X-ray absorption spectroscopy studies of reactions of technetium, uranium and neptunium with mackinawite

A Critical Look at Colloid Generation, Stability, and Transport in Redox-Dynamic Environments: Challenges and Perspectives

Colloid generation, stability, and transport are important processes that can significantly influence the fate and transport of nutrients and contaminants in environmental systems. Here, we critically review the existing literature on colloids in redox-dynamic environments and summarize the current state of knowledge regarding the mechanisms of colloid generation and the chemical controls over colloidal behavior in such environments. We also identify critical gaps, such as the lack of universally accepted cross-discipline definition and modeling infrastructure that hamper an in-depth understanding of colloid generation, behavior, and transport potential. We propose to go beyond a size-based operational definition of colloids and consider the functional differences between colloids and dissolved species. We argue that to predict colloidal transport in redox-dynamic environments, more empirical data are needed to parametrize and validate models. We propose that colloids are critical components of element budgets in redox-dynamic systems and must urgently be considered in field as well as lab experiments and reactive transport models. We intend to bring further clarity and openness in reporting colloidal measurements and fate to improve consistency. Additionally, we suggest a methodological toolbox for examining impacts of redox dynamics on colloids in field and lab experiments.

Review of Theoretical and Computational Studies of Bulk and Single Atom Catalysts for H2 S Catalytic Conversion

Catalytic conversion of hydrogen sulfide (H2 S) plays a vital role in environmental protection and safety production. In this review, recent theoretical advances for catalytic conversion of H2 S are systemically summarized. Firstly, different mechanisms of catalytic conversion of H2 S are elucidated. Secondly, theoretical studies of catalytic conversion of H2 S on surfaces of metals, metal compounds, and single-atom catalysts (SACs) are systematically reviewed. In the meantime, various strategies which have been adopted to improve the catalytic performance of catalysts in the catalytic conversion of H2 S are also reviewed, mainly including facet morphology control, doped heteroatoms, metal deposition, and defective engineering. Finally, new directions of catalytic conversion of H2 S are proposed and potential strategies to further promote conversion of H2 S are also suggested: including SACs, double atom catalysts (DACs), single cluster catalysts (SCCs), frustrated Lewis pairs (FLPs), etc. The present comprehensive review can provide an insight for the future development of new catalysts for the catalytic conversion of H2 S.

Degradation of Nitrobenzene by Mackinawite through a Sequential Two-Step Reduction and Oxidation Process

Mackinawite (FeS) has gained increasing interest due to its potential application in contaminant removal by either reduction or oxidation processes. This study further demonstrated the efficiency of FeS in degrading nitrobenzene (ArNO2) via a sequential two-step reduction and oxidation process under neutral conditions. In the reduction stage, FeS rapidly reduced ArNO2 to aniline (ArNH2), with nitrosobenzene (ArNO) and phenylhydroxylamine (ArNHOH) serving as the intermediates. X-ray photoelectron spectroscopy (XPS) analysis indicated that both Fe(II) and S(II) in FeS contributed electrons to the reduction of ArNO2. In the subsequent oxidation stage with oxygen, by addition of 0.5 mM tripolyphosphate (TPP), ArNH2 generated in the reduction process could be effectively oxidized to aminophenols by hydroxyl radicals (•OH), which would undergo eventual mineralization via ring-cleavage reactions. TPP exerted a favorable role in enhancing •OH production for ArNH2 degradation by promoting the formation of the dissolved Fe(II)-TPP complex, thus enhancing the homogeneous Fenton reaction. Additionally, TPP adsorption inhibited the surface oxidation reactivity of FeS due to the change of Fe(II) coordination. Finally, the effective degradation of ArNO2 by FeS in actual groundwater was demonstrated by using this sequential reduction and oxidation approach. These research findings provide a theoretical basis for a new FeS-based remediation approach, offering an alternative way for comprehensive removal of ArNO2.

Removal of radionuclide 99Tc from aqueous solution by various adsorbents: A review

Technetium isotope 99Tc is a main radioactive waste produced in the process of nuclear reaction, which has the characteristics of long half-life and strong environmental mobility, and can be bio-accumulated in organisms, resulting in serious threat to human health and ecosystem. Adsorption method is widely used in the field of removing radionuclides from water due to the advantages of high treatment rate, simple and mature industrial application. In this review paper, the recent advances in research and application of various adsorption materials for 99Tc pollution treatment were summarized and analyzed for the first time, including inorganic adsorbents, such as activated carbon, zero-valent iron, metallic minerals, clay minerals, layered double hydroxides (LDHs), tin-based materials, and sulfur-based materials; organic adsorbents, such as porous organic polymers (POPs), covalent-organic frameworks (COFs), metal-organic frameworks (MOFs), and ion exchange resin; and biological adsorbents, such as biopolymers (chitosan, cellulose, alginate), and microbial cells. The performance characteristics and the adsorption kinetics and isotherms of various adsorption materials were discussed. This review could deepen the understanding of the adsorptive removal of 99Tc from aqueous solution, and provide a reference for the future research in this field.

Consider the Anoxic Microsite: Acknowledging and Appreciating Spatiotemporal Redox Heterogeneity in Soils and Sediments

Reduction-oxidation (redox) reactions underlie essentially all biogeochemical cycles. Like most soil properties and processes, redox is spatiotemporally heterogeneous. However, unlike other soil features, redox heterogeneity has yet to be incorporated into mainstream conceptualizations of soil biogeochemistry. Anoxic microsites, the defining feature of redox heterogeneity in bulk oxic soils and sediments, are zones of oxygen depletion in otherwise oxic environments. In this review, we suggest that anoxic microsites represent a critical component of soil function and that appreciating anoxic microsites promises to advance our understanding of soil and sediment biogeochemistry. In sections 1 and 2, we define anoxic microsites and highlight their dynamic properties, specifically anoxic microsite distribution, redox gradient magnitude, and temporality. In section 3, we describe the influence of anoxic microsites on several key elemental cycles, organic carbon, nitrogen, iron, manganese, and sulfur. In section 4, we evaluate methods for identifying and characterizing anoxic microsites, and in section 5, we highlight past and current approaches to modeling anoxic microsites. Finally, in section 6, we suggest steps for incorporating anoxic microsites and redox heterogeneities more broadly into our understanding of soils and sediments.

Sulfidation and Reoxidation of U(VI)-Incorporated Goethite: Implications for U Retention during Sub-Surface Redox Cycling

Over 60 years of nuclear activity have resulted in a global legacy of contaminated land and radioactive waste. Uranium (U) is a significant component of this legacy and is present in radioactive wastes and at many contaminated sites. U-incorporated iron (oxyhydr)oxides may provide a long-term barrier to U migration in the environment. However, reductive dissolution of iron (oxyhydr)oxides can occur on reaction with aqueous sulfide (sulfidation), a common environmental species, due to the microbial reduction of sulfate. In this work, U(VI)-goethite was initially reacted with aqueous sulfide, followed by a reoxidation reaction, to further understand the long-term fate of U species under fluctuating environmental conditions. Over the first day of sulfidation, a transient release of aqueous U was observed, likely due to intermediate uranyl(VI)-persulfide species. Despite this, overall U was retained in the solid phase, with the formation of nanocrystalline U(IV)O₂ in the sulfidized system along with a persistent U(V) component. On reoxidation, U was associated with an iron (oxyhydr)oxide phase either as an adsorbed uranyl (approximately 65%) or an incorporated U (35%) species. These findings support the overarching concept of iron (oxyhydr)oxides acting as a barrier to U migration in the environment, even under fluctuating redox conditions.

