Hao Zhangabc,
Weihua Liuc,
Feng Tianc,
Zhongfeng Tang*bc and
Haitao Lin*a
aCollege of Biological and Chemical Engineering, Guangxi University of Science and Technology, Liuzhou, Guangxi, China 545006. E-mail: lhthost@163.com
bState Key Laboratory for Modification of Chemical Fibers and Polymer Materials, Donghua University, Shanghai 201620, China
cShanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai, China 201800. E-mail: tangzhongfeng@sinap.ac.cn; Fax: +86-21-39194681; Tel: +86-21-39194681
First published on 11th November 2019
Treating water that has been polluted with chemical dyes is an important task related to water resources. Advanced oxidation processes are highly efficient for the destruction of organic contaminants. In this study, a Co3O4/graphene oxide (GO)/polyacrylonitrile (PAN) filter membrane was prepared through hydrothermal synthesis followed by vacuum filtration. The samples were characterised using different methods. The results showed that the Co3O4/GO sheets securely entered the voids of the PAN nanofibres. The Co3O4/GO/PAN filter membrane demonstrated the effective degradation of the organic dye Orange II, with a degradation rate of 93.5949%. The degradation rate remained at a high level after five cycles. The Co3O4/GO/PAN filter membrane has huge potential for application in industrial dye wastewater treatment.
Among these methods, there has been increased interest in AOPs in the research and development (R&D) of wastewater treatment technologies. AOPs can be broadly defined as aqueous phase oxidation methods based on the intermediary of highly reactive species, such as (primarily, but not exclusively) hydroxyl radicals, in the mechanisms leading to the destruction of the target pollutant. During the past 30 years, there have been immense R&D involving AOPs, particularly for two reasons, namely (i) the diversity of technologies involved and (ii) the areas of potential application.5 Although water and wastewater treatment is by far the most common area for R&D, AOPs have also found applications as diverse as groundwater treatment,6 soil remediation,7 municipal wastewater sludge conditioning,8 ultrapure water production,9 volatile organic compound treatment,10 and odour control.11
The Fenton reagent is a typical system used with AOPs because it results in the generation of highly reactive hydroxyl radicals through the coupling of a transition metal with an oxidant. The fundamental Fenton reaction initially involved the addition of dilute hydrogen peroxide (H2O2) to a degassed solution of iron(II).12 The reaction provides a very powerful oxidizing agent and is used in several industrial applications for the treatment of contaminated wastewater. However, several significant limitations exist that require resolution: the reaction is effective only at pH values close to 3. At a higher pH value of 6, the efficiency of the Fenton reagent is dramatically decreased due to iron speciation and precipitation.13
The system of generating sulfate radicals through the transition metal (e.g., cobalt) decomposition of peroxymonosulfate is a modification of the Fenton reagent. Some research shows that sulfate radicals, generated by the conjunction of transition metals with peroxymonosulfate, are even more efficient oxidants than hydroxyl radicals, and experimentation revealed that the reactivity of the sulfate radicals (such as Co/PMS) was sustained at high pH values, where the efficiency of the Fenton reagent is known to be diminished.14,15 Attributes such as efficiency, wide applicability, cost-effectiveness, and no need for an extra energy supply (such as UV) are attractive to the water purification industry. However, when transition metals are formulated into nanometal oxide powders to increase their specific surface area and subsequently improve the catalytic efficiency, this causes harm to the natural environment.16 Therefore, an environmentally friendly solution that has been utilized is their immobilization on the surface of other materials for practical uses without affecting the degradation efficiency.
