Alkali-modified halloysite for enhanced cadmium/lead vapor capture: Focus on silicic surface via experimental and computational investigations

We attempted to activate the Si surface of Halloysite (Hal) to enhance its adsorption capacity for heavy metal (HM, CdCl2/PbCl2) vapors through thermal-alkali modification. A combination of experimental and simulation methods was used to analyze the stabilization mechanism of CdCl2/PbCl2 comprehensively. FTIR, 1H MAS NMR, and XPS results confirm the generation of Si-OH groups on the alkali-treated Hal (AHal) surface. The adsorption performance of adsorbents was investigated in a two-stage high-temperature adsorption setup, and the results show that a relatively short alkali treatment time (20 min) improved the adsorption capacity of Hal, while a longer treatment time (60 min) led to a negative impact. XPS results revealed that the adsorption of CdCl2/PbCl2 involved both physical and chemical adsorption. Grand Canonical Monte Carlo simulations demonstrated an increased adsorption potential of AHal and enhanced reactivity with the HMs. Density functional theory was employed to investigate the adsorption mechanism of Cd/Pb compounds on the Si surface. Compared to raw Hal, AHal exhibited higher adsorption energy for HMs after introducing Si-OH groups, resulting in a more stable configuration. The stabilization of Cd/Pb monomers on the Si surface primarily relies on promoting the H migration from Si-OH to interlayer O and then forming covalent bonds with the remaining active O atoms, while Si-OH played a role in facilitating the occurrence of De-HCl reaction, thereby transforming the adsorption of metal chlorides into more stable metal monomers adsorption.