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Scalable fabrication of bimetal modified polyacrylonitrile (PAN) nanofibrous membranes for photocatalytic degradation of dyes
Journal Pre-proofs Scalable Fabrication of Bimetal Modified Polyacrylonitrile (PAN) Nanofibrous Membranes for Photocatalytic Degradation of Dyes Shixiong Yi, Sheng Sun, Yuangsong Zhang, Yushan Zou, Fangyin Dai, Yang Si PII: DOI: Reference:
S0021-9797(19)31197-X https://doi.org/10.1016/j.jcis.2019.10.018 YJCIS 25512
To appear in:
Journal of Colloid and Interface Science
Received Date: Revised Date: Accepted Date:
18 August 2019 1 October 2019 6 October 2019
Please cite this article as: S. Yi, S. Sun, Y. Zhang, Y. Zou, F. Dai, Y. Si, Scalable Fabrication of Bimetal Modified Polyacrylonitrile (PAN) Nanofibrous Membranes for Photocatalytic Degradation of Dyes, Journal of Colloid and Interface Science (2019), doi: https://doi.org/10.1016/j.jcis.2019.10.018
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Scalable Fabrication of Bimetal Modified Polyacrylonitrile (PAN) Nanofibrous Membranes for Photocatalytic Degradation of Dyes Shixiong Yi,† Sheng Sun,† Yuangsong Zhang,† Yushan Zou,† Fangyin Dai,† Yang Si,*,‡ †College
of Textile and Garment, Southwest University, Chongqing, 400715, P.R.
China ‡Innovation
Center for Textile Science and Technology, Donghua University, Shanghai
200051, China.
ABSTRACT Fabricating nanostructured fibrous membranes as photocatalyst would facilitate the effective treatment of dyeing effluents. However, creating such photocatalytic nanofibrous membranes has proven tremendously challenging. In this work, we report a scalable strategy to prepare copper-iron bimetal modified polyacrylonitrile (PAN) nanofibrous membranes by electrospinning technology. The resultant PAN membranes exhibit an ultra-fine fiber diameter (600 nm), large surface area (5.34 m2 g-1) and excellent photocatalytic capacity for reactive blue 19, reactive red 195 and acid orange 7 ( ＞ 99.99%) within 60 min. The successful synthesis of such intriguing materials could provide a versatile platform for further exploring nano-sized photocatalyst fibrous membranes for degradation of dyes. Keywords: Bimetal, Polyacrylonitrile, Membranes, Photocatalytic, Degradation, Dyes
1. Introduction Dyeing wastewater from the textile factory is the main producer of contaminated aqueous waste streams1-4. In recent years, Fenton technology was used as highly effective method for the treatment of these dyes5-8. The heterogeneous Fenton catalysts were prepared by coordination of Fe3+ ions on the materials9-12. In previous reports, we have noted that polyacrylonitrile (PAN) fiber can be easily grafted by nitrile groups13. Thus, the PAN yarns could be grafted to construct the photocatalysts through Fe3+
16
ions immobilization. In previous studies, Han et al. created the copper-iron bimetal modified PAN fibers for photocatalytic degradation of anionic dyes. The catalytic activity of Fe3+ ions catalysts could be improved by adding other metals17-20. But drawbacks in the use of such materials are found to be the big fiber diameter and low surface area of fibers, which limit the catalytic property. Therefore, some attempts have been made to develop the nanofibrous membranes as catalyst21-24. Electrospun nanofibers, as advanced fibrous materials, possess extra-fine fiber diameters, a large surface area and surface functional groups25-28. These fabricated functionalized nanofibrous membranes could be used for different industrial application such as adsorption, filtration, sensing and photocatalytic degradation29-32. In this work, we demonstrate a scalable methodology to construct copper-iron bimetal modified PAN nanofibrous membranes (BM-PANNM) by electrospinning technology, as shown in Scheme 1. In our study, nanofibrous membranes are reconstructed. The prepared BM-PANNMs possessed a large surface area (5.34 m2 g1)
and excellent photocatalytic capacity for reactive blue 19, reactive red 195 and acid
orange 7 ( ＞ 99.99%) within 60 min. The fabrication of such PAN nanofibrous membranes may provide a versatile platform for further exploring nano-size photocatalyst fibrous membrane materials for degradation of dyes.
