Effect of alkaline metals (Na, Ca) on heavy metals adsorption by kaolinite during coal combustion: Experimental and DFT studies

Abstract

Kaolinite can adsorb heavy metals (Cu, Zn, Pb, and Cd) and alkaline metals (Na and Ca) during coal combustion. However, the effect of alkaline metals on the heavy metals (HMs) adsorption by kaolinite is not clear. In this paper, in order to minimize the interference of associated minerals in the coal during the HMs adsorption by kaolinite, demineralized coal and kaolinite were assembled as base coal. Then experimental and density functional theory (DFT) were combined to analyze the effect of NaCl, Na2CO3, and CaO on the adsorption of Cu, Zn, Pb, and Cd by kaolinite. The results showed that the addition of NaCl decreased the fixation rate of Pb from 46.54% to 4.27% at 1000 °C. The fixation rates of HMs gradually decreased with the addition of Na2CO3. However, the addition of CaO did not cause a significant reduction in the fixation rates of HMs. Meanwhile, the alkaline metals increased the leaching risk of HMs from the ash. From 800 °C to 1200 °C, the fixation rates of Pb and Cd fell significantly with increasing temperatures at a CaO loading amount of 6 wt%. However,the fixation rates of Cu and Zn were little affected by increasing temperatures. The DFT calculations indicated that the adsorption of HMs on the metakaolin(001) surface was inhibited by NaCl and Na2O. Effective adsorption of HMs was still feasible after the combination of CaO and metakaolin, and Ca was the adsorption site for O (from oxidized HMs) atoms. This work revealed the effect and mechanisms of alkaline metals on the HMs adsorption by kaolinite.