Solubility Of Uranium Research Articles

Dissolution behavior of calcium uranate under oxidizing and reducing conditions

Calcium uranate solid phase is a secondary mineral found in geological environments. It may form in the residues of high-level radioactive liquid waste and in the fuel debris of the TEPCO's Fukushima Daiichi Nuclear Power Plant under high-temperature conditions. To assess the chemical stability of CaUO4 in different aqueous environments, static immersion tests were performed under various redox conditions and carbonate ion concentrations. pH and Eh values, as well as concentrations of uranium, calcium, and total carbonate in the solutions, were measured after the supernatants were filtered through a membrane with a 10 kDa molecular weight cutoff. The components of the solid phase were also evaluated using X-ray diffraction and X-ray absorption fine structure analyses. The dissolution mechanism of CaUO4 was examined using data from solid and liquid analyses, along with chemical thermodynamic calculations. Under reducing conditions and without carbonate, U(VI) in CaUO4 was reduced to U(V) and the mineral was converted into non-stoichiometric CaUO4−x. The dissolved uranium was then further reduced to U(IV) in the aqueous media, forming UO2(am), which controlled the U solubility. Under oxidizing conditions and in the absence of carbonate, dissolved uranium formed metaschoepite ((UO3)·2H2O(cr)) at pH ≤ 7 and sodium diuranate (Na2U2O7·H2O(cr)) at pH > 7, which controlled uranium solubility. In oxidizing conditions with carbonate present, the apparent solubility of U was lower than predicted by the solid-phase solubility calculations. The concentration of U was constrained to levels similar to that of calcium when the calcium concentration reached saturation with CaCO3. Additionally, the dissolution of calcium from CaUO4 influenced uranium dissolution.

Uranium(VI) hydrolysis up to 250 °C and its geological implications

The hydrolysis of uranium(VI) has been investigated by solubility experiments conducted on sodium uranate (Na2U2O7) over a pHT range of 4.5–8 at 200 and 250 °C. Equilibrium constants for the UO2OH+, UO2OH20, UO2OH3- and UO2OH42- complexes have all been derived at both temperatures. Alongside this investigation a comprehensive literature review has been conducted to document previous investigations into U(VI) hydrolysis at temperatures above 25 °C. Using the new data reported here alongside those available in the literature, modified Ryzhenko-Bryzgalin equation of state parameters have been generated for the UO2OH+, UO2OH20, UO2OH3- and UO2OH42- complexes along with the polynuclear UO22OH22+ and UO23OH5+ complexes. This permits future works to extrapolate thermodynamic properties for these complexes to pressure–temperature conditions beyond those for which experimental data are available. Results reported here highlight the potential inadequacies of using the Ryzhenko-Bryzgalin and revised Helgeson-Kirkham-Flowers equations of state to extrapolate thermodynamic properties solely from room temperature data, with discrepancies ranging over 2–6 orders of magnitude being observed between extrapolations and the equilibrium constants experimentally derived at 200/250 °C. Using the newly derived properties, we report the results of thermodynamic calculations of uranium solubility and aqueous speciation under hydrothermal conditions relevant to a range of geological systems. These calculations suggest that uranyl hydroxide complexes may be responsible for uranium transport in a wide range of crustal fluids potentially obviating the need for high ligand concentrations.

Hydrothermal synthesis of (Zr,U)SiO4: an efficient pathway to incorporate uranium into zircon.

The preparation of synthetic (Zr,U)SiO4 solid solution is challenging, as the conventional high-temperature solid-state method limits the solubility of uranium (4 ± 1 mol%) in the orthosilicate phase due to its thermodynamic instability. However, these compounds are of great interest as a result of (Zr,U)SiO4 solid solutions, with uranium contents exceeding this concentration, being observed as corium phases formed during nuclear accidents. It has been identified that hydrothermal synthesis pathways can be used for the formation of the metastable phase, such as USiO4. The investigation carried out in this study has indeed led to the confirmation of metastable (Zr,U)SiO4 compounds with high uranium contents being formed. It was found that (Zr,U)SiO4 forms a close-to-ideal solid solution with uranium loading of up to 60 mol% by means of hydrothermal treatment for 7 days at 250 °C, at pH = 3 and starting from an equimolar reactant concentration equal to 0.2 mol L-1. A purification procedure was developed to obtain pure silicate compounds. After purification, these compounds were found to be stable up to 1000 °C under an inert atmosphere (argon). The characterisation methods used to explore the synthesis and thermal stability included powder X-ray diffraction (PXRD), Fourier transform infrared (FTIR) and Raman spectroscopies, scanning electron microscopy (SEM) and thermogravimetric analysis (TGA).