Sulfidation of ferric (hydr)oxides and its implication on contaminants transformation: a review

Rapid industrialization and urbanization have resulted in elevated concentrations of contaminants in the groundwaters and subsurface soils, posing a growing hazard to humans and ecosystems. The transformation of most contaminants is closely linked to the mineralogy of ferric (hydr)oxides. Sulfidation of ferric (hydr)oxides is one of the most significant biogeochemical reactions in the anoxic environments, causing reductive dissolution and recrystallization of ferric (hydr)oxides and further affecting the transformation of iron-associated contaminants. This paper provides a comprehensive review on the sulfidation process of ferric (hydr)oxides and the transformation of relevant contaminants. This review presents detailed reaction mechanisms between ferric (hydr)oxides and dissolved sulfide, and elucidates the factors (e.g. crystallinity of ferric (hydr)oxides, the ratio of sulfide concentration to the surface area concentration of ferric (hydr)oxides) that control the formation of surface associated Fe(II), iron sulfide minerals, as well as transformation of secondary minerals. Then, we summarized the transformation mechanisms of a variety of typical environmentally relevant contaminants existing in groundwater and subsurface soils, including heavy metals, metal(loid) oxyanions (arsenic, antimony, chromium), radionuclides (uranium, technetium), organic contaminants and phosphate/nitrate species. The general mechanisms of contaminant transformation involve a combination of release, reduction and re-adsorption/incorporation processes, the specific pathway of which is highly dependent on the properties of the contaminant itself and the extent of sulfidation. Moreover, the challenge of extending our knowledge towards in situ remediation, as well as further research needs are identified.

Neptunium and Uranium Interactions with Environmentally and Industrially Relevant Iron Minerals

Neptunium (237Np) is an important radionuclide in the nuclear fuel cycle in areas such as effluent treatment and the geodisposal of radioactive waste. Due to neptunium’s redox sensitivity and its tendency to adsorb strongly to mineral phases, such as iron oxides/sulfides, the environmental mobility of Np can be altered significantly by a wide variety of chemical processes. Here, Np interactions with key iron minerals, ferrihydrite (Fe5O8H·4H2O), goethite (α-FeOOH), and mackinawite (FeS), are investigated using X-ray Absorption Spectroscopy (XAS) in order to explore the mobility of neptunyl(V) (Np(V)O2+) moiety in environmental (radioactive waste disposal) and industrial (effluent treatment plant) scenarios. Analysis of the Np LIII-edge X-ray Absorption Near-Edge Structure (XANES) and Extended X-ray Absorption Fine Structure (EXAFS) showed that upon exposure to goethite and ferrihydrite, Np(V) adsorbed to the surface, likely as an inner-sphere complex. Interestingly, analysis showed that only the first two shells (Oax and Oeq) of the EXAFS could be modelled with a high degree of confidence, and there was no clear indication of Fe or carbonate in the fits. When Np(V)O2+ was added to a mackinawite-containing system, Np(V) was reduced to Np(IV) and formed a nanocrystalline Np(IV)O2 solid. An analogous experiment was also performed with U(VI)O22+, and a similar reduction was observed, with U(VI) being reduced to nanocrystalline uraninite (U(IV)O2). These results highlight that Np(V) may undergo a variety of speciation changes in environmental and engineered systems whilst also highlighting the need for multi-technique approaches to speciation determination for actinyl (for example, Np(V)O2+) species.

Sulfidation of magnetite with incorporated uranium

Uranium (U) is a radionuclide of key environmental interest due its abundance by mass within radioactive waste and presence in contaminated land scenarios. Ubiquitously present iron (oxyhydr)oxide mineral phases, such as (nano)magnetite, have been identified as candidates for immobilisation of U via incorporation into the mineral structure. Studies of how biogeochemical processes, such as sulfidation from the presence of sulfate-reducing bacteria, may affect iron (oxyhydr)oxides and impact radionuclide mobility are important in order to underpin geological disposal of radioactive waste and manage radioactively contaminated land. Here, this study utilised a highly controlled abiotic method for sulfidation of U(V) incorporated into nanomagnetite to determine the fate and speciation of U. Upon sulfidation, transient release of U into solution occurred (∼8.6% total U) for up to 3 days, despite the highly reducing conditions. As the system evolved, lepidocrocite was observed to form over a period of days to weeks. After 10 months, XAS and geochemical data showed all U was partitioned to the solid phase, as both nanoparticulate uraninite (U(IV)O₂) and a percentage of retained U(V). Further EXAFS analysis showed incorporation of the residual U(V) fraction into an iron (oxyhydr)oxide mineral phase, likely nanomagnetite or lepidocrocite. Overall, these results provide new insights into the stability of U(V) incorporated iron (oxyhydr)oxides during sulfidation, confirming the longer term retention of U in the solid phase under complex, environmentally relevant conditions.

Exploring the Evolution Mechanism of Sulfur Vacancies by Investigating the Role of Vacancy Defects in the Interaction between H2S and the FeS(001) Surface

Vacancy defects are inherent point defects in materials. In this study, we investigate the role of Fe vacancy (V_Fe) and S vacancy (V_S) in the interaction (adsorption, dissociation, and diffusion) between H₂S and the FeS(001) surface using the dispersion-corrected density functional theory (DFT-D2) method. V_Fe promotes the dissociation of H₂S but slightly hinders the dissociation of HS. Compared with the perfect surface (2.08 and 1.15 eV), the dissociation energy barrier of H₂S is reduced to 1.56 eV, and HS is increased to 1.25 eV. Meanwhile, S vacancy (V_S) significantly facilitates the adsorption and dissociation of H₂S, which not only reduces the dissociation energy barriers of H₂S and HS to 0.07 and 0.11 eV, respectively, but also changes the dissociation process of H₂S from an endothermic process to a spontaneous exothermic one. Furthermore, V_Fe can promote the hydrogen (H) diffusion process from the surface into the matrix and reduce the energy barrier of the rate-limiting step from 1.12 to 0.26 eV. But it is very hard for H atoms gathered around V_S to diffuse into the matrix, especially the energy barrier of the rate-limiting step increases to 1.89 eV. Finally, we propose that V_S on the FeS(001) surface is intensely difficult to form and exist in the actual environment through the calculation results.

