Enhanced catalytic degradation of methylene blue by α-Fe2O3/graphene oxide via heterogeneous photo-Fenton reactions
Graphical abstract
The proposed degradation diagram of MB in α-Fe2O3@GO + H2O2 + UV system.
Introduction
Dye wastewater pollution has been a critical issue in recent years owing to the massive production and applications. For instance, more than 100,000 commercially available dyes are produced at an estimated annual rate of over 7 × 105 tons globally [1]. Industries, including textile, food, paper, printing, leather and cosmetic, are the main sources of dye wastewater [2]. Because of their toxicity and potentially carcinogenic nature, dyes pose serious threat to the human health and the environment [3]. Therefore, it is important to remove these dye pollutants from wastewater effluents using efficient processes [4], [5].
As one of the important advanced oxidation processes (AOPs), the conventional homogeneous Fenton process holds great promise in the treatment of the refractory organic compounds due to power of generating highly reactive hydroxyl radicals (OH) by activating H2O2 with Fe2+ [6], [7], [8]. However, the major drawbacks that limit the practical applications of the traditional Fenton process include: (i) a narrow working pH range (pH <3); (ii) high iron concentrations in the final effluent that requires costly removal treatment and generate iron-containing sludge; and (iii) a high demand for H2O2 [9]. To overcome these drawbacks, heterogeneous Fenton processes have emerged as promising alternatives because of their high efficient activity, wide application pHs and durability [10]. Nevertheless, heterogeneous iron oxide Fenton catalysts (e.g., Fe3O4 [11], α-Fe2O3 [12], γ-Fe2O3 [13], α-FeOOH [14], [15], β-FeOOH [16], [17], and γ-FeOOH [18]) usually have lower activities in decomposing H2O2 than their homogeneous counterparts do, because of the potential diffusion resistance for reactants enter the surface of the catalysts. Moreover, some catalysts have poor durability or stability due to metal leaching in oxidation conditions [19]. Therefore, developing effective and durable catalysts remains a challenge.
To date, nanostructured iron oxides have been widely used as heterogeneous Fenton catalysts, primarily because of their abundance, environmental benign, and the effective generation of surface iron complex and hydroxyl radicals under UV irradiation [20]. Among them, α-Fe2O3 is the most common crystalline polymorph of Fe2O3 and an n-type semiconductor with small band gap of approximately 2.2 eV that can absorb light with a wavelength up to 560 nm and capable for oxide organic substrates. Besides, α-Fe2O3 is more chemically and thermally stable with low iron release or dissolution when used as a heterogeneous Fenton catalyst in diverse fields [21]. Unfortunately, the reaction activity of α-Fe2O3 in Fenton system is usually lower than that of γ-Fe2O3, γ-FeOOH, α-FeOOH and Fe3O4 due to the high electron-hole recombination rate [21], [22]. Practical applications of α-Fe2O3 have also been hampered by aggregation. To improve the photochemical and catalytic properties, different supports were hybridized with α-Fe2O3, such as activated carbon [23], alginate [24], clay [25], bentonite [26], kaolin [27], zeolite [28], and CNTs [29]. Comparing with aforementioned supports, Graphene would endow α-Fe2O3 more improved catalytic activity due to its two dimensional (2D) carbonaceous mono-layered structure, excellent thermal conductivity, electronic properties and high surface area (e.g. 2600 m2/g) [30], [31]. The multiple oxygen-containing functional groups (carboxyl, hydroxyl, and epoxy) covalently attach to its layers, resulting in a negatively charged surface [32]. The oxygen functional groups on graphene oxide sheet serve as activation sites for nucleation and growth of iron precursors to form graphene supported metal oxide-containing nanocomposites [33], [34] aiming to fabricate α-Fe2O3@graphene composites with unique properties and applications [35], [36]. So far, α-Fe2O3@graphene nanocomposites in many past studies are primarily used as anode materials for lithium-ion batteries [37], advanced energy storage devices [38], supercapacity [39], and electrochemical sensor [40], [41]. Only a few studies reported the catalytic applications in the treatment of dye pollutants [42]. The previously reported synthesis methods often involved the use of toxic organic chemicals (e.g., hydrazine), high temperatures or high pressures, which may reduce the scalability and cost effectiveness of the applications. For example, Qiu et al. reported that Fe2O3 nanocrystals were in situ grown on the surface of graphene aerogels by a Stober-like method using ethanol and acetonitrile as cosolvents at a high temperature and a high pressure [43]. Xu et al. reported the fabrication of Fe2O3@Graphene microsphere through a two-step spray-drying process under 150–280 °C [41]. Zhang et al. used GO, FeCl2, and urea to synthesize Fe2O3@graphene under microwave heater with N2 protection [44]. Wang et al. synthesized Fe2O3@graphene by coating ferric on graphene oxide and then reduced via H2 within 12 h [45]. Zhu et al. reported that Fe2O3@graphene was synthesized involved the precipitation of ferric with urea and then regeneration of GO by hydrazine [46]. Additionally, there is a lack of detailed investigations on the mechanisms of the enhanced degradation efficiency or pathways of dye pollutants [41]. For example, the enhanced heterogeneous Fenton degradation of MB was investigated by nanoscale zero valent iron (nZVI) assembled on magnetic Fe3O4/reduced graphene oxide (Fe–Fe3O4@rGO) [33]. In this study, the optimum operation pH was 3, above which the decolorization ratio of MB sharply decreased. Therefore, this new catalyst was actually able to overcome the drawbacks of homogeneous Fenton reaction. Also, the decolorization ratio of MB catalyzed by Fe–Fe3O4@rGO decreased from ∼93% in first cycle decreased to 69% in fifth cycles [33]. Moreover, MB degradation mechanisms, photodegradation pathways and toxicity of the intermediates were largely overlooked on these novel hybrid catalysts, which is essential to fundamentally understand the unique strength and potential limitations in dye wastewater treatment.
