U(VI) sorption during ferrihydrite formation: Underpinning radioactive effluent treatment

Computational study of the interactions of tetravalent actinides (An = Th-Pu) with the α-Fe13 Keggin cluster

In recent years, evidence has emerged that actinide (An) uptake at the enhanced actinide removal plant (EARP) at the UK's Sellafield nuclear site occurs via An interactions with an α-Fe13 Keggin molecular cluster during ferrihydrite formation. We here study theoretically the substitution of aquo complexes of the actinides Th-Pu onto a Na-decorated α-Fe13 Keggin cluster using DFT at the PBE0 level. The optimised Pu-O and Pu-Fe distances are in good agreement with experiment and Na/An substitutions are significantly favourable energetically, becoming more so across the early 5f series, with the smallest and largest ΔrG° being for Th and Pu at -335.7 kJ mol-1 and -396.0 kJ mol-1 respectively. There is strong correlation between the substitution reaction energy and the ionic radii of the actinides (Δrε0R2 = 0.97 and ΔrG° R2 = 0.91), suggesting that the principal An-Keggin binding mode is ionic. Notwithstanding this result, Mulliken and natural population analyses reveal that covalency increases from Th-Pu in these systems, supported by analysis of the occupied Kohn-Sham molecular orbitals where enhanced An(5f)-O(2p) overlap is observed in the Np and Pu systems. By contrast, quantum theory of atoms in molecules analysis shows that U-Keggin binding is the most covalent among the five actinides, in keeping with previous studies.

Adsorption of octahedral mono-molybdate and poly-molybdate onto hematite: A multi-technique approach

Molybdenum (Mo) is a key trace element and a contaminant in many environments including mine tailings and acid mine drainage systems. Under oxic conditions Mo exists in a number of forms, including mono-molybdate (Mo(VI)O₄^2-) and various poly-molybdate species (e.g. Mo(VI)₇O₂₄^6-) depending on the geochemical conditions (e.g. pH). The mobility and bioavailability of Mo is often controlled by sorption to mineral surfaces, including iron (oxyhydr)oxides e.g. hematite (Fe₂O₃). This study uses adsorption isotherms, PHREEQC geochemical modeling, Attenuated Total Reflection-Fourier Transform Infrared Spectroscopy (ATR-FTIR), and X-ray Absorption Spectroscopy (XAS) to holistically characterise the molecular scale adsorption of molybdate to hematite as a function of pH (3-12) and Mo(VI) concentration (0.01 × 10^-4 - 2 × 10^-3 M). PHREEQC and ATR-FTIR indicated both pH and Mo concentration are important variables when forming mono- vs. poly- molybdate and suggest low pH (≤ 4) and high Mo(VI) concentration (≥ 5 × 10^-4 M) contribute to the formation of a poly-molybdate surface species on the hematite surface. XAS found Mo adsorbed to hematite via an inner-sphere corner-sharing bidentate binuclear complex with an octahedral mono-molybdate structure at a Mo concentration of 0.6 × 10^-4 M across the pH range, and at a Mo(VI) concentration of 5 × 10^-4 M and pH over 5. This is the first direct observation of octahedrally coordinated Mo(VI) adsorption species on hematite, and this information has broad implications for the mobility and transport of Mo as a contaminant in the environment.

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.

Extracellular electron transfer influences the transport and retention of ferrihydrite nanoparticles in quartz sand coated with Shewanella oneidensis biofilm

Microbial biofilm has been found to impact the mobility of nanoparticles in saturated porous media by altering physicochemical properties of collector surface. However, little is known about the influence of biofilm's biological activity on nanoparticle transport and retention. Here, the transport of ferrihydrite nanoparticles (FhNPs) was studied in quartz sands coated with biofilm of Shewanella oneidensis MR-1 that is capable of reducing Fe(III) through extracellular electron transfer (EET). It was found that MR-1 biofilm coating enhanced FhNPs' deposition under different pH/ionic strength conditions and humic acid concentrations. More importantly, when the influent electron donor (glucose) concentration was increased to promote biofilm's EET activity, the breakthrough of FhNPs in biofilm-coated sands was inhibited. A lack of continuous and stable supply of electron donor, on the contrary, led to remobilization and release of the originally retained FhNPs. Column experiments with biofilm of EET-deficient MR-1 mutants (ΔomcA/ΔmtrC and ΔcymA) further indicated that the impairment of EET activity decreased the retention of FhNPs. It is proposed that the effective surface binding and adhesion of FhNPs that is required by direct EET cannot be neglected when evaluating the transport of FhNPs in sands coated with electroactive biofilm.

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.

