Coupling of Adsorption Site and Cation Ratio Regulates the Adsorption of Cs+ and Na+ at the Surface of Clay Mineral

Abstract

Na+ and Cs+ ions have always co-existed in natural Cs+-bearing soil environments, and a vast number of research works have investigated their adsorption at clay surface. However, in fact, which one has a stronger adsorption affinity and what is the underlying mechanism are still controversial. In the present work, adsorption of Cs+/Na+ with five different ratios (ϕ1 ~ ϕ5) at the surface of illite was systematically investigated, using molecular dynamics simulations. Site-specific adsorption phenomenon occurred for both Cs+ and Na+. Cs+ ions have three adsorption sites (TA, TB, and TC) in the siloxane cavity while Na+ ions have one more that close to the substitution site (TD). Though Na+ had a substantially stronger adsorption affinity compared to Cs+ at TA/TB/TC sites, its adsorption at TD site was significantly weaker. Considering the specificity of different adsorption site, the total effective adsorption affinity increased as Na+ (ϕ5) < Cs+ (ϕ1) based on their pure solutions while increased as Cs+ < Na+ when they were mixed together (ϕ2 ~ ϕ4). Therefore, the relative adsorption affinity of Cs+ and Na+ should be further measured by considering the coupling effect of adsorption site and Cs+/Na+ ratio, besides the effect of ionic radius and hydration energy. It is likely that the common understanding on the adsorption of Cs+ and Na+ relies too much on pure system. In this way, this work provided a new clue to understand the affinity of mixed cations at the clay-water interface, which may be useful for assessing the transport and bioavailability of elements in soils.