Visible light-assisted heterogeneous Fenton with ZnFe2O4 for the degradation of Orange II in water
Graphical abstract
Introduction
Recently, the increasing use of dyes in numerous industries has led to a rise in the disposal of large amounts of dye wastewater [1], [2]. Among these dyes, up to 70% are azo compounds [3], [4]. Orange II (C16H11N2NaO4S), also called Acid Orange 7, is an anionic azo dye widely employed in various industries, including paper, printing, food, cosmetics, textile and leather [5], [6]. Generally, Orange II presents good resistance to UV or solar light irradiation and microbial attack and sometimes may produce more hazardous intermediates during the degradation process [7]. Therefore, research on the development of effective and practical treatment processes for removal of Orange II has attracted increasing interest in recent years.
Nowadays, advanced oxidation processes (AOPs) based on the generation of highly reactive hydroxyl radicals (OH, E0 = 2.80 V/SHE) are commonly recognized as potential alternative technologies for the degradation of recalcitrant contaminants [8]. Among AOPs, the Fenton process is simple, inexpensive, and easy to run under mild conditions of temperature and pressure [3]. During the Fenton reaction, hydrogen peroxide (H2O2) is activated by Fe2+ under acidic solution (pH 3) to generate OH (Eq. (1)) [6]. However, the recovery of Fe2+ through Eq. (2) is extremely slow [9]. Thus, ultraviolet (UV) light or electrochemical technology is frequently employed to the Fenton system for the regeneration of Fe2+ [1], [10], [11]. In the presence of UV irradiation, Fe2+ can be recycled and more hydroxyl radicals can be formed through Eq. (3) [12].H2O2 + Fe2+ → Fe3+ + OH + OH−Fe3+ + H2O2 → Fe2+ + HO2− + H+Fe(OH)2+ + hν → Fe2+ + OH
Unfortunately, it is well known that only 4–5% of solar energy belongs to UV light, while visible (Vis) light covers about 45% of the energy [13], [14]. In addition, the major drawbacks of homogeneous photo-Fenton process, such as the tight range of pH and further treatment of iron sludge, would finally increase the overall cost for water treatment [15], [16]. Therefore, exploring highly efficient and low cost Vis light responsive heterogeneous photocatalysts is currently considered as a hot research topic [14], [17]. In recent years, many studies have been reported for the heterogeneous photochemical removal of organic dyes by Vis-light/catalyst/H2O2 process [18], [19], [20], [21], and the decomposition rate of H2O2 is considered as one of the crucial factors on the photocatataytic degradation efficiency [13]. Thus, the development of photocatalysts with enhanced Vis-light-driven activity is of high interest.
Among various materials applied as photocatalysts with sunlight, the spinel ferrites are proposed as potential catalysts because of their narrow band gap and high stability [14]. Particularly, zinc ferrite (ZnFe2O4), which is non-expensive with a relatively narrow band gap of 1.9 eV, has drawn widespread attention in photocatalytic treatment of contaminants [22]. Additionally, ZnFe2O4 could react with H2O2 to produce OH due to its intrinsic peroxidase-like activity [23]. Hence, it can be expected that ZnFe2O4 might show better performance under Vis-light/H2O2 process.
It was reported that the synthesis condition and process had great influence on the physical and chemical properties such as the size and structure of spinel ferrites [19]. Up to now, various synthesis methods have been applied for ZnFe2O4, such as co-precipitation, sol–gel, hydrothermal, solvothermal, thermal decomposition, microwave irradiation, and combustion methods [24], [25]. Among these routes, hydrothermal synthesis has been considered as one of the effective and economic approaches due to its relative low cost, short process time, homogeneity, reproducibility, energy efficiency, environmental friendliness, and simplicity, compared to other conventional methods [26], [27]. Besides, many heterogeneous catalysts suffer from leaching problems. The concentration of Fe ions is usually high due to a significant degree of leaching out from the heterogeneous catalysts during the photo-Fenton process [28]. Therefore, a relatively facile method for the fabrication of smaller and uniform ZnFe2O4 catalyst via reduction–oxidation route was improved in this study, which included easily achievable reduction at ambient atmosphere. Then, the iron nuclei were oxidized in a separate hydrothermal process [25], [26].
Herein, in this study, the Vis-light-assisted catalyst ZnFe2O4 was synthesized by a novel reduction–oxidation method mentioned above and the catalyst was characterized by transmission electron microscope (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy with an energy dispersive spectrometer (SEM–EDS) before and after reaction. The photocatalytic activity of ZnFe2O4 was investigated using Orange II as a target contaminant under Vis-light irradiation together with H2O2. The influence of important variables such as initial pH, catalyst dosage, H2O2 concentration and visible light power were investigated. A possible mechanism for degradation of Orange II in the Vis/ZnFe2O4/H2O2 process was elucidated. The intermediate products were determined by GC–MS and a possible degradation pathway of Orange II was suggested. The mineralization of Orange II in terms of total organic carbon (TOC) removal efficiency and chemical oxygen demand (COD) removal efficiency together with the variation of toxicity to activated sludge were also studied.
Section snippets
Reagents and materials
The chemicals were used as purchased without further purification. Orange II (C16H11N2NaO4S) with reagent purity grade was purchased from Shanghai No. 3 Reagent Factory (China). Other reagents such as ferric nitrate (Fe(NO3)3·9H2O), zinc nitrate (Zn(NO3)2·6H2O) and sodium borohydride (NaBH4) were of analytical grade and the solutions were prepared using deionized water (the specific conductivity was less than 5 μs/cm), which was obtained via a water purification system (Youpu Instrument Co.,
Characterization of ZnFe2O4 catalysts
The crystalline nature and composition of the as-synthesized ZnFe2O4 catalyst and the corresponding samples after 5 cycles of photochemical reaction for degradation of Orange II were first characterized by XRD. As depicted in Fig. 1(a), the peaks at 2θ values of 78.4°, 73.5°, 70.5°, 62.2°, 56.6°, 53.1°, 42.8°, 36.8°, 35.2°, 29.9° and 18.2° could be ascribed to (4 4 4), (5 3 3), (6 2 0), (4 4 0), (5 1 1), (4 2 2), (4 0 0), (2 2 2), (3 1 1), (2 2 0) and (1 1 1) crystal planes of cubic ZnFe2O4 (JCPDS No. 82-1049),
Conclusions
The ZnFe2O4 photocatalyst was successfully synthesized by a two-step method. The catalyst displayed high photocatalytic activity for splitting H2O2 and for the removal of Orange II under irradiation by Vis light (λ > 420 nm). Orange II could be quickly degraded in a wide pH0 range (3–7) because of the apparent production of sufficient OH in this system. The catalyst exhibited good activity and stability under Vis irradiation. The decolorization efficiency was still over 94% after five reuse cycles
Acknowledgement
This work was supported by Natural Science Foundation of Hubei Province, China (Grant No. 2012FFA089). The analysis of XPS, XRD, SEM, EDS, AAS, and GC–MS was partially supported by Large-scale Instrument and Equipment Sharing Foundation of Wuhan University. D.D. Dionysiou also acknowledges support from the University of Cincinnati through a UNESCO co-Chair Professor position on “Water Access and Sustainability”.
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