Gel Techniques for in situ Measurement in Natural Waters, Soils and Sediments

Abstract

The diffusive equilibrium in thin films (DET) and diffusive gradient in thin films (DGT) gel techniques have been shown as a new approach to in situ measurements of trace metals and their species in aquatic systems. This review briefly demonstrates the ability of the DET technique to provide information about the depth profiles of solutes in sediment pore water with high resolution. The DGT technique, based on Fick´s First diffusion law, is introduced as a robust technique, which is easy to use for in situ measurement of metal species in natural waters. The DGT technique uses a simple device, which, after passage through a hydrogel, accumulates solutes on a sorbent, such as Chelex-100, acting as a well defined diffusion layer. By simply controlling the mass transport to the sorbent, it is possible to quantify the accumulated metals by measurement of flux or concentration. The quality of diffusive layer is the major factor in determination of the measured species. Metal ions are preconcentrated in situ in the sorbent layer; hence, very low detection limits are possible. The deployment times can vary from 1 h to several months in natural waters. The DGT provides a time-averaged solute concentration and could be used for measuring mean concentrations over a time period. The other parameters of natural waters like temperature, pH and ionic strength do not affect the measurement. When used for assessment of element availability in soils and sediments, the DGT results are interpreted as fluxes from the solid phase to solution. Both the described gel techniques are very useful tools for monitoring of pollution as well as in study of metal availability in soils and sediments and trace metal circulation in the environment.