Fabrication and characterization of hydrogen-treated tungsten oxide nanofibers for cationic dyes removal from water

Journal Pre-proof Fabrication and characterization of hydrogen-treated tungsten oxide nanofibers for cationic dyes removal from water

Nemat Tahmasebi, Sahar Sezari, Parisaa Zaman PII:

S1293-2558(19)31009-X

DOI:

https://doi.org/10.1016/j.solidstatesciences.2019.106073

Reference:

SSSCIE 106073

To appear in:

Solid State Sciences

Received Date:

25 August 2019

Accepted Date:

18 November 2019

Please cite this article as: Nemat Tahmasebi, Sahar Sezari, Parisaa Zaman, Fabrication and characterization of hydrogen-treated tungsten oxide nanofibers for cationic dyes removal from water, Solid State Sciences (2019), https://doi.org/10.1016/j.solidstatesciences.2019.106073

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Graphical abstract

RhB

Qe(mg/g)

qmax= 312.5 mg/g

Ce (mg/l)

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Fabrication and characterization of hydrogen-treated tungsten oxide nanofibers for cationic dyes removal from water Nemat Tahmasebi1,*, Sahar Sezari1, Parisaa Zaman1 1Department

of Physics, Jundi-Shapur University of Technology, Dezful, I.R. Iran

*Corresponding

author: [email protected] (Nemat Tahmasebi) Tel: +98936 070 9042

Abstract The pollution of water resources by various pollutants is one of the most common challenges in all over the world, which attracts much research interest in wastewater purification. In this work, WO3 nanofibers have been used as an adsorbent for the removal of RhB molecules from aqueous solution. The nanofibers were synthesized by an electrospinning method, followed by a hydrogen annealing in the temperature of 350 ºC to create oxygen vacancies within WO3. It is observed the hydrogen-treated WO3 nanofibers exhibit higher adsorption performance than untreated nanofibers. Our results show that the effect of hydrogen treatment on the crystal structure, morphology, and specific surface area of the nanofibers is negligible. However, the surface of hydrogen-treated WO3 nanofibers was more negatively charged due to the induced oxygen vacancies, which enhances the surface adsorption of cationic dye molecules due to the electrostatic interaction between WO3 nanofibers surface and RhB molecules. Also, the mechanism of dye adsorption on nanofibers was investigated. The results indicated that the adsorption kinetics and adsorption isotherm could be described by pseudo-second order and Langmuir models, respectively. Furthermore, the maximum equilibrium adsorption capacity of hydrogen-treated nanofibers as adsorbent for RhB was 312.5 mg/g, which is higher than the previously reported values. Keywords: WO3; Nanofibers; Hydrogen treatment; Adsorption 1

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1.

Introduction With the development of science and technology, high amount of pollutants such as heavy

metals, organic and inorganic compounds were discharged into the water [1-4]. During the last decades, the removal of various pollutants from wastewater has received much research interest [1-2,5]. Up to now, different methods such as photodegradation, oxidation, photocatalyst, membrane separation and adsorption have been applied for water purification [5-10]. Among them, the adsorption is considered as more favorable method due to its simple operation, high efficiency, low cost and low consumption of energy [11-12]. Activated carbons which have high specific surface area are the most study adsorbents for the removal of organic dyes from wastewater [13-16]. However, the high cost and its high regeneration temperature limited its practical application [17]. It has recently been reported that the metal oxide nanostructures such as MoS2, MoO3, NiO, ZnCo2O4, and WO3 display better adsorption performance for dye removal from wastewater due to their unique physical and chemical properties [17-20]. Among them, WO3 is an n-type semiconductor that has received much research interest for removal of organic dye from water because of very low toxicity, naturally abundant, high chemical stability and environmentally friendly nature [20,23]. To date, the dye adsorption properties of various tungsten oxide nanostructures such as hierarchical, nanosheets, nanorods, nanowires and nanotubes have been reported [24-29]. Recently, the one-dimensional nanofibers synthesized by electrospinning technique as a simple and economical method receive extensive attention for water treatment due to their high surface to volume ratio and porous structure [30-32]. Also, the change of morphology from nanoparticles to nanofibers decrease the agglomeration of particles, and the separation of nanofibers is easier than nanoparticles [33]. To the best of our knowledge, the dye adsorption 2

