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Journal Articles

Improved immobilization of Re(VII) from aqueous solutions bimetallic Ni/Fe $^{0}$ nanoparticles; Implications towards Tc(VII) removal

Maamoun, I.; Tokunaga, Kohei; Dohi, Terumi; Kanno, Futoshi*; Falyouna, O.*; Eljamal, O.*; Tanaka, Kazuya

Frontiers in Nuclear Engineering (Internet), 2, p.1142823_1 - 1142823_17, 2023/03

https://doi.org/10.3389/fnuen.2023.1142823

Recently, the rapid development of nuclear power technologies and the continuous energy demand around the world exhibited massive amounts of contaminated water with radionuclides. Technetium-99 ( $^{99}$ Tc) is one of the long-lived radionuclides with a highly mobile anionic form (Tc(VII)) in aqueous solutions. Hence, the rapid and efficient Tc(VII) removal from radioactive water has emerged as a challenging issue. In this work, bimetallic nickel/iron nanoparticles (Ni/Fe $^{0}$ ) were prepared to enhance the immobilization of rhenium (Re(VII)) from aqueous solutions, as the surrogate of Tc(VII).

Journal Articles

Improvement of the stability of IO $_{3}$ $^{-}$ -, SeO $_{3}$ $^{2-}$ -, and SeO $_{4}$ $^{2-}$ -coprecipitated barite after treatment with phosphate

Tokunaga, Kohei; Tanaka, Kazuya; Takahashi, Yoshio*; Kozai, Naofumi

Environmental Science & Technology, 57(8), p.3166 - 3175, 2023/02

https://doi.org/10.1021/acs.est.2c08939

Coprecipitation of radionuclides with barite has been studied to remove radionuclides from radioactive liquid waste because of its excellent removal efficiency; however, little information exists concerning the stability of the ions coprecipitated with barite. This study systematically investigated the stability of iodate, selenite, and selenate coprecipitated with barite via leaching tests. These oxyanions were gradually leached from the oxyanion-bearing barite into ultrapure water over time. Leaching of the oxyanions significantly increased in leaching solutions containing NaCl (pH5.3), NaNO $_{3}$ (pH5.9), and Na $_{2}$ SO $_{4}$ (pH5.7). Conversely, leaching of the oxyanions was suppressed in KH $_{2}$ PO $_{4}$ solution (pH8.5), indicating that phosphate stabilized the oxyanion-bearing barite. The effect of phosphate treatment on oxyanion-bearing barite was further investigated. The results showed that the barite surface was modified with phosphate, and a thin surface layer of a barium phosphate-like structure was formed. The amount of oxyanions leached from the phosphate-treated samples into leaching solutions containing NaCl or NaNO $_{3}$ was much lower than the amounts leached from the untreated barite samples into ultrapure water. The barite coprecipitation combined with subsequent phosphate treatment may be a promising method to efficiently remove iodate, selenite, and selenate from wastewater and stabilize them as barite coprecipitates.

Journal Articles

Arsenic and iron speciation and binding in the surface soils in Ningyo-toge mill tailings pond using X-ray absorption fine spectroscopy

Tokunaga, Kohei; Takahashi, Yoshio*; Kozai, Naofumi

Journal of Nuclear and Radiochemical Sciences (Internet), 23, p.14 - 19, 2023/00

Journal Articles

Nested antiferromagnetic spin fluctuations and non-Fermi-liquid behavior in electron-doped CeCo $_{1-x}$ Ni $_{x}$ In

Sakai, Hironori; Tokunaga, Yo; Kambe, Shinsaku; Zhu, J.-X.*; Ronning, F.*; Thompson, J. D.*; Kotegawa, Hisashi*; To, Hideki*; Suzuki, Kohei*; Oshima, Yoshiki*; et al.

