Determination of the chemical speciation of trace metals in aqueous systems by the Wageningen Donnan Membrane Technique

In order to determine the 'free' metal ion concentration in aqueous solutio ns the so-called Wageningen Donnan Membrane Technique (WDMT) has been devel oped. This involves a continuous flow system in which the donor side and th e acceptor side of the WDMT cell are continuously flushed with solution acr oss the membrane. The new cell design allows pseudo equilibrium to be reach ed for the free metal ions via a Donnan equilibrium across a negatively cha rged ion-exchange membrane within a reasonable time span. The donor solutio n contains both 'free' and complexed metal ions. The concentration of the c ations in the acceptor solution is either equal to the concentration of the 'free' cation concentration in the donor solution, or it can be calculated using simple correction factors. The optimization experiments have been pe rformed with cadmium and copper, in the presence of various (in)organic com plexing agents, at various pH values, and different salt concentrations. in multi-component systems, Like the systems used for the experiments with ca lcium, cadmium, copper, protons and EDTA, the results are in good agreement with the speciation calculations. The transport of the different cations a nd anions across the membrane can be very well explained by the simplified theory presented. Effects of difference in salt concentration on metal conc entrations can be corrected. For the more complicated systems with natural dissolved organic matter (e.g. humic acid) the concentrations measured in t he acceptor solution are also in good agreement with the speciation calcula tions performed for the donor solution. (C) 2000 Elsevier Science B.V. All rights reserved.