報告書番号

U01063

タイトル（和文）

環境中における放射性核種の挙動に及ぼす微生物影響評価（その１） --プルトニウムとネプツニウムのベントナイトへの吸着挙動に及ぼす地下微生物の還元作用およびシデロフォアの影響--

タイトル（英文）

Microbial impact on the behavior of radionuclides in the environment (1) -Adsorption behavior of Pu(IV) and Np(V) by bentonite under the influence of microbial reduction and siderophore-

概要 （図表や脚注は「報告書全文」に掲載しております）

放射性廃棄物処分の安全性確保の観点から，核種の環境中での挙動を把握することは重要である。そのため，代表的な嫌気微生物である硫酸還元菌の存在下でのネプツニウムのベントナイトへの吸着挙動について検討を行った。好気条件では溶存態として安定である同元素は，菌体の存在する嫌気条件下において，還元され，ベントナイトへ吸着あるいは溶解度の低下により沈殿し，固相へと移行した。化学的嫌気条件下の場合に比べ，菌体存在下における還元速度は速いことから，菌体が直接的にネプツニウムを還元していることが示唆された。また，プルトニウムの吸着挙動に及ぼす微生物由来の鉄キレーター（シデロフォア）の影響についても検討を行った結果，シデロフォア濃度の増大に伴い，吸着量は低下した。しかしながら，同濃度が低い場合，ベントナイトから溶出する鉄イオン等がシデロフォアとプルトニウムの錯体形成を阻害するため，その影響は小さくなると考えられた。

概要 （英文）

It is essential to understand the behavior of actinide in the environment to determine if the repositories can safely contain high-level radioactive waste. In the meantime, microbes contribute to the number of geochemical reactions in the subsurface environment, and some microorganisms can interact with actinides directly and/or indirectly (e.g., biotransformation, biosorption, bioaccumulation). From this point of view, we first investigated experimentally the microbial influence on the adsorption behavior of neptunium, which element is highly mobile in the environment because of the chemical form, NpO2+. With the cells(Desulfovibrio desulfaricans), Np in suspension was decreased to 5 % or less for 10 min, whereas the reductive adsorption by reducing agent Na2S, i.e., without cells, was much slower. This may show the microorganism reduce mobile Np(V) enzymatically to immobile Np(IV). Secondly, we studied the effect of iron-chelator(Hydroxamate siderophore, Desferrioxamate B(DFOB)) on plutoniumIV adsorption behavior. The Pu adsorption decreased with increasing concentration of DFOB and reduced to almost 0% at DFOB concentrations of 100mM. However, at the low concentrations of DFOB and Pu (less than 1 x 10-6 mol/l and 3.7 x 10-11 mol/l respectively), there was little effect of DFOB on the adsorption of Pu. This result shows the DFOB effect on adsorption of Pu depends on the concentration of DFOB. At the low DFOB concentration, Pu would not chelate with DFOB because DFOB in the solution is not enough to form the complexes. DFOB actually dissolved impurities associated with bentonite, and the concentration of dissolved metal, e.g., Fe3+, was increasing with an increase of DFOB concentration in the suspension. These metal ions would compete with actinides, and the metal exchange may occur in a system with actinide-DFOB complexes.These results show that microorganisms can influence the behavior of actinides in the environment. Therefore, it is getting more important to understand and predict the microbial impact on the migration of radionuclides from waste repositories, as well as developing remediation and decontamination strategies for contaminated sites.

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