Diffusive Gradients In Thin Films Technique Research Articles

Prediction of wheat response to an application of phosphorus under field conditions using diffusive gradients in thin-films (DGT) and extraction methods

The ability of the Diffusive Gradients in Thin Films (DGT) technique and two other established testing methods (Colwell, resin) to predict wheat responsiveness to applied P from 35 field trials across southern Australia was investigated. Regression analysis of relative early dry matter production and grain yield responses demonstrated that the DGT method predicted plant responsiveness to applied P more accurately than Colwell P and resin P at sites where maximum yields were reached with P rates used (20 out of 35). The measured concentration in soils at the DGT surface, CDGT, explained 74% of the variation in response for both early dry matter and grain, compared to 7% for early dry matter and 35% for grain using the resin P method. No significant relationships could be obtained for Colwell P although modifying the Colwell test data using Phosphorus Buffering Index resulted in a correct response prediction for 11 of the 20 field sites compared to 18 for DGT and 14 for resin P. These observations suggest that the DGT technique can assess plant available P in soils with significantly greater accuracy than traditional soil P testing methods. The critical P threshold, expressed as CDGT, was 255 μg L−1 for early dry matter and 66 μg L−1 for grain.

Metals pollution tracing in the sewerage network using the diffusive gradients in thin films technique

Diffusive Gradients in Thin-films (DGT) is a quantitative, passive monitoring technique that can be used to measure concentrations of trace species in situ in solutions. Its potential for tracing metals pollution in the sewer system has been investigated by placing the DGT devices into sewage pumping stations and into manholes, to measure the concentration of certain metals in the catchment of a sewage treatment works with a known metals problem. In addition the methodology and procedure of using the DGT technique in sewers was investigated. Parameters such as temperature and pH were measured to ensure they were within the limits required by the DGT devices, and the optimum deployment time was examined. It was found that although the results given by the DGT technique could not be considered to be fully quantitative, they could be used to identify locations that were showing an excess concentration of metals, and hence trace pollution back to its source. The DGT technique is 'user friendly' and requires no complicated equipment for deployment or collection, and minimal training for use. It is thought that this is the first time that the DGT technique has been used in situ in sewers for metals pollution tracing.

Sodium polyacrylate as a binding agent in diffusive gradients in thin-films technique for the measurement of Cu 2+ and Cd 2+ in waters

An aqueous solution containing sodium polyacrylate (PA, 0.0030 M) was used in diffusive gradients in thin-films technique (DGT) to measure DGT-labile Cu 2+ and Cd 2+ concentrations. The DGT devices (PA DGT) were validated in four types of solutions, including synthetic river waters containing metal ions with or without complexing EDTA, natural river water (Hun River, Shenyang, China) spiked with Cu 2+ and Cd 2+, and an industrial wastewater (Shenyang, China). Results showed that only free metal ions were measured by PA DGT, recovery = 98.79% for Cu 2+ and recovery = 97.80% for Cd 2+ in solutions containing only free metal ions, recovery = 51.02% for Cu 2+ and recovery = 51.92% for Cd 2+ in solution with metal/EDTA molar ratio of 2:1 and recovery = 0 in solutions with metal/EDTA molar ratio of 1:1 and 1:2. These indicated that the complexes of Cu–EDTA and Cd–EDTA were DGT-inert or not DGT-labile. The DGT performance in spiked river water (recovery = 8.47% for Cu 2+ and recovery = 27.48% for Cd 2+) and in industrial wastewater (recovery = 14.16% for Cd 2+) were also investigated. Conditional stability constants (log K) of PA–Cu and PA–Cd complexes were determined as 6.98 and 5.61, respectively, indicating strong interaction between PA and the metals.

