Reduction rate of neptunium(V) in heterogeneous solution with magnetite

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.

Sorption behaviour of neptunium in marine and fresh water bottom sediments in Far East area of Russia (Lake Khanka and Amur Bay)

The concentration and sorption behavior of 237Np on the bottom sediments of water bodies in the Far East region of Russia (Lake Khanka and Peter the Great Bay) were studied for the first time. The 237Np concentrations vary from 1.06 × 10-6 to 4.43 × 10-5 mBq g-1 in the bottom sediments of Lake Khanka and from 1.05 × 10-4 to 2.52 × 10-3 mBq g-1 for Amur Bay. The experiment on the adsorption of Np on marine and lake sediment showed that it is sorbed through complexation with silicates (albite, leucite). The Np sorption isotherm on marine sediments is described by the Langmuir equation; the distribution coefficients (Kd) of Np vary from 57 to 588 mL g-1. For lake sediments, the isotherm is described by the Henry equation; the Kd value reaches 935 mL g-1.

Oxidation and Nanoparticle Formation during Ce(III) Sorption onto Minerals

The sorption of Ce(III) on three abundant environmental minerals (goethite, anatase, and birnessite) was investigated. Batch sorption experiments using a radioactive ¹³⁹Ce tracer were performed to investigate the key features of the sorption process. Differences in sorption kinetics and changes in oxidation states were found in the case of the sorption of Ce(III) on birnessite compared to that on other minerals. Speciation of cerium onto all of the studied minerals was investigated using spectral and microscopic methods: high-resolution transmission electron microscopy (HRTEM), electron energy loss spectroscopy (EELS), and X-ray absorption spectroscopy (XAS) in conjunction with theoretical calculations. It was found that during the sorption process onto birnessite, Ce(III) was oxidized to Ce(IV), while the Ce(III) on goethite and anatase surfaces remained unchanged. Oxidation of Ce(III) by sorption on birnessite was also accompanied by the formation of CeO₂ nanoparticles on the mineral surface, which depended on the initial cerium concentration and pH value.

Retention of immobile Se(0) in flow-through aquifer column systems during bioreduction and oxic-remobilization

Selenium (Se) is a toxic contaminant with multiple anthropogenic sources, including ⁷⁹Se from nuclear fission. Se mobility in the geosphere is generally governed by its oxidation state, therefore understanding Se speciation under variable redox conditions is important for the safe management of Se contaminated sites. Here, we investigate Se behavior in sediment groundwater column systems. Experiments were conducted with environmentally relevant Se concentrations, using a range of groundwater compositions, and the impact of electron-donor (i.e., biostimulation) and groundwater sulfate addition was examined over a period of 170 days. X-Ray Absorption Spectroscopy and standard geochemical techniques were used to track changes in sediment associated Se concentration and speciation. Electron-donor amended systems with and without added sulfate retained up to 90% of added Se(VI)_(aq), with sediment associated Se speciation dominated by trigonal Se(0) and possibly trace Se(-II); no Se colloid formation was observed. The remobilization potential of the sediment associated Se species was then tested in reoxidation and seawater intrusion perturbation experiments. In all treatments, sediment associated Se (i.e., trigonal Se(0)) was largely resistant to remobilization over the timescale of the experiments (170 days). However, in the perturbation experiments, less Se was remobilized from sulfidic sediments, suggesting that previous sulfate-reducing conditions may buffer Se against remobilization and migration.

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.

Reduction of Hg(II) by Fe(II)-Bearing Smectite Clay Minerals

Aluminosilicate clay minerals are often a major component of soils and sediments and many of these clays contain structural Fe (e.g., smectites and illites). Structural Fe(III) in smectite clays is redox active and can be reduced to Fe(II) by biotic and abiotic processes. Fe(II)-bearing minerals such as magnetite and green rust can reduce Hg(II) to Hg(0); however, the ability of other environmentally relevant Fe(II) phases, such as structural Fe(II) in smectite clays, to reduce Hg(II) is largely undetermined. We conducted experiments examining the potential for reduction of Hg(II) by smectite clay minerals containing 0–25 wt% Fe. Fe(III) in the clays (SYn-1 synthetic mica-montmorillonite, SWy-2 montmorillonite, NAu-1 and NAu-2 nontronite, and a nontronite from Cheney, Washington (CWN)) was reduced to Fe(II) using the citrate-bicarbonate-dithionite method. Experiments were initiated by adding 500 µM Hg(II) to reduced clay suspensions (4 g clay L−1) buffered at pH 7.2 in 20 mM 3-morpholinopropane-1-sulfonic acid (MOPS). The potential for Hg(II) reduction in the presence of chloride (0–10 mM) and at pH 5–9 was examined in the presence of reduced NAu-1. Analysis of the samples by Hg LIII-edge X-ray absorption fine structure (XAFS) spectroscopy indicated little to no reduction of Hg(II) by SYn-1 (0% Fe), while reduction of Hg(II) to Hg(0) was observed in the presence of reduced SWy-2, NAu-1, NAu-2, and CWN (2.8–24.8% Fe). Hg(II) was reduced to Hg(0) by NAu-1 at all pH and chloride concentrations examined. These results suggest that Fe(II)-bearing smectite clays may contribute to Hg(II) reduction in suboxic/anoxic soils and sediments.

