A multi-technique approach for the study of speciation of nickel, cadmium, copper, lead, and zinc in freshwaters.

摘要

Several techniques for the determination of metal speciation in freshwaters were employed. Model solutions of nickel and Humic Acid (HA) were studied using the kinetics-based Competing Ligand Exchange Method (CLEM) to determine the dissociation rate coefficients of Ni(II)-HA complexes as a function of pH and [Ni]/[HA] mole ratio. These results were compared with the previous results of Ni(II)-complexes with a well-characterized Fulvic Acid (FA). It was found that Ni(II)-HA complexes were stronger than Ni(II)-FA complexes and had smaller dissociation rate coefficients than Ni(II)-FA complexes. Kinetics-based CLEM and equilibrium-based speciation methods employing Cathodic or Anodic Stripping Voltammetry were used to determine the free-metal ion and labile metal-complex concentrations of Ni, Zn, Cd, and Cu in some metal-impacted lake surface waters from Rouyn-Noranda, Quebec. The measured free-metal ion and labile metal-complex concentrations were compared with the predictions of a widely-used computer speciation code, the Windermere Humic Aqueous Model (WHAM), versions V and VI, to determine the capability of WHAM in modelling the above metals in freshwaters. WHAM was also used to model the effect of the presence or absence of Al and Fe on the free-metal ion concentration. Both versions of WHAM were more successful in modelling Ni, Cd, and Zn in the freshwaters, but were not as successful in modelling Cu. WHAM VI was generally better or equal to WHAM V for predicting the free-metal ion concentrations. The modelled competition by Al and Fe in the lake surface waters showed that in most cases, copper was most affected by this competition. WHAM VI predicts a larger effect from the Al and Fe competition than does WHAM V, probably because WHAM VI allows tridentate binding to occur. Rotating Disk Electrode Voltammetry (RDEV) was employed for the study of speciation parameters such as diffusion coefficients and free-metal ion concentrations of model solutions of Zn-, Cd- and Cu-HA and of freshwaters as a function of pH. The RDEV study showed that increasing pH resulted in smaller diffusion coefficients in both the model solutions and the freshwater samples, suggesting increased aggregation of the metal complexes.