Uranium Flux Research Articles

Changes of active particulate uranium under the Water-Sediment Regulation Scheme in the lower Yellow River: Potential impact to the uranium flux into the global ocean

With the annual Water-Sediment Regulation Scheme (WSRS) transporting large amounts of suspended particulate matter (SPM) to the sea within several days, the behavior of uranium in the Yellow River during the WSRS is crucial to better understand the uranium flux. In this study, the active forms (the exchangeable, carbonate bounded, Fe/Mn oxides bounded, organic matter bounded) and the residual form of particulate uranium were extracted by the sequential extraction method and their uranium contents were measured respectively. Results show that the content of total particulate uranium was 1.43–2.56 μg/g and the active forms accounts for 11–32 %. Particle size and redox environment are the two main factors controlling the active particulate uranium. The flux of active particulate uranium at Lijin was 4.7 tons during 2014 WSRS, which was about 50 % of the dissolved uranium flux for the same period. Thus, the terrestrial uranium flux is significantly modified by artificial regulation.

Climate-driven fluxes of organic-bound uranium to an alpine lake over the Holocene

Uranium (U) isotopic signatures and concentration in sediments are widely used as paleo-redox proxies, as the behavior of U is often controlled by bottom water oxygenation. Here, we investigated the processes controlling U accumulation in the sediments of Lake Nègre (Mediterranean Alps, South-East France) over the past 9200 years. Exceptionally high natural U concentrations (350–1250 μg·g−1) allowed the measurement of U along with other elements by high-resolution X-Ray Fluorescence core-scanning. Weathering and erosion proxies (Ti content, Zr/Al and K/Ti ratios) indicate that sedimentary inputs were controlled by Holocene climatic variations. After a period of low erosion during the Holocene Climatic Optimum, a major regime shift was recorded at 4.2 kyr BP when terrigenous fluxes consistently increased until present with high sensitivity to centennial-scale climatic events. Sedimentary organic matter (OM) inputs were dominated by terrigenous OM from the catchment soils until 2.4 kyr BP, as attested by carbon to nitrogen (C/N) and bromine to organic carbon (Br/TOC) ratios. From 2.4 kyr BP to present, lake primary production and soils equally contributed to sedimentary OM. Uranium fluxes to the sediments were well correlated to terrigenous OM fluxes from 7 kyr BP to present, showing that U supply to the lake was controlled by U scavenging in the soils of the watershed followed by transport of U bound to detrital organic particles. Higher U/OM ratios before 7 kyr BP likely reflect the development of the upstream wetland. The fluctuations of U sedimentary inputs appear to be independent of bottom water oxygenation, as estimated from constant Fe/Mn ratios and δ238U isotopic signatures, and rather controlled by the production, erosion and sedimentation of terrigenous OM. This finding confirms that the use of U (and potentially other metals with high affinity to OM) concentrations alone should be used with caution for paleo-redox reconstructions.

AXIAL FLUX PROFILE IN THE ADVANCED TEST REACTOR

This work demonstrates an approach to determine probability of perturbation of the axial profile of the thermal neutron flux in the Advanced Test Reactor. The axial flux profile is expected to follow a theoretical cosine shape, due to the minimal use of vertically-withdrawn shims. Reactivity is normally controlled by rotation of Outer Shim Control Cylinders, uniformly affecting neutron flux at all axial locations. The Advanced Test Reactor routinely accepts for irradiation experiments of a variety of designs. Among the analyses required by the safety basis approved by the United States Department of Energy is the characterization of a new experiment’s potential for perturbing the axial flux, which could exacerbate power peaking in the driver fuel. However, this perturbation can be more or less severe in different locations within the fuel. Therefore, the best characterization of axial flux perturbation requires knowledge of baseline axial flux. Such information is obtained by measuring decay in activated uranium flux wires irradiated at known positions in cooling channels in plate-type fuel elements. Due to variability in measured axial flux, it is not usually clear whether a given anomalous measurement is caused by an actual perturbation. Assuming normality in random measurement errors, the probability of an actual perturbation is quantified.

