The diffusive gradients in thin films (DGT) technology provides a novel approach for the in situ measurement of the labile forms of chemical elements, such as phosphorus (P), sulphur (S), arsenic (As) and metals in waters, sediments and soils. It was invented in Lancaster in 1993. The simple device uses a hydrogel binding layer impregnated with Chelex resin or other binding agents to accumulate ions. The binding layer is overlain by a diffusive layer of hydrogel and a filter. Ions have to diffuse through the filter and diffusive layer to reach the binding layer. It is the establishment of a constant concentration gradient in the diffusive layer that forms the basis for measuring chemical element concentrations in solutions quantitatively. The effect of temperature can be predicted from the known temperature dependence of the diffusion coefficient.
Compared with conventional methods, DGT has significant advantages:
- In situ measurement
- Time averaged concentrations
- High spatial resolution
The in situ measurement avoids the artificial influences including contamination of sample collection and treatment which may change the forms of chemicals. The time averaged concentration reflects representative measurement over a period of time. The high-resolution information captures biogeochemical heterogeneity of interested elements distributed in microenvironments, such as in rhizosphere and the vicinity of the sediment-water interface. Moreover, DGT is a dynamic technique by simultaneously considering the diffusive of solutes and their kinetic resupply from the solid phases. All the advantages of DGT significantly promote the collection of “true” information of the bioavailable or labile forms of chemicals in the environment, with potential applications in agriculture, environmental monitoring and mining industry.
The fundamental theory behind DGT is Fick’s first law of diffusion. For deployment, the unit is emerged in water or inserted into sediments or in close contact with wet soils. The labile forms of chemical elements diffuse through the filter and diffusive gel, adsorbed on the binding gel, and then quantified.
The analytes that can be measured by DGT are determined by the binding agent in use. The binding agent for the first DGT was Chelex resin for the measurements of metal ions. After that, the ferrihydrite gel was used to measure phosphorus, and silver iodide was included in the gel to take up sulphide. Recently the Zr-oxide gel was developed to measure phosphorus and inorganic arsenic with high capacities. The agents are also combined to enable simultaneous measurements of multiple analytes. For example, the hydrous zirconium oxide (Zr-oxide) has been combined with silver iodide to measure both phosphorus and sulphide, and combined with Chelex to measure phosphorus and iron.
Another significant development in DGT is the 2D high resolution measurement, which provides new evidences for the micro-scale geochemical heterogeneity. The scales have generally reached sub-millimetre level using various technologies, including proton induced X-ray emissions (PIXE), computer-imaging densitometry (CID), laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), and 2D slicing.
The field applications are still at the early testing stage. Further studies are needed to properly interpret the DGT measured results under complex environmental conditions, and standard procedures and guideline values based on DGT are required to pave the way for its routine applications in environmental monitoring.
Contact: Dr. Chaosheng Zhang, School of Geography and Archaeology, National University of Ireland, Galway, Ireland; Prof. Shiming Ding, Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences, Nanjing, China.