Uranium Transport Research Articles

Radiological Analyses of 226Ra and 238U in Surface Water and Sediments from the Jackpile Member of the Morrison Formation, Pueblo of Laguna, New Mexico.

Surface water and sediments from the Jackpile mine, St. Anthony mine, Rio Paguate, Rio Moquino, and Mesita Dam areas near Pueblo of Laguna, New Mexico, were analyzed for 226Ra and U using gamma (γ) spectroscopy and inductively coupled plasma mass spectroscopy, respectively. Activity ratios for 226Ra/238U for solid samples range from 0.34 ± 0.13 to 16 ± 2.9, which reflect uranium transport and accumulation (<1), relatively pristine material in secular equilibrium (1), and removal of uranium by weathering (>1). Concentrations ranging from 80 to 225 μg L-1 U were detected in unfiltered water samples near the Jackpile mine. Water samples upstream and downstream from the mine contained concentrations ranging from 12 to 15 μg L-1 U. Water samples collected from the North Pit standing pond in the Jackpile mine contained as much as 1560 pCi L-1 of 226Ra, and passing the water through a 0.2 μM filter did not substantially reduce the activity of 226Ra in the water. 234Th and 226Ra are in secular equilibrium in this water, while radon gas was lost from the water. The results of the current study provide insight into the distribution of U-series radionuclides in the Pueblo of Laguna area, including detection of high levels of radioactivity in water at some locations within the Jackpile mine.

Uranium mineralization in the Thrace Basin, NW Türkiye: Evidence from radiation-induced defects in detrital quartz and synchrotron XRF/XANES analysis

The Paleogene-Neogene Thrace Basin in northwestern Türkiye has long been known to host economic gas and oil resources and has recently been reported to potentially host sandstone-type uranium deposits in the Oligocene Süloğlu Formation. The latter discovery raises questions about the source and deposition mechanism of uranium mineralization in the basin. This contribution reports on the results of a detailed electron paramagnetic resonance (EPR) spectroscopic study of detrital quartz from four sandstone and one mudstone samples in the Süloğlu Formation and documents the distribution and speciation of uranium using combined microbeam synchrotron X-ray fluorescence maps (μsXRF) and microbeam X-ray near edge structure spectroscopy (μsXANES). The EPR spectra of quartz separates are characterized by the presence of diagnostic radiation-induced defects (i.e., silicon-vacancy hole centers H′3, H′4, and H′7 with gmax = 2.049, 2.034, and 2.018, respectively, and the oxygen-vacancy electron center E′1), formed by the bombardment of alpha particles emitted from uranium, thorium, and their unstable progenies. Moreover, notable decreases in the intensity of silicon-vacancy hole centers in the EPR spectra of quartz separates after partial dissolution with hydrofluoric acid, provide compelling evidence for the circulation of uranium-bearing fluids in the Thrace Basin. The μsXRF and μsXANES data reveal the occurrences of mixed U6+ and U4+ species in hematite partially replacing pyrite aggregates but dominantly U4+ in disseminated pyrite and illite in sandstones of the Süloğlu Formation. These results provide new insights into uranium transport, reduction, and deposition mechanisms, with important implications for better understanding sandstone-type uranium deposits in general and further exploration in the Thrace Basin.

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.

Phosphate and illite colloid pose a synergistic risk of enhanced uranium transport in groundwater: A challenge for phosphate immobilization remediation of uranium contaminated environmental water

The phosphorus-containing reagents have been proposed to remediate the uranium contaminated sites due to the formation of insoluble uranyl phosphate mineralization products. However, the colloids, including both pseudo and intrinsic uranium colloids, could disturb the environmental fate of uranium due to its nonnegligible mobility. In this work, the transport pattern and micro-mechanism of uranium coupled to phosphate and illite colloid (IC) were investigated by combining column experiments and micro-spectroscopic evidences. Results showed that uranium transport was facilitated in granular media by forming the intrinsic uranyl phosphate colloid (such as Na-autunite) when the pH > 3.5 and CNa+ < 10 mM. Meanwhile, the mobility of uranium depended greatly on the typical water chemistry parameters governing the aggregation and deposit of intrinsic uranium colloids. However, the attachment of phosphate on illite granule increased the repulsive force and enhanced the dispersion stability of IC in the IC-U(VI)-phosphate ternary system. The non-preequilibrium transport and retention profiles, HRTEM-mapping, as well as TRLFS spectra revealed that the IC enhanced uranium mobility by forming the ternary IC-uranyl phosphate hybrid, and acted as the coagulation preventing agent for uranyl phosphate particles. This observed facilitation of uranium transport resulted from the formation of intrinsic uranyl phosphate colloids and IC-uranyl phosphate hybrids should be taken into consideration when evaluating the potential risk of uranium migration and optimizing the in-situ mineralization remediation strategy for uranium contaminated environmental water.