Nitrate stimulated microbial and viral activity and the subsequent influence on uranium mobility in sedimentary systems

Mobilization of naturally-occurring uranium(U) has been recognized to give rise to geogenic U groundwater contamination in aquifers. In addition to carbonate ligand complexation, nitrate has been demonstrated to play a role in controlling U mobility by altering uranium solubility through redox reactions. Nitrate is a common anthropogenic contaminant often prevalent at high concentrations in alluvial aquifers overlaying managed land. Alluvial deposition processes that form these aquifers create a lithologically heterogeneous subsurface with defined contacts between sands, silts, and clays. This leads to deposition of organic carbon and accumulation of reduced metals/radionuclides, including U(IV), in the finer grained silts and clays. The addition of high nitrate porewater into uranium-bearing alluvial aquifer silt sediments stimulated a nitrate reducing microbial community capable of catalyzing U(IV) oxidation and mobilization of U into porewaters. However, metadata from an aquifer wide study and a subsequent experiment revealed that this result is concentration dependent. Low concentrations of nitrate bearing pore-water added into organic-rich, uranium bearing sediments and resulted in a decrease in dissolved U(VI), consistent with reduction. XANES analysis of sediments supported U(VI) reduction with the precipitation of U(IV). U(VI) reduction activity occurred concurrent with an increase in dissolved organic carbon (DOC) and cell and virus abundance and activity. Metagenome assembled genomes from the microbial community revealed the metabolic potential indicating complex carbon degradation, fermentation, mineralization as well as the potential for anaerobic respiration of nitrate, metal/radionuclides, and sulfate. The virome recovered from the samples indicated a change in viral community in response to nitrate amendments and viral-encoded carbohydrate active enzymes were upregulated indicating a coupled response of both viral and microbial community regulating nitrate stimulated carbon biogeochemical cycling. These data together suggest that the addition of an electron acceptor in to organic carbon reduced sediments stimulates not only microbial but also viral activity leading to upregulation of genes associated with carbon biogeochemical cycling in sedimentary systems. While genes associated with metal oxidation are observed, net reduction of uranium prevails leading to uranium immobilization at low nitrate concentrations. Thus together these data indicate a tipping point whereby the influx of nitrate into the reduced environment can influence uranium mobility in DOC and carbon cycling supporting microbial activity and reducing conditions subsurface systems.

Genetic Mechanism of Tabular-Shaped Orebody of the Hailijin Sandstone-Type Uranium Deposit in the Songliao Basin: Constraints on the Clay Mineralogy of Ore-Bearing Sandstone