2. Materials and methods 2.1 Materials and Reagents The PAN yarns were supplied from Chongqing Biebei Yihe Cultural Co., China. The anionic anthraquinone dyes (reactive blue 19) and azo dyes (reactive red 195 and acid orange 7) were provided by Chengdu Xinda chemicals Co., China. The chemical structures of dyes were presented in Scheme 2. Hydroxylamine hydrochloride, N, Ndimethylformamide (DMF), ferric chloride, sodium hydroxide, copper sulfate and hydrogen peroxide were obtained from Aldrich Co. (MO, USA). Double distilled was used throughout this study. 2.2 Fabrication of PAN Nanofibrous Membranes (PANNM) The PAN yarn was dissolved in DMF by stirring at 25 ℃ for 24 h. Then, the asprepared 15 wt% PAN solution was transferred to the syringe needle. The electrospinning experiment was carried out. The voltage of 20 kV was supplied. The receiving distance from spinneret to collector was 15 cm. 2.3 Chemical Modification of PANNM PAN nanofibrous membranes were amidoximated in the presence of hydroxylamine hydrochloride and sodium hydroxide at 2.0 h at 65℃. Afterward, the amidoximated PAN nanofibrous membranes (AO-PANNM) was washed thoroughly and dried. The obtained AO-PANNM was put in the metal salt solutions of copper sulfate and ferric chloride for producing the copper-iron bimetal modified PAN nanofibrous membranes (BM-PANNM). The concentrations of the metal ions were 0.10 mol L−1. 2.4 Catalytic Procedure and Analysis 0.20 g of BM-PANNM was immersed into 50 mL test solutions containing given concentration of dyes and H2O2 in the vessel at room temperature. The solution in the
vessel was exposed to the irradiation of UV 365 nm lamp in the reaction system (shown in Figure 1). At time intervals, 2 mL of the dye aqueous solution was taken off. Then, the dye concentration was calculated at λmax of dyes used (590 nm for reactive blue 19, 523 nm for reactive red 195, and 480 nm for acid orange 7). The degradation rate of the dye was expressed as equation (1). To evaluate the degradation kinetics, the degradation process of dye solutions at four initial concentrations were performed, respectively. Finally, the change of dye concentration was observed at different reaction time. The pseudo-first-order kinetic equation was applied to fit experimental data. The kinetic equation is demonstrated as equation (2) 33-34. The total organic carbon (TOC) removal percentage (TOCR%) of the test solution was calculated as equation (3). 𝐷% = (1 − Ct/C0) × 100% ln(C0/Ct) = kt + const
(1) (2)
TOCR% = (1−TOCt/TOC0) × 100%
(3)
where C0 and Ct are the concentrations of the solution at initial and interval time, 𝐷% is the degradation rate of dyes, k is the corresponding degradation rate, and t is the degradation time, TOC0 and TOCt are the TOC values (mg L−1) at initial and interval time, respectively. 2.5 Reusability Measurements To evaluate the economic cost, the BM-PANNM were used and washed to remove the adsorbed dyes. Subsequently, the BM-PANNM were dried and reused for next cycle by using the same experimental method. 2.6 Characterization Surface morphologies of PANNM and BM-PANNM were observed by using a Hitachi S-4800 SEM system. The Fourier transform infrared (FT-IR) spectra of fibrous membranes were obtained by a Magna-560 spectrometer (Thermo Nicolet Co., USA).
The surface area was determined using a Micromeritics ASAP2460 surface area analyzer. The UV-visible spectra of dyes were determined using a scanning UV-visible spectrophotometer (Jasco model V-530, Japan). Electron spin resonance (ESR) signals were determined on a JES-FA ESR spectrometer (JEOL, Tokyo, Japan). Scanning transmission electron microscopy (STEM) and energy-dispersive X-ray (EDX) mappings were acquired on a JEOL 2500 at 200 kV with a Thermo Electron Pioneer EDX detector. The total organic carbon of the test dye solution was measured by total organic carbon analyzer (TOC-L CPH/CPN, H544356, SHIMADZU, Japan).
3. Results and discussion 3.1 Morphology and Structure We designed and fabricated the BM-PANNM on the basis of four principles: (1) highly tortuous open-porous structure and large surface area structure, (2) stable physical and chemical structure, (3) excellent photocatalytic capacity, (4) the membranes can be reused. The first principles and second principles were satisfied by electrospinning technique28. To meet the other two requirements, we fabricate the eletrospun PANNM coordinated with copper-iron metal, which could produce the hydroxyl radicals. Firstly, the SEM images of PANNM and BM-PANNM are demonstrated in Figure 2a and 2b. It indicated that the physical structure of the membranes was stable after modification. It was seen from Figure 2c and 2d that PANNM and BM-PANNM possessed an irregular nonwoven structure with an average diameter of 580 nm and 600 nm, which was much smaller than PAN fiber (about 17 μm). It was necessary to further study the porous structure of the PANNM for its attractive fiber morphologies. The N2 adsorption-desorption isotherms of PANNM and BM-PANNM were shown in Figure 2e. It demonstrated the type IV isotherm and a H3 hysteresis loop,
indicating the characteristics of mesopores of BM-PANNM35-36. Brunauer-EmmettTeller (BET) surfaces areas analysis indicated that the PANNM and BM-PANNM exhibited similar surface areas of 5.52 and 5.34 m2 g-1, respectively. In order to evaluate porous structure of membranes, Frenkel-Halsey-Hill (FHH) theory was employed to perform the fractal analysis. Figure 2f indicated the FHH plots possessed two different linear regions. The calculated fractal dimension (D) of PANNM and BM-PANNM were 2.64 and 2.58, respectively, confirming the irregular porous feature, as demonstrated by FE-SEM observations37-38. The STEM and EDX were employed to examine the element distribution of BMPANNM. It was proved from Figure 3a-3e that the Fe and Cu elements were homogeneously distributed throughout the PAN membranes, as well as C and O elements. Based on the measurement of EDX (Figure 3f), it was demonstrated that the high metal content of Fe and Cu were achieved after chemical modification of membranes39. The water wetting properties of PANNM after modification was improved for the increasing of substantial amino groups, as demonstrated in Figure 3g. The infiltration time of 26 s of BM-PANNM was obviously shorter than that of PANNM (36 s). The chemical structure change of PANNM, AO-PANNM and BMPANNM was evaluated by FT-IR spectra in Figure 3h, respectively. There is a strong absorbance at 2244 cm−1 of PANNM corresponding to the stretching vibration of C≡N group. After modification, the intensity of C≡N group of AO-PANNM decreased with hydroxylamine, indicating the chemical reaction between C≡N group and hydroxylamine hydrochloride18. The new peaks of AO-PANNM appeared at 3500-3000 cm−1, 1662 cm−1 and 931 cm−1 attributed to the stretching vibration of N–H, O–H, C=N and N–O groups in amidoxime, respectively18. These results indicated the formation of amidoxime groups. Moreover, the C–N characteristic bands of BM-PANNM at 1120
