Takuya Nankawa

Synthesis, Structure, and Redox Properties of [{(5-C5H5)Co(S2C6H4)}2Mo(CO)2], a Novel Metalladithiolene Cluster

Metal-metal bond formation by a cobaltadithiolene complex was observed for the first time in the reaction of [Co(η(5) -C5 H5 )(S2 C6 H4 )] with [Mo(CO)3 (py)3 ] and BF3 to give the Co-Mo-Co cluster 1. Cyclic voltammetry reveals that 1 undergoes two one-electron reduction steps at the Co centers, which is indicative of transmission of the Co-Co electronic interaction through the Mo center.

Thermodynamics and Mechanism Studies on Electrochemical Removal of Cesium Ions from Aqueous Solution Using a Nanoparticle Film of Copper Hexacyanoferrate

Nanoparticle (NPs) film of copper hexacyanoferrate (CuHCF(III)) was developed for electrochemically cesium separation from wastewater. Different form the electro- or chemical deposited films, CuHCF(III) NPs were firstly covered with ferrocyanide anions, so that they can be well dispersed in water and formed ink. Then CuHCF(III) NPs can be uniformly coated by simple wet printing methods, so it is feasible to prepare NPs film of any sizes, or any patterns at low cost. This process provided a promising technology for preparing large scale electrodes for sequential removal of Cs from wastewater in the columns. Cs separation can be controlled by an electrically switched ion exchange (ESIX) system. Effect of temperatures, and ionic strength on Cs removal was investigated. Thermodynamics results showed that Cs adsorption process was exothermic in nature and favored at low temperature. Ionic strength study indicated the CuHCF(III) film can selectively separate Cs in wide ionic strength range from 1 × 10(-4) to 1 × 10(-1) M Na(+). XPS results demonstrated that the electrochemical oxidation-reduction of Fe (II/III) made contributions to Cs separation.

Reduction behavior of uranium in the presence of citric acid

We examined the reduction behavior of UO22+ in citrate media at pH 2.0−7.0 by column electrodes. At pH 2.0, UO22+ was reduced to U(IV) through a one-step reduction process, while it was reduced to U(IV) through a two-step reduction process at pH 3.0−5.0. The reduction potential of UO22+ shifted to lower values with an increase in pH from 2.0 to 7.0. At pH 6.0 and 7.0, UO22+ was not reduced to U(IV) completely at the electrode potential above -0.8 V vs . silver/silver chloride electrode. Ultraviolet-visible spectroscopy and speciation calculation of UO22+ in citrate media indicated that uranium existed as a mixture of UO22+, [UO2Cit]- and [(UO2)2Cit2]2- at pH 2−3, and a predominant species at pH 3−5 was [(UO2)2Cit2]2- (H3Cit: citric acid). At pH 5−7, polymeric complexes, probably, [(UO2)3Cit3]3- and [(UO2)6Cit6(OH)10]16- were present. These findings suggest that the reduction of UO22+ is more difficult by polymerization of UO22+ with citric acid at higher pHs. Absorption spectra of the reduced complexes showed that U(IV) forms soluble complexes with citric acid at pH 2.0−5.0, and presence of U(V) species was not observed during the reduction of UO22+.

Association of Actinides with Microorganisms and Clay: Implications for Radionuclide Migration from Waste-Repository Sites

