Title：Photocatalytic Degradation of Deoxynivalenol by Graphene/Zinc Oxide Hybrids in Aqueous Suspensions
Journal: Applied Catalysis B: Environmental
IF: 20.3
Original link：http://doi.org/10.1016/j.apcatb.2016.11.010
Reporter：Wanru Ge-24-master
Water-soluble deoxynivalenol (DON) pose a major threat as a potential organic pollutant to water environmental quality. DON is a toxic secondary metabolite produced by molds of the Fusarium genus and one of the most important mycotoxins in cereal commodities, which can be enriched from the contaminated grain by deoxynivalenol in the process of wet processing. In this work, graphene/ZnO hybrids has been successfully prepared via a simple one-step hydrothermal method and exhibited superior photocatalytic activity for the photodegradation of DON under the irradiation of UV light. The UV light photocatalytic activity of graphene/ZnO hybrid GZ0.3 was 3.1 times than pure ZnO and about 99% of DON (15 ppm) could be photodegraded within 30 min totally while there were three peaks of intermediate products appeared. The ESI/MS analysis confirmed the presence of DON and degradation product with the secondary mass spectrogram in the positive ESI mode. Overall, this work could provide new insights into the fabrication of graphene/ZnO hybrids composite as high performance photocatalysts and facilitate their application in the mycotoxin detoxification and environmental protection issues.
Water-soluble deoxynivalenol as a potential organic pollutant poses a significant threat to the quality of water environment. During wet processing, deoxynivalenol can enrich soluble toxin molecules from contaminated grain, posing a potentially serious threat to the aquatic environment. Deoxynivalenol belongs to the class of trichothecenes, a group of more than 180 structurally related sesquiterpene mycotoxins produced primarily by Fusarium fungi growing on basic commodities such as wheat, corn and barley. The group is subdivided into type A trichothecenes, such as T-2 toxin, and type B trichothecenes, such as deoxynivalenol, with the difference being that the latter have a ketone substitution at the C-8 site. Although type B trichothecenes are generally less acutely toxic than type A, C, and d trichothecenes, which are the most prevalent congeners in cereals, DONs have structural and toxicological features that may cause chemical reactions - an epoxy ring, hydroxyl groups, and an unsaturated carbonyl group. To date, although many conventional physical, chemical and biological detoxification methods have been tested, none of them have really achieved the necessary efficacy and safety. Meanwhile, the degradation effect is limited and the mechanism is still unclear, which requires further in-depth research. Therefore, the development of green and efficient detoxification technologies is essential to improve the decontamination efficiency of soluble deoxynivalenol and reduce the treatment cost.
Photocatalytic degradation technology has received increasing attention in the field of pollutant treatment, especially for mycotoxin detoxification, which can be applied to wastewater containing small amounts of difficult-to-biodegrade organic matter. The technology has several advantages over competing methods. They are (1) complete mineralization, (2) no waste disposal problems, (3) low cost, and (4) only mild temperature and pressure conditions are required. Although photocatalytic technology shows great potential to degrade organic pollutants, it is still challenging to obtain high photocatalytic efficiency. Photo-induced charge recombination is a key obstacle to the photocatalytic quantum efficiency. Therefore, a new method to improve photocatalytic efficiency by reducing the rate of recombination is urgently needed. Graphene sheet (GS) is an excellent electron collector and transporter and has been used to improve the performance of various energy conversion and storage devices such as photovoltaic devices, supercapacitors, fuel cells and lithium-ion batteries.ZnO semiconductor, with the advantages of non-toxicity, low cost, and high reactivity, is an important photocatalyst. It has been widely used to degrade environmental pollutants in air or water, as well as to convert selective organic pollutants into non-toxic small molecules or even into CO2/H2O.In recent years, functionalized graphene-based semiconductor photocatalysts have attracted much attention due to their good electronic conductivity, large specific surface area and high adsorption. Although it has been previously shown that graphene-based semiconductors exhibit stronger photocatalytic performance than simple photocatalysts, there are still some issues that hinder the further application of existing nanocomposites.
