Speciation and lability of zinc(II) in river waters

Dynamic voltammetric experiments on a number of European river water sample s reveal the presence of labile zinc(ll) complexes with stability constants in the range between 10(6.4) and 10(7) M-1. Theoretical reconstruction of the labilities on the basis of the elementary association/dissociation rate constants confirms the experimental findings. Taking into account (i) the differences in diffusion coefficients between free metal ions and metal com plexes, and (ii) the lability of the complexes, the distribution of zinc(II ) over free metal and complexed species is found to be roughly 30% and 70%, respectively. Consequently the operational bioavailability of zinc(ll),i.e ., the flux of zinc(II) into an organism, is reduced to 30% if the actual i nterfacial uptake process is rate-determining. The reduction amounts to 50% if the diffusional supply of zinc(ll) to the biointerphase is rate-determi ning.