Effect of metal/fulvic acid mole ratios on the binding of Ni(II), Pb(II), Cu(II), Cd(II), and Al(III) by two well-characterized fulvic acids in aqueous model solutions

The kinetics of dissociation of Ni(II), Pb(II), Cu(II), and Cd(II)–fulvic acid (FA, Armadale), and Al(III)–FA (Suwannee River) complexes in aqueous model solutions was studied by the competing ligand exchange method (CLEM). Chelex-100 cation exchange resin was used as the competing ligand and inductively-coupled plasma mass spectrometry (ICP-MS) or graphite furnace atomic absorption spectrometry (GFAAS) was used to determine the rate of dissociation of the metal–FA complexes in the model solutions. At low metal concentrations and at very low [metal]/[FA] mole ratios, the metals formed inert complexes (dissociation rate coefficient k d ≈ 10 −5 s −1) with the FAs. The percentage of inert complexes increased as the [metal]/[FA] mole ratio was decreased at a constant concentration of the metal. The percentage of labile metal–FA complexes (dissociation rate coefficient k d ≈ 10 −2–10 −3 s −1) increased as the [metal]/[FA] mole ratio in the model solutions was increased. The results were similar for all the metals studied. The two well-characterized FAs showed similar behavior despite their marked differences in the binding capacity and in the source and nature of the two FAs — the Armadale FA is pedogenic, and the Suwannee River FA is aquogenic. The results have special significance for the natural environment in which potentially toxic metals are often present at trace levels. In such a situation, experiments at the environmentally relevant [metal]/[FA] mole ratios are necessary to determine the effect of the [metal]/[FA] mole ratios on dissociation of metal–FA complexes and release of metal–aquo complexes which are reported to be toxic.