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A dynamic numerical model to characterize labile metal complexes collected with diffusion gradient in thin films devices

Abstract : 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 zetaD(ML)/D-M. D-M and D-ML are the diffusion coefficients of the free and metal complex, respectively, and zeta 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 zeta 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 DML). Overall, the fraction of metal complex extracted by the DGT decreases when DML 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 ATtau(1/2) used with voltammetric macroelectrodes is similar to the lability criteria zeta, 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.
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M.H. Tusseau-Vuillemin, R. Gilbin, M. Taillefert. A dynamic numerical model to characterize labile metal complexes collected with diffusion gradient in thin films devices. Environmental Science and Technology, American Chemical Society, 2003, 37 (8), pp.1645-1652. ⟨10.1021/es025839o⟩. ⟨hal-02581397⟩



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