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properties of one-dimensional tungsten oxide nanofibers synthesized by electrospinning technique have never been reported. On the other hand, the specific surface area, the interaction between adsorbent and adsorbate, and surface charge are the main parameters in determining the adsorption capacity of an adsorbent. However, the adsorption capacity of a metal oxide adsorbent was mostly determined by its surface charge [34,35]. Many reports indicated that the oxygen vacancies on metal oxide surface could provide a negatively charged surface, which may enhance the surface adsorption of cationic dye molecules due to the electrostatic interaction between metal oxide surface and dye molecules [14, 35]. Different methods have been used to induce oxygen vacancies within metal oxide nanostructures [36,37]. The thermal treatment in a hydrogen atmosphere is a novel method to construct oxygen vacancies on the surface or within the bulk of metal oxides [14,38-40]. However, the hydrogen treatment at elevated temperatures can simultaneously modify the crystal structure, specific surface area and electronic properties of metal oxides, which may have an adverse effect on the dye removal performance [38-40]. Thus, it is difficult to clearly indicate the effect of oxygen vacancies alone on the adsorption capacity of hydrogen-treated tungsten oxide. In this study, the WO3 nanofibers were annealed under a hydrogen atmosphere at a relatively low temperature 350 ºC to create surface oxygen vacancies. The experimental data indicated the surface charge of hydrogen-treated nanofibers is more negative than that of untreated nanofibers due to the creation of surface oxygen vacancies. While, the hydrogen treatment has a negligible effect on the crystal structure, morphology and specific surface area of nanofibers. It is proposed the oxygen vacancies on hydrogen-treated WO3 nanofibers provided a negatively charged surface, enhancing the surface adsorption of cationic dye molecules. Moreover, the adsorption mechanism of RhB on WO3 nanofibers was also investigated in detail. 3

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Experimental Citric acid (C6H8O10), hydrochloric acid (HCl), tungstic acid (H2WO4), polyvinylpyrrolidone (PVP, Mw=1,300,000) rhodamine B (RhB) and methylene blue (MB) and methyl orange (MO) were purchased from Sigma Aldrich. All chemical reagents are analytical grade and used without further purification. Typically, solution 1 was prepared by adding 3.75 g PVP into 3.84 mL ethanol under magnetic stirring. Meanwhile, 0.61 g of H2WO4 was added into 2.5 ml distilled water under magnetic stirring for 30 min, then 0.52 g HCl was dissolved in above solution. The mixed solution was marked as solution 2. The solution 3 was prepared by adding 0.63 g citric acid into 1.75 ml distilled water. Subsequently, the solution 2 was added into the solution 3. Then, 5 g of this solution was transferred into the PVP aqueous solution (solution 1). The mixture was further stirred for 15 h to obtain a homogenous sol for the electrospinning process. The precursor sol was subsequently added into a 5 ml plastic syringe with a stainless needle, and then ejected from the needle with a voltage of 19 kV with flow rate of 0.2 µl/h. An aluminum foil was used to collect the electrospun nanofibers. The distance between tip and collector was 10 cm. The as-spun nanofibers were calcined at 500 ºC for 1 h with a heating rate of 1 ºC/min from room temperature. Then, the synthesized tungsten oxide nanofibers were annealed in a hydrogen atmosphere (H2/Ar = 5%) at a temperature of 350 ºC for 2 h with the flow rate of hydrogen at 10 ml/h. The crystal structures of the products were investigated by X-ray diffraction (XRD, X’Pert PRO, CuKα radiation, λ = 1.544187 Å). The morphology and chemical composition of the products were investigated by scanning electron microscopy (SEM, TESCAN VEGA model) and energy-dispersive X-ray spectroscopy (EDS). Fourier transform infrared spectra (FT-IR) ranging from 4000–400 cm-1 were recorded using a PerkinElmer spectrum RXI. Ultraviolet-visible (UV– Vis) diffused reflectance spectroscopy (DRS) was recorded using an Avaspec-2048-TEC 4