Dissolved organic matter and trace element variability in a blackwater-fed bay following precipitation

Dissolved organic matter (DOM) often forms complexes with trace metals. The main objective of this study was to identify the sources of trace elements to a coastal bay that is fed by two blackwater rivers using DOM compositions. Surface water samples from twelve sites in Weeks Bay, Alabama were collected during four field trips and a bottom sediment sample was collected during one of the trips. Spectroscopic measurements in tandem with parallel factor analysis and multivariate statistics were used to derive DOM compositions and inductively coupled plasma mass spectrometry was used for determining trace metal concentrations. DOM chemistry and trace element concentrations together with precipitation and discharge, watershed land use and land cover data, and physicochemical parameters were used to determine the source of trace elements in the adjoining areas of the watershed to the bay and finally settling into the bay sediments. Arsenic, copper, and uranium fluxes increased while fluxes of mercury, zinc, manganese, and iron decreased following precipitation events. Concentration of all trace elements analyzed were 10,000 times higer in the bay botton sediments than their concentrations in the water colunmn. Microbially reprocessed and soil derived DOM components increased along with the trace elements and nutrients in the bay as a result of precipitation whereas concentration of terrestrial humic-like DOM component remained the same during both dry (spring) and wet (summer and fall) periods. Principal component analysis revealed significant correlations between microbially reprocessed DOM and the trace elements including arsenic, copper and uranium indicating urban, pasture, and agricultural areas in the watershed were the sources for these trace metals. On the other hand, multiple sources were identified for manganese, zinc, and iron. Overall it was found that DOM compositions are useful for inferring the possible sources and mobility of trace elements in aquatic systems.

Spatial and temporal variability of the transformation of dissolved matter runoff in the Mezen’ River estuary

Spatial and temporal variability of the transformation of dissolved matter runoff in the Mezen’ River estuary is studied by results of the complex hydrological-hydrochemical researches lead in 2009 and 2015. The conservative behavior of major ions and dissolved forms of Li, Rb, Cs, Sr, B, F, As, Sb, and Mo is demonstrated. The additional input into solution reaching 93 and 32–38% of content in the river water mass is determined for phosphates and silicon. This is caused, apparently, by mobilization of these nutrients from pore waters of regularly tide-stirred bottom sediments and vertical mixing of the water column. The desorption flux of barium and uranium due to long-term interaction of terrigenic material with saline waters exceeded their input with a continental runoff reaching 180–380 and 90–150% of content of these elements in the river waters. Up to 50, 43, 29, 32, 44, 50, and 45% of Fe, Pb, Y, La, Ce, Pr, and Nd supplying with river runoff which are present in the form of strong organic complexes are removed from solution at the beginning of the mixing zone due to coagulation of colloids. It is drawn a conclusion on spatial homogeneity and long-term stability of transformation features of dissolved matter runoff in the Mezen’ River estuary. Peculiar characteristic of migration of dissolved phosphates, silicon, barium, and uranium are caused by hydrological features of the estuary.

Spatial and Temporal Variability of the Transformation of Dissolved Matter Runoff in the Mezen River Estuary

Spatial and temporal variability of the transformation of dissolved matter runoff in the Mezen’ River estuary is studied by results of the complex hydrological-hydrochemical researches lead in 2009 and 2015. The conservative behavior of major ions and dissolved forms of Li, Rb, Cs, Sr, B, F, As, Sb, and Mo is demonstrated. The additional input into solution reaching 93 and 32–38% of content in the river water mass is determined for phosphates and silicon. This is caused, apparently, by mobilization of these nutrients from pore waters of regularly tide-stirred bottom sediments and vertical mixing of the water column. The desorption flux of barium and uranium due to long-term interaction of terrigenic material with saline waters exceeded their input with a continental runoff reaching 180–380 and 90–150% of content of these elements in the river waters. Up to 50, 43, 29, 32, 44, 50, and 45% of Fe, Pb, Y, La, Ce, Pr, and Nd supplying with river runoff which are present in the form of strong organic complexes are removed from solution at the beginning of the mixing zone due to coagulation of colloids. It is drawn a conclusion on spatial homogeneity and long-term stability of transformation features of dissolved matter runoff in the Mezen’ River estuary. Peculiar characteristic of migration of dissolved phosphates, silicon, barium, and uranium are caused by hydrological features of the estuary.

The exchange relationship of surface water-groundwater and uranium flux in the Gas Hure Salt Lake of northwest Qaidam Basin, China

The exchange relationship of surface water-groundwater and uranium flux in the Gas Hure Salt Lake of northwest Qaidam Basin, China

Uranium isotopic compositions of the crust and ocean: Age corrections, U budget and global extent of modern anoxia

The 238U/235U isotopic composition of uranium in seawater can provide important insights into the modern U budget of the oceans. Using the double spike technique and a new data reduction method, we analyzed an array of seawater samples and 41 geostandards covering a broad range of geological settings relevant to low and high temperature geochemistry. Analyses of 18 seawater samples from geographically diverse sites from the Atlantic and Pacific oceans, Mediterranean Sea, Gulf of Mexico, Persian Gulf, and English Channel, together with literature data (n=17), yield a δ238U value for modern seawater of −0.392±0.005‰ relative to CRM-112a. Measurements of the uranium isotopic compositions of river water, lake water, evaporites, modern coral, shales, and various igneous rocks (n=64), together with compilations of literature data (n=380), allow us to estimate the uranium isotopic compositions of the various reservoirs involved in the modern oceanic uranium budget, as well as the fractionation factors associated with U incorporation into those reservoirs. Because the incorporation of U into anoxic/euxinic sediments is accompanied by large isotopic fractionation (ΔAnoxic/Euxinic-SW=+0.6‰), the size of the anoxic/euxinic sink strongly influences the δ238U value of seawater. Keeping all other fluxes constant, the flux of uranium in the anoxic/euxinic sink is constrained to be 7.0±3.1Mmol/yr (or 14±3% of the total flux out of the ocean). This translates into an areal extent of anoxia into the modern ocean of 0.21±0.09% of the total seafloor. This agrees with independent estimates and rules out a recent uranium budget estimate by Henderson and Anderson (2003). Using the mass fractions and isotopic compositions of various rock types in Earth’s crust, we further calculate an average δ238U isotopic composition for the continental crust of −0.29±0.03‰ corresponding to a 238U/235U isotopic ratio of 137.797±0.005. We discuss the implications of the variability of the 238U/235U ratio on Pb–Pb and U–Pb ages and provide analytical formulas to calculate age corrections as a function of the age and isotopic composition of the sample. The crustal ratio may be used in calculation of Pb–Pb and U–Pb ages of continental crust rocks and minerals when the U isotopic composition is unknown.

The terrestrial uranium isotope cycle.

Changing conditions on the Earth's surface can have a remarkable influence on the composition of its overwhelmingly more massive interior. The global distribution of uranium is a notable example. In early Earth history, the continental crust was enriched in uranium. Yet after the initial rise in atmospheric oxygen, about 2.4billion years ago, the aqueous mobility of oxidized uranium resulted in its significant transport to the oceans and, ultimately, by means of subduction, back to the mantle. Here we explore the isotopic characteristics of this global uranium cycle. We show that the subducted flux of uranium is isotopically distinct, with high (238)U/(235)U ratios, as a result of alteration processes at the bottom of an oxic ocean. We also find that mid-ocean-ridge basalts (MORBs) have (238)U/(235)U ratios higher than does the bulk Earth, confirming the widespread pollution of the upper mantle with this recycled uranium. Although many ocean island basalts (OIBs) are argued to contain a recycled component, their uranium isotopic compositions do not differ from those of the bulk Earth. Because subducted uranium was probably isotopically unfractionated before full oceanic oxidation, about 600 million years ago, this observation reflects the greater antiquity of OIB sources. Elemental and isotope systematics of uranium in OIBs are strikingly consistent with previous OIB lead model ages, indicating that these mantle reservoirs formed between 2.4 and 1.8 billion years ago. In contrast, the uranium isotopic composition of MORB requires the convective stirring of recycled uranium throughout the upper mantle within the past 600 million years.

Concentrations and fluxes of uranium in two major Chinese rivers: The Changjiang River and the Huanghe River

We collected samples from January 2010 to December 2011 to determine the concentrations and fluxes of uranium in the Changjiang (Yangtze) and Huanghe (Yellow) Rivers in China. The dissolved U concentrations (DUC) were measured by inductively coupled plasma mass spectrometry (ICP-MS) at the freshwater end members of the Changjiang and Huanghe Rivers. The DUCs ranged from 1.32 to 4.06 nmol/L and 13.85 to 29.99 nmol/L in the Changjiang and Huanghe Rivers, respectively. The temporal variations of DUC followed the seasonal change, with high values in the two rivers occurring during the dry seasons and low values during the flood seasons. A strong negative correlation was observed between DUC and discharge in the Changjiang River (R2 = 0.69), but a weak correlation (R2 = 0.35) was found in the Huanghe River. The correlations between the major ions and the U in the rivers indicated that the primary source of uranium was from the weathering of carbonate and evaporite in the Changjiang Basin. The weathering of evaporite-bearing sequences and the erosion of loess dominated the U sources of the Huanghe River. Carbonate ligands in the dry season and phosphate ligands in the wet season were the primary factors controlling the accumulation and transportation of dissolved uranium in the Changjiang River. The soils of the Huanghe Basin contained five times more leachable uranium compared to the soils of the Changjiang Basin, which may explain the high DUC in the Huanghe River. The weighted-mean-concentrations of uranium were 2.78 nmol/L in the Changjiang River and 22.07 nmol/L in the Huanghe River. This lead to annual dissolved U fluxes (DUF) of 2.3 × 106 mol/yr in the Changjiang River and 4.1 × 105 mol/yr in the Huanghe River. The sum of the U fluxes in the two rivers represented 11.9% of the global U riverine flux into the sea comparing with 2.5% of the global runoff into the sea. The 234U/238U activity ratio of the Huanghe River had higher values (1.455–1.418) compared to the Changjiang River (1.424–1.374) from the dry season to the wet season, respectively.

Enrichment of uranium, arsenic, molybdenum, and selenium in sediment cores from boreal lakes adjacent to northern Saskatchewan uranium mines

Assessment of flux rates and concentrations of potentially toxic elements in sediment cores from 5 lakes adjacent to the McClean Lake uranium mines in northern Saskatchewan (impact lakes) compared to results from a set of 5 upwind and distant lakes (reference lakes) clearly indicated enrichment of several elements in lakes close to the mining operations. The flux of uranium to the sediments of the 5 impact lakes was significantly higher post-1960 from the mean pre-1960 background conditions by 1.5 to 23 times. Flux rates of arsenic, molybdenum, and selenium also indicated significant enrichment from background conditions in the majority of the impact lakes. In contrast, large and significant increases in flux only occurred in 1 of the 5 reference lakes for arsenic. The highest levels of enrichment were found in the 3 lakes either in closest proximity to the tailings management facility, waste-rock piles, or downstream from the treated effluent management system. High flux rates and concentrations of elements such as arsenic and vanadium in both the reference lakes and in pre-mining samples of the impact lakes indicated that potentially toxic elements can occur naturally in this region. Sediment core analyses of elemental flux and concentration in both reference lakes and pre-1960 conditions in the impact lakes provided an estimate of natural background conditions. Our data suggest that the McClean Lake mine may be a source for the enrichments of toxic elements in the lakes in close proximity to the mining operations.

Simplified behaviors from increased heterogeneity: I. 2-D uranium transport experiments at the decimeter scale

Intermediate scale tank studies were conducted to examine the effects of physical heterogeneity of aquifer material on uranium desorption and subsequent transport in order to bridge the scaling gap between bench and field scale systems. Uranium contaminated sediment from a former uranium mill field site was packed into two 2-D tanks with internal dimensions of 2.44×1.22×0.076 m (tank 1) and 2.44×0.61×0.076 m (tank 2). Tank 1 was packed in a physically homogenous manner, and tank 2 was packed with long lenses of high and low conductivities resulting in different flow fields within the tanks. Chemical gradients within the flow domain were altered by temporal changes in influent water chemistry. The uranium source was desorption from the sediment. Despite the physical differences in the flow fields, there were minimal differences in global uranium leaching behavior between the two tanks. The dominant uranium species in both tanks over time and space was Ca2UO2(CO3)3(0). However, the uranium/alkalinity relationships varied as a function of time in tank 1 and were independent of time in tank 2. After planned stop-flow events, small, short-lived rebounds were observed in tank 1 while no rebound of uranium concentrations was observed in tank 2. Despite appearing to be in local equilibrium with respect to uranium desorption, a previously derived surface complexation model was insufficient to describe uranium partitioning within the flow domain. This is the first in a pair of papers; the companion paper presents an intermediate scale 3-D tank experiment and inter-tank comparisons. For these systems, physical heterogeneity at or above the decimeter scale does not affect global scale uranium desorption and transport. Instead, uranium fluxes are controlled by chemistry dependent desorption patterns induced by changing the influent ionic composition.

Landscape control of uranium and thorium in boreal streams – spatiotemporal variability and the role of wetlands

Abstract. The concentrations of uranium and thorium in ten partly nested streams in the boreal forest region were monitored over a two-year period. The investigated catchments ranged from small headwaters (0.1 km2) up to a fourth-order stream (67 km2). Considerable spatiotemporal variations were observed, with little or no correlation between streams. The fluxes of both uranium and thorium varied substantially between the subcatchments, ranging from 1.7 to 30 g km−2 a−1 for uranium and from 3.2 to 24 g km−2 a−1 for thorium. Airborne gamma spectrometry was used to measure the concentrations of uranium and thorium in surface soils throughout the catchment, suggesting that the concentrations of uranium and thorium in mineral soils are similar throughout the catchment. The fluxes of uranium and thorium were compared to a wide range of parameters characterising the investigated catchments and the chemistry of the stream water, e.g. soil concentrations of these elements, pH, TOC (total organic carbon), Al, Si and hydrogen carbonate, but it was concluded that the spatial variabilities in the fluxes of both uranium and thorium mainly were controlled by wetlands. The results indicate that there is a predictable and systematic accumulation of both uranium and thorium in boreal wetlands that is large enough to control the transport of these elements. On the landscape scale approximately 65–80% of uranium and 55–65% of thorium entering a wetland were estimated to be retained in the peat. Overall, accumulation in mires and other types of wetlands was estimated to decrease the fluxes of uranium and thorium from the boreal forest landscape by 30–40%, indicating that wetlands play an important role for the biogeochemical cycling of uranium and thorium in the boreal forest landscape. The atmospheric deposition of uranium and thorium was also quantified, and its contribution to boreal streams was found to be low compared to weathering.

Environmental and seasonal controls on riverine dissolved uranium in the Hudson, James, and Ungava bays region, Canada

This study documents the spatiotemporal variability of riverine uranium contents and fluxes in rivers discharging into the Hudson, James, and Ungava bays (HJUB). Samples retrieved during a monitoring program of the Koksoak, Great Whale, La Grande, and Nelson rivers were analyzed for dissolved uranium concentration [U] and activity ratio (234U/238U). Field surveys conducted during baseflow and snowmelt in six other rivers of the HJUB basin provided complementary data. It is estimated that altogether, the studied rivers export 3.4 × 105 mol/year of U towards the HJUB, with a discharge weighted average (234U/238U) of 1.27. Two main factors appear to control seasonal fluctuations in dissolved U exports from HJUB rivers: snowmelt and anthropogenic flow control. Under natural hydrological regimes, the dilution of U caused by snowmelt does not compensate for the associated increase in discharge, and riverine U fluxes are intensified during springtime. Contrastingly, the timing of riverine U exports is decoupled from hydroclimatic conditions in rivers affected by flow-control structures. Despite the seasonal variations in riverine U contents, the sampling profiles carried along two of the monitored rivers reveal that within the study region, sample locality is at least as important as sampling time for evaluating riverine U fluxes. In addition, a compilation of data from North American rivers reveals that spatial variations in riverine U contents seem to respond to a prominent lithological control, as rivers draining sedimentary rocks (with abundant carbonates) tend to present overall higher U contents and lower (234U/238U) variability than the rivers of the Canadian Shield.

Transport and separation of uranium(VI) by a polymer inclusion membrane based on di-(2-ethylhexyl) phosphoric acid

Abstract Di-(2-ethylhexyl) phosphoric acid (D2EHPA) is a commonly used extraction reagent for the separation of uranium from aqueous solutions. Previous work showed the suitability of a polymer inclusion membrane (PIM) based on D2EHPA and poly(vinyl chloride) (PVC) for the extraction and transport of uranium(VI) from sulfate solutions. This study details the improvement of the membrane transport by optimising the composition of the source and receiving solutions on one hand and membrane composition with the use of a plasticizer on the other. Initial fluxes of uranium as high as 3.0 × 10−6 mol m−2 s−1 corresponding to permeability of 7.2 × 10−6 m s−1 were recorded using a membrane composed of 35% (m/m) D2EHPA, 10% (m/m) o-nitrophenyloctyl ether and 55% (m/m) PVC from a solution containing 100 mg L−1 U(VI) in 0.1 mol L−1 H2SO4 into a solution containing 6 mol L−1 H2SO4, although transport across a membrane containing 45% (m/m) D2EHPA and 55% (m/m) PVC was only slightly slower. The effect of the acid anion in the source and receiving solutions on the extraction and back extraction of U(VI) is discussed and the membranes are tested for durability over repeated cycles of extraction and back-extraction. Using a 45% (m/m) D2EHPA and 55% (m/m) PVC PIM (m/m), U(VI) is completely separated thermodynamically from a range of common metal cations and successfully separated from Fe(III) kinetically.

Radium-based pore water fluxes of silica, alkalinity, manganese, DOC, and uranium: A decade of studies in the German Wadden Sea

A decade of studies of metal and nutrient inputs to the back-barrier area of Spiekeroog Island, NW German Wadden Sea, have concluded that pore water discharge provides a significant source of the enrichments of many components measured in the tidal channels during low tide. In this paper we add studies of radium isotopes to help quantify fluxes into and out of this system. Activities of radium isotopes in surface water from tidal channels in the back-barrier area exhibit pronounced changes in concert with the tide, with highest activities occurring near low tide. Other dissolved components: silica, total alkalinity (TA), manganese, and dissolved organic carbon (DOC) exhibit similar changes, with patterns matching the Ra isotopes. Uranium follows a reverse pattern with highest concentrations at high tide. Here we use radium isotope measurements in water column and pore water samples to estimate the fluxes of pore waters that enter the tidal channels during low tide. Using a flushing time of 4 days and the average activities of 224Ra, 223Ra, and 228Ra measured in the back-barrier surface and pore waters, we construct a balance of these isotopes, which is sustained by a deep pore water flux of (2–4)×108L per tidal cycle. This flux transports Ra and the other enriched components to the tidal channels and causes the observed low tide enrichments. An independent estimate of pore water recharge is based on the depletion of U in the tidal channels. The U-based recharge is about two times greater than the Ra-based discharge; however, other sinks of U could reduce the recharge estimate. The pore waters have wide ranges of enrichment in silica, alkalinity, manganese, DOC, and depletion of U with depth. We estimate concentrations of these components in pore water from the depth expected to contribute the majority of the pore water flux, 3.5m, to determine fluxes of these components to the tidal channels. Samples from this depth have minimum concentrations of silica, alkalinity, manganese, and DOC. We also estimate the exports of these components (and import of U) due to mixing based on average measured concentrations in the tidal creeks and the 4-day flushing time. A comparison of these estimates reveals that the exports (negative in the case of U) equal or exceed the pore water fluxes. By using values slightly higher than the minimum concentrations at 3.5m to calculate inputs, the two estimates could be forced to match. We conclude that pore water drainage is the major factor regulating fluxes of Ra isotopes, silica, alkalinity, manganese, DOC, and uranium in this system.

Evaluation and application of anion exchange resins to measure groundwater uranium flux at a former uranium mill site

Laboratory tests and a field validation experiment were performed to evaluate anion exchange resins for uranium sorption and desorption in order to develop a uranium passive flux meter (PFM). The mass of uranium sorbed to the resin and corresponding masses of alcohol tracers eluted over the duration of groundwater installation are then used to determine the groundwater and uranium contaminant fluxes. Laboratory based batch experiments were performed using Purolite A500, Dowex 21K and 21K XLT, Lewatit S6328 A resins and silver impregnated activated carbon to examine uranium sorption and extraction for each material. The Dowex resins had the highest uranium sorption, followed by Lewatit, Purolite and the activated carbon. Recoveries from all ion exchange resins were in the range of 94–99% for aqueous uranium in the environmentally relevant concentration range studied (0.01–200 ppb). Due to the lower price and well-characterized tracer capacity, Lewatit S6328 A was used for field-testing of PFMs at the DOE UMTRA site in Rifle, CO. The effect on the flux measurements of extractant (nitric acid)/resin ratio, and uranium loading were investigated. Higher cumulative uranium fluxes (as seen with concentrations > 1 ug U/gram resin) yielded more homogeneous resin samples versus lower cumulative fluxes (<1 ug U/gram resin), which caused the PFM to have areas of localized concentration of uranium. Resin homogenization and larger volume extractions yield reproducible results for all levels of uranium fluxes. Although PFM design can be improved to measure flux and groundwater flow direction, the current methodology can be applied to uranium transport studies.

Extraction of uranium(VI) from sulfate solutions using a polymer inclusion membrane containing di-(2-ethylhexyl) phosphoric acid

In this study a polymer inclusion membrane (PIM) for the extraction and separation of uranium (UO 2 2+) from acidic sulfate solutions is detailed. Poly(vinyl chloride) (PVC) based PIMs containing the following commercial extractants were screened for their ability to extract uranium: Alamine 336, Cyanex 272, Aliquat 336, di-(2-ethylhexyl) phosphoric acid (D2EHPA), and tri- n-butyl phosphate (TBP). The membrane containing 40% (m/m) D2EHPA is shown to be the most effective in the quantitative extraction and back extraction of uranium using sulfuric acid solutions. Facilitated transport of uranium across the membrane against its concentration gradient is demonstrated with uranium fluxes as high as 4.85 × 10 −7 mol m −2 s −1. A new method for determining the stoichiometry of extracted complexes in both membrane and solvent extraction systems has revealed the composition of the UO 2 2+–D2EHPA complex to be UO 2·A 2·(HA) 2 where HA and A − are D2EHPA and its conjugated base, respectively.

Evolution of the percutaneous penetration and distribution of uranyl nitrate as a function of skin-barrier integrity: an in vitro assessment

As recommended by OECD Guidelines, percutaneous penetration studies consider intact skin, but rarely injured skin. Recent years have witnessed a growing concern for these two types of dermal exposure in the industry, particularly in the nuclear industry. The aim of this study was to show that a method based on an in vitro device can be used to realistically assess how skin-barrier alterations caused by occupational accidents can modify the percutaneous penetration and distribution of radionuclides, particularly uranium. Wounds encountered in the nuclear industry (i.e., nitric acid burns and abrasion) were simulated on hairless rat skin. Skin-barrier alterations were characterized by means of a histological study and by measuring transepidermal water loss (TEWL) and skin thickness. The percutaneous penetration of uranyl nitrate through intact or injured skin biopsies was then measured in vitro. The maximum uranium flux values obtained for intact skin, skin abrasion with stratum corneum removal, and skin exposed to 2 N HNO3, 5 N HNO3, and 14 N HNO3 were, respectively, 0.6 ± 0.02, 1.2 ± 0.03, 1.2 ± 0.04, 42.0 ± 1.0, and 174.0 ± 8.7 ng.cm−2.h−1. These results demonstrated that the percutaneous absorption of uranium increased with the increased impairment of the stratum corneum. TEWL, combined with maximum uranium flux values measured in vitro, yielded a good prediction of the percutaneous penetration of uranium through injured skin, previously observed in vivo. To conclude, this in vitro assay provides a conservative estimate of the percutaneous diffusion of uranium through intact or injured skin, making it a good alternative method for toxicological studies and risk assessments.

Effect of Rapidly Changing River Stage on Uranium Flux through the Hyporheic Zone

Measurement of ground water/surface water interaction within the hyporheic zone is increasingly recognized as an important aspect of subsurface contaminant fate and transport. Understanding the interaction between ground water and surface water is critical in developing a complete conceptual model of contaminant transport through the hyporheic zone. At the Hanford Site near Richland, Washington, ground water contaminated with uranium discharges to the Columbia River through the hyporheic zone. Ground water flux varies according to changes in hydraulic gradient caused by fluctuating river stage, which changes in response to operation of dams on the Columbia River. Piezometers and continuous water quality monitoring probes were installed in the hyporheic zone to provide long-term, high-frequency measurement of hydraulic gradient and estimated uranium concentrations. Subsequently, the flux of water and uranium was calculated for each half-hour time period over a 15-month study period. In addition, measurement of water levels in the near-shore unconfined aquifer enhanced the understanding of the relationship between river stage, aquifer elevation, and uranium flux. Changing river stage resulted in fluctuating hydraulic gradient within the hyporheic zone. Further, influx of river water caused lower uranium concentrations as a result of dilution. The methods employed in this study provide a better understanding of the interaction between surface and ground water in a situation with a dynamically varying vertical hydraulic gradient and illustrate how the combination of relatively standard methods can be used to derive an accurate estimation of water and contaminant flux through the hyporheic zone.