Toshihiko Ohnuki

• Doctor of Philosophy
• Laboratory Head at Japan Atomic Energy Agency

Microbial exudates including siderophore, which changes chemical species of actinides and lanthanides. We have investigated effects of desferrioxamine B (DFOB; one of the siderophores) and siderophore-like organic molecules (SLOM) on the adsorption of lanthanides by microbial cells, aluminium oxide (Al2O3), and manganese (Mn) oxides. When DFOB was...

In this study, we report chemical species of Cs and I in condensed vaporized particles (CVPs) produced by melting experiments using nuclear fuel components containing CsI with concrete. Analyses of CVPs by SEM with EDX showed the formation of many round particles containing Cs and I of diameters less than ∼20 μm. X-ray absorption near-edge-structur...

Azoarcus sp. DN11 was previously isolated from gasoline-contaminated groundwater as an anaerobic benzene-degrading bacterium. Genome analysis of strain DN11 revealed that it contained a putative idr gene cluster (idrABP1P2), which was recently found to be involved in bacterial iodate (IO3⁻) respiration. In this study, we determined if strain DN11 p...

Radioactive Cs-rich microparticles (CsMPs) released from the Fukushima Daiichi Nuclear Power Plant (FDNPP) are a potential health risk through inhalation. Little has been documented on the occurrence of CsMPs, particularly their occurrence inside buildings. In this study, we quantitatively analyze the distribution and number of CsMPs in indoor dust...

A novel magnetic Bi2WO6/TiO2/Fe3O4 photocatalyst was synthesized by a hydrothermal approach. The pattern, structure, elemental composition, light-absorbing properties, and magnetism of Bi2WO6/TiO2/Fe3O4 were characterized and analyzed. The performance, influencing factors, and mechanism of Bi2WO6/TiO2/Fe3O4 towards bisphenol A (BPA) degradation wer...

Biogenic manganese (Mn) oxides occur ubiquitously in the environment including the uranium (U) mill tailings at the Ningyo-toge U mine in Okayama, Japan, being important in the sequestration of radioactive radium. To understand the nanoscale processes in Mn oxides formation at the U mill tailings site, Mn2+ absorption by a basidiomycete fungus, Cop...

Nanopollutants are nanoparticles that have escaped into the environment and can include engineered nanoparticles as well as nanoparticles that are products of degradation (e.g. nanoplastics) or other processes. As many of these particles are only recently developed or described there is still a lot to learn about where they come from, where they en...

Due to the increasing of industrial plastic waste and its refractory characteristics, it is extremely urgent to develop new degradation technology and environmentally friendly catalyst for industrial plastic waste. Manganese oxides are one of the most promising candidates for the catalytic degradation of plastic wastes. However, an improved underst...

Remnant nuclear fuel debris in the damaged nuclear reactors at the Fukushima Daiichi Nuclear Power Plant (FDNPP) has contacted the groundwater containing microorganisms for over ten years. Herein, we report the possibility of bacterial alteration of fuel debris. We investigated the physical and chemical changes of fuel debris simulants (FDS) in the...

We investigated the sorption of Pu(IV) on biogenic Mn oxide, composed of Mn(IV) oxide and hyphae, produced by Mn(II)-oxidizing fungus. The sorption of Pu(IV) on biogenic Mn oxide was similar to that of U(VI) and different from that of Th(IV), possibly due to oxidation of Pu(IV) to Pu(VI). When Pu(IV) was sorbed on hyphae only, it was desorbed into...

Boron carbide control rods remain in the fuel debris of the damaged reactors in the Fukushima Daiichi Nuclear Power Plant, potentially preventing re-criticality; however, the state and stability of the control rods remain unknown. Sensitive high-resolution ion microprobe analyses have revealed B–Li isotopic signatures in radioactive Cs-rich micropa...

The diffusion of components of nuclear fuel debris formed during the Fukushima Daiichi nuclear accident to cooling water is the root cause of the contaminated water. This study aims to investigate the role of microorganisms in the dissolution process of fuel debris. Mixed powder of UO2 and Fe(0) was used to simulate the fuel debris because it has b...