cm−1 and 1038 cm−1 revealed that Fe3+ ions and Cu2+ ions were coordinated with amino groups18. 3.2 Catalytic property Figure 4a indicated the effect of H2O2 initial concentration on degradation process of reactive blue 19. The D% values was increased with increasing reaction time. It was noted that the D% values was enhanced at higher H2O2 concentration. This is because H2O2 molecules could not generate enough ∙OH radicals at lower H2O2 concentration, which caused the limited decoloration of dyes. When the H2O2 concentration was increased, the dyes could be sufficiently degraded. Thus, the higher D% values were obtained. To examine the stability of BM-PANNM as photocatalyst at various pH values, the D% values at different pH values were investigated. Figure 4b showed that the D% values varied significantly with pH values. The degradation reaction of reactive blue 19 could be occurred under acidic or alkaline conditions. It should be pointed out that the highest D% value was achieved when pH was 6. This is because the precipitation was produced by the hydrolysis of Fe ions and Cu ions at alkaline medium. In contrast, the stronger capture between hydrogen ion and hydroxyl radicals was obtained at acidic medium, and the degradation of dyes was improved. Figure 4c showed that the concentration of reactive blue 19 decreased under light irradiation in the presence of H2O2 with different PANNM. It is worth pointing out that the reactive blue 19 degraded very slowly by using Cu-PANNM as catalyst. Fe3+ ions presented better catalytic activity than Cu2+ ions loaded on PANNM under same conditions. For bimetal catalysts, the catalytic activity could be usually improved for the creation of defects and novel active sites. Thus, the BM-PANNM exhibited an excellent catalytic activity at the same time. It was shown from Figure 4d that the degradation degree was increased with increased intensity of light. This was because the stronger light power
caused the photolysis of H2O2 in the reaction system and improved the production of the ∙OH radicals. Additionally, the aggregation of the dyes was broken, and the photocatalytic degradation of dye molecules was improved40. The degradation tests were carried out with four initial concentrations of reactive blue 19, as shown in Figure 5a. It showed that the decoloration of reactive blue 19 increased with increased light time. The higher decoloration rate of dyes was obtained at the lower initial concentration of the dye. Figure 5b and Table 1 showed that the reaction process of reactive blue 19 well followed the pseudo-first-order model. The degradation rate (k) decreased with increasing initial concentration of dyes. These results indicated that the initial concentration of dyes had an important influence on the degradation process under the same conditions. Figure 5c showed that the absorbance at 255 nm and 590 nm was decreased with increased reaction time for reactive blue 19. The characteristic peaks at 255 nm was attributed to the anthraquinone structure of the anthraquinone dyes. The characteristic peaks at 592 nm was attributed to chromophore structure of dyes. The peaks at 255 nm and 590 nm were decreased, indicating that the anthraquinone and rings parts of dyes were completely destroyed after 60 min41. The color of reactive blue 19 became from blue to colorless. Also, the ESR technique was used to detect the generated •OH radicals for BM-PANNM. It was shown from Figure 5d that the TOCR% values increased with increasing reaction time. There is a linear relationship between TOCR% value and reaction time. It is worth noting that the TOCR% value reaches 60.25% within 60 min. It was proved that reactive blue 19 molecules can be destructed and degraded for the production of •OH radicals, which was detected by the ESR technique. The intensity peaks of 1:2:2:1 were obtained during light irradiation, which was due to the appearance of DMPO-•OH adducts (insert, in Figure 5d). The peaks were intensified with increasing irradiation time, confirming the production of
•OH radicals for BM-PANNM18. To further evaluate the photocatalytic activity of BMPANNM for other dyes, the UV-Vis spectra of azo dyes including reactive red 195 and acid orange 7 were measured, respectively. The results were shown in Figure 5e and 5f. It indicated that the characteristic peaks of reactive red 195 in UV region (296 nm) and in visible region (523 nm) decreased with increased reaction time. The color became from red to colorless. The characteristic peaks of acid orange 7 at 303 nm and 480 nm exhibited the same trend. The color became from yellow to colorless. Usually, the characteristic peaks at 400−800 nm represent the n → π* transition of the azo and hydrazine groups, while the characteristic peaks at 200−400 nm represent the n → π* transition of benzene and the naphthalene ring42. Thus, the characteristic peaks in visible region was measured to observe the decolorization process of dyes. The characteristic peaks in the UV region was measured to observe the broken process of its aromatic groups. These results implied that the azo dyes such as reactive red 195 and acid orange 7could be degraded and decolored by the catalytic activity of BM-PANNM, which is similar to that of anthraquinone dyes (reactive blue 19). 3.3 Cyclic catalytic performance To evaluate the economic cost of BM-PANNM, it is necessary to study its reusability. The reuse evaluation of BM-PANNM was performed as demonstrated in Figure 6a. Fantastically, after 5 cycles, the BM-PANNM still exhibited good photocatalytic performance for reactive blue 19. Figure 6b demonstrated the surface morphology of BM-PANNM with no noticeable difference in the structure. These results indicated that BM-PANNM could improve its practical application for photocatalytic degradation of anionic dyes.
