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In many European countries (e.g., France, Switzerland), thick steel casks are foreseen for the containment of high-level radioactive waste in deep geological repositories. In contact with pore-water, steel corrodes forming mixed iron oxides, mainly magnetite (Fe3O4). After tens of thousands of years, the casks may breach allowing for leaching of the radionuclides (e.g., Tc and Pu) into pore-water. The radionuclides can be retarded by corrosion products either by adsorption or via structural incorporation [1,2]. The molecular scale mechanisms of these phenomena are investigated by ab initio simulations and X-ray absorption spectroscopy (XAS). The dominant low-index surfaces of magnetite particles and their termination at the relevant conditions were identified based on density functional theory (DFT), using the open-source CP2K code [3]. The DFT+U method was employed for the strongly correlated 3d- and 5f- electrons of Fe and Pu, respectively. After benchmarking the model setup, the surface energies of the (111) facet with different surface terminations and water coverage were analysed as a function of redox conditions and pH. The Eh and pH predominance diagram were used to predict the most stable mineral surfaces under real repository conditions [4]. Further, the stability of nanocrystals with approximately 2 nm size were evaluated. Subsequently, ab initio molecular dynamics (MD) were applied to simulate sorption structures of radionuclides on the expected magnetite (111) surfaces based on experimental findings [2,5]. Complementary, wet-chemical coprecipitation experiments of magnetite nanocrystals in the presence of Tc or Pu were conducted. The resulting complexes represent the expected incorporation and sorption structures [1,2,5] and were analysed by the XAS techniques (EXAFS, XANES) available at the European Synchrotron Radiation Facility. Finally, the synergy of the experimental and theoretical studies contributes to the understanding of the retention mechanism

Understanding processes regulating thioarsenate (HxAsSnO4−n3−x; n = 1 – 3; x = 1 – 3) mobility is essential to predicting the fate of arsenic (As) in aquatic environments under anoxic conditions. Under such conditions, natural organic matter (NOM) is known to effectively sorb arsenite and arsenate due to metal cation-bridged ternary complexation with the NOM. However, the extent and mechanism of thioarsenate sorption onto NOM via similar complexation has not been investigated. By equilibrating monothioarsenate (representative of thioarsenate) with a peat (model NOM) with different Fe(III) loadings, this study shows that NOM can sorb monothioarsenate considerably via Fe(III)-bridging. Iron and As K-edge XAS analysis of the monothioarsenate-treated Fe-loaded peats revealed that monothioarsenate forms bidentate mononuclear edge-shared (1E) (RAs···Fe: 2.89 ± 0.02 Å) and bidentate binuclear corner-shared (2C) (RAs···Fe: 3.32 Å) complexes with organically bound Fe(O,OH)6 octahedra, in addition to direct covalent bonds with oxygen-containing functional groups (e.g., –COOH and –OH) (RAs···C: 2.74 ± 0.02 Å), upon equilibration with the Fe(III)-loaded peat. However, the extent of monothioarsenate sorption was considerably less than that of its precursor As species, arsenite, due to higher electrostatic repulsion between the negatively charged monothioarsenate and peat. This study implies that thioarsenate formation under anoxic conditions would increase As mobility by decreasing its sorption onto the NOM.

Magnetite nanoparticles (MNPs) play an important role in geological and environmental systems because of their redox reactivity and their ability to sequester a wide range of metals and metalloids. X-ray absorption spectroscopy conducted at metal and metalloid edges has suggested that the magnetite 111 faces of octahedrally-shaped nanoparticles play a dominant role in the redox and sorption processes of these elements. However, studies directly probing the magnetite surfaces - especially in their fully solvated state - are scarce. Therefore, we investigate here the speciation and stability over a wide Eh/pH range of octahedrally shaped MNPs of 2 nm size by means of KohnSham Density Functional Theory with Hubbard correction (DFT+U). By altering the protonation state of the crystals, a redox-sensitive response of the octahedrally coordinated Fe could be achieved. Further, the preferential H distribution could be identified highlighting the difference between edges, vertices and facets of the nanocrystals. Subsequently, the interactions of the MNPs with a solvent of pure water or 0.5 M NaCl solution were studied by classical molecular dynamics (MD) simulations. Finally, a comparison of the corresponding macroscopic magnetite (111) surface with the investigated MNPs was conducted.

The safety assessment of repositories for high-level radioactive waste, mostly spent nuclear fuel, requires a detailed understanding of the interactions of radionuclides with corroded phases in the near-field of a given repository. Zirconia (ZrO2), the main corrosion product of the zircaloy cladding material of nuclear fuel rods, constitutes a first barrier against the release of radionuclides into the environment. A multimethod approach was pursued to gain a thorough understanding of the Np(V) sorption processes at the water−zirconia interface. For the macroscopic description, pH-dependent batch sorption experiments (varying ionic strength, Np(V) concentration, and solid-to-liquid ratio) as well as sorption isotherms experiments at pH 4.5 and 6.0 were performed. The uptake of Np(V) was pH-dependent, with an increased sorption starting from pH 3 and being at a maximum at pH 6 and above. The Np(V) sorption was independent of ionic strength, suggesting the predominance of Np(V) inner-sphere complexes on the zirconia surface. This was supported by electrokinetic measurements in the absence and presence of neptunium, where Np(V) caused a shift to higher pH values of the isoelectric point of the neat ZrO2. The Np(V) sorption edge was shifted towards lower pH values with increasing solid-to-liquid ratio, indicating the presence of different kinds of sorption sites (strong and weak), which was also corroborated by the shape of the sorption isotherms. Molecular level information was derived from in situ attenuated total reflection Fourier-transform infrared spectroscopy and extended X-ray absorption fine structure spectroscopy (EXAFS), which confirmed the presence of Np(V) inner-sphere complexes. EXAFS experiments revealed the formation of a bidentate inner-sphere surface complex in the weak sorption site regime. The derived information at the macroscopic and molecular levels was used to parametrize a charge distribution multi-site complexation (CD-MUS

Magnetite is a common mixed Fe(II,III) iron oxide in mineral deposits and the product of (anaerobic) iron corrosion. In various Earth systems, magnetite surfaces participate in surface mediated redox reactions. The reactivity and redox properties of the magnetite surface depend on the surface speciation, which varies with the environmental conditions. In this study, Kohn-Sham density functional theory (DFT+U method) was used to examine the stability and speciation of the magnetite crystal face {111} in a wide range of pH and Eh conditions. The simulations reveal that oxidation state and speciation of the surface depend strongly on imposed redox conditions and, in general, differ from those of the bulk state. Corresponding predominance phase diagrams for the surface speciation and structure were calculated from first principles. 1The obtained knowledge of surface structure and oxidation state of iron is essential for modeling retention of redox-sensitive nuclides. Further, classical molecular dynamics (MD) simulations were conducted investigating the mobility of water near the magnetite surface.

Interactions of selenium (Se), a trace element bio-essential at low concentrations but highly toxic at high concentrations, with the most abundant mineral in the Earth's crust, namely pyrite, was investigated over a wide range of time scales. At the nanosecond scale, selenate Se(VI)O42– adsorption onto the net pyrite surface is shown by ab-initio computations to proceed via the formation of a chemical bond between an oxyanion oxygen atom and a surface Fe atom, weakening the other Se-O bonds and reducing Se atom oxidation state. At the hour-to-day scale, adsorption and coprecipitation of selenate and selenite, Se(IV)O32–, were investigated through wet chemical batch experiments at various pH values at different sulfide concentrations. Selenium removal from solution is slower and weaker for selenate than for selenite. After 24h, only 10% of selenate, against 60% for selenite (or even 100% in the presence of sulfide), is removed by the pyrite surface. Independently of its original oxidation state, adsorbed Se is completely reduced to elemental selenium via adsorption or coprecipitation, as shown by XANES spectroscopy. Our EXAFS results, compared to published data on Se-rich pyrite, show a Se to S substitution within the pyrite structure. The reductive coprecipitation mechanism of selenium with pyrite represents valuable new insights for improving our understanding of modern and ancient biogeochemical cycles involving Se. In addition, several industries can benefit from direct applications of our findings, such as water treatment, green technologies and sustainable mining.

