Modelling the Distributions of Dissolved Zn and Ni in the Tamar Estuary Using Hydrodynamics Coupled with Chemical Kinetics

Abstract

Suspended particulate matter (SPM) from the Tamar Estuary was separated into permanent SPM (PSPM) and temporary SPM (TSPM) by settling experiments. Both particulate fractions were chemically characterized and used to study the kinetics of adsorption and desorption of Zn and Ni. The results showed that the adsorption of Zn and Ni at salinity=0 was biphasic, with an initial rapid step, completed in <5min, followed by a slower progression to equilibrium. The PSPM adsorbed relatively more Zn and Ni than the TSPM, which had a smaller specific surface area and lower acetic acid-leachable Fe and Mn content. Labile Zn and Ni desorbed biphasically, when the metal-laden particles were suspended in seawater (salinity=30) and a greater amount of metal was desorbed from TSPM than PSPM. The sorption mechanisms were described by reversible, first order reactions, from which reaction constants were estimated for the slow step. The response time for the adsorption of dissolved Zn during the slow step (i.e. time to achieve 63% of the new equilibrium) was 3–5h, whereas during desorption of Zn it was approximately 25h. For dissolved Ni, the response time for the slow step was about 13h and in the range 33–50h for the desorption reaction. Sorption reaction constants for Zn and Ni were incorporated into the estuarine contaminant simulator (ECoS) and used to predict dissolved Zn and Ni distributions in the Tamar Estuary. The predicted trends in dissolved metal concentrations were in general agreement with published field observations.