Technetium immobilization by chukanovite and its oxidative transformation products: Neural network analysis of EXAFS spectra

The uptake of the fission product technetium (Tc) by chukanovite, an Fe^II hydroxy carbonate mineral formed as a carbon steel corrosion product in anoxic and carbonate-rich environments, was studied under anoxic, alkaline to hyperalkaline conditions representative for nuclear waste repositories in deep geological formations with cement-based inner linings. The retention potential of chukanovite towards Tc^VII is high in the pH range 7.8 to 12.6, evidenced by high solid-water distribution coefficients, log R_d ~ 6, and independent of ionic strength (0.1 or 1 M NaCl). Using Tc K-edge X-ray absorption spectroscopy (XAS) two series of samples were investigated, Tc chukanovite sorption samples and coprecipitates, prepared with varying Tc loadings, pH values and contact times. From the resulting 37 XAS spectra, spectral endmembers and their dependence on chemical parameters were derived by self-organizing (Kohonen) maps (SOM), a neural network-based approach of machine learning. X-ray absorption near-edge structure (XANES) data confirmed the complete reduction of Tc^VII to Tc^IV by chukanovite under all experimental conditions. Consistent with mineralogical phases identified by X-ray diffraction (XRD), SOM analysis of the extended X-ray absorption fine-structure (EXAFS) spectra revealed the presence of three species in the sorption samples, the speciation predominately controlled by pH: Between pH 7.8 and 11.8, TcO₂-dimers form inner-sphere sorption complexes at the surface of the initial chukanovite as well as on the surface of secondary magnetite formed due to redox reaction. At pH ≥ 11.9, Tc^IV is incorporated in a mixed, chukanovite-like, Fe/Tc hydroxy carbonate precipitate. The same species formed when using the coprecipitation approach. Reoxidation of sorption samples resulted in a small remobilization of Tc, demonstrating that both the original chukanovite mineral and its oxidative transformation products, magnetite and goethite, contribute to the immobilization of Tc in the long term, thus strongly attenuating its environmental transport.

Technetium immobilization by materials through sorption and redox-driven processes: A literature review

The objective of this review is to evaluate materials for use as a barrier or other deployed technology to treat technetium-99 (Tc) in the subsurface. To achieve this, Tc interactions with different materials are considered within the context of remediation strategies. Several naturally occurring materials are considered for Tc immobilization, including iron oxides and low solubility sulfide phases. Synthetic materials are also considered, and include tin-based materials, sorbents (resins, activated carbon, modified clays), layered double hydroxides, metal organic frameworks, cationic polymeric networks and aerogels. All of the materials were evaluated for their potential in-situ and ex-situ performance with respect to long-term Tc uptake and immobilization, environmental impacts and deployability. Other factors such as the technology maturity, cost and availability were also considered. Given the difficulty of evaluating materials under different experimental conditions (e.g., solution chemistry, redox conditions, solution to solid ratio, Tc concentration etc.), a subset of these materials will be selected, on the basis of this review, for subsequent standardized batch loading tests.

Oxidative Degradation of Organic Contaminants by FeS in the Presence of O2

Reductive transformation of organic contaminants by FeS in anoxic environments has been documented previously, whereas the transformation in oxic environments remains poorly understood. Here we show that phenol can be efficiently oxidized in oxic FeS suspension at circumneutral pH value. We found that hydroxyl radicals (•OH) were the predominant reactive oxidant and that a higher O₂ content accelerated phenol degradation. Phenol oxidation depended on •OH production and utilization efficiency, i.e., phenol degraded per •OH produced. Low FeS contents (≤1 g/L) produced less •OH but higher utilization efficiency, while high contents produced more •OH but lower utilization efficiency. Consequently, the most favorable conditions for phenol oxidation occurred during the long-term interaction between dissolved O₂ and low levels of FeS (i.e., ≤1 g/L). Mössbauer spectroscopy suggests that FeS oxidation to lepidocrocite initially produced an intermediate Fe(II) phase that could be explained by the apparent preferential oxidation of structural S(-II) relative to Fe(II), rendering a higher initial •OH yield upon unit of Fe(II) oxidation. Trichloroethylene can be also oxidized under similar conditions. Our results demonstrate that oxidative degradation of organic contaminants during the oxygenation of FeS can be a significant but currently underestimated pathway in both natural and engineered systems.

New Insights into 99Tc(VII) Removal by Pyrite: A Spectroscopic Approach

⁹⁹Tc(VII) uptake by synthetic pure pyrite at 21 °C was studied in a wide pH range from 3.50 to 10.50 using batch experiments combined with scanning electron microscopy, X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS), and Raman microscopy. We found that pyrite removes Tc quantitatively from solution (log K_d = 5.0 ± 0.1) within 1 day at pH ≥ 5.50 ± 0.08. At pH < 5.50 ± 0.08, the uptake process is slower, leading to 98% Tc removal (log K_d = 4.5 ± 0.1) after 35 days. The slower Tc uptake was explained by higher pyrite solubility under acidic conditions. After 2 months in contact with oxygen at pH 6.00 ± 0.07 and 10.00 ± 0.04, Tc was neither reoxidized nor redissolved. XAS showed that the uptake mechanism involves the reduction from Tc(VII) to Tc(IV) and subsequent inner-sphere complexation of Tc(IV)-Tc(IV) dimers onto a Fe oxide like hematite at pH 6.00 ± 0.07, and Tc(IV) incorporation into magnetite via Fe(III) substitution at pH 10.00 ± 0.04. Calculations of Fe speciation under the experimental conditions predict the formation of hematite at pH < 7.50 and magnetite at pH > 7.50, explaining the formation of the two different Tc species depending on the pH. XPS spectra showed the formation of TcS_x at pH 10.00 ± 0.04, being a small fraction of a surface complex, potentially a transient phase in the total redox process.

Formation of a U(VI)-Persulfide Complex during Environmentally Relevant Sulfidation of Iron (Oxyhydr)oxides

Uranium is a risk-driving radionuclide in both radioactive waste disposal and contaminated land scenarios. In these environments, a range of biogeochemical processes can occur, including sulfate reduction, which can induce sulfidation of iron (oxyhydr)oxide mineral phases. During sulfidation, labile U(VI) is known to reduce to relatively immobile U(IV); however, the detailed mechanisms of the changes in U speciation during these biogeochemical reactions are poorly constrained. Here, we performed highly controlled sulfidation experiments at pH 7 and pH 9.5 on U(VI) adsorbed to ferrihydrite and investigated the system using geochemical analyses, X-ray absorption spectroscopy (XAS), and computational modeling. Analysis of the XAS data indicated the formation of a novel, transient U(VI)-persulfide complex as an intermediate species during the sulfidation reaction, concomitant with the transient release of uranium to the solution. Extended X-ray absorption fine structure (EXAFS) modeling showed that a persulfide ligand was coordinated in the equatorial plane of the uranyl moiety, and formation of this species was supported by computational modeling. The final speciation of U was nanoparticulate U(IV) uraninite, and this phase was evident at 2 days at pH 7 and 1 year at pH 9.5. Our identification of a new, labile U(VI)-persulfide species under environmentally relevant conditions may have implications for U mobility in sulfidic environments pertinent to radioactive waste disposal and contaminated land scenarios.

Metaschoepite Dissolution in Sediment Column Systems-Implications for Uranium Speciation and Transport

Metaschoepite is commonly found in U-contaminated environments and metaschoepite-bearing wastes may be managed via shallow or deep disposal. Understanding metaschoepite dissolution and tracking the fate of any liberated U is thus important. Here, discrete horizons of metaschoepite (UO₃·nH₂O) particles were emplaced in flowing sediment/groundwater columns representative of the UK Sellafield Ltd. site. The column systems either remained oxic or became anoxic due to electron donor additions, and the columns were sacrificed after 6- and 12-months for analysis. Solution chemistry, extractions, and bulk and micro/nano-focus X-ray spectroscopies were used to track changes in U distribution and behavior. In the oxic columns, U migration was extensive, with UO₂²⁺ identified in effluents after 6-months of reaction using fluorescence spectroscopy. Unusually, in the electron-donor amended columns, during microbially mediated sulfate reduction, significant amounts of UO₂-like colloids (>60% of the added U) were found in the effluents using TEM. XAS analysis of the U remaining associated with the reduced sediments confirmed the presence of trace U(VI), noncrystalline U(IV), and biogenic UO₂, with UO₂ becoming more dominant with time. This study highlights the potential for U(IV) colloid production from U(VI) solids under reducing conditions and the complexity of U biogeochemistry in dynamic systems.

