Bentonite porewater chemistry

ベントナイト間隙水化学モデリング

Ochs, M.*; Lothenbach, B.*; 柴田 雅博 ; 油井 三和

Ochs, M.*; Lothenbach, B.*; Shibata, Masahiro; Yui, Mikazu

処分環境条件下での人工バリアシステムにおけるベントナイト間隙水化学を明らかにするため、間隙水化学のモデルシミュレーションを実施した。シミュレーションに用いた間隙水化学モデルは、Na型スメクタイトの表面反応についてイオン交換反応と表面錯体形成反応を考慮し、ベントナイトを構成する随伴鉱物および可溶性不純物の溶解反応を組み合わせた熱力学的地球化学モデルである。本資料では、(1)スメクタイト・水界面におけるイオン交換反応モデルのレビュー、(2)ベントナイト・地下水の相互作用のモデル化について実験データとの比較検証を行った結果、(3)ベントナイトモデルに対する主要因子の影響解析結果を報告する。そして、これら知見を基に、間隙水化学における支配的反応の定量化・モデルの選定、間隙水化学に対する支配的反応の長期的な寄与の推測を行い、(4)可能性のある変動幅を考慮した、処分環境におけるベントナイト間隙水化学のシミュレーション結果を報告する。本研究によって、ベントナイト間隙水組成が炭酸塩鉱物や硫酸塩の溶解反応、スメクタイトの表面反応の影響を強く受け、長期的にはこれら反応の進行の度合いによって変動することが計算された。

Porewater composition in a compacted bentonite is dependent on the composition of the surrounding groundwater, and on the characteristics of the bentonite itself. Two mechanisms through which bentonite influences the respective porewater composition are distinguished: sulface chemical reactions (ion exchange, surface complexation) on smectite and dissolution of minerals and soluble impurities included in bentonite. This report provides the results of different activities related to the definition of porewater chemistry through the use of geochemical models: (1) review of thermodynamic model on ion exchange reaction, (2) modeling of bentonite-water interactions under aerobic conditions, (3) performance of sensitivity analyses of key parameters in the bentonite model, (4) model simulation of bentonite porewater chemistry in the engineered barrier system under repository conditions. Experimental information of bentonite-water interaction allowed the determination of soluble impurities in the bentonite, and the knowledge of these impurities is important for predictive modeling. For the impurities of Kunigel-V1, 0.38% of CaSO, 0.0011% of NaCl and 0.0044% KCl were determined. The sensitivity analyses resulted in that the presence of calcite, CaSO and pyrite strongly influences the pH in the compacted bentonite, and the pH in compacted bentonite is buffered by the acid/base equilibria at the Na-smectite surface as well. Through the model calculations, some remarks on the expected trends for the long term behavior can be made like that the pH in compacted bentonite is expected to increase with increasing number of water exchange cycles, as long as CaCO contributes to the pH buffering capacity, due to slow depletion of the soluble impurities in the bentonite. The pH of the porewater, however, lies in all cases (but in the presence of CaCO) between 5.6 and 9.5. Based on the findings discussed above, a large number of calculations were carried ...