Zirconia (ZrO₂), the main corrosion product of the zircaloy cladding material of nuclear fuel rods, might potentially act as the first barrier for radionuclides. Thus, the interactions of radionuclides, such as the long-lived actinide neptunium, with zirconia have to be considered in the safety assessment process of a repository for radioactive waste. The sorption of Np(V) onto zirconia (ZrO₂) was investigated in the absence of carbonate at the macroscopic and molecular scale. For the macroscopic description, pH-dependent batch sorption experiments under varying ionic strength (0.1 and 0.01 mol∙L⁻¹ NaCl), Np(V) concentration (1∙10⁻⁶ or 6∙10⁻⁶ mol∙L⁻¹) and solid-to-liquid ratio (m/V = 0.5 or 4 g∙L⁻¹ ZrO₂) were conducted. Np(V) sorption isotherms at pH 4.5 and 6.0 were additionally obtained at 0.01 mol∙L⁻¹ NaCl. The Np(V) uptake on zirconia strongly depends on pH, with sorption starting from acidic pH and maximum sorption was reached at pH 6 and above. Increasing the m/V ratio caused a significant shift of the sorption edge towards lower pH values. This indicates the presence of different kinds of sorption sites, which was supported by the results of the Np(V) sorption isotherms, where the shape of the isotherm suggested the presence of strong and weak sorption sites. The Np(V) uptake was independent of ionic strength, suggesting the presence of inner-sphere Np(V) surface complexes on zirconia. This was also supported by zeta potential measurements where a shift of the isoelectric point of the pristine zirconia towards higher pH values in the presence of Np(V) was observed. Molecular level investigations by means of spectroscopic techniques, namely in situ attenuated total reflection Fourier transform Infrared Spectroscopy (ATR FT-IR) and extended X-ray absorption fine structure spectroscopy (EXAFS), confirmed the predominant presence of Np(V) inner-sphere complexes on the zirconia surface. EXAFS experiments conducted in the weak sorption site regime revealed the form

Selenium immobilization is of high importance due to its high toxicity and the mobility of the soluble oxyanion selenite. Fe-bearing clay minerals are major redox-active ingredients of Earth’s critical zone and constitute an important component of the barrier in (radioactive and other) waste repositories, having the potential to reduce selenite to insoluble forms under anaerobic condition. Our research focuses on a systematical investigation of the kinetics of selenite sorption and reduction, considering variations in structural Fe(II) content of clay minerals, redox potential and pH. Combining batch experiments and X-ray absorption fine structure (XAFS) spectroscopy, we examine how these factors influence the selenite reduction process and kinetics by identifying the reduced Se species over time. We observed that selenite adsorption and reduction are kinetically controlled. Selenite reduction is redox potential dependent and presents a slower reduction rate at pH 7 compared to pH 5. Concerning Fe(II) content in clay minerals, higher amount of structural Fe(II) results in faster selenite reduction kinetics, and only elemental Se(0) was observed as reduced species, while the absence of soluble Fe prevented precipitation of Fe selenide. Moreover, we observed a transformation of amorphous red Se(0) to metallic trigonal grey Se(0) in the reduction species as the reaction time increases. This insight into the selenite reduction mechanism provides valuable information for the development of effective approaches for selenite immobilization.

Thick steel casks are used for radioactive waste disposal in deep geological repositories. However, it is expected that steel corrodes over time. The corrosion products are expected to form mixed iron oxides, mainly magnetite. After tens of thousands of years, casks may breach allowing leaching of the radiotoxic elements, such as plutonium and technetium, by host rock pore-water. The dissolved radionuclides can then interact with the steel corrosion products and be adsorbed or incorporated into the solids [1]. But since these interaction mechanisms are poorly understood at the atomistic scale, our goal is to better understand them by using computer simulations alongside experiments [2]. In this computational study we identified the dominant low index surfaces on nano-magnetite particles and their termination at the relevant conditions based on Kohn-Sham density functional theory (DFT). This was done using the open source CP2K code and, for highly correlated chemical elements such as iron or plutonium, utilizing the DFT+U method because of the highly localized d and f electrons. The U parameter was determined by comparing experimental cell constants and band gaps to our result [3]. With this revised model, we examined the preferential magnetite crystal orientation plane (111) with different surface terminations as a function of oxygen and water fugacity. Based on our modelling, we found the most stable magnetite (111) surfaces under real repository conditions being Fe_oct1-O-H and Fe_tet1-O-H. Further, we used classical and ab initio MD simulations to investigate the behaviour of radionuclides at the water-magnetite interface in deep geological repositories. [1] Dumas, T.; Fellhauer, D.; Schild, D.; Gaona, X.; Altmaier, M.; Scheinost, A. C. Plutonium Retention Mechanisms by Magnetite Under Anoxic Conditions: Entrapment Versus Sorption. ACS Earth and Space Chemistry 2019, 3 (10), 2197–2206. [2] Yalçıntaş, E.; Scheinost, A. C.; Gaona, X.; Altmaier, M. Systematic XAS St

Techntium-99 (⁹⁹Tc) is one of the most concerning fission products due to its long half-life (2.13∙10⁵ years) and the high mobility of the anion pertechnetate (TcO₄⁻) [1]. Tc migration decreases significantly when Tc(VII) is reduced to Tc(IV). This scavenging step can be induced by Fe(II) minerals, which have been widely studied due to their versatility, low cost, and ubiquity [2]. In addition, Fe(II) minerals will play an important role in the near-field of the nuclear waste repository, in case that corrosion of the waste canisters will occur and radioactive material be leaked in the environment. Green rust is formed when Fe²⁺ interacts with Fe(III) minerals [3]. Thus, its presence is expected in both the near- and far-field of a repository. It is a Fe(II)-Fe(III) hydroxide that can immobilize radionuclides by adsorption, anion exchange, and reduction mechanisms. A previous work reports the interaction of green rust with Tc, but the results are limited to very narrow experimental conditions [4]. Thus, further studies are needed to both identify the optimal Tc scavenging conditions by green rust and the mechanism responsible of Tc retention. Our studies consisted of a combination of batch contact studies, microscopic and spectroscopic analysis. Batch contact studies were performed under a wide range of conditions, i.e. pH (3.5 - 11.0), Tc concentration (nM - mM), and ionic strength (0.0 - 0.1 M). X-ray powder diffraction, Raman microscopy, X-ray photoelectron spectroscopy (XPS), and X-ray absorption spectroscopy provided information on Tc oxidation state and speciation as well as on secondary redox products related to the Tc interaction with chloride green rust (GR(Cl)). In addition, re-oxidation experiments have been performed for one year to analyze the Tc retention reversibility. The results show that GR(Cl) removes Tc from solution with efficiencies between 80% (Kd = 8.0∙10³ mL/g) and ≈ 100% (Kd = 9.9∙10⁵ mL/g) for pH > 6.0 (Figure 1). In contrast, Tc removal fo

Sorption of hazardous metals on clay minerals is a key process contributing to the safety of repository sys-tems. The existence of high- and low-affinity adsorption sites on smectites edge surfaces, responsible for met-als uptake has been revealed in previous studies [1,2]. The exact molecular nature of these adsorption sites has not been fully resolved. Similar to the adsorption of Ni2+ on montmorillonite [1], Ni2+ on saponite shows a non-linear adsorption be-havior, which, by analogy, suggests the existence of strong and weak sorption sites (Fig.1). Based on the sorp-tion isotherms obtained on saponite, self-oriented clay films were prepared, with Ni loadings corresponding to adsorption dominated by strong and weak sites, and then polarized Ni K-edge EXAFS spectra were recorded. For samples with low Ni2+ loadings (below 4 mmol/kg), P-EXAFS results confirmed the existence of strong sorption sites. An increase of Ni loading (up to 40 mmol/kg) leads to decreasing in Ni-Mg bond length and an increase in Ni-Si coordination number, indicating the formation of a secondary phase. The exact structure of the new phase is still under evaluation, but this solid phase will certainly mask Ni sorbed to weak sites. The molecular structure of cations adsorbed on the water-edge interfaces for the most stable surfaces was modeled using ab initio MD, which enable us to obtain a theoretical estimation of the free energies of Zn-Ni cation exchange reactions between weak and strong sites on (100) and (130) edges of saponite. Theoretical values obtained for Zn-Ni exchange are equal to 13.5±0.4 and 5.50±0.03 kJ/mol for (100) and (130) surfaces, respectively. These results are in a good agreement with experimental results for montmorillonite (9.1 kJ/mol) – a trustworthy proxy and suggest a stronger affinity of Zn to strong sites than Ni.

The geochemical behavior of antimony (Sb), a priority pollutant of increasing concern, is closely linked to the redox cycling of iron (Fe). Microbial reduction of Fe(III) oxides and production of soluble Fe(II) under anoxic conditions has been shown to release co-associated Sb (occurring as Sb(V) and/or Sb(III)). In redox-dynamic environments, Fe(II) can be re-oxidized and precipitate as Fe(III) oxides. The extent to which these processes affect Sb mobility, however, is still poorly understood and likely depends on an array of factors such as pH, Sb species and the nature of the newly formed Fe(III) oxides. Here, we investigated the effect of Fe(II) oxidation on Sb sequestration and on the mineralogy of the resulting Fe(III) precipitates in a pH range typical of Sb-contaminated systems (i.e. pH 6, 6.5, and 7). To initiate the oxidation reaction, 0.5 mmol L-1 Fe(II) was added to an oxygen-saturated electrolyte solution containing 0 – 50 µmol L-1 Sb(V) or Sb(III). Changes in aqueous Sb and Fe(II) were tracked during the experiments, and solid phase samples were collected at the end of the oxidation reaction for characterization by diffractometric, spectroscopic, and microscopic techniques. Iron(II) oxidation kinetics and Sb sequestration were a function of pH and Sb species, with Sb(III) retarding the oxidation reaction. In the absence of Sb, Fe K-edge extended X-ray absorption fine structure (EXAFS) analysis revealed lepidocrocite as the only solid-phase product. In presence of Sb, feroxyhyte and goethite formed instead of lepidocrocite. The shift in Fe oxide assemblage was more pronounced when Sb was present as Sb(V) and at a lower pH. Antimony EXAFS analyses showed an almost complete oxidation of Sb(III) to Sb(V) followed by the incorporation of Sb(V) into the structure of the Fe oxides precipitates. Our results allow for a better understanding of Sb geochemistry in redox-dynamic environments as they demonstrate that Sb itself can influence the pathways of secondar

