Performance And Mechanism Of Transition Metal Sulfides For Capturing High Concentration Gaseous Elemental Mercury From Smelting Flue Gas

Mercury pollution has increasingly become a hot environmental issue of global concern.As a party to the Minamata Convention and the country with the largest mercury emissions in the world,China is facing tremendous pressure to reduce mercury emissions.The metal smelting industry is the third largest source of anthropogenic mercury pollution in my country,and it is imperative to implement mercury pollution reduction.Mercury mainly exists in three forms of gaseous elemental mercury(Hg⁰),divalent mercury(Hg²⁺)and particulate mercury(Hg^p)in smelting flue gas.Since gaseous elemental mercury has high volatility and low water solubility,it is difficult to remove by existing flue gas purification devices.Therefore,the key to controlling mercury pollution in the non-ferrous metal smelting industry is to control the emission of high-concentration gaseous elemental mercury in the smelting flue gas.The Boliden-Norzink method is a common method to remove high-concentration gaseous elemental mercury in smelting flue gas,but it uses a highly toxic mercury chloride solution as an absorption liquid,and the process is complicated and has potential safety risks.To this effect,our research group has proposed a new idea of using renewable monolithic titanium dioxide supported transition metal sulfides to recover high-concentration gaseous elemental mercury in smelting flue gas through adsorption.The development of transition metal sulfide adsorbents with excellent gaseous elemental mercury adsorption performance is the key to the implementation of this smelting flue gas mercury pollution control technology.The transition metal sulfides CuS_x/TiO₂ and FeS_x/TiO₂ had been widely used in the removal of high-concentration gaseous elemental mercury in smelting flue gas.However,their gaseous elemental mercury adsorption characteristics were significantly different,and the mercury adsorbed on the CuS_x/TiO₂ surface would be spontaneously desorbed.To this end,this study compared the adsorption characteristics of gaseous elemental mercury on CuS_x/TiO₂ and FeS_x/TiO₂ surfaces,established an adsorption/desorption kinetic model based on the adsorption mechanism of gaseous elemental mercury on CuS_x/TiO₂ and FeS_x/TiO₂ surfaces,and compared the adsorption/desorption kinetic parameters which revealed the reason why CuS_x/TiO₂ and FeS_x/TiO₂ adsorbed gaseous elemental mercury with different adsorption characteristics.Further more,by changing the precursor of CuS_x/TiO₂ to improve the performance of CuS_x/TiO₂ in adsorbing gaseous elemental mercury while inhibiting the desorption of gaseous elemental mercury,a transition metal sulfide that can realize the recovery of high-concentration gaseous elemental mercury under simulated conditions had been developed.The main research results of this article are as follows:(1)CuS_x/TiO₂ and FeS_x/TiO₂ exhibited different characteristics of adsorbing gaseous elemental mercury.The HgS adsorbed on the surface of CuS_x/TiO₂ would be spontaneously released in the form of gaseous elemental mercury.During the oxidation of elemental mercury in the physical adsorption state,the Cu-S bond on the CuS_x/TiO₂ surface would not be completely broken,while the S-S bond on the FeS_x/TiO₂ surface would.Therefore,the activation energy of CuS_x/TiO₂ oxidation physical adsorption state elemental mercury was much smaller than FeS_x/TiO₂,so CuS_x/TiO₂ exhibited better oxidation physical adsorption state elemental mercury performance than FeS_x/TiO₂.However,as the HgS adsorbed on the CuS_x/TiO₂ surface would share S^2-with Cu,the Hg-S bond of HgS on the CuS_x/TiO₂ surface was very weak,resulting in poor thermal stability.Therefore,the HgS adsorbed on the CuS_x/TiO₂ surface is metastable and could be spontaneously desorbed in the form of gaseous elemental mercury.(2)The amount of gaseous elemental mercury adsorbed by CuS_x/TiO₂ synthesized with CuSO₄ as the precursor(CuS_x/TiO₂-S)was 1.2-1.4 times that of CuS_x/TiO₂ synthesized with Cu(NO₃)₂ as the precursor(CuS_x/TiO₂-N).The amount of gaseous elemental mercury desorption from CuS_x/TiO₂-S was 4.4-17.2% of the adsorbed amount,which was much smaller than CuS_x/TiO₂-N.Because CuSO₄ had excellent thermal stability,it could maintain its inherent structure after calcination at 500 ?,while Cu(NO₃)₂ would decompose into CuO and Cu₂O.Therefore,during the vulcanization process,more active S species(S₂^2-)would be generated on the surface of CuS_x/TiO₂-S.At the same time,the number of unsaturated coordination on the surface of CuS_x/TiO₂-S was far greater than that of CuS_x/TiO₂-N.Therefore,CuS_x/TiO₂-S not only showed better gaseous elemental mercury adsorption performance than CuS_x/TiO₂-N,but also had less desorption of gaseous elemental mercury.Therefore,CuS_x/TiO₂-S could realize the recovery of high-concentration gaseous elemental mercury in smelting flue gas under simulated conditions.