Unraveling pH-Dependent Changes in Adsorption Structure of Uranyl on Alumina (012).

Mitigating uranium transport in groundwater is imperative for ensuring access to clean water across the globe. Here, in situ resonant anomalous X-ray reflectivity is used to investigate the adsorption of uranyl on alumina (012) in acidic aqueous solutions, representing typical UVI concentrations of contaminated water near mining sites. The analyses reveal that UVI adsorbs at two distinct heights of 2.4-3.2 and 5-5.3 Å from the surface terminal oxygens. The former is interpreted as the mixture of inner-sphere and outer-sphere complexes that adsorb closest to the surface. The latter is interpreted as an outer-sphere complex that shares one equatorial H2O with the terminal surface oxygen. With increasing pH, we observe an increasing prevalence of these outer-sphere complexes, indicating the enhanced role of the hydrogen bond that stabilizes adsorbed uranyl species. The presented work provides a molecular-scale understanding of sorption of uranyl on Al-based-oxide surfaces that has implications for environmental chemistry and materials science.

Colloids facilitate transport of cadmium and uranium in arable soils, which is undetected by suction cups in the field

AbstractColloids can facilitate the transport of trace elements in soil, but earlier studies fell short either because of sampling artefacts or because no contrasting soils were used; both factors hamper a better understanding of where this occurs. Here, leaching in undisturbed and unsaturated soil columns was observed to identify colloidal transport of cadmium (Cd) and uranium (U) in soils with contrasting properties. Crucially, it was examined how such column data relate to more traditional in situ data obtained with porous suction cups. Soils were collected from three agricultural fields in Switzerland (soil pH 5.1–7.3), leached during 36 days (0.6 pore volumes, ~1.6 mm day−1), and elements trapped in the PTFE suction plates were recovered and were ascribed to colloids. Of the total mobile mass (sum percolate and plate), 18%–53% (Cd) or 8–89% (U) was retained by plates, the range depending on the field where soil was sampled and that fraction depends on soil pH and for U also on dissolved inorganic carbon. The total mobile Cd or U mass divided by the percolated volume indicates the metal concentration in the average leachate and was logically highest for Cd in the most acidic soil and for U in the most alkaline soil. The concentrations of Cd and U in these column leachates were &gt;10‐fold higher than corresponding concentrations in the suction cups in the field for those soils and elements where colloids dominated leaching. In contrast, they were within a factor of two where colloids are &lt;20% of the mobile mass. Flow Field‐Flow Fractionation (FlFFF) analyses showed an association of colloidal U with oxyhydroxides, clays and organic matter and, for colloidal Cd, an association with organic matter. These data show that suction cups or filtered percolates can underestimate the leaching of Cd and U by up to an order of magnitude.

Modeling evaluation of the impact of residual source material on remedial time frame at a former uranium mill site

Groundwater contamination at legacy uranium processing sites is an ongoing global challenge. Plumes at many uranium-contaminated sites are more persistent than originally predicted by groundwater modeling. Previous investigations of uranium plume persistence identified residual and secondary sources that contribute to plume longevity, but there is a remaining need to revise forecasted cleanup times using information about these ongoing sources. The purpose of this study is to investigate the quantitative impact of residual vadose zone sources of uranium on groundwater remediation time frame. This objective was approached by applying numerical uranium transport simulations and uncertainty analysis to a former uranium mill site in the southwestern United States. Information from recent site investigations provided details about the distribution and release characteristics of uranium accumulations in the vadose zone. The residual uranium characteristics were incorporated as decaying source terms in the transport model. A stochastic approach using an iterative ensemble smoother was applied for history matching, and the transport model was used to assess the impact of multiple remedial alternatives on forecasted time frame. The forecasted time frame to achieve the groundwater remediation goal for uranium by monitored natural attenuation is on the order of thousands of years, and treatment of the dissolved plume does not reduce the projected time frame. The large proportion of residual uranium mass remaining in the vadose zone and the gradual leaching rate due to the site's semiarid climate create a long-lived source that can sustain a dissolved plume for thousands of years despite an estimated 99% mass removal achieved during mill tailings disposal. Residual uranium in vadose zone sediments beneath former tailings impoundments could present comparable uranium plume persistence and remediation challenges at other legacy uranium mill sites in semiarid climates. Other remaining uranium-impacted sites are similarly complex, and forecasted remedial time frames are needed to effectively achieve compliance, manage risk, assess the benefits of additional treatment, manage and project costs, and support beneficial site reuse.