We conducted a series of basic studies on the microbial accumulation of actinides to elucidate their migration behavior around backfill materials used in the geological disposal of radioactive wastes. We explored the interactions of U(VI) and Pu(VI) with Bacillus subtilis, kaolinite clay, and within a mixture of the two, directly analyzing their association with the bacterium in the mixture by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The accumulation of U by the mixture rose as the numbers of B. subtilis cells increased. Treating the kaolinite with potassium acetate (CH 3 COOK) removed approximately 80% of the associated uranium while only 65% was removed in the presence of B. subtilis. TEM-EDS analysis confirmed that most of the U taken from solution was associated with B. subtilis. XANES analyses revealed that the oxidation state of uranium associated with B. subtilis, kaolinite, and with the mixture containing both was U(VI). The amount of Pu sorbed by B. subtilis increased with time, but did not reach equilibrium in 48 h; in kaolinite alone, equilibrium was attained within 8 h. After 48 h, the oxidation state of Pu in the solutions exposed to B. subtilis and to the mixture had changed to Pu(V), whereas the oxidation state of the Pu associated with both was Pu(IV). In contrast, there was no change in the oxidation state of Pu in the solution nor on kaolinite after exposure to Pu(VI). SEM-EDS analysis indicated that most of the Pu in the mixture was associated with the bacteria. These results suggest that U(VI) and Pu(VI) preferentially are sorbed to bacterial cells in the presence of kaolinite clay, and that the mechanism of accumulation of U and Pu differs. U(VI) is sorbed directly to the bacterial cells, whereas Pu(VI) first is reduced to Pu(V) and then to Pu(IV), and the latter is associated with the cells. These results have important implications on the migrations of radionuclides around the repository sites of geological disposal. Microbial cells compete with clay colloids for radionuclides accumulation, and because of their higher affinity and larger size, the microbes accumulate radionuclides and migrate much slower than do the clay colloids. Additionally, biofilm coatings formed on the fractured rock surfaces also accumulate radionuclides, thereby retarding radionuclide migration.

Redox behavior of Ce(IV)/Ce(III) in the presence of nitrilotriacetic acid: A surrogate study for An(IV)/An(III) redox behavior

Abstract Using cyclic voltammetry, we investigated the redox behavior of Ce(IV)/Ce(III), which is a surrogate for An(IV)/An(III) (An=actinides), in a solution of nitrilotriacetic acid (NTA) at 25 °C. The cyclic voltammogram of Ce in a 0.1 M NTA solution at pH 6 showed a reversible one-electron redox reaction for Ce(IV)/Ce(III) at 0.51 V vs. Ag/AgCl. This redox potential was much lower than that obtained in 1 M nitric acid, indicating that Ce(IV) was preferentially stabilized by complexation with NTA. The redox potential in the NTA solution was independent of the Ce concentration from 2 to 20 mM, NTA concentration from 5 to 200 mM and pH between 3 and 7. These results indicated that no polymerization and no additional coordination of NTA and OH− to the Ce(III)-NTA complex took place during the redox reaction. As the speciation calculation of Ce(III) in the NTA solution showed that the predominant species was CeIII(nta)23− (H3nta=NTA), the redox reaction of the Ce-NTA complex was expressed by the following: CeIV(nta)22−+e−⇋CeIII(nta)23−. The logarithm of the stability constant of CeIV(nta)22− was calculated to be 38.6±0.8 for I=0 from the redox potential shift of Ce(IV)/Ce(III) in the NTA solution. The value was in good accordance with the stability constant of the NpIV(nta)22− complex, demonstrating that the aqueous coordination chemistry of Ce(IV) with NTA is quite similar to that of An(IV). These results strongly suggest that a negative shift of the Pu(IV)/Pu(III) redox potential in the NTA solution should make Pu(IV) more stable than Pu(III) even in a reducing environment.

Modeling of the Interaction of Pu(VI) with the Mixture of Microorganism and Clay

A model analysis of the transformation of Pu(VI) in a mixture of a common soil bacterium, Bacillus subtilis, and kaolinite clay was carried out. A simplified kinetic model, where the adsorptions of Pu(VI) and Pu(V) are assumed to be in instantaneous equilibrium and the two-step reduction of Pu(VI) to Pu(V) and then to Pu(IV) is described by a first-order rate law, was proposed. When we assumed in the model analysis that the reduction rate of Pu(V) to Pu(IV) was higher for B. subtilis than for kaolinite, the estimated fractions of Pu in the solution and in the mixture, and the oxidation states of Pu in the solution and in the mixture were in good agreement with the measured ones. On the contrary, the assumption that the reduction rate of Pu(V) to Pu(IV) for kaolinite was the same as that for B. subtilis gave a wrong prediction of Pu association with the mixture. These results strongly suggest that Pu(V) on the bacterial cell is reduced to Pu(IV) by the bacterially mediated electron transfer process during the accumulation of Pu(VI) in the mixture.

