As organic dyes are the main pollutants in water pollution, seeking effective removal solutions is urgent for humans and the environment. A novel environmentally friendly three-dimensional CoFe-LDHs (3D CoFe-LDHs) catalyst was synthesized by one-step hydrothermal method. Scanning electron microscopy, energy dispersive spectroscopy, Fourier transform infrared spectra, X-ray diffraction, X-ray photoelectron spectroscopy, Brunauer-Emmett-Teller technique as well as UV-Vis diffuse reflectance spectra were used to characterize the prepared samples. The experimental results revealed that 3D CoFe-LDHs exhibited a rapid decolorization of methyl orange and Rhodamine B by heterogeneous photo-Fenton process after reaching the adsorption equilibrium, and the final decolorization efficiency reached 91.18% and 93.56%, respectively. On the contrary, the decolorizing effect of 3D CoFe-LDHs on neutral blue was relatively weak. The initial concentrations of azo dyes, pH and H2O2 concentration affected the decolorization of dyes and the catalyst maintained excellent reusability and stability after reuse over five cycles. The quenching experiments found that •OH, •O2− and h+ were the main active substances and reaction mechanisms were further proposed. The study suggests that the synergistic effect of photocatalysis and Fenton oxidation process significantly improved the removal of azo dyes and the synthesized catalyst had potentially promising applications for difficult-to-biodegrade organic pollutants in wastewater.
3D CoFe-LDHs was fabricated and used in efficient removal of organic dyes by photo-Fenton process; the decolorization of MO is highly pH and H2O2 dependent.
Possible reaction mechanism and the stability of 3D CoFe-LDHs were investigated.
OH, •O2− and h+ formed during photo-Fenton reaction are effective to enhance the decolorization of MO.
With the rapid development of industrialization and urbanization, environmental problems have gradually attracted great public attention, and water pollution is the typical one. It is well known that the textile printing and dyeing industry relies heavily on organic dyes. If these toxic dyes are directly discharged without treatment, it will greatly harm the natural water environment. Especially, azo dyes and their degradation products are carcinogenic, teratogenic and mutagenic even at extremely low concentrations, which poses a huge threat to human health and ecosystems (Govindan et al. 2019; Sarma et al. 2019). However, the common methods for removing azo dyes, which include coagulation (Luo et al. 2019), adsorption (Guo et al. 2019), biodegradation (Liu et al. 2019) and membrane separation (Lin et al. 2015), are not complete solutions due to the complex molecular structure and biological resistance of azo dyes. Therefore, how to deal with the effluents containing dyes is looming ahead.
Fenton oxidation process is one of the most widely used methods of advanced oxidation processes (AOPs), the advantage of which is that strong oxidizing free radicals (•OH) are generated and used to treat pollutants in water and soil (Dos Santos et al. 2020). Nevertheless, the activity of the Fenton oxidation process is affected by the strict pH range, low Fe2+ regeneration and excessive ferric sludge. Based on the above, the heterogeneous photo-Fenton procedure occurred as an alternative method for its advantages of wider pH range, less iron-containing sludge and higher oxidation efficiency than the homogeneous photo-Fenton procedure (Dos Santos et al. 2020; Goncalves et al. 2020). In addition, heterogeneous photo-Fenton catalysts not only retain the advantages of traditional photo-Fenton catalysts, such as being cheap, non-toxic, and easy to separate, but also make up for the shortcomings of having few active sites, poor reusability, low catalytic efficiency and low mineralization efficiency.
