Incorporation and retention of 99-Tc(IV) in magnetite under high pH conditions

Radionuclide biogeochemistry: from bioremediation toward the treatment of aqueous radioactive effluents

Civilian and military nuclear programs of several nations over more than 70 years have led to significant quantities of heterogenous solid, organic, and aqueous radioactive wastes bearing actinides, fission products, and activation products. While many physicochemical treatments have been developed to remediate, decontaminate and reduce waste volumes, they can involve high costs (energy input, expensive sorbants, ion exchange resins, chemical reducing/precipitation agents) or can lead to further secondary waste forms. Microorganisms can directly influence radionuclide solubility, via sorption, accumulation, precipitation, redox, and volatilization pathways, thus offering a more sustainable approach to remediation or effluent treatments. Much work to date has focused on fundamentals or laboratory-scale remediation trials, but there is a paucity of information toward field-scale bioremediation and, to a lesser extent, toward biological liquid effluent treatments. From the few biostimulation studies that have been conducted at legacy weapon production/test sites and uranium mining and milling sites, some marked success via bioreduction and biomineralisation has been observed. However, rebounding of radionuclide mobility from (a)biotic scale-up factors are often encountered. Radionuclide, heavy metal, co-contaminant, and/or matrix effects provide more challenging conditions than traditional industrial wastewater systems, thus innovative solutions via indirect interactions with stable element biogeochemical cycles, natural or engineered cultures or communities of metal and irradiation tolerant strains and reactor design inspirations from existing metal wastewater technologies, are required. This review encompasses the current state of the art in radionuclide biogeochemistry fundamentals and bioremediation and establishes links toward transitioning these concepts toward future radioactive effluent treatments.

Efficient removal and transformation of Cr(VI) from alkaline wastewater to form a ferrochromium spinel multiphase via a modified ferrite process

Chromium-containing wastewater causes serious environmental pollution due to the harmfulness of Cr(VI). The ferrite process is typically used to treat chromium-containing wastewater and recycle the valuable chromium metal. However, the current ferrite process is unable to fully transform Cr(VI) into chromium ferrite under mild reaction conditions. This paper proposes a novel ferrite process to treat chromium-containing wastewater and recover valuable chromium metal. The process combines FeSO4 reduction and hydrothermal treatment to remove Cr(VI) and form chromium ferrite composites. The Cr(VI) concentration in the wastewater was reduced from 1040 mg L-1 to 0.035 mg L-1, and the Cr(VI) leaching toxicity of the precipitate was 0.21 mg L-1 under optimal hydrothermal conditions. The precipitate consisted of micron-sized ferrochromium spinel multiphase with polyhedral structure. The mechanism of Cr(VI) removal involved three steps: 1) partial oxidation of FeSO4 to Fe(III) hydroxide and oxy-hydroxide; 2) reduction of Cr(VI) by FeSO4 to Cr(III) and Fe(III) precipitates; 3) transformation and growth of the precipitates into chromium ferrite composites. This process meets the release standards of industrial wastewater and hazardous waste and can improve the efficiency of the ferrite process for toxic heavy metal removal.

Stability and Speciation of Hydrated Magnetite {111} Surfaces from Ab Initio Simulations with Relevance for Geochemical Redox Processes

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 environmental conditions. In this study, Kohn-Sham density functional theory (DFT + U method) was used to examine the stability and speciation of the prevalent magnetite crystal face {111} in a wide range of pH and Eh conditions. The simulations reveal that the oxidation state and speciation of the surface depend strongly on imposed redox conditions and, in general, may differ from those of the bulk state. Corresponding predominant phase diagrams for the surface speciation and structure were calculated from first principles. Furthermore, classical molecular dynamics simulations were conducted investigating the mobility of water near the magnetite surface. The obtained knowledge of the surface structure and oxidation state of iron is essential for modeling retention of redox-sensitive nuclides.

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.

Design and Application of Materials for Sequestration and Immobilization of 99Tc

⁹⁹Technetium (⁹⁹Tc) is a hazardous radionuclide that poses a serious environmental threat. The wide variation and complex chemistries of liquid nuclear waste streams containing ⁹⁹Tc often create unique, site specific challenges when sequestering and immobilizing the waste in a matrix suitable for long-term storage and disposal. Therefore, an effective management plan for ⁹⁹Tc containing liquid radioactive wastes (such as storage (tanks) and decommissioned wastes) will likely require a variety of suitable materials/matrixes capable of adapting to and addressing these challenges. In this review, we discuss and highlight the key developments for effective removal and immobilization of ⁹⁹Tc liquid waste in inorganic waste forms. Specifically, we review the synthesis, characterization, and application of materials for the targeted removal of ⁹⁹Tc from (simulated) waste solutions under various experimental conditions. These materials include (i) layered double hydroxides (LDHs), (ii) metal-organic frameworks (MOFs), (iii) ion-exchange resins (IERs) as well as cationic organic polymers (COPs), (iv) surface modified natural clay materials (SMCMs), and (v) graphene-based materials (GBMs). Second, we discuss some of the major and recent developments toward ⁹⁹Tc immobilization in (i) glass, (ii) cement, and (iii) iron mineral waste forms. Finally, we present future challenges that need to be addressed for the design, synthesis, and selection of suitable matrixes for the efficient sequestration and immobilization of ⁹⁹Tc from targeted wastes. The purpose of this review is to inspire research on the design and application of various suitable materials/matrixes for selective removal of ⁹⁹Tc present globally in different radioactive wastes and its immobilization in stable/durable waste forms.

