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RISS 인기검색어
- 검색키워드
- 저자 : Zachara, John M. (검색결과 3 건)
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Reduction of pertechnetate [Tc(VII)] by aqueous Fe(II) and the nature of solid phase redox products
Zachara, John M.,Heald, Steve M.,Jeon, Byong-Hun,Kukkadapu, Ravi K.,Liu, Chongxuan,McKinley, James P.,Dohnalkova, Alice C.,Moore, Dean A. Elsevier 2007 Geochimica et cosmochimica acta Vol.71 No.9
<P><B>Abstract</B></P><P>The subsurface behaviour of <SUP>99</SUP>Tc, a contaminant resulting from nuclear fuels reprocessing, is dependent on its valence (e.g., IV or VII). Abiotic reduction of soluble Tc(VII) by Fe(II)<SUB>(aq)</SUB> in pH 6–8 solutions was investigated under strictly anoxic conditions using an oxygen trap (<7.5×10<SUP>−9</SUP>atmO<SUB>2</SUB>). The reduction kinetics were strongly pH dependent. Complete and rapid reduction of Tc(VII) to a precipitated Fe/Tc(IV) form was observed when 11μmol/L of Tc(VII) was reacted with 0.4mmol/L Fe(II) at pH 7.0 and 8.0, while no significant reduction was observed over 1 month at pH 6.0. Experiments conducted at pH 7.0 with Fe(II)<SUB>(aq)</SUB>=0.05–0.8mmol/L further revealed that Tc(VII) reduction was a combination of homogeneous and heterogeneous reaction. Heterogeneous reduction predominated after approximately 0.01mmol/L of Fe(II) was oxidized. The heterogeneous reaction was more rapid, and was catalyzed by Fe(II) that adsorbed to the Fe/Tc(IV) redox product. Wet chemical and Fe–X-ray absorption near edge spectroscopy measurements (XANES) showed that Fe(II) and Fe(III) were present in the Fe/Tc(IV) redox products after reaction termination. <SUP>57</SUP>Fe-Mössbauer, extended X-ray adsorption fine structure (EXAFS), and transmission electron microscopy (TEM) measurements revealed that the Fe/Tc(IV) solid phase was poorly ordered and dominated by Fe(II)-containing ferrihydrite with minor magnetite. Tc(IV) exhibited homogeneous spatial distribution within the precipitates. According to Tc-EXAFS measurements and structural modeling, its molecular environment was consistent with an octahedral Tc(IV) dimer bound in bidentate edge-sharing mode to octahedral Fe(III) associated with surface or vacancy sites in ferrihydrite. The precipitate maintained Tc(IV)<SUB>aq</SUB> concentrations that were slightly below those in equilibrium with amorphous Tc(IV)O<SUB>2</SUB>·<I>n</I>H<SUB>2</SUB>O<SUB>(s)</SUB>. The oxidation rate of sorbed Tc(IV) in the Fe/Tc precipitate was considerably slower than Tc(IV)O<SUB>2</SUB>·<I>n</I>H<SUB>2</SUB>O<SUB>(s)</SUB> as a result of its intraparticle/intragrain residence. Precipitates of this nature may form in anoxic sediments or groundwaters, and the intraparticle residence of sorbed/precipitated Tc(IV) may limit <SUP>99</SUP>Tc remobilization upon the return of oxidizing conditions.</P>
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Influence of calcium on microbial reduction of solid phase uranium(VI)
Liu, Chongxuan,Jeon, Byong-Hun,Zachara, John M.,Wang, Zheming John Wiley & Sons 2007 Biotechnology and bioengineering Vol.97 No.6
<P>The effect of calcium on the dissolution and microbial reduction of a representative solid phase uranyl [U(VI)], sodium boltwoodite (NaUO<SUB>2</SUB>SiO<SUB>3</SUB>OH · 1.5H<SUB>2</SUB>O), was investigated to evaluate the rate-limiting step of microbial reduction of the solid phase U(VI). Microbial reduction experiments were performed in a culture of a dissimilatory metal-reducing bacterium (DMRB), Shewanella oneidensis strain MR-1, in a bicarbonate medium with lactate as electron donor at pH 6.8 buffered with PIPES. Calcium increased the rate of Na-boltwoodite dissolution and U(VI) bioavailability by increasing its solubility through the formation of a ternary aqueous calcium-uranyl-carbonate species. The ternary species, however, decreased the rates of microbial reduction of aqueous U(VI). Laser-induced fluorescence spectroscopy (LIFS) and transmission electron microscopy (TEM) collectively revealed that microbial reduction of solid phase U(VI) was a sequentially coupled process of Na-boltwoodite dissolution, U(VI) aqueous speciation, and microbial reduction of dissolved U(VI) to U(IV) that accumulated on bacterial surfaces/periplasm. Under studied experimental conditions, the overall rate of microbial reduction of solid phase U(VI) was limited by U(VI) dissolution reactions in solutions without calcium and limited by microbial reduction in solutions with calcium. Generally, the overall rate of microbial reduction of solid phase U(VI) was determined by the coupling of solid phase U(VI) dissolution, U(VI) aqueous speciation, and microbial reduction of dissolved U(VI) that were all affected by calcium. Biotechnol. Bioneg. 2007;97: 1415–1422. © 2007 Wiley Periodicals, Inc.</P>
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