WHICH AQUEOUS SPECIES CONTROL THE RATES OF TRACE-METAL UPTAKE BY AQUATIC BIOTA - OBSERVATIONS AND PREDICTIONS OF NONEQUILIBRIUM EFFECTS

Over the past 2 decades, great progress has been made in understanding the relationship of metal speciation and biotic metal uptake in aquat ic systems. Most work in this area has adopted the 'free ion model' (F IM) to express the dependence of metal uptake rates on medium chemistr y. Coupled with recent advances in analytical chemistry, this approach has led to successful predictions of metal uptake by aquatic organism s 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 an d bound to cell surface transporter sites, has been proved incorrect. In order to take these exceptions into account, this paper explores th e implications of reaction kinetics and diffusion for the uptake of me tals. In particular, it shows how the apparent aqueous metal species c ontrolling the rates of metal uptake will depend on whether any of the se two rate-limiting factors become important. Guidelines are suggeste d for predicting when they may be important factors. (C) 1998 Elsevier Science B.V. All rights reserved.