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.

A key question for the practical application of DGT (Diffusive Gradients in Thin films) as dynamic sensors in the environmental monitoring of trace metals is the influence of pH and dissolved ligands over the linear accumulation regime. Protons compete with metal ions for the binding to the DGT resin sites at relatively low pH, whereas high affinity dissolved ligands compete with resin sites for the binding of metals. Any of the two phenomena can lead to a departure from the linear accumulation regime and an underestimation of the actual species concentration in solution. These effects are studied here through numerical simulation of the diffusion-reaction processes in both gel and resin domains using a detailed chemical model of metal ions and protons interacting with resin sites. Results were tested successfully against experimental data of the Cd-NTA representative system. Charts to delimitate the range of experimental conditions (pH, ligand concentration and strength) where the linear accumulation regime prevails, can be helpful for designing sampling strategies in field conditions. For example, it is foreseen that perturbations of linear regime within 10 h of deployment are negligible above pH 5 and weak complexation (log K' < 0) or above pH 7 and strong complexation (log K' < 3), where K' is the effective stability constant. These plots can also be approximately used for partially labile systems whenever the time is replaced with the product lability degree times t.

The penetration of complexes into the resin domain of the DGT devices has a large influence on the lability degree of these complexes, since the reaction layer (the layer where there is net dissociation) extends from the diffusive gel into the resin domain. Numerical simulation shows that, typically, the contribution to the metal accumulation from dissociation of complexes inside the resin domain is dominant. As a consequence, in excess of ligand, the influence of the ligand concentration on the lability degree is much reduced, in comparison with this effect in the voltammetric sensors. The presence of a mixture of ligands leads to parallel complexes that mutually influence their lability degrees. In general, the interaction between the complexes has an impact on the lability degree of each one, but the total metal accumulation is less sensitive due to cancellation (mutually opposite effects of a couple of complexes). This result paves the way to predict the metal accumulation from the lability degree available for each complex in a single ligand system. Maximum discrepancies of 10% have been found in these predictions which can still be reduced if thicker resin gels are used.

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Environmental contextThe availability of trace metals to aquatic organisms is influenced by the natural ligands present in water. We investigate the influence of the composition of the system on the availability of metal cations as nutritive or toxic species. The focus is on clarifying whether availability measured in single-ligand systems with diffusive gradients in thin film devices can be used to predict accumulation in mixtures. AbstractNatural waters contain mixtures of ligands, which collectively affect the availability of trace metals. The individual contribution of each complex to the overall metal flux received by a sensor can be described in terms of its lability degree. The question arises as to whether the mixture entails specific non-additive effects, i.e. to what extent is it possible to predict the collective behaviour of the mixture from the values of the lability degree of each single ligand system (SLS). For this reason, a series of experiments with diffusion gradients in thin films (DGT) devices were carried out to measure nickel accumulation from synthetic media comprising either nitrilotriacetic acid (NTA), ethylenediamine (EN) or mixtures of both ligands. The results were compared with numerical simulations. It is shown that NiNTA becomes more inert in the mixture than in the SLS that contains the same concentration of free Ni and NiNTA, whereas the opposite is true for the Ni bound to EN, which becomes more labile in the mixture than in the SLS. This unprecedented behaviour arises when one of the ligands (NTA, forming strong and partially labile complexes) is present under non-excess conditions. As NiNTA and NiEN have an opposite influence on the lability degree of each other, the sum of partial fluxes calculated from the lability degrees obtained in SLSs yields a reasonable estimate of DGT performance in the mixture. Experimental accumulations in the mixture are just slightly below the predicted values, with errors lower than 11 % when NTA concentrations vary from 20 to 100 % of the total Ni concentration.