In recent years, types of heterogeneous photo-Fenton catalysts have been used to remove various pollutants in water, such as CdS, FeS2, MnOx, MOFs, TiO2 and layered double hydroxides (LDHs) (Espinoza et al. 2016; Zhao et al. 2017; Xu et al. 2020). Among them, LDHs have increasingly been focused on as catalytic materials, separation and adsorption materials and biomedical materials due to their large specific surface area, high ion exchange capacity, excellent thermal stability, low preparation cost and environmental friendliness (Zong et al. 2018). Yet, the limited active sites of conventional LDHs powders/nanopowders make it difficult to mineralize difficult-to-biodegrade organic pollutants. Nowadays, owing to their higher specific surface area for adsorption, more abundant active sites for pollutant reaction, and a fractal architecture that facilitates an outstanding ability to trap incident light at any angle, three-dimensional LDHs (3D-LDHs) are considered to be ideal heterogeneous photo-Fenton catalysts. Zhang et al. (2019) reported 3D MgAl-LDHs showed excellent adsorption performance for Methyl Orange (MO) and Rhodamine B (RhB), and the removal mechanisms were surface adsorption and anion exchange. Xue et al. (2018) synthesized 3D NiAl-LDHs/porous anodic aluminum oxide templates that exhibited excellent decolorization of MO. Dai et al. (2019) investigated 3D ZnFe-LDHs composite sponge that exhibited high adsorption ability for As(V) and the maximum adsorption capacity reached 85.7 mg·g−1. Wang et al. (2018) built a Fenton system containing H2O2 and CuNiFe-LDHs to effectively mineralize phenol. However, a high Fe content will destroy the stable hydrotalcite structure and lead to the formation of magnetite particles (Ma et al. 2016). Hence, it is necessary to determine the content of Fe in LDHs. To the best of our knowledge, papers about the decolorization of azo dyes by individual 3D-LDHs without templates are rare.
Herein, the aims of the work are: (1) to synthesize an example of flower-like 3D CoFe-LDHs catalyst via a simple one-step hydrothermal method; (2) to characterize the morphological features, structures and physical and chemical properties of the as-prepared catalyst by SEM, XRD, FT-IR, XPS, BET and UV-Vis DRS; (3) to investigate the decolorization of MO, RhB and neutral blue, the influencing factors of decolorization and reaction mechanisms under the synergistic effect of photocatalytic and Fenton oxidation process. The results can lay a theoretical foundation for the practical application of 3D-LDHs in the field of difficult-to-biodegrade organic wastewater.
MATERIAL AND METHODS
Chemicals and materials
Co(NO3)2·6H2O, Fe(NO3)3·9H2O, NH4F, urea, and anhydrous methanol (≥99.5%) were of analytical grade and purchased from Shanghai Macklin Biochemical Co., Ltd. The azo dyes MO (≥85%), RhB (96%) and neutral blue (≥82%) were analytical reagents and purchased from Sinopharm Chemical Reagent Co., Ltd. All reagents were used directly without further purification.
Preparation of 3D CoFe-LDHs
3D CoFe-LDHs were synthesized via a hydrothermal method according to literature reported by Gao et al. (2019). In a typical synthesis, Co(NO3)2·6H2O (2.619 g) and Fe(NO3)3·9H2O (1.212 g) were dissolved in methanol anhydrous (30 mL) to form a clear salt solution with Co2+/Fe3+ molar ratio of 2.0. Then, urea (1.261 g) and NH4F (0.1 g) were immediately added into the salt solution and vigorously stirred for 2 h. Afterward, the solution was transferred to a 100 mL Teflon-lined stainless-steel autoclave. The autoclave was sealed and kept at 120 °C for 12 h. A brown solid precipitate was collected by centrifugation and washed with distilled water and ethanol several times. Finally, the product was dried in a vacuum oven at 70 °C overnight.
Combined with a variety of characterization methods, the surface morphology, structure, and physical and chemical properties of 3D CoFe-LDHs were investigated. The crystallographic structure of the catalyst was measured by power X-ray diffractometer with the diffraction angle range from 5° to 70° (XRD). The morphology and structure of the composite was studied by scanning electron microscopy (SEM, JSM-6510). The material composition and functional groups were examined by recording the Fourier transform infrared spectra (FT-IR, IFS 66 V/S, Germany). The elemental analysis was analyzed by energy dispersive spectroscopy (EDS) attached to SEM. X-ray photoelectron spectroscopy (XPS) results of catalysts were determined through a Rigaku 2500/PC spectrometer. The Brunauer-Emmett-Teller (BET) surface area of catalysts was determined on a Micromeritics Tristar II3020 apparatus. The decolorization experiments of azo dyes were observed using an Ultraviolet-visible (UV-Vis) spectrophotometer (UVmini-1240, SHIMADZU). UV-Vis diffuse reflectance spectra (UV-Vis DRS) of the catalyst was obtained on a U-4100 UV-Vis spectrophotometer. Total organic carbon of the dye was analyzed using a TOC analyzer (AA-6000CF).