4. Conclusions
In summary, we have described a novel approach for fabrication of copper-iron bimetal modified polyacrylonitrile (PAN) nanofibrous membranes by electrospinning technology. The as-prepared bimetal modified PAN nanofibrous membranes exhibited the prominent photocatalytic capacity towards reactive blue 19, reactive red 195 and acid orange 7 (＞99.99%) within 60 min, and good reusability. We predict that such PAN nanofibrous membranes will provide a versatile platform for further exploring nano-size photocatalyst fibrous membranes for degradation of dyes.
Author information Corresponding Authors *E-mail: [email protected]. ORCID Yang Si: 0000-0001-7209-6206 Notes The authors declare no competing financial interest.
Acknowledgements This research is supported by Fundamental Research Funds for the Central Universities (No.
XDJK2019B016)
and
Natural
Science
Foundation
of
Chongqing
(cstc2016jcyjA0210).
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Table 1 Results from linear regression of the plots for the pseudo-first-order kinetic equation. Dye (mg L-1)
Rate equation
k (1/min)
R2
25 50 75 100
ln(C0/Ct) = 0.0574t + 0.1788 ln(C0/Ct) = 0.0312t + 0.1094 ln(C0/Ct) = 0.0235t + 0.0722 ln(C0/Ct) = 0.0148t + 0.0511
0.0574 0.0312 0.0235 0.0148
0.9958 0.9978 0.9983 0.9948
Scheme 1. Scheme of the design, processing and photocatalytic functions of BM-PANNM.
Reactive blue 19:
Reactive red 195:
Acid orange 7:
Scheme 2. Chemical structure of three dyes.
Figure 1. Schematic diagram of photoreaction system.
Figure 2. SEM images of (a) PANNM, (b) BM-PANNM. (c) Diameter distribution of PANNM. (d) Diameter distribution of BM-PANNM. (e) N2 adsorption-desorption isotherms of PANNM before and after modification. (f) FHH plots of ln(V/Vmono) against ln(ln(p0/p)) reconstructed from the relevant N2 adsorption isotherms.
Figure 3. (a-e) STEM images of BM-PANNM with corresponding elemental mapping images of (b) C, (c) O, (d) Fe, and (e) Cu, respectively, on a single fiber. (f) The EDX spectrum of BMPANNM. (g) Dynamic photographic measurements of water permeation on the surface of PANNM and BM-PANNM. (h) FT-IR spectra of PANNM and BM-PANNM.
Figure 4. (a) Effect of H2O2 initial concentration on degradation of reactive blue 19. (b) Degradation rate of reactive blue 19 with BM-PANNM at different pH values. (c) Degradation process of reactive blue 19 with different fibrous membranes. (d) Effect of irradiation intensity on the degradation process.
Figure 5. (a) The degradation performance of BM-PANNM towards reactive blue 19 at different initial concentration. (b) The kinetic linear fitting curves of reactive blue 19 degradation performance of BM-PANNM. (c) UV-vis absorption spectra of reactive blue 19 at time intervals. (d) The TOCR% values of reactive blue 19 solution at different time, insets: ESR signals of the DMPO-•OH adducts for BM-PANNM. (e) and (f) UV-vis absorption spectra of reactive red 195 and acid orange 7 at time intervals.
Figure 6. (a) Reversibility capacity of BM-PANNM for 5 cycles. (b) SEM images of BM-PANNM after 5 cycles.