Herein, graphene oxide (GO) was chosen as the medium used to fix Co nanoparticles to polyacrylonitrile (PAN) electrospun fibres to prepare Co3O4/GO/PAN nanofiber membranes. There are many advantages to using graphene oxide (GO), with a one-atom-thick 2D individual sheet structure.17 Compared with graphene, GO is hydrophilic and can achieve immense interfacial contact with metal oxide nanoparticles because of its various oxygen-containing functional groups, such as hydroxyls, epoxides, carbonyls, and carboxyls.18 GO also exhibits a high specific surface area. Therefore, aggregation of nanoparticles can be prevented by loading the nanoparticles onto GO, which provides a large specific surface area for composites19 (there are many studies describing the synergistic catalysis of metal oxides and graphene in S1†). Electrospun polyacrylonitrile (PAN) nanofiber mats exhibit high porosity, stable chemical properties, low density, large specific surface area, good flexibility, and toughness.20 Polar nitrile groups contained in PAN molecular chains can also interact with the oxygen-containing groups located on GO sheets. Thus, PAN nanofiber mats used as the basic material to prepare Co3O4/GO/PAN combine the advantages of PAN and GO.
In this study, Co3O4/GO sheets were prepared by loading Co3O4 nanoparticles on GO sheets through hydrothermal synthesis. Then, they were deposited onto an electrospun PAN nanofiber mat through vacuum filtration. The obtained Co3O4/GO/PAN nanofiber membrane was characterized in detail through scanning electron microscopy (SEM)/Fourier transform infrared (FTIR) spectroscopy/wide angle X-ray diffraction (WAXD), and its catalytic performance was evaluated through Orange II dye degradation. Orange II reagent is generally used as a model substrate for aromatic azo dyes, which are common contaminants found in wastewater because of their wide industrial uses.21 Orange II is a widely used synthetic azo dye that does not decompose through biological methods and resists light irradiation and chemical oxidation. Additionally, the degradation mechanism was investigated. The developed Co3O4/GO/PAN filter membrane exhibited outstanding degradation ability and reusability.
The PAN nanofiber mat was obtained through electrospinning technology. PAN/DMF solution (10.0 wt%) was obtained by dissolving PAN in DMF with stirring at 80.0 °C. Electrospinning was performed at 16 kV voltage and 0.9 mL h−1 spinning rate. The distance between the electrode and the collector was maintained at 10.0 cm. PAN nanofiber was deposited on the aluminium foil of a rolling collector as a ground electrode at the speed of 500.0 rpm.
The prepared Co3O4/GO sheets were ultrasonically shaken with 400.0 mL deionized (DI) water for 4.0 h, forming the aqueous dispersion of Co3O4/GO sheets. The PAN nanofiber mat was cut into a circle and fixed into a vacuum filtration device, where the aqueous dispersion of Co3O4/GO fragments was poured. Vacuum filtration was maintained for 6.0 h, and then, the Co3O4/GO/PAN filter membrane was fabricated. The Co3O4/GO/PAN filter membrane was rinsed with DI water to remove the Co3O4/GO sheets that were not immobilized.
The Co3O4/GO/PAN filter membrane was used as a filter paper with a filtration flask to evaluate its usability, as shown in Fig. 1b. Orange II dye solution (50.0 mL) (0.1 mM) flowed through the filter by gravity in each recycle for approximately 30 min. The concentration of Orange II in the filtrate was measured by using an ultraviolet-visible (UV-Vis) spectrophotometer (Hitachi U-3900) at 486.0 nm for each run. The Co3O4/GO/PAN filter membrane was washed with distilled water and dried at 60 °C in a vacuum after each recycle.
Fig. 1 (a) A photo of the filtration device, and (b) a photo of the fabricated Co3O4/GO/PAN filter membrane. |
WAXD was conducted using a 16B Beamline equipped with a Rayonix-2M detector at the Shanghai Synchrotron Radiation Facility. The samples were systematically sandwiched between two pieces of Kapton tape for the preferred orientation effect. The chosen wavelength was 0.124 nm. The distance between sample and the detector was 12.8 cm, and the data were collected for 20 s for all the measurements.
The concentration of Orange II was determined through UV-Vis spectrophotometry using a U-3900 spectrophotometer (Hitachi, Japan). The concentration of Orange II was measured on the basis of the Beer–Lambert law:23.