In order to estimate the reliable risk and safety required for retrieval of the fuel debris from Fukushima Daiichi NPP damaged reactors, it is important to understand the degradation of the fuel debris. Because groundwater merges into the contaminated water through cracks caused by PCV damage, microorganisms in the groundwater were introduced into...

Radioactive iodine is one of the mobile radionuclides contained in the contaminated water generated by cooling of the fuel debris at Fukushima Daiichi nuclear power plant (FDNPP). It is reported that chemical species of iodine transform between iodate, iodide, and organo-I in the environments. In FDNPP, the contaminated water is merged with ground...

Biogenic Mn oxides are reactive and ubiquitous in many Earth surface environments, yet their role in radionuclide sequestration at U mill-tailings sites still require an improved understanding at the nano- and molecular-scales. This study concerns the uptake of Ba, utilized as a safe and chemically appropriate surrogate for radioactive Ra, by bioge...

Following the NPP accident, several hundred tons of heat-generating corium and fuel debris have been cooled permanently by millions of m³ of flowing. Knowledge on the interaction with water is crucial for any decommissioning planning. Starting from knowledge on the evolutions of the accident in the three reactor cores and associated fuel debris for...

This study contributes toward developing measures for the disposal of radiocesium-contaminated sewage sludge ash (SSA). Here, we prepared two types of solidified bodies containing 30 wt% radiocesium-bearing SSA. The material used for the two solidified bodies were alkaline-reacted metakaolinite (geopolymer) and ordinary Portland cement (OPC). Cemen...

A large amount of iodine-containing solution (ICS) is produced in both nuclear medicine and the atomic energy industry, which may cause serious health hazards in case if it enters aquatic ecosystem without sufficient treatment. The treatment of ICS by silver demonstrates great potential for iodine capture and retention. Nano-scale silver and its ox...

A contaminated zone elongated toward Futaba Town, north-northwest of the Fukushima Daiichi Nuclear Power Plant (FDNPP), contains highly radioactive particles released from reactor Unit 1. There are uncertainties associated with the physio-chemical properties and environmental impacts of these particles. In this study, 31 radioactive particles were...

Ferrihydrite, a hydrous ferric oxide, is ubiquitous in silica-rich mine drainage sites, although some uncertaity remains concerning its nanoscale texture and colloidal properties such as its association with amorphous silica and reactions with contaminant metals. To understand the mechanisms of natural attenuation of contaminants governed by ferrih...

Traces of Pu have been detected in material released from the Fukushima Daiichi Nuclear Power Plant (FDNPP) in March of 2011; however, to date the physical and chemical form of the Pu have remained unknown. Here we report the discovery of particulate Pu associated with cesium-rich microparticles (CsMPs) that formed in and were released from the rea...

Ce anomaly is an important indicator for many geochemical processes. Although the role of strong ligand such as siderophore desferrioxamine B (DFOB) in facilitating negative Ce anomaly formation on Mn (hydr)oxide has been well studied, the effect of ubiquitous weak ligands such as polysaccharides on the mobility of Ce have not been considered. Here...

Understanding migration of radionuclides in subsurface environments is of critical importance to risk assessment at nuclear legacy sites. The chapters in this volume cover various phenomena in the groundwater system around the area of Lake Karachai of the Mayak Production Association site and highlight the important role of microorganisms and natur...

This chapter shows findings obtained from a microbial ecological study carried out on groundwater contaminated with radionuclides and nitrate in the area of Lake Karachai in southern Ural. This study was conducted to answer the questions raised from previous studies at the same site on the difference in migration patterns between strontium-90 and n...

High nitrate concentration together with high salinity of the groundwater in the area of Lake Karachai may enhance the activity of halophilic and denitrifying microorganisms. Those microorganisms affect transformation of chemical species of radionuclides through the function of cellular compounds as carboxyl and phosphoryl functional groups. Coordi...

