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

Abstract

Trace metal solid-solution partitioning, represented by K Ds, has been studied along a salinity gradient created by batch mixing of end-member samples taken from the Humber Estuary. The K Ds 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 K Ds for Zn were relatively invariant, there is evidence to suggest that a reduction in K D with salinity would be effected under more acidic (pH < 7·6) conditions. An increase in K D 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 K Ds 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 K Ds 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 K Ds into a sorption model.