Physical Review B, 106(23), p.235152_1 - 235152_8, 2022/12

https://doi.org/10.1103/PhysRevB.106.235152

Times Cited Count：0 Percentile：0(Materials Science, Multidisciplinary)

We investigate the electronic state of Ni-substituted CeCo $_{1-x}$ NiIn by nuclear quadrupole and magnetic resonance (NQR/NMR) techniques. The heavy fermion superconductivity below $T_{rm c} = 2.3$ K for is suppressed by Ni substitutions, and $T_{rm c}$ reaches zero for . The $^{115}$ In NQR spectra for and 0.25 can be explained by simulating the electrical field gradient that is calculated for a virtual supercell with density functional theory. The spin-lattice relaxation rate indicates that Ni substitution weakens antiferromagnetic correlations that are not localized near the substituent but instead are uniform in space. The temperature () dependence of $(T_1T)^{-1}$ for shows a maximum around $T_{rm g} = 2$ K and $(T_1T)^{-1}$ decreases toward almost zero when temperature is further reduced as if a gap might be opening in the magnetic excitation spectrum; however, the magnetic specific heat and the static magnetic susceptibility evolve smoothly through $T_{rm g}$ with a dependence. The peculiar T dependence of $(T_1T)^{-1}$ and non-Fermi-liquid specific heat and susceptibility can be interpreted in a unified way by assuming nested antiferromagnetic spin fluctuations in a quasi-two-dimensional electronic system.

Journal Articles

Urea-introduced ionic liquid for the effective extraction of Pt(IV) and Pd(II) ions

Ueda, Yuki; Eguchi, Ayano; Tokunaga, Kohei; Kikuchi, Kei*; Sugita, Tsuyoshi; Okamura, Hiroyuki; Naganawa, Hirochika

Industrial & Engineering Chemistry Research, 61(19), p.6640 - 6649, 2022/05

https://doi.org/10.1021/acs.iecr.2c00304

Times Cited Count：0 Percentile：0(Engineering, Chemical)

no abstracts in English

Journal Articles

A Storm-induced flood and associated nearshore dispersal of the river-derived suspended $^{137}$ Cs

Uchiyama, Yusuke*; Tokunaga, Natsuki*; Azuma, Kohei*; Kamidaira, Yuki; Tsumune, Daisuke*; Iwasaki, Toshiki*; Yamada, Masatoshi*; Tateda, Yutaka*; Ishimaru, Takashi*; Ito, Yukari*; et al.

Science of the Total Environment, 816, p.151573_1 - 151573_13, 2022/04

https://doi.org/10.1016/j.scitotenv.2021.151573

Times Cited Count：5 Percentile：72.85(Environmental Sciences)

no abstracts in English

Journal Articles

Nanoscale heterogeneity induced by nonmagnetic Zn dopants in the quantum critical metal CeCoIn; $^{115}$ In NQR/NMR and $^{59}$ Co NMR study

Sakai, Hironori; Tokunaga, Yo; Kambe, Shinsaku; Zhu, J.-X.*; Ronning, F.*; Thompson, J. D.*; Ramakrishna, S. K.*; Reyes, A. P.*; Suzuki, Kohei*; Oshima, Yoshiki*; et al.

Physical Review B, 104(8), p.085106_1 - 085106_12, 2021/08

https://doi.org/10.1103/PhysRevB.104.085106

Times Cited Count：3 Percentile：32.35(Materials Science, Multidisciplinary)

Antiferromagnetism in a prototypical quantum critical metal CeCoIn is known to be induced by slight substitutions of non-magnetic Zn atoms for In. In nominally 7% Zn substituted CeCoIn, an antiferromagnetic (AFM) state coexists with heavy fermion superconductivity. Heterogeneity of the electronic states is investigated in Zn doped CeCoIn by means of nuclear quadrupole and magnetic resonances (NQR and NMR). Site-dependent NQR relaxation rates indicate that the AFM state is locally nucleated around Zn substituents in the matrix of a heavy fermion state, and percolates through the bulk at the AFM transition temperature $T_{rm N}$ . At lower temperatures, an anisotropic superconducting (SC) gap below the SC transition temperature $T_{rm c}$ , and the SC state permeates through the AFM regions via a SC proximity effect. Applying an external magnetic field induces a spin-flop transition near 5 T, reducing the volume of the AFM regions. Consequently, a short ranged inhomogeneous AFM state survives and coexists with a paramagnetic Fermi liquid state at high fields.