Using porous iron composite (PIC) material to immobilize rhenium as an analogue for technetium

Technetium (⁹⁹Tc), a uranium-235 (²³⁵U) and plutonium-239 (²³⁹Pu) fission product, is a primary risk driver in low level radioactive liquid waste at U.S. Department of Energy sites. Previous studies have shown success in using Zero Valent Iron (ZVI) to chemically reduce and immobilize redox sensitive groundwater contaminants. Batch and column experiments were performed to assess the ability of a novel porous iron composite material (PIC) to immobilize Tc(VII) in comparison with two commercial Fe oxide sorbents and reagent grade ZVI in the presence and absence of NO₃^-, a competing oxidized species that is often found in high concentrations in liquid nuclear waste. Perrhenate (ReO₄^-) was used as a non-radioactive chemical analogue for pertechnetate (TcO₄^-) under both oxic and anoxic test conditions. The PIC powder was the most effective at immobilizing Re(VII) under all batch test conditions. The presence of nitrate (NO₃^-) slowed the removal of ReO₄^- from solution, presumably through chemical reduction and precipitation. Even so, the PIC and ZVI were effective at removing both Re(VII) and NO₃^- completely from solution. Nitrate was reduced to NH₃ with very little nitrite (NO₂^-) buildup during equilibration. Significant Re immobilization was observed in the column tests containing PIC sorbent, even though inlet solutions were in equilibrium with O₂. The presence of NO₃^- hastened Re breakthrough, while NO₃^- reduction to NH₃ was observed. The results suggest that PIC and ZVI would be the most effective at the removal of TcO₄^- from contaminated groundwater sites.

The impact of iron nanoparticles on technetium-contaminated groundwater and sediment microbial communities

Iron nanoparticles are a promising new technology to treat contaminated groundwater, particularly as they can be engineered to optimise their transport properties. Technetium is a common contaminant at nuclear sites and can be reductively scavenged from groundwater by iron(II). Here we investigated the potential for a range of optimised iron nanoparticles to remove technetium from contaminated groundwater, and groundwater/sediment systems. Nano zero-valent iron and Carbo-iron stimulated the development of anoxic conditions while generating Fe(II) which reduced soluble Tc(VII) to sparingly soluble Tc(IV). Similar results were observed for Fe(II)-bearing biomagnetite, albeit at a slower rate. Tc(VII) remained in solution in the presence of the Fe(III) mineral nano-goethite, until acetate was added to stimulate microbial Fe(III)-reduction after which Tc(VII) concentrations decreased concomitant with Fe(II) ingrowth. The addition of iron nanoparticles to sediment microcosms caused an increase in the relative abundance of Firmicutes, consistent with fermentative/anoxic metabolisms. Residual bacteria from the synthesis of the biomagnetite nanoparticles were out-competed by the sediment microbial community. Overall the results showed that iron nanoparticles were highly effective in removing Tc(VII) from groundwater in sediment systems, and generated sustained anoxic conditions via the stimulation of beneficial microbial processes including Fe(III)-reduction and sulfate reduction.

Technetium and iodine aqueous species immobilization and transformations in the presence of strong reductants and calcite-forming solutions: Remedial action implications

At the Hanford Site in southeastern Washington, discharge of radionuclide laden liquid wastes resulted in vadose zone contamination, providing a continuous source of these contaminants to groundwater. The presence of multiple contaminants (i.e., ⁹⁹Tc and ¹²⁹I) increases the complexity of finding viable remediation technologies to sequester contaminants in situ and protect groundwater. Although previous studies have shown the efficiency of zero valent iron (ZVI) and sulfur modified iron (SMI) in reducing mobile Tc(VII) to immobile Tc(IV) and iodate incorporation into calcite, the coupled effects from simultaneously using these remedial technologies have not been previously studied. In this first-of-a-kind laboratory study, we used reductants (ZVI or SMI) and calcite-forming solutions to simultaneously remove aqueous Tc(VII) and iodate via reduction and incorporation, respectively. The results confirmed that Tc(VII) was rapidly removed from the aqueous phase via reduction to Tc(IV). Most of the aqueous iodate was transformed to iodide faster than incorporation into calcite occurred, and therefore the I remained in the aqueous phase. These results suggested that this remedial pathway is not efficient in immobilizing iodate when reductants are present. Other experiments suggested that iodate removal via calcite precipitation should occur prior to adding reductants for Tc(VII) removal. When microbes were included in the tests, there was no negative impact on the microbial population but changes in the makeup of the microbial community were observed. These microbial community changes may have an impact on remediation efforts in the long-term that could not be seen in a short-term study. The results underscore the importance of identifying interactions between natural attenuation pathways and remediation technologies that only target individual contaminants.

Toxicity and Transcriptome Sequencing (RNA-seq) Analyses of Adult Zebrafish in Response to Exposure Carboxymethyl Cellulose Stabilized Iron Sulfide Nanoparticles

Increasing utilization of stabilized iron sulfides (FeS) nanoparticles implies an elevated release of the materials into the environment. To understand potential impacts and underlying mechanisms of nanoparticle-induced stress, we used the transcriptome sequencing (RNA-seq) technique to characterize the transcriptomes from adult zebrafish exposed to 10 mg/L carboxymethyl cellulose (CMC) stabilized FeS nanoparticles for 96 h, demonstrating striking differences in the gene expression profiles in liver. The exposure caused significant expression alterations in genes related to immune and inflammatory responses, detoxification, oxidative stress and DNA damage/repair. The complement and coagulation cascades Kyoto encyclopedia of genes and genomes (KEGG) pathway was found significantly up-regulated under nanoparticle exposure. The quantitative real-time polymerase chain reaction using twelve genes confirmed the RNA-seq results. We identified several candidate genes commonly regulated in liver, which may serve as gene indicators when exposed to the nanoparticles. Hepatic inflammation was further confirmed by histological observation of pyknotic nuclei, and vacuole formation upon exposure. Tissue accumulation tests showed a 2.2 times higher iron concentration in the fish tissue upon exposure. This study provides preliminary mechanistic insights into potential toxic effects of organic matter stabilized FeS nanoparticles, which will improve our understanding of the genotoxicity caused by stabilized nanoparticles.

Reductive immobilization of pertechnetate in soil and groundwater using synthetic pyrite nanoparticles

Radioactive technetium (⁹⁹Tc) is of intense concern because of its toxicity and high mobility in the environment. Reduction of Tc(VII) to Tc(IV) decreases the mobility and availability of technetium in soil and groundwater. In this study, pyrite nanoparticles (FeS₂) were synthesized, characterized and tested for immobilizing/removing ⁹⁹Tc(VII) in soil and groundwater through batch and column experiments. Influences of particle dosage, dissolved organic matter (DOM), and pH on the reductive immobilization kinetics were examined. At a dosage of 0.28 g/L as Fe, the pyrite nanoparticles were able to rapidly and completely remove 4.88 × 10^-7 M of Tc(VII) by converting it to insoluble Tc(IV), with a retarded first-order rate constant of 0.30 h^-1. The presence of high concentrations of DOM only moderately inhibited the reduction effectiveness, and acidic pH was more favorable for Tc(VII) reduction. Column experiments showed that embedding a 0.8 cm pyrite layer of the material in a soil bed, simulating a permeable reactive barrier, was able to retard technetium transport 710 times more than a model sandy soil. The results demonstrated that the pyrite particles may serve as a long-lasting reactive material to remediate Tc-contaminated soil, groundwater and solid wastes.

