Molecular studies of Cs adsorption sites in inorganic layered materials

A number of measures and restrictions have been enacted since the Fukushima crisis. For this reason, a lot of attention has been put on decontamination of sites affected by the radioactive Cs. According to the Japan’s Ministry of environment, decontamination has been effective helping reduce dose levels significantly in the special decontamination areas. According to the stipulated decontamination procedure, approximately 5 cm of the soil surface must be scraped away in a bid to realize appreciable results. This has however yielded a huge volume of contaminated soil.

The urgency of containing the high volume of contaminated soil has continued to put pressure on the ongoing works along with reducing decontamination costs. To reduce the volume of contaminated soil, water treatment implementing the water circulation equipment is adopted. The basic principle is that the radioactive Cs dissolved in water strongly adsorbs onto the surface of 2-dimensional nanosheets, which is a basic clay structural unit.

The tiny particles of the contaminated soil tend to have a higher specific surface area for the adsorption in aqueous solution, thus associating with a huge amount of the Cs. Therefore, the circulation of an amount of contaminated soil in aqueous solution is able to filter the proportions containing higher Cs doses. Therefore, there is a need for an understanding of the molecular-scale Cs adsorption in aqueous solution with different doses of the radioactive Cs. This is important especially with regards to reusing and recycling contaminated soil.

Professors Kiminori Sato at Tokyo Gakugei University in collaboration with Professor Michael Hunger from University of Stuttgart investigated the adsorption of Cs in aqueous solution across varying Cs⁺ doses for saponite mineral, by adopting the typical elution test, radiocesium interception potential, and the Cs magic-angle-spinning nuclear magnetic resonance. Their research work is published in peer-reviewed journal, Physical Chemistry Chemical Physics.

In view of the recently developed analytical method, the concentrations of nanosheet edges, and surfaces, and the incoming hexagonal cavities actively involved in Cs adsorption were established successfully to be in the range of a few mmol kg^-1. The nanosheet edges, surfaces and the incoming hexagonal cavities were identified in this study. This a potential Cs adsorption site in the geological environment.

Sato and Hunger also observed that the concentrations of the three molecular sites increased with Cs concentration in aqueous solution. This indicated that a high portion of the Cs⁺ ions could be adsorbed there in highly concentrated Cs solutions. In addition, the concentration of the incoming hexagonal cavities involved in Cs adsorption was observed to increase with Cs⁺ molarity of the aqueous solution slowly as opposed to that of nanosheet surfaces. This was indicative that the adsorption of Cs into the hexagonal cavities was more difficult than that onto the surfaces.

Nanosheet edges concentration was observed to increase slowly with increasing Cs⁺ molarity of the aqueous solution. This was due to the fact that Cs chemisorption occurred. The concentration of Cs at the nanosheet edges extensively taking part in Cs adsorption was reported to be between 13 and 40%. Sato and Hunger concluded that these sites ought to be targeted for future decontamination.