Preparation,Characterization And Wet Process Removal Of Elemental Mercury By Modified BiOI-based Photocatalysts

Mercury has become a global problem due to the acute toxicity of mercury and its ability to circulate in ecosystems and bioaccumulation in the food chain.At present,anthropogenic mercury emissions are mainly from coal-fired power plants.More than 140 nations around the word adopted the first legally binding international treaty to set enforceable limits on emissions of mercury and exclude,phase out,or restrict some products that contain mercury in January2013.Mercury in flue gas contains three forms:elemental mercury?Hg⁰?,oxidized mercury(Hg²⁺)and particle-bound mercury?Hg^p?.Hg²⁺and Hg^p can be removed by the existing wet flue gas desulfurization,electrostatic precipitator and bag filter.Compared with Hg²⁺and Hg^p,Hg⁰ is much more difficult to capture due to its high volatility and low solubility in water.The effective removal of Hg⁰ is a difficult problem in the field of atmospheric mercury pollution.In this paper,Bi₅O₇I,BiOI-EDTA and series of Ag?x?PANI/BiOI photocatalysts were prepared.The effects of preparation method,Ag content,fluorescent light irradiation,reaction temperature,pH value,flue gas composition,cycling tests and scavenger addition on Hg⁰removal using these photocatalysts were investigated in detail on a wet bubbling photocatalytic reactor.The as-synthesized photocatalysts were characterized by N₂ adsorption-desorption,SEM,TEM,XRD,DRS,XPS,ESR,PL and photocurrent response.The main results of the work are summarized as follows:?1?The two Bi₅O₇I photocatalysts obtained from modifying BiOI with NaOH solution have different effects on the removal of Hg⁰.The mechanism for higher Hg⁰ removal efficiency using Bi₅O₇I-HAc photocatalyst was analyzed by series of experiments and characterization.?2?BiOI-EDTA photocatalysts with different mole ratio of I/Bi were prepared with the aid of EDTA solvent.It was found that there is a big difference on Hg⁰ removal efficiency when changing the I/Bi mole ratio.Among them,BiOI-E1 with a molar ratio of 1:1 showed the best Hg⁰ removal performance.The addition of EDTA-2Na would lead to the appearance of metal Bi and the increase of chemical oxygen content,which probably was the main reason for its enhanced Hg⁰ removal.?3?The Hg⁰ removal performance of BiOI photocatalyst modified by PANI was significantly improved in comparison to pure BiOI.Moreover,Hg⁰ removal activity of series of Ag?x?PANI/BiOI photocatalysts was also greatly enhanced after adding Ag⁰ nanoparticles.The optimal Ag?4%?PANI/BiOI possessed the highest Hg⁰ removal efficiency.The presence of Ag⁰nanoparticles can enhance the separation of e^--h⁺pair on the photocatalyst surface,thereby leading to much superior mercury removal performance.?4?The presence of SO₂ and NO would react with the reactive spescies,resulting in a decrease in mercury removal efficiency.When SO₂ and NO stream were cut off,Hg⁰ removal efficiency could be returned to anout their initial values,indicating that photocatalyst structure would be not damaged in the researched reaction time.?5?The reative species such as^·O₂^-and h⁺played crutial roles in the process of Hg⁰ catalytic oxidation,and the fluorescent lamp irradiation was essential for the efficient removal of Hg⁰.