Fate of Adsorbed U(VI) during Sulfidization of Lepidocrocite and Hematite

The impact on U(VI) adsorbed to lepidocrocite (γ-FeOOH) and hematite (α-Fe₂O₃) was assessed when exposed to aqueous sulfide (S(-II)_aq) at pH 8.0. With both minerals, competition between S(-II) and U(VI) for surface sites caused instantaneous release of adsorbed U(VI). Compared to lepidocrocite, consumption of S(-II)_aq proceeded slower with hematite, but yielded maximum dissolved U concentrations that were more than 10 times higher, representing about one-third of the initially adsorbed U. Prolonged presence of S(-II)_aq in experiments with hematite in combination with a larger release of adsorbed U(VI), enhanced the reduction of U(VI): after 24 h of reaction about 60-70% of U was in the form of U(IV), much higher than the 25% detected in the lepidocrocite suspensions. X-ray absorption spectra indicated that U(IV) in both hematite and lepidocrocite suspensions was not in the form of uraninite (UO₂). Upon exposure to oxygen only part of U(IV) reoxidized, suggesting that monomeric U(IV) might have become incorporated in newly formed iron precipitates. Hence, sulfidization of Fe oxides can have diverse consequences for U mobility: in short-term, desorption of U(VI) increases U mobility, while reduction to U(IV) and its possible incorporation in Fe transformation products may lead to long-term U immobilization.

Oxidizing Impact Induced by Mackinawite (FeS) Nanoparticles at Oxic Conditions due to Production of Hydroxyl Radicals

Mackinawite (FeS) nanoparticles have been extensively tested for reducing contaminants under anoxic conditions, while the oxidizing impact induced by FeS under oxic conditions has been largely underestimated. In light of previous findings that hydroxyl radicals (·OH) can be produced from oxygenation of sediment Fe(II), herein we revealed that ·OH can be produced efficiently from FeS oxygenation at circumneutral conditions, yielding 84.7 μmol ·OH per g FeS. Much more ·OH was produced from the oxygenation of FeS compared with siderite, pyrite, and zerovalent iron nanoparticles under the same conditions. The oxidation of FeS was a surface-mediated process, in which O₂ was transformed by the structural Fe(II) on FeS surface to ·OH with the generation of H₂O₂ intermediate. A small proportion of Fe(II) was regenerated from the reduction of Fe(III) by FeS and S(-II), but this proportion did not significantly contribute to ·OH production. We further validated that the ·OH produced from FeS oxygenation considerably contributed to the oxidation of arsenic. As the change of redox conditions from anoxic to oxic is common in both natural and artificial processes, our findings suggest that the oxidizing impact induced by FeS at oxic conditions should be concerned due to ·OH production.

Application of iron sulfide particles for groundwater and soil remediation: A review

Rapid industrialization and urbanization have resulted in elevated concentrations of hazardous inorganic and organic contaminants in groundwater and soil, which has become a paramount concern to the environment and the public health. In recent years, iron sulfide (FeS), a major constituent of acid-volatile sulfides, has elicited extensive interests in environmental remediation due to its ubiquitous presence and high treatment efficiency in anoxic environment. This paper provides a comprehensive review on recent advances in: (1) synthesis of FeS particles (including nanoscale FeS); and (2) reactivity of FeS towards a variety of common environmental contaminants in groundwater and soil over extended periods of time, namely, heavy metals (Hg(II), Cu(II), Pb(II), and Cr(VI)), oxyanions (arsenite, arsenate, selenite, and selenate), radionuclides (e.g., uranium (U) and neptunium (Np)), chlorinated organic compounds (e.g., trichloroethane, trichloroethylene, and p-chloroaniline), nitroaromatic compounds, and polychlorinated biphenyls. Different physiochemical and biological methods for preparing FeS with desired particle size, structure, and surface properties are discussed. Reaction principles and removal effectiveness/constraints are discussed in details. Special attention is placed to the application of nanoscale FeS particles because of their unique properties, such as small particle size, large specific surface area, high surface reactivity, and soil deliverability in the subsurface. Moreover, current knowledge gaps and further research needs are identified.

4-Component relativistic calculations of L3ionization and excitations for the isoelectronic species UO22+, OUN+and UN2

4-Component relativistic calculations explore uranium 2p_3/2ionization and excitation in the isoelectronic series UO₂²⁺, OUN⁺and UN₂.

Systematic XAS study on the reduction and uptake of Tc by magnetite and mackinawite

The mechanisms for the reduction and uptake of Tc by magnetite (Fe₃O₄) and mackinawite (FeS) are investigated using X-ray absorption spectroscopy (XANES and EXAFS), in combination with thermodynamic calculations of the Tc/Fe systems and accurate characterization of the solution properties (pH_m, pe, [Tc]).

Technetium Incorporation into Goethite (α-FeOOH): An Atomic-Scale Investigation

During the processing of low-activity radioactive waste to generate solid waste forms (e.g., glass), technetium-99 (Tc) is of concern because of its volatility. A variety of materials are under consideration to capture Tc from waste streams, including the iron oxyhydroxide, goethite (α-FeOOH), which was experimentally shown to sequester Tc(IV). This material could ultimately be incorporated into glass or alternative low-temperature waste form matrices. However, questions remain regarding the incorporation mechanism for Tc(IV) in goethite, which has implications for predicting the long-term stability of Tc in waste forms under changing conditions. Here, quantum-mechanical calculations were used to evaluate the energy of five different charge-compensated Tc(IV) incorporation scenarios in goethite. The two most stable incorporation mechanisms involve direct substitution of Tc(IV) onto Fe(III) lattice sites and charge balancing either by removing one nearby H(+) (i.e., within 5 Å) or by creating an Fe(III) vacancy when substituting 3 Tc(IV) for 4 Fe(III), with the former being preferred over the latter relative to gas-phase ions. When corrections for hydrated references phases are applied, the Fe(III)-vacancy mechanism becomes more energetically competitive. Calculated incorporation energies and optimized bond lengths are presented. Proton movement is observed to satisfy undercoordinated bonds surrounding Fe(III)-vacancies in the goethite structure.

The surface chemistry of NO(x) on mackinawite (FeS) surfaces: a DFT-D2 study

We present density functional theory calculations with a correction for the long-range interactions (DFT-D2) of the bulk and surfaces of mackinawite (FeS), and subsequent adsorption and dissociation of NO(x) gases (nitrogen monoxide (NO) and nitrogen dioxide (NO2)). Our results show that these environmentally important molecules interact very weakly with the energetically most stable (001) surface, but adsorb relatively strongly onto the FeS(011), (100) and (111) surfaces, preferentially at Fe sites via charge donation from these surface species. The NOx species exhibit a variety of adsorption geometries, with the most favourable for NO being the monodentate Fe-NO configuration, whereas NO2 is calculated to form a bidentate Fe-NOO-Fe configuration. From our calculated thermochemical energy and activation energy barriers for the direct dissociation of NO and NO2 on the FeS surfaces, we show that NO prefers molecular adsorption, while dissociative adsorption, i.e. NO2 (ads) → [NO(ads) + O(ads)] is preferred over molecular adsorption for NO2 onto the mackinawite surfaces. However, the calculated high activation barriers for the further dissociation of the second N-O bond to produce either [N(ads) and 2O(ads)] or [N(ads) and O2(ads)] suggest that complete dissociation of NO2 is unlikely to occur on the mackinawite surfaces.

