Rotating disk electrode voltammetry/anodic stripping voltammetry for chemical speciation of lead and cadmium in freshwaters containing dissolved organic matter

Abstract

AbstractRotating 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 the direct determination (i.e., without preconcentration) of lead and cadmium speciation in aqueous solutions containing dissolved organic matter. The RDEV technique has been used to estimate dissociation rate constants and diffusion coefficients; pseudopolarograms have indicated the identity of the metal complexes undergoing electrode reductions at the corresponding plateaus of the pseudopolarograms in a number of model solutions containing Cd or Pb and known amounts of fulvic acid. It has been found that the effect of dissolved organic material is measurable when the concentrations of fulvic acid are equal to or less than 20 μg/mL (which is below the concentration of naturally occurring dissolved organic carbon in most freshwaters). The effective electrode surface area has been found to be considerably less than the geometric surface area, and if this fact is not explicitly taken into consideration, the result would be an erroneous estimation of dissociation rate constants and diffusion coefficients. Pseudopolarograms of aqueous samples containing Cd or Pb and fulvic acid and samples of Rideau river surface water clearly show that NCTMFEs effectively block the transport of large inert metal‐FA complexes to the electrode surface, resulting in interference‐free determination, and giving well‐defined pseudopolarograms, and yet do not hinder the transport of small metal‐FA complexes to the electrode surface. It is concluded that RDEV/ASV offers promise of being a valuable technique for determining chemical speciation of lead and cadmium in river surface waters.