Construction And Activity Study Of Novel Bismuth Oxychloride And Titanium Dioxide-based Systems For Efficient Photodegradation Of Tetracycline

Tetracycline（TC）is a widely used broad-spectrum antibiotic that plays an important therapeutic role in human life and medical fields,especially in the prevention and control of bacterial infections.However,this drug is often detected to exist in water and soil environments.Once TC enters the environment,its degradation process is extremely difficult,and it may also produce more toxic degradation intermediates during this process.More seriously,TC can induce microorganisms to gradually develop drug resistance,leading to the continuous accumulation of drug-resistant bacteria.This situation not only poses a serious threat to the environment,but also poses a potential threat to human health.To solve this issue,various technologies including physical adsorption,biological treatment and semiconductor photocatalytic oxidation have been developed to deal with TC wastewaters.Semiconductor photocatalytic technology,as an important component of advanced oxidation processes,has attracted widespread social attention due to its excellent ability to degrade multiple pollutants without causing secondary pollution.So far,many kinds of semiconductor photocatalysts have been widely studied and successfully applied in the field of photocatalytic degradation of organic pollutants.However,single component semiconductor materials have significant shortcomings in visible light response and photogenerated carrier separation efficiency,which greatly limits their practical application effectiveness.Therefore,in order to address the increasingly severe environmental problems,it is particularly crucial and important to explore and develop new photocatalytic materials with excellent solar energy utilization and high photogenerated electron hole pair separation efficiency.In this doctoral dissertation,we mainly focus on bismuth oxychloride（BiOCl）and titanium dioxide（TiO₂）-based composites to develop novel photodegradation systems of TC with high efficiency,recyclability,environmental protection,low cost,and excellent photocatalytic performance by modifying transition metal cocatalysts,constructing heterojunction structures,and coupling easily recyclable floating lightweight carriers or magnetic ferric oxide nanospheres.The main research contents and conclusions are as follows:（1）In recent years,bismuth oxychloride（BiOCl）,as an important photocatalyst,has been paid close attention by many researchers because of its unique layered structure,excellent photocatalytic activity and green characteristics.Its unique layered structure works in tandem with the indirect electron transition mode to effectively inhibit the internal recombination of photogenerated electron-hole pairs,thus significantly improving the photogenerated carrier migration and separation efficiency.At the same time,it is easy to develop composite photocatalysts based on defects such as surface oxygen vacancies because of the unique structure of the[Bi₂O₂]²⁺plate that interlayers Cl ions between bilayers,so that the Bi-O bond is easily broken to produce OVs.In this work,the BiOCl nanoplates were synthesized by a simple hydrothermal method,which mainly exposed the（001）and（110）crystal faces.Then,the BiOCl-Ru composites were prepared by in-situ modification of metallic ruthenium nanoclusters with a mean size of～0.91 nm into BiOCl by photodeposition.It has been found that the as-constructed BiOCl-Ru photocatalyst can generate a large number of surface OVs under in-situ light radiation,and the induced OVs can not only significantly reduce the bandgap width（e.g.,the impurity level under the conduction band）,but also greatly enhance the response ability of the photocatalyst to visible light.At the same time,they also act as effective electron traps,which can trap photogenerated electrons,greatly promoting the efficient separation of photogenerated charges.Furthermore,it can serve as an electron trap,capturing photogenerated electrons and thereby facilitating the efficient separation of photogenerated charges.In addition,the experiments and density functional theory（DFT）calculations confirmed that the modified Ru nanoclusters tend to possess a high adsorption energy on the（110）facet of BiOCl.This unique single selective crystal plane co-catalyst modification can improve the space charge separation efficiency from the level of single particle catalyst,and greatly improve the photocatalytic activity.Thanks to the synergistic effect of surface OVs and Ru nanoclusters,the optimized composite photocatalyst BiOCl-Ru-0.93 demonstrated an excellent photodegradation activity of TC,with a removal rate of up to 90.2%in 75 min of photocatalytic reaction,which is higher than that of the single BiOCl.Finally,combining electron spin resonance spectroscopy（EPR）free radical assay,active free radical scavenging assay and liquid-mass coupling assay（LC-MS）,the reasonable photocatalytic mechanism and TC photodegradation conversion path were proposed.In this study,the developed photocatalytic system provides a valuable research strategy for further exploring the field of photocatalysis of organic pollutants over single particle nanocatalysts.（2）Traditional powder photocatalysts suffer from many problems such as easy agglomeration,difficult recovery,high cost,limited light utilization and slow transport of gaseous reactants,which greatly limit the practical application of photocatalysts.Therefore,in this work,the S-scheme heterojunction photocatalyst of PS@TiO₂/Bi₂O₃ with a rough litchi like surface was constructed by modifying and coupling TiO₂/Bi₂O₃ composites with floating polystyrene spheres（PS）as lightweight carriers and a novel gas-liquid-solid photocatalytic TC degradation system was developed.Owing to the floating advantage of the photocatalyst,oxygen in the air can be rapidly diffused to the solid-water interface to promote the formation of superoxide free radicals（·O₂^-）.The swift transportation of O₂ facilitates the rapid consumption of electrons,thereby effectively restraining the recombination of photogenerated e^-/h⁺pairs and significantly enhancing the photocatalytic activity.Combined with the advantages of floatable and S-scheme heterojunction,the well-designed composite of PS@TiO₂/Bi₂O₃ can remove 88.4%of TC at 1 h photoreaction,demonstrating an excellent photodegradation activity of TC.Finally,in situ X-ray photoelectron spectroscopy（XPS）,EPR free radical monitoring and LC-MS experiments were employed to confirm the unique S-scheme heterogeneous structure of the photocatalyst,and a reasonable photodegradation and transformation path of TC was proposed.The high efficiency recyclable composite photocatalyst developed in this work provides a promising research strategy for the treatment of antibiotic wastewater.（3）To address and resolve the challenges posed by traditional powder photocatalysts,including limited visible light photoactivity,inefficient separation of photogenerated carriers,and difficulties in effective recovery from reaction media,innovative approaches and materials are urgently needed.Herein,a magnetic composite of Fe₃O₄/TiO₂-Co was prepared by combining hydrothermal and sodium borohydride reduction method.Coupled magnetic Fe₃O₄nanospheres can not only be used as magnetic carriers to solve the fundamental challenges of low recovery and reusability of TiO₂ photocatalysts in practical applications through appropriate magnetic separation means,but also serve as transfer channels and acceptors for TiO₂ photogenerated electrons,improving the separation efficiency of TiO₂ photogenerated carriers.Moreover,the modified metal Co nanoparticles can improve the visible light response of the Fe₃O₄/TiO₂-Co composite,and can also act as electron traps to capture photogenerated electrons,further reacting with O₂ to generate abundant active free radicals of·O₂^-,greatly improving the photodegradation activity of TC.Thanks to the synergies of various components,the composite of Fe₃O₄/TiO₂-Co exhibits an excellent photodegradation activity and efficient magnetic recyclability.The experimental results show that the removal rate of TC by the optimized Fe₃O₄@TiO₂-Co-2.7 catalyst is as high as 92.4%after 2 h photoreaction as well as the photocatalytic activity remains unchanged after 5 consecutive recoverings by magnetic separation.In this work,the development of such a structurally stable,highly efficient and recyclable magnetic composite photocatalyst provides a new research idea for solving the problem of antibiotic micro-pollution of water.