Application of the KD Concept to the Study of Trace Metal Removal and Desorption During Estuarine Mixing

Trace metal solid-solution partitioning, represented by KDs, has been studied along a salinity gradient created by batch mixing of end-member samples taken from the Humber Estuary. The KDs decrease with salinity for Cd, due to complexation of the dissolved form with chloride ions, and Cs, due to competition from sea water cations for particle sorption sites. Although KDs for Zn were relatively invariant, there is evidence to suggest that a reduction in KD with salinity would be effected under more acidic (pH < 7•6) conditions. An increase in KD for Mn with salinity by more than an order of magnitude is related to heterogeneous oxidative precipitation onto pre-existing particles. The extent of metal desorption is a function of end-member KDs the concentration of seaward advecting riverine particles and the reversibility of sorption. Thus, mass-balance calculations predict the extent of desorption of Cd which is loosely held to particle surfaces, but cannot be applied to Cs because a significant fraction (70%) of ions is irreversibly lodged within particle matrices. Although KDs for Cd, Cs and Zn are independent of particle concentration, maintenance of a constant partitioning with change in particle concentration requires a shift in the relative proportions of metal in solution and particle bound (w/w). This effect is manifested by the pronounced removal of trace constituents in an estuarine turbidity maximum and can be quantified by incorporating KDs into a sorption model.