Fabrication Of Bismuth-based Composite Photocatalyst And Study On The Degradation Performance Of Organic Pollutants In Water

The global energy-demand crisis and severe ecological environment contamination have led to an ever-growing requirement for clean and renewable solar energy in recent years.The semiconductor photocatalytic process has been recognized as an efficient and sustainable technology for the degradation and removal of organic pollutants because it can convert solar energy to chemical energy in environmentally friendly energy circulation.Thus,it is crucial to develop new visible-light-induced catalysts in practical application to utilize sustainable solar energy better.As a bismuth-based material with excellent performance,Bi₅O₇I has attracted widespread attention due to its suitable energy band position and unique layered structure.However,the low photogenerated carrier transfer rate and the high electron-hole recombination rate make the traditional single Bi₅O₇I catalytic system seriously hinder the conversion of solar energy to chemical energy.In order to overcome these shortcomings and optimize the photocatalytic ability of Bi₅O₇I material,this paper adopts the strategy of in-situ growth to construct the Z-scheme heterojunction structure to prepare the bismuth-based semiconductor photocatalyst Bi₅O₇I/BiOIO₃.On the one hand,the direct contact interface formed by in-situ growth can use the energy level difference between semiconductors to accelerate the charge transfer,and on the other hand,it also significantly improves the stability of the chemical reaction of the catalyst.In addition,iodate（IO₃^-/I^-）can be used as a redox mediator to form a Z-type heterojunction structure with a unique electron migration path with Bi₅O₇I/BiOIO₃,which can increase the separation efficiency of electrons and holes while still maintaining high redox capacity,thus achieving the goal of improving photocatalytic performance.At the same time,the abundant reserves and low cost of bismuth-based materials also make Bi₅O₇I/BiOIO₃ catalysts have great potential in practical applications.The successful preparation of catalysts also provides a unique strategy for the modification of bismuth-based photocatalytic materials.Based on this,this paper prepared a binary composite Bi₅O₇I/BiOIO₃photocatalytic material,characterized,and discussed the material’s crystal structure,photochemical properties,and photocatalytic mechanism through various characterization techniques.Furthermore,the HPLC-MS/MS detection method was established and optimized for analyzing the photocatalytic performance of the composite material and simulating the antibiotic substrate’s degradation path.The main contents of the paper are as follows:（1）Preparation of binary composite photocatalytic material:The 3D Bi₅O₇I/BiOIO₃heterojunction composite photocatalytic material was prepared using the nanorod-shaped Bi₅O₇I prepared by the hydrolysis alkalization method as a template for in-situ growth.Then tetracycline（TC）,norfloxacin（NFX）and reactive blue（RB-19）were used as target pollutants to optimize the preparation process.The results showed that when the p H value of the mixed reaction was adjusted to 3,the reaction temperature was 150°C,the reaction time was 6 h,and the mass reaction ratio of Bi₅O₇I:BiOIO₃ was 1:1（w%）,the resulting Bi₅O₇I:BiOIO₃ composite catalyst（BiOIBi-1:1）has the highest photoreactivity.（2）Discussion on the structure,optical properties and reaction mechanism of the photocatalyst:XRD,XPS,SEM,HRTEM and other analysis results indicated that BiOIO₃nanosheets of uniform size were successfully loaded on the surface of the three-dimensional nanorod-shaped Bi₅O₇I catalyst,which resulted in the increase of the diameter of the Bi₅O₇I nanorods and the roughness of the surface.Photoelectrochemical tests（UV-vis DRS,PL,EIS,transient photocurrent）display that the formation of composite photocatalysts significantly increased the visible light response range.At the same time,the introduction of heterojunction structure improved the separation efficiency of electron-hole pairs and promoted the rapid transfer of carriers to the surface of photocatalyst to participate in the redox reaction.The photocatalytic performance of the catalyst was evaluated by degrading four antibiotics（tetracycline,chlortetracycline,norfloxacin,and lomefloxacin）and the results showed that the removal rate of Bi₅O₇I/BiOIO₃ on various antibiotics reached more than 90%.Moreover,the photocatalytic reaction mechanism was inferred through Mott-Schottky curve combined with electron spin resonance experiments.The above results indicated that the Z-scheme heterojunction catalytic system has been successfully constructed.In this system,IO₃^-/I^-could act as an electron transfer mediator in coordination with the direct contact interface between different catalysts,which dramatically shortened the photogenerated carrier transport path and accelerated the separation of photogenerated electron-hole pairs.Finally,the h⁺and·O₂^-active free radicals with strong redox ability in the heterojunction system were retained in the valence band and conduction band potential,thereby significantly improving photocatalytic performance.The experiment of degrading tetracycline through five cycles showed high stability during the degradation process.（3）To separate and analyze four standard mixed antibiotic mimic substrate solutions of tetracycline,chlortetracycline,norfloxacin,and lomefloxacin,the analytical method of high-performance liquid chromatography combined with multistage mass spectrometry was established and optimized.Specifically,a Sharpsil-C18（2.1×100 mm,3μm）chromatographic column was selected as the chromatographic conditions,and the mobile phases were acetonitrile and an aqueous solution containing 0.1%formic acid,and gradient elution was performed at a flow rate of 0.3 m L/min.Then,coupled with mass spectrometry to perform multi-reaction detection scanning in positive ion mode,the baseline separation of the four mixed antibiotics was achieved.After that,the established method was validated with parameters such as linear range,detection limit and quantification limit,precision and accuracy,and its application in actual water quality.The results showed that the content of the four target antibiotics in actual water quality was mainly distributed in the range of 100-2000 ng·m L^-1,the established method has excellent performance in practical application.（4）The established HPLC-MS/MS method was used to monitor the concentration changes of Bi₅O₇I/BiOIO₃ complex in the process of photocatalytic degradation of high-concentration mixed antibiotics simulation solution and actual water quality antibiotic residues.Afterwards,four antibiotic degradation pathways were inferred by using the obtained m/z information of the intermediate product of antibiotic degradation,as well as the information of the structural functional group and bond energy of the antibiotic.Specifically,the degradation pathways of tetracycline and chlortetracycline mainly include the cleavage of characteristic functional groups and ring-opening reactions,and the degradation of norfloxacin mainly involves the oxygenation of the piperazine ring and the cleavage of the quinolone ring.Lomefloxacin was mainly transformed into small molecules through defluorination,decarboxylation and piperazine ring conversion.The small-molecule organic substances produced above were finally mineralized to form inorganic compounds such as CO₂,H₂O,etc.