Insight into the high-temperature adsorption of PbO/PbCl2 by kaolinite: An experimental and DFT study on the role of H2O/–OH

Abstract

Water vapor and surface hydroxyl groups were considered to participate in the high-temperature adsorption of metal vapor by kaolinite; however, the effect and the mechanisms were not clear yet. In this study, we investigated the effect of H2O on PbO/PbCl2 adsorption by kaolinite at high temperatures by drop tube furnace (DTF) experiments at 800-1300℃, and further investigated the role of H2O/–OH in adsorption by DTF experiments and density functional theory (DFT) calculations. It was found that H2O inhibited the PbO adsorption but enhanced the PbCl2 adsorption. No significant interactions between H2O and PbO/PbCl2 were found before adsorption, so the interactions between H2O and kaolinite surface were considered to be responsible for the effect of H2O. At high temperatures, H2O inhibited the conversion of –OH on kaolinite surface to H2O mainly by the reverse conversion of H2O to –OH. The residual –OH on kaolinite surface inhibited the adsorption of PbO molecule and PbCl2 molecule by the occupation of the adsorption sites. However, dechlorination was another important step for PbCl2 adsorption, which made the unstable chloride adsorption (adsorption energy ≈ 200 kJ/mol) transform to stable oxide adsorption (adsorption energy > 500 kJ/mol). –OH on the surface could lower the dechlorination energy from around 400 kJ/mol to around 200 kJ/mol. Therefore, H2O enhanced the adsorption of PbCl2 by increasing the amount of –OH on kaolinite surface. The optimum adsorption temperatures of PbO adsorption and PbCl2 adsorption were 1200℃ and 1000℃, respectively.