Predicting availability of mineral elements to plants with the DGT technique: a review of experimental data and interpretation by modelling

Environmental context. Total concentrations of mineral elements in soil bear little relation to their availability for plants. The DGT (diffusive gradients in thin-films) technique has been found to be a good predictor of trace metal uptake and P deficiency, though not consistently in all studies for all elements. This review examines the fundamental basis for the relation between DGT fluxes and plant uptake and assesses under which conditions this relation may break down. Abstract. In the DGT technique, elements are accumulated on a binding gel after their diffusive transport through a hydrogel. In this paper, we explore in more detail why – and under which conditions – DGT correlates with plant uptake. The theoretical considerations are illustrated with experimental results for metal uptake and toxicity, and for phosphorus deficiency. Strong correlations between DGT and plant uptake are predicted if the diffusive transport of the element from soil to the plant roots is rate-limiting for its uptake. If uptake is not limited by diffusive transport, DGT-fluxes and plant uptake may still correlate provided that plant uptake is not saturated. However, competitive cations may affect the plant uptake under these conditions, whereas they have no effect on the DGT flux. Moreover, labile complexes are not expected to contribute to the plant uptake if diffusion is not limiting, but they are measured with DGT. Therefore, if plant uptake is not limited by diffusion, interpretation of the observed correlation in terms of the labile species measured by DGT is inappropriate.

In situ measurement of trace metals in estuarine waters of Patos Lagoon using diffusive gradients in thin films (DGT)

The diffusive gradients in thin-films (DGT) technique was used to determine labile metals in situ during different hydrological conditions in the Patos Lagoon estuary. Water samples were also analyzed for Chelex-labile fractions of Cd, Co, Cu, Mn, Ni and Zn using Chelex-100 resin. DGT-labile concentrations of Cd, Cu, Ni and Zn were lower than Chelex-labile concentrations, possibly because colloids were trapped in the Chelex resin column. There was a decrease with increasing deployment time in the concentrations of all elements measured by DGT, reflecting changes in metal speciation in solution caused by the decrease in the salinity and associated with change in pH in the estuary. The results demonstrated the sensitivity of DGT to metal speciation and the feasibility of using the DGT technique in highly dynamic estuarine waters to obtain a time-integrated record of labile trace metals.

Using diffusive gradients in thin-films for in situ monitoring of dissolved phosphate emissions from freshwater aquaculture

The high concentrations of phosphorus and suspended matter in freshwater aquaculture are often an environmental concern and given the difficulties for sample preservation, an in situ monitoring tool would be most useful. The diffusive gradients in thin-films (DGT) technique can be used for on site monitoring and also has the advantage to integrate measurements over time and thus partly record ephemeral concentrations peaks. In this study, several DGT deployments were done in fisheries in different geographical areas: Québec, Canada and Lancashire, UK. The results obtained using the DGT technique were compared with standard measurements. The deployment periods varied from 24 h to 21d. The results show that the DGT technique can be used to measure reactive phosphorus in freshwater aquaculture up to 4-day deployment periods before biofilm buildup on the DGT device begins to interfere with diffusion and measurements. We obtained good reproducibility in the phosphorus determinations and moreover, this technique integrates daily and hourly variations of phosphorus and the DGT sampler is easy to use and deploy.

Inputs of total and labile trace metals from wastewater treatment plants effluents to the Seine River

The Seine river basin has long been impacted by metal inputs from the Paris area, but the water quality has been gradually improving for the last 20 years. Among all metal pollution sources (surface runoff, industries), urban wastewater discharge has been shown to significantly contribute, during low-flow periods, to metal fluxes of the River Seine. This paper assesses the current wastewater contribution to metal inputs in the Seine river basin, based on sampling of nine wastewater treatment plants (WWTPs). Seven metals were targeted (Al, Cd, Cr, Cu, Fe, Ni and Pb) during dry weather periods. Since total and dissolved concentrations alone are not relevant enough for an ecological risk assessment, labile metals (free + weekly complexed) were also measured by means of DGT (diffusive gradient in thin film technique). Results show that WWTPs greatly reduce total metal concentrations but reduce labile metal concentrations only slightly. Estimations made for direct total metal inputs in the River Seine via treated effluent discharge confirm the decrease observed for the 1994–1995 period. Labile metals released by WWTP were also considered by comparing fluxes in the effluent discharge of two different WWTPs to those flowing in the receiving river. Fluxes discharged by the largest plant were similar to those measured in the river during low-flow periods whereas they were negligible for the smaller one. Nevertheless, labile metal concentrations in both discharges were similar and the wastewater discharge’s contribution to labile fluxes in receiving waters seems to depend mostly on the relative significance of the discharge flow compared to the receiving water flow.

