Study On The Performance Of Modified BiOI/BiOBr Photocatalyst For Removal Of Gaseous Elemental Mercury

Mercury mainly exists in three chemical forms:elemental mercury（Hg⁰）,oxidized mercury(Hg²⁺)and particulate mercury（Hg^p）.Among them,Hg^p and Hg²⁺can be effectively removed by existing pollution control technologies.In contrast,Hg⁰ is difficult to handle due to its unique physical properties.Therefore,the study of effective Hg⁰ removal methods is one of the hotspots in the field of environmental pollution control.Mercury removal by photocatalytic oxidation technology is considered to be a green and efficient pollutant treatment method,and has been widely studied in pollutant treatment.In this paper,BiOI/BiOBr,Ag Br-BiOI/BiOBr and Ag₂WO₄-BiOI/BiOBr series photocatalysts were prepared by co-precipitation method,and the performance of wet photocatalytic removal of elemental mercury was studied from simulated flue gas.The effects of experimental factors such as silver-based loading,photocatalyst dosage,initial p H value of reaction solution,fluorescent lamp irradiation,SO₂,NO and radical trapping on the mercury removal performance of photocatalysts were investigated.The physical and chemical structure characteristics of the samples were studied by means of characterization methods such as X-ray diffraction,N₂ adsorption-desorption,scanning electron microscopy,X-ray photoelectron spectroscopy,electron spin resonance and UV-visible diffuse reflection spectrum.On the basis of experiments and characterization analysis,the density functional theory（DFT）calculations were carried out to explore the mechanism of photocatalytic mercury removal.The main conclusions are as follows:（1）The photocatalytic activity of BiOI/BiOBr samples was significantly improved compared with that of single BiOI and BiOBr.When the molar ratio of BiOI and BiOBr was 2:8,the photocatalyst had the best mercury removal ability（98%）,which meant that the ratio of BiOBr to BiOI was the best and there was a significant synergy between them.With the increase of BiOI content in the composite,the diffraction peak intensity of BiOBr gradually decreased,while the diffraction peak intensity of BiOI gradually increased.The BiOI/BiOBr heterojunction photocatalyst retained the three-dimensional microsphere morphology of BiOBr,and micron-sized BiOI flakes were clearly observed,indicating a good contact between the two species.According to the free radical trapping experiments,^·O₂^-and h⁺were important active species in the efficient photocatalytic oxidation of Hg⁰.（2）The mercury removal activities of the ternary photocatalysts were greatly improved compared with the binary materials BiOI/BiOBr.The photocatalytic activity order was:Ag Br（5%）-BiOI/BiOBr>Ag Br（7%）-BiOI/BiOBr>Ag Br（3%）-BiOI/BiOBr>Ag Br（1%）-BiOI/BiOBr.Density functional theory calculations showed that the electron density of BiOI and Ag Br was reduced due to the electron transfer,and the electron density of BiOBr was enhanced,and an internal electric field of BiOI→BiOBr and Ag Br→BiOBr was formed at the interface,which accelerated the transfer of photogenerated electrons.The efficient mercury removal mechanism of the ternary Ag Br-BiOI/BiOBr photocatalyst followed the Z-type heterojunction charge transfer law.（3）When the loading amount of Ag₂WO₄ was 5%,the photocatalytic performance of elemental mercury removal reached 95.4%.SO₂ had a significant inhibitory effect on Ag₂WO₄（5%）-BiOI/BiOBr photocatalyst.Illumination was necessary for the excellent performance of the catalyst.The Ag₂WO₄（5%）-BiOI/BiOBr composite photocatalyst possessed a large pore size and specific surface area,which can provide more active sites,thereby enhancing the photocatalytic performance.