Chemistry of the Interaction and Retention of TcVII and TcIV Species at the Fe3O4(001) Surface

The pertechnetate ion Tc^VIIO₄ ^- is a nuclear fission product whose major issue is the high mobility in the environment. Experimentally, it is well known that Fe₃O₄ can reduce Tc^VIIO₄ ^- to Tc^IV 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 Tc^VIIO₄ ^- and Tc^IV species at the Fe₃O₄(001) surface through a hybrid DFT functional (HSE06) method. We studied a possible initiation step of the Tc^VII reduction process. The interaction of the Tc^VIIO₄ ^- ion with the magnetite surface leads to the formation of a reduced Tc^VI species without any change in the Tc coordination sphere through an electron transfer that is favored by the magnetite surfaces with a higher Fe^II content. Furthermore, we explored various model structures for the immobilized Tc^IV final products. Tc^IV can be incorporated into a subsurface octahedral site or adsorbed on the surface in the form of Tc^IVO₂·xH₂O chains. We propose and discuss three model structures for the adsorbed Tc^IVO₂·2H₂O 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.

Recent advances toward structural incorporation for stabilizing heavy metal contaminants: A critical review

Heavy metal pollution has resulted in serious environmental damage and raised significant public health concerns. One potential solution in terminal waste treatment is to structurally incorporate and immobilize heavy metals in some robust frameworks. Yet extant research offers a limited perspective on how metal incorporation behavior and stabilization mechanisms can effectively manage heavy metal-laden waste. This review sets forth detailed research on the feasibility of treatment strategies to incorporate heavy metals into structural frameworks; this paper also compares common methods and advanced characterization techniques for identifying metal stabilization mechanisms. Furthermore, this review analyses the typical hosting structures for heavy metal contaminants and metal incorporation behavior, highlighting the importance of structural features on metal speciation and immobilization efficiency. Lastly, this paper systematically summarizes key factors (i.e., intrinsic properties and external conditions) affecting metal incorporation behavior. Drawing on these impactful findings, the paper discusses future directions in the design of waste forms that efficiently, effectively treat heavy metal contaminants. By examining tailored composition-structure-property relationships in metal immobilization strategies, this review reveals possible solutions for crucial challenges in waste treatment and enhances the development of structural incorporation strategies for heavy metal immobilization in environmental applications.

Reductive removal of pertechnetate and chromate by zero valent iron under variable ionic strength conditions

Radioactive technetium-99 (Tc) present in waste streams and subsurface plumes at legacy nuclear reprocessing sites worldwide poses potential risks to human health and environment. This research comparatively evaluated efficiency of zero-valent iron (ZVI) toward reductive removal of Tc(VII) in presence of Cr(VI) from NaCl and Na₂SO₄ electrolyte solutions under ambient atmospheric conditions. In both electrolytes, anticorrosive Cr(VI) suppressed oxidation of ZVI at elevated concentrations resulting in the delay of initiation of Tc(VII) reduction to Tc(IV). In the absence of Cr(VI), no delay was observed in the analogous systems. At low ionic strength (IS), retarded ZVI oxidation inhibited Tc(VII) reduction. Higher IS favored reduction of both Tc(VII) and Cr(VI), which followed second-order reaction rates in both electrolytes attributed to the more efficient iron oxidation as evident from solids characterization studies. Magnetite was the primary iron oxide phase, and its higher fraction in the SO₄^2- solutions facilitated reductive removal of Tc(VII) and Cr(VI). In the Cl^- matrix, Cr(VI) promoted further oxidation of magnetite as well as formation of chromite diminishing overall reductive capacity of this system and resulting in less effective removal of Tc(VII) compared to the SO₄^2- solutions.

Exploring the Reduction Mechanism of 99Tc(VII) in NaClO4: A Spectro-Electrochemical Approach

Technetium (Tc) is an environmentally relevant radioactive contaminant whose migration is limited when Tc(VII) is reduced to Tc(IV). However, its reaction mechanisms are not well understood yet. We have combined electrochemistry, spectroscopy, and microscopy (cyclic voltammetry, rotating disk electrode, X-ray photoelectron spectroscopy, and Raman and scanning electron microscopy) to study Tc(VII) reduction in non-complexing media: 0.5 mM KTcO₄ in 2 M NaClO₄ in the pH from 2.0 to 10.0. At pH 2.0, Tc(VII) first gains 2.3 ± 0.3 electrons, following Tc(V) rapidly receives 1.3 ± 0.3 electrons yielding Tc(IV). At pH 4.0-10.0, Tc(IV) is directly obtained by transfer of 3.2 ± 0.3 electrons. The reduction of Tc(VII) produced always a black solid identified as Tc(IV) by Raman and XPS. Our results narrow a significant gap in the fundamental knowledge of Tc aqueous chemistry and are important to understand Tc speciation. They provide basic steps on the way from non-complexing to complex media.