Technetium-99 (⁹⁹Tc) is a long-lived fission product (2.13∙10⁵ years) of uranium-235 (²³⁵U) and plutonium-239 (²³⁹Pu) and therefore, of great concern for the long-term safe management of nuclear waste. The migration of Tc in the environment is highly influenced by the redox conditions since Tc may be present in various oxidation states. Depending on the chemical properties of environmental relevant systems Tc is expected to mainly occur as Tc(VII) and as Tc(IV) under oxidizing and reducing conditions, respectively. The anion pertechnetate (Tc(VII)O₄⁻) is known to barely interact with mineral surfaces; this, in turn, enhances its migration in groundwater and favours its entry into the biosphere. On the contrary, the formation of Tc(IV) limits the migration of Tc since it forms a low soluble solid (TcO₂) and/or species whose interaction with minerals is more favourable. In the last decades Tc migration has been focused on the reduction of Tc(VII) to Tc(IV) by various reductants, such as Fe(II), Sn(II) or S(-II) either present in solution, taking part in mineral structures, (Pearce et al., 2019) or metabolically induced by microbial cascades (Newsome et al., 2014). We have studied the immobilization of Tc by various Fe(II)-containing phases: Fe²⁺ pre-sorbed on alumina nanoparticles (Mayordomo et al., 2020), Fe(II)-Al(III) layered double hydroxide (Mayordomo et al., 2021), and Fe(II) sulfides (Rodriguez et al., 2020; Rodríguez et al., 2021). We have combined sorption experiments, with microscopic and spectroscopic techniques (scanning electron microscopy, Raman microscopy, X-ray photoelectron spectroscopy, infrared spectroscopy, and X-ray absorption spectroscopy) to elucidate the mechanisms responsible of Tc(VII) reductive immobilization. Those works have been focused on binary systems i.e., studies of the interaction of Tc with a given reductant. However, the environment is a complex system where different components often depend on and modify each other. Thus, Tc migr

99Tc is a fission product with a long half-life of 2.14 × 105 years. Its migration and bioavailability strongly depend on its oxidation state and speciation in aqueous solution. Under oxidizing conditions, Tc mainly exists as pertechnetate, TcVIIO4, a highly water-soluble anion with s negligible sorption to most minerals. Under reducing conditions, TcIV prevails, whose main species, TcO2 xH2O, is a polymer of low solubility. As the presence of reductants like Fe2+ in the near-field of a nuclear waste repository is expected due to canister corrosion, several studies consider 99TcVII reductive immobilization by minerals containing reductant moieties, such as magnetite (FeIIFe2IIIO4) or mackinawite (FeS) [1] Pyrite (cubic FeS2) is a redox sensitive sulfide mineral that has been identified as a good sorbent for TcVII from soil and groundwater [2]. Under repository conditions, both pyrite and marcasite (orthorhombic FeS2) are expected to form by corrosion processes and microbial interaction [3]. Moreover, both iron sulfides are also accessory minerals in granitic and argillaceous rocks. Therefore, reliable data on 99TcVII retention by both minerals and their mixtures is relevant for the safe disposal of nuclear waste. We have studied the Tc retention by pure pyrite and by a mixture of marcasite and pyrite (60:40) using a combination of batch experiments and spectroscopy (Raman microscopy, X-ray photoelectron spectroscopy and X-ray absorption spectroscopy) at pH 6 and 10. [4, 5]. We confirm the 99TcVII reduction and subsequent 99TcIV retention on the mineral surfaces and shed new light on different retention mechanisms for pyrite and marcasite at pH 10. We acknowledge funding from the German Federal Ministry of Economic Affairs and Energy (BMWi) for VESPA II project (02E11607B), and from the German Federal Ministry of Education and Research (BMBF) for the NukSiFutur TecRad young investigator group (02NUK072). [1] Yalçıntaş E, et al. 2016 Dalton Trans. 45 17874 [2] Huo L

Thick steel casks are used for radioactive waste disposal in deep geological repositories. However, it is expected that steel corrodes over time. The corrosion products are expected to form mixed iron oxides, mainly magnetite. After tens of thousands of years, casks may breach allowing leaching of the radiotoxic elements, such as plutonium and technetium, by host rock pore-water. The dissolved radionuclides can then interact with the steel corrosion products and be adsorbed or incorporated into the solids [1]. But since these interaction mechanisms are poorly understood at the atomistic scale, our goal is to better understand them by using computer simulations alongside experiments [2]. In this computational study we identified the dominant low index surfaces on nano-magnetite particles and their termination at the relevant conditions based on Kohn-Sham density functional theory (DFT). This was done using the open source CP2K code and, for highly correlated chemical elements such as iron or plutonium, utilizing the DFT+U method because of the highly localized d and f electrons. The U parameter was determined by comparing experimental cell constants and band gaps to our result [3]. With this revised model, we examined the preferential magnetite crystal orientation plane (111) with different surface terminations as a function of oxygen and water fugacity. Based on our modelling, we found the most stable magnetite (111) surfaces under real repository conditions being Fe_oct1-O-H and Fe_tet1-O-H. Further, we used classical and ab initio MD simulations to investigate the behaviour of radionuclides at the water-magnetite interface in deep geological repositories. [1] Dumas, T.; Fellhauer, D.; Schild, D.; Gaona, X.; Altmaier, M.; Scheinost, A. C. Plutonium Retention Mechanisms by Magnetite Under Anoxic Conditions: Entrapment Versus Sorption. ACS Earth and Space Chemistry 2019, 3 (10), 2197–2206. [2] Yalçıntaş, E.; Scheinost, A. C.; Gaona, X.; Altmaier, M. Systematic XAS St

Techntium-99 (⁹⁹Tc) is one of the most concerning fission products due to its long half-life (2.13∙10⁵ years) and the high mobility of the anion pertechnetate (TcO₄⁻) [1]. Tc migration decreases significantly when Tc(VII) is reduced to Tc(IV). This scavenging step can be induced by Fe(II) minerals, which have been widely studied due to their versatility, low cost, and ubiquity [2]. In addition, Fe(II) minerals will play an important role in the near-field of the nuclear waste repository, in case that corrosion of the waste canisters will occur and radioactive material be leaked in the environment. Green rust is formed when Fe²⁺ interacts with Fe(III) minerals [3]. Thus, its presence is expected in both the near- and far-field of a repository. It is a Fe(II)-Fe(III) hydroxide that can immobilize radionuclides by adsorption, anion exchange, and reduction mechanisms. A previous work reports the interaction of green rust with Tc, but the results are limited to very narrow experimental conditions [4]. Thus, further studies are needed to both identify the optimal Tc scavenging conditions by green rust and the mechanism responsible of Tc retention. Our studies consisted of a combination of batch contact studies, microscopic and spectroscopic analysis. Batch contact studies were performed under a wide range of conditions, i.e. pH (3.5 - 11.0), Tc concentration (nM - mM), and ionic strength (0.0 - 0.1 M). X-ray powder diffraction, Raman microscopy, X-ray photoelectron spectroscopy (XPS), and X-ray absorption spectroscopy provided information on Tc oxidation state and speciation as well as on secondary redox products related to the Tc interaction with chloride green rust (GR(Cl)). In addition, re-oxidation experiments have been performed for one year to analyze the Tc retention reversibility. The results show that GR(Cl) removes Tc from solution with efficiencies between 80% (Kd = 8.0∙10³ mL/g) and ≈ 100% (Kd = 9.9∙10⁵ mL/g) for pH > 6.0 (Figure 1). In contrast, Tc removal fo

The mobility of antimony (Sb), a toxic metalloid of increasing concern, is closely linked to the biogeochemical cycling of iron (Fe). For example, the microbial production of soluble Fe(II) under oxygen-free conditions has been shown to result in the release of co-associated Sb (occurring as Sb(V) and/or Sb(III)). In redox-dynamic environments, Fe(II) may be re-oxidized and precipitate as Fe(III) oxides. The extent to which Fe(III) precipitation by Fe(II) oxidation immobilizes Sb, however, is still poorly understood and likely depends on an array of factors such as pH, Sb species and the nature of the newly formed Fe(III) oxides. This study aims at investigating the effect of Fe(II) oxidation on Sb sequestration and on the mineralogy of the resulting Fe(III) precipitates in a pH range typical of Sb-contaminated systems (i.e. pH 6 – 7). To initiate the oxidation reaction, 0.5 mmol L-1 Fe(II) are added to an oxygen-saturated electrolyte solution containing 0 – 50 µmol L-1 Sb(V) or Sb(III). Changes in aqueous Sb and Fe(II) concentration are monitored during the experiment. Resulting solid phase samples are collected and characterized using a combination of spectroscopic, microscopic, and wet chemical extraction techniques. First results show that Fe(II) oxidation kinetics and Sb sequestration are highly pH-dependent. At circumneutral pH and in the absence of Sb, X-ray diffractometry revealed lepidocrocite as the only solid-phase product. In contrast, the presence of Sb(V) partially inhibited lepidocrocite precipitation, and additionally resulted in formation of feroxyhyte – a rarely reported FeOOH polymorph. Sb EXAFS analysis indicated the incorporation of Sb(V) into the Fe oxide structure. Our results are important for a robust understanding of Sb geochemistry in redox-dynamic environments as they demonstrate that Sb itself influences the pathways of secondary Fe oxide formation. This study also provides important information for the development of adequate remediatio

