Removal Of SeO₂ From Flue Gas Utilizing Fenton-like Reaction

Selenium is one of the most volatile trace metal emissions in coal burning and utility boilers.In biology,selenium is an essential trace element,but is toxic in larger doses^[1].Acute selenium poisoning occurs when the human body is exposed to high concentration of Se O₂^[2].Long-term accumulation of selenium in the human body can also lead to chronic poisoning and various systemic symptoms^[3].China is one of the countries with high incidence of selenium poisoning.In Shaanxi and Hubei province,selenium poisoning incidents have occurred in people and livestock^[4].China’s coal-based energy production makes coal-burning become the primary anthropogenic source of selenium in ambient air.In coal combustion conditions,selenium is predominantly present in the elemental state（Se）and the oxide form（Se O₂）^[5][6][7].Se can be efficiently removal by the current air pollution control devices,such as electrostatic precipitator and wet flue gas desulfurization systems,respectively.The high volatility and its existence in the form of vapor make Se O₂ emission control very difficult by the conventional air pollution control devices^[8][9].Considering selenate has less impact on the environment and that it is the most stable oxidizing form of selenium in natural environment,oxidation of Se O₂ to selenate is a promising method for the removal of Se O₂^[4].In this study,Fenton-like catalyst was used in the bubbler reactor to achieve the conversion of Se⁴⁺to Se⁶⁺,so as to solve the problem that Se O₂is difficult to be removed by smoke pollution control equipment.:（1）The Fenton-like reaction was used to remove Se O₂ in the bubbler reactor,and the removal efficiency of Se O₂ was improved by changing the catalytic conditions.（2）Fe_3-xCr_xO₄ was prepared by hydrothermal synthesis method,and the catalyst was characterized by SEM,EDS,XRD and XPS.The results showed that there were redox pairs of Fe_surf2+/Fe³⁺_surf and Cr²⁺_surf/Cr³⁺_surf on the catalyst surface,which generated hydroxyl radical（·OH）by decomposing H₂O₂ and promoted the removal of Se O₂.（3）The Se O₂ oxidation removal in simulated flue gas was studied in the reaction device,and the effects of Cr content,H₂O₂ dosage,Fe_3-xCr_xO₄ dosage,p H values,temperature and different flue gas components on the treatment effect were discussed.The results showed that the average removal rate of Se O₂ reached 88.5%under the optimal experimental conditions of Fe_3-xCr_xO₄ with x value of 0.6,Fe_3-xCr_xO₄ dosage of 1 g·L^-1,H₂O₂ concentration of 5 mmol·L^-1,reaction temperature of 50℃and solution p H of 5.6.In addition,high concentration of SO₂ and NO inhibited Se O₂removal due to Se O₂’s competition with two coexistence gases,while low concentration of NO slightly promoted Se O₂ removal.The stability of the catalyst was tested through continuous experiments.After 5 catalyst cycles,the removal rate of Se O₂ remained at 83%,proving that Fe_3-xCr_xO₄ was highly recyclable.（4）the quenching experiment of hydroxyl radical（·OH）with tert-butanol proved that·OH was the main oxidizing agent.The vast majority of H₂O₂ reacts with Fe and Cr redox pairs on the catalyst surface,and decompose into hydroxyl radical（·OH）with strong oxidization and Se O₂,so as to remove Se O₂.The product was analyzed by XPS,and it was confirmed that Se⁴⁺was oxidized to Se⁶⁺.（5）According to the characterization results and relevant literature,the reaction mechanism of Se O₂ and heterogeneous Fenton-like reagent was preliminarily speculated.The results will provide a new method for the removal of Se O₂ by Fenton reagent.