Biogeochemical Cycling of 99Tc in Alkaline Sediments

⁹⁹Tc will be present in significant quantities in radioactive wastes including intermediate-level waste (ILW). The internationally favored concept for disposing of higher activity radioactive wastes including ILW is via deep geological disposal in an underground engineered facility located ∼200-1000 m deep. Typically, in the deep geological disposal environment, the subsurface will be saturated, cement will be used extensively as an engineering material, and iron will be ubiquitous. This means that understanding Tc biogeochemistry in high pH, cementitious environments is important to underpin safety case development. Here, alkaline sediment microcosms (pH 10) were incubated under anoxic conditions under "no added Fe(III)" and "with added Fe(III)" conditions (added as ferrihydrite) at three Tc concentrations (10^-11, 10^-6, and 10^-4 mol L^-1). In the 10^-6 mol L^-1 Tc experiments with no added Fe(III), ∼35% Tc(VII) removal occurred during bioreduction. Solvent extraction of the residual solution phase indicated that ∼75% of Tc was present as Tc(IV), potentially as colloids. In both biologically active and sterile control experiments with added Fe(III), Fe(II) formed during bioreduction and >90% Tc was removed from the solution, most likely due to abiotic reduction mediated by Fe(II). X-ray absorption spectroscopy (XAS) showed that in bioreduced sediments, Tc was present as hydrous TcO₂-like phases, with some evidence for an Fe association. When reduced sediments with added Fe(III) were air oxidized, there was a significant loss of Fe(II) over 1 month (∼50%), yet this was coupled to only modest Tc remobilization (∼25%). Here, XAS analysis suggested that with air oxidation, partial incorporation of Tc(IV) into newly forming Fe oxyhydr(oxide) minerals may be occurring. These data suggest that in Fe-rich, alkaline environments, biologically mediated processes may limit Tc mobility.

Advances in metal(loid) oxyanion removal by zerovalent iron: Kinetics, pathways, and mechanisms

Metal(loid) oxyanions in groundwater, surface water, and wastewater can have harmful effects on human or ecological health due to their high toxicity, mobility, and lack of degradation. In recent years, the removal of metal(loid) oxyanions using zerovalent iron (ZVI) has been the subject of many studies, but the full scope of this literature has not been systematically reviewed. The main elements that form metal(loid) oxyanions under environmental conditions are Cr(VI), As(V and III), Sb(V and III), Tc(VII), Re(VII), Mo(VI), V(V), etc. The removal mechanisms of metal(loid) oxyanions by ZVI may involve redox reactions, adsorption, precipitation, and coprecipitation, usually with one of these mechanisms being the main reaction pathway and the other playing auxiliary roles. However, the removal mechanisms are coupled to the reactions involved in corrosion of Fe(0) and reaction conditions. The layer of iron oxyhydroxides that forms on ZVI during corrosion mediates the sequestration of metal(loid) oxyanions. This review summarizes most of the currently available data on mechanisms and performance (e.g., kinetics) of removal of the most widely studies metal(loid) oxyanion contaminants (Cr, As, Sb) by different types of ZVI typically used in wastewater treatment, as well as ZVI that has been sulfidated or combination with catalytic bimetals.

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.

99TcO4- Separation through Selective Crystallization Assisted by Polydentate Benzene-Aminoguanidinium Ligands

⁹⁹Tc is one of the most abundant radiotoxic isotopes in used nuclear fuel with a high fission yield and a long half-life. Effective removal of pertechnetate (TcO₄^-) from an aqueous solution is important for nuclear waste separation and remediation. Herein, we report a series of facilely obtained benzene-linked guanidiniums that could precipitate TcO₄^- and its nonradioactive surrogate ReO₄^- from a high-concentration acidic solution through self-assembly crystallization. The resulting perrhenate and pertechnetate solids exhibit exceptionally low aqueous solubility. The benzene-linked guanidiniums hold one of the highest TcO₄^- removal capacities (1279 mg g^-1) among previously reported materials and possess a removal percentage of 59% for ReO₄^- in the presence of Cl^- over 50 times. The crystallization mechanism was clearly illustrated by the single-crystal structures and density functional theory calculations, indicating that TcO₄^- is captured through a charge-assisted hydrogen bonding interaction and stabilized by π-π stacking layers. In addition, the removal process is easily recycled and no toxic organic reagents are introduced. This work provides a green approach to preliminarily separate TcO₄^- from high-level nuclear wastes.