Uranium (U) mining and milling activities, as well as mineral processing plants, raise environmental concerns due to the possible release of radioactive and other potentially toxic elements. To understand their fate in the environment and evaluate their potential impact, the main scientific challenge calls for identifying their solubility, mobility and bioavailability in the environment. Around former U mining and processing plants, wetlands prove to be specific zones with significant amounts of U. This is partly explained by the reduction of the mobile U(VI) into U(IV) due to strongly reducing conditions related to the microbial activity and/or by complexation with the organic matter occurring with high concentrations in wetlands. At the center of the ancient mining district of Lachaux in France (45.994°N, 3.596°E), the site of Rophin (within the ZATU: Uranium Working Zone = Long Term Socio-Ecological Research Tool of CNRS, Fig.1.a) is characterized by a wetland area with large U concentrations up to 16 g.kg-1 of dry mass of soil [1]. Several cross-analyses indicate that U was transported in particulate forms into the wetland during the exploitation of U(VI) phosphate minerals [1]. The Rophin site therefore provides the opportunity to study the stability of these U minerals over almost 70 years in a non-manipulated wetland since the closure of the mine. In this context, the main challenge is to describe the behavior of U (and decay products of interest) in the wetland using a predictive model that combines transport and chemical speciation. The overall adopted scientific approach is to propose a mechanistic description of the mobility of these elements, from the molecular scale (speciation) to in natura behavior (lability), by coupling field investigations and laboratory experiments. A simplified three-layer model describes the soil profile of the Rophin wetland, with variable U concentrations and specific physico-chemical soil properties (Fig.1.b). Analyses carrie

Thick steel casks are used for radioactive waste disposal in deep geological repositories. However, it is expected that steel corrodes over time. The corrosion products are expected to form mixed iron oxides, mainly magnetite. After tens of thousands of years, casks may breach allowing leaching of the radiotoxic elements, such as plutonium and technetium, by host rock pore-water. The dissolved radionuclides can then interact with the steel corrosion products and be adsorbed or incorporated into the solids [1]. But since these interaction mechanisms are poorly understood at the atomistic scale, our goal is to better understand them by using computer simulations alongside experiments [2]. In this computational study we identified the dominant low index surfaces on nano-magnetite particles and their termination at the relevant conditions based on Kohn-Sham density functional theory (DFT). This was done using the open source CP2K code and, for highly correlated chemical elements such as iron or plutonium, utilizing the DFT+U method because of the highly localized d and f electrons. The U parameter was determined by comparing experimental cell constants and band gaps to our result [3]. With this revised model, we examined the preferential magnetite crystal orientation plane (111) with different surface terminations as a function of oxygen and water fugacity. Based on our modelling, we found the most stable magnetite (111) surfaces under real repository conditions being Fe_oct1-O-H and Fe_tet1-O-H. Further, we used classical and ab initio MD simulations to investigate the behaviour of radionuclides at the water-magnetite interface in deep geological repositories. [1] Dumas, T.; Fellhauer, D.; Schild, D.; Gaona, X.; Altmaier, M.; Scheinost, A. C. Plutonium Retention Mechanisms by Magnetite Under Anoxic Conditions: Entrapment Versus Sorption. ACS Earth and Space Chemistry 2019, 3 (10), 2197–2206. [2] Yalçıntaş, E.; Scheinost, A. C.; Gaona, X.; Altmaier, M. Systematic XAS St

In many European countries (e.g., Switzerland, France, Sweden), thick steel casks are foreseen for the containment of high-level radioactive waste in deep geological repositories. In contact with pore-water, steel corrodes forming mixed iron oxides, mainly magnetite at the surface (Fe3O4). After tens of thousands of years, casks may breach allowing for leaching of the radionuclides by pore-water. Magnetite can retard dissolved radionuclides either by adsorption or structural incorporation [1,2]. But since these interaction mechanisms are poorly understood at the atomistic scale, our goal is to better understand them by using computer simulations alongside experiments [3]. In this computational study, we identified the dominant low-index surfaces of magnetite particles and their termination at the relevant conditions based on Kohn-Sham density functional theory (DFT). This was done using the open-source code CP2K. The DFT+U method was employed for the strongly correlated 3d and 5f electrons of Fe and Pu, respectively. After revising our model and determining the Hubbard U parameter [4], we examined the preferential magnetite crystal orientation plane (111) allowing for different surface terminations, water coverage and redox conditions. Comparing the surface energy, the most stable surface can be deduced and we found the most stable magnetite (111) surfaces under real repository conditions. Subsequently, we used ab initio molecular dynamics (MD) to simulate sorption structures on the expected magnetite (111) surfaces. [1] T. Dumas et al. (2019). Plutonium Retention Mechanisms by Magnetite under Anoxic Conditions: Entrapment versus Sorption. ACS Earth & Space Chemistry, 2019, 3(10), 2197. [2] R. Kirsch et al. (2011). Oxidation State and Local Structure of Plutonium Reacted with Magnetite, Mackinawite, and Chukanovite. Environmental Science & Technology, 2011, 45(17), 7267. [3] E. Yalçintaş et al. (2016). Systematic XAS study on the reduction and uptake of Tc by magnet

Sorption of hazardous metals on clay minerals is a key process contributing to the safety of repository sys-tems. The existence of high- and low-affinity adsorption sites on smectites edge surfaces, responsible for met-als uptake has been revealed in previous studies [1,2]. The exact molecular nature of these adsorption sites has not been fully resolved. Similar to the adsorption of Ni2+ on montmorillonite [1], Ni2+ on saponite shows a non-linear adsorption be-havior, which, by analogy, suggests the existence of strong and weak sorption sites (Fig.1). Based on the sorp-tion isotherms obtained on saponite, self-oriented clay films were prepared, with Ni loadings corresponding to adsorption dominated by strong and weak sites, and then polarized Ni K-edge EXAFS spectra were recorded. For samples with low Ni2+ loadings (below 4 mmol/kg), P-EXAFS results confirmed the existence of strong sorption sites. An increase of Ni loading (up to 40 mmol/kg) leads to decreasing in Ni-Mg bond length and an increase in Ni-Si coordination number, indicating the formation of a secondary phase. The exact structure of the new phase is still under evaluation, but this solid phase will certainly mask Ni sorbed to weak sites. The molecular structure of cations adsorbed on the water-edge interfaces for the most stable surfaces was modeled using ab initio MD, which enable us to obtain a theoretical estimation of the free energies of Zn-Ni cation exchange reactions between weak and strong sites on (100) and (130) edges of saponite. Theoretical values obtained for Zn-Ni exchange are equal to 13.5±0.4 and 5.50±0.03 kJ/mol for (100) and (130) surfaces, respectively. These results are in a good agreement with experimental results for montmorillonite (9.1 kJ/mol) – a trustworthy proxy and suggest a stronger affinity of Zn to strong sites than Ni.

The geochemical behavior of antimony (Sb), a priority pollutant of increasing concern, is closely linked to the redox cycling of iron (Fe). Microbial reduction of Fe(III) oxides and production of soluble Fe(II) under anoxic conditions has been shown to release co-associated Sb (occurring as Sb(V) and/or Sb(III)). In redox-dynamic environments, Fe(II) can be re-oxidized and precipitate as Fe(III) oxides. The extent to which these processes affect Sb mobility, however, is still poorly understood and likely depends on an array of factors such as pH, Sb species and the nature of the newly formed Fe(III) oxides. Here, we investigated the effect of Fe(II) oxidation on Sb sequestration and on the mineralogy of the resulting Fe(III) precipitates in a pH range typical of Sb-contaminated systems (i.e. pH 6, 6.5, and 7). To initiate the oxidation reaction, 0.5 mmol L-1 Fe(II) was added to an oxygen-saturated electrolyte solution containing 0 – 50 µmol L-1 Sb(V) or Sb(III). Changes in aqueous Sb and Fe(II) were tracked during the experiments, and solid phase samples were collected at the end of the oxidation reaction for characterization by diffractometric, spectroscopic, and microscopic techniques. Iron(II) oxidation kinetics and Sb sequestration were a function of pH and Sb species, with Sb(III) retarding the oxidation reaction. In the absence of Sb, Fe K-edge extended X-ray absorption fine structure (EXAFS) analysis revealed lepidocrocite as the only solid-phase product. In presence of Sb, feroxyhyte and goethite formed instead of lepidocrocite. The shift in Fe oxide assemblage was more pronounced when Sb was present as Sb(V) and at a lower pH. Antimony EXAFS analyses showed an almost complete oxidation of Sb(III) to Sb(V) followed by the incorporation of Sb(V) into the structure of the Fe oxides precipitates. Our results allow for a better understanding of Sb geochemistry in redox-dynamic environments as they demonstrate that Sb itself can influence the pathways of secondar

Vorlesung im Rahmen der KRIMI Winter School an der TU Berlin

The redox reaction between natural Fe-containing clay minerals and its sorbates is a fundamental process controlling the cycles of carbon, nutrients, and inorganic as well as organic pollutants in Earth’s critical zone. While the structure of natural clay minerals under oxic conditions is well known, this is not true for anoxic conditions, thereby impeding a full understanding of the mechanisms of clay-driven redox reactions especially under reducing conditions. Here we investigate the structure of an Fe-rich natural clay mineral, nontronite, under different redox conditions, and compare several methods for the determination of iron redox states. Nontronite was gradually reduced chemically with the Citrate-Bicarbonate-Dithionite (CBD) method. All methods used to determine the Fe redox state, i.e. 57Fe Mössbauer spectrometry, X-ray photoelectron spectroscopy (XPS), X-ray absorption near edge structure (XANES) spectroscopy including its pre-edge, extended X-ray absorption fine structure (EXAFS) spectroscopy, and mediated electrochemical oxidation and reduction (MEO/MER), provided consistent Fe(II)/Fe(III) ratios. By combining X-ray diffraction (XRD) and Transmission electron microscopy (TEM), we show that the long-range structure of nontronite at the highest obtained reduction degree of 44% is not different from that of fully oxidized nontronite except for a slight basal plane dissolution on the external surfaces. The short-range order probed by EXAFS spectroscopy suggests, however, an increasing structural disorder and Fe clustering with increasing reduction of structural Fe.

