Francine Carrot

Chemical forms of selenium in the metal-resistant bacterium Ralstonia metallidurans CH34 exposed to selenite and selenate.

ABSTRACT Ralstonia metallidurans CH34, a soil bacterium resistant to a variety of metals, is known to reduce selenite to intracellular granules of elemental selenium (Se0). We have studied the kinetics of selenite (SeIV) and selenate (SeVI) accumulation and used X-ray absorption spectroscopy to identify the accumulated form of selenate, as well as possible chemical intermediates during the transformation of these two oxyanions. When introduced during the lag phase, the presence of selenite increased the duration of this phase, as previously observed. Selenite introduction was followed by a period of slow uptake, during which the bacteria contained Se0 and alkyl selenide in equivalent proportions. This suggests that two reactions with similar kinetics take place: an assimilatory pathway leading to alkyl selenide and a slow detoxification pathway leading to Se0. Subsequently, selenite uptake strongly increased (up to 340 mg Se per g of proteins) and Se0 was the predominant transformation product, suggesting an activation of selenite transport and reduction systems after several hours of contact. Exposure to selenate did not induce an increase in the lag phase duration, and the bacteria accumulated approximately 25-fold less Se than when exposed to selenite. SeIV was detected as a transient species in the first 12 h after selenate introduction, Se0 also occurred as a minor species, and the major accumulated form was alkyl selenide. Thus, in the present experimental conditions, selenate mostly follows an assimilatory pathway and the reduction pathway is not activated upon selenate exposure. These results show that R. metallidurans CH34 may be suitable for the remediation of selenite-, but not selenate-, contaminated environments.

Preliminary results on the leaching process of phosphate ceramics, potential hosts for actinide immobilization

Abstract A mixed zirconium–lanthanum phosphate La 1/3 Zr 2 (PO 4 ) 3 , noted LaZrP, is studied as a potential host for actinides issued from high-level nuclear wastes. Chemical durability is evaluated and compared with two other phases: a monazite phase LaPO 4 , noted LaP, and a mixed oxide phase La 0.1 ZrO 2.15 , noted LaZrO. Leaching tests are performed and followed by solution and solid analyses. Static tests are performed with a low ratio between surface of ceramic and volume of solution, S / V (about 0.1 cm −1 ). For LaZrP, the phosphate release rate decreases from 10 −2 to 10 −3 g / m 2 / day , while zirconium and lanthanum releases remain very low ( −5 g / m 2 / day ). For the LaP phase, the phosphate release is about 10 times lower than for LaZrP and remains stationary, while the rate of the lanthanum release is below 10 −6 g / m 2 / day . Tests performed at high S / V (about 20 cm −1 ) reveal that the lanthanum release rates after 2 weeks were, respectively, at 10 −9 , 10 −6 and 10 −4 g / m 2 / day for LaP, LaZrP and LaZrO phases. It is shown that NZP or monazite type ceramics are highly resistant to the leaching process.

Relevance of NH4F in acid digestion before ICP-MS analysis.

In order to implement a simpler, less expensive and more safe sample dissolution procedure, we have substituted the HF-HClO(4) mixture by NH(4)F. By testing three certified reference materials, lichen 336, basalt BE-N, soil 7, it was found that the three-reagents digestion without HF and HClO(4) (HNO(3)+H(2)O(2)+NH(4)F was used) was very effective for the pretreatment of ICP-MS measurement. The comparison was based on the measurement results and their uncertainties. All are reference material for amount contents of different trace elements. The accuracy and precision of the developed method were tested by replicate analyses of reference samples of established element contents. The accuracy of the data as well as detection limits (LODs) vary among elements but are usually very good (accuracy better than 8%, LODs usually below 1 microg/g in solids). ICP-MS capabilities enable us to determine routinely 13 and 16 minor and trace elements in basalt and soil.

Uranium(VI) complexation in cell culture medium : influence of speciation on Normal Rat Kidney (NRK-52E) cell accumulation

Summary Uranium bioavailability and toxicity are closely linked to the metals speciation in solution. However in biological fluids or in media classically used for cell culture – and subsequently for in vitro cell exposure –, uranium is rarely present as free-ion since these media contain non-negligible concentrations of potential ligands such as phosphate and bicarbonate but also co-ions such as calcium which can cause U(VI) complexes precipitation. The chemical form of uranium that is internalized in cells and interferes with biological processes is of major concern. Uranium toxicity and accumulation were evaluated in vitro on NRK-52E cells, model for rat renal proximal tubule. Uranium intracellular accumulation begins after 12 h exposure to 600 μM U(VI); toxicity appears as soon as cells accumulated 25 to 30 mg U/g protein. Modification of uranium speciation in the exposure medium induces great changes in toxicity and cell accumulation. Comparison of toxicity and accumulation results to theoretical uranium speciation, calculated with the J-Chess computer program, shows that free-ion concentration can not explain the total uranium intracellular accumulation. Low molecular weight U(VI) complexes, such as UO2(CO3)34− but also UO2PO4− could be implicated in U(VI) cellular accumulation and toxicity.

Investigation of cadmium toxicity on renal epithelial cells using nuclear microprobe analysis

Abstract Cadmium is a highly toxic metal that causes well-known severe renal damages. Its toxicity is frequently investigated in vitro using numerous epithelial models. The accumulation and transport of cadmium in cultured renal epithelial cells has been studied by means of nuclear microscopy (micro-PIXE coupled with micro-RBS) for cell monolayer analyses, and by ICP-MS for culture medium analyses. Cell viability, measured by biochemical tests, was used as toxicity indicator. Dependence on cadmium concentration (1–100 μM) and exposure time (1–24 h) was found. Micro-PIXE reveals a strong anti-correlation of intra-cellular cadmium concentration with zinc concentration, a biological metal, suggesting substitution mechanism of both metals.

Alteration of glasses by micro-organisms

Abstract Micro-organisms are suspected to play a basic part in materials alteration. Obviously, they will be present in nuclear waste repositories, either introduced by technological activity or laid by fluids circulation. Their metabolism may induce chemical modifications to the surrounding media and then affect the durability of storage materials. Biodegradation of glasses is studied in the Pierre Sue Laboratory. In the frame of a collaboration with microbiologists interested in stained glasses alteration, leaching experiments with various species of bacteria and fungi are carried out. Ion beam analysis techniques are performed to quantify surface modification of glasses and elemental incorporation in micro-organisms. Analyses of the solutions will lead to a complete assessment of elemental exchanges between glass sample, culture media and micro-organisms. In this paper, preliminary results on characterisation of glasses and micro-organisms and the first results of leaching experiments are presented.

Behaviour of lanthanide aluminosilicate glass in water: Preliminary results

Abstract Lanthanide aluminosilicate glasses have been developed for their high mechanical properties. Nevertheless, little is known about their chemical durability and the role of rare-earth elements on the dissolution mechanisms of such glasses. This type of glass appears to be a suitable complement waste forms of borosilicate glass for the specific immobilisation of trivalent actinides. The first part of this paper deals with the synthesis and the characterisation of glasses in the system SiO2–Al2O3–Y2O3–Ln2O3 (with Ln=La or Ce) and the second part with the study of their leachability in deionised water using a reflux column at 100°C. Hydrated glass surfaces have been examined with scanning electron microscopy coupled with X-ray microanalysis and nuclear microanalysis. Leachates have been characterised by using ICP-MS and UV-visible spectrophotometry. After leaching, the formation of a surface layer slightly depleted in Si and Al and containing Y and La hydroxicarbonates and hydroxides is discussed.