Synthesis Of ZnO Based Magnetic Photocatalysts And Study On The Photocatalytic Degradation On Tetracyclines

With the substantial improvements in human health and living quality, Pharmaceuticaland Personal Care Products (PPCPs) are widely used in daily life, and there are large load ofthe effluents discharged into the environment. Because the PPCPs like antibiotics haveenvironmental effects, genotoxic effects and toxicity of physiology and ecology, publicconcern has been aroused of the wastewater treatment technique on removing these emergingcontaminants for their possible threats to aquatic environment and human health. Currently,the traditional techniques including biological treatment, physical adsorption and photolysisdegradation have restricted by the water condition with disadvantages of low degradabilityand intricate procedure. Advanced oxidation processes make use of highly reactive hydroxylradical degrading the persistent and refractory biodegradation organic compound, which arewidely used in the sewage treatment process.Advanced Oxidation of semiconductor photocatalyst is typically based on n-typesemiconductor, the photogenerated electrons and holes generated from the energy band underirradiation at certain wavelength range, which lead to the degradation of pollutants absorbedon the surface of photocatalyst by the redox reaction. ZnO has a wide bandgap and lowproduction cost standing out of the semiconductor photocatalysts. However, the lowutilization of sunlight and electron–hole pair recombination reduced the the catalytic activityand recycling of ZnO. In this paper, four kind of ZnO-based magnetic photocatalysts designedby changing the ZnO nanostructures, hybrid the different band structure composite materialsand bring in the magnetic materials prepared by low-temperature hydrothermal method. Byarious of characterization, the crystalline phase, morphology, chemical composition, opticaland magnetic properties and the reusability were investigated. Meanwhile, their behaviours ofphotocatalytic degradation of three tetracyclines antibiotics were studied by the batchphotoreaction operations.The main conclusions included the following items:1. Hydrothermal synthesis of ZnO nanomaterials on silicon substrate and theirphotocatalytic properties (1) Hollow hexahedral ZnO nanocrystals were prepared by using a facile two-step ofhydrothermal procedure on a Si(100) substrate. The structure, morphology, photocatalytic andmagnetic properties of the products were examined. The results demonstrated that theuniformly sized ZnO of hexahedral structure is200nm and a hole with diameter of100nm,when the zinc source is Zinc nitrate and the concentration of KOH is1.6M. Hollow ZnOnanocrystals were proved having ferromagnetic property, the Ms is2.38×10-2emu/g at5Kand0.42×10-2emu/g at300K. Further investigation on the photocatalytic activity of theproducts showed the degradation of TC, OTC and DC is96.44%，77.83%and84.83%, whichcan be described by the frst order kinetic model, the rate constant is2.013×10-2min-1,1.272×10-2min-1and1.561×10-2min-1, respectively.(2) Flower-like ZnO micro and nano structures were prepared by using a facile two-stepof hydrothermal procedure on a Si(111) substrate. The structure, morphology, photocatalyticand magnetic properties of the products were examined. The results showed that flower-likestructure nanoparticles with the size of1μm can be obtained at100oC with1.5M KOH,which contains multiple petals with the diameter of500nm and a flower cluster center.Flower-like ZnO structures showed positive ferromagnetic property, the Ms is3.6×10-2emu/gtested at room temperature. Further investigation on the photocatalytic activity of the productsshowed the degradation of TC, OTC and DC under mercury lamp is96.47%,63.77%and91.15%, respectively, which can be described by the frst order kinetic model, and the rateconstant is2.414×10-2min-1,1.03×10-2min-1and1.976×10-2min-1, respectively.2. Preparation of ZnO/Fe3O4magnetic core-shell structure composite and theirphotocatalytic propertiesMagnetic Fe3O4/ZnO core-shell nanomaterials have been successfully prepared by usingtwo-step method with coprecipitation and annealing treatment. The material