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The speciation of metal species extracted by diffusion gradient in thin films (DGT) devices during experimental deployments in simple metal-ligand synthetic solutions was numerically modeled, analyzed at steady state, and simulated in the dynamic regime. The modeled speciation of two well-known complexes (Cu-citrate and Cu-EDTA) are in good agreement with experimental data obtained in NaNO3 solutions. For any metal complex, the rate at which the metal accumulates on the chelating resin is proportional to the concentration of free metal in solution plus a fraction of the metal complex concentration in solution equivalent to xiD(ML)/D(M). D(M) and D(ML) are the diffusion coefficients of the free and metal complex, respectively, and xi characterizes the complex lability: it is defined as the fraction of metal complex lost when diffusing from the bulk solution to the chelating resin. Numerical simulations were used to explore the variations of xi as a function of several operational and chemical parameters. Lability increases when the dissociation rate constant or the residence time of the metal complex within the hydrogel increase (i.e., by either increasing the thickness of the hydrogel or decreasing D(ML)). Overall, the fraction of metal complex extracted by the DGT decreases when D(ML) decreases, which confirms the ability of diffusion-restrictive hydrogels to separate the free metal fraction from complex solutions. Although the DGT model does not exactly comply with the voltammetric formalism, the lability criteria deltatau(1/2) used with voltammetric macroelectrodes is similar to the lability criteria xi, except that it is valid for high metal-ligand ratio. To determine free metal concentrations in aquatic systems using DGT, it is necessary to account for the presence of numerous ligands with different complexing properties. Soon, the numerical model will be improved to include additional ligands competing for the metal, and new experiments will be designed to discriminate between different labile complexes. Experimental results obtained with different DGT devices (of different hydrogel thicknesses, for example) could be interpreted in terms of conditional kinetic characteristics of the dominant metal-ligand complex in solution.

It is often difficult to evaluate the level of contamination in small urban rivers because pollution is mainly diffuse, with low levels of numerous substances. The use of a coupled approach using both chemical and biological measurements may provide an integrated evaluation of the impact of micro-pollution on the river. Zebra mussels were transplanted along a metal and organic pollution gradient in spring 2008. For two months, mussels and water samples were collected from two sites every two weeks and analyzed for metal and PAH content as well as water physicochemical parameters. Diffusive gradients in thin film (DGT) were also used to assess levels of labile metals. Exposure of mussels to contaminants and potential impact were evaluated using physiological indices and various biomarkers including condition index (CI), defense mechanisms (glutathione-S-transferase: GST), digestive enzymes (amylase and cellulase) and genotoxicity (micronucleus test: MN and comet assay: CA). For most contaminants, the water contamination was significantly higher downstream. Bioaccumulation in zebra mussels was related to water contamination in the framework of the biodynamic model, which allowed us to take into account the biological dilution that was caused by the growth of soft tissue downstream. Thus, metal influxes were on average two times higher downstream than upstream in particular for Zn, Cr, Cu and Cd. Significant differences in condition index were observed (final CI was 0.42 ± 0.03 downstream and 0.31 ± 0.03 upstream) reflecting a better food availability downstream. Moreover a significant decrease of GST activity and digestive enzymes activity in the cristalline style was observed downstream. Interpreting this decrease requires considering not only micro-pollution but also the trophic status related to the water's physicochemistry. The MN test and the CA on gill cells highlighted genotoxicity in mussels transplanted downstream compared to upstream.

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