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Francisco, P. C. M.; 松村 大樹; 菊池 亮佑*; 石寺 孝充; 舘 幸男
Environmental Science & Technology, 56(5), p.3011 - 3020, 2022/03
被引用回数：2 パーセンタイル：29.93(Engineering, Environmental)The radionuclide selenium-79 (Se-79) is predicted to be a key contributor to the long-term radiologic hazards associated with geological high-level waste (HLW) repositories, hence its release is of pertinent concern in the safety assessment of repositories. In this study, we examined the immobilization mechanisms of Se(-II)-prevalent under anoxic and reducing repository conditions-during interaction with aqueous Fe(II) and freshly precipitated Fe(OH)
at circumneutral and alkaline conditions, respectively, its response to changes in pH, and its behavior during aging at 90
C. Using microscopic and spectroscopic techniques, we observed that Se(-II) was readily immobilized via iron selenide precipitation regardless of whether Se(-II) reacts with aqueous species or solid phases and that modifying the pH following initial immobilization did not immediately remobilize Se(-II). These observations indicate that Se(-II) migration beyond the overpack can be effectively and rapidly retarded via interaction with Fe(II) species arising from overpack corrosion. Thermodynamic calculations, however, showed that at alkaline conditions, iron selenides immediately became metastable and will likely dissolve in the long term. Aging experiments at 90C showed that Se(-II) can be completely retained via the crystallization of ferroselite at circumneutral conditions, while it will be largely remobilized at alkaline conditions. Our results show that Se(-II) mobility can be significantly influenced by its interactions with the corrosion products of the steel overpack and that these behaviors will have to be considered in repository safety assessments.
Francisco, P. C. M.; 舘 幸男
ACS Earth and Space Chemistry (Internet), 4(12), p.2366 - 2377, 2020/12
被引用回数：2 パーセンタイル：14.2(Chemistry, Multidisciplinary)Immobilization of Se(IV) by adsorption on iron oxides exerts a key control on its mobility in a number of natural and engineered settings; however, the presence of other coexisting ligands may influence the extent and stability of adsorption. In this work, we investigated the impacts of Si, which is an abundant component of many natural waters, on Se(IV) adsorption and retention on ferrihydrite. Using in situ ATR-IR spectroscopy, we obtained time-resolved information on Se(IV) adsorption on ferrihydrite in the presence of Si at pH 5, 7, and 9. The results show that Si inhibited the adsorption of Se(IV) at all pH if Si was initially adsorbed on ferrihydrite. This was attributed to the oligomerization of Si on ferrihydrite, which blocked Se(IV) from binding to ferrihydrite. In experiments where Se(IV) was initially adsorbed, Si was found to induce Se(IV) desorption. Competitive adsorption experiments, where Se(IV) and Si were present in the same solution, showed that Si negligibly affected Se(IV) adsorption at pH 5 and 7, likely because of lower Si adsorption and the lack of considerable oligomerization. On the other hand, at pH 9, Se(IV) was initially adsorbed and then subsequently desorbed with increasing Si adsorption and oligomerization. Hence, Si affects Se(IV) adsorption over a narrower pH range than when it is preadsorbed on ferrihydrite. These results show that Si can enhance Se(IV) mobility and highlight the influence of pH and the timing of Si adsorption in inhibiting Se(IV) adsorption on ferrihydrite. This work offers the first direct insights into the dynamic processes underpinning Se(IV) and Si competitive adsorption, which may be useful in understanding Se(IV) behavior in a broad range of environmental settings.
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：4 パーセンタイル：45.45(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 (
; m
kg
) of Se(-II) indicated ionic strength independence and slight pH dependence. The 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 values for Se(-II).
and Si under anoxic and reducing conditions; Structural characteristics of ferrous silicate co-precipitatesFrancisco, 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：15 パーセンタイル：76.71(Geochemistry & Geophysics)The interaction of Fe and S
-silicate phases, which play important roles in regulating the solubility and bioavailability of both Fe 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 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 
-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.