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spectrometer in the wavelength range of 200–800 nm. The nitrogen adsorption-desorption isotherms were measured using a Belsorp mini II instrument. Zeta potential was measured using zeta potential analyzer (Malvern; ZEN3600). For the X-ray photoelectron spectroscopy (XPS) experiment, an Al anode X-ray source (~ 1486.6 eV) was employed with a concentric hemispherical analyzer (Specs Company, model EA10 plus) to analyze the energy of the emitted photoelectron from the surface. The energy scale was calibrated against the carbon binding energy at 285 eV. The dye adsorption performances of as-prepared and hydrogen treated WO3 nanofibers were investigated by the adsorption of RhB in aqueous solution. 20 mg of powder sample was added into 60 ml of RhB aqueous solution (30 mg/L) in the dark and at room temperature under stirring. Then, at certain time intervals, 2 mL of solution is sampled and centrifuged to remove the adsorbent. The same condition was used to determine the adsorption kinetics of the samples for RhB. Furthermore, for the adsorption isotherm experiment, 3 mg of sample was added into 40 ml of RhB with different concentrations (5- 50 mg/L), then the solution was stirred for 24 h. The maximum absorption peak of RhB at 554 nm was used to determine the concentration of RhB within aqueous solution. The removal percentage of RhB molecules and the amount of RhB adsorption by nanofibers (mg/g) were determined using the below equations: Removal (%) = qe =

(C 0 ― C ) C0

(1)

× 100

(C 0 ― C 𝑒 ) 𝑉

(2)

m

5

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where C0 (mg/L), C (mg/L) and Ce (mg/L) are the concentration of RhB at the initial time, different times and equilibrium, respectively, V (L) is the volume of dye solution and m (g) is the amount of adsorbent. 2. Results and discussion Thermal gravimetric analysis (TGA) was used to evaluate the thermal decomposition behavior of as-spun precursor nanofibers during the calcination process. Fig. 1 shows the TGA curve of asspun precursor nanofibers. The first weight loss in the temperature range of 40-140 ºC is attributed to the desorption of residual solvent and water. The weight loss between 190-340 ºC is ascribed to the decomposition of organic compounds. The weight loss between 380-470 ºC is owing to the decomposition of PVP [41]. The last weight loss in the temperature range of 470-550 ºC could be

Weight (%)

attributed to the crystallization of WO3.

Temperature (ºC)

Fig. 1 the TGA curve of as-spun precursor nanofibers.

The as-spun precursor nanofibers were first calcined in air at 500 ºC for 1 h to form WO3 nanofibers. Then, the nanofibers were annealed under a hydrogen atmosphere at 350 ºC to create oxygen vacancies in WO3. The XRD analysis was applied to determine the crystal structure and 6

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phase change of tungsten oxide nanofibers during the hydrogen treatment. Fig. 2 displays the diffraction pattern of as-prepared WO3 nanofibers and after hydrogen treatment at the temperature of 350 ºC. All the diffraction peaks of nanofibers before hydrogen treatment can well be indexed to the orthorhombic phase of WO3 (JCPDS 20-1324) with the lattice constants of a = 7.3840 Å, b = 7.5120 Å, c = 3.8460 Å and α = β = γ = 90º. No observable peaks of other phases could be

Intensity (a.u.)

