Specific elevated adsorption and stability of cations in the interlayer compared with at the external surface of clay minerals

Abstract

Clay minerals in soils contain ubiquitously abundant interlayer spaces; however, the vital role the interlayer plays in determining the adsorption and stability of aqueous species is still enigmatic. The aim of this work is to systematically address the specific elevated adsorption characteristics and underlying mechanism in the clay interlayer by carefully comparing with those at the external surface. A series of cations (Na+, K+, Cs+, Ca2+, Pb2+, Cd2+, Ba2+, and Zn2+) are considered and the molecular dynamics methods are conducted. The interlayer of clay minerals is highly compacted, and the electric double layer (EDL) structures of neighboring surfaces extensively overlap. This results in specific elevated adsorption and stability of cations confined in the interlayer, as compared to the external surface. The adsorption modes remained the same whereas the interfacial distributions clearly changed, and the adsorption numbers increased. The local structure alteration was obviously more pronounced for divalent (M2+) rather than monovalent (M+) cations, and M2+ ions were coordinated stably to approximately six surface oxygens on each side of the interlayer surface. Extremely condensed time-evolution trajectories of the interlayer cations predicted an enhanced stability, which was further evidenced by the generally increased averaged residence time with surface oxygens and substantially reduced diffusion coefficients. These results address the vital role of the clay interlayer in determining the adsorption and retention of metal ions in a soil solution, which may further help to manage radioactive and heavy metal ion pollution.