Journal Articles

Effective removal of iodate by coprecipitation with barite; Behavior and mechanism

Tokunaga, Kohei; Takahashi, Yoshio*; Tanaka, Kazuya; Kozai, Naofumi

Chemosphere, 266, p.129104_1 - 129104_10, 2021/03

https://doi.org/10.1016/j.chemosphere.2020.129104

Times Cited Count：5 Percentile：47.38(Environmental Sciences)

Radioactive iodine ( $^{129}$ I) is of great concern owing to its high mobility in the environment and long-term radiotoxicity, but there is a lack of effective techniques for removing iodate (IO $_{3}$ $^{-}$ ) from aqueous solution. The aim of this study is to develop a new technique for removing radioactive iodate from contaminated solution by using barite (BaSO $_{4}$ ). In the present study, we examined the coprecipitation mechanism of iodate by barite at the molecular level for determining optimum conditions for iodate removal. The results showed that iodate was effectively removed from aqueous solution by coprecipitation, even in the presence of competitive anions in solution. Comparing our method with previous studies, iodate removal efficiency by barite was determined to be about two orders of magnitude greater than that by hydrotalcite-like layered double hydroxide at Cl $^{-}$ concentration of 10 mmol L $^{-1}$ . Extended X-ray absorption fine structure (EXAFS) analysis indicated that incorporated iodate was strongly bound in the crystal lattice of barite by substituting the sulfate site in the structure when the iodine concentration was low. The charge compensation problem from the IO $_{3}$ $^{-}$ substitution in SO $_{4}$ $^{2-}$ site is achieved by the substitution of Na $^{+}$ -IO $_{3}$ $^{-}$ pairs at the nearest Ba $^{2+}$ site. Therefore, considering high removal efficiency and strong binding of iodate in barite, coprecipitation with barite is a promising material for removing radioactive iodate from various aqueous solutions contaminated with iodate.

Journal Articles

A Fluorous phosphate for the effective extraction of Ln $^{III}$ from nitrate media; Comparison with a conventional organic phosphate

Ueda, Yuki; Kikuchi, Kei*; Tokunaga, Kohei; Sugita, Tsuyoshi; Aoyagi, Noboru; Tanaka, Kazuya; Okamura, Hiroyuki

Solvent Extraction and Ion Exchange, 39(5-6), p.491 - 511, 2021/00

https://doi.org/10.1080/07366299.2021.1874115

Times Cited Count：0 Percentile：0(Chemistry, Multidisciplinary)

no abstracts in English

Journal Articles

H $_{2}$ production from methane decomposition by fullerene at low temperature

Tokunaga, Tomoharu*; Kuno, Kohei*; Kawakami, Takumi*; Yamamoto, Takahisa*; Yoshigoe, Akitaka

International Journal of Hydrogen Energy, 45(28), p.14347 - 14353, 2020/05

https://doi.org/10.1016/j.ijhydene.2020.03.178

Times Cited Count：1 Percentile：3.76(Chemistry, Physical)

To understand the catalytic behavior of graphite and carbon black with mainly 6-membered rings with sp $^{2}$ bonds for H $_{2}$ production by CH $_{4}$ decomposition, fullerenes with 6-membered rings and also those comprising 5- and 7-membered rings with sp $^{2}$ bonds was investigated using gas chromatography, XPS and TEM analysis. From these analysis, it is anticipated that the ring structures without 6-membered rings in carbon materials with sp $^{2}$ bonding contribute to the catalytic behavior for CH $_{4}$ decomposition at a low temperature of 400 $^{circ}$ C.

