Adsorption Mechanism of PbO/PbCl2 on Kaolinite Surfaces during Coal Combustion Based on Frontier Orbital Theory

In the field of heavy-metal emission control during coal combustion, kaolinite can be used as an additive in furnaces to effectively capture semivolatile heavy-metal elements. To investigate the microscopic mechanism of the interaction between PbO/PbCl2 and kaolinite/metakaolin, quantum chemistry computations were utilized to examine the interaction among the frontier orbitals during the adsorption process and the active variation occurring during the thermal conversion of kaolinite. The calculations indicate that metakaolin possesses a stronger adsorption activity than that of kaolinite. The lowest unoccupied molecular orbital (LUMO) of kaolinite (DeOH 001) and the highest occupied molecular orbital (HOMO) of PbO/PbCl2 effortlessly interact in the adsorption process. PbO is more conveniently adsorbed on the surface of kaolinite (DeOH 001) than PbCl2. During the dehydroxylation of kaolinite, the active sites of adsorption move to the vicinity of V/IV-coordinated Al, which is favorable for the adsorption of PbO/PbCl2. Nevertheless, III-coordinated Al in metakaolin and the thermal decomposition products (mullite and cristobalite) lowers the adsorption activity. During the adsorption process, PbO/PbCl2 and the surface of kaolinite (DeOH 001) form stable bonding orbitals and unstable antibonding orbitals, respectively. In addition, charge transfer mainly takes place in the p orbitals. We expect that the results can provide effective guidance and theoretical support for the emission control of the heavy metals PbO/PbCl2 by kaolinite during coal combustion and the modification of kaolinite additives in high-temperature furnaces.