A = abc |
Fig. 2 FTIR spectra of (a) electrospun PAN nanofibers, (b) Co3O4/GO, and (c) the Co3O4/GO/PAN filter membrane. |
The IR spectrum for Co3O4/GO showed a strong peak at 663 cm−1 because of the formation of Co3O4 spinel oxide,31 confirming the existence of Co3O4. Apparently, all the characteristic peaks from PAN, GO, and Co3O4 were observed in the IR spectrum for the Co3O4/GO/PAN filter membrane (curve c), indicating that the Co3O4/GO sheets were successfully attached to the PAN nanofiber. No new peaks were observed in the IR spectrum for Co3O4/GO/PAN. Co3O4/GO was loaded on the PAN nanofibers, but not through chemical bonds.
The chemical state of the elements in Co3O4/GO is provided by XPS measurement. The binding energies of samples obtained via XPS analyses were corrected for specimen charging by referencing the C 1s peak to 284.7 eV. Fig. 3a shows the C 1s, O 1s, and Co 2p peaks for Co3O4/GO. The Co 2p XPS spectra illustrated in Fig. 3b show two major peaks with binding energies at 780.5 eV and 786.1 eV, assigned to Co 2p3/2 and Co 2p1/2, respectively.
Fig. 3 (a) XPS survey spectrum for Co3O4/GO, (b) the Co 2p XPS spectrum for Co3O4/GO, (c) the C 1s XPS spectrum for GO and Co3O4/GO, and (d) the Raman spectrum for Co3O4. |
Two shake-up satellite peaks located around the main peaks at approximately 803.9 eV and 787.7 eV are characteristic of a Co3O4 phase I.29,32,33 The C 1s peak shown in Fig. 3c clearly indicates a considerable degree of oxidation, with three components that correspond to carbon atoms in different functional groups: the carbon in C–C, CC, and C–H at 284.7 eV.; the carbon in C–O at 286.3 eV; and the carbonyl carbon (CO) at 288.3 eV,34 respectively. This proves that carbon in the form of GO exists in the sample. Raman ID/IG ratios (where ID and IG are the D-band and G-band Raman intensities, respectively) are widely used to evaluate the quality of carbon materials. As shown in Fig. 3d, the relative intensity (ID/IG) ratio of the D (1345 cm−1) and G bands (1598 cm−1) for Co3O4/GO is 1.09, close to the graphene nanoribbons (1.0–1.5) synthesized,35 and it is also highly consistent with the reports in the literature.34
SR-WAXD is usually used to investigate the crystalline properties of materials. Fit2D (v12.077) software was used to process SR-WAXD patterns. 2D WAXD results for PAN nanofibers were obtained. The Co3O4/GO/PAN filter membrane is shown in Fig. 4a and b, and the integrated 1D WAXD patterns are shown in Fig. 4c. For PAN, a broad diffraction peak at 2θ = 28.26° was observed, which corresponded to the pristine graphite27 for Co3O4/GO/PAN. No new peak was observed, indicating that no new crystal structure was formed between PAN and GO in the Co3O4/GO/PAN membrane. Combined with the FTIR analysis, this indicated that the loading of the Co3O4/GO sheets on the PAN nanofibers occurred via physical interaction.
Fig. 4 2D-WAXD patterns of (a) electrospun PAN nanofibers, and (b) the Co3O4/GO/PAN filter membrane; (c) the WAXD spectra of 1D-WAXD, processed by FIT2D. |
Typical SEM images of PAN nanofibers, Co3O4/GO, and the Co3O4/GO/PAN filter membrane are shown in Fig. 5. PAN nanofibers had a diameter of approximately 300 nm and exhibited high permeability and porosity. Co3O4 particles were dispersed on the surface of the GO without obvious aggregation (Fig. 5c). Co3O4/GO sheets were entrapped inside the 3D network formed by the PAN nanofibers. The EDS spectra of the Co3O4/GO/PAN filter membrane (Fig. 6) confirmed the presence of cobalt and carbon, indicating that Co3O4 particles were successfully loaded onto the PAN nanofibre mat.