The pertechnetate ion TcVIIO4− is a nuclear fission product whose major issue is the high mobility in the environment. Experimentally, it is well-known that Fe3O4 can reduce TcVIIO4− to TcIV species and retain such products quickly and completely, but the exact nature of the redox process and products is not completely understood. Therefore, we investigated the chemistry of TcVIIO4− and TcIV species at the Fe3O4(001) surface through a hybrid DFT functional (HSE06) method. We studied a possible initiation step of the TcVII reduction process. The interaction of the TcVIIO4− ion with magnetite surface leads to the formation of a reduced TcVI species without any change in the Tc coordination sphere, through an electron transfer that is favored by the magnetite surfaces with a higher FeII content. Furthermore, we explored various model structures for the immobilized TcIV final products. TcIV can be incorporated into a subsurface octahedral site or adsorbed on the surface in the form of TcIVO2·xH2O chains. We propose and discuss three model structures for the adsorbed TcIVO2·2H2O chains in terms of relative energies and simulated EXAFS spectra. Our results suggest that the periodicity of the Fe3O4(001) surface matches that of the TcO2·2H2O chains. The EXAFS analysis suggests that in experiments TcO2·xH2O chains were probably not formed as an inner-shell adsorption complex with the Fe3O4(001) surface.

The reactivity of Fe(II) sorbed on three Ca-exchanged smectite clays has been probed via U(VI) reduction at pH 6.0 (± 0.2) under CO2-free, anoxic (O2 <1 ppmv) condition. The clays varied with regard to structural Fe content from Fe-free (0 wt.%) montmorillonite (MONT), to Fe-poor (2.6 wt.%) montmorillonite (Fe-MONT), to Fe-rich (25.8 wt.%) nontronite (NAu-2). U LIII-edge XANES spectra showed no reduction of U(VI) in presence of Fe(II) sorbed on either Fe-MONT or NAu-2 after 72 h but a partial reduction (21%) on MONT after 24 h. U LIII-edge EXAFS spectra further showed the formation of a soddyite-like surface precipitate on MONT in presence of sorbed Fe(II) after 72 h. The Mössbauer data further reveals that 10 (± 2)% of the total sorbed Fe(II) was oxidized in presence of MONT before and an additional 6 (± 2)% after U(VI) addition. The mechanism of U(VI) reduction involves Fe(II) specifically sorbed on oxidized and reduced strong edge sites of MONT. The non-reactivity of Fe(II) sorbed on Fe-MONT and NAu-2 towards U(VI) reduction is likely linked to the inter-valence charge transfer (IVCT) between surface Fe(II) and structural Fe(III). The present study demonstrates that the reduction capacity of surface bound Fe strongly depends on the nature of clay and correspondingly on the oxidation state of the sorbed Fe. Thus Fe(II) sorbed clay may not be always considered as “universal reductant” for U(VI).

99Tc is a fission product with a long half-life of 2.14 × 105 years. Its migration and bioavailability strongly depend on its oxidation state and speciation in aqueous solution. Under oxidizing conditions, Tc mainly exists as pertechnetate, TcVIIO4, a highly water-soluble anion with s negligible sorption to most minerals. Under reducing conditions, TcIV prevails, whose main species, TcO2 xH2O, is a polymer of low solubility. As the presence of reductants like Fe2+ in the near-field of a nuclear waste repository is expected due to canister corrosion, several studies consider 99TcVII reductive immobilization by minerals containing reductant moieties, such as magnetite (FeIIFe2IIIO4) or mackinawite (FeS) [1] Pyrite (cubic FeS2) is a redox sensitive sulfide mineral that has been identified as a good sorbent for TcVII from soil and groundwater [2]. Under repository conditions, both pyrite and marcasite (orthorhombic FeS2) are expected to form by corrosion processes and microbial interaction [3]. Moreover, both iron sulfides are also accessory minerals in granitic and argillaceous rocks. Therefore, reliable data on 99TcVII retention by both minerals and their mixtures is relevant for the safe disposal of nuclear waste. We have studied the Tc retention by pure pyrite and by a mixture of marcasite and pyrite (60:40) using a combination of batch experiments and spectroscopy (Raman microscopy, X-ray photoelectron spectroscopy and X-ray absorption spectroscopy) at pH 6 and 10. [4, 5]. We confirm the 99TcVII reduction and subsequent 99TcIV retention on the mineral surfaces and shed new light on different retention mechanisms for pyrite and marcasite at pH 10. We acknowledge funding from the German Federal Ministry of Economic Affairs and Energy (BMWi) for VESPA II project (02E11607B), and from the German Federal Ministry of Education and Research (BMBF) for the NukSiFutur TecRad young investigator group (02NUK072). [1] Yalçıntaş E, et al. 2016 Dalton Trans. 45 17874 [2] Huo L

Technetium-99 (⁹⁹Tc) is a long-lived fission product (2.13∙10⁵ years) of uranium-235 (²³⁵U) and plutonium-239 (²³⁹Pu) and therefore, of great concern for the long-term safe management of nuclear waste. The migration of Tc in the environment is highly influenced by the redox conditions since Tc may be present in various oxidation states. Depending on the chemical properties of environmental relevant systems Tc is expected to mainly occur as Tc(VII) and as Tc(IV) under oxidizing and reducing conditions, respectively. The anion pertechnetate (Tc(VII)O₄⁻) is known to barely interact with mineral surfaces; this, in turn, enhances its migration in groundwater and favours its entry into the biosphere. On the contrary, the formation of Tc(IV) limits the migration of Tc since it forms a low soluble solid (TcO₂) and/or species whose interaction with minerals is more favourable. In the last decades Tc migration has been focused on the reduction of Tc(VII) to Tc(IV) by various reductants, such as Fe(II), Sn(II) or S(-II) either present in solution, taking part in mineral structures, (Pearce et al., 2019) or metabolically induced by microbial cascades (Newsome et al., 2014). We have studied the immobilization of Tc by various Fe(II)-containing phases: Fe²⁺ pre-sorbed on alumina nanoparticles (Mayordomo et al., 2020), Fe(II)-Al(III) layered double hydroxide (Mayordomo et al., 2021), and Fe(II) sulfides (Rodriguez et al., 2020; Rodríguez et al., 2021). We have combined sorption experiments, with microscopic and spectroscopic techniques (scanning electron microscopy, Raman microscopy, X-ray photoelectron spectroscopy, infrared spectroscopy, and X-ray absorption spectroscopy) to elucidate the mechanisms responsible of Tc(VII) reductive immobilization. Those works have been focused on binary systems i.e., studies of the interaction of Tc with a given reductant. However, the environment is a complex system where different components often depend on and modify each other. Thus, Tc migr

Thick steel casks are used for radioactive waste disposal in deep geological repositories. However, it is expected that steel corrodes over time. The corrosion products are expected to form mixed iron oxides, mainly magnetite. After tens of thousands of years, casks may breach allowing leaching of the radiotoxic elements, such as plutonium and technetium, by host rock pore-water. The dissolved radionuclides can then interact with the steel corrosion products and be adsorbed or incorporated into the solids [1]. But since these interaction mechanisms are poorly understood at the atomistic scale, our goal is to better understand them by using computer simulations alongside experiments [2]. In this computational study we identified the dominant low index surfaces on nano-magnetite particles and their termination at the relevant conditions based on Kohn-Sham density functional theory (DFT). This was done using the open source CP2K code and, for highly correlated chemical elements such as iron or plutonium, utilizing the DFT+U method because of the highly localized d and f electrons. The U parameter was determined by comparing experimental cell constants and band gaps to our result [3]. With this revised model, we examined the preferential magnetite crystal orientation plane (111) with different surface terminations as a function of oxygen and water fugacity. Based on our modelling, we found the most stable magnetite (111) surfaces under real repository conditions being Fe_oct1-O-H and Fe_tet1-O-H. Further, we used classical and ab initio MD simulations to investigate the behaviour of radionuclides at the water-magnetite interface in deep geological repositories. [1] Dumas, T.; Fellhauer, D.; Schild, D.; Gaona, X.; Altmaier, M.; Scheinost, A. C. Plutonium Retention Mechanisms by Magnetite Under Anoxic Conditions: Entrapment Versus Sorption. ACS Earth and Space Chemistry 2019, 3 (10), 2197–2206. [2] Yalçıntaş, E.; Scheinost, A. C.; Gaona, X.; Altmaier, M. Systematic XAS St

