Removal Of Gaseous Elemental Mercury By Modified Bismuth Oxybromide Photocatalysts

At present,heavy metal pollution has become one of the hot issues in the world,among which mercury has been recognized as a highly toxic heavy metal pollutant because of its high toxicity,volatile and persistent bioaccumulation.Mercury emissions from coal-fired power plants account for about a quarter of anthropogenic mercury emissions due to the large use of coal each year.Of the three forms of mercury emitted(elemental mercury Hg⁰,oxidized mercury Hg²⁺,and particulate bound mercury Hg^p),Hg²⁺could be removed by wet flue gas desulfurization（WFGD）units,and most of the Hg^p was removed by electrostatic precipitators and bag filter.Elemental mercury Hg⁰ is difficult to remove because of its insolubility,volatility and stability.The greatest difficulty in limiting mercury emissions from industrial flue gas is the removal of Hg⁰.In this paper,Bi₂O₂CO₃/BiOBr,BiOBr/BiOCl and Ag₂Cr O₄-BiOBr/BiOCl series photocatalysts were prepared and tested in the wet flue gas mercury removal setup to test the performance of Hg⁰ removal from simulated flue gas.The activity of the samples was comprehensively investigated by changing the experimental conditions such as light irradiation,inorganic anions,solution temperature,p H value,dosage,flue gas composition,number of cycles and capture of reactive radicals.The physicochemical properties of the samples were characterized by N₂ adsorption-desorption,SEM,TEM,XRD,DRS,XPS,ESR,etc.The reaction mechanism for the efficient removal of Hg⁰ was explored.The main conclusions were as follows:（1）Bi₂O₂CO₃/BiOBr and BiOBr/BiOCl series catalysts were prepared by two different methods via the modification of BiOBr.Compared with pure BiOBr,the composite catalyst could significantly improve the catalytic performance under fluorescent lamp and exhibit a better ability to remove Hg⁰.Among them,Bi₂O₂CO₃-3and BiOBr_0.2Cl_0.8 catalysts both showed more than 90%mercury removal effect.However,both catalysts have the disadvantages of poor tolerance to SO₂ and NO,and high sample dosage for mercury removal.（2）After the modification of BiOBr_0.2Cl_0.8 sample using Ag₂Cr O₄,0.3 g of the obtained 3%Ag₂Cr O₄-BiOBr_0.2Cl_0.8 sample could achieve a mercury removal efficiency of more than 90%and possess good anti-sulfur and anti-nitrification properties.The acid environment showed little effect on the photocatalyst.The strong alkaline environment reduced the mercury removal effect of the photocatalyst because the active species in the reaction system will be consumed by OH^-.（3）NO₃^-ion had almost no effect on the photocatalytic activity of the three catalysts,while SO₄^2-ion slightly inhibited the mercury removal effect of Bi₂O₂CO₃-3.Cl^-ion possessed an obvious inhibition effect on BiOBr_0.2Cl_0.8.And CO₃^2-ions demonstrated a inferior effect on the catalytic activity of the three catalysts.The mercury removal performances of the three photocatalysts were greatly inhibited by SO₂,fortunately their mercury removal performances can be recovered after removing SO₂.Compared with SO₂,NO showed a less effect on the mercury removal,but the mercury removal activities cannot be recovered after NO was closed.（4）The experimental and mechanism analysis show that light irradiation is essential in the efficient mercury removal process of photocatalyst,and the main active species involved in the mercury removal reaction are anionic superoxide radical^·O₂^–and holes h⁺.There are 48 Figures,3 Tables and 130 References in the thesis.