The use of ultra-thin diffusive gradients in thin-films (DGT) devices for the analysis of trace metal dynamics in soils and sediments: a measurement and modelling approach

Abstract

Environmental contextThe recently developed diffusive gradients in thin-films (DGT)-planar optode sandwich probe uses extremely thin resin binding layers, separated from the medium of interest by a very thin material diffusive layer. This work investigates how these changes to the physical nature of a DGT probe are likely to change the interpretation of trace metal measurements in solutions, soils and sediments by using a combination of experimental measurements in well characterised solutions and spiked soils, and advanced reactive transport modelling. AbstractThe interpretation of diffusive gradients in thin-films (DGT) measurements of trace metals in aquatic systems has developed from studies using DGT devices with standard dimensions, but increasingly ultra thin devices are being used for measurements in sediments. This work investigates their performance and the suitability of using traditional data interpretation. The relationship between the concentration of DGT-labile trace metal and the mass of Cu and Cd bound by a 50 µm-thick suspended particulate reagent–imidodiacetate resin binding layer was found to be linear when the total mass of the metals bound by the resin was less than 3 µg cm–2, demonstrating that the capacity is adequate for measurements in uncontaminated environments. An ultra thin DGT probe using a 50 µm-thick resin gel and a 0.01 mm-thick material diffusion layer (MDL), was deployed in soil to demonstrate the spatial resolution in trace metal measurements that can be achieved using this approach. DGT probes with extremely thin (0.01 mm) and more conventional MDLs (0.8 mm) were used to investigate if the mechanisms traditionally used to describe DGT uptake of Cu and Cd from solutions and soil porewaters apply for ultra-thin probes. Interpretation of the results using a dynamic numerical model demonstrated that the processes determining the supply of Cu and Cd to a DGT probe in these media are unlikely to differ for conventional and ultra thin probes, despite the higher metal fluxes to the latter probes. Overall, the results establish that measurements made using ultra-thin DGT devices in soils and sediments can be meaningfully interpreted using an extension of the existing theory.