Oscillating oxic/anoxic conditions induces processes at mineral-Kuid interfaces aeecting the speciation and fate of redox active elements. Such Kuctuations occur in natural environments (at watershed and delta water table or in lake water chemocline) and in engineered systems (in oil reDnery drainage water peatbogs or nuclear waste repository). In presence of aqueous sulDde or ferrous iron ions, Se, Sb, Pu and Re undergo heterogeneous reductive precipitation. At the nanosecond scale, selenate adsorption onto the net pyrite surface is shown by ab-initio computations to proceed via the formation of a chemical bond between the oxyanion oxygen atom and a surface Fe atom, weakening the other Se-O bonds and reducing Se atom oxydation state. At hour-to-day scale, mobile Se(VI), Se(IV), Sb(V), Pu(V) and Re(VII) oxyanions are shown by strictly anoxic wet chemistry experiments, XAFS and Mossbauer spectroscopy [1] to be reduced, in presence of nanosized particles (iron sulDdes (mackinawite or pyrite) [2], oxides (magnetite) [3] [4] [5] or clays [6]) and aqueous iron (II) or S(-II), to solids (Se(0), Sb O , PuO , Re0 , ReS or Re(0)) or to highly speciDc Sb(III) [7] and Pu(III) [8] sorption complexes. STEM-HAADF microscopy demonstrates the formation of Se(0) trigonal gray selenium nanowires after co-adsorption of selenate and ferrous iron ions on 10 nm nanomagnetite [4], a reaction which results from a complex, kinetically controlled interplay of intermediate reduced surface complexes and transport of these reduced species to the tip of the nanowires. These processes in far-from-equilibrium conditions are of prime importance in the safety assessment of petroleum industry and geological repositories, as well as in the bioavailability to crop or human beings. [1] Charlet et al., 2022, J. Mat. Res. DOI: 10.1557/s43578-022-00823-8 [2] Son et al., 2022, Geochim. Cosmochim. Acta 338 : 220–228. [3] Goberna-Feron et al., 2021, Env. Sci. & Tech. 55, 3021−3031. [4] Poulain et al., 2022, E

Oxide minerals (include oxides, hydroxides, oxyhydroxides and hydrated oxides) are primary and secondary minerals of Si, Fe, Mn, Al and Ti. Secondary oxides, particularly of Fe, Al and Mn, are perhaps the most reactive and important components in many soils due to their high specific surface area and strong sorption capacity for many essential and potentially toxic elements. Iron and Mn oxides also play key roles in many redox reactions and have major influence on the transformation and availability of organic and inorganic contaminants in soils. This chapter provides an overview of the oxide minerals in soils, with an emphasis on their structure and composition, environmental conditions for their formation and their properties.

Uranium (U) mining and milling activities, as well as mineral processing plants, raise environmental concerns due to the possible release of radioactive and other potentially toxic elements. To understand their fate in the environment and evaluate their potential impact, the main scientific challenge calls for identifying their solubility, mobility and bioavailability in the environment. Around former U mining and processing plants, wetlands prove to be specific zones with significant amounts of U. This is partly explained by the reduction of the mobile U(VI) into U(IV) due to strongly reducing conditions related to the microbial activity and/or by complexation with the organic matter occurring with high concentrations in wetlands. At the center of the ancient mining district of Lachaux in France (45.994°N, 3.596°E), the site of Rophin (within the ZATU: Uranium Working Zone = Long Term Socio-Ecological Research Tool of CNRS, Fig.1.a) is characterized by a wetland area with large U concentrations up to 16 g.kg-1 of dry mass of soil [1]. Several cross-analyses indicate that U was transported in particulate forms into the wetland during the exploitation of U(VI) phosphate minerals [1]. The Rophin site therefore provides the opportunity to study the stability of these U minerals over almost 70 years in a non-manipulated wetland since the closure of the mine. In this context, the main challenge is to describe the behavior of U (and decay products of interest) in the wetland using a predictive model that combines transport and chemical speciation. The overall adopted scientific approach is to propose a mechanistic description of the mobility of these elements, from the molecular scale (speciation) to in natura behavior (lability), by coupling field investigations and laboratory experiments. A simplified three-layer model describes the soil profile of the Rophin wetland, with variable U concentrations and specific physico-chemical soil properties (Fig.1.b). Analyses carrie

Thick steel casks are used for radioactive waste disposal in deep geological repositories. However, it is expected that steel corrodes over time. The corrosion products are expected to form mixed iron oxides, mainly magnetite. After tens of thousands of years, casks may breach allowing leaching of the radiotoxic elements, such as plutonium and technetium, by host rock pore-water. The dissolved radionuclides can then interact with the steel corrosion products and be adsorbed or incorporated into the solids [1]. But since these interaction mechanisms are poorly understood at the atomistic scale, our goal is to better understand them by using computer simulations alongside experiments [2]. In this computational study we identified the dominant low index surfaces on nano-magnetite particles and their termination at the relevant conditions based on Kohn-Sham density functional theory (DFT). This was done using the open source CP2K code and, for highly correlated chemical elements such as iron or plutonium, utilizing the DFT+U method because of the highly localized d and f electrons. The U parameter was determined by comparing experimental cell constants and band gaps to our result [3]. With this revised model, we examined the preferential magnetite crystal orientation plane (111) with different surface terminations as a function of oxygen and water fugacity. Based on our modelling, we found the most stable magnetite (111) surfaces under real repository conditions being Fe_oct1-O-H and Fe_tet1-O-H. Further, we used classical and ab initio MD simulations to investigate the behaviour of radionuclides at the water-magnetite interface in deep geological repositories. [1] Dumas, T.; Fellhauer, D.; Schild, D.; Gaona, X.; Altmaier, M.; Scheinost, A. C. Plutonium Retention Mechanisms by Magnetite Under Anoxic Conditions: Entrapment Versus Sorption. ACS Earth and Space Chemistry 2019, 3 (10), 2197–2206. [2] Yalçıntaş, E.; Scheinost, A. C.; Gaona, X.; Altmaier, M. Systematic XAS St

In many European countries (e.g., Switzerland, France, Sweden), thick steel casks are foreseen for the containment of high-level radioactive waste in deep geological repositories. In contact with pore-water, steel corrodes forming mixed iron oxides, mainly magnetite at the surface (Fe3O4). After tens of thousands of years, casks may breach allowing for leaching of the radionuclides by pore-water. Magnetite can retard dissolved radionuclides either by adsorption or structural incorporation [1,2]. But since these interaction mechanisms are poorly understood at the atomistic scale, our goal is to better understand them by using computer simulations alongside experiments [3]. In this computational study, we identified the dominant low-index surfaces of magnetite particles and their termination at the relevant conditions based on Kohn-Sham density functional theory (DFT). This was done using the open-source code CP2K. The DFT+U method was employed for the strongly correlated 3d and 5f electrons of Fe and Pu, respectively. After revising our model and determining the Hubbard U parameter [4], we examined the preferential magnetite crystal orientation plane (111) allowing for different surface terminations, water coverage and redox conditions. Comparing the surface energy, the most stable surface can be deduced and we found the most stable magnetite (111) surfaces under real repository conditions. Subsequently, we used ab initio molecular dynamics (MD) to simulate sorption structures on the expected magnetite (111) surfaces. [1] T. Dumas et al. (2019). Plutonium Retention Mechanisms by Magnetite under Anoxic Conditions: Entrapment versus Sorption. ACS Earth & Space Chemistry, 2019, 3(10), 2197. [2] R. Kirsch et al. (2011). Oxidation State and Local Structure of Plutonium Reacted with Magnetite, Mackinawite, and Chukanovite. Environmental Science & Technology, 2011, 45(17), 7267. [3] E. Yalçintaş et al. (2016). Systematic XAS study on the reduction and uptake of Tc by magnet

The mobility of antimony (Sb), a toxic metalloid of increasing concern, is closely linked to the biogeochemical cycling of iron (Fe). For example, the microbial production of soluble Fe(II) under oxygen-free conditions has been shown to result in the release of co-associated Sb (occurring as Sb(V) and/or Sb(III)). In redox-dynamic environments, Fe(II) may be re-oxidized and precipitate as Fe(III) oxides. The extent to which Fe(III) precipitation by Fe(II) oxidation immobilizes Sb, however, is still poorly understood and likely depends on an array of factors such as pH, Sb species and the nature of the newly formed Fe(III) oxides. This study aims at investigating the effect of Fe(II) oxidation on Sb sequestration and on the mineralogy of the resulting Fe(III) precipitates in a pH range typical of Sb-contaminated systems (i.e. pH 6 – 7). To initiate the oxidation reaction, 0.5 mmol L-1 Fe(II) are added to an oxygen-saturated electrolyte solution containing 0 – 50 µmol L-1 Sb(V) or Sb(III). Changes in aqueous Sb and Fe(II) concentration are monitored during the experiment. Resulting solid phase samples are collected and characterized using a combination of spectroscopic, microscopic, and wet chemical extraction techniques. First results show that Fe(II) oxidation kinetics and Sb sequestration are highly pH-dependent. At circumneutral pH and in the absence of Sb, X-ray diffractometry revealed lepidocrocite as the only solid-phase product. In contrast, the presence of Sb(V) partially inhibited lepidocrocite precipitation, and additionally resulted in formation of feroxyhyte – a rarely reported FeOOH polymorph. Sb EXAFS analysis indicated the incorporation of Sb(V) into the Fe oxide structure. Our results are important for a robust understanding of Sb geochemistry in redox-dynamic environments as they demonstrate that Sb itself influences the pathways of secondary Fe oxide formation. This study also provides important information for the development of adequate remediatio