Immobilization of perrhenate using synthetic pyrite particles: Effectiveness and remobilization potential

Radioactive pertechnetate (TcO4^-) has been detected in nuclear waste affected soil and groundwater, posing significant effect on human health and the environment. Yet, cost-effective remediation of Tc-contaminated soil and groundwater remains challenging. To address this critical technology need, we prepared a class of pyrite (FeS₂) particles for effective immobilization of pertechnetate. Using perrhenate (ReO₄^-) as a non-radioactive surrogate of TcO₄^-, we tested the immobilization effectiveness of the material through batch kinetic experiments, and evaluated the remobilization potential of immobilized Re under anoxic (sealed from air) and oxic (exposed to air) conditions and in the presence of humic acid (HA), EDTA, nitrate, and a Chinese loess soil. The results showed that more acidic pH gave faster Re(VII) removal due to more abundant electron sources (Fe²⁺ and S₂^2-). X-ray diffraction (XRD) and/or X-ray photoelectron spectroscopy (XPS) analyses confirmed formation of ReO₂/ReS₂ as the major reduction products. The immobilized Re remained highly stable when aged for 360 days under anoxic conditions at different influence factors. Yet, the immobilized Re was vulnerable to oxygen oxidation, and about 78% of Re was remobilized after 40 days of exposure to air regardless of the initial pH (3.5-9.0) due to excessive pyrite oxidation and the associated pH drop (~2). HA at 120 mg/L inhibited Re remobilization under oxic conditions, which lowered the Re remobilization by ~21% after 40 days of oxic aging. The presence of EDTA facilitated dissolution of Fe but inhibited the dissolution of Re under oxic conditions. Nitrate showed negligible effect on Re remobilization. The presence of a Chinese loess soil effectively inhibited Re remobilization under both oxic and anoxic conditions, lowering the leachable Re by ~32% under oxic conditions. The findings may guide engineered application of pyrite particles as a long-lasting reducing material for immobilization pertechnetate or similar redox-active contaminants in soil and water.

Spontaneous redox continuum reveals sequestered technetium clusters and retarded mineral transformation of iron

The sequestration of metal ions into the crystal structure of minerals is common in nature. To date, the incorporation of technetium(IV) into iron minerals has been studied predominantly for systems under carefully controlled anaerobic conditions. Mechanisms of the transformation of iron phases leading to incorporation of technetium(IV) under aerobic conditions remain poorly understood. Here we investigate granular metallic iron for reductive sequestration of technetium(VII) at elevated concentrations under ambient conditions. We report the retarded transformation of ferrihydrite to magnetite in the presence of technetium. We observe that quantitative reduction of pertechnetate with a fraction of technetium(IV) structurally incorporated into non-stoichiometric magnetite benefits from concomitant zero valent iron oxidative transformation. An in-depth profile of iron oxide reveals clusters of the incorporated technetium(IV), which account for 32% of the total retained technetium estimated via X-ray absorption and X-ray photoelectron spectroscopies. This corresponds to 1.86 wt.% technetium in magnetite, providing the experimental evidence to theoretical postulations on thermodynamically stable technetium(IV) being incorporated into magnetite under spontaneous aerobic redox conditions.

Covalent Organic Frameworks in Separation

In the wake of sustainable development, materials research is going through a green revolution that is putting energy-efficient and environmentally friendly materials and methods in the limelight. In this quest for greener alternatives, covalent organic frameworks (COFs) have emerged as a new generation of designable crystalline porous polymers for a wide array of clean-energy and environmental applications. In this contribution, we categorically review the merits and shortcomings of COF bulk powders, nanosheets, freestanding thin films/membranes, and membranes on porous supports in various separation processes, including separation of gases, pervaporation, organic solvent nanofiltration, water purification, radionuclide sequestration, and chiral separations, with particular reference to COF material pore size, host–guest interactions, stability, selectivity, and permeability. This review covers the fabrication strategies of nanosheets, films, and membranes, as well as performance parameters, and provides an overview of the separation landscape with COFs in relation to other porous polymers, while seeking to interpret the future research opportunities in this field.

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.

Technetium retention by gamma alumina nanoparticles and the effect of sorbed Fe2

Technetium (Tc) retention on gamma alumina nanoparticles (γ-Al₂O₃ NPs) has been studied in the absence (binary system) and presence (ternary system) of previously sorbed Fe²⁺ as a reducing agent. In the binary system, γ-Al₂O₃ NPs sorb up to 6.5% of Tc from solution as Tc(VII). In the ternary system, the presence of previously sorbed Fe²⁺ on γ-Al₂O₃ NPs significantly enhances the uptake of Tc from pH 4 to pH 11. Under these conditions, the reaction rate of Tc increases with pH, resulting in a complete uptake for pHs > 6.5. Redox potential (Eh) and X-ray photoelectron spectroscopy (XPS) measurements evince heterogeneous reduction of Tc(VII) to Tc(IV). Here, the formation of Fe-containing solids was observed; Raman and scanning electron microscopy showed the presence of Fe(OH)₂, Fe(II)-Al(III)-Cl layered double hydroxide (LDH), and other Fe(II) and Fe(III) mineral phases, e.g. Fe₃O₄, FeOOH, Fe₂O₃. These results indicate that Tc scavenging is predominantly governed by the presence of sorbed Fe²⁺ species on γ-Al₂O₃ NPs, where the reduction of Tc(VII) to Tc(IV) and overall Tc retention is highly improved, even under acidic conditions. Likewise, the formation of additional Fe solid phases in the ternary system promotes the Tc uptake via adsorption, co-precipitation, and incorporation mechanisms.

