The adsorption mechanism of heavy metals from coal combustion by modified kaolin: Experimental and theoretical...

Experimental and theoretical studies are combined to analyze the adsorption properties of modified kaolin for heavy metal (Pb, Cd, Zn and Cr) from coal combustion. The results indicate that the retention effect of kaolin for Pb, Cd, Zn and Cr has been significantly enhanced after intercalation-exfoliation combined with acid/alkali modification, which is mainly attributed to more active sites for adsorption, richer porosity and more effective in retarding coking of coal ash. The higher oxygen concentration is positive to the enrichment of heavy metals at 900-1200 ℃, while the coking of coal ash and the thermal conversion of additives become the main factors affecting the absorption at 1200-1300 ℃. The acid/alkali modification effectively promotes the inductive effect of electron transfer between modified kaolin and heavy metals to form stable chemical adsorption. The electron transfer induction of modified kaolin for Pb, Cd is higher than Zn, Cr at 900-1000 ℃, while the adsorption activity of mullite and cristobalite for Zn, Cr is stronger than Pb, Cd at 1200-1300 ℃. In addition, Pb, Cd and Zn are more readily adsorbed as oxides by additives at 900-1300 °C. The results shed new light on strengthening the adsorption activity of kaolin to Pb, Cd, Zn and Cr in high temperature.