In situ trace metal speciation in a eutrophic lake using the technique of diffusion gradients in thin films (DGT)

Abstract

We evaluated the application of DGT (diffusion gradients in thin films) as a tool to determine Cu, Zn, Ni, Cd, Pb and Mn concentrations and speciation in a hardwater eutrophic lake. This technique was used in situ during six sampling periods over one year in Lake Greifen. The DGT-labile species of Cu and Ni amounted to 15–25% of the total dissolved concentrations. Speciation by ligand-exchange/DPCSV indicated that Cu and Ni were predominantly organically complexed (>99%). Thus, the DGT-labile species for Cu and Ni were much more abundant than the free ionic and inorganic species determined by ligand-exchange/DPCSV. The results can be explained by incomplete metal exchange of very strong complexes with the chelating resin in the DGT devices, metal exchange of less abundant weaker complexes, and by slow diffusion of exchangeable organic complexes. For Zn (36 to >90% DGT-labile) and Mn (50 to 100% DGT-labile), the results indicated that these metals are less strongly organically complexed. A larger fraction of Zn occurred in DGT-labile species in the hypolimnion than it did in the surface water, probably due to a larger concentration of strong ligands in the productive surface water. DGT-labile Cd- (0.01–0.02 nM) and Pb-species (0.03–0.06 nM) were detected at very low levels. The combination of measurements of dissolved and DGT-labile species showed a decrease of Cu, Zn, Cd and Mn concentration at 2.5 m from June to August, which was probably linked to intensive sedimentation of organic matter during summer stagnation. Mixing and oxygenation of the lake in winter-spring led to an increase in dissolved and DGT-labile Ni, Zn and Cd, whereas Mn decreased in the hypolimnion.