Column study on electrochemical separation of cesium ions from wastewater using copper hexacyanoferrate film

Nanoparticle film of copper hexacyanoferrate (CuHCFIII) was developed and coated on the rolled sheet electrodes for column cesium separation from wastewater. Results indicated well balance of Cs adsorption and desorption can be achieved switching the potentials between anodes and cathodes. XPS and isotherm studies indicated the electrochemical oxidation–reduction of Fe(II/III) on the monolayer of the CuHCFIII film made contributions to reversible Cs separation. Minimization of secondary waste, simple regeneration and uniformly coating for any size, suggest a promising column technology for sequential removal of Cs from actual wastewater.

Yeast Genes Involved in Uranium Tolerance and Uranium Accumulation: A Functional Screening Using the Nonessential Gene Deletion Collection

We screened 4908 non-essential gene deletion mutant yeast strains for uranium sensitivity and low accumulation by growth in agar medium containing uranium. All mutant strains grew successfully on agar media containing 0 or 0.2 mM uranium for one week at 30o C. Thirteen strains with single gene deletions showed reduced growth in the agar medium containing 0.5 mM uranium and were identified as uranium-sensitive mutant strains. The phosphate transporter genes of PHO86, PHO84, PHO2, and PHO87 were among the deleted genes in the uranium-sensitive mutant strains, suggesting that genes concerned with phosphate transport contribute to uranium tolerance. Seventeen single-deletion strains showed lower uranium accumulation than the wild-type after exposure to agar medium containing 0.5 mM uranium, and were identified as mutant strains with low uranium accumulation. Among the deleted genes in these strains were cell membrane proteins, phospholipid-binding proteins, and cell wall proteins, suggesting that cell surface proteins contribute to uranium accumulation.

Electrochemical Studies on Uranium in the Presence of Organic Acids

We examined electrochemical redox reactions of UO2 2+ in perchlorate and organic acid (oxalic, malonic, succinic, adipic, L-malic, and L-tartaric acids) solutions using cyclic voltammetry to reveal the effects of complex formation with organic acids on the redox behavior. In the perchlorate and organic acid solutions, a redox reaction of UO2 2+/UO2 + and an oxidation reaction of U(IV) produced by a disproportionation of UO2 + were observed. The peak potentials of the UO2 2+ reduction showed a good linear relationship with the stability constants of 1:1 UO2 2+-organic complexes. In the presence of malonic acid, the redox potential for UO2 2+/UO2 + was constant at pH 1-2 and 5-6 while it decreased with an increase in pH from 2 to 5. Additionally, it was independent of malonate concentration at 0.1–0.5 M while it decreased with an increase in the concentration from 0.005 to 0.1 M. Based on the experimental and the speciation calculation results, we determined the redox reactions of UO2 2+-malonate complexes as a function of pH and malonate concentration. We also determined the redox reactions of UO2 2+-oxalate complexes in the same way.

High Proton Conductivity of Zinc Oxalate Coordination Polymers Mediated by a Hydrogen Bond with Pyridinium

A novel metal-organic framework, (Hpy)2[Zn2(ox)3]·nH2O (n = 0, 1), having a pyridinium cation, was newly synthesized, and the crystal structures were determined. The hydrated compound shows a high proton conductivity of 2.2 × 10(-3) S cm(-1) at 298 K and 98% relative humidity. Single crystal XRD analysis revealed a rotational displacement factor for the hydrated pyridinium ring and elongated water site that is thought to cause the high proton conductivity.