Metal oxide aerogels: Preparation and application for the uranium removal from aqueous solution

The low density CeO₂, Pr₂O₃ and Nd₂O₃ aerogels were synthesized by a novel solution-freeze-drying-calcination route. The bulk densities of the CeO₂, Pr₂O₃ and Nd₂O₃ aerogels were calculated to be 8.10, 10.67 and 9.80 mg/cm³, respectively. Moreover, the structure of metal oxide aerogels was similar due to the same template materials, which made them possible to be appealing materials for adsorption. The maximum adsorption capacity for U(VI) of the CeO₂, Pr₂O₃ and Nd₂O₃ aerogels reached 481.5, 840.6 and 587.3 mg/g (pH = 7, T = 25 °C), respectively, which were much higher than most of other modified metal oxides. Moreover, at low concentration of U(VI) (5 mg/L), it was completely dislodged by CeO₂ aerogel and the remaining U(VI) was below 0.006 mg/L, which was lower than the effluent standards of United States Environmental Protection Agency and World Health Organization. Besides, after five cycles, the adsorption efficiency of metal oxide aerogels almost remained at a high level. Due to the excellent adsorption performance and high reusability, the three metal oxide aerogels would be promising adsorbents for the removal of U(VI).

Uranium and neptunium retention mechanisms in Gallionella ferruginea/ferrihydrite systems for remediation purposes

The ubiquitous β-Proteobacterium Gallionella ferruginea is known as stalk-forming, microaerophilic iron(II) oxidizer, which rapidly produces iron oxyhydroxide precipitates. Uranium and neptunium sorption on the resulting intermixes of G. ferruginea cells, stalks, extracellular exudates, and precipitated iron oxyhydroxides (BIOS) was compared to sorption to abiotically formed iron oxides and oxyhydroxides. The results show a high sorption capacity of BIOS towards radionuclides at circumneutral pH values with an apparent bulk distribution coefficient (Kd) of 1.23 × 104 L kg-1 for uranium and 3.07 × 105 L kg-1 for neptunium. The spectroscopic approach by X-ray absorption spectroscopy (XAS) and ATR FT-IR spectroscopy, which was applied on BIOS samples, showed the formation of inner-sphere complexes. The structural data obtained at the uranium LIII-edge and the neptunium LIII-edge indicate the formation of bidentate edge-sharing surface complexes, which are known as the main sorption species on abiotic ferrihydrite. Since the rate of iron precipitation in G. ferruginea-dominated systems is 60 times faster than in abiotic systems, more ferrihydrite will be available for immobilization processes of heavy metals and radionuclides in contaminated environments and even in the far-field of high-level nuclear waste repositories.

Biogeochemical Modelling of Uranium Immobilization and Aquifer Remediation Strategies Near NCCP Sludge Storage Facilities

Nitrate is a substance which influences the prevailing redox conditions in groundwater, and in turn the behaviour of U. The study of groundwater in an area with low-level radioactive sludge storage facilities has shown their contamination with sulphate and nitrate anions, uranium, and some associated metals. The uranyl ion content in the most contaminated NO3–Cl–SO4–Na borehole is 2000 times higher (1.58 mg/L) than that in the background water. At the same time, assessment of the main physiological groups of microorganisms showed a maximum number of denitrifying and sulphate-reducing bacteria (e.g., Sulfurimonas) in the water from the same borehole. Biogenic factors of radionuclide immobilization on sandy rocks of upper aquifers have been experimentally investigated. Different reduction rates of NO3−, SO42−, Fe(III) and U(VI) with stimulated microbial activity were dependent on the pollution degree. Moreover, 16S rRNA gene analysis of the microbial community after whey addition revealed a significant decrease in microbial diversity and the activation of nonspecific nitrate-reducing bacteria (genera Rhodococcus and Rhodobacter). The second influential factor can be identified as the formation of microbial biofilms on the sandy loam samples, which has a positive effect on U sorption (an increase in Kd value is up to 35%). As PHREEQC physicochemical modelling numerically confirmed, the third most influential factor that drives U mobility is the biogenic-mediated formation of a sulphide redox buffer. This study brings important information, which helps to assess the long-term stability of U in the environment of radioactive sludge storage facilities.

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.

Plutonium(IV) Sorption during Ferrihydrite Nanoparticle Formation

Understanding interactions between iron (oxyhydr)oxide nanoparticles and plutonium is essential to underpin technology to treat radioactive effluents, in cleanup of land contaminated with radionuclides, and to ensure the safe disposal of radioactive wastes. These interactions include a range of adsorption, precipitation, and incorporation processes. Here, we explore the mechanisms of plutonium sequestration during ferrihydrite precipitation from an acidic solution. The initial 1 M HNO₃ solution with Fe(III)_(aq) and ²⁴²Pu(IV)_(aq) underwent controlled hydrolysis via the addition of NaOH to pH 9. The majority of Fe(III)_(aq) and Pu(IV)_(aq) was removed from solution between pH 2 and 3 during ferrihydrite formation. Analysis of Pu-ferrihydrite by extended X-ray absorption fine structure (EXAFS) spectroscopy showed that Pu(IV) formed an inner-sphere tetradentate complex on the ferrihydrite surface, with minor amounts of PuO₂ present. Best fits to the EXAFS data collected from Pu-ferrihydrite samples aged for 2 and 6 months showed no statistically significant change in the Pu(IV)-Fe oxyhydroxide surface complex despite the ferrihydrite undergoing extensive recrystallization to hematite. This suggests the Pu remains strongly sorbed to the iron (oxyhydr)oxide surface and could be retained over extended time periods.

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.