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

Abstract

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 difficult; 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.

Magnetic properties of synthetic fluorophlogopite mica crystals

“…It is worth noting that in 2 : 1 layered phyllosilicates both the tetrahedrally and octahedrally coordinated cations are prone to considerable substitution by other metal ions. In particular, this concerns the interchangeable Al 3+ and Si 4+ cations in the tetrahedral sites and the substitutions by Fe 2+ and Ti 4+ of octahedral Mg 2+ , 31,[41][42][43] and the resulting charges can be eventually balanced by the cations in the interlayer positions (see Fig. 1).…”

Machine-learning-accelerated multimodal characterization and multiobjective design optimization of natural porous materials

“…10 Moreover, clay morphology is critically affected by the pervasive substitutions in the lattice structure and by exchangeable cations affecting the layer charge and swelling properties. 10,17 The structural dissimilarities between various clays are reflected in the surface reactivity, which can be approximated by Brøensted and Lewis acid-base characters. [18][19][20] Generally, the active sites reside predominantly in the particle edges, in the hydroxylic termination of octahedral sheets and on the basal oxygen of tetrahedral sheets.…”

“…23,24 Furthermore, defects in the chemical structure may cause charge vacancies compensated by diverse ions influencing the catalytic behavior or the affinity with certain molecules. 17 Such modification of natural nanoclays may drastically change the morphology and surface activity, and in some cases it may also lead to loss of crystallity. 14,25,26 Fig.…”

Effect of Crystal Chemistry Properties on the Distribution Characteristics of H2O and Na+ in Na-Montmorillonite Interlayer Space: Molecular Dynamics Simulation Study

“…The structure of MT is one Al-O(OH) octahedral sheet sandwiched between two Si-O tetrahedral sheets [2]. Isomorphic substitution in the structure of MT is a requirement, that is Al 3+ are substituted by Mg 2+ in the octahedron and Si 4+ are substituted by Al 3+ in the tetrahedron [3]. As a result of heterovalent isomorphic substitution, the layers of MT are permanently negatively charged.…”