Graphical Abstract
Abstract: The copper-iron bimetal modified PAN nanofibrous membranes possess the integrated properties of ultrathin fiber diameter, large surface area and high porosity, which can release hydroxyl radicals under light irradiation and exhibit an excellent photocatalytic degradation capacity of dyes, and good reusability performance. Keywords: Bimetal, Polyacrylonitrile, Membranes, Photocatalytic, Degradation, Dyes Cover Art:
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:
S0021-9797(19)31197-X https://doi.org/10.1016/j.jcis.2019.10.018 YJCIS 25512
To appear in:
Journal of Colloid and Interface Science
Received Date: Revised Date: Accepted Date:
18 August 2019 1 October 2019 6 October 2019
Please cite this article as: S. Yi, S. Sun, Y. Zhang, Y. Zou, F. Dai, Y. Si, Scalable Fabrication of Bimetal Modified Polyacrylonitrile (PAN) Nanofibrous Membranes for Photocatalytic Degradation of Dyes, Journal of Colloid and Interface Science (2019), doi: https://doi.org/10.1016/j.jcis.2019.10.018
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© 2019 Published by Elsevier Inc.
Scalable Fabrication of Bimetal Modified Polyacrylonitrile (PAN) Nanofibrous Membranes for Photocatalytic Degradation of Dyes Shixiong Yi,† Sheng Sun,† Yuangsong Zhang,† Yushan Zou,† Fangyin Dai,† Yang Si,*,‡ †College
of Textile and Garment, Southwest University, Chongqing, 400715, P.R.
China ‡Innovation
Center for Textile Science and Technology, Donghua University, Shanghai
200051, China.
ABSTRACT Fabricating nanostructured fibrous membranes as photocatalyst would facilitate the effective treatment of dyeing effluents. However, creating such photocatalytic nanofibrous membranes has proven tremendously challenging. In this work, we report a scalable strategy to prepare copper-iron bimetal modified polyacrylonitrile (PAN) nanofibrous membranes by electrospinning technology. The resultant PAN membranes exhibit an ultra-fine fiber diameter (600 nm), large surface area (5.34 m2 g-1) and excellent photocatalytic capacity for reactive blue 19, reactive red 195 and acid orange 7 ( ＞ 99.99%) within 60 min. The successful synthesis of such intriguing materials could provide a versatile platform for further exploring nano-sized photocatalyst fibrous membranes for degradation of dyes. Keywords: Bimetal, Polyacrylonitrile, Membranes, Photocatalytic, Degradation, Dyes
1. Introduction Dyeing wastewater from the textile factory is the main producer of contaminated aqueous waste streams1-4. In recent years, Fenton technology was used as highly effective method for the treatment of these dyes5-8. The heterogeneous Fenton catalysts were prepared by coordination of Fe3+ ions on the materials9-12. In previous reports, we have noted that polyacrylonitrile (PAN) fiber can be easily grafted by nitrile groups13. Thus, the PAN yarns could be grafted to construct the photocatalysts through Fe3+
16
ions immobilization. In previous studies, Han et al. created the copper-iron bimetal modified PAN fibers for photocatalytic degradation of anionic dyes. The catalytic activity of Fe3+ ions catalysts could be improved by adding other metals17-20. But drawbacks in the use of such materials are found to be the big fiber diameter and low surface area of fibers, which limit the catalytic property. Therefore, some attempts have been made to develop the nanofibrous membranes as catalyst21-24. Electrospun nanofibers, as advanced fibrous materials, possess extra-fine fiber diameters, a large surface area and surface functional groups25-28. These fabricated functionalized nanofibrous membranes could be used for different industrial application such as adsorption, filtration, sensing and photocatalytic degradation29-32. In this work, we demonstrate a scalable methodology to construct copper-iron bimetal modified PAN nanofibrous membranes (BM-PANNM) by electrospinning technology, as shown in Scheme 1. In our study, nanofibrous membranes are reconstructed. The prepared BM-PANNMs possessed a large surface area (5.34 m2 g1)
and excellent photocatalytic capacity for reactive blue 19, reactive red 195 and acid
orange 7 ( ＞ 99.99%) within 60 min. The fabrication of such PAN nanofibrous membranes may provide a versatile platform for further exploring nano-size photocatalyst fibrous membrane materials for degradation of dyes.