Francisco, P. C. M.; 三ツ井 誠一郎; 石寺 孝充; 舘 幸男
no journal, ,
This study describes batch synthesis experiments of Fe(II)-Si phases under anoxic and reducing conditions as a function of pH. Solutions of Fe(II) and Si were mixed, base titrated and aged for up to 20 days at high temperature to induce crystallization. The results of the aging experiments show the formation of both Fe(II) oxides as well as Fe(II)-Si phases. The pH appear to control the transformation products, with higher pH favoring crystallization. These results may be useful in validating thermodynamic databases that predict the formation of Fe(II)-Si phases.
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 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.
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.
Francisco, P. C. M.; 松村 大樹; 菊池 亮佑*; 石寺 孝充; 舘 幸男
no journal, ,
The radionuclide Selenium-79 (Se-79) is predicted to be a key contributor to the long-term radiologic hazards associated with geological high-level waste (HLW) repositories. Se exists predominantly as selenide Se(-II) under anoxic conditions in the deep subsurface, and is likely immobilized via precipitation with, or adsorption on, Fe(II) minerals. However, the initial immobilization mechanisms with metastable Fe(II) minerals, and Se(-II)s subsequent response to iron phase transformation in the long term remain poorly understood. In this work, we investigated the retention and mobilization behavior of Se(-II) as a function of pH and the mode of its initial interaction with aqueous or solid Fe(II) phases. We carried out batch precipitation and transformation experiments under N atmosphere and reducing conditions. We examined two cases: in the first, Se(-II) was reacted with aqueous Fe(II), while in the second, Fe(OH) was first precipitated before reaction with Se(-II); both experiments were carried out at pH 8 and 12. In both cases, Se(-II) introduction resulted in the immediate precipitation of black particles. EXAFS and TEM characterization showed that Se(-II) precipitated as iron selenide nanoparticles in both cases regardless of pH. Aging of these initial precipitates at 
C resulted in magnetite crystallization. Regardless of whether Se(-II) was reacted with aqueous Fe(II) or solid phases, it was completely retained as discrete, crystalline FeSe at pH8. At pH12, Se(-II) was mostly remobilized; however, we observed evidence of partial retention via the precipitation of nanocrystalline iron selenide (Fe
Se
or Fe
Se
) on the surface of magnetite, as well as by incorporation in defects on the edges of magnetite crystals. These results show that pH controls long-term Se(-II) behavior and that magnetite crystallization may play a role in the retention of Se(-II), particularly at high pH.
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(
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。
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。
Francisco, P. C. M.; 菊池 亮佑*; 松村 大樹; 塩飽 秀啓; 石寺 孝充; 舘 幸男
no journal, ,
Selenium-79 (Se-79) will be a significant contributor to the long-term radiologic hazards in geological repositories for high-level wastes due to its long half-life. Thus, understanding its behavior in anoxic and reducing subsurface environments prevalent in geological repositories is a key concern in repository safety assessments. There is limited information on primary reaction products arising from the interaction of aqueous Se species with either aqueous or solid Fe(II) phases, which are present in either as geological repositories as products of steel overpack corrosion or as accessory phases in buffer materials or host rocks. In this work, we investigated the interaction of aqueous Se(-II) and Se(IV), potentially the most mobile Se species in repositories, with aqueous Fe(II) and solid Fe(OH) at circumneutral and alkaline conditions, respectively. Using electron microscopy and XAFS spectroscopy, we identified beta-FeSe as the primary reaction product at circumneutral and alkaline conditions between Se(-II) and either aqueous Fe(II) and Fe(OH). For Se(IV), elemental Se(0) was the main reaction product at circumneutral conditions while iron selenides only became dominant at alkaline conditions; these results show rapid and efficient reduction of Se(IV). Aging the solids at 90C for 1 week induced the transformation of beta-FeSe and elemental Se(0) to FeSe at circumneutral conditions, while a proportion of Se was released back into solution at alkaline conditions. These results provide constraints on the initial behavior of Se upon release from the waste, and may be useful in validating long-term thermodynamic predictions. *This work was part of "The project for validating near-field assessment methodology in geological disposal (FY2021, Grant Number: JPJ007597)" supported by the Ministry of Economy, Trade and Industry of Japan.