Interfacial reaction of Sn(II) on mackinawite (FeS)

The interaction of Sn(II) with metastable, highly reactive mackinawite is a complex process due to transient changes of the mackinawite surface in the sorption process. In this work, we show that tin redox state and local structure as investigated by Sn-K X-ray absorption spectroscopy (XAS) change with pH. We observe at pH<7 that divalent Sn forms two short (2.38 Å) Sn-S bonds to the S-terminated surface of mackinawite, and two longer (2.59 Å) Sn-S bonds pointing most likely towards the solution phase, in line with a SnS4 innersphere sorption complex. Precipitation of SnS or formation of a solid solution with mackinawite could be excluded. At pH>9, Sn(II) is completely oxidized to Sn(IV) by an Fe(II)/Fe(III) (hydr)oxide, most likely green rust, forming on the surface of mackinawite. Six O atoms at 2.04 Å and 6 Fe atoms at 3.29 Å indicate a structural incorporation by green rust, with Sn(IV) substituting for Fe in the crystal structure. The transition between Sn(II) and Sn(IV) and between sulfur and oxygen coordination takes place at a pH of 7 to 8 and an Eh of -250 mV, close to the thermodynamically predicted transitions from mackinawite to Fe (hydr)oxide and from sulfide to sulfate. The uptake processes of Sn(II) by mackinawite are largely in line with the uptake processes of divalent cations with soft Lewis-acid character like Cd, Hg and Pb, and lead to a strong retention of Sn with logRd values from 5 to 7 across the investigated pH range of 5 to 11.

Simultaneous reduction of arsenic(V) and uranium(VI) by mackinawite: role of uranyl arsenate precipitate formation

Uranium (U) and arsenic (As) often occur together naturally and, as a result, can be co-contaminants at sites of uranium mining and processing, yet few studies have examined the simultaneous redox dynamics of U and As. This study examines the influence of arsenate (As(V)) on the reduction of uranyl (U(VI)) by the redox-active mineral mackinawite (FeS). As(V) was added to systems containing 47 or 470 μM U(VI) at concentrations ranging from 0 to 640 μM. In the absence of As(V), U was completely removed from solution and fully reduced to nano-uraninite (nano-UO2). While the addition of As(V) did not reduce U uptake, at As(V) concentrations above 320 μM, the reduction of U(VI) was limited due to the formation of a trögerite-like uranyl arsenate precipitate. The presence of U also significantly inhibited As(V) reduction. While less U(VI) reduction to nano-UO2 may take place in systems with high As(V) concentrations, formation of trögerite-like mineral phases may be an acceptable reclamation end point due to their high stability under oxic conditions.

Influence of alkaline co-contaminants on technetium mobility in vadose zone sediments

Pertechnetate was slowly reduced in a natural, untreated arid sediment under anaerobic conditions (0.02 nmolg(-1)h(-1)), which could occur in low permeability zones in the field, most of which was quickly oxidized. A small portion of the surface Tc may be incorporated into slowly dissolving surface phases, so was not readily oxidized/remobilized into pore water. In contrast, pertechnetate reduction in an anaerobic sediment containing adsorbed ferrous iron as the reductant was rapid (15-600 nmolg(-1)h(-1)), and nearly all (96-98%) was rapidly oxidized/remobilized (2.6-6.8 nmolg(-1)h(-1)) within hours. Tc reduction in an anaerobic sediment containing 0.5-10mM sulfide showed a relatively slow reduction rate (0.01-0.03 nmolg(-1)h(-1)) that was similar to observations in the natural sediment. Pertechnetate infiltration into sediment with a highly alkaline water resulted in rapid reduction (0.07-0.2 nmolg(-1)h(-1)) from ferrous iron released during biotite or magnetite dissolution. Oxidation of NaOH-treated sediments resulted in slow Tc oxidation (∼0.05 nmolg(-1)h(-1)) of a small fraction of the surface Tc (13-23%). The Tc remaining on the surface was Tc(IV) (by XANES), and autoradiography and elemental maps of Tc (by electron microprobe) showed Tc was present associated with specific minerals, rather than being evenly distributed on the surface. Dissolution of quartz, montmorillonite, muscovite, and kaolinite also occurred in the alkaline water, resulting in significant aqueous silica and aluminum. Over time, aluminosilicates, cancrinite, zeolite and sodalite were precipitating. These precipitates may be coating surface Tc(IV) phases, limiting reoxidation.

Oxidative remobilization of technetium sequestered by sulfide-transformed nano zerovalent iron

Our previous study showed that formation of TcS2-like phases is favored over TcO2 under sulfidic conditions stimulated by nano zerovalent iron. This study further investigates the stability of Tc(IV) sulfide upon reoxidation by solution chemistry, solid phase characterization, and X-ray absorption spectroscopy. Tc dissolution data showed that Tc(VII) reduced by sulfide-transformed nZVI has substantially slower reoxidation kinetics than Tc(VII) reduced by nZVI only. The initial inhibition of Tc(IV) dissolution at S/Fe = 0.112 is due to the redox buffer capacity of FeS, which is evidenced by the parallel trends in oxidation-reduction potentials (ORP) and Tc dissolution kinetics. The role of FeS in inhibiting Tc oxidation is further supported by the Mössbauer spectroscopy and micro X-ray diffraction data at S/Fe = 0.112, showing persistence of FeS after 24-h oxidation but complete oxidation after 120-h oxidation. X-ray absorption spectroscopy data for S/Fe = 0.011 showed significantly increasing percentages of TcS2 in the solid phase after 24-h oxidation, indicating stronger resistance of TcS2 to oxidation. At S/Fe = 0.112, the XAS results revealed significant transformation of Tc speciation from TcS2 to TcO2 after 120-h oxidation. Given that no apparent Tc dissolution occurred during this period, the speciation transformation might play a secondary role in hindering Tc oxidation. Collectively, the results indicate that sequestrating Tc as TcS2 under stimulated sulfate reduction is a promising strategy to improve the long-term stability of reduced Tc in subsurface remediation.

Growth of Desulfovibrio vulgaris when respiring U(VI) and characterization of biogenic uraninite

The capacity of Desulfovibrio vulgaris to reduce U(VI) was studied previously with nongrowth conditions involving a high biomass concentration; thus, bacterial growth through respiration of U(VI) was not proven. In this study, we conducted a series of batch tests on U(VI) reduction by D. vulgaris at a low initial biomass (10 to 20 mg/L of protein) that could reveal biomass growth. D. vulgaris grew with U(VI) respiration alone, as well as with simultaneous sulfate reduction. Patterns of growth kinetics and solids production were affected by sulfate and Fe(2+). Biogenic sulfide nonenzymatically reduced 76% of the U(VI) and greatly enhanced the overall reduction rate in the absence of Fe(2+) but was rapidly scavenged by Fe(2+) to form FeS in the presence of Fe(2+). Biogenic U solids were uraninite (UO2) nanocrystallites associated with 20 mg/g biomass as protein. The crystallite thickness of UO2 was 4 to 5 nm without Fe(2+) but was <1.4 nm in the presence of Fe(2+), indicating poor crystallization inhibited by adsorbed Fe(2+) and other amorphous Fe solids, such as FeS or FeCO3. This work fills critical gaps in understanding the metabolic utilization of U by microorganisms and formation of UO2 solids in bioremediation sites.

