SEGH Articles

Diffusive gradients in thin films (DGT): An emerging technique for bioavailability assessment of chemicals in the environment

08 April 2014
As a rapidly developing passive sampling method for the labile forms of chemicals in waters, sediments and soils, the diffusive gradients in thin films (DGT) technique has significant advantages over conventional methods: in situ measurement, time averaged concentrations and high spatial resolution.

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.

 

 

Keep up to date

Submit Content

Members can keep in touch with their colleagues through short news and events articles of interest to the SEGH community.

Science in the News

Latest on-line papers from the SEGH journal: Environmental Geochemistry and Health

  • Agro-ecological suitability assessment of Chinese Medicinal Yam under future climate change 2019-10-15

    Abstract

    Chinese Medicinal Yam (CMY) has been prescribed as medicinal food for thousand years in China by Traditional Chinese Medicine (TCM) practitioners. Its medical benefits include nourishing the stomach and spleen to improve digestion, replenishing lung and kidney, etc., according to the TCM literature. As living standard rises and public health awareness improves in recent years, the potential medicinal benefits of CMY have attracted increasing attention in China. It has been found that the observed climate change in last several decades, together with the change in economic structure, has driven significant shift in the pattern of the traditional CMY planting areas. To identify suitable planting area for CMY in the near future is critical for ensuring the quality and supply quantity of CMY, guiding the layout of CMY industry, and safeguarding the sustainable development of CMY resources for public health. In this study, we first collect 30-year records of CMY varieties and their corresponding phenology and agro-meteorological observations. We then consolidate these data and use them to enrich and update the eco-physiological parameters of CMY in the agro-ecological zone (AEZ) model. The updated CMY varieties and AEZ model are validated using the historical planting area and production under observed climate conditions. After the successful validation, we use the updated AEZ model to simulate the potential yield of CMY and identify the suitable planting regions under future climate projections in China. This study shows that regions with high ecological similarity to the genuine and core producing areas of CMY mainly distribute in eastern Henan, southeastern Hebei, and western Shandong. The climate suitability of these areas will be improved due to global warming in the next 50 years, and therefore, they will continue to be the most suitable CMY planting regions.

  • Application of stable isotopes and dissolved ions for monitoring landfill leachate contamination 2019-10-15

    Abstract

    We evaluated groundwater contamination by landfill leachate at a municipal landfill and characterized isotopic and hydrogeochemical evidence of the degradation and natural attenuation of buried organic matter at the study site. Dissolved ion content was generally much higher in the leachate than in the surrounding groundwater. The leachate was characterized by highly elevated bicarbonate and ammonium levels and a lack of nitrate and sulfate, indicating generation under anoxic conditions. Leachate δD and δ13CDIC values were much higher than those of the surrounding groundwater; some groundwater samples near the landfill showed a significant contamination by the leachate plume. Hydrochemical characteristics of the groundwater suggest that aquifer geology in the study area plays a key role in controlling the natural attenuation of leachate plumes in this oxygen-limited environment.

  • Lead transfer into the vegetation layer growing naturally in a Pb-contaminated site 2019-10-10

    Abstract

    The lead was one of the main elements in the glazes used to colour ceramic tiles. Due to its presence, ceramic sludge has been a source of environmental pollution since this dangerous waste has been often spread into the soil without any measures of pollution control. These contaminated sites are often located close to industrial sites in the peri-urban areas, thus representing a considerable hazard to the human and ecosystem health. In this study, we investigated the lead transfer into the vegetation layer (Phragmites australis, Salix alba and Sambucus nigra) growing naturally along a Pb-contaminated ditch bank. The analysis showed a different lead accumulation among the species and their plant tissues. Salix trees were not affected by the Pb contamination, possibly because their roots mainly develop below the contaminated deposit. Differently, Sambucus accumulated high concentrations of lead in all plant tissues and fruits, representing a potential source of biomagnification. Phragmites accumulated large amounts of lead in the rhizomes and, considering its homogeneous distribution on the site, was used to map the contamination. Analysing the Pb concentration within plant tissues, we got at the same time information about the spread, the history of the contamination and the relative risks. Finally, we discussed the role of natural recolonizing plants for the soil pollution mitigation and their capacity on decreasing soil erosion and water run-off.