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.

Comparative analysis of ZVI materials for reductive separation of 99Tc(VII) from aqueous waste streams

Technetium-99 (Tc) is a long-lived radioactive contaminant present in legacy nuclear waste streams and contaminated plumes of the nuclear waste storage sites worldwide that poses risks for human health and the environment. Pertechnetate (TcO₄^-), the most common chemical form of Tc under oxidative conditions, is of particular concern due to its high aqueous solubility and mobility in the subsurface. One approach to treatment and remediation of TcO₄^- is reduction of Tc⁷⁺ to less soluble and mobile Tc⁴⁺ and its removal from the contaminated streams such as liquid secondary waste generated during vitrification of the Hanford low activity tank waste. Zero valent iron (ZVI) is a common reactive agent for reductive treatment of environmental contaminants, including reducible heavy metal ions, which can offer a potential solution to this challenge. Here, we present a comparative study of eleven commercial ZVI materials manufactured by different methods that were evaluated for the reductive removal of TcO₄^- from an aqueous 80 mM NaCl solution at near neutral pH representing low activity waste off-gas condensate. Performance of ZVI materials was analyzed in relation to time-dependent Fe²⁺ dissolution as well as pH and ORP profiles of the contact solution. Large variability in the efficiency and kinetics of Tc⁷⁺ reduction by different ZVI materials was contingent on their origin. ZVI materials manufactured by electrolytic method exhibited superior performance, and the kinetics of the Tc⁷⁺ reduction correlated to particle size. ZVI materials manufactured by iron pentacarbonyl reduction with hydrogen were ineffective for Tc⁷⁺ reduction. In general, our results highlight the need for thorough performance analysis of commercial ZVI materials for any contaminant of interest.

99Tc immobilization from off-gas waste streams using nickel-doped iron spinel

Technetium-99 (⁹⁹Tc) incorporation within stable spinel phases is a novel method for ⁹⁹Tc removal and immobilization from waste streams. In this study, transformation of Ni-doped Fe(OH)₂(s) to spinel minerals, e.g. trevorite (NiFe₂O₄), is explored as a method for removing ⁹⁹Tc from Hanford Waste Treatment and Immobilization Plant (WTP) primary off-gas waste stream simulant. The Fe(OH)₂(s) transformation process was found to reduce ⁹⁹Tc(VII) to ⁹⁹Tc(IV) and incorporate reduced Tc(VI) into the produced spinel simultaneously. Nickel doping was applied in the mineral transformation to inhibit potential reoxidation of ⁹⁹Tc(IV). Solid phase characterization by XRD and XANES confirmed the formation of nickel substituted ferric-spinel, and suggest incorporation of ⁹⁹Tc(IV) in the final spinel. Furthermore, in the primary off-gas stream, which contains both redox-sensitive contaminants Cr(VI) and ⁹⁹Tc(VII), results from solution analysis and solid digestion indicate that nearly 100% Cr and over 80% ⁹⁹Tc can be simultaneously removed by adding Fe(OH)₂(s) to solution with a solid to solution ratio of 5 g/L under near neutral and alkaline conditions. The ⁹⁹Tc removal approach developed herein provides an alternative treatment method to eliminate the proposed recycle process of the off-gas waste stream, which ultimately can reduce WTP mission cost and operation time.

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.

Cr(VI) Effect on Tc-99 Removal from Hanford Low-Activity Waste Simulant by Ferrous Hydroxide

Here, Cr(VI) effects on Tc-immobilization by Fe(OH)₂(s) are investigated while assessing Fe(OH)₂(s) as a potential treatment method for Hanford low-activity waste destined for vitrification. Batch studies using simulated low-activity waste indicate that Tc(VII) and Cr(VI) removal is contingent on reduction to Tc(IV) and Cr(III). Furthermore, complete removal of both Cr and Tc depends on the amount of Fe(OH)₂(s) present, where complete Cr and Tc removal requires more Fe(OH)₂(s) (∼200 g/L of simulant), than removing Cr alone (∼50 g/L of simulant). XRD analysis suggests that Fe(OH)₂(s) reaction and transformation in the simulant produces mostly goethite (α-FeOOH), where Fe(OH)₂(s) transformation to goethite rather than magnetite is likely due to the simulant chemistry, which includes high levels of nitrite and other constituents. Once reduced, a fraction of Cr(III) and Tc(IV) substitute for octahedral Fe(III) within the goethite crystal lattice as supported by XPS, XANES, and/or EXAFS results. The remaining Cr(III) forms oxide and/or hydroxide phases, whereas Tc(IV) not fully incorporated into goethite persists as either adsorbed or partially incorporated Tc(IV)-oxide species. As such, to fully incorporate Tc(IV) into the goethite crystal structure, additional Fe(OH)₂(s) (>200 g/L of simulant) may be required.

