Thursday 7 October, 4 PM CET
JORDI SANS DUÑÓ
ABSTRACT: DGT (Diffusive Gradients in Thin-films) is an environmentally-oriented technique based on the accumulation of a target analyte after diffusion through a diffusive gel. Metals are mainly present in natural waters as complexes with simple (Cl-, SO42-,HCO3-,...) or macromolecular ligands (humic or fulvic acids,..). Assuming that only the free metal can be bound to the resin, the accumulation in the DGT devices results from the contribution of all the metal species, with metal complexes contributing according to their lability degree (ξ) (1). The lability degree of a system can be experimentally measured as the ratio of the actual complex contribution to the metal accumulation over the maximum contribution that would be achieved if dissociation was fast enough to reach instantaneously equilibrium with the free metal at all the relevant spatial domain. The contribution of each metal species depends on the configuration of the DGT device (characterized by the thickness of the resin and of the diffusive gels used), so that independent and complementary information on the dynamic characteristics of the sample can be obtained from measurements of accumulation with different DGT configurations (2).
In this work, the accumulation of Ni in solutions containing different nitrilotriacetic acid (NTA) concentrations has been measured with several DGT devices containing combinations of resin(s) and diffusive gel(s) resulting in total different thicknesses. It is shown that ξ increases as either the thickness of the diffusive gel (δg) or of the resin (δr) increase (3). These results are reproduced with a simulation tool that considers the diffusion reactions in the DGT device by solely fitting the kinetic dissociation constant of the NiNTA complex. Diffusion coefficients have been measured in a diffusion cell and speciation has been obtained with VMINTEQ.
In conclusion, the thickness of the resin and/or diffusive gels of a DGT device can be optimized to improve the correlation with bioavailability. Likewise, using a set of DGT configurations to measure the contribution of complexes in natural waters, a set of equations can be written to recover dynamic characteristics of individual complexes or some fractions with similar chemical properties determined by chemical speciation (e.g. inorganic and organic pools).