To develop a facile and scalable method for Fe2O3@graphene with higher durability, this study presented a new synthesis method for the heterogeneous catalyst, α-Fe2O3 anchored on graphene oxide nanosheets (α-Fe2O3@GO). The photodegradation of methylene blue (MB) was studied under ultraviolet light irradiation in presence of H2O2. MB was chosen as a typical model cationic dye, as it represents a class of non-biodegradable dye and is widely used in the textile industry [47]. We also explored the effectiveness of decolorization and mineralization mechanisms of the MB dye and other typical dye or emerging contaminants (i.e., OII, OG, 2-NP, and E2) with α-Fe2O3@GO in a heterogeneous photo-Fenton process under different experimental variables such as pH, H2O2 levels, initial MB concentration, various organic structures, repeated use and scaling up experiments.
Section snippets
Reagents and materials
MB with a molecular formula C16H18ClN3S·3H2O, Fe(NO3)3·9H2O and H2O2 used in this study was purchased from Runjie Chemical Ltd. (Shanghai, China); Orange II (OII), Orange G (OG), rhodamine B (RhB), 2-nitrophenol (2-NP) and phenol, were all purchased from Sinopharm Chemical Reagent Ltd. (Shanghai, China). 17β-estradiol (E2) was purchased from Eppendorf Company (Hamburg, Germany). Natural graphite was purchased from Guangfu fine chemical research institute (Tianjin, China), and used without
Characterization of the catalyst
Fig. 1 shows the typical characteristic peaks of the synthesized α-Fe2O3@GO nanocomposite, GO, and graphite. For graphite, there was only one peak at 2θ = 25.6°, which disappeared after being converted to GO. A distinguished peak at 2θ of 10.6° showed up for GO, indicative of the successful conversion from natural graphite to GO. The main diffraction peaks of α-Fe2O3@GO particles were indexed to the XRD pattern of pure orthorhombic phase α-Fe2O3 (JCPDS No. 33-0664), indicating the formation of
Conclusion
α-Fe2O3 impregnated graphene oxide (α-Fe2O3@GO) nanocomposites were synthesized and characterized. The heterogeneous Fenton catalyst α-Fe2O3@GO operated at a wider pH range (from 3 to 12) and exhibited faster decolorization ratio of MB than those of other combinations such as Degussa P25 TiO2 + UV and α-Fe2O3 + H2O2 + UV. The catalyst was highly stable with consistently high decolorization and negligible iron leaching after 10 cycles of consecutive uses. Luminous bacteria toxicity evaluation showed
Acknowledgements
This work is supported by China's NSF (No. 21377039) and the Shanghai international cooperation research projects (No. 14230710900).
References (75)
- et al.
Bioresour. Technol.
(2001) - et al.
Appl. Catal. B: Environ.
(2009) - et al.
Chemosphere
(2001) - et al.
Appl. Catal. B: Environ.
(2015) - et al.
Bioresour. Technol.
(2007) - et al.
Sol. Energy Mater. Sol. Cells
(2002) - et al.
Appl. Catal. B: Environ.
(2013) - et al.
Appl. Catal. B: Environ.
(2008) - et al.
Appl. Catal. B: Environ.
(2015) - et al.
Appl. Catal. B: Environ.
(2001)
J. Hazard. Mater.
Appl. Catal. B: Environ.
Ultrason. Sonochem.
Chem. Eng. J.
Sci. Total Environ.
Appl. Clay Sci.
Appl. Catal. B: Environ.
Appl. Catal. B: Environ.
J. Alloys Compd.
Carbon
J. Water Process Eng.
J. Alloys Compd.
J. Alloys Compd.
J. Alloys Compd.
Talanta
J. Alloys Compd.
Adv. Colloid Interface Sci.
Carbon
Appl. Catal. B: Environ.
Appl. Catal. B: Environ.
Catal. Today
Chem. Eng. J.
Appl. Catal. B: Environ.
Appl. Catal. A: Gen.
Appl. Catal. B: Environ.
Water Res.
Sci. Rep.
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