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.