Equilibrium Models in Seawater: Applications and Limitations

Abstract

A knowledge of chemical species is fundamental to understanding the behavior of substances in environmental processes. Thermodynamic data provide a basis for predicting the equilibrium state of a chemical system. There are three basic steps in formulating an equilibrium model for trace constituents in seawater. The composition of the medium with respect to the major ions must be specified on a conventional concentration scale such as molality. The interactions among the major ions must be considered in order to determine the free-ligand concentrations or activities which are available to interact with the trace constituents. Finally, the complexation of a trace element by the ligands in seawater can be calculated. This procedure is illustrated using an ion-pair model for the major ions and considering copper, iron(II), iron(III), cadmium, and lead as examples of trace elements. Equilibrium models have been particularly successful in accounting for the chemical behavior of many solution-phase reactions such as ionic interactions, weak acid dissociation, and some aspects of trace-metal complexation. Caution must be used in applying this approach to redox reactions and heterogeneous phase reactions because kinetic factors often limit the attainment of equilibrium. Biological systems are inherently far from equilibrium. Nevertheless, considerations of the equilibrium state can be useful even in systems that are not in equilibrium; departures from equilibrium indicate the important role of kinetic factors. Substantial advances have been made in recent years in obtaining a suitable base of thermodynamic data for marine systems, but limitations exist because data are lacking for many trace-element reactions involving some of the important oxyanions in seawater. The existing equilibrium data base is inadequate to prediet the effects of temperature and pressure on most reactions in the marine environment. A variety of analytical methods are available to test the predictions of equilibrium models.