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Collective Structural Changes in Vermiculite Clay Suspensions Induced by Cesium Ions

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Collective Structural Changes in Vermiculite Clay Suspensions Induced by Cesium Ions

Ryuhei Motokawa et al. Sci Rep.

Abstract

Following the Fukushima Daiichi nuclear disaster in 2011, Cs radioisotopes have been dispersed over a wide area. Most of the Cs has remained on the surface of the soil because Cs(+) is strongly adsorbed in the interlayer spaces of soil clays, particularly vermiculite. We have investigated the microscopic structure of an aqueous suspension of vermiculite clay over a wide length scale (1-1000 Å) by small-angle X-ray scattering. We determined the effect of the adsorption behavior of Cs(+) on the structural changes in the clay. It was found that the abruption of the clay sheets was induced by the localization of Cs(+) at the interlayer. This work provides important information for predicting the environmental fate of radioactive Cs in polluted areas, and for developing methods to extract Cs from the soil and reduce radioactivity.

Figures

Figure 1
Figure 1. Structural changes in the vermiculite/Cs+ suspension on the nano scale.
Double-logarithmic plots of the SAXS profiles obtained for Cs+ adsorbed on vermiculite in aqueous suspension at different wCs for samples 1 (black circles), 2 (red circles), 3 (green circles), 4 (blue circles), and 5 (pink circles). The peaks indicated by P1 and P2 originate from the interference between the stacked clay sheets. The peak indicated by P3 is attributed to long-range inhomogeneity in the stacking of the clay sheets.
Figure 2
Figure 2. Numerical analyses of SAXS intensity distribution.
The SAXS profiles are vertically shifted to avoid overlap by an offset constant of 1, and the constant values for samples 1–5 are 0, 1, 2, 3, and 4, respectively. The scattering profile of sample 1 is shown on a net intensity scale of log[Inor(q)]. The solid black lines are best-fit theoretical profiles obtained by using equation (2) together with the characteristic parameters and error (Table 2).
Figure 3
Figure 3. Structural changes in the crystal domain of vermiculite.
(a) Di and (b) N with the error plotted as a function of wCs on logarithmic and linear scales, respectively. Dashed lines in (a) and (b) are a visual guide and single exponential function, exp(-wCs/wCs*), respectively. The arrow in (a) is a visual guide. Errors in Di in the numerical SAXS analyses are within ±3% accuracy. Error bars of Di are omitted for clarity.
Figure 4
Figure 4. Collective structural changes induced by Cs+ adsorption to vermiculite clay.
Schematic illustrations of the crystal domain of vermiculite clay with Cs+. (a) Collective intercalation (localization) of Cs+ in the selective layer spaces and (b) segmentation of the crystal domain of vermiculite clay, providing fresh planar adsorption sites for Cs+.

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