the degree of amidoximation of polyacrylonitrile fibers and its effect on their capacity to adsorb uranyl ions. Ind Eng Chem Res 54, 3101−3106. Zheng Y., Liu Y., Wang A., 2012. Kapok fiber oriented polyaniline for removal of

RI PT

sulfonated dyes. Ind Eng Chem Res 51, 10079−10087. Zhou X., Zhou X., 2014. The unit problem in the thermodynamic calculation of

adsorption using the Langmuir equation, Chem. Eng. Commun. 201, (11)1459– 1467.

SC

Zubavichus Y., Zharnikov M., Shaporenko A., Fuchs O., Weinhardt L., Heske C., Umbach E., Denlinger J.D., Grunze M., 2004. Soft X-ray Induced

AC C

EP

TE D

Chem A 108, 4557-4565.

M AN U

Decomposition of Phenylalanine and Tyrosine: A Comparative Study. J Phys

23

ACCEPTED MANUSCRIPT TABLE CAPTIONS Table 1. Deconvolution of C 1s and N 1s peaks of the EDTA-EDA-PAN nanofibers. Table 2: Isotherm parameters for MO and RR molecules adsorption on the EDTAEDA-PAN nanofibers at 25 ºC (dye concentration = 150 mgL-1, adsorbent dose = 2 gL1

, period = 24 h, pH = 4).

RI PT

Table 3. Parameters of the kinetic models for the adsorption of dyes on the EDTA-

EDA-PAN nanofibers at 25 ºC (dye concentration = 150 mgL-1, adsorbent dose = 2 gL1

, period = 24 h, pH = 4).

Table 4: Thermodynamic parameters calculated from Langmuir isotherm constant

M AN U

SC

(KL).

FIGURE CAPTIONS

Fig. 1. SEM micrographs (at 10 000 magnification) of the (a) electrospun PAN, (b) EDA-PAN, and (c) EDTA-EDA-PAN nanofibers.

Fig. 2. FT-IR spectra of the as-prepared and modified electrospun PAN nanofibers. Fig. 3. Wide range XPS spectrum of EDTA-EDA-PAN nanofibers.

TE D

Fig. 4: High resolution deconvoluted C 1s peak of EDTA-EDA-PAN nanofibers. Fig. 5. High resolution deconvoluted N 1s peak of EDTA-EDA-PAN nanofibers. Fig. 6. Zeta (ξ) potentials of PAN and EDTA-EDA-PAN nanofibers as a function of pH

EP

of the solution (with no inclusion of background electrolyte). Fig 7. Effect of pH on the adsorption of dyes by EDTA-EDA-PAN and PAN nanofibers (inset graph) using 0.05 g nanofibers in 25 mL of 50 mgL-1 dye solutions

AC C

(where, MO – methyl orange and RR – reactive red 120). Fig. 8. Effect of contact time on the adsorption of MO and RR on EDTA-EDA-PAN nanofibers using 0.05 g nanofibers in 25 mL 150 mgL-1 dye solutions. Fig. 9. Effect of initial concentration and temperature for the adsorption of MO (a) and RR (b) on EDTA-EDA-PAN nanofibers using 0.05 g nanofibers in 25 mL of 10 ̴ 300 mgL-1 dye solutions. Fig. 10. Langmuir, Freundlich and Temkin models for the adsorption of (a) MO and (b) RR onto EDTA-EDA-PAN nanofibers. Fig. 11. Adsorption kinetic of MO and RR onto the EDTA-EDA-PAN nanofibers using (a) pseudo-first and (b) pseudo-second order models. 24

ACCEPTED MANUSCRIPT Fig. 12. Adsorption/desorption cycles of MO and RR dye molecules onto the EDTAEDA-PAN nanofibers. Fig. 13. Adsorption of synthetic wastewater solutions of MO and RR dye molecules onto the EDTA-EDA-PAN nanofibers.

