Adsorption structures and bonding states of cesium and barium adsorbed on various sites of vermiculite

Yamaguchi, Akiko; Okumura, Masahiko; Kawamura, Naomi*; Takahashi, Yoshio*

Science of the Total Environment, 964, p.178585_1 - 178585_13, 2025/02

https://doi.org/10.1016/j.scitotenv.2025.178585

There are many unresolved issues in the adsorption reactions of clay minerals. One of the reasons is the existence of multiple adsorption sites. It is known that the contribution of each adsorption site depends on the concentration of adsorbed ions, and there is a challenge in comprehensively correlate the results of atomic-level simulations that employ limited number of atoms and inevitably deal high-concentration samples with actual environmental samples where concentrations are orders of magnitude lower. In this study, we combined experiments using synchrotron radiation and first-principles calculations to comprehensively elucidate the systematic changes in the local structure of adsorption sites and adsorbed ions based on the adsorption concentration at the atomic level, and demonstrated that the interaction between adsorbed ions and clay minerals involves ionic bonding.

Evolution of radionuclide transport and retardation processes in uplifting granitic rocks, Part 1; Key processes, conceptual models and scenario

Metcalfe, R.*; Tachi, Yukio; Sasao, Eiji; Kawama, Daisuke*

Science of the Total Environment, 957, p.177375_1 - 177375_17, 2024/12

https://doi.org/10.1016/j.scitotenv.2024.177375

A safety case for an underground radioactive waste repository must show that groundwater will not in future transport radionuclides from the repository to the near-surface environment (the biosphere) in harmful quantities. Safety cases are developed step-wise throughout a programme to site and develop a repository. At early stages, before a site is selected, safety cases are generic and based on simplified safety assessment models of the disposal system that have conservative parameter values. Later, when site-specific conditions are known, more realistic models are needed for the long-term geo-environmental evolution and their impacts on radionuclide migration/retention. Uplift is one such environmental change, which may be particularly important in countries near active tectonic plate boundaries, such as Japan. Here we review the state of knowledge about how the properties of fractured granitic rocks evolve during uplift, based on studies in Japan. Hence, we present conceptual models and a generic scenario for mass transport and retardation processes in uplifting granitic rocks as a basis for realistic numerical models to underpin safety assessment.

Root endophytic bacterial and fungal communities in a natural hot desert are differentially regulated in dry and wet seasons by stochastic processes and functional traits

Taniguchi, Takeshi*; Isobe, Kazuo*; Imada, Shogo*; Eltayeb, M. M.*; Akaji, Yasuaki*; Nakayama, Masataka; Allen, M. F.*; Aronson, E. L.*

Science of the Total Environment, 899, p.165524_1 - 165524_13, 2023/11

https://doi.org/10.1016/j.scitotenv.2023.165524

Dryland ecosystems experience seasonal cycles of severe drought and moderate precipitation. Desert plants typically have patchy distributions, and many may develop symbiotic relationships with root endophytic microbes to survive under the repeated wet and extremely dry conditions. Although community coalescence has been found in many systems, the colonization by functional microbes and its relationship to seasonal transitions in arid regions are not well understood. Here we examined root endophytic microbial taxa, and their traits in relation to their root colonization, during the dry and wet seasons in a hot desert of the southwestern United States. We used high-throughput DNA sequencing of 16S rRNA and ITS gene profiling of five desert shrubs, and analyzed the seasonal change in endophytic microbial lineages. In summer, Actinobacteria increased, although this was not genus-specific. For fungi, Glomeraceae selectively increased in summer. In winter, Gram-negative bacterial genera, including those capable of nitrogen fixation and plant growth promotion, increased. Neutral model analysis revealed a strong stochastic influence on endophytic bacteria but a weak effect for fungi, especially in summer. The taxa with higher frequency than that predicted by the neutral model shared environmental adaptability and symbiotic traits, whereas the frequency of pathogenic fungi was at or under the predicted value. These results suggest that community assembly of bacteria and fungi is regulated differently. The bacterial community was affected by stochastic and deterministic processes via the bacterial response to drought (response trait) and beneficial effect on plants (effect trait). For fungi, mycorrhizal fungi were selected by plants in summer. The regulation of beneficial microbes by plants in both dry and wet seasons suggests the presence of plant-soil positive feedback in this natural desert ecosystem.

A New approach to extracting biofilm from environmental plastics using ultrasound-assisted syringe treatment for isotopic analyses

Battulga, B.; Atarashi-Andoh, Mariko; Nakanishi, Takahiro; Koarashi, Jun

Science of the Total Environment, 849, p.157758_1 - 157758_11, 2022/11

https://doi.org/10.1016/j.scitotenv.2022.157758

Characterizing plastic-associated biofilms is key to the better understanding of organic material and mineral cycling in the "Plastisphere"-the thin layer of microbial life on plastics. In this study, we propose a new method to extract biofilms from environmental plastics, in order to evaluate the properties of biofilm-derived organic matter through stable carbon (C) and nitrogen (N) isotope signatures and their interactions with radionuclides especially radiocesium (Cs). After ultrasound-assisted separation from the plastics, biofilm samples were successfully collected via a sequence of syringe treatments. Biofilm-derived organic matter samples (14.5-65.4 mg) from four river mouths in Japan showed Cs activity concentrations of 75 to 820 Bq kg biofilm (dw), providing evidence that environmental plastics, mediated by developed biofilms, serve as a carrier for Cs in the coastal environment. Significant differences in the (C and N signatures were also obtained for the biofilms, indicating the different sources, pathways, and development processes of biofilms on plastics.

