Isomorphic Substitutions in Clay Materials and Adsorption of Metal Ions onto External Surfaces: A DFT Investigation

摘要

Clay materials show particular adsorption performances while the mechanisms and influencing factors remain largely elusive, which are presently tackled by DFT calculations. Isomorphic substitutions pervasive in clay materials may bring about critical impacts on the adsorption processes. Octahedral substitutions in montmorillonite occur preferentially at specific T sites (referred to as octahedral or tetrahedral sites) or evenly at different T sites, which depend strongly on the ionic radii of heteroatoms. Structural perturbations resulting from isomorphic substitutions and substitution energies generally increase with ionic radii, except Be2+/Al3+ substitutions that cause structural collapses. Octahedral rather than tetrahedral substitutions are preferred, and, compared with montmorillonite, octahedral substitutions in hectorite are more facile while tetrahedral substitutions are more difficu furthermore, hectorite exhibits superior adsorption performances. The second heteroatoms are always more difficult to incorporate, and their locations depend significantly on the first heteroatoms. After analyzing the adsorption structures and energies, the various factors affecting the adsorption performances (identity of heteroatoms, crystallographically distinct T sites, structural alterations, quantity of negative charges, distance from charge centers to metal ions and source of negative charges) are discussed, with their respective contributions being estimated. Quantity of negative charges is the foremost factor that controls the adsorption performances, while other factors in certain circumstances can also play a critical role. Results are beneficial to understand the particular adsorption behaviors of clay materials.