The long-lived anionic radionuclide of ⁹⁹Tc (pertechnetate ion TcO4⁻) is highly mobile under oxic conditions, and therefore long-term removal and immobilization of TcO4⁻ from radioactively contaminated water have been major challenges. Mg–Al layered double-hydroxides (LDH) have been often investigated to remove anions from waste water. In this stud...

Low-level radioactive wastes are commonly immobilized in cementitious materials, where cement-based material can incorporate radionuclides into their crystal structure. Specifically, ettringite (Ca6Al2(OH)12(SO4)3∙26H2O) is known to stabilize anionic species, which is appealing for waste streams with radioactive iodine (129I) that persists as iodid...

The abundance and distribution of highly radioactive cesium-rich microparticles (CsMPs) that were released from the Fukushima Daiichi Nuclear Power Plant (FDNPP) during the first stage of the nuclear disaster in March 2011 are described for 20 surface soils collected around the FDNPP. Based on the spatial distribution of the numbers (particles/g) a...

Growth of the nuclear industry has encouraged us to look for techniques to treat large volumes of nuclear waste. 60Co is one of the most problematic radioactive wastes in the nuclear industry. In this study, a Mn slag-based geopolymer (MSG) was prepared, which exhibited better Co immobilization performance than the ordinary metakaolin-based geopoly...

The fate of radioactive Cs deposited after the Fukushima nuclear power plant accident and its associated radiological impacts are largely dependent on its mobility from surface soils to forest ecosystems. We measured the accumulation of radioactive Cs in the fruit bodies of wild fungi in the forest at Iidate, Fukushima, Japan. The transfer factors...

Permanganate treatment is widely used for disinfection of bacteria in surface-contaminated water. In this paper, the fate of the dissolved permanganate in aqueous solution after contact with cells of Pseudomonas fluorescens was studied. Concomitant accumulation of divalent cations of Mg2+, Zn2+, and Co2+ during precipitation of Mn oxides was also s...

To understand the chemical durability of highly radioactive cesium-rich microparticles (CsMPs) released from the Fukushima Daiichi Nuclear Power Plant in March 2011, we have, for the first time, performed systematic dissolution experiments with CsMPs isolated from Fukushima soils (one sample with 108 Bq and one sample with 57.8 Bq of 137Cs) using t...

Following identification of radioactive Cs microparticles (CsMPs) in aerosol samples from the Fukushima Daiichi Nuclear Power Plant (FDNPP), numerous reports on CsMPs have been published. This paper reviews recent progress in the measurement and characterization of CsMPs by advanced analytical techniques, including advanced transmission electron mi...

Porous materials of hydrotalcite-like layered double hydroxides (LDHs) have been used for removal of anionic contaminants from solution. However, local coordination structures of anions adsorbed on LDHs are not fully understood because of the lack of spectroscopic studies. In this study, we utilized X-ray absorption fine structure spectroscopy to c...

We used the spent mushroom substrata (SMSs) which are a kind of by-product after growing edible mushrooms for the in-situ investigation of radioactive Cs mobility in litter zone in a forest of Fukushima prefecture, Japan. The powder SMS was filled in a plastic net bag of 0.35 × 0.55 m, then was placed in a forest for ~6 months under three kinds of...

Silver-impregnated zeolite (AgIZ) has been used for removing radioiodine from contaminated groundwater and nuclear waste streams and the worldwide inventory of such secondary waste is rapidly increasing. The objective of this study was to 1) quantify the effectiveness of two grout waste forms for disposing of the used AgIZ, and 2) determine the I s...

Humic acid (HA) in the environment may exist in either dissolved or fixed forms. However, laboratory studies usually take only the former into account. Here we synthesized a hybrid of HA and aluminum hydroxide (Al(OH)3) to mimic fixed HA, compared the effects of fixed and dissolved HA on Eu(III)/Yb(III) adsorption on Al(OH)3, and analyzed the adsor...

Although the adsorption of cesium (Cs) onto phyllosilicate minerals has been widely studied, the effect of Cs on redox-sensitive biogenic Mn oxide is not well understood. In this study, the structural transformation of biogenic birnessite stimulated by commonly occurring natural heavy metals, and the influence of those metals on the adsorption beha...