The Hailijin (HLJ) sandstone-type uranium deposit was newly discovered in the southwestern Songliao Basin in recent years. Different from the roll-front orebody of the sandstone-type uranium deposits with (phreatic oxidation) interlayer redox origin (or phreatic oxidation), the orebody of the HLJ uranium deposit is tabular-shaped and multi-stratiform. The kaolinite content in ore-controlling gray sandstones is significantly higher than that in oxidized sandstones, which have the highest kaolinite content in the less oxidized zone of sandstone-type uranium deposits in the basins of western China (such as Yili Basin and Turpan-Hami Basin). In order to identify the properties of ore-forming fluids and the genesis of the tabular-shaped orebody of the HLJ uranium deposit, trace element, scanning electron microscopy (SEM), X-ray diffraction (XRD), and uranium mineral electron probe (EPMA) analyses of different geochemical zone sandstones in ore-bearing strata were carried out. As a result, kaolinite, illite, and illite/smectite formation (I/S) appear to alternate with one another in ore-controlling gray sandstones, and the content of kaolinite is the highest in ores. SEM analysis also suggests that uranium minerals are commonly adsorbed on the surface of foliated and vermicular kaolinite or trapped within micropores of kaolinite. In this case, it is inferred that kaolinite in ore-controlling gray sandstones is of epigenetic origin, and the ore-bearing sandstones have undergone at least one transformation of acidic fluids. Combined with the regional paleoclimate, regional tectonics, and regional burial history, it is concluded that the acidic fluid originated from the uranium-rich source rocks of the Lower Cretaceous Jiufotang Formation, and the tabular-shaped orebody of the HLJ uranium deposit was formed by exudative metallogeny. When the uranium-rich acidic organic fluids exuded upward from deep levels along the faults to the target strata, the solubility of uranium and other polymetallic elements decreased because of the decrease in temperature and pressure, and uranium eventually precipitated and accumulated in sandstones with suitable permeability and porosity. However, it cannot be ruled out that the superimposition and transformation of uranium mineralization was caused by phreatic oxidation or local interlayer redox during the interval of exudative metallogeny.

The Immobility of Uranium (U) in Metamorphic Fluids Explained by the Predominance of Aqueous U(IV)

The solubility of uranium (U) in hydrothermal fluid is thought to be controlled by oxidation. In general, uranium is mainly transported as U(VI) in oxidized fluid, but precipitated as U(IV) in reduced fluid. However, many geological observations indicate that metamorphic fluids, which are buffered by metamorphic rocks with oxidized protoliths such as oxidized pelite or altered marine basalt, are not enriched in U. To explore the reason of the low solubility of U in metamorphic fluids, we simulated the hydrous speciation and solubility of U in fluids that are in equilibrium with rocks. The simulations were conducted at pressure–temperature (P-T) conditions of greenschist and amphibolite facies metamorphism. The results show that U is mainly dissolved as U(IV), instead of U(VI), in metamorphic fluids. The solubility of U remains at a low level of ~10−12 molal, and is not significantly influenced by metamorphic temperature, pressure, and fluid salinity. This result is consistent with geological observations and, thus, can explain the low-U nature of natural metamorphic fluids. The simulation also shows high solubility of U(VI) (1.3 × 10−7 molal) in oxidized pelite-buffered fluids at low temperature (<250 °C), consistent with the geological fact that U can be mobilized by low-temperature geofluids.

Uranium(VI) Sorption onto Hardened Cement Paste under High Saline and Alkaline Conditions

Evaluation of the mobility behaviour of radionuclides under highly saline and alkaline conditions is a major concern for the performance assessment of radioactive waste disposal. The aim of this study was to determine the effect of up to 2.8 mol/kgsolution content of NaNO3, on the solubility and the retention of U(VI) at 22 °C onto a hardened cement paste (HCP) prepared from ordinary Portland cement (CEM I). To avoid the interference of the high salt concentration and ionic strength, and because of the expected low solubility of uranium under such alkaline conditions, time-resolved laser fluorescence spectroscopy (TRLFS) was selected to accurately measure U(VI) concentration in solution using the standard addition method in 85% H3PO4. This allows both limiting the dilution and matrix effects and determining the resulting [U(VI)] in solution with acceptable precision for the distribution factor (Rd) in both sorption and desorption experiments. The operational solubility limit measured at high ionic strength lowered by a factor of three compared to the reference cementitious condition, and its Rd values decreased by a factor ca. four. The sorption of U(VI) appears to be reversible under these conditions.