出井 俊太郎; 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中に含まれるセレン(Se;約30万年)は、岩盤への収着分配係数が小さいため、地層処分の核種移行評価上は遅延効果が期待できない核種として位置付けられている。一方で、二次鉱物生成に伴う共沈反応は、セレンの移行遅延に寄与する反応として期待できる可能性がある。本研究では、セレンの移行遅延に関する天然事例研究として、北海道幌延地域で得られた深部堆積岩を対象にセレンの長期的な収着・保持メカニズムを明らかにすることを目的とした。セレンの存在形態および局所構造を明らかにするために実施した逐次抽出試験とXAFS分析の結果から、セレンが黄鉄鉱の結晶構造中の硫黄と置換する形で取り込まれていることが明らかになり、黄鉄鉱の共沈反応によってセレンが岩盤中に保持されていることが示された。以上の結果より、深部堆積岩中では、共沈反応によりセレンが移行しづらい形態で岩盤に保持される可能性が示された。
Francisco, P. C. M.; 石寺 孝充; 塩飽 秀啓; 菊池 亮佑*; 舘 幸男
no journal, ,
As a long-lived fission product, Se-79 is predicted to account for a significant proportion of radioactivity long after the closure of a geological repository for high-level wastes. Predicting its behavior for long-term safety assessment of repositories thus requires a detailed understanding of its reactions with various geological repository components. Interaction with ferrous and sulfide aqueous species and solid phases could help attenuate Se concentrations under anoxic and reducing conditions in repositories. We performed four sets of coprecipitation experiments where Se(-II) was reacted with aqueous Fe(II) and with either monosulfides or polysulfides, at Fe/Stot ratios of 1 and 0.5. The precipitates were aged under anoxic conditions at ~25C for up to 5 months. The monosulfide coprecipitation experiments produced mackinawite and efficiently sequestered Se(-II) from solutions. XANES analyses showed that Se retained its -II oxidation state and EXAFS fitting results were consistent with Se being incorporated in the mackinawite lattice. Uptake was similarly efficient in the polysulfide experiments, though lesser Se was sequestered than in the monosulfide experiments. Unlike in the monosulfide experiments, Se was initially oxidized and precipitated as elemental Se(0). In the experiment at Fe/S 1, Se became incorporated in pyrite. Despite the very low abundance of pyrite, it was clear that it hosted most of the Se. In summary, monosulfide coprecipitation sequesters and retains Se in mackinawite, while reaction with polysulfides sequesters Se first as Se(0), then incorporates it in pyrite. Because sulfide speciation is tied to redox conditions, the results provide fundamental insights into the processes governing Se retention across a broad range of redox environments, which may aid in predicting Se migration behavior in geological repositories.
出井 俊太郎; 舘 幸男; 天野 由記; 杉浦 佑樹; Francisco, P. C. M.; 高橋 嘉夫*
no journal, ,
Se is one of the key radionuclides for safety assessment of the high-level radioactive waste (HLW) due to its long half-life and complex speciation depending on the redox condition. Since selenium is considered to be easily migrates as an anion, it is important to understand the retention mechanism of Se in the subsurface environment. The aim of this study is to elucidate the retention mechanisms of Se in the deep subsurface by clarifying the speciation of Se.
Francisco, P. C. M.; 塩飽 秀啓; 石寺 孝充; 舘 幸男
no journal, ,
The long-lived fission product Se-79 is a significant contributor to predicted radioactivity in geological repositories long after their closure. One possible retardation mechanism in repositories is by incorporation into Fe-sulfides like pyrite (FeS2). In this study, we clarify the role of polysulfides in the incorporation of Se into pyrite. Polysulfides are intermediate-oxidation sulfide species that are likely produced by diagenetic and microbial processes in repository host rocks. The present results show that reaction of Se(-II) with polysulfides resulted in the initial formation of elemental selenium. Aging at anoxic conditions led to the incorporation of Se into pyrite. These results highlight the role of polysulfides in retaining Se. We also investigate the effects of aging in air to determine the effects of changes in the redox conditions. The results show that oxidation did not affect Se retention.