2. Materials and methods 2.1 Materials and Reagents The PAN yarns were supplied from Chongqing Biebei Yihe Cultural Co., China. The anionic anthraquinone dyes (reactive blue 19) and azo dyes (reactive red 195 and acid orange 7) were provided by Chengdu Xinda chemicals Co., China. The chemical structures of dyes were presented in Scheme 2. Hydroxylamine hydrochloride, N, Ndimethylformamide (DMF), ferric chloride, sodium hydroxide, copper sulfate and hydrogen peroxide were obtained from Aldrich Co. (MO, USA). Double distilled was used throughout this study. 2.2 Fabrication of PAN Nanofibrous Membranes (PANNM) The PAN yarn was dissolved in DMF by stirring at 25 ℃ for 24 h. Then, the asprepared 15 wt% PAN solution was transferred to the syringe needle. The electrospinning experiment was carried out. The voltage of 20 kV was supplied. The receiving distance from spinneret to collector was 15 cm. 2.3 Chemical Modification of PANNM PAN nanofibrous membranes were amidoximated in the presence of hydroxylamine hydrochloride and sodium hydroxide at 2.0 h at 65℃. Afterward, the amidoximated PAN nanofibrous membranes (AO-PANNM) was washed thoroughly and dried. The obtained AO-PANNM was put in the metal salt solutions of copper sulfate and ferric chloride for producing the copper-iron bimetal modified PAN nanofibrous membranes (BM-PANNM). The concentrations of the metal ions were 0.10 mol L−1. 2.4 Catalytic Procedure and Analysis 0.20 g of BM-PANNM was immersed into 50 mL test solutions containing given concentration of dyes and H2O2 in the vessel at room temperature. The solution in the
vessel was exposed to the irradiation of UV 365 nm lamp in the reaction system (shown in Figure 1). At time intervals, 2 mL of the dye aqueous solution was taken off. Then, the dye concentration was calculated at λmax of dyes used (590 nm for reactive blue 19, 523 nm for reactive red 195, and 480 nm for acid orange 7). The degradation rate of the dye was expressed as equation (1). To evaluate the degradation kinetics, the degradation process of dye solutions at four initial concentrations were performed, respectively. Finally, the change of dye concentration was observed at different reaction time. The pseudo-first-order kinetic equation was applied to fit experimental data. The kinetic equation is demonstrated as equation (2) 33-34. The total organic carbon (TOC) removal percentage (TOCR%) of the test solution was calculated as equation (3). 𝐷% = (1 − Ct/C0) × 100% ln(C0/Ct) = kt + const
(1) (2)
TOCR% = (1−TOCt/TOC0) × 100%
(3)
where C0 and Ct are the concentrations of the solution at initial and interval time, 𝐷% is the degradation rate of dyes, k is the corresponding degradation rate, and t is the degradation time, TOC0 and TOCt are the TOC values (mg L−1) at initial and interval time, respectively. 2.5 Reusability Measurements To evaluate the economic cost, the BM-PANNM were used and washed to remove the adsorbed dyes. Subsequently, the BM-PANNM were dried and reused for next cycle by using the same experimental method. 2.6 Characterization Surface morphologies of PANNM and BM-PANNM were observed by using a Hitachi S-4800 SEM system. The Fourier transform infrared (FT-IR) spectra of fibrous membranes were obtained by a Magna-560 spectrometer (Thermo Nicolet Co., USA).
The surface area was determined using a Micromeritics ASAP2460 surface area analyzer. The UV-visible spectra of dyes were determined using a scanning UV-visible spectrophotometer (Jasco model V-530, Japan). Electron spin resonance (ESR) signals were determined on a JES-FA ESR spectrometer (JEOL, Tokyo, Japan). Scanning transmission electron microscopy (STEM) and energy-dispersive X-ray (EDX) mappings were acquired on a JEOL 2500 at 200 kV with a Thermo Electron Pioneer EDX detector. The total organic carbon of the test dye solution was measured by total organic carbon analyzer (TOC-L CPH/CPN, H544356, SHIMADZU, Japan).
3. Results and discussion 3.1 Morphology and Structure We designed and fabricated the BM-PANNM on the basis of four principles: (1) highly tortuous open-porous structure and large surface area structure, (2) stable physical and chemical structure, (3) excellent photocatalytic capacity, (4) the membranes can be reused. The first principles and second principles were satisfied by electrospinning technique28. To meet the other two requirements, we fabricate the eletrospun PANNM coordinated with copper-iron metal, which could produce the hydroxyl radicals. Firstly, the SEM images of PANNM and BM-PANNM are demonstrated in Figure 2a and 2b. It indicated that the physical structure of the membranes was stable after modification. It was seen from Figure 2c and 2d that PANNM and BM-PANNM possessed an irregular nonwoven structure with an average diameter of 580 nm and 600 nm, which was much smaller than PAN fiber (about 17 μm). It was necessary to further study the porous structure of the PANNM for its attractive fiber morphologies. The N2 adsorption-desorption isotherms of PANNM and BM-PANNM were shown in Figure 2e. It demonstrated the type IV isotherm and a H3 hysteresis loop,
indicating the characteristics of mesopores of BM-PANNM35-36. Brunauer-EmmettTeller (BET) surfaces areas analysis indicated that the PANNM and BM-PANNM exhibited similar surface areas of 5.52 and 5.34 m2 g-1, respectively. In order to evaluate porous structure of membranes, Frenkel-Halsey-Hill (FHH) theory was employed to perform the fractal analysis. Figure 2f indicated the FHH plots possessed two different linear regions. The calculated fractal dimension (D) of PANNM and BM-PANNM were 2.64 and 2.58, respectively, confirming the irregular porous feature, as demonstrated by FE-SEM observations37-38. The STEM and EDX were employed to examine the element distribution of BMPANNM. It was proved from Figure 3a-3e that the Fe and Cu elements were homogeneously distributed throughout the PAN membranes, as well as C and O elements. Based on the measurement of EDX (Figure 3f), it was demonstrated that the high metal content of Fe and Cu were achieved after chemical modification of membranes39. The water wetting properties of PANNM after modification was improved for the increasing of substantial amino groups, as demonstrated in Figure 3g. The infiltration time of 26 s of BM-PANNM was obviously shorter than that of PANNM (36 s). The chemical structure change of PANNM, AO-PANNM and BMPANNM was evaluated by FT-IR spectra in Figure 3h, respectively. There is a strong absorbance at 2244 cm−1 of PANNM corresponding to the stretching vibration of C≡N group. After modification, the intensity of C≡N group of AO-PANNM decreased with hydroxylamine, indicating the chemical reaction between C≡N group and hydroxylamine hydrochloride18. The new peaks of AO-PANNM appeared at 3500-3000 cm−1, 1662 cm−1 and 931 cm−1 attributed to the stretching vibration of N–H, O–H, C=N and N–O groups in amidoxime, respectively18. These results indicated the formation of amidoxime groups. Moreover, the C–N characteristic bands of BM-PANNM at 1120
