Extending the Use of Diffusive Gradients in Thin Films (DGT) to Solutions Where Competition, Saturation, and Kinetic Effects Are Not Negligible

Abstract

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.

Diffusive Gradients in Thin Films Can Predict the Toxicity of Metal Mixtures to Two Microalgae: Validation for Environmental Monitoring in Antarctic Marine Conditions

“…The Chelex resin does not act as a perfect metal sink at subsaturation concentrations. This is an underreported finding but in agreement with previous studies (Degryse et al ; Jiménez‐Piedrahita et al ). A Chelex‐100 resin has a capacity for divalent cations of 10 to 15 µmol per DGT piston (Bio‐Rad Laboratories ; Tankéré‐Muller et al ).…”

In situ measurements of micronutrient dynamics in open seawater show that complex dissociation rates may limit diatom growth

“…Moreover, of all elements that we determined, it has the lowest affinity for the iminodiacetic groups of the Chelex resin. This makes its measurement by DGT the most susceptible to competition and saturation effects at the salinity of seawater, which could possibly explain the difference between the fast and classic DGT results 60 . In the last decade, soluble Mn has been re-evaluated to include soluble Mn (III)-complexes 61 and recently Oldham et al .…”

Coupling mixture reference models with DGT-perceived metal flux for deciphering the nonadditive effects of rare earth mixtures to wheat in soils

“…3D, H; Table S3). In the DGT device, the binding gel was generally regarded as an infinite sink with no competition at the gel, however, competition effects were indeed observed in some situations (Jimenez-Piedrahita et al, 2017). In our study, a dynamic competition between Y 3+ and Ce 3+ for the binding sites at the surface of the binding layer might exist as they possessed similar selectivity for the Chelex resin (Oyvind Aaberg et al, 2003).…”