Kinetic speciation of lead and cadmium in freshwaters using square-wave anodic stripping voltammetry with a thin mercury film rotating disk electrode

Square-wave anodic stripping voltammetry with a thin mercury film electrode was used to investigate the kinetic speciation of lead and cadmium in freshwaters. Diffusion coefficients and dissociation rate coefficients of lead and cadmium species were determined in an ultrafiltered sample of Rideau River surface water (RRSW). The RRSW sample was first filtered through 0.45 μm cellulose acetate filters and then fractionated into <50 000 D, <10 000 D and <1000 D molecular weight cut-off (MWCO) fractions by cascade (sequential) ultrafiltration. Diffusion coefficients of lead and cadmium complexes in the RRSW samples were significantly lower (<50%) than those for the lead and cadmium aquo complexes. Two groups of kinetically distinguishable lead and cadmium species were determined in the RRSW samples: one of fast-dissociating complexes and another of slowly-dissociating complexes. Dissociation rate coefficients of the: fast-dissociating complexes of lead and cadmium increased with decreasing MWCO fractions. Dissociation rate coefficients of the slowly-dissociating complexes of lead and cadmium had the smallest values in the 10 000 D MWCO and 1000 D MWCO fractions, respectively. The above results agree with a model of the metals forming complexes with humic substances, which are ubiquitous in freshwaters; the polyfunctional and the oligoelectrolytic properties of humic substances mainly determined the thermodynamic stability and kinetic lability of the complexes.