Chromium-Binding Ability of Tannin in Water-Extracts from Withered Oak Leaves

Reaction mixtures containing constant amounts of water-extract from withered oak leaves and various concentrations of potassium chromate were incubated, and each reaction mixture was fractionated by a Sephadex G-25 column. The concentrations of the Cr-tannin obtained in the fractions were determined. Then the Cr-binding ability of the tannin was examined by apparent stability constants and Cr-complexing capacities, following construction of a Scatchard plot which adopts a two-site model. The tannin was shown to have two-classes of Cr-binding sites. Based on the Cr-complexing capacities, 1 mg tannin was shown to bind about 0.5 mg of reduced Cr. Moreover, functional groups contributing to the complexation with Cr were investigated by FTIR and pH titration. The relationship between the Cr-complexing ligans in the two sites and the functional groups in the tannin was also investigated. Finally, Cr ion species which emerged during the reaction between chromate and the tannin were determined by EPR. The spectra showed a sharp signal attributable to Cr(V) and a broad one assignable to Cr(III). Thus, it can be speculated that the tannin most probably forms a final Cr(III)–tannin via Cr(V) species.