Journal Articles

A New technique for removing strontium from seawater by coprecipitation with barite

Tokunaga, Kohei; Kozai, Naofumi; Takahashi, Yoshio*

Journal of Hazardous Materials, 359, p.307 - 315, 2018/07

https://doi.org/10.1016/j.jhazmat.2018.07.044

Times Cited Count：25 Percentile：69.17(Engineering, Environmental)

In the present study, we looked at the removal efficiency of Sr by using barite (BaSO $_{4}$ ) under various experimental conditions to develop techniques for the direct removal of Sr from seawater. The uptake of Sr by barite was found to be dependent on pH, saturation state, and [Ba $^{2+}$ ]/[SO $_{4}$ $^{2-}$ ] ratio in initial aqueous solution, showing that most of the aqueous Sr can be removed from the aqueous solution by adjusting these parameters. However, the effects of ionic strength and competitive ions were negligible, suggesting the effectiveness of its application to removal of Sr from seawater. Batch experiments were also conducted in a seawater system, and a rather high removal efficiency of Sr from seawater (more than 90%) was achieved. Considering its high removal and retention efficiency of Sr in seawater systems, barite is a reliable material for the removal of Sr from seawater.

Journal Articles

Effective removal of selenite and selenate ions from aqueous solution by barite

Tokunaga, Kohei*; Takahashi, Yoshio*

Environmental Science & Technology, 51(16), p.9194 - 9201, 2017/08

https://doi.org/10.1021/acs.est.7b01219

Times Cited Count：40 Percentile：82.6(Engineering, Environmental)

In the present study, we explore a new application of barite (BaSO $_{4}$ ) as a sequestering phase for selenite (Se(IV)) and selenate (Se(VI)) ions from aqueous solutions due to the low solubility and high stability of barite. The uptake of Se(IV) and Se(VI) during coprecipitation with barite was investigated through batch experiments to understand the factors controlling effective removal of Se(IV) and Se(VI) from polluted water to barite. The uptake of Se(IV) by barite is dependent on pH, coexistent calcium ion, and sulfate concentration in the initial solution, possibly due to their effects on the chemical affinity and structural similarity. On the other hand, the uptake of Se(VI) by barite was strongly dependent on sulfate concentration in the initial solution, which is only related to the structural similarity. This study provides a good estimate of its ability to effectively remove Se(IV) and Se(VI) from aqueous solutions (more than 80%) under optimized experimental parameters.

Journal Articles

Ligand exchange adsorption and coordination structure of Pd on -MnO $_{2}$ in NaCl solution

Tanaka, Kazuya; Tanaka, Masato*; Watanabe, Naoko*; Tokunaga, Kohei*; Takahashi, Yoshio*

Chemical Geology, 460, p.130 - 137, 2017/06

https://doi.org/10.1016/j.chemgeo.2017.04.022

Times Cited Count：7 Percentile：31.94(Geochemistry & Geophysics)

Pd is highly accumulated in ferromanganese nodules and crusts relative to its concentration in seawater but the mechanism by which Pd(II) is incorporated remains poorly understood. We investigated the local coordination structure of Pd(II) adsorbed on -MnO $_{2}$ , using X-ray absorption fine structure spectroscopy. X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) analyses indicated that Pd was adsorbed on -MnO $_{2}$ through ligand exchange, from Cl coordination to O coordination. Furthermore, curve fitting of EXAFS spectra demonstrated the formation of two different inner-sphere complexes, bidentate-mononuclear and bidentate-binuclear complexes, and this finding was supported by density functional theory calculations. The formation of inner-sphere complexes is reasonable given the relatively large distribution coefficients obtained from adsorption experiments.

Oral presentation

NQR/NMR study on Zn-doped CeCoIn

Sakai, Hironori; Hattori, Taisuke; Tokunaga, Yo; Kambe, Shinsaku; Ronning, F.*; Zhu, J.-X.*; Suzuki, Kohei*; Oshima, Yoshiki*; Yokoyama, Makoto*

no journal, ,

Dilute substitutions of non-magnetic dopants Zn can induce a long-ranged antiferromagnetic ordering to a heavy fermion superconductor CeCoIn. We have microscopically investigated the Zn-doped CeCoIn by means of the nuclear quadrupole/magnetic resonance (NQR/NMR) techniques.

