Effect of the Interaction of Tannins with Coexisting Substances. II. Reduction of Heavy Metal Ions and Solubilization of Precipitates

Upon examination of the precipitate-formation in the solution of geraniin, punicalin or tannic acid JP, mixed with that of Cd2+, Cr6+, Cu2+, Fe3+, Hg2+, Mn2+, Pb2+, or Zn2+, at pH 5.4, the amount of precipitate generally increased with an increase in concentration of tannin, in the experiments with lower tannin-concentration. However, the amount of precipitate decreased with an increase in concentration of punicalin or tannic acid JP, of 1×10-3-5×10-3M, in several experiments. The precipitates once formed in these experiments were also solubilized upon further increase of the tannin concentration. Simultaneously, the amount of heavy metal in the supernatant liquor, and also the ratio of tannin to heavy metal in the precipitate increased. Extensive reduction of Cr6+, Fe3+ and Cu2+, along with the complex formation, occurred in the presence of tannins (geraniin, punicalagin and tannic acid JP) and (-)-epigallocatechin gallate. Each metal ion was determined by colorimetry, and the formation of soluble complex was exhibited by paper electrophoresis. These results indicate that the toxicity of metal ions could be reduced in the presence of tannins and polyphenols.