Determination of the free ion concentration of trace metals in soil solution using a soil column Donnan membrane technique

Accurate measurement of the free metal ion is difficult, especially for trace metals present in very small concentrations (less than micromolar) in natural systems: The recently developed Donnan membrane technique can measure the concentrations in solution in the presence of inorganic and organic complexing agents. We have developed this method further to make it applicable for analysing soil samples. The major development is the linkage of a soil column with the Donnan cell. The operational aspects of the method, including equilibrium time, buffering capacity and correction for differences in ionic strength, were investigated and optimized. The method was applied to determine concentrations of free Cu2+, Cd2+, Pb2+ and Zn2+ in the soil solution of 15 soil samples (pH 2.9-7.1, organic C<2.9-137.4 g kg(-1), clay 0.2-51.6%). Compared with other speciation methods, the Donnan membrane technique has the advantage of allowing the measurement of several elements simultaneously and it minimizes the disturbance of substrate. The detection limit of the technique is about 10(-9) M. This method can be applied to study the biogeochemical behaviour of metals in soils, sediments and other solid materials.