Assignment of Raman-active vibrational modes of tetragonal mackinawite: Raman investigations and ab initio calculations

We report on the first assignment of the Raman-active vibrational modes of mackinawite using Density Functional Perturbation Theory and direct methods with BLYP + dispersion correction. Based on experimental data and calculation results, the Raman bands were assigned as 236 cm⁻¹ (B_1g), 256 cm⁻¹ (E_g), 376 cm⁻¹ (A_1g) and 395 cm⁻¹ (E_g).

Uranium(VI) interactions with mackinawite in the presence and absence of bicarbonate and oxygen

Mackinawite, Fe(II)S, samples loaded with uranium (10(-5), 10(-4), and 10(-3) mol U/g FeS) at pH 5, 7, and 9, were characterized using X-ray absorption spectroscopy and X-ray diffraction to determine the effects of pH, bicarbonate, and oxidation on uptake. Under anoxic conditions, a 5 g/L suspension of mackinawite lowered 5 × 10(-5) M uranium(VI) to below 30 ppb (1.26 × 10(-7) M) U. Between 82 and 88% of the uranium removed from solution by mackinawite was U(IV) and was nearly completely reduced to U(IV) when 0.012 M bicarbonate was added. Near-neighbor coordination consisting of uranium-oxygen and uranium-uranium distances indicates the formation of uraninite in the presence and absence of bicarbonate, suggesting reductive precipitation as the dominant removal mechanism. Following equilibration in air, mackinawite was oxidized to mainly goethite and sulfur and about 76% of U(IV) was reoxidized to U(VI) with coordination of uranium to axial and equatorial oxygen, similar to uranyl. Additionally, uranium-iron distances, typical of coprecipitation of uranium with iron oxides, and uranium-sulfur distances indicating bidentate coordination of U(VI) to sulfate were evident. The affinity of mackinawite and its oxidation products for U(VI) provides impetus for further study of mackinawite as a potential reactive medium for remediation of uranium-contaminated water.

Abiotic reductive immobilization of U(VI) by biogenic mackinawite

During subsurface bioremediation of uranium-contaminated sites, indigenous metal and sulfate-reducing bacteria may utilize a variety of electron acceptors, including ferric iron and sulfate that could lead to the formation of various biogenic minerals in situ. Sulfides, as well as structural and adsorbed Fe(II) associated with biogenic Fe(II)-sulfide phases, can potentially catalyze abiotic U(VI) reduction via direct electron transfer processes. In the present work, the propensity of biogenic mackinawite (Fe 1+x S, x = 0 to 0.11) to reduce U(VI) abiotically was investigated. The biogenic mackinawite produced by Shewanella putrefaciens strain CN32 was characterized by employing a suite of analytical techniques including TEM, SEM, XAS, and Mössbauer analyses. Nanoscale and bulk analyses (microscopic and spectroscopic techniques, respectively) of biogenic mackinawite after exposure to U(VI) indicate the formation of nanoparticulate UO2. This study suggests the relevance of sulfide-bearing biogenic minerals in mediating abiotic U(VI) reduction, an alternative pathway in addition to direct enzymatic U(VI) reduction.

Sorption speciation of lanthanides/actinides on minerals by TRLFS, EXAFS and DFT studies: a review

Lanthanides/actinides sorption speciation on minerals and oxides by means of time resolved laser fluorescence spectroscopy (TRLFS), extended X-ray absorption fine structure spectroscopy (EXAFS) and density functional theory (DFT) is reviewed in the field of nuclear disposal safety research. The theoretical aspects of the methods are concisely presented. Examples of recent research results of lanthanide/actinide speciation and local atomic structures using TRLFS, EXAFS and DFT are discussed. The interaction of lanthanides/actinides with oxides and minerals as well as their uptake are also of common interest in radionuclide chemistry. Especially the sorption and inclusion of radionuclides into several minerals lead to an improvement in knowledge of minor components in solids. In the solid-liquid interface, the speciation and local atomic structures of Eu(III), Cm(III), U(VI), and Np(IV/VI) in several natural and synthetic minerals and oxides are also reviewed and discussed. The review is important to understand the physicochemical behavior of lanthanides/actinides at a molecular level in the natural environment.

Technetium incorporation into hematite (alpha-Fe2O3)

Quantum-mechanical methods were used to evaluate mechanisms for possible structural incorporation of Tc species into the model iron oxide, hematite (alpha-Fe2O3). Using periodic supercell models, energies for charge-neutral incorporation of Tc4+ or TcO4- ions were calculated using either a Tc4+/Fe2+ substitution scheme on the metal sublattice, or by insertion of TcO4- as an interstitial species within a hypothetical vacancy cluster. Although pertechnetate incorporation is found to be invariably unfavorable, incorporation of small amounts of Tc4+ (at least 2.6 wt %) is energetically feasible. Energy minimized bond distances around this impurity are provided to aid in future spectroscopic identification of these impurity species. The calculations also show that Fe2+ and Tc4+ prefer to cluster in the hematite lattice, attributed to less net Coulombic repulsion relative to that of Fe3+-Fe3+. These modeling predictions are generally consistent with observed selective association of Tc with iron oxide under reducing conditions, and in residual waste solids from underground storage tanks at the U.S. Department of Energy Hanford Site (Washington, U.S.). Here, even though relatively high pH and oxidizing conditions are dominant, Tc incorporation into iron oxides and (oxy)hydroxides is prospectively enabled by prior reduction of TcO4- to Tc4+ via interaction with radiolytic species.

Pertechnetate immobilization in aqueous media with hydrogen sulfide under anaerobic and aerobic environments

The basic chemistry for the immobilization of pertechnetate (TcO4 −) by hydrogen sulfide was investigated in aqueous solution under both aerobic and anaerobic environments. Pertechnetate immobilization was acid dependent, with accelerated rates and increased immobilization yields as the acid concentration increased. Oxygen had no effect under acidic conditions. Under anaerobic alkaline conditions, the pH, and therefore the speciation of sulfide, was the determining factor on the immobilization of pertechnetate. Only 53% of the TcO4 − was immobilized at pH 8, while the yield increased to 83% at pH 9 as HS− became the dominant sulfide species. The immobilization yield then decreased to 73% at pH 13. No reaction was observed between TcO4 − and sulfide under aerobic alkaline conditions, indicating that oxygen suppressed this reaction.

Pertechnetate immobilization was found to be first order with respect to both sulfide and pertechnetate in acidic solutions, and in alkaline solution under anaerobic conditions. The results of stoichiometry studies and product analysis under alkaline anaerobic environments indicated that Tc2S7 was obtained at pH 9. EXAFS (extended X-ray absorption fine structure) and XANES (X-ray absorption near edge structure) studies suggested that the samples obtained from acidic, aerobic solution and alkaline anaerobic solution were both Tc2S7. The stability of Tc2S7 is affected by O2 with accelerated dissolution at high pH.