The design of a nuclear waste repository is a challenging technological entreprise. The reasons are intrinsically related to the nature of the problem: the storage of a variety of radioactive materials for a period longer than human history. The potential consequences of a repository failure are dramatic because they lead to high environmental impact and economical costs of remediation [1]. Repository implementation should therefore rely on deep process understanding and intrinsic passive safety of geological systems. Modern approaches to the geological storage of nuclear wastes deal with a careful site selection and a multi-barrier system. The Swiss waste disposal program is currently in the final stage of the site selection process. Three geological location identified as potential disposal sites are located in Opalinus Clay formations. The multi-barrier disposal concept is heavily relying on the physical and chemical properties of clay minerals, in particular, their interaction with radionuclides [2]. The minerals clays of interest (illite and montmorillonite) were studied over years to derive empirically the sorption properties necessary for the safety assessments. Natural systems are heterogeneous in terms of mineralogical and chemical composition. The experimental studies are limited to a finite number of scenarios, simplified chemistry and short timescale. A mechanistic description of the sorption processes is necessary to transfer the data obtained from simplified reference systems to complex natural environments using computational models based on physical and chemical process understanding. A synthetic clay, saponite, was selected as the customizable and chemically pure starting material. Sorption experiments were carried out to obtain sorption isotherms for a divalent transition metal (nickel) and a trivalent lanthanide (lutetium). The mechanistic information about the sorption processes was achieved by X-ray spectroscopy techniques that have been carried

The reactivity of Fe(II) sorbed on three Ca-exchanged smectite clays has been probed via U(VI) reduction at pH 6.0 (± 0.2) under CO2-free, anoxic (O2 <1 ppmv) condition. The clays varied with regard to structural Fe content from Fe-free (0 wt.%) montmorillonite (MONT), to Fe-poor (2.6 wt.%) montmorillonite (Fe-MONT), to Fe-rich (25.8 wt.%) nontronite (NAu-2). U LIII-edge XANES spectra showed no reduction of U(VI) in presence of Fe(II) sorbed on either Fe-MONT or NAu-2 after 72 h but a partial reduction (21%) on MONT after 24 h. U LIII-edge EXAFS spectra further showed the formation of a soddyite-like surface precipitate on MONT in presence of sorbed Fe(II) after 72 h. The Mössbauer data further reveals that 10 (± 2)% of the total sorbed Fe(II) was oxidized in presence of MONT before and an additional 6 (± 2)% after U(VI) addition. The mechanism of U(VI) reduction involves Fe(II) specifically sorbed on oxidized and reduced strong edge sites of MONT. The non-reactivity of Fe(II) sorbed on Fe-MONT and NAu-2 towards U(VI) reduction is likely linked to the inter-valence charge transfer (IVCT) between surface Fe(II) and structural Fe(III). The present study demonstrates that the reduction capacity of surface bound Fe strongly depends on the nature of clay and correspondingly on the oxidation state of the sorbed Fe. Thus Fe(II) sorbed clay may not be always considered as “universal reductant” for U(VI).

Thick steel casks are used for radioactive waste disposal in deep geological repositories. However, it is expected that steel corrodes over time. The corrosion products are expected to form mixed iron oxides, mainly magnetite. After tens of thousands of years, casks may breach allowing leaching of the radiotoxic elements, such as plutonium and technetium, by host rock pore-water. The dissolved radionuclides can then interact with the steel corrosion products and be adsorbed or incorporated into the solids [1]. But since these interaction mechanisms are poorly understood at the atomistic scale, our goal is to better understand them by using computer simulations alongside experiments [2]. In this computational study we identified the dominant low index surfaces on nano-magnetite particles and their termination at the relevant conditions based on Kohn-Sham density functional theory (DFT). This was done using the open source CP2K code and, for highly correlated chemical elements such as iron or plutonium, utilizing the DFT+U method because of the highly localized d and f electrons. The U parameter was determined by comparing experimental cell constants and band gaps to our result [3]. With this revised model, we examined the preferential magnetite crystal orientation plane (111) with different surface terminations as a function of oxygen and water fugacity. Based on our modelling, we found the most stable magnetite (111) surfaces under real repository conditions being Fe_oct1-O-H and Fe_tet1-O-H. Further, we used classical and ab initio MD simulations to investigate the behaviour of radionuclides at the water-magnetite interface in deep geological repositories. [1] Dumas, T.; Fellhauer, D.; Schild, D.; Gaona, X.; Altmaier, M.; Scheinost, A. C. Plutonium Retention Mechanisms by Magnetite Under Anoxic Conditions: Entrapment Versus Sorption. ACS Earth and Space Chemistry 2019, 3 (10), 2197–2206. [2] Yalçıntaş, E.; Scheinost, A. C.; Gaona, X.; Altmaier, M. Systematic XAS St

Thick steel casks are used for radioactive waste disposal in deep geological repositories. However, it is expected that steel corrodes over time. The corrosion products are expected to form mixed iron oxides, mainly magnetite. After tens of thousands of years, casks may breach allowing leaching of the radiotoxic elements, such as plutonium and technetium, by host rock pore-water. The dissolved radionuclides can then interact with the steel corrosion products and be adsorbed or incorporated into the solids [1]. But since these interaction mechanisms are poorly understood at the atomistic scale, our goal is to better understand them by using computer simulations alongside experiments [2]. In this computational study we identified the dominant low index surfaces on nano-magnetite particles and their termination at the relevant conditions based on Kohn-Sham density functional theory (DFT). This was done using the open source CP2K code and, for highly correlated chemical elements such as iron or plutonium, utilizing the DFT+U method because of the highly localized d and f electrons. The U parameter was determined by comparing experimental cell constants and band gaps to our result [3]. With this revised model, we examined the preferential magnetite crystal orientation plane (111) with different surface terminations as a function of oxygen and water fugacity. Based on our modelling, we found the most stable magnetite (111) surfaces under real repository conditions being Fe_oct1-O-H and Fe_tet1-O-H. Further, we used classical and ab initio MD simulations to investigate the behaviour of radionuclides at the water-magnetite interface in deep geological repositories. [1] Dumas, T.; Fellhauer, D.; Schild, D.; Gaona, X.; Altmaier, M.; Scheinost, A. C. Plutonium Retention Mechanisms by Magnetite Under Anoxic Conditions: Entrapment Versus Sorption. ACS Earth and Space Chemistry 2019, 3 (10), 2197–2206. [2] Yalçıntaş, E.; Scheinost, A. C.; Gaona, X.; Altmaier, M. Systematic XAS St

The mobility of 79Se, a long half-life radioisotope and fission product of 235U, and contaminant of drainage waters from black shale mountains and from coal mines, is an important parameter in the safety assessment of radioactive nuclear waste disposal systems. Highly mobile and soluble in its high oxidation states (Se(VI)O42-, Se(IV)O32-), selenium oxyanions can interact with magnetite, a mineral present in anoxic natural environments and in steel corrosion products, and be precipitated by reduction, and thus immobilized. Here, the sorption and reduction capacity of synthetic nanomagnetite towards Se(VI) was investigated at neutral and acidic pH, under reducing, oxygen free conditions. The additional presence of Fe(II)aq, released during magnetite dissolution at pH 5, is shown to have an effect on the reduction kinetics. XANES analyses revealed that, at pH 5, trigonal gray Se(0) formed, and that outer-sphere Se(IV) complexes existed at the nanoparticle surface at longer reaction times. The Se(0) nanowires grew during the reaction, which points to a complex transport mechanism of reduced species or to active reduction sites at the tip of the Se(0) nanowires. The concomitant uptake of aqueous Fe(II) and Se(VI) ions is interpreted as a consequence of small pH oscillations that result from the Se(VI) reduction, leading to a re- adsorption of aqueous Fe(II) onto the magnetite, renewing its reducing capacity. This effect is not observed at pH 7, indicating that the presence of aqueous Fe(II) may be an important factor to be considered when examining the environmental reactivity of magnetite.

The interaction of highly mobile radioactive elements in the spent fuel with the different technical and geological barriers of a nuclear waste repository needs quantification and mechanistic understanding to allow a reliable safety assessment. One of the most concerning mobile fission products is Tc-99. It is a long-lived radionuclide (half-life of 0.213 million years) that is expected to occur as Tc(VII) under oxidizing conditions and as Tc(IV) under reducing conditions. The anion pertechnetate (TcO₄⁻) is the main species of Tc(VII) and it is known to be a highly mobile species since it barely interacts with mineral surfaces. On the contrary, TcO₂ is the main species of Tc(IV) and it is a hardly soluble solid. Therefore, the reduction of Tc(VII) to Tc(IV) limits the mobility of Tc in water and is triggered by reducing agents such as Fe(II) or Sn(II). [1] In a previous work, we have observed that pre-sorption of Fe(II) on alumina enabled the Tc(VII) reduction at the interface, even at low pH values when Tc(VII) reduction by Fe(II) was expected to be limited due to the low sorption of Fe(II) on alumina. [2] In this study we focus on the impact of Sn(II). We have performed sorption experiments following a stepwise strategy to ensure that Tc(VII) reduction by Sn(II) occurred at the interface (heteroreduction). i) Sn(II) was sorbed on alumina, ii) the Sn(II) pre-sorbed on alumina solid was isolated and dried, iii) a solution of Tc(VII) was added to this modified alumina, and iv) the yield of Tc removal by Sn(II) pre-sorbed on alumina was analyzed. The resulting Tc-containing solid was analyzed by X-ray absorption spectroscopy (XAS) at the Rossendorf Beamline (ROBL) at the European Synchrotron Radiation Facility in Grenoble (France). Re-oxidation experiments were performed in samples where Tc(VII) reduction by Sn(II) was obtained by different pathways: i) Tc(VII) direct reduction by dissolved Sn(II) (homoreduction) and ii) Tc(VII) reduction by Sn(II) pre-sorbed on alumi

