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論文

Sorption behavior of selenide on montmorillonite

杉浦 佑樹; 戸村 努*; 石寺 孝充; 土井 玲祐; Francisco, P. C. M.; 塩飽 秀啓; 小林 徹; 松村 大樹; 高橋 嘉夫*; 舘 幸男

Journal of Radioanalytical and Nuclear Chemistry, 324(2), p.615 - 622, 2020/05

 被引用回数:0 パーセンタイル:100(Chemistry, Analytical)

Batch sorption experiments were performed to investigate the sorption mechanism of Se on montmorillonite under reducing conditions in deep geological environments. Based on Eh-pH diagrams and ultraviolet-visible spectra, Se was dissolved as selenide (Se(-II)) anions under the experimental conditions. The distribution coefficients ($$K_{rm d}$$; m$$^{3}$$ kg$$^{-1}$$) of Se(-II) indicated ionic strength independence and slight pH dependence. The $$K_{rm d}$$ values of Se(-II) were higher than those of Se(IV), which also exists as an anionic species. X-ray absorption near edge spectroscopy showed that the oxidation state of Se-sorbed on montmorillonite was zero even though selenide remained in the solution. These results suggest that Se(-II) was oxidized and precipitated on the montmorillonite surface. Therefore, it is implied that a redox reaction on the montmorillonite surface contributed to high $$K_{rm d}$$ values for Se(-II).

論文

Interaction of Fe$$^{II}$$ and Si under anoxic and reducing conditions; Structural characteristics of ferrous silicate co-precipitates

Francisco, P. C. M.; 三ツ井 誠一郎; 石寺 孝充; 舘 幸男; 土井 玲祐; 塩飽 秀啓

Geochimica et Cosmochimica Acta, 270, p.1 - 20, 2020/02

 被引用回数:1 パーセンタイル:100(Geochemistry & Geophysics)

The interaction of Fe$$^{II}$$ and Si is at the heart of many critical geochemical processes in diverse natural and engineered environments. The interaction of these elements results in the formation of Fe$$^{II}$$-silicate phases, which play important roles in regulating the solubility and bioavailability of both Fe$$^{II}$$ and Si, as well as serve as sinks for trace elements. Therefore, a detailed understanding of their structural characteristics may provide insights that may help in predicting their reactivity and stability under different conditions. In this work, co-precipitates with different Si/Fe$$^{II}$$ ratios (0.5, 1.0 and 2.0) were synthesized under anoxic and reducing conditions at different solution pH (7, 9 and 11). The co-precipitates were studied using X-ray diffraction (XRD), infrared (IR) spectroscopy and Fe $$K$$-edge X-ray absorption spectroscopy (XAS). The results show the immediate and rapid formation of phyllosilicate-like local structures from solution. These incipient structural units lack long-range order but may serve as the precursors of crystalline phases.

口頭

Se(-II) immobilization in Fe(II)-(hydro)oxides; Coprecipitation and post-transformation retention behavior

Francisco, P. C. M.; 石寺 孝充; 舘 幸男

no journal, , 

As a key long-lived fission product in high-level wastes (HLW), Se-79 presents a significant contribution to the long-term radiological hazards associated with geological HLW disposal. Thus, its behavior is important for the performance assessment of geological HLW repositories. Under reducing conditions Se exists as Se(-II). A possible retardation pathway for Se(-II) species is via interaction with aqueous Fe(II) from the corrosion of the steel canister enclosing the waste matrix. There is relatively little information on Se(-II) uptake involving coprecipitation with poorly crystalline iron phases and subsequent behavior during phase transformation. Batch coprecipitation and crystallization experiments of Se(-II) with Fe(II) phases under N$$_{2}$$ atmosphere and reducing conditions were carried out. Rapid Se(-II) uptake was observed during coprecipitation. During aging, Se(-II) was observed both as discrete phases and surface complexes.

口頭

Impacts of Si on Se(IV) immobilization on ferrihydrite; An In-situ ATR-IR spectroscopic investigation

Francisco, P. C. M.; 舘 幸男

no journal, , 

The long-lived Se-79 is one of the key radionuclides of interest in radioactive waste management due to its long half-life and radiologic and chemical toxicity. The migration of the highly mobile Se(IV) species may be mitigated with iron phases present in the disposal environment. However, the presence of Si derived from the clay buffer or host rocks may interfere with the sorption and retention of Se(IV) on iron phases. In this work, we conducted in situ ATR-IR to investigate the interaction of Se(IV) and Si with the surface of ferrihydrite, a poorly crystalline iron phase. We observed that Si reduces the sorption of Se(IV) on ferrihydrite and promotes its remobilization by blocking sorption sites. This impact of Si on Se(IV) sorption must then be taken into account in modeling studies for the safety assessment of radioactive waste repositories.

口頭

北海道幌延地域における堆積岩中のセレンの存在形態

出井 俊太郎; 宮川 和也; 笹本 広; 舘 幸男; 天野 由記; Francisco, P. C. M.; 杉浦 佑樹; 高橋 嘉夫*

no journal, , 

高レベル放射性廃棄物の地層処分の安全性評価において、セレン($$^{79}$$Se;半減期: 300kyr)は人工バリアや天然バリアである岩盤に収着されにくいと考えられる。一方で、セレンは鉄水酸化物に収着することが室内試験より報告されている。本研究では、深部地下環境におけるセレンの長期的な収着挙動を理解するために、北海道幌延地域の堆積岩を対象にセレンの化学形態分析を実施した。堆積岩中のセレンの化学形態について、逐次抽出試験およびXAFS分析を実施した結果、セレンは-I価または0価の還元的な形態で堆積岩中に存在することが明らかになった。$$mu$$XRF分析より、セレンはパイライトに濃集していることが示唆された。セレンはパイライト中の硫黄と置換し、固体中に取り込まれることが報告されており、同様の形態で初期続成作用時からセレンが長期間保持されている可能性が考えられる。今後、堆積岩中のパイライトへのセレン収着メカニズムを明らかにする予定である。

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