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.
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