| Abstract
| - Competition of Ca(II) and Mg(II) with Ni(II) ions forbinding sites of a well-characterized fulvic acid (FA) inmodel solutions at constant pH and ionic strength wasinvestigated. The Competing Ligand Exchange Method withChelex-100 and dimethyl glyoxime as the competingligands was employed to measure the rate of free Ni2+ion release using graphite furnace atomic absorptionspectrometry and adsorptive cathodic stripping voltammetry,respectively. The Windermere Humic Aqueous Modelwas used to predict the effect of competition of Ca(II) andMg(II) on the binding of Ni(II) by the FA. The resultsshow that the presence of high concentrations of Ca(II)and Mg(II) in model solutions has considerable effects onthe binding of Ni(II) by the FA. Since the concentrationsof Ca(II) and Mg(II) used are 4 orders of magnitude higherthan those of Ni(II), Ca(II) and Mg(II) can outcompeteNi(II) for sites where electrostatic interactions dominate,resulting in Ni(II) forming weak Ni(II)−FA complexes that arelabile. The significance is that in freshwaters containinghumic substances and trace quantities of nickel and majorcations, Ca2+ and Mg2+, the competition of Ca2+ andMg2+ with Ni2+ for binding sites of humic substancesproduces weak Ni(II)−humate complexes that are labile,releasing free Ni2+ ions.
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