Fe(II) Induced Reduction of Incorporated U(VI) to U(V) in Goethite

Uranium immobilization via sulfur-modified Fe0 nanoparticles: U(VI) trapping kinetics and long-term stability evaluation

The modification of nanoscale zero-valent iron (nZVI) by loading or incorporating sulfur into the iron crystal lattice can augment their efficacy in the removal of hydrophobic contaminants from wastewater. Nevertheless, the reactivity of sulfur-embedded nZVI (SnZVI) in immobilizing hydrophilic uranyl ions and the long-term stability of the sequestered uranium has received little attention. This study employed Na2S2O4 to modify the nZVI with different S/Fe molar ratios (0.1 and 0.3), following one-step and two-step approaches to create SnZVI-1 and SnZVI-2, respectively. Both experimental and theoretical calculation results revealed that the U(VI) ions exhibited low affinity for the surface of SnZVI. Additionally, the hindered electron transfer between the electron donors of SnZVI and U(VI) led to a diminished U(VI) reduction efficiency for SnZVI-1 (50.71 %∼67.74 %) and SnZVI-2 (68.03 %∼86.89 %), inferior to that of nZVI (78.63 %∼90.78 %). Consequently, the uranium detachment ratios of SnZVI (0.04 %∼0.85 %) during the 210-day stability assessment were higher compared to those of nZVI (0.04 %∼0.34 %). Hence, this study offered novel insights into how sulfur affected the adsorptive and redox properties of nZVI for U(VI) immobilization through solid and aqueous samples analyses, complemented by theoretical calculations. The findings are instrumental in designing SnZVI for effective and environmentally sound treatment of uranium-contaminated radioactive wastewater.

Role of Valency and Defects in the Incorporation of Uranium into the Goethite [010] Surface: An Embedded Cluster Density Functional Theory Study

Incorporation of actinide species into iron (oxyhydr)oxides could present an environmentally secure method for preventing the release of actinides over an extended period, as would be the case in a number of radioactively contaminated land situations including surface, near-surface, and subsurface disposal and storage. Uranium is known to incorporate into iron (oxyhydr)oxides, including goethite, in a number of valence states, but the atomistic structures of these processes are unclear. In particular, it is increasingly reported that iron-containing minerals can reductively incorporate and stabilize the +V state of uranium, an oxidation state that is known to be unstable with respect to disproportionation. Here, we use density functional theory within the Periodic Electrostatic Embedded Cluster Method to model U(IV), U(V), and U(VI) incorporation into the pristine and iron-vacancy [010] surface and near-surface region of goethite. Solvated and unsolvated surfaces are studied, and the role of electron transfer from the lattice to uranium ions is explored. Comparisons are made with published X-ray absorption spectroscopic data, and we conclude that, based on the expected conditions for surface and near surface storage sites, both U(VI) and U(V) would incorporate into goethite as it transforms from ferrihydrite, forming two distinct structural types. We find that U(VI) incorporated into goethite may be reduced to U(V), where electron transfer occurs from oxygens surrounding iron vacancies and the incorporated uranium, reducing the U(VI) species to U(V). Both U(VI) and U(V) can incorporate into the surface of goethite with an adjacent iron vacancy, or U(V) can uniquely incorporate into the structure within the near-surface region, containing local but not immediately adjacent iron vacancies for charge compensation. Both of these incorporation schemes are little affected by the presence of a monolayer of surface water, suggesting that incorporation into goethite is a viable method to prevent uranium release into the aqueous surroundings.

Efficient remediation and synchronous recovery of uranium by phosphate-functionalized magnetic carbon-based flow electrode capacitive deionization

Through the design of flow electrodes, flow electrode capacitive deionization (FCDI) enables the efficient remediation of uranium-contaminated water to meet World Health Organization (WHO) standards (uranium ≤ 30 ppb), while concurrently facilitating the recovery of uranium from the flow electrode slurry. In this work, the phosphate-functionalized magnetic carbon-based flow electrode (OMPAC) was synthesized by simply co-precipitation and oxygen plasma treatment. The enhanced conductivity of OMPAC accelerated the efficient remediation of surface water contaminated with multiple nuclides, due to the improved charge-transfer capability facilitated by the introduced magnetic particles (Fe, Fe3O4, Fe3C) and heteroatoms (O, P). The uranium in feed solution was selectively adsorbed by OMPAC in flow electrode slurry, benefiting from the multiple strong sorption interactions between U(VI) and C=O/P=O/P-O groups, as well as the redox reactions between U(VI) and Fe (0/II). After four batch cycles, the average uranium removal rate by OMPAC was maintained at 97.84 %, while the recovery rate of uranium from OMPAC reached 78.2 %, demonstrating the excellent long-term performance and synchronous uranium recovery capability in FCDI. This study provides feasibility guidance for the remediation of radioactive pollution and the strategic reuse of resources via the FCDI technology.

