Research Of Gaseous Elemental Mercury Removal Through Electric Catalytic Via Gas Diffusion Electrode

Elemental mercury（Hg⁰）is the only one of heavy metal pollutants in gaseous form in the atmosphere,which is very easily transport by atmosphere and cause atmospheric mercury pollution in the globe.It can cause great harm to plants,animals and human bodies through the biological accumulation,therefore controlling mercury emission is imminent.This study adopts the electric catalytic reactor with a self-made gas diffusion electrode（GDE）to oxidize Hg⁰ from simulated gas.We have discussed H₂O₂ production,Hg⁰ the removal efficiency via GDE under different conditions and their mechanisms.Besides,the composite oxide systems such as Electric-Fenton System and Electric-Photocatalytic Oxidation System are also studied to remove Hg⁰.Firstly,the GDE who has smooth surface,well permeability and large "gas-liquid-solid"contact surface was prepared.The reaction to produce H₂O₂ was conducted in a cylindrical reactor,self-made GDE used as cathode,Ti/IrO₂ as anode.Simulated gas entered in from the bottom of the reactor and diffused under certain pressure through GDE.When in an energized state,O₂ from simulated gas reacted through oxygen reduction reaction（ORR）to produce a large amount of H₂O₂ on the three phase interface.We studied the effect factors of H₂O₂ production,such as the variety of carbon,electrolytic voltage,concentration of electrolyte and so on.The results show that the GDE prepared by carbon black（Vulcan XC-72）has better H₂O₂ production and current efficiency than made by graphite.After 120 minours’ reaction,H₂O₂ centration was 24 mmol/L under 4 V and its current efficiency is above 60%,while the voltage up to 6 V,the concentration was triple as 3 V.When the electrolyte concentration increases to 0.8 mol/L,the output of H₂O₂ is 51.6 mmol/L.When the oxygen content increased feom 6%to 21%,the change of H₂O₂ is less than 5%.The optimal pH value is 7,acid or basic are unbeneficial to the increase H₂O₂ amount.Too high temperature is not conducive to the accumulation of H₂O₂ in the solution,when it reached to 70 ℃,H₂O₂ concentration was only 1.9 mmol/L.Secondly,the GDE which was efficiently produced H₂O₂ was adopted to remove Hg⁰ through electric catalytic oxidation.The simulated gas is composed of N₂,21%O₂ and certain amount of Hg⁰.It was found that it can remove Hg⁰ efficiently and repeatable.We discussed the effect factors above and found that GDE made by carbon black is better than the one made by graphite,which can remove 33.9%Hg⁰ under the condition of initial Hg⁰ concentration was 350 μg/m3.When the voltage increased from 3 V to 6 V,Hg⁰ remove rate raise from 21%to 46%.Increased electrolyte concentration can also benefit Hg⁰ removal,the removal efficiency can up to 72%when the electrolyte is 0.8 mol/L.It turned out that relatively high H₂O₂ concentration was beneficial to Hg⁰ removal.However,Hg⁰ removal rate was not only relied on the concentration of H₂O₂,but more affected by the strong oxidizing free radicals,which were mainly derived from ORR reaction.Electrolyte pH affects the production of H₂O₂ and its stability.The optimal pH is 11,Hgo removal rate was 60%.High temperature will also promote the decomposition of H₂O₂.While the electrolyte temperature is 70 ℃,the corresponding removal rate reached to 91%.Thirdly,certain amount of Fe²⁺ was added to the electrolyte to form Electric-Fenton System to enhance the oxidation.Its Hg⁰ removal efficiency was greatly improved compared with the electric catalytic oxidation system.When adding 10 mmol Fe²⁺,Hg⁰ removal rate reached 82.6%,which increased three times as the one without Fe²⁺ and the optimal pH is 3.However,during the reaction,iron mud appeared and blocked some narrow channel on GDE,which led to the poor repeatability electrode.Lastly,by introducing an ultraviolet light into the electric catalytic system to form an UV-electric oxidation system to strengthen Hg⁰ removal.The complex system is mainly used the energy of UV,which can excite Hg⁰,meanwhile,it promotes H₂O₂ to be further decomposed into hydroxyl radicals with strong oxidizing property.Therefore,the composite system can remove Hg⁰ efficiently.Turn on UV lamp under 4 V,0.1 mol/LNa₂SO₄ as electrolyte,Hg⁰ remove rate reached to 70%.Besides,its remove efficiency increased with the increment of voltage,gas flow rate,electrolyte concentration and so on.In order to improve the oxidation of the system,electric system and UV system were combined in series was composed and 86%Hg⁰ could be removed in such secondary treatment.