RI PT

SCHEME CAPTION

Scheme 1. Chemical reactions for the modification of PAN nanofibers (where, ethylenediamine (EDA), ethylediaminetetraacetic acid dianhydride (EDTAD),

SC

tetrahydrofuran (THF), triethyleneamine (Nt3); the monomers l, k and m correspond to

AC C

EP

TE D

M AN U

93, 6 and 1%, respectively).

25

ACCEPTED MANUSCRIPT TABLES, FIGURES AND SCHEME CAPTIONS

TABLE CAPTIONS

Peak area 26118

Peak area % conc. 33978 68.66

287.6

49529

42.97

401.6

15510

31.34

289.6

39609

34.37

-

-

-

SC

% conc. 22.67

N 1s BE (eV) 400.0

M AN U

C 1s BE (eV) 286.3

RI PT

Table 1. Deconvolution of C 1s and N 1s peaks of the EDTA-EDA-PAN nanofibers.

Table 2: Isotherm parameters for MO and RR molecules adsorption on the EDTAEDA-PAN nanofibers at 25 ºC (dye concentration = 150 mgL-1, adsorbent dose = 2 gL, period = 24 h, pH = 4).

TE D


,. (mg g-1)

MO

RR

99.15

110.0

Parameters

Langmuir (L)

 (mg g-1)

82.31

123.4

(Lmg-1)

0.9326

0.08955

R2

0.7329

0.9326



0.971

0.0896



35.16

25.39

n

0.221

0.3539

R2

0.8250

0.9604



2.157

15.5

EP

Isotherm models

Freundlich (F)

AC C

1

Temkin (Te)

11



1.39e

7.696

R2

0.2775

0.9022

ACCEPTED MANUSCRIPT Table 3. Parameters of the kinetic models for the adsorption of dyes on the EDTAEDA-PAN nanofibers at 25 ºC (dye concentration = 150 mgL-1, adsorbent dose = 2 gL1

, period = 24 h, pH = 4).

Dyes Parameters

MO

Pseudo-first-order

 . (mg g-1)

63.85

 . (mg g-1)

10.01

 (min-1)

-0.0022

R2

0.7561

 . (mg g-1)

63.85

65.65

 . (mg g-1)

63.94

67.57

 (min-1)

3.530

1.449

R2

0.9998

0.9980

65.65

28.17

-0.0030

SC

0.9357

M AN U

Pseudo-second-order

RR

RI PT

Kinetic models

( )

TE D

Table 4: Thermodynamic parameters calculated from the Langmuir isotherm constant

Temperature KL (Lmg-1)

298

318

RR

MO

RR

∆H° (KJmol-1) ∆S° (KJmol-1) MO

0.9326 0,08955 -31.66 -28.97 -33.59 0,4185 0,1860

AC C

308

MO

EP

(K)

∆G° (KJmol-1)

0,457

0,3928

-30.29 -31.82 -31.50 -34.83

RR 58.28

MO 7.9

RR -292.7

ACCEPTED MANUSCRIPT

SC

RI PT

FIGURE CAPTIONS

M AN U

Fig. 1

E D A -P A N

TE D

% T ra n sm ita n c e

E D T A -E D A -P A N

AC C

EP

PAN

4000

Fig. 2

3500

3000

2500

2000

1500 -1

W aven u m b er (cm )

1000

500

ACCEPTED MANUSCRIPT

35000

C 1s 30000

25000

O 1s

RI PT

In te n sity (c p s)

N 1s 20000

15000

10000

SC

5000

0 200

400

600

800

M AN U

0

1000

1200

B in d in g en erg y (eV )

30000

C 1s

CC–N, N–C=N

CCOOH

EP

20000

CC=O

15000 10000

AC C

Intensity (cps)

25000

TE D

Fig. 3

5000 0

280

Fig. 4

282

284

286

288

290

Binding energy (eV)

292

294

296

ACCEPTED MANUSCRIPT 25000

N 1s

23000

N–NH2 and –NH–

19000

NN–C=O

17000 15000 13000 11000 9000 7000 5000 396

398

400

402

Binding energy (eV)

M AN U

Fig. 5

40 30

-1 0 -2 0

AC C

-3 0

PAN E D T A -E D A -P A N

TE D

0

406

EP

Z e ta p o te n tia l (m V )

