Takeshi Ohnuki

Sorption behavior of europium(III) and curium(III) on the cell surfaces of microorganisms

Summary We investigated the association of europium(III) and curium(III) with the microorganisms Chlorella vulgaris, Bacillus subtilis, Pseudomonas fluorescens, Halomonas sp., Halobacterium salinarum, and Halobacterium halobium . We determined the kinetics and distribution coefficients (Kd) for Eu(III) and Cm(III) sorption at pH 3-5 by batch experiments, and evaluated the number of water molecules in the inner-sphere (NH₂O) and the degree of strength of ligand field (RE/M) for Eu(III) by time-resolved laser-induced fluorescence spectroscopy (TRLFS). Exudates from C. vulgaris, Halomonas sp., and H. halobium had an affinity for Eu(III) and Cm(III). The log Kd of Eu(III) and Cm(III) showed that their sorption was not fully due to the exchange with three protons on the functional groups on cell surfaces. The halophilic microorganisms (Halomonas sp., Halobacterium salinarum, H. halobium) showed almost no pH dependence in log Kd, indicating that an exchange with Na+ on the functional groups was involved in their sorption. The Δ NH₂O(=9-NH₂O) for Eu(III) on C. vulgaris was 1-3, while that for the other microorganisms was over 3, demonstrating that the coordination of Eu(III) with C. vulgaris was predominantly an outer-spherical process. The RE/M for Eu(III) on halophilic microorganisms was 2.5-5, while that for non-halophilic ones was 1-2.5. This finding suggests that the coordination environment of Eu(III) on the halophilic microorganisms is more complicated than that on the other three non-halophilic ones.

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+.

Interactions of trivalent and tetravalent heavy metal-siderophore complexes with Pseudomonas fluorescens

Summary We investigated the interactions of the Fe(III)-, Eu(III)-, and Hf(IV)-desferrioxamine B (DFO) complexes with the Gram-negative aerobic bacterium Pseudomonas fluorescens. Potentiometric titration of 1:1 Fe(III)-, Eu(III)-, and Hf(IV)-DFO complexes showed that Hf(IV) formed a strong complex with DFO whose stability was comparable to that of the Fe(III)-DFO complex, while Eu(III) formed a weaker one. DFO in a growth medium was not degraded by P. fluorescens. Contact of P. fluorescens cells with the Fe(III)-, Eu(III)-, and Hf(IV)-DFO complexes at pH 4-9 revealed that there was negligible adsorption of Hf(IV) and Fe(III), whereas Eu(III) was dissociated from DFO and was readily adsorbed by the cells. These results suggest that Fe(III) and Hf(IV) form stable complexes with DFO and are not adsorbed by P. fluorescens cells. Europium(III) forms a weaker complex with DFO than Fe(III) and Hf(IV) do and its DFO complex is readily dissociated in the presence of the cells.

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.