Effect of the competition of copper and cobalt on the lability of Ni(II)–organic ligand complexes, Part II: in freshwaters (Rideau River surface waters)

Abstract

Competitive binding of Cu 2+, Co 2+ and Ni 2+ ions by naturally-occurring organic complexants in the Rideau River surface water (RRSW) samples has been investigated employing the competing ligand exchange method (CLEM). It is well known that 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 metals, being 3×10 4 s −1 for Ni 2+ ion, 1×10 9 s −1 for Cu 2+ ion and 2×10 6 s −1 for Co 2+ ion. Naturally-occurring, heterogeneous, organic complexants in freshwaters are mainly humic substances, which are polyfunctional, and are ubiquitous in freshwaters and constitute ∼70–90% of the dissolved organic carbon. They have minor (∼1–10%) strong binding sites and major (∼99–90%) weak binding sites. The above metals first occupy strong binding sites and after all the strong binding sites are occupied, the remaining metals are bound to the weak binding sites of the organic complexants. Since the RRSW contains the above metals in concentrations below their limits of quantification by graphite furnace atomic absorption spectrometry (GFAAS), the RRSW samples have been spiked with various concentrations of Ni(II), Cu(II), and Co(II). In CLEM, a cation-exchange resin Chelex-100 has been used as the competing ligand, and GFAAS has been employed for monitoring the rate of uptake of the metals by Chelex-100. The results have revealed that in the absence of Cu(II) and Co(II), Ni(II) occupies strong binding sites of the naturally-occurring organic complexants, forming strong complexes, which are inert. When Cu(II) and Co(II) are present in sufficient concentrations, they occupy all the strong binding sites, with the result that Ni(II) binds to the weak binding sites, forming weak complexes, which are labile. The lability of the nickel complexes in the presence of Cu(II) and Co(II) is thus the result of successful competition of Cu(II) and Co(II) with Ni(II) for strong binding sites of the organic complexants (humic substances). The lability of nickel-complexes is enhanced more by copper than by cobalt. The environmental significance of this work for freshwaters is that even at relatively low nickel loadings, the metals usually associated with nickel in nickel ores, such as copper and cobalt, may successfully compete with nickel for strong binding sites of the naturally-occurring organic complexants in freshwaters with the consequence that nickel may exist as free nickel ions i.e. Ni(II)–aquo ions, which are reported to be toxic.