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)

Competitive binding of Cu2+, Co2+ and Ni2+ ions by naturally-occurring orga nic complexants in the Rideau River surface water (RRSW) samples has been i nvestigated employing the competing ligand exchange method (CLEM). It is we ll known that the reaction rate between a metal ion and a ligand is depende nt on the rate of exchange of the coordinated water, which varies greatly f or metals, being 3 x 10(4) s(-1) for Ni2+ ion, 1 x 10(9) s(-1) for Cu2+ ion 2 x 10(6) s(-1) for Co2+ ion. Naturally-occurring, heterogeneous, organic complexants in freshwaters are mainly humic substances, which are polyfunct ional, and are ubiquitous in freshwaters and constitute similar to 70-90% o f the dissolved organic carbon. They have minor (similar to 1-10%) strong b inding sites and major (similar to 99-90%) weak binding sites. The above me tals first occupy strong binding sites and after all the strong binding sit es are occupied, the remaining metals are bound to the weak binding sites o f the organic complexants. Since the RRSW contains the above metals in conc entrations below their limits of quantification by graphite furnace atomic absorption spectrometry (GFAAS), the RRSW samples have been spiked with var ious concentrations of Ni(II), Cu(II), and Co(II). In CLEM, a cation-exchan ge resin Chelex-100 has been used as the competing ligand, and GFAAS has be en 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 complexant s, forming strong complexes, which are inert. When Cu(II) and Co(II) are pr esent in sufficient concentrations, they occupy all the strong binding site s, with the result that Ni(II) binds to the weak binding sites, forming wea k 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 co mplexants (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, m ay successfully compete with nickel for strong binding sites of the natural ly-occurring organic complexants in freshwaters with the consequence that n ickel may exist as free nickel ions i.e. Ni(II)-aquo ions, which are report ed to be toxic. (C) 1999 Elsevier Science B.V. All rights reserved.