cm−1 and 1038 cm−1 revealed that Fe3+ ions and Cu2+ ions were coordinated with amino groups18. 3.2 Catalytic property Figure 4a indicated the effect of H2O2 initial concentration on degradation process of reactive blue 19. The D% values was increased with increasing reaction time. It was noted that the D% values was enhanced at higher H2O2 concentration. This is because H2O2 molecules could not generate enough ∙OH radicals at lower H2O2 concentration, which caused the limited decoloration of dyes. When the H2O2 concentration was increased, the dyes could be sufficiently degraded. Thus, the higher D% values were obtained. To examine the stability of BM-PANNM as photocatalyst at various pH values, the D% values at different pH values were investigated. Figure 4b showed that the D% values varied significantly with pH values. The degradation reaction of reactive blue 19 could be occurred under acidic or alkaline conditions. It should be pointed out that the highest D% value was achieved when pH was 6. This is because the precipitation was produced by the hydrolysis of Fe ions and Cu ions at alkaline medium. In contrast, the stronger capture between hydrogen ion and hydroxyl radicals was obtained at acidic medium, and the degradation of dyes was improved. Figure 4c showed that the concentration of reactive blue 19 decreased under light irradiation in the presence of H2O2 with different PANNM. It is worth pointing out that the reactive blue 19 degraded very slowly by using Cu-PANNM as catalyst. Fe3+ ions presented better catalytic activity than Cu2+ ions loaded on PANNM under same conditions. For bimetal catalysts, the catalytic activity could be usually improved for the creation of defects and novel active sites. Thus, the BM-PANNM exhibited an excellent catalytic activity at the same time. It was shown from Figure 4d that the degradation degree was increased with increased intensity of light. This was because the stronger light power
caused the photolysis of H2O2 in the reaction system and improved the production of the ∙OH radicals. Additionally, the aggregation of the dyes was broken, and the photocatalytic degradation of dye molecules was improved40. The degradation tests were carried out with four initial concentrations of reactive blue 19, as shown in Figure 5a. It showed that the decoloration of reactive blue 19 increased with increased light time. The higher decoloration rate of dyes was obtained at the lower initial concentration of the dye. Figure 5b and Table 1 showed that the reaction process of reactive blue 19 well followed the pseudo-first-order model. The degradation rate (k) decreased with increasing initial concentration of dyes. These results indicated that the initial concentration of dyes had an important influence on the degradation process under the same conditions. Figure 5c showed that the absorbance at 255 nm and 590 nm was decreased with increased reaction time for reactive blue 19. The characteristic peaks at 255 nm was attributed to the anthraquinone structure of the anthraquinone dyes. The characteristic peaks at 592 nm was attributed to chromophore structure of dyes. The peaks at 255 nm and 590 nm were decreased, indicating that the anthraquinone and rings parts of dyes were completely destroyed after 60 min41. The color of reactive blue 19 became from blue to colorless. Also, the ESR technique was used to detect the generated •OH radicals for BM-PANNM. It was shown from Figure 5d that the TOCR% values increased with increasing reaction time. There is a linear relationship between TOCR% value and reaction time. It is worth noting that the TOCR% value reaches 60.25% within 60 min. It was proved that reactive blue 19 molecules can be destructed and degraded for the production of •OH radicals, which was detected by the ESR technique. The intensity peaks of 1:2:2:1 were obtained during light irradiation, which was due to the appearance of DMPO-•OH adducts (insert, in Figure 5d). The peaks were intensified with increasing irradiation time, confirming the production of
•OH radicals for BM-PANNM18. To further evaluate the photocatalytic activity of BMPANNM for other dyes, the UV-Vis spectra of azo dyes including reactive red 195 and acid orange 7 were measured, respectively. The results were shown in Figure 5e and 5f. It indicated that the characteristic peaks of reactive red 195 in UV region (296 nm) and in visible region (523 nm) decreased with increased reaction time. The color became from red to colorless. The characteristic peaks of acid orange 7 at 303 nm and 480 nm exhibited the same trend. The color became from yellow to colorless. Usually, the characteristic peaks at 400−800 nm represent the n → π* transition of the azo and hydrazine groups, while the characteristic peaks at 200−400 nm represent the n → π* transition of benzene and the naphthalene ring42. Thus, the characteristic peaks in visible region was measured to observe the decolorization process of dyes. The characteristic peaks in the UV region was measured to observe the broken process of its aromatic groups. These results implied that the azo dyes such as reactive red 195 and acid orange 7could be degraded and decolored by the catalytic activity of BM-PANNM, which is similar to that of anthraquinone dyes (reactive blue 19). 3.3 Cyclic catalytic performance To evaluate the economic cost of BM-PANNM, it is necessary to study its reusability. The reuse evaluation of BM-PANNM was performed as demonstrated in Figure 6a. Fantastically, after 5 cycles, the BM-PANNM still exhibited good photocatalytic performance for reactive blue 19. Figure 6b demonstrated the surface morphology of BM-PANNM with no noticeable difference in the structure. These results indicated that BM-PANNM could improve its practical application for photocatalytic degradation of anionic dyes.
