The speciation of lead in seawater by pseudopolarography using a vibrating silver amalgam microwire electrode

Abstract

A novel electrode based on a vibrating silver amalgam microwire was used for pseudopolarography of lead (Pb) in seawater. Pseudopolarography is a method to study the stability of metal species by examination of their reduction potential. The reduction potential of kinetically irreversible species is related to the thermodynamic complex stability constant, whilst for labile species it is related to the conditional stability constant. The new electrode enables the speciation of Pb at picomolar level in uncontaminated seawater. The potential shift of electrochemically irreversible lead complexes was calibrated against the complex stability of several ligands in pH8 seawater. The method was compared against the mercury drop electrode at various concentrations of lead and various deposition times, and the response was in general agreement with the theory. The method is tested on samples from oceanic (Atlantic), coastal (Irish Sea) and estuarine (Mersey) origins at Pb concentrations between 30pM and 2nM. The pseudopolarograms were found to resolve the speciation of lead in terms of labile and irreversibly bound Pb species. The lead in the samples was largely in the labile component of the pseudopolarograms, indicating that it was reversibly complexed with values for log α′Pb2+L of 2 for the oceanic sample, and of 4 for the coastal sample. A smaller fraction was resolved as stable, irreversibly bound species with values for the thermodynamic stability constant of log KPb2+L=15–16. The speciation method was able to resolve metal species occurring at levels of only a fraction of that of the metal. Unlike copper, where organic complexation is strong and electrochemically irreversible, and able to significantly decrease its bioavailability, the Pb mostly occurs in the labile complexed fraction and is therefore readily available for geochemical scavenging or biological uptake reactions.