Analysis of the dynamic features of diffusion gradients in thin film devices (DGT) indicates that the penetration of complexes into the resin layer dramatically increases their lability. This should be taken into account when interpreting DGT measurements in terms of the dynamics of solution speciation. The experimental accumulation of Cd by DGT sensors in Cd-NTA systems confirmed these theoretical analyses. A computational code, which allows a rigorous digital simulation of the diffusion-reaction processes in the gel and resin layers, was used to model the results and to demonstrate the effect of the complex penetration into the resin layer on the lability degree. These findings suggest that DGT renders all complexes much more labile than if the resin-diffusive gel interface was considered as a perfect planar sink, explaining why DGT often measures a high proportion of the metal in a natural water. This information is relevant since some studies have stressed the importance of labile complexes as a source of bioaccumulated metal.

Penetration of complexes into the resin layer can dramatically increase the contribution of complexes to the metal flux measured with a DGT (diffusive gradients in thin films) sensor, but equations to describe this phenomenon were not available. Here, simple approximate analytical expressions for the metal flux, the lability degree and the concentration profiles in a DGT experiment are reported. Together with the thickness of the reaction layer in the gel domain, the effective penetration distance into the resin layer that would be necessary for full dissociation of the complex (λ(ML)) plays a key role in determining the metal flux. An increase in the resin-layer thickness (r) effectively increases the metal flux and the lability degree until r ≈ 3λ(ML). For the usual DGT configuration, where the thickness of the gel layer exceeds that of the resin layer, the complex is labile if r > (D(ML)/k(d))½, where D(ML) is the diffusion coefficient of the metal complex and k(d) its dissociation rate constant. A general procedure for estimating the lability of any complex in a standard DGT configuration is provided.

As the measurement of metals by DGT (diffusion gradients in thin films) in low salinity media has been controversial, a thorough study of the impact of ionic strength (I) is timely. DGT accumulations of Cd, Co, and Ni in the presence of NTA at pH 7.5 with I in the range from 10(-4) to 0.5 M were obtained. An observed decrease in the metal accumulation as the ionic strength of the system decreased is partially explained by the electrostatic repulsion between the negatively charged resin domain and the dominant negatively charged complex species M-NTA. This electrostatic effect reduces the complex penetration into the resin domain, especially for nonlabile complexes, which do not fully dissociate in the gel domain. Analytical expressions, based on the Donnan model, were able to quantify these electrostatic effects. Additionally, the data indicate that the kinetic dissociation constant of M-NTA complexes in the resin layer is higher than Eigen predictions, suggesting a ligand-assisted dissociation mechanism. As the ionic strength decreases, the rate of reaction in the resin layer decreases due to the repulsion between the negatively charged resin sites and the complex species. This decrease contributes to the decrease in metal accumulation. These novel, previously unconsidered, effects of ionic strength and the ligand-assisted dissociation mechanism in the resin domain will affect DGT measurements made in freshwaters and soils.

The penetration of metal complexes into the resin layer of DGT (diffusive gradients in thin films) devices greatly influences the measured metal accumulation, unless the complexes are either totally inert or perfectly labile. Lability criteria to predict the contribution of complexes in DGT measurements are reported. The key role of the resin thickness is highlighted. For complexes that are partially labile to the DGT measurement, their dissociation inside the resin domain is the main source of metal accumulation. This phenomenon explains the practical independence of the lability degree of a complex in a DGT device with respect to the ligand concentration. Transient DGT regimes, reflecting the times required to replenish the gel and resin domains up to the steady-state profile of the complex, are also examined. Low lability complexes (lability degree between 0.1 and 0.2) exhibit the longest transient regimes and therefore require longer deployment times to ensure accurate DGT measurements.