Adsorption of Cesium at the External Surface of TOT Type Clay Mineral: Effect of the Interlayer Cation and the Hydrated State

Abstract

The adsorption at the clay−water interface plays a critical role in many chemical, environmental, and geological processes as well as technological applications. Adsorption at the external surface and in the clay interlayer has been widely studied, whereas the relationship between them is always overlooked. Here, we specifically addressed the effect of the interlayer on the adsorption of Cs + ions at the external surface of 2:1 tetrahedral SiO 4 (T) and octahedral AlO 6 (O) sheet (TOT)-type clay mineral using molecular dynamics simulation, considering the absence and presence of the clay interlayer, three interlayer cations, and four hydrated states. It is not recommended to use a single clay layer to predict the adsorption at the external surface of TOT-type clay mineral because the absence of the clay interlayer will overestimate the adsorption affinity. Further studies suggested that adsorption of Cs + at the external surface depends sensitively on the nature of the interlayer cation and the hydrated state. The stability of external Cs + ions was affected by the interlayer cation and increased as Na + < K + < Cs + , which is caused by the different interactions of the interlayer cation and clay layers. The hydrated state of the clay interlayer also exerts a significant effect and the stability of Cs + at the external surface is enhanced in the order of 1W < 2W < 3W, determined by the interlayer water rather than the interlayer cation according to results of energy analysis. The present work may provide a new insight into the understanding of interfacial chemistry between clay minerals and aqueous solutions and further the accurate assessment of geochemical behavior of elements.

Adsorption of cations at the illite–water interface and its effect on intrinsic potassium ions

“…A total of 48 Al 3+ /Si 4+ isomorphic substitutions were randomly introduced into the tetrahedral sheets, avoiding the possibility of substituting two vertex-sharing tetrahedra, following the treatment described in a previous paper (Sakuma & Kawamura, 2011), resulting in a negative charge, À1.5 eÁuc À1 (uc = unit cell). On this basis, the chemical formula of illite in this study was Al 4 (Si 6.5 Al 1.5 )O 20 (OH) 4 (Li, Liu, & Zhang, 2019). Second, another layer of illite was duplicated, and 48 K + ions were inserted into the interlayer region to compensate for part of the negative charges from each layer of illite.…”

“…The potential of mean force (PMF) for Na + /K 0+ (Figure 7a,b) and Cs + /K 0+ (Figure 7c,d) adsorption onto the surface of illite was calculated in order to analyse the effect of concentration on the stability of adsorbed cations. Cations with higher adsorption free energy correspond to weaker adsorption affinity and thus have lower stability on the surface (Li et al, 2018;Li, Liu, & Zhang, 2019). During the adsorption of Na + onto the surface, three local minima were located at ~0.19, ~0.39 and ~0.61 nm, corresponding to IS, OS and extended OS species, respectively, in which the last two adsorption types were not obvious in the ADPs (Figure 2).…”

“…The available experimental tools cannot easily exclusively describe the adsorption at the external surface of clay minerals, which is one of the major advantages of computational modelling (Li, Liu, & Zhang, 2019;Loganathan, Yazaydin, Bowers, Kalinichev, & Kirkpatrick, 2016). Molecular dynamics simulation, as a computational modelling technique, can examine the interfacial mineral-water environment, dynamic adsorption process and thermodynamic mechanisms and has thus received increasing attention from researchers (Daniel, Uddin, Harper, & Henry, 2019).…”

“…Two characteristics could be obtained from the studies of co-adsorption or competition of cations at the surface of micas or illite. First, the adsorption of exogenous cations was studied in the absence of intrinsic potassium ions, because intrinsic potassium ions were completely exchanged before experiments or molecular modelling (Baek et al, 2019;Brugman et al, 2020;Jin et al, 2020;Lammers et al, 2017;Lee et al, 2020;Li, Liu, & Zhang, 2019;Yang et al, 2020). Second, intrinsic potassium ions in micas or illite were not exchanged by exogenous cations in some experiments and molecular simulations.…”

New Approach To Understanding the Experimental 133Cs NMR Chemical Shift of Clay Minerals via Machine Learning and DFT-GIPAW Calculations

“…Computational approaches to the analysis of Cs adsorption on clay minerals have mainly been carried out using molecular dynamics (MD) simulations − and ab initio calculations. − The former directly simulates the dynamical behavior of Cs in water–clay systems, adsorption at edge sites, and exchange between inter- and intralayer spaces. Improved computational performance and highly accurate potential functions have made MD simulations involving many clay layers possible.…”