Speciation and lability of zinc(II) in river waters

Dynamic voltammetric experiments on a number of European river water samples reveal the presence of labile zinc(II) complexes with stability constants in the range between 106.4 and 107 M-1. Theoretical reconstructio- n of the labilities on the basis of the elementary association/ dissociatio- n rate constants confirms the experimental findings. Taking into account (i) the differences in diffusion coefficients between free metal ions and metal complexes, and (ii) the lability of the complexes, the distributi- on of zinc(II) over free metal and complexed species is found to be roughly 30% and 70%, respectively. Consequently the operational bioavailabi- lity of zinc(II), i.e., the flux of zinc(II) into an organism, is reduced to 30% if the actual interfacial uptake process is rate-determining. The reduction amounts to 50% if the diffusional supply of zinc(II) to the biointerphase is rate-determining.