(b)

(a)

JCPDS no. 20-1324

2 Theta (degree)

Fig. 2 XRD patterns of (a) as-prepared and (b) hydrogen-treated tungsten oxide nanofibers

detected after hydrogen annealing. It can be concluded that the hydrogen annealing at this temperature doesn’t change the crystal structure of WO3 nanofibers. Fig. 3 (a) and (b) show the SEM images of as-prepared WO3 nanofibers and after hydrogen treatment at the temperature of 350 °C. It is observed the average diameter and the length of asprepared WO3 nanofibers are about 400 nm and several micrometers, respectively. Moreover, the surface of nanofibers is nearly rough, which may provide significant active sites for dye adsorption. After hydrogen treatment at the temperature of 350 °C, no considerable change

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observes in nanofibers morphology. The EDS analysis was used to study the chemical composition of the nanofibers. Fig. 3 (c) and (d) display the EDS spectrum of as-prepared WO3 nanofibers and hydrogen-treated nanofibers at temperature of 350 ºC. The EDS results indicate that only O and W elements observe in the spectra which confirm the high purity of the samples. (b)

(a)

10 µm

10 µm

(c) 1400

(d)

W M

1300

W M

1000

1200 900

1100

800

Intensity (a.u.)

900 800 700 600

Element

Line

W%

A%

O

Ka

27.21

81.12

W

La

72.79

18.88

700

Element

Line

W%

A%

600

O

Ka

21.35

75.72

500

W

La

78.65

24.28

Intensity (a.u.)

1000

400

500 400

300

300 O K

200 O K

200

W Ll W L W L2 W L11 W L W L1

100 0 0

0

5

5

10

10 Energy (KeV)

W L W L2 W L11 W L W L1

100

keV 15

15

20

20

W Ll 0 0

0

5

5

10

10 Energy (KeV)

keV 15

15

20

20

Fig. 3 SEM image and EDS spectrum of (a and c) as-prepared WO3 nanofibers, and (b and d) hydrogen-treated WO3 nanofibers.

The effect of hydrogen annealing on the structure of WO3 nanofibers was also investigated by FTIR spectra. Fig. 4 shows the FTIR spectra of WO3 nanofibers and after hydrogen treatment at the temperature of 350 °C. It is observed all the FTIR spectra are nearly identical. The strong absorption band in the range of 545 – 1010 cm-1 is attributed to the vibration of tungsten-oxygen bonds [25-26]. Moreover, the broad absorption band at 3420 cm-1 and the absorption band at 1620 cm-1 can be ascribed to the O-H stretching vibration and bending vibration of coordinated water, respectively [42].

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Intensity (a.u.)

(b)

(a)

Wavenumber (cm-1)

Fig. 4 FTIR spectra of (a) as-prepared and (b) hydrogen-treated WO3 nanofibers

It is well known that the specific surface area of adsorbent and the electrostatic attraction between adsorbent and dye molecules are two crucial factors that determine the adsorption capacity of an adsorbent during dye removal from wastewater. Fig. 5 shows the N2 adsorption/desorption isotherms and pore size distribution curves of samples. It can be seen the samples exhibit a type IV isotherm with a H3-hysteresis loop in the region of 0.75 < P/P0 < 1, corresponding to the mesoporous materials [28,35]. The specific surface areas of as-prepared WO3 nanofibers and hydrogen-treated nanofibers calculated by Brunauer-Emmett-Teller (BET) method

9

dV/drp

Quantity adsorbed (cm3g-1)

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(a) (b)

(a)

rp(nm)

(b) Relative pressure (P/P0)

Fig. 5 N2 adsorption/desorption isotherm and pore size distribution curve of (a) as-prepared and (b) hydrogen-treated WO3 nanofibers.

