Magnetic imprinted N-doped P25/Fe3O4-graphene oxide (MIGNT) was prepared with methyl orange as the dummy template and pyrrole as functional monomer for catalytic degradation of Congo red (CR). Hummers method and the hydrothermal method were used to synthesize Fe3O4-GO and N-doped P25, respectively. The results of adsorption and degradation experiments showed that the adsorption capacity and catalytic degradation ability of the imprinted composite for CR were obviously higher than those of a non-imprinted one. Moreover, the effect factors on degradation efficiency of CR, such as the initial concentration of CR, catalysis time, pH of the solution and temperature, were investigated. The MIGNT was characterized by X-ray diffraction, Fourier transform infrared spectroscopy, transmission electron microscopy, a physical property measurement system and a thermal gravimetric analyzer. The degradation products of CR were detected with high performance liquid chromatography and a mass spectrometer. The MIGNT was a brand-new imprinted composite and had high degradation efficiency for CR under dark ambient conditions. The MIGNT could be recycled conveniently, due to its magnetic property, and could be used as an effective, environmentally friendly and low-cost catalytic degradation material for the treatment of water contaminated by CR.
Environmental pollution caused by dyes has become a global challenge faced by human beings. Owing to their large and complex molecular structures, the high recalcitrance to degradation of dyes has been a severe problem for color removal (Anliker 1977). Among these dyes, azo dyes are highly toxic and even carcinogenic to animals and humans. Since Congo red (CR) is one of the well-known azo dyes that are not readily degraded, it was used as the dye model (Bagheri et al. 2014). Efficient and reliable methods are clearly indispensable for the removal and degradation of CR from wastewater.
Some methods have been developed and used for the removal of dye contaminants, including coagulation/flocculation (Huang et al. 2016), advanced oxidation processes, ozonation, membrane filtration and biological treatment (Walker et al. 2003). However, the remarkable results obtained from these treatment methods required high operation and maintenance cost (Forgacs et al. 2004). Recently, the removal of dyes from wastewater through heterogeneous photocatalysis has drawn increasing attention (Senthilraja et al. 2014).
Titanium dioxide (TiO2) is usually used to remove the dyes through heterogeneous photocatalysis. However, TiO2 has a large band gap of 3.2 eV, which severely limits absorption of visible light (Linsebigler et al. 1995). To develop a highly efficient and low-cost catalyst system with activity extended in the visible region, approaches like sensitization and/or doping with cations or anions have been attempted (Chen & Mao 2007; Wang et al. 2009). Among these approaches, N-doped TiO2 (N-doped P25) exhibited the highest photocatalytic activity in the visible range (Chen et al. 2008; Kuvarega et al. 2011).
In recent years, research into graphene oxide (GO) as an adsorbent has attracted intense interest (Sitko et al. 2013). However, it is hard to separate GO from aqueous solution (Yazici et al. 2017). To overcome this drawback, GO was modified with Fe3O4 nanoparticles. The prepared magnetic GO combined the magnetic property of Fe3O4 with the huge specific surface area of GO (Gao et al. 2009). Moreover, the magnetic property could solve the recycling problem of the catalytic degradation materials.
In addition, molecular imprinting has been proven to be an effective strategy for improving the catalytic selectivity of materials in complex systems (Gupta et al. 2014). Due to the capabilities of efficiently donating electrons and transporting cavities under visible light excitation, the conductive polymer, polypyrrole (PPy), could not only be used as the structure material for molecular imprinting (Huang et al. 2015), but also as a stable photosensitizer to modify TiO2. Thus, a molecularly imprinted coating, PPy, could be used for enhancing the catalytic degradation activity of TiO2 under visible light and its catalytic selectivity (Wei et al. 2015). However, the main limitation of this method lies in the demand for a greater amount of light.
Dark catalytic oxidation methods have aroused people's attention (Lafaye et al. 2015). However, some of them require additional chemicals, high electrical power or other extreme conditions. Compared with other methods, catalytic wet air oxidation (CWAO) is a very promising advanced oxidation process to efficiently degrade the dyes under relatively mild conditions. In experiments, CWAO has provided enough free oxygen for the catalytic process of TiO2 and significantly improved the oxidation rate of the target molecules under dark ambient conditions. Moreover, due to the low price of the oxidant and great availability everywhere in the world, CWAO was adopted to solve the light problem of the TiO2 catalyst (Anushree et al. 2015).
In this paper, magnetic imprinted N-doped P25/Fe3O4 graphene oxide composite (MIGNT) was prepared for the catalytic degradation of CR. The magnetic GO was used to solve the recycling and separation problem of the MIGNT. In addition, the method of CWAO was adopted to solve the light problem of the TiO2 catalyst. CR could be degraded when used as the template molecule in the imprinting process. Thus, methyl orange (MO), a structural analogue of CR that was stable in the imprinting procedure was necessary as the dummy template. A series of characterizations of MIGNT were investigated. Moreover, the degradation products of CR were detected with high performance liquid chromatography (HPLC) and a mass spectrometer (MS).