Microbial removal of uranyl from aqueous solution by Leifsonia sp. in the presence of different forms of iron oxides

Immobilization of uranyl by indigenous microorganisms has been proposed as an economic and clean in-situ approach for removal of uranium, but the potential mechanisms of the process and the stability of precipitated uranium in the presence of widespread Fe(III) (hydr)oxides remain elusive. The potential of iron to serve as a reductant and/or an oxidant of uranium indicates that bioemediation strategies which mainly rely on the reduction of highly soluble U(VI) to poorly soluble U(IV) minerals to retard uranium transport in groundwater may be enhanced or hindered under different environmental conditions. This study purposes to determine the effect of ubiquitous Fe(III) (hydr)oxides (two-line ferrihydrite, hematite and goethite) on the removal of U(VI) by Leifsonia sp. isolated from an acidic tailings pond in China. The removal mechanism was elucidated via SEM-EDS, XPS and Mössbauer. The results show that the removal of U(VI) was retarded by Fe(III) (hydr)oxides when the initial concentration of U(VI) was 10 mg/L, pH was 6, temperature was 25 °C. Particularly, the retardatory effect of hematite on U(VI) removal was blindingly obvious. Also, it is worth noting that the U(VI) in the precipitate slow-released in the Fe(III) (hydrodr) oxide treatment groups, accompanied by an increase in Fe(II) concentration. SEM-EDS results demonstrated that the ferrihydrite converted to goethite may be the reason for U(VI) release in the process of 15 days culture. Mössbauer spectra fitting results further imply that the metastable iron oxides were transformed into stable Fe3O4 state. XPS measurements results showed that uranium product is most likely a mixture of Iron-U(IV) and Iron-U(VI), which indicated that the hexavalent uranium was converted into tetravalent uranium. These observations imply that the stability of the uranium in groundwater may be impacted on the prevailing environmental conditions, especially the solid-phase Fe(III) (hydr)oxide in groundwater or sediment.

Water-rock interactions of uranium deposits: A field investigation and laboratory batch experiment

Interactions between bedrock and groundwater in the far-field of a deep geological repository (DGR) for high-level radioactive waste significantly affect the transport of uranium, which is the major radionuclide in a DGR. Therefore, the present study evaluated the geochemical behavior of uranium by assessing the hydrogeochemical characteristics of groundwater, surface water, mine water, and quarry runoff water in the Okcheon metamorphic belt, which represents a natural analogue study site in Korea. A batch experiment was also conducted to further elucidate the interactions between the uranium-containing rock and groundwater in the study area. The results of the field investigation indicated that uranium in groundwater formed aqueous complexes in the form of calcium uranyl tricarbonate. The concentrations of uranium in groundwater were low, at less than 1 μg L−1, which seems to be attributable to the dissolutions of uraninite, coffinite, UO2(am), and U4O9(c) under the moderately reduced condition and subsequent control by co-precipitation with iron (hydr)oxides. The results of the batch experiment showed that uranium concentrations may be controlled by adsorption or surface complexation with iron oxides. The field study demonstrates the equilibrium state maintained in the current geochemical conditions at the study site, while the batch experiment provides implications regarding the reaction processes over time. Overall, this study showed that uranium forms complexes with carbonate and sulfate under the geochemical conditions of the study site, and that its mobility could be restricted through co-precipitation or adsorption/surface complexation. This study is expected to contribute to a comprehensive understanding of how the behavior of uranium is affected by the evolution of geochemical conditions around the DGR.

