Study On Adsorption/Capture Mechanism And High Temperature Sintering Solidification Of Heavy Metal Arsenic

Fly ash is fine ash collected from the flue gas after coal combustion,and it is one of the main solid wastes discharged from coal-fired power plants.Long-term storage of a large amount of fly ash will not only pollute the atmosphere,water sources and soil,but also endanger human health through various ways.The preparation of ceramsite from fly ash can not only reduce the threat to the environment and human health to a great extent,but also promote the development of circular economy.However,in the actual production process of fly ash ceramsite,especially in the high-temperature calcination process,the volatile heavy metal arsenic originally enriched in fly ash will enter the flue gas again in the form of gaseous arsenic,resulting in poisoning and deactivation of the denitration catalyst.On the other hand,fly ash ceramsite used as building materials is often eroded by acid rain,and the heavy metal arsenic in it may leach out and pollute the surrounding environment through migration and diffusion.Therefore,it is necessary to study the adsorption and removal methods of arsenic in high temperature flue gas and aqueous solution.In addition,considering the long service life of fly ash ceramsite,it is necessary to clarify the relationship between the structural characteristics of ceramsite and the solidification effect of heavy metal arsenic and conduct corresponding environmental risk assessment.Based on the above considerations,in this paper,aiming at the environmental and technical problems existing in the preparation and use of ceramsite,the following research work has been carried out:Aiming at the possible arsenic pollution in air and water during the preparation and use of ceramsite,the corresponding arsenic pollution removal research was carried out respectively.First of all,in order to construct a capture method suitable for gaseous arsenic in high-temperature flue gas and reveal its fixation mechanism,calcium oxide and iron oxide were taken as the research objects,and the adsorption and removal of gaseous arsenic in high-temperature flue gas were studied.For calcium oxide,the effects of adsorption temperature,flue gas components（SO₂,NO）,oxygen concentration and other factors on arsenic capture performance were investigated and optimum working conditions of arsenic removal reaction was determined.By analyzing the speciation of key active metal and heavy metal arsenic on the surface of materials before and after adsorption,the fixation mechanism of gaseous arsenic in flue gas on the surface of calcium oxide adsorbent was revealed.In addition,in the study of gas-phase arsenic adsorption,it was found that adsorbent materials were easy to be sintered and deactivated in high temperature environment,which led to the decline of gas-phase arsenic capture performance.In order to solve this problem effectively,iron oxide with hollow sphere structure was designed and constructed directionally.Combined with structural and morphological analysis techniques,it could be seen that even in the high temperature environment of 1200°C,iron oxide with hollow sphere structure still retained developed pore structure,and the surfaces of its inner and outer spheres could provide a large number of adsorption sites for gas-phase arsenic oxide molecules.Based on this structural advantage,iron oxide with hollow sphere structure showed good sintering resistance and arsenic capture effect in high temperature flue gas.The adsorption mechanism of arsenic was analyzed from the molecular level by first-principles calculation,and the adsorption mode and configuration of gaseous arsenic oxide on the surface of iron oxide were determined.This part not only illustrated the adsorption/fixation process of gaseous arsenic on the surfaces of calcium oxide and iron oxide in high temperature flue gas,but also provided a theoretical basis for revealing the interaction between heavy metal arsenic and calcium-based and iron-based mineral components during ceramsite sintering and clarifying the solidification mechanism of heavy metal arsenic.Secondly,the experimental study on adsorption and removal of arsenic in aqueous solution was carried out to explore the adsorption behavior of ionic arsenic-containing compounds on the surface of materials and reveal the reaction mechanism.Fe-Ce bimetallic oxide nanosheets with exposed specific crystal faces were prepared by hydrothermal method,and the relationship between structural characteristics and arsenic adsorption effect of the material was established,so as to conduct directional control and performance optimization of the material.By investigating the influence of main working conditions（p H,coexisting ions）on the adsorption effect of heavy metal arsenic,its applicability in practical environment was evaluated.In addition,the relationship between morphology,physical and chemical properties and oxidation-complexation properties of materials was clarified.The existence and relative contents of arsenic and oxygen in the reaction products were analyzed and determined by XPS and ICP-AFS,and the reaction path was deduced and analyzed by quantum chemical calculation method.This work clarified the adsorption process of arsenic compounds in water on the surface of iron oxide,revealed the bonding mode of arsenic atoms on the surface of material,and provided theoretical basis for studying the control method of arsenic compounds in water.Aiming at the technical problems existing in the preparation and calcination process of fly ash ceramsite,that is,the burning loss rate of fly ash ceramsite is too high,and the solidification of heavy metal arsenic needs to be investigated,the relationship between its main element composition,crystal composition and structural characteristics and the solidification effect of heavy metal arsenic was established,so as to systematically evaluate the structural stability and durability of lightweight aggregate and optimize the preparation process of ceramsite.According to the results of thermodynamic calculation and thermogravimetric analysis,the chemical reactions and phase changes involved in the calcination process were analyzed in detail to deeply explore the volatilization behavior and solidification process of heavy metal arsenic.In addition,the gas-solid distribution of heavy metal arsenic during high temperature calcination of fly ash ceramsite was studied by quantitative analysis.The results showed that the leaching concentration of heavy metal arsenic in ceramsite could be greatly reduced to below the environmental standard value by adjusting the raw material ratio and calcination temperature,but a small amount of gas-phase arsenic would still volatilize into the flue gas during calcination,which might cause atmospheric environmental pollution.After completing the optimization of ceramsite preparation process and the study on the volatilization behavior of heavy metal arsenic,the research on the fixation mechanism of heavy metal arsenic in ceramsite continued.Based on the Environmental Protection Agency Method 1313（EPA Method 1313）,an experiment was carried out to characterize the leaching equilibrium of main elements and heavy metal arsenic from ceramsite,and a geochemical speciation model of heavy metals in aggregate medium was established to reveal the solubility and leaching control mechanism of heavy metal arsenic at different p H values.According to this model,the leaching process of heavy metal arsenic is controlled by calcium hydroxide arsenate（Ca₅（OH）（As O₄）₃）and iron arsenate（Fe As O₄）in ceramsite.In addition,the environmental risk of fly ash ceramsite and its long-term effect of heavy metal arsenic solidification were also evaluated,which proved the safety of ceramsite in neutral and alkaline environment,but in acidic environment,the leaching concentration of heavy metal arsenic in ceramsite was high,which was easy to cause water pollution.