Sample

SBET (m2g-1)

Pore volume (cm3g-1)

Pore diameter (nm)

(a)

4.71

0.023

19.90

(b)

3.63

0.017

19.66

Table 1. The BET surface area, total pore volume and mean pore diameter of (a) as-prepared and (b) hydrogen-treated WO3 nanofibers.

are 4.71 and 3.63 cm3g-1, respectively, which exhibit that the BET surface areas of two samples are nearly the same (Table 1). XPS analysis was carried out to investigate the surface chemical composition and the valance states of W atoms on nanofibers surface. Fig. 6 (a) displays the survey XPS spectrum of hydrogentreated WO3 nanofibers. It is observed the sample contains W4f, W4d, C1s, O1s, and O (KLL) Auger transition peaks. The carbon is attributed to the adsorption of contamination on the nanoparticles’ surface due to exposure to ambient air. Fig. 6 (b) and (c) show the W4f spectra of 10

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as-prepared and hydrogen-treated tungsten oxide nanofibers. It is observed the W4f spectrum of as-prepared nanofibers exhibits two main peaks at binding energy of 35.60 eV and 37.72 eV, which can be attributed to the W4f7/2 and W4f5/2 of W6+ chemical state, respectively. While, the W4f spectrum of hydrogen-treated nanofibers can be deconvoluted into four peaks; two (a) (a)

Intensity (a.u.)

O1s O (Auger) C1s

W4d

W4f

Binding energy (eV)

W6+

W4f

O1s

OL

Intensity (a.u.)

Intensity (a.u.)

OV OA

Binding energy (eV)

(c)

Binding energy (eV) W6+

Intensity (a.u.)

W4f

O1s

W5+

Binding energy (eV)

OL

Intensity (a.u.)

(b)

OV OA

Binding energy (eV)

Fig. 6 (a) The survey XPS spectrum of hydrogen treated WO3 nanofibers, and High-resolution spectra of W4f and O1s peaks for (b) as-prepared and (c) hydrogen-treated WO3 nanofibers.

peaks centered at 35.60 eV and 37.72 eV attributed to W6+ states, and the other two peaks observed at 34.30 eV and 37.00 eV can be assigned to the W4f7/2 and W4f5/2 of W5+ states. As a result, the existence of W5+ suggests the presence of oxygen vacancies within hydrogen-treated nanofibers 11

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[14,35]. Furthermore, as shown in Fig. 6, the O1s spectra can be deconvoluted into three peaks. The observed peaks at 530.41 eV (OL) and 531.97 eV (OV) can be attributed to the lattice oxygen bonded to W (O-2) and the O-1 in the vicinity of oxygen vacancies, respectively [43,44]. Moreover, the peak at 533.40 eV (OA) corresponds to the adsorbed H2O on the surface [43,44]. The ratio of OV/OL for the hydrogen-treated sample is larger than that for the untreated sample, which confirms the hydrogen-treated sample has more oxygen vacancies. The optical properties were investigated by UV-Vis diffuse reflectance spectroscopy. Fig. 7 shows the UV-Vis absorption spectra of samples. It can be seen the hydrogen-treated sample displays a strong optical absorption tail in the visible and near-infrared region, while in this spectral

(αhv)1/2 (eV)^1/2

(B)

(b) (a) Energy (eV)

Fig. 7 (A) DRS spectra, and (B) (αhv)1/2 versus hv curves of (a) as-prepared and (b) hydrogentreated WO3 nanofibers

region no optical absorption is observed for as-prepared WO3 nanofibers. The optical absorption tail in the visible and near-infrared can be attributed to the creation of oxygen vacancies within WO3 structure under hydrogen treatment [14,45]. The optical band gaps of samples are estimated from the Tauc plots obtained from UV-Vis diffuse reflectance spectroscopy. From Fig. 7 (B), it can be seen the band gap of as-prepared WO3 nanofibers and after hydrogen annealing at 350 ºC are nearly 3.11 eV and 2.69 eV, respectively, which exhibits a red shift. As previously reported, 12

