Competition of Ca(II) and Mg(II) with Ni(II) for binding by a well-characterized fulvic acid in model solutions

Competition of Ca(II) and Mg(II) with Ni(II) ions for binding sites of a we ll-characterized fulvic acid (FA) in model solutions at constant pH and ion ic strength was investigated. The Competing Ligand Exchange Method with Che lex-100 and dimethyl glyoxime as the competing ligands was employed to meas ure the rate of free Ni2+ ion release using graphite furnace atomic absorpt ion spectrometry and adsorptive cathodic stripping voltammetry, respectivel y. The Windermere Humic Aqueous Model was used to predict the effect of com petition of Ca(II) and Mg(II) on the binding of Ni(II) by the FA. The resul ts show that the presence of high concentrations of Ca(II) and Mg(II) in mo del solutions has considerable effects on the binding of Ni(II) by the FA. Since the concentrations of Ca(II) and Mg(II) used are 4 orders of magnitud e higher than those of Ni(II), Ca(II) and Mg(II) can outcompete Ni(II) for sites where electrostatic interactions dominate, resulting in Ni(II) formin g weak Ni(II)-FA complexes that are labile. The significance is that in fre shwaters containing humic substances and trace quantities of nickel and maj or cations, Ca2+ and Mg2+, the competition of Ca2+ and Mg2+ with Ni2+ for b inding sites of humic substances produces weak Ni(II)-humate complexes that are labile, releasing free Ni2+ ions.