Temporal variability of Cs concentrations in coastal sediments off Fukushima

Suzuki, Shotaro*; Amano, Yosuke*; Enomoto, Masahiro*; Matsumoto, Akira*; Morioka, Yoshiaki*; Sakuma, Kazuyuki; Tsuruta, Tadahiko; Kaeriyama, Hideki*; Miura, Hikaru*; Tsumune, Daisuke*; et al.

Science of the Total Environment, 831, p.154670_1 - 154670_15, 2022/07

https://doi.org/10.1016/j.scitotenv.2022.154670

Contamination processes of tree components in Japanese forest ecosystems affected by the Fukushima Daiichi Nuclear Power Plant accident Cs fallout

Ota, Masakazu; Koarashi, Jun

Science of the Total Environment, 816, p.151587_1 - 151587_21, 2022/04

https://doi.org/10.1016/j.scitotenv.2021.151587

In forests affected by the Fukushima Daiichi Nuclear Power Plant accident, trees became contaminated with Cs. However, Cs transfer processes determining tree contamination (particularly for stem wood, which is a prominent commercial resource in Fukushima) remain insufficiently understood. This study proposes a model for simulating the dynamic behavior of Cs in a forest tree-litter-soil system and applied it to two contaminated forests (cedar plantation and natural oak stand) in Fukushima. The model-calculated results and inter-comparison of the results with measurements elucidated the relative impact of distinct Cs transfer processes determining tree contamination. The transfer of Cs to trees occurred mostly ( 99%) through surface uptake of Cs directly trapped by leaves or needles and bark during the fallout. By contrast, root uptake of Cs from the soil was unsubstantial and several orders of magnitude lower than the surface uptake over a 50-year period following the accident. As a result, the internal contamination of the trees proceeded through an enduring recycling (translocation) of Cs absorbed on the tree surface at the time of the accident. A significant surface uptake of Cs at the bark was identified, contributing 100% (leafless oak tree) and 30% (foliated cedar tree; the remaining surface uptake occurred at the needles) of the total Cs uptake by trees. It was suggested that the trees growing at the study sites are currently (as of 2021) in a decontamination phase; the activity concentration of Cs in the stem wood decreases by 3% per year, mainly through radioactive decay of Cs and partly through a dilution effect from tree growth.

Remobilisation of radiocaesium from bottom sediments to water column in reservoirs in Fukushima, Japan

Funaki, Hironori; Tsuji, Hideki*; Nakanishi, Takahiro; Yoshimura, Kazuya; Sakuma, Kazuyuki; Hayashi, Seiji*

Science of the Total Environment, 812, p.152534_1 - 152534_10, 2022/03

https://doi.org/10.1016/j.scitotenv.2021.152534

Reservoir sediments generally act as a sink for radionuclides derived from nuclear accidents, but under anaerobic conditions, several radionuclides remobilise in bioavailable form from sediment to water columns, which may contribute to a long-term contamination in aquatic products. This study systematically investigated the Cs activities between sediment and pore water, which is a direct evidence of the remobilisation of bioavailable Cs from sediments, in two highly contaminated reservoirs affected by the Fukushima Dai-ichi Nuclear Power Plant accident. Our results strongly indicate a competitive ion exchange process between Cs and NH via a highly selective interaction with the frayed edges sites of phyllosilicate minerals to be the major reason for the variability of the Kd values between sediment and pore water, even in the Fukushima case.

Impacts of freeze-thaw processes and subsequent runoff on Cs washoff from bare land in Fukushima

Igarashi, Yasunori*; Onda, Yuichi*; Wakiyama, Yoshifumi*; Yoshimura, Kazuya; Kato, Hiroaki*; Kozuka, Shohei*; Manome, Ryo*

Science of the Total Environment, 769, p.144706_1 - 144706_9, 2021/05

https://doi.org/10.1016/j.scitotenv.2020.144706

Distribution and settling behavior of americium-241 in the tropical East Pacific

Kinoshita, Norikazu*; Nagaoka, Mika; Nakanishi, Takashi*

Science of the Total Environment, 753, p.142087_1 - 142087_10, 2021/01

https://doi.org/10.1016/j.scitotenv.2020.142087

The distribution of the anthropogenic radionuclide americium-241 (Am), a decay product of Pu discharged from atmospheric tests of nuclear weapons, was investigated to resolve its horizontal and vertical migration in the Tropical East Pacific. We analyzed Am concentrations in seawater samples collected in 2003. On comparing the Am concentrations with the previously determined concentrations of Pu in the same samples, the vertical profiles of Am were found to be similar to those of Pu. At some stations, the maximum concentration of Am occurred 100-200 m deeper than that of Pu. The Am/Pu ratios in the North Pacific and South Pacific were comparable to one another, and were typical of the ratio for the Pacific. The Am distribution was influenced by the water mass at depths below 400 m. The Am data support the view there is a current flowing at depths of 400-3000 m from the North Pacific through the equator to the South Pacific. In addition, the Am vertical profile was explained by using a box model that considers the decay of Pu and adsorption and scavenging by suspended particles. The different depths for the maximum concentrations of Am and Pu observed at some stations were well explained by the model and by the distribution of CaCO particles. The residence time of Am in the Pacific was also estimated by using the model.

Reservoir sediments as a long-term source of dissolved radiocaesium in water system; A Mass balance case study of an artificial reservoir in Fukushima, Japan

Funaki, Hironori; Sakuma, Kazuyuki; Nakanishi, Takahiro; Yoshimura, Kazuya; Katengeza, E. W.*

Science of the Total Environment, 743, p.140668_1 - 140668_9, 2020/11

https://doi.org/10.1016/j.scitotenv.2020.140668