Solid solubility of uranium in Ca-phosphate glass and its impact on heat capacity and Tg of the glass matrix

Solid solubility of uranium in Ca-phosphate glass and its impact on heat capacity and Tg of the glass matrix

Influence of synthesis atmosphere on the solid solubility of uranium at B-site of Nd2Zr2O7 pyrochlore

In this study, the solid solubility of uranium in Nd2Zr2O7 pyrochlore and the ensuing phase evolution have been explored for nuclear waste immobilization and inert matrix fuel applications. To investigate the phase evolution upon U-substitution at the B-site of Nd2Zr2O7 pyrochlore, Nd2Zr2-xUxO7 (0 ≤ x ≤ 2) samples have been synthesized by a solid-state route under reducing and oxidizing conditions and thoroughly characterized by X-ray diffraction and Raman spectroscopy. XRD studies reveal that uranium forms a solid solution throughout the composition range adopting the pyrochlore structure at x ≤ 0.8 and the fluorite-type structure at x ≥ 1.2 under reducing conditions at 1873 K. The pyrochlore-type phase-field shrinks (x ≤ 0.4) with the concomitant extension of the fluorite-type phase-field (0.8 ≤ x ≤ 2) upon oxidation of these reduced samples in air at 1473 K. The samples synthesized under oxidizing conditions (air) at 1873 K exhibit transformation of the pyrochlore-type phase to a fluorite-type phase through a biphasic phase-field. The pyrochlore-type ordering diminishes for the oxidized samples compared to that of the reduced counterpart, which could be attributed to the oxidation of U4+ to smaller-sized U5+/U6+ and the concomitant incorporation of additional oxygen anions in the lattice for charge balance. Raman spectroscopy has been used to shed light on the local structures. Chemical analysis indicates an O/U ratio of 2.00(3) and 2.66(3) for the reduced and oxidized samples, respectively.

U(VI) binding onto electrospun polymers functionalized with phosphonate surfactants

We previously observed that phosphonate functionalized electrospun nanofibers can uptake U(VI), making them promising materials for sensing and water treatment applications. Here, we investigate the optimal fabrication of these materials and their mechanism of U(VI) binding under the influence of environmentally relevant ions (e.g., Ca2+ and CO32-). We found that U(VI) uptake was greatest on polyacrylonitrile (PAN) functionalized with longer-chain phosphonate surfactants (e.g., hexa- and octadecyl phosphonate; HDPA and ODPA, respectively), which were better retained in the nanofiber after surface segregation. Subsequent uptake experiments to better understand specific solid-liquid interfacial interactions were carried out using 5 mg of HDPA-functionalized PAN mats with 10 μM U at pH 6.8 in four systems with different combinations of solutions containing 5 mM calcium (Ca2+) and 5 mM bicarbonate (HCO3-). U uptake was similar in control solutions containing no Ca2+ and HCO3- (resulting in 19 ± 3% U uptake), and in those containing only 5 mM Ca2+ (resulting in 20 ± 3% U uptake). A decrease in U uptake (10 ± 4% U uptake) was observed in experiments with HCO3-, indicating that UO2-CO3 complexes may increase uranium solubility. Results from shell-by-shell EXAFS fitting, aqueous extractions, and surface-enhanced Raman scattering (SERS) indicate that U is bound to phosphonate as a monodentate inner sphere surface complex to one of the hydroxyls in the phosphonate functional groups. New knowledge derived from this study on material fabrication and solid-liquid interfacial interactions will help to advance technologies for use in the in-situ detection and treatment of U in water.

Solubility of uranium oxide in ternary aluminosilicate glass melts

Uranium solubility was measured in melts belonging to the CaO-Al2O3-SiO2 (CAS) and MgO-Al2O3-SiO2 (MAS) systems using the Pt wire loop technique, enabling independent control of the temperature (1400 °C), glass composition, and oxygen fugacity (-16.1<log(fO2)<-0.7). The low sample masses allowed the equilibrium state to be reached quickly and rapid quenching of the glasses was performed in order to immobilize each system as close as possible to the molten state. The compositions of the different quenched glasses were analyzed by EDS. Uranium solubility decreased with decreasing oxygen fugacity, highlighting the lower solubility of uranium at reduced oxidation states. For each system, uranium solubility was constant from log(fO2)<-9.7. Moreover, different uranium behavior was evidenced between the two ternary systems. Modification of the Al content affected only uranium solubility in the CAS compositions, while uranium volatilization for oxidizing conditions was noted in the MAS system. This difference in behavior may be attributed to structural changes and probably to the variable proportions of [5]Al in each glass system.

