An Electrochemical Investigation of Complexation of Pb(II) by a Well‐Characterized Fulvic Acid in Model Systems – Effect of Competition with Major Cations and Trace Metals

Abstract

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...

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