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the oxygen vacancies in metal oxide can produce unoccupied localized states within the forbidden gap as polaron states [45]. These states can shift the optical band gap of the metal oxide to lower band gap, and also increase the optical absorption in the visible and near-infrared regions. Thus, it is suggested the creation of oxygen vacancies within the WO3 structure can provide a negatively charged surface, which may enhance the electrostatic interaction between nanofibers surface and cationic dye molecules [14,35,38]. To determine the surface charge of the samples, the zeta potential measurement was carried out. The results are shown in Fig. 8. It is observed the zeta potential of as-prepared and hydrogen-treated WO3 nanofibers are – 28.74 and – 74.80 mV, respectively. Thus, the zeta potential of hydrogen treated nanofibers is more negative than that of

Intensity (a.u.)

untreated nanofibers.

(b)

(a)

Zeta Potential (mV)

Fig. 8 Zeta potential curves for (a) as-prepared and (b) hydrogen-treated WO3 nanofibers.

In order to investigate the effect of hydrogen treatment on the adsorption capacity of WO3 nanofibers, the ability of synthesized powder samples to adsorb the dye molecules from wastewater was measured. RhB as a cationic dye was used as model pollutant. 20 mg of powder sample was added into 60 ml of RhB solution with the initial concentration of 15 or 30 mg/L. Fig. 9 (a) and (b) show the time evolution of UV-Vis absorption of RhB aqueous solution (15 mg/L) in

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the presence of untreated and hydrogen-treated tungsten oxide nanofibers under dark condition, respectively. It is observed the intensity of absorption peak at 554 nm decreases with increasing (b)

Absorbance (a.u.)

Absorbance (a.u.)

(a)

Wavelength (nm)

Wavelength (nm)

(d)

(c) As-prepared Hydrogen-treated Commercial WO3

Time (min)

% Removal

% Removal

As-prepared Hydrogen-treated Commercial WO3

Time (min)

Fig. 9 Time evolution of UV-Vis absorption of RhB aqueous solution (15 mg/L) in the presence of (a) untreated and (b) hydrogen-treated tungsten oxide nanofibers, and the removal percentage of RhB solution with initial concentration of (c) 15 mg/L and (d) 30 mg/L in the presence 20 mg of different adsorbents as function of contact time.

contact time, which confirms the ability of both samples to remove the RhB molecules from solution. Fig. 9 (c) and (d) display the removal percentage of RhB molecules within the aqueous solution with initial concentration of 15 mg/L and 30 mg/L in the presence of different adsorbents as a function of contact time. The results indicate that after 120 min dark adsorption 85% and 47% of RhB molecules in the presence of as-prepared WO3 nanofibers, and also 100% and 96% of RhB molecules in the presence of hydrogen-treated nanofibers are removed. Thus, the hydrogen-treated sample is more efficient for the removal of RhB. For comparison, the removal percentage of RhB in aqueous solution in the presence of a commercial tungsten oxide powder under the same

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condition has been added to the Fig. 9 (c) and (d). It can be seen the ability of commercial tungsten oxide powder to remove the RhB molecules is negligible. In addition, the promising adsorbent should have good reusability for practical applications in water treatment. Thus, the reusability of hydrogen-treated WO3 nanofibers for RhB was investigated. After each cycle, the sample was separated and washed with water and ethanol for three times. Fig. 10 shows the ability of nanofibers to adsorb RhB molecules for five successive

Removal (%)

cycles. It can be seen the adsorption performance slightly decrease from 100 to 88 after five cycles.

Cycle number

Fig. 10 Removal efficiency of RhB onto hydrogen-treated WO3 nanofibers in five successive cycles (20 mg within 60 ml dye solution (15 mg/L).

