Environmental exposure to uranium in a population living in close proximity to gold mine tailings in South Africa.

BACKGROUND: Gold mining activities in South Africa resulted in contamination of residential environment with uranium-rich wastes from mine tailings. Health of the people living around the mine tailings could be affected by uranium exposure due to its hazardous chemotoxic and radiological properties. METHODS: We conducted a cross-sectional study to assess i) uranium (U) concentrations in individual hair samples of children and adults living in close proximity to mine tailings in Northeast- Soweto in Johannesburg, South Africa, and ii) the association between U concentrations in hair and various factors, including zone of residence, socio-demographic and housing characteristics. Sampling sites were divided into three zones based on the distance between a dwelling and a cluster of mine tailings (zone 1: <= 500 m, zone 2: 2-3 km away, zone 3: 4-5 km away). U concentrations in hair samples were measured using inductively coupled plasma mass spectrometry. To test the association between U concentrations and selected factors we used robust regression models with log-transformed U concentrations. RESULTS: Among 128 subjects with available U measurements, 63 (49%) were children (ages 7-15 years) of which 38 were girls, the remaining 65 (51%) were adult females. Mean (median) U concentration in hair samples was 143 (92) µg/kg. In the mutually adjusted analyses, only an inverse association between age and U concentration in hair remained statistically significant, with geometric mean in children being 2.1 times higher compared to adults (P < 0.001). There was no evidence of an association between zones and U concentration (P = 0.42). CONCLUSIONS: There was little evidence of association between U concentration in hair and distance from the mine tailings within the 5 km range, but overall concentrations were elevated compared to general population samples in other parts of the world. Children had statistically significantly higher geometric mean of uranium concentration in hair compared to adults. The results are important for improvement of mining waste policies and implementation of health monitoring and protective measures in populations at risk. ARTICLE CATEGORY: Research Article.

A comprehensive study on the interaction of Eu(III) and U(VI) with plant cells (Daucus carota) in suspension.

Daucus carota suspension cells showed a high affinity towards Eu(III) and U(VI) based on a single-step bioassociation process with an equilibrium after 48-72 h. Cells responded with an increased metabolic activity towards heavy metal stress. Luminescence spectroscopy pointed to multiple species for both f-block elements in the culture media, providing initial hints of their interaction with cells and released metabolites. Using nuclear magnetic resonance spectroscopy, we could prove that malate, as an released metabolite in the culture medium, was found to complex with U. Luminescence spectroscopy also showed that Eu(III)-EDTA species are interacting with the cells. Furthermore, Eu(III) and U(VI) coordination is dominated by phosphate groups provided by the cells. We found that Ca ion channels of D. carota cells were involved in the uptake of U(VI), which led to a bioprecipitation of U(VI) in the vacuole of the cells, most probably as uranyl(VI) phosphates along with an intracellular sorption of U(VI) on biomembranes by lipid structures. Eu(III) could be found locally concentrated in the cell wall and in the cytoplasm with a co-localization with phosphorous and oxygen.

Endocytosis is a significant contributor to uranium(VI) uptake in tobacco (Nicotiana tabacum) BY-2 cells in phosphate-deficient culture.

Endocytosis of metals in plants is a growing field of study involving metal uptake from the rhizosphere. Uranium, which is naturally and artificially released into the rhizosphere, is known to be taken up by certain species of plant, such as Nicotiana tabacum, and we hypothesize that endocytosis contributes to the uptake of uranium in tobacco. The endocytic uptake of uranium was investigated in tobacco BY-2 cells using an optimized setup of culture in phosphate-deficient medium. A combination of methods in biochemistry, microscopy and spectroscopy, supplemented by proteomics, were used to study the interaction of uranium and the plant cell. We found that under environmentally relevant uranium concentrations, endocytosis remained active and contributed to 14% of the total uranium bioassociation. Proteomics analyses revealed that uranium induced a change in expression of the clathrin heavy chain variant, signifying a shift in the type of endocytosis taking place. However, the rate of endocytosis remained largely unaltered. Electron microscopy and energy-dispersive X-ray spectroscopy showed an adsorption of uranium to cell surfaces and deposition in vacuoles. Our results demonstrate that endocytosis constitutes a considerable proportion of uranium uptake in BY-2 cells, and that endocytosed uranium is likely targeted to the vacuole for sequestration, providing a physiologically safer route for the plant than uranium transported through the cytosol.

Bioassociation of U(VI) and Eu(III) by Plant (Brassica napus) Suspension Cell Cultures-A Spectroscopic Investigation.

In this study, we investigated the interaction of U(VI) and Eu(III) with Brassica napus suspension plant cells as a model system. Concentration-dependent (0-200 µM) bioassociation experiments showed that more than 75% of U(VI) and Eu(III) were immobilized by the cells. In addition to this phenomenon, time-dependent studies for 1 to 72 h of exposure showed a multistage bioassociation process for cells that were exposed to 200 µM U(VI), where, after initial immobilization of U(VI) within 1 h of exposure, it was released back into the culture medium starting within 24 h. A remobilization to this extent has not been previously observed. The MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) assay was used to correlate the bioassociation behavior of Eu and U with the cell vitality. Speciation studies by spectroscopy and in silico methods highlighted various U and Eu species over the course of exposure. We were able to observe a new U species, which emerged simultaneously with the remobilization of U back into the solution, which we assume to be a U(VI) phosphate species. Thus, the interaction of U(VI) and Eu(III) with released plant metabolites could be concluded.

Uranium(VI) toxicity in tobacco BY-2 cell suspension culture - A physiological study.

For the first time, the physiological and cellular responses of Nicotiana tabacum (BY-2) cells to uranium (U) as an abiotic stressor were studied using a multi-analytic approach that combined biochemical analysis, thermodynamic modeling and spectroscopic studies. The goal of this investigation was to determine the U threshold toxicity in tobacco BY-2 cells, the influence of U on the homeostasis of micro-macro essential nutrients, as well as the effect of Fe starvation on U bioassociation in cultured BY-2 cells. Our findings demonstrated that U interferes with the homeostasis of essential elements. The interaction of U with BY-2 cells confirmed both time- and concentration-dependent kinetics. Under Fe deficiency, a reduced level of U was detected in the cells compared to Fe-sufficient conditions. Interestingly, blocking the Ca channels with gadolinium chloride caused a decrease in U concentration in the BY-2 cells. Spectroscopic studies evidenced changes in the U speciation in the culture media with increasing exposure time under both Fe-sufficient and deficient conditions, leading us to conclude that different stress response reactions are related to Fe metabolism. Moreover, it is suggested that U toxicity in BY-2 cells is highly dependent on the existence of other micro-macro elements as shown by negative synergistic effects of U and Fe on cell viability.

Plant cell (Brassica napus) response to europium(III) and uranium(VI) exposure.

Experiments conducted over a period of 6 weeks using Brassica napus callus cells grown in vitro under Eu(III) or U(VI) stress showed that B. napus cells were able to bioassociate both potentially toxic metals (PTM), 628 nmol Eu/gfresh cells and 995 nmol U/gfresh cells. Most of the Eu(III) and U(VI) was found to be enriched in the cell wall fraction. Under high metal stress (200 µM), cells responded with reduced cell viability and growth. Subsequent speciation analyses using both metals as luminescence probes confirmed that B. napus callus cells provided multiple-binding environments for Eu(III) and U(VI). Moreover, two different inner-sphere Eu3+ species could be distinguished. For U(VI), a dominant binding by organic and/or inorganic phosphate groups of the plant biomass can be concluded.

Calorimetrically Determined U(VI) Toxicity in Brassica napus Correlates with Oxidoreductase Activity and U(VI) Speciation.

Radioecological studies depend on the quantitative toxicity assessment of environmental radionuclides. At low dose exposure, the life span of affected organisms is barely shortened, enabling the transfer of radionuclides through an almost-intact food chain. Lethality-based toxicity estimates are not adequate in this regime because they require higher concentrations. However, increased radionuclide concentration alters its speciation, rendering the extrapolation to the low dose exposure chemically inconsistent. Here, we demonstrate that microcalorimetry provides a sensitive real-time monitor of toxicity of uranium (in the U(VI) oxidation state) in a plant cell model of Brassica napus. We introduce the calorimetric descriptor "metabolic capacity" and show that it correlates with enzymatically determined cell viability. It is independent of physiological models and robust against the naturally occurring fluctuations in the metabolic response to U(VI) of plant cell cultures. In combination with time-resolved laser-induced fluorescence spectroscopy and thermodynamic modeling, we show that the plant cell metabolism is affected predominantly by hydroxo-species of U(VI) with an IC50 threshold of ∼90 µM. The data emphasize the yet-little-exploited potential of microcalorimetry for the speciation-sensitive ecotoxicology of radionuclides.

Complex formation of uranium(VI) with L-phenylalanine and 3-phenylpropionic acid studied by attenuated total reflection Fourier transform infrared spectroscopy.

Uranyl complexes with phenylalanine and the analogous ligand phenylpropionate were investigated in aqueous solution by attenuated total reflection (ATR) Fourier transform infrared (FT-IR) spectroscopy. The assignment of the observed bands to vibrational modes was accomplished using spectra of the pure ligands recorded at different pH values and spectra of the 15N labeled analogous compounds of the amino acid. The results presented in this work provide a detailed description of the binding states of the uranyl complexes in solution. A bidentate binding of the carboxylate group to the actinide ion was observed by the characteristic shifts of the carboxylate modes. From the spectra the presence of the protonated amino group in the actinide complex can be derived. Due to these findings, contributions of the amino group to the binding to the uranyl ion in the amino acid complex can be ruled out.

U(VI)-kaolinite surface complexation in absence and presence of humic acid studied by TRLFS.

Time-resolved laser-induced fluorescence spectroscopy (TRLFS) was applied to study the U(VI) surface complexes on kaolinite in the presence and absence of humic acid (HA). Two uranyl surface species with fluorescence lifetimes of 5.9 +/- 1.4 and 42.5 +/- 3.4 micros and 4.4 +/- 1.2 and 30.9 +/- 7.2 micros were identified in the binary (U(VI)-kaolinite) and ternary system (U(VI)-HA-kaolinite), respectively. The fluorescence spectra of adsorbed uranyl surface species are described with six and five fluorescence emission bands in the binary and ternary system, respectively. The positions of peak maxima are shifted significantly to higher wavelengths compared to the free uranyl ion in perchlorate medium. HA has no influence on positions of the fluorescence emission bands. In the binary system, both surface species can be attributed to adsorbed bidentate mononuclear surface complexes, which differ in the number of water molecules in their coordination environment. In the ternary system, U(VI) prefers direct binding on kaolinite rather than via HA, but it is sorbed as a uranyl-humate complex. Consequently, the hydration shell of the U(VI) surface complexes is displaced with complexed HA, which is simultaneously distributed between kaolinite particles. Aluminol binding sites are assumed to control the sorption of U(VI) onto kaolinite.

Migration of uranium(IV)/(VI) in the presence of humic acids in quartz sand: a laboratory column study.

The migration behavior of U(IV) and U(VI) in the presence of humic acid was studied in a quartz sand system. Laboratory column experiments were performed using humic acid, U(VI) in humic acid absence, U(IV) and U(VI) in humic acid presence, and for comparison a conservative tracer. In experiments using humic acid, both redox species of U migrate nearly as fast as the conservative tracer. Humic acid accelerates the U(VI) breakthrough compared to the humic acid-free system. There are strong indications for a similar effect on the U(IV) transport. At the same time, a part of U(IV) and U(VI) associated with the humic acid is immobilized in the quartz sand due to humic colloid filtration thus producing a delaying effect. Tailing at a low concentration level was observed upon tracer elution. The experimental breakthrough curves were described by reactive transport modeling using equations for equilibrium and kinetic reactions. The present study demonstrates that humic acids can play an important role in the migration of actinides. As natural organic matter is ubiquitous in aquifer systems, the humic colloid-borne transport of actinides is of high relevance in performance assessment.