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 a well-characterized fulvic acid

Abstract

The competitive binding of Cu(II), Co(II) and Ni(II) ions by a well-characterized fulvic acid (FA) in model aqueous solutions has been investigated by employing the competing ligand exchange method (CLEM) with Chelex-100 as the competing ligand. The reaction rate between a metal ion and a ligand is dependent on the rate of exchange of the coordinated water, which varies greatly for the above metals. Fulvic acid is a polyfunctional, polyelectrolytical, physically-heterogeneous, organic complexant having binding sites that can be roughly classified into two categories: minor (∼1–10%), strong sites, and major (∼99–90%), weak sites. The strong binding sites are first occupied, and after all the strong binding sites are occupied, 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 spectrometry and inductively-coupled plasma mass spectrometry were employed to 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 compete 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 occupied by Ni(II), forming strong complexes, which are inert, whereas, in the presence of Cu(II) and Co(II), the strong binding sites of the FA are occupied by Cu(II); Co(II), Ni(II), and the remaining Cu(II) occupy the weak binding 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) indicates that Cu(II) and Co(II) successfully compete with Ni(II) for the strong binding sites of the FA.