In this paper, we demonstrate for the first time that graphene/ZnO hybridized photocatalysts can be used for the detoxification of DON in aqueous suspensions, investigate the possible structures of the intermediates, and systematically elucidate the effect of graphene on the photocatalytic activity of DON degradation. Moreover, the UV (254 nm, 365 nm) photoactivity of ZnO photodegradation of DON was enhanced by graphene surface hybridization. The results showed that the photocatalytic activity of ZnO/graphene hybrids for DON degradation depended on the content of graphene and exhibited a stable efficiency for the photodegradation of DON over a longer period of time.The photocatalytic activity and the intermediates of DON degradation could provide new ideas for green and efficient decontamination technology to improve the decontamination efficiency of mycotoxins and to reduce the cost of treatment.
1. Material synthesis
Graphene oxide was prepared by oxidizing natural graphite powder using the modified Hummers method. Briefly, graphite (3.0 g) was added to concentrated sulfuric acid (70 mL) with stirring at room temperature, and then sodium nitrate (1.5 g) was added and cooled to 0 °C. Under vigorous stirring, potassium permanganate (9.0 g) was added slowly, keeping the temperature of the suspension below 20◦ C. Sequentially, the reaction system was transferred to a 35-40◦ C water bath for about 0.5 h to form a thicker paste. Then, 140 mL of water was added and stirred for another 15 min. Another 500 mL of water was added and then 20 mL of H2O2 (30%) was added slowly to change the color of the solution from brown to yellow. The mixture was filtered and washed with 1:10 aqueous HCl (250 mL) to remove metal ions, then washed repeatedly with water and centrifuged to remove acid. The solid obtained was dispersed in water by ultrasonication for 1 h to make an aqueous dispersion of graphene oxide (0.5 wt%). The resulting brown dispersion was then centrifuged at 4000 rpm for 30 min to remove any aggregates. Finally, the remaining salt impurities were removed by dialysis purification for 1 week and used for subsequent experiments. Graphene/ZnO hybrids were synthesized as follows: suitable GO was dispersed in distilled water, and then the dispersion was dispersed by ultrasonication for 60 min to strip the GO. The resulting brown dispersion was then centrifuged at a low speed of 5000 rpm for 30 min to remove any unstripped GO.Subsequently, the suspension was centrifuged at a high speed of 10000 rpm to remove the residual supernatant. The obtained exfoliated GO was dispersed in 100 mL of water and an amount of pure ZnO was added to the GO dispersion, respectively. The ZnO nanoparticles and GO mixture were dispersed by ultrasonication for 30 min and stirred for 3 h. The homogeneous mixed dispersion was then sealed in a 16 mL polytetrafluoroethylene hot press kettle and kept at 180 °C for 6 h. The hot press kettle was then naturally cooled down to room temperature, and the prepared samples were taken out. It was washed with deionized water and ethanol sequentially to remove surface impurities for subsequent experiments. The content of graphene oxide in the composites was adjusted by changing the addition amount of graphite oxide. Graphene/ZnO hybrids with graphene mass ratio of 0.1% ~ 10.0% were prepared according to the above method. The wt% of graphene/ZnO-X hybridized photocatalysts was labeled as GZX, and X was labeled as graphene/ZnO hybrids with mass ratios of 0.1, 0.3, 3.0, 5.0, 8.0 and 10.0.
