EQUILIBRIUM ION-EXCHANGE METHOD - METHODOLOGY AT LOW IONIC-STRENGTH AND COPPER(II) COMPLEXATION BY DISSOLVED ORGANIC-MATTER IN A LEAF-LITTER EXTRACT

The equilibrium ion exchange method (EIM) is a powerful tool for the i nvestigation of metal cation complexation by dissolved organic matter (DOM) in natural systems. Tests with different ion exchange resins dem onstrated that under low ionic strength conditions (0.01 mol/kg) and i n the presence of DOM, equilibration times of at least 24 h are requir ed for experiments with Cu(II). The classical approach to the EIM was modified by using nonlinear reference adsorption isotherms in order to expand the method to a broader range of experimental conditions. For Cu(II) at low ionic strength (0.01 mol/kg), the reference isotherms be tween pH 4 and 6 were identical and were mathematically modeled in ter ms of Langmuir adsorption parameters. The EIM using nonlinear referenc e isotherms was Validated between pH 4 and 6 by the correct determinat ion of the stability constants for the complexes CuOxalate and Cu(Oxal ate)(2). Then the method was used to quantitatively characterize the C u(II) complexation behavior of DOM in an aqueous chestnut leaf litter extract between pH 4 and 6. In contrast to the classical approach to t he EIM, data were analyzed by using plots [Cu](bound)/[Cu](free) vs: [ Cu](bound). This allowed the determination of both, conditional stabil ity constants and metal binding capacities for two different binding s ite classes. The logarithmic values of the stability constants were ab out 8 for the strong binding sites and 5.5-6 for the weak binding site s. The total Cu(II) binding capacity increased from 0.22 mol/(kg C) at pH 4 to 2.85 mol/(kg C) at pH 6.