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RISS 인기검색어
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- 저자 : Xavier Gaona (검색결과 3 건)
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Lee, Jun-Yeop,Vespa, Marika,Gaona, Xavier,Dardenne, Kathy,Rothe, Jö,rg,Rabung, Thomas,Altmaier, Marcus,Yun, Jong-Il De Gruyter 2017 RADIOCHIMICA ACTA Vol.105 No.3
<P><B>Abstract</B></P><P>The formation of ternary Mg-UO<SUB>2</SUB>-CO<SUB>3</SUB>complexes under weakly alkaline pH conditions was investigated by time-resolved laser fluorescence spectroscopy (TRLFS) and extended X-ray absorption fine structure (EXAFS) and compared to Ca-UO<SUB>2</SUB>-CO<SUB>3</SUB>complexes. The presence of two different Mg-UO<SUB>2</SUB>-CO<SUB>3</SUB>complexes was identified by means of two distinct fluorescence lifetimes of 17±2 ns and 51±2 ns derived from the multi-exponential decay of the fluorescence signal. Slope analysis in terms of fluorescence intensity coupled with fluorescence intensity factor as a function of log [Mg(II)] was conducted for the identification of the Mg-UO<SUB>2</SUB>-CO<SUB>3</SUB>complexes forming. For the first time, the formation of both MgUO<SUB>2</SUB>(CO<SUB>3</SUB>)<SUB>3</SUB><SUP>2−</SUP>and Mg<SUB>2</SUB>UO<SUB>2</SUB>(CO<SUB>3</SUB>)<SUB>3</SUB>(aq) species was confirmed and the corresponding equilibrium constants were determined as log β<SUP>0</SUP><SUB>113</SUB>=25.8±0.3 and log β<SUP>0</SUP><SUB>213</SUB>=27.1±0.6, respectively. Complementarily, fundamental structural information for both Ca-UO<SUB>2</SUB>-CO<SUB>3</SUB>and Mg-UO<SUB>2</SUB>-CO<SUB>3</SUB>complexes was gained by extended EXAFS revealing very similar structures between these two species, except for the clearly shorter U-Mg distance (3.83 Å) compared with U-Ca distance (4.15 Å). These results confirmed the inner-sphere character of the Ca/Mg-UO<SUB>2</SUB>-CO<SUB>3</SUB>complexes. The formation constants determined for MgUO<SUB>2</SUB>(CO<SUB>3</SUB>)<SUB>3</SUB><SUP>2−</SUP>and Mg<SUB>2</SUB>UO<SUB>2</SUB>(CO<SUB>3</SUB>)<SUB>3</SUB>(aq) species indicate that ternary Mg-UO<SUB>2</SUB>-CO<SUB>3</SUB>complexes contribute to the relevant uranium species in carbonate saturated solutions under neutral to weakly alkaline pH conditions in the presence of Mg(II) ions, which will induce notable influences on the U(VI) chemical species under seawater conditions.</P>
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Chemical Behavior of Niobium(V) in Cementitious Conditions: Solubility and Sorption
Yongheum Jo,Nese Cevirim-Papaioannou,Krassimir Garbev,Markus Fuss,Karsten Franke,Oliver Dieste Blanco,Benny de Blochouse,Marcus Altmaier,Xavier Gaona 한국방사성폐기물학회 2022 한국방사성폐기물학회 학술논문요약집 Vol.20 No.2
Niobium (Nb) is present in Ni-based alloys and stainless steels used in nuclear reactors as structural materials. Nb-93 is a naturally occurring and stable isotope of niobium and Nb-94 (half-life = 20,000 years) is produced by neutron activation of Nb-93. Nb-94 can be present in waste streams from dismantling of nuclear power plants and treatment of the primary coolant circuit. Hence, the radioactive wastes containing active Nb-94 are disposed of in the repositories for low- and intermediate-level waste (LILW). Nb predominantly exhibits a pentavalent oxidation state (i.e., +V) within the stability field of water. Cementitious materials (concrete, mortar, and grout) are extensively utilized in LILW disposal systems as structural components and chemical agents for the stabilization of waste. Solubility defines the source term (i.e., upper concentration limit) in the repository system. However, the solubility behavior of Nb in cementitious systems at high pH remains ill-defined, and information available on the Nb solid phases controlling the solubility is scarce and often ambiguous. Sorption on cementbased materials is one of the main mechanisms controlling the retention of niobium(V) in a LILW repository, and distribution coefficients (Rd) are necessary to evaluate the retention capacity by sorption in the safety assessment of disposal systems. Available sorption data of Nb(V) on cement showed a large discrepancy in Rd, moreover, no sorption data is available for Nb(V) under conditions characterizing the first degradation stage of cement (young cement condition) at pH 13 – 13.5. In this context, the solubility of Nb was extensively investigated in porewater conditions representative of the cement degradation stage I, as well as in CaCl2-Ca(OH)2 systems. Special focus was given to the accurate characterization of the solubility-controlling solid niobium phases. We also studied the sorption of Nb(V) by hardened cement pastes (HCP) and calcium silicate hydrates (CSH, major hydrate of HCP). This work provides the results on Rd, sorption isotherm and sorption mechanisms of Nb(V). Besides, the impact of ISA (polyhydroxycarboxylic acid generated by the degradation of cellulose) on Nb(V) sorption and the dissolution of cement materials was investigated.
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