Effects of EDTA and Bicarbonate on U(VI) Reduction by Reduced Nontronite

Widespread Fe-bearing clay minerals are potential materials capable of reducing and immobilizing U(VI). However, the kinetics of this process and the impact of environmental factors remain unclear. Herein, we investigated U(VI) reduction by chemically reduced nontronite (rNAu-2) in the presence of EDTA and bicarbonate. U(VI) was completely reduced within 192 h by rNAu-2 alone, and higher Fe(II) in rNAu-2 resulted in a higher U(VI) reduction rate. However, the presence of EDTA and NaHCO3 initially inhibited U(VI) reduction by forming stable U(VI)-EDTA/carbonato complexes and thus preventing U(VI) from adsorbing onto the rNAu-2 surface. However, over time, EDTA facilitated the dissolution of rNAu-2, releasing Fe(II) into solution. Released Fe(II) competed with U(VI) to form Fe(II)-EDTA complexes, thus freeing U(VI) from negatively charged U(VI)-EDTA complexes to form positively charged U(VI)-OH complexes, which ultimately promoted U(VI) adsorption and triggered its reduction. In the NaHCO3 system, U(VI) complexed with carbonate to form U(VI)-carbonato complexes, which partially inhibited adsorption to the rNAu-2 surface and subsequent reduction. The reduced U(IV) largely formed uraninite nanoparticles, with a fraction present in the rNAu-2 interlayer. Our results demonstrate the important impacts of clay minerals, organic matter, and bicarbonate on U(VI) reduction, providing crucial insights into the uranium biogeochemistry in the subsurface environment and remediation strategies for uranium-contaminated environments.

Determination of Uranium Central-Field Covalency with 3d4f Resonant Inelastic X-ray Scattering

Understanding the nature of metal-ligand bonding is a major challenge in actinide chemistry. We present a new experimental strategy for addressing this challenge using actinide 3d4f resonant inelastic X-ray scattering (RIXS). Through a systematic study of uranium(IV) halide complexes, [UX6]2-, where X = F, Cl, or Br, we identify RIXS spectral satellites with relative energies and intensities that relate to the extent of uranium-ligand bond covalency. By analyzing the spectra in combination with ligand field density functional theory we find that the sensitivity of the satellites to the nature of metal-ligand bonding is due to the reduction of 5f interelectron repulsion and 4f-5f spin-exchange, caused by metal-ligand orbital mixing and the degree of 5f radial expansion, known as central-field covalency. Thus, this study furthers electronic structure quantification that can be obtained from 3d4f RIXS, demonstrating it as a technique for estimating actinide-ligand covalency.

Enhanced uranium removal from aqueous solution by core-shell Fe0@Fe3O4: Insight into the synergistic effect of Fe0 and Fe3O4

In this study, Fe0@Fe3O4 was synthesized and used to remove U(VI) from groundwater. Different experimental conditions and cycling experiments were used to investigate the performance of Fe0@Fe3O4 in the U(VI) removal, and the XRD, TEM, XPS and XANES techniques were employed to characterize the Fe0@Fe3O4. The results showed that the U(VI) removal efficiency of Fe0@Fe3O4 was 48.5 mg/g that was higher than the sum of removal efficiency of Fe0 and Fe3O4. The uranium on the surface of Fe0@Fe3O4 mainly existed as U(IV), followed by U(VI) and U(V). The Fe0 content decreased after reaction, while the Fe3O4 content increased. Based on the results of experiments and characterization, the enhanced removal efficiency of Fe0@Fe3O4 was attributed to the synergistic effect of Fe0 and Fe3O4 in which Fe3O4 accelerated the Fe0 corrosion that promoted the progressively formation of Fe(II) that promoted the reduction of adsorbed U(VI) to U(IV) and incorporated U(VI) to U(V). The performance of Fe0@Fe3O4 at near-neutrality condition was better than at acidic and alkalic conditions. The chloride ions, sulfate ions and nitrate ions showed minor effect on the Fe0@Fe3O4 performance, while carbonate ions exhibited significant inhibition. The metal cations showed different effect on the Fe0@Fe3O4 performance. The removal efficiency of Fe0@Fe3O4 decreased with the number of cycling experiment. Ionizing radiation could regenerate the used Fe0@Fe3O4. This study provides insight into the U(VI) removal by Fe0@Fe3O4 in aqueous solution.