Oral presentation

NQR/NMR study of Zn-doped CeCoIn

Sakai, Hironori; Hattori, Taisuke; Higa, Nonoka; Tokunaga, Yo; Kambe, Shinsaku; Ronning, F.*; Zhu, J.-X.*; Suzuki, Kohei*; Oshima, Yoshiki*; Yokoyama, Makoto*

no journal, ,

We have used Nuclear Quadrupole Resonance (NQR) to probe microscopically the response of prototypical quantum critical metal CeCoIn to substitutions of small amounts of Cd or Zn for In. Such non-magnetic Cd or Zn substitutions induce a long range antiferromagnetism. In the case of Cd substitutions, approximately half of the Cd substituents induce local Ce moments in their close proximity, as observed by site-dependent longitudinal nuclear spin relaxation rates . In the paramagnetic state, the values of at the main peak position are similar to those in the pure CeCoIn, while those are enhanced at the skirt position of spectrum. We will discuss about the heterogeneous electronic state realized in such a Zn substituted system and discuss about the coexistence of superconductivity with the antiferromagnetism, as well.

Oral presentation

Development of removal method for radioactive elements by coprecipitation with barite (BaSO $_{4}$ )

Tokunaga, Kohei; Kozai, Naofumi; Takahashi, Yoshio*; Ohara, Yoshiyuki

no journal, ,

In the present study, we explore a new application of barite (BaSO $_{4}$ )) as a sequestering phase for selenite (Se(IV)) and selenate (Se(VI)) ions from aqueous solutions. Barite is a common phase in many geological environments, and it can be used to remove toxic and/or radioactive elements from aqueous solutions, but it has not been widely used in environmental studies. This study describes the mechanisms of Se distribution between barite and water, thus providing a good estimate of its ability to effectively remove Se(IV) and Se(VI) from aqueous solutions (more than 80%) using the optimized experimental parameters examined here.

Oral presentation

A New technique for removing radioactive nuclides from aqueous solution by coprecipitation with barite

Tokunaga, Kohei; Takahashi, Yoshio*; Kozai, Naofumi

no journal, ,

In the present study, we explore a new application of barite (BaSO $_{4}$ ) as a sequestering phase for radioactive nuclides from aqueous solutions. Barite is a common phase in many geological environments, and it can be used to remove toxic and/or radioactive elements from aqueous solutions, but it has not been widely used in environmental studies. This study describes the mechanisms of SeO $_{3}$ $^{2-}$ , SeO $_{4}$ $^{2-}$ , IO $_{3}$ $^{-}$ , and Sr $_{2}$ $^{+}$ distribution between barite and water, thus providing a good estimate of its ability to effectively remove these ions from aqueous solutions (more than 80%) using the optimized experimental parameters examined here.

Oral presentation

A New technique for removing radioactive nuclides from aqueous solution by coprecipitation with barite

Tokunaga, Kohei; Takahashi, Yoshio*; Kozai, Naofumi

no journal, ,

Oral presentation

A New technique for removing oxyanions from aqueous solution by coprecipitated with barite

Tokunaga, Kohei; Takahashi, Yoshio*; Kozai, Naofumi

no journal, ,

In the present study, we explore a new application of barite as a sequestering phase for radioactive oxyanions, such as selenium-79, technetium-99, and iodine-129, from aqueous solutions. Barite is a common phase in many geological environments, and it can be used to remove toxic and/or radioactive elements from aqueous solutions, but it has not been widely used in environmental studies. This study mainly describes the mechanisms of selenite and selenate distribution between barite and water, thus providing a good estimate of its ability to effectively remove these ions from aqueous solutions (more than 80%) using the optimized experimental parameters examined here.

Oral presentation

Effective removal of oxyanions from aqueous solution by coprecipitation with barite

Tokunaga, Kohei; Kozai, Naofumi; Takahashi, Yoshio*

no journal, ,

no abstracts in English