Progress in understanding the use of diffusive gradients in thin films (DGT) – back to basics

Abstract

Environmental context. It is now nearly 20 years since the introduction of the technique of diffusive gradients in thin films, which can provide information on solute concentrations and dynamics in sediments, soils and water. The interpretation of these measurements in terms of concentrations relies on simple equations and associated assumptions. This review examines how well they have stood the test of time.Abstract. Diffusive gradients in thin films (DGT) is now widely used to measure a range of determinands in waters, soils and sediments. In most cases the mass accumulated is interpreted in terms of a labile form of the component being measured using a simple equation that applies to steady-state conditions. During the past decade several publications have revealed phenomena that question some of the assumptions necessary for use of the simple equation. This review systematically examines the available evidence relating to appropriate geometry, possible charge effects, binding of solutes and ligands to the diffusive gel and filter, the rate of reaction with the binding layer, the effects of solution complexation and kinetic limitation, necessary time for deployment and the measurement of nanoparticles. DGT emerges as a robust monitoring tool for labile components in solution. Although there is evidence, for some conditions, of binding of metals and, more moderately, humic substances to the diffusive gel and filter membrane, this is unlikely to affect DGT measurement in natural waters for deployment times exceeding a few days. Detailed speciation and kinetic studies require a more thorough interpretation of the mass accumulated by DGT. A coherent theory has emerged for relatively simple solutions, but systems with complex heterogeneous ligands, as is the case for natural waters, are challenging. The size discrimination of DGT is still poorly known. Systematic measurements with well characterised nanoparticles are required to define the distribution of pore sizes in the gels and to establish the contribution of natural colloids to the DGT measurement.

Evaluation of Discrete and Passive Sampling (Diffusive Gradients in Thin-films – DGT) Approach for the Assessment of Trace Metal Dynamics in Marine Waters – a Case Study in a Small Harbor

“…[7,24,43] Accordingly, the variability of DGT-labile concentrations over the examined period (5 months) is expected to be lower compared to dissolved trace metal concentrations, as observed in this study ( Figure 2, blue lines). Relatively low inter-variability (good repeatability) between the two DGT devices (side by side) were obtained in most cases (small error bars plotted in red).…”

“…[7,45,55,56] Thus, the thermodynamic consideration of the speciation calculations does not necessarily reflect the real processes at DGT/water interface which are partially or fully under kinetical control. [55] Accordingly, the DGT technique is considered "operational" because it depends on the actual physico-chemical conditions.…”

“…Due to the complexity of the aquatic environment, the speciation of trace metals is a very challenging, and different approaches have been applied. [5][6][7][8] The most common approach providing sufficient information to understand general behavior of trace metals distribution is based on discrete sampling and the measurement of dissolved and total metal concentrations. [2,4,9] However, for a greater understanding of the behavior and the interactions of trace metals with other constituents (including biota), specific techniques were developed and widely applied, providing valuable information on the trace metal speciation.…”

“…It has a satisfactory detection limit, and allows measurement of free ions and labile organic/inorganic complexes in water, soil or sediment. [7,8,10,11,21,23] Since DGTs can be deployed in water over a defined period of time (from a few days to approximately two months), the resulting concentration represents the time weighted average (TWA) concentration of reactive species. [7,24] Concentrations determined by this technique are known as DGT-labile and represent potentially bioavailable metal forms.…”

“…[7,8,10,11,21,23] Since DGTs can be deployed in water over a defined period of time (from a few days to approximately two months), the resulting concentration represents the time weighted average (TWA) concentration of reactive species. [7,24] Concentrations determined by this technique are known as DGT-labile and represent potentially bioavailable metal forms. [25] Currently, the only methodology which respects metal speciation for the assessment of the water quality in fresh waters is Biotic Ligand Model (BLM) [26] introduced by the US Environmental Protection Agency (US EPA) for the estimation of water quality criteria on Cu.…”