As is well known, the specific surface area and surface charge are the effective factors in determining the adsorption performance of a metal oxide adsorbent. However, based on our experimental data, the specific surface area of as-prepared and hydrogen-treated WO3 nanofibers are nearly the same. While, the surface charge of hydrogen treated nanofibers is more negative than that of untreated nanofibers, due to the oxygen vacancies formed in WO3 under hydrogen treatment. Thus, the higher adsorption performance of hydrogen-treated WO3 nanofibers toward cationic dyes may be attributed to the more negatively charged surface. To confirm the effect of 15

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surface charge of nanofibers on adsorption properties, the adsorption performance of hydrogentreated nanofibers to remove RhB and MB as cationic dyes and MO as an anionic dye from water has been evaluated in Fig. 11 (a). It is observed nearly all of the RhB and MB molecules as cationic dyes were removed from the water, whereas the removal of MO as an anionic dye was negligible. In other words, the electrostatic interaction between hydrogen-treated WO3 nanofibers surface and cationic dyes (RhB and MB) enhances the adsorption of cationic dye molecules compared with anionic dyes such as MO [14, 35]. A schematic diagram for the adsorption of cationic dye molecules on hydrogen-treated WO3 nanofibers is presented in Fig. 11 (b).

Removal (%)

(a)

Adsorption

(b) H2/Ar treatment at 350 ºC

RhB +

Adsorption of RhB +

Electrostatic interaction

+ + + + +

+ + + +

+

Fig. 11 (a) The adsorption percentage of different dyes onto hydrogen-treated WO3 nanofibers (20 mg within 60 ml dye solution (30 mg/L)) and (b) a schematic diagram for the adsorption of cationic dye molecules on hydrogen-treated tungsten oxide nanofibers.

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In order to describe the adsorption rate and the mechanism of the dye adsorption process, the pseudo first-order (equation 3) and pseudo second-order (equation 4) models were applied to fit the RhB adsorption experimental data of hydrogen-treated nanofibers (Fig. 12 (a) and (b)). These models are described as [46,47]: ln (qe ― qt) = lnqe ― k1t

(3)

t 1 t = + qt k2q2e qe

(4) (a) (a)

Ln (qe-qt)

t/qt (mn.g.mg-1)

(b) (b)

t (min)

t (min)

Qe(mg/g)

(c) (c)

Ce (mg/l)

Fig. 12 (a) Pseudo first-order and (b) pseudo second-order kinetic models for the RhB adsorption onto hydrogen-treated nanofibers (with initial concentration of 15 and 30 mg/L), and (c) fitting adsorption isotherms for RhB adsorption onto hydrogen-treated WO3 nanofibers corresponding to Langmuir and Freundlich models.

where qe and qt are the amounts of RhB adsorption by nanofibers in equilibrium and at time t (min), respectively. k1 and k2 are the rate constants for pseudo first-order and pseudo second-order, 17

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respectively. The kinetic parameters for adsorption of RhB from aqueous solution (15 and 30 mg/l) onto as-prepared nanofibers and hydrogen-treated nanofibers are presented in table 2. The results indicated that the pseudo second-order model provides better fitting to the experimental data with a regression coefficient of 0.990, indicating that the rate of RhB adsorption was controlled by chemical adsorption. 1. Kinetic parameters Parameters

First order model

Second order model

k1

qe

R2

k2

qe

R2

15 mg/L

0.034

31.871

0.972

0.007

45.045

0.997

30 mg/L

0.039

74.963

0.962

0.002

88.495

0.990

2. Isotherm parameters Parameters Value

Langmuir isotherm

Freundlich isotherm

qm

b

R2

kf

1/n

R2

312.50

1.454

0.994

161.305

0.229

0.896

Table 2. Adsorption kinetics and isotherm parameters for RhB adsorption onto hydrogen-treated WO3 nanofibers. For adsorption kinetic the initial concentration of RhB was 15 mg/L and 30 mg/L.