Fig. 6 An SEM image of (a) the Co3O4/GO/PAN filter membrane, the SEM-EDS mapping of (b) C elements and (c) Co elements, and (d) the corresponding chemical compositions. |
Fig. 6d shows the chemical composition of the tested samples. The content of carbon was 89%, which was higher than that of pure PAN (at approximately 68%). GO contributed to the high carbon content, signifying that the Co3O4/GO sheets were securely combined with PAN to enter the void of the PAN nanofibers. The results were consistent with those of FTIR and also the SEM morphology.
After 1 h degradation, no changes were observed in the Orange II dye solution that was in the presence of the PAN nanofiber mat. At the same time, the solution in the presence of the Co3O4/GO/PAN membrane essentially became colourless, as shown in Fig. 7a. This condition indicated that the PAN nanofiber did not contribute to the degradation of Orange II in the presence of Oxone. The Co3O4/GO/PAN membrane exhibited good catalytic activity for the degradation of Orange II. Hence, Co3O4/GO sheets degraded Orange II in water by advanced oxidation technology based on sulphate radicals.
Five recycling runs were performed to investigate the reusability of the Co3O4/GO/PAN membrane during practical applications. In the recycling experiments, Co3O4/GO/PAN or the PAN mat was used as the filter membrane in a setup with a filtration flask (Fig. 1), and 50.0 mL Orange II solution flowed through the membrane by gravity. Each run took approximately 30.0 min. The solution became colourless when the cycle completed.
The degradation rates for five runs are presented in Fig. 7b. The degradation rate was more than 90.0% for the first run, although the retention time was short for the solution, indicating the high catalytic activity of Co3O4/GO/PAN. The activity of the membrane slightly decreased compared with the fresh catalyst, which may be related to the loss of catalyst during washing and drying after each run. (The result of spectroscopic characterization of the membrane before and after degradation is shown in S2†). For pure PAN, no catalytic effect upon degradation nor absorption effect of Orange II was observed. PAN was used as the loading material to facilitate the recycling of the catalyst because of its high toughness and chemical stability.
Compared with common catalysts of metal oxide materials (such as Fe ions36), the Co3O4/GO/PAN filter membrane showed a high degradation rate and good reusability. The dye degradation process did not require an extra light source, rendering it suitable for many applications.
Fig. 8 A schematic diagram of the fabrication process and the proper working mechanism of Co3O4/GO/PAN. |
As shown in Fig. 9, seven-line spectra were observed, which were primarily assigned to the DMPO-SO4 formed by the addition of DMPO to SO4−˙.37,38 SO4−˙ radicals were formed within 3 min, and the concentration of [SO4−˙] radicals initially increased and then gradually decreased. The peak of the ESR spectrum slightly deviated from the data in the reported literature.39 This condition may be caused by the combination of signals from the addition product of DMPO to ·OH. Combined with the above characterization information, several reactions occurred during the degradation of Orange II, which are described as follows:15,29,40
Co2+ + HSO5− → Co3+ + SO4−˙ + OH− |
Co3+ + HSO5− → Co2+ + SO5−˙ + H+ |
SO4−˙ + H2O → SO42− + ·OH + H+ |
SO4−˙ + organic dyes [Orange II] → intermediate products → CO2 + H2O |
·OH + organic dyes [Orange II] → intermediate products → CO2 + H2O |
Fig. 9 ESR spectra of Co3O4/GO as a catalyst in the presence of PMS at room temperature at different times. |
The catalytic effect of the Co3O4/GO/PAN filter membrane was validated. The formed radicals were detected through ESR by using DMPO as a radical spin-trapping agent. DMPO (3 μL) was added to 3 mL organic dye (Orange II) with 1 mM PMS and 0.1 g L−1 Co3O4/GO sheets.
Degradation experiments showed that the filter membrane can effectively degrade organic dyes, such as Orange II, with a degradation rate of 93.5949%. The degradation rate remained at a high level after five cycles. The Co3O4/GO/PAN filter membrane showed high catalytic activity for the degradation of Orange II in water through advanced oxidation technology based on sulphate radicals. The Co3O4/GO/PAN filter membrane has great potential for application in industrial dye wastewater treatment.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c9ra06656j |
This journal is © The Royal Society of Chemistry 2019 |