Low-consumption with efficient capture and characteristics of heavy metals from coal combustion by modified kaolin: Experimental and simulation studies

Abstract

Heavy metals produced by coal combustion tend to bring environmental pollution. The modified additives were prepared by kaolin to capture heavy metals from coal combustion and analyzed by XRD, XRF, FT-IR, SEM and BET. The experimental results showed that the retention rates of heavy metals were ranked as Cr > Zn > Pb > Cd. The best retention of heavy metals was achieved at 1000 °C, while the retention capacity gradually decreased from 1000 to 1300 °C. The trapping effect of heavy metals in dynamic experiment is better than static experiment at 1300 °C. The general ranking of the adsorption capacity of the additives for the four heavy metals was pseudo boehmite-metakaolin (PBK) > aluminum sulfate impregnated metakaolin (AlSK) > metakaolin (MKA) > original kaolin (OKA). The PBK was the best adsorbent with the greatest retention rates of 77.35 %, 74.68 %, 83.35 % and 80.41 % for Pb, Cd, Cr and Zn, respectively. In addition, PBK showed the most significant improvement in the trapping of four heavy metals, with an average improvement of 37.79 % compared to no additives. The geometry models of heavy metal compounds and adsorbents were optimized. The heavy metal oxides were more stable than the chlorides and were more easily adsorbed. There is competitive adsorption between oxides and chlorides of heavy metals. The Al-O-Si-H and Al-O-Si are the best adsorption surfaces for OKA and MKA, respectively. However, CdO and CdCl2 are hardly adsorbed on the Si-O surface of OKA. The frontline orbitals of four heavy metal oxides/chlorides, θ-Al2O3 and α-Al2O3 have been simulated. Computer simulations show that the mechanism of heavy metal oxide/chloride adsorption on additives is a stronger electron transfer induced interaction between the highest occupied molecular orbital of the adsorbate and the lower unoccupied molecular orbital of the adsorbent, which facilitates the formation of stable chemical bonds.