Degradation Of Antibiotic Pollutants In Aqueous Solution By Photo-fenton Oxidation Over Solid Fe-based Catalysts

The extensive use of antibiotics induces the emergence of drug-resistant bacteria and thus endangers aquatic organisms,posing a serious environmental threat to aquatic and terrestrial ecosystems.Therefore,removing antibiotics from water has become an important issue.Photo-Fenton oxidation process is a kind of new hybrid advanced oxidation technology.Hydroxyl radicals,super oxygen free radicals,and holes et.al.are produced under irradiation,which show strong oxidizing property.Compared with biological degradation and physical adsorption method,photo-Fenton process not only possesses faster reaction rate,higher degradation efficiency and no secondary pollution,but also not affected by water quality conditions.Therefore,it has a broad application prospect.Proper catalyst is the key of photo-Fenton oxidation reaction to remove antibiotics.Fe-based solid catalytic materials(including manganese ferrite and iron coordination compounds)have attracted much attention due to their excellent photo-Fenton catalytic performance.The specific research work and results of this paper are carried out on the oxidative degradation of antibiotics by photo-Fenton process over iron-based solid catalytic materials,including the following four parts:(1)Photo-Fenton degradation of tetracycline over MnFe₂O₄-MoS₂composites with different structureMnFe₂O₄-MoS₂(FM)composites with different morphology and structure were prepared by hydrothermal method and solvothermal method.Solvothermal method used ethylene glycol as solvent.Scanning electron microscopy,X-ray diffraction,BET,etc were used to observe the structure and morphology of FM composites.It was found that FM prepared via hydrothermal method(FMW)showed flower-like structure and clusters of laminated shape.Moreover,flower-like FMW exhibited strong adsorption performance owing to the mesoporous structure,and can provide enough open active site,which is beneficial for mass transfer reaction.The FM prepared by solvothermal method(FMW)showed bar structure,spheres and clusters composed of nanorods without pores.It was found that the adsorption ability of FMG was smaller than that of FMW.To investigate the effect of structure on photo-Fenton reaction performance,a series of experiments on tetracycline(TC)degradation with adjusting H₂O₂content,light intensity,reaction time,concentration of tetracycline were done.The intermediates of TC degradation were measured by liquid chromatography mass spectrometry.Capture experiments were used to explore the role of radicals in the photo-Fenton process.The results showed that flower-like 1FMW(mass ratio of MnFe₂O₄to MoS₂=1:1)showed the highest degradation performance with degrading 80.9%of TC within 1 h.Moreover,owing to the magnetic property,80.4%of 1FMW can be easily recycled,and some was lost in the experimental,material washing and packaging process.The introduction of MoS₂promotes the electron transport in the photo-Fenton system,thus improving the photo-Fenton performance.(2)H₂O₂-free photo-Fenton degradation of antibiotics over oxygen enriched carbon nitride-MnFe₂O₄compositesBased on the results of the 1st part,the photo-Fenton system of light/FM/H₂O₂can effectively degrade TC,and the used FM catalysts can be rapidly separated from water by an external magnetic field.However,the system still needs adding H₂O₂,which is a burden for the practical application.Therefore,H₂O₂-free photo-Fenton system with magnetic catalysts was explored in this study.Oxygen enriched carbon nitride(OCN)was chosen to combine with MnFe₂O₄to form OMF composite.OCN was synthesized by introducing oxygen-containing functional groups into carbon nitride.Under visible light irradiation,oxygen-containing functional group promoted the two-electron reduction reaction to reduce oxygen molecules to H₂O₂.H₂O₂produced by OMF can meet the requirements of photo-Fenton system.TC,oxytetracycline and ciprofloxacin were chosen as target pollutants to study the performance and universality of H₂O₂-free photo-Fenton system over OMF.86.5%of TC,82.2%of oxytetracycline and 42.7%of ciprofloxacin could be degraded via this H₂O₂-free photo-Fenton process.In addition,the effects of p H,water quality and reused materials on the photo-Fenton system were also studied.The surface morphology,structure,surface element composition and valence state of OMF were observed by scanning electron microscopy and X-ray electron spectroscopy.Combined with experimental data and characterization results,the mechanism of H₂O₂-free photo-Fenton degradation of antibiotics over OMF was analyzed.The generated H₂O₂participated in the reduction of trivalent Fe and Mn,and promoted the photo-Fenton oxidation reaction.(3)Photo-Fenton degradation on TC and rhodamine B over three-dimensional flower-like Hemin-Bi₂WO₆composites with biomimetic propertyBased on the research results of 1st and 2nd part,MnFe₂O₄-based composites can effectively catalyze the photo-Fenton oxidation for antibiotics degradation,especially the flower-like catalyst.MnFe₂O₄-based catalysts can be easily separated through an external magnetic field.However,magnetic separation is not a simple operation in practical application.For example,it is difficult to wash the magnetic particles adsorbed by the magnetic media in backwash process owing to the magnetic remanence of the medium,which affects the next cycle;To improve the magnetic field gradient,it is necessary to choose the magnetic gathering medium with high magnetic saturation,which has some technical difficulties and increases the cost of operation;Some catalysts still remain in water,causing a burden to the environment.Therefore,it is of great research significance to develop environment-friendly catalysts and reduce the adverse effects of exogenous materials on ecological environment.In this part,three-dimensional flower-like H-Bi₂WO₆composites were constructed by the bionic Hemin and perovskite Bi₂WO₆.Hemin and Bi₂WO₆were combined via non-covalent forces like van der Waals force.The performance of photo-Fenton system over H-Bi₂WO₆was studied via the degradation experiments of TC and rhodamine B,while the universality of this photo-Fenton system for organics degradation was also tested.The results showed that the photo-Fenton system showed high catalytic performance,which can degrade 81.2%TC and 99.5%of rhodamine B within 1 h.Additionally,H-Bi₂WO₆showed high mineralization capacity,the removal of total organic carbon reached 70%.Based on the experimental results and characterization analysis,it was speculated that Hemin played two main roles in the degradation process:firstly,Hemin acted as an electron shuttle that transferred the photogenerated electrons from CB of Bi₂WO₆to the surface to be trapped by dissolved molecular oxygen to form strong oxidative radicals·O₂^-;Secondly,photogenerated electrons participated in the Fenton circulation of Hemin with the assistance of trace H₂O₂.(4)H₂O₂-free photo-Fenton degradation of antibiotics over two-dimensional biomimetic Hemin-Bi₂WO₆compositeBased on the above studies,the use of biomimetic materials is expected to reduce the adverse effects of exogenous materials on the environment while ensuring the efficient degradation of antibiotic pollutants and H₂O₂-free photo-Fenton system is more conducive to the practical application.Therefore,it is proposed to design a catalyst with ideal structure.Two-dimensional Hemin-Bi₂WO₆(HBWO)composites were prepared to achieve H₂O₂free.The microstructure and surface properties of HBWO were observed by atomic force microscopy and other characterization techniques.It was found that the two-dimensional HBWO was layered structure with layer spacing at about 0.35 nm.In the degradation experiment,TC was used as the target antibiotic contaminant to study the mechanism of this photo-Fenton reaction.Compared with three-dimensional structure,two-dimensional structure has more open surface area,which is conducive to the dispersion and bonding of materials,and effectively enhances the stability of the composites.On the other hand,two-dimensional structure can shorten the distance of photogenerated carriers from inside to surface and speed up the electron transfer rate.It was found that 36?M H₂O₂was generated in the photo-Fenton process,and the degradation experiment results showed that the two-dimensional HBWO photo-Fenton process showed a more efficient degradation performance than the three-dimensional H-Bi₂WO₆in the 3rd part.The degradation rate of TC reached 86.4%within 1 hour.Photo-Fenton system constructed by light and HBWO well meets the requirement of green development.In this paper,we systematically studied the mechanism of photo-Fenton reaction in removing antibiotics from water,which provided a lot of valuable information for further development of photo-Fenton technology in the field of organic pollutants treatment,and contributed to the practical engineering application.