Effect of the competition of copper and cobalt on the lability of Ni(II)-organic ligand complexes. Part I. In model solutions containing Ni(II) and awell-characterized fulvic acid

The competitive binding of Cu(II), Co(II) and Ni(II) ions by a well-charact erized fulvic acid (FA) in model aqueous solutions has been investigated by employing the competing ligand exchange method (CLEM) with Chelex-100 as t he competing ligand. The reaction rare between a metal ion and a ligand is dependent on the rate of exchange of the coordinated water, which varies gr eatly for the above metals. Fulvic acid is a polyfunctional, polyelectrolyt ical, physically-heterogeneous, organic complexant having binding sites tha t can be roughly classified into two categories: minor (similar to 1-10%), strong sites, and major (similar to 99-90%), weak sites. The strong binding sites are first occupied, and after all the strong binding sites are occup ied, the weak binding sites are occupied. Experiments have been done using model aqueous solutions containing various concentrations of Cu, Co and Ni, and the above well-characterized FA, Graphite furnace atomic absorption sp ectrometry and inductively-coupled plasma mass spectrometry were employed t o monitor the rate of uptake of the metals by Chelex-100. The above metals, in the metals/FA mole ratios used in this study, have been found to compet e with one another for the relatively small number of strong binding sites of the FA, and Cu(II) and Co(II) which have much greater rate constant for water exchange than that of Ni(II) win the competition over Ni(II). In the absence of Cu(II) and Co(II), the strong binding sites of the FA are occupi ed by Ni(II), forming strong complexes, which are inert, whereas, in the pr esence of Cu(II) and Co(II), the strong binding sites of the FA are occupie d by Cu(II); Co(II), Ni(II), and the remaining Cu(II) occupy the weak bindi ng sites of the FA, forming weak complexes, which are labile, The enhanced lability of the Ni-FA complexes in the presence of Cu(II) and Co(II) indica tes that Cu(II) and Co(II) successfully compete with Ni(II) for the strong binding sites of the FA. (C) 1999 Elsevier Science B.V. All rights reserved .