Investigation of the dissolution kinetics of lanthanide complexes with the technique of diffusive gradients in thin films (DGT)

Understanding the physico-chemical fate and bioavailability of metals requires knowledge of the forms that the metals occur in (the speciation). Diffusive gradients in thin films (DGT) is a technique for passive sampling of metals and it has in situ spec iation capabilities in that it collects only free metal ions and labile metal complexes. The fraction of metal complexes that will be collected depends on their rates of diffusion and their kinetic characteristics. This means that a precise understanding of what DGT measures requires an appreciation of the dissociation kinetics of complexes in solution. For any one ligand , the rate of dissociation of complexes of metals in the lanthanide series is expected to show a regular decrease across the series of 15 metals. This property of the lanthanide complexes can be exploited in a study of the capability of DGT to discriminate between complexes with different kinetic characteristics, provided that suitable ligands can be found. The criteria is that the ligan ds form so strong bonds with the lanthanides that the rate of dissociation will affect the uptake of lanthanides in the DGT sampler. Influence of pH and ionic strength will be tested and experiments with synthetic and natural organic ligands will be perf ormed. Such information is necessary to assess the applicability of DGT to field conditions, as its in situ deployment could be used to obtain conditional kinetic information.