Pertechnetate immobilization with amorphous iron sulfide

The reduction of pertechnetate (TcO4−) with freshly prepared amorphous iron sulfide was investigated. The amorphous iron sulfide (FeS) was shown to have an elemental composition of FeS0.97for all of the size fractions and a point of zero charge of pHpzc=7.4. Solubility studies of FeS in various buffers indicated that in the pH range 6.1–9.0, the concentrations of dissociated Fe2+and S2−were negligible. The reductive immobilization of TcO4−with FeS was shown to be accelerated by increasing ionic strength and strongly pH dependent. At pH values below the pHpzc, the positively charged FeS surface reacted much faster with TcO4−and had higher immobilization yields relative to the negatively charged FeS surface at pH values above pHpzc. The TcO4−−FeS reaction is consistent with a surface mediated reaction through ligand exchange. The TcO4−−FeS reductive immobilization reaction product was characterized by X-ray absorption near edge spectroscopy (XANES), extended X-ray absorption fine structure (EXAFS), Fourier transform infrared spectroscopy (FT-IR), and energy dispersive X-ray spectroscopy (EDS) and found to be predominantly TcO2. Studies on the reductive capacity of the FeS and the long term stability of the TcO4−−FeS reaction product under both anaerobic and aerobic environments shows the potential utility of thein situgaseous (hydrogen sulfide gas) immobilization technology in solidification of TcO4−by creating a FeS permeable reaction barrier in the vadose zone.

Technetium reduction and reoxidation behaviour in Dounreay soils

Technetium is a radioactive contaminant found in groundwaters at sites where nuclear wastes have been processed or stored. The redox chemistry of technetium is a major control on its environmental mobility. Under oxic conditions, technetium exists as the pertechnetate ion, Tc(VII)O4 −, which is poorly sorbed by minerals across a wide range of environmentally relevant pH values. Under reducing conditions pertechnetate is converted to lower valency species, of which Tc(IV) tends to predominate. Typically, the Tc(IV) oxidation state readily precipitates as insoluble hydrous Tc(IV) oxides or, at low concentrations, is removed from solution by association with mineral surfaces. Here, we present novel X-ray absorption spectroscopy (XAS) data examining Tc associations with reduced Dounreay soils. In reduced unamended microcosms and in microcosms where we added the co-contaminants ethylenediaminetetraacetic acid (EDTA) or bicarbonate to investigate their effect on Tc biogeochemistry, Tc was removed from solution on exposure to the reduced sediments and was present on solids as hydrous Tc(IV)O2-like phases. Furthermore, to investigate the long term stability and remobilization of solid phase associated Tc in reduced soils, we reoxidized reduced, Tc(IV)-labeled soils, in the presence of air and nitrate. The extent of remobilization of Tc to solution was dependent on the oxidant used. After reoxidation with air for 60 d, (42±6)% of the initial soil bound Tc was resolubilized. In the presence of 25 or 100 mmol L−1 nitrate as an oxidant, negligible microcosm reoxidation or remobilization of Tc to solution occurred. XAS analysis of soils treated with the two oxidants showed that in both systems, the remaining soil associated Tc was present as hydrous TcO2-like phases. The recalcitrance of Tc remobilization under reoxidizing conditions has implications for the fate of Tc in contaminated environments.

Biogenic U(IV) oxidation by dissolved oxygen and nitrate in sediment after prolonged U(VI)/Fe(III)/SO(4)(2-) reduction

Sediment column experiments were performed to quantify the effect of biogenic iron sulfide precipitates on the stability of bioreduced uranium during and after a simulated bioremediation scenario. In particular, this study examined the effect of different oxidants (dissolved oxygen and nitrate) on biogenic U(IV) oxidation in sediment that experienced significant sulfate reduction in addition to Fe(III) and U(VI) reduction. The experimental set-up included five replicate columns (each 5 cm in diameter, 15 cm long and packed with background sediment from a site contaminated with uranium) that were bioreduced for 70 days by injecting a nutrient media containing 3 mM acetate and 6 mM sulfate prior to oxidation. Upon oxidation, iron sulfide precipitates formed during bioreduction acted as a buffer to partially prevent biogenic U(IV) oxidation. The iron sulfides were more effective at protecting biogenic U(IV) from oxidation when dissolved oxygen was the oxidant compared to nitrate. A constant supply of 0.25 mM and 1.6 mM nitrate over a 50 day period resulted in uranium resolubilization of 11% and 60%, respectively, while less than 1% of the uranium was resolubilized in the column supplied 0.27 mM dissolved oxygen during the same time period. Over time, oxidation increased pore water channeling, which was more pronounced during oxidation with nitrate. Increased channeling with time of oxidation could affect the transport of an oxidant through the previously reduced zone, and hence the oxidation dynamics of the reduced species.

X-ray absorption and photoelectron spectroscopy investigation of selenite reduction by FeII-bearing minerals

The long-lived radionuclide 79Se is one of the elements of concern for the safe storage of high-level nuclear waste, since clay minerals in engineered barriers and natural aquifer sediments strongly adsorb cationic species, but to lesser extent anions like selenate (SeVIO4(2-)) and selenite (SeIVO3(2-)). Previous investigations have demonstrated, however, that SeIV and SeVI are reduced by surface-associated FeII, thereby forming insoluble Se0 and Fe selenides. Here we show that the mixed FeII/III (hydr)oxides green rust and magnetite, and the FeII sulfide mackinawite reduce selenite rapidly (< 1 day) to FeSe, while the slightly slower reduction by the FeII carbonate siderite produces elemental Se. In the case of mackinawite, both S(-II) and FeII surface atoms are oxidized at a ratio of one to four by producing a defective mackinawite surface. Comparison of these spectroscopic results with thermodynamic equilibrium modeling provides evidence that the nature of reduction end product in these FeII systems is controlled by the concentration of HSe(-); Se0 forms only at lower HSe(-) concentrations related to slower HSeO3(-) reduction kinetics. Even under thermodynamically unstable conditions, the initially formed Se solid phases may remain stable for longer periods since their low solubility prevents the dissolution required for a phase transformation into more stable solids. The reduction by Fe2+-montmorillonite is generally much slower and restricted to a pH range, where selenite is adsorbed (pH < 7), stressing the importance of a heterogeneous, surface-enhanced electron transfer reaction. Although the solids precipitated by the redox reaction are nanocrystalline, their solubility remains below 6.3 x 10(-8) M. No evidence for aqueous metal selenide colloids nor for Se sorption to colloidal phases was found. Since FeII phases like the ones investigated here should be ubiquitous in the near field of nuclear waste disposals as well as in the surrounding aquifers, mobility of the fission product 79Se may be much lower than previously assumed.

Selenite reduction by mackinawite, magnetite and siderite: XAS characterization of nanosized redox products

Suboxic soils and sediments often contain the Fe(II)-bearing minerals mackinawite (FeS), siderite (FeCO3) or magnetite (FesO4), which should be able to reduce aqueous selenite, thereby forming solids of low solubility. While the reduction of selenate or selenite to Se(O) by green rust, pyrite and by Fe2+ sorbed to montmorillonite is a slow (weeks), kinetically limited redox reaction as demonstrated earlier, we show here that selenite is rapidly reduced within one day by nanoparticulate mackinawite and magnetite, while only one third of selenite is reduced by micrometer-sized siderite. Depending on Fe(II)-bearing phase and pH, we observed four different reaction products, red and gray elemental Se, and two iron selenides with structures similar to Fe7Se8 and FeSe. The thermodynamically most stable iron selenide, ferroselite (FeSe2), was not observed. The local structures of the reaction products suggest formation of nanoscale clusters, which may be prone to colloid-facilitated transport, and may have a higher than expected solubility.