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Diffusion and sorption of neptunium(V) in compacted montmorillonite; Effects of carbonate and salinity

圧縮モンモリロナイト中のNp(V)の収着・拡散挙動:炭酸と塩濃度の影響

舘 幸男; 中澤 俊之*; Ochs, M.*; 四辻 健治; 陶山 忠宏; 清田 佳美; 山田 憲和*; 油井 三和

Tachi, Yukio; Nakazawa, Toshiyuki*; Ochs, M.*; Yotsuji, Kenji; Suyama, Tadahiro; Seida, Yoshimi; Yamada, Norikazu*; Yui, Mikazu

放射性廃棄物地層処分の安全評価において重要となる圧縮ベントナイト中の核種の収着・拡散挙動を把握・評価するため、圧縮モンモリロナイト中のNp(V)の収着・拡散挙動に及ぼす炭酸濃度と塩濃度の影響を、実験とモデルの両面から調査した。密度800kg/m$$^{3}$$の圧縮モンモリロナイト中のNp(V)の実効拡散係数($$D$$$$_{rm e}$$)と分配係数($$K$$$$_{rm d}$$)が、塩濃度(0.05, 0.5MのNaCl)と炭酸濃度(0, 0.01MのNaHCO$$_{3}$$)の異なる4種類の条件下で取得した。炭酸が存在しない系では$$D$$$$_{rm e}$$は塩濃度とともに減少し、炭酸共存系では逆の傾向を示した。$$K$$$$_{rm d}$$は炭酸が共存する系で1桁減少する結果が得られた。これらの収着・拡散挙動は、地球化学平衡計算,イオン交換と表面錯体反応を考慮した収着モデル,狭隘間隙中の電気二重層を考慮した拡散モデルによって解釈された。現象論収着・拡散モデルが、圧縮系での複雑な化学種の収着・拡散挙動の予測評価に有効であることを示した。

Diffusion and sorption of radionuclides in compacted bentonite are the key processes in the safe geological disposal. The effects of carbonate and salinity on Np(V) diffusion and sorption in compacted montmorillonite were investigated by experimental and modeling approaches. Effective diffusion coefficients ($$D$$$$_{rm e}$$) and distribution coefficients ($$K$$$$_{rm d}$$) of Np in montmorillonite compacted to dry density of 800 kg/m$$^{3}$$ were measured under four conditions with different salinities (0.05/0.5 M NaCl) and carbonate concentrations (0/0.01 M NaHCO$$_{3}$$). The $$D$$$$_{rm e}$$ for carbonate-free conditions decreased as salinity increased, and those for carbonate conditions showed the opposite dependency. The $$K$$$$_{rm d}$$ decreased by one order of magnitude under high carbonate condition. Diffusion and sorption behaviors were interpreted by coupling the thermodynamic aqueous speciation, the thermodynamic sorption model based on ion exchange and surface complexation, and the diffusion model based on electrical double layer theory in narrow pores. The mechanistic model could be useful in predicting the sorption and diffusion behavior of complex species in compacted systems.

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パーセンタイル:17.38

分野:Chemistry, Inorganic & Nuclear

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