Highly radioactive cesium-rich microparticles (CsMPs) were released from the Fukushima Daiichi nuclear power plant (FDNPP) to the surrounding environment at an early stage of the nuclear disaster in March of 2011; however, the quantity of released CsMPs remains undetermined. Here, we report a novel method to quantify the number of CsMPs in surface...

To understand the competing effects of the components in extracellular substances (ES), polymeric substances (PS) and low-molecular-weight small substances (SS) <1 kDa derived from microorganisms, on the colloidal stability of cerium dioxide nanoparticles (CeNPs), we investigated their adsorption to sparingly soluble CeNPs at room temperature at pH...

Microorganisms play an important role in the mineralization of heavy metals in different environments. Previous studies have reported the phosphate mineralization of light (Ce) and heavy (Yb) rare earth elements with yeast. However, little is known about differences in the biomineralization process of middle rare earth elements (including Sm, Eu, G...

Trace U was released from the Fukushima Daiichi Nuclear Power Plant (FDNPP) during the meltdowns, but the speciation of the released components of the nuclear fuel remains unknown. We report, for the first time, the atomic-scale characteristics of nano-fragments of the nuclear fuels that were released from the FDNPP into the environment. Nano-fragm...

Highly radioactive cesium-rich microparticles (CsMPs) released from the Fukushima Daiichi Nuclear Power Plant (FDNPP) provide nano-scale chemical fingerprints of the 2011 tragedy. U, Cs, Ba, Rb, K, and Ca isotopic ratios were determined on three CsMPs (3.79–780 Bq) collected within ~10 km from the FDNPP to determine the CsMPs’ origin and mechanism...

This study investigated the interaction of inorganic aqueous Eu(III), Pb(II), and U(VI) with Paramecium sp., a representative single-celled protozoan that lives in freshwater. Living and prekilled Paramecium cells were tested. The prekilled cells were killed with a fixative. After 24 h exposure of the cells to inorganic aqueous solutions containing...

The reversibility of cesium adsorption in contaminated soil is largely dependent on its interaction with micaceous minerals, which may be greatly influenced by various cations. Herein, we systematically investigated the effects of NH4+, K+, Mg2+, and Ca2+ on the adsorption/desorption of Cs+ into different binding sites of vermiculitized biotite (VB...

In light of the widespread Cs pollution that may follow nuclear disasters, the decontamination of post-accident soil has earned much attention due to the difficulty of Cs removal for its super-retention in micaceous minerals. Herein, we successfully used solvated Mg2+ to desorb Cs from Cs-saturated vermiculitized biotite (VB) to clarify the microsc...

Radioactive strontium (⁹⁰Sr) leaked into saline environments, including the ocean, from the Fukushima Daiichi Nuclear Power Plant after a nuclear accident. Since the removal of ⁹⁰Sr using general adsorbents (e.g., zeolite) is not efficient at high salinity, a suitable alternative immobilization method is necessary. Therefore, we incorporated solubl...

Although the Cs adsorption onto soil minerals has been widely studied, the structural factor of Mn oxides affecting the adsorption behavior of trace amounts of Cs is unclear. In order to elucidate the adsorption mechanisms of radioactive Cs at trace levels and the role of Mn oxides on Cs migration in the terrestrial environment, the Cs adsorption o...

In order to understand the adsorption preferences of extracellular polymeric substances (EPS) components derived from fungus Saccharomyces cerevisiae on sparingly soluble CeO2 nanoparticles (CeNPs), the adsorption experiments of the EPS including organic matter with low molecular weight have been performed at pH 6.0 at room temperature (25 ± 1 °C)....

Sorbed Cs on vermiculitized biotite in saturation amount was desorbed by extractions with 1 M K⁺ and Mg²⁺ at varied temperatures. At room temperature, Cs⁺ was readily desorbed by leaching with K⁺ (ca. 100%) but not by Mg²⁺ (ca. 50%). However, the contrary was true for desorption at high temperatures, in which Mg²⁺ desorbed far more Cs⁺ (ca. 100%) t...

