Study On The Mechanism Of Gas-Phase Arsenic Adsorption On Manganese-Fe-Based Adsorbents

Gas-phase arsenic released from coal combustion has attracted much attention due to its large total amount,high toxicity and non-degradable characteristics.Gas-phase arsenic in coal-fired flue gas mainly exists in the form of As₂O₃,and the capture of As₂O₃in flue gas by metal oxide adsorbent is an effective control method.Manganese-iron adsorbent（Mn-Fe₂O₃）has the advantages of low cost,strong temperature adaptability,and excellent adsorption performance of As₂O₃,but its adsorption mechanism for As₂O₃is still unclear.Therefore,the quantum chemical calculation method is used to simulate the microscopic reaction mechanism of As₂O₃adsorption and oxidation on Mn-Fe₂O₃.The main work contents and results are as follows:Firstly,the adsorption mechanism of As₂O₃by Mn-Fe₂O₃adsorbent was studied by analyzing the formation energy,surface structure,electronic properties and the adsorption and oxidation performance of As₂O₃on the surface of Mn-Fe₂O₃（001）.The results show that Mn on the surface of Mn-Fe₂O₃（001）can weaken the bonding strength of the lattice O atoms and the surface,enhance the surface activity of the adsorbent,and convert the surrounding Fe to a high valence state to promote the adsorption of As₂O₃.Fe_3f(or Mn_3f)atoms and lattice O atoms on the surface of Mn-Fe₂O₃（001）are the active sites for As₂O₃adsorption;Mn in Mn-Fe₂O₃is beneficial to the activation of lattice O atoms,thereby promoting the adsorption and oxidation of As₂O₃;However,the introduction of Mn has little effect on improving the As₂O₃oxidation performance of the adsorbent.Furthermore,by calculating the adsorption and dissociation of O₂on the surface of Mn-Fe₂O₃（001）and its effect on the adsorption and oxidation of As₂O₃,studied the mechanism of the effect of O₂on the adsorption of As₂O₃by Mn-Fe₂O₃adsorbent.The results are as follows:O₂in the flue gas will compete with As₂O₃for adsorption,forming adsorbed O₂molecules on the surface of Mn-Fe₂O₃（001）,which is not conducive to the adsorption of As₂O₃;however,the adsorbed O atoms formed by the dissociation of O₂molecules can be very good.Mn in the adsorbent can promote the dissociation of O₂,and sufficient adsorbed O atoms are the key to promoting the adsorption and oxidation of As₂O₃;the process of the adsorption and oxidation of As₂O₃by Mn-Fe₂O₃is:O₂oxidizes the low-valence Mn in the adsorbent to a high-valence state,self-dissociated and reduced to O^-with high As₂O₃adsorption activity,after As₂O₃adsorption,it undertakes the task of accepting electrons and transporting electrons to Mn⁴⁺and Fe³⁺,thereby oxidizing the adsorbed As（III）into a less toxic and endangering As（V）with a more stable state exists,the adsorption of As₂O₃follows the Eley-Rideal mechanism.Finally,by calculating the adsorption,dissociation and oxidation of SO₂on the surface of Mn-Fe₂O₃（001）and the effect on the adsorption active sites of As₂O₃,the effect of the combined action of O₂and SO₂on the adsorption mechanism of As₂O₃on Mn-Fe₂O₃was studied.The results show that the mechanism of SO₂on As₂O₃adsorption is mainly as follows:SO₂in flue gas will undergo oxidation reaction with adsorbed O₂molecules that have no As₂O₃adsorption activity on the surface of Mn-Fe₂O₃（001）to generate easily desorbed SO₃and an adsorbed state.O atoms increase the active sites for As₂O₃adsorption;In addition,the S and O active sites formed by the dissociation of SO₂on the surface of Mn-Fe₂O₃（001）have high adsorption activity of As₂O₃,and also play a certain role in promoting the adsorption of As₂O₃.