Interaction mechanism between unburned carbon in coal-fired fly ash and arsenic in flue gas based on the density functional theory

The interaction mechanism between the unburned carbon in fly ash and the arsenic pollutants in flue gas such as As, AsO, AsO₂ and As₂O₃ was studied based on the density functional theory. The results show that the elemental arsenic is preferentially adsorbed at the carbon bridge site, with an adsorption energy in the range (-5.95)-(-5.88) eV; the AsO molecule preferentially combines with the unburned carbon in a way that the arsenic and oxygen atoms are bound with the surface carbon atoms respectively, forming a most stable configuration with an adsorption energy of -7.87 eV. When AsO₂ is dissociated on the unburned carbon surface and form an AsO molecule and a surface reactive oxygen species, the system is the most stable, possessing an adsorption energy of -10.65 eV. While once the two oxygen atoms in a trigonal bipyramid As₂O₃ molecule first collide with the unburned carbon surface, it will be dissociated to small molecules of AsO and AsO₂, forming a covalent bond with surface carbon. The adsorption energy is significantly reduced to -10.64 eV, compared with the undissociated case. The unburned carbon in fly ash is easy to bind with AsO or AsO₂ small molecules, which locally tends to form a special five-member ring structure. Compared with As, AsO and AsO₂, the most toxic trivalent arsenic As₂O₃ is chemically stable and not easy to adsorb. Catalytic pyrolysis of As₂O₃ into small molecules of AsO and AsO₂ is expected to be a feasible measure to control the arsenic pollution in coal-fired power plants flue gas.