Precipitation of Metal Ions by Plant Polyphenols:  Optimal Conditions and Origin of Precipitation

Precipitation of copper(II) and zinc(II) by low molecular weight phenols, penta-O-galloyl-β-d-glucose, and commercial tannins is studied at pH 5. The extent of copper precipitation, higher than that of zinc, depends on the initial concentration of both copper and phenolic compounds and on the control of acidification which results from complexation. The copper/phenol ratio in the precipitate, determined by elemental analysis, is independent of the initial concentration of catechol. However, it increases with chestnut tannin when the metal concentration is increased to a value corresponding to complexation with all o-dihydroxyphenyl chelating groups in the molecule. Stability of the precipitates obtained with coppper(II) and various polyphenols in water, acetic acid, and ethylenediaminetetraacetic acid solutions is determined. Models for metal/polyphenol precipitation are proposed. Keywords: Polyphenols; tannins; metal ions; copper; zinc; complexation; precipitation