Studies on chemical speciation of some trace metals in the aquatic and the terrestrial environment

New analytical techniques and methods for determining chemical speciation of some trace metals in the aquatic and the terrestrial environment have been investigated and developed. Rotating disk electrode voltammetry (RDEV) in conjunction with anodic stripping voltammetry (ASV) with a thin mercury film electrode (TMFE), or with a Nafion-coated thin mercury film electrode (NCTMFE) has been evaluated for direct determination of lead and cadmium speciation in aqueous solutions containing dissolved organic matter. The RDEV technique has been also employed to estimate dissociation rate constants and diffusion coefficients. Pseudopolarographic analysis of aqueous samples containing Cd or Pb and a well-characterized fulvic acid, and of samples of Rideau river surface water show that NCTMFEs provide interference-free determination, and give well-defined pseudopolarograms for determination of stability constants. For determination of dissociation rate constants of nickel complexes, Competitive Ligand Exchange/Adsorptive Cathodic Stripping Voltammetry (CLE/AdCSV) using high-frequency square-wave voltammetry has been developed as Anodic Stripping Voltammetry cannot take advantage of high-frequency square-wave voltammetry to achieve the extremely high sensitivity required for determining trace concentrations of nickel in unpolluted waters. Application of the CLE/AdCSV method to aqueous samples containing dissolved organic matter shows that complexes having dissociation rate constants from 10−3 s−1 to 10−6 s−1 can be resolved from very labile complexes, and also from inert complexes. For determination of chemical speciation of Zn and Cd in soil, High Performance Liquid Chromatography—Microfiltration Method was developed for determination of solution phase, labile-bound, and nonlabile Zn and Cd species. Extraction of metals from soil solids was accomplished by using online microfiltration which trapped soil solids from an injection of a soil slurry and allowed the mobile phase to extract metals and carry them to the chromatographic column for separation. The results of this kinetic study of cadmium and zinc sorption/desorption on a well-characterized soil show that the metal sorption on the soil is a two-step process, involving an initial fast binding followed by a much slower sorption. Experimental differentiation of the Cd and Zn species on soil was found to be useful for understanding the mechanisms of sorption/desorption reactions of Zn(II) and Cd(II) on soil.