2. Morphology of graphene/ZnO photocatalysts
A series of graphene/ZnO hybrid photocatalysts were obtained by simple treatment of ZnO nanoparticles and graphene oxide using a hydrothermal method, and a typical sample was noted as GZ0.3. Under hydrothermal conditions, this process easily reduced GO to GR, while the ZnO particles were dispersed on graphene sheets. Digital photographs and electron microscope images of the final solid product GZ0.3 with graphene addition of 0.3 wt.%. The color of GZ0.3 appears off-white, and a similar phenomenon was observed for other weights of graphene-added materials in the GZX series. The morphology of the typical products was investigated using SEM and TEM.The SEM and TEM images of GZ0.3 show that the ZnO nanoparticles are uniform and tightly wrapped by the graphene sheets without aggregation as can be seen in the figures. The above observations further demonstrate that the presence of graphene flakes contributes to the agglomeration of ZnO nanoparticles during the growth of graphene/ZnO hybrids and makes these oxide particles well wrapped morphologically by graphene flakes.
3. Enhanced photocatalytic activity
Due to the balance between charge separation and light trapping, the photocatalytic activity of graphene/ZnO hybrids firstly increases and then decreases with the addition of graphene. Under UV light (365 nm), graphene/ZnO hybrid GZ0.3 can degrade DON rapidly.In the presence of graphene/ZnO hybrid photocatalysts, DON molecules may be degraded into small segmented molecules by ring-opening, and the fragmented structure may be further disrupted at higher excitation energies.
4. Structure of graphene-ZnO hybrids
The broad XRD peaks of lyophilized SGH indicate that the graphene sheets are poorly ordered along their stacking direction, reflecting the fact that the backbone of SGH consists of few layers of stacked graphene sheets.There is stacking between the graphene sheets in the SGH, and residual oxygen-containing functional groups are present on the reduced GO sheets as well. The diffraction peak widths of the graphene/ZnO hybrids were similar to those of the prepared pure ZnO, indicating smaller ZnO particle size. In addition, the diffraction peaks of graphene/ZnO hybrids are unchanged compared to pure ZnO, which suggests that the lattice constants of ZnO are unchanged due to the surface hybridization of carbonaceous materials. Compared with graphene, it can be found that graphene introduces many new peaks in the FTIR spectra after hybridization. In the FTIR spectrum of GZ3.0, these new peaks were observed at 1572 cm-1, 1236 cm-1 and 1073 cm-1 and appeared to be of significant intensity, suggesting a strong chemical interaction between graphene and ZnOmolecules.
5. Mechanism of Enhanced Photocatalytic Activity of DON and Possible Degradation Products
The mechanism of photo-oxidation occurring on the surface of graphene/ZnO hybrids may involve the direct oxidation of DON with superoxide radicals (--O2) and holes.TIC and MS2 analyses of DON confirmed the formation of the degradation products, with the m/z values in the positive ionic mode of 281.8747, 333.0932 and 298.8996, indicating that DON was photodegraded and the new products obtained were DOM-1, [m+H++2H2O]+],and [m+H]+, respectively.
In summary, ZnO/graphene monolayer hybrids with high photocatalytic performance were successfully prepared directly by a simple one-step hydrothermal method. The hybrid photocatalyst has a larger specific surface area, a wider photoresponse range, and a stronger light intensity, and exhibits excellent photocatalytic activity for the photodegradation of DON under UV light irradiation.The UV photocatalytic activity of the ZnO/graphene heteroaromatic compound, GZ0.3, is 3.1 times higher than that of pure ZnO, and it can degrade about 99% (15 ppm) of DON in 30 min, with the appearance of three peaks of the intermediate product. ESI/MS analysis confirmed the photodegradation of DON, and secondary mass spectrometry of the degradation products in positive ESI mode yielded m/z values of 281.8747, 333.0932, and 298.8996, which indicated that DON was photodegraded, and new products were formed as DOM-1, [m +H++2H2O]+], and [m +H]+], respectively. The excellent photocatalytic activity of the hybrids may be due to the direct oxidation reaction of DON with superoxide radicals (-O2)) and holes. The graphene/ZnO hybrid material can be considered as an ideal and effective material for the removal of DON molecules from aqueous solutions due to its synergistic or competitive effects of adsorption and photocatalysis. This work could provide an important inspiration for the development of graphene-based photocatalysts for mycotoxin detoxification and environmental remediation applications.