Experimental, Grand canonical Monte Carlo, and Density Functional Theory studies for comparing halloysite and kaolinite on heavy metals fate during pyrolysis of solid waste

Abstract

In this study, we utilized low-cost halloysite (Hal) for the first time to enhance the solid-phase enrichment and stability of heavy metals (HMs) during solid waste pyrolysis through experimental and theoretical methods, and compared with kaolinite (Kao). Experimental results demonstrated that Hal was superior to Kao in improving the solid-phase enrichment of HMs. Specifically, the solid-phase enrichment of Cd increased by 32.6 % (500 °C) and 25.94 % (600 °C), while that of Pb and Zn increased by 17.37 %/16.83 % and 19.82 %/22.37 % (700/800 °C), respectively. Adding Hal reduced the proportion of HMs in the unstable fraction (F1 + F2), consequently lowering the environmental risk of biochar and the extractable state of HMs. Through Grand canonical Monte Carlo and Density Functional Theory (DFT) simulations, we analyzed the adsorption amounts, adsorption sites, and adsorption mechanisms of Cd/Pb compound on Hal/Kao surfaces, revealing that the primary factor influencing the adsorption performance of Hal and Kao was the difference in specific surface area. The adsorption amounts of HMs by Hal were significantly higher than Kao and decreased with increasing temperature, while the difference in adsorption performance caused by structural bending was negligible. The DFT results indicated that Cd and Pb monomers were stabilized by establishing covalent bonds with OH or reactive O atoms on the Al-(001) surface, whereas the covalent bonds with ionic bonding properties formed between Cl atoms and unsaturated Al atoms played a crucial role in stabilizing HM chlorides. Furthermore, the adsorption energy of Hal on HMs increased with the removal rate of OH. Our study highlights the potential of Hal in stabilizing HMs during pyrolysis without requiring any modifications, thereby avoiding the generation of modified waste solutions and unnecessary cost loss.