composition,morphology and magnetic properties were examined and verified. The results showed that thestrongly-magnetic materials have the diameter of100nm, the Fe3O4nanoparticles work as themagnetic core while ZnO is the photocatalytic shell, and the nanocomposites can be recycledunder magnetic field since the Ms is46.89emu/g. The recombination action of Fe3O4reducedthe bandgap energy to2.83eV, leading to high photocatalytic degradation on TC, DC andOTC, and the degradation rate is81.02%,70.94%,63.67%, respectively. The photocatalysis of Fe3O4/ZnO can be described by the frst order kinetic model, the low reaction rate requiredlong reaction process. Meanwhile, The nano-Fe3O4was modified by surfactants working asan magnetic adsorbent to remove TC from aqueous solutions. The nano-Fe3O4crystalline anddispersion was improved significantly by PEG-4000, the particle diameter of unmodifiedFe3O4, PVP-Fe3O4and PEG-4000-Fe3O4is25nm,20nm and10nm, respectively. The resultsof batch adsorption experiments suggested hydrogen bonds formation between hydroxylgroups played a leading role in removing TC, and PEG-4000-Fe3O4has the greatestabsorption capacity(47.62mg/g) comparing with PVP-Fe3O4(36.1mg/g) and Fe3O4(13.45mg/g), the equilibrium data was fitted to the Langmuir isotherm model better than theFreundlich model The kinetic property of modified Fe3O4was well described by theintraparticle diffusion model, dominated by surface adsorption and intraparticle diffusionacted as auxiliary adsorption.3. Hydrothermal synthesis of hollow cone-like ZnO/CoFe2O4heterostructures andtheir photocatalytic propertiesHeterostructures of hollow cone-like ZnO/CoFe2O4nanocomposites are successfullyfabricated by a two-step hydrothermal route with assistance of Si(100) substrate. The materialstructure, composition, morphology, specific surface area, the magnetic and optical propertieswere investigated. The findings illustrated that through preferential etching process and theattractive effects of magnetic nanoparticles, hollow cone-like nanocomposites were assembledby CoFe2O4coated on the ZnO bone-structure with diameter of200-300nm in length and200nm in hole. Band gap energy of the nanocomposites (2.68eV) is lower than that of pure ZnO(3.22eV). This special heterostructures brought novel surface area of88.1593m2/g and thetotal porosity of0.18cm3/g. The degradation on TC, DC and OTC of nanocomposites underxenon light reaches72.42%,66.18%and55.92%, and the photocatalysis can be described bythe frst order kinetic model.4. Preparation of CdS/ZnO and CdS/CoFe2O4/ZnO composite photocatalyst andtheir photocatalytic properties(1) ZnO/CdS nanocomposites were synthesized by hydrothermal method with optimumthe ratio of raw materials and reaction time. The structure, morphology and photocatalyticproperty of the products were examined. The results demonstrated that the ZnO/CdS of optimal morphology was obtained at zinc and cadmium molar ratio of25:1reacted for10h,there are several CdS nanoparticles adhere to the surface of porous ZnO rod skeleton. Theband gap energy of nanocomposites is about2.87eV, leading to the degradation on TC, DCand OTC under xenon light reached81.65%,70.68%and54.61%respectively. Moreover,ZnO/CdS catalyst almost completely removed three antibiotics under UV irradiation, provedhigh catalytic efficiency of the composite catalyst.(2) Using ZnO, CdS and CoFe2O4as precursor solution, ZnO/CdS/CoFe2O4nanocomposites were synthesized by hydrothermal method. The morphology, magnetic andoptical properties of nanomaterials were characterized, also the photocatalytic efficiency andreusability were tested. The results showed that the cylindrical ZnO with the size of500nmwas coated by a large number of block CdS/CoFe2O4nanoparticles with the diameter of100nm. The complex photocatalysts have a narrow bandgap of2.07eV because of themulti-componets, while the saturation magnetization of the composite material is6.65emu/g.TC, DC and OTC were photocatalyzed by ZnO/CdS/CoFe2O4under xenon light, anddegradation rate were82.96%,68.93%and58.53%, respectively. The degradation reactionrate worked in line with first-order kinetics. The TC degradability of ZnO/CdS/CoFe2O4remained above65%after repeated three cycles.