Trace metal ion binding to fulvic acids in model systems and freshwaters

Bioavailability of trace metals is known to be often correlated with free metal ion activity in freshwaters. The free metal ion activity of trace metals is dependent on the dissociation of metal complexes with humic susbstances which are ubiquitous in freshwaters. Trace metal complexation by humic substances determines at least to some extent solubility, mobility and transport of trace metals in freshwaters. An experimental study of dissociation of fulvic acid complexes of selected trace metals in model systems and in freshwaters is described.;Metal binding characteristics were investigated as a function of the [metal]/[FA] mole ratio by Competing Ligand Exchange Method using Chelex-100 as a competing ligand and inductively-coupled plasma mass spectrometry to measure the first-order dissociation of metal complexes. The kinetic data of metal-complex dissociation were analyzed by non-linear regression analysis. The model systems consisted of Ni(II), Cu(II), Co(II), Pb(II), and Zn(II) and two well-characterized fulvic acids. The freshwater samples were unpolluted river surface water from the Rideau River, and metal-polluted streams from Sudbury (Canada) mines area.;The dissociation rate coefficient and the percentage of the faster kinetically distinguishable component increased with increasing [metal]/[FA] mole ratio. The percentage of the faster kinetically distinguishable component of Ni(II) complexes in freshwaters from the Sudbury mines area increased with increasing [Ni(II]/[DOC] ratios. The order of increase in the lability of the complexes in the model systems and the freshwater samples was Co(II) > Zn(II) > Ni(II) > Cu(II). Even in the presence of excess of the FA, Cu(II) and Co(II) competed with Ni(II) for strong binding sites of the FA. Since fulvic acids are heterogeneous complexants, the dissociation rate coefficients of their metal complexes are modelled more satisfactorily as a distribution function. Decay curves of the dissociation of metal fulvic acid complexes were analyzed using a Distribution Analysis Method. Simulated data were first subjected to the Distribution Analysis Method to ferret out its capabilities and limitations. Analysis of the experimental data by the Distribution Analysis Method shows that metal-FA complexes have a broad range of dissociation rate coefficients rather than a single value.