RESULTS AND DISCUSSION
The morphology, structure and elemental constitution of 3D CoFe-LDH are shown in Figure 1. X-ray diffraction data (Figure 1(a)) manifest that the reflection peaks in the pattern for the catalyst at 2θ = 11.05°, 22.44°, 33.11°, 33.88°, 38.16°, 45.44°, 59.05° and 60.68° correspond to planes of (003), (006), (101), (012), (015), (018), (110) and (113) respectively, suggesting a highly crystalline structure and a high purity of the LDH phase, which is consistent with a previous report (Arif et al. 2019). Figure 1(b) shows that we successfully synthesized a flower-like microporous sample. These microspheres are assembled into a discrete layer structure from many irregularly and randomly arranged nanosheets. Also, there are a large number of interconnected macroporous networks between layers and hierarchies. On the one hand, the flower-like structure provides more reactive sites, which is expected to increase the adsorption capacity of the catalyst and significantly enhance the removal of pollutants (Sanchez et al. 2005). On the other hand, combined with the biomimetic and bioinspired thinking, the flower-like structure makes full use of solar energy through multiple internal reflections and improves the light-trapping capability of the catalyst (Serrà et al. 2019). In Figure 1(c), the catalyst sample shows a typically broad band at around 3,424 cm−1 corresponding to interlayer water molecules where hydrogen is bonded to the metal-OH group (Ma et al. 2016). The band at about 1,631 cm−1 corresponds to the bending mode of water molecules. The characteristic bands at around 673 and 492 cm−1 in the low wavenumber region relate to the brucite-like structure and can be attributed to metal-OH bending and metal-O stretching vibrations in the octahedral hydroxyl sheets, respectively (Cheng et al. 2014). The typical absorption peaks of carbonate ions are approximately 1,380 cm−1. In the meantime, two weak bands at 2,850 and 2,920 cm−1 can be ascribed to the stretching vibration of C-H (Xia et al. 2015), indicating that a little methanol is still remaining in the product. These assignments are in common with previously reported results (Yang et al. 2019). A typical EDS spectrum of catalyst in Figure 1(d) obviously proved the overlapping of corresponding elements of Co, Fe and O, authenticating the uniform dispersion of 3D CoFe-LDHs. In order to further confirm the surface electronic state and elemental composition of the catalyst, XPS test were performed. Figure 2(a) displays that the survey XPS spectrum contains Co, Fe, O and C, which is consistent with EDS mapping. For the Co 2p spectrum in Figure 2(b), the peaks located at 783.9/801.3 eV and 782.1/798.0 eV can be asssigned to the main peaks of Co2+ (2p3/2/2p1/2) and Co3+ (2p3/2/2p1/2), accompanied by the satellite peaks of Co and Co2+ salt at 787.4/804.0 eV (Zhao et al. 2020). The Fe 2p spectrum (Figure 2(c)) features multiple remarkable peaks at 711.9/723.3 eV, 714.5/727.0 eV and 719.2 eV, corresponding to 2p3/2 and 2p1/2 orbits of Fe2+ and Fe3+ as well as Fe Sat. orbits, respectively (Hao et al. 2019). The BET surface area of the catalyst is observed in Figure S2 (supplementary information). The large BET surface area (103.7 m2·g−1) provides more reaction sites for the catalyst to remove polluntants. Moreover, the UV-vis DRS spectra of the catalyst was analyzed by a UV-vis spectrophotometer with a wavelength range of 200–800 nm (Figure S3). It can be seen that the prepared catalyst had strong absorption in the ultraviolet region and the visible region, and the absorption range was wide, indicating that the catalyst is expected to be used as a heterogeneous photo-Fenton catalyst.
