Availability of magnesium is a matter of concern due to its role in many environmental and biological processes. Diffusive Gradients in Thin Films (DGT) devices can measure Mg availability in situ. This work shows that Mg accumulation in water largely increases when ionic strength (I) decreases. This phenomenon can be explained from (i) the increase of both the association equilibrium (K) and rate (k) constants for the reaction between Mg cations and resin sites, and (ii) the growing contribution of the partitioning of Mg cations at the resin-gel interface, as I decreases. Two theoretical models that take into account electrical interactions among Mg cations, background electrolyte, and resin sites can successfully be used to determine k and K at each I. Both models yield similar k values, which fulfill an expression for the kinetic salt effect. For freshwater (with a typical salinity of 10 mM and circumneutral pH), the binding of Mg is so fast and strong that the simplest perfect-sink DGT expression can be helpful to predict (overestimation lower than 5%) the accumulation in solutions with Mg concentrations up to 1 mM whenever the deployment time is below 9 h. Perfect sink conditions can still be applied for longer times, in systems with either a lower I or a lower Mg concentration.

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DGT (Diffusion Gradients in Thin films) was designed to sample trace metals in situ at their natural concentrations. The setup and the experimental deployment conditions were established to allow interpretation of a linear accumulation of metal with time, using a simple expression based on a steady-state flux under perfect sink conditions. However, the extension of DGT to a wide range of analytes and its use under varied conditions has shown that, in some situations, these conditions are not fulfilled, so that accumulations with time are nonlinear. Previously, when such curvature was observed, concentrations in solution could not be reliably calculated. Here, we present fundamentally derived equations that reproduce the time accumulation for three situations: (i) kinetic limitations in the binding to the resin, (ii) saturation or equilibrium effects, or (iii) non-negligible competitive effects. We show how the accumulations can be quantified, in terms of the required kinetic and thermodynamic constants, and provide practical guidance for their use to obtain reliable estimates of solution concentrations. Solutions containing Mg or Mn, where all three situations can prevail, are used as examples. Calculated concentrations show reasonable agreement with the experimentally known values and with the results of a numerical model of the system, significantly improving the estimations based on perfect sink conditions. Such an approach opens up the possibility of using DGT more widely in challenging systems and allows DGT data to be interpreted more fully.

Binding resin beads used in DGT (diffusion gradients in thin films) tend to settle to one side of the resin during casting. This phenomenon might be relevant for metal accumulation when partially labile complexes dominate the metal speciation, especially after recognizing the important role played by complex dissociation in the resin domain. The influence of the inhomogeneity of the binding agent distribution on metal accumulation is here assessed by numerical simulation of DGT devices with binding beads in only one half of the resin disc, as a reasonable model of the standard resin discs. Results indicate that a decrease in mass accumulation of less than 13% can arise in these inhomogeneous devices (as compared with an ideal disc with homogeneous dispersion of the resin beads) when complexes with stability constant K<10(2)m(3)mol(-1) (K<10(5)Lmol(-1)) dominate the metal speciation. The loss increases as K increases, but the percentage of mass loss always remains lower than the volume fraction of resin disc without beads. For very labile or inert complexes, the impact of the inhomogeneous distribution of binding resin beads is negligible. As kinetic dissociation constants of complexes can be estimated from the distribution of the metal accumulation in a DGT device with a stack of two resin discs, the influence of the inhomogeneity on the recovered kinetic constant is also assessed. For the cases studied, the recovered kinetic dissociation constant, kd,recovered, retains the correct order of magnitude, being related to the true kd by kd≈f(-1)kd,recovered, quite independently of K and kd values, being f the fraction of volume of the resin disc where resin beads are dispersed.

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.