20 10

404

SC

394

RI PT

Intensity (cps)

21000

-4 0 -5 0

Fig. 6

2

4

6

pH

8

10

ACCEPTED MANUSCRIPT

35

MO RR

30

2 1

-1

q e (m g g )

0 4

RI PT

qe (m g g

-1

)

3

6

8

pH

SC

25

3

4

5

M AN U

20 6

7

8

9

pH

TE D

Fig. 7

55

-1

AC C

q e (m g g )

60

EP

65

50

45

MO RR 40 0

5

10

15

T im e (h )

Fig. 8

20

25

30

ACCEPTED MANUSCRIPT

125

RR 4 5 ºC 3 5 ºC 2 5 ºC

RI PT

100

(b )

SC

-1

q e (m g g )

75

25

0 0

50

100

M AN U

50

150

200

250

300

250

300

-1

TE D

In itial co n cen tratio n (m g L )

MO 4 5 ºC 3 5 ºC 2 5 ºC

75

-1

AC C

q e (m g g )

100

(a )

EP

125

50

25

0 0

50

100

150

200 -1

In itial co n cen tratio n (m g L )

Fig. 9

ACCEPTED MANUSCRIPT

120

(a) 100

60 40 20 L

0 0

10

20

30

40

50

60

70

140

100

90

100

80

TE D

qe (mg/g)

80

(b)

120

60

20 0

10

AC C

0

EP

40

Fig. 10

Te

M AN U

Ce (mg/L)

F

SC

qe

RI PT

qe (mg/g)

80

20

qe 30

40

Ce (mg/L)

L 50

F 60

70

Te 80

ACCEPTED MANUSCRIPT 1 ,5

MO RR

(a) 1 ,0

RI PT

lo g (q e - q t)

0 ,5

0 ,0

SC

-0 ,5

0

200

400

M AN U

-1 ,0 600

800

1000

1200

1400

T im e (m in u tes)

25

MO RR

TE D

20

AC C

EP

t/q t

15

10

(b )

5

0

0

200

400

600

800

T im e (m in )

Fig. 11

1000

1200

1400

ACCEPTED MANUSCRIPT

MO

RR

RI PT

100

50

SC

% R em oval

75

0

1 st cyc le

2 n d c ycle

3 rd cycle

AC C

EP

TE D

Fig. 12

M AN U

25

4 th cyc le

5 th c ycle

ACCEPTED MANUSCRIPT

90

MO

RR

80 70

RI PT

% R em oval

60 50 40 30

SC

20 10

M AN U

0

In d ivid u al d yes

In d ivid u al d ye + m etals

TE D

Fig. 13

SCHEME CAPTION

M ixtu re of d yes

O R

NH2

R

O

EP

NH2

O

HN

O

k

l

95 ºC, 2 h

m

O

AC C

CN

O

O

k

x

N

R=

m CN

O

O

NH

l-x

EDTAD THF/Nt3 r.t. O

m

k

x

NH

O

HOOC

HN

NH

l-x

EDA

NH

NH

O

CN O

NH

HN

HN

NH2

NH NH2

O

HN

COOH

N

COOH

R R

Scheme 1. Chemical reactions for the modification of PAN nanofibers (where, ethylenediamine (EDA), ethylediaminetetraacetic acid dianhydride (EDTAD), tetrahydrofuran (THF), triethyleneamine (Nt3); the monomers l, k and m correspond to 93, 6 and 1%, respectively).

O

ACCEPTED MANUSCRIPT HIGHLIGHTS

•

Polyscrylonitrile (PAN) nanofibers were prepared through electrospinning;

•

PAN nanofibers were chemically modified with EDTA using ethylenediamine crosslinker; Fabricated nanofibers have enhanced surface chemistry and nanofibrous

RI PT

•

structure; •

High adsorption capacities up to 110 mg g-1 toward dye molecules were observed;

The modified PAN nanofibers can be reused 5 times without losing their

SC

•

AC C

EP

TE D

M AN U

performance.