Modelling Estuarine Chemical Dynamics of Trace Metals

Yang Ping Liu

September 1996

Department of Environmental Sciences, University of Plymouth

in collaboration with

Plymouth Marine Laboratory, Plymouth, U.K.

Surveys of dissolved and particulate trace metals and of estuarine master parameters have been undertaken in the Tamar Estuary and Restronguet Creek, in order to understand the transport and fate of trace metals in estuaries. In the Tamar Estuary, the concentrations of dissolved Zn and Ni were in the range 5-30 and 1.5-6.4 mg l-1 respectively; they had a minimum in the fresh water of the turbidity maximum zone, with increased concentrations on passing the saline water interface. The extent of removal and release of dissolved Zn and Ni was attributed to sorption reactions. In addition, the observed partition coefficients for Zn (10 - 100 x 103 l kg-1) and Ni (1 - 32 x 103 l kg-1) behaved differently from predictions using a laboratory derived equation (Bale, 1987), suggesting that desorption from particulate phase due to salinity could play a key role in the addition of dissolved Zn and Ni in the Tamar Estuary. In the River Carnon, a linear relationship between the apparent equilibrium constant and pH has been observed. This function could be incorporated an estuarine chemical modelling frame work, resulting in simulation and prediction of trace metal behaviour in the River Carnon and Restronguet Creek system.

Chemically characterised Tamar and Carnon particles were used to study the kinetics of adsorption and desorption of Zn and Ni under controlled conditions. The effect of SPM concentration (60 - 400 mg l-1) and salinity (0 - 35) on the uptake of Zn and Ni, the reversibility of the adsorption process and the effect on the desorption of Zn and Ni of individual cations Ca2+, Mg2+ and anions Cl- and SO42- were examined. Rate constants for Zn and Ni sorption were calculated by separating the reaction into multisteps, with an instantaneous process, followed by a kinetically controlled migrating process. The calculated rate constants (h-1) for Zn (k1 0.15 - 0.60, k-1 0.06 - 0.20) and Ni (k1 0.03 - 0.34, k-1 0.04 - 0.26) allow the time scale of chemical reactions to be compared with physical residence time of the water.

A chemical modelling frame work has been built including adsorption and desorption kinetics. The comparisons of simulated concentrations of dissolved Zn, between using equilibrium partitioning and kinetic sorption processes, demonstrated that kinetic adsorption has potential in scavenging trace metals, whilst kinetic desorption attributed to the observed maximum of dissolved metal and that particulate type is an important factor in controlling the extent of trace metal sorption.

This work has demonstrated a strong coupling between the physical and chemical timescales. The good agreement between the simulated and observed concentrations of dissolved Zn and Ni suggested that kinetic approach is of value in the refinement of modelling of estuarine trace metal chemical processes with a dynamic model.

© 1996 by Yang Ping Liu. All Rights Reserved

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