Trace-Metal Biogeochemistry in the Mediterranean Thau Lagoon, a Shellfish Farming Area

The present study contributes to the knowledge of the biogeochemistry of Pb, Cd, Cu, and Ni in the Mediterranean Thau Lagoon, southern France, which is an important shellfish farming system. The concentrations of the metals were determined in sediment cores and the overlying waters using inductively coupled plasma mass spectrometry. Particular attention was given to the determination of dissolved Cu species because of their dual role as essential nutrient and toxicant to planktonic organisms. Dissolved Cu speciation was determined using the diffusive gradient in thin-film technique (DGT) and competitive ligand exchange–adsorptive cathodic stripping voltammetry (CLE-ACSV). Our data indicated a significant historical contamination of the sediments, which commenced in the second half of the 19th century, with trace metal inputs persisting until the end of the 20th century. In recent years a decrease in metal contamination has become apparent. The maxima observed for Pb, Cd, and Cu profiles probably indicate the occurence of anoxia crises. A strong complexation of the dissolved Cu species was observed in the waters of the Thau Lagoon, which reduced the bioavailability of Cu. The dissolved Cu2 concentrations were probably too low to cause direct toxic effects on shellfish, but the highest concentration (5.29 pM) observed in this study can potentially influence phytoplankton communities. A comparison between the Cu speciation data indicates that up to 50% of the complexed Cu determined using CLE-ACSV was DGT labile.

Accuracy of the Diffusive Gradients in Thin-Films Technique: Diffusive Boundary Layer and Effective Sampling Area Considerations

When using the diffusive gradients in thin-films (DGT) technique in well-stirred solutions, the diffusive boundary layer has generally been ignored on the assumption that it is negligibly thin compared to the total thickness of delta g, i.e., the sum of the thickness of the prefilter and diffusive gel. Deployment of devices with different diffusive layer thicknesses showed that the thickness of the DBL was approximately 0.23 mm in moderate to well-stirred solutions, but substantially thicker in poorly or unstirred solutions. Measurement of the distribution of Cd in the DGT resin gel at high spatial resolution (100 microm) using laser ablation inductively coupled plasma mass spectrometry showed that the effective sampling window had a larger diameter (2.20 cm) than the geometric diameter of the exposure window (2.00 cm). Lateral diffusion in the gel, which had previously been neglected, therefore increased the effective surface area of the device by approximately 20%. The concentrations measured by DGT agreed well with the known concentrations in standard solutions for all diffusion layer thicknesses, when the effective area and the appropriate diffusive boundary layer (DBL) were used. The extent of the error associated with neglecting the DBL and using the geometric window area depends on the gel layer thickness and the true thickness of the DBL, as determined by the deployment geometry and flow regime. When DGT measurements were made in well-stirred solutions using a 0.80-mm diffusive gel, the effect of neglecting the DBL and using the inappropriate geometric area offset each other, with the error being <+/-10%. For precise measurements, and especially work involving speciation or kinetic measurements, where DGT devices with different diffusive gel layer thicknesses are deployed, it is necessary to use the effective area and the appropriate DBL thickness in the full DGT equation, which allows for the use of layer-specific diffusion coefficients.

Analysis of Micro-Nutrient Behaviour in the Rhizosphere using a DGT Parameterised Dynamic Plant Uptake Model

Mathematical models of micro-nutrient uptake by plants have been based on models designed for uptake of major nutrients. This poses a number of problems, as the behaviour of micro-nutrients in the rhizosphere is different, particularly with respect to interactions between solid and solution compartments. There is no standard procedure for the measurement of soil parameters that affect the supply of micro-nutrient from the soil. The Dynamic Plant Uptake Model (DPUM) presented here can use soil parameters for micro-nutrients that are measured under similar conditions to those that apply during plant uptake, as, for example, by the technique of DGT (Diffusive Gradients in Thin-films). The construction and practical applications of the model are discussed in the context of mathematical simulations of realistic situations. The simulations have been used to investigate conditions under which uptake of zinc by a zinc hyperaccumulator (Thlaspi caerulescens), and a non-accumulator plant (Thlaspi arvense), is limited by either processes occurring in the plant or in the soil. The size of the solid phase reservoir of zinc, and the rate at which the solid phase reservoir responds to zinc depletion in the solution phase, are important in determining the soil solution concentration at which the plant uptake kinetics begin to limit the uptake of the metal. The relative importance of advective transport of zinc from the surrounding soil to the root–soil interface depends on both the size of the solid phase reservoir and the speed at which the soil responds to zinc depletion in the pore water. These results have been used to assess conditions where DGT is likely to behave as a reliable surrogate for the transport and metal release processes that affect plant uptake.

