Identification of Multiple Cs⁺ Adsorption Sites in a Hydroxy-interlayered Vermiculite-like Layered Silicate through ¹³³Cs MAS NMR Analysis

Abstract

133Cs magic-angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy revealed two Cs sites in Cs-adsorbed aluminium-altered phlogopite (Cs Al-Phl), but only one site for Cs-adsorbed magnesium-altered phlogopite (Cs Mg-Phl). Cesium ions in hydroxy-interlayered vermiculite-like layered silicate reside at sites in both mica zones and in interlayer wedge zones of Al-Phl. It is likely that chemical exchange of Cs + ions occurs between these two zones. Natural vermiculite formed by weathering of biotite or phlogopite (Phl) has an interstratified structure consisting of anhydrous layers containing potassium ions and hydrous layers containing magnesium ions. Previous studies have determined that artificially modified phlogopites by topotactic reactions showed controllable adsorption capabilities. 5 In particular, phlogopite samples altered with hydrated ions such as Na + , Mg 2+ , and Ca 2+ , denoted by Na-Phl, Mg-Phl, and Ca-Phl, exhibited the same high Cs + -adsorption capacity and Langmuirtype adsorption as natural vermiculite. Thus, artificially modified phlogopites saturated with hydrated ions have the potential for increasing the Cs adsorption capacity.On the other hand, studies have shown that phlogopite altered with hydrated Al 3+ , (Al-Phl), formed hydroxy-interlayered vermiculite (HIV)-like structures, which exhibited wedge zones with partially enlarged interlayer spacings, typical of natural vermiculite. The Al 3+ in these Al-Phl structures underwent minimal replacement with Cs + ions, but the Al-Phl exhibited high partition coefficients (K d ) at lower Cs concentrations, indicating significant selectivity for Cs. 5,7 When the remnant mica transforms to vermiculite, the wedges in the silicate interlayers are splayed. These vermiculite interlayer wedges have been considered equivalent to those frayed edges of less thoroughly weathered mica particles, and should be similarly selective for large-ion alkali metals such as Cs + (Figure 1). Because sorption and retention of cesium radioisotopes in soil is such a catastrophic problem in Japan, it is important to understand the diverse mechanisms by which hydroxylated aluminium in the interlayers may trigger adsorption of Cs + ions. In this study, we investigated the local structure of Cs MgPhl and Cs Al-Phl using 133 Cs magic-angle spinning nuclear magnetic resonance (MAS NMR) measurements. Solid-state NMR spectroscopy is a powerful method for elucidating the local structure of measured ions in solids.812 Among a number of Cs-adsorbed clays, it is reported that Cs-adsorbed vermiculite and a weathered synthetic F-rich phlogopite differ from others in 133 Cs MAS NMR results due to the dehydrated Cs + ions in interlayer sites. 810Hydrothermal treatment of phlogopite was conducted according to a procedure detailed in a previous publication, 5 and Cs-adsorbed Mg-Phl and Al-Phl were also obtained. (F 0.11 ,OH 1.89 ), and their cation-exchange capacity (CEC, the amounts of the interlayer cations including K + ions) estimated by the IPC data are 213 and 234 meq/...

New Insights into the Cs Adsorption on Montmorillonite Clay from 133Cs Solid-State NMR and Density Functional Theory Calculations

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Effects of NH4+, K+, Mg2+, and Ca2+ on the Cesium Adsorption/Desorption in Binding Sites of Vermiculitized Biotite

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