A calcium (Ca)-deficient hydroxyapatite was investigated for its potential to remove Sr2+ from environmentally relevant water. We conducted sorption tests on solutions containing magnesium ion (Mg2+) and calcium ion (Ca2+) as competing cations at a strontium ion (Sr2+) concentration of 0.05 mmol/L. The Ca-deficient hydroxyapatite maintained a high...

The sorption of Np(V) by microbe consortia and by a single pure culture of the Fe-reducing bacterium Shewanella putrefaciens was studied at pH levels between 3 and 7 under resting cell conditions. The sorption of Np(V) by S. putrefaciens obtained under inert conditions and by the consortia in the aerated condition was higher than that by S. putrefa...

The nuclear disaster at the Fukushima Daiichi Nuclear Power Plant (FDNPP) in March 2011 caused partial meltdowns of three reactors. During the meltdowns, a type of condensed particle, a caesium-rich micro-particle (CsMP), formed inside the reactors via unknown processes. Here we report the chemical and physical processes of CsMP formation inside th...

The reduction of uranium hexavalent (U(VI)) to tetravalent (U(IV)) is an important reaction because of the change in its mobility in the natural environment. Although the flavin mononucleotide (FMN) has acted as an electron shuttle for the U(VI) reduction in vivo system, which is called an electron mediator, only the rate constant for the electron...

Adsorption of cesium (Cs) on phyllosilicates has been intensively investigated because natural soils have strong ability of immobilizing Cs within clay minerals resulting in difficulty of decontamination. The objectives of present study are to clarify how Cs fixation on vermiculite is influenced by structure change caused by Cs sorption at differen...

Ambient temperature geopolymerization of paper sludge ashes (PS-ashes) discharged from paper mills was studied by means of scanning electron microscopy (SEM), X-ray diffraction (XRD), induction coupled plasma atomic emission spectrometry (ICP-AES), and X-ray absorption near edge structure (XANES). Two varieties of alkaline liquors were used in the...

This paper presents the accumulation process of radioactive Cs in edible mushrooms. We here first report the direct accumulation pathway of radioactive Cs from contaminated wood logs to the fruit-bodies of shiitake mushrooms through the basal portion of the stipe. In this pathway, radioactive Cs is not transported through the hyphae. This pathway r...

The migration and dispersion of radioactive Cs (mainly ¹³⁴Cs and ¹³⁷Cs) are of critical concern in the area surrounding the Fukushima Daiichi Nuclear Power Plant (FDNPP). Considerable uncertainty remains in understanding the properties and dynamics of radioactive Cs transport by surface water, particularly during rainfall-induced flood events to th...

Anthropogenic radionuclides contaminate a range of environments as a result of nuclear activities, for example, leakage from waste storage tanks/ponds (e.g. Hanford, USA or Sellafield sites, UK) or as a result of large scale nuclear accidents (e.g. Chernobyl, Ukraine or Fukushima, Japan). One of the most widely applied remediation techniques for co...

Anthropogenic radionuclides contaminate a range of environments as a result of nuclear activities, for example, leakage from waste storage tanks/ponds (e.g. Hanford, USA or Sellafield sites, UK) or as a result of large scale nuclear accidents (e.g. Chernobyl, Ukraine or Fukushima, Japan). One of the most widely applied remediation techniques for co...

We found that root endophytes of 137Cs accumulator plant produce siderophores, resulting in the desorption of 137Cs from the contaminated soil collected at Fukushima, Japan. We selected an endemic Japanese deciduous tree, Eleutherococcus sciadophylloides (Franch. et Sav), that accumulates high concentrations of 137Cs and Mn. Root endophytic bacteri...

Although microorganisms possess high sorption capability for lanthanides, the effect of their biological response on lanthanides migration is unclear. Using active fungus Acremonium strictum KR21-2, supplied with nutrients, this study compared the transformation of lanthanides during the biooxidation of Mn(II) in the absence and presence of trisodi...

