J. P. Pinheiro

ON LABILITY OF CHEMICALLY HETEROGENEOUS SYSTEMS. COMPLEXES BETWEEN TRACE METALS AND HUMIC MATTER

Abstract Complexation of trace metals by humic matter is an important issue, as it determines to a large extent their bioavailability, toxicity and mobility. In a complexing medium the supply of metal species towards an interface is always related to the kinetics of the complex association/dissociation reactions, irrespective of whether one is dealing with living organisms in natural media, or with an electrode in voltammetry, or with a membrane in a separation process. In voltammetric studies lability criteria have been developed for steady-state and dynamic experiments, for homogeneous and chemically heterogeneous systems. In this work these criteria are reformulated on a more rigorous level and used to investigate the lability of cadmium and lead complexes in different samples of fulvic and humic matter (chemically heterogeneous systems). It is explained how lability varies with the stability of the chemically heterogeneous complex. The discussion further shows that, amongst other things, an increasing degree of chemical heterogeneity leads to a decrease of the lability.

Electrochemical Methods for Speciation of Trace Elements in Marine Waters. Dynamic Aspects

The contribution of electrochemical methods to the knowledge of dynamic speciation of toxic trace elements in marine waters is critically reviewed. Due to the importance of dynamic considerations in the interpretation of the electrochemical signal, the principles and recent developments of kinetic features in the interconversion of metal complex species will be presented. As dynamic electrochemical methods, only stripping techniques (anodic stripping voltammetry and stripping chronopotentiometry) will be used because they are the most important for the determination of trace elements. Competitive ligand exchange-adsorptive cathodic stripping voltammetry, which should be considered an equilibrium technique rather than a dynamic method, will be also discussed because the complexing parameters may be affected by some kinetic limitations if equilibrium before analysis is not attained and/or the flux of the adsorbed complex is influenced by the lability of the natural complexes in the water sample. For a correct data interpretation and system characterization the comparison of results obtained from different techniques seems essential in the articulation of a serious discussion of their meaning.

Lability criteria for metal complexes in micro-electrode voltammetry

Theoretical expressions are derived for the voltammetric lability criteria of metal complexes in the micro-electrode regime. The treatment includes three limiting situations: (i) the macro-electrode limit, where both the diffusion layer and the dissociation reaction layer are linear; (ii) an intermediate case with a hemispherical diffusion layer and a linear reaction layer; and (iii) the micro-electrode limit with hemispherical diffusion and reaction layers. The results show that the operational lability of complexes will generally decrease with decreasing electrode size in going from (i) to (ii), but that it increases again when (iii) is approached. Results for some types of metals with different kinetic characteristics are presented.

THE PH EFFECT IN THE DIFFUSION COEFFICIENT OF HUMIC MATTER : INFLUENCE IN SPECIATION STUDIES USING VOLTAMMETRIC TECHNIQUES

Abstract The dynamic aggregation properties of humic matter and the influence of intramolecular electrostatic interaction lead to an increase of polydispersity at pH > 5 in two humic samples studied (“Purified Peat Humic Acid”, PPHA and “Laurentian Fulvic acid”, LFA). The smaller entities formed at pH increasing above 5 are responsible for the increase of the diffusion coefficient as “seen” by voltammetry. The voltammetric results show that the diffusion coefficient of the humic complex inmplex increases from 5 × 10−12 m2 s−1 (pH≤5) up to 2 × 10−11 m2 s−1 (pH=6) for Cd PPHA and from 6 × 10−11 m2 s−1 (pH Pb LFA . Systematic shifts in voltammetric results with changing ligand concentrations often attributed to adsorption of humic matter on the electrode and/or heterogeneity effects, may also be due to incorrect diffusion coefficient values used in speciation calculations. The aim of this paper is to show that the diffusion coefficient of metal-humic complexes is a function of pH. It is necessary to take this into account to obtain more accurate speciation data from voltammetric results and interpret the physicochemical background of the metal humic interaction.

Kinetics of adsorption of humic matter on mercury.

Simulations of 0 vs t1i2 curves for an unstirred medium and an electrode of constant area were set assuming that the global adsorption process of humic matter on mercury is controlled by both steps, diffusion and adsorption kinetics a t the interface. Experimental 0 vs N 2 results obtained by alternating current voltammetry for different concentrations of humic matter a t pH 2.5 and pH 5.0 were successfully fitted to theoretical curves if an association reaction of humic matter a t the surface of the electrode and repulsive interactions that strongly affect high 0 values were considered after a certain degree of surface coverage.