Study On Performance And Mechanism Of Silver-based Modified BiOI Photocatalysts For Elemental Mercury Removal From Flue Gas

As a global pollutant,mercury has attracted worldwide attention due to its portability,persistence and biological accumulation in the environment.Coal-fired power plants are the main source of anthropogenic mercury emissions in the atmosphere.Mercury mainly exists in three forms:elemental mercury（Hg⁰）,mercuric oxide(Hg²⁺)and granular mercury（Hg^p）.Among them,Hg²⁺and Hg^p can be easily removed by existing devices.However,Hg⁰ is volatile and insoluble in water,which is difficult to be removed by existing pollutant equipment.Thus,the study of mercury pollution control technology from flue gas has important application value and broad development prospect.Semiconductor photocatalysis technology has potential applications in the fields of environment and energy.Photocatalytic technology based on semiconductor materials can not only use solar energy to generate available energy such as hydrogen energy,electricity and organic fuel,but also control pollutants through oxidative degradation.Bismuth iodideoxide（BiOI）is a new semiconductor material with square crystal structure,which is composed of[Bi₂O₂]sheet layer and two layers of halogen elements.Being a cheap materaial and a very potential visible light catalyst,BiOI is suitable for large-scale commercial applications.Based on BiOI material,three types of Ag₂WO₄/BiOI/CoFe₂O₄,Ag IO₃/BiOI/CoFe₂O₄ and Ag₂O-Ag I/BiOI/CoFe₂O₄ series photocatalysts were prepared by a facile deposition-precipitation method.To study the physical and chemical structure characteristics of the samples,many characterization methods such as the X-ray diffraction,N₂ adsorption-desorption,ultraviolet-visible diffuse reflection spectra,scanning electron microscope,transmission electron microscopy,X-ray photoelectron spectroscopy,electron spin resonance,hysteresis loop,transient photocurrent response,photoluminescence spectra and electrochemical impedance studied.Moreover,the effects of active component loading,photocatalyst dosage,fluorescent lamp irradiation,solution anion,pH value,SO₂ and NO on Hg⁰ removal performance of photocatalyst were studied in a photocatalytic reactor.Through characterization analysis and experimental conclusion,the following main conclusions are drawn:（1）Compared with monocomponent and binary component photocatalysts,Hg⁰removal activity of ternary Ag₂WO₄（0.1）BiOI/CoFe₂O₄ photocatalyst increases greatly.The Hg⁰ removal rate constant of the photocatalyst is 0.479 min^-1,and its removal efficiency can reach as high as 100%after 15 minutes.Fluorescent lamp irradiation is very important for efficient Hg⁰ removal of photocatalyst.After 20 cycles,the samples still have good stability and reusability.The characterization results show that Ag₂WO₄and CoFe₂O₄ are highly dispersed on the BiOI surface,and the binding energy migration in XPS indicates the formation of heterogeneous structures between ternary materials.The combination of ternary Ag₂WO₄（0.1）BiOI/CoFe₂O₄ photocatalyst can enhance the separation efficiency of photogenerated electron hole pairs in comparison to its monocomponent and binary component photocatalyst.The free radical capture experiments show that^·O₂^-and h⁺are the main active species in the removal process of Hg⁰.（2）The as-synthesized Ag IO₃（7%）/BiOI/CoFe₂O₄ shows a removal efficiency of about 99%using BiOI with a standing time of 12 hours as the precursor,and the reaction rate constant value that could be reached within 10 minutes was 0.316 min^-1.SEM result shows that the photocatalyst is a microsphere structure composed of nanocrystals.DRS ananlysis shows that photocatalysts exhibit an enhanced light absorption in the visible region.The presence of magnetic CoFe₂O₄ makes the photocatalyst easy to be separated and recovered.At the reaction solution temperature of 75°C,Hg⁰ removal efficiency of the photocatalyst is significantly reduced to 9%,and CO₃^2-have a strong inhibitory effect on photocatalytic activity.The result of transient photocurrent analysis shows that the teranary photocatalyst has higher separation efficiency of photogenerated carriers.（3）Compared with BiOI material,Hg⁰ removal activity of quaternary Ag₂O（6%）Ag I/BiOI/CoFe₂O₄ increase siginifcantly,and it shows the highest reaction rate constant(0.215 min^-1).When the pH value of the solution is 12,the photocatalyst can achieve a removal efficiency of 99%,which indicates that the photocatalyst has wide range of pH.SO₂ have a little inhibition on Hg⁰ removal activity of photocatalyst,while NO shows a slightly positive effect on Hg⁰ removal.The instantaneous photocurrent response,photoluminescence spectroscopy and electrochemical impedance characterization methods prove that the quaternary photocatalyst could significantly enhance the transfer and separation of photoexcited carriers.The free radical capture experiment shows that^·O₂^-and h⁺are the main active species in the efficient oxidation process of Hg⁰.