Bioavailable cadmium during the bioremediation of phenanthrene-contaminated soils using the diffusive gradients in thin-film technique

To study the impact of fungal bioremediation of phenanthrene on trace cadmium solid-solution fluxes and solution phase concentration. The bioremediation of phenanthrene in soils was performed using the fungus Penicillium frequentans. Metal behaviour was evaluated by the techniques of diffusive gradient in thin-films (DGT) and filtration. Fluxes of cadmium (Cd) show a significant (P < 0.002) increase after the start of bioremediation, indicating that the bioremediation process itself releases significant amount of Cd into solution from the soil solid-phase. Unlike DGT devices, the solution concentration from filtration shows a clear bimodal distribution. We postulate that the initial action of the fungi is most likely to breakdown the surface of the solid phase to smaller, 'solution-phase' material (<0.45 microm) leading to a peak in Cd concentration in solution. Phenanthrene removal from soils by bioremediation ironically results in the mobilization of another toxic pollutant (Cd). Bioremediation of organic pollutants in contaminated soil will likely lead to large increases in the mobilization of toxic metals, increasing metal bio-uptake and incorporation into the wider food chain. Bioremediation strategies need to account for this behaviour and further research is required both to understand the generality of this behaviour and the operative mechanisms.

Trace metal speciation measurements in waters by the liquid binding phase DGT device

The speciation measurements of trace metals by the diffusive gradients in thin-films technique (DGT) using a poly(4-styrenesulfonate) (PSS) aqueous solution as a binding phase and a cellulose dialysis membrane (CDM) as a diffusive layer, CDM-PSS DGT, were investigated and showed good agreement with computer modelling calculations. The diffusion coefficients of ethylenediaminetetraacetic acid (EDTA) complexes with Cd 2+ and Cu 2+ were measured and compared with those of the inorganic metal ions. CDM-PSS DGT device was tested for speciation measurement in sample solutions containing EDTA, tannic acid (TA), glucose (GL), dodecylbenzenesulfonic acid (DBS) and humic acid (HA) as complexing ligands forming organic complexes with varying stability constants. Lower percentages of DGT labile copper concentrations over total filterable copper concentrations obtained from the deployments in freshwater sites indicated that copper complexes with organic matter were basically not measured by the devices.

Development and Performance of the Diffusive Gradients in Thin-Films Technique for the Measurement of Technetium-99 in Seawater

A novel technique for obtaining time-integrated 99Tc concentrations in seawater has been developed, using diffusive gradients in thin films (DGT). The performance of TEVA resin as a binding agent for 99Tc was investigated via laboratory experiments. The accumulated 99Tc activity per unit area of resin-gel was proportional to both the bulk solution activity and the exposure time for deployments of up to 2 weeks. The response of DGT was found to be independent of solution chemistries over the pH range 3-8 and ionic strength range 0.01-1.3 M. Seawater has pH 8 and ionic strength of approximately 0.7 M; therefore, the potential of the technique for field deployment in seawater was demonstrated. Detection limits of 0.05 and 0.025 Bq L(-1), for 2- and 4-week DGT deployments, respectively, were calculated for 99Tc measurement by liquid scintillation spectrometry. Using quadrupole ICPMS to measure bound 99Tc could reduce these detection limits to 0.125 mBq L(-1) for a 4-week deployment. These detection limits are sufficiently low for monitoring contaminated environments, including the Irish Sea. This method is simpler and faster than other 99Tc analysis methods and represents the only means of obtaining time-integrated data.

