Effect of Gel Interactions with Dissolved Organic Matter on DGT Measurements of Trace Metals

The technique of diffusive gradients in thin films (DGT) has emerged as a powerful technique for obtaining information on the dynamics of chemical speciation and partitioning of a diverse range of solutes in natural waters. However, there have been concerns that adsorption of dissolved organic matter (DOM) to the diffusive hydrogel could affect measurements, particularly of trace metals. Diffusive gels were exposed to solutions containing DOM from eight different sources and the adsorption of organic matter measured spectrophotometrically, both by its removal from solution and its accumulation on the gel. Both fulvic and humic acid extracted from water adsorbed slightly, but there was much more marked adsorption of humic acid extracted from a peaty soil. There was virtually no evidence of adsorption of DOM in filtered natural waters of various sources, including a lake, river and the porewaters of soils, even when the latter had high concentrations of humic substances. It appears that humic substances in solution, rather than those extractable from the solid phase of soils, do not adsorb markedly to the DGT diffusive hydrogel. DGT measurements of Cd were unaffected if their diffusive gels were pre-exposed to DOM in soil and surface waters, but DGT measurements of Cu were suppressed, especially if the pre-exposure had been to a water containing high concentrations of DOM. This effect on the DGT measurement is likely to be minimised by use of longer deployment times, but further work is required to establish the extent of such effects for a range of realistic conditions.