Competitive binding of metal ions by humic substances in model systems and in freshwaters

Metal complexation reactions involving dissolved organic carbon (DOC) in freshwaters play a key role in regulating trace metal speciation, mobility, bioavailability and toxicity. In freshwaters having multimetals and multiligands milieu, many different metal ions are in competition with the same complexant, and many different complexants are in competition with the same metal ion. The above competitions determine the speciation of trace metals and their bioavailability.; Competitive binding of Cu(II), Co(II) and Ni(II) by humic substances in model systems and in freshwaters from the Rideau River surface area was investigated employing the Competing Ligand Exchange Method (CLEM). CLEM is a kinetic speciation method based on ligand-exchange kinetics. The CLEM was applied using a cation-exchange resin, Chelex-100, as the competing ligand, and Graphite Furnace Atomic Absorption Spectrometry for measurement of the rate of free Ni2+ ion release from the dissociation of Ni(II) complexes. It was found that even at relatively low Ni(II) loadings, Cu(II) and Co(II), which were usually associated with nickel in nickel ores, successfully competed with Ni(II) for binding sites of humic substances, and released bound Ni(II) as free Ni2+ ions (toxic).; The competition of major cations, Ca2+ and Mg2+ , with the target trace metal, Ni(II), in model systems and in nickel-polluted natural waters from the Sudbury area (Canada) was also investigated using the same analytical technique as above. It was found that the release of bound Ni(II) as free Ni2+ ions is critically dependent on the competition by the major cations. Bioassay toxicity tests of the water samples from the Sudbury area showed that the released free Ni2+ ion was bioavailable and toxic.; The effect of metal competition was also studied using the equilibrium approach involving measurement of stability constants of metal complexes, which gave a measure of the availability of free metal ions at equilibrium. Square Wave Anodic Stripping Voltammetry (SWASV) at a static mercury drop electrode was employed to determine stability constants of metal complexes.; The environmental significance for freshwaters is that it has demonstrated the importance of competition of trace metals and major cations in release of DOC-bound target trace metals as free metal ions (toxic).