Uranium in groundwaters: Insights from the Leinster granite, SE Ireland

Uranium concentrations in groundwaters from >100 private wells in the Caledonian Leinster Granite Batholith of SE Ireland range from a few μg/l to >300 μg/l. Approximately 10% of the wells exhibit uranium concentrations that exceed the current WHO provisional guideline of 30 μg/l and a further 10% exceed the original WHO guideline value of 15 μg/l. New whole-rock geochemical analyses and laboratory batch leaching experiments confirm the granite as the ultimate source of the groundwater uranium, but carbonate-bearing glacial deposits that overlie the granite play an important role in facilitating the generation of soluble and mobile anionic uranyl carbonate complexes. Whilst the granite has average uranium concentrations broadly similar to other Irish and British Caledonian granites, it is enriched locally in uranium, with whole-rock values up to c. 30 mg/kg. Uraniferous zircon, apatite, monazite and associations with secondary Fe-bearing oxide minerals were detected in drillcore whole-rock samples that had the highest uranium concentrations, but uraninite was not detected. Broad positive correlations between groundwater uranium concentrations and bicarbonate alkalinity and electrical conductivity are found in both the sampled groundwaters and in laboratory leachates of the granite and surficial glacial deposits. Leachates from the granite had approximately an order of magnitude higher uranium concentrations compared with those from the overlying limestone-bearing glacial tills. (234U/238U) ratios for the groundwaters and the leachates always exceed unity, indicating preferential leaching of loosely bound radiogenic 234U from mineral surfaces or grain boundaries into the groundwaters, rather than wholescale dissolution of the primary uraniferous minerals in the granite. Strontium isotopes ratios in the groundwaters constrain the balance between limestone and granite-derived Sr, and by extension the sources of dissolved Ca. These data indicate that calcite dissolution is the main source of dissolved calcium, highlighting the role of weathering of overlying limestone-bearing glacial tills during recharge in determining the geochemistry of the groundwaters. Geochemical modelling indicates that in the oxidising groundwaters of the study area U6+ is dominant, in the form of UO2(CO3)3−4 at the high pH values (7.49–7.69) of waters from wells overlain by limestone-bearing till. By contrast, in a small number of wells overlain by granite-bearing till, modelling indicates that the dominant species is UO2(CO3)2−2 reflecting their lower pH (6.22–6.30). Overall, carbonate species play a major role in uranium solubility and mobility in these granite-hosted groundwaters.

Phase evolution in [Nd1-xUx]2Zr2O7+δ system in oxidizing and reducing conditions: A nuclear waste form

Nd 2 Zr 2 O 7 pyrochlore has been proposed as a promising candidate for the immobilization of actinides and inert matrix fuel in the nuclear industry. In view of this, uranium substituted Nd 2 Zr 2 O 7 samples with general formula [Nd 1-x U x ] 2 Zr 2 O 7+δ (0.0 ≤ x ≤ 0.5) have been synthesized under reducing and oxidizing conditions by a solid-state route and thoroughly characterized by X-ray diffraction, Raman and IR spectroscopy to determine uranium solubility in Nd 2 Zr 2 O 7 . All the characterization studies confirm the formation of pyrochlore/fluorite-type phases in reduced samples at x ≤ 0.5 indicating 50 at.% uranium solubility. However, Rietveld analysis in conjunction with Raman spectroscopic studies indicates that the concentration of F-type domains increases with increasing uranium substitution in the largely pyrochlore-type phase (x ≤ 0.3). The solubility of uranium decreases when the samples are synthesized in an oxidizing atmosphere as phase separation is observed at x = 0.5. Rietveld analysis also indicates the increase of F-type phase fractions in the oxidizing atmosphere compared to that of the reduced counterpart. This increase of the F-type phase has been attributed to the formation of smaller-sized U 5+ /U 6+ (as compared to U 4+ ) and the concomitant incorporation of additional oxygen anions in the lattice in oxidizing conditions. The thermogravimetry method has been used to study the oxidation of reduced samples in a flowing oxygen atmosphere. The weight gain corresponds to the oxidation of U 4+ to higher oxidation states and the oxygen to uranium ratio (O/U) of the oxidized samples is close to 2.67 similar to U 3 O 8 .

