Title: An Assessment of the Feasibility of Using DGT Procedures to Measure Trace Metals and Radionuclides in Rivers
Author: W Davison
Author: W Hutchinson
Author: Environment Agency
Document Type: Monograph
Annotation: Environment Agency Project ID:EAPRJOUT_655, Representation ID: 220, Object ID: 1987
Background Sampling and analysis of waters for trace metals and radionuclides is a difficult, costly procedure requiring skilled personel. Monitoring is restricted to a series of point samples whereas ideally a continuous or integrated record is required. The availability of trace metals and radionuclides to biota depends on their chemical speciation and this may change during the sampling and handling steps. There is a requirement for an in situ procedure which can provide a quantitative integrated record of well-defined solution species of trace metals and radionuclides. The development of quality standards and risk assessments protocols for the impacts of metals in contaminated waters is hampered by the absence of a simple chemical diagnostic for potential metal toxicity. Measurements of total metals are inappropriate because toxicity depends on the availability of the metal to the biota, and hence the amount of free or weakly bound metal. There is a requirement for a device capable of acting as a surrogate for bioavailable metal. The new procedure of diffusive gradients in thin-films (DGT) has recently emerged. It collects solutes on an immobilized binding agent after diffusion through a thin-film of gel. Because the geometry is well defined the measured mass of metal accumulated after a known deployment time can be used to calculate the time averaged mean concentrations of solutes in the exposure solution. Concentrations can be calculated theoretically, overcoming the difficult problems of in situ calibration. DGT responds to labile metal. Like biota it continuously removes the metal from solution, so it may be a good chemical surrogate for bioavailability. Objectives Prior to this contract, DGT had only been partially tested for zinc and cadmium and its application was restricted to seawater The aims of the research were to develop DGT into a practical device and to establish its feasibility for measuring several metals in freshwater, including cadmium and mercury. Particular attention was to be placed on the verification of the basic principles and to establishing practical performance characteristics. Results A simple plastic assembly capable of manufacture by injection moulding was developed. It was tested principally with solutions of cadmium in synthetic freshwater. The concentration measured using DGT was in good quantitative agreement with the known concentration in solution.The measured mass of accumulated metal in the resin-gel layer increased linearly with exposure time and was inversely proportional to the diffusion layer thickness. The measurement was independent of pH between 5 and 9. Temperature dependence was governed solely by the known diffusion coefficient of the solute and was therefore predictable. The DGT response was almost independent of the flow rate in the solution. Theoretical responses were observed over wide ranges of concentrations and R and D Technical Report P92 3 deployment times, up to 1 month. By using ICP-MS for the final laboratory measurement it was possible to measure zinc, cadmium, copper, lead, cobalt and nickel simultaneously. Mercury gave the theoretical response at high concentrations. When DGT assemblies were deployed in situ in the Trent - Mersey canal, sensible and reproducible concentrations of cadmium and mercury were obtained. The capabilities for high sensitivity while avoiding contamination were demonstrated. The potential for DGT to measure radionuclides was also demonstrated. Chelex can not be used to measure cesium. When a general cationic exchange resin was incorporated in the resin layer, both stable strontium and cesium and the radionuclides Sr-90 and Cs- 137 could be measured. Use of such a non selective resin limits the deployment time of DGT in natural waters to a few hours, but the principle is established for selective resins which should allow deployment times of several weeks. Conclusion DGT is a cheap reliable device which can be used to measure quantitatively, in situ labile species of trace metals and potentially radionuclides. Further work is required to test DGT devices in a range of field conditions, to improve its use for mercury, to extend its use to anionic species and nutrients and to test its ability for use as a surrogate test for bioavailability. Key words: in situ measurement, polyacrylamide gel, trace metals, cadmium, mercury, diffusive gradients in thin-films (DGT), ion-exchange resin. R and D Technical Report P92 4 1.
Publisher: Environment Agency
Subject Keywords: Trace metals; Cadmium; Mercury; In situ measurement; Polyacrylamide gel; Diffusive gradients in thin-films (dgt); Ion-exchange resin.
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