Insights into the long-term immobilization performances and mechanisms of CMC-Fe0/FeS with different sulfur sources for uranium under anoxic and oxic aging

Nanoscale zerovalent iron (Fe0)-based materials have been demonstrated to be a effective method for the U(VI) removal. However, limited research has been conducted on the long-term immobilization efficiency and mechanism of Fe0-based materials for U(VI), which are essential for achieving safe handling and disposal of U(VI) on a large scale. In this study, the prepared carboxymethyl cellulose (CMC) and sulfurization dual stabilized Fe0 (CMC-Fe0/FeS) exhibited excellent long-term immobilization performances for U(VI) under both anoxic and oxic conditions, with the immobilization efficiencies were respectively reached over 98.0 % and 94.8 % after 180 days of aging. Most importantly, different from the immobilization mechanisms of the fresh CMC-Fe0/FeS for U(VI) (the adsorption effect of -COOH and -OH groups, coordination effect with sulfur species, as well as reduction effect of Fe0), the re-mobilized U(VI) were finally re-immobilized by the formed FeOOH and Fe3O4 on the aged CMC-Fe0/FeS. Under anoxic conditions, more Fe3O4 was produced, which may be the main reason for the long-term immobilization U(VI). Under oxic conditions, the production of Fe3O4 and FeOOH were relatively high, which both played significant roles in re-immobilizing U(VI) through surface complexation, reduction and incorporation effects.

Unveiling the crucial role of iron mineral phase transformation in antimony(V) elimination from natural water

Antimony (Sb) in natural water has long-term effects on both the ecological environment and human health. Iron mineral phase transformation (IMPT) is a prominent process for removing Sb(V) from natural water. However, the importance of IMPT in eliminating Sb remains uncertain. This study examined the various Sb-Fe binding mechanisms found in different IMPT pathways in natural water, shedding light on the underlying mechanisms. The study revealed that the presence of goethite (Goe), hematite (Hem), and magnetite (Mag) significantly affected the concentration of Sb(V) in natural water. Elevated pH levels facilitated higher Fe content in iron solids but impeded the process of removing Sb(V). To further our understanding, polluted natural water samples were collected from various locations surrounding Sb smelter sites. Results confirmed that converting ferrihydrite (Fhy) to Goe significantly reduced Sb levels (<5 μg/L) in natural water. The emergence of secondary iron phases resulted in greater electrostatic attraction and stabilized surface complexes, which was the most likely cause of the decline of Sb concentration in natural water. The comprehensive findings offer new insights into the factors governing IMPT as well as the Sb(V) behavior control.

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.

Application of High-Energy-Resolution X-ray Absorption Spectroscopy at the U L3-Edge to Assess the U(V) Electronic Structure in FeUO4

FeUO₄ was studied to clarify the electronic structure of U(V) in a metal monouranate compound. We obtained the peak splitting of spectra utilizing high-energy-resolution fluorescence detection-X-ray absorption near-edge structure (HERFD-XANES) spectroscopy at the U L₃-edge, which is a novel technique in uranium(V) monouranate compounds. Theoretical calculations revealed that the peak splitting was caused by splitting of the 6d orbital of U(V) in FeUO₄, which would be used to detect minor U(V) species. Such distinctive electronic states are of major interest to researchers and engineers working in various fields, from fundamental physics to the nuclear industry and environmental sciences for actinide elements.

Persistence of the Isotopic Signature of Pentavalent Uranium in Magnetite

Uranium isotopic signatures can be harnessed to monitor the reductive remediation of subsurface contamination or to reconstruct paleo-redox environments. However, the mechanistic underpinnings of the isotope fractionation associated with U reduction remain poorly understood. Here, we present a coprecipitation study, in which hexavalent U (U(VI)) was reduced during the synthesis of magnetite and pentavalent U (U(V)) was the dominant species. The measured δ²³⁸U values for unreduced U(VI) (∼-1.0‰), incorporated U (96 ± 2% U(V), ∼-0.1‰), and extracted surface U (mostly U(IV), ∼0.3‰) suggested the preferential accumulation of the heavy isotope in reduced species. Upon exposure of the U-magnetite coprecipitate to air, U(V) was partially reoxidized to U(VI) with no significant change in the δ²³⁸U value. In contrast, anoxic amendment of a heavy isotope-doped U(VI) solution resulted in an increase in the δ²³⁸U of the incorporated U species over time, suggesting an exchange between incorporated and surface/aqueous U. Overall, the results support the presence of persistent U(V) with a light isotope signature and suggest that the mineral dynamics of iron oxides may allow overprinting of the isotopic signature of incorporated U species. This work furthers the understanding of the isotope fractionation of U associated with iron oxides in both modern and paleo-environments.