Mechanical properties and thermal conductivity of (U,Zr)SiO4

During the core meltdown at the Fukushima Daiichi Nuclear Power Plant (1F), molten core materials penetrated the reactor pressure vessel (RPV), and they may have reacted with the concrete at the bottom of the primary containment vessel (PCV). Through this molten core concrete interaction (MCCI), uranium could potentially dissolve into ZrSiO4 and form a (U,Zr)SiO4 solid solution. For fuel debris retrieval to proceed smoothly, detailed information on the physical properties and critically characteristic of all possible fuel debris, including MCCI products, is necessary. However, no studies have reported on the physical properties of uranium in ZrSiO4 because it is difficult to synthesize. In this study, we fabricated dense (U,Zr)SiO4 samples through the spark plasma sintering (SPS) of SiO2 and (U,Zr)O2 and analyzed the solubility of uranium in ZrSiO4. The solubility limit of uranium in ZrSiO4 was found to be within 1.6 – 2.6 at%. The mechanical properties of the (U,Zr)SiO4 solid solution were calculated using the samples’ sound velocities. To understand the effect zirconium substitution by uranium has on heat transport, the thermal conductivity was calculated using the thermal diffusivity data measured with laser flash. This is the first study on the thermophysical properties of (U,Zr)SiO4, and it provides us with a better understanding of the properties of MCCI products.

Studies on chemical state of uranium in modified sodium borosilicate glass using X-ray photoelectron spectroscopy and X-ray absorption spectroscopy

The modified sodium borosilicate glass was synthesized by incorporating CaO, TiO2 and Al2O3 to the primary glass forming constituents (SiO2, B2O3 and Na2O) and on melting the mixture at 1373–1473K/3h by melt quench method. Uranium loaded analogues were synthesized by adding varying amounts of UO3 (2,4,6 and 8 mol %) to the borosilicate glass forming reagents then on melting at 1373–1473K. The U-loaded matrices were characterized by XRD to ensure the formation of glass matrices. Solubility of uranium in sodium borosilicate glass has been investigated. Further, the glasses were characterized by X-ray photoelectron spectroscopy (XPS) to unravel the oxidation states of uranium in the modified glass composition. X-ray absorption spectroscopy (EXAFS) revealed the chemical environment of uranium in the glass matrix indicating the coordination number of U in glass.

Uranium solubility and speciation in reductive soda-lime aluminosilicate glass melts

Uranium solubility in aluminosilicate melts of the Na2O-CaO-Al2O3-SiO2 system with two different Na/Ca ratios was studied at temperatures of 1250–1400°C and under various redox conditions. A closed thermochemical reactor was used to control the alkali metal activity (sodium oxide content) and the oxygen fugacity imposing the reducing environment on the glass melt (10−5 atm < fO2 < 10−15 atm). The compositions of the quenched glasses were analyzed by scanning electron microscopy and electron probe microanalysis. It appeared that uranium solubility decreased with decreasing oxygen fugacity, elucidating the roles of the different valences of uranium. To account for the respective effects of theses valences, we proposed a method to determine the proportion of each uranium oxidation state in the glass sample. The coexisting UVI, UV, and UIV species have been characterized for the first time in glass samples using U M4 edge high energy resolution X-ray absorption near-edge structure. Results showed that the lowest solubility values, of approximately 1 mol% UO2, were obtained under strongly reducing conditions, and thus with UIV as the main valence. Under higher oxygen fugacity, uranium solubility was controlled and drastically enhanced by the UVI concentration in the melt.