Radioactive Cs isotopes (137Cs, t1/2 = 30.07 years and 134Cs, t1/2 = 2.062 years) occur in severely contaminated soils within a few kilometer of the Fukushima Daiichi nuclear power plant at concentrations that range from 4 × 105 to 5 × 107 Bq/kg. In order to understand the mobility of Cs in these soils, both bulk and submicron-sized particles elutr...

Sorption of Ce by mixtures of synthetic Mn oxides and microbial cells of Pseudomonas fluorescens was investigated to elucidate the role of microorganisms on Ce(III) oxidative migration in the environment. The mixtures, upon which Ce was sorbed following exposure to solutions containing 1.0 × 10−4 or 1.0 × 10−5 mol L−1 Ce(III), were analyzed by scan...

Humic substances (HSs) are ubiquitous in various aquatic systems and play important roles in many geochemical processes. There are increasing evidences on the presence of HSs in deep groundwater; nevertheless, their ion binding properties are largely unknown. In this study we investigated the physicochemical and ion-binding properties of humic and...

The accumulation of Cs by unicellular fungus of Saccharomyces cerevisiae in the presence of minerals has been studied to elucidate the role of microorganisms in the migration of radioactive Cs in the environment. Two different types of experiments were employed: experiments using stable Cs to examine the effect of a carbon source on the accumulatio...

To understand the effects of nanoparticles on microorganisms, we experimentally investigated the effects of CeO2 nanoparticles (CeNPs) on yeast (S. cerevisiae) focusing on microbial metabolites and intracellular proteins. The yeast were harvested from a yeast extract peptone dextrose medium containing 0, 10, 100, and 250 ppm of CeNPs and incubated...

The radioactive fallout cesium ((137)Cs) in the sewage sludge ashes (SSAs) produced in Japan after the Fukushima Daiichi Nuclear Accident was tested. Five samples of SSAs produced in 2011 and 2012 were tested. Two of the samples contained (137)Cs (23 and 9.6 kBq/kg, respectively) above the radioactivity criterion (8 kBq of radioactive Cs/kg of soli...

Uptake of uranium by higher fungi, such as mushroom is little elucidated. We have studied the interaction of uranium with Pleurotus sp. (a mushroom) in pure culture over a wide range of U concentration (50-3000 mg/L). The Pleurotus sp. was cultured in two different media. One was rice bran medium, and the other was agar (yeast extract, peptone and...

To elucidate the sorption behavior of americium(III) on bentonite, which is a mixture of montmorillonite clay, quartz and other minerals, simplified desorption experiments were applied to the solid phases collected after the sorption experiments. The sorption–desorption behavior was examined in the final pH range from 2 to 8. The desorption experim...

In this study, we have examined the chemical states change of lanthanides (Lns) of La, Ce, Pr, Eu, and actinides (Ans) of Th(IV), Pu(IV) in the complexes with organic acids of citric acids or desferrioxamine B (DFO) by the interaction with Pseudomonas fluorescens cells under the resting condition at pH 4 – 9. In the adsorption experiments the distr...

The surface reactivity of biogenic birnessite is attributed to its structure. However, structural control of heavy metal adsorption on biogenic birnessite is not well understood. Here a poorly-crystalline birnessite was produced by the fungus Paraconiothyrium sp. WL-2 strain under ambient pH and temperature conditions. The structure was characteriz...

Oxidation of Co by Mn oxide has been investigated using abiotically synthesized Mn oxide. However, oxidation of Co by biogenic Mn oxide is not well known. In this study, we isolated a Mn-oxidizing bacterium (Pseudomonas sp.), designated as strain NGY-1, from stream water. Sorption experiments on Co were carried out using biogenic Mn oxide produced...

The local area distribution and relocation of radioactive cesium deposited in trees after the 2011 tsunami-related accident at the Fukushima Daiichi Nuclear Power Plant (FDNPP) have been studied by measuring the spatial distribution of cesium on/in trees by autoradiography analysis. Samples of trees were collected from places located between 4 and...