Heavy Metal Binding by Tannic Acid: A Voltammetric Study

The interactions between tannic acid (TA) and the heavy metal ions Cu(II), Pb(II), Cd(II), and Zn(II), have been studied by several voltammetric techniques as a model for the metal binding of tannin substances and also to evaluate the possible effects of such compounds on the voltammetric analysis of natural samples. The study has shown quite a different behavior of the systems considered: Cd(II) is practically not complexed by TA, whereas Pb(II) and Zn(II) produce labile complexes and Cu(II) seems to form an inert complex with TA. In the cases of the Zn(II)+TA and Pb(II)+TA systems, it was possible to give a rough estimation of the stability constants and the relative diffusion coefficients of the complexes. From these values, it appears that the affinity for TA increases in the order Cd(II)<Zn(II)<Pb(II). In contrast, the possible stabilization of Cu(I) species during the voltammetric measurement hindered a quantitative study of the complexation of Cu(II) by TA.