Metal speciation dynamics and bioavailability: Inert and labile complexes

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 considers the simultaneous effects of bioconversion kinetics and metal transport in the medium coupled with metal complex dissociation kinetics. For the two kinetically limiting situations of inert and fully labile complexes, the bioavailabilities of bioinactive metal complexes are analyzed under conditions where (i) the actual biouptake follows a Michaelis-Menten type of steady-state flux and (ii) the supply of free metal is governed by diffusion of tree metal or coupled diffusion of the different labile metal species. The resulting steady-state fluxes are given in terms of two fundamental quantities, i.e., the relative bioaffinity parameter (a) and the ratio between the limiting uptake flux and the limiting transport flux (b). For labile complexes, these variables are differentiated by a complexation parameter defined by the ratio between the free metal ion activity and the total labile metal activity. Limits of the uptake flux for extreme values of the bioaffinity parameter a and the limiting flux ratio b are easily derived from the general flux expression. The analysis precisely 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.