To further determine the maximum capacity of hydrogen-treated nanofibers, the Langmuir and Freundlich adsorption isotherms were applied to fit the experimental adsorption data for RhB adsorption. The Langmuir adsorption model (equation 5) describes that all the adsorption sites are homogeneous and an ideal monolayer is formed. Whereas the Freundlich model (equation 6) proposed that the adsorption is a multilayer heterogeneous adsorption. These models are represented as below [48,49]:

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qmbCe

(5)

qe = 1 + bCe 1

(6)

qe = kfCne

where qe (mg/g) is the amount of RhB adsorption by nanofibers in equilibrium, Ce (mg/L) is the concentration of RhB at equilibrium, b is Langmuir isotherm model constant, and qm is the

Dye

qm (mg/g)

References

MoS2 – flower-ljke

RhB

291

[7]

SnS2 - hierarchical

RhB

200

[8]

MoO3 - hierarchical

RhB

204.08

[9]

W18O49 - nanourchins

RhB

82

[14]

WO3 hydrate

MB

247.3

[20]

WO3 - nanorods

RhB

57

[22]

WO3 - nanorods

MB

64.2

[25]

W18O49 - nanowires

RhB

120

[27]

WO3 - nanorods

RhB

64

[29]

WO3 - nanoparticles

MB

57.7

[34]

WO3.H2O - nanosheets

MB

451

[42]

Hydrogen-treated WO3 - nanofibers

RhB

312.5

This work

Adsorbent

Table 3. Comparison of adsorption capacity of different adsorbents in previous reports.

maximum adsorption capacity. kf (mg/g) is the Freundlich adsorption equilibrium constant and 1/n is the adsorption intensity. Fig. 12 (c) displays the fitting results to the experimental data obtained from RhB adsorption onto nanofibers. It can be seen the regression coefficient (R2) of Langmuir model (R2 = 0.99) is higher than that from Freundlich model (R2 = 0.89), suggested that the Langmuir model is a better model to describe the RhB adsorption process onto hydrogen-treated

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tungsten oxide nanofibers (table 2). Based on the Langmuir equation, the value of qm is calculated to be 312.5 mg/g. This amount is much higher than those of other reports (table 3). 3. Conclusion In summary, one-dimensional tungsten oxide nanofibers with average diameter of ~ 400 nm were successfully fabricated by the electrospinning method. The effect of hydrogen treatment at the temperature of 350 ºC on the crystal structure, chemical composition, optical properties and RhB adsorption performance of nanofibers was investigated. The adsorption experiments indicated that the hydrogen-treated WO3 nanofibers exhibit higher performance toward RhB adsorption as compared to untreated nanofibers. After 120 min dark adsorption, 100% and 96% of RhB molecules in aqueous solution with the initial concentration of 15 mg/L and 30 mg/L adsorbed on nanofibers, respectively. This value is about 85% and 47% for as-prepared WO3 nanofibers. Results indicated that the crystal structure, chemical band and BET surface area of as-prepared and hydrogen-treated nanofibers are nearly the same. Based on XPS and zeta potential results, the higher adsorption capacity of hydrogen-treated WO3 nanofibers is attributed to the more negative charges, which enhance the surface adsorption of cationic dye molecules due to the electrostatic interaction between nanofibers surface and dye molecules. In addition, the dye adsorption mechanisms investigations indicated that adsorption kinetics and isotherms could be described by pseudo second-order and Langmuir models, respectively. Based on the Langmuir model, the maximum equilibrium adsorption capacity of hydrogen-treated nanofibers for RhB was 312.5 mg/g, which is higher than other reported values.

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The authors declare no conflict of interest.

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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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 The WO3 nanofibers were successfully synthesized by electrospinning technique.  The hydrogen treatment had increased the oxygen vacancies and negative surface charges of nanofibers.  The hydrogen-treated WO3 nanofibers exhibited higher adsorption performance than untreated nanofibers.  The maximum capacity of hydrogen-treated nanofibers was 312.5 mg/g.  The mechanism of dye adsorption on nanofibers was investigated.