Article pubs.acs.org/est
A DGT Technique for Plutonium Bioavailability Measurements Ruslan Cusnir,† Philipp Steinmann,‡ François Bochud,† and Pascal Froidevaux*,† †
Institute of Radiation Physics, Lausanne University Hospital, Lausanne, Switzerland Division of Radiation Protection, Federal Office of Public Health, Bern, Switzerland
‡
S Supporting Information *
ABSTRACT: The toxicity of heavy metals in natural waters is strongly dependent on the local chemical environment. Assessing the bioavailability of radionuclides predicts the toxic effects to aquatic biota. The technique of diffusive gradients in thin films (DGT) is largely exploited for bioavailability measurements of trace metals in waters. However, it has not been applied for plutonium speciation measurements yet. This study investigates the use of DGT technique for plutonium bioavailability measurements in chemically different environments. We used a diffusion cell to determine the diffusion coefficients (D) of plutonium in polyacrylamide (PAM) gel and found D in the range of 2.06− 2.29 × 10−6 cm2 s−1. It ranged between 1.10 and 2.03 × 10−6 cm2 s−1 in the presence of fulvic acid and in natural waters with low DOM. In the presence of 20 ppm of humic acid of an organic-rich soil, plutonium diffusion was hindered by a factor of 5, with a diffusion coefficient of 0.50 × 10−6 cm2 s−1. We also tested commercially available DGT devices with Chelex resin for plutonium bioavailability measurements in laboratory conditions and the diffusion coefficients agreed with those from the diffusion cell experiments. These findings show that the DGT methodology can be used to investigate the bioaccumulation of the labile plutonium fraction in aquatic biota.
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INTRODUCTION The release of artificial radionuclides into the environment as a result of nuclear accidents, nuclear weapon tests (NWT), and nuclear waste disposal contributes to the radioactive contamination of the environment and provides an additional exposure risk factor via ingestion through water and food. Plutonium is of particular concern since it is an alpha-particle emitter with a long half-life and its stockpile is continuously increasing as a byproduct of nuclear power production.1 Although most of current environmental plutonium originates from the global fallout of the NWT,2 specific areas were significantly affected by accidental releases of plutonium.3 The presence of residual plutonium contamination in Palomares soils has been confirmed more than 30 years later following the 1966 plane crash involving two nuclear bombs, even though considerable remediation efforts toward contamination removal were undertaken.4 More recently, a continuous series of major radioactive water leaks from the damaged Fukushima Daiichi nuclear plant5 arouse concern regarding the potential radioactive contamination of the ocean and ground waters and set a necessity for a reliable risk assessment tool in case of accidental radioactive discharge. Plutonium, as a low-soluble, strongly sorbing contaminant, is considered a highly immobile species. However, recent laboratory and field studies have demonstrated that colloidfacilitated plutonium transport over significant distances occurs in subsurface environments and groundwater systems.6,7 An enhanced mobility of fallout plutonium and its excess compared to 241Am and 137Cs have been demonstrated in the waters and © 2014 American Chemical Society
mosses from the Venoge spring of the karst system in the Swiss Jura Mountains, evoking the possible role of colloidal transport and enhanced bioavailability.8 Naturally occurring colloids in these environments include humic acids, mineral colloids and particles, as well as heterogeneous aggregates of humic acidcoated iron hydroxide particles and organic nuclei coated by a Fe−Ca rich layer.9 The association of plutonium with colloid species can increase its mobility, but will potentially reduce the bioavailability of the radionuclide. The technique of diffusive gradients in thin films (DGT) is widely used for passive sampling of trace metals. It allows the determination of time-weighted average concentrations of bioavailable contaminant species.10,11 A DGT sampler device consists of a diffusive gel layer, permeable for labile contaminant species, providing a surrogate of aquatic organisms, and a binding gel layer containing the binding agent, with both layers embedded into a plastic housing. The DGT technique has been successfully applied for determining radioelements such as U, 226Ra, 137Cs, and 134Cs in natural waters.12−15 To our knowledge, DGT has not yet been applied for plutonium speciation measurements and the diffusion coefficient for plutonium in polyacrylamide (PAM) gels is unknown. Received: Revised: Accepted: Published: 10829
March 7, 2014 August 20, 2014 August 20, 2014 August 20, 2014 dx.doi.org/10.1021/es501149v | Environ. Sci. Technol. 2014, 48, 10829−10834
Environmental Science & Technology
Article
by Zhang and Davison.16 Since plutonium can be present in several different oxidation states (III−VI) in environmental conditions, we chose to adjust the oxidation state of plutonium to IV to simplify the interpretation of our experimental data to the diffusion of the 239Pu (IV) species only. The oxidation state of plutonium was adjusted to IV by evaporation and calcination of the suitable aliquot of 239Pu tracer solution with NaHSO4 and H2SO4 as recommended by Bajo et al.20 The residue was dissolved in 75 mL of 10 mM MOPS, resulting in a solution of 10 mM Na2SO4, adjusted at pH 6.50−7.00 and left overnight exposed to the atmosphere for equilibration with CO2 prior to the diffusion experiment. Some experiments were designed to check for the oxidation state of plutonium during and after the diffusion experiment. In one experiment designed to control the +IV oxidation state of plutonium, all the solutions were degassed with N2 until the oxygen probe (CellOx 325) read