The fine-scale remobilization of metals in the surface sediment of the North-East Atlantic

Vertical profiles of trace metals were measured at 1-mm intervals by deploying DGT probes in sediment cores collected from the deep Atlantic Ocean (Feni Drift) before and after the spring phytoplankton bloom. DGT (diffusive gradients in thin-films) accumulates metals on a chelating resin after their diffusive transport through a layer of hydrogel. The mean concentration of metal in the porewaters at the surface of the device during its deployment was derived from the measured mass of metal. Well-defined laboratory systems were used to establish the reproducibility and accuracy of the DGT technique for measuring metals at 1-mm intervals. The Fe and Mn profiles showed the expected redox sequence with depth, with additional fine-scale (2–5mm) features. The close correspondence between the Mn and Co profiles, on both coarse and fine scales, showed that their chemistry is closely coupled in sediments as well as water columns. Changes in the background concentrations of Cd, Cu, Ni and Zn with depth were well correlated with Mn and Co, but the four metals also displayed mm-scale maxima that were largely independent of Mn, Co or Fe. Two-dimensional measurements at a 100-μm resolution on a DGT probe, deployed in situ with an autonomous lander, showed that the maxima were most likely associated with near-spherical, highly localised sources. The results were consistent with release of Cd, Cu, Ni and Zn from organic and planktonic skeletal material. Where there was associated release of Mn and/or Fe, through their oxides acting as electron acceptors in the decomposition of the organic matter, trace metals could also be supplied by release from the oxides. The microniches responsible for high local concentrations of metals in the porewaters were hypothesised to be due to downward mixing of material by burrowing organisms, known to be prevalent in these sediments. Much of this material appeared to be present as faecal pellets of various origins, with sizes in the range of 50μm to 1cm.

DGT as an in Situ Tool for Measuring Radiocesium in Natural Waters

The application of diffusive gradients in thin-films (DGT) samplers for the measurement of cesium radionuclides in solution, using an ammonium molybdophosphate (AMP) binding agent, was tested under both laboratory and field conditions. In the former they proved able to reproduce known 134Cs concentrations (60 Bq L(-1)) with a high degree of accuracy and precision over periods up to approximately 1 d, in freshwaters over a wide range of pH and temperature, and in saline water. In field trials in a freshwater lake receiving nuclear power station discharges, mean concentrations of 137CS (47-61 mBq L(-1)) were measured over periods from 5 d to 1 month. These agreed, within error, with mean concentrations determined from grab samples but rigorous field validation of long-term (month) deployments of DGT devices proved impossible using conventional sampling procedures, due to loss of 137Cs to container walls. Identified limitations of the DGT technique included probable AMP degradation over longer periods and calibration problems if large changes in temperature and concentration occurred together. Potential limitations due to biofilm growth were considered not to be significant. Despite the limitations, the technique appears to measure concentrations accurately for deployment times of 1 month or less. It has several advantages over traditional sampling methods for monitoring radionuclides in the solution/dissolved phase, including its simplicity, provision of time-averaged mean concentrations, and automatic in-situ concentration onto a medium with ideal counting geometry for gamma spectrometry.

Diffusional characteristics of hydrogels used in DGT and DET techniques

Hydrogels are increasingly being used to control mass transport in the analytical determination of trace elements. Quantitative use of the techniques of diffusive gradients in thin-films (DGT) and voltammetry at gel covered microelectrodes relies on precise knowledge of the diffusion coefficients in the gels. Two simple procedures, one relying on establishing pseudo steady state (Fick’s first law) and the other on a dynamic response (Fick’s second law), have been used to measure the diffusion coefficients of trace metals and fulvic and humic substances in five different hydrogels. To obtain consistent results it was essential to treat gels with an electrolyte prior to measurement. The different procedures agreed to within 5% with each other and with measurements made by DGT. For polyacrylamide gels cross-linked with an agarose derivative (APA) and with Bis acrylamide at 5% monomer concentration (BPA2) and for pure agarose gel (AGE), there was no significant difference between the diffusion coefficients of trace metals measured in the gels and those reported for water. Bis cross-linked gels containing 15% monomer (BPA1) and the constrained gel (CGa) impeded the diffusion of metal ions compared to water. There was little evidence for the AGE gel impeding the diffusion of fulvic substances, but it appeared to increasingly restrict the diffusion of humic substances as molecular weight increased. There was clear restriction of fulvic and humic substances by the APA gel and much greater restriction by the constrained gel (CGa). The results are consistent with the known open structures of AGE and APA gels and the dependence of the structure of BPA gels on monomer concentration. The much greater retardation of humic and fulvic substances in the constrained gels compared to trace metals opens up the possibility of using gel techniques to discriminate organically and inorganically complexed metals in natural waters.