Forschungs- & Entwicklungsinformationsdienst der Gemeinschaft - CORDIS

Model predictions of metal speciation in freshwaters measured by in situ techniques

This result presents a test of the predictive capabilities of two commercially available equilibrium speciation models by comparison with measurements of trace metal species in situ in a soft-water river, a hard-water lake and a hard-water stream. Diffusive gradients in thin films (DGT) and voltammetry at a gel-integrated microelectrode (GIME) were used to measure dynamic species. The Donnan membrane technique (DMT) and hollow fibre permeation liquid membrane (HF-PLM) were used to measure free ion activities. Speciation calculations used WHAM 6, incorporating Humic Ion Binding Model VI and Visual MINTEQ incorporating NICA-Donnan.

Predictions of dominant species using the two models agreed reasonably well, even when colloidal oxide components were considered. Measurements of Cd, Cu and Ni using DGT and GIME agreed reasonably well with each other and with model predictions of the metal transported through the gel layer. Some lower values for GIME were attributed to it having a shorter time window that excluded some species with slower rates of dissociation. For the Pb in soft-water with high dissolved organic carbon DGT and GIME measurements agreed well, but were an order of magnitude lower than model predictions, which appeared to under estimate colloidal Pb. Model predictions of the free ion activity of Ni agreed well with DMT measurements. Predictions of the free ion concentrations of Cu and Pb using Visual MINTEQ were substantially lower than those made using WHAM 6, but both predictions were substantially less than the values using DMT and HF-PLM, which were in good agreement. The results highlight the need to test further model predictions of free ion activities in natural waters rather than in test solutions containing isolated humic substances.

The limiting conditions under which equilibrium models may be applicable for freshwaters are identified. The result is useful for any end user concerned with predicting trace metal speciation in freshwaters, e.g. other research, water quality control agencies.

Reported by

Department of Environmental Sciences, Bailrigg
United Kingdom
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