Stability, Composition, and Core-Shell Particle Structure of Uranium(IV)-Silicate Colloids

Uranium is typically the most abundant radionuclide by mass in radioactive wastes and is a significant component of effluent streams at nuclear facilities. Actinide(IV) (An(IV)) colloids formed via various pathways, including corrosion of spent nuclear fuel, have the potential to greatly enhance the mobility of poorly soluble An(IV) forms, including uranium. This is particularly important in conditions relevant to decommissioning of nuclear facilities and the geological disposal of radioactive waste. Previous studies have suggested that silicate could stabilize U(IV) colloids. Here the formation, composition, and structure of U(IV)-silicate colloids under the alkaline conditions relevant to spent nuclear fuel storage and disposal were investigated using a range of state of the art techniques. The colloids are formed across a range of pH conditions (9-10.5) and silicate concentrations (2-4 mM) and have a primary particle size 1-10 nm, also forming suspended aggregates <220 nm. X-ray absorption spectroscopy, ultrafiltration, and scanning transmission electron microscopy confirm the particles are U(IV)-silicates. Additional evidence from X-ray diffraction and pair distribution function data suggests the primary particles are composed of a UO₂-rich core and a U-silicate shell. U(IV)-silicate colloids formation correlates with the formation of U(OH)₃(H₃SiO₄)₃^2- complexes in solution indicating they are likely particle precursors. Finally, these colloids form under a range of conditions relevant to nuclear fuel storage and geological disposal of radioactive waste and represent a potential pathway for U mobility in these systems.

Uranium Dioxides and Debris Fragments Released to the Environment with Cesium-Rich Microparticles from the Fukushima Daiichi Nuclear Power Plant

Trace U was released from the Fukushima Daiichi Nuclear Power Plant (FDNPP) during the meltdowns, but the speciation of the released components of the nuclear fuel remains unknown. We report, for the first time, the atomic-scale characteristics of nanofragments of the nuclear fuels that were released from the FDNPP into the environment. Nanofragments of an intrinsic U-phase were discovered to be closely associated with radioactive cesium-rich microparticles (CsMPs) in paddy soils collected ∼4 km from the FDNPP. The nanoscale fuel fragments were either encapsulated by or attached to CsMPs and occurred in two different forms: (i) UO_2+X nanocrystals of ∼70 nm size, which are embedded into magnetite associated with Tc and Mo on the surface and (ii) Isometric (U,Zr)O_2+X nanocrystals of ∼200 nm size, with the U/(U+Zr) molar ratio ranging from 0.14 to 0.91, with intrinsic pores (∼6 nm), indicating the entrapment of vapors or fission-product gases during crystallization. These results document the heterogeneous physical and chemical properties of debris at the nanoscale, which is a mixture of melted fuel and reactor materials, reflecting the complex thermal processes within the FDNPP reactor during meltdown. Still CsMPs are an important medium for the transport of debris fragments into the environment in a respirable form.

Facile incorporation of technetium into magnetite, magnesioferrite, and hematite by formation of ferrous nitrate in situ: precursors to iron oxide nuclear waste forms

The fission product, ⁹⁹Tc, presents significant challenges to the long-term disposal of nuclear waste due to its long half-life, high fission yield, and to the environmental mobility of pertechnetate (TcO₄⁻), the stable Tc species in aerobic environments.

Impacts of Repeated Redox Cycling on Technetium Mobility in the Environment

Technetium is a problematic contaminant at nuclear sites and little is known about how repeated microbiologically mediated redox cycling impacts its fate in the environment. We explore this question in sediments representative of the Sellafield Ltd. site, UK, over multiple reduction and oxidation cycles spanning ∼1.5 years. We found the amount of Tc remobilised from the sediment into solution significantly decreased after repeated redox cycles. X-ray Absorption Spectroscopy (XAS) confirmed that sediment bound Tc was present as hydrous TcO₂-like chains throughout experimentation and that Tc's increased resistance to remobilization (via reoxidation to soluble TcO₄^-) resulted from both shortening of TcO₂ chains during redox cycling and association of Tc(IV) with Fe phases in the sediment. We also observed that Tc(IV) remaining in solution during bioreduction was likely associated with colloidal magnetite nanoparticles. These findings highlight crucial links between Tc and Fe biogeochemical cycles that have significant implications for Tc's long-term environmental mobility, especially under ephemeral redox conditions.