Decolorization of azo dyes under different systems
MO (anionic dye), RhB (cationic dye) and neutral blue (neutral dye) were selected as the target pollutants to evaluate the removal effect of 3D CoFe-LDHs on organic pollutants. Figure S4 reveals 3D CoFe-LDHs showed stronger decolorization of MO and RhB than neutral blue. This is probably because the prepared catalyst is composed of positively charged metal cations and negatively charged anions between the layers. Thus, the electrostatic interaction between the charge on the surface of the adsorbent and the molecular structure of the cationic dye/anionic dye is stronger than that of the neutral dye, resulting in the adsorption effect of 3D CoFe-LDHs on MO and RhB being much better than that of neutral blue. Moreover, RhB has photosensitivity under the simulated sunlight, and the photo-stability of MO is higher than RhB, so the decolorization of RhB is higher than that of MO.
Control experiments were conducted to deeply investigate the decolorization of azo dyes under different systems. The experimental results are shown in Figure 3.
Furthermore, an obvious change in total organic content (TOC) of MO was observed during the heterogeneous photo-Fenton process. Figure 3(b) displays that the 3D CoFe-LDHs had a high mineralization degree of MO. It was also found that the decolorization of MO was always faster than the mineralization. For example, the corresponding TOC value decreased from 2.992 to 1.024 mg/L and reached about 65.78% after an hour, whereas the decolorization of MO was 91.18% at the same time. In summary, the synergistic effect of photocatalytic and Fenton oxidation processes significantly improves the decolorization and mineralization of MO, which is in good agreement with the results reported previously (Wang et al. 2020).
To further study the decolorization of azo dyes in the real water environment, two kinds of actual wastewater were collected from Sichuan FAW Toyota Motor Co., Ltd, which respectively corresponded to degreasing wastewater and electrophoretic wastewater. The quality parameters of the two kinds of wastewater and the decolorization behavior of the MO in them are shown in Table S1 and Figure S5, respectively. The results suggest that the decolorization of MO in electrophoretic wastewater was significantly higher than that of degreasing wastewater under simulated sunlight, and the specific decolorization efficiency reached 83.53% and 27.2%, respectively. The reason for this phenomenon may be that the higher pH of the degreasing wastewater is not conducive to the reaction, so the decolorization efficiency is weaker than that of electrophoretic wastewater. In addition, the chemical oxygen demand (COD) of electrophoretic wastewater is higher than that of degreasing wastewater, and the COD is larger; the pollution of water bodies by organic compounds is more serious. Under the synergy of various organic and inorganic substances, the decolorization of MO in electrophoretic wastewater was better.
Effect of reaction conditions on the removal of MO
Effect of MO concentrations
Three concentrations (2, 5 and 10 mg/L) of MO were selected to investigate the effect of different initial concentrations of MO on their decolorization.
As shown in Figure 4, the study found that as the initial concentration of MO increased from 2 to 5 mg/L, the removal rate increased from 84.12% to 91.18%. This is because with the concentration of MO augmenting, the corresponding MO content adsorbed on the surface of the catalyst increased, promoting the decolorization. Nevertheless, when the concentration of MO continued to increase to 10 mg/L, the decolorization efficiency decreased. It might be because intermediate products generated during the reaction would gradually accumulate on the surface of the catalyst, covering the active sites of the catalyst, ultimately leading to a slight decrease in the decolorization. The results are similar to previously reported results (Chen et al. 2017).
Effect of initial pH
It is well known that the pH value of the solution has a greater impact on the heterogeneous photo-Fenton process (Yuan et al. 2018). The effect of pH on the decolorization of MO is illustrated in Figure 5. The different pH of the MO solution were adjusted to 2.1, 3.2, 7.2, and 11.7 with H2SO4 and NaOH, while other conditions were kept the same with the above experiment.
