Which aqueous species control the rates of trace metal uptake by aquatic biota? Observations and predictions of non-equilibrium effects

Over the past 2 decades, great progress has been made in understanding the relationship of metal speciation and biotic metal uptake in aquatic systems. Most work in this area has adopted the `free ion modelʹ (FIM) to express the dependence of metal uptake rates on medium chemistry. Coupled with recent advances in analytical chemistry, this approach has led to successful predictions of metal uptake by aquatic organisms in natural systems. There are some cases that have proved difficult to reconcile with the FIM and one case in which the central assumption of the FIM, that pre-equilibrium exists between metals in solution and bound to cell surface transporter sites, has been proved incorrect. In order to take these exceptions into account, this paper explores the implications of reaction kinetics and diffusion for the uptake of metals. In particular, it shows how the apparent aqueous metal species controlling the rates of metal uptake will depend on whether any of these two rate-limiting factors become important. Guidelines are suggested for predicting when they may be important factors.