Determination of diffusion coefficients of uranium in liquid gallium by GITT

In the electrowinning or chemical reductive extraction process, those mass transfer kinetic parameters, for instance the diffusion coefficient in the liquid cathode or medium, have a strong impact upon the practical extraction rate and separation efficiency. This work aims to calculate the diffusion coefficient of uranium in liquid gallium with galvanostatic intermittent titration technique (GITT) and to deduce its corresponding activation energy. By analyzing the data of GITT, before U saturation in liquid gallium at the temperature range of 673-873 K, the temperature dependence of diffusion coefficients of uranium in liquid gallium was measured as ln D U Ga = − 6.90 ± 1.03 − 1.2 ± 0.8 × 10 3 T and the corresponding activation energy was determined as E a = −11.0 ± 6.7 kJ/mol. The possible source of errors to calculate the diffusion coefficients, such as reference electrode, small solubility of uranium in gallium, side reactions and geometrical approximation of cathode in the theory formula are also presented and discussed.

Solubility and activity coefficients of uranium in bimetallic Ga–In and Ga–Al liquid alloys

The equilibrium potentials of U3+/U couple, triple U–Ga–In and U–Ga–Al alloys versus Cl−/Cl2 reference electrode in the temperature range 723–823 K in fused LiCl–KCl eutectic in inert atmosphere were carried out by open-circuit potentiometry. The apparent standard potentials of U3+/U couple and triple alloys were determined. The activity, solubility and activity coefficients of uranium in bimetallic liquid Ga–In and Ga–Al eutectic alloys were calculated. The results show the high interaction between uranium and liquid metals due to the formation of intermetallic compounds. The principle thermodynamic properties of triple U–Ga–In and U–Ga–Al alloys were determined.

Recovery of uranium from low-grade tailings by electro-assisted leaching

Approximately 24 billion tons of existing hazardous radioactive waste, such as uranium tailings can yield nearly 9.6 million tons of uranium, which can be made available for nuclear industrial production for almost the next 100 years as per the current consumption levels. However, the extraction of uranium from tailings is highly challenging owing to the high proportion of the gangue content and relatively low content of the soluble U(VI). In this study, we engineered an innovative electro-assisted leaching (EAL) method, based on the dilute acid leaching (AL) method integrated with an electric field, for uranium recovery from low-grade uranium tailings. The proposed method offers advantages in terms of efficiency and environmental pollution control. In our experiments, the EAL method displayed a higher uranium extraction capacity (95%) than the traditional methods (55%) under the same conditions, during a rapid leaching of uranium tailings (containing 0.008% uranium) collected from a radioactive tailings dam in China. The excellent performance of EAL stemmed from (i) the continuous supply of oxidants (ferric ions), which ensured a high uranium solubility (i.e., transformation of U(IV) to U(VI) and (ii) the effect of the electric field on ion migration, which allowed for efficient dissociation of uranium from the complicated multi-mineral composition. In addition, these results demonstrated that EAL could be an efficient and environment-friendly scalable method for extracting valuable metals from low-grade ores or tailings.

Assessment of arsenic and uranium co-occurrences in groundwater of central Gangetic Plain, Uttar Pradesh, India

Arsenic and uranium in the aquatic environment are recognized as a major catastrophic problem worldwide and have natural as well as human-made sources such as mining, industry and agriculture. The severity of this problem is further accelerated by in-situ physio-chemical factors in the fluvial environment which enhances the concentration of arsenic and uranium in groundwater that feeds millions of people in Central Gangetic Plain in India. This study aims to establish a better understanding of the processes responsible for the co-occurrence of arsenic and uranium along with the factors controlling their solubility and mobility in the groundwater of central Gangetic plain. This study is an attempt to bring out the (a) the spatial distribution pattern of arsenic and uranium, (b) insight into the hydrogeochemical characteristics of aquifers that are controlling their co-occurrence and provides (c) information of their source which are validated by statistical tools. Silicate weathering controls U mobilization and distribution whereas carbonate dissolution and ion exchange process controls As in groundwater. The land use and land cover pattern of the North-eastern (NE) part of the study area is agriculture dominated and is composed of Holocene older alluvium. However, the South and South-western areas which are closer to the river are formed by the deposition of river born newer alluvial. Newer alluvium in the SW part has unoxidized organic-rich clay that enhances arsenic mobilization by reductive dissolution of Fe-oxyhydroxide. In the NE region, the abundance of fertilizers in the subsurface oxidizing environment augments the solubility and mobility of uranium.