Nuclear activities of 1945-1975 have released in the environment significant amounts of radioactive elements. Three main sources contributed to contamination with plutonium: nuclear weapon tests, discharges from nuclear facilities and nuclear accidents. Global atmospheric fallout has marked pristine environments with trace amounts of 239Pu and 240Pu. Plutonium is an alpha-particle emitter with long half life and it is dangerous when incorporated in the organism. The amount of Pu which can be assimilated by living organisms depends on its chemical form and on local physico-chemical characteristics of the environment. To study the bioavailability of plutonium in aquatic environments, we developed a technique of diffusive gradients in thin films (DGT), which allows measuring free and labile Pu species only, diffusing through a thin layer of a polymer gel. DGT devices exposed in freshwaters in the mineral Venoge spring and organic-rich Noiraigue Bied brook have demonstrated that the major fraction of Pu in these waters is fully available for biouptake. Similarly, the ultrafiltration technique has shown that in both mineral and organic-rich water the major fraction of Pu is found in the dissolved phase. Laboratory experiments using the DGT technique revealed a relatively high dissociation rate constant for Pu complexes with natural organic matter, suggesting that such complexes in natural environment can contribute to biouptake of Pu by aquatic organisms. Sequential elution of Pu from different compartments of aquatic mosses (Venoge) and plants (Noiraigue) indicated that the major fraction of Pu was co-precipitated with calcite on their extracellular parts. The development and testing of the DGT technique for Pu measurements in the frame of this project paves the way for further applications of DGTs to study the bioavailability and speciation of Pu in contaminated and marine environments.
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Les activités nucléaires des années 1945-1975 ont relâché dans l'environnement d'importantes quantités d'éléments radioactifs. Trois sources principales ont contribué à la contamination par le plutonium: les essais d'armes nucléaires, les rejets des installations nucléaires et les accidents nucléaires. Les retombées atmosphériques globales ont marqué l'environnement avec des traces de 239Pu et 240Pu. Le plutonium est un émetteur de particules alpha de longue période et il est dangereux lorsqu'il est incorporé dans l'organisme. La quantité de Pu pouvant être assimilée par les organismes vivants dépend de sa forme chimique et des caractéristiques physico-chimiques locales de l'environnement en question. Pour étudier la biodisponibilité du plutonium dans les milieux aquatiques, nous avons développé une technique du gradient de diffusion en couche mince (DGT), qui permet de mesurer seules les espèces libres de Pu, capables de diffuser à travers une mince couche d'un gel polymère. Les dispositifs DGT exposés dans les eaux douces de la source de la Venoge et du Bied de Noiraigue, riche en matière organique, ont démontré que la majeure fraction de Pu dans ces eaux est entièrement disponible pour l'assimilation biologique. De même, la technique d'ultrafiltration a démontré que dans l'eau minérale et dans l'eau riche en matière organique, le Pu se trouve principalement dans la phase dissoute. Des expériences de laboratoire utilisant la technique DGT ont révélé une constante de dissociation relativement élevée pour des complexes de Pu avec la matière organique naturelle, ce qui suggère que ces complexes en milieu naturel peuvent contribuer à l'accumulation de Pu par les organismes aquatiques. L'élution séquentielle de Pu de différents compartiments de mousses aquatiques (Venoge) et de plantes (Noiraigue) a montré que la majeure partie du Pu a été co-précipité avec la calcite sur leurs parties extracellulaires. Le développement et l'application de la technique DGT pour les mesures de Pu dans le cadre de ce projet ouvre la voie à de nouvelles applications de DGTs pour étudier la biodisponibilité et la spéciation du Pu dans des environnements marins contaminés.
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The speciation and the bioavailability of trace elements in the aquatic environments are the key limiting factors defining the toxic effects to the aquatic biota. Radioactive elements found in the environment arouse particular interest since they can exert radiotoxic effects at low concentrations, yet insufficient to show any chemotoxic effects. Plutonium (Pu) is a heavy artificial radionuclide of particular concern due to its long half-life and alpha- particle emission. Assessing the speciation and the bioavailability of Pu in natural waters is essential in order to predict its environmental risks.
The overall goal of this research work was to develop the technique of the diffusive gradients in thin films (DGT) for in-situ bioavailability measurements of Pu. We first aimed to determine the diffusion coefficients (D) of Pu in the diffusive polyacrylamide (PAM) gel. Afterwards, we intended to test in laboratory experiments the feasibility of the DGT technique for Pu measurements, using Chelex resin as a binding phase. The positive outcome of the initial experiments with Pu allowed us to expand the application of the DGT technique to further studies on the molecular interactions of Pu with natural organic matter (NOM) and on the mobility of Pu in +IV and +V oxidation states. To measure the bioavailable fraction of Pu in natural waters, DGT samplers with large surface area were fabricated and deployed in two, chemically distinct, karstic freshwater environments. The results of DGT measurements were complemented with ultrafiltration technique, providing the distribution of Pu between colloid-bound and truly dissolved fractions. Finally, the determination of Pu in different compartments of aquatic mosses and plants enabled us to integrate all the results into a model of Pu speciation in karstic freshwater environments.
The determination of diffusion coefficients of Pu in solutions of different chemical composition demonstrated that the mobility of Pu species in natural waters depends on the content of natural colloids and natural organic matter. Diffusion experiments yielded higher diffusion coefficients for Pu in the simple buffered solutions, compared to solutions containing Pu in the presence of fulvic or humic substances. Similarly, the diffusion of Pu was less restrained in water from a mineral spring, compared to the Pu diffusion in organic-rich water. Enriching natural waters with colloids by ultrafiltration also reduced the diffusion coefficients of Pu. The effects of the redox speciation of Pu were also quantified using the DGT technique. As demonstrated, Pu (V) was more mobile compared to Pu (IV) in both raw buffered solutions and in the presence of humic substances.
The interaction of Pu with natural organic matter results in the formation of soluble complexes, altering its speciation in natural waters. The rate of dissociation of these complexes determines the fraction of free Pu, available for biouptake. Multiple DGT devices with different diffusive layer thicknesses were used to estimate the dissociation constant, kdis, of Pu-NOM complexes and the extent of their contribution to the biouptake of Pu. Using a mathematical dynamic model, we found a relatively high kdis for Pu-NOM complexes (kdis
7.5 × 10-3 s-1), suggesting that in the organic-rich natural waters Pu is fully available for biouptake.
The findings obtained in laboratory experiments were thereafter confirmed in the field studies. DGT samplers deployed in the mineral water of the Venoge spring (NOM < 1 mg L-1) measured the major fraction of Pu as bioavailable. In the organic-rich water (NOM = 15 mg L-1) of the Noiraigue Bied brook the fraction of free Pu was lower compared to the Venoge spring. However, the DGT devices with thicker diffusive gel layer accumulated twice as much Pu, as expected from the theory for labile complexes. This indicated that the dissociation of soluble Pu-NOM complexes in this water contributed to the uptake of Pu.
The distribution of Pu eluted from different compartments of the aquatic mosses and plants demonstrated that the major fraction of Pu is co-precipitated with calcite on the leaves surfaces. Pu concentrations determined in the mosses (Fontinalis antypiretica) from the Venoge spring suggest that Pu is possibly found in a chemical form similar to a soluble uranyl-carbonate complex in water; this complex can further co-precipitate with calcite when CO2 is degassing from the water. Aquatic plants (Phragmites australis) in the Noiraigue Bied brook contained even a greater fraction of Pu within the calcite. However, the presence of organic matter at high concentration can complex Pu and contribute to a higher biouptake of Pu in this water since the diffusion and dissociation of the Pu-NOM complexes will create additional flux to the plant.