Reduction and Simultaneous Removal of 99Tc and Cr by Fe(OH)2(s) Mineral Transformation

Technetium (Tc) remains a priority remediation concern due to persistent challenges, including mobilization due to rapid reoxidation of immobilized Tc, and competing comingled contaminants, e.g., Cr(VI), that inhibit Tc(VII) reduction and incorporation into stable mineral phases. Here Fe(OH)₂(s) is investigated as a comprehensive solution for overcoming these challenges, by serving as both the reductant, (Fe(II)), and the immobilization agent to form Tc-incorporated magnetite (Fe₃O₄). Trace metal analysis suggests removal of Tc(VII) and Cr(VI) from solution occurs simultaneously; however, complete removal and reduction of Cr(VI) is achieved earlier than the removal/reduction of comingled Tc(VII). Bulk oxidation state analysis of the final magnetite solid phase by XANES shows that the majority of Tc is Tc(IV), which is corroborated by XPS measurements. Furthermore, EXAFS results show successful, albeit partial, Tc(IV) incorporation into magnetite octahedral sites. Cr XPS analysis indicates reduction to Cr(III) and the formation of a Cr-incorporated spinel, Cr₂O₃, and Cr(OH)₃ phases. Spinel (modeled as Fe₃O₄), goethite (α-FeOOH), and feroxyhyte (δ-FeOOH) are detected in all reacted final solid phase samples analyzed by XRD. Incorporation of Tc(IV) has little effect on the spinel lattice structure. Reaction of Fe(OH)₂(s) in the presence of Cr(III) results in the formation of a spinel phase that is a solid solution between magnetite (Fe₃O₄) and chromite (FeCr₂O₄).

Tracking the Superefficient Anion Exchange of a Dynamic Porous Material Constructed by Ag(I) Nitrate and Tripyridyltriazole via Multistep Single-Crystal to Single-Crystal Transformations

To avoid the instability and inefficiency for anion-exchange resins and layered double-hydroxides materials, we present herein a flexible coordination network [Ag(L²⁴³)](NO₃)(H₂O)(CH₃CN) (L²⁴³ = 3-(2-pyridyl)-4-(4-pyridyl)-5-(3-pyridyl)-1,2,4-triazole) with superefficient trapping capacity for permanganate, as a group-7 oxoanion model for radiotoxic pertechnetate pollutant. Furthermore, a high-throughput screening strategy has been developed based on concentration-gradient design principle to ascertain the process and mechanism for anion exchange. Significantly, a series of intermediates can be successfully isolated as the qualified crystals for single-crystal X-ray diffraction. The result evidently indicates that such a dynamic material will show remarkable breathing effect of the three-dimensional host framework upon anion exchange, which mostly facilitates the anion trapping process. This established methodology will provide a general strategy to discover the internal secrets of complicated solid-state reactions in crystals at the molecular level.

Long-Term Immobilization of Technetium via Bioremediation with Slow-Release Substrates

Radionuclides are present in groundwater at contaminated nuclear facilities with technetium-99, one of the most mobile radionuclides encountered. In situ bioremediation via the generation of microbially reducing conditions has the potential to remove aqueous and mobile Tc(VII) from groundwater as insoluble Tc(IV). However, questions remain regarding the optimal methods of biostimulation and the stability of reduced Tc(IV) phases under oxic conditions. Here, we selected a range of slow-release electron donor/chemical reduction based substrates available for contaminated land treatment, and assessed their potential to stimulate the formation of recalcitrant Tc(IV) biominerals under conditions relevant to radioactively contaminated land. These included a slow-release polylactate substrate (HRC), a similar substrate with an additional organosulfur ester (MRC) and a substrate containing zerovalent iron and plant matter (EHC). Results showed that Tc was removed from solution in the form of poorly soluble hydrous Tc(IV)-oxides or Tc(IV)-sulfides during the development of reducing conditions. Reoxidation experiments showed that these phases were largely resistant to oxidative remobilization and were more resistant than Tc(IV) produced via biostimulation with an acetate/lactate electron donor mix in the sediments tested. The implications of the targeted formation of recalcitrant Tc(IV) phases using these proprietorial substrates in situ is discussed in the context of the long-term management of technetium at legacy nuclear sites.

Incorporation of Technetium into Spinel Ferrites

Technetium (⁹⁹Tc) is a problematic fission product for the long-term disposal of nuclear waste due to its long half-life, high fission yield, and to the environmental mobility of pertechnetate, the stable species in aerobic environments. One approach to preventing ⁹⁹Tc contamination is using sufficiently durable waste forms. We report the incorporation of technetium into a family of synthetic spinel ferrites that have environmentally durable natural analogs. A combination of X-ray diffraction, X-ray absorption fine structure spectroscopy, and chemical analysis reveals that Tc(IV) replaces Fe(III) in octahedral sites and illustrates how the resulting charge mismatch is balanced. When a large excess of divalent metal ions is present, the charge is predominantly balanced by substitution of Fe(III) by M(II). When a large excess of divalent metal ions is absent, the charge is largely balanced by creation of vacancies among the Fe(III) sites (maghemitization). In most samples, Tc is present in Tc-rich regions rather than being homogeneously distributed.