Effect of H2O2 concentrations
The effect of H2O2 dosage on MO degradation was investigated with different concentrations of H2O2 under simulated sunlight. The results are presented in Figure 6.
Recycling of catalyst
Repeated avaliability of the heterogeneous photo-Fenton catalyst is an important property for its application, so recycling reactions were implemented. In each test, the catalyst was collected and reused after washing with water and drying. The situation of cycling for five times under simulated sunlight is shown in Figure 7.
During the experiments, the initial concentrations of MO, H2O2 and 3D CoFe-LDHs were 5 mg/L, 2.54 mM and 20 mg respectively. The reaction pH value and time were 3.2 and 60 min. The recycling experiments results revealed that MO degradation during five cycles can still reach more than 75%, indicating that 3D CoFe-LDHs possessed good stability and activity in heterogeneous photo-Fenton process, which is much better than the results reported before (Li et al. 2017). Thence, 3D CoFe-LDHs is an excellent candidate to remove azo dyes for its reutilization. Meanwhile, the XRD and XPS spectrums of the samples after five cycles of recycling were determined. It can be seen from Figure 1(a) and Figure S6, there is basically no obvious shift in the position of the peak, but the intensity of the peak has a downward trend. This may indicate that the structure of the catalyst after the reaction hardly changed, showing excellent stability.
Studies have shown that the removal of organic pollutants is mainly because of a series of active substances generated during the reaction process. In order to further explore the decolorization mechanism of MO during heterogeneous photo-Fenton process, quenching experiments of active substances were performed. Isopropyl alcohol (IPA), benzoquinone (BQ), sodium azide (NaN3), silver nitrate (AgNO3) and ammonium oxalate ((NH4)2C2O4) were added as quenchers for hydroxyl radicals (•OH), superoxide radicals (•O2−), singlet oxygen (1O2), electron (e−) and holes (h+) respectively (Guan et al. 2017). During the experiments, 0.8 mM of IPA, BQ, NaN3, AgNO3 or (NH4)2C2O4 was added to the system.
In Figure 8, it can be observed that the decolorization efficiency without quencher, IPA, BQ, NaN3, AgNO3 and (NH4)2C2O4 was 96.15%, 38.49%, 67.28%, 95.36%, 90.53% and 53.88%, respectively. The decolorization of MO was almost not affected by the addition of NaN3. On the contrary, the decolorization of MO was sharply decreased with the addition of IPA, BQ and (NH4)2C2O4, which indirectly showed that •OH, •O2− and h+ generated during the heterogeneous photo-Fenton process were the main anticipants.
In this work, a novel flower-like catalyst 3D CoFe-LDHs was synthesized via one-step hydrothermal method. It has been demonstrated that during the initial period of heterogeneous photo-Fenton process, the adsorption effect of 3D CoFe-LDHs on MO and RhB is obvious and about 40–55% of dyes was removed due to its high specific surface area. Then a highly efficient decolorization of MO and RhB under simulated sunlight was shown. Experiments revealed that the decolorization of 3D CoFe-LDHs is more effective for removing MO and RhB than neutral dye, and is highly dependent on pH and concentrations of dyes. The addition of different concentrations of H2O2 and different kind of quenchers both proved that •OH, •O2− and h+ are effective to promote the decolorization of dyes under the synergistic effect of photocatalysis and Fenton oxidation process. What is more, the catalyst showed excellent reusability and stability. All in all, the novel 3D CoFe-LDHs is expected to seek to remove difficult-to-biodegrade organic pollutants from wastewater and realize high mineralization under the synergistic effect of photocatalytic and Fenton oxidation process.
This work was financially supported by Thirteenth Five-Year Program for Science and Technology of Education Department of Jilin Province (JJKH20200803KJ), the Youth Fund of Changchun University of Science and Technology (XQNJJ-2019-14), Natural Science Foundation Project of Jilin Province (20200201082JC).
The Supplementary Material for this paper is available online at https://dx.doi.org/10.2166/wst.2020.293.