4. Conclusions
In summary, we have described a novel approach for fabrication of copper-iron bimetal modified polyacrylonitrile (PAN) nanofibrous membranes by electrospinning technology. The as-prepared bimetal modified PAN nanofibrous membranes exhibited the prominent photocatalytic capacity towards reactive blue 19, reactive red 195 and acid orange 7 (＞99.99%) within 60 min, and good reusability. We predict that such PAN nanofibrous membranes will provide a versatile platform for further exploring nano-size photocatalyst fibrous membranes for degradation of dyes.
Author information Corresponding Authors *E-mail: [email protected]. ORCID Yang Si: 0000-0001-7209-6206 Notes The authors declare no competing financial interest.
Acknowledgements This research is supported by Fundamental Research Funds for the Central Universities (No.
XDJK2019B016)
and
Natural
Science
Foundation
of
Chongqing
(cstc2016jcyjA0210).
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Table 1 Results from linear regression of the plots for the pseudo-first-order kinetic equation. Dye (mg L-1)
Rate equation
k (1/min)
R2
25 50 75 100
ln(C0/Ct) = 0.0574t + 0.1788 ln(C0/Ct) = 0.0312t + 0.1094 ln(C0/Ct) = 0.0235t + 0.0722 ln(C0/Ct) = 0.0148t + 0.0511
0.0574 0.0312 0.0235 0.0148
0.9958 0.9978 0.9983 0.9948
Scheme 1. Scheme of the design, processing and photocatalytic functions of BM-PANNM.
Reactive blue 19:
Reactive red 195:
Acid orange 7:
Scheme 2. Chemical structure of three dyes.
Figure 1. Schematic diagram of photoreaction system.
Figure 2. SEM images of (a) PANNM, (b) BM-PANNM. (c) Diameter distribution of PANNM. (d) Diameter distribution of BM-PANNM. (e) N2 adsorption-desorption isotherms of PANNM before and after modification. (f) FHH plots of ln(V/Vmono) against ln(ln(p0/p)) reconstructed from the relevant N2 adsorption isotherms.
Figure 3. (a-e) STEM images of BM-PANNM with corresponding elemental mapping images of (b) C, (c) O, (d) Fe, and (e) Cu, respectively, on a single fiber. (f) The EDX spectrum of BMPANNM. (g) Dynamic photographic measurements of water permeation on the surface of PANNM and BM-PANNM. (h) FT-IR spectra of PANNM and BM-PANNM.
Figure 4. (a) Effect of H2O2 initial concentration on degradation of reactive blue 19. (b) Degradation rate of reactive blue 19 with BM-PANNM at different pH values. (c) Degradation process of reactive blue 19 with different fibrous membranes. (d) Effect of irradiation intensity on the degradation process.
Figure 5. (a) The degradation performance of BM-PANNM towards reactive blue 19 at different initial concentration. (b) The kinetic linear fitting curves of reactive blue 19 degradation performance of BM-PANNM. (c) UV-vis absorption spectra of reactive blue 19 at time intervals. (d) The TOCR% values of reactive blue 19 solution at different time, insets: ESR signals of the DMPO-•OH adducts for BM-PANNM. (e) and (f) UV-vis absorption spectra of reactive red 195 and acid orange 7 at time intervals.
Figure 6. (a) Reversibility capacity of BM-PANNM for 5 cycles. (b) SEM images of BM-PANNM after 5 cycles.
Graphical Abstract
Abstract: The copper-iron bimetal modified PAN nanofibrous membranes possess the integrated properties of ultrathin fiber diameter, large surface area and high porosity, which can release hydroxyl radicals under light irradiation and exhibit an excellent photocatalytic degradation capacity of dyes, and good reusability performance. Keywords: Bimetal, Polyacrylonitrile, Membranes, Photocatalytic, Degradation, Dyes Cover Art:
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: