Development of the DGT technique for in-situ Pu speciation measurements

Toxic effects of artificial radionuclides are strongly dependent on the surrounding chemical environment which determines the bioavailability of contaminant species. Speciation of plutonium in the environment is of particular interest since it is a long-life actinide contributing to the dose exposure via ingestion with water and food in case of radioactive discharge. Furthermore, natural colloids present in waters, as main carriers of dissolved trace metals, can transport plutonium over significant distances from contaminated sites. The diffusive gradients in thin films (DGT) technique is an efficient instrument for passive sampling of trace metals. It allows for in-situ monitoring of mean concentrations of bioavailable contaminant species (1). A DGT sampler consists of two gel layers: a polyacrylamide hydrogel diffusion layer allowing to diffuse labile metal species to the second, binding layer, impregnated with ion-exchange resin. Resin elution and further analysis allow determining the amount of the bioavailable fraction of trace metals in the bulk solution, which can be calculated from the relationship: Cbulk= (M x Δg)/(D x t x A) where M is the measured metal species inventory in the resin, Δg the thickness of the diffusion layer, D the diffusion coefficient of the species in the gel, t the time of deployment, A the diffusion area. To our knowledge, the DGT has not yet been applied for Pu speciation measurements and the D value is unknown. Here we propose the use of DGT to monitor the dissolved phase and labile complexes of plutonium in the aquatic environment. We have first measured the diffusion coefficient of plutonium in the hydrogel. Experiments were carried out in a diffusion cell (2) and with commercially available DGT samplers exposed in standardized solutions containing 239Pu at pico-molar concentrations. Both approaches give comparable D values in the range of 2.30 x 10-6 - 2.45 x 10-6 cm2 s-1. We then studied plutonium diffusion in the presence of naturally occurring ligands - fulvic and humic acids - and in natural waters sampled at a mineral spring and an organic-rich brook of a karst system in the Swiss Jura Mountains, in which the increased mobility of 239+240Pu compared to 241Am and 137Cs has been recently observed (3). Diffusion experiments in simulated natural conditions show different mobility of the radionuclide in fresh waters with different DOM content, supporting the idea that the speciation of plutonium and the abundance of free ionic form in particular, are of prime importance to better estimate the bioavailability and the toxicity of plutonium. (authors)