Natural flake graphite powder (99.0% purity) was purchased from Sigma–Aldrich. Titanium dioxide (P25, 20% rutile and 80% anatase) was purchased from Degussa (Frankfurt, Germany). Iron (II) chloride tetrahydrate (FeCl2·4H2O, 98%), iron (III) chloride hexahydrate (FeCl3·6H2O, 97%), ammonium hydroxide (28%), (NH4)2Fe(SO4)2·6H2O and NH4Fe(SO4)2·12H2O were all of analytical grade supplied by Tianjin Institute of Fine Chemicals (Tianjin, China). Pyrrole (98%), methyl orange (MO, 98%) and Congo red (CR, 98%) were all purchased from J&K Scientific (Shanghai, China). All other substances were of analytical grade from Yong Chang Reagent (Harbin, China).
Preparation of Fe3O4-GO
GO was prepared with a modified Hummers method. Natural flake graphite powder (2 g), H2SO4 (50 mL), NaNO3 (2 g) and KMnO4 (6 g) were sequentially added into a 500 mL three necked flask immersed in an ice water bath, then the mixture was stirred for 10 min. After that, the flask was transferred to a water bath at 28 °C, and the mixture was stirred for 20 h. After successively mixed dropwise with 80 mL distilled water and 20 mL H2O2, the mixture was incubated for 30 min. Then the obtained solid was washed with 0.1 mol L−1 HCl aqueous solution, distilled water and absolute ethyl alcohol, respectively. Finally, the desirable solid (GO) was dried for 8 h at room temperature under static air.
After dissolution of (NH4)2Fe(SO4)2·6H2O (5.8 g) and NH4Fe(SO4)2·12H2O (10.7 g) into 50 mL distilled water with stirring for 10 min, the solution was adjusted with 20 mL of 28% ammonium hydroxide to obtain Solution A. In the meantime, GO (1 g) was dispersed into 50 mL distilled water under ultrasound for 30 min to obtain GO suspensions. After that, GO suspensions were slowly added into Solution A at 80 °C, accompanied with stirring for 30 min. The product (Fe3O4-GO) was washed with distilled water and absolute ethyl alcohol, respectively, then dried in an oven at 70 °C for 12 h.
Preparation of N-doped P25
N-doped P25 (NT) was synthesized by a hydrothermal method according to previous reports (Chen et al. 2008; Kuvarega et al. 2011). The specific steps were as follows: P25 (0.5 g) was dispersed into 80 mL ethylenediamine in a 100 mL beaker under ultrasound for 60 min, then stirred for 30 min. The suspensions were moved to a 100 mL Teflon-lined stainless-steel autoclave and heated at 120 °C for 10 h after being sealed. The NT was washed with 0.1 mol L−1 HCl aqueous solution, distilled water and absolute ethyl alcohol, respectively. Then, NT was dried for 8 h at 60 °C under static air.
Preparation of MIGNT
NT (0.5 g), Fe3O4-GO (0.1 g), 45 mL distilled water and 25 mL ethyl alcohol were mixed in a 100 mL beaker. The suspensions were stirred for 30 min and then dispersed under ultrasound evenly. After hydrothermal reaction for 12 h at 130 °C, the composite object (NT/Fe3O4-GO) was washed with 0.01 mol L−1 HCl aqueous solution, distilled water and absolute ethyl alcohol, respectively, and then dried for 10 h at 70 °C under static air.
MO (0.12 mmol) and pyrrole (35.0 μL) were dissolved in 10 mL methanol aqueous solution (1:1, v/v) under nitrogen atmosphere for 40 min. NT/Fe3O4-GO (0.65 g) was dispersed in the 0.01 mol L−1 HCl aqueous solution (100 mL) under nitrogen atmosphere for 30 min. The above two solutions were mixed at low temperature with stirring for 30 min. At the same time, FeCl3·6H2O (0.3 g) was dissolved in 2 mL HCl aqueous solution (pH = 2), and then this solution was mixed with the above mixture drop by drop. Under the protection of nitrogen, the reaction was stirred for 4 h at room temperature. The product was washed with 1% ammonium hydroxide, 0.01 mol L−1 HCl aqueous and distilled water, respectively. Then, the product (MIGNT) was dried for 12 h at 70 °C under static air.
Moreover, the magnetic non-imprinted NT/Fe3O4-GO graphene oxide (MNGNT) was prepared and processed similarly as above, except that MO was not added.
X-ray diffraction (XRD) patterns of the prepared materials were taken by a DX-2600 X-ray diffractometer (Dandong, China) with Cu Kα radiation (λ¼0.154 nm) operated at 30 mA and 40 kV. Transmission electron microscope (TEM) images were acquired on a Hitachi H-7650 transmission electron microscope (Matsudo, Japan). Fourier transform infrared spectroscopy (FTIR) analysis of the samples was recorded on a Nicolet Avatar 360 spectrophotometer (Nicolet, Madison, WI, USA) using conventional KBr pellets. The element contents of materials were measured by an elemental analyzer (vario EL, Elementar Analysensysteme GmbH, Germany). The absorbancy of the CR aqueous solution at 496 nm was recorded on a TU UV-1901 ultraviolet and visible spectrophotometer (UV–vis, Shanghai, China) equipped with an integrating sphere using water as reference. Thermal gravimetric analysis (TGA) was performed by an STA 6000-SQ8 analytical instrument with a heating rate of 10 °C min−1 and a flow rate of 60 mL min−1 under N2 environment (PE Analytical Instruments Inc., USA). The magnetic property was determined with a Model 6000 physical property measurement system (PPMS, American Quantum Design Inc., USA).
