Study On Electrochemcial Detection Of Heavy Metals In Water Environment Based On C/Bi₄O₅Br₂ And The Photoelectric Synergistic Mechanisms

With the increasing development of industrialization,mining,metal smelting,etc.have produced a large amount of heavy metal contaminated wastewater during the production process.Heavy metal contaminated wastewater that has not been properly treated is discharged into the natural water environment,which poses a great threat to the ecological environment.Therefore,the realization of rapid,sensitive and accurate detection of trace heavy metals in water is of great significance to pollution early warning and environmental monitoring.Traditional instrumental analysis methods such as atomic absorption spectrometry have high detection accuracy,but the instruments are expensive,require professional maintenance and operation.The electrochemical detection technology has become the focus of research in the field of trace heavy metals detection due to its high sensitivity,easy to operate,etc.The construction of a new functional material modified electrode with good conductivity and high-efficiency heavy metal chelating ability is the basis for realizing a highly sensitive electrochemical heavy metal detection system.This work innovatively constructs a composite electrochemical sensing electrode based on C/Bi₄O₅Br₂ to realize the synchronous high-sensitivity detection of Pb²⁺,Cd²⁺,and Zn²⁺in the water environment,and explore the mechanisms of the inherent difference in the detection performance of the new type of electrode for three heavy metals,by coupling the photoelectric effect to further improve the heavy metal detection sensitivity of the new electrode,and study the photoelectric coordination mechanism of heavy metal detection,providing a new technical path for the improvement of the electrochemical detection performance of heavy metals in water environment.The main research content and results of this thesis include the following parts:（1）Preparation of C/Bi₄O₅Br₂ and study on the electrochemical detection performance of heavy metals in water environment.This study proposes a highly repeatable,convenient,simple,and low-cost method for synthesizing Bi₄O₅Br₂.Citric acid（C₆H₈O₇）as a carbon source,KBr as a template and the Br source,were heated in a tube furnace under a nitrogen atmosphere and produced a three-dimensional sheet-like C/Bi₄O₅Br₂ composite material.The KBr content of the material was optimized,and the results showed that the best performance was achieved when the relative mass ratio of KBr to Bi（NO₃）₃·5H₂O was 20:1.Characterizations of microscopic morphology and physical properties with ESEM,XPS,FTIR,XRD showed that Bi₄O₅Br₂ nanoparticles were successfully incorporated into the three-dimensional carbon nanosheets,with high crystal purity and complete structure.In addition,the key conditions of the electrochemical detection process are optimized,the sensitivity and detection limit of typical heavy metal detection in water can reach:Cd²⁺was the highest(0.4179μAμg^-1 L,0.33μg L^-1),Pb²⁺was the second(0.30754μAμg^-1 L,0.99μg L^-1),and Zn²⁺was the lowest(0.03618μAμg^-1 L,12.81μg L^-1)under optimized conditions.This thesis also evaluatds the stability,repeatability and anti-interference of the electrochemical sensor and the results showed that the detection of Cd²⁺and Pb²⁺with C/Bi₄O₅Br₂ electrode showed good stability and high repeatability,and the detection of Zn²⁺was relatively poor.In addition,in the actual detection of tap water and lake water,typical heavy metal standard recovery rates could reach 97.99%(Pb²⁺,tap water),94.76%(Pb²⁺,lake water),102.69%(Cd²⁺,tap water)and 94.86%(Cd²⁺,lake water).（2）Research on the mechanisms of heavy metal detection mechanism with C/Bi₄O₅Br₂electrode by density functional theory.The two key factors affecting the detection of heavy metals are the adsorption ability of heavy metals on the surface of the electrode material and their diffusion rates on the electrode surface.Therefore,this work calculates the adsorption energy and diffusion energy barrier of Pb²⁺,Cd²⁺,and Zn²⁺on the Bi OBr（-101）surface,the amount of heavy metal ions reduced and deposited on the electrode surface were directly proportional to the adsorption energy,and the amount of oxidation and dissolution were inversely proportional to the diffusion energy barrier.The adsorption energy were:Pb²⁺（-11.6e V）>Cd²⁺（-8.2 e V）>Zn²⁺（-7.7 e V）,and the diffusion energy barrier were:Pb（1.58 e V）>Zn（0.32 e V）>Cd（0.16 e V）.The calculation results showed that the amount of heavy metal deposition in the electrochemical deposition stage were in the sequence of:Pb>Cd>Zn.In the dissolution stage,the oxidation dissolution of heavy metals were in the sequence of:Cd>Pb>Zn.The diffusion energy barrier of Pb was about 10 times that of Cd,and its adsorption energy was about 1.5 times that of Cd,indicating that the oxidation dissolution of Cd during the electrochemical detection process was higher than that of Pb,which provided a solid theoretical explaination for the difference in the detection performance of the three heavy metal ions with C/Bi₄O₅Br₂ electrode,showing the delection sensitivity with the sequence of Cd>Pb>Zn.（3）Study on the synergistic photoelectric mechanism of the photosensitive C/Bi₄O₅Br₂electrode for the electrochemical detection of heavy metals.In order to explore the influence of stoichiometric ratio of Br on the detection performance,CNTs/Bi OBr composite was also prepared.The optical properties of the semiconductor and its energy band structure were characterized via UV-Vis DRS,Mott-Schottky,PL,i-t method and other photoelectrochemical tests,where Bi₄O₅Br₂ has a better response to light,and the electron-hole recombination rate is lower than Bi OBr.Using a mercury lamp as the excitation light source,the electrochemical detection of Cd²⁺was studied under four conditions:with light irradiation in the deposition stage,with light irradiation in the desorption stage,with light light irradation in both of the deposition and dissolution stage,and with no light throughout the test.The detection sensitivity of C/Bi₄O₅Br₂ and CNTs/Bi OBr electrodes was improved in the three light cases,and it showed that light has a synergistic effect on the electrochemical detection of Cd²⁺.Based on the energy band structure characteristics of Bi₄O₅Br₂ and Bi OBr,during light in the disorption stage,the reaction would happen:Cd+h⁺→Cd²⁺,promoting the detection of Cd²⁺.DFT calculations showed that Cd²⁺was deposited on the semiconductor surface,the band gap energy is significantly reduced,and electrons on the semiconductor surface continued to migrate to the Cd（0）surface under light,forming a Schottky barrier,inhibiting the recombination of electron-hole pairs,and the reaction would happen:Cd²⁺+e^-→Cd,which ultimately improved the electrochemical detection performance on heavy metals.