An in situ dynamic technique for better understanding of chemical speciation, bioavailability and biogeochemical processes in aquatic systems

Séminaire de Hao Zhang (Lancaster University)

Abstract:

This presentation examines the characteristics and strengths of an in situ dynamic speciation technique, namely DGT (diffusive gradients in thin films). DGT measures free metal and inorganic and/or organic complexes which can readily dissociate (labile). This measurement of labile species is important in assessing the potential bioavailability and toxicity of metals in natural waters. DGT labile concentrations have been used as parameters for trace metal accumulation in variety of biotas, both in microcosm and field experiments. Good correlations between DGT labile concentrations and the amount of metals accumulated in biota suggest that DGT may have a general capability for assessing bioavailability. 

The in situ DGT deployments in lakes and rivers have advanced our understanding of the interaction, the dynamic processes and the kinetic signature of metals and organics, including dissolved organic carbon and biotic ligands.

DGT can be used simply to measure chemical concentrations from in situ deployments, without the need to calibrate on site or to include internal reference compounds. Moreover, it can be used to measure a much wider range of analytesthan other techniques because its construction allows for easy fabrication of many different binding agents. Metal speciation with different oxidation states can be measured using DGT with different specific binding layers. DGT has also been established for measuring nutrients, oxyanion metals, radionuclides and a wide range of organic pollutants such as pesticides and antibiotics. 

When DGT devices are deployed directly in sediments, the devices perturb the environments by providing a sink for the solute of interest. By simply measuring the amount of solute, such as metals or nutrients, which is accumulated by DGT it is therefore possible to gain information about the concentration of solute in solution, its potential flux from the solid phase and the extent of the solid phase reservoir that can readily supply the solute. Thu,s this simple measurement provides valuable information on dynamic sediment processes. A numerical model has been developed for obtaining kinetic parameters of solution and solid-phase interactions based on DGT measurements. Multiple spatial high-resolution measurements can be obtained from a single sediment DGT probe to understand the behaviors of trace metals at microinche scale and the heterogeneity nature of the sediment.

Undoubtedly, novel and useful information will be obtained by DGT technique. The challenge is to fully harness the new information and new knowledge to advance our holistic understanding of biogeochemical processes.