Catalytic degradation experiments
The catalytic degradation experiments of MIGNT and MNGNT for CR aqueous solution were carried out as follows.
For degradation kinetics, MIGNT or MNGNT (13 mg) was added into 15 mL CR aqueous solutions with different initial concentrations ranging from 150 mg L−1 to 200 mg L−1, and then the obtained suspensions were continuously shaken at the rate of 300 rpm in the dark which was created with a shading cloth at 25 °C for 45 min. Finally, the adsorbent was separated by an external permanent magnet. The remaining CR on MIGNT was eluted with methanol and the eluent together with the residual CR in the supernatant was detected by UV-vis at a wavelength of 496 nm. According to the calibration curve, the total residual concentration of CR was calculated for instant time. The concentration of CR degraded by MIGNT could be calculated by subtracting its total residual concentration from the initial concentration of CR. According to the first-order reaction equation, the concentrations of CR degraded by MIGNT were used for degradation kinetics of CR on MIGNT.
RESULTS AND DISCUSSION
Preparation of MIGNT
MIGNT was prepared for the catalytic degradation of CR. Hummers method and the hydrothermal method were used to synthesize Fe3O4-GO and NT, respectively. The molecular imprinting technique is notable in endowing the surface sites catalytic activity and selectivity concurrently by creating specific molecular recognition sites in MIGNT with the use of target substances as template molecules during the growth of the PPy. As a result, abundant recognition sites were retained on the surface of the MIGNT after the template was removed. However, the MNGNT was prepared similarly to the MIGNT, except that the template molecule was not added. Thus, there were no recognition sites on the surface of the MNGNT. Furthermore, the MIGNT had higher adsorption capacity than MNGNT. Because CR could be degraded when used as the template molecule in the imprinting process, MO, a structural analogue of CR that was stable in the imprinting procedure, was used as the dummy template. Since the catalytic reactions predominantly occurred on the surface of MIGNT, the adsorption of pollutants on MIGNT played an important role in the catalytic degradation process (Liu et al. 2017).
An elemental analysis of the NT was performed. The quality percentage of nitrogen was 1.12%, which is evidence of the successful preparation of N-doped P25.
Binding experiments of MO and CR
A binding experiment for CR as a control experiment is shown in Figure 6(b). It was also observed that the adsorption phenomenon of CR was similar to MO and the binding amount of MIGNT toward CR was higher than that of the MNGNT at the initial concentration range of 120–220 mg L−1. Furthermore, the MIGNT has a higher adsorption capacity and catalytic degradation ability toward CR than the MNGNT.
Effect factors for the catalytic degradation ability of MIGNT
The initial concentration of CR
The catalysis time could also affect the degradation efficiency of CR. In Figure 7(b), the catalysis time was segmented into 20, 30, 40, 45, 50 and 60 min to carry out a test. It can be seen that the degradation efficiency of the MIGNT and MNGNT increased rapidly in the range of 20–45 min and the MIGNT turned steady at 99.3%, but the MNGNT changed to 40.6% after 45 min. The results prove that the MIGNT had a better catalytic degradation ability in a short time.
The pH of solution
The effect of temperature on degradation of CR was also studied. Different common temperatures of water (15, 20, 25 and 30 °C) were investigated as depicted in Figure 8(b). The degradation efficiency of CR increased obviously with the increase of temperature and then reached a maximum, which agreed well with the theoretical trend of the influence of temperature on degradation efficiency.
Repeatability of the MIGNT
Degradation kinetics of CR on MIGNT
|Equation of first-order||R2||K (min−1)||Maximum degradation efficiency (%)||Maximum degradation amount (μg mg−1)|
|Equation of first-order||R2||K (min−1)||Maximum degradation efficiency (%)||Maximum degradation amount (μg mg−1)|
Degradation pathway and identification of degradation products
An easy and effective route was adopted for the fabrication of MIGNT which was an environmentally friendly hybrid nanocomposite and prepared with a novel molecularly imprinting and hydrothermal method. Microscopy studies revealed that magnetic P25 was incorporated on the GO surface and molecularly imprinted material was in the outer layer. Spectroscopic studies showed that the optimal assembly and interfacial coupling were between the GO and P25. Thermodynamic analysis evinced that the molecularly imprinted material showed stable adhesion at normal temperature. The dummy imprinting technique effectively alleviated the decomposition of the template molecule in the synthesis process of MIGNT. Moreover, the MIGNT was easy to recycle and capable of rapid degradation of CR without light illumination and additional chemicals. This type of environmentally friendly, low-cost and efficient material could promote environmental protection and material recycling.
This work was financially supported by the Program for New Century Excellent Talents in University (NCET-13-0710) and the National Natural Science Foundation of China (31570572).