Enhanced irreversible fixation of cesium by wetting and drying cycles in soil

Estimation of rooting depth of 137Cs uptake by plants

Understanding the soil-to-plant transfer process of ¹³⁷Cs is essential for predicting the contamination levels of plants in contaminated areas. The rooting depth is considered one of the key factors explaining the difference in the activity concentration of ¹³⁷Cs in different plant species. In this study, the distributions of ¹³⁷Cs and ¹³³Cs in soils and plants were investigated, and the plants' rooting depth of ¹³⁷Cs uptake was estimated using the ¹³⁷Cs/¹³³Cs ratios in exchangeable fractions of soils and biological samples. The results showed that different plant species accumulate different levels of ¹³⁷Cs and ¹³³Cs. The ¹³⁷Cs/¹³³Cs ratios were fairly constant in plants of the same species. The average ¹³⁷Cs/¹³³Cs ratios in bamboo grasses and ferns were 0.015 ± 0.009 (n = 5) and 0.13 ± 0.04 Bq ng^-1 (n = 10) in Yamakiya, respectively. The percentage of ¹³⁷Cs in the exchangeable fraction of the uppermost soil layer was lower than that in the deeper soil layers. The activity concentrations of ¹³⁷Cs in the soil profiles decreased sharply with depth, whereas the depth distributions of ¹³³Cs were uniform. Therefore, the ¹³⁷Cs/¹³³Cs ratios were driven mainly by the ¹³⁷Cs activity concentrations in soil. The plants' rooting depths of ¹³⁷Cs uptake were estimated on the basis of the relationships between the averaged ¹³⁷Cs/¹³³Cs ratio in the soil layer and the ¹³⁷Cs/¹³³Cs ratio in the plant. The results indicate that the deeper-rooted species such as bamboo grasses have a lower accumulation of ¹³⁷Cs than the superficial-rooting species such as ferns. The soil-to-plant transfer factors would be determined using rooting depth by calculating the averaged activity concentration of ¹³⁷Cs within the estimated rooting depth.

Desorption technologies for remediation of cesium-contaminated soils: a short review

This review summarizes the mechanisms for desorbing and extracting cesium (Cs⁺) from clay minerals and soil. Most techniques use ion exchange with acids, cations, polymers, and surfactants. Some improve desorption of Cs⁺ from clay minerals, while surfactants and polymers expand the interlayer. Mixtures of acids/polymers, acids/surfactants, cations/polymers, and cations/surfactants are therefore more effective agents for desorption of Cs⁺ from clay minerals. Hydrothermal treatment plays a role similar to that of polymers and surfactants in expanding the interlayer of clay minerals. The primary desorption mechanism expands the interlayer and desorbs Cs⁺, but multiple sequential extractions based on these techniques can more effectively desorb Cs⁺ from clay minerals and field-contaminated soils. Desorption techniques for Cs⁺ based on multiple sequential extractions can reportedly achieve an efficiency greater than 90%, and such approaches are likely to be important technologies for remediation of Cs⁺-contaminated soils and industrial accident sites, as well as the dismantling of nuclear power plants.

Interpretation of the interaction between cesium ion and some clay minerals based on their structural features

Cesium (Cs⁺) is known to have a strong interaction with various clay minerals; however, it is not interpreted from the structure of clay minerals and the adsorption isotherm. The adsorption interactions between Cs⁺ and hydrobiotite (H-Bio), biotite (Bio), vermiculite (Verm), and exfoliated vermiculite (E-Verm) were evaluated by analyzing adsorption isotherm, basal spacing, and adsorption/desorption experiments. The Cs⁺ adsorption of H-Bio and Verm fitted well to the Langmuir adsorption isotherm, while the Cs⁺ adsorption of Bio and E-Verm fitted well to the Freundlich adsorption isotherm. The basal spacing of H-Bio and Verm was approximately 1.4 nm, while Bio and E-Verm basal spacing was 1.0 nm. The adsorption experiment results for Cs⁺ under the coexistence of Ca²⁺ and K⁺ indicated that the contribution of the interlayer sites to Cs⁺ adsorption on H-Bio and Verm was 25-40%, while the contribution of the interlayer sites to that on Bio and E-Verm was almost 0%. The adsorption isotherms reflected this interlayer contribution to Cs⁺ adsorption, which was dependent on the basal spacing. Therefore, the basal spacing of clay minerals is one of the key structural properties controlling both the adsorption capacity and the adsorption mechanism of Cs⁺ in clay minerals.

Effects of organic amendments on the natural attenuation of radiocesium transferability in grassland soils with high potassium fertility

Organic amendments affect the behavior of radiocesium in soil-plant systems in a complex way; they can inhibit radiocesium fixation by clay minerals by blocking selective sorption sites, whereas K supplied to the soil solution by amendments can reduce Cs uptake by plant roots. Here, we investigated the influence of inorganic and organic amendments on the transferability of radiocesium from soil to grass seedling in a humus-rich Andosol with high exchangeable K content. Soil samples were spiked with a¹³⁷Cs tracer, treated with N, N-P-K, compost (cattle manure using rice straw), or no amendment (control soil), and subjected to repeated two-week wetting and air-drying treatments for one year in an artificial climate chamber. Small-scale cultivations of orchard grass were performed four times during the experimental period to assess temporal changes of availability of ¹³⁷Cs in the soils. The ¹³⁷Cs transfer factor (TF), defined as the ¹³⁷Cs concentration in the plant divided by that in the soil, decreased with time in the control soil. The soil treated with compost showed higher TFs than the control soil in each cultivation and a slower attenuation of ¹³⁷Cs transferability. By comparing the extractability of ¹³⁷Cs, NH₄⁺, and K⁺ with the observed TFs, we show that K released from the compost was not effective in reducing root uptake of ¹³⁷Cs, but enhanced ¹³⁷Cs desorption from the soil under K-rich conditions. This result suggests that organic amendment is ineffective in reducing root uptake of radiocesium under high exchangeable K concentrations, and may instead enhance the long-term availability of radiocesium in soils.