Effect of different physico-chemical factors on sorption of Pa(V) on iron oxides

The effects of different physico-chemical parameters on sorption of protactinium (Pa) on three different iron oxides (goethite, hematite and magnetite) were studied. The sorption of Pa(V) on all the three iron oxides was low at pH 1, increased to ~ 90% at neutral pH and then decreased slightly above pH 9. Presence of humic acid increases the sorption of Pa(V) on iron oxides in acidic medium whereas sorption is reduced in basic medium. The classical methods like ionic strength, time and temperature dependent sorption studies have been used for the elucidation of the sorption mechanism of Pa(V) on iron oxides. Physisorption in acidic pH and chemisorption in basic pH are the most probable mechanisms. The study demonstrates that classical method alone can be used for the sorption mechanism investigations in the circumstances where it is difficult to have spectroscopic evidences.

Kinetics of neptunium sorption and desorption in the presence of aluminum (hydr)oxide minerals: Evidence for multi-step desorption at low pH

Kinetics analyses of sorption and desorption provide important insight into reaction mechanisms occurring at the mineral-water interface. They are also needed to determine when equilibrium is achieved, identify intermediate chemical species, and inform models describing neptunium mobility. Neptunium sorption to and desorption from four different aluminum (hydr)oxides - bayerite (α-Al(OH)₃), gibbsite (γ-Al(OH)₃), corundum (α-Al₂O₃), and γ-alumina (γ-Al₂O₃) - were investigated as a function of mineral concentration (5 - 170 m² L^-1), neptunium concentration (10^-9 - 10^-7 M), and pH (5.5 - 10.5). Neptunium sorption was characterized by a two-step reaction with an initial fast sorption step occurring within minutes followed by a slower equilibrium process, which was attributed to initial sorption of neptunium to a small number of strong sorption sites followed by sorption of neptunium to a larger number of weak sorption sites. The kinetics data were modeled using the linear and non-linear forms of the pseudo-first and pseudo-second order rate equations and the goodness of fit parameters were compared. Non-linear pseudo-second order rate constants described neptunium sorption to aluminum (hydr)oxides most accurately and were used to determine the reaction orders with respect to mineral concentration and [H⁺]. Neptunium desorption experiments demonstrated that the desorption mechanism changed as a function of pH and that the forward and reverse reactions were not equivalent. At pH ≥ 7.5, desorption reached steady-state within an hour and was accurately described by the non-linear pseudo-second order rate equations. A desorption plateau was observed at pH 5.5 that could not be described by either pseudo-first or -second order kinetics, suggesting the possibility of a multi-step desorption reaction. The comparatively slow desorption kinetics observed here suggests that sorbed neptunium could be slowly released back into the aqueous phase and act as a continuous source of contamination to the environment.

Neptunium(V) and Uranium(VI) Reactions at the Magnetite (111) Surface

Neptunium and uranium are important radionuclides in many aspects of the nuclear fuel cycle and are often present in radioactive wastes which require long term management. Understanding the environmental behaviour and mobility of these actinides is essential in underpinning remediation strategies and safety assessments for wastes containing these radionuclides. By combining state-of-the-art X-ray techniques (synchrotron-based Grazing Incidence XAS, and XPS) with wet chemistry techniques (ICP-MS, liquid scintillation counting and UV-Vis spectroscopy), we determined that contrary to uranium(VI), neptunium(V) interaction with magnetite is not significantly affected by the presence of bicarbonate. Uranium interactions with a magnetite surface resulted in XAS and XPS signals dominated by surface complexes of U(VI), while neptunium on the surface of magnetite was dominated by Np(IV) species. UV-Vis spectroscopy on the aqueous Np(V) species before and after interaction with magnetite showed different speciation due to the presence of carbonate. Interestingly, in the presence of bicarbonate after equilibration with magnetite, an unknown aqueous NpO2+ species was detected using UV-Vis spectroscopy, which we postulate is a ternary complex of Np(V) with carbonate and (likely) an iron species. Regardless, the Np speciation in the aqueous phase (Np(V)) and on the magnetite (111) surfaces (Np(IV)) indicate that with and without bicarbonate the interaction of Np(V) with magnetite proceeds via a surface mediated reduction mechanism. Overall, the results presented highlight the differences between uranium and neptunium interaction with magnetite, and reaffirm the potential importance of bicarbonate present in the aqueous phase.

