XRD and HRTEM Evidence for Fixation of Cesium Ions in Vermiculite Clay

Abstract

X-ray diffraction and high-resolution transmission electron microscopy showed that cesium ions (Cs + 's) in a vermiculite clay formed a segregated monoionic layer in the interlayer spaces. Each ion was fixed simultaneously at the centers of hexagonal rings in the upper and lower silicate tetrahedral sheets. No Cs + was eluted even when Cs + vermiculite was kept at 80°C for 48 h in the presence of a large excess of Al(NO 3 ) 3 or [Co(NH 3 ) 6 ]Cl 3 . 137 Cs is a major radioactive component of high-level nuclear wastes. After the accident in the nuclear power plants at Fukushima last March, it has been widely dispersed in the northern Kanto district of Japan and is thought to be mostly fixed in soils.1 Accordingly the removal of Cs + from soils is an urgent problem for lowering the radioactivity level in that area.As is well known, Cs + is selectively and strongly adsorbed by the phyllosilicate fraction of soils.2,3 A number of works have been reported on the interactions of Cs + with clays and its environmental fate.47 Some of the conceptual models are presented to explain the high affinity of Cs + toward clays such as a frayed edge-planar site. 810 No evidence, however, has been presented so far concerning its fixed state at an atomic scale. The understanding of the detailed structure of immobilized Cs + can be a first step to develop methods of purifying radioactively contaminated soils.We applied X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) to study the interaction of Cs + with a clay. The results gave insight for understanding its extremely high stability.As a phyllosilicate clay, magnesium vermiculite (product from Transvaal, South Africa: denoted as Mg-Verm) 11 was used (Supporting Information; SI). 21 Its physical and chemical properties have been reported elsewhere.12 Actually it is the interstratification of almost the same amount of the vermiculite layer with hydrated Mg 2+ and K + -mica layer with K + at the interlayer spaces, as shown below. This mineral was recently used as an adsorbate to remove Cs + in sea water. 13 1.0 g of MgVerm was suspended in 5.0 mL of aqueous solution containing 18, 45, or 100 mmol L ¹1 CsNO 3 (denoted as samples (I), (II), and (III), respectively). The suspensions were stirred for 48 h at room temperature. After they were filtrated off, the clay particles were dried and subjected to XRD, FT-IR, and HRTEM. The filtrates were analyzed by ICP measurements. The specimens for HRTEM were prepared according to the method described in our previous work.14 Clay samples were embedded in epoxy resin, applying pressure so that the (001) planes of the clay fragments were oriented. Then they were sliced perpendicular to the (001) planes, polished mechanically, and thinned to electron transparency by argon ion-milling. Finally the samples were carboncoated before they were examined by TEM. HRTEM observation was performed at 200 kV using a JEOL JEM-2010UHR electron microscope (Cs: 0.5 mm) with a LaB 6 filament. Table 1 shows the results of the ICP ...

Cs–sorption in weathered biotite from Fukushima granitic soil

“…However, several unsubstituted vermiculite layers are still present in the vicinity. This is similar to the result reported by Kogure et al (2012) for Cs-substituted Transvaal vermiculite, and also supports the result of the XRD analysis (Fig. 3b).…”

“…Owing to the large radius of Cs, the Cs-substituted vermiculite layer has the basal spacing of ca 10.6 Å . However, unlike in previous works (e.g, Kogure et al, 2012;Tamura et al, 2014), the influence of such larger basal spacing on the XRD pattern, for instance shift of the biotite peak, was not observed in Figure 2. This is probably due to the smaller proportion of the vermiculite layers in the present biotite than in Transvaal vermiculite used in these previous experiments.…”

Concentration and vertical flux of Fukushima-derived radiocesium in sinking particles from two sites in the Northwestern Pacific Ocean

“…The Chernobyl fallout was rapidly (within days) solubilized in seawater (Whitehead et al, 1988). However, a part of the cesium can be adsorbed onto or incorporated into aluminosilicates (clay minerals), similar to potassium (Qin et al, 2012, and references therein), and the adsorption or incorporation of cesium is not easily reversed (e.g., Kogure et al, 2012;Otosaka and Kobayashi, 2012). To determine the form of the radiocesium associated with sinking particles, we computed correlations between the concentration and flux of radiocesium with those of organic carbon, biogenic opal, CaCO 3 , lithogenic materials (LM) and trace elements.…”

Analysis of External Surface Irregularities on Fukushima-Derived Fallout Particles

“…Studies report preferential Cs sorption onto the interlayers (edge sites) both mica and clay-type minerals (Kogure et al, 2012;Mukai et al, 2014;Saito et al, 2014), where it is difficult to disassociate, and is subsequently transported, carried by the host particulate material. Additional studies have also identified Cs within larger material, including 1-2 µm diameter spherical particles (Adachi et al, 2013;Abe et al, 2014;Kogure et al, 2016).…”