Modelling metal desorption kinetics in estuaries

The kinetics of desorption of Cd, Cu, Ni and Zn from riverine and estuarine sediments have been determined in deionised water and seawater (salinity=30). The time-dependent release of Cd, Cu and Ni from riverine sediments in both media showed an increase in dissolved metal concentrations within a few minutes, followed by a plateau after several hours of incubation. Desorption of Zn from riverine sediments in seawater was characterised by a maximum dissolved concentration after several minutes, followed by readsorption onto the suspended sediment. The extent of metal desorption from estuarine sediment (relative to a chemical digest of available particulate metal), in both deionised water and seawater, was less than that from the fluvial sediments, even though the available particulate metal concentrations were higher. The desorption kinetics of the metals were modelled using a mechanism consisting of reversible, first-order consecutive reactions. The dissolved metals were assumed to desorb into the bulk solution from exchangeable surface sites and from binding sites located within the particle matrix. A reasonable fit to the observed time-dependent desorption behaviour was simulated by integrating the differential equations resulting from the mechanism, assuming that the exchangeable particulate metal was defined by the chemical digest. The results are discussed within the context of improved understanding of the non-conservative behaviour of dissolved metals in estuaries.