In situ speciation measurements. Using diffusive gradients in thin films (DGT) to determine inorganically and organically complexed metals

Abstract

The possibilities of using diffusive gradients in thin films (DGT) and anodic stripping voltammetry (ASV) to perform speciation measurements in natural waters are discussed. Both techniques measure labile species, but different approaches have been used to discriminate organic (Corg) and inorganic (Cinorg) metal complexes. In DGT, metals are bound to a resin after passing through a hydrogel that serves as a well-defined diffusion layer. DGT devices with different hydrogels that impede the diffusion of humic substances by different amounts were deployed in solutions of copper and humic substances. Devices with a gel composition that greatly restricted the diffusion of humic substances, but only retarded the diffusion of Cu ions slightly, could be used directly to determine Cinorg. By using different, more open pored gels, which allowed some passage of humic substances, it was possible to determine both Corg and Cinorg. The two separate measurements of Cinorg obtained using the two DGT approaches agreed well. At the high concentrations of Cu used there was good agreement with the predicted distribution from the speciation code WHAM. At the lowest Cu concentration, the proportion of Cinorg estimated using DGT was higher than with WHAM. Possibilities of errors in the DGT or modeling approaches are discussed.