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Competition of Ca(II) and Mg(II) with Ni(II) ions for binding sites of a well-characterized fulvic acid (FA) in model solutions at constant pH and ionic strength was investigated. The Competing Ligand Exchange Method with Chelex-100 and dimethyl glyoxime as the competing ligands was employed to measure the rate of free Ni2+ ion release using graphite furnace atomic absorption spectrometry and adsorptive cathodic stripping voltammetry, respectively. The Windermere Humic Aqueous Model was used to predict the effect of competition of Ca(II) and Mg(II) on the binding of Ni(II) by the FA. The results show that the presence of high concentrations of Ca(II) and Mg(II) in model 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 magnitude 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) forming weak Ni(II)−FA complexes that are labile. The significance is that in freshwaters containing humic substances and trace quantities of nickel and major cations, Ca2+ and Mg2+, the competition of Ca2+ and Mg2+ with Ni2+ for binding sites of humic substances produces weak Ni(II)−humate complexes that are labile, releasing free Ni2+ ions.

Adsorptive cathodic stripping voltammetry (AdCSV) in conjunction with the competing ligand-exchange method (CLEM) was investigated as a tool for measuring dissociation rate coefficients of nickel complexes in model systems. Dimethylglyoxime (DMG) was used as the competing ligand. Citric acid (CA) and a well-characterized fulvic acid (FA) were used as model ligands. The rate coefficients were calculated, and the consistency of equilibrium and kinetic data was discussed. The contributions of the disjunctive pathway (proceeding by the dissociation of the initial complex) and the adjunctive pathway (proceeding by the formation of an intermediate complex as a result of direct attack of the competing ligand on the initial complex) on the overall reactions were investigated. The reactions of Ni-CA or Ni-FA complexes with DMG were demonstrated to proceed by both disjunctive and adjunctive pathways. The predominant pathway for the overall reaction depends on the nickel-to-initial ligand and the DMG-to-initial ligand ratios. The reactions follow predominantly the disjunctive pathway for [DMG] > or = 3 mM and Ni-to-dissolved organic carbon (DOC) ratios greater than 10 nM Ni 2+/g of DOC. Since free nickel ion in freshwaters is reported to be toxic, its rate and pathway of formation are of environmental concern.

Effect of competition of major cations, Ca 2 , Mg 2 , and trace metals, Cu 2 , Ni 2 and Zn 2 , on the stability of the Pb(II)-fulvic acid (FA) complexes in model systems at constant pH and ionic strength was investigated by squarewave anodic stripping voltammetry (SWASV). The results show that the trace metals compete with the target trace metal, Pb(II), for binding sites of the FA even when they are present in the same concentration as that of Pb(II), whereas the major cations compete with Pb(II) only when they are present in massive excess.Keywords: SWASV, Competition, Trace metals, Major cations, Conditional stability constant Information about the competition between trace metals for naturally-occurring organic complexants in freshwaters is scarce. Recent experimental results in our laboratories have clearly demonstrated that in model systems and in freshwaters, competition of other trace metals and major cations with target trace metals for strong (ca. 1 ± 10% of the total sites) and weak (ca. 99 ± 90% of the total sites) binding sites of polyfunctional, polyelectrolytical, macromolecular, heterogeneous, organic complexants, such as humic substances, determines the release of bound metal as free metal ions [1 ± 3]. However, all these results were based on the kinetic approach. However, interpretation of metal complexation by naturally-occurring, heterogeneous, organic complexants, and assessment of their relationship to biogeochemical processes require combined equilibrium and kinetic approaches [4,5]. Here, the effect of trace and major metal competition on the stability of the Pb(II)-FA complexes in model systems has been investigated using the equilibrium approach. Xue et al. [6], Ghosh et al. [7] and Cao et al. [8] have reported on the importance of competitive complexation of trace metals with humic acids on the metal release. Donat et al. Electrochemical methods such as polarography and voltammetry are widely used in trace element speciation in natural water systems because of their metal specificity, high sensitivity and their ability for direct (i.e., without external preconcentration) determination [12 ± 14]. Although the specific potential of voltammetric techniques for characterization of complexation reactions of electroactive metal ions has been recognized for a long time [15], application of voltammetry to metal speciation in freshwaters is quite recent.The objective of this research is to investigate the binding of Pb(II) by a well-characterized fulvic acid (FA) in model systems when major cations, Ca(II) and Mg(II), and other trace metals, Cu(II), Ni(II) and Zn(II), compete with Pb(II) for binding sites of the FA.The method of Hoop et al. [16] has been adapted as described below. The complexation scheme in which an electroactive metal ion, M (charge omitted for simplicity), associates with a ligand or site, L, to form an electroinactive complex, ML, can be presented as follows:where M 0 denotes the metal atom, and k a and k d are association and dissociation rate coefficients, respec...