Trivalent Actinide Uptake by Iron (Hydr)oxides

The retention of Am(III) by coprecipitation with or adsorption onto preformed magnetite was investigated by X-ray diffraction (XRD), solution chemistry, and X-ray absorption spectroscopy (XAS). In the coprecipitation experiment, XAS data indicated the presence of seven O atoms at 2.44(1) Å, and can be explained by an Am incorporation at Fe structural sites at the magnetite surface. Next-nearest Fe were detected at distances suggesting that Am and Fe polyhedra share corners in geometries ranging from bent to close to linear Am-O-Fe bonds. After aging for two years, the coordination number and the distance to the first O shell significantly decreased, and atomic shells were detected at higher distances. These data suggest a structural reorganization and an increase in structural order around sorbed Am. Upon contact with preformed Fe₃O₄, Am(III) forms surface complexes with cosorbed Fe at the surface of magnetite, a possible consequence of the high concentration of dissolved Fe. In a separate experiment, chloride green rust (GR) was synthesized in the presence of Am(III), and subsequently converted to Fe(OH)₂(s) intermixed with magnetite. XAS data indicated that the actinide is successively located first at octahedral brucite-like sites in the GR precursor, then in Fe(OH)₂(s), an environment markedly distinct from that of Am(III) in Fe₃O₄. The findings indicate that the magnetite formation pathway dictates the magnitude of Am(III) incorporation within this solid.

Impeding (99)Tc(IV) mobility in novel waste forms

Technetium (⁹⁹Tc) is an abundant, long-lived radioactive fission product whose mobility in the subsurface is largely governed by its oxidation state. Tc immobilization is crucial for radioactive waste management and environmental remediation. Tc(IV) incorporation in spinels has been proposed as a novel method to increase Tc retention in glass waste forms during vitrification. However, experiments under high-temperature and oxic conditions show reoxidation of Tc(IV) to volatile pertechnetate, Tc(VII). Here we examine this problem with ab initio molecular dynamics simulations and propose that, at elevated temperatures, doping with first row transition metal can significantly enhance Tc retention in magnetite in the order Co>Zn>Ni. Experiments with doped spinels at 700 °C provide quantitative confirmation of the theoretical predictions in the same order. This work highlights the power of modern, state-of-the-art simulations to provide essential insights and generate theory-inspired design criteria of complex materials at elevated temperatures.

Technetium-99 retention in spinel-containing glass is a promising strategy for radioactive waste management, but volatility is still an issue. Here, the authors show that doping magnetite with 1st row transition metals enhances technetium retention by altering the redox capacity of the Tc-containing spinel.

Computational Investigation of Technetium(IV) Incorporation into Inverse Spinels: Magnetite (Fe3O4) and Trevorite (NiFe2O4)

Iron oxides and oxyhydroxides play an important role in minimizing the mobility of redox-sensitive elements in engineered and natural environments. For the radionuclide technetium-99 (Tc), these phases hold promise as primary hosts for increasing Tc loading into glass waste form matrices, or as secondary sinks during the long-term storage of nuclear materials. Recent experiments show that the inverse spinel, magnetite [Fe(II)Fe(III)2O4], can incorporate Tc(IV) into its octahedral sublattice. In that same class of materials, trevorite [Ni(II)Fe(III)2O4] is also being investigated for its ability to host Tc(IV). However, questions remain regarding the most energetically favorable charge-compensation mechanism for Tc(IV) incorporation in each structure, which will affect Tc behavior under changing waste processing or storage conditions. Here, quantum-mechanical methods were used to evaluate incorporation energies and optimized lattice bonding environments for three different, charge-balanced Tc(IV) incorporation mechanisms in magnetite and trevorite (∼5 wt % Tc). For both phases, the removal of two octahedral Fe(II) or Ni(II) ions upon the addition of Tc(IV) in an octahedral site is the most stable mechanism, relative to the creation of octahedral Fe(III) defects or increasing octahedral Fe(II) content. Following hydration-energy corrections, Tc(IV) incorporation into magnetite is energetically favorable while an energy barrier exists for trevorite.

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]).

Redox Interactions of Tc(VII), U(VI), and Np(V) with Microbially Reduced Biotite and Chlorite

Technetium, uranium, and neptunium are contaminants that cause concern at nuclear facilities due to their long half-life, environmental mobility, and radiotoxicity. Here we investigate the impact of microbial reduction of Fe(III) in biotite and chlorite and the role that this has in enhancing mineral reactivity toward soluble TcO4(-), UO2(2+), and NpO2(+). When reacted with unaltered biotite and chlorite, significant sorption of U(VI) occurred in low carbonate (0.2 mM) buffer, while U(VI), Tc(VII), and Np(V) showed low reactivity in high carbonate (30 mM) buffer. On reaction with the microbially reduced minerals, all radionuclides were removed from solution with U(VI) reactivity influenced by carbonate. Analysis by X-ray absorption spectroscopy (XAS) confirmed reductive precipitation to poorly soluble U(IV) in low carbonate conditions and both Tc(VII) and Np(V) in high carbonate buffer were also fully reduced to poorly soluble Tc(IV) and Np(IV) phases. U(VI) reduction was inhibited under high carbonate conditions. Furthermore, EXAFS analysis suggested that in the reaction products, Tc(IV) was associated with Fe, Np(IV) formed nanoparticulate NpO2, and U(IV) formed nanoparticulate UO2 in chlorite and was associated with silica in biotite. Overall, microbial reduction of the Fe(III) associated with biotite and chlorite primed the minerals for reductive scavenging of radionuclides: this has clear implications for the fate of radionuclides in the environment.

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.