Interplay of legacy irrigation and nitrogen fertilizer inputs to spatial variability of arsenic and uranium within the deep vadose zone

The vadose zone is a reservoir for geogenic and anthropogenic contaminants. Nitrogen and water infiltration can affect biogeochemical processes in this zone, ultimately affecting groundwater quality. In this large-scale field study, we evaluated the input and occurrence of water and nitrogen species in the vadose zone of a public water supply wellhead protection (WHP) area (defined by a 50-year travel time to groundwater for public supply wells) and potential transport of nitrate, ammonium, arsenic, and uranium. Thirty-two deep cores were collected and grouped by irrigation practices: pivot (n = 20), gravity (n = 4) irrigated using groundwater, and non-irrigated (n = 8) sites. Beneath pivot-irrigated sites, sediment nitrate concentrations were significantly (p < 0.05) lower, while ammonium concentrations were significantly (p < 0.05) higher than under gravity sites. The spatial distribution of sediment arsenic and uranium was evaluated against estimated nitrogen and water loading beneath cropland. Irrigation practices were randomly distributed throughout the WHP area and presented a contrasting pattern of sediment arsenic and uranium occurrence. Sediment arsenic correlated with iron (r = 0.32, p < 0.05), uranium negatively correlated to sediment nitrate (r = −0.23, p < 0.05), and ammonium (r = −0.19 p < 0.05). This study reveals that irrigation water and nitrogen influx influence vadose zone geochemistry and mobilization of geogenic contaminants affecting groundwater quality beneath intensive agricultural systems.

Uranium migration and radioactive characteristics of the Sarıçiçek and Sarıhan Granodiorites

The radionuclide concentrations of eU (ppm), eTh (ppm), K (%) and dose rate values were measured in Sarıçiçek (Gümüşhane) and Sarıhan (Bayburt) granodiorites for a duration of 5 minutes at each of 532 measurement points. The radioelement ratios (eU/eTh, eU/K, and eTh/K) indicating the origins of the rocks, the geochemical indicators (Ume, F parameter, and eU-(eTh/3,5) rate) showing the uranium mobility and the radioelement concentrations were calculated and mapped within the study areas. The average K, eU, and eTh concentrations were calculated as 2.98%, 3.15 ppm, and 12.45 ppm for Sarıçiçek granodiorite, and 1.83%, 2.73 ppm, and 13.6 ppm for Sarıhan granodiorite, respectively. Higher radioactivity values were observed in basaltic, sedimentary, and ultramafic rock combinations within the granodiorite masses. In the classification according to radioelement ratios, it was concluded that the rocks in the study areas formed as a mixture of upper mantle and crustal materials. In both study areas, there was uranium transport from the granodioritic masses into the surrounding rocks, and accordingly, the rocks in the surrounding formations were enriched in uranium. As a result, radioactivity levels, rock formation origins, and uranium transport of both granodioritic masses and rocks in the surrounding formations were determined by evaluation with radioelement concentration values and ratios and migration parameters. The study areas were characterized by associating them with geology in light of radioactive data.

Natural attenuation of uranium in a fluvial Wetland: Importance of hydrology and speciation

A nuclear fuel fabrication facility released 43,500 kg of uranium into a riparian wetland located on the Savannah River Site between 1955 and 1988. Studies were undertaken to evaluate hydrological and geochemical processes influencing uranium accumulation in the wetland. Gamma-radiation-mapping surveys were conducted by systematically walking over the contaminated wetland with backpacks equipped with global positioning systems and NaI gamma detectors. Based on maps compiled from >700,000 gamma spectra and eight sediment uranium depth profiles, it was determined that 94% of the released uranium remained in the wetland. The uranium in the wetland is concentrated in five multi-hectare areas along the stream, accounting for ∼11% of the land area adjacent to the stream. While land type (upland or wetland) and topography provided a reasonable first approximation of where much of the uranium was deposited, hydrological watershed modeling revealed that the stream velocity was especially slow through many of the hot spots. Using autoradiography combined with SEM/EDX measurements of contaminated sediments, surprisingly few hot particles were detected. Instead, uranium was evenly distributed throughout the sampled sediment, suggesting that dissolved uranium had bound to sediment particles that became suspended and later deposited in low energy (low flow velocity) portions of the stream. EXAFS suggested that U atoms were present as individual ions in disordered complexes within the sediment. Furthermore, linear combination analyses suggested that the predominant component of the U(VI) was adsorbed to sediment minerals (∼70%) and a minor component (∼30%) was associated with organic matter phases. These studies show that wetlands can be extraordinarily effective at binding and retaining uranium, thereby providing a natural barrier to the transport of uranium out of a watershed. However, significant anthropogenic or climatic changes to wetlands, such as those associated with flooding, forest fires, or land use, may disrupt the complex hydrological and biogeochemical balance necessary to maintain long-term immobilization of uranium.

