Abstract
Adsorption states of alkali metal ions in three kinds of 2:1 type clay minerals are systematically investigated via first-principles-based metadynamics. Our reconstructed free energy surfaces in a two-dimensional space of coordination numbers specifically employed as collective variables for describing the interlayer cations show that an inner-sphere (IS) complex is preferentially formed for Cs(+) in the 2:1 type trioctahedral clay minerals with saponite-like compositions, where lighter alkali metal ions show a tendency to form an outer-sphere one instead. The strong preference for an IS complex observed for Cs(+) is found to result partially from the capability of recognizing selectively Cs(+) ions at the basal O atoms with the Lewis basicity significantly enhanced by the isomorphic substitution in tetrahedral sheets.
Highlights
Adsorption states of interlayer cations in swelling clay minerals such as the smectite and vermiculite groups are difficult to characterize quantitatively using experimental techniques at the atomic level
Many simulation studies found in the literature suggest that adsorbed cations form either an inner-sphere (IS) or outersphere (OS) complex.[1−10] In the former complex, interlayer cations are directly associated with isomorphic substitution (ISub) sites in tetrahedral (Th) sheets; the resulting cations are strongly bound to the clay surface
The free energy profile for the adsorption of a sodium ion onto the surface of a smectite clay evaluated by using constrained firstprinciples molecular dynamics (FPMD)[5] shows that the lowest free-energy region corresponds to a Na+ bound to only one basal O (Ob) atom, accompanied by the shift of the position of a Na+ ion by 1.5 Å from the midplane of the interlayer toward the Th sheet including the ISub
Summary
Adsorption states of interlayer cations in swelling clay minerals such as the smectite and vermiculite groups are difficult to characterize quantitatively using experimental techniques at the atomic level. Many simulation studies found in the literature suggest that adsorbed cations form either an inner-sphere (IS) or outersphere (OS) complex.[1−10] In the former complex, interlayer cations are directly associated with isomorphic substitution (ISub) sites in tetrahedral (Th) sheets; the resulting cations are strongly bound to the clay surface. Radiocesium isotopes 134Cs and 137Cs released in the Fukushima Daiichi nuclear disaster occurring in 2011 were observed to aggregate in particular clay minerals of weathered biotite constituting soils around Fukushima.[14] detailed mechanisms for aggregating Cs+ ions in the interlayer of particular clay minerals are not yet understood to date In this computational study, we describe the detailed structure of adsorbed alkali metal ions in 2:1 type clay minerals by reconstructing the free energy surface (FES). The structures of our clay systems including the ISub in either Th or octahedral (Oh) sheets only are oversimplified compared to the actual ones, the present systematic study reveals the underlying fundamental sciences in varied adsorption states of a series of alkali metal ions in the interlayer of our model clays, providing a clue to understanding the cesium aggregation in particular clay minerals
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