Nanoparticles (NPs) are relevant in medicine, catalysis and environmental remediation. Among them, magnetite (Fe(II)Fe(III)2O4) NPs are especially interesting due to their redox and magnetic properties and their tunability of size and surface properties, which makes them suited for the removal of many redox-active pollutants. Tc is of great concern for the safety assessment of nuclear waste repositories, since 99Tc is a fission product with a long half-life (t1/2 = 2.1∙105 years). Under oxidative conditions Tc forms an anionic species, pertechnetate (Tc(VII)O4-), which is mobile due to its weak interactions with minerals. Under anaerobic conditions, pertechnetate is reduced by reducing agents to Tc(IV), which sorbs on minerals, forms insoluble oxides like TcO2, or is structurally incorporated by stable natural minerals. [1] Previous studies by Yalcintas et al. [2] suggested that Tc(VII) reduction by magnetite resulted in the precipitation and surface adsorption of TcO2-like oligomers at pH 9, i.e. close to the pH of magnetite solubility minimum, while reduction at lower pH of 6 7 resulted in a partial incorporation of Tc(VI) in octahedral Fe sites of magnetite [3]. A working hypothesis was that the incorporation happens only at higher magnetite solubility, while the final retention mechanism remains enigmatic. Thus, our investigations are aimed to carry out a systematic approach covering a wide pH range (3 13), initial Tc concentration ([Tc] = µM-mM) and equilibration time (teq = 1 210 days). The results show that magnetite removes at least 98 % dissolved Tc. To characterize the molecular geometry of the Tc vicinity, mainly X-ray absorption spectroscopy (XAS) has been used. XANES analysis reveals the predominance of Tc(IV) at all evaluated pH values, supporting that reductive Tc immobilization is the main retention mechanism. A detailed EXAFS analysis with different preparation methods (sorption, coprecipitation, Fe(II)-recrystallization) is currently underway to el

Thick steel casks are used for radioactive waste disposal in deep geological repositories. However, it is expected that steel corrodes over time. The corrosion products are expected to form mixed iron oxides, mainly magnetite. After tens of thousands of years, casks may breach allowing leaching of the radiotoxic elements, such as plutonium and technetium, by host rock pore-water. The dissolved radionuclides can then interact with the steel corrosion products and be adsorbed or incorporated into the solids [1]. But since these interaction mechanisms are poorly understood at the atomistic scale, our goal is to better understand them by using computer simulations alongside experiments [2]. In this computational study we identified the dominant low index surfaces on nano-magnetite particles and their termination at the relevant conditions based on Kohn-Sham density functional theory (DFT). This was done using the open source CP2K code and, for highly correlated chemical elements such as iron or plutonium, utilizing the DFT+U method because of the highly localized d and f electrons. The U parameter was determined by comparing experimental cell constants and band gaps to our result [3]. With this revised model, we examined the preferential magnetite crystal orientation plane (111) with different surface terminations as a function of oxygen and water fugacity. Based on our modelling, we found the most stable magnetite (111) surfaces under real repository conditions being Fe_oct1-O-H and Fe_tet1-O-H. Further, we used classical and ab initio MD simulations to investigate the behaviour of radionuclides at the water-magnetite interface in deep geological repositories. [1] Dumas, T.; Fellhauer, D.; Schild, D.; Gaona, X.; Altmaier, M.; Scheinost, A. C. Plutonium Retention Mechanisms by Magnetite Under Anoxic Conditions: Entrapment Versus Sorption. ACS Earth and Space Chemistry 2019, 3 (10), 2197–2206. [2] Yalçıntaş, E.; Scheinost, A. C.; Gaona, X.; Altmaier, M. Systematic XAS St

Thick steel casks are used for radioactive waste disposal in deep geological repositories. However, it is expected that steel corrodes over time. The corrosion products are expected to form mixed iron oxides, mainly magnetite. After tens of thousands of years, casks may breach allowing leaching of the radiotoxic elements, such as plutonium and technetium, by host rock pore-water. The dissolved radionuclides can then interact with the steel corrosion products and be adsorbed or incorporated into the solids [1]. But since these interaction mechanisms are poorly understood at the atomistic scale, our goal is to better understand them by using computer simulations alongside experiments [2]. In this computational study we identified the dominant low index surfaces on nano-magnetite particles and their termination at the relevant conditions based on Kohn-Sham density functional theory (DFT). This was done using the open source CP2K code and, for highly correlated chemical elements such as iron or plutonium, utilizing the DFT+U method because of the highly localized d and f electrons. The U parameter was determined by comparing experimental cell constants and band gaps to our result [3]. With this revised model, we examined the preferential magnetite crystal orientation plane (111) with different surface terminations as a function of oxygen and water fugacity. Based on our modelling, we found the most stable magnetite (111) surfaces under real repository conditions being Fe_oct1-O-H and Fe_tet1-O-H. Further, we used classical and ab initio MD simulations to investigate the behaviour of radionuclides at the water-magnetite interface in deep geological repositories. [1] Dumas, T.; Fellhauer, D.; Schild, D.; Gaona, X.; Altmaier, M.; Scheinost, A. C. Plutonium Retention Mechanisms by Magnetite Under Anoxic Conditions: Entrapment Versus Sorption. ACS Earth and Space Chemistry 2019, 3 (10), 2197–2206. [2] Yalçıntaş, E.; Scheinost, A. C.; Gaona, X.; Altmaier, M. Systematic XAS St

The interactions of long-lived actinides, such as the transuranium element neptunium, with corrosion products in the near-field of a repository are important processes that have to be considered when assessing the safety of a nuclear waste repository. As a main corrosion product of the zircaloy cladding material of spent nuclear fuel rods, zirconia (ZrO₂) constitutes a first possible barrier against the release of radionuclides. To gain a detailed understanding of the surface processes in the Np(V)−zirconia system, a comprehensive, multi-method approach was applied. The Np(V)−ZrO₂ system has been studied on the macroscopic level by conducting pH-dependent batch sorption experiments under varying conditions (ionic strength, Np(V) concentration, and solid-to-liquid ratio (m/V)). In addition, a Np(V) sorption isotherm at pH 6 was collected. The results revealed that Np(V) sorption onto ZrO₂ was affected by pH, Np(V) concentration, and solid-to-liquid ratio. Uptake of Np(V) increased with pH, starting around pH 3 with maximum sorption reached from pH 6. The shift of the sorption edge towards lower pH with increase of the m/V ratio points to the presence of different kinds of sorption sites. This is supported by the Np(V) sorption isotherm results, where the shape suggests strong and weak binding sites. Furthermore, Np(V) uptake was found to be independent of ionic strength and zeta potential measurements revealed a shift towards higher pH values of the isoelectric point of the neat ZrO₂ in the presence of Np(V). Hence, the formation of Np(V) inner-sphere surface complexes is indicated. Molecular information about the surface species were obtained by Extended X-ray Absorption Fine Structure Spectroscopy (EXAFS) and in situ Infrared (IR) Spectroscopy, revealing the predominant formation of inner-sphere Np(V) surface complexes. A short Np−Zr distance derived from EXAFS suggests the presence of Np(V) bidentate inner-sphere complexes on the ZrO₂ surface. These information ob

Nanoparticles (NPs) are relevant in medicine, catalysis and environmental remediation. Among them, magnetite (Fe(II)Fe(III)2O4) NPs are especially interesting due to their redox and magnetic properties and their tunability of size and surface properties, which makes them suited for the removal of many redox-active pollutants. Tc is of great concern for the safety assessment of nuclear waste repositories, since 99Tc is a fission product with a long half-life (t1/2 = 2.1∙105 years). Under oxidative conditions Tc forms an anionic species, pertechnetate (Tc(VII)O4-), which is mobile due to its weak interactions with minerals. Under anaerobic conditions, pertechnetate is reduced by reducing agents to Tc(IV), which sorbs on minerals, forms insoluble oxides like TcO2, or is structurally incorporated by stable natural minerals. [1] Previous studies by Yalcintas et al. [2] suggested that Tc(VII) reduction by magnetite resulted in the precipitation and surface adsorption of TcO2-like oligomers at pH 9, i.e. close to the pH of magnetite solubility minimum, while reduction at lower pH of 6 7 resulted in a partial incorporation of Tc(VI) in octahedral Fe sites of magnetite [3]. A working hypothesis was that the incorporation happens only at higher magnetite solubility, while the final retention mechanism remains enigmatic. Thus, our investigations are aimed to carry out a systematic approach covering a wide pH range (3 13), initial Tc concentration ([Tc] = µM-mM) and equilibration time (teq = 1 210 days). The results show that magnetite removes at least 98 % dissolved Tc. To characterize the molecular geometry of the Tc vicinity, mainly X-ray absorption spectroscopy (XAS) has been used. XANES analysis reveals the predominance of Tc(IV) at all evaluated pH values, supporting that reductive Tc immobilization is the main retention mechanism. A detailed EXAFS analysis with different preparation methods (sorption, coprecipitation, Fe(II)-recrystallization) is currently underway to el