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.

NeptuniumV Retention by Siderite under Anoxic Conditions: Precipitation of NpO2-Like Nanoparticles and of NpIV Pentacarbonate

The Np^V retention by siderite, an Fe^II carbonate mineral with relevance for the near-field of high-level radioactive waste repositories, was investigated under anoxic conditions. Batch sorption experiments show that siderite has a high affinity for aqueous Np^VO₂⁺ across pH 7 to 13 as expressed by solid-water distribution coefficients, log R_d, > 5, similar to the log R_d determined for the (solely) tetravalent actinide Th on calcite, suggesting reduction of Np^V to Np^IV by siderite. Np L₃-edge X-ray absorption near edge (XANES) spectroscopy conducted in a pH range typical for siderite-containing host rocks (7-8), confirmed the tetravalent Np oxidation state. Extended X-ray absorption fine-structure (EXAFS) spectroscopy revealed a local structure in line with NpO₂-like nanoparticles with diameter < 1 nm, a result further corroborated by high-resolution transmission electron microscopy (HRTEM). The low solubility of these NpO₂-like nanoparticles (∼10^-9 M), along with their negligible surface charge at neutral pH conditions which favors particle aggregation, suggest an efficient retention of Np in the near-field of radioactive waste repositories. When Np^V was added to ferrous carbonate solution, the subsequent precipitation of siderite did not lead to a structural incorporation of Np^IV by siderite, but caused precipitation of a Np^IV pentacarbonate phase.

Effects of Titanium Doping in Titanomagnetite on Neptunium Sorption and Speciation

Neptunium-237 is a radionuclide of great interest owing to its long half-life (2.14 × 10(6) years) and relative mobility as the neptunyl ion (NpO2(+)) under many surface and groundwater conditions. Reduction to tetravalent neptunium (Np(IV)) effectively immobilizes the actinide in many instances due to its low solubility and strong interactions with natural minerals. One such mineral that may facilitate the reduction of neptunium is magnetite (Fe(2+)Fe(3+)2O4). Natural magnetites often contain titanium impurities which have been shown to enhance radionuclide sorption via titanium's influence on the Fe(2+)/Fe(3+) ratio (R) in the absence of oxidation. Here, we provide evidence that Ti-substituted magnetite reduces neptunyl species to Np(IV). Titanium-substituted magnetite nanoparticles were synthesized and reacted with NpO2(+) under reducing conditions. Batch sorption experiments indicate that increasing Ti concentration results in higher Np sorption/reduction values at low pH. High-resolution transmission electron microscopy of the Ti-magnetite particles provides no evidence of NpO2 nanoparticle precipitation. Additionally, X-ray absorption spectroscopy confirms the nearly exclusive presence of Np(IV) on the titanomagnetite surface and provides supporting data indicating preferential binding of Np to terminal Ti-O sites as opposed to Fe-O sites.

Sorption coefficients and molecular mechanisms of Pu, U, Np, Am and Tc to Fe (hydr)oxides: a review

Pu, U, Np, Am and Tc are among the major risk drivers at nuclear waste management facilities throughout the world. Furthermore, uranium mining and milling operations have generated an enormous legacy of radioactively contaminated soils and groundwater. The sorption process of radionulcides onto ubiquitous Fe (hydr)oxides (FHOs; hematite, magnetite, goethite and ferrihydrite) is one of the most vital geochemical processes controlling the transport and fate of radionuclides and nuclear wastes in the subsurface zones. Meanwhile, understanding molecular-level chemical speciation of radionuclides onto FHOs is crucial to model their behavior in subsurface environments, and to develop new technologies for nuclear waste treatment and long-term remediation strategies for contaminated soils and groundwater. This review article aims (1) to provide risk or performance assessment modelers with macroscopic distribution coefficient (K(d)) data of Pu, U, Np, Am and Tc onto FHOs under different conditions (pH, radionuclide concentration, solution ion strength, sorbent loading, partial pressure of CO(2) (P CO(2)), equilibrium time) pertinent to environmental and engineered systems, and (2) to provide a microscopic or molecular-level understanding of the chemical speciation and sorption processes of these radionuclides to FHOs.

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.