Analysis of Uranium Sorption in a Laboratory Column Experiment Using a Reactive Transport and Surface Complexation Model

The transport and retardation of radioactive elements in hyper-alkaline conditions of radioactive waste repositories is a challenging field that is still poorly understood. In this study, the transport and attenuation of uranium in a column experiment was modelled by considering kinetic reactions, advection–dispersion and chemical/physical retardation processes. The modelling was first performed for three alluvium samples from Yucca Mountain in circumneutral pH to moderately alkaline conditions. Sorption of uranyl ({mathrm{UO}}_{2}^{2+} ({U}_{mathrm{VI}})) was found to strongly depend on the surface complexation model assumed, with no significant removal of {U}_{mathrm{VI}} by precipitation or ion exchange process. The surface/edge site reaction of Al-hydroxyl group in kaolinite was shown to have a high affinity for uranyl adsorption, while the hydrous ferric oxide edge on hematite adsorbed most of the uranyl ions. The model was then used to interpret uranium transport in a laboratory column filled with Hollington sandstone under hyper-alkaline (pH 13) conditions. The simulation results show that uranium adsorption on the Al-hydroxyl edge of kaolinite exceeds adsorption by the calcium silicate hydrate phase. This result may reflect the lack of surface complexation parameters for calcium silicate hydrate minerals. Hence, further studies are required in the field of surface complexation reactions for calcium silicate hydrate phases.Graphical

Radionuclide Transport Simulations Supporting Proposed Borehole Waste Disposal in Israel

A scientific collaboration between the U.S. and Israel is underway to assess the suitability of a potential site for subsurface radioactive waste disposal in the Negev Desert, Israel. The Negev Desert has several favorable attributes for geologic disposal, including an arid climate, a deep vadose zone, interlayered low-permeability lithologies, and carbonate rocks with high uranium-sorption potential. These features may provide a robust natural barrier to radionuclide migration. Geologic and laboratory characterization data from the Negev Desert are incorporated into multiphase flow and transport models, solved using PFLOTRAN, to aid in site characterization and risk analysis that will support decision-making for waste disposal in an intermediate-depth borehole design. The lithology with the greatest uranium sorption potential at the site is phosphorite. We use modeling to evaluate the ability of this layer to impact uranium transport around a proposed disposal borehole. The current objective of the simulations is focused on characterizing hypothetical leakage from waste canisters and subsequent uranium migration under three infiltration scenarios. Here, we describe a hydrogeologic model based on data from a local exploratory borehole and present results for uranium flow and transport simulations under varying infiltration scenarios. We find that under the current climate conditions, it is likely that uranium will remain in the near-field of the borehole for thousands of years. However, under a hypothesized extreme climate scenario representing an increase in infiltration by a factor of 300x above present-day values, uranium may break through the phosphorite layer and exit the base of the model domain (~200 m above the water table) within 1000 years. Simulation results have direct implications for the planning of nuclear waste disposal in the Negev Desert, and specifically in intermediate-depth boreholes.

Colloids facilitate cadmium and uranium transport in an undisturbed soil: A comparison of soil solution isolation methods

Accurate data of cadmium (Cd) and uranium (U) leaching are needed in the context of identifying their mass balances in agricultural soils. There is some controversy related to sampling methods and the contribution of colloid facilitated transport. Here, leaching was measured in undisturbed unsaturated soils and the impact of colloids was measured with due attention to solution sampling protocols. Soils were sampled in an arable, pH neutral silty loam soil. The columns (n = 8) were irrigated and PTFE suction plates (1 μm pores) at the bottom ensured unsaturated flow. New here is that both percolates and associated suction plates were collected, the elements in the plates were recovered with acid digestion and used as a lower estimate of colloidal forms. The fraction of elements collected in the plates were 33 % (Cd) and 80 % (U) of the total mobility (=percolates + plates), illustrating colloidal transport. Composition of pore water extracted by soil centrifugation varied largely between initial and final samples and showed that colloids increased as a result of reduced solution calcium after leaching two pore volumes with low calcium water. Flow Field-Flow Fractionation (FIFFF) of the pore water and percolates revealed co-elution of U with colloidal organic matter, oxyhydroxides and clay, illustrating colloidal transport of U by these vectors. Colloidal transport of Cd was less pronounced and was dominated by organic matter. Soil extracts with 0.01 M CaCl2 have lower colloid concentration and consequently underestimate mobile U. In contrast, Cd concentrations in 0.01 M CaCl2 extracts exceed that of percolates due to chloride complexation and higher calcium, mobilizing Cd. Soil leaching experiments better indicate potential leaching losses than a single pore water composition because the former yields the time integrated data. Suction plates and/or bottom filters need to be analysed in leaching studies to account for metal transport by colloids.

Reactive transport model of uranium by CO2 + O2 in situ leaching

Uranium is an important strategic resource, and its safe and efficient development and utilization are of great significance to ensuring a nation's energy supply and strategic security. Sandstone-type uranium ore is commonly mined by CO2 + O2 in situ leaching, for which understanding the coupling mechanism between the hydrodynamic and chemical fields is key to predicting uranium leaching. This study focused on a coal-uranium ore deposit in China. A convection and dispersion model of the solute transport by in situ leaching was constructed in COMSOL, and a thermodynamic model of CO2 + O2 in situ leaching in sandstone-type uranium ore was constructed in PHREEQC. The two models were coupled to simulate the reactive transport and dynamic leaching processes of uranium by CO2 + O2 in situ leaching. A sensitivity analysis was performed to quantitatively analyze the effects of different model parameters on the uranium leaching efficiency and uranium contamination remediation. The results showed that the coupled model could simulate and predict the reaction and transport of uranium. The sensitivity analysis indicated that the production rate and the injected CO2 and O2 concentrations are the key parameters that control the uranium leaching efficiency, followed by the formation permeability and injection rate. The uranium leaching efficiency does not increase monotonically with the formation permeability and production rate. The results also indicated that natural dilution is insufficient for remediating uranium-contaminated groundwater, but the remediation efficiency can be improved by increasing solution extraction. The results of this study can be used to develop guidelines for the safe and efficient development and utilization of uranium ore while protecting the ecological environment.

Sandstone-hosted uranium deposits of the Colorado Plateau, USA

More than 4,000 sandstone-hosted uranium occurrences host over 1.2 billion pounds of mined and in situ U3O8 throughout the Colorado Plateau. Most of the resources are in two distinct mineral systems with deposits hosted in the Triassic Chinle and Jurassic Morrison Formations. In the Chinle mineral system, base metal sulfides typically accompany mineralization. The Morrison mineral system is characterized by V/U ratios up to 20. The uranium source was likely volcanic ash preserved as bentonitic mudstones in the Brushy Basin Member of the Morrison Formation, and lithic volcanic clasts, ash shards, and bentonitic clay in the lower part of the Chinle Formation. Vanadium originated from two possible sources: iron–titanium oxides that are extensively altered in bleached rock near deposits or from similar minerals in variably bleached red beds interbedded with and beneath the Morrison. In Chinle-hosted deposits, in addition to volcanic ash, a contributing source of both vanadium and uranium is proposed here for the first time to be underlying red beds in the Moenkopi and Cutler Formations that have undergone a cycle of reddening-bleaching-reoxidation. Transport in both systems was likely in groundwater through the more permeable sandstones and conglomerate units. The association of uranium minerals with carbonate and more rarely apatite, suggests that transport of uranium was as a carbonate or phosphate complex. The first comprehensive examination of paleoclimate, paleotopography, and subsurface structure of aquifers coupled with analysis of the geochronology of deposits suggests that that there were distinct pulses of uranium mineralization/redistribution during the period from about 259 Ma to 12 Ma when oxidized mineralizing fluids were intermittently rejuvenated in the Plateau in response to changes in tectonic regime and climate. Multiple lines of evidence indicate that deposits formed at ambient temperatures of about 25 °C to no greater than about 140 °C. In both systems, deposits formed where groundwater flow slowed and was subject to evaporative concentration. Stagnant conditions allowed for prolonged interaction of U- and V-enriched groundwater with ferrous iron-bearing reductants, such as illite and iron–titanium oxides, and more rarely organic material such as plant debris. Paragenetically late in the sequence, reducing fluids introduced additional organic matter to some deposits. Reducing fluids and introduced organic matter (now amorphous and altered by radiolysis) may originate from regional petroleum systems where peak oil and gas generation was from ∼ 82 to ∼ 5 Ma. Our novel analysis indicates that these reducing fluids bleached rock and protected affected deposits from remobilization during exposure and weathering that followed uplift of the Plateau (∼80 to 40 Ma).

Towards a Mineral Systems Model for Surficial Uranium Mineralization Based on Deposits in the Erongo District of Namibia

Surficial deposits in Namibia’s Erongo district contain substantial but low-grade resources of uranium and vanadium (nearly 500 Mlb U3O8), hosted in palaeochannels. This review attempts to develop a mineral systems model for the deposit type, but it is emphasised that research into this important class of deposit has been minimal since the nineteen eighties, largely as a result of a limited investment in uranium exploration. The deposits are the result of groundwater movement in aquifers developed within Cenozoic palaeochannels. The source of uranium was probably granitic rocks traversed by these palaeodrainages, particularly black-quartz rich pegmatites similar to those that make up the hard-rock alaskite deposits of the region. Transport of uranium is generally assumed to have occurred in aqueous uranium species after palaeochannels became filled with sediment. U-enriched clasts within the palaeochannels have yet to be investigated as a local source of uranium. The localised deposition of uranium occurred after regionally extensive carbonate cementation of the palaeochannel sediments, which was the result of climate change (aridification). Pre-uranium calcite may have acted as a chemical buffer (pH) and probably influenced palaeochannel hydrology, restricting groundwater flow to the deeper portions. Uranium is paragenetically related to Mg clays and dolomite, suggesting that the groundwater evolved to a more Mg-rich composition during uranium deposition, probably as a result of more extreme evaporation. The controls on the localisation of mineralisation remain unclear and unpredictable, as are the controls on uranium grade—the fundamental determinant of economic viability. There are few absolute age determinations for any of the deposits, but none occur in rocks likely to be older than the Miocene. This reflects low preservation potential. For example, the Langer Heinrich deposit is incised by active drainage with attendant erosion and probable removal of mineralised material.

Membrane-Based Electrochemical Detection of Uranium: A Review

The determination of uranium in environmental samples has always been a crucial environmental issue due to its adverse impacts on human life. Electrochemical detection is one of the most suitable methods for directly determining uranium because of its portable instrument and quick response characteristics. The ion-selective membrane in the working electrodes is selectively responsible for uranium transport and separation. This mini-review provides a general overview of the membrane-based ion-selective electrodes in detecting uranium ions reported in the literature. The ion-selective membranes are classified according to their ionophore categories. Furthermore, the limits and outlook are also discussed to provide a reference for further developing membrane-based electrochemical uranium sensors.

Cellular transport of uranium and its cytotoxicity effects on CHO-k1 cells

Uranium is a radioactive heavy metal and a significant public health concern; however, its associated underlying toxicological mechanisms remain largely unknown. In this work, the uptake and efflux processes of uranium in CHO-k1 cells were studied and the cytotoxicity effects were explored. It was found that both the uptake and efflux processes took place rapidly and half of the internalized uranium was expelled within 8 h. The uranium exposure caused a decrease of cell viability and adhesion ability in a dose-dependent manner and blocked the cell cycle at the G1 stage. In addition, gene expression analysis revealed relative changes in the transcription of metabolism related genes. Further studies revealed that the cytotoxicity of uranium could be alleviated by exposing cells to a lower temperature or by the addition of amantadine-HCl, an endocytosis inhibitor. Interestingly, after uranium exposure, needle-like precipitates were observed in both intracellular and extracellular regions. These findings collectively suggest that the cellular transport of uranium is a rapid process that disturbs cell metabolism and induces cytotoxicity, and this impact could be reduced by slowing down endocytic processes.