Metal speciation dynamics and bioavailability. 2. Radial diffusion effects in the microorganism range

The free ion activity model for the biouptake of metals from complex media is limited to cases where mass transfer is not flux determining. This paper follows a previous paper (Van Leeuwen, H. P. Environ. Sci. Technol. 1999, 33, 3743) where speciation dynamics and bioavailability of metals are analyzed in terms of bioconversion kinetics and simultaneous metal transport in the medium coupled with dissociation kinetics. Such analysis shows under what conditions labile complex species contribute to the biouptake process or, equivalently, under what conditions the free ion activity model is not obeyed. The present work addresses the theoretical extension of the expressions for the metal flux in the medium by a radial diffusion term so that these are also applicable in the microorganism size range. The transition from macroscopic to microscopic surfaces affects not only the nature of the flux but also the extent of lability of complex species (Van Leeuwen, H. P.; Pinheiro, J. P. J. Electroanal. Chem. 1999, 471, 55), and this can have a dramatic influence on the rate of biouptake of metal ions. Labilities of metal complexes and the ensuing limiting metal fluxes are therefore systematically analyzed for various dimensions of the uptaking surface. Different conditions of bioaffinities and bioconversion capacities are considered, and a number of examples of metal complexes with specified kinetic features are discussed.