Design Of ZnO-based Catalytic Materials And Photocatalytic Degradation Of DOM In Leachate

With the rapid development of economy and the improvement of people’s living standards,the production of solid waste continues to grow and the total amount of landfill leachate continues to increase.Dissolved organic matter(DOM)is an important organic contaminant in landfill leachate,which contains many organic functional groups and can interact with organic and inorganic pollutants in the leachate,thus causing environmental pollution risk to the air,soil and groundwater environment around the landfill site.Advanced oxidation process and membrane separation technology are widely used in the treatment of DOM in landfill leachate,but their power consumption is large and the operation cost is high.Photocatalytic technology has been widely concerned in solving environmental problems,especially in the removal of organic pollutants.As a photocatalyst,ZnO has the characteristics of wide band gap(～3.37 eV)in the near ultraviolet region,strong oxidation ability,excellent photocatalytic performance,environmental protection and low cost.It is one of the green photocatalytic materials with great application potential.In this thesis,ZnO-based photocatalytic materials are used as the object to study the photocatalytic performance of methylene-blue and DOM in leachate through morphological regulation,coupling with reduced graphene oxide(rGO),recombination with TiO2 and construction of heteroj unction system and other microstructural regulation methods to explore the relevant photocatalytic degradation mechanism.The main research work is as follows:(1)Rod-like,flower-like,needle-like and rod array-like ZnO photocatalysts were synthesized by adjusting hydrothermal reaction conditions.The phase,morphology,optical absorption and carrier separation of the photocatalyst were analyzed,and the degradation activity and reaction mechanism of the photocatalyst to methylene blue and leachate DOM were studied.Compared with other morphologies of ZnO photocatalysts,rod-like ZnO has the most oxygen vacancies and higher carrier separation performance.The degradation process of methylene blue and TOC by the prepared ZnO photocatalyst conforms to the quasi-first-order kinetics equation.Rod-like ZnO showed the best photocatalytic activity,and the degradation efficiency of methylene blue reached 81.1%,and the degradation efficiency of leachate TOC and fulvic acid substances reached 37%and 59.49%,respectively.After four cycles,the fluorescence intensities of protein-like peak,visible fulvic-like and UV fulvic-like peaks in the leachate treated with rod-like ZnO decreased by 11.0%,29.6%and 37.4%respectively.The mechanism study shows that ·OH-plays a major role in the photocatalytic reaction process.(2)Rod-like,flower-like and needle-like ZnO-rGO heterojunction photocatalysts were prepared by two-step hydrothermal method.The phase,morphology,optical absorption performance and carrier separation performance of ZnO-rGO photocatalyst were analyzed by characterization test.The photocatalytic activity and reaction mechanism of ZnO-rGO photocatalyst for DOM in leachate were investigated by 3D-EEM,synchronous fluorescence and UV-vis spectra.The results show that the rod-like ZnO-rGO photocatalyst has more oxygen vacancies and higher carrier separation performance.The degradation efficiency of fulvic acid-like substances in leachate by ZnO-rGO photocatalyst is significantly higher than that of pure ZnO.The rod-like ZnO-rGO photocatalyst exhibited much higher photocatalytic activity,the fluorescence intensity of the visible fulvic-like and UV fulvic-like peaks decreased by more than 80%after 90 min photocatalytic reaction.After 4th cycles of test,the fluorescence intensity of the two peaks decreased by 47.9%and 49.6%,respectively.Photocatalytic mechanism analysis shows that·OH-is the main active species in the reaction process of ZnO-rGO photocatalyst.The presence of heterostructures in ZnO-rGO photocatalyst is beneficial to accelerating electron separation and restraining electron recombination.(3)TiO2/ZnO heterojunction photocatalysts with different mass ratios were prepared by hydrothermal and solid-state reaction method.The photoluminescence spectrum,EIS and transient photocurrent test show that TiO2/ZnO heterojunction photocatalyst has higher carrier separation performance than pure ZnO and TiO2.Photocatalytic test shows that the degradation efficiency of TiO2/ZnO photocatalyst to methylene blue is above 84.6%,which is obviously higher than that of pure ZnO and TiO2 photocatalyst.After 60 min photocatalytic reaction,the fluorescence intensity of tryptophan-like peak,visible fulvic-like and UV fulvic-like peaks in TiO2/ZnO(1:1)photocatalyte decreased by 11.6%,92.8%and 33.2%,respectively.The correlation between characteristic parameters of UV-vis spectrum and peak intensity of synchronous fluorescence spectrum indicates that the photocatalytic process reduces the molecular weight of organic matter and the number of benzene rings in the leachate.The mechanism study shows that ·O2-is the main active species in the photocatalytic reaction process of TiO2/ZnO(1:1)catalyst.ZnO and TiO2 have good matching of conduction and valence bands,and the formation of a direct Z-type heterojunction can effectively promote carrier separation and improve photocatalytic activity.(4)To improve the catalytic activity of TiO2/ZnO photocatalysts,TiO2/ZnO-rGO composite heterojunction photocatalysts with different volume rGO contents were prepared by hydrothermal method.The degradation efficiency of the prepared TiO2/ZnO-rGO photocatalyst for methylene blue reached 90%,and its degradation process conformed to the quasi-first-order kinetics equation.The degradation efficiency of TiO2/ZnO-rGO photocatalyst for TOC in leachate was up to 65.1%.After 60 min photocatalytic reaction,the fluorescence intensity of both visible fulvic-like and UV fulvic-like peaks in leachate decreased by more than 81.8%.The molecular weight of organic matter,aromaticity and the number of benzene ring compounds in leachate decreased with the increase